NO118198B - - Google Patents

Description

Process for the production of potassium sulfate.

It is known that potassium sulfate can be produced from calcium sulfate and potassium chloride.

Many ways have been proposed

to do this on, where the calcium sulfate is used as it is, after activation or converted to other intermediate sulfates, such as e.g. magnesium sulfate or ammonium sulfate.

Among the methods by which calcium sulfate is used as it is, some of them work in aqueous solution, but the reaction between KC1 and calcium sulfate takes place with greater or lesser yield with the formation of double salts, such as syngenite Ca.SO4.K2SO4.H-O or penta -the salt 5 Ca.SO4.K-SO4.H2O, which is then split to give K2SO4.

It is also known that in the presence of an aqueous ammonia solution, one obtains either a double salt, if a small excess of potassium chloride is used compared to the stoichiometric, but the yields are better than when the reaction takes place in an aqueous solution or one obtains K2SO4 directly by application of a large excess of potassium chloride. But in the latter case, the large quantities of mother liquor will contain a significant amount of potassium chloride, which can only be extracted with the help of complicated precipitation of double salts and their further processing.

Furthermore, it is known from the French patent no. 1,042,187 (Sociéte Potasse & Produits Chimiques) that potassium sulfate can be obtained directly by allowing potassium chloride to react with the calcium sulfate in practically stoichiometric proportions at ordinary temperature in a concentrated ammoniacal environment, when the solution contains approx. . 50 percent more ammonia, i.e. using a sufficiently high pressure, namely approx. 3 kg/cm<2> absolutely.

The inventors have now found that it is possible to reduce this minimum concentration of ammonia in the aqueous solutions when working at lower temperatures than normal temperature, preferably by reducing the mentioned minimum concentration to 40 percent when the temperature is lower than 5° C.

This surprising improvement of the method according to the French patent no. 1,042,187 also entails other important advantages: 1) It is possible to use substantially smaller amounts of ammonia solution as the ratio ammonia solution/CaSOi is always less than 2.60. 2) The amount of ammonia used to convert the same amount of calcium sulfate is smaller for two reasons, as the amount of solution and its NH3 concentration are reduced at the same time.

Both of these advantages, which are achieved without a stoichiometric excess of KC1, are of significant importance for the recovery of NH3 from the mother liquors, either by distillation or by the per se known precipitation of CaCl2-ammoniacates and their decomposition.

Furthermore, the temperature and pressure conditions used make it possible to carry out the reaction at atmospheric pressure, which contributes to the ammonia losses being significantly smaller, while at the same time avoiding the difficulties of introducing solids into containers that are under pressure and separating liquids and solids under pressure after the reaction has ended.

It has also been shown that even better results can be achieved when K2SO4 is produced continuously and in countercurrent as follows: In a first work step, CaSOi and a deficit of KC1 are added to an ammoniacal solution of KC1 and CaCb, hit -resulting from a next work step, which will be described in the following.

At the end of the first stage, i.e. when the conversion rate becomes low, the solid product is sucked off. The mother liquors are distilled and the separated solid, moist material is treated in a next step with a fresh ammonia solution and a new amount of KC1, the amount of KC1 added together in the first two steps corresponds stoichiometrically to the amount of CaSOt, which is added in the first step. By adding fresh components in this way in the second step, a significantly greater conversion rate is achieved during the last part of the reaction, compared to when the reaction is carried out in one step. The accumulation of CaCb in the mother liquors inhibits the reaction between CaS04 and KC1 to a high degree, and this disadvantage is avoided by adding the partially converted moist filter cake to the fresh ammoniacal solution.

If the temperature in this way of working is kept below 5° C, it is also achieved that the amount of NH3 required to convert part of the calcium sulphate can be reduced to a significant extent.

In the method described above, calcium sulphate can be used in any form: Gypsum, hemihydrate or anhydrite.

In the following, two examples of the implementation of the method according to the invention are given:

Example 1.

One allows:

100 parts gypsum with a content of 79 parts CaSOé 87 » KC1

113 » water and

93 » NH3

react with each other under atmospheric pressure at -3° C for 3 hours with moderate stirring.

The mother liquor is then removed by filtration and the filter cake, which consists of K2SO4, is washed with ammonia water.

The yield in the conversion of gypsum to K2SO4 is 98 percent, since 2 parts of KC1 accompanied the mother liquor.

If, on the other hand, the above procedure is carried out under the same concen-

sion conditions (same amount of ammoniacal solution and same amount of NH3 in the solution), but at normal temperature (20-25° C), i.e. under a pressure of 2 kg/ cm<2> abs. after washing with ammonia water, a solid substance consisting of a mixture of syngenite, CaSO4.K2SO4.H2O and KC1 is obtained and the yield from the conversion of gypsum to K2SO4 is equal to 50 per cent.

Under the same concentration conditions, under which a double salt of calcium and potassium is precipitated at ordinary temperatures, by lowering the temperature, K2SO4 will be obtained directly with excellent yield.

If you work at normal temperature to obtain K2SO4 directly with a good yield, you must, according to French patent no. 1,042,187, simultaneously increase the amount of ammonia solution in relation to the amount of CaS04 being treated and this solution's NHi concentration. You then use for 100 parts gypsum: 101 parts water and 127 » NH3.

Compared to this method, which enables the direct production of K2SO4 at ordinary temperatures, by working at low temperatures a significant saving of ammonia is achieved. If you compare the ratios CaS04/NH3 in the two cases, the ratio is

at -3° C equal to 0.85 and

» ordinary temperature equal to 0.62.

i.e. a saving of 37 per cent.

Example 2.

In a first step, one allows:

108 parts natural gypsum with a content of 92.6 percent CaS04.H20

65 » KCL (58-60 percent K2O) and mother liquor from the next reaction step with a content of

105 parts water

86 » NH3

21.5 » CaCb and

3 » KC1

react with each other.

After less than an hour's reaction at -3° C under atmospheric pressure and slow stirring, approx. 72 per cent of the gypsum was converted and after filtration you get: a) a moist filter cake, which contains:

28 pieces of plaster

72.7 » K2SO4

8 » unresolved

4.7 » KC1

II » water

7 » NH3

6 » CaCb and

b) a mother liquor containing: 109 parts water 79 » NH.-i 62 » CaCb

0.8 » KC1.

NH.> is recovered from the mother liquor. in ways known per se, either by precipitation of CaCb-ammonia and cleavage thereof.

In the next step, the moist filter cake is taken up in: 95 parts water, 78 » NH:l

20.8 » KC1.

After an hour's reaction at -3° C under atmospheric pressure and slow stirring, approx. 90 per cent of the gypsum was converted, and by filtration you get: a) a moist filter cake containing: 2 parts gypsum 99.2 » K2SO4 8 » undissolved 6.5 » water 5 » NH.3 1.3 » CaCb and b ) mother liquor containing 105 parts water 80 » NHm 3 » KC1 21.5 » CaCb which is fed back into the process after enrichment with NH:;.

Under these working conditions, the ratio CaSO,i/NH:t is equal to 0.92, which corresponds to an NH.!-saving of 8 per cent., compared to the one-step process at ordinary temperature.

Claims (2)

1. Process for producing potassium sulphate from calcium sulphate and potassium chloride using an aqueous ammonia solution as the reaction medium, characterized in that the reaction is carried out at a temperature below 5° C in an aqueous ammonia solution containing at least 10% by weight. NH3. 2. Method according to claim 1, characterized in that the method is carried out continuously in a two-step process.