8 BULLETIN 415, U. S. DEPARTMENT OF AGRICULTURE. 
potassium sulphate and collecting it in some suitable manner. It is 
claimed that the addition of carbon to the mineral during ignition 
facilitates the volatilization 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 metallic aluminum. 
Potash is now being recovered as a by-product from blast-furnace 
operations and in the manufacture of cement from potash-bearing 
silicates, lime being used to convert the potash into oxide, in which 
form it is readily driven off. The residue then consists of a clinker 
having the composition and properties of hydraulic cement, a readily 
salable product. 
While the volatilization process may be applicable to the purer 
grades of alunite, it seems likely that a fixation and consequent loss 
of potash would ensue if this practice were applied 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 difficulty 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 
metallic aluminum, Cameron x 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 alum in a 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 crystallized 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 
public-service patent application 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 
arid (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. 
