THE CARBONATION PROCESSES 



285 



have come into general use. These have developed from the cement kilns, 

 the design of which they closely follow. The kiln consists of a rotary 

 cylinder slightly inclined from the horizontal. The limestone and products 

 of combustion travel in the same direction, the lime being removed at that 

 end of the cylinder remote from the burners. 



Capacity of Lime Kilns. Very widely variant capacities are given in 

 standard works. Ware states that the Belgian kiln will readily afford 

 500 kilos burnt lime per day per cubic metre capacity. This reduces to I 4 

 Ibs. per hour per cu. ft. Geerligs, however, referring to practice in Java, 

 gives the capacity as 16 Ibs. lime per day per cu. ft., or only 0-66 Ibs. per 

 hour per cu. ft. 



The maximum quantity of lime used in any form of carbonation process 

 is 3 per cent, on cane, or 60 Ibs. per ton. 

 Following on which of the above two capa- 

 cities is selected as a basis of design, the 

 cubic contents of a kiln should be either 

 43 or 91 cu. ft. per ton-cane-hour. Four 

 factories in Java, of which the writer has 

 data, had 55, 66, 75 and 108 cu. ft. per 

 ton-cane-hour, or an average of 75 cubic 

 feet. 



Fuel required in Lime Kilns. For the 

 decomposition of 100 Ibs. of commercial 

 limestone of 95 per cent, purity 6 Ibs. of 

 gas coke are required. Generally, in 

 European beet practice 9 Ibs. of coke are 

 required, and under very good control 

 this may be reduced to 7-5 Ibs. Reduced 

 to volume measurements in actual work, 

 from 3 to 4-5 volumes of limestone are 

 used to i volume of coke. 



Action of the Lime Kiln. In the lime 

 kiln as usually operated, four zones are to 

 be recognised. The upper zone is occupied 

 entirely by the produced gases, and serves 

 as a regulating zone and reservoir, whence 

 the pump draws. Below this is the heating 

 and drying zone, where water is removed 

 from the materials, which are also raised 

 to the decomposition temperature. This temperature is of the order 1,000 

 C., and when the materials in their downward passage reach this temperature 

 the decomposition zone is reached. Here the temperature varies from 1,000 

 C. to 1,300 C., and below it is reached the fourth zone or zone of c.ooling 

 extending to the lowest part of the kiln. 



Of these zones, that devoted to cooling occupies about half the total 

 capacity of the kiln, the decomposition zone occupying one quarter, and the 

 heating and regulating zones one-eighth each. In operating a kiln a high 

 percentage of carbon dioxide in the gas is required, together with absence 

 of carbon monoxide, which should not exceed 0-5 per cent. The absolute 

 maximum of carbon dioxide is, with coke 10 per cent, on limestone, 38 to 39 

 per cent., and a percentage of not less than 30 per cent, is considered satis- 



FIG. 169 



