286 CHAPTER XV 



factory. Accompanying this will be up to 2 per cent, of oxygen, the balance 

 being nitrogen. The excess of oxygen is necessary, since in the upward 

 passage of carbon dioxide and water through the incandescent coke some 

 decomposition into carbon monoxide occurs, which has afterwards to be 

 burnt to the dioxide in the upper portion of the kiln. The temperature 



at which calcium carbonate begins to dissociate is about 400° C, but the 

 reaction is a balanced one, definite positions of equilibrium determined by 

 the temperature and pressure obtaining. Accordingly, temperatures much 

 higher than the dissociation temperature must be maintained, and experience 

 has found that a temperature in the decomposition zone of 1,000 °-i, 200° C, 

 is economically proper. Higher temperatures are inadvisable, since at about 

 1,300° C. the carbon dioxide decomposes into carbon monoxide and oxygen, 

 which pass into the gas aspirate from the kiln. A second result of a too high 

 temperature results in the formation of " dead " lime, which requires an 

 abnormally long time to slake. This phenomenon is usually attributed to 

 the formation of a skin of fused silica on the lime, but it is more probably 

 due to the formation at high temperatures of an allotropic form of lime, 

 which only slowly passes into the normal form. 



Choice of Limestone. — The objectionable constituents which occur in 

 limestone are silica, alumina, magnesia and sulphate of lime. If either of 

 the two former is present during the calcination, fusible silicates and alumin- 

 ates of lime and magnesia are formed, giving rise to what is known as scaffold- 

 ing in the kiln — i.e., a fused mass is formed preventing the descent of the 

 lime. In addition, their presence maybe a cause of slow slaking of the burnt 

 lime. Silica also may dissolve in the juice and be precipitated both as 

 scale in the evaporators, besides causing filtration difficulties. Magnesia 

 and sulphate of lime are also likely to cause scale in the evaporators. 



Below are given analyses by Gallois and Dupont of different types of 

 limestone : — 



Material. 



Moisture 



Sand, clay, and insoluble matter 



Organic matter 



Soluble silica ... 



Oxides of iron and alumina 



Calcium carbonate (limestone) 



Magnesium carbonate 



Soda and potash 



Undetermined 



The inefficient working of a kiln may arise from the following points : — 



1. .Scaffolding, which may as already mentioned be caused by the 

 presence of siHca or alumina, and also by careless work in changing or in 

 mixing the limestone and fuel. 



2. Withdrawal of unburnt lime when too little fuel is used or when 

 combustion is too rapid. 



3. Presence of carbon monoxide, due to too little air being admitted for 

 complete combustion, or to too low a temperature in the kiln. 



4. Presence of air due to leaks in the masonry or to air sucking back, or 

 to working the pump too fast. 



The composition of the gas from the kilns varies within wide limits, the 



