Cement, Energy and Environment

The following is an example of compensation for ball charge attrition using the mill automatic speed control system. According to the statistics of grinding bodies wear of 1kg/t of cement being ground, the additional charge schedule (after 9 days) and the Davis volumetric-weight ball wear theory, the company has built an approximate ball charge wear vs. grinding time function <p =f (t) (Figure 3). Based on this theory, the ball mass in the beginning of any n-th interval is equal to its initial mass G 1 multiplied by constant factor q raised to n less one . G n-1 G n = q 1-----(4) Where q is the common ratio of the geometric progression q = G:IG1. Formula (4) holds not only for one ball , but also for the entire fill of grinding bodies. Based on the grinding bodies wear vs. grinding time function , conversions yield the productivity and specified rotational speed dependence on grinding bodies wear (Figure 4 ), i.e. Q = f(<p) at n =0.65 ncr = constant and Q=f(<p) at n = var. They are needed to obtain the productivity vs. time graphs for operating conditions with a continuously-controlled mill rotational speed. The dependence Oopt = f (<p) at n = var. and Q = f (<p) at n = 0.65 ncr =constant are given in Figure 4. They allow defining mill productivity growth due to an optimal rotational speed condition. In this case, the variation of the rotational speed setting in an automatic control system is defined by grinding bodies wear, it can be effected by the Q. t/hr 15 ~ 85 1''\. / 14 13 \_ ~ ? 80 ....- 'x v 75 v ' / ", 70 / ~, 1c ll 10 ", 65 oJc8 OJ32 Figure 4. Mill productivity and preset rotational speed vs. grinding bodies wear. Q, t/hr 15 ~ ~ ~ ~ C\1 14 - " "-.... --- -J) (") - :::::: - r- ........ r - r-- ) opt - .......... .......... 13 ~ ........... ~ ~ ........ 1'-Q ~ .g . IJc 12 11 'Q "' I' ' .C "' u I I' 0 4 8 12 16 t, day Figure 5. Mill product ivity vs. grinding bodies wear. programme set point nset = ~<p) shown in Figure 4 as a dashed line. Treatment of experimental curves (Figure 5) yields an estimate of productivity Q growth when controlling the rotational speed with changing controller setting n* as the balls wear. The growth value is 6.2%. A variable - speed electric drive not only dramatically increases the grinding unit productivity, but additionally compensates for perturbations related to a changing grinding bodies fill factor. Optimal mill rotational speed control maintains the mill operating conditions in the suboptimal zone in the time between successive additional charges. Hence, optimal TBM operating conditions at a preset LID ratio uniquely and essentially depend on the combined interaction of the mill rotational speed and the ball fill factor. As a control action, the mill rotational speed allows the 16