166 CHEMICAL FERTILIZERS 



The manufacture of potassium chloride is chiefly carried 

 out by re-crystallization from crude carnallite, which is a 

 more difficult process. The composition of crude carnallite 

 is shown in Table 19. 



TABLE 19. 

 COMPOSITION OF CRUDE CARNALLITE. 



The remaining 2 or 3 % consists of traces of insoluble matter, 

 calcium chloride and a little bromine. The true mineral 

 carnallite contains no sodium at all, but consists of potassium 

 chloride and magnesium chloride only. True carnallite is 

 unstable in the presence of much water, even at a low temper- 

 ature, and is decomposed into a magma of very fine crystals 

 of potassium chloride and a solution of magnesium chloride 

 with a little potassium chloride in solution. When the 

 solution is heated a much larger quantity of potassium 

 chloride is dissolved, and on cooling the latter salt separates 

 out in large crystals, so long as the amount of magnesium 

 chloride in solution is not more than three times that of the 

 potassium chloride. When this proportion is exceeded, the 

 liquor deposits carnallite. Sodium chloride is much less 

 soluble in hot magnesium chloride solution than potassium 

 chloride, consequently much is left behind in the insoluble 

 form. 



The first operation in practice consists in dissolving the 

 crude carnallite, which is usually treated with waste liquors 

 obtained later in the process. The carnallite should be 

 treated as quickly as possible, so that unnecessary solution 

 of sodium chloride is avoided. Most works use the hot 

 liquors at the ordinary atmospheric pressure, or very little 

 above it, with mechanical agitation. With high pressures 

 much more sodium chloride dissolves, and with finely powdered 

 material the same difficulties arise as in treating the fine 

 materials in the manufacture of sodium nitrate described on 



