During the design of gold grinding circuit the project metallurgist is focused on how to achieve and maintain the most efficient circuit operation with minimal expense of grinding time and media war. One simple way is to relate the pressure developed between the tumbling mill and bearings. Basically, metallurgists well informed of grinding process and their parameters, agree that optimum performance is influenced on an accurate control of slurry density and ore to media ration inside the SAG mill or Ball mill. Slurry density is properly controlled either by determining percent solids and regulation the addition of water and the other possibility is to relate the water to the ore delivered to the crushing circuit. These conditions are important and gold mills have a better of their operating factors.
It is important to indicate that several procedures of grinding circuit control have been studied for several years. Some of these are based on determining the circulating load, mill power draw, percent solids measurement and some combinations. The application of any method has been limited because there is an inherent inability to consistently and exactly identify changes in the ball mill loadings. So far, the bearing pressure measurement method has been developed and employed with successful due to the method is highly sensitive to changes in ball mill inventory. Essentially, the procedure considers that SAG mill or ball mill loading is obtained by measuring the pressure developed in the bearing. At this point, the mechanical and hydraulic pressures are the fundamentals of the method.
In this way, the information obtained from grinding circuits help to identify and define the influence of ore feed variability, slurry density, retention time, bearing temperature, grinding target and type of grinding media on the bearing pressure during optimized grinding circuit control. Essentially, the bearing pressure control may perfectly maintain a mill inventory independent of variations in ore grindability or circulating loads. The method is successfully applied to SAG mills due to they are equipped with a grate, which has an influence on the ore discharge rate. Grinding processes that employ bearing pressure control needs periodic updating of the desired mill inventory level to compensate for inherent wear of the grinding media. The frequency of adjustment is related to the rate of media wear and changes with the mineralogy of the material sent to the plant. It has been noted that the presence of high content of clay, could favor the formation of sticky slurry inside the SAG mill. This condition may be critical in some circumstances.