Method to make phosphoric acid and ammonium phosphates from phosphate minerals

Abstract

A method of producing ammonium phosphates from at least one mineral containing phosphate and an element which is calcium, magnesium, iron, or aluminum. The method includes contacting the at least one mineral (or a combination of them) with a cation exchanger for a time and at a temperature sufficient to yield phosphoric acid from the mineral.

Claims

1. A method of producing ammonium phosphates comprising: contacting at least one mineral with a cation exchanger for a time and at a temperature sufficient to yield an ammonium phosphate from the mineral, wherein the mineral comprises phosphate and at least one element selected from the group consisting of calcium, magnesium, iron, and aluminum, and wherein the mineral requires from 30 to 1000 mass parts of water of pH 7.0 to one mass part of the mineral to dissolve the mineral and has increased solubility in acidic water having a pH range from about 2.0 to less than 7.0, and wherein the mineral is brushite.

2. The method of claim 1, wherein the cation exchanger comprises a solid substrate functionalized with a cation exchange moiety selected from the group consisting of carboxylic acid, sulfonic acid, phosphonic acid, carboxyalkyl, sulfoalkyl, phosphoalkyl, sulfoxyalkyl, and orthophosphate.

3. The method of claim 1, wherein the at least one mineral is isolated from a water stream.

4. The method of claim 1, wherein the cation exchanger is acid-charged.

5. The method of claim 1, wherein the ammonium phosphate produced is selected from the group consisting of mono-ammonium phosphate, di-ammonium phosphate, and ammonium poly phosphate.

6. A method of producing ammonium phosphates comprising: (a) isolating from a water stream at least one mineral, wherein the mineral comprises phosphate and at least one element selected from the group consisting of calcium, magnesium, iron, and aluminum, and wherein the mineral requires from 30 to 1000 mass parts of water of pH 7.0 to one mass part of the mineral to dissolve the mineral and has increased solubility in acidic water having a pH range from about 2.0 to less than 7.0, and wherein the mineral is brushite; and (b) contacting the at least one mineral with a cation exchanger for a time and at a temperature sufficient to yield an ammonium phosphate from the mineral.

7. The method of claim 6 wherein the cation exchanger of step (b) comprises a solid substrate functionalized with a cation exchange moiety selected from the group consisting of carboxylic acid, sulfonic acid, phosphonic acid, carboxyalkyl, sulfoalkyl, phosphoalkyl, sulfoxyalkyl, and orthophosphate.

8. The method of claim 6, wherein step (a) comprises isolating the at least one mineral from a waste-water stream.

9. The method of claim 6, wherein the cation exchanger is acid-charged.

10. The method of claim 6, wherein the ammonium phosphate produced is selected from the group consisting of mono-ammonium phosphate, di-ammonium phosphate, and ammonium poly phosphate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1. United States phosphorus fertilizer use: di-ammonium phosphate (DAP), ammonium poly phosphate (APP), mono-ammonium phosphate (MAP), triple super phosphate (TSP), single super phosphate (SSP). (From Havlin et al., 2014. Soil Fertility and Fertilizers: an introduction to nutrient management, 8th Ed. Pearson Inc., New Jersey.)

(2) FIG. 2. World phosphorus fertilizer use: mono-ammonium phosphate (MAP), di-ammonium phosphate (DAP), single super phosphate (SSP), other phosphorus and nitrogen fertilizer (Other NP), other phosphate fertilizer (Other P), and rock phosphate (RP). (Havlin et al., supra.)

(3) FIG. 3. Flow diagram of rock phosphate ore processing through various stages of end products with a description of their uses.

DETAILED DESCRIPTION

Abbreviations and Definitions

(4) APP=ammonium polyphosphate. DAP=diammonium phosphate. MAP=monoammonium phosphate. Other NP=other phosphorus and nitrogen fertilizer. Other P=other phosphate fertilizer. RP=rock phosphate. SSP=single superphosphate. TSP=triple superphosphate.

(5) Ion exchanger is broadly defined herein to refer to any inert resin, support, or substrate that is dimensioned, configured, and capable of acting as a medium for ion exchange, without limitation. Anionic exchangers exchange one type of anion for another (i.e., exchange a first negatively charged ion for a second negatively charged ion). Cation exchangers exchange one type of cation for another (i.e., exchange a first positively charged ion for a second positively charged ion). An ion exchanger comprises an insoluble inert matrix (or support) of any shape or size (but preferably in the form of small beads ranging from roughly 0.1 mm to 1 mm), fabricated from any suitable inert material (typically an organic polymer). As used herein, the term resin is used synonymously with matrix and support and includes both organic, polymeric supports and inorganic supports. The matrix is preferably porous, thus providing a large surface area for ion exchange to take place. Most commercial ion exchangers use a substrate comprising a cross-linked polystyrene matrix that has been functionalized to includes an ion exchange moiety. The substrate can be further functionalized to provide improved or specific ion exchange functionality. A very large number of ion exchangers, both anionic and cationic, are available commercially from global suppliers. Typical cation exchangers are functionalized to contain an acidic moiety that will exchange hydrogen cations, H.sup.+ or H.sub.3O.sup.+, for cations present in an aqueous solution. The acidic moiety may be, for example (and not by way of limitation), carboxylic acid/carboxylate (COO.sup.), sulfonic acid/sulfonate (SO.sub.3.sup.), or phosphonic acid/phosphonate (H.sub.2PO.sub.3.sup. [pK.sub.a 1.3], HPO.sub.3.sup.2 [pK.sub.a 6.7]). A non-exclusive list of commercially available cationic ion exchangers that can be used in the present method include those having a sulfonate-based ion exchange group, such as MonoS, MiniS, Source 15S and 30S, SP SEPHAROSE FAST FLOW, and SP SEPHAROSE High Performance, all from GE Healthcare Bioprocess R&D AB (Uppsala, Sweden), TOYOPEARL SP-650S and SP-650M from Tosoh Corporation (Yamaguchi, Japan), Macro-Prep High S from BioRad Laboratories, Inc. (Hercules, California, USA), Ceramic HYPERD S, TRISACRYL M and LS SP and Spherodex LS SP from Pall Corporation (Port Washington, New York, USA); a sulfoethyl-based ion exchange group, such as FRACTOGEL SE from Merck KGAA (Darmstadt, Germany) and POROS S-10 and S-20 from Applied Biosystems, LLC (Foster City, California, USA); a sulfopropyl-based exchange group, such as TSK Gel SP 5PW and SP-5PW-HR from Tosoh Corporation, POROS HS-20 and HS 50 from Applied Biosystems; a sulfo-isobutyl-based group, such as FRACTOGEL EMD SO.sub.3 from Merck KGAA; a sulfoxyethyl-based group, for example SE52, SE53 and Express-Ion S from Whatman plc (a wholly owned subsidiary of GE Healthcare, Little Chalfont, Buckinghamshire, England), a carboxymethyl-based ion exchange group, such as CM SEPHAROSE Fast Flow from GE Healthcare, Hydrocell CM from BioChrom Labs Inc. (Foster City, California, USA), Macro-Prep CM from BioRad, Ceramic HYPERD CM, TRISACRYL M CM, TRISACRYL LS CM, from Pall Corporation, Matrex CELLUFINE C500 and C200 from JNC Corporation (Tokyo, Japan), CM52, CM32, CM23 and Express-Ion C from Whatman, TOYOPEARL CM-650S, CM-650M and CM-650C from Tosoh Corporation; sulfonic- and carboxylic acid-based groups, such as BAKERBOND Carboxy-Sulfon from Avantor Performance Materials, LLC (Center Valley, Pennsylvania, USA); a carboxylic acid-based group, for example WP CBX from Avantor, DOWEX MAC-3 from Dow Chemical Company (Midland, Michigan, USA), AMBERLITE-brand weak cation exchange resins (Rohm and Hass Company, Philadelphia, Pennsylvania, USA), DOWEX weak cation exchangers (Dow Chemical), DIAION-brand cation exchangers from Mitsubishi Chemical Corporation, and FRACTOGEL COO from Merck KGAA; a sulfonic acid-based ion exchange group, such as Hydrocell SP from BioChrom Labs, DOWEX Fine Mesh Strong Acid Cation Resin from Dow Chemicals, UNOsphere S and WP Sulfonic from Avantor, SARTOBIND S-brand membrane from Sartorius Stedim Biotech GmbH (Goettingen, Germany), AMBERLITE-brand strong cation exchangers (Rohm and Hass), DOWEX strong cation exchangers (Dow Chemical) and DIAION-brand cation exchangers from Mitsubishi Chemical; and an orthophosphate-based group, such as pI 1-brand resins from Whatman. A cation exchange membrane may be used (instead of a beads or other discrete particles), for example SARTOBIND S-brand membranes from Sartorius.

(6) Solubility is defined by the International Union of Pure and Applied Chemistry as follows: The analytical composition of a saturated solution, expressed in terms of the proportion of a designated solute in a designated solvent, is the solubility of that solute. The solubility may be expressed as a concentration, molality, mole fraction, mole ratio, etc. (See IUPAC. Compendium of Chemical Terminology, 2nd ed. (the Gold Book). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8.) Solubility is described herein using the long-established set of descriptive terms set forth in the U.S. Pharmacopoeia (see usp.org and Pharmacopeia of the United States of America, 32nd revision and the National Formulary, 27th edition, 2009, p. 1-12.)

(7) TABLE-US-00001 Mass parts of solvent required to dissolve 1 mass Term part of solute Very soluble <1 Freely soluble 1 to 10 Soluble 10 to 30 Sparingly soluble 30 to 100 Slightly soluble 100 to 1,000 Very slightly soluble 1,000 to 10,000 Practically insoluble or insoluble >10,000

(8) Numerical ranges as used herein are intended to include every number and subset of numbers contained within that range, whether specifically disclosed or not. Further, these numerical ranges should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 2 to 8, from 3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, and so forth.

(9) All references to singular characteristics or limitations of the present method shall include the corresponding plural characteristic or limitation, and vice-versa, unless otherwise specified or clearly implied to the contrary by the context in which the reference is made. The indefinite articles a and an mean one or more.

(10) All combinations of method or process steps as used herein can be performed in any order, unless otherwise specified or clearly implied to the contrary by the context in which the referenced combination is made.

(11) The method disclosed herein can comprise, consist of, or consist essentially of the essential steps and/or limitations of the method as described, as well as any additional or optional steps or limitations described herein or otherwise useful in isolating phosphorus-containing solutions.

(12) The Method:

(13) As noted above, a growing interest in removing phosphorus from wastewater for water quality improvement and nutrient recycling has led to two major processes to recover phosphorus using crystallization reactions: the struvite (magnesium ammonium phosphate hexahydrate) process and the brushite (dicalcium phosphate dihydrate) process. In the struvite process, magnesium is added to an anaerobic digest, usually as its chloride salt but sometimes as a base (magnesium oxide/hydroxide) to raise the pH of the digest. Phosphorus is then recovered in the form of struvite, which precipitates from the reaction mixture. In the brushite process, the effluent from an organic acid digester is reacted with calcium hydroxide to raise the pH of the effluent and to supply an excess concentration of calcium for complete brushite formation (which precipitates from the reaction).

(14) Use of Brushite and Struvite as a Slow-Release High-Grade Phosphorus Fertilizer:

(15) Since the 1970's, the consumption of phosphorus fertilizers in the U.S. has approximately doubled. See FIG. 1. Over the same time period, global consumption has increased approximately six times. See FIG. 2. Regarding the sharp increase in global fertilizer use, this is due to increased global distribution of fertilizers, coupled with the conversion of large areas of land in tropical zones to agricultural use. Phosphorus needs in tropical climates are higher than in temperate regions, and made higher still when agricultural land is managed more intensively to raise productivity.

(16) The current U.S. and international phosphorus fertilizer markets are dominated by monoammonium phosphate (MAP), diammonium phosphate (DAP), Triple Superphosphate (TSP, monocalcium phosphate) and ammonium polyphosphate (APP) (US consumption only; see FIG. 1), along with single superphosphate (SSP) (international consumption only; see FIG. 2.). These materials have a high P.sub.2O.sub.5 content (48-61%, 46-53%, 39-53%, 34-37%, and 16-20% P.sub.2O.sub.5 respectively), high water solubility, and a high availability to plants. December 2017 prices were $385/ton for DAP and $278/ton for TSP, while low phosphorus-availability rock phosphate ore had a 2017 value of $80/ton (Index Mundi. 2017. Rock Phosphate Monthly PriceUS Dollars per Metric Ton. On the web at: http://www.indexmundi.com/(verified Feb. 28, 2017).)

(17) At present, neither brushite nor struvite are recognized phosphorus fertilizers in the US nor globally, although they contain 41% and 29% P.sub.2O.sub.5, respectively. Relatively little scientific literature, and no field trials, are currently available to make a clear assessment of the value of brushite as a phosphorus source for plants. Greenhouse experiments have shown one-to-one equivalency with conventional phosphorus fertilizers. (See Anderson, T. 2015. Production and Evaluation of Wastewater-Derived Brushite As Fertilizer, University of WisconsinMadison, MS Thesis.) Brushite is only sparingly soluble in water (solubility product of 1.8710.sup.7 mol L.sup.1); at pH 6.5 brushite can sustain levels of 0.43 mM phosphorus or 13 mg phosphorus L.sup.1 (13 ppm) in water.

(18) (See Koutsoukos, P. G. and Valsami-Jones, E. 2004. Phosphorus in Environmental Technologies: Principles and Applications, pp. 193-248, IWA Publishing, London, UK.) In comparison, Matula (2011) reports that 0.8 ppm phosphorus in soil solution is adequate for growth of barley and measured a maximum soil solution phosphorus of 4.3 ppm for 72 soils with widely varying agronomic characteristics. (Matula, J. 2011. Relationship between phosphorus concentration in soil solution and phosphorus in shoots of barley. Plant Soil and Environment 57:307-314.) More literature exists on stuvite as a fertilizer, mostly at the greenhouse level, again showing a near one-to-one equivalency with standard phosphorus fertilizers. (Kataki, S, H West, M Clarke and D C Baruah. 2016. Phosphorus recovery as struvite: Recent concerns for use of seed, alternative Mg source, nitrogen conservation and fertilizer potential. Resources, Conservation and Recycling 107:142-156.).

(19) Using brushite and struvite recovered from wastewater directly as phosphorus fertilizers is likely the most desirable use of upcycled phosphorus because it requires the least additional processing. However, their use in commercial quantities is predicated upon: 1) brushite and struvite being at least as effective as the conventional phosphorus fertilizers currently in use; and 2) the willingness of farmers to switch to brushite or struvite if they are demonstrably equivalent in performance and price.

(20) Use of Upcycled Brushite and Struvite to Manufacture High-Grade Phosphorus Fertilizers:

(21) Brushite and struvite may be considered as ore for the manufacture of standard phosphorus fertilizers such as TSP, DAP and MAP. Currently, commercial production of phosphate fertilizers uses mined phosphate rock as a raw material. Although more than 300 phosphate minerals are known, only minerals in the apatite group occur in sufficient abundance and concentration to serve as commercial sources of the element (D. E. C Cobridge, Phosphorus: Chemistry, Biochemistry, and Technology, 6.sup.th ed. CRC Press, Boca Raton, FL, 2010). Apatite has a crystal formula of Ca.sub.10(PO.sub.4).sub.6(OH, F, Cl).sub.2 and mineralogical endmembers are referred to as hydroxylapatite, fluorapatite, and chlorapatite, respectively. Such minerals are known to have very low solubility. The raw ore is processed into phosphoric acid using strong acid. The phosphoric acid is then further reacted with rock phosphate to make TSP, or is reacted with anhydrous ammonia to make MAP and DAP. See FIG. 3.

(22) Using brushite and struvite as phosphate ores to replace phosphate rock in the production of conventional phosphorus fertilizers has many benefits. These benefits include: 1) Using less sulfuric acid for acidulation and therefore producing less gypsum byproduct; 2) using a softer raw material having a fine particle size; there is no need for additional grinding of the brushite and struvite; 3) no extra processing to remove naturally occurring contaminants found in rock phosphate ores, such as fluorine or radionuclides; 4) similarly, no extra processing to remove metal contaminants (Pb, Cd, Hg, etc.) found in rock phosphate ores; and 4) using a raw material with a naturally low iron, magnesium and aluminum content, thereby yielding a purer phosphoric acid product with far less processing.

(23) In the conventional wet process for making phosphoric acid, rock phosphate is slaked with a strong aqueous solution of sulfuric acid. The resulting phosphoric acid is produced by the following reaction:
Ca.sub.3(PO.sub.4).sub.2(s)+3H.sub.2SO.sub.4+6H.sub.2O=>2H.sub.3PO.sub.4+3CaSO.sub.4.Math.2H.sub.2O(s)[1]

(24) Here the relatively low solubility of gypsum, CaSO.sub.4.Math.2H.sub.2O (2.55 g/L) is employed to remove calcium and sulfate from solution. In practice, though, separating the liquid phosphoric acid, H.sub.3PO.sub.4, from the large mass of fine gypsum solids is technically challenging. Centrifugation is required, which consumes large amounts of energy, and a significant amount of phosphate is retained in the gypsum discard. (In fact, the discarded gypsum is referred to in the industry as phosphogypsum. Despite its relative safety, the U.S. E.P.A. prohibits use of phosphogypsum in gypsum products such as wall board and plaster due to its radium content as comparted to natural, mined gypsum. See http://www.fipr.state.fl.us/about-us/phosphate-primer/phosphogypsum-and-the-epa-ban/; verified Jan. 4, 2019.) The phosphoric acid produced is typically too dilute to be sold as is. It must be concentrated by evaporation (also energy intensive) to yield a salable product.

(25) The conventional wet process of making phosphoric acid is used to make the vast majority of the phosphoric acid and phosphate fertilizers sold worldwide. The wet process, however, produces approximately 5 tons of phosphogypsum for every ton of P.sub.2O.sub.5. As a result, the wet process has been described (ironically) as a method of making phosphogypsum, with phosphoric acid as a valuable byproduct. (Elgafi, M.I. 2017. Phosphoric Acid and Phosphate FertilizersVolume I: State of the Art and Future Trends, Pacific Academic Press (Forest Grove, Oregon, USA), ISBN 978-0692926062.) Great effort is exerted to filter the phosphoric acid from the mass of gypsum, including using belt filters, tilting pan filters and/or rotating table filters, and washing the gypsum to remove entrained phosphoric acid.

(26) Applying the conventional wet process to brushite (i.e., treating brushite with aqueous sulfuric acid) gives the following reaction:
CaHPO.sub.4.Math.2H.sub.2O(s)+H.sub.2SO.sub.4=>H.sub.3PO.sub.4+CaSO.sub.4.Math.2H.sub.2O(s)[2]

(27) Although only two-thirds of the sulfuric acid is required for reaction [2] as for reaction [1] and only two-thirds of the amount of gypsum is generated per unit of phosphoric acid produced, here too the soluble phosphoric acid product is entrained in a mass of fine particles of gypsum and the required solid/liquid separation is difficult.

(28) Treating struvite with sulfuric acid to produce phosphoric acid follows reaction [3]:
NH.sub.4MgPO.sub.4.Math.6H.sub.2O(s)+3/2H.sub.2SO.sub.4=>H.sub.3PO.sub.4+NH.sub.4.sup.++Mg.sup.2++3/2SO.sub.4.sup.2+6H.sub.2O[3]

(29) Upon acidulation of struvite with sulfuric acid, the high solubility of magnesium sulfate351 g/L at 20 C., approximately 100 times the solubility of gypsumleaves high concentrations of magnesium and sulfate ions in the reaction mix. The wet process does not satisfactorily accommodate adding of struvite to the rock phosphate.

(30) Treating Brushite and Struvite with Solid-Phase Ion Exchangers:

(31) Ion exchangers are insoluble materials carrying reversibly fixed ions that may be stoichiometrically exchanged for other ions of the same sign. Ion exchangers, particularly currently commercially available ion exchangers, are cross-linked polymers carrying fixed functional groups or sites. Among the cation exchange resins, sulfonate (SO.sub.3.sup.) and carboxylate (CO.sub.2.sup.) groups lead to strong and weak acid cation exchangers, respectively. Ion exchange resins are commonly available as beads of defined sizes, ranging from micrometers to millimeters in diameter.

(32) A cation exchange resin in the acid form may be denoted as RH.sup.+ and the reaction of brushite with a cation exchange resin in the acid form follows the following overall reaction:
CaHPO.sub.4.Math.2H.sub.2O(s)+2RH.sup.+(s)=>H.sub.3PO.sub.4+2RCa.sup.2+(s)+2H.sub.2O[4]

(33) This reaction does not introduce the sulfate ion with sulfuric acid and no gypsum is formed. Further, the calcium ions derived from the brushite are removed by the ion exchange process, which is preferential for divalent ions over monovalent ions such as H.sup.+ (except in the case of weak acid exchangers that have a higher specificity for H+). Solid/liquid separation is therefore greatly simplified because the only step required is a simple filtration to remove the millimeter-sized resin beads and a small amount of insoluble residue from the phosphoric acid.

(34) The Ca.sup.2+-charged cation exchange resin can be regenerated by treating it with any strong acid, concentrated or not, including sulfuric acid:
2RCa.sup.2+(s)+H.sub.2SO.sub.4+2H.sub.2O=>2RH.sup.+(s)+CaSO.sub.4.Math.2H.sub.2O(s)[5]

(35) The overall reaction between brushite and sulfuric acid with the cation exchange resin as the intermediary is the same as the reaction without the cation exchange resin. However, the cation exchange resin employed in this manner prevents the mixing of the phosphoric acid product with the gypsum byproduct and thereby removes the need for and difficulty of a solid/liquid separation.

(36) Partial acidulation of struvite with a cation exchanger follows the reaction:
NH.sub.4MgPO.sub.4.Math.6H.sub.2O(s)+2RH.sup.+(s)=>NH.sub.4.sup.++H.sub.2PO.sub.4.sup.+2RMg.sup.2+(s)+6H.sub.2O[6]

(37) Here the soluble product is monoammonium phosphate, MAP, a well-known and high-value fertilizer.

(38) Complete acidulation of struvite proceeds by reaction [7]:
NH.sub.4MgPO.sub.4.Math.6H.sub.2O(s)+3RH.sup.+(s)=>H.sub.3PO.sub.4+2RMg.sup.2+(s)+RNH.sub.4.sup.+(s)+6H.sub.2O[7]

(39) Reactions of the ion exchanger with brushite and struvite take advantage of the fact that both minerals are sparingly soluble. The ion exchange process replaces soluble Ca.sup.2+, in the case of brushite, or Mg.sup.2+ and NH.sub.4.sup.+ in the case of struvite, with equivalent amounts of H.sup.+, which lowers the pH of the reaction mix and in turn increases the solubility of these minerals. This increase in solubility releases yet more of their constituent cations until the minerals are fully dissolved. The phosphoric acid produced in one round of brushite and struvite dissolution could be added to a subsequent round to speed dissolution, after which the acid-charged ion exchanger could be added to remove the cations produced by mineral dissolution.

Experimental Results

(40) Experiments were conducted with lab-synthesized brushite. To 10 mmoles of RH.sup.+ (DOWEX-brand 50W-X8; Dow Chemicals), amounts of brushite were added that corresponded to molar ratios of 0.5, 1, 2, and 4 moles brushite per mole RH.sup.+. The mixes were agitated and left overnight to react, after which they were filtered and the filtrates analyzed.

(41) As seen in the table below, supplying two (2) moles of RH.sup.+ for each mole of brushite reaches the maximum P solubilization (67%,) and that the soluble calcium concentration is low due to the ion exchange between H.sup.+ and Ca.sup.2+.

(42) TABLE-US-00002 P Ca P Brushite Molar Concn, Concn, Solubilized, Added, g ratio mM mM % 20 0.5 4.9 2.3 25% 10 1 5.1 2.4 51% 5 2 3.4 0.6 67% 2.5 4 1.6 0.0 66%

(43) Additional experiments with a NH.sub.4.sup.+-charged cation exchanger showed that only about 15% of the brushite was dissolved. With 1:1 RNH.sub.4.sup.++R:H.sup.+, it was found that phosphate solubilization followed the amount of RH.sup.+ added. Thus, using a H.sup.+-charged cation exchanger (alone or in combination with other types of cation exchangers) in the method is preferred (but not required) to yield maximum dissolution of the brushite.

(44) Follow-up experiments were run using brushite collected from the Woodridge Greene Valley phosphate recovery pilot conducted by Nutrient Recovery and Upcycling, LLC (Madison, Wisconsin, USA). Both raw and calcined (heated to a temperature of 500 C. overnight) brushite were used. Calcining the brushite removes most of the organic matter and also dehydrates the brushite to dicalcium phosphate, CaHPO.sub.4. To both, a stoichiometric amount of hydrogen cation exchange resin (DOWEX-brand 50W-X8) was added, allowed to react overnight at ambient temperatures, and filtered the following day.

(45) Removing the ion exchange resin beads by filtration proceeded smoothly. The orange-red resin beads were easily captured on 8-m filter paper and the gypsum-free residue captured on 1.5-m filter paper. The filtration was much easier with the hydrogen resin treatment than a parallel treatment of brushite with sulfuric acid because there was no co-production of gypsum (which tended both to clog the filters and to pass through a 1.5-m filter).

(46) A translucent brown acid (pH 2) filtrate was produced. The hydrogen resin treatment with raw brushite produced a 20% P.sub.2O.sub.5 filtrate with a 69% recovery of the brushite phosphorus, and a 7% P.sub.2O.sub.5 filtrate with 89% recovery using the calcined brushite, on a mass basis.

(47) It is envisioned that as a commercial process in batch mode, the cation exchange resin beads would be sieved off, regenerated with low-quality acid, and recycled back into the process, or put into a column and recharged, much as in a water softener. Alternatively, fluidized bed or expanded bed modes could be used for contacting ion exchange resins with solid-containing solutions. See, for example, Zagorodni, AA. 2007. Ion Exchange Materials: Properties and Applications. Elsevier, Amsterdam, ISBN: 978-0080445526. The phosphoric acid produced can be reacted with ammonia or with ammonium bicarbonate to make mono- or di-ammonium phosphates.

(48) The choice of ion exchanger is not critical to the functionality of the method and is not limited to the specified carboxylic and sulfonic groups used in the experiments.

(49) Market:

(50) The disclosed method is of commercial interest to those businesses currently or potentially in the market to produce brushite and struvite from wastewater at water treatment plants and to process it into conventional fertilizers. Such conventional fertilizers derived from wastewater would not be organic, per se, but could certainly be truthfully labeled as sustainably produced or 100% post-consumer recycled materials. Thus, the method creates a unique market that does not exist at present. The fertilizer so produced could be priced at a premium compared to conventional fertilizers produced from virgin materials.