86 CHEMICAL FERTILIZERS 



left unacted on, when the agricultural value of this by- 

 product becomes small. The ammonia gas coming out of the 

 main still now passes away through a wide pipe to the satura- 

 tor, which is a lead-lined vessel. Sulphuric acid is admitted 

 to the saturator in a slow, steady stream, at such a rate as 

 to neutralize the ammonia coming in. Ammonium sulphate 

 separates out, and is blown up by steam into the wide exit 

 pipe, where crystals of ammonium sulphate and a strong 

 solution of ammonium sulphate are forced out together in 

 a soup-like condition into large wooden reservoirs, where the 

 ammonium sulphate rapidly crystallizes, and the small 

 amount of mother liquor drains away and is returned to the 

 saturator. The gas coming out of the main still contains, 

 as described above, hydrogen sulphide, carbon dioxide and 

 hydrogen cyanide in addition to the ammonia. On passing 

 through the sulphuric acid these added gases are not absorbed, 

 but pass away through the exit. As the combination of 

 ammonia and sulphuric acid gives rise to much heat, the 

 gas passing away from the saturator is far above 100 C. 

 and can be used to pre-heat the ammonia liquor. The gases 

 passing away are then usually sent through an iron-oxide 

 purifier box, smaller but similar to those employed in the 

 purification of coal gas. When the iron oxide contains a 

 sufficient amount of sulphur, it is burnt in the pyrites burners 

 in a sulphuric acid plant, to recover the sulphur. A portion 

 of these gases, containing hydrogen sulphide and carbon 

 dioxide, with traces of prussic acid, is used to purify the 

 sulphuric acid. Pyrites sulphuric acid always contains 

 arsenic, which can be precipitated by hydrogen sulphide, 

 even in the presence of strong sulphuric acid. The yellow 

 arsenic sulphide is filtered off through special porous bricks 

 or through a sand bed. At present no satisfactory use has 

 been found for this arsenic sulphide. If arsenic had been 

 left in the sulphuric acid, a yellow arsenic sulphide would 

 have been precipitated in the saturator, and would have 

 contaminated the ammonium sulphate. From the immedi- 

 ate point of view of the producer, discoloration reduces 

 the market value, but, in addition, the presence of arsenic 



