REQUISITES AND TREATMENT OF KILN FUELS. 519 



screen-frames wire cloth of such mesh as shall be shown by the grinda- 

 bility of the material to pass the proper capacity for pulverization in 

 the tube mill. No. 7 ball mill requires at the maximum 18 horse-power 

 and uses a floor space of 11 feet 4 inches by 15 feet 7 inches. Like the 

 tube mill, the ball mill is a slow-speed machine, making 22 revolutions 

 per minute. 



"The parts most subjected to wear are the grinding-plates, the side 

 linings, the gear, and the pinion. Depending entirely upon the char- 

 acter of the material, the grinding-plates should give a wear of from 

 six to eighteen months. The side linings do not wear so rapidly. As 

 the gear and pinion are operated at slow speed the wear on these is 

 relatively small. Gears are seldom replaced short of one or two years 

 and pinions have a life of from six to twelve months. Of course gears 

 and pinions wear in proportion to the attention given to lubrication 

 and protection from dust." 



Power and output in coal-grinding. There is probably greater 

 diversity in coal-grinding practice than in the grinding of either raw 

 material or clinker. Grinding-machines of many different types are 

 in use; the coal reaches the plant in various sizes from slack to lump, 

 and is ground to different finenesses. All this makes it difficult to esti- 

 mate closely on the power requirements and output of the coal-grind- 

 ing mill, but the following data may be of use in this connection. 



A Williams mill employed at an Illinois cement-plant, working on 

 Illinois coal from the dryer and preparing it for the tube mills, ground 

 six tons of coal per hour to the following fineness. 



Mesh of sieve 20 50 100 200 



Per cent residue 6.9 43.3 76.2 87.3 



Per cent passing 93.1 56.7 23.8 12.7 



If the results of this test be compared with those given on p. 

 for the same mill working on raw materials, it will be seen that coal 

 is very readily crushed to 20-mesh, and quite easily to 50-mesh. But 

 the percentages of coal passing the 100-mesh and 200-mesh sieves 

 respectively are very much lower than the percentages of raw mix 

 passing the same sieves. 



A Griffin mill grinding coal from rolls or small crushers will reduce 

 about two tons per hour to a fineness of 95 per cent through a 100- 

 mesh screen, taking 25 to 30 H.P. in doing so. 



Slack coal for three kilns is ground at one plant in a single No. 16 

 Davidsen tube mill, the product being about three tons per hour. 



The above data show a considerable variation in the power required 



