68 CHEMICAL FERTILIZERS 



At first carbon dioxide is absorbed with the production 

 of calcium carbonate and calcium hydrogen sulphide, the 

 latter being subsequently decomposed by more carbon 

 dioxide, with the formation of more calcium carbonate and 

 hydrogen sulphide. The hydrogen sulphide is removed as 

 gas by the stream of nitrogen left from the kiln gas. The 

 equations which represent these parts of the process are 



(1) CavS + H 2 + C0 2 = CaC0 3 + Ca(HS) 2 



(2) Ca(HS) 2 + H 2 + C0 2 = CaCO 3 + 2H 2 S 

 During equation (i) no hydrogen sulphide is removed as 

 gas and the gas passing away is discharged. During 

 equation (2) the effluent gas contains a much higher 

 percentage of hydrogen sulphide than did the incoming 

 gas contain of carbon dioxide. If the incoming gas 

 contains 20 % of carbon dioxide, the effluent gas contains 

 over 30 % of hydrogen sulphide. The hydrogen sulphide 

 is sometimes burnt to sulphur and water in a special 

 furnace, and sometimes burnt completely to sulphur dioxide, 

 which is then passed through leaden chambers and con- 

 verted into sulphuric acid. The residual calcium carbonate 

 is run out into heaps, and allowed to dry by drainage and 

 natural wind action. It does not pay to spend any money 

 over this material, but, by merely erecting a few simple 

 sheds to keep off the rain and provide the necessary accom- 

 modation for the men employed, a fairly satisfactory form of 

 waste lime can be produced, to be subsequently sold for agri- 

 cultural purposes under the name of lime mud or Chance 

 mud. This material, as put on the rail in trucks and delivered 

 to the farmer, may contain about two-thirds of its weight 

 of calcium carbonate, the remainder being chiefly water, 

 together with small quantities of calcium sulphate and organic 

 matter, derived originally from the coal used in the black 

 ash process. The material so obtained has the disadvantage 

 that when it becomes thoroughly dry it blows about in the 

 wind. It is therefore best applied to the land in a condition 

 of partial dampness, from 20-30 % of water being a 

 suitable condition for handling. A larger amount of water 

 adds to the weight and makes the material too sticky, but 



