Cement and Energy

The mill load level is calculated by using factors and comparing various signals, including the total fresh feed, the recirculating !low rate, the mill sound, and the discharge bucket elevator power as well as the mill motor rower. The factors that are taken into account for determination or mill load level are shown in Figure 1. FRESH FEED WEIGHFEEDERS Figure I Facwrs.for detennination. of mil/load level Operation of closed circuit ball mill Normally during the operation of the mill , mill sound level measured by microphone and mill output measured by bucket elevator power and/or returns flow meter are the predom inant control signals. These signals determine the conditions in the grinding circuit and control the fresh Iced to bring the mill power into the optimal power range. As the mill power approaches and enters the ortimal power range, the mill kW signal becomes the predominant measurement and as the sound and bucket elevator power become mor~ stable, their effect on the operation of the mill reduces. For each type or product, there is an optimum mill power level. When the mi ll is operating in the range of optimum power level , the mill motor power determines the operating set point and then mill load control strategy controls the rate of the fresh feed to satisfy the set point. The strategy continuously scans the conventional sensors (sound, bucket elevator power, etc) to aid in establishing the mill load level and the circulating load and adjusts the set point as necessary to accommodate changes in grinding media charge, feed material characteristics and product fineness. The characteristic power curve for ball mills is shown in Figure 2. KW MaximumkW f~1···-~ Optimal mill power Load Figure 2 Characteristic power curve for ball mills Control philosophy The heart of the strategy is a standard PID algorithm for control, therefore it is not rule based or considered an expert system. Because of this, there is no requirement for special hardware or additional computers. The control strategy is integrated into the plant's existing control system. The strategy is con figured in the same control hardware as the rest of the mill circuit controls. Although it does require some complex calculations, the strategy js generally compact enough to be integrated into the same controller as the rest of the anal og mill control. . A comparison of the mill load control logic with the conventi onal logic is shown in Table I. Table l Comparison of Mill Load Control Logic with Conventional Logic Mill Load Control Logic Conventional Logic Uses PlD and Algori thms Uses PID only Set point is derived Set point is manual Mill motor power has CO!}trol function Mill motor power has no control function Correction is au tomatic Automatic correction is not possible Mill load is maintained based on the evaluation of Mill control .is based on some particular various parameters parameters only 2