PREPARING THE MIXTURE FOR THE KILN. 417 



it is difficult to secure the material. In a minor sense location is still 

 an important factor, for marl deposits necessarily and invariably are 

 found in depressions; and the mill must, therefore, just as necessarily 

 be located at a higher level than its source of raw material, which involves 

 increased expense in transporting the raw material to the mill. 



Glacial clays, which are usually employed in connection with marl, 

 commonly carry a much larger proportion of sand and pebbles than 

 do the sedimentary clays of more southern regions. 



The effect of the water carried by the marl has been noted in an 

 earlier paper. The material as excavated will consist approximately 

 of equal weights of lime carbonate and of water. This on the face of 

 it would seem to be bad enough as a business proposition; but we 

 find that in practice more water is often added to permit the marl to 

 be pumped up to the mill. 



On the arrival of the raw materials at the mill the clay is often dried, 

 in order to simplify the calculation of the mixture. The reduction of 

 the clay is commonly accomplished in a disintegrator or in edge-runner 

 mills, after which the material is further reduced in a pug-mill, suffi- 

 cient water being here added to enable it to be pumped readily. It is 

 then ready for mixture with the marl, which at some point in its course 

 has been screened to remove stones, wood, etc., so far as possible. The 

 slurry is further ground in pug-mills or wet-grinding mills of the disk 

 type, while the final reduction takes place commonly in wet-tube mills. 

 The slurry, now containing 30 to 40 per cent of solid matter and 70 

 to 60 per cent of water, is pumped into storage-tanks, where it is kept 

 in constant agitation to avoid settling. Analyses of the slurry are 

 taken at this point, and the mixture in the tanks is corrected if found 

 to be of unsatisfactory composition. After standardizing, the slurry 

 is pumped into the rotary kilns. Owing to the large percentage of 

 water contained in the slurry the fuel consumption per barrel of finished 

 cement is 30 to 50 per cent greater and the output of each kiln corre- 

 spondingly less than in the case of a dry mixture. This point will, how- 

 ever, be further discussed in a later chapter. 



At a plant working a rather stiff slurry carrying only 40 per 

 cent of water two Bonnot tube mills, using 25 H.P. each, handled 

 together 4000 cubic feet of slurry in twenty-four hours, equivalent 

 to a production of 300 or more barrels of cement per day. This is 

 equivalent to a power consumption of a little less than 4 H.P. hours per 

 barrel of finished cement. The clay contained in the slurry had been 

 passed through a dry-pan and the slurry was then mixed and ground 

 to some extent in a pug-mill. 



