10 



BULLETIN 572, TJ. S. DEPARTMENT OF AGRICULTURE. 



tilized potash with the silicates in the dust. In some plants where 

 coal is used for burning the extent to which the potash occurs in this 

 "recombined " form may be considerable, while in certain other nlants 

 where oil is used for fuel this combination of the potash occurs in com- 

 paratively small amount. In Table II the percentages of the 

 different forms of potash occurring in a dust from a plant which uses 

 oil for fuel and also from one which uses coal are given: 



Table II. — Percentages of the different combinations of potash in flue-dust samples 



from different sources. 



Combination. 



Cement dust from- 



Riverside 



Portland 



Cement 



Co. 



Security 



Cement 



and Lime 



Co. 



Aeid-insoluble potash, K 2 



Slowly soluble potash 



Water-soluble potash 



Total 



Per cent. 



0.2 



.7 



9.8 



Per cent. 

 0.6 

 4.1 

 6.8 



10.7 



11.4 



The determinations of the soluble potash in these samples were 

 made by boiling 10 grams of the sample in 500 cubic centimeters of 

 water for one-half hour as directed in the Methods of the Associa- 

 tion of Official Agricultural Chemists. The insoluble potash was 

 assumed to be that portion of the total potash which remained 

 insoluble after boiling in a 5 per cent solution of hydrochloric acid. 

 The difference between the total potash and the sum of the soluble 

 and insoluble portions was then taken as the proportion of the 

 potash which is slowly soluble in water. 



In a previous publication it was shown, as already stated, that the 

 greater part of the potash in feldspar may be made to pass into solu- 

 tion by digesting with lime under a steam pressure of 10 to 15 atmos- 

 pheres. In cement dust as it escapes from the kilns the slowly soluble 

 and the insoluble potash already are associated with a considerable 

 percentage of free lime and consequently it was thought that the 

 greater part of these constituents might be recovered in soluble form 

 by digesting the dust with steam alone under pressure. This has been 

 found to be the case, and with the Security cement dust it has been 

 found possible to recover in this way in soluble form and with little 

 increase in pressure upward of 95 per cent of the total potash present. 

 If it be assumed, therefore, that it would be practicable to recover, 

 or render available, say, 95 per cent x of the potash in the recover- 

 able dust of cement plants, then the recoverable and available 

 potash escaping from the cement plants of this country amounts 



i In the case of coal-fired plants it may not be possible, with present commercial apparatus, to render 

 soluble as much as 95 per cent of the collected potash, but there is no doubt that as details of commer- 

 cial apparatus are perfected this value will represent a fair average for the different plants of the country. 



