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Title:
HIGH-YIELD INTEGRATED PROCESS FOR THE MANUFACTURE OF POTASSIUM SULFATE FROM KAINITE
Document Type and Number:
WIPO Patent Application WO/2018/229758
Kind Code:
A1
Abstract:
An improved method for production of K2SO4 from mixed potassium salts such as kainite or carnallite. A mixed potassium salt such as kainite is obtained by solution mining, and the brine thereby obtained is mixed with a sulfate-rich brine in order to increase the potassium/sulfate molar ratio to its optimum value of 2: 1. The kainite is thus converted to schoenite and/or leonite from which K2SO4 is produced. By recycling end brines and ensuring that the potassium to sulfur ratio is at its optimum value, the method disclosed has higher efficiency and smaller losses than processes known in the art.

Inventors:
KESELMAN, Hugo (180/2 Eshkolot St, 01 Carmiel, 2197201, IL)
FRIM, Ron (6A Goldman St, 3 Haifa, 854113, IL)
HABIF, Yosef (7 HaGanim St, 07 Kiryat Bialik, 2720607, IL)
Application Number:
IL2018/050640
Publication Date:
December 20, 2018
Filing Date:
June 12, 2018
Export Citation:
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Assignee:
IMI TAMI INSTITUTE FOR RESEARCH AND DEVELOPMENT LTD (1 Deshanim Road, Industrial Zone Kiriyat Ata 28226, 8 Haifa, 854123, IL)
International Classes:
C01D5/00
Foreign References:
US2895794A1959-07-21
US3634041A1972-01-11
Attorney, Agent or Firm:
BRESSLER, Eyal et al. (Dr. Eyal Bressler Ltd, 11 Tuval St.Lazrom House, 26 Ramat Gan, 5252226, IL)
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Claims:
CLAIMS

We claim:

1. A method for production of potassium sulfate, comprising:

extracting at least one salt selected from the group consisting of kainite, carnallite, and sylvinite by solution mining with water, thereby obtaining a brine comprising at least one brine selected from the group consisting of kainite/lower carnallite brine, sylvinite brine, and upper carnallite brine;

transferring said brine to an evaporation pond;

mixing said brine with a first sulfate-rich brine in a proportion sufficient to obtain a potassium to sulfate ratio of between 1.1 and 2.2, thereby obtaining a mixed brine; precipitating at least one potassium-containing double salt from said mixed brine in said evaporation pond;

precipitating at least one potassium mixed salt from said mixed brine;

treating said potassium mixed salt with a second sulfate-rich brine, thereby converting kainite and carnallite to crude leonite/schoenite and precipitating additional potassium salts;

reducing the NaCl content of said crude leonite/schoenite by a method selected from the group consisting of:

performing flotation on said crude leonite/schoenite, thereby obtaining sodium chloride and low-NaCl leonite/schoenite; and,

leaching said NaCl from said crude leonite/schoenite;

filtering said low-NaCl leonite/schoenite, thereby obtaining filtered low-NaCl leonite/schoenite and flotation liquor;

contacting said low-NaCl leonite/schoenite with a leaching liquor in an amount sufficient to dissolve said NaCl, thereby obtaining a slurry of highly pure leonite/schoenite in said leaching liquor;

separating said highly pure leonite/schoenite from said leaching liquor;

transferring said highly pure leonite/schoenite flotation liquor to said evaporating pond; contacting said highly pure leonite/schoenite with water to leach magnesium sulfate therefrom, thereby obtaining a slurry of potassium sulfate in a potassium sulfate

(SOP) mother liquor;

separating said potassium sulfate from said SOP mother liquor, thereby obtaining potassium sulfate cake; and, washing said potassium sulfate cake with water, thereby obtaining washed potassium sulfate cake.

2. A method for production of potassium sulfate, comprising:

extracting at least one salt selected from the group consisting of kainite, carnallite, and sylvinite by solution mining with water, thereby obtaining a brine comprising at least one brine selected from the group consisting of kainite/lower carnallite brine, sylvinite brine, and upper carnallite brine;

transferring said brine to an evaporation pond;

mixing said brine with a first sulfate-rich brine in a proportion sufficient to obtain a potassium to sulfate ratio of between 1.1 and 2.2, thereby obtaining a mixed brine; precipitating at least one potassium-containing double salt from said mixed brine in said evaporation pond;

precipitating at least one potassium mixed salt from said mixed brine;

treating said potassium mixed salt with a second sulfate-rich brine, thereby converting kainite and carnallite to crude leonite/schoenite and precipitating additional potassium salts;

reducing the NaCl content of said crude leonite/schoenite by a method selected from the group consisting of:

performing flotation on said crude leonite/schoenite, thereby obtaining sodium chloride and low-NaCl leonite/schoenite; and,

leaching said NaCl from said crude leonite/schoenite;

filtering said low-NaCl leonite/schoenite, thereby obtaining filtered low-NaCl leonite/schoenite and flotation liquor;

transferring said highly pure leonite/schoenite flotation liquor to said evaporating pond; contacting said highly pure leonite/schoenite with water to leach magnesium sulfate therefrom, thereby obtaining a slurry of potassium sulfate in a potassium sulfate (SOP) mother liquor;

separating said potassium sulfate from said SOP mother liquor, thereby obtaining potassium sulfate cake; and,

washing said potassium sulfate cake with water, thereby obtaining washed potassium sulfate cake.

3. The method according to either one of claim 1 or claim 2, wherein said step of extracting at least one salt comprises extracting simultaneously a plurality of salts. 4. The method according to either one of claim 1 or claim 2, wherein said step of extracting at least one salt comprises extracting separately a plurality of salts.

5. The method according to claim 4, wherein said step of extracting at least one salt comprises extracting salts successively from different mineral layers.

6. The method according to either one of claim 1 or claim 2, wherein said potassium mixed salt comprises KC1.

7. The method according to either one of claim 1 or claim 2, wherein said step of contacting said brine with a first sulfate-rich brine comprises contacting said brine with said first sulfate- rich brine in a proportion required to obtain a potassium to sulfate ratio of between 1.9 and 2.1.

8. The method according to either one of claim 1 or claim 2, wherein said step of mixing said brine with a first sulfate-rich brine comprises mixing at least one brine selected from the group consisting of said sylvinite brine and said carnallite brine with flotation liquor.

9. The method according to either one of claim 1 or claim 2, wherein said step of mixing said brine with a first sulfate-rich brine is performed in situ in said evaporating pond.

10. The method according to either one of claim 1 or claim 2, wherein said step of precipitating at least one potassium-containing double salt from said mixed brine comprises precipitating at least one salt selected from the group consisting of leonite, schoenite, kainite, and carnallite from said mixed brine.

11. The method according to either one of claim 1 or claim 2, wherein said step of precipitating at least one potassium mixed salt from said mixed brine comprises precipitating potassium chloride from said mixed brine.

12. The method according to either one of claim 1 or claim 2, wherein said step of performing flotation on said crude leonite/schoenite comprises performing direct flotation.

13. The method according to either one of claim 1 or claim 2, wherein said step of performing flotation on said crude leonite/schoenite comprises performing reverse flotation.

14. The method according to either one of claim 1 or claim 2, where at least one of said first sulfate-rich brine and said second sulfate-rich brine comprises said flotation liquor.

15. The method according to either one of claim 1 or claim 2, wherein said low-NaCl leonite/schoenite contains less than 15% (w/w) NaCl. 16. The method according to claim 1, wherein said leaching liquor comprises SOP mother liquor.

17. The method according to claim 1, wherein said step of contacting said low-NaCl leonite/schoenite with a leaching liquor in an amount sufficient to dissolve said NaCl comprises contacting said low-NaCl leonite/schoenite with an amount of leaching liquor just sufficient to dissolve said NaCl.

18. The method according to either one of claim 1 or claim 2, wherein said step of contacting said highly pure leonite/schoenite with water comprises contacting said highly pure leonite/schoenite with water at a temperature of 20 °C - 60 °C.

19. The method according to either one of claim 1 or claim 2, wherein said step of contacting said highly pure leonite/schoenite with water comprises contacting said highly pure leonite/schoenite with water for 1.5 - 3.5 hours.

20. The method according to either one of claim 1 or claim 2, comprising:

drying said washed potassium sulfate cake, thereby obtaining dried potassium sulfate; and at least one step selected from the group consisting of:

screening said dried potassium sulfate; and,

compacting said dried potassium sulfate.

21. The method according to either one of claim 1 or claim 2, wherein said step of transferring said brine to an evaporation pond comprises transferring said kainite lower carnallite brine to said evaporation pond, and wherein said method comprises:

transferring any mixture of brines remaining from said solution mining to at least one tailing pond;

precipitating at least one potassium-containing double salt from said mixture of brines in said tailing pond; and,

mixing said at least one potassium-containing double salt in said tailing pond with said at least one potassium-containing double salt obtained in said evaporation pond.

22. The method according to either one of claim 1 or claim 2, wherein said step of transferring said brine to an evaporation pond comprises:

transferring said kainite lower carnallite brine to a first evaporation pond;

transferring said sylvinite/upper carnallite brine to a second evaporation pond; precipitating halite and sylvinite from said sylvinite/upper carnallite brine;

if sulfate is present, precipitating kainite and/or carnallite from said sylvinite/upper carnallite brine; and said method comprises:

separating potassium salts in said first evaporation pond and said second evaporation pond from said brines; and,

mixing said potassium salts.

23. The method according to either one of claim 1 or claim 2, comprising recycling at least part of at least one brine selected from the group consisting of brine obtained from said evaporation pond and MgCh-rich kainite brine to said evaporation ponds, thereby promoting precipitation of NaCl precipitation and acting to limit precipitation of leonite/schoenite.

24. The method according to either one of claim 1 or claim 2, comprising partially evaporating or cooling said SOP mother liquor, thereby precipitating additional leonite/schoenite.

25. The method according to claim 24, comprising decomposing said additional leonite/schoenite by treating said additional leonite/schoenite with hot water, thereby obtaining fully soluble potassium sulfate.

26. The method according to either one of claim 1 or claim 2, comprising:

dissolving said washed potassium sulfate cake in hot water, thereby obtaining a potassium sulfate solution;

filtering said potassium sulfate solution; and,

crystallizing said potassium sulfate from said potassium sulfate solution.

27. The method according to either one of claim 1 or claim 2, comprising:

optionally, drying said washed potassium sulfate cake, thereby obtaining dry potassium sulfate;

mixing said washed potassium sulfate cake and/or said dry potassium sulfate with a solution of sulfuric acid in a proportion sufficient to obtain a K2SO /H2SO4 ratio of between IS and 200 (w/w), thereby obtaining a mixed K2SO4 H2SO4 solution; and, heating said mixed K2SO4/H2SO4 solution, thereby obtaining dry acid potassium sulfate.

28. The method according to claim 27, wherein said step of mixing said washed potassium sulfate cake and/or said dry potassium sulfate comprises mixing said washed potassium sulfate cake and/or said dry potassium sulfate with a solution of sulfuric acid in a proportion sufficient to obtain a K2SO4 H2SO4 ratio of between 50 and 100 (w/w).

29. The method according to claim 27, wherein said step of drying comprises drying at a temperature of between 50 °C and 300 °C. 30. The method according to either one of claim 1 or claim 2, comprising:

optionally, drying said washed potassium sulfate cake, thereby obtaining dry potassium sulfate;

mixing said washed potassium sulfate cake and/or said dry potassium sulfate with KHSO4 in a proportion sufficient to obtain a K3SO4/KHSO4 ratio of between 10 and 140 (w/w), thereby obtaining a mixed K2SO4/KHSO4 solution; and,

drying said mixed K2SO4/KHSO4 solution, thereby obtaining acid potassium sulfate.

31. The method according to claim 30, wherein said step of mixing said washed potassium sulfate cake and/or said dry potassium sulfate with KHSO4 comprisesmixing said washed potassium sulfate cake and/or said dry potassium sulfate with KHSO4 in a proportion sufficient to obtain a K2SO4/KHSO4 ratio of between 35 and 70 (w/w).

32. The method according to either one of claim 1 or claim 2, comprising:

drying said washed potassium sulfate cake, thereby obtaining dry potassium sulfate; and,

separating said dry potassium sulfate into a coarse fraction and a fine fraction.

33. The method according to claim 32, wherein said step of separating is performed by a method selected from the group consisting of sieving and separating in a cyclone separator.

34. The method according to claim 32, wherein said step of separating said dry potassium sulfate comprises separating said dry potassium sulfate into a >200 mesh coarse fraction and a <200 mesh fine fraction.

35. The method according to either one of claim 1 or claim 2, wherein said step of performing flotation on said crude leonite/schoenite is followed by a step of performing an additional flotation on NaCl-containing waste obtained in said step of performing flotation.

36. The method according to either one of claim 1 or claim 2, comprising performing flotation on crude kainite/carnallite prior to said step of treating said potassium mixed salt with a second sulfate-rich brine, thereby separating potassium-containing salts from NaCl.

37. The method according to claim 36, wherein said crude kainite/carnallite contains between 12% and 28% NaCl (w/w).

38. The method according to claim 36, wherein said step of performing flotation comprises performing direct flotation. 39. The method according to claim 36, wherein said step of performing flotation comprises performing reverse flotation.

40. The method according to claim 36, wherein said step of performing flotation comprises performing flotation using a brine selected from the group consisting of:

end brine from a pond containing kainite; and,

brine of similar composition to said end brine from a pond containing kainite.

41. The method according to either one of claim 1 or claim 2, wherein said potassium-containing double salts are mixed with crude sylvinite containing up to 70% halite prior to said step of performing flotation.

42. The method according to either one of claim 1 or claim 2, wherein said step of treating said potassium mixed salt with a second sulfate-rich brine comprises adding a substance selected from the group consisting of solid sylvinite containing less than 10% NaCl (w/w) and KC1 in a proportion sufficient to yield a molar ratio between KC1 and leonite/schoenite of between 1.1 and 2.2.

43. The method according to 42, wherein said step of treating said potassium mixed salt with a second sulfate-rich brine comprises adding a substance selected from the group consisting of solid sylvinite containing less than 10% NaCl (w/w) and KC1 in a proportion sufficient to yield a molar ratio between KC1 and leonite/schoenite of between 1.9 and 2.1.

Description:
HIGH-YIELD INTEGRATED PROCESS FOR THE MANUFACTURE OF POTASSIUM

SULFATE FROM ΚΑΙΝIΓΕ

REFERENCE TO RELATED PUBLICATIONS

[0001] This invention claims priority from U.S. Provisional Pat. Appl. No. 62/518,616, filed 13 June 2017.

FIELD OF THE INVENTION

[0002] This invention relates in general to processes for the production of potassium sulfate. It relates in particular to processes for the production of potassium sulfate from solution mining of potassium minerals, particularly kainite, carnallite, and sylvinite.

BACKGROUND OF THE INVENTION

[0003] Kainite is a double salt of potassium chloride and magnesium sulfate (KClMgS0 4 «2.75H20). Kainite usually occurs in mineral deposits closely associated with sodium chloride. It is found extensively in Sicilian potash mines, as well as in Ethiopia, Eritrea, and in smaller quantities or mixtures in some German, Russian and other deposits. In these deposits, kainite is frequently found layered with other potassium minerals such as sylvinite (KCl/NaCl), carnallite (KClMgCl2 e 6H20), etc. These minerals are frequently found mixed with other minerals such as calcium sulfate, magnesium sulfate, potassium chloride, sodium chloride, polyhalite, etc. A typical structure of such layers is shown in FIG. 1, with the overall structure shown in FIG. 1A and an expanded view of the layers in which minerals such as kainite, sylvinite, and carnallite are typically found shown in FIG. IB. The potassium-containing minerals can be recovered by solution mining (extraction with water). Each layer (e.g. kainite, carnallite, sylvinite) can be recovered separately and in succession.

[0004] The production of potassium sulfate (K2SO4) requires half a mole of sulfate for each mole of potassium. Kainite, which contains one mole of SO4 2" per mole of K + , thus has a 50% excess of sulfate that will be lost during production in the waste brine as MgS0 4 with a theoretical maximum sulfate yield of 50%. As it is very difficult to separate potassium from a stream containing MgS0 4 , the presence of the excess sulfate leads to a significant reduction in the yield of potassium recovered from kainite, typically on the order of 20 to 30%.

[0005] Three patents to the Great Salt Lake Minerals and Chemical Co. disclose methods for recovery of kainite from natural brines. U.S. Pat. No. 3589871 discloses a method that comprises evaporating the brine at ambient temperature to concentrate dissolved potassium, adjusting the brine by mixing it with a solution that contains magnesium and potassium but with an Mg:K ratio that is higher than that of the natural brine, evaporating the brine to precipitate NaCl, and precipitating kainite from the residual brine. A similar method is disclosed in U.S. Pat. No. 3S9261S, but the precipitation is performed by producing the brines during the summer and storing them until the winter, at which points they naturally cool, precipitating kainite from the cold brine. U.S. Pat. No. 361S2S9 discloses a method in which the solar evaporation zone is divided into subzones arranged for series flow. After part of the crystal crop is deposited in a first solar evaporation area, the brine is decanted and transferred to the next zone; the process is continued until the effluent brine has a desired bitterns composition.

[0006] U.S. Pat. No. 3634041, also to the Great Salt Lake Minerals & Chemical Co., discloses a cyclical method for production of potassium sulfate from potassium-containing double salts of magnesium sulfate via an intermediate step involving the production of essentially pure schoenite (K2S04*MgS04*6H20) without the necessity of removing NaCl from the schoenite prior to the leaching. In the process disclosed therein, MgS0 4 is leached from schoenite, K2SO4 is separated from the resulting leach liquor, the leach liquor added to an evaporation pond in the amount just sufficient to dissolve soluble impurities, thereby producing a slurry of essentially pure schoenite, and separating and recycling the schoenite. While this patent discloses a method for obtaining K2SO4 low in NaCl from schoenite or leonite, it does not disclose any method for increasing the potassium to sulfate ratio.

[0007] U.S. Pat. Nos. 7041268 and 8182784 to the Council of Scientific and Industrial Research (India) disclose a process for recovery of K2SO4 from sulfate-rich bittern that uses bittern and lime as raw materials. Kainite is obtained by fractional crystallization of bittern and then converted to schoenite with simultaneous removal of NaCl by processing with water and end liquor obtained from the reaction of schoenite. The end liquor from the conversion to schoenite (SEL) is desulfated and supplemented with MgCh using end bittern generated in the process of making carnallite. The carnallite is decomposed to obtain crude potash, and the liquor obtained from the decomposition is reacted with hydrated lime to produce CaCh and Mg(OH) 2 ; the CaCl 2 solution is then used to desulfate the SEL.

[0008] PCT (International) Pat. Appl. Pub. No. WO2014/033687, also to the Council of Scientific and Industrial Research, discloses an integrated process for production of potassium sulfate and ammonium sulfate from kainite mixed salt. The method is broadly similar to that disclosed in U.S. Pat. No. 7041268, except that CaCl 2 is produced by adding HC1 in CaC03 to produce CaCh and pressurized CO2, and the gypsum and CO2 produced by desulfating the SEL are reacted with ammonia to produce ammonium sulfate solution and solid CaC0 3 , the calcium carbonate being recycled to the CaCh production step.

[0009] German Pat. No. 10 2009 018 956 discloses a method for producing potassium sulfate from sea saline mother liquor by multistage evaporation in solar ponds. This method also involves conversion of kainite into schoenite and decomposing the schoenite to potassium sulfate. The bittern is processing the initial salt mixture into a salt mixture suitable for potassium sulfate production and then converting this salt mixture into potassium sulfate in aqueous solution. The mixture is gradually evaporated in a series of evaporation steps during which kainite is converted to schoenite.

[0010] A method of producing high-purity potassium sulfate that is high-yield, low-cost, does not involve unnecessary reactants, and avoids the inevitable losses of methods known in the art thus remains a long-felt but as yet unmet need.

SUMMARY OF THE INVENTION

[0011] The method disclosed herein was devised to meet this need. The initial brine is treated so as to increase the potassium/sulfate molar ratio to its optimum value of 2:1 by production of leonite/schoenite in an intermediate step. Potassium sulfate is then obtained from the leonite/schoenite in high yield with minimal losses.

[0012] It is therefore an object of this invention to disclose an improved, highly efficient method for producing potassium sulfate, comprising: (a) extracting at least one salt selected from the group consisting of kainite, carnallite, and sylvinite by solution mining with water, thereby obtaining a brine comprising at least one brine selected from the group consisting of kainite lower carnallite brine, sylvinite brine, and upper carnallite brine; (b) transferring said brine to an evaporation pond; (c) mixing said brine with a first sulfate-rich brine in a proportion sufficient to obtain a potassium to sulfate ratio of between 1.1 and 2.2, thereby obtaining a mixed brine; (d) precipitating at least one potassium-containing double salt from said mixed brine in said evaporation pond; (e) precipitating at least one potassium mixed salt from said mixed brine; (f) treating said potassium mixed salt to with a second sulfate-rich brine, thereby converting kainite and carnallite to crude leonite/schoenite and precipitating additional potassium salts; (g) reducing the NaCl content of said crude leonite/schoenite by a method selected from the group consisting of: (i) performing flotation on said crude leonite/schoenite, thereby obtaining sodium chloride and low-NaCl leonite/schoenite; and, (ii) leaching said NaCl from said crude leonite/schoenite; performing flotation on said crude leonite/schoenite, thereby obtaining sodium chloride and low-NaCl leonite/schoenite; (h) filtering said low-NaCl leonite/schoenite, thereby obtaining filtered low-NaCl leonite/schoenite and flotation liquor; (i) contacting said low-NaCl leonite/schoenite with a leaching liquor in an amount sufficient to dissolve said NaCl, thereby obtaining a slurry of highly pure leonite/schoenite in said leaching liquor; (j) separating said highly pure leonite/schoenite from said leaching liquor; (k) transferring said highly pure leonite/schoenite leaching liquor to said evaporating pond; (1) contacting said highly pure leonite/schoenite with water to leach magnesium sulfate therefrom, thereby obtaining a slurry of potassium sulfate in a potassium sulfate (SOP) mother liquor; (m) separating said potassium sulfate from said SOP mother liquor, thereby obtaining potassium sulfate cake; and, (n) washing said potassium sulfate cake with water, thereby obtaining washed potassium sulfate cake.

[0013] In some preferred embodiments of the invention, said step of contacting said low- NaCl leonite/schoenite with a leaching liquor in an amount sufficient to dissolve said NaCl comprises contacting said low-NaCl leonite/schoenite with an amount of leaching liquor just sufficient to dissolve said NaCl. In some preferred embodiments of the invention, said leaching liquor comprises SOP mother liquor.

[0014] It is an additional object of this invention to disclose an improved, highly efficient method for producing potassium sulfate, comprising: (a) extracting at least one salt selected from the group consisting of kainite, carnallite, and sylvinite by solution mining with water, thereby obtaining a brine comprising at least one brine selected from the group consisting of kainite lower carnallite brine, sylvinite brine, and upper carnallite brine; (b) transferring said brine to an evaporation pond; (c) mixing said brine with a first sulfate-rich brine in a proportion sufficient to obtain a potassium to sulfate ratio of between 1.1 and 2.2, thereby obtaining a mixed brine; (d) precipitating at least one potassium-containing double salt from said mixed brine in said evaporation pond; (e) precipitating at least one potassium mixed salt from said mixed brine; (f) treating said potassium mixed salt to with a second sulfate-rich brine, thereby converting kainite and carnallite to crude leonite/schoenite and precipitating additional potassium salts; (g) performing flotation on said crude leonite/schoenite, thereby obtaining sodium chloride and highly pure leonite/schoenite; (h) filtering said highly pure leonite/schoenite, thereby obtaining filtered highly pure leonite/schoenite and flotation liquor; (i) transferring said filtered highly pure leonite/schoenite flotation liquor to said evaporating pond; (j) contacting said highly pure leonite/schoenite with water to leach magnesium sulfate therefrom, thereby obtaining a slurry of potassium sulfate in a potassium sulfate (SOP) mother liquor; (k) separating said potassium sulfate from said SOP mother liquor, thereby obtaining potassium sulfate cake; and, (1) washing said potassium sulfate cake with water, thereby obtaining washed potassium sulfate cake.

[0015] In some embodiments, said step of extracting at least one salt comprises extracting simultaneously a plurality of salts. In some embodiments, said step of extracting at least one salt comprises extracting separately a plurality of salts. In some preferred embodiments, said step of extracting at least one salt comprises extracting salts successively from different mineral layers.

[0016] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of contacting said brine with a first sulfate-rich brine comprises contacting said brine with said first sulfate-rich brine in a proportion required to obtain a potassium to sulfate ratio of between 1.9 and 2.1.

[0017] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of mixing said brine with a first sulfate-rich brine comprises mixing at least one brine selected from the group consisting of said sylvinite brine and said carnallite brine with flotation liquor.

[0018] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of mixing said brine with a first sulfate-rich brine is performed in situ in said evaporating pond.

[0019] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of precipitating at least one potassium-containing double salt from said mixed brine comprises precipitating at least one salt selected from the group consisting of leonite, schoenite, kainite, and carnallite from said mixed brine.

[0020] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of precipitating at least one potassium mixed salt from said mixed brine comprises precipitating potassium chloride from said mixed brine.

[0021] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of performing flotation on said crude leonite/schoenite comprises performing direct flotation. [0022] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of performing flotation on said crude leonite/schoenite comprises performing reverse flotation.

[0023] It is an object of this invention to disclose the method as defined in any of the above, where at least one of said first sulfate-rich brine and said second sulfate-rich brine comprises said flotation liquor.

[0024] It is an object of this invention to disclose the method as defined in any of the above, wherein said low-NaCl leonite/schoenite contains less than 15% (w/w) NaCl.

[0025] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of contacting said highly pure leonite/schoenite with water comprises contacting said highly pure leonite/schoenite with water at a temperature of 20 °C - 60 °C.

[0026] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of contacting said highly pure leonite/schoenite with water comprises contacting said highly pure leonite/schoenite with water for 1.5 - 3.5 hours.

[0027] It is an object of this invention to disclose the method as defined in any of the above, comprising: drying said washed potassium sulfate cake, thereby obtaining dried potassium sulfate; and at least one step selected from the group consisting of: (a) screening said dried potassium sulfate; and, (b) compacting said dried potassium sulfate.

[0028] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of transferring said brine to an evaporation pond comprises transferring said kainite lower carnallite brine to said evaporation pond, and wherein said method comprises: (a) transferring any mixture of brines remaining from said solution mining to at least one tailing pond; (b) precipitating at least one potassium-containing double salt from said mixture of brines in said tailing pond; and, (c) mixing said at least one potassium- containing double salt in said tailing pond with said at least one potassium-containing double salt obtained in said evaporation pond.

[0029] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of transferring said brine to an evaporation pond comprises: (i) transferring said kainite/lower carnallite brine to a first evaporation pond; (ii) transferring said sylvinite/upper carnallite brine to a second evaporation pond; (iii) precipitating halite and sylvinite from said sylvinite/upper carnallite brine; and (iv) if sulfate is present, precipitating kainite and/or carnallite from said sylvinite/upper carnallite brine, and said method comprises: (a) separating potassium salts in said first evaporation pond and said second evaporation pond from said brines; and, (b) mixing said potassium salts.

[0030] It is an object of this invention to disclose the method as defined in any of the above, comprising recycling at least part of at least one brine selected from the group consisting of brine obtained from said evaporation pond and MgCb-rich kainite brine to said evaporation ponds, thereby promoting precipitation of NaCl precipitation and acting to limit precipitation of leonite/schoenite.

[0031] It is an object of this invention to disclose the method as defined in any of the above, comprising partially evaporating or cooling said SOP mother liquor, thereby precipitating additional leonite/schoenite. In some preferred embodiments of the method, it comprises decomposing said additional leonite/schoenite by treating said additional leonite/schoenite with hot water, thereby obtaining fully soluble potassium sulfate.

[0032] It is an object of this invention to disclose the method as defined in any of the above, comprising: (a) dissolving said washed potassium sulfate cake in hot water, thereby obtaining a potassium sulfate solution; (b) filtering said potassium sulfate solution; and, (c) crystallizing said potassium sulfate from said potassium sulfate solution.

[0033] It is an object of this invention to disclose the method as defined in any of the above, comprising: (a) optionally, drying said washed potassium sulfate cake, thereby obtaining dry potassium sulfate; (b) mixing said washed potassium sulfate cake and/or said dry potassium sulfate with a solution of sulfuric acid in a proportion sufficient to obtain a K2SO4/H2SO4 ratio of between 15 and 200 (w/w), thereby obtaining a mixed K2SO4 H2SO4 solution; and, (c) heating said mixed K2SO4 H2SO4 solution, thereby obtaining dry acid potassium sulfate.

[0034] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of mixing said washed potassium sulfate cake and/or said dry potassium sulfate comprises mixing said washed potassium sulfate cake and/or said dry potassium sulfate with a solution of sulfuric acid in a proportion sufficient to obtain a K2SO4 H2SO4 ratio of between SO and 100 (w/w). In some preferred embodiments of the invention, said step of drying comprises drying at a temperature of between 50 °C and 300 °C.

[0035] It is an object of this invention to disclose the method as defined in any of the above, comprising: (a) optionally, drying said washed potassium sulfate cake, thereby obtaining dry potassium sulfate; (b) mixing said washed potassium sulfate cake and/or said dry potassium sulfate with KHSO4 in a proportion sufficient to obtain a K2SO4 KHSO4 ratio of between 10 and 140 (w/w), thereby obtaining a mixed K2SO4 KHSO4 solution; and, (c) drying said mixed K2SO4/KHSO4 solution, thereby obtaining acid potassium sulfate. In some preferred embodiments of the invention, said step of mixing said washed potassium sulfate cake and/or said dry potassium sulfate with KHSO4 comprises mixing said washed potassium sulfate cake and/or said dry potassium sulfate with KHSO4 in a proportion sufficient to obtain a K2SO4 KHSO4 ratio of between 35 and 70 (w/w).

[0036] It is an object of this invention to disclose the method as defined in any of the above, comprising: (a) drying said washed potassium sulfate cake, thereby obtaining dry potassium sulfate; and, (b) separating said dry potassium sulfate into a coarse fraction and a fine fraction. In some preferred embodiments of the invention, said step of separating is performed by a method selected from the group consisting of sieving and separating in a cyclone separator. In some preferred embodiments of the invention, wherein said step of separating said dry potassium sulfate comprises separating said dry potassium sulfate into a >200 mesh coarse fraction and a <200 mesh fine fraction.

[0037] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of performing flotation on said crude leonite/schoenite is followed by a step of performing an additional flotation on NaCl-containing waste obtained in said step of performing flotation.

[0038] It is an object of this invention to disclose the method as defined in any of the above, comprising performing flotation on crude kainite/carnallite prior to said step of treating said potassium mixed salt with a second sulfate-rich brine, thereby separating potassium- containing salts from NaCl. In some preferred embodiments of the invention, said crude kainite/carnallite contains between 12% and 28% NaCl (w/w). In some preferred embodiments of the invention, said step of performing flotation comprises performing direct flotation. In some preferred embodiments of the invention, said step of performing flotation comprises performing reverse flotation. In some preferred embodiments of the invention, said step of performing flotation comprises performing flotation using a brine selected from the group consisting of (i) end brine from a pond containing kainite and (ii) brine of similar composition to said end brine from a pond containing kainite.

[0039] It is an object of this invention to disclose the method as defined in any of the above, wherein said potassium-containing double salts are mixed with crude sylvinite containing up to 70% halite prior to said step of performing flotation. [0040] It is an object of this invention to disclose the method as defined in any of the above, wherein said step of treating said potassium mixed salt with a second sulfate-rich brine comprises adding a substance selected from the group consisting of solid sylvinite containing less than 10% NaCl (w/w) and KC1 in a proportion sufficient to yield a molar ratio between KC1 and leonite/schoenite of between 1.1 and 2.2. In some preferred embodiments of the invention, said step of treating said potassium mixed salt with a second sulfate-rich brine comprises adding a substance selected from the group consisting of solid sylvinite containing less than 10% NaCl (w/w) and KC1 in a proportion sufficient to yield a molar ratio between KC1 and leonite/schoenite of between 1.9 and 2.1.

BRIEF DESCRIPTION OF THE DRAWING

[0041] The invention will now be described with reference to the drawing, wherein:

[0042] FIG. 1 shows a typical geological structure of a mineral deposit that includes a kainite layer, FIG. 1A showing an overall view and FIG. IB a detailed view of the "Houston Formation" in which layers containing minerals of interest such as sylvinite, carnallite, and kainite are found.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] In the following description, various aspects of the invention will be described. For the purposes of explanation, specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent to one skilled in the art that there are other embodiments of the invention that differ in details without affecting the essential nature thereof. In some cases, for clarity or conciseness, individual process steps of the method herein disclosed are discussed individually. Nonetheless, any combination of individual elements disclosed herein that is not self-contradictory is considered by the inventors to be within the scope of the invention.

[0044] As used herein, the abbreviation "SOP" stands for "sulfate of potash," i.e. potassium sulfate.

[0045] Unless specifically described otherwise, all concentrations and relative amounts are given as w/w.

[0046] Potassium-containing minerals can be extracted from mineral deposits by solution mining by extraction with water. Non-limiting examples of salts that can be mined in this manner include leonite (K2S04«MgS04«4H20)/Schoenite (K2S04 e MgS04"6H20), laiinite (KClMgS04 « 2.75H 2 0) 5 camallite (KClMgCl2 « 6H 2 0) and potassium chloride ( C1), all of which will have some halite (NaCl) content that may differ from layer to layer. These salts frequently contain other materials as impurities, such calcium sulfate, kieserite, bischofitite, etc.

[0047] The extraction of all of the minerals of interest can be done simultaneously by solution mining of the entire deposit. In some embodiments of the invention, each layer of the deposit (e.g. kainite, camallite and sylvinite) is extracted separately by extracting successively each layer.

[0048] In the invention herein disclosed, a combination of different potassium brines (including brine obtained directly from solution mining and internally recycled brines from the process itself) is fed to a system of solar evaporation ponds in order to precipitate potassium-containing salts. The main objective of the brine mixture is to increase the potassium content in the brine to the optimum potassium/sulfate molar ratio of 2:1. In this case, the end brine of the process will be primarily an aqueous solution of MgCk with a very low potassium and magnesium sulfate content. The relative compositions of the salt products will depend on the initial brine composition, content of the recycled solutions, temperature, etc. For example, if end brine is partially recycled, and the concentration of MgCh in the initial brine is sufficiently increased, precipitation of leonite/schoenite will be avoided. In addition, halite will be salted out prior to the precipitation of the leonite/schoenite, leading to a reduced sodium content in the potassium salt products.

[0049] In processes for production of K2SO4 known in the art, the salt harvested from the solar evaporation ponds is ground, crushed, and wet ball milled, and is converted into leonite/schoenite and solid NaCl by contacting the potassium-containing brine with brine rich in sulfates, followed by floating the solid in order to separate the potash salt from the NaCl. The flotation can be performed in a direct form (flotation of potassium salts) or inverse form (flotation of NaCl). In these processes, if the NaCl content of the potassium salt (leonite/schoenite) is too high (typically, 5% is the acceptable upper limit), then a further leaching to remove NaCl is necessary in order to prevent co-precipitation of glasserite (Na2S04*K2SO-i) with the potassium sulfate.

[0050] After the separation of the leonite/schoenite from the brine, it is typically decomposed by addition of hot water (typically 20 °C - 60 °C). In some embodiments, KC1 is added in order to improve the reaction yield; in this case no heating is necessary. [0051] The mother liquor from the above reaction (SOP mother liquor), which contains MgS0 4 and K2SO4, is used for leaching the remaining NaCl from the leonite/schoenite after the flotation. This SOP mother liquor is saturated in sulfate, and hence will not dissolve leonite/schoenite, but it will dissolve NaCl remaining in the brine.

[0052] In preferred embodiments, the SOP mother liquor (after leaching) is used to treat kainite-carnallite and to convert it to leonite/schoenite. Surprisingly, this process step yields more leonite/schoenite than would be expected according to stoichiometry. This unexpected increase is due a process of salting out. During the transformation of kainite-carnallite to leonite/schoenite, MgCh is produced, causing precipitation of part of the potassium salt that was dissolved in the SOP mother liquor.

[0053] After the conversion of the salts to leonite/schoenite, the same brine is also used for the flotation step. If necessary, the concentration of solids in the brine is reduced prior to its use in the flotation step.

[0054] This brine is now saturated in NaCl and contains about 40-70% leonite/schoenite. The brine is then fed to the conversion reactor, the excess of sulfate (as MgS0 4 ) and the MgCb being derived from the conversion of the kainite-carnallite to leonite/schoenite. In order to recover the potassium content of the brine in high yield, this brine must be evaporated in ponds and the precipitated salts are treated together with the rest of the harvested potassium salt. In some non-limiting embodiments of the invention, this precipitation is performed in a separate tailing pond with the addition of extra KCl from sylvinite or carnallite brine. In other non-limiting embodiments of the invention, the evaporation is performed by recycling the brine back to the evaporation ponds containing fresh kainite brine and sylvinite or carnallite.

[0055] In these ponds, the MgS0 4 in the SOP mother liquor will react with the KCl of the sylvinite-carnallite and will (after evaporation) lead to the precipitation of kainite or leonite/schoenite according to one or both of the following reactions:

[0056] The present invention thus discloses an improved method of production of K2SO4 from mineral deposits containing kainite. By reacting the initial brine obtained from solution mining with a sulfate-rich brine that can be recycled mother liquor from production of K 2 SO 4 , the potassium to sulfate ratio in the brines is raised, thereby increasing the efficiency and the yield of potassium sulfate recovery.