154 CEMENTS, LIMES, AND PLASTERS. 



Effects of heating magnesite. If magnesite (MgCO 3 ) be strongly 

 heated, the effect (as with lime carbonate) is to drive off the carbon 

 dioxide (CO 2 ), leaving magnesia (MgO) as a white solid. A curious 

 and technologically important phenomenon connected with the tem- 

 perature employed is to be noted. If the calcination be carried on 

 quickly at a red heat, the magnesia resulting will have a specific gravity 

 of 3.00 to 3.07; while if the calcination is long continued or carried 

 on at a higher temperature the resulting magnesia will be much denser, 

 possessing a specific gravity of 3.61 to 3.80. 



The technologic importance of the two forms of magnesia lies in 

 the fact that the lightly burned magnesia will slake with water and 

 if then exposed to air will finally recarbonate and harden slowly, just 

 as lime does. The denser, higher-burned magnesia, however, will not 

 take up either water or carbon dioxide from the atmosphere. Another 

 difference of commercial interest lies in the fact that the light form 

 of magnesite possesses a certain amount of plasticity, so that it can 

 be molded into shape under heavy pressure, while the dense form of 

 magnesia is entirely devoid of plasticity. 



Methods of burning magnesite. For calcining magnesite at low 

 temperature, so as to obtain lightly burned magnesia, kilns closely similar 

 to ordinary lime-kilns are employed in California. The kilns in use at 

 one California magnesia-plant are built in the form of a frustum of a 

 cone, the broader part downwards. These kilns are about 19 feet in 

 height, 3 feet in diameter at the top, and 7 feet in diameter at the base. 

 Drawing-doors are placed at the base, while draft is obtained by suction, 

 air being drawn through a flue near the top of the kiln. These kilns are 

 charged with coke and magnesite mixed, in about the proportion of 

 300 Ibs. magnesite to 20 Ibs. of coke. The product is the light form 

 of magnesite, and is probably not entirely decarbonated. This fuel 

 consumption would amount to about 14 per cent on the weight of mag- 

 nesia produced. 



When the dead-burnt or heavy magnesia is required, the burning 

 must take place at much higher temperatures. This kind of magnesia 

 may be prepared in reverberatory furnaces, in cupolas lined with silicious 

 material, or in highly heated gas-kilns.* 



The practice in Greece is described t as follows : 



" At the Greek magnesite mines, until recently roughly built kilns 

 fired by wood were employed for catcining the ore, which required a, 



* Proc. Inst. Civil Engineers, vol. 112, p. 381. 1893. 

 t Engineering and Mining Journal, Feb. 28, 1903 



