THE CARBONATION PROCESS. 



of 100 Ibs. limestone of 95 per cent, purity, a theoretical minimum of 6 Ibs. 

 coke is required ; this corresponds to one volume coke to six volumes of lime- 

 stone. In practice, from four to five volumes of limestone are used with one 

 volume of coke, and some authorities allow three volumes. 



Coke gives twice the calorific value of average wood, and accordingly 12 

 Ibs. wood to 100 Ibs. limestone would be required. 



The following are the points to be observed to obtain a good result in 

 the kiln : 



1. The limestone should contain a minimum of silica and alumina. 



2. Washed coke, containing not more than 7 per cent, of ash, should 

 be used. 



3. The proportion of coke and limestone should be carefully regulated. 



4. The charges should be frequent and regular. 



5. The lime should be drawn successively from the doors in turn. 



6. Only two-thirds of the volume of the kiln should be occupied, 



7. The aspirating pump should work slowly and regularly. 



8. Air should not be allowed to suck back through the gas pipe. 



The objectionable constituents which occur in limestone are silica, alumina, 

 magnesia, and sulphate of lime. If either of the two former are present 

 during the calcination, fusible silicates and aluminates of lime and magnesia 

 are formed, giving rise to what is known as scaffolding in the kiln >., a 

 fused mass is formed, preventing the descent of the lime. In addition, their 

 presence prevents the lime slaking properly; furthermore, silica introduced 

 into the juice may dissolve in the presence of alkalies and be precipitated as 

 scale in the evaporators, besides causing difficulties in the filtration. Magnesia 

 ;md sulphate of lime also cause scale in the evaporators. 



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

 of limestone : 



Material. Bad. Passable. Excellent. 



Moisture .. .. 4-10 .. 6'25 .. 1-21 



Sand, clay, and insoluble matter.. 4'50 .. 3'17 .. 0*55 



Organic matter 1-20 .. 1-12 .. 0-41 



Soluble silica .. 2-10 .. 0'64 .. 0*20 



Oxides of iron and alumina . . . . 0*37 . . O'lo . . 0-23 



Calcium carbonate (limestone) . . 85'86 . . 87'93 . . 96'58 



Magnesium carbonate 0-95 . . O'o3 . . O'oO 



Soda and potash . . 0'05 . . . . 



Undetermined .. .. 0'87 .. 0'24 .. 0'32 



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