Improper size distribution of the grinding media in a multicompartment mill can be a source of many operation problems encountered. Inefficient use of the power expended for grinding, and an undesirable product size distribution are two frequent resultants.
The objective then is to provide a method which would be practical to use and accurate enough to create an operating data base. It should be noted that information on the form relating to actual wear rates, power consumption and mill efficiency are not necessary to the ball size distribution and make-up table. These are provided as indicators for excessive wear and increased power consumption. A steady rise in wear rate with a decrease in power consumption may indicate degradation of the ball charge. As the average ball size in the mill decreases the surface area of the ball charge (while maintaining the same volume) is increasing resulting in higher wear rates. At the same time, the cascading action of the the charge is decreasing with a drop in the mill power demand. With the power input to the mill decreasing, the product rate is usually headed in the same direction. To compound this problem, the kwh/ton requirement begins to rise due to the inefficient action of the ball charge.
Measurement of Ball Charge
An accurate measurement of the ball charge can be obtained easily and at frequent time intervals. The distance to the level ball charge from the inside top of the mill must be found, and the average diameter and length of the compartment known. A chart such as depicted in figure 1 can be made to relate depth to percent filling, or the weight of the ball charge. Subtraction of this weight from the previous known (or calculated) weight yields the loss in the compartment due to wear. Applying the percentages and the wear ratios of the different sized media results in the amount of media being transferred to the next smaller size.
Weight Loss Factors
The amount of the ball charge lost to the next smaller size is a function of the amount of material lost from the media to product. As wear takes place, the sizes of the individual pieces of media are diminishing. There is a definite limit to the largest and smallest size of media in the mill. The top size of the media is governed by the size used for make-up. The smallest size is limited by the opening in the compartment discharge screen.
The calculations were based on the volume change which occurs to the individual pieces of grinding media, not the aggregate charge. In this manner the numbers may be applied to media of various densities. Examination of the figures indicates a negligible change in the ratio of volume loss between varying sized media (i.e., a 50 mm (2″) diameter ball will lose approximately twice the percentage of its initial weight in the same time period compared to a 100 mm (4″) diameter ball). The-change in ratio is even less significant since the mills are not charged with one size of grinding balls. To further substantiate the use of the average loss ratio of the four cases selected, a loss of 4 mm (5/32″) in ball diameter is relative to a reduction of approximately 15% by weight of a nominal ball charge. It is a safe assumption that new media would be added to the charge within a range of 4.0% to 15.0% weight loss.
Mill Performance Summary
When grinding to a variety of product sizes, the specific power required also varies, (kWh/t). Calculation of the average product size produced will allow an adjustment to the specific power to a common or standard product size.
