CRUSHING AND PULVERIZING MACHINERY. 



457 



"The No. 8 mill will take pieces up to 6 inches in diameter, but the 

 feeding device usually used is designed to take material up to 2^ inches 

 only, which is as large as required for cement purposes in this country. 



"In grinding, the balls in consequence of the peculiar arrangement 

 of the grinding-plates, on rotation of the drum not only fall on the plate 

 and roll forwards, but also fall over one another, whereby the mate- 

 rial is rapidly broken up and finely crushed. After being thus ground 

 for a sufficient time, the material falls through the apertures of the 

 grinding-plates onto the inner or fore sieves, from this onto the fine 

 sieves and through this finally, completely ground, into the lower part 

 of the dust casing, whence it is discharged through the outlet. The 

 material not yet sufficiently ground, and retained by the inner or the 

 fine sieve is carried back by the return scoops into the drum for further 

 grinding. This method of working gives rise to very little dust, as 

 the material being ground as soon as it is sufficiently finely crushed, 

 is immediately discharged automatically." 



A Krupp 5'X21' .ball mill, run on ^-inch clinker, gave a product 

 of 7914 Ibs. per hour. This product showed sieve results as follows: 



Tube mills. Unlike the ball mill, the tube mill has been an unquali- 

 fied success as a fine grinder. Several manufacturers have placed these 

 mills on the market, those most generally used being the Davidsen 

 (F. L. Smidth & Co.), the Gates (Allis-Chalmers Company), the Krupp, 

 and the Bonnot. The latter is rarely used in dry-process plants, but 

 is common in wet plants. 



The following description of the tube mill, published by the manu- 

 facturers of the Davidsen type, contains much of interest on the group 

 in general. 



"Davidsen tvJbe mill. The tube mill is simple, effective, and eco- 

 nomical in both operation and maintenance. It consists of a wrought 

 tube mounted as a shaft by the attachment of dome-shaped ends so 

 formed as to make shafts, which rest in bearings at both ends. A large 

 gear attached to the tube and a pinion attached to the pulley-shaft 

 make the actuating device. The tube is lined with stone or chilled 



