The Giesecke mill
The Giesecke mill, a combination ball and tube mill was tried on the Rand and at the time it was predicted by enthusiasts that stamps were doomed to extinction, but new stamps have been erected and not ball mills. This mill may be described as a modified tube mill some 24 ft. long divided into two sections (separated by a screen) of approximately one-fourth and threefourths of its length, the smaller section being 7^ and the longer 6 ft. in diameter. The mill revolved at the rate of 25 r.p.m. and the grinding was done by steel balls Y in. in diameter in the smaller section and 2^ to 4 in. in the longer section. While the mill showed great capacity and the first cost of the mill was comparatively low, yet the cost of operating and metallurgical considerations proved that the stamps in use in South Africa had all the advantage of the ball mill. Mr. H. Stadler says, in regard to the results of the investigations of the Mines Trial Committee of South Africa, that "it appears that the merits of the three methods of crushing, single stamp, stamps and tube mills combined, and single tube mill grinding (Giesecke) are so close, as regards the mechanical reduction efficiency, that the final decision in each case depends upon other considerations, such as metallurgical requirements, nature of the ore, local conditions, etc. In any case, the investigation clearly proved that nothing could be gained by the promotion of the tube mill from auxiliary to primary grinders."
Inventors and manufacturers are now trying every variation of shape and every means of screening to attain capacity with the least expenditure of power, so it is possible that from this mass of new trials a machine may be evolved that will be able to stand the test of time. Hardinge has adopted the principle of the sizing action of a revolving cone; Abbe, Fig. 2, places a screen at the discharge end, returning the oversize; Herman, Fig. 3, uses a screen on the periphery of the cylinder; Chalmers and Williams have an open screen on the discharge end of their ball mills, Fig. 4; while the Marcy ball mill, Fig. 5, has an adjustable quick discharge somewhat similar to that used on the Chalmers and Williams tube mill.
The Herman mill is particularly suited for dry grinding to a fineness that will readily pass through a screen. The size of the product is determined by the mesh of the screen covering the slots on the periphery of the cylinder. A mill of this sort will naturally give a product with but little undersize.
The "Compeb" mill, Fig. 6, lately brought out by the Allis- Chalmers Mfg. Co., is a combination mill having two compartments separated by a grating. The mill is made in different sizes for both wet and dry grinding. The size most used for dry grinding is 7 ft. in diameter and 22 ft. long, with a rolled-steel shell and cast-steel heads. The lining is chilled cast iron about 2J in. thick in the preliminary chamber and 1J^ in. thick in the finishing chamber. Forged-steel balls 5 in. in diameter and smaller are used in the preliminary chamber and %-in. diameter special-alloy iron balls in the finishing chamber. The manufacturers have recently been recommending for use in the finishing chamber their special patented shape, the "Concavex." The concavex, as its name implies, has both concave and convex surfaces, and preferably consists of a sphere with two opposite sides spherically recessed, the radii of the recessed surfaces being approximately equal to the radius of the sphere, the object being to attain a mortar and pestle effect.
The charge of balls in the preliminary chamber is maintained by the addition of 5-in. balls. Where concavex are used in the finishing chamber they are generally 1J^ in. in diameter. This special shape has the advantage that, when it wears, the concave surfaces tend to flatten slightly so that these surfaces, on concavex which have been reduced in size through wear, fit the convex surface of the full-size concavex.
The ball wear in the preliminary chamber is approximately 75 Ib. per 250 tons of cement clinker. The wear of the grinding medium in the finishing chamber is about two-thirds of this amount.
The mill is operated by a direct-connected motor through herringbone gears or plain-cut teeth-spur gears, also by belt drive through cast teeth-spur gears. The fineness of discharge varies with size and quantity of material fed to the mill, but averages 97 per cent, through 100-mesh screen and 81 per cent, through 200-mesh screen.
The mill carries a charge of 40 tons of balls and requires 375 hp. to drive it. Its capacity for grinding cement clinker is stated to be 70 bbl. per hour, or say 330 tons a day.
