98 CEMENTS, LIMES, AND PLASTERS. 



GROUP B. Magnesian limes: Limes containing over 5 per cent (usually 30 

 per cent or over) of magnesia. These limes are all slower slaking and 

 cooler than the high-calcium limes of the preceding group, and they 

 appear to make a stronger mortar. They are, however, less plastic 

 or " smooth", and in consequence are disliked by workmen. As com- 

 mercially produced, they usually carry over 30 per cent of magnesia. 



Methods and Costs of Lime-burning. 



t 



Compared with the complicated processes employed in the manu- 

 facture of Portland cement, lime-making is a very simple industry, 

 the only distinctive operation requiring attention being the burning 

 of the limestone. In the present section the types of lime-kilns employed 

 at various localities will be considered, detailed descriptions of several 

 of the more important styles being given. A few brief notes on the 

 utilization of a hitherto practically unused by-product (carbon dioxide) 

 will then be given, after which the costs of lime-manufacture will be 

 considered. 



Heat requirements in lime-burning. In burning limestone to lim- 

 heat is required for three purposes: 



(a) Evaporating any water contained in the limestone. 



(6) Heating the limestone to its dissociation temperature. 



(c) Driving off carbon dioxide from the lime (and magnesium) 



carbonate. 



The water in the limestone, however, aids in the dissociation, so 

 that the first heat requirement may be neglected here. Heating the 

 limestone from the air temperature (say 60 F.) to its point of disso- 

 ciation (about 1300 F. for non-magnesian limestones), assuming that 

 within this range the specific heat of limestone is 0.22, would require, 

 for one ton of limestone 2000 X 0.22 X (1300 -60) = 545,600 B.T.U. For 

 a magnesian limestone, which loses its carbon dioxide at a lower tem- 

 perature, this amount would be considerably reduced. 



The heat used in the actual dissociation is known quite accurately. 

 One pound of lime carbonate (CaCOs) will require 784 B.T.U. for its 

 dissociation, while one pound of magnesium carbonate would require 

 only 381 B.T.U. Using, then, these data in connection with those 

 quoted in the preceding paragraph, the following tabulated statement 

 can be made concerning the total heat requirements in burning lime- 

 stones of different composition. In actual practice, of course, the fuel 

 consumption is always far in excess of these theoretical requirements. 



