88 CHEMICAL FERTILIZERS 



thus maintaining more perfect agitation and so preventing 

 local conditions of alkalinity. As iron always occurs in the 

 sulphuric acid and is reduced to the ferrous condition by 

 hydrogen sulphide, the ammonium sulphate will always 

 contain crystals of ammonium ferrous sulphate, which 

 slowly oxidize in air to led ferric compounds. Hence a 

 very pure, white ammonium sulphate may be obtained in 

 the first instance, which slowly changes to a pale yellow 

 colour on exposure to air. This action is, however, usually 

 only superficial, very slow and very slight. All the pipes 

 in, approaching to and leaving the saturator must be lead, 

 or at least lead-lined, but the latter portion of the exit pipe is 

 often replaced by a wooden trough. 



A good many variations occur in the details of con- 

 struction in this apparatus. Floor space can be saved by 

 constructing the main still on top of the secondary still. 

 The saturator, as represented on p. 84, may be replaced 

 by a half-open saturator, in which the ammonium sulphate 

 is fished out by a perforated ladle, but in all cases the exit 

 gases containing hydrogen sulphide must be removed, and 

 either burnt or the sulphur recovered. The sulphate of 

 ammonia may be separated from its mother liquor by a 

 centrifugal separator. 



The acid used, as a rule, is fairly strong about 1 7 specific 

 gravity but in some methods slightly weaker acids are used. 

 The resulting ammonium sulphate usually contains between 

 20 % and 21 % of nitrogen, or 24-25 % of ammonia, 

 equivalent to from 93 to 99 % of pure ammonium sulphate. 

 Generally it contains from between o*i-o'5 % of free 

 sulphuric acid, but special efforts are now being made to 

 reduce the acidity to as low as 0*02 % with some consider- 

 able success. Some of the gas companies are producing 

 a fine dry sulphate of ammonia which neither dries nor 

 cakes and contains 21 "i % of nitrogen, no free acid and only 

 0*03 % of water. Owing to the fact that the saturator is 

 at a higher temperature than the still, there is little tendency 

 for any volatile organic matter to accumulate in the saturator, 

 hence there is no trouble with discoloration by tarry 



