IL47518A - Process for the production of potassium sulphates - Google Patents

Description

Process for the production of Potassium Sulphates The present invention relates to an Improved method for the production of potassium sulphates. More specifically, the present Invention provides a process for the manufacture of potassium sulphates from sulphur dioxide and potassium-bearing brines.

A number of methods have been proposed for the direct recovery of potassium values from potassium-bearing brines by precipitation as an insoluble compound. In one such proposal, by George D.R. et al., (Chem. Eng. Progress 64 (5), 1968» pp. 96-99), an aqueous solution of sodium perchlorate is added at a low temperature to a potassium-bearing brine so as to precipitate potassium perchlorate. The precipitate is dissolved in hoti'.water, and converted to potassium chloride, sulphate or carbonate, by being treated with the corresponding sodium salt in a cation exchange system, using a resin cation exchanger, thereby recovering sodium perchlorate for re-use In the precipitation step. This could be an interesting process for production of potassium sulphate, if a cheap source of sodium sulphate were available, but this is not normally the case. Furthermore, the use of resin ion exchangers in continuous processing, while undoubtedly feasible, is cumbersome.

Potassium sulphate is a widely used potassium fertiliser for those applications in which the presence of chloride is undesirable. In those regions of the world where the naturally occurring sources of potassium contain also the sulphate ion, potassium sulphate may be manufactured simply by suitably processing said naturally occurring sources; but elsewhere, some synthetic chemical treatment is required, wherein potassium chloride is reacted with a suitable sulphur-bearing compound. Known processes involving such chemical treatment of potassium chloride are the Mannheim process, 1n which potassium chloride 1s reacted at a high temperature with sulphuric add to produce potassium sulphate and hydrogen chloride gas, and the Hargreaves process, 1n which potassium chloride 1s reacted, also at a high temperature, with sulphur dioxide and air, to produce potassium sulphate and a gaseous mixture containing, among others, also hydrogen chloride. Processes have also been developed, for example the one claimed in Israel Patent 25 691, to Improve the economics of the said Mannheim process by conducting the reaction of potassiu chloride and sulphuric acid in such a way as to produce chlorine gas as by-product Instead of hydrogen chloride.

All of said synthetic methods for the production of potassium sulphate suffer, however, from a number of disadvantages, e.g., they require high temperatures , with the attendant corrosion problems, and require as starting material a source of potassium chloride of a high degree of purity, as all the impurities con-r talned 1n the potassium chloride feed are carried through to the potassium sulphate product. In addition, the pr&ce differential between potassium sulphate and chloride is such that in many cases the additional revenue which could be obtained by conversion of potassium chloride to sulphate, is not sufficient to cover the cost of the conversion process.

It Is also an object of this Invention, to provide a process for the manufacture of potassium bisulphate KHSO^, which 1s a useful chemical intermediate for processes which require the presence of a strong acid and of potassium ion, e.g., the process described by Drechsel, E. . ACSjFertll izer and Soil Chemistry Meeting Aug. 28, 1973 wherein potassium phosphates are produced directly from phosphate rock by treatment with potassium bisulphate. Potassium bisulphate Is normally produced by conducting at about 200°C, a reaction in accordance with the equation: C1 + H S0 →· KHS0 + HC1 (g) said method is the requirement of the use of sulphuric acid since the Hargreaves process mentioned above cannot be used to make KHSO^, which sulphuric acid represents a considerable part of the cost of production of KHSO^.

It is thus readily apparent that a process for preparing potassium sulphates which can employ sulphur dioxide rather than sulphuric acid, can use an unpurified form of potassium chloride, and does not require elevated temperatures, would be a highly desirable and economically attractive process.

According to the present invention there is now provided a process for the production of potassium sulphates from potassium bearing brines comprising the steps of selectively precipitating the potassium in the form of an Insoluble or slightly soluble salt, reacting said salt with sulphur dioxide in the presence of a liquid amine extractant to form a potassium sulphite salt in aqueous solution and oxidizing said sulphite to sulphate.

It should be noted at this point that the potassium-bearing brines referred to in this patent specification generally contain, in the form of dissolved salts, the chlorides of sodium, calcium and/or magnesium, as well as of potassium, thus the anion suitable for precipitation of potassium should be one whose sodium, calcium, and magnesium salts, are very soluble in water and chloride brines, so that the desired separation of potassium from the other cations contained in said brines can be satisfactorily, achieved and it is 1n this context that the term selective precipitation 1s used. ' In a preferred embodiment of the present invention the agent used to precipitate the potassium from its brines 1s any anion A" characterized by that a) the potassium salt KA of said anion is only slightly soluble In water or saline solutions, whereas those of sodium, calcium ^ and magnesium are very soluble, : b) the anion A~ has a higher affinity than the sulphite 1ons HS03" and SO3 for liquid anion exchangers , and c) the anion A" shall not Itself oxidise, or otherwise react with the sulphite Ions HS03" or SO3 .

Thus said preferred embodiment of the present Invention makes use of the discovery that a number of the 1ons which form Insoluble salts with potassium possess a considerable affinity for liquid ion exchangers, the affinities being considerably higher than for sulphite or bisulphite 1ons.

Such anions Include permanganate, perrhenate, perchlorate, chlorate, c oro platlnate, fluoborate and tetraphenyl borate; and combinations thereof can also be used. However, In practice, chlorate and permanganate Ions could not be used, 1n view of their known oxidising tendencies. It Is thus seen to be possible to proceed directly from a potassium-bearing brine to a potassium sulphite salt, by means of two principal operations: a) selective precipitation of potassium 1on from said brine as an Insoluble salt, and b) reaction of the said Insoluble salt with sulphur dioxide In the presence of a liquid amine extractant, to form a potassium sulphite salt. In aqueous solution, the amine being subsequently regeneratable by treatment with a base.

To convert the potassium sulphite or bisulphite, obtained by the above-mentioned operations of potassium 1on precipitation and anion exchange, to the corresponding sulphate, use 1s made of the discovery that whereas the oxidation of sulphur dioxide to tri oxide requires a catalyst, a sulphite salt In aqueous solution can be oxidised to the corresponding sulphate salt merely by contacting with air 1n a suitably agitated device and no catalyst 1s required. Such an oxidation step thus represents the third principal operation In the process according to the present Invention, the overall result of which m l t s can be roduced at ambient tem eratures from The following examples which Include preferred embodiments, will serve to illustrate the practice of this Invention, it being > understood that the invention is not limited to those specific examples, vWch are intended merely to illustrate a few preparations performed employing the process of the present invention.

Example 1 - Preparation of potassium bisulphite An aqueous solution of potassium bisulphite was prepared by contacting, in a mixer-settler device, approximately equimolar proportions of potassium perchlorate, sulphur dioxide, and (o) Alamine ' 336 (a commercially available mixed C8"*CT 0 *ert1"ary amine). The quantities used were as follows : 140 gms CIO^ in solid form, 62.3 gms SOg dissolved in 500 ml water, and an amine-diluent mixture containing 500 ml amine and 500 ml diluent, said diluent being 85% xylenes and 15% 1,2 dichloro-ethane. After mixing for 3 hrs and separation of phases, it was found that 29.8 gm KCIO^ remained unreacted, I.e., about 20% of the KCIO^ fed to the system. The analysis of the aqueous phase obtained was, in gm/Hter: +62.1; HSO3 112.4; SO* 8.4; CIO" 2.1; thus, some atmospheric oxidation to sulphate is seen to have occurred already during the 1 iquid-anion-exchange operation.

The pH of the said aqueous phase was between 4 and 5. The following mass balance illustrates the success of the experiment, in which almost 89% conversion of CIO^ to potassium bisulphite and sulphate, was achieved in a single stage* To avoid confusion resulting from partial oxidation of sulphite to sulphate, the mass balance for S02, sulphite and sulphate, 1s given in terms of sulphur S.

K in entering solid phase 39.4 gm in exit " " 8.4 gm K in " aqueous " 31.0 gm CIO^ 1n entering solid phase 100.0 gm C104 1n exit solid phase 21.4 gm CIO^ 1n exit organic phase 76.8 gm CIO^ in exit aqueous phase 1.1 gm S 1n entering aqueous phase 31.1 gm S 1n exit organic phase 5.7 gm S 1n exit aqueous phase 23.6 gm Polishing of organic phase. To reduce the bisulphite and sulphate concentrations 1n the organic phase obtained 1n the experiment described above, pert of said organic phase was contacted in a second mixer-settler device for 30 minutes with solid potassium perchlorate and water. After separation of phases, more than 70% of the bisulphite and sulphate contained in said organic phase were found to have transferred to the aqueous phase, leaving said device.

Example 2 - Conversion of potassium bisulphite to sulphite, and oxidation to sulphate The aqueous phase obtained in the first experiment described in Example 1 was contacted with a fresh quantity of amine-diluent mixture for 30 minutes 1n a further mixer-settler device, the molar ratio of amine to bisulphite content in the entering aqueous phase being about 2. The results of this operation are Indicated by the following analyses (in gms/ liter): entering aqueous phase: HSOg - 112.4; S0° 8.4; CIO" 2.1 pH 4.5. exit aqueous phase: HSOj/SO" 8.6; S0~ 21.1; CIO^ traces. pH 6.

As 1n example 1, some oxidation to sulphate is seen to have occurred already during the aniontexchange conversion.

To the aqueous phase thus obtained a small quantity of potassium hydroxide was added, to raise the pH to 9, after which hrs. A further oxidation was thereby achieved, and a solid precipitate obtained, which was essentially pure potassium sulphate.

Example 3 - Direct conversion to potassium sulphite by use of excess amine extractant The experiment of example 1 was repeated, but at a higher molar ratio of amine to SO,,, 3:1 Instead of 1:1, so as to achieve conversion of potassium perchlorate directly to sulphite, In one stage. The following materials were fed to a mixer-settler device: ClO^t 42 gm (0.3 moles); amlne-dlluent mixture, as 1n ex. 1, 300 ml, (0.3 moles amine); SOg 6.6 gm (0.1 mole) dissolved 1n 150 ml water.

After contacting for 3 hrs and subsequent phase separation, the aqueous phase obtained had the following composition (1n gms per liter) : as ions: + 37.8 ^SOg SO" 51.1; SOj 13.2; ClS^ 2.8. as salts: KHSO3 71.8; KgSOg 5.2; 2S04 23.9; KC104 3.9.

The pH of the aqueous phase was 7. It 1s evident from these results that a further stage would be needed to Increase the conversion of HS03 to KgSO^, and to extract the residual perchlorate 1on. As 1n the preceding examples, some oxidation of sulphite to sulphate occurred already during the anion exchange operation, in the mixer-settler device; th s Is Interesting, as the duty of any subsequent oxidation operation 1s thereby reduced.

A solid phase was also obtained during the said phase separation, conslstlngof 21.4 gm (o,15 moles) unreacted KCIO^ this 1s not surprising, as the potassium perchlorate Input to the said mixer-settler device was 1n excess - even 1f all the S02 Input had been converted to K2S03» or K2S0 » not more than 0.2 moles C10. could have reacted.

In order that the present Invention 1n Its broader aspects may be understood aore fully there Is now provided a aore general description of alternative processes and procedures according to the present nvention and general considerations governing the same.

General Example 4 A potassium bearing brine 1s mixed In any suitable contacting device with an aqueous solution of the calcium and/or magnesium salt of any anion A" satisfying the following requirements: a) the potassium salt of anion A" 1s only slightly soluble In water or brines while the sodium, calcium and magnesium salts are very soluble, b) the anion A has a higher affinity than the sulphite Ions HS0~ or S0° for liquid anion exchangers, c) the anion A" shall not Itself oxidise, or otherwise react with, the sulphite Ions HSOjj or SO^ , and the precipitated salt KA Is separated from the mother liquor by any known means. The following table gives the solubility of a number of potassium salts which meet requirements (a) and (b): solubility gm/kg water, at: 0eC 10°C 20°C 30°C KC10, 33.0 50.0 74.0 105.0 KC10 7.6 10.8 4 16.7 25.0 KMn04 28.4 44.0 65.0 91.0 K e04 4.8 5.8 10.1 15.0 K2PtCl6 7.4 9.0 11.2 14.0 KBF4 2.5 6.5 9.0 KBPh 4 0.05 but In practice the choice of suitable anions Is limited. The chlorate and permanganate 1ons do not satisfy requirement (c), while 1ons such as BPh^" or PtClg* are not desirable for economic reasons. The most convenient, and therefore preferred anion s perchlorate CIO^" , as make-up requirements can be produced on site by electrolytic oxidation of chloride brines.

Nevertheless, any anion, or combination of anions, satisfying the above requirements, are to be regarded as falling within the scope of this Invention.

The said precipitation of potassium salt KA occurs in accordance with the equation 2K+(aq) ♦ Ca/MgA2(aq) 2KA+ ♦ Ca/Mg++(aq) . . . (1) The quantities of the said calcium or magnesium salt of anion A" to be added to the potassium-bear ng brine may be more, less, or equal, to the stochlometrlc requirements equivalent to the potassium content of sa d brine. If excess of anion A" 1s used, the recovery of potassium values from said brine will be Improved, but the mother liquor or filtrate after separation of the precipitated salt KA w ll contain undesirably large quantities of residual anion A~. These can be recovered for re-use by known means such as anion exchange or extraction, but such a recovery step will of necessity involve additional expenditure. As generally the value of anion A", whichever one of those satisfying the said two requirements (a) and (b) be used, exceeds tflt of the equivalent amount of potassium ion +, the preferred mode of operation Is to carry out the precipitation of salt KA under conditions of maximum usage of anion A", I.e., the quantity of calcium and/or magnesium salt of anion A" used should be less than equivalent to the potassium content of the said brine. In said preferred mode of operation, the recovery of potassium values will not be complete, but the residual content of anion A" n the said mother Hquor or filtrate will be reduced to a minimum. This residual quantity of anion A" may be recovered, If desired, by known methods, but 1t will generally not be economical. Jo do so.

Notwithstanding the above, all proportions of said calcium and/or magnesium salt to the potassium content of said brine are to be regarded as Included within the scope of this Invention.

As stated in the preceding paragraph, the preferred mode of operation 1s such as to minimise the residual content of anion A" 1n the mother liquor or filtrate after separation of the precipitated salt KA. A further way of achieving this aim 1s to carry out the precipitation of salt KA at reduced temperature. The solubilities of potassium salts in water are, generally, very temperature-dependent, so a considerable reduction 1n residual content of anion A" In the said mother Hquor or filtrate can thus be obtained. Any temperature from ambient down to the freezing temperature, I.e., temperature of ice formation of the brine, can be used, though the preferred range is from 10° to -10°C, as above 10°C the losses of residual anion A" will be high, whereas below -10°C the additional cost of cooling to such low temperature brings diminishing returns, and in addition the higher viscosity of brines at such low temperatures could increase the difficulty of separating the precipitated salt KA from Its mother Hquor.

The cooling required to reach the chosen operating temperature can be accomplished by any known means, but should Include a heat exchange device to transfer heat between Incoming t bearing brine and the aqueous solution of calcium and/or magnesium salt of anion A" may be cooled prior to mixing, or may be mixed ambient temperature forming a slurry containing some solid salt In the latter case, t Is possible to separate the solid salt KA from Its mother Hquor, which 1s subsequently cooled to the chosen temperature of operation thereby precipitating a further crop of solid salt KA, or alternately, the said slurry may be cooled to the chosen low temperature to complete the precipitation, all the precipitated salt KA being then removed In one operation. The advantage of prior mixing and partial precipitation of salt KA at ambient temperature 1s that some of the heat of crystallisation of salt KA s thus dissipated at ambient temperature, nstead of at the chosen lower temperature of operation, thereby reducing the refrigeration duty for the process.

It 1s to be understood that throughout this patent specification unless otherwise Indicated, the term "amine extractant" shall be taken to mean an organic water-1mm1sc1ble amine, dissolved In an Inert organic solvent which acts as diluent, the choice of amine and diluent being specified hereinafter. Likewise, the term "extraction device" shall be taken to mean any form of equipment wherein aqueous and organic: immiscible liquid phases, and possibly also an Inorganic solid phase, can be contacted, so as to bring about mass transfer and/or chemical reaction, with arrangements for subsequent separation of the phases. The said device may consist of one or more stages, the flow of the phases being counter-current, co-current, cross-current, or any combination of such modes of flow.

The conversion of the potassium salt KA, as obtained by the means described above, to a potassium sulphite salt by means of liquid anion exchange, Is the heart of the process according to the present nvention, and a number of methods are available for h e t be re arded as Included within the scope of this Invention. The choice of which procedure 1s to be adopted will depend on whether potassium or bisulphate 1s the desired product, and on the source of sulphur dioxide available. For example, 1f the sulphur dioxide available 1s fairly concentrated. I.e., 1f obtained from burning of sulphur or roasting of sulphide ores 1n air, one of the following procedure will be adopted: a) sulphur dioxide Is absorbed In water at atmospheric pressure, the solution thus obtained then being fed to an extraction device together with potassium salt KA and amine extractant, In the proportions expressed by the equation KA (c, aq) + HgSOg (aq) + Rg (org) * KHS03(aq) + RgNH+A"(org) (2) or, by the equation: 2KA(c,aq) + HgSOg q) + 2R3N(org) * KgSOg q) + 2R3NH+A" (org) (3) depending on whether potassium bisulphate or sulphate, 1s ultimately desired. b) sulphur dioxide Is absorbed In water as In procedure (a), the solution thus obtained Is reacted with amine extractant to form amine sulphite or bisulphite, this 1n turn being reacted with potassium salt KA In the presence of water 1n an extraction device to form potassium sulphite or bisulphite In aqueous solution and the amine salt R3NH+A"(org). This procedure may be preferable to procedure (a) If the aqueous solution of sulphur dioxide 1s very dilute -procedure (a) would require that all the solvent water 1s carried through to the potassium sulphite or bisulphite output from the extraction device, whereas procedure (b) makes 1t possible to obtain a concentrated solution of potassium sulphite or bisulphite 1n spite of a dilute starting solution of sulphur dioxide. c) sulphur dioxide 1s absorbed directly, from the gaseous phase, by amine extractant, 1n the presence of some water or water vay*»½¾ n accordance with the equations: S02(g) + H20 + R3N(org) R3NH+HS03(org) (4) or: S02(g) + H20 + 2R3N(org) (R3NH*)2S03 (org) (5) the amine sulphite salt thus obtained being subsequently reacted with potassium salt A as In procedure (b). This procedure s not desirable 1f the gases containing sulphur dioxide are hot, as contact of amine extractant with hot gases could cause excessive volatilisation losses of amine, and especially of diluent.

If the sulphur dioxide 1s available n dilute form only, e.g., from the combustion of sulphur-containing fuels or of waste products from the paper Industry, one of the following procedures may be adopted: d) the gases containing sulphur dioxide may be processed by any of the known methods used in anti-pollution technology to produce a concentrated stream of sulphur dioxide gas, which 1s then used as In procedures (a), (b), or (c), above; e) sulphur dioxide 1s absorbed Into an aqueous solution of potassium sulphite and the potassium bisulphite solution thus obtained 1s treated with potassium salt KA and amine extractant, in an extraction device, in accordance with the equation: KA(c, aq) + KHS03(aq) + R3N(org) + K2S03(aq) + R3NH+A"(org) (6) some of the potassium sulphite thereby obtained being recycled to absorb more sulphur dioxide, and some being processed further, as described below, to produce potassium sulphate.

If the sulphur dioxide 1s available as an essentially pure liquid, refrigerated or under pressure. It may be injected direc'*^ nto an extraction device to which potassium salt A and amine extractant are also fed; the conversion to potassium sulphite or bisulphite then proceeds In accordance with equations (2) or (3), above.

It should be noted that n all the above procedures (a) to (e), f essentially complete conversion to sulphite Is required, an excess of amine extractant, more than that required by equations (3)» (5), or (6), should be present.

The amine extractant used 1n this process, formulated for convenience as In equations (2) to (6) above, may be one, or more than one, used 1n combination, of a wide variety of types. Secondary or tertiary amines may be used. The amines are generally aliphatic In character, although they may be partially aromatic. The aliphatic hydrocarbon groups can be straight chained or branch chained, saturated or unsaturated, and the two or three hydrocarbon chains need not necessarily be Identical. The amine or amines selected should be liquid, stable and of low volatility at ambient temperature, and both 1t and Its salts should be Insoluble 1n, and Immiscible with, ^water. It Is readily seen that a variety of amines fall within the scope of this Invention, but the preferred type Is a tertiary straight- or branch-chain aliphatic amine, the chain length being from 8 to 12 carbon atoms. Compounds of this type are readily available on the market.

A variety of organic solvents will normally be compatible with the selected amine, specified as above, and its salts, so the choice of diluent s wide. The diluent, or combination of diluents, should be Immiscible with water and should be sufficiently different in density from water and from aqueous solutions of potassium sulphites so that no difficulty will arise In phase separation. high selectivity for anion A as compared to sulphite 1ons, yet at the same time. 1f the anion exchange Is carried out In two steps as in procedure (b) above, should have a high extractive power for sulphurous akceirdosfernoem> Its aqueous solutions. Materials which are preferred are/ aromatic hydrocarbons such as toluene or any xylene, or chlorinated hydrocarbons such as chloroform or 1,2 dlchloroethane. Oxygenated compounds such as Isoamyl alcohol may also be used, but such compounds generally have a considerable solubility in water, which Increases solvent losses unless a costly recovery step 1s incorporated In the process. It 1s to be understood that all diluents satisfying the requirements of this paragraph, used singly or 1n combination, fall within the scope of this Invention.

The amine concentration In the selected diluent can vary from as low as IX to an upper limit governed by the solubility of the amine salts with the anion A~ or with the sulphite 1ons HSOg and SO^ In the said diluent. However, the preferred concentration 1s not below 10%, otherwise, very large equipment Is required for a given amine throughout.

The proportion of water to potassium salt KA fed to the anion exchange system can vary over a wide range. Theoretically, 1t 1s possible to dissolve the salt KA completely 1n water, and so conduct the anion exchange as expressed by equations (2), (3), or (6), with only liquid phases present. This could simplify the equipment required for the said anion exchange, e.g., a plate extraction column could be used, but the aqueous extract would be an extremely dilute solution of potassium sulphite salts, which would require considerable and costly evaporation or concentration by other means to obtain a solid product. It 1s therefore preferable to reduce the amount of water fed to the anion exchange system, to less than that required for complete dissolution of salt KA, the lower limit being that amount of water needed to ensure that the potassium sulphites produced by said anion exchange are completely dissolved.

At the beginning of the anion exchange reaction* the salt KA then exists In slurry form. I.e., a partially dissolved state.

The mass transfer of Ions actually takes place between the two liquid phases, but as the anion exchange proceeds, anions A~ transfer from the aqueous to the organic phase, so that the reactions expressed by the equations KA (c) ♦ aq — K* + A" (aq) -°J¾ K+(aq) + A"(org)... (7) are shifted to the right, thus more solid KA can dissolve. Ultimately, then, the aqueous phase consists of an aqueous solution of potassium sulphite salts, generally containing some residual A" Ion, which can be reduced to a level as low as desired by Increasing the number of counter-current stages 1n the anion exchange system.

The temperature at which the anion exchange 1s carried out 1s not critical. Use of a higher temperature would tend to Increase the solubility of salt KA 1n water, which, as can be seen from equation (7), assists the anion exchange, I.e., more 1on transfer would be achieved per contact stage, but the stability of some of the possible anions from which anion A" 1s selected, Is reduced with Increasing temperature, likewise the volatility of the selected amine extractant and, particularly, of the diluent, will Increase with temperature. Thus the Increased losses of reactants and solvents may well offset any gains resulting from Improved anion exchange. The preferred temperature Is therefore approximately ambient temperature.

The organic extract from the anion exchange system as described above 1s now treated with a suitable base so as to regenerate the free amine for reuse. Suitable materials are sodium hydroxide, sodium carbonate, alkaline earth oxides or hydroxides, but for The contacting may be in one or more stages, and the ratio of base to amine should be not less than stochiometric. The reactio proceeds according to the equation: 2R3NH*A"(org) + Ca/Mg(0H)2 * 2R3N(org) + Ca/MgA2(aq) + 2HgO (8) The free amine Is returned to the anion exchange system, and the aqueous solution of the calcium and/or magnesium salt of anion A" 1s recycled for preci itation of potassium salt KA In accordance with equation (1) above.

It will be realized from the above description that uany variations of the process of the present Invention can be carried out. Thus for example a process of the present Invention could be characterized as one in which the liquid anion exchange Is carried out by a sequence of steps wherein: a) a mixture of free amine and diluent 1s contacted with sulphur dioxide in any available form - gaseous, liquid, or aqueous solution - and with an aqueous solution or slurry of a potassium salt KA as defined in any suitable extraction device comprising one or more contact stages, 1n approximately stochiometric proportions, such that the aqueous extract is essentially a solution of potassium bisulphite, sulphite or a combination thereof and the organic phase consists essentially of the amine salt of the anion A" as defined, dissolved 1n the said diluent; b) the organic phase leaving step (a) 1s brought Into conter-current or cross-current contact in any suitable contacting device comprising one or more stages, with a base in the form of solid, aqueous slurry or aqueous solution, the ratio of base to amine salt being not less than stochiometric, and the base being any base capable of decomposing an amine salt as obtained in step (a); and c) the products of step (b) are separated Into an organic phase consisting essentially of free amine and diluent which Is returned to ste a for re-use thereb com letin the amine cycle In the and/or magnesium salt of anion A" which 1s re-used for further precipitation of salt KA, thereby completing the cycle of said ^ anion A" 1n the process.

Similarly the process could be performed by a variation of the above process wherein step (a) Is carried out 1n the following manner: a mixture of free amine and diluent 1s contacted with an aqueous solution of sulphur dioxide 1n any suitable extraction device, whereby an organic phase 1s obtained consisting essentially of amine bisulphite dissolved in the said diluent, an aqueous phase also being obtained which may be discarded, the said organic phase being subsequently contacted with an aqueous solution or slurry of potassium salt KA, as specified In step (a).

In said variation the sulphur dioxide can be absorbed directly from a gaseous mixture containing thessame, by contacting with said mixture of free amine and diluent, the liquid organic phase thus obtained being subsequently contacted with an ?queous solution or slurry of said potassium salt KA.

In any of the above described processes the proportions of reagents - amine, sulphur dioxide and potassium salt KA can be such that the ratio of potassium to sulphite SO^ 1n the aqueous extract from the anion exchange 1s more than that dictated by the formula KHSO^, the said aqueous extract thus containing a mixture of potassium bisulphite and sulphite In any proportion from 1% up to 100% sulphite.

Yet another variation of the process described above could be one wherein step (a) Is carried out n the following manner: sulphur dioxide, present 1n low concentrations 1n waste combustion gases or similar gaseous streams, 1s first absorbed from said gaseous streams by means of contact, In a suitable device, with an aqueous solution of potassium sulphite, or of a mixture of potassium bisulphite and sulphite In any proportion from 12 up to 100% sulphite, the aqueous solution of potassium bisulphite thus obtained being subsequently T contacted with potassium salt KA, In form of solid, aqueous or aqueous solution* and with a mixture of free amine and diluent 1n any suitable extraction device comprising one or more contact stages, such that the organic phase obtained consists essentially of the amine salt of anion A" dissolved 1n said diluent, and the aqueous extract 1s a solution of potassium sulphite, or a mixture of potassium bisulphite and sulphite 1n any proportion from 1% up to 100% sulphite, some of this ι aqueous phase being recycled so as to absorb more sulphur dioxide, while the rest represents the aqeuous output from the anion exchange operation.

The aqueous extract from the anion exchange system can be treated by a variety of means to obtain solid products, the methods to be adopted depending, Inter alia, on whether the said aqueous extract contains potassium sulphite, bisulphite, or a mixture of both, and en whether the desired product Is potassium sulphate or bisulphate. Thus, 1f the said extract contains bisulphite only, and potassium bisulphate 1s required, or If said extract contains sulphit only, and potassium sulphate Is required, the extract 1s treated with any available oxidising agent so as to achieve the required conversion. The agent selected should preferably be one which adds no undesirable contaminant to the potassium sulphate or bisulphate product, thus hydrogen peroxide, gaseous oxygen, or even atmospheric air, may be used; for economic reasons, the use of air 1s preferred, but care must then be taken to achieve good contact between the air and the aqueous solution being oxidised. The aqueous stream leaving the oxidation unit 1s now treated by known means so as to obtain solid potassium bisulphate or sulphate, respectively. Such means Include evaporation, vacuum crystallisation, spray-drying, or, In the case of potassium sulphate, outsaltlng with organic water-mlsc ble agents such as lower alcohols or acetone.

Thus according to the present Invention an aqueous solution of potassium bisulphite can be oxidised by any known means to b1-sulphate, suitable oxidising agents being, for example, oxygen, hydrogen peroxide, or air, though air 1s preferred and the aqueous solution of potassium blsulphate thus obtained being treated by known means to obtain solid potassium blsulphate.

If the aqueous extract from the anion exchange system contains potassium bisulphite only, or a mixture of sulphite and bisulphite, but the product required 1s potassium sulphate, one, or more than one used 1n combination, of the following methods, may be adopted: a - potassium carbonate, b1 -carbonate or hydroxide may be added to convert all remaining bisulphite Ion to sulphite ,the aqueous solution of potassium sulphite thus obtained being oxidised and subsequently treated to obtain solid potassium sulphate, as described In the precedin paragraph and thus the process can be one In which the aqueous extract or output from the anion exchange operation, containing potassium bisulphite or a mixture of bisulphite and sulphite, Is treated with potassium hydroxide, carbonate, or bicarbonate, so as to convert all bisulphite to sulphite; b - the said aqueous extract may be heated, so as to decompose the HSO^ Ion-gaseous sulphur dioxide 1s evolved which 1s returned to the anion exchange system, and an aqueous solution of potassium sulphite 1s obtained, which Is treated as 1n method (a); c - 11 me or limestone may be adds! to bring about the following reaction: 2KHS03(aq) + CaO → CaS03+ + I^SO-, (aq) + H20 (9) -Hre- Vqueous solution of potassium sulphite being treated as In method (a) wherein conversion of bisulphite to sulphite 1s effected by treating the said aqueous extract or output with calcium oxide, hydroxide, or carbonate, thus obtaining a precipitate of calcium sulphite which 1s separated, and an aqueous solution of potassium . sulphite.

This method represents a waste of part of the S02 Input to the anion exchange system, and therefore 1s economic only 1f the S02 Is available to the process at no, or Uttle, cost; for example, 1n the form of waste combustion gases. d - 1f the aqueous extract contains both sulphite and bisulphite, 1t may undergo a partial oxidation, as a result of which the aqueous solution will contain potassium sulphate and bisulphite. Due to the lower solubility of potassium sulphate, concentration of said aqueous solution by evaporation or other means, and/or the addition of an organic outsalting agent such as methanol or acetone, will cause the precipitation of potassium sulphate crystals, the residual solution of potassium bisulphite being treated, If applicable, to recover the organic outsalting agent, and recycled to the anion exchange system.

It will be evident to those skilled In the art that the Invention 1s not limited to the details of the foregoing Illustrative embodiments and examples and that the present Invention may be embodied 1n other specific forms without departing from the spirit or essential attributes thereof, and 1t Is therefore desired that the present embodiments be considered in all respects as Illustrative and not restrictive, reference being made to the appended claims, rather than to the foregoing description, 1n which 1t Is Intended to claim all modifications coming within the scope of the invention.

Claims (25)

47518/2 WHAT IS CLAIMED IS:

1. A process for the production of potassium sulphates from potassium bearing brines comprising the steps of selectively pre- ^ dp tatlng the potassium In the form of an Insoluble or slightly soluble salt, reacting said salt with sulphur dioxide In the presence of a liquid amine extractant to form a potassium sulphite salt In aqueous solution and oxidizing said sulphite to sulphate.

2. A process according to claim 1, wherein the agent, used to precipitate the potassium from Its brines 1s any anion A" characterized by that a) the potassium salt KA of said anion Is only slightly soluble 1n water or saline solutions, whereas those of sodium, calcium and magnesium are very soluble, b) the anion A~ has a higher affinity than the sulphite Ions HSOg and SOg for liquid anion exchangers, and c) the anion A* shall not Itself oxidise, or otherwise react with the sulphite Ions HSO^ or SOg

3. A process according to claim 2 wherein the anion A" Is selected from the group consisting of perrhenate, perchlorate, chloroplatlnate , fluoborate, tetraphenyl borate or a combination thereof.

4. A process according to claim 2 wherein said precipitating agent Is a perchlorate anion.

5. A process according to claim 3 wherein the precipitation Is carried out by mixing a potassium-bearing brine and an aqueous solution of a calcium and/or magnesium salt of anion A" In any suitable contacting device or sequence of devices, the ratio of said calcium and/or magnesium salt to potassium content of the said brine being not higher than stoichiometric and the precipitated salt KA being separated from Its mother Hquor by any known means.

6. A process according to claim 5 wherein the precipitation of potassium salt A Is carried out at ambient temperature or at any lower temperature above the freezing point. I.e., 1ce formation temperature, of the brine being treated.

7. A process according to claim 6 wherein said temperature 1s between -10°C and +10eC.

8. A process according to claim 2 wherein the precipitation of potassium salt KA Is carried out In two steps, the first taking place at ambient temperature and the second at a lower temperature of between -10°C and +10°C. said

9. A process according to claim 2 wherein -a-n organic amine extractant 1s used in a liquid anion exchange In combination with a liquid Inert diluent which 1s Immiscible with water and Is compatible with said amine In all Its possible forms of free amine, sulphite or bisulphite salt, or salt with the anion A* .

10. A process according to claim 9 wherein said amine Is a long-chain secondary and/or tertiary amine Immiscible with water.

11. A process according to claim 9 wherein said amine 1s a tertiary straight- or branch-chain aliphatic amine, or mixture of amines, with chain lengths from 8 to 12 carbon atoms.

12. A process according to claim 9 wherein said diluent 1s an aliphatic hydrocarbon such as kerosene, an aromatic hydrocarbon such as toluene or xylene, a chlorinated hydrocarbon such as chloroform or 1,2 dichloroethane , or any mixture thereof.

13. A process according to claim 9 wherein the concentration of said amine in said diluent 1s any value from a minimum of about 1% amine to a maximum governed by the requirement that the amine salts with the sulphite anions HSO^ and SO^ and with "

14. A process according to claim 13 In which the liquid anion exchange 1s carried out by a sequence of steps wherein: a) a mixture of free amine and diluent 1s contacted with sulphur dioxide and with an aqueous solution or slurry of a potassium salt KA 1n any suitable extraction device comprising one or more contact stages, In approximately stoichiometric proportions* such that the aqueous extract Is essentially a solution of potassium sulphite, potassium bisulphite of a mixture thereof and the organic phase consists essentially of the amine salt of the anion A", dissolved 1n the said diluent; b) the organic phase leaving step (a) 1s brought Into counter-current or cross-current contact 1n any suitable contacting device comprising one or more stages, with a base 1n the form of solid, aqueous slurry or aqueous solution, the ratio of base to amine salt being not less than stoichiometric, and the base being any base capable of decomposing an amine salt as obtained In step (a); and c) the products of step (b) are separated Into an organic phase consisting essentially of free amine and diluent which 1s returned to step (a) for re-use thereby completing the amine cycle 1n the process, and an aqueous phase comprising a solution of calcium and/or magnesium salt of anion *A" which Is re-used for further precipitation of salt KA, thereby completing the cycle of said anion A" Γ 1n the process.

15. A process as claimed 1n claim 14 1n which the liquid anion exchange 1s carried out by a sequence of steps wherein: a) a mixture of free amine and diluent Is contacted with an aqueous solution of sulphur dioxide 1n any suitable extraction device, whereby an organic phase Is obtained consisting essentially of amine bisulphite dissolved 1n the said diluent, an aqueous phase 1s obtained which may be discarded, and aald organic phase 1s subsequently contacted with an aqueous solution or slurry of potassium salt KA, as specified 1n step (a) of claim (14); and b) the organic phase leaving step (a) 1s treated as 1n step (b) and (c) of claim 14.

16. A process as claimed In claim 15, wherein sulphur dioxide Is first absorbed from a gaseous mixture containing the same, by contacting with said mixture of free amine and diluent, and the liquid organic phase thus obtained Is subsequently contacted with an aqueous solution or slurry of said potassium salt KA.

17. A process as claimed In claim 14, wherein the proportions of amine, sulphur dioxide and potassium salt KA are such that the ratio of potassium to sulphite SOg n the aqueous extract from the anion exchange Is more than that dictated by the formula KHSOg, the said aqueous extract thus containing a mixture of potassium bisulphite and sulphite 1n any proportion from 1¾ up to 100% sulphite.

18. A process as claimed in claim 14 1n which the liquid anion exchange 1s carried out by a sequence of steps wherein: a) sulphur dioxide, present In low concentrations in waste combustion gases or similar gaseous streams, 1s first absorbed from said gaseous streams by means of contact, In a suitable device, with an aqueous solution of potassium sulphite, or of a mixture of potassium bisulphite and sulphite 1n any proportion from 1% up to 100% sulphite, the aqueous solution of potassium bisulphite thus obtained

19. Is subsequently contacted with potassium salt KA, 1n form of solid, aqueous slurry or aqueous solution, and with a mixture of free amine and diluent 1n any suitable extraction device comprising one or more contact stages, such that the organic phase obtained consists essentially of the amine salt of anion A" dissolved 1n said diluent, and the aqueous extract Is a solution of potassium sulphite, or a mixture of potassium bisulphite and sulphite in any proportion from 1¾ up to 100% sulphite, some of said aqueous phase being recycled so as to absorb more sulphur dioxide, while the rest represents the aqueous output from the anion exchange operation; and b) the organic phase leaving step (a) 1s treated as In steps (b) and (c) of claim 14. 1Θ. A process as claimed in claim 14, 15 and 16 In which an aqueous solution of potassium bisulphite Is oxidised to bisuiphate using suitable oxidising agents and the aqueous solution of potassium bisuiphate thus obtained 1s then treated to obtain solid potassium bisuiphate.

20. A process according to any of claims 14, 15, 16, 17 or 18 wherein the aqueous extract or output from the anion exchange operation, containing potassium bisulphite or a mixture of bisulphite and sulphite, is treated with potassium hydroxide, carbonate, or b1 -carbonate , so as to convert all bisulphite to sulphite.

21. A process as claimed 1n claim 20, wherein conversion of bisulphite to sulphite is effected by treating the said aqueous extract or output with calcium oxide, hydroxide, or carbonate whereby there 1s obtained a precipitate of calcium sulphite which Is separated, and an aqueous solution of potassium sulphite. 47518/ 2

22. A process as claimed 1n claim 20, wherein conversion of bisulphite to sulphite Is effected by heating said aqueous extract or output to a temperature sufficient for the bisulphite ion to decompose, whereby there Is obtained potassium sulphite in solution, and a gaseous stream of S02 which 1s recycled to the anion exchange operation.

23. A process as in claim 17, 18, 20, 21 or 22 wherein a solution of potassium sulphite obtained 1s oxidised by any known means to sulphate using suitable oxidising agents and the aqueous solution of potassium sulphate thus obtained Is treated with water-mlsdble organic otrtsaltlng agents selected from the group consisting of acetone or lower aliphatic alcohols to obtain solid potassium sulphate.

24. A process as 1n claim 17 or 18 wherein an aqueous solution of potassium bisulphite and sulphite, leaving the anion exchange operation, undergoes only partial oxidation and the resulting solution of potassium bisulphite and sulphate 1s treated by evaporation and/or outsaltlng with water-mlsdble organic substances selected from acetone and lower aliphatic alcohols to obtain a precipitate of potassium sulphate, which Is separated, and an aqueous solution of potassium bisulphite which 1s recycled to the anion exchange operation.

25. A process for the production of potassium sulphate or blsulphate from potassium-bearing brines, substantially as herein described and with reference to the given examples. For the Applicants Wolff, Bregman and Goller ....