Abstract:
A process for producing KNO3 from polyhalite to is disclosed. In a preferred embodiment, the process comprises steps of (a) contacting polyhalite with HNO3; (b) adding Ca(OH)2 to the solution, thereby precipitating as CaSO4 at least part of the sulfate present in said solution; (c) precipitating as Mg(OH)2 at least part of the Mg2+ remaining in said solution by further addition of Ca(OH)2 to the remaining solution; (d) concentrating the solution, thereby precipitating as a sulfate compound at least part of the sulfate remaining in solution; (e) separating at least part of the NaCl from the solution remaining; and (f) crystallizing as solid KNO3 at least part of the K+ and NO3-contained in the solution. The process enables direct conversion of polyhalite to KNO3 of purity exceeding 98.5% and that is essentially free of magnesium and sulfate impurities.

Description:
REFERENCE TO RELATED PUBLICATION 
       [0001]    This application claims priority from U.S. provisional application 61/220,230, dated 25 Jun. 2009, which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention concerns methods for production of KNO 3 , in particular, methods that use polyhalite as the starting material. 
       BACKGROUND OF THE INVENTION 
       [0003]    Potassium nitrate (KNO 3 ) is a commercially important chemical with uses from explosives to fertilizers. Polyhalite (K 2 SO 4 .MgSO 4 .2CaSO 4 .2H 2 O) is a widely-distributed and readily available source of potassium. Reaction of polyhalite with acids (e.g. HNO 3 ) is known in the literature as a means of producing crude salt mixtures containing “complex mineral fertilizer.” For example, U.S. Pat. No. 4,246,019 discloses a method for production of a. mixture containing 53.54% KNO 3 , 39.87% Mg(NO 3 ) 2 , 5.48% CaSO 4 , and 1.01% H 2 O from the reaction of polyhalite with HNO 3 . Russian Pat. No. 2,276,123 discloses a method for production of a solution containing a mixture of K 2 SO 4 , MgSO 4  and NH 4 NO 3  from reaction of polyhalite with HNO 3  followed by neutralizaion with ammonia. Thus the processes described in the literature allow to produce a solution containing a mixture of substances, including potassium and magnesium nitrates, as well as gypsum that had not been separated in early stages of the process. 
         [0004]    Several well-known processes, such as the Southwest Potash and IMI processes, are known for the production of KNO 3  by reaction of KCl with HNO 3  ( Ullman&#39;s Agrochemicals, vol.  1; Weinheim: Wiley-VCH, 2007, pp. 334-336). Production of essentially pure KNO 3  directly from the reaction of polyhalite with strong acid remains unknown, however. Thus, there remains a long-felt need for a process that can produce essentially pure KNO 3  from polyhalite without the complications of known processes such as production of complicated product mixtures and necessity of neutralization with ammonia. 
       SUMMARY OF THE INVENTION 
       [0005]    The process disclosed in the present invention produces KNO 3  from polyhalite with almost total recovery, without any necessity for preliminary thermal treatment and without the necessity for washing out of NaCl from the polyhalite. The separation of NaCl from the KNO 3  is effected by the different temperature dependences of the solubility of the two substances: the solubility of NaCl changes very little with temperature, while that of KNO 3  strongly increases with increasing temperature. In the process herein disclosed, Mg is recovered as Mg(OH) 2 , which is precipitated with lime. The Mg(OH) 2  thus recovered can be used directly as a slurry with water, dried or transformed into Mg salts such as MgSO 4 .H 2 O. 
         [0006]    It is therefore an object of the present invention to disclose a process for producing KNO 3  wherein the starting material for the process is polyhalite. 
         [0007]    It is a further object of the present invention to disclose a process for producing KNO 3  from polyhalite, comprising steps of (a) contacting polyhalite with a substance comprising NO 3   − ; (b) adding at least one inorganic base to the solution obtained in the step of contacting polyhalite with HNO 3 , thereby precipitating as a solid at least part of the sulfate present in said solution; (c) precipitating as Mg(OH) 2  at least part of the Mg 2+  remaining in said solution by adding at least one basic compound to the remaining solution; (d) concentrating the solution obtained after said step of precipitating at least part of the Mg 2+  remaining in said solution; (e) precipitating at least part of the NaCl derived from said polyhalite, if any, from the solution obtained after said step of concentrating the solution obtained after said step of precipitating at least part of the Mg 2+  remaining in said solution; (f) separating said precipitated NaCl, if any, from the reaction stream, and (g) separating as solid KNO 3  at least part of the K +  and NO 3   −  contained in the solution remaining after the step of precipitating at least part of the NaCl derived from said polyhalite. It is within the essence of the invention wherein said process is adapted to produce commercially usable KNO 3  from polyhalite. 
         [0008]    It is a further object of this invention to disclose such a process, further including an additional step of washing said polyhalite prior to said step of contacting polyhalite with a substance comprising NO 3   − , thereby removing at least a part of the NaCl contained within said polyhalite. 
         [0009]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of contacting polyhalite with a substance comprising NO 3   −  takes place at a temperature between about 60° C. and about 90° C. 
         [0010]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said substance comprising NO 3   −  is chosen from the group consisting of (a) HNO 3 ; (b) Ca(NO 3 ) 2 ; (c) any combination of the above. 
         [0011]    It is a further object of this invention to disclose such a process, wherein said substance comprising NO 3   −  is HNO 3 , and further wherein said step of contacting polyhalite with a substance containing NO 3   −  further includes an additional step of contacting polyhalite with a quantity of HNO 3  sufficient that the amount of HNO 3  in the solution thus obtained is at least 0.5% (w/w). 
         [0012]    It is a further object of this invention to disclose such a process, wherein said substance comprising NO 3   −  is HNO 3 , and further wherein said step of contacting polyhalite with a substance containing NO 3   −  further includes an additional step of contacting polyhalite with a quantity of HNO 3  sufficient that the amount of HNO 3  in the solution thus obtained is at least 5% (w/w). 
         [0013]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of contacting polyhalite with a substance comprising NO 3   −  further includes an additional step of contacting polyhalite with 60% HNO 3 . 
         [0014]    It is a further object of this invention to disclose such a process as defined in any of the above, further including an additional step of recycling into the reaction vessel at least part of the solution remaining after said step of separating solid KNO 3 . 
         [0015]    It is a further object of this invention to disclose such a process, wherein at least a part of said substance comprising NO 3   −  is obtained from said solution recycled into said reaction vessel. 
         [0016]    It is a further object of this invention to disclose such a process as defined in any of the above, further including an additional step removing from the reaction stream at least part of the solid produced during said step of contacting polyhalite with HNO 3 . 
         [0017]    It is a further object of this invention to disclose such a process, further including an additional step of removing by filtration at least part of the solid produced during said step of contacting polyhalite with HNO 3 . 
         [0018]    It is a further object of this invention to disclose such a process, further including an additional step of washing said solid. 
         [0019]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of adding at least one inorganic base to the solution obtained in the step of contacting polyhalite with a substance comprising NO 3   −  further includes an additional step of adding at least one inorganic base chosen from the group consisting of Ca(OH) 2 , CaCO 3 , and CaO. 
         [0020]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said wherein said step of adding at least one inorganic base to the solution obtained in the step of contacting polyhalite with a substance comprising NO 3   −  further includes an additional step of adding sufficient inorganic base to reduce substantially the SO 4   2−  content of said solution. 
         [0021]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of adding at least one inorganic base to the solution obtained in the step of contacting polyhalite with a substance comprising NO 3   −  further includes an additional step of adding sufficient inorganic base to reduce the SO 4   2−  content of said solution by at least 85%. 
         [0022]    It is a further object of this invention to disclose such a process, wherein said inorganic base is chosen from the group consisting of (a) basic Ca compounds; (b) basic Ba compounds; (c) any combination of the above. 
         [0023]    It is a further object of this invention to disclose such a process as defined in any of the above, further including an additional step of removing from the reaction stream at least part of the insoluble sulfate produced during said step of contacting polyhalite with HNO 3 . 
         [0024]    It is a further object of this invention to disclose such a process as defined in any of the above, further including an additional step of separating by filtration at least part of the insoluble sulfate produced during said step of contacting polyhalite with HNO 3 . 
         [0025]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of precipitating as Mg(OH) 2  at least part of the Mg 2+  remaining in said solution further comprises an additional step of adding a sufficient amount of at least one basic Ca compound to precipitate more than 50% of the Mg 2+  remaining in said solution as Mg(OH) 2 . 
         [0026]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of precipitating as Mg(OH) 2  at least part of the Mg 2+  remaining in said solution further comprises an additional step of adding a sufficient amount of at least one basic Ca compound to precipitate more than 85% of the Mg 2+  remaining in said solution as Mg(OH) 2 . 
         [0027]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of precipitating as Mg(OH) 2  at least part of the Mg 2+  remaining in said solution further comprises an additional step of adding at least one basic Ca compound chosen from the group consisting of Ca(OH) 2  and CaO. 
         [0028]    It is a further object of this invention to disclose such a process as defined in any of the above, further including an additional step of removing from the reaction stream at least part of said Mg(OH) 2  obtained in said step of precipitating as Mg(OH) 2  at least part of the Mg 2+  remaining in said solution. 
         [0029]    It is a further object of this invention to disclose such a process, further including an additional step of washing said Mg(OH) 2 . 
         [0030]    It is a further object of this invention to disclose such a process, wherein said Mg(OH) 2  is at least 92% pure. 
         [0031]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of concentrating the solution remaining after said step of precipitating at least part of the Mg 2+  remaining in said solution further comprises a step chosen from the group consisting of (a) using a multiple effect evaporator to concentrated said solution and (b) concentrating said solution by mechanical vapor recompression. 
         [0032]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of precipitating at least part of the NaCl further includes an additional step of precipitating NaCl by evaporative crystallization. 
         [0033]    It is a further object of this invention to disclose such a process, wherein said step of precipitating NaCl by evaporative crystallization occurs at a temperature exceeding about 60° C. 
         [0034]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of separating as solid KNO 3  at least part of the K +  and NO 3   −  contained in the solution remaining after said step of adding a basic Ca compound further includes an additional step of crystallizing KNO 3  from said solution. 
         [0035]    It is a further object of this invention to disclose such a process, wherein said step of crystallizing KNO 3  from said solution further includes an additional step of cooling said solution in order to affect crystallization of KNO 3 . 
         [0036]    It is a further object of this invention to disclose such a process, wherein said step of cooling said solution includes a further step of cooling said solution to a temperature below 40° C. 
         [0037]    It is a further object of this invention to disclose such a process as defined in any of the above, further including an additional step of purifying said KNO 3  obtained in said step of separating KNO 3 . 
         [0038]    It is a further object of this invention to disclose such a process, wherein said step of purifying said KNO 3  further includes an additional step of purifying said KNO 3  by at least one method chosen from the group consisting of (a) washing said KNO 3 ; (b) pulping with a substantially pure KNO 3  solution; and (c) recrystallization. 
         [0039]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein the purity of said KNO 3  exceeds 98.5%. 
         [0040]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of precipitating at least part of the Mg 2+  is carried out prior to said step of separating KNO 3 . 
         [0041]    It is a further object of this invention to disclose such a process as defined in any of the above, wherein said step of precipitating at least part of the Mg 2+  is carried out subsequent to said step of separating KNO 3 . 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURE 
         [0042]      FIG. 1  shows a schematic flowchart of the process herein disclosed. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    The present invention is described hereinafter with reference to the drawings and examples, in which preferred embodiments are 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. Therefore the invention is not limited by that which is illustrated in the figures and described in the specification, but only as indicated in the accompanying claims, with the proper scope determined only by the broadest interpretation of said claims. 
         [0044]    In the detailed description that follows, formulas indicating water of hydration are given according to conventional literature practice. As such, no claims are made regarding the specific level of hydration of the compounds (including those for which no water of hydration is indicated explicitly), and the invention herein disclosed is not limited to the specific levels of hydration given. 
         [0045]    The fundamental chemistry involved in the process herein disclosed can be summarized as follows: 
         [0000]      K 2 SO 4 .MgSO 4 .2CaSO 4 .2H 2 O+2HNO 3 +4H 2 O+Ca(OH) 2 [+Ca(NO 3 ) 2 ]→2KNO 3 +Mg(NO 3 ) 2 +4(CaSO 4 .2H 2 O)
 
         [0046]    The Mg(NO 3 ) 2  produced is precipitated as Mg(OH) 2  by further reaction with Ca(OH) 2 : 
         [0000]      Mg(NO 3 ) 2 +Ca(OH) 2 →Mg(OH) 2 +Ca(NO 3 ) 2  
 
         [0047]    In the present invention, in contrast to methods known in the art, nitrate ion and calcium ion are added in separate independent steps of the process. In some embodiments of the process, Ca(NO 3 ) 2  recovered from later stages of the process is recycled and reacted with the polyhalite. 
         [0048]    In a preferred embodiment of the invention herein disclosed, the following series of steps is employed to produce a commercial grade of KNO 3  from the reaction of polyhalite with HNO 3 . The various “stages” are listed with reference to the schematic flowchart given in  FIG. 1  for a preferred embodiment 10 of the invention herein disclosed. 
         [0049]    In some embodiments of the invention, NaCl is washed from the polyhalite. This step is entirely optional; there is no requirement to wash out the NaCl from the polyhalite, nor is there any need for preliminary thermal treatment of the polyhalite. 
         [0050]    In Stage 1 of the process ( 1010 ), polyhalite  101  is contacted with a substance comprising NO 3   −   102 . In preferred embodiments, substance  102  is HNO 3 ; in most preferred embodiments, the HNO 3  is provided in 60% concentration. In preferred embodiments of the invention, a sufficient amount of 60% HNO 3  is added such that concentration of HNO 3  in the resulting solution (i.e. including the polyhalite) is at least 5% (w/w). In preferred embodiments of the invention, the polyhalite and HNO 3  are brought into contact at a temperature of between about 60° C. and about 90° C. In other embodiments of the invention, temperatures outside of the range used in preferred embodiments are used. In other embodiments, substance  102  may comprise another nitrate salt such as Ca(NO 3 ) 2 , or a mixture of a nitrate salt and HNO 3 . In typical embodiments in which Ca(NO 3 ) 2  is used, it is added at the beginning of stage 1 in addition to or instead of the HNO 3  added at the beginning of the stage and the Ca compound added at the end of this stage, described below. 
         [0051]    In preferred embodiments of the invention, at least part of the solution ( 210 ) obtained in stage 5 (described below) is recycled into the reaction vessel in which the contact between polyhalite and the substance comprising NO 3   −  takes place. 
         [0052]    The reaction mixture is then brought into contact with a base (in preferred embodiments, a Ca(OH) 2  slurry ( 105 )); in preferred embodiments, sufficient slurry is added to bring the pH to substantially neutral. Addition of the Ca(OH) 2  slurry thereby yields a solution comprising primarily K + , Mg 2+ , Ca 2+ , Na + , NO 3   − , and Cl − , along with solid CaSO 4  (gypsum). In preferred embodiments, sufficient base is added to precipitate at least 85% of the SO 4   −  present in the solution. 
         [0053]    In preferred embodiments of the invention, solid gypsum precipitated during the reaction between the polyhalite and the HNO 3  is filtered and washed with wash water  103  (in preferred embodiments, by counter-current washing on a filter  1015 ) in order to reduce the nitrate content. The mother liquor ( 106 ) is transferred to stage 2, while the wet gypsum after washing ( 201 ) is discharged from the system. The wash filtrate  104  is then returned to the reaction vessel in which stage 1 takes place. 
         [0054]    In Stage 2 of the process ( 1020 ), additional Ca(OH) 2  slurry ( 105 ) is added to the solution obtained in Stage 1 ( 106 ) after removal of solid gypsum in order to precipitate the major part of the Mg 2+  contained in the solution as Mg(OH) 2  ( 202 ); in preferred embodiments, sufficient Ca(OH) 2  is added to precipitate at least 50% of the Mg 2+  present. The Mg(OH) 2  is washed ( 1025 ) and removed. After precipitation of Mg(OH) 2 , a solution comprising primarily Ca 2+ , K + , Na + , NO 3   − , Cl −  and residual Mg 2+  remains. In some embodiments, this stage is carried out after Stage 5 (described below) on the solutions to be recycled in stage 1. The chemical purity of the Mg(OH) 2  produced is dependent on the purity of the CaO or Ca(OH) 2  used. In preferred embodiments, Mg(OH) 2  with a purity exceeding 92% is obtained. 
         [0055]    The process then proceeds to Stage 3 ( 1030 ), in which the solution obtained in Stage 2 is concentrated. In preferred embodiments, the concentration is effected by evaporation using any technique known in the art, e.g., a multiple effect evaporator or by mechanical vapor recompression. In a preferred embodiment, at least part of the residual CaSO 4  thus precipitated is separated from the supernatant solution at the exit of the vessel in which the concentration takes place. The evaporation can be also carried out by solar evaporation in an evaporation pond and thus the calcium sulfate precipitated can be left on the bottom of the pond. 
         [0056]    The process then continues to Stage 4 ( 1040 ), in which NaCl ( 203 ) and a small part of the CaSO 4  ( 204 ) present in the solution are partially separated from the solution remaining after Stage 3 by crystallization in an evaporative crystallizer at a temperature exceeding 60° C. The solids are separated (in preferred embodiments, by filtration  1045 ) and removed. 
         [0057]    In Stage 5 of the process ( 1050 ), KNO 3  is crystallized from the solution by cooling the solution remaining from Stage 4. The crystallization can be carried out by any technique known in the art, e.g., in a cooling crystallizer of the various types existing, including cooling disc crystallizer. In typical embodiments of the invention, the purity of the white KNO 3  product obtained after washing in the tests exceeds 98.5%. In typical embodiments of the invention, the main impurities are Ca (&lt;0.2%), (&lt;1000 ppm); Na (˜500 ppm); SO 4   2−  (˜200 ppm); Mg (˜10 ppm); and Sr (˜10 ppm). The KNO 3  thus produced can be further purified by any technique known in the art, for example, by repulping with a pure KNO 3  solution or by recrystallization. 
         [0058]    The solution  210  remaining from Stage 5 is recycled to the vessel in which Stage 1 takes place. The Ca(NO 3 ) 2  contained in the solution remaining from Stage 5 reacts with the sulfate in the solution in stage 1 to precipitate gypsum. 
       EXAMPLE 1 
       [0059]    Polyhalite (unwashed, crushed and screened to −0.5 mm, 400 g) was added to a stirred mixture of nitric acid (59%, 146.7 g) and recycled solution (1090 g, made from combining mother liquor from KNO3 crystallization presented in Example 3 and gypsum wash water from a previous batch). The concentration of the nitric acid is modified by dilution with wash water from previous runs in order to maintain a constant nitrate concentration of 15-16% in the final filtrate. The reaction mixture was heated to 65° C. and stirred for 3 h. After that time, milk of lime (169.4 g, 30% in water) was added dropwise via pump over a 1 h period to the hot mixture in order to neutralize the acidity of the slurry. When the mixture reached pH of 5.5-6.5 the addition was stopped and the mixture was filtered while hot under vacuum. The gypsum cake (700 g, 60.8% solids) was then washed with water (3×350 g) so that the nitrate content of the cake was satisfactorily low. The wet, washed gypsum (575.6 g, 73.9% solids) was then dried overnight in an oven at 60° C. yielding 425.5 g of gypsum (CaSO 4 .2H 2 O&gt;98.5%, K&lt;0.4%, Mg&lt;0.2%, NO 3   −  &lt;100 ppm). The filtrate (1094.7 g, K=4.7%, Mg =1.5%, Ca=0.6%, SO 4   2− =0.3%, NO 3   − =15.5%) was used as the basis for the Mg(OH) 2  separation step (see Example 2 below) while the wash water was combined with the recycled solution for the next batch. 
       EXAMPLE 2 
       [0060]    A sample of solution obtained after completion of the reaction presented in Example 1 above (720-900 g of solution were treated at a temperature of 60-70° C. with 15% solution of milk of lime (300 g). As a result of this treatment, the Mg concentration decreased from 1.5% to less than 0.2%. The solids precipitated were settled and, afterwards, filtered and washed. The dry solids contained more than 92% Mg(OH) 2 . The main impurities were Ca (&lt;5%), SO 4   =  (2%), NO 3   −  (0.2%) and Cl −  (0.05%). 
       EXAMPLE 3 
       [0061]    A sample of the solution of remaining after the precipitation of Mg(OH) 2  described in Example 2 above, comprising (concentrations on w/w basis relative to the total solution) 2.2% Ca, 4.4% K, 1.9% Na, 0.01% Mg, 13.3% NO 3   − , 3.1% Cl − , and 0.08% SO 4   2−  was concentrated by evaporation at a temperature exceeding  80 ° C. The total concentration of dissolved salts increased by &gt;80% as a result of the concentration. The NaCl thus crystallized was separated at a temperature exceeding 80° C. and its purity after washing exceeded 98%. 
         [0062]    The remaining solution was then cooled down to a temperature &lt;40° C., leading to precipitation of KNO 3 , which was then separated from the mother liquor and washed. Rhe purity of the KNO 3  obtained exceeded 99.5%, while the concentration of dissolved salts in the mother solution to be recycled to the reaction was in the range of 55-60%.