BAUXITE 271 



precipitating; sewage or coloring; matter in water; and, in 

 general, for all purposes in which alum was formerly used. 



Potassium alum(K3SO,,Alo(SO,)3,24HoO), is known also 

 as potash alum and common alum. The manufacture of 

 alum from bauxite involves the preparation of a pure solu- 

 tion of aluminum sulphate to which is added the proper pro- 

 portion of an alkali sulphate in order to form the special alum 

 desired. Thus potassium sulphate is used to form potassium 

 alum and sodium sulphate for sodium alum. AH alums 

 crystallize from solutions perfectl}^, forming very pure crystals 

 even from impure solutions, and it is because of this property 

 that the alums are so extensively used in the arts. The chief 

 uses of common alum are as a mordant in dyeing ; in preparing 

 size in paper making; in tawing skins; in making pigment 

 lakes; for clarifying turbid liquors; for precipitating sewage 

 waters; and for hardening plaster of Paris casts and other 

 forms. For these uses, however, as mentioned above, alum- 

 inum sulphate is generally preferred because of its greater 

 strength and solubility. 



The manufacture of aluminum hydroxide from bauxite 

 is of great importance, because by a simple calcination it 

 yields aluminum oxide (alumina), w^hich is the chief crude 

 material used for the manufacture of the metal aluminum. 

 In making aluminum hydroxide, bauxite is roasted, pul- 

 verized, and mixed with calcined soda ash in the proportion 

 of 1 part of ALO3 to 1.1 parts of NaoO, or greater if silica 

 be present in the ore. The mixture is calcined at a white 

 heat for three or four hours until all traces of carbon dioxide 

 and w^ater have been expelled. The calcined product is 

 then ground and lixiviated with hot water ; this process yields 

 a solution of sodium aluminate from which the aluminum 

 hydroxide is precipitated by passing carbon dioxide gas 

 through it. The impurities, silica and iron oxide, remain 

 dissolved in the mother hquor. 



In order to avoid the costly and tedious chemical process 

 of obtaining pure aluminum hydroxide, an electric furnace 

 method has been recently patented by Mr. Charles M. Hall, 

 whereby the impurities in the bauxite, mainly iron oxide and 

 silica are removed, and the bauxite is thus purified for the 



