16 BULLETIN 572, U. S. DEPARTMENT OF AGRICULTURE. 



then the total dust escaping from the kilns of this plant amounts 

 to about 4 per cent of the charge fed into the kilns. If it be assumed 

 that the dust thus collected has undergone about 50 per cent calci- 

 nation, then the true loss of raw mix in the form of dust will amount 

 to 5 per cent, which is recognized to be a fair average for the plants 

 of this country using the dry process. In calculating the values 

 given in the last column of Table I, the dust lost from the kilns in 

 plants using this process was accordingly taken as 4 per cent. 



On this basis of a 4 per cent loss as dust, the potash contained 

 in the dust from the Security plant should amount to 10.9 per cent. 

 It happens, however, that 27 per cent of the dust, containing about 

 4.5 per cent of potash, is collected in the flue stacks which connect 

 the kilns with the treaters, and, therefore, the remaining dust, 

 assuming a 90 per cent precipitation of the potash, should contain 

 11.1 per cent of potash. A sample of this dust analyzed in this 

 laboratory was found to contain 11.4 per cent of potash. 



The quantity of dust lost from plants using the wet process is 

 dependent on a number of factors, such as the dimensions of the 

 kiln, the amount of water in the slurry, and the plasticity of the 

 raw mix. Estimates which have been given the writers vary be- 

 tween rather wide limits. On the basis of the most reliable infor- 

 mation obtained, it is probable that a loss as dust amounting to 

 2 per cent of the raw mix is a fair average. This value was there- 

 fore used in estimating the percentage of potash in the dust from 

 wet-process plants. The actual potash content of the dust escaping 

 from any plant will be greater or less than the values given according as 

 the true percentage of dust lost is less or greater than the value cited. 



As shown in experiments which are to be described in detail in a 

 subsequent publication, the usual constituents of the water extract 

 of freshly collected flue dust are sulphates, chlorides, sulphides, and 

 polysulphides of potash, soda, and lime. Chlorides occur only in 

 small amounts. The percentage of sulphides and polysulphides in 

 the dust from different plants may vary greatly, and in some cases 

 they may be entirely absent. Free lime is found in all cement dust, 

 and in the water extract of all samples analyzed the basic radicals 

 were found to be in excess. It follows, therefore, that the potash 

 in the water extract of freshly collected flue dust will be present to 

 a greater or less extent in the form of the hydroxide. As the dust 

 becomes carbonated either by chemical treatment or by exposure to 

 the air, the hydroxides present will change into carbonates, and the 

 water-soluble potash then will be obtained principally in the form of 

 the sulphate, which is the form universally recognized as of most 

 value for use as a fertilizer. 



The flue dust as collected by electrical precipitation may be (1) 

 used directly as a source of cement material; (2) disposed of as col- 

 lected for use as a fertilizer or in fertilizer manufacture; or (3) leached 



