M. D. Schlesinger, S. Nazaruk, and Leslie Reggel
Pittsburgh Coal Research Center Bureau of Mines
Pittsburgh, Pennsylvania
I
Laboratory Cylinder-Mill
for Small Samples
M o s t laboratory workers have been confronted a t one time or another with the problem of preparing a small quantity of powdered solid. In the smallest ball mill jars, a gram or two of material is just about enough to coat the balls and wall, and little size reduction occurs after extended periods of rotation; also, recovery of the charge is difficult. This problem has been solved in our laboratory by the design of a small mill that can grind as little as a half gram of coal or other solids; and when opened, the entire volume is easily cleaned.
Figure 1.
Cylinder mill.
In principle, the mill is a rotating thin cylinder in which smaller cyclinders roll. The rolling cylinders are in contact, and a differential grinding action takes place between them as well as crushing between them and the periphery of the mill. Figure 1 shows the tempered Nitralloy cylinder halves. When they are bolted together, a 3-in. by 27/32-in.cavity, having a volume of less than 100 ml, is formed. The mill is rotated by the assembly shown in Figure 2. A small gear reduction motor1 drives a pulley set which rotates the mill arbor. Speed control between 0 and 120 rpm is provided by a 500 W bench type variable voltage transformer in the power line to the motor. For convenience of assembling and removal of the mill from the drive system, a handle is bolted to the cover plate. The extension on the other half of the mill fits into a hole in the base plate. The motor and arbor support block are mounted on a steel base plate. The entire unit can be placed inside a dry box. The sample (for example 0.5-10 g of 60-mesh coal) is placed in the right half of the mill; one t,o four BuBodine Company, Type V-IOR. Reference to specific brands or m&ers of equipment is made to facilitate understanding and does not imply endorsement by the Bureau of Mines.
546
/
rundumQ2 grinding cylinders are placed upright; and the halves are bolted together. The halves are always oriented toward each other in the same manner by alignment of the dowel index and hole. The assembled mill is snapped into the drive shaft, and rotation is started. The speed is slowly increased to a point where the cylinders roll smoothly without lifting and dropping. After grinding, the mill is removed from the drive shaft, placed in a horizontal position, and opened. The sample is removed easily; the interior can be cleaned with a camel's hair brush. At the end of a 4 hr grinding, all of a coal sample is usually finer than 325-mesh. Commercial Burundnm cylinders are approximately ' a / 1 6 in. X 1 3 / 1 e in. There is some small size variation in a particular lot, so some care should he taken in selecting the cylinders used. If too long, they will bind against the end plates and not roll freely. The undersize length is not critical, but should be a reasonably good fit so that the cylinders will not tilt and bind. In the design presented here, a clearance of in. is provided. Care should be taken in the machining to provide the radius corner on the internal circumferenceof the mill. This conforms approximately
Journal of Chemical Education
Figure 2.
Mill assembly.
to the curvature of the cylinder and eliminates a potential dead volume. Cylinders made of steel, brass, or other materials can be used. The unit described has been in continuous use for about 5 years and its popularity has prompted the construction of additional units. Engineering drawings are available from the authors. Acknowledgment is hereby made to the Bureau's Engineering Design staff a t Bruceton, Pennsylvania, who worked out the many small construction details. 1 U.S. Stoneware Company, Akron, Ohio. The selection of Borundum was based on its high density and hard ceramic snrface (9+ on the Mohs aesle).
