AN ALTERNATIVE APPROACH TO CONTROLLING A MILL WITH AN INDEPENDENTLY DRIVEN PRESSURE FEEDER

ACTIVE mill torque control has been used on Australian milling units since the 1970s. A typical mill installation in the 1970s consisted of a mill geared to a pressure feeder so that the pressure feeder to mill ratio was constant and the feed chute was fitted with an adjustable flap to control mill torque. Since the 1980s, more than 30 milling units have had independent pressure feeder drives installed. While milling units with fixed pressure feeder to mill ratios need to control mill speed and mill torque, mills with independently driven pressure feeders need to control pressure feeder speed and pressure feeder torque in addition to mill speed and mill torque. The most common control system for mills of this type uses mill speed to control chute level, pressure feeder speed to control mill torque and the feed chute flap to control pressure feeder torque. While this system works well in many installations, some installations have problems due to interactions between chute level and mill torque control loops. An alternative control system is described which eliminates the interactions between the chute level and mill torque control loops. This control system uses pressure feeder speed to control chute level and mill speed to control mill torque. The system has been successfully used at Harwood factory on all milling units for several years and was introduced in 2003 on the final mill at Rocky Point factory. It is advisable, when using the alternative control system, to cater for the special situations where there is a gap in the feed to the mill and when the mill starts under fully loaded conditions.