Methods for the production of potassium sulphate from potassium-containing ores at high ambient temperatures

Abstract

A method for the production of potassium sulphate comprising contacting an aqueous potassium- and sulphate-containing composition with magnesium chloride (MgCl.sub.2), thereby obtaining a composition comprising kainite; optionally concentrating the kainite from the composition; reacting the kainite with magnesium sulphate (MgSO.sub.4) and potassium sulphate (K.sub.2SO.sub.4) so as to convert the kainite into leonite (K.sub.2SO.sub.4.MgSO.sub.4.4H.sub.2O); optionally contacting the leonite with water to remove excess MgSO.sub.4; and contacting the leonite with water so as to leach the MgSO.sub.4, contained in the leonite, and to at least substantially selectively precipitate potassium sulphate (K.sub.2SO.sub.4). The method can be operated at higher temperatures, in particular, at temperatures above 35 C., and does not require a cooling step at 20 to 25 C. The method produces potassium sulphate with a low amount of chloride.

Claims

1. A method for the production of potassium sulphate comprising the consecutive steps of: Ia) contacting an aqueous potassium- and sulphate-containing composition with magnesium chloride (MgCl.sub.2), thereby obtaining a composition comprising kainite (KCl.MgSO.sub.4.2.75H.sub.2O); IIa) optionally, concentrating the kainite from the composition, obtained in step Ia; IIIa) reacting the kainite, obtained in step Ia or IIa, with magnesium sulphate (MgSO.sub.4) and potassium sulphate (K.sub.2SO.sub.4) at a temperature of 45 C. to 55 C., so as to convert the kainite into leonite (K.sub.2SO.sub.4.MgSO.sub.4.4H.sub.2O), thereby obtaining a composition comprising leonite, wherein leonite is present in the composition comprising leonite at a concentration of at least 90% by weight; IVa) optionally, contacting the leonite, obtained in step Ma, with water to remove excess MgSO.sub.4; and Va) contacting the leonite obtained in step Ma or IVa, with water so as to leach the MgSO.sub.4 contained in the leonite and to at least substantially selectively crystallize potassium sulphate (K.sub.2SO.sub.4).

2. The method of claim 1, wherein the aqueous potassium- and sulphate-containing composition is a solution mining brine.

3. The method of claim 2, further comprising contacting one or more potash-containing ores with water so as to obtain the aqueous potassium- and sulphate-containing composition.

4. The method according to claim 1, wherein the aqueous potassiumand sulphate-containing composition comprises about 5 to about 200 g/l of K ion.

5. The method according to claim 1, wherein the aqueous potassiumand sulphate-containing composition comprises about 5 to about 100 g/l of SO.sub.4.sup.2 ion.

6. The method according to claim 1, wherein the aqueous potassiumand sulphate-containing composition comprises about 5 to about 150 g/l of Mg.sup.2+ ion.

7. The method according to claim 1, wherein contacting the aqueous potassium- and sulphate-containing composition with magnesium chloride is carried out by contacting the aqueous potassium- and sulphate-containing composition with an aqueous composition comprising the magnesium chloride.

8. The method of claim 7, wherein the composition comprising the magnesium chloride comprises about 5 to about 300 g/l of Mg.sup.2+ ion.

9. The method according to claim 1, wherein the method comprises controlling the concentration of sodium chloride present in the composition comprising kainite so as to maintain the concentration of sodium chloride below about 10% by weight on dry matter basis.

10. The method according to claim 1, wherein the concentration of sodium chloride in the kainite composition is controlled by flotation.

11. The method according to claim 1, wherein the controlling of the concentration of sodium chloride, present in the composition comprising kainite, is effective for obtaining a concentration of kainite of above 50% by weight on dry matter basis.

12. The method according to claim 1, wherein the method avoids formation of bloedite.

13. The method according to claim 1, wherein the leonite obtained in step Ma comprises less than 5% by weight of bloedite.

14. The method according to claim 1, wherein the method avoids formation of schoenite.

15. The method according to claim 1, wherein the leonite obtained in step Ma comprises less than 5% by weight of schoenite.

16. The method according to claim 1, wherein the crystallized potassium sulphate obtained contains less than 10% by weight of impurities.

17. The method of claim 1, wherein step Va provides crystallized potassium sulfate, and the method further comprises separating the crystallized potassium sulfate from a water leach by means of a solid-liquid separation, wherein the water leach comprises potassium sulphate and magnesium sulphate.

18. The method according to claim 17, further comprising recycling the water leach by reacting kainite with the water leach to convert the kainite into leonite.

19. The method according to claim 1, wherein the crystallization of the potassium sulphate is carried out at a temperature of 45 C. to 60 C.

Description

DETAILED DESCRIPTION OF THE DRAWINGS

(1) In the following drawings, which represent by way of example only, various embodiments of the disclosure:

(2) FIG. 1 shows a block diagram of an example of a process according to the present invention

(3) According to one aspect of the invention, the brines (or salt compositions) that can be used in the methods of the present disclosure can be either naturally occurring, as in lakes, springs, or subsurface brine deposits, or produced by actively solution-mining deeper, more consolidated deposits. The brine can be concentrated in solar evaporation ponds by evaporation and the composition of the brine, as it progresses through a series of ponds, can be controlled by the use of recycled brine from subsequent steps in the process so as to produce salts comprising kainite, halite (NaCl), possibly carnallite (KMgCl.sub.3.6(H.sub.2O)) and hydrated magnesium sulphate salts, other than leonite or schoenite, such as MgSO.sub.4.6H.sub.2O in the solar ponds. For example, by management of the solar ponds, the salts eventually harvested can be limited to halite, magnesium sulphate and kainite, for example, by the use of recycle brine.

(4) Solar salts from the harvest ponds comprising kainite and halite can have a kainite concentration above about 50% by weight, or above about 59% by weight. For example, the salts can be sized to smaller than 400 microns by, for example, crushing and slurrying. For example, the concentration of kainite can be increased by means of flotation and/or leaching with suitable brine, where the species to be rejected are halite and hydrated magnesium sulphate salts, such that concentrated salts are obtained.

(5) The concentrated salts can have a kainite concentration of above 65% or 70% by weight, and they can then be reacted (conversion) at a temperature above about 35 C., or of about 35 C. to about 65 C., with recycled brine from subsequent steps in the process (also called mother liquor) to convert the kainite into leonite. The use of this recycled brine (mother liquor), which can contain a significant concentration of potassium sulphate, results in more leonite being produced than the potassium ion in the kainite feed alone would permit. For example, depending on the temperature of the conversion, other MgSO.sub.4 contaminants may be precipitated, as well as leonite, and the leonite resulting from this reaction, if necessary to achieve a purity which is suitable for a feed to a potassium sulphate crystallization circuit, may be leached with suitable brine (leonite wash) and subjected to known solid-liquid separation techniques. At temperatures above about 35 C. or above about 45 C., the formation of schoenite was not observed.

(6) The magnesium sulphate, contained in the leonite, can then be subjected to selective leaching with water ((for example water added (or added to water) and crystallization), for example, in a vessel or vessels designed to promote crystal growth, whereby substantially all of the magnesium sulphate and a portion of the potassium sulphate contained in the leonite are taken into solution (or leached), with the remaining portion of the potassium sulphate produced as crystalline material. This crystallization can be conducted at a temperature of about 45 C. to about 60 C. For example, and without wishing to be bound by such a theory, leonite can be dissolved substantially at the same time the K.sub.2SO.sub.4 crystallization occurs.

(7) For example, clear brine from this step can be used in earlier steps of the process where additional leonite may be precipitated. For example, it can be used for reacting magnesium sulphate in the kainite conversion reaction step into leonite. The clear brine can have a magnesium to potassium weight ratio of about 0.4 to about 0.7 or of about 0.5 to about 0.6. Potassium sulphate, remaining in brine streams, eventually recycled to the solar evaporation ponds, can again be captured as solid kainite and recovered. The potassium sulphate solids can be withdrawn from the crystallization equipment and may or may not be leached with additional water before being subjected to known solid-liquid separation techniques, where they may or may not be washed with water.

(8) The high purity potassium sulphate solids can then be dried, sized and either granulated to meet market specifications or sold as produced.

(9) Brines containing ions of K, Mg, Na, CI and SO.sub.4 can be concentrated by solar evaporation and by the use of recycle brines caused to precipitate salts comprising kainite, halite, carnallite and one or more hydrated magnesium sulphate salt.

(10) The methods of the present disclosure can be directed to the production of high purity potassium sulphate, encompassing a maximized recovery of potassium sulphate in the crystallization step, by a process including conversion of kainite to high purity leonite in a system operating at high ambient temperature (for example temperatures above about 35 C.; temperatures of about 35 C. to about 65 C.; or about 35 C. to about 55 C.). At temperatures above about 35 C., formation of schoenite was not observed.

(11) When tests were conducted to confirm conversion of kainite, containing appreciable amounts of halite and hydrated magnesium sulphate, to leonite in reaction with brine from the potassium sulphate crystallization step at a temperature above about 35 C., the resulting leonite was contaminated with what at first appeared to be an unacceptable level of bloedite (Na.sub.2Mg(SO.sub.4).4H.sub.2O) not removable by washing. It was subsequently discovered that this is related to a high concentration of sodium ions in solution which results in bloedite forming, not as a separate discrete species, but as crystal lattice replacement within the leonite crystals (a solid solution of the two species). Without wishing to be bound by such a theory, this is likely the result of the similarity between leonite and bloedite crystal structure; they are analogs in that both are four water hydrates of a magnesium sulphate double salt, with very little difference in size between the potassium and sodium ions (1.33 and 0.96 Angstrom respectively). The inventors found that contamination of leonite with bloedite by this mechanism may be controlled by maintaining the concentration of sodium ion in the conversion reaction brine low, say, for example, below about 10% by weight, below about 8% by weight, or below about 6% by weight, and controlling the degree of super saturation created in the reaction vessels.

(12) Without wishing to be bound by such a theory, it is believed that this crystal lattice replacement phenomenon is analogous to the contamination of sodium carbonate decahydrate crystal by crystal lattice inclusions of sodium sulphate decahydrate, experienced by the inventors in previous work. For the sodium carbonatesodium sulphatewater system, the degree of contamination is directly proportional to the concentration of sulphate ion in the mother liquor. There was also an apparent correlation observed with the degree of super saturation created in the crystallizerhigher super saturation level and more rapid crystal formation accompanied by more sulphate in the crystal latticealthough this was difficult to prove beyond question, as was an apparent correlation with temperature.

(13) The presence of magnesium sulphate, not associated with the potassium sulphate ion, requires higher water to potassium sulphate ratio to dissolve all the magnesium sulphate contained in the leonite feed to the potassium sulphate crystallizer; this results in a higher percentage of the potassium sulphate contained in the leonite being taken into solution. Put in another way, the result is lower recovery of potassium as solid potassium sulphate and higher recycle brine flow because more water is used per unit of potassium sulphate produced, and larger evaporation ponds and plant are required for any given production capacity.

EXAMPLES

(14) The following example illustrates the method according to the invention. Optimization was not performed but the gist of the invention is shown hereunder. All process steps are performed in the laboratory on a laboratory scale.

(15) Step I was not performed. The salt mixture used in the laboratory testing was made in the laboratory. The kainite salt was produced from a laboratory brine, made from commercially available halite and magnesium sulphates.

(16) All testing was done in a bench scale range of 1-8 kg. However, the figures in the tables below are adjusted to reflect a starting solid of 100 kg to Step II (kainite concentration).

(17) Step II: Concentrating Kainite and Removal of Halite

(18) A salt mixture of 57 weight % kainite, 18 weight % halite, 22 weight % magnesium sulphate and 6 weight % bishofite (MgCl.sub.2.6H.sub.2O) was slurried in a flotation brine (composition: NaCl, KCl, MgCl.sub.2, MgSO.sub.4.7H.sub.2O and water). A frother aid and a flotation aid was added and the frothy supernatant was collected, filtered to remove remaining brine and kept for further processing in Step III. The salt mixture was ground to a P80 of about 350 microns). Flotation was carried out at 45 C. Recovery of K was 90%.

(19) TABLE-US-00001 Salt in (from Flotation Brine Flotation concentrate Step II) (slurry fraction) (top fraction) 100 kg 370 kg 64 kg K 9.0% 1.0% 13% Mg 8.6% 6.6% 10% S 10.5% 2.5% 12% Cl 21.3% 17.6% 18% Na 6.8% 1.8% 2% All % based on weight.

(20) Step III: Conversion of Kainite into Leonite

(21) The process was performed in semi continuous mode to prevent problems with super-saturation and sudden precipitation. The solids from step II and SOP-mother liquor brine from step V (synthetically made) was added in increments to a starting brine having the composition for an continuous process. The process was maintained at 45 C. and the retention time was 1 hour. The slurry was filtered and the solids were kept for further processing in Step IV. Leonite was added to seed the precipitation.

(22) TABLE-US-00002 SOP-Mother Starting Starting Leonite Salt in liquor brine Brine Leonite solids 64 kg 170 kg 177 kg 62 kg 180 kg K 13% 5.7% 1.9% 19.7% 18.7% Mg 10% 3.5% 5.5% 6.9% 7.5% S 12% 6.9% 4.5% 17.2% 16.7% Cl 18% 0.1% 10.4% 0% 2.9% Na 2% 0.05% 1.8% 0.03% 0.8% All % based on weight.

(23) Step IV: Washing of Leonite

(24) The solids from step III were reslurried in leach brine to dilute entrained brine from the conversion reactor for 60 min (leach brine=SOP-mother liquor almost saturated with MgSO.sub.4, similar to purge brine). It was then filtered and washed with brine from SOP crystallizer (SOP-mother liquor). The filtered solids were kept for further processing in Step V.

(25) TABLE-US-00003 SOP mother Leonite Salt in Leach brine liquor brine solids Kilogram 180 kg 440 kg 127 kg 163 kg K 18.7% 2.8% 5.7% 20.2% Mg 7.5% 5.3% 3.5% 6.9% S 16.7% 8.3% 6.9% 18.4% Cl 2.9% 0.1% 0.1% 0.1% Na 0.8% 0.05% 0.05% 0.3% All % based on weight.

(26) Step V: SOP Crystallization

(27) This process was performed in a semi-continuous mode. The crystallizer was loaded with a starting brine made from 0.49 weight % of the water and 59 weight % of the solid (leonite). The remaining salts and water were added in increments, while clear liquid was removed to keep the amount constant. The procedure lasted approximately 6 hours. The slurry was then centrifuged and dried. The potassium sulphate produced had a K.sub.2O content over 50%, and a CI content below 1%, which reflects the standard grade of chlorine free potassium sulphate.

(28) TABLE-US-00004 Leonite solids Water Filtrate (total) (total) SOP solids (SOP ML) 147 kg 181 kg 24 kg 304 kg K 20.8% 41.9% 6.0% Mg 7.0% 0.4% 3.3% S 18.7% 17.5% 7.0% Cl 0% Na 0.2% 0.1% 0% All % based on weight.

(29) Overall recovery is about 48% for this laboratory scale experiment. Although the recovery is somewhat low, the method can be optimized to achieve recoveries of 60% and more.

(30) While a description was made with particular reference to the specific embodiments, it will be understood that numerous modifications thereto will appear to those skilled in the art. The scope of the claims should not be limited by specific embodiments and examples provided in the present disclosure and accompanying drawings, but should be given the broadest interpretation consistent with the disclosure as a whole.