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The Need for Lime : How to Meet It. 



[Jan., 



the centre parts of the larger pieces of liniestone which have 

 not given up the whole of their carbon dioxide. Although it 

 is necessary to raise the stone to a high temperature in order 

 to complete the reaction, it is not always safe to reach the upper 

 limit given, for too much heat opens the door to several different 

 kinds of trouble. In the first place, a stone which contains 

 any appreciable percentage of impurities, such as silica, alumina 

 or iron, will be found partially to fuse and to combine with the 

 impurities in such a way that it is very slow to slake, and, even 

 if it w^ill slake, may be very granular and insoluble. Moreover, 

 even a pure calcium carbonate stone if over-burnt will yield a 

 granular hydrate, which is much less satisfactory than the 

 impalpable powder formed when water in the correct propor- 

 tions is added to a w^ell-burnt stone. It is an interesting fact 

 that, owing to the combination between the magnesium and 

 calcium carbonates in a dolomite breaking up below the 

 temperature at which the carbon dioxide is given off, the result- 

 ing hydrate is often of the very finest grain, provided of course, 

 that the dolomite is free from fusible impurities. 



Amount of Heat required. — Despite the claims of phenomenal 

 efficiency put forward by the proprietors of special types of kiln, 

 there is a certain minimum amount of heat which must 

 necessarily be supplied to burn a given weight of lime, and 

 below this minimum it is utterly impossible to go, no matter 

 how w^onderfully efficient the special kiln recommended may be. 

 In the first place, there is the heat required to drive off the 

 carbon dioxide from the stone; this amount is exactly known, 

 and amounts to 3,255,000 British Thermal Units (B.Th.U.) 

 per ton of lime, and represents the burning of about 2 cwt. of 

 good average coal. This factor is unalterable by skill in kiln 

 design, or care in operation. Secondly, there is a certain 

 amount of heat required to drive off any moisture which may 

 be present in the charge and to heat it up to the temperature 

 at w^hich the reaction takes place. It is true that some of this 

 heat (but never as much as 50 per cent.) may be recover- 

 able from the burnt lime as it is cooled in the lower part of the 

 kiln. Next, some heat is needed to warm the air used for 

 combustion of the fuel to the temperature of the reaction zone. 

 Some of this is supplied by the heat in the cooling lime, but 

 some also has to be provided by the fuel itself, and although 

 in the upper part of the kiln a fair proportion of this heat may 

 be recovered in warming the stone before it reaches the reaction 

 zone, much heat always escapes with the gases from the top of 

 the kiln. 



