RECOVERY OF POTASH IN THE CEMENT INDUSTRY. 9 



this country be now taken in round numbers as 90,000,000 barrels l 

 and if the same average conditions be assumed to prevail in the few 

 plants from which no samples were received as was found on an 

 average for all other plants, then it may be estimated that the total 

 potash (K 2 0) escaping from all the cement plants in this country 

 as at present operated amounts to 86,850 tons annually. 



The process at present most generally considered in connection 

 with the recovery of the potash that escapes from cement kilns is 

 electrical precipitation. The extent to which the dust may be 

 recovered in this way is dependent, assuming proper installation, on 

 the voltage used and the linear velocity with which the gases pass 

 through the treater pipes. The volume of gases treated being con- 

 stant, the degree of precipitation of the dust will be dependent on 

 the extent of the installation. With present installations a recovery 

 of approximately 99 per cent 2 of the dust has been obtained. It 

 happens, however, that the finest portion of the dust escaping from 

 any cement plant contains relatively the highest percentage of potash, 

 and a 99 per cent recovery of the dust represents a somewhat less 

 efficient recovery of the potash. 



In the plant of the Security Cement and Lime Co., where the Cottrell 

 process of electrical precipitation has been installed, it is found that 

 under normal working conditions, and with a daily output of 2,500 

 barrels, the dust collected in the kiln stacks amounts to 16,000 pounds 

 and in the treaters 45,000 pounds every 24 hours. The former con- 

 tains on an average 4.5 per cent potash, and the latter, as analyzed in 

 this laboratory, 11.4 per cent, making a total of 5,850 pounds of 

 potash recovered daily. As shown in Table I, the potash lost from the 

 kilns during the same period amounts to 6,525 pounds. On this basis 

 of calculation the potash recovery in this plant amounts to 90 per cent 

 of the total. If this value be accepted as the efficiency of recovery of 

 the potash in the most economic installations, then the total recover- 

 able potash in the cement plants of this country under present work- 

 ing conditions amounts to 78,165 tons annually. 



The potassium compounds occurring in cement dust may be 

 divided into three groups, as follows: (1) Those which are readily 

 soluble in water; (2) those which are slowly soluble; and (3) those 

 which are insoluble. 



The insoluble potash represents the combinations occurring in the 

 original silicates of the raw mix carried over mechanically in the dust 

 before being subjected to a sufficiently high temperature to bring 

 about decomposition. The form of combination which is slowly 

 soluble in water is supposed to be due to a recombination of the vola- 



i Burchard, E. F. Loe. cit. 



2 Schmidt, W. A., Paper presented at the meeting of the American Institute of Mining Engineers, 

 Globe, Ariz., Sept. 21, 1916. 



103837°— 17— Bull. 572 2 



