168 CHEMICAL FERTILIZERS 



carried out in vacuum pans. The finished salts are washed 

 with water and the wash water utilized for dissolving the 

 crude salts ; even then the washed salts still contain a fair 

 amount of sodium chloride. After the crystallization of the 

 artificial carnallite, the liquors running away contain very 

 little potassium chloride, but large amounts of magnesium 

 chloride. 



The Purification of Crude Potassium Chloride. - 

 The crude material is washed at a low temperature, whereby 

 some sodium chloride and magnesium chloride wash out. 

 The washing takes place in iron vessels, provided with false 

 bottoms, on which the salts rest, and water is sprinkled on. 

 The washing operation is not practical unless the crude salts 

 contain more than 50 % of potassium chloride. The resulting 

 salts still contain a considerable quantity of water, which is 

 removed by drying. Many systems of drying result in some 

 crusts being formed, which have a different composition from 

 the main part of the crystals. By these means various 

 grades of salts from 80 to 98 % of potassium chloride can 

 be obtained. 



The Manufacture of Potassium Sulphate. Potassium 

 sulphate is generally made either from kainite or by acting 

 on potassium chloride with sulphuric acid. Most of the 

 potassium sulphate in the mines occurs in the form of kainite, 

 which consists of potassium sulphate, magnesium sulphate 

 and sodium chloride. The first step in preparing potassium 

 sulphate is to manufacture an artificial schoenite by a system 

 of re-crystallization. A cold saturated solution of kainite 

 is employed for extracting fresh kainite. The solution is 

 clarified, and on cooling deposits schoenite, i.e. potassium 

 magnesium sulphate. Or mixtures of kainite and sylvinite 

 are treated with a hot solution of kainite, when almost pure 

 schoenite crystallizes out. This latter is very often used 

 directly for agricultural purposes. When potassium sulphate 

 is made from schoenite, a saturated solution of hot schoenite 

 is run on to dry powdered potassium chloride, when the follow- 

 ing reaction takes place : 



KgS0 4 . MgS0 4 + 3KC1 = 2K 2 S0 4 + KC1 . MgCl 2 



