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Home , Potassium cyanate, Potassium sulfate, Potassium thiocyanate

Patented Mar. 9, 1943 2,313,680

UNITED STATES PATENT OFFICE 2,313,680 MANUFACTURE OF THOCYANATES Lee B. Smith, Woodbridge, N.J., assignor to Gen eral Chemical Company, New York, N. Y., a corporation of New York No Drawing. Application January 26, 1942, Serial No. 428,222. 10 Claims. (C. 23-5) This invention is directed to processes for mak by procedure according to the practice ing alkali metal . While the prin of which it is possible to produce alkali metal ciples of the improvements are applicable to crystals of greater purity than here manufacture of thiocyanate, for con tofore, and further to effect such production of venience the invention is discussed largely in 5 high purity alkali metal thiocyanates without connection with production of thiocy necessity of recrystallization, and without dis anate. card from the system of substantial quantities of In the most commonly used commercial meth mother liquor after separation of Crystallized al od for making , a sodium cy kali metal thiocyanates. A further object of the anide Solution is reacted with in aqueous 0 invention is provision of methods by which the Suspension to form crude sodium thiocyanate liq foregoing object may be attained and at the same uor containing Substantial quantities of sodium time so-called off-grade alkali metal , compound impurities mostly as sodium carbonate, containing relatively large amounts of alkali net sodium and sodium hydroxide. In the prior all carbonate, sulfide, and hydroxide impurities, practice, such sodium thiocyanate solution is con may be used as Sources of alkali metal . centrated and solid sodium thiocyanate subse Since the sought-for alkali metal, thiocyanate quently recovered in crystal form. On account productS are formed by crystallization out of a of the high impurity content of the crude sodium liquor, this mode of Solid product, formation nec thiocyanate solution, the sodium thiocyanate essarily causes contamination of the crystals with Crystals obtained by crystallization are contam 20 impurities if the mother liquor from which the inated with substantial quantities of sodium car alkali metal thiocyanate is crystallized contains bonate and sodium sulfide. For example, first Substantial quantities of impurities. Hence, the Crop Crystals manufactured by this procedure problem presented is minimization of the im often contain 0.7-1.4% sodium carbonate and as purity content of the alkali metal thiocyanate so high as 0.02% sodium sulfide. If more nearly lution from which alkali metal thiocyanate crys pure sodium thiocyanate crystals are desired, it tals are formed. Such minimization in turn re is necessary to resort to expensive and cumber quires conversion of the alkali metal impurities Some recrystallization operations. Furthermore, in the Crude alkali netal thiocyanate liquor to a in the prior art method the sodium carbonate, form in which most of such impurities may be sulfide and hydroxide impurities build up in the 30 removed from the liquor before alkali metal thio mother liquor, from which sodium thiocyanate Crystallization is effected. is Crystallized, to such an extent that substantial In accordance with this invention, it has been quantities of mother liquor must be discarded found that by treating a crude alkali metal thio from time to time in order to obtain a sodium thio cyanate solution-containing alkali metal com cyanate product of reasonably suitable purity. pound impurities principally in the form of car It has not been feasible in the past to remove bonate, sulfide and hydroxide-with sodium compound impurities from crude sodium in a manner hereinafter described, it is possible thiocyanate liquors, prior to sodium thiocyanate to convert alkali metal compound impurities crystallization, by treating the crude iiquor partly to the form of gases which may be expelled with reagents such as nitrate and lead 40 by heating and partly to alkali metal nitrate which precipitate carbonate and sulfide which may be removed from the alkali metal of the impurities as calcium carbonate and lead thiocyanate liquor, prior to crystallization of al sulfide because the Sodium nitrate simultaneously kali metal thiocyanate, to such an extent that formed is very soluble and cannot be taken out the alkali metal thiocyanate crystals subsequently of the liquor before crystallizing the sodium thio crystallized out of the liuqor are contaminated cyanate. Hence, the sodium nitrate remains in to only a very Small degree with alkali metal the sodium thiocyanate liquor and contaminates sulfate. Subsequently crystallized sodium thiocyanate to Returning to description of the invention in an extent comparable with the contamination connection with manufacture of sodium thiocy brought about by the presence of sodium carbon 50 anate, anhydrous or the dihydrate, the sulfuric ate, sodium sulfide and sodium hydroxide in liq acid treatment and subsequent steps of the inven uors from which sodium thiocyanate is crystal tion are applicable to treatment of crude sodium lized. thiocyanate solutions made according to any The primary object of this invention is provis known or convenient method. It will be under sion of methods for making alkali metal thio-. 55 stood the crude, sodium thiocyanate solutions

m 2 2,318,680 formed by reaction of sodium cyanide and Sulfur acid treated liquor to 6.3, and preferably to 5 contain, even if pure sodium cyanide were used, which find to be the most satisfactory working substantial quantities of Sodium hydroxide and figure. While pH lower than 4.2 may be used sodium sulfide, the sodium hydroxide being in some instances, greater acidity is not required, formed by hydrolysis of sodium cyanide, and the and for pH values below 4.2 there is an apprecia sodium sulfide by side reactions involving reac ble tendency to decompose sodium thiocyanate tion of sulfur and sodium hydroxide. Since in in Some Operations. In this sulfuric acid treat practice the sodium cyanide employed is com ment step, Sodium carbonate is converted to so mercial sodium cyanide which contains substan dium sulfate and CO2, sodium sulfide to sodium tial quantities of Sodium carbonate, Sodium hy O Sulfate and H2S, and sodium hydroxide to sodi droxide and sodium cyanate impurities, the re um sulfate and water. I find that by so pro Sulting crude Sodium thiocyanate liquor Con ceeding, the sodium compound impurities present tains, in addition to the sodium hydroxide and in the crude thiocyanate liquor may be converted sodium sulfide formed during the Sodium cyanide partly to gases which are driven off as such and sulfur reaction, sodium carbonate and sodium partly to soluble which is an im hydroxide impurities brought in with the Com purity having characteristics Such that most of mercial sodium cyanide and also sodium carbon it may be removed from the sodium thiocyanate ate formed by decomposition of sodium cyanate. liquor prior to crystallization of the sought-for Thus crude Sodium thiocyanate Solutions contain sodium thiocyanate products. Further, I am en as the major impurities sodium carbonate, So 20 abled to accomplish impurity removal from crude dium sulfide and Sodium hydroxide. While the sodium thiocyanate solution without decomposs sodium compound impurity content of the crude ing sodium thiocyanate. sodium thiocyanate liquor may obviously vary In carrying out the acid treatment step, Sul over a rather wide range because of differing Op furic acid of any strength may be employed. erating conditions and varying Sodium compound 25 However, to facilitate addition of sulfuric acid impurities contained in the initial SOdium cya to the crude sodium thiocyanate solution with anide, it may be said that typical, crude sodium . out control of rate of acid addition in order to thiocyanate solutions contain from 0.25 to 2.00% avoid local decomposition of sodium thiocyanate, Na2CO3, from 0.02 to 0.25% Na2S, and from nil it is preferred to use sulfuric acid of strength 30 not exceeding 40%. Following completion of ad to In0.25% manufacture NaOH. of sodium cyanide, it often dition of the sulfuric acid, any residual CO2 and happens that there are produced substantial HaS in the liquor may be expelled by agitation quantities of sodium cyanide in which the sodium or aeration preferably while the liquor is warm carbonate, Sodium hydroxide and sodium cyanate. or hot. The liquor is then preferably treated content is too high to satisfy commercial specif 35 with decolorizing carbon and filtered. cations. The nature of the present improvements The Kext step in the process comprises separa is such that this so-called off-grade sodium cy tion of sodium sulfate from the sodium thiocyan anide may be readily used as the Source of NaCN ate liquor. This is accomplished by evaporating in production of crude sodium thiocyanate Solt the liquor to a concentration not in excess of tion. 40 that at which, on cooling of the evaporated liq While the known sodium cyanide-sulfur reac uor to a temperature close to but above the tran tion is being carried out, temperatures are such sition point of anhydrous Sodium thiocyanate to as to decompose most of any sodium cyanate in the dihydrate, the thus cooled liquor is not sat purity present to sodium carbonate, and annonia, urated with respect to anhydrous sodium thio and CO2 both of which pass off as gases. On cyanate. - Sodium sulfate crystallized during completion of reaction, the mass is filtered to evaporation and cooling is then separated out of separate out excess Sulfur. If the filtrate con the liquor. tains any polysulfide, indicated by yellow color Appreciable decomposition of sodium thiocyan ation, the filtrate is heated, if cool, to tempera ate begins to take place at temperatures of about . ture of 85-105 C., and enough sodium cyanide 50 105 C. Hence, during evaporating and concen is added to reduce polysulfide to the colorless trating operations, temperatures in excess of 105 monosulfide, Na2S. To insure completion of de C. should be avoided, and preferably evaporation. composition of sodium cyanate, it is preferred to and concentration are carried out under pressure boil the filtrate after the reduction of polysulfide sufficiently reduced so that liquor temperatures to monosulfide until there is no discernible trace 55 do not exceed about 95. C. Of ammonia in the vapors over the liquor, this To bring about crystallization of sodium sul condition indicating completion of sodium cyan fate without crystallizing sodium thiocyanate, ate decomposition. The liquor is then parrmitted the clarified Sodium thiocyanate-sodium sulfate to cool to about 85-75 C. in order to avoid too filtrate is evaporated by heating to a concentra violent evolution of CO2 during the subsequent tion close to but not in excess of a concentration treatment with sulfuric acid. Crude sodium thio 60 corresponding to 38 Bé. at 80° C. More particul Cyanate solutions of the type described are larly, the filtrate may be concentrated under re strongly alkaline, the pH varying from about 0.5 duced pressure of about 110 mm. of Hg to a boil to 9.5. The sodium compound impurity contents, ing point of about 80 C. Liquor concentration principally sodium carbonate, sodium sulfide and at this stage corresponds to 38° Bé. at 80° C., and Sodium hydroxide, Vary as previously indicated. the liquor is saturated with respect to sodium Concentration of these crude solutions, at 20° C., Sulfate, a large portion of the sodium sulfate may range from 23 Bé, to 28° Bél having been already salted out during evapora In accordance with the invention, the crude tion. The vacuum is broken and the concen sodium thiocyanate solution is treated with sul 70 trated liquor is cooled down close to but not be furic acid in quantity to convert all of the sodium low the transition point (about 30° C.) of anhy Compound impurities contained in the liquor to droussodium thiocyanate to the dihydrate. By Sodium sulfate, water, CO2 and H2S. This may evaporating the liquor to a concentration corre be accomplished When the amount of sulfuric acid sponding to 38 Bé, at 80° C., concentration of the used is such as to lower the pH of the resulting 75 evaporated liquor with respect to sodium thiocy.

2,313,680 3 anate is such that when the liquor is cooled to a Washed lightly with distilled water and prefer temperature, e.g. 35 C., providing a convenient ably dried under a vacuum of from 550 mm. to working temperature above the transition point 760 mm. of Hg at temperature ranging from 80 of sodium thiocyanate to the dihydrate, the liq to 105 C. The product crystals obtained from uor is not saturated with respect to sodium thio the indicated preferred operating conditions cyanate, and consequently no crystallization of analyze above 99% NaCNS, Na2SO4 content is sodium thiocyanate takes place. The not in excess of about 0.2%, and the Nas, NaCO of Sodium thiocyanate dihydrate is substantially contents are substantially nil. less than that of anhydrous sodium thiocyanate If Sodium thiocyanate dihydrate is desired as and, if the temperature on cooling is permitted 10 the final product, the concentrated sodium thio to drop below the transition point of sodium thi Cyanate liquor is cooled down to below the tran ocyanate to the dihydrate, an appreciable quan sition point of NaCNS to NaCNS.2H2O, and the tity of Sodium thiocyanate dihydrate is likely to dihydrate crystals are filtered out. crystallize out along with the sodium sulfate. In the manufacture of , Hence minimum temperature of cooling is kept the procedure followed is substantially, the same above the indicated transition point, as that already described in connection with pro Crystallized sodium sulfate, which at such tem duction of sodium thiocyanate up to and includes peratures is also in the anhydrous form, is filtered ing clarification of sodium thiocyanate-sodium out of the SOdium thiocyanate liquor at a tem Sulfate solution by treatment with decolorizing perature above the transition point of sodium 20 carbon and filtration. Separation of potassium thiocyanate to the dihydrate. Where the initial Sulfate from clarified potassium thiocyanate-po sodium thiocyanate-sodium sulfate liquor has tassium sulfate solution differs somewhat from been evaporated to the degree indicated, i. e., as Separation of sodium sulfate from clarified so high or almost as high as a concentration corre dium thiocyanate-sodium sulfate solution be sponding with 38 Bé. at 80°C., the concentration 5 cause of different of the potassium of the evaporated liquor, with respect to sodium Salts and because potassium thiocyanate dihy sulfate, has been effected to an extent such that, drate apparently does not exist. after cooling and separation of sodium sulfate What has been noted previously as to maximum Crystals, the sodium thiocyanate filtrate has a evaporating and concentrating temperatures very low concentration of sodium sulfate, less 30 than 0.25%. In order to separate sodium sulfate With respect to sodium thiocyanate likewise ap in Such a Way that the sodium thiocyanate fil plies in production of potassium thiocyanate. trate, after separation of sodium sulfate crystals, To separate out of initial does not contain more than about 0.3% sodium clarified potassium thiocyanate-potassium sul sulfate, it is preferred to evaporate the initial fate liquor, the liquor. is evaporated to a con sodium thiocyanate-sodium sulfate liquor to a centration not in excess of that at Which, on concentration not less than that corresponding cooling to a suitable working temperature which to 36° Bé, at 80° C. Thus when evaporating the may be from say 20 to 30° C., the cooled solu sodium thiocyanate-sodium sulfate liquor to a tion is not saturated with respect to potassium concentration not less than that corresponding 40 thiocyanate. Such concentration may be ef. to 36 Bé, at 80'. C. and not in excess of a con fected by evaporating the liquor to a concentra centration corresponding to 38 Bé. at 80°C, it is tion close to but not in excess of a Concentration possible on cooling to about say 35 C., to avoid Corresponding to 42 Bé, at 72° C. For example, Crystallization of sodium thiocyanate and effect the initial liquor may be Concentrated under crystallization of sodium Sulfate to such a degree...4. reduced pressure of about 110 mm. of Hg to a that the mother liquor, after separation of the boiling point of about 72°C., this Condition corre sodium sulfate crystals, contains not more than . Sponding with a concentration of about 42B. at about 0.3% sodium sulfate, 2 C. The vacuum is broken and the concen Following separation of the sodium sulfate trated liquor is cooled. Since minimum tempera crystals, the sodium thiocyanate filtrate is ther ture of cooling is not dependent pOn prevention concentrated under any suitable conditions to of crystallization of potassium thiocyanate di form a substantially saturated solution of So hydrate, minimum temperature of cooling is dium thiocyanate at a temperature preferably Inore or less one of operating convenience, and below 95° C. For example, the sodium thiocya hence the liquor may be cooled to between say nate filtrate may be concentrated at reduced 20 and 30 C. Concentration of the potassium pressure of about 110 mm. of Hg to a boiling thiocyanate-potassium sulfate liquor to concen point of about 87° C., at which point the sodium tration corresponding to 42° Bé. at 720 C. Creates thiocyanate solution is substantially saturated. conditions such that on cooling the Concentrated The vacuum is then broken and the liquor is iguor to a temperature not lower than about cooled. Minimum temperature of cooling is de 20 C, the cooled liquor is not saturated With pendent upon whether it is desired to recover respect to potassium thiocyanate and none of this the product as anhydrous sodium thiocyanate or material crystallizes. Potassium sulfate crys as sodium thiocyanate dihydrate. The preferred tallizes during evaporating and cooling, and the sodium product of the invention is anhydrous resulting anhydrous potassium sulfate is filtered sodium thiocyanate, and consequently the liquor his out While the cooled liquor is at about room ten is cooled to a temperature close to, but above the perature. By this procedure the potassium thio transition point of sodium thiocyanate to the Cyanate filtrate contains not more than about dihydrate, and the resulting anhydrous Sodium 0.25% K2SO4. When evaporating the initial thiocyanate crystals are separated out of the potassium thiocyanate-potassium sulfate liquor, mother liquor at a temperature above such tran it is preferred to evaporate to at least a concen. sition point. The mother liquor may be returned tration corresponding to 40° Bé. at 72° C. By preferably to the sodium thiocyanate evapora-. SC doing, the liquor is concentrated sufficiently, ... tor, for reprocessing with a succeeding batch, With respect to potassium sulfate, so that after The anhydrous sodium thiocyanate crystals are 75 separation of potassium sulfate crystals the

4. 2,318,880 value, e.g., 35 C., above such transition point, the potassium thiocyanate another liquor filtrate coin liquor is not saturated with respect to anhydrous tains not more than about 0.3% K2SO4, sodium thiocyanate and there is no crystallization The potassium thiocyanate filtrate is then con of this material. At liquor temperature above centrated under conditions to form a substan about 30°. C., the precipitated sodium sulfate is tially saturated solution of potassium thiocyanate 5 in the form of anhydrous Na2SO4 which is filtered at temperature less than about 95° C. For exam out at temperature above the transition point of ple, the potassium thiocyanate solution may be anhydrous sodium thiocyanate to the dihydrate. concentrated at reduced pressure of about 110 The anhydrous sodium sulfate is dried as con mm. of Hg to a boiling point of 85 C. The vacu pletely as possible on the filter without washing, um is then broken and the liquor is cooled. Here 10 and the sodium sulfate amounts to about 60 parts . again, minimum temperature to which the Con by weight. The sodium thiocyanate filtrate re centrated liquor is cooled is one of operating con sulting from separation of the anhydrous Na2SO4, venience, and may be anywhere from say 20 to Containing not more than about 0.25% Na2SO4 30° C. Potassium thiocyanate crystals are filtered in solution, is again concentrated at reduced pres out, washed lightly with distilled water and pref Sure of about 110 mm. of mercury at a boiling erably dried under a vacuum of from 550 mm. to point of about 87° C., corresponding to a concen 760 mm. of Hg at temperatures varying from 80 tration of about 42 Bé. at 87° C. At this point, to 105° C. When proceeding in accordance with the liquor is substantially saturated with sodium the indicated preferred operating conditions, the thiocyanate, the vacuum is broken and the liquor product crystals analyze about 99%. KCNS, 20 is cooled down close to but not below the trans KaSO4 content is not in excess of about 0.2%, and tion point of anhydrous sodium thiocyanate to the KS and K2CO3 contents are substantially nil. the dihydrate. . The sodium thiocyanate crystals Following are specific examples of practice of are filtered out at a temperature above such the invention. transition point so that anhydrous sodium thio Eacample 1 2:5 cyanate product is recovered. The anhydrous In this example, directed to the manufacture of crystals are Washed lightly with a mist of distilled anhydrous sodium thiocyanate, the sodium cya water and dried under a vacuum. The anhy nide used is an off-grade material analyzing drous sodium thiocyanate crystals analyze over 93.7% NaCN, 3.31% Na2CO3, 1.03% NaOH and 99% NaCNS, the Na2SO4 content is 0.2% or less. 0.75% NaCNO. This sodium cyanide is dissolved 30 The Crystals contain substantially no Na2S or in water in quantity to form about a 30% NaCN Na2CO3. solution, approximately a saturated sodium cya Eacample 2 nide solution, 535 parts by weight of flowers of sulfur are mixed with Sufficient Water to form a In this example, directed to manufacture of 30% slurry, and the slurry is heated to boiling. 35 anhydrous potassium thiocyanate, the potas Sodium cyanide solution is added to the sulfur sium cyanide used is off-grade material analyz slurry, the total amount of 100% NaCN being 773 ing 93.8% KCN, 3.1% K2CO3, 0.42% KOH, and parts by weight. Temperature of the mass dur 2.33% KCNO. This is dis ing addition of the Sodium cyanide Solution is solved in water sufficient to form about a 35% about boiling, i. e., 103-105° C. but may be 80° C. KCN solution, approximately a saturated Solu or lower. The crude sodium thiocyanate solu tion of KCN. 480 parts of flowers of Sulfur are tion, after removal of excess elemental sulfur by mixed with sufficient water to form a 30%, slurry filtration, is then boiled until the vapors contain which is heated to boiling. Potassium cyanide no ammonia, indicating completion of decomposi solution is added, the total amount of 100% KCN tion of sodium cyanate to Sodium carbonate and used being 910 parts by Weight. Temperature of ammonia. After NH3 expulsion the solution is the maSS during addition of the cyanide Solution cooled to about 80° C. in order to prevent too is ordinarily about boiling, i. e., 103-105° C. but violent evolution of CO2 when sulfuric acid is sub may be 80' C. or lower. The crude potassium sequently added. The pH of the crude sodium thiocyanate solution, after removal of unreacted thiocyanate is about 10.5, and the solution con elemental Sulfur, is boiled until the vapors con tains sodium carbonate, Sodium sulfide and so tain no ammonia, this condition indicating com dium hydroxide as the major impurities. pletion of decomposition of Sulfuric acid of about 30% strength is added inpurity to Sodium carbonate and ammonia. until the pH of the resulting acid treated liquor the potassium thiocyanate liquor at this point is reduced to 5. About 35 parts of 100% H2SO4 has a pH of around 9.5-10 and contains potas are needed. The sulfuric acid converts the so sium carbonate, , and potas dium carbonate, sodium sulfide and sodium hy sium hydroxide as the major impurities. The droxide impurities to sodium sulfate, water, CO2 liquor is then cooled down to about 80° C. to pre and H2S. After completion of sulfuric acid addi vent too violent evolution of CO2 during the sub tion the liquor is agitated and boiled until all H2S 80 Sequent treatment of the liquor with sulfuric and CO2 have been expelled. The solution is acid. then clarified by addition of decolorizing carbon The potassium compound impurities in the liq followed by filtration. The concentration of the uor are converted to sulfate by treating the liqs filtrate is about 25 Bé. at 20° C. uor with Sulfuric acid of about 30% strength in The clear scdium thiocyanate filtrate, contain 85 quantity to bring the pH of the acid treated liq ing sodium sulfate in solution, is concentrated uor down to about 5. About 30 parts by weight under reduced pressure of about 110 mm. Of mer of 100% H2SO4 are needed. The liquor mass is cury to a boiling point of about 80 C. corre agitated and boiled until all H2S and CO2 have sponding to a concentration of 38° Bé. at 80° C. been expelled. The solution is clarified by treat The vacuum is broken and the liquor is cooled 70 ment with decolorizing carbon and Subsequent down close to but not below the transition point filtration. (about 30° C.) of anhydrous sodium thiocyanate The clear potassium thiocyanate filtrate, con to sodium thiocyanate dihydrate. The concen taining potassium sulfate in solution, is concen tration operation described is such that when the trated by evaporation at reduced pressure of about liquor is cooled down as indicated to a working .75 110 mm. of mercury to a boiling point of about

2,818,680 S 72° C., corresponding to a concentration of about recovering alkali metal thiocyanate in solid form 42 Bé. at 72 C. The vacuum is broken, and the from said liquor. liquor is cooled down to about roqm tempera 4. The' process for making alkali metal thio ture, e.g. 25-30°C. The concentration operation cyanate which comprises treating an alkali metal described is such that when the liquor is cooled thiocyanate solution, containing at least one im down to the indicated convenient working tem purity of the group consisting of alkali metal perature, 25-30 C, the liquor is not Saturated carbonate, alkali metal sulfide and alkali metal with respect to anhydrous potassium thiocyanate, hydroxide, with sulfuric acid in quantity suff and there is no crystallization of this material. cient to convert substantially all alkali metal im The anhydrous potassium sulfate crystals formed O purity contained in said solution to alkali metal by concentration and cooling are filtered out and sulfate, crystallizing alkali metal sulfate and dried as completely as possible on the filter with separating such sulfate crystals from the re outwashing. The moist sulfate amounts to about sulting liquor while maintaining substantially all 65 parts by weight. of the alkali metal thiocyanate in solution in After removal of the potassium sulfate crys 5 said liquor, then crystallizing alkali metal thio tals, the filtrate containing not more than 0.25% cyanate from residual liquor, and separating KaSO4 is again concentrated under reduced pres alkali metal thiocyanate crystals therefrom, sure of about 10 mm Hg to a boiling point of 5. In the process of purifying alkali metal thio about 85 C., corresponding with a concentra cyanate solution containing at least one impurity tion of 50 Bé, at 85°C. The liquor is substan 20 of the group consisting of alkali metal carbonate, tially saturated with potassium thiocyanate at alkali metal sulfide and alkali metal hydroxide, this temperature. The vacuum is broken and the the step comprising treating said solution with liquor is cooled to around room temperature, e. g. sulfuric acid in quantity sufficient to convert 25-30 C., and anhydrous potassium thiocyanate substantially all alkali metal impurity contained crystals are filtered out, washed lightly with a 25 in said solution to alkali metal sulfate. mist of distilled Water and dried under Vacuum. 6. In the process of purifying alkali metal thio Anhydrous potassium thiocyanate crystals ob cyanate solution containing at least one impurity tained analyzed about 99.5% KNCS, the KSO4. of the group consisting of alkali metal carbonate, content is not over 0.2% and the product Con alkali metal sulfide and alkali metal hydroxide, tains substantially no KzS or K2CO3. 30 the step comprising treating the solution with In the previous discussion and in the appended sulfuric acid in quantity sufficient to convert sub claims, the term "alkali metal' is intended to stantially all alkali metal impurity contained in exclude annonia, Sadd solution to alkali metal sulfate but insuffi I claim: cient to reduce the pH of the resulting solution 1. The process for making alkali metal thio 35 lower than 4.2. cyanate which comprises treating an alkali metal 7. In the process of purifying alkali metal thio thiocyanate Solution, containing at least one in cyanate solution containing at least one impurity purity of the group consisting of alkali metal of the group consisting of alkali metal carbonate, carbonate, alkali metal Sulfide and alkali metal alkali metal sulfide and alkali metal hydroxide, hydroxide, with sulfuric acid in quantity suff 40 the Step comprising treating said Solution with cient to convert substantially all alkali metal Sulfuric acid of strength not greater than 40% impurity contained in said solution to alkali HaSO4 and in quantity sufficient to convert sub metal Silfate, separating alkali metal Sulfate from stantially all alkali metal impurity contained in the resulting liquor while maintaining alkali metal Said solution to alkali metal Sulfate. thiocyanate in Solution in Said liquor, and then 45 8. The process for making sodium thiocyanate recovering alkali metal thiocyanate in solid form which comprises treating a sodium thiocyanate from said liquor. solution, containing at least One impurity of the 2. The process for making alkali metal thio group consisting of sodium carbonate, Sodium cyanate which comprises treating an alkali metal sulfide and Sodium hydroxide, with sulfuric acid thiocyanate solution, containing at least one in in quantity sufficient to convert substantially all purity of the group consisting of alkali metal car Sodium compound impurity contained in said bonate, alkali metal Sulfide and alkali metal Solution to Sodium sulfate, concentrating the re hydroxide, with Sulfuric acid in quantity suff Sulting liquor by heating to a concentration not cient to reduce the pH of the resulting liquor to less than that corresponding to 36 Be... at 80° C. not lower than 4.2 and not higher than 6.3 Where 5 5 and not in excess of a concentration correspond by alkali metal impurity contained in said so ing to 38° Be... at 80° C., cooling the liquor to a ution is converted to alkali metal Sulfate, Sep temperature above the transition point of anhy arating alkali metal sulfate from said resulting drous sodium thiocyanate to the dihydrate, sepa liquor while maintaining alkali metal thiocyanate rating crystallized sodium sulfate from the liq in solution in said liquor, and then recovering 60 uor while maintaining the same at temperature alkali metal thiocyanate in solid form from Said above said transition point, concentrating the liquor. residual liquor by heating, cooling such liquor to crystallize Sodium thiocyanate, and separating 3. The process for making alkali metal thio Sodium thiocyanate crystals from such liquor. cyanate which comprises treating an alkali metal 9. The process for making anhydrous sodium thiocyanate solution, containing at least one im thiocyanate which comprises treating a sodium purity of the group consisting of alkali metal thiocyanate Solution, containing at least one im carbonate, alkali metal sulfide and alkali metal purity of the group consisting of sodium carbon hydroxide, with sulfuric acid of strength not ate, Sodium sulfide and sodium hydroxide, with greater than 40% H2SO4 and in quantity sufi 70 Sulfuric acid in quantity sufficient to convert sub cient to convert substantially all alkali metal in stantially all sodium compound impurity con purity contained in said solution to alkali metal tained in said solution to sodium sulfate, concen sulfate, separating alkali metal sulfate from the trating the resulting liquor by heating to a con resulting liquor while maintaining alkali metal centration not less than that corresponding to

thiocyanate in Solution in said liquor, and then 75 36 Bé. at 80° C. and not in excess of a concen 3. 2,33,680 tration corresponding to 38 Bé. at 80° C., cooling sium carbonate, potassium sulfide and potassium the liquor to a temperature above the transition hydroxide, with sulfuric acid in quantity Suff point of anhydrous sodium thiocyanate to the cient to convert substantially all potassium com dihydrate, separating crystallized sodium sulfate pound impurity contained in said solution to from the liquor while maintaining the same at potassium sulfate, concentrating the resulting temperature above said transition point, concen liquor by heating to a concentration not less trating the residual liquor by heating, cooling than that corresponding to 40 Bé. at 72° C. and such liquor to a temperature above the transition not in excess of a concentration corresponding to point of anhydrous sodium thiocyanate to the 42 Bé, at 72°C., cooling the liquor to effect crys dihydrate, and separating anhydrous Sodium 10 tallization of potassium Sulfate, Separating potas thiocyanate from such liquor at a temperature sium sulfate from the liquor, concentrating the above said transition point. residual liquor by heating, cooling Such liquor to 10. The process for making anhydrous potas effect crystallization of anhydrous potassium sium thiocyanate which Comprises treating a po thiocyanate, and separating anhydrous potassium tassium thiocyanate solution, containing at least 5 thiocyanate from Such liquor. one impurity of the group consisting of potas LEEB, SMITH,