POTASSIUM FERTILIZERS 161 



from forming, and most of the potash is returned as soluble 

 potassium sulphate, whilst nearly all the fluorine is recovered 

 in the filter press as a cake of calcium fluoride, mixed with 

 calcium sulphate. Nearly all the fluorine used can be 

 returned to the furnace in the form of this cake. 



About 90 % of all the potash present in the materials is 

 volatilized, from which must be subtracted about 10 % lost 

 in the collectors and 5 % lost in washing. Hence about 

 three-quarters of all the potash in the materials is recovered 

 as potassium sulphate. Potassium sulphate is a more valu- 

 able salt for fertilizing purposes than potassium chloride and 

 fetches a higher price. 



The Manufacture of Pure Salts from Potash 

 Minerals. Before considering the special case of the manu- 

 facture of commercially pure specimens of the more impor- 

 tant potassium salts, it is necessary to discuss some of 

 the general properties of salts in aqueous solution. All 

 substances are soluble to some slight extent in water, but in 

 many cases the amount dissolved is so minute that it may be 

 neglected, and it is therefore common to classify salts as 

 soluble, sparingly soluble and insoluble in water. In the 

 case of every salt there is a maximum quantity which can be 

 dissolved at any particular temperature, but this quantity 

 varies when the temperature is altered, usually increasing 

 with the temperature. In some particular cases, such as 

 that of potassium chloride, the solubility increases step by 

 step with the temperature, but in general the solubility of 

 salts increases with increasing pace as the temperature rises. 

 That is to say, if the solubilities be plotted on squared paper 

 with reference to the temperature, the equation to the curve 

 in the case of potassium chloride is of the form S = a -}- bt, 

 where S represents solubility and t represents temperature. 

 For most salts, a further figure of ct 2 must be added, and 

 perhaps occasionally even ft takes part in the equation. 

 The solubility of sodium chloride is expressed on squared 

 paper as an almost straight line. Temperature has, in 

 this last case, very little eifect on the solubility, which is 

 nearly constant from o-ioo C. (32-2i2 2 F.). The 

 v. ii 



