GB2224726A - Potassium persulphate - Google Patents

Description

2224726 V PROCESS FOR PREPARING POTASSIUM PERSULFATE OF HIGH PURITY

Background of the Invention

The present invention relates to a process for preparing a solid potassium persulfate of high purity with a reduced content of potassium sulfate in the production of potassium persulfate by the reaction of ammonium persulfate and potessium 1ydroxide. _40n of L pot in the prelarat Lassium persulfate, during the reaction of ammonium persilfate and potassium hydroxide, during separation of the resulting potassium persulfate crystals, and during storarin- n-c the mother liquor after separation of the crystals, the Potassium persulfate decomposes to form potassium sulfate in the presence of dissolved amnonia or heavy metal ions. This potassium sulfate contaminates the potassium persulfate product, causing a lowering of its purity.

In orC-er to solve the above-mentioned problem, in the preparation of a corresponding persulfate by reacting ammonium persulfate with a hydroxide of an alkali metal or alkaline earth metal, various proposals have been made about the process-for preparing a high purity persulfate by decreasing the decomposition rate of the persulfate to suppress the formation of.sulfate.

1 In Japanese Patent No. 1,022,912 there is proposed a process wherein the ammonia formed in the reaction of ammonium persulfate and an alkali metal or alkaline earth metal hydroxide as well as part of the vater present in the reaction system are evaporated in the same system, then the resulting aqueous solution or slurry of persulfate with minimized ammonia concentration is 7::- en i - --In c.- t.-, e r e a c -L a- c n an a c o ri C e n Z r a Z i o n e s -.;- s t - were conducted, to crvstallize the persulfate.

d 0,2 e 12 2: E, G, 4 1 19 Tj 6 there is proposed a process wherein the ammonia formed in the reaction-o-F ammonium persulfate and sodium hydroxide or potassium hydroxide-is removed using a countercurrent gas stripper to maintain the ammonia concentration in the reaction solution lower than 0.5 wt%.

According to those proposed processes, however, although the decomposition rate"of persulfate decreases due to lowering of the ar=nia concentration, it is impossible to prevent the decomposition of persulfate caused by other factors, so that the sulfate in the mother liquor which is recycled is accumulated gradually and finally precipitates together with the persulfate, thus reducing the purity of the product obtained.

2 - -1 i 1 Further, as a method for removing the sodium sulfate resulting from the decomposition of sodium persulfate, Japanese Patent Laid Open No. 161907/1983 proposes a method wherein partaf the mother liquor is diluted and cooled to precipitate sodium sulfate decahydrate, which is then separated and removed. However, this method is disadvantageous in that when applied to the production of potassi -,- _-21-fate, there occurs Gf:, - 7=- "one cr tocether wir- 'Dotassium zotass= perss-_ ___e al sulfate.

t::, -ne prcr ar--, 'or preven:ng the incorporation of potassium sulfate into the potassium the persulfate product, there is adopted a method where-L reaction mother liquor is partially discarded, then diluted with water and recycled, or a method wherein the whole amount of the mother liauor is discarded just before the concentration at which potassium sulfate precipitates is reached. However, these methods are ve,_-disadvantageous from the economic point of view.

It is the object of the present invention to solve the above-mentioned problems of the prior art and more particularly to provide a process for preparing a high purity potassium sulfate while keeping the loss of mother liquor to a minimum.

Summary of the Invention

Having made extensive studies for attaining the above-mentioned object, the present inventors found out a process capable of precipitating and removing potassium sulfate from mother liquor by adding a starting potassium hydroxide to the mother liquor after separation of potassium persulfate obtained by the reaction of ammonium persulfate and potassium hydroxide, and thus accomplished the present invention.

More specifically, the present invention resides in a process for preparing a high purity potassium persulfate with a reduced content of potassium sulfate by the reaction of ammonium persulfate and potassium hydroxide, characterized in that at least part of the potassium hydroxide used as a starting material is added to at least part of the mother liquor after separation of potassium persulfate crystals obtained by the said reaction to precipitate and remove potassium sulfate formed by the decomposition of potassium persulfate, and the mother liquor is recycled.

In the potassium persulfate produc.ts obtained by the conventional processes there is contained not less than 0.5 wt% olf potassium sulfate, while by adopting the process of the present invention it is possible to reduce the potassium sulfate content to not higher than 0.1.wt%.

4 - i f i i 1 i T Detailed Description of the Invention

The process of the present invention will now be described more concretely. Ammonium persulfate and potassium hydroxide are reacted in an aqueous solution; the resulting ammonia is removed by a vacuum evaporation method involving evaporation for concentration in a vacuum or- by a stripper method involving concentration using a large amount of air at atmospheric pressure; ter, the precipitated potassium persulfate is thereal.

separated using, for example, a centrifugal separator; then, potassium hydroxide is added to a part or the whole of the mother liquor after separation of the potassium persulfate crystals to crystallize and remove the potassium sulfate contained in the mother liquor; and ammonium persulfate is dissolved in the so-treated mother liquor for re-use as the reaction solution.

The reaction of arffmnia persulfate and potassium hydroxide is performed in a known manner. Usually, both are reacted in water medium at a temperature in the range of about 00 to about 45'C for a period of time sufficient for the formation of potassium persulfate. After the reaction, the resulting potassium persulfate crystals are separated and the mother liquor is re- used in the process of the present invention.

- Where the amount of potassium persulfate decomposed in the preparation step is large, it is desirable to treat the whole amount of the mother liquor, but when the amount of potassium persulfate decomposed is small, part of the mother liquor may be treated and thereafter recycled together with the untreated mother liquor.

As to the addition of potassium hydroxide to the mother i-iauor afL--er separation of the Potassium per-sulfate crystals, 10 to 60 wt%, preferably 40 to 55 wt%, of potassium hydroxide is added to the mo therliquor in -o a part or the whole of its an amount corresponding t amount used as a starting material. An optimum range of the amount of potassium hydroxide to be added differs depending on the amount of potassium sulfate contained in the mother liquor, etc., even if its amount is small, a certain effect will be exhibited. Usually, its amount LE larger than 10 wt% of its amount used as a starting material.

The addition of potassium hydroxide, precipitation of potassium sulfate and separation of the resulting crystals are performed at temperatures in the range of 5 to 60C, preferably 150 to 30C.

The following examples are given to illustrate the present invention in more detail, but it is to be understood that the invention is not limited thereto.

I Example 1 g of an aqueous 50 wt% potassium hydroxide solution was added to 100 g of a mother liquor consisting of 2.3 wt% potassium persulfate and 7.5 wt% potassium sulfate which mother liquor had been obtained by the reaction of ammonium persulfate and potassium hydroxide in a known manner and subsequent evaporation of ammonia in The temperature was held at 250C to allow pozassLum sulfate to be precipitated. After separation of the precipitated potassium sulfate crystals, the mother liquor was fed into a reaction vessel. Further, a - solution in 86.5 g ammonium persulfate of 770 g of a rnoil-ier liquor having the above composition and not subjected to the potassium sulfate removing operation was fed to the reaction vessel together with 76 g of 50% potassium hydroxide. The reaction vesssel was held at 250C, 18 mmHg for 30 minutes. Ammonia formed by the reaction was removed and 13.6 g of water was evaporated.

The resulting potassium persulfate slurry was fed to a centrifugal separator for the separation of crystals and the mother liquor, then the mother liquor was used in the reaction under repetition of the same operation as above. The potassium persulfate crystals obtained were pure to such extent as shown in Table 1.

Comparative Example 1 Reaction was performed in the same way as in Example 1 except that the mother liquor was used repeatedly E , i without being subjected to the potassium sulfate removing operation. The resulting potassium persulfate crystals were found to have such purities as shown in Table 1.

Table 1

No.. cf: ti.nes of Crys,Cal purity Crystal purity in repeated use in Example 1 ComiDarative Example 1 1 991.9 or more 99.7 2 99.4 3 99.0 97.9 1 i i i 1 1 i 1 9 i 1 - 9

Claims (5)

1. A process for preparing a high purity potassium persulfate with a reduced content of potassium sulfate by the reaction of ammonium persulfate and p otassium hydroxide, characterized in that at least part of the potassium hydroxide used as a starting material is' added to at least part of the mother liquor after separation of r),Dt a S, 41 crystals olbi-ne, =, persu fate decomposition of potassium perrsulfate, and the mother li-guor is recycled.
2. 2. 7 process as set forth in Claim 1, wherein the potassium hydroxide added to the mother liquor after separation of the potassium persulfate crystals is in the form of a 10-60 weight % aqueous solution.
3. 3. A process as set forth in Claim 1, wherein the operating temperature is in the range of 50 to 600C.
4. 4. A process as claimed in claim 1, substantially as hereinbefore described and as illustrated in the Examples.
5. 5. High purity potassium persulfate when obtained by the process claimed in any one of the preceding claims.

   Published 1990 at The Patent Office. State House, 66 71 High Holborn. London WClR4TP-Purther copies maybe obtainedfrom The Patent OfficeSales Branch. St Mary Cray. Orpington. Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray. Kent, Con. 1187