8 BULLETIISr 415; XJ. S. DEPARTMENT OF AGRICULTURE. 



potassixun sulphate and collecting it in some suitable manner. It is 

 claimed that the addition of carbon to the mineral during ignition 

 facihtates the volatiHzation of the potash, causing it to take place at 

 a much lower temperature than that specified above. The residue, 

 it is said, is suitable for the manufacture of metaUic aluminum. 



Potash is now being recovered as a by-product from blast-furnace 

 operations and in the manufacture of cement from potash-bearing 

 sihcates, lime being used to convert the potash into oxide, in which 

 form it is readily driven off. The residue then consists of a chnker 

 having the composition and properties of hydraulic cement,. a readily 

 salable product. 



While the volatilization process may be apphcable to the purer 

 grades of alunite, it seems likely that a fixation and consequent loss 

 of potash would ensue if this practice were appHed to alunite high 

 in silica. The addition of coal or coke to the charge, as. recom- 

 mended by the inventor of this process, would also tend to decrease 

 the value of the residue remaining in the furnace. 



In order to avoid the difiiculty of obtaining a complete extraction 

 of the potash from highly heated alunite, and also for the purpose of 

 obtaining alumina in a pure form suitable for the manufacture of 

 metaUic alxuninum, Cameron ^ has proposed a process, the first steps 

 of which are similar to the old method of making potash alum. 

 The mineral is first ignited at a relatively low temperature (500° to 

 550° C.) sufficient only to decompose it into alumina and the double 

 salt potassium aluminum sulphate. The latter compound, which is 

 much more soluble than potassium sulphate, is leached out with 

 water, and alum is crystaUized from the resulting solution. The alum 

 is then heated till it decomposes, yielding pure alumina and potassium 

 sulphate, which are separated by solution of the latter. The fumes 

 of sulphur dioxide and trioxide evolved during the decomposition of 

 the alum are conducted into a suspension of the alumina from the 

 first ignition, forming aluminum sulphate and sulphite. The salts 

 in solution are then filtered free from any insoluble residue and 

 either sold directly as precipitants or mordants, or they are evapo- 

 rated to dryness and ignited to produce more alumina. 



Another method of treating alunite has been the subject of a 

 pubhc-service patent appHcation by Mr. Paul J. Fox, of this bureau. 

 In this process the aim is to separate the potassium as sulphate and 

 leave the aluminum as fluoride in a condition suitable for recovery 

 by electrolysis. The sulphuric acid is not recovered. Alunite and 

 fluorspar or some other fluoride are powdered and intimately mixed 

 (or, better, are ground together) and heated at such a temperature 

 that the hydrofluoric acid (of the fluoride) is set free by the sulphuric 

 acid (or sulphur trioxide) yielded by the alunite on heating. The 



1 An application for a patent on this process has been filed by the inventor. 



