EFFECT OF DRY HEAT. 
The commercial utilisation of alunite in the past has been 
based on the formation of potash alum through the dissociation 
of the mineral by heat and then by wetting the roasted product. 
The theory of the process needs verification and, in the opinion 
of the author, the effect of heating alunite is not to form a true 
alum, but a basic sulphate of alumina and alkalis. The following 
experiments were carried out with a view to ascertaining the effect 
of dry heat at varying temperatures: — 
100° Centigrade . 
The sample of alunite (D) used in this experiment was crushed 
to pass a 30 mesh screen, the major portion passing a 90 mesh. 
The mineral was dried in a water oven at 98° C. to remove hygro- 
scopic water, cooled, and then -weighed: It was then reheated for 
one hour and again weighed, the loss being equal to 0.01 per cent. 
It was again placed in the water oven for two hours and the weight 
again taken, showing a loss of only 0.002 per cent. It is evident, 
therefore,, that at the above temperature, the mineral is prac- 
tically unaffected. 
200° Centigrade . 
The mineral used in the previous experiment was heated to a 
temperature of 200°C and weighed; it was again heated for one 
hour at 200 °C, and again weighed. No loss at all was recorded. 
Alunite is, therefore,, unaffected at temperatures up to 200°C. 
300° Centigrade. 
A sample of alunite (D) containing 0.42 per cent, of hygro- 
scopic water was heated to a temperature of 300° C., and showed a 
loss of 0.49 per cent. 
The mineral is, therefore, unaffected at temperatures up to 
300° C. 
418° Centigrade. 
One gm. of a sample (D) containing 99 per cent, alunite, was 
floated in a gas muffle gradually to the temperature of the melting 
point of pure zinc (418°0.). To prevent any oxidation of the zinc 
indicators, the metal was enclosed in sealed combustion glass tubes. 
The heating was stopped as soon as the zinc melted, the charge 
cooled and weighed. The tests were repeated until a further loss 
was inappreciable. 
The successive losses in per cent, noted were:— 2.36 per cent., 
3.06 per cent.; 5.80 per cent.; 7.48 per cent.; 8.34 per cent. ^ 8.9.) 
per cent.; 9.29 per cent.; 9.42 per cent,; 9.63 per cent,; 9.67 per 
cent. e 
The total loss recorded was equivalent to 4 Vo mols. of water, 
if it was water only, and not a mixture ot ELO and bO s . 
