RECOVERY OF POTASH IN THE CEMENT INDUSTRY. 3 



effective. The presence of sulphur dioxide in the furnace when the 

 ignitions were made was found to have a retarding effect on the 

 evolution of the potash. 



Since potash occurs in the form of feldspar or other silicates in the 

 raw materials used in cement manufacture, it might be inferred from 

 the experiments made on the ignition of feldspar mixtures and of 

 cement materials that more or less complete volatilization of the 

 potash would take place in the burning of cement. 



It happens, however, that the clinkering zone of a rotary kiln 

 constitutes only a comparatively short portion of its length, and 

 while the time taken for the charge to pass through the kiln may 

 vary from about 1 J to 2\ hours, depending on the length of the kiln, 

 the time that the charge is subjected to a clinkering temperature 

 does not exceed one-half hour. The length of a rotary kiln must 

 also act disadvantageously on the escape of the potash. That some 

 potash is volatilized in cement burning has long been known, and 

 in the case of one plant a quantitative study of the amount lost in 

 this way has been made by R. K. Meade 1 from analyses of the raw 

 material fed into the kiln, the resulting clinker, and the coal used 

 for burning. The results of three separate tests showed that the 

 potash volatilized ranged from 46 to 52 per cent. 



POTASH IN RAW -MATERIALS AND LOSS THROUGH VOLATILIZATION 



IN CEMENT MANUFACTURE. 



With a view to securing more extended information in this direc- 

 tion, an investigation was undertaken about five years ago to 

 ascertain the percentage of potash occurring in the raw materials 

 used in the different cement plants of this country and also the per- 

 centage of the total potash that is volatilized in each case. Repre- 

 sentative samples of raw mix and ground clinker were collected from 

 the different cement plants in the United States and in Canada, with 

 a view to analyzing each sample for potash. With the data thus 

 obtained, and knowing the ratio between the raw mix and the cement 

 produced, and the output of the latter, it was thought possible to 

 calculate for each plant the approximate quantity of potash that 

 escapes daily from the kilns. After the work was partly completed it 

 had to be abandoned for a time. A few months ago the problem 

 was taken up again and entirely new samples were collected. The 

 work now has been completed, with the results given in Table I. 



This table shows that the potash occurring in the raw mix used by 

 the different cement plants in this country varies from 0.20 to 1.16 

 per cent. The raw materials used in the mills of the Universal Port- 

 land Cement Co. consisting of blast-furnace slag and a high-grade 

 limestone contain considerably less potash than the average for 

 the other cement plants of the country. 



i Chem. Engineer, 2, 221 (1905). 



