RECOVERY OF POTASH IN THE CEMENT INDUSTRY. 17 



with a view to preparing concentrated potash salts. The value of the 

 dust as a source of cement material is estimated usually at about 75 

 cents per ton. The value of potash in the form of sulphate depends 

 on the concentration and varies at normal wholesale prices from about 

 70 cents per unit for low-grade material to about 90 cents per unit in 

 material containing a minimum of 48 per cent potassium sul- 

 phate, or to 95 cents in material containing a minimum of 90 per 

 cent potassium sulphate. Thus it is seen that a comparatively low 

 percentage of potash in flue dust is sufficient to give it more value 

 as a source of potash than as cement material. By separating and 

 concentrating the potash occurring in flue dust the price thereof per 

 unit is increased, the cost of transportation is reduced, the residue 

 becomes available for cement manufacture, and if the process of 

 separation involves some such treatment as steaming under pressure, 

 the percentage of available potash in the dust of some plants at least 

 may be increased considerably. Against these advantages incident 

 to the separation of the dust must be balanced the relative cost of 

 the operation, which will be dependent on the percentage of potash 

 in the dust. The best method of disposing of the dust is thus de- 

 pendent on a number of factors which will vary with conditions, and 

 it thus happens that all of the three procedures referred to for utiliz- 

 ing the dust are now being given practical application in plants 

 where the dust is being collected by electrical precipitation. 



The work so far done on the recovery of potash from cement plants 

 apparently has been limited to plants using the dry process. No 

 installation for the electrical recovery of the dust has yet been made 

 in plants using the wet process, but it is probable that the electrical 

 method would prove just as applicable to plants of this kind as to 

 those using the dry process. At the temperature at which the gases 

 leave the kilns no precipitation of water would take place, and as long 

 as the temperature of the gases is maintained above 100° while pass- 

 ing through the treater pipes little condensation of moisture should 

 occur. The dust in all probability would be precipitated either in a 

 dry state or as a mud or slurry. If precipitated in the form of a 

 slurry, no electrode hammering system would then be necessary, for 

 the precipitated material would flow down the pipes continuously 

 and a concentrated solution of salts would thus be obtained directly 

 without the necessity of any special treatment. 



RATIO OF POTASH TO SODA IN CEMENT DUST. 



The extent to which the potash salts in flue dust may be concen- 

 trated conveniently is dependent in a large measure on the proportion 

 of sodium salts also present in the dust. It was, therefore, thought 

 advisable to determine the proportion to be expected for a number of 

 plants with a view to gaining some idea as to the quantity of soda that 

 might be contained in the dust. The results are given in Table V. 



