PHOSPHORUS FERTILIZERS 129 



A charge of pyrites in the burners usually lasts for about 

 24 hours, and the kilns are charged in regular succession, while 

 the quantity of air entering the kiln is carefully regulated by 

 a well-fitting door. Certain furnaces, of a mechanical type, 

 have also been designed to deal with powdered pyrites. 

 The fumes passing away consist of sulphur dioxide, nitrogen 

 and oxygen, which are then drawn by a powerful draught 

 through nitre pots containing a mixture of sodium nitrate 

 and sulphuric acid heated by the gases proceeding from the 

 pyrites furnace. The necessary draught is obtained by a 

 tall chimney, sometimes assisted by a fan. The gases then 

 pass up the Glover tower and meet a stream of weak acid from 

 the leaden chambers mixed with the strong acid running out 

 of the Gay Lussac tower, which delivers a strong acid impreg- 

 nated with nitric oxide recovered from the waste gases 

 passing away out of the leaden chambers. In the Glover 

 tower interaction takes place between sulphur dioxide 

 and nitrogen oxides, with the production of a certain amount 

 of sulphuric acid and the evolution of nitric oxide. At the 

 same time the heat from the pyrites burners evaporates 

 water, in consequence of which the Glover tower is a means 

 of concentrating the sulphuric acid produced. The acid 

 running out of the Glover tower has been deprived of all its 

 oxides of nitrogen and much of its water so that it can be 

 used directly for the manufacture of superphosphate without 

 any further concentration. The gases then pass away from 

 the Glover tower through four or five leaden chambers, 

 in which the oxidation of the sulphur dioxide is completed. 

 The common explanation of the changes is given in the 

 following equations : 



(1) 2S0 2 + 3N0 2 + H 2 = NO + 2HO . SO 2 . NO 2 



(2) 2NO + O 2 =2NO 2 



(3) 4HO.SO 2 .NO 2 + 2H 2 O-j-O 2 =4HO.SO 2 .OH -f 4NO 2 

 or 



(1) NO + N0 2 ^N 2 3 



(2) 2S0 2 + N 2 3 + 2 + H 2 = 2HO.S0 2 .N0 2 



(3) 2HO . SO 2 . NO 2 + H 2 O = 2HO . SO 2 . OH + N 2 O 3 

 Steam is blown into the leaden chambers, so as to supply the 



v. 9 



