SOURCES AND PREPARATION OF MAGNESIA. 



155 



large quantity of fuel. In recent years, however, modern shaft cal- 

 ciners have been built and a soft lignite coal is used. When calcined^ 

 magnesite falls into powder and is apt to choke the lower or cooler 

 portion of the kiln, preventing the access of air and heated gases to 

 the upper portion. The shaft furnaces are constructed to overcome 

 this result. The quantity of fuel required is from 15 to 20 per cent 

 of the weight of magnesite, equivalent to a fuel consumption of 30 to 40 

 per cent on the weight of magnesia produced. In some cases the cal- 

 cining is done in a double-hearth reverberatory furnace, where the flame 

 is brought into direct contact with the freshly charged magnesite on 

 the upper hearth, the operation being completed on the lower hearth, 

 which is the hotter of the two." 



Composition of the product. The analyses given in Table 64 will 

 serve to show the composition of the burned product, which naturally 

 varies according to that of the magnesite from which it is made. 



TABLE 64. 

 ANALYSES OF CALCINED MAGNESITE ( = MAGNESIA). 



1, 2. Burned Hungarian magnesite. Iron Age, Jan. 15, 1903, pp. 20, 21. 

 3, 4, 5. ' Mineral Industry, vol. 10, p. 439. 



6. " Styrian (Austrian) magnesite. Proc. Inst. C. E., vol. 112, p. 381. 



7. " Grecian magnesite. Proc. Inst. C. E., vol. 112, p. 381. 



Use of magnesite for preparation of carbonic acid, etc. California 

 practice in the manufacture of carbonic acid from magnesite is de- 

 scribed as follows in a recent report : * 



" In the manufacture of carbonic-acid gas, the gas is extracted from 

 the magnesite by calcining and the remaining calcined material is sold 

 to the manufacturers of wood-pulp paper. The best English coke is 

 used for calcining the magnesite. From one short ton of magnesite, 

 after removing the gas, they obtain about 1200 Ibs. of residue, which 

 is partly calcined magnesite still carrying some 20 per cent of gas. In 

 the process about 500 Ibs. of gas is obtained when finally compressed 



* Mineral Resources of the U. S. for 1903, p. 1133. 1904. 



