CHAPTER VII. 

 LIME-BURNING. 



BEFORE taking up the discussion of manufacturing methods it will 

 be of value to consider briefly the chemical principles upon which these 

 methods are based and the classes of products which result. 



The burning of a pure non-magnesian limestone. An absolutely pure 

 limestone, free from both magnesia and other impurities, corresponds 

 in composition to calcium carbonate (CaC0 3 ). If a pure limestone 

 be heated to 800 C. or over, this carbonate is dissociated, the carbon 

 dioxide (CO2) being driven off as a gas, while the calcium oxide (CaO) 

 is left behind as a white solid, known as quicklime or caustic lime. This 

 decarbonation may be expressed in a formula as follows: 



Limestone (CaCOs)+ heat = CaO (lime) +C02 (carbon dioxide). 



As the original lime carbonate consisted of 56 parts by weight of 

 CaO to 44 parts of CC>2, the formula may be given a commercial quanti- 

 tative value, as below: 



100 Ibs. limestone + heat = 56 Ibs. quicklime + 44 Ibs. carbon dioxide. 



In the process of burning from limestone to quicklime, the material 

 has therefore lost 44 per cent in weight. It has also decreased in bulk, 

 but in a much smaller ratio, the decrease varying from 12 to 20 per cent. 

 Pure limestone has a specific gravity of 2.715, while the true specific 

 gravity of pure quicklime is 3.09 to 3.15, though much less in lumps. 



The dissociation of limestone on heating begins at a temperature 

 of about 750 C., but is usually not complete until 900 C. or there- 

 abouts is reached. As the expulsion of the carbon dioxide is hindered 

 by the presence of the gas itself dissociation is accomplished more rapidly 

 if either (a) the carbon-dioxide gas, as fast as it is formed, is removed 

 from the kiln by a pump, or (6) a jet of steam or water is introduced, 

 the effect being to form a mixture of steam and carbon dioxide, which 

 exerts less pressure than would the pure gas alone. In practice the 

 pumping method is rarely used, but the injection of water or steam 

 is quite a common practice. 



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