THE RECOVERY OF POTASH FROM ALUNITE. 11 
especially marked in the case of the samples containing large per- 
centages of silica, indicating that at high temperatures a fixation of 
the soluble potash takes place in the presence of this substance. 
The leached residues from the highly heated samples, 3b, 7, and 10, 
were, therefore, submitted for a microscopic examination by Mr. 
W. H. Fry, of this bureau, but owing to the amorphous nature of the 
material no compounds of silica and potash could be identified. 
It is possible also that there might be a loss of potash through 
volatilization, but analyses of the leached residues 2 and 5 showed 
there had been no such loss. 
In order to leach the highly heated samples free from sulphates 
only 75 c. c. to 115 c. c. of hot water were required, while the sam- 
ples heated to 550° to 650° C. required from 280 to 400 c. c. of 
water. The difficulty in extracting the sulphates in the latter case 
was due in part no doubt to the gelatinous nature of the residue 
and partly also to the greater quantities of basic sulphates present. 
The nitrates from these latter residues were quite cloudy on account 
of the separation of basic aluminum sulphate, and upon the addition 
of ammonia considerable alumina was precipitated. The filtrates from 
the highly heated samples, on the other hand, were only slightly cloudy 
and contained but very little alumina, showing that the decomposition 
of the double salt potassium aluminum sulphate was practically 
complete. 
The four samples of alunite (Nos. 1, 2, 5, and 6) which were heated 
from 750° to 775° C, a temperature only slightly above that at which 
complete decomposition of alunite takes place, 1 yielded up their 
potash quite readily, less than 100 c. c. of hot water being required 
to wash the residues free from sulphates. Moreover, the extraction 
of potash was more complete than from the same samples heated 
at either a higher or lower temperature. The filtrates were somewhat 
opalescent, but gave only a slight precipitate with ammonia, showing 
that the decomposition of the double salt potassium aluminum 
sulphate was almost complete. 
INFLUENCE OF FINE GRINDING ON THE EXTRACTION OF POTASH FROM 
ALUNITE. 
In factory practice material may be ground so that 80 to 90 per 
cent will pass a 60-mesh sieve at comparatively small cost, but the 
grinding of relatively hard rock to a greater degree of fineness than 
the above not only consumes considerably more time but also 
increases appreciably the cost of production. 
In the extraction of potash from alunite the question has been 
raised as to the degree of fineness to which the material should be 
i Circular 70, Bureau of Soils, U. S. Dept. Agr. (1912). 
