E, Sorel*, whose description of 1 lie La Tolfa. method of treat- 
ment of alunite is the most detailed on record, says; — 
The treatment of the alunite rock begins with a very moderate roasting, which 
by dehydrating the excess of the alumina, renders it insoluble. But it is 
necessary to be very careful not to push the temperature too fa r, for the sul- 
phate of aluminium would he partially decomposed and would set free a mix- 
ture of sulphurous and sulphuric anhydrides with oxygen. 
It is probable, when the proportion of sulphuric acid is greater than that 
which corresponds to the quantity of alum equivalent to the potash present, 
that there is formed, under the action of heat, an insoluble basic sulphate 
of aluminium, GA1 2 0 3 - 2S0 3 +. Under the action of a temperature sufficient 
to liberate the vapours of sulphuric anhydride, one would obtain potassium 
sulphate in excess, alum, and a still more basic sulphate of aluminium, 7 A 1 9 0 8 * 
5S0 3 . 
From the results obtained by t lie writer, it appeal’s that on 
heating’ alunite at temperatures up to 500° Centigrade, free potas- 
sium sulphate and a basic sulphate of aluminium are formed and 
not an anhydrous alum. This would also explain I he necessity for 
exposing the calcined mineral to air and moisture for many weeks 
for the production of potash alum after calcining’ at the above tem- 
peratures. 
SOI ° Centigrade. 
On heating’ the mineral to a temperature of 801° C. (melting 
point of common salt), the whole of the water and three-quarters 
of the sulphur trioxide is driven off, leaving a residue of potassium 
sulphate and alumina. 
One gram of the mineral heated in a gas muffle at the above 
temperature gave the following losses: — 
(i) (a) 
41.44 41.76 
The equation is — 
K 2 S0 4 • 3H0A1S0 4 • 3A1 (OH)„ — > K 2 S0 4 + 3A1 2 0 S + 3SO s + 6H 2 0. 
In this and previous quotations, Na,S0 4 replaces KJ30 4 to an extent 
proportional to the N a which substitutes K in the original mineral. 
The theoretical loss to satisfy this equation is 42.0 per cent. 
Tlie potassium sulphate formed is readily dissolved in water 
and the solution shows no tendency to produce a precipitate on 
warming. With a view to determining the conditions under which the 
whole of the potassium sulphate could be leached out of the calcin- 
ed mass, several lots of the alunite ore were calcined under similar 
conditions and leached with water for varying lengths of time, 
* La Grande Industrie Ohemique Mincrnle, p. 716. 
f 2 is probably a misprint in the original for 5. 
