CA1217027A - Process for producing and decomposing syngenite for producing k.sub.3h(so.sub.4).sub.2 crystals and potassium sulfate crystals, and for producing potassium nitrate - Google Patents

Abstract

ABSTRACT
A process for producing K2SO4 from potassium chloride salts, calcium sulphate salts and another sulphate source wherein syngenite is formed and then decomposed. In one embodiment, syngenite is decomposed to produce crystalline K3H(SO4)2. The K3H(SO4)2 crystals are recrystallized to produce K2SO4 crystals. In another embodiment, syngenite is decomposed to produce KNO3 in solution and solid CaSO4. The solution containing the KNO3 is separated from the CaSO4 precipitate and subjected to a crystallization step, followed by recovery of the solid crystalline KNO3.

Description

~7~27 PROC~SS FOR PRODUCING AND DECOMPOSING SYNGE~ITE, FOR PRODUCING K3~(SO4)2 CRYSTALS AND
POT~SSIUM SULFATE CRYSTALS, AND FOR
PRODUCING POTASSIUM NITRATE
BACKGROUND OF THE INVENTION
Potassium chloride, the major form in which potassium is used in fertilizers, has been known for many years to have agronomic disadvantages when compared with certain other potassium salts. Thus, currently the sulphate and nitrate are widely used on crops such as tobacco, tomatoes, and potatoes, especially for those to be used in the production of potato chips (crisps).
The chloride ion, if allowed to build to sufficiently high levels, is toxic to most plant species, and its elimination is a desirable aim for the fertilizer industry. In arid areas, totally dependent upon irrigation for their water, for example, the build-up OL chloride ions in the soil can become a major factor in producing a reduced crop yield. At such times, major quantities of water are required to flush out the chloride. Such flushin~ not only wastes large quantities of valuable water, but, at the same time, flushes out necessary fertilizer constituents in the soil.
The major, if not the sole, factor which has caused the continued use of potassium chloride under these circumstances, is the ready availability and consequently low cost of the chloride as compared with other potassium salts.
The most common substitute for potassium chloride is potassiu~ sulphate. This salt exists in various mineral ~orms in a number of places, but its separation, usually by crystallization techniques, is more complex and more expensive than that for the chloride. It is produced directly as a double sulphate salt along with magnesium sulphate in the Western UoS~A~ but such material, although not expensive, per se, is low in potassium concentration and hence more costly to transport and store.

For many years, the ch]oride sal~ of potassium has been converted into the sulphate by high temperature reaction with sulfuric acid, and considerable quantities are manu-factured in this way, particularly in Belgium. U.S. Patent 4,342,737 discloses one such process. The major factors restricting further production by this method are threefold:
1. The high energy requirement.

2. The highly corrosive nature of the reactants and the by-product hydrogen chloride.

3. The need for a local market for the hydrogen chloride produced - otherwise, it must be neutralized at considerable cost before it can be discarded.
For many years, varying routes have been described to convert the chloride using calcium sulphate or sodium sulphate, but none has been used commercially up to the present.
Many of the routes proposed produce glaserite, a double salt of potassium and sodium sulphate, Na2SO4 3K2SO4, as an intermediate and subsequently react with excess potassium chloride to convert the sodium sulphate to potassium sulphate.
The product may be recovered, for example, by evaporation and recrystallization. U.S. Patent 4,215,100 is directed to such a process. Other routes produce the calcium double salt, syngenite, CaSO4 K2SO4 H2O as intermediary. This may be decomposed by water at elevated temperature and pressure, as disclosed in British Patent 435,772, or by concentrated ammonia at low temperature, as disclosed in French Patent 787,713. In British Patent 2,068,918, sylvinite, a double salt of potassium and sodium chloride of variable composition, and calcium sulphate are reacted with aqueous ammonia to produce the potassium sulphate, sodium sulphate double salt;
the double salt is reacted with sylvinite or additional sylvinite in aqueous ammonia to produce potassium sulphate crystals.

All such routes are complexl costly, major energy users and may require operation under undesirable conditions.
Thus, a need has continued to exist for a process of producing K2S04 using readily available raw materials of low cost, said process being relatively uncomplicated, highly energy efficient, and requiring no substantial equipment cost outlay.
Potassium nitrate is largely produced by one of two processes. In the first, which is the only one currently practiced in the U.S.A., potassium chloride is reacted with nitric acid at a high temperature to produce potassium nitrate and chlorine, utilizing the following reactions:
3 > 3KNo3 + C12 + NOCL + 2H20 2NOCl + 4HN03 ---~ 6N02 + Cl + 2H O
4N2 + 2 ~ 2H2 4HNo3 The corrosive nature of the various reagents with consequent design and maintenance problems and the need to dispose of the by-product chlorine have prevented wider usP
of the process.
In recent years, an increasing tonnage of potassium nitrate has been imported into the U.S. A major source is Israel where a process is operating whereby nitric acid and potassium chloride are reacted in solution-and the co-product hydrogen chloride is removed from the solution by solvent extraction. The viability of such a process is depen~-~ent upon the economic disposal of the hydrogen chloride for which no major demand exists in many parts of the world.
Thus, a need has continued to exist for a process of producing XN03 by an alternative method without either the maintenance and construction, or by-product disposal problems.

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SUMMARY OF THE INVENTION
.
It is an object of this invention to produce an agronomically acceptable form o~ potassium.
It is a further object of this invention to produce potassium in the form of potassium sulphate or potassium nitrate.
It is another object of this invention to produce potassium sulphate or potassium nitrate by a process which is relatively uncomplicated and highly energy efficient to operate.
It is still another object of this invention to produce potassium sulphate or potassium nitrate from syngenite.
It is yet another object of this invention to produce potassium sulphate utilizing a process wherein various of the process by-products are returned to the production cycle.
These and other objects of the invention, as will hereinafter become more readily apparent, have been accomplished by a simple process which produces, and then decomposes, syngenite to yield potassium sulphate or potassium nitrate.
In reaction stage 1, potassium chloride and a sulphate are a~itated with a calcium sulphate or penta salt suspension which may be recycled from a subsequent decomposition, stage 2, to produce sodium chloride and syngenite in suspension according to the equation:
2KCl + Na2S4 + CaS04 + H ~ CaSO K SO H O + 2NaCl or 8KCl + 4Na2S04 + 5CaS04 K2S04 H2 2 ~ 5(CaS04~K2S04-H20) + 8NaCl After separation, the syngenite is fed to reaction stage 2 where it is reacted with a suitable acid containing solution such as a hot sulphuric acid containing solution ~1 ~ ~fll'r3 and thereby decomposed into calcium sulphate or penta salt and potassium sulphate, depending on reaction conditions.
The stoichiometry is as follows:
2 o4 CaS04 K2S04 H20----~CaS04 + 2KHS04 + H20 or 4H2S04 + 5(CaS04 K2S4 H2 ) 5CaS04 K2S04 H20 + 8KHS04 ~ 4H20 The suspension so produced is separated at the reaction temperature, and the calcium sulphate or penta salt removed may be recycled to stage 1 for further syngenite synthesis. The resultant hot solution containing potassium, sulphate, and bisulphate ions, at appropriate concentrations, is cooled to crystallize K3H(S04)2, a double salt of potassium sulphate and bilsuphate. This is removed by filtration or other suitable means and the mother liquor recovered may be reheated and recycled to sta~e 2 to decompose further syngenite.
Crystallization of an aqueous solution of the double salt, K3H(S04)2, yields crystals of potassium sulphate, the desired product, and a mother liquor containing potassium bisulphate and free sulphuric acid which may be separated and recycled to stage 2 to decompose further syngenite.
In the case of decomposing syngenite with nitric acid rather than sulphuric acid, potassium, hydrogen, bi-sulphate and nitrate ions remain in solution, after removal of calcium sulphate.
CaS04-K2S04 H20 + 2H + 2N03 > CaS04~ +
HS04 + 2K + 2N03 + H+ + H20 To maximize decomposition using nitric acid, therefore, the nitric acid content in solution must be sufficient to convert all the potassium sulphate into potassium bisulphate solutions.

. .

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The addition of lime or calcium carbonate, in appropriate amounts, to such a solution results in the precipitation of further calcium sulphate leaving a solution containing effectively potassium and nitrate ions, from which potassium nitrate may be crystallized.
2K + HS04 ~ H + 2N03 + CaC03 3

4~ + 2~ ~ 2No3 + H20 + C02 ~
The presence of excess nitric acid may be advantageous as this lowers the solubility of the potassium nitrate and results in a lower potassium recycle. Alternatively, calcium nitrate may be added to the solution to precipitate calcium sulphate and after crystallization and removal of potassium nitrate, the nitric acid solution remaining, containing some potassium nitrate, may be recycled to the syngenite decomposition stage.
2X + HS04 ~ H + ~N03 + Câ + 2No3 >
CaS04 + 2K + 4N03 + 2H+
2K+ + 4N0 + 2H+ Crystallize_ 2KNo + 2N0 ~ + 2H+
While developing the above processes using nitric acid, it became clear that any process, which reduces the sulphate level in reactant solutions to a level below that which is in equilibrium with syngenite, will result in the liberation of potassium ions from the syngenite even in the absence of hydrogen ions.
Thus, the addition of calcium ions to such a solution does, in fact, result in the precipitation of calcium sulphate and liberation of potassium ions.
4 2 4 2 + Ca __~ 2Cas04~ + 2K~ + H20 The addition of calcium ions in the form of calcium nitrate, which may be produced "in situ" from nitric acid and calcium carbonate or hydroxide, results in the production of a -Q2~

potassium nitrate solution from which solid KN03 may be crystallized, i.e., 4 2 4 2 Ca N03 --~ 2CaS04 ~+ 2K + 2N03 + H20 .
This may be interpreted as:
4 ~ 4 2 + Ca(N03)2-~ 2CaS04 ~ + 2KNo ~ H O

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a flow diagram of the process of this invention wherein sylvinite is converted to potassium sulphate.
Figure 2 is a process flow diagram representing one embodiment wherein syngenite is decomposed with HN03 and additional sulphate ion precipit~ted by addition of calcium in the form of calcium carbonate, calcium hydroxide or calcium nitrate, fol-lowed by recovery of crystalline KN03.
Figure 3 is a process flow diagram representing another embodiment wherein syngenite is decomposed by addition of calcium ion in the form of calcium nitrate, followed by recovery of crystalline KN03.

DESCRIPTION OF THE PREFERRED E~BODIMENT
.
Much of the potassium chloride mined in the world is in the form of sylvinite, a double salt of sodium and potassium chloride of variable composition. The method used to prepare syngenite from potassium chloride can be readily modified to use sylvinite as starting material. In fact, an advantage exists in using sylvinite in that the sodium chloride generated by the syngenite synthesis can be discarded along with the sodium chloride in the sylvinite without separate segregation or evaporation.

The data presented by Hill, A.E., J. Am. Chem.
Soc. 56, 1071-8 (1934) ibid. 59 2242-4 (1937) and Bodaleva, N.V. and Lepeshkov, I.N., Zh. Neorgan. Khim 1, 995-1007 (1956) shows that the stability of syngenite decreases as the temperature increases from 40C to 100C. This shows that syngenite is best synthesized at lower temperatures and decomposed into calcium and potassium sulphate at higher temperatures.
This same data further indicates that in order to produce stable syngenite, the concentration of potassium sulphate in the solution must be in excess of about 4~ w/w at 40C increasing to about 11% w/w at 100C.
Any process synthesizir.g syngenite from potassium sulphate solutions must then operate within these parameters.
We have shown that it is possible with solid calcium sulphate to obtain syngenite from solutions containing potassium ions, added as potassium chloride, and sulphate ions, added as sodium sulphate, ammonium sulphate, or other soluble sulphates, in concentrations which satisfy the solubility product for syngenite at the reaction temperature.
Further, we have shown, based on data of Cornec, E. and Krombach, H., Compt. Rend. 194, 714-6 (1932) that satisfactory concentrations of potassium chloride can be obtained directly from sylvinite by taking advantage of the mutual solubilities of sodium and potassium chloride.
The compositions of solutions in equilibrium with both sodium and potassium chloride at various temperatures can be plotted from the Cornec et al data. Thus, a solution in equilibrium at 20C when heated to 80C and used to leach sylvinite, will dissolve potassium chloride and discard solid sodium chloride until it attains equilibrium with both sodium and potassium chloride.
If such a solution at 80C is now contacted with the appropriate amou.,t of calcium sulphate and an amount of ~2~7(~7 sodium sulphate equivalent to the calcium sulphate is added to the solution, which is allowed to cool to 20C, syngenite is formed.
The concentration of potassium chloride can be reduced to that at the invariant or equilibrium point at 20C by choosing the appropriate amounts of calcium and sodium sulphate. The sodium chloride produced by the reaction CaS04 + 2KCl + Na2S04 ~ H2 ~~~ 4 2 4 2 is in excess of that solution at the 20C invariant so that the excess will precipitate along with the syngenite while the remainder will stay in solution at the invariant or equilibrium point at 20C. After separation of the precipitated solids, the solution may be heated to ~0C and recycled to leach sylvinite and dissolve resh potassium chloride while rejecting the remainder of the sodium chloride generated by the syngenite synthesis step. This sodium chloride is then discarded along with that remaining after solution of the potassium chloride from the sylvinite. Alternatively, potassium chloride alone may be added to the solution at 80C, where it will dissolve until equilibrium is reached, and sodium chloride will be precipitated and can be removed.
Phase considerations also restrict the conditions for the decomposition of the syngenite and for separation of the potassium sulphate so produced. The phase diagram for K2SO4-H2SO4 in water can be plotted from data of D'Ans, J.z., Anorg. Chem. 63, 225-9 (1909), Stortenbecker, W., Rec.
Trav. Chem. 21, 407 (1909), and Babaewa, A.W., Trans. Inst.
Pure Chem. Reagents (Moscow) llI 114 (1931).
Our studies have shown that in the presence of hydrogen ion at elevated temperature ( >60C), syngenite decomposes into either calcium sulphate or penta salt (5CaSO4 K2SO4 H2O), dependent upon conditions. ~he mechanism ~2~'7~

appears to depend upon the conversion of sulphate ion in solution into bisulphate ion with consequent dissolution of K2S04 from the syngenite to try to restore the sulphate concentration in solution.
If sufficient hydrogen ion is present in the form of sulphuric acid, or other acid with a dissociation constant greater than the second dissociation constant of sulphuric acid, 1.2 x 10 2, the syngenite will totally decompose into solid CaSO4 and potassium and bisulphate ions in solution.
To maximize decomposition using sulphuric acid, therefore, the sulphuric acid content in solution must be sufficient to convert all the potassium sulphate, both any in solution originally and that liberated from the syngenite, into potassium bisulphate solutions.
Solutions, a~ter syngenite decomposition, with compositions with a K:SO4 sulphate molar ratio greater than 1:1 containing excess potassium sulphate compared with a solution of potassium bisulphate, will be technically feasible but economically unsatisfactory r since the amount of potassium sulphate liberated from the syngenite is directly equivalent to the amount of free sulphuric acid available in the decomposition solutiont since it is the hydrogen ions which convert sulphate to bisulphate ions.
The data of D Ans, Stortenbecker and Babaewa shows that even solutions containing free sulphuric acid in excess of that required to form bisulphate upon cooling will, under appropriate conditions, yield crystals, not of potassium bisulphate, but of the double salt X3H(SO4)2. However, at low starting temperatures ( >30~C), bisulphate composition solutions will, in fact, yield potassium sulphate on cooling.
The mother liquor will contain free acid and may be recycled to decompose fresh syngenite.
The data of D'Ans, Stortenbecker and Babaewa also shows that t at any given temperature t with sulphuric acid ~17C~7 concentrations lower than that at the invariant point (i.e., where the solid double salt K3H(SO4)2 along with solid potassium sulphate are in equilibrium with the solution) the solid phase in equilibrium is potassium sulphate. Thus, for example, at 30CC the addition of double salt to solutions containing less than 20 grams of H2SO4 per 100 grams water will result in the crystallization of potassium sulphate until such additions bring the sulphuric acid concentration in the mother liquor to 20 grams per 100 grams water. After separation of the solid potassium sulphate, such mother liquor may be recycled to syngenite decomposition after suitable concentratlon by evaporation. At higher temperatures, up to the boiling point of the solution, a similar result is obtained, but with h:.gher concentration solutions being recycled. This reduces the amount of evaporation required.
Although such isothermal crystallization of potassium sulphate is preferred, the double salt, K3H(SO4)2, may be dissolved in hot water. Such solution, upon cooling, will yield potassium sulphate as the solid phase, and the acid rich mother liquor may be recycled to syngenite decomposition.
The preferred temperature for this solution step is at, or near, the boiling point. Typically, a solution containing 46.5% K3H(SO4)2 on a weight to weight basis (87.5 grams of K3H(SO~)2/100 grams of water) at 95C will, when cooled to 0C, precipitate K2SO4 crystals [68.5 grams of K2SO4 - 187.5 grams original solution].
In order to operate the process in its most financially economical way, it is desirable to minimize plant equipment size, which means operating at conditions which will give maximum through-put for a given unit size.
For this reason, it is clearly best to leach sylvinite at the highest feasible temperature, about 100C, thereby extracting the maximum amount of potassium chloride and obtainin~ the invariant solution of highest concentration.
Assuming tha~ we wish to maintain the potassium chloride concentration, after synthesis, greater than 3.5%, this defines the amount of KCl which can be reacted and hence the amount of NaCl produced. It is clear that at this portion of the solubility curve, temperature is unimportant.
The lowest potassium concentration quoted above is derived from examination of the data presented by Hill and Bodaleva et al discussed above, which defines the lowest K2S04 concentration in equilibrium with syngenite at 40C as about 4 grams per 100 grams solution which is equivalent in potassium concentration to 3.5 grams KCl per 100 grams solution. In practice, we have shown that at 20C, KCl concentrations as low as 2.9 grams per 100 grams are satis-factory. The data presented by Hill and Bodaleva et al also shows that the stability range of syngenite is greater at lower temperatures, which defines the most useful synthesis temperature range as being below 60C, probably below 40C, which has, in fact, been confirmed by experiment.
As already mentioned, the data of D'Ans, Stortenbecker and Babaewa shows that the most efficient syngenite decompo-sition conditions are ones where the composition of the solution, after decomposition, corresponds to a K:S04 molar ratio of 1:1. These compositions should also yield the double salt, K3H(S04)2, upon cooling, and the highest concen-tration solution meeting these criteria is that at a concen-tration of about 31.6 grams K2S04 and 17.8 grams H2S04 per 100 grams solution which, upon cooling to 0C, yields a solution at a concentration of about 12 grams K2S04 and 18.5 grams H2S04 per 100 grams solution. The cooling, therefore, crystallizes out double salt containing the equivalent of 22.86 grams K2S04, along with 4.3 grams H~S04 per 100 grams of initial solution. No other composition of solution has ~L2~

such a favorable yield of K2S04 with as good a K2S04/H2S04 ratio.
The invention may best be described by reference to Figure 1. A potassium chloride salt, such as sylvinite or KCl, and recycle solution A, to be described below, are contacted, in leach system 1, wherein potassium chloride is dissolved and solid sodium chloride precipitated. The sodium chloride is discarded. The preferred temperatures for this step are in the range between about ambient and the boiling point of the solution. Eighty degrees centigrade (80C) to the boiling point is the most preferred temperature range.
The leach solution B is now fed to the syngenite preparation unit 2, wherein it is allowed to cool while reacting with a calcium sulphate salt and additional sulphate.
Suitable additional sulphate sources include, but are not limited to, (NH4)2S04 and alkali metal sulphates. Suitable calcium sulphate salts include, but are not limited to, CaS04 and the penta salt, 5CaS04 K2S04 H20. At least a part of this addition may come from the by-product recovery of the subsequent syngenite decomposition. The syngenite slurry, D, so produced, is separated by, for example, filtra-tion and washed to remove sodium chloride in unit 3. The mother liquor and washings, E, may be concentrated to remove the wash water in concentrator 4 and recycled to the leach stage as the concentrate A.
Solid syngenite, F, is fed to the syngenite decomposition vessel 5, maintained at elevated temperature, along with a mineral acid. The mineral acid may be supplied in the recycled liquor, G and H, from later stages in the process. "Gypsum" slurry, J, is separated by filtration in separation unit, 6, and the solid "gypsum", I, washed in 7 with water. This "gypsum" may be recycled to syngenite preparation, 2, as C.

7C~

Suitable temperatures for the acid decomposition of syngenite are in the range of ambient to boiling, with temperatures above about 70C preferred. Eighty degrees centigrade (80C) or above are the most preferred tempera-tures. The reaction goes to completion at atmospheric pressure, usually re~uiring a time of about 30 minutes to 2 hours. Appropriate concentrations for proceeding to the next step are 31.6~ w/w K2SO4 and 17.8% w/w X2SO4.
The mother liquor, K, from 6, containing K~SO4, is cooled to 0C in crystallizer 8 to precipitate K3HSO4 crystal slurry L which is separated by, for example, filtration in 9. It is preferred that the solution containing KHSO4 be cooled to a temperature in the range of about 0C
to 45C, preferably in the range of about 0C to 30C. The most preferred range is about 0C to 20C. The mother liquor Q may be recycled to syngenite decomposition in 5.
The double salt K3H(SO4)2 obtained from the cooling step above is slurried at 100C in the wash liquors N and O in crystallizer 10. Wash liquor N is recycled from the gypsum wash 7, while wash liquor O is recycled from a subsequent potassium sulphate wash.
Potassium sulphate crystals so produced, P, are separated in 11 and the mother liquor, Q, after concentration in evaporator 12, is recycled to syngenite decomposition 5.
The crude potassium sulphate crystals R are washed in 13 with saturated potassium sulphate solution S before being passed as wet potassium sulphate T to the drier 1~, from which the product X2SO4 is obtained~
Referring now to Figure 2, syngenite 136, recycle solution 1~8 and the appropriate amount of fresh nitric acid 137 are reacted in syngenite decomposition vessel 138.
Suitable temperatures for the acid decomposition of syngenite are in the range of ambient to boiling, with temperatures above 70C preferred. Eighty degrees centrigrade (80C) or 7C~7 above are the most preferred temperatures. The reaction goes to completion at atmospheric pressure, usually requiring a time of about 5 minutes to 2 hours.
The slurry of calcium sulphate 140 so produced is separated, for example, by filtration, in first stage separator 142. The calcium sulphat~ solids 144 are washed at 132 with water 134 and the washings 139, recycled through evaporator 146 as part of solution 148 to vessel 138. The washed calcium sulphate 130 is discarded to recycled to syngenite synthesis, as appropriate.
The solution 143 separated in 142 is reacted in the second stage calcium sulphate precipitator 152 with the appropriate amounts of calcium carbonate, calcium hydroxide, or calcium nitrate 150. The slurry of calcium sulphate 154 so produced is separated, for example, by filtration, in second stage separator 156. The calcium sulphate 158 is washed at 150 with water 162 and discarded, or recycled to syngenite preparation as appropriate at 164. The wash liquor 161 is recycled through evaporator 146 as part of solution 148 to syngenite decomposition vessel 138.
The separated solution 180 is cooled in crystallizer 182 to crystallize solid potassium nitrate which is separated from the mother liquor in the slurry 184 in separator 186.
The solution 180 is typically cooled in crystallizer 182 to a temperature in the range of about 0C to 45C, more typically in the range of about 0C to 30C. The preferred range is about 0C to 20C. The filtrate 198 is recycled through evaporator 146 as part of solution 148 to syngenite decomposition.
In Figure 3~ syngenite 236 and recycle solution 248 are reacted at the same temperatures as in Figure 1 with the equivalent amount of calcium nitrate 237 in syngenite decomposer 238. The slurry of calcium sulphate 240 so produced is fed to separator 242 where the calcium sulphate - 15 ~

~ \

244 is removed and washed with water 234 at 232 before being discarded or recycled, for example, to syngenite preparation 230.
The wash liquor 239 is fed to evaporator 246 and thence as part of recycle solution 248 recycled to 238.
The solution 254 separated from the calcium sulphate is now cooled in crystallizer 282 to crystallize potassium nitrate. The crystallizer 282 is operated at the same temperatures as the crystallizer 182 in Figure 1. The slurry of potassium nitrate 284 so p~oduced is separated in separator 286. The mother liquor 298 is fed to evaporator 246 before being recycled as part of solution 248 to syngenite decomposition 238. The potassium nitrate produced is washed and dried as appropriate to yield KNO3 crystals 296.
Having generally described the invention, a better understanding can be obtained by reference to certain specific preliminary examples, which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.

Example 1 This example shows that potassium chloride and sodium sulphate can be used to prepare syngenite.
A saturated solution of potassium chloride was prepared by dissolving 859 grams of potassium chloride in 2500 grams water at 23C. To this solution was added 115 grams potassium chloride, 216.5 grams sodium sulphate and 186 grams anhydrous calcium sulphate. This mixture was reacted at 23C for two hours. The slurry was filtered, and the product washer and dried. The product contained 22.3%
potassium and 60.43~ sulphate. The 414.2 grams of dried product represents a yield of 92.3~ based on the calcium sulphate.

~ 16 -7()~7 E _ ple 2 This example demonstrates that ammonium sulphate can be used for the preparation of syngenite by reaction with potassium chloride and calcium sulphate.
A calcium sulphate slurry was prepared by adding 77 grams of calcium hydroxide to 730 grams of water and reacting this mixture with 101.6 grams of concentrated sulphuric acid (97~) and cooling to 10C.
Syngenite was prepared from the above mixture by adding 149.1 grams potassium chloride, 132 grams ammonium sulphate and 520 grams additional water. This mixture was stirred for one hour at 10C, filtered, and the product washed with approximately 600 grams water.
The washed product, after drying, weighed 266.9 grams (81% yield), and had potassium, sulphate, calcium, ammonium (as nitrogen) and chloride ion contents of 18.46~, 62.65%, 14.17~, 0.0% and 0.07%, respectively. This represents 100% conversion based on potassium, allowing for the potassium sulphate remaining in the solution and washings, in equilibrium, with the syngenite.

Example 3 This example illustrates that syngenite can be prepared from a sylvinite leach liquor mutually saturated with potassium and sodium chloride, containing potassium chloride, sodium chloride and water contents of 20%, 17.5%
and 62.5%, respectively.
One kilogram of the above clear solution at 80C
was placed in a reactor and to it was added, in order, 71 grams sodium sulphate and 6~ grams calcium sulphate~ The mixture was reacted at 80C for 30 minutes, then cooled to 23C. The total reaction time was about two hours~ The reaction slurry was filtered, washed and dried. The dried product, 140.7 grams, contained 22.3% potassium, 59.1%

7~7 sulphate and 0% chloride ion. The 140.7 grams of product represents a 96% yield based on the calcium sulphate.

Example 4 This e~ample shows that syngenite can be effectively decomposed with sulphuric acid.
Filtrate solution, 476 grams, from a prior decomposition containing 29.2% potassium sulphate and 14.7% sulphuric acid was reacted with 47.9 grams sulphuric acid (97%) and 140 grams syngenite for 2 hours at lOO~C, the resultant slurry was filtered hot (90C). The residue was washed and dried to yield 43.2 grams of calcium sulphate solids containing 2.6% potassium, 64.2% sulphate and 0% chloride. The decomposition efficiency, when adjusted for 5.9 grams of potassium sulphate that was contained in the solution held by the wet gypsum residue before drying, was about 100%.
r Example_5 This example shows that nitric acid will also decompose syngenite. 241 grams of syngenite was reacted with 500 grams of a 1.51 molar HNO3 acid solution containing 48.3 grams of HNO3 for 2 hours at 60C.
The resultant slurry was -filtered, the 166 grams wet solids were washed and dried. The 116.8 grams of dry solids contained 9.5~ potassium and 63.9% sulphate. The decomposition efficiency to calcium sulphate under these conditions was 81% based on the potassium remaining in the decomposition residues, and, to the penta salt, 95%.

Example 6 This example demonstrates the crystallization of double salt, K3H(SO4)2, from solutions with a sulphuric acid to potassium sulphate molar ratio of 1 to 1/ equivalent to potassium bisulphate. 300 grams of potassium sulphate and ~7~

180.9 grams of sulphuric acid (94%) were dissolved in 519.1 grams water at 50C. The solution was cooled to 0C and the solids which crystallized were removed by filtration to yield 283.6 grams wet solids, which, on drying, yielded 260.8 grams dry crystals of the double salt, analyzing K, 40.1%, and SO4, 61.8~. (K3H(SO4)2 required K, 37.7%; SO4 61.9%). Yield based on potassium sulphate, 77.8%.

Example 7 This example illustrates the isothermal crystalliza-tion of K2SO4 from K3H(SO4)2 solution at 30C. K3H(SO4)2, 250 grams, is added to water, 200 grams, at 30C and the slurry so produced agitated for 2 hours. At the end of this time, the solids were removed by filtration and dried, to yield, after water washing, 148.9 grams dry solids analyzing K, 44.4%, SO4 54.7%. (K2SO4 requires K, 44.8%; SO4, 55.2%).
The mother liquor contained K 8.4%, and SO4 19.9%, corresponding to 26.2 grams K2SO4/100 grams H2O and 13.7 grams H2SO4/100 grams H2O.

Example 8 This demonstrates the separation of potassium sulphate crystals from solutions of the double salt, K3H(SO4)2.
225 grams of the double salt produced in Example 6 were dissolved in 250 grams of water at approximately 100C.
When cooled to 0C, the solution yielded 152.4 grams wet potassium sulphate crystals which gave a dry weight of 144.3 grams of material analyzing K, 44.9% and SO4, 54.9%. (K2SO4 requires K, 44.8%; SO4 55.2%). Yield based on potassium sulphate, 72.0%.

.

Example 9 This example demonstrates that a solution from a prior syngenite preparation can be regenerated b~ leaching a sylvini~e-type ore composite, prior to reuse.
845.~ grams of a solution containing 6% potassium, 0.91% sulphate and 17.2~ chloride or approximately 11.5~ KCl and 20% NaCl was mixed with 175 grams potassium chloride, and 132 grams of sodium chloride at 80C for 1 hour. The slurry was filtered yielding 253.3 grams wet solid and 831.9 grams of solution. The wet solid was dried to 244 grams and contained 21.8% potassium, 2.11% sulphate and 55.0% chloride.
The resultant leach solution contain2d 10.2~ potassium, 19.5% chloride and 0.8% sulphate or 19.4% KCl and 16.9%
NaCl.
This solution composition, as shown in Example 3, is satisfactory for syngenite preparation.

Example 10 This example shows the use of nitric acid and calcium carbonate.
Syngenite, 256g., containing 67g. K was agitated at 100C for two hours in a solution of nitric acid containing 155g of 69% HNO3 nitric acid and 345g. water. The calcium sulphate produced was removed by filtration and, after washing with water and drying, weighed 113g. (the washings were discarded). The mother liquor, 535g., was neutralized with stirring, to pH 7, by the addition of calcium carbonate at 50C. Filtration of the slurry so produced yielded 89g.
of dry, washed calcium sulphate and 306g. of filtrate ~the washings were discarded), which on cooling to 0C yielded 67g. wet potassium nitrate crystals, which dried to 61.5g.
of product analyzing K, 38.23% (KNO3 required K, 38.67%).

Example 11 This example shows the use of nltric acid and calcium nitrate. Syngenite, 149g., was agitated at 100C
for two hours with a solution of nitric acid, 137g., at 69%
in 458g. water. The calcium sulphate so produced was removed by filtration and after washing and drying, weighed 52g.
(the washings were discarded). Mother liquor, 593g., was agitated for thirty minutes with 147.5g. of Ca(NO3)2 4H2O
(calcium nitrate tetrahydrate) at 50C. Filtration of the slurry so produced yielded 54g. of dry, washed calcium sulphate and 574g. of filtrate ~the washings were discarded).
On cooling to 0C, the solution yielded 70g. of wet potassium nitrate crystals, which dried to 38.5g. product analyzing K, 38.5~ IKNO3 requires K, 38.67%).

Example 12 This example shows the direct decomposition of syngenite using calcium nitrate.
Syngenite, 149g., was agitated at 100C for two hours with a solution of calcium nitrate tetrahydrate, 147.5g., in 500g. water.
The calcium sulphate produced was removed by filtration and, after washing with water and drying, weighed 122g. The washings were discarded. After coolin~ to 0C, the filtrate, 426g., yielded 33.7g. of wet crystals which, on drying, yielded 25.2g. of potassium nitrate, analyzing K, 38.1%.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made without departing from the spirit and scope of the invention as set forth herein.

~ .

Claims (6)

Claims

1. A process for producing K2SO4, comprising:
(a) forming syngenite;
(b) decomposing the syngenite formed in step (a) with sulfuric acid to produce a solution of K+, H+ and sulfate and bisulfate ions expressed as SO4= in a molar ratio of 1:1:1 and solid calcium sulfate or the penta salt;
(c) separating the solution produced in step (b) at the reaction temperature from the solid calcium sulfate or penta salt;
(d) controlling the concentration of said solution as produced in step (b) such that when the solution is cooled to a first temperature of about 0° to 45°C a double salt K3H(SO4)2 crystallizes;
(e) cooling the KHSO4 solution produced in step (d) to the first predetermiend temperature and separating the K3H(SO4)2 crystals thus produced;
(f) forming K2SO4 by contacting the separated K3H(SO4)2 crystals separated in step (d) with water or a recycled aqueous wash liquor containing water and low concentrations of K+, H+ and sulfate and bisulfate ions at a second temperature in the range of about 0° to 100°C but above that of step (d) such that the crystalline phase in equilibrium is K2SO4;
and (g) recovering K2SO4.

2. The process of claim 1 wherein the first temperature is 0° to 30°C.

3. The process of claim 1 wherein the first temperature is 0° to 20°C.

4. The process of claim 1 and further comprising recycling the mother liquor produced in step (d) to step (b).

5. The process of claim 1 and further comprising concentrating the mother liquor produced in step (f) and recycling it to step (b).

6. The process of claim 1 wherein the second temperature is 30° to 100°C.