Influencing the size of product
Ball mill that works on the principle of impact is an efficient technique of size reduction through grinding. The size reduction is an important factor for many industrial applications as this process is carried out to increase the surface area, to intimate mixing and to improve dissolution rate, solubility, binding strength and dispersion properties.
Therefore it is important to maintain the size of products there are many factors that influence the size of product, some of these factors are;
- Feed rate; with a high feed rate, less size reduction is effected since in this case the material is present in the mill for a shorter time.
- Properties of the feed material; with a hard material, a smaller size reduction is achieved.
Weight of balls; with a heavy charge of balls, there is production of fine product. One can increase the weight of the charge by increasing the number of balls or by using a ball material of high density. Optimum grinding conditions are obtained when the volume of the balls is equal to 50 percent that of the ball mill. So the variation in the weight of balls is done by using materials of different densities.
Speed of rotation of mill; at low speeds, the balls simply roll over one another and little grinding is obtained while at very high speeds, the balls are simply carried along the walls of the shells and little or no grinding takes place, so for an effective grinding, the ball mill should be operated at a speed that is optimum speed equal to 50 to 75 percent of the critical speed (the minimum speed at which centrifuging occurs is called the critical speed of the ball mill, and under these conditions, centrifugal force will be exactly balanced by the weight of the ball, little or no grinding takes place when mill is centrifuging, therefore to operate practically the mill, the operating speed must be less than the critical speed).
Level of the material in the mill; a low level of material in the ball mill reduce the power consumption. If the level of material is raised, the cushioning action increases and power is wasted by the production of undersized material in an excessive quantity.