Background to optimal volumetric filling
Experienced SAG mill operators are familiar with the concept that an optimal volumetric mill filling exists, which maximizes the mill throughput rate of a set feed material. The grind curve work of Powell and Mainza (2006) and Powell et al (2009) demonstrated that the optimal volumetric mill filling for maximum federate does not necessarily align with maximum power draw, especially for high aspect SAG mills. It follows that SAG mill process control systems that target maximum SAG motor power will not necessarily result in the maximum milling rate. Instead, SAG mill process control systems that aim to maximize SAG mill federate should target operation at the optimal volumetric mill filling for the prevailing feed.
Despite the importance of volumetric mill filling, a measurement is not traditionally provided to the mill operator or metallurgist by instrumentation (Toor, 2015). However, an inferred measurement can be made using a SAG mill weight measurement, using either bearing pressure transducers or load cells. Details of various SAG process control systems have been published extensively in literature (Karageorgos, 2006; Baas, 2014; Ruel, 2013; Smith et al, 2004). A common approach across these systems is to provide the mill operator with a SAG weight setpoint. Mill feedrate and mill speed for mills equipped with variable speed drives are manipulated by the control system to control mill weight to this setpoint. As mill feed competency and feed size changes, the optimal volumetric mill filling for maximum feedrate changes. The weight setpoint is adjusted by either an experienced operator or an advanced process control system in response to the feed changes.
The SAG mill weight measurement combines mill shell mass, charge mass and liner mass. Powell (2006; 2009), via a series of mill charge inspections at various operating mill weights, confirmed that strong relationships can be developed between volumetric mill filling and mill weight. These relationships will only hold in the short term if ball charge is constant with time (changing charge density) and will not hold over the longer term due to wearing of mill liners (wearing liner mass) (Toor, 2015). Consequently to maintain volumetric filling within a set operating range, continual review of the weight setpoint is required due to changes in SAG mill charge density (rock:ball ratio) and the wearing of mill liners. As examples, Figure 5 presents the characteristic reduction of SAG mill weight setpoint across the mill shell liners lives of the Telfer Gold Mine Train 2 SAG mill and the Lihir Gold Mine FGO SAG mill.