Cement, Energy and Environment

The same laws govern BM grinding; however, each industry has its specific features. In the productivity formulas, they have to be reflected by different factors rather than by the formula structure. Comparing (1) through (3) results in the following conclusions: • Generally, the mill productivity is a function of the grinding space because all other parameters are constant and their values have little significance. • (2) and (3) show that not only is the mill grinding space significant, but also the UD ratio within this volume. • Only (2) indicates a minor impact of the mill rotational speed on grinding, whereas in (1) and (3), this parameter is completely absent. Here, parameter n°· 8 at a constant rotational speed of classical mills is nothing else than constant factor k. • (1) and (2) indicate a minor impact of the ball charge, whereas in (3) this parameter is not accounted for at all. Hence, all industries fail to account for one phenomenon, i.e. the impact of the mill rotational speed and the ball charge. They are the critical parameters of the grinding process with the given LID ratio , whereas the formulas themselves do not reflect the grinding process in full . Table-1 Operation of coal tube-ball mills (TBM) and cement tubular mills. · Coal TBM Cement ball mills Sh-SO Sh-SOA with closed-cycle grinding Dimensions (m) 4.0/8.0 3.7/8.5 3.95/6.4 3.65/10.3 Grinding space(m 3 ) 100.0 91.5 100.0 104.0 Ratio UD 2.0 2.3 1.62 2.82 Ball charge (t) 100 105 123.0 141 .0 Ball fill factor 0.22 0.25 0.27 0.3 Productivity (tph) 50.0 50.0 43.0 45.0 Power (kW) 2460 2000 1490.0 1862.0 Electric power 49.2 40.0 34.7 41.4 Consum"'::on (kWh/t) Table-2 Ball Charging Parameters Ball charge in three chambers of tubular mills 1 Working condition Crushing Grinding Re-milling Ball dropping Waterfall Waterfall- Cascade velocity paths (impact) cascade (attrition) (crushing) Ball dia. (mm) 100-60 60-30 30-20 Weighted average 74.5 44.8 25.1 ball (mm) Ball fill factor(%) 30 27 24 Gain of number of ~q> = 3% =const. balls as per chambers The effect of the UD ratio on grinding efficiency Formulas (2) and (3) imply that productivity depends on the mill inner dia. D far more than on its length L. Here, the critical condition - the limits of validity and applicability of these parameters - is not specified. Thus, on the contrary to the technical sense, it is maintained that maximum productivity for the given grinding space can be provided at D-+ 00 and L-+0. Since the grinding process is very specific, it is best to consider known cases in different industries. Let us review the operation of coal tube-ball mills (TBM) and cement tubu lar mills and compare the results (Table 1). Analysis of tube-ball mill Sh B M 400/800 (Sh-50)has shown its inefficiency, and only contrary to formulas (2 and 3) after redistribution of LID dependency to increase the length and decrease the diameter, Sh B M- 370/850 (Sh-50A) was commissioned in the power industry as a base one in the CIS countries. This reduced electric power consumption by 9.2%. The UD ratio in Sh-50A could have been chosen bigger were it not for the constraint on mill length. The mill performance was improved by reducing the ball lifting height, increasing the turnover of the ball charge and the time of drying wet coal. Actually, by drawing on the experience of operating cement mills, coal TBMs are approaching the dimensions of tubular mills. Note that the efficiency of mill Sh-50 with a variable-speed drive will exi:eed that of the upgraded mill Sh-50A. Two cement mills (Table 1) with closed-cycle grinding, having practically the same grinding spaces and productivity, 12