THE EECOVEKY OF POTASH FEOM 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, 35, 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 sihca and potash could be identified. 



It is possible also that there might be a loss of potash through 

 volatiUzation, 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 filtrates from these latter residues were quite cloudy on account 

 of the separation of basic aluminum sulphate, and upon the addition 

 of anmionia considerable alumina was precipitated. The filtrates from 

 the highly heated samples, on the other hand, were only sHghtly 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, ^ 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 ffitrates 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 fuieiiess to which the material should be 



1 circular 70, Bureau of Bolls, U. 8. Dept. Agr. (1912). 



