Systems and processes for the recovery of .SUP.226 .Ra from phosphogypsum

Abstract

Methods of processing phosphogypsum (PG) to recover Radium (e.g., .sup.226Ra), and/or other constituents from PG, are described. PG (stockpiled PG, fresh PG or a combination) is combined with a leach solution, allowed to react for 2-6 hours (e.g., a single leaching step), at a temperature in the range of about 40-70 C. to obtain leachate and leach residue. Further processing by subjecting the leachate and/or the leach residue to one or more separation techniques, such as ion exchange, enables the recovery of one or more constituents of interest. By separating .sup.226Ra, rare earth elements (REE) and/or other constituents from this secondary resource (e.g., waste PG), gypsum can be purified for use in the construction industry, the recovery of .sup.226Ra can be used to produce dedicated isotopes like .sup.223Ra and/or .sup.225Ac for life-saving cancer medication, and raw materials can be provided for the high-tech industry, agriculture and the building industry.

Claims

1. A method of processing phosphogypsum (PG), the method comprising: combining phosphogypsum (PG) with a leach solution comprising Na(NO.sub.3).sub.2 and/or Ca(NO.sub.3).sub.2; reacting the leach solution with the PG for a time and temperature sufficient to obtain i) leachate comprising an amount of radium and/or .sup.226Ra extracted from the PG and ii) solids; and separating the leachate from the solids.

2. The method of claim 1, comprising not repeating the reacting step, thereby performing only a single leaching step.

3. The method of claim 2, wherein: the leachate comprises at least 70% of the radium and/or .sup.226Ra from the PG; or the solids comprise a non-detectable amount of radium and/or .sup.226Ra, or a concentration of no more than about 10 picocuries per gram (pCi/g) of radium and/or .sup.226Ra.

4. The method of claim 1, further comprising: separating an amount of radium and/or .sup.226Ra from the leachate by ion exchange.

5. The method of claim 4, wherein the separating by ion exchange is performed by contacting at least a portion of the leachate with an ion exchange resin.

6. The method of claim 1, further comprising: prior to the reacting, separating a fine fraction from the PG and performing the reacting with the fine fraction to obtain the leachate and the solids; wherein at least 75% of particles of the fine fraction are less than 25 m in diameter.

7. The method of claim 6, further comprising: separating an amount of radium and/or .sup.226Ra from the leachate by way of ion exchange.

8. The method of claim 7, wherein the separating by ion exchange is performed by contacting at least a portion of the leachate with an ion exchange resin.

9. The method of claim 6, further comprising: grinding particles of the fine fraction prior to or during the reacting and/or the separating.

10. The method of claim 9, further comprising subjecting at least a portion of the leachate and/or at least a portion of the solids to one or more separation techniques.

11. The method of claim 10, wherein one or more of the separation techniques is chosen from separation by precipitation, ion exchange, solvent extraction, or membrane technologies.

12. The method of claim 10, wherein: one or more of the separation techniques is capable of separating one or more of the following constituents from the leachate and/or the solids: one or more alkaline earth metals; technologically enhanced naturally occurring radioactive material (TENORM); gypsum; and/or heavy metals.

13. The method of claim 10, wherein one or more of the separation techniques comprises: precipitation and/or ion exchange; and/or precipitation with hydroxide; and/or precipitation of Ca(OH).sub.2; and/or precipitation with sulfate; and/or precipitation of (Ba,Ra) SO.sub.4.

14. The method of claim 13, wherein one or more of the separation techniques comprises: reducing Ca content of the leachate to provide a Ca-reduced leachate; and separating an amount of radium and/or .sup.226Ra from the Ca-reduced leachate by way of selective precipitation, ion exchange, solvent extraction, and/or membrane technologies.

15. The method of claim 10, wherein one or more of the separation techniques comprises: ion exchange with acidic gel type cation exchange resin; or ion exchange with one or more cation exchange resin; wherein the cation is H.sup.+; and/or wherein the cation exchange resin comprises sulfonic acid functional groups.

16. The method of claim 10, further comprising: subjecting the leachate to ion exchange by contacting at least a portion of the leachate with an ion exchange resin and separating an amount of radium and/or .sup.226Ra from the leachate to provide a recyclable leach solution; recycling and combining the recyclable leach solution with a second batch of PG comprising radium and/or .sup.226Ra; allowing the recycled leach solution to react with the second batch of PG to obtain a second batch of leachate and a second batch of solids; separating the second batch of leachate from the second batch of solids; subjecting the second batch of leachate to ion exchange by contacting at least a portion of the second batch of leachate with an ion exchange resin; and separating an amount of radium and/or .sup.226Ra from the second batch of leachate.

17. The method of claim 10, further comprising: processing at least a portion of the solids into NORM-free gypsum having a non-detectable amount, or a concentration of no more than about 10 picocuries per gram (pCi/g) of radium and/or .sup.226Ra.

18. The method of claim 1, wherein the leach solution comprises Na(NO.sub.3).sub.2.

19. The method of claim 1, wherein the leach solution comprises Ca(NO.sub.3).sub.2.

20. The method of claim 19, wherein: the time the leach solution is reacted with the material PG is for about 2-6 hours; and the temperature is in the range of about 40-70 C.

21. The method of claim 1, wherein the leach solution comprises about 10-30% Ca(NO.sub.3).sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings illustrate certain aspects of embodiments of the present invention, and should not be used to limit the invention. Together with the written description, the drawings serve to explain certain principles of the invention.

(2) FIG. 1 is a block diagram of a representative leaching method according to embodiments of the invention (in combination with an exemplary recovery method).

(3) FIG. 2A is a graph showing the particle size distribution (PSD) of PG of a sample.

(4) FIG. 2B is a graph showing the mineral size distribution (MSD) of Ba phase particles of the PG sample of FIG. 2A.

(5) FIG. 3 is a graph showing evaluation of .sup.226Ra leachability based on comparison of the activity of .sup.226Ra in the feed sample PG-mix with that of the leach residue (Table 11) of the corresponding BRT test 1-11.

(6) FIG. 4A is a graph showing leaching efficiencies .sup.PLS of the BRTs with neutral salt lixiviants.

(7) FIG. 4B is a graph showing leaching efficiencies .sup.PLS of the BRTs with caustic and acidic lixiviants.

(8) FIG. 5A is a graph showing .sup.226Ra leaching efficiency by Ca(NO.sub.3).sub.2 concentration.

(9) FIG. 5B is a graph showing .sup.226Ra leaching efficiency by temperature.

(10) FIG. 6 is a graph showing .sup.226Ra leaching efficiency as a function of nitrate concentration.

(11) FIG. 7A is a graph showing leaching efficiencies .sup.PLS of the BRTs with increasing Ca(NO.sub.3).sub.2 concentration.

(12) FIG. 7B is a graph showing leaching efficiencies .sup.PLS of the BRTs with different pulp densities.

(13) FIG. 8A is a graph showing leaching efficiencies PLS based on temperature.

(14) FIG. 8B is a graph of leaching efficiencies .sup.PLS based on HNO.sub.3 concentration.

(15) FIG. 9A is a graph of the leaching efficiency .sup.LR of fresh PG compared to PG mix.

(16) FIG. 9B is a graph of leaching efficiency .sup.PLS of the BRTs using fresh PG.

(17) FIG. 10 is a graph showing .sup.226Ra-leaching trends with temperature.

(18) FIG. 11 is a graph showing leaching efficiencies PLS of the verification tests.

(19) FIG. 12A Loading of IX resin in % is shown in FIG. 12A and Ra load depending of PLS/resin ratio in 12B

(20) FIG. 12B Loading of IX resin in % is shown in FIG. 12A and Ra load depending of PLS/resin ratio in 12B

(21) FIG. 13 is a graph showing load of Ra per WSR depending on the ratio between PLS and WSR.

DETAILED DESCRIPTION OF VARIOUS

Embodiments of the Invention

(22) The inventors have discovered an inventive hydrometallurgical method for the processing of PG as by-product of the phosphoric acid production from phosphate rock to separate, extract and/or recover .sup.226Ra. The process is selective for separation of .sup.226Ra and as such provides for the capability of being able to utilize other valuable constituents of the PG, such as to produce NORM-free gypsum, phosphor as fertilizer, rare earth elements (REEs) and other critical raw materials (CRMs).

(23) Due to the large amount of PG available as waste of the fertilizer production from phosphate minerals, and the large amount of gypsum (as compared to the target elements) comprised in such waste, the inventors have discovered selective leach procedures that enable further use of the purified/processed gypsum (e.g., as construction material) by selectively targeting and separating .sup.226Ra and/or other radioactive materials or tracer elements and/or contaminants from PG. The inventive selective leaching procedures focus on separation of .sup.226Ra from PG, while minimizing the effect on, or separation of, other constituents of the PG. Use of the inventive selective leaching processes provide not only for i) a .sup.226Ra-PLS but also ii) a PG with reduced radioactive contamination, both of which can be used for downstream applications, such as .sup.226Ra for medical uses and purified PG for building products.

(24) Selective Leaching (see, e.g., EXAMPLE 1). Embodiments of the invention are directed to methods of selective leaching of .sup.226Ra from PG, such as from PG tailings, stockpiled PG or fresh PG. One such embodiment is a selective leaching process comprising one or more of the following parameters: use of Ca(NO.sub.3).sub.2 as lixiviant, e.g., at a concentration of 20%, a pulp density of 10%, a temperature of between room temperature and 70 C., and a retention/residence time in the range of 2-24 hours.

(25) FIG. 1 shows a process block diagram of a representative leaching method according to embodiments of the invention (in combination with an exemplary recovery method).

(26) Although a particular combination of inventive methods for leaching, separating, recovering/extracting .sup.226Ra from PG is shown in FIG. 1, it should be noted that the leaching and/or recovery methods can be substituted and/or modified according to the disclosure herein. For example, other lixiviants can be used in place of the exemplified lixiviant(s) and other process parameters can be changed according to the guidance provided by this disclosure. One of ordinary skill in the art would further understand that any of the recovery/extraction methods described herein and/or variations thereof may be used in combination with any of the leaching methods described herein and/or variations thereof.

(27) Further, for example, although process steps may be included in the exemplified process, one or more process steps can be omitted. For example, any one or more of the steps relating to benefaction, leaching, filtration, purification, extraction, .sup.226Ra production and/or recycling can be performed in any combination and/or in any order. For example, a purification step is optional and if performed can be performed before or after leaching. Further for example, leaching and extraction can be performed for the production of purified PG and .sup.226Ra production. Additionally or alternatively, additional steps can be included, such as by including a physical separation step for focusing on a particular size fraction of the PG. Additionally, the methods can be performed on other feed material alternatively to PG and/or can be performed to separate and/or recover other elements of interest from any material of interest. For example, the methods can be applied to any material for selective separation of Barium or Uranium from the material or for the separation of any one or more REEs, CRMs and/or heavy metals.

(28) Specific embodiments include for example selective leaching of PG and extraction of .sup.226Ra for .sup.226Ra production for various applications, including medical applications. Embodiments also include selective leaching of PG and production of purified PG, for example as a construction material. Embodiments also include leaching of PG (by any method) and extraction of .sup.226Ra for .sup.226Ra production and/or production of purified PG. Embodiments include selective leaching of PG, separation of .sup.226Ra from the leachate, recycling of reduced-.sup.226Ra raffinate for performing additional selective leaching of PG. Further embodiments include physically separating the PG to obtain a desired fines fraction and performing any of the methods described herein on the fines fraction. Additionally, the methods of the invention can be performed on stockpiled PG and/or fresh PG.

(29) Material characterization (using QEMScan) identified gypsum/anhydrite as the dominant mineral in the PG-mix sample demonstrated in the Examples. Other mineral phases identified were quartz, barite, fluorite, silicates, Ca-phosphates, pyrite, and Fe/Ti-oxides. No discrete .sup.226Ra or U phases were found. Since the barium content of the sample is relatively high compared to the literature and .sup.226Ra and barium have similar solution chemistry, it is very likely that co-precipitation of .sup.226Ra with barite, mainly by way of an inclusion (lattice replacement) process, is the main mechanism controlling the radium content in the PG samples. Additionally, .sup.226Ra leaching is almost exclusively accompanied by leaching of other alkaline earth metals, with Ca, Ba, and Sr being the main impurities, and a correlation between .sup.226Ra and Ba leaching was found, which supports the that .sup.226Ra is bound within (Ba,Ra) SO.sub.4.

(30) Additionally, a majority of PG particles have a size of between about 10 and 50 m. Since barite has very small grains with 83%<10 m compared to most of the particles, and due to co-precipitation of .sup.226Ra with barite, it is expected that .sup.226Ra enrichment will occur in the fine fraction. In addition, the mineral phase liberation analysis revealed that most of the barite grains are either completely liberated or bound to the surface of gypsum or quartz particles, thus, according to embodiments of the invention low-intensity attrition by sieving can be used to liberate the remaining barite grains and produce radium enrichment through barite accumulation in the fine fraction of a screening.

(31) Fresh PG (see, e.g., EXAMPLE 2). Leaching of fresh PG was tested with an exemplary Ca(NO.sub.3).sub.2 lixiviant system to compare it with the results from stockpiled PG-mix. The .sup.226Ra leaching efficiency of fresh PG was in the range of 56% for all tests and appeared to be independent of the lixiviant tested. For a 2-stage leach regime (water and Ca(NO.sub.3).sub.2), a slight increase in .sup.226Ra leaching to 67% was observed in the second stage. Thus, in fresh PG, it is believed that .sup.226Ra is mainly present in a water-soluble form and that the additional leaching with Ca(NO.sub.3).sub.2 led to a leaching of the sulfate-bound .sup.226Ra. Any one or more of the selective leaching methods of embodiments of the invention can be performed on stockpiled PG and/or on fresh PG.

(32) Upscaled Leaching Process (see, e.g., EXAMPLE 3). Verification tests were used to verify that the leaching conditions are applicable on a larger scale and to produce a larger volume of PLS for testing subsequent process steps. Additionally, the positive effect of increased temperature to increase the .sup.226Ra leaching efficiency was validated, and it was found that the retention time could be reduced from 24 hours to 4 hours with the same or even better leaching efficiency. In addition, the tests were evaluated by assaying the PLS and the leach residue. A good mass balance and accountability of the .sup.226Ra could be proven within the measurement accuracy of the -spectroscopy. In total, 50 L of PLS were produced and the PLS' of 4 tests were combined for the .sup.226Ra recovery work, with the mixed PLS having a .sup.226Ra concentration of 36689 Bq/L.

(33) Recovery of .sup.226Radium (see, e.g., EXAMPLE 4) and Other Elements. In addition and/or alternatively, embodiments of the invention include methods for the separation/recovery/extraction of .sup.226Ra from a liquid, such as recovering .sup.226Ra from a pregnant leach solution (PLS) or leachate obtained from the selective leaching methods of the invention and/or other separation/leaching methods. Exemplary recovery methods for recovering/extracting .sup.226Ra include recovery/extraction methods based on or comprising ion exchange (IX), selective precipitation, solvent extraction (SX) and/or membrane technologies, but are not limited to these.

(34) .sup.226Ra separation was tested with the PLS obtained as a result of the leaching process from EXAMPLE 1. Preferred method of efficient and economically relevant .sup.226Ra separation is ion-exchange (IX). The specific characteristics like high salinity and a high Ca/Ra ratio were considered for the selection of suitable .sup.226Ra-separation technology. Thus, in some embodiments, selective precipitation and ion exchange (IX) are suitable. For example, in embodiments, .sup.226Ra can be extracted/separated/recovered from the PLS in high yields using ion exchange technology.

(35) Selective precipitation of (Ba,Ra) SO.sub.4 is demonstrated using two different precipitants. Tests with the addition of concentrated precipitants showed insufficient precipitation efficiencies, and the highest .sup.226Ra precipitation (23%) was achieved with concentrated H.sub.2SO.sub.4 (90%) in an under-stochiometric dosage (e.g., SP_3). The sulfate dosage of this test was 56% of the stoichiometric amount. The higher precipitation efficiency demonstrates that the low efficiencies are not controlled by the sulfate dosage but kinetics and equilibration. It was concluded that a further dilution and longer retention times are favorable for a more selective .sup.226Ra-precipitation. The precipitation of Ca(OH).sub.2 to reduce the Ca/Ra-ratio by NaOH addition showed a good selectivity at pH=12 for Ca. Only minor losses of .sup.226Ra, Ba, and Sr could be found and were mainly assigned to adherent solution and slight co-precipitation. However, this approach consumes a high amount of NaOH and replaces high Ca concentrations by Na. Selective precipitation can be used alone or in combination with other separation techniques, such as a purification step prior to or after IX.

(36) Ion exchange demonstrated the selectivity of the IX resin used for .sup.226Ra extraction. The resin LEWATIT MDS 200 H, commonly used for water treatment, was used. By increasing the volume of PLS per WSR (wet settled resin) an increasing selectivity for Ba.sup.2+ and Ra.sup.2+ was determined and a maximum resin loading of 3.7 nmol/L.sub.WSR Ra.sup.2+ (36309 Bq/L.sub.WSR) could be achieved. At the same time, the Ca loading was significantly reduced with increasing PLS volume. Thus, the resin is highly selective for Ra which makes it applicable for Ra separation.

(37) The loaded IX resin is eluted in a chromatographic manner to enhance the purity of the Ra eluate. Thereby liquid fractions containing Ca, Sr, and Ba are obtained as a waste stream. The purified and concentrated .sup.226Ra eluate can be further processed, for example by way of a .sup.226Ra production process. (Chiarizia 1999-Chiarizia, R.; Dietz, M. L.; Horwitz, E. P.; Burnett, W. C.; Cable, P. H. Radium Separation Through Complexation by Aqueous Crown Ethers and Ion Exchange or Solvent Extraction*. Separation Science and Technology 1999, 34 (6-7), 931-950. DOI: 10.1080/01496399908951074.) Thus, the elution step can be adapted to the preferred .sup.226Ra production process.

(38) In embodiments, the raffinate obtained as a result of the IX process is ideally free of .sup.226Ra, or substantially free of .sup.226Ra, and shows only low concentrations of Ba and Sr, making it possible to use (e.g., recycle) the raffinate solution as a lixiviant (or to combine the raffinate with fresh lixiviant) for subsequent .sup.226Ra leaching. Recycling is desirable for reducing chemical costs and the amount of waste from the process. The raffinate can be recycled to the leaching step and used with one or more second, subsequent and/or additional batches of PG (optionally used in combination with additional recycled or fresh lixiviant, wherein the original or first lixiviant (i.e., lixiviant used to leach the original or first batch of PG) and/or recycled lixiviant(s) and/or fresh lixiviants and/or second, subsequent and/or additional lixiviants can be the same or different lixiviants). When recycling raffinate as lixiviant (whether used alone or with other lixiviants), additional adjustment of the solution properties of the raffinate or raffinate+lixiviant might be needed.

(39) As noted above, in embodiments, an additional purification step can be performed to enhance the selectivity of the IX process for .sup.226Ra. This purification can be achieved by way of a hydroxide precipitation at pH of about 7, by the addition of Ca(OH).sub.2 or an alternative caustic reagent (e.g., NaOH, see above selective precipitation methods). Using purification and a subsequent filtration, a solid waste containing metal hydroxides (e.g., Fe and Al) is produced. The purified PLS is transferred to the IX process where .sup.226Ra is extracted. Additionally, the obtained raffinate is free of or substantially free of .sup.226Ra and can be recycled to the leaching step to be used with one or more second, subsequent and/or additional batches of PG (optionally used in combination with additional recycled or fresh lixiviant, wherein the original or first lixiviant (i.e., lixiviant used to leach the original or first batch of PG) and/or recycled lixiviant(s) and/or fresh lixiviants and/or second, subsequent and/or additional lixiviants can be the same or different lixiviants). When recycling raffinate as lixiviant (whether used alone or with other lixiviants), additional adjustment of the solution properties of the raffinate or raffinate+lixiviant might be needed.

(40) In addition or alternatively, embodiments of the invention include methods of purifying PG by removing one or more contaminants/elements, such as heavy metals, REEs, CRMs and/or radioactive elements or tracer elements from PG, such as fresh PG, waste PG or stockpiled PG. The separation/removal of one or more constituents (selective or non-selective for a particular constituent) of PG can be performed simultaneously or sequentially. For example, selective leaching of .sup.226Ra from PG can be performed, followed by a separation method used to separate any one or more of the remaining constituents of the leach residue, such as the separation of Cadmium. For example, selective leaching of .sup.226Ra can be performed on PG, followed by separation of Cadmium from the leach residue for providing gypsum with a reduced level or non-detectable level of Cadmium.

(41) In embodiments, one or more of the separation techniques is capable of separating one or more of the following constituents from any of the batch of PLS; the first, second, additional, or subsequent batch of PLS; and/or the solids (leach residue), the first, second, additional, or subsequent batch of solids (leach residue): i) one or more alkaline earth metals, including for example radium, calcium, barium; ii) one or more of thorium, uranium, fluorine, phosphorus, phosphate; iii) naturally occurring radioactive material (NORM); iv) technologically enhanced naturally occurring radioactive material (TENORM); v) gypsum; vi) heavy metals, such as Zn, Cr, Mn, Ni, Pb, Cd, As, Hg, Ag, Cu, Fe, Pd, Pt; vii) rare earth elements (REEs), such as: Sb, Be, Co, Ga, Ge, Mg, In; viii) Platinum Group Elements (PGMs), such as platinum, palladium, rhodium, ruthenium, iridium, osmium; ix) Nb and Ta; x) lanthanides (such as light REEs (LREEs), including La, Ce, Pr, Nd, Pm, Sm, and heavy rare earth elements (HREEs), including Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu); xi) Sc, Y (an HREE).

EXAMPLE 1 (Leaching of PG-Mix Sample)

(42) With respect to processes for leaching of .sup.226Ra from PG, ten lixiviants for the processing of stockpiled PG-mix are demonstrated in this EXAMPLE 1, with the highest .sup.226Ra leaching efficiencies obtained for Ca(NO.sub.3).sub.2 (15%), HNO.sub.3 (28%), oxalic acid (H.sub.2Ox) (19%), and ammonium acetate (NH.sub.4+Ac) (13%).

(43) Many .sup.226Ra salts are sparingly soluble. However, Ra(NO.sub.3).sub.2, Ra oxalate, RaCO.sub.3, and Ra(OH).sub.2 are more soluble than the corresponding calcium salts. The inventors have discovered that conversion of the existing .sup.226Ra compound into one of these salts can enable selective leaching of .sup.226Ra. For example, in embodiments, the methods can employ any one or more of Ca(NO.sub.3).sub.2, HNO.sub.3, oxalic acid (H.sub.2Ox) and/or ammonium acetate (NH.sub.4Ac) as lixiviant, alone or in combination with any other lixiviant, including water. In embodiments, the leaching processes can comprise one or more or all of the lixiviants disclosed herein for leaching of .sup.226Ra.

(44) Particular embodiments, for example, can comprise HNO.sub.3 and/or Ca(NO.sub.3).sub.2 as lixiviant(s) for selective leaching of .sup.226Ra from PG. In embodiments, the leach solution (lixiviant) comprises Ca(NO.sub.3).sub.2, such as 10-30% Ca(NO.sub.3).sub.2, or about 20-25% Ca(NO.sub.3).sub.2.

(45) Further of note is that particular temperatures, residence times and processing parameters for leaching and/or extraction can be selected according to the disclosures provided herein. For single, double or multi-step leaching, the leaching can be performed using a residence time (contact time of lixiviant and PG) in the range of up to about 24 hours, such as about 2-6 hours, such as about 2-5 hours, such as about 4 hours and/or at a temperature in the range of about room temperature (RT) to about 70 C., such as about 25-70 C., or about 30-55 C., or about 35-45 C., or about 35-50 C. Optionally, mechanical stirring of the PG and lixiviant can be employed so as to contribute a grinding effect, which may further enhance .sup.226Ra leachability.

(46) According to embodiments of the invention, optional processing techniques can be included, such as grinding of the PG feed material and/or separation of an .sup.226Ra-rich PG fraction by physical means (e.g., sieve) and performing the leaching on the fine fraction.

(47) Exemplary leach systems, for example, using Ca(NO.sub.3).sub.2 and/or HNO.sub.3 as lixiviant(s), are described in greater detail below. It will be understood by persons of ordinary skill in the art that the exemplary process conditions/parameters demonstrated in the examples can be modified according to the teachings elsewhere in this disclosure. For example, with respect to the lixiviants exemplified, any one or more lixiviants can be substituted for any other lixiviant described herein. Further, with respect to the temperature of the leaching and/or the residence time for leaching, any temperature or time can be substituted for any other temperature or time within the ranges exemplified. Additionally, any lixiviant or lixiviant combination can be used in a leaching process with any temperature or time disclosed herein.

(48) Abbreviations for the following terms used in this disclosure and/or used in the EXAMPLES in particular are defined as follows:

(49) TABLE-US-00003 TABLE 3 Definitions Acronym/ Abbreviation Term BRT Bottle roll tests CRM Critical raw materials IX Ion exchange LR Leach residue NORM Naturally occurring radioactive material NORM-free Free of (or within specified limits of, or a non-detectable amount of) Naturally Occurring Radioactive Material or Technically Enhanced Naturally Occurring Radioactive Material (TENORM), or comprising radioactive material at a level below a specified limit, e.g., a material having a radium concentration of no more than about 20 picocuries per gram (pCi/g), such as about 15 pCi/g, or about 10 pCi/g, such as about 7 pCi/g or about 5 picocuries per gram (pCi/g) or less) NH.sub.4Ac Ammonium acetate NH.sub.4CH.sub.3COO NORM Natural occurring radioactive material PG Phosphogypsum PLS Pregnant leach solution REE Rare earth elements SLR Solid to liquid ratio SX Solvent extraction WSR WSR

(50) Five sealed bags of air-dried PG of 4-5 kg (23.9 kg of stockpile sample material) and 1.1 kg fresh, moist PG material were obtained. The dried samples were light beige in color and fine-grained with a few hard, round agglomerates. The moist sample was brown, pulpy, and fine-grained. The five sample bags of the stockpiled PG material were combined and homogenized by placing them in a round drum and mixing for 1 h.

(51) Samples were analyzed by various analytical methods. Stockpiled PG sample material was homogenized, analyzed by XRF (X-ray fluorescence spectroscopy), -spectroscopy, QEMScan (Erzlabor), and ICP-MS/OES (Inductively coupled plasma mass spectrometry/optical emission spectroscopy) (Eurofins). The chemical composition was determined by XRF and ICP-MS/OES, while the QEMScan gave an understanding about mineralogical composition and associations. The radioactive nuclides were quantified by -spectroscopy.

(52) In Table 4, the evaluation of the feed samples is summarized. The .sup.226Ra activity of the PG-mix samples was determined as A.sub.s=1233.6257.1 Bq/kg.

(53) TABLE-US-00004 TABLE 4 Determination of the specific .sup.226Ra Activity of the feed samples E N(E) (E) () Sample in in tL in in m in A.sub.s in A.sub.s in ID keV N(E) % in s % % g Bq/kg Bq/kg PG-mix 186 23839 12.8 336763 2.71 0.41 57.46 1233.6 257.1 Fresh PG 186 54894 12.4 546083 2.71 0.41 70.82 1439.8 296.6

(54) For the experimental data sets, the activities of the specific samples are given in the Tables below (e.g., Tables 11, 12, 13, 20, 22).

(55) Sample characterization of stockpiled sample material using ICP-MS/OES and XRF analyses showed that the elemental data were well within the typical range of elemental composition for this type of material compared to literature data. The main elements of the sample are Ca and S. The main radium tracer element, uranium, was measured as being present in an amount of about 3-4%. The only outlier compared to the literature data was the barium content being 248 ppm, which indicates the presence of a Ba mineral such as barite.

(56) Elemental Analysis. XRF measurements (Thermo Scientific Niton FXL 950 Field X-Ray Lab) were performed. The uncalibrated XRF measurements (i.e., without calibration using ICP-MS) are listed in Tables 5 and 6. The ranges of XRF measurements were comparable to those reported in literature, except for higher Ti and Sc concentrations. The REE were measured separately by a handheld Niton XLp 522. More specifically, Table 5 shows the uncalibrated data of XRF measurement (middle) (LOD Limit of detection), compared with literature research (left), and ICP-MS/OES measurement (right).

(57) TABLE-US-00005 TABLE 5 Main element concentration of the homogenized PG sample Literature ICP-MS/ Element Data in % XRF in % OES in % CaO 24-34 27 31.8 SO.sub.4 48-58 33 51.0 SiO.sub.2 0.5-18 2.8 4.3 Na.sub.2O 0.12-10 n.d. <0.1 C (organic) 0.1-2.5 n.d. 0.1 F 0.1-1.8 n.d. 0.345 P.sub.2O.sub.5 0.05-8 0.08 0.75 Al.sub.2O.sub.3 0.05-0.6

(58) QEMScan analyses were performed (FEI Quanta 650F scanning electron microscope (SEM) equipped with two Bruker Quantax X-Flash 5030 energy dispersive X-ray spectrometers). QEMScan is also used to access the particle size distribution. The specific particle size of each grain is estimated by calculating the ECD (equal circle diameter), which represents the diameter of a circle that has the same area as the measured grain.

(59) Material characterization using QEMScan identified gypsum/anhydrite as the dominant mineral in the sample (PG-mix sample from stockpiled PG). Other mineral phases were quartz, barite, fluorite, silicates, Ca-phosphates, pyrite, and Fe/Ti-oxides. No discrete .sup.226Ra or uranium phases were found. Since the barium content of the sample is relatively high compared to the literature and .sup.226Ra and barium have similar solution chemistry, it is very likely that coprecipitation of .sup.226Ra with barite (BaSO.sub.4), mainly via an inclusion (lattice replacement) process, is the main mechanism controlling the .sup.226Ra content in the PG samples.

(60) The chemical composition (main elements) calculated from the QEMScan measurement is in good agreement with the XRF and ICP-MS/OES data (Table 6).

(61) TABLE-US-00006 TABLE 6 Main elements of the homogenized PG sample XRF ICP-MS/ Calculated from Element in % OES QEMScan CaO 27 31.8 32.8 SO.sub.4 33 51.0 55.8 SiO.sub.2 2.8 4.3 2.2 Na.sub.2O n.d. <0.1 0.0004 C (organic) n.d. 0.1 n.d. F n.d. 0.345 0.14 P.sub.2O.sub.5 0.08 0.75 0.07 Al.sub.2O.sub.3

(62) A back scattered electron (BSE) and QEMScan false color overview of the measured PG sample area was performed and showed the PG consists mainly of gypsum and/or anhydrite. Significant amounts of quartz and fluorite were observed, while no discrete .sup.226Ra or uranium phases were found. In addition to the high Ba contents from XRF and ICP-MS/OES, several Ba-bearing phases were detected. However, the most likely barium phase in this environment is barite.

(63) .sup.226Ra and barium have similar solution chemistry, and one of the main reasons for this is the similarity in effective ionic radii, which are equal to 1.42 for Ba.sup.2+ and 1.48 for Ra.sup.2+ (in 8-fold coordination) (Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides. Acta crystallographica section A: crystal physics, diffraction, theoretical and general crystallography 1976, 32 (5), 751-767), making the incorporation of .sup.226Ra in barite most probable for the PG. Predominantly, .sup.226Ra is rapidly dissolved in leaching and co-precipitates in the form of Ba(Ra) SO.sub.4. (Paige, C. R.; Kornicker, W. A.; Hileman, O. E.; Snodgrass, W. J. Solution Equilibria for Uranium Ore Processing: The BaSO4-H2SO4-H2O System and the RaSO4-H2SO4-H2O System. Geochimica et Cosmochimica Acta 1998, 62 (1), 15-23. DOI: 10.1016/S0016-7037 (97) 00320-7.) Therefore, co-precipitation of .sup.226Ra with barite (BaSO.sub.4), mainly via an inclusion (lattice replacement) process, is considered the main controlling mechanism for the .sup.226Ra content in PG. (Zhang, T.; Gregory, K.; Hammack, R. W.; Vidic, R. D. Co-Precipitation of Radium with Barium and Strontium Sulfate and Its Impact on the Fate of Radium during Treatment of Produced Water from Unconventional Gas Extraction. Environ. Sci. Technol. 2014, 48 (8), 4596-4603. DOI: 10.1021/es405168b.)

(64) Concentrations of accessory elements of the homogenized PG sample is as follows:

(65) TABLE-US-00007 TABLE 7 Accessory elements of the homogenized PG sample Element Literature (ppm) XRF (ppm) ICP-MS/OES (ppm) Ti 26-470 1227 90.7 Co 0.05-2.3 7.5

(66) The concentration of REEs is shown in Table 8, with the concentrations for Sc, Sm, Eu, Ho, Er, Tm, Yb, Lu, and Th not listed as the values were below the detection limit:

(67) TABLE-US-00008 ID La ppm Ce ppm Pr ppm Nd ppm Gd ppm Dy ppm PG 31.5 23.0 5.6 21.0 5.5 5.3

(68) The comparison of the literature data with the ICP-MS/OES and XRF analyses shows that the element concentrations of the samples obtained fit well within the typical elemental composition range of these types of materials. The main elements of the sample are Ca and S, which form the main sample minerals gypsum and anhydrite. All other elements are present only in traces, in agreement with the literature. The main .sup.226Ra tracer element, U, was measured at 3-4%, which is in the lower range of the literature data. The Ba content of 248 ppm indicates the presence of a Ba mineral such as barite. Na.sub.2O, C (organic), and F could not be measured with XRF, but ICP-MS/OES data are comparable to the literature. Comparing the results of ICP-MS/OES and XRF, the values for U, CaO, SiO.sub.2, Cl, MgO, and Fe.sub.2O.sub.3 agree well. SO.sub.4, P.sub.2O.sub.5, Sr, and Ba were underestimated by XRF compared to ICP-MS/OES while high Ti and Sc contents from XRF were not confirmed by ICP-MS/OES (overestimation by XRF).

(69) The grouping of the detected material phases led to the following main phases within the PG (Table 9): Gypsum/anhydrite, quartz, barite, fluorite, silicates, Ca-phosphates, pyrite, Fe/Ti-oxides, and others.

(70) TABLE-US-00009 TABLE 9 PG composition in wt % and average size of each phase Composition Average Mineral in wt % size in m Gypsum/Anhydrite 97.23 28 Quartz 2.25 34 Fluorite 0.36 14 Barite 0.07 8 Ca-Phosphate 0.04 16 Fe/Ti-oxides 0.02 19 Silicates 0.02 25 Pyrite <0.01 8 Others <0.01 16 Gypsum 87.07 21 Anhydrite 10.16 8

(71) The particle size distribution determined by QEMScan is dominated by the size of the anhydrite- and gypsum-bearing particles, since they are the main constituents of the sample. FIG. 2A shows the particle size distribution (PSD) of PG and FIG. 2B shows the mineral size distribution (MSD) of Ba phase particles of the PG sample.

(72) The majority of PG particles in the sample have a size of between 10-50 m. By applying mechanical sieving, the percentage of particles of this size can be controlled in specific samples, such as by filtering out larger particles to increase the percentage of particles of this size in the sample. Additionally or alternatively, mechanical sieving can be performed to isolate any particular fraction of the PG, such as particles sized less than about 50 m, or less than about 30 m, or less than about 25 m, or less than about 20 m, or in the range of from about 10-20 m, or less than about 10 m, and so on.

(73) Of particular note, the QEMScan revealed that Barite, for example, has very small grains, 83%<10 m, compared to the majority of particles. If radium occurs primarily in barite, it is plausible to assume that .sup.226Ra is enriched in the fine fraction of below 25 m.

(74) Additionally, the QEMScan revealed that with respect to the association of barite particles to anhydrite/gypsum, most of the barite grains are either completely liberated or bound to the surface of gypsum or quartz particles. Consequently, the barite is not intergrown with most particles and is can be easily liberated with low-intensity attrition.

(75) Bottle Roll Tests (BRTs). Various lixiviants can be used according to the leaching methods described herein, including one or more of water, sodium chloride solutions, solutions comprising sodium chloride and barium chloride, calcium nitrate, ammonium acetate (NH.sub.4CH.sub.3COO), lime water, sodium hydroxide, sulfuric acid, nitric acid, solutions comprising calcium nitrate and/or nitric acid, and/or oxalic acid.

(76) In embodiments, the leach solution (lixiviant) comprises one or more of: i) H.sub.2O (water, de-ionized water, tap water, rain water, etc.); ii) NaCl (e.g., 1-20%, such as 5%, 10% or 15% NaCl, or any range in between); iii) a solution of NaCl+BaCl.sub.2 (such as 10% NaCl+0.1% BaCl.sub.2, or 20% NaCl+0.2% BaCl.sub.2, or 1-20% NaCl+0.05-0.5% BaCl.sub.2, or any range in between); iv) Ca(NO.sub.3).sub.2 (such as 2-40% Ca(NO.sub.3).sub.2, such as 5%, 10%, 15%, 20%, 25%, 30%, or 35% Ca(NO.sub.3).sub.2, or any range in between); v) NH.sub.4Ac (such as 2-30% NH.sub.4Ac, such as 5%, 10%, 15%, 20%, or 25% NH.sub.4Ac, or any range in between); vi) lime water (e.g., saturated lime water); vii) NaOH (such as 5-30% NaOH, such as 5%, 10%, 15%, 20%, or 25% NaOH, or any range in between); viii) H.sub.2SO.sub.4 (such as 2-20% H.sub.2SO.sub.4, such as 5%, 10%, or 15% H.sub.2SO.sub.4, or any range in between); ix) HNO.sub.3 (such as 2-20% HNO.sub.3, such as 3%, 4%, 5%, 10%, or 15% HNO.sub.3, or any range in between); x) H.sub.2Ox (such as 2-20% H.sub.2Ox, such as 5%, 10%, or 15% H.sub.2Ox, or any range in between); xi) a solution of Ca(NO.sub.3).sub.2+HNO.sub.3 (such as 5% Ca(NO.sub.3).sub.2+3% HNO.sub.3, such as 10% Ca(NO.sub.3).sub.2+5% HNO.sub.3, such as 15% Ca(NO.sub.3).sub.2+2-10% HNO.sub.3, such as 5% Ca(NO.sub.3).sub.2+5-15% HNO.sub.3, such as 10% Ca(NO.sub.3).sub.2+2-10% HNO.sub.3, or any range in between); one or more nitrate salts, such as NaNO.sub.3; xii) or any combination of any one or more of these.

(77) In the work demonstrated herein, the highest .sup.226Ra leaching efficiencies were found for calcium nitrate (Ca(NO.sub.3).sub.2) (15%), nitric acid (HNO.sub.3) (28%), oxalic acid (H.sub.2Ox) (19%), and ammonium acetate (NH.sub.4Ac) (13%). Variations of the leaching system using Ca(NO.sub.3).sub.2 are further exemplified to demonstrate .sup.226Ra leaching efficiency of this system in particular. The inventors have found that higher nitrate concentrations and elevated temperatures lead to an increased efficiency in leaching of .sup.226Ra. Leaching systems according to embodiments of the invention, for example, can be performed at a temperature of between RT70 C. with a lixiviant comprising calcium nitrate (Ca(NO.sub.3).sub.2), such as at a concentration of about 20-25% and a pulp density of about 10%, with or without stirring and/or grinding during all or a portion of the reaction. .sup.226Ra leaching efficiencies in the range of up to about 73% and up to about 79% were found for these conditions.

(78) Several samples were processed according to the methods described herein and were based on 10.sup.2 g scale (100-200 g per batch). The bottle roll tests (BRTs) were conducted as bottle roll tests under ambient conditions using the following as leach solutions: Water (duplicate), NaCl, Ca(NO.sub.3).sub.2, diluted H.sub.2SO.sub.4 (<0.5 mol/L), diluted HNO.sub.3, oxalic acid (H.sub.2Ox), diluted NaOH, and ammonium acetate (NH.sub.4Ac). Other methods of leaching can alternatively or in addition be employed, including heap leaching or bio leaching.

(79) Leaching methods of embodiments of the invention optionally can include one or more of the following in addition to or as substitution of one or more steps of a leaching processes disclosed herein: i) pretreatment of PG with hot water or NaOH; ii) carbonation of PG by hydrothermal Na.sub.2CO.sub.3 leaching (Matyskin, A. V.; Ebin, B.; Tyumentsev, M.; Allard, S.; Skarnemark, G.; Ramebck, H.; Ekberg, C. Disassembly of Old Radium Sources and Conversion of Radium Sulfate into Radium Carbonate for Subsequent Dissolution in Acid. J Radioanal Nucl Chem 2016, 310 (2), 589-595. DOI: 10.1007/s10967-016-4927-x) and subsequent dissolution with acid if appropriate; iii) carbonation of PG with NH.sub.3-water and CO.sub.2 (Mukaba, J.-L.; Eze, C. P.; Pereao, O.; Petrik, L. F. Rare Earths' Recovery from Phosphogypsum: An Overview on Direct and Indirect Leaching Techniques. Minerals 2021, 11 (10), 1051. DOI: 10.3390/min11101051) and subsequent dissolution with acid if needed; and/or iv) direct leaching with mineral acids (Mukaba 2021).

(80) The general experimental procedure for the bottle roll tests was as follows: setup of 1 L glass leach vessel, preparation of lixiviant with 800 g water, addition of 200 g PG, monitoring of vessel for foaming and continue mixing at target temperature, after target retention time, filtration of suspension through 1-2 m filter paper, then drying the sample of leach residue at 60 C. to determine weight loss.

(81) The processes performed at ambient temperature were mixed in an overhead shaker, while the processes performed at elevated temperature were mixed by a magnetic stirrer stored in a heating cabinet to ensure constant conditions over time. Either mixing process can be used for any of the leaching methods disclosed herein, regardless of temperature.

(82) The bottle roll tests include examples of the leaching methods using various lixiviants, at various concentrations and/or with various combinations of lixiviants and show results of the leaching methods as applied to stockpiled PG and fresh PG. A summary of the bottle roll tests is provided in Table 10:

(83) TABLE-US-00010 TABLE 10 Parameters of BRT conducted with constant retention time (24 h) ID Ore Lixiviant Temperature BRT_1 Stockpiled PG H.sub.2O_1 RT BRT_2 Stockpiled PG H.sub.2O_2 RT BRT_3 Stockpiled PG 10% NaCl RT BRT_4 Stockpiled PG 10% NaCl + RT 0.1% BaCl.sub.2 BRT_5 Stockpiled PG 10% Ca(NO.sub.3).sub.2 RT BRT_6 Stockpiled PG 10% NH.sub.4Ac RT BRT_7 Stockpiled PG Lime Water RT (saturated) BRT_8 Stockpiled PG 5% NaOH RT BRT_9 Stockpiled PG 5% H.sub.2SO.sub.4 RT BRT_10 Stockpiled PG 5% HNO.sub.3 RT BRT_11 Stockpiled PG 5% H.sub.2Ox RT BRT_12 Stockpiled PG 5% Ca(NO.sub.3).sub.2 RT BRT_13 Stockpiled PG 20% Ca(NO.sub.3).sub.2 RT BRT_14 Stockpiled PG 5% Ca(NO.sub.3).sub.2 + RT 3% HNO.sub.3 BRT_15 Stockpiled PG 10% Ca(NO.sub.3).sub.2 RT BRT_16 Fresh PG H.sub.2O RT BRT_17 Fresh PG 10% Ca(NO.sub.3).sub.2 RT BRT_18 Fresh PG 5% HNO.sub.3 RT BRT_20 Stockpiled PG 20% Ca(NO.sub.3).sub.2 40 C. BRT_21 Stockpiled PG 20% Ca(NO.sub.3).sub.2 70 C. BRT_22 Stockpiled PG 25% Ca(NO.sub.3).sub.2 RT BRT_23 Fresh PG H.sub.2O RT BRT_23b LR 20% Ca(NO.sub.3).sub.2 RT DMT_BRT_23

(84) BRTs with different salt, acidic or caustic solutions (BRT_1 to BRT_11) were conducted in parallel to compare the behavior of the slurry samples. The color of the suspensions varied considerably between the individual tests. The tests with water and neutral salt solution showed all the same color, which corresponded to the suspended PG. The acidic tests with sulfuric, nitric and oxalic acid (H.sub.2Ox) showed a significant change in color, and after the settling of the PG, a strongly dark discolored and turbid supernatant solution was left. The caustic tests with sodium hydroxide and lime water obtained a bright and turbid supernatant solution which suggests that a substantial amount of fine fraction is present.

(85) The filtration time of all tests was comparable, and the adherent moisture of the neutral tests was in the range of 12-15%. For the caustic and acidic tests, the remaining moisture in the leach residue was slightly higher in the range of 16-30%. The filtrates of all tests were clear and almost color less. Only NaOH and HNO.sub.3 showed a light yellowish staining. Thus, the brownish color of the acidic suspension is caused by a solid fraction of the leach residue (LR). The experimental data of the first test series are summarized in Table 11:

(86) TABLE-US-00011 TABLE 11 Experimental data for BRT_1 to BRT_11 ORP EC in Mass A .sup.PLS .sup.PLS (SHE) mS/ Moist loss in in Ra Ra Test ID Medium pH in mV cm in % % Bq A in % in % PG-mix 1.8 247 51.4 BRT_1 H.sub.2O (1) 6.26 433 (641) 2.28 15.0 3.1 268 76.5 0.0 22.7 BRT_2 H.sub.2O (2) 6.29 438 (646) 2.28 12.2 3.2 254 71.6 0.0 21.4 BRT_3 NaCl 5.51 471 (679) 139.40 14.0 3.1 240 62.1 2.6 20.3 BRT_4 NaCl + 5.17 462 (670) 143.10 13.4 1.0 229 47.0 7.2 19.3 BaCl.sub.2 BRT_5 Ca(NO.sub.3).sub.2 5.30 304 (513) 77.20 11.7 1.2 210 74.1 15.0 17.7 BRT_6 NH.sub.4Ac 6.74 280 (489) 81.50 14.1 5.4 214 65.1 13.3 18.1 BRT_7 Lime 12.06 93 (303) 5.78 16.1 3.1 229 62.0 7.3 19.3 Water BRT_8 NaOH 12.44 77 (132) 84.70 24.4 23.0 354 87.0 0.0 29.9 BRT_9 H.sub.2SO.sub.4 0.40 520 (728) 231.00 30.2 17.8 326 78.7 0.0 27.5 BRT_10 HNO.sub.3 0.45 754 (964) 248.00 17.8 7.0 178 49.9 28.0 15.0 BRT_11 H.sub.2Ox 0.62 380 (589) 215.00 24.0 4.8 201 57.5 18.7 16.9

(87) The neutral test of water and salt lixiviants did not show significant changes in the process parameters pH, ORP (Oxidation reduction potential measured vs. Ag/AgCl, in mV; SHE (Oxidation reduction potential vs. standard hydrogen electrode, given in mV)), and EC (electric conductivity). The duplicated water leach tests are in good agreement and show a good reproducibility. The mass loss of the leach residues of water, NaCl, and NH.sub.4Ac are in the same range within the experimental error and can be explained by the dissolution of gypsum up to the saturation of the solution. The tests of NaCl with addition of BaCl.sub.2 and Ca(NO.sub.3).sub.2, both show a slight gain in weight which is due to the formation of sparingly soluble BaSO.sub.4 and likely a gypsum formation from the excess Ca and Ba of the lixiviant. The caustic lixiviants show a significant difference in mass loss. The mass loss in lime water (Ca(OH).sub.2 solution, saturated at room temperature) was only slight and comparable to water. In the NaOH solution however, the mass loss was 23% which can be assigned to the dissolution of gypsum and the formation of a saturated Ca(OH).sub.2 solution (Equation (1)).
CaSO.sub.4.Math.2H.sub.2O+2NaOH.fwdarw.Ca(OH).sub.2+Na.sub.2SO.sub.4+2H.sub.2O(Equation 1)

(88) Sulfuric and nitric acid also show a high mass loss, whereas oxalic acid (H.sub.2Ox) only showed a slight loss compared to water. The mass loss is due to the formation of CaHSO.sub.4.sup.+ under acidic conditions. Ca and oxalic acid form a sparingly soluble calcium oxalate, which is precipitated as soon as gypsum is dissolved. Thus, the gypsum is converted and only a slight mass loss is detected.

(89) The leach efficiency specifies the portion of the target element that was transferred to the solution (e.g., Ra, Ca, Sr, etc.). It evaluates the leaching step itself and was calculated from the results of the XRF and ICP-MS analyses, as well as the -spectroscopy of (i) the leachate (PLS) sampled from the filtrates, and (ii) the leach residue (LR) each related to the ICP-MS analysis and -spectroscopy of the PG feed (Equations 2 and 3):

(90) $\begin{array}{cc}\begin{array}{cc}{}_{\mathrm{el}}^{\mathrm{PLS}}=\frac{c_{el}^{\mathrm{PLS}}.Math.m_{Lix}.Math.{}_{Lix}}{c_{\mathrm{el}}^{\mathrm{ore}}.Math.m_{\mathrm{ore}}}.Math.100\%& \mathrm{el}\end{array}=\mathrm{element}& \mathrm{Equation}\left(2\right)\end{array}$$\begin{array}{cc}\begin{array}{cc}{}_{\mathrm{el}}^{\mathrm{LR}}=100\%-\left(\frac{c_{el}^{LR}.Math.m_{LR}}{c_{\mathrm{el}}^{\mathrm{ore}}.Math.m_{\mathrm{ore}}}.Math.100\%\right)& \mathrm{el}=\mathrm{element}\end{array}& \mathrm{Equation}\left(3\right)\end{array}$

(91) with .sup.i leach efficiency of the element el calculated from i=PLS or LR, ci concentration of el in j=PLS, LR and ore, m.sub.k mass of k=Lixiviant (Lix), LR and initial ore feed, respectively, .sub.j density of the j=Lix. For simplification, the efficiency was set to 100% if the elemental concentration in the PLS was below detection limit and to 0% if a negative value was calculated. For the evaluation of the .sup.226Ra leaching efficiency the specific activity in Bq/kg was considered instead of the concentration.

(92) FIG. 3 shows evaluation of .sup.226Ra leachability based on comparison of the activity of .sup.226Ra in the feed sample PG-mix with the leach residue (Table 11) of the corresponding test.

(93) The leaching efficiency was calculated according to Eq. (3). The activities measured of most LRs are in the range of uncertainty of the determination of the PG-mix activity, however, the measurements are consistent with each other and can be compared to show trends.

(94) All PLS samples were analyzed by ICP-MS to determine the leaching efficiency of the accompanying elements according to Eq. 2, with results shown in FIG. 4A and FIG. 4B.

(95) Again, the results of the duplicated water leach are in good agreement and no .sup.226Ra was dissolved in these tests. In addition, the Ca and Sr leaching efficiency was between 1.2% and 1.3%, which is consistent with the expectation that since the stockpile material has been exposed to alteration processes (e.g., rain) over a long period of time, the water-soluble fraction of .sup.226Ra is expected to be washed out and only the main component of the PG, gypsum and other alkali earth sulfates, were leached to the solubility equilibrium.

(96) An increase in salinity by NaCl caused a slight dissolution of .sup.226Ra and the addition of BaCl.sub.2 was beneficial and increased the leaching efficiency by 5%. These tests showed an increased leaching efficiency of Ca, Ba and P compared to water and the addition of BaCl.sub.2 increased the leaching efficiency slightly further. The Sr leaching increased with NaCl and decreased with BaCl.sub.2 addition. This might indicate the precipitation of Sr in the presence of Ba. The formation of and (Ba,Sr) SO.sub.4 might be possible. (E. Brower and J. Renault. Solubility and Enthalpy of the Barium-Strontium Sulfate Solid Solution Series: Circular 116. New Mexico State Bureau of Mines and Mineral Resources 1971, No. 116.)

(97) The leaching with Ca(NO.sub.3).sub.2 showed a further increase in .sup.226Ra leaching, whereby 15% of .sup.226Ra could be leached. In addition, Ca, Sr, and Ba were dissolved. This preferential leaching of alkali earth metals can be explained by the high solubility of the corresponding nitrate salts. The expected reaction is shown in Eq. (4):
RaSO.sub.4+2Ca(NO.sub.3).sub.2.fwdarw.Ra(NO.sub.3).sub.2+CaSO.sub.4Eq. (4)

(98) Similar reactions for Sr-, Ba- and Ca- sulfate are considered. However, the predominance of Ca in the lixiviant will suppress CaSO.sub.4 leaching. In addition, the solubility of Ra(NO.sub.3).sub.2 is higher compared to Ba(NO.sub.3).sub.2. (Brown, P. L.; Matyskin, A. V.; Ekberg, C. The Aqueous Chemistry of Radium. Radiochimica Acta 2022, 110 (6-9), 505-513. DOI: 10.1515/ract-2021-1141), which favors the dissolution of .sup.226Ra.

(99) The NH.sub.4Ac leaching showed a Ra leaching efficiency of 13.3%. The alkali earth acetates show a good solubility, which probably is the driving force for the dissolution of .sup.226Ra, Ca, Sr, and Ba in this lixiviant.

(100) The caustic lixiviants had only minor effect on the .sup.226Ra-leachability and only small amounts of Ca and Sr were dissolved. Therefore, the assumed conversion of RaSO.sub.4 to Ra(OH).sub.2 did not take place or took place only to a limited extent. High measured activity of the LR from NaOH leaching shows the accumulation of .sup.226Ra inside the LR. The overestimation of activity is caused by the high mass loss (23%) during the test and the associated measurement error.

(101) The leaching with mineral acids H.sub.2SO.sub.4 and HNO.sub.3 caused a significant increase in leaching of metals. Al, Fe, Zn, REEs and U are leached. Ca, Sr, and Ba only show a minor leaching efficiency. .sup.226Ra was preferably leached by HNO.sub.3. H.sub.2SO.sub.4 did not result in the dissolution of .sup.226Ra. The leaching of metals indicates that these are bound to a different mineral phase than .sup.226Ra, presumably an oxide or a phosphate mineral such as monazite. The .sup.226Ra leaching is completely suppressed by H.sub.2SO.sub.4. This indicates that .sup.226Ra is present as a sulfate within the PG. This can be further confirmed by the high leachability with HNO.sub.3. Here it can be considered that a similar leaching mechanism underlies as already discussed for Ca (Eq. (4)).

(102) Oxalic acid is a diprotic organic acid. The anion forms chelating ligands and is therefore a good complexing agent. Leaching in oxalic acid leads to high leaching efficiencies for Al, Fe, and Zn. These metals form oxalate complexes with high stability constants and solubilities in water. Thus, they are preferably/selectively leached. In addition, the P concentration increases. Therefore, it can be concluded that these elements are associated with phosphate phases. The leaching efficiencies for the alkaline earth elements differ between the elements. .sup.226Ra and Sr show an increased leaching efficiency whereas Ca and Ba are only slightly dissolved.

(103) The highest .sup.226Ra leaching efficiency was found for HNO.sub.3, H.sub.2Ox, Ca(NO.sub.3).sub.2 and NH.sub.4Ac. In practical applications of processing stockpiled PG, the leaching process will be expected to have a very high mass flow of PG in order to extract .sup.226Ra as efficiently as possible. Thus, in embodiments lixiviant(s) are selected that are available in large quantities at low cost, have good recyclability within the process, and are as environmentally friendly as possible if the lixiviant cannot be completely removed from the leach residue.

(104) Embodiments of the invention include methods of processing PG comprising: A) combining fresh and/or stockpiled PG with a lixiviant (leach solution) chosen from i) H.sub.2O (water, de-ionized water, tap water, rain water, etc.); ii) NaCl (e.g., 1-20%, such as 5%, 10% or 15% NaCl, or any range in between); iii) a solution of NaCl+BaCl.sub.2 (such as 10% NaCl+0.1% BaCl.sub.2, or 20% NaCl+0.2% BaCl.sub.2, or 1-20% NaCl+0.05-0.5% BaCl.sub.2, or any range in between); iv) Ca(NO.sub.3).sub.2 (such as 2-40% Ca(NO.sub.3).sub.2, such as 5%, 10%, 15%, 20%, 25%, 30%, or 35% Ca(NO.sub.3).sub.2, or any range in between); v) NH.sub.4Ac (such as 2-30% NH.sub.4Ac, such as 5%, 10%, 15%, 20%, or 25% NH.sub.4Ac, or any range in between); vi) lime water (e.g., saturated lime water); vii) NaOH (such as 5-30% NaOH, such as 5%, 10%, 15%, 20%, or 25% NaOH, or any range in between); viii) H.sub.2SO.sub.4 (such as 2-20% H.sub.2SO.sub.4, such as 5%, 10%, or 15% H.sub.2SO.sub.4, or any range in between); ix) HNO.sub.3 (such as 2-20% HNO.sub.3, such as 3%, 4%, 5%, 10%, or 15% HNO.sub.3, or any range in between); x) H.sub.2Ox (such as 2-20% H.sub.2Ox, such as 5%, 10%, or 15% H.sub.2Ox, or any range in between); xi) a solution of Ca(NO.sub.3).sub.2+HNO.sub.3 (such as 5% Ca(NO.sub.3).sub.2+3% HNO.sub.3, such as 10% Ca(NO.sub.3).sub.2+5% HNO.sub.3, such as 15% Ca(NO.sub.3).sub.2+2-10% HNO.sub.3, such as 5% Ca(NO.sub.3).sub.2+5-15% HNO.sub.3, such as 10% Ca(NO.sub.3).sub.2+2-10% HNO.sub.3, or any range in between); xii) one or more nitrate salts, such as NaNO.sub.3; xiii) or any combination of any one or more of these; B) allowing the leach solution to react with the PG for a residence time of up to about 24 hours, such as for about 2-6 hours, such as for about 3-5 hours, such as for about 4 hours and at a temperature in the range of about RT70 C., such as from about 25-55 C., such as from about 30-50 C., such as from about 35-45 C., or about 40 C. to obtain pregnant leach solution (PLS) comprising .sup.226Ra separated from the PG and solids; and C) separating the PLS from the solids.

(105) It will be understood by persons of ordinary skill in the art that the exemplary process conditions/parameters demonstrated in the Examples can be modified according to the teachings elsewhere in this disclosure. For example, with respect to the lixiviants exemplified, any one or more lixiviants can be substituted for any other lixiviant described herein. Further, with respect to the temperature of the leaching and/or the residence time for leaching, any temperature or time can be substituted for any other temperature or time within the ranges exemplified. Additionally, any lixiviant or lixiviant combination can be used in a leaching process with any temperature or time disclosed herein.

(106) Exemplary leach systems, for example, using Ca(NO.sub.3).sub.2 and/or HNO.sub.3 as lixiviant(s), are described in greater detail below.

(107) Nitrate System. Additional leaching processes for the nitrate system, Ca(NO.sub.3).sub.2 and/or HNO.sub.3, designed similar to the first, were further explored, with results shown in Table 12:

(108) TABLE-US-00012 TABLE 12 Experimental data for BRT_12 to BRT_15 and BRT_20 to BRT_22 ORP EC in Mass .sup.PLS .sup.PLS (SHE) mS/ Moist loss A in Ra Ra Test ID Medium pH in mV cm in % in % Bq A In % in % PG-mix 247 51.4 BRT_12 5% Ca(NO.sub.3).sub.2 5.49 601 46.20 18.1 2.0 268 76.5 22.8 16.1 (811) BRT_13 20% Ca(NO.sub.3).sub.2 4.06 573 112.90 13.6 6.9 254 71.6 51.5 10.1 (784) BRT_14 5% Ca(NO.sub.3).sub.2 0.35 795 183.00 13.6 2.7 240 62.1 27.1 15.2 3% HNO.sub.3 (1005) BRT_15 10% 5.08 595 77.40 13.6 0.7 229 47.0 41.8 12.1 Ca(NO.sub.3).sub.2 (805) BRT_20 40 C., 4.31 302 13.70 10.1 1.8 214 65.1 72.5 5.7 20% Ca(NO.sub.3).sub.2 (511) BRT_21 70 C., 3.81 296 13.83 17.1 7.1 229 62.0 73.2 5.6 20% Ca(NO.sub.3).sub.2 (505) BRT_22 25% Ca(NO.sub.3).sub.2 4.44 368 14.00 10.2 39.5 354 87.0 79.2 4.3 (577)

(109) Table 12 summarizes the experimental data relating to various nitrate leaching systems. Only a slight mass loss or even a gain in mass was observed. Using BRT_22, 25% Ca(NO.sub.3).sub.2, as lixiviant showed an enormous increase in mass. This might be due to the ongoing precipitation of the lixiviant and formation of gypsum.

(110) HNO.sub.3 as lixiviant resulted in a decrease of the pH and an increase of the ORP as expected. Furthermore, it could be observed that with increasing temperature both the pH value and the ORP of the resulting PLS decreased compared to the leaching at room temperature.

(111) An increase in .sup.226Ra leaching with increasing Ca(NO.sub.3).sub.2 content in the lixiviant was observed as shown in FIG. 5A. FIG. 5B shows .sup.220Ra leaching efficiency by temperature.

(112) When comparing the leachability in Ca(NO.sub.3).sub.2 solution with and without HNO.sub.3, no significant effect of the acid could be detected. .sup.226Ra leaching showed rather a correlation with total nitrate content as shown in FIG. 6 (.sup.226Ra leaching efficiency as a function of nitrate concentration).

(113) The 2% Ra leaching had its maximum 25% Ca(NO.sub.3).sub.2, however, the highest gain in mass was also observed. The temperature dependent assays showed an increasing .sup.226Ra leaching with increasing temperature, wherein the increase was highest between ambient conditions and 40 C. and only a small difference between 40 C. and 70 C. was observed. Since the tests at elevated temperature were mixed with a magnetic stirrer instead of the overhead shaker, the magnetic stir bar might have a grinding effect which enhances the .sup.226Ra-leaching.

(114) Leaching efficiencies of accompanying elements are summarized in FIG. 7A, FIG. 7B, FIG. 8A and FIG. 8B. FIG. 7A shows leaching efficiencies .sup.PLS of the BRTs with increasing Ca(NO.sub.3).sub.2 concentration and FIG. 7B shows leaching efficiencies .sup.PLS of the BRTs with different pulp densities. FIG. 8A shows leaching efficiencies .sup.PLS based on temp, while FIG. 8B shows leaching efficiencies .sup.PLS based on HNO.sub.3 concentration.

(115) With increasing Ca(NO.sub.3).sub.2 concentration, an increase in Ba, Ca and Sr leaching is observed, in addition, leaching of zinc occurs at the highest Ca(NO.sub.3).sub.2 concentration (FIG. 7A, FIG. 8A). A comparison of the tests with 10% Ca(NO.sub.3).sub.2 shows that a higher leaching efficiency can be achieved with a lower pulp density (FIG. 7B). This supports the hypothesis that leaching is dependent on the nitrate to PG ratio. With increasing temperature, an increase in Ba, Ca and Sr leaching as well as Al, P and Zn leaching is also observed (FIG. 8B).

(116) Leaching under acidic conditions again shows an increasing leaching of impurity elements. Since this does not occur in pure Ca(NO.sub.3).sub.2 solution, the pure and thus neutral salt solution is to be preferred for leaching of radium.

(117) From the BRTs with stockpiled PG-mix the following observations are noted: .sup.226Ra leaching ability increases with increasing nitrate content of the lixiviant.

(118) No additional positive effect of an acidified lixiviant could be found. It is also to be expected that too high acidity leads to a decrease of radium nitrate solubility (Kirby & Salutsky, 1964, The radiochemistry of radium, in National Academy of Sciences, Nuclear Science Series NAS-NS 3057).

(119) A high calcium nitrate concentration in solution leads to precipitation of calcium in the form of gypsum and thus to a strong gain in mass of the leach residue.

(120) With increasing temperature, the .sup.226Ra leaching efficiency could be increased with only small differences between 40 C. and 70 C.

(121) Increased .sup.226Ra leaching is accompanied by increased leaching efficiency of alkaline earth elements such as Ba, Ca, and Sr. Thus, .sup.226Ra is associated with the sulfates of the alkaline earth elements. Solid solution series between Ba and .sup.226Ra as well as between Sr and .sup.226Ra or mixtures of all three elements are possible (Hedstrm, H., Persson, I., Skarnemark, G., Ekberg, C., 2013, Characterization of Radium Sulfate, Journal of Nuclear Chemistry, vol 2013, article ID 940701, http://dx.doi.org/10.1155/2013/940701; Zhang, T., Gregory, K., Hammack R. W., Vidic, R. D., 2014, Co-precipitation of Radium and Strontium Sulfate and its impact on the fate of Radium during treatment of produced water from unconventional gas extraction, Environ. Sci. Technol., vol. 48, no. 8, p. 4596-4603).

(122) From the BRTs the following conditions could be determined:

(123) The inventors have demonstrated that higher nitrate concentrations, such as 20% Ca(NO.sub.3).sub.2 and/or elevated temperature (between 40 C. and 70 C.) result in increased .sup.226Ra leaching. Under these conditions, .sup.226Ra leaching efficiencies of at least about 65% can be achieved. .sup.226Ra-separation efficiency can be enhanced by a further increase in nitrate concentration of the lixiviant. Thus, alternative salts to Ca(NO.sub.3).sub.2 can in addition or alternatively be used so as to avoid consumption of the medium by the formation of gypsum, but a higher gypsum dissolution is expected for a Ca-free lixiviant. Additionally, no further improvement results from increasing acidity during the leaching process. In fact, the solubility of Ra(NO.sub.3).sub.2 decreases with increasing acidity. (Kirby, H. W., Salutsky, Murrell L., Grace, W. R. The Radiochemistry of Radium; NAS-NS No. 3057: Springfield, Virginia, 1964.) Furthermore, the increase in acidity causes the leaching of impurities.

EXAMPLE 2 (Fresh PG)

(124) In addition to the PG-mix sample (EXAMPLE 1), a sample of fresh PG was processed using embodiments of leaching methods of the invention, including determining .sup.226Ra leaching efficiency of fresh PG with Ca(NO.sub.3).sub.2 and HNO.sub.3 as lixiviants. More particularly, the lixiviants included i) H.sub.2O (BRT_16), ii) 10% Ca(NO.sub.3).sub.2 (BRT_17), iii) 5% HNO.sub.3 (BRT_18) and iv) sequential leaching with H.sub.2O and 10% Ca(NO.sub.3).sub.2 (BRT_23). It is noted that although the leaching of fresh PG was investigated using the nitrate system, like stockpiled PG, any of the leaching regimes described herein can be used with fresh PG as well.

(125) The optical impression was similar to the acidic leaching tests with PG-mix, and the supernatant suspension was darkly discolored, but the leach filtrate was clear and colorless. The Ra leaching efficiency was in the range of 56% for H.sub.2O (BRT_16), 10% Ca(NO.sub.3).sub.2 (BRT_17), and 5% HNO.sub.3 (BRT_18) and thus appeared to be independent of the lixiviant tested. For a 2-stage leach regime (water and Ca(NO.sub.3).sub.2, BRT_23) a slight increase in .sup.226Ra leaching to 67% was observed. The experimental data for leachability of fresh PG are summarized in Table 13:

(126) TABLE-US-00013 TABLE 13 Experimental data for BRT_16 to BRT_18 and BRT_23 ORP EC in Mois Mass .sup.PLS .sup.PLS (SHE) mS/c t in loss in A in Ra Ra Test ID Medium pH in mV m % % Bq A In % in % PG fresh 33.2 288 59.3 BRT_16 H.sub.2O 2.38 470 (679) 63.00 18.0 5.3 125 33.4 56.3 9.0 BRT_17 10% Ca(NO.sub.3).sub.2 1.00 585 (793) 78.40 20.0 5.9 130 43.5 55.4 9.3 BRT_18 5% HNO.sub.3 0.33 798 (1006) 237.00 18.3 8.6 120 34.5 58.7 8.6 BRT_23 H.sub.2O 2.31 430 (639) 4.94 10% Ca(NO.sub.3).sub.2 4.31 302 (511) 13.70 16.2 1.7 87 21.6 67.2 6.8

(127) Of note, the leaching efficiency of the accompanying elements is higher than for the corresponding tests with PG-mix and can be attributed to the acidity of the leaching solutions caused by remaining phosphoric acid in fresh PG. Additionally, the fresh PG had an initial moisture of 33.4%, which resulted in the solid liquid separation during long storage and having to be resuspended. The residual moisture of the leach residue was between 16 and 20%, slightly higher than for the tests with PG-mix (Table 11). The tests with water (BRT_16) and Ca(NO.sub.3).sub.2 (BRT_17) resulted in a slight gain in mass. The leaching with HNO.sub.3 (BRT_18) resulted in a mass decrease. Successive leaching with water and Ca(NO.sub.3).sub.2 (BRT_23) leads to a slight decrease in mass. Due to the high moisture of the starting material of these tests, the indication of the mass loss is strongly subject to error because the weighing of the starting material with a constant moisture content was difficult, which may have resulted in different amounts of solids.

(128) The pH of the leaching solutions was already strongly acidic both for leaching with HNO.sub.3 and for leaching with water and neutral salt. This is an indicator that the fresh PG is still contaminated with phosphoric acid. The acidity can be washed out with water which is shown by the higher pH of the PLS of BRT_23 (4.31 pH). The ORP of the PLSs is in accordance with the lixiviant and complies with expectations.

(129) For the evaluation of the leaching efficiency, the fresh PG was initially analyzed, with the elemental composition provided in Table 14:

(130) TABLE-US-00014 TABLE 14 Elemental composition of fresh PG based on the composition of PG-mix mg/kg As Al Ba Ca Fe P Si Sr Ti Zn PG-mix 0.70 349 248 227294 283 3270 20057 868 90.7 7 water soluble 0 0 0 4611 23 1665 922 44 0 2 Fresh PG (wet) 0.47 233 166 156441 212 3849 14320 624 60.6 7 mg/kg Y La Ce Pr Nd Gd Dy U PG-mix 68.5 31.5 23.0 5.55 21.0 5.50 5.30 3.05 water soluble 0 0 0 0 0 0 0 0.90 Fresh PG (wet) 45.7 21.0 15.3 3.71 14.0 3.67 3.54 2.94

(131) Elemental composition was calculated based on the composition of PG-mix and the water-soluble fraction from the fresh PG of test BRT_16. For this calculation, the assumption is made that PG-mix is a leached fresh PG. The weathering was simulated by the water leaching so that the calculated composition should approximately reflect the real composition of the fresh PG.

(132) The specific activity of the fresh PG was determined and is higher than the activity of PG-mix with A.sub.s(fresh PG)=1439.84296.62 Bq/kg (Table 4).

(133) FIG. 9A and FIG. 9B show .sup.226Ra leachability and elemental leaching. In particular, FIG. 9A shows leaching efficiencies .sup.LR of fresh PG compared to PG-mix, while FIG. 9B shows leaching efficiencies .sup.PLS of the BRTs conducted with fresh PG.

(134) For the fresh PG, the results show that .sup.226Ra leaching efficiency was independent of the lixiviant: H.sub.2O (BRT_16) (56.3%), 10% Ca(NO.sub.3).sub.2 (BRT_17) (55.4%), and 5% HNO.sub.3 (BRT_18) (58.7%). But for the leach regime using H.sub.2O and 10% Ca(NO.sub.3).sub.2 (BRT_23) with fresh PG, a slight increase in .sup.226Ra leaching of 10.9% to 67.2% was observed. This leads to the assumption that .sup.226Ra is present in fresh PG mainly in water-soluble form. Additional leaching with Ca(NO.sub.3).sub.2 leads to a leaching of the sulfate-bound .sup.226Ra.

(135) The additional leaching step(s) can be performed sequentially by leaching with water then another lixiviant, or can be performed sequentially by leaching with a first lixiviant then water. In embodiments, it is not critical the order of operation or the number of leaching steps.

(136) For example, the water-soluble form of .sup.226Ra can be leached with a selected lixiviant and the water-insoluble form of .sup.226Ra can be leached with a different lixiviant, regardless of which lixiviant is exposed to the PG first or if the lixiviants are exposed to the PG simultaneously. Further, for example, such 2-step or multi-stage leaching can involve one or more leaching steps performed in a manner to separate either or both the soluble and insoluble forms of .sup.226Ra from PG. In particular examples, fresh or stockpiled PG (or a combination) can be treated with a first lixiviant, then treated with a different lixiviant. Fresh or stockpiled PG (or a combination) can be treated with a first lixiviant, then treated with a different lixiviant, then treated with the first lixiviant again. Fresh or stockpiled PG (or a combination) can be treated with a first lixiviant, then treated with another lixiviant, then treated with the first lixiviant again, then treated with the other lixiviant again. Fresh or stockpiled PG (or a combination) can be treated with a first lixiviant, then treated with another lixiviant, then treated with the first lixiviant again, then treated with the other lixiviant again, and this pattern repeated any number of times, such as from 2-10 times, such as 3, 4, 5, 6, 7, 8, or 9 times. Fresh or stockpiled PG (or a combination) can be treated with a first lixiviant, then treated with a second lixiviant, then treated with another lixiviant, which is the same or different as the first or second lixiviant, optionally this pattern can be repeated any number of times, such as from 2-10 times, such as 3, 4, 5, 6, 7, 8, or 9 times.

(137) Embodiments of the invention include methods of processing PG comprising: A) combining fresh and/or stockpiled PG with a lixiviant (leach solution) chosen from i) Ca(NO.sub.3).sub.2 (such as 2-40% Ca(NO.sub.3).sub.2, such as 5%, 10%, 15%, 20%, 25%, 30%, or 35% Ca(NO.sub.3).sub.2, or any range in between); ii) NH.sub.4Ac (such as 2-30% NH.sub.4Ac, such as 5%, 10%, 15%, 20%, or 25% NH.sub.4Ac, or any range in between); iii) HNO.sub.3 (such as 2-20% HNO.sub.3, such as 3%, 4%, 5%, 10%, or 15% HNO.sub.3, or any range in between); iv) H.sub.2Ox (such as 2-20% H.sub.2Ox, such as 5%, 10%, or 15% H.sub.2Ox, or any range in between); v) a solution of Ca(NO.sub.3).sub.2+HNO.sub.3 (such as 5% Ca(NO.sub.3).sub.2+3% HNO.sub.3, such as 10% Ca(NO.sub.3).sub.2+5% HNO.sub.3, such as 15% Ca(NO.sub.3).sub.2+2-10% HNO.sub.3, such as 5% Ca(NO.sub.3).sub.2+5-15% HNO.sub.3, such as 10% Ca(NO.sub.3).sub.2+2-10% HNO.sub.3, or any range in between); vi) or any combination of any one or more of these; B) allowing the leach solution to react with the PG for a residence time of up to about 24 hours, about 2-6 hours, such as for about 3-5 hours, such as for about 4 hours and at a temperature in the range of about RT70 C., such as from about 25-55 C., such as from about 30-50 C., such as from about 35-45 C., or about 40 C. to obtain leachate/pregnant leach solution (PLS) comprising .sup.226Ra separated from the PG, and solids; and C) separating the PLS from the solids.

(138) Further of note is that particular temperatures, residence times and processing parameters for leaching and/or extraction can be selected according to the disclosures provided herein. For single, double or multi-step leaching, the leaching can be performed using a residence time (contact time of lixiviant and PG) in the range of up to about 24 hours, such as about 2-6 hours, such as about 2-5 hours, such as about 4 hours and/or at a temperature in the range of about room temperature (RT) to about 70 C., such as about 25-70 C., or about 30-55 C., or about 35-45 C., or about 35-50 C. Optionally, mechanical stirring of the PG and lixiviant can be employed so as to contribute a grinding effect, which may further enhance .sup.226Ra leachability.

(139) It is noticeable that there is no direct correlation between the .sup.226Ra and the Ba or Sr leaching. Still, theses alkaline earth metals are leached by nitrate lixiviants. The observation suggests that .sup.226Ra is not completely associated with barite in fresh PG.

(140) The test results show a higher initial .sup.226Ra leachability of fresh PG suggesting that 60% of .sup.226Ra is in water soluble form. The leachability can be increased by Ca(NO.sub.3).sub.2 re-leaching. Based on the results of the previous BRTs, this shows that 40% of the .sup.226Ra in fresh PG is sulfate bound. This percentage could be due to an aging process in which the crystallization of sulfate phases proceeds. These observations are in accordance with the findings of Robert A. Zielinski, Mohammad S. Al-Hwaiti, James R. Budahn and James F. Ranville, 2011, Radionuclides, trace elements and radium residence in phosphogypsum of Jordan, Environ Geochem Health, vol. 33, p. 149-165 (Zielinski et al., 2011).

EXAMPLE 3Upscaling of Process

(141) Verification Tests. Select processes were repeated in 10.sup.3 g-scale to verify the results and produce a larger amount of solution for .sup.226Ra separation and refining tests. In particular, the inventors demonstrate leaching methods using Ca(NO.sub.3).sub.2 as an exemplary lixiviant which provides for enhanced .sup.226Ra leaching efficiency, however, embodiments of the invention are by no means limited to this particular lixiviant, nor particular concentration of lixiviant.

(142) The verification tests were used to verify on a larger scale select leaching conditions and to produce a larger volume of pregnant leach solution (PLS) for .sup.226Ra separation tests therefrom. The positive effect of increased temperature to increase the .sup.226Ra leaching efficiency was validated, as well as a reduction in retention time. It was determined that the retention time could be reduced from 24 h to 4 h with the same or even better leaching efficiency. In total, 50 L of PLS were produced. The PLS of 4 tests were combined and had a .sup.226Ra concentration of 36689 Bq/L.

(143) For the experimental setup a 20 L double-walled stir tank reactor equipped with reflux condenser and gas scrubber was chosen. The temperature was set with an oil bath thermostat and monitored with a thermometer in the reactor. Evaporating water was recovered via a reflux condenser coupled to a cryostat to minimize mass loss. Additional exhaust gases were cleaned with a gas scrubber and vented through the ventilation system. The reactor was fed via a handhole and drained via a bottom valve. The suspension was filtrated in 3 batches with the help of a vacuum filtration unit in 5 L scale.

(144) The experimental procedure was as follows: i) Setup of a 20 L leach vessel (double walled stirred tank reactor); ii) Preparation of the lixiviant at ambient temperature; iii) Start heating to target temperature; iv) Addition of PG; v) Mixing for target retention time; vi) Discharge of suspension; vii) Vacuum filtration through 2.5 m filter paper in 5 L batches; viii) Dry sample of leach residue at 60 C., determine weight loss.

(145) The analytical program consisted of: i) pH, ORP, EC determination of the solution samples at ambient conditions; ii) Elemental analysis of PLS and leach residue by XRF; iii) Elemental analysis by ICP-MS/OES of selected samples, iv) -spectroscopy of leach residue.

(146) The experimental parameters of the leach conditions of the five batches were further varied. An overview of the large-scale verification tests is given in Table 15.

(147) TABLE-US-00015 TABLE 15 Experimental parameters of verification tests Temper- Leaching ID Feed Lixiviant ature SLR time VT_1 Stockpiled PG 20% Ca(NO.sub.3).sub.2 70 C. 3/12 24 h VT_2 Stockpiled PG 20% Ca(NO.sub.3).sub.2 60 C. 3/12 24 h VT_3 Stockpiled PG 20% Ca(NO.sub.3).sub.2 50 C. 3/12 24 h VT_4 Stockpiled PG 20% Ca(NO.sub.3).sub.2 70 C. 3/12 4 h VT_5 Stockpiled PG 20% Ca(NO.sub.3).sub.2 70 C. 3/12 8 h

(148) The experimental results are summarized in Tables 16 and 17.

(149) TABLE-US-00016 TABLE 16 Experimental data for VT_1 to VT_5 Test Parameter ORP (SHE) EC in Moist Mass loss ID (T, t) pH in mV mS/cm in % in % PG-mix PG-mix VT_1a 70 C., 4 h 2.52 525 (734) 112.00 VT_1b 70 C., 24 h 2.69 460 (669) 110.70 14.1 4.4 VT_2 55 C., 24 h 4.10 468 (677) 118.20 14.8 1.9 VT_3 50 C., 24 h 4.15 304 (602) 117.00 15.5 13.0 VT_4 70 C., 4 h 3.48 380 (587) 118.10 16.0 5.5 VT_5 70 C., 8 h 3.09 491 (700) 117.70 11.1 3.1

(150) TABLE-US-00017 TABLE 17 Balancing of .sup.226Ra activities of the verification tests A.sup.LR A.sup.LR A.sup.PLS A.sup.PLS Account- Account- Parameter in in .sup.LR.sub.el .sup.LR.sub.el in in ability in ability Test ID (T, t) Bq/kg Bq/kg in % in % Bq/L Bq/L % in % PG-Mix 3701 771 VT_1a 70 C., 4 h 1231 483 67 7 VT_1b 70 C., 24 h 2266 464 39 13 1523 403 102 27 VT_2 55 C., 24 h 865 345 77 5 3473 719 117 33 VT_3 50 C., 24 h 1292 407 65 7 2515 734 103 31 VT_4 70 C., 4 h 942 569 75 5 3686 709 125 36 VT_5 70 C., 8 h 615 243 83 3 3507 705 111 31

(151) The larger-scale leaching tests exhibited similar behavior to the BRTs, with the suspension showing comparable stirring and settling patterns. Filtration could also be performed similarly without encountering any difficulties. The moisture content of the leach residue is comparable across all five tests. The mass loss, however, showed a significant difference for VT_3 at 50 C. All tests showed a slight gain in weight (2-5%), whereas VT_3 had an increase of 13%. The test was conducted at 50 C. At this temperature the transition between gypsum and anhydrite as thermodynamic stable phase occurs (Freyer, D and Voigt, W., 2003, Crystallization and phase stability of CaSO.sub.4 and CaSO.sub.4-based salts, Monatshefte fr Chemie, vol. 134, p. 693-719). For the other tests at higher temperatures, anhydrite is expected to be the stable phase. However, the formation of anhydrite at higher temperatures is kinetically hindered and hemihydrate is formed, which shows a much higher solubility (Freyer, Voigt, 2003). As a result, the CaSO.sub.4 phase formed remains in solution and does not precipitate. The weight gain at 50 C. can be attributed to the precipitation of gypsum from the leaching solution, as observed previously in the case of BRT 22 (Table 12).

(152) The pH of the leach solutions varied and decreased with increasing temperature. VT_1 already had a slightly lower initial pH since a different feedstock of salt was used. The ORP of the tests did not show a trend with time or temperature. However, it varies strongly between 600 and 700 mV. It is assumed that this is an effect of the absolute concentration of the PLS. It was observed that the volumes of the filtrates varied greatly. In some cases, there were differences of up to 4 L. This effect is due to the loss through evaporation. The lixiviant was heated prior to leaching in order to be able to perform short leaching periods safely at the target temperature. Depending on the length of this heating and holding period, evaporation of water occurred which could not be completely prevented by the reflux condenser.

(153) .sup.226Ra leachability was determined based on the analysis of the leach residue and the PLS. The determination of the mass balance and accountability (Table 17) allowed for the evaluation of all tests. The accountability across all tests was within a similar range, with a slight tendency to overestimate .sup.226Ra. This discrepancy can be attributed to experimental errors and the inherent measurement uncertainty of -spectroscopy.

(154) The .sup.226Ra-leaching trends with temperature are shown in FIG. 10 and more particularly, .sup.226Ra leaching efficiency depending on temperature and Ca(NO.sub.3).sub.2 concentration with the verification tests represented by circles and the BRTs represented by squares.

(155) The leaching efficiencies of all VTs were in the range of 65-83%, with the exception of VT_1b (24 h), which showed a significantly decreased efficiency of .sup.LR=39%. The 4 h sample of the same test at 70 C. (VT_1a), however, resulted in an efficiency of .sup.LR=67%. The decrease in .sup.226Ra concentration of the VT_1b PLS is believed to be due to a secondary precipitation of Ra.

(156) The dependency of the leaching efficiency on the retention time was tested in VT_4 and VT_5. Both VT_4 and VT_5 showed very high .sup.226Ra leaching efficiency of .sup.LR=75-83%. Thus, a reduced retention time does not affect the leaching efficiency. Accordingly, in embodiments of the invention the retention time of the phosphogypsum in contact with the lixiviant(s) can be in the range of up to about 24 hours, such as in the range of about 2-6 hours, or in the range of about 3-5 hours, such as about 4 hours.

(157) The accompanying elemental leaching is summarized in FIG. 11, showing leaching efficiencies PLS of the verification tests.

(158) Similar elemental concentrations were detected for all VTs. In general, the leaching efficiency was highest for VTs at 70 C. and decreased with temperature. Again, Ba, Ca, and Sr were the main elements leached, and the leaching corresponds with the trends in .sup.226Ra leaching. The reduced retention time in VT_4 and VT_5 resulted in the highest Ba leaching efficiency. Since it is assumed that Ra is associated with barite, this is in accordance with expectations. The VT_1 test does not match expectations. A different salt feedstock was used here, which lead to lower .sup.226Ra and alkaline earth element leaching. The feed solution used showed elevated concentrations of Mg, Ba, Al, and Fe, which may have hindered the leaching. Further, the inventors have demonstrated that higher nitrate concentrations and elevated temperatures (e.g., temperatures above room temperature) lead to an increased efficiency in leaching of .sup.226Ra and have found no further improvement from increasing acidity during the leaching process. In fact, the solubility of Ra(NO.sub.3).sub.2 decreases with increasing acidity. (Kirby, H. W., Salutsky, Murrell L., Grace, W. R. The Radiochemistry of Radium; NAS-NS No. 3057: Springfield, Virginia, 1964.) In addition, the increase in acidity causes the leaching of impurities.

(159) In addition to the solution analysis, the LR of VT_4 was sampled and re-suspended in water. The suspension was vacuum filtrated, and the residue was washed on the filter. The elemental analysis of the dried solid as an example for a purified PG is given in Table 18.

(160) TABLE-US-00018 TABLE 18 Elemental composition of the washed LR of VT_4 vs. PG-mix PG- LR VT_4 Difference Element mix (washed) in % CaO in % 31.8 36.4 14 SO.sub.4 in % 51.0 47.8 6 SiO.sub.2 in % 4.3 3.7 13 Na.sub.2O in % <0.1 <0.1 C (organic) in % 0.1 n.d. F in % 0.345 0.377 9 P.sub.2O.sub.5 in % 0.75 0.81 9 Al.sub.2O.sub.3 in % 0.07 0.16 145 Cl in % <0.01 n.d. MgO in % <0.005 <0.005 Fe.sub.2O.sub.3 in % 0.04 0.06 59 Ti in mg/kg 90.7 84.35 7 Co in mg/kg <1 <1 Cu in mg/kg <1 <1 Zn in mg/kg 7.0 4.45 36 Hg in mg/kg <0.07 <0.07 Pb in mg/kg <2 <2 As in mg/kg 0.7 <0.8 Ba in mg/kg 248 166.5 33 Sr in mg/kg 868 898.5 4 Th in mg/kg <20 <2 U in mg/kg 3.1 2.85 7 REE in mg/kg 91.8 77.25 16 Sc in mg/kg <50 <10 Y in mg/kg 68.5 63.2 8

(161) While there are no major changes in the elemental composition of LR VT_4 relative to the PG-mix sample, the Ba content was strongly reduced (33%). Thus, it can be shown that leaching of .sup.226Ra is very selective and has only negligible effect on the overall composition of PG.

(162) 50 L of PLS was produced from the verification tests, and the PLS' of VT_2 to VT_5 were combined as a mixed PLS, having a .sup.226Ra concentration of 330.351.5 Bq/L.

(163) Fresh PG already shows high leaching efficiencies with water, indicating that only a small portion of the .sup.226Ra in fresh PG is associated with barite. In contrast, QEMScan analysis for aged PG suggests an association with barite, which is also related to reduced leachability.

EXAMPLE 4Extraction/Recovery of .SUP.226.Ra

(164) Once .sup.226Ra has been separated/removed from the phosphogypsum into the PLS/leachate, one or more various techniques can be used to extract the .sup.226Ra for example for the purpose of isolating .sup.226Ra. Although ion exchange (IX) is noted in FIG. 1 as an extraction method that can be used in combination with the exemplified leaching processes, one of ordinary skill in the art equipped with the teachings of this disclosure will understand/know that other extraction methods can be substituted instead. More details regarding these exemplary extraction methods are provided below.

(165) In this Example, 2% Ra separation/extraction was performed using the pregnant leach solution (PLS) obtained from the verification tests above. The recovery specifies the portion of the target element (e.g., 2% Ra) transferred into the final filtrate. It evaluates the process step including the loss of solution within the process step, e.g., by filtration or transferring solution. The recovery is calculated similar to the efficiencies from the results of the XRF and ICP-MS analysis and -spectroscopy of the PLS sampled from the filtrates related to the ICP-MS analysis of the ore feed (Equation 5):

(166) $\begin{array}{cc}\begin{array}{cc}{}_{\mathrm{el}}^{\mathrm{PLS}}=\frac{c_{\mathrm{el}}^{\mathrm{PLS}}.Math.m_{\mathrm{PLS}}.Math.{}_{\mathrm{PLS}}}{c_{\mathrm{el}}^{\mathrm{ore}}.Math.m_{\mathrm{ore}}}.Math.100\%& \mathrm{el}=\mathrm{element}\end{array}& \mathrm{Equation}\left(5\right)\end{array}$

(167) With n recovery of the element el calculated from i=PLS, c concentration of el in j=PLS and ore, m.sub.k mass of k=PLS and initial ore feed, respectively, .sub.j density of the j=PLS. For simplification, the recovery was set to 100% if the elemental concentration in the PLS was below detection limit and to 0% if a negative value was calculated.

(168) The error propagation of the .sup.226Ra leachability was calculated due to the partly very high measurement uncertainty of the -spectroscopy caused by the low activity of some samples according to Equation 6:

(169) $\begin{array}{cc}\left(x,y,.Math.\right)=\sqrt{{\left(\frac{}{x}\right)}^2{x}^2+{\left(\frac{}{y}\right)}^2{y}^2+.Math.}& \mathrm{Equation}\left(6\right)\end{array}$

(170) The following assumptions were made for the evaluation of the derived data: For the calculation of the leaching efficiency it is assumed that the total volume of the lixiviant is not affected by the leaching.

(171) Specific characteristics of the PLS, such as high salinity and/or a high Ca/Ra ratio, should be considered when selecting an appropriate Ra-separation technology. Described herein are exemplary methods for extracting Radium from a liquid, such as PLS/leachate. Extraction methods can include extracting/separating Ra (or other constituent of interest) by way of selective precipitation and/or ion exchange (IX) and/or solvent extraction (SX) and/or membrane technologies.

(172) For selective precipitation of (Ba,Ra) SO.sub.4, various precipitants such as NaSO.sub.4 or H.sub.2SO.sub.4 can be used. Notably, the inventors have discovered that selective Ra-precipitation as sulfate is kinetically hindered and that equilibration of the solution can be enhanced using longer retention times and an increase in temperature.

(173) Batch tests using ion exchange (IX) were conducted to demonstrate the selectivity of the IX resin used for .sup.226Ra extraction. The resin, LEWATIT MDS 200 H, commonly used for water treatment, was employed for the ion exchange. With increasing volume of PLS per wet settled resin (WSR), selectivity for Ba.sup.2+ and Ra.sup.2+ also increases. It was determined that a maximum resin loading of 3.68 nmol/L.sub.WSR Ra.sup.2+ (36308 Bq/L.sub.WSR) could be achieved. At the same time, Ca.sup.2+ loading was significantly reduced with increasing PLS volume. With increased selectivity for Ba.sup.2+ and Ra.sup.2+ but decreased selectivity for Ca.sup.2+, it was demonstrated that the LEWATIT MDS 200 H resin is highly selective for radium, making it suitable for .sup.226Ra separation.

(174) According to embodiments of the invention, for Ra separation from the PLS when the raffinate obtained is free of, or essentially free of, Radium and/or when the raffinate comprises only low concentrations of Ba and Sr, such raffinates can then be recycled in the process as a lixiviant for radium leaching again. This recycling option when implemented in the process would provide the additional benefits of reduced chemical consumption and wastewater production.

(175) In embodiments, the process can include aging the PLS and/or using an aged PLS as the feed solution for the .sup.226Ra extraction/separation process, such as ion exchange.

(176) In embodiments, the IX feed solution can be treated with one or more purification step, such as prior to subjecting the feed solution to ion exchange.

(177) The PLS obtained from the .sup.226Ra-selective leaching process described herein had high salinity and ionic strength, high density, gypsum saturation, and a high Ca/Ra ratio. In addition, due to the high Ca(NO.sub.3).sub.2 dosage (about 1 kg per kg PG) a process is envisaged in which the leaching solution can be recycled efficiently. Thus, selective precipitation and ion exchange (IX) were considered as suitable processes.

(178) For the selective precipitation, the (Ba,Ra) SO.sub.4 solid solution applies as a potential precipitate due to its low solubility in water (Brown, P. L., Matyskin, A. V., Ekberg, C., 2022, The aqueous chemistry of Radium, Radiochim. Acta, vol. 110, no. 6-9, p. 505-513). In addition, precipitation of hydroxides is also possible. The solubility of Ra(OH).sub.2 is the highest among the alkaline earth elements (Brown 2022) and, in addition, the precipitation of the different hydroxides is strongly pH dependent. Thus, a fractional precipitation of the single alkaline earth hydroxides might be possible. This can help to reduce the Ca/Ra ratio so that alternative separation processes may apply.

(179) .sup.226Ra Separation/Extraction by Selective Precipitation. .sup.226Ra separation by selective precipitation was performed on the PLS by two precipitation techniques, sulfate precipitation and hydroxide precipitation. The experimental procedure was identical for both. The general procedure included i) preparation of a 1 L glass vessel with feed solution (e.g., PLS); ii) adding precipitant (NaSO.sub.4 or H.sub.2SO.sub.4); intensive mixing for 24 h at room temperature by magnetic stirrer on a stirring plate; iii) after target retention time, filtration of suspension through 1-2 m filter paper; and iv) drying the leach residue at 105 C. and determining weight loss.

(180) In embodiments, the precipitant is chosen from one or more of NaOH, NaSO.sub.4 and/or H.sub.2SO.sub.4. In embodiments, the precipitant is concentrated or dilute.

(181) The experimental parameters of the precipitation tests are summarized in Table 19.

(182) TABLE-US-00019 TABLE 19 Parameters of selective precipitation with constant retention time (24 h) and temperature (RT) Dosage SO.sub.4.sup.2 PLS Source Precipitant (mmol/L) SP_1 VT_F-mix Na.sub.2SO.sub.4 (S) 0.76 SP_2 VT_F-mix Na.sub.2SO.sub.4 (S) 3.78 SP_3 VT_F-mix H.sub.2SO.sub.4 (90%) 0.76 SP_4 VT_F-mix H.sub.2SO.sub.4 (90%) 3.78 SP_5 VT_F-mix NaOH (s + 50%) addition to pH = 12 SP_6 VT_F-mix H.sub.2SO.sub.4 (10%) 1.45

(183) The dosing at SP_1 and SP_3 corresponds to 57% of the stochiometric required amount of sulfate, while the dosage of test SP_2 and SP_4 was 287% of the stochiometric amount.

(184) The experimental data is summarized in Table 20. The observed changes in pH, ORP and EC compared to the feed correspond to the expectation.

(185) TABLE-US-00020 TABLE 20 Experimental data for SP_1 to SP_6 ORP EC in A Test Pre- (SHE) mS/ in .sup.PLS.sub.Ra .sup.PLS.sub.Ra ID cipitant pH in mV cm Bq A in % in % VT- 3.57 445 118.5 385 94 F_mix (654) DMT_ Na.sub.2SO.sub.4 3.61 451 117.9 310 58 19.3 19.7 SP_1 (659) DMT_ Na.sub.2SO.sub.4 3.46 450 117.9 314 75 18.3 19.9 SP_2 (658) DMT_ H.sub.2SO.sub.4 2.20 514 118.2 296 88 23.0 18.8 SP_3 (723) DMT_ H.sub.2SO.sub.4 1.42 561 119.0 304 68 21.1 19.3 SP_4 (769) DMT_ NaOH 12.05 77 170.2 283 51 26.3 18.0 SP_5 (285) DMT_ H.sub.2SO.sub.4 1.96 475 118.0 340 59 11.7 21.5 SP_6 (682)

(186) The precipitation feed for the sulfate and hydroxide precipitation assays was prepared by mixing the filtrates from VT_2 to VT_4. The .sup.226Ra concentration was determined with A.sub.s=36689 Bq/L by -spectroscopy.

(187) Sulfate precipitation assays were performed using Na.sub.2SO.sub.4 and H.sub.2SO.sub.4, for formation of (Ba,Ra) SO.sub.4. During the tests with concentrated precipitants (SP_1 to SP_4) a chunky precipitate had formed immediately. To resuspend it and to force the quickest possible equilibration, the tests were stirred very intensively. A milky suspension had formed but still some larger agglomerates remained over in the filter cake. With diluted H.sub.2SO.sub.4, these agglomerates were not observed.

(188) .sup.226Ra precipitation for the sulfate precipitation assays using concentrated precipitant (SP_1-4) was low (e.g., respectively 19.3%, 18.3%, 23.0%, 19.3%). A slight correlation to Ba could be found. The highest .sup.226Ra precipitation of the sulfate precipitation assays was achieved for SP_3 with concentrated H.sub.2SO.sub.4 (90%) in an under-stochiometric dosage (i.e., 57% under stochiometric). The higher precipitation efficiencies are believed to demonstrate, however, that it is not an effect of sulfate dosage but kinetics and equilibration. As such, it is expected that further dilution and longer retention times would favor a more selective .sup.226Ra-precipitation and, thus, equilibration of the solution can be enhanced using a sulfate dosage close to stoichiometry, longer retention times and an increase in temperature.

(189) The hydroxide precipitation aimed for a Ca reduced solution for further separation. The addition of NaOH was conducted in three steps. For the first two steps, solid NaOH was used and for the last addition, a 50% NaOH solution was used. A white slurry formed, and the filtration process was slow. During the NaOH addition, the pH and Ca concentration (by XRF) were monitored. Even only a slight addition of NaOH caused a significant increase in pH. With further NaOH addition, the Ca concentration reduced significantly and only minor changes in pH could be measured. This trend in pH is very typical for the hydroxide precipitation since the hydroxide is consumed by the formed Ca(OH).sub.2.

(190) The selectivity of this precipitation is seen in the trends of the precipitation efficiencies of the alkaline earth elements, which precipitated in the order of increasing atomic numbers (Mg, Ca, Sr, Ba, Ra). The precipitation efficiency of Sr, Ba, and Ra is in the same order of magnitude and is probably due to adherent solution in the filter cake and a slight co-precipitation. With the precipitation efficiency of Ca being much higher, the cut between Ca and that of Sr, Ba and Ra is very clear. Thus, selective precipitation of Ca with hydroxide is a viable option.

(191) .sup.226Ra Separation/Extraction by Ion Exchange. Another technique that can be used for extraction/separation of .sup.226Ra is ion exchange (IX). With ion exchange, the selectivity of an IX resin is used to remove .sup.226Ra from the solution, which is then enriched in an elution step. The separation effect is based on similar mechanisms as that of solvent extraction (SX). Compared to this, however, IX has the advantage in the present application of being able to process large solution volumes efficiently. In addition, the separation effect in an IX column is given in two dimensions since the concentration of the solution changes over the column length. As shown by Bi et al. a strong acidic cation exchange resin is suitable for the separation of .sup.226Ra from high-total dissolved solid (TDS) brines (Bi, Y.; Zhang, H.; Ellis, B. R.; Hayes, K. F. Removal of Radium from Synthetic Shale Gas Brines by Ion Exchange Resin. Environmental Engineering Science 2016, 33 (10), 791-798. DOI: 10.1089/ees.2016.0002.) The enhanced selectivity for Ra.sup.2+ is attributed to the resin's greater affinity for larger cations, which possess lower hydration energy and can more effectively release water molecules during the ion exchange process (Bi, 2016).

(192) .sup.226Ra separation by ion exchange was performed on the PLS. The experimental procedure included: i) washing of resin with water to remove contaminants; ii) preparation of a reaction vessel with the feed solution (PLS); iii) adding resin (MDS 200H); iv) mixing for 24 h at room temperature with an overhead shaker; v) after target retention time, filtration of suspension through 5-13 m fluted filter. A 10 L large scale test was conducted in a larger vessel with a mechanical PTFE stirrer. The stirring speed was set low to ensure gentle agitation of the resin. The experimental parameters of the ion exchange separation are summarized in Table 21.

(193) TABLE-US-00021 TABLE 21 Test parameters of IX tests conducted with constant retention time (24 h) and temperature (RT) Feed Resin Volume ID Solution Volume in L Resin in mL DMT_IX_1 DMT_VT 1.05 MDS 200H 10.25 DMT_IX_2 DMT_VT 1.05 MDS 200H 102.6 DMT_IX_3 DMT_VT 10.00 MDS 200H 10.38 DMT_IX_4 DMT_VT 0.53 MDS 200H 202.6

(194) The analytical program consisted of: i) pH, ORP, EC determination of the solution samples at ambient conditions; ii) Elemental analysis of filtrate by XRF; iii) Elemental analysis by ICP-MS/OES of selected samples; and iv) -spectroscopy of the filtrate.

(195) Experimental data were evaluated to determine the efficacy of .sup.226Ra separation from impurities and alkaline earth elements. The following parameters were calculated to evaluate the process.

(196) For the IX test work a strongly acidic gel type cation exchange resin in its H.sup.+ form, LEWATIT MDS 200 H, was used. The functional group of this resin is sulfonic acid, and it is a resin for IX chromatography with very small beads of a uniform size. This resin is similar to the Purolite C100E cation exchange resin used by Bi at al. (Bi, 2016).

(197) To evaluate the selectivity of the resin towards .sup.226Ra, batch tests with varying volumes of wet settled resin (WSR) and feed solution were conducted. The aim of these tests was not the complete extraction of .sup.226Ra but to achieve the maximum loading of the resin with .sup.226Ra to determine the applicability of the selected resin for the separation of .sup.226Ra.

(198) Table 22 summarizes the experimental data of the tests:

(199) TABLE-US-00022 TABLE 22 Experimental data for IX_1 to IX_4 ORP EC in Test (SHE) mS/ A.sub.s in .sup.PLS.sub.Ra .sup.PLS.sub.Ra ID Resin pH in mV cm Bq/L A.sub.s in % in % VT- 3.57 445 118.5 366.2 89.3 F_mix (654) DMT_ MDS 1.15 504 120.5 251.7 71.4 31.8 16.6 IX_1 200H (714) DMT_ MDS 0.23 727 140.5 270.3 58.1 27.4 17.7 IX_2 200H (936) DMT_ MDS 2.16 536 118.70 332.8 64.2 10.3 21.9 IX_3 200H (743) DMT_ MDS 0.25 825 201.00 250.6 58.6 31.6 16.7 IX_4 200H (1033)

(200) It can be seen that the acidity of the obtained solution increases due to the exchange of protons of the resin with cation of the solution. The ORP increased with decreasing pH due to the formation of HNO.sub.3.

(201) Based on the elemental composition of the extraction raffinates and the theoretical capacity of the resin (2.3 eq/L.sub.WSR) the relative load of the resin was calculated (cf. FIG. 12A). Loading of IX resin in % is shown in FIG. 12A and Ra load depending of PLS/resin ratio in 12B.

(202) In FIG. 12B it can be seen that the reduction of available binding sites (by increasing the volume of PLS per WSR) leads to an increase in selectivity for Ba.sup.2+ and Ra.sup.2+. In test IX_1, a resin loading of 1.21 nmol/L.sub.WSR Ra.sup.2+ (11928 Bq/L.sub.WSR) could be achieved. Due to the small effect of the exchange reaction on the overall Ca concentration, the extraction efficiencies of test IX_1 and IX_3 were .sup.PLS=0. Therefore, the Ca concentration of the solutions was calculated assuming that Ca occupied all binding sites not occupied by the other particular ions. This assumption ignores the competition with H ions, therefore the indicated amount of Ca extracted is the sum of Ca.sup.2+ and H.sup.+ at the resin. For Test IX_4 (2.6 L PLS/L.sub.WSR) this assumption did not apply. The Ca extraction is .sup.PLS=18% which leads to a difference of 6.9% with respect to the capacity of the resin. This difference can be explained by the increasing acidity of the solution and the associated increased concurrence to H. In addition, there is the relatively high influence of the analysis error of the Ca determination.

(203) FIG. 13 shows the load of Ra per WSR depending on the ratio between PLS and WSR. In particular, for IX_3 a resin loading of 3.69 nmol/L.sub.WSR Ra.sup.2+ (36309 Bq/L.sub.WSR).

(204) At the same time, the Ca loading was greatly reduced. Thus, the resin is highly selective for Ra which makes is applicable for Ra separation.

(205) In an IX process, the resin is a packed column over which the feed solution is passed. In this case, the exchange process retains all ions in the order of their affinity and the raffinate is free of these ions and strongly acidic due to the exchange of the H. As the load increases, the concentration of ions in the raffinate increases until the feed concentration is reached. This is the so-called breakthrough of the respective ion. Based on the results of tests IX_3, at least 1000 L of PLS can be processed with 1 L of WSR to remove Ra.sup.2+ from solution.

(206) In addition to further verification tests, the prior purification of the PLS is recommended to remove the competing ions such as Fe.sup.3+ and Al.sup.3+. A pH adjustment to pH=7 could already be sufficient for this.

(207) After loading the resin with ions, they can be eluted with a salt solution or a strong acid. Based on the results of the present tests, the different affinity of the ions to the resin can also be exploited and these can be fractionally washed out of the resin with, for example, a pH gradient, thus obtaining an additionally purified .sup.226Ra fraction. For this step, the type of acid is selectable and can be adjusted to the .sup.226Ra production process that would follow.

(208) In embodiments, IX shows high affinity of the Ra to the resin and the obtained IX raffinate is ideally free of Ra and shows only low concentrations of Ba and Sr. Thus, optionally after neutralization of the increased acidity, this solution can again be used (recycled) as a lixiviant for .sup.226Ra leaching. This recycling reduces chemical costs and reduces waste streams of the process.

EXAMPLE 5Purifying Phosphogypsum

(209) Methods of embodiments of the invention include methods of purifying phosphogypsum. For example, PG (fresh or aged/stockpiled) can be treated with a leach solution (lixiviant) to remove .sup.226Ra from the PG. Such lixiviants include one or more of: H.sub.2O (water, de-ionized water, tap water, rain water, etc.); NaCl (such as 1-20%, such as 5%, 10% or 15% NaCl); a solution of NaCl+BaCl.sub.2 (such as 10% NaCl+0.1% BaCl.sub.2, or 20% NaCl+0.2% BaCl.sub.2, or 1-20% NaCl+0.05-0.5% BaCl.sub.2,); Ca(NO.sub.3).sub.2 (such as 2-40% Ca(NO.sub.3).sub.2, such as 5%, 10%, 15%, 20%, 25%, 30%, or 35% Ca(NO.sub.3).sub.2); NH.sub.4+Ac (such as 2-30% NH.sub.4Ac, such as 5%, 10%, 15%, 20%, or 25% NH.sub.4Ac); lime water (e.g., saturated lime water); NaOH (such as 5-30% NaOH, such as 5%, 10%, 15%, 20%, or 25% NaOH); H.sub.2SO.sub.4 (such as 2-20% H.sub.2SO.sub.4, such as 5%, 10%, or 15% H.sub.2SO.sub.4); HNO.sub.3 (such as 2-20% HNO.sub.3, such as 3%, 4%, 5%, 10%, or 15% HNO.sub.3); H.sub.2Ox (such as 2-20% H.sub.2Ox, such as 5%, 10%, or 15% H.sub.2Ox); a solution of Ca(NO.sub.3).sub.2+HNO.sub.3 (such as 5% Ca(NO.sub.3).sub.2+3% HNO.sub.3, such as 10% Ca(NO.sub.3).sub.2+5% HNO.sub.3, such as 15% Ca(NO.sub.3).sub.2+2-10% HNO.sub.3, such as 5% Ca(NO.sub.3).sub.2+5-15% HNO.sub.3, such as 10% Ca(NO.sub.3).sub.2+2-10% HNO.sub.3). In particular embodiments, the lixiviant comprises one or more of Ca(NO.sub.3).sub.2, HNO.sub.3. NH.sub.4Ac, and/or H.sub.2Ox.

(210) The lixiviant can be allowed to react with the PG for 2-6 hours at a temperature in the range of about 40-70 C. to obtain a pregnant leach solution (PLS) (leachate comprising .sup.226Ra) and solids. The solids (leach residue) can be separated from the leachate/PLS by one or more physical or chemical processes, such as filtering. At least a portion of the solids can be further processed to provide NORM-free gypsum and/or NORM-free fertilizer. For example, the NORM-free gypsum and/or fertilizer can be processed in a manner such that the .sup.226Ra concentration is in an amount of no more than about 20 picocuries per gram (pCi/g), no more than about 15 pCi/g, no more than about 10 pCi/g, no more than about 7 pCi/g, or about 5 picocuries per gram (pCi/g) or less. In particular embodiments, the .sup.226Ra concentration of the NORM-free gypsum and/or fertilizer is in an amount of about 10 pCi/g or less.

(211) Additionally or alternatively, the NORM-free gypsum and/or fertilizer can have a concentration of Pb, Cr and/or Cd (or other contaminants, such as heavy metals) below a selected limit. For example, one or more separation techniques can be performed to remove or reduce the amount of any one or more of the following constituents: one or more alkaline earth metals, including for example radium, calcium, barium; one or more of thorium, uranium, fluorine, phosphorus, phosphate; naturally occurring radioactive material (NORM); technologically enhanced naturally occurring radioactive material (TENORM); gypsum; heavy metals, such as Zn, Cr, Mn, Ni, Pb, Cd, As, Hg, Ag, Cu, Fe, Pd, Pt; rare earth elements (REEs), such as: i) Sb, Be, Co, Ga, Ge, Mg, In; ii) Platinum Group Elements (PGMs), such as platinum, palladium, rhodium, ruthenium, iridium, osmium; iii) Nb and Ta; iv) lanthanides (such as light REEs (LREEs), including La, Ce, Pr, Nd, Pm, Sm, and heavy rare earth elements (HREEs), including Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu); and/or to remove/reduce the amount of Sc, Y (an HREE).

(212) In particular embodiments for NORM-free fertilizer, the fertilizer can be processed in such manner to comprise no more than 20 mg kg.sup.1 P.sub.2O.sub.5.

(213) For example, as described by Weizhen Liu, et al., Effective Extraction of Cr(VI) from Hazardous Gypsum Sludge via Controlling the Phase Transformation and Chromium Species, Environ. Sci. Technol. 2018, 52, 22, 13336-13342 (Oct. 24, 2018), DOI: 10.1021/acs.est.8b02213, the dissolution-recrystallization of CaSO.sub.4.Math.2H.sub.2O into CaSO.sub.4 can be used to release encapsulated Cr(VI) from phosphogypsum, with a persulfate salt providing H and SO.sub.4.sup.2 ions. The H ions provide for an acidic condition for transforming CrO.sub.4.sup.2 into Cr.sub.2O.sub.7.sup.2 with less similarity to SO.sub.4.sup.2, which prevents the recombination of the Cr(VI) with gypsum. The SO.sub.4.sup.2 ions accelerate crystal growth of calcium sulfate and enhance Cr(VI) extraction. Such removal of Cr can be performed before or after removal of .sup.226Ra from the phosphogypsum.

(214) Lead can be removed before or after either of .sup.226Ra or Cr removal. Techniques for removing/reducing the amount of lead, if present, can include the remediation of lead contaminated gypsum sludge by washing the hazardous waste with hydrochloric acid (HCl) and ethylenediaminetetraacetic acid (EDTA) solutions. In some cases, EDTA is more efficient than HCl in lead extraction, but either can be used. Further, based on its high lead removal efficiency and lesser dissolution of solids, 0.1 M EDTA is considered as a most effective washing agent for removing lead from contaminated gypsum sludge. See, e.g., GC Raju, et al., Remediation of Lead Contaminated Gypsum Sludge, Geotechnical Special Publication, DOI: 10.1061/40970 (309) 71, Conference: GeoCongress (March 2008).

(215) The present disclosure has described particular implementations having various features. In light of the disclosure provided herein, it will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope or spirit of the disclosure. One skilled in the art will recognize that the disclosed features may be used singularly, in any combination, or omitted based on the requirements and specifications of a given application or design. When an implementation refers to comprising certain features, it is to be understood that the implementations can alternatively consist of or consist essentially of any one or more of the features. Other implementations will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure.

(216) It is noted in particular that where a range of values is provided in this specification, each value between the upper and lower limits of that range is also specifically disclosed. The upper and lower limits of these smaller ranges may independently be included or excluded in the range as well. The singular forms a, an, and the include plural referents unless the context clearly dictates otherwise. It is intended that the specification and examples be considered as exemplary in nature and that variations that do not depart from the essence of the disclosure fall within the scope of the disclosure. Further, all of the references cited in this disclosure including patents, published applications, and non-patent literature are each individually incorporated by reference herein in their entireties and as such are intended to provide an efficient way of supplementing the enabling disclosure as well as provide background detailing the level of ordinary skill in the art.