128 CHEMICAL FERTILIZERS 



the manufacture of sulphuric acid has other objects besides 

 that of the production of superphosphate, nevertheless most 

 of the sulphuric acid made in Great Britain is devoted to 

 the manufacture of fertilizers, and only a small amount 

 to other purposes. About 60 % of the total consumption 

 of sulphuric acid is devoted to superphosphates and sulphate 

 of ammonia. In the majority of superphosphate works, 

 sulphuric acid is produced on the spot, and therefore the 

 sulphuric acid industry is closely connected with the manu- 

 facture of artificial manures. As a result of this close con- 

 nection, the production of sulphuric acid in Great Britain 

 has followed directions which are especially suitable for the 

 fertilizer trade. In the fertilizer trade, a weak and not a 

 strong acid is required, and for this purpose leaden chambers 

 are convenient. 



The Manufacture of Sulphuric Acid for Fertilizers. 

 The first part of the process consists in burning pyrites 

 in kilns. The lumps of pyrites are broken up into nuts, as free 

 from dust as possible, and burnt in a long furnace. The 

 heat of combustion of the pyrites is sufficient to keep the 

 material alight, and no outside heat is necessary. Pyrites 

 contains 40-50 % of sulphur, much iron, a little copper 

 and some arsenic. The copper is left in the burnt pyrites, 

 and is subsequently recovered at other works. The arsenic 

 volatilizes and finds its way into the final product, sulphuric 

 acid, where it remains as an impurity, unless subsequently 

 removed (see p. 86). The waste iron oxide from gas 

 purifiers can also be burnt in the same way. In recent years 

 the use of zinc has increased. Zinc blende containing 30-40% 

 of sulphur forms a difficult material for the manufacture 

 of sulphuric acid, but as the manufacture of metallic zinc 

 demands that first of all the ore should be deprived of its 

 sulphur, sulphur dioxide is a waste product, and must be 

 used somehow. The operation of burning zinc sulphide 

 requires additional heat, usually supplied by the combustion 

 of fuel which adds carbon dioxide to the gases passing into 

 the leaden chambers. Carbon dioxide retards the chemical 

 changes taking place in the leaden chambers. 



