Optimization of mill performance by using online ball and pulp measurements

Generally, crash stops are performed to obtain measurement of charge filling and slurry loading. The crash stop involves running the mill under steady state then cutting off all feed streams to the mill as it is being stopped. Sufficient time is required to obtain all the important measurements correctly during the crash stop. The mill filling should be measured at a minimum of 3 points along the mill. Excess of slurry could also be estimated. To get rid of pulp and rocks in the charge, a mill grind-out (no ore feed) of 10 to 20 minutes is also performed before mill inspection or relining. The complete grind-out is required to obtain the accurate ball load measurement or the percentage by volume of balls in the mill. This is usually performed soon after a crash stop. The basic principle is to measure the height ‘H’ from the charge to the shell and the internal mill diameter ‘Di’. By calculating the ratio ‘H/Di’ and using the graph below (Figure 1), the charge filling degree in volume could be estimated. The number of visible plates on the shell liner could also be counted and an angle could be calculated (α) but the accuracy of this method is far lower than the measurement of the vertical height from the charge to the roof. Although crash stops and grind-outs provide important information, they have several disadvantages: