

LIME-BURNING. 97 



The burning of a magnesian limestone. If the limestone, though 

 otherwise pure, contains magnesium carbonate, the effects produced 

 by burning will vary somewhat from those discussed above. Suppose, 

 for example, that a limestone consisting of 60 per cent lime carbonate 

 +40 per cent magnesium carbonate be burned until dissociated. The 

 original limestone consisted of 



60 Ibs. CaC0 3 ( = CaO + C0 2 )+40 Ibs. MgCO 3 (MgO + CO 2 ). 



While lime carbonate is made up of 56 per cent CaO plus 44 per cent 

 C0 2 , magnesium carbonate contains 47.6 per cent MgO plus 52.4 per 

 cent CO 2 . The bulk composition of the original limestone may, 

 therefore, be expressed quantitatively as follows: 



60 Ibs. lime carbonate - 



40 " magnesium carbonate = .......... { 2o'96 " CO 



The original rock, therefore, carries in 100 Ibs. 33.60 Ibs. of lime (CaO), 

 19.04 Ibs. of magnesia (MgO), and 47.36 Ibs. of carbon dioxide (C0 2 ). 



If a rock of this composition be burned, the carbon dioxide will be 

 driven off, as in the case of a pure limestone, but the solid mass remain- 

 ing will consist partly of lime (CaO) and partly of magnesia (MgO). 

 In addition to this difference, a difference in loss of weight is to be noted. 

 In discussing the burning of a pure non-magnesian limestone it was 

 stated that the driving off of the carbon dioxide meant the loss of 44 

 per cent in weight. In the case of the particular magnesian limestone 

 here discussed it can be seen that the expulsion of the carbon dioxide 

 is equivalent to a loss of 47.36 per cent in weight. 



Dissociation, in the case of a magnesian limestone, appears to be 

 effected at a somewhat lower temperature * than when a non-mag- 

 nesian limestone is burned, but no accurate data on this point are avail- 

 able. 



Classification of limes. For commercial purposes limes carrying less 

 than 5 per cent of magnesia can be marketed as pure or high-calcium 

 limes; but those containing over 5 per cent differ so markedly in their 

 properties that it is necessary to class them separately. The groups 

 are, therefore, as follows: 



GROUP A. High-calcium limes: Limes containing less than 5 per cent of 

 magnesia. The limes of this group differ among themselves according 

 to the amount of silica, alumina, iron, etc., contained. A lime carrying 

 less than 5 per cent of such impurities is a "fat" or "rich" lime, as 

 distinguished from the more impure "lean" or "poor" limes. 



* Probably about 600-700 C. 



