Mance: Quarry Industry of Southern Indiana 143 



be calculated with all investment charges, including interest, 

 depreciation, wear and tear, and repairs, at about 8.455 cents per 

 ton in a plant handling 200 tons per day; and this figure will be 

 decreased with an increase of output and increased with a decrease 

 of output. The Sturtevant Mill Company is putting these 

 mills on the market and the above figures were taken from their 

 catalogs. 



The cost of screening and elevating the crushed stone is a 

 more difiicult proposition on which to make calculations, because 

 conditions differ so much in various plants. It is therefore almost 

 impossible to give anything more than the most approximate 

 calculations. Screens are named from the motion they have: 

 as, shaking screens or riddles, rotating screens or trommels, and 

 inclined separators. These do not need defining since the names 

 are self-explanatory. Of these the trommel is the type most 

 used in fertilizing plants, altho the Newaygo separator of the 

 Sturtevant Mill Company, which is an inclined separator, is 

 rapidly coming into use. The chief advantage of the latter lies 

 in the small amount of power consumed, the largest sizes taking 

 less than one horse-power. The vibration in this type of screen 

 is imparted to the wire cloth by a number of small hammers 

 while it is held taut. The inclination of the screen allows a coarse 

 wire to be used even when a fine product is desired. In the 

 trommel the material is fed in at one end and the coarse particles 

 pass out of the other end while the fine material passes thru the 

 sides. These screens are rotated and slightly inclined. The 

 greater the inclination the finer the screening that can be done. 

 They use but little power and are rotated from 16 to 20 times per 

 minute. Screening is improved by faster rotation and greater 

 slope; but the decrease in output of screened material under these 

 conditions is very rapid and the limit is soon reached. Plates 

 with slits are often recommended in place of wire cloth, and are 

 more lasting, but the percentage of openings is necessarily less, 

 and the screening is slower. The cost of screening will, when all 

 charges are taken into account, be about 1 cent per ton when 

 the output handled is 200 tons of hmestone per day to 20-mesh. 

 Another thing to be taken into account when deciding which 

 type of screen to purchase is the size of machine necessary to do 

 the work. The trommel must necessarily have a larger screening 

 surface than a vibrating screen, for only a small portion of its 

 screening area is in use at once while the entire surface of the 

 vibrating screen can be in operation at one time. The other 



