Cement, Energy and Environment

Table-3 Certificate data on grinding cement of one grade with industrial mills of different designs. Parameters Series tubular mill, MAAG stage 1 dimensions 2.0 x mill 2.3/1 .8 x11 m 10.5m I Equivalent dia. (m) 2.0 1.96 Grinding space (m 3 ) 28.0 27.5 Ball charge (t) 32.0 29.0 Power consumption 540.0 365.0 (kW) Cement productivity (tph) Chromite and magnesite Productivity (tph) Power consumption Per 1 t of material (kWh/t) Reduced efficiency For comparison, Table-3 gives the certificate data on grinding cement of one grade with industrial mills of different designs. At the refractory plant, these mills grind chromite and magnesite. This additionally and unambiguously warrants the validity of results obtained. The two compared mills have almost the same initial conditions. However, the grinding space of the stage mill is smaller by 2% and the ball fill is 10% less than in the compared mill , thus creating an advantage for the tubular mill. The same productivity ratio of the compared mills when they grind cement and chromite- magnesite (21.9:12 = 10:5.4) excludes randomness of the results analysed and additionally proves their val idity. In spite of the tubular mill's advantages with regard to initial conditions, the efficiency (the reduced ratio of electric power consumption per tonne of ground product) of the stage mill is 2.71 times higher than that of the classical design mill , and the specific electric power consumption is 16.6kWh/t. 12.0 21 .9 5.4 10.0 45.0 16.6 1.0 2.71 Hence, a fundamental result has been obtained, which proves the efficiency of mills with a variable LID ratio. Such mills partially implement the following : separation of grinding bodies, variable ball filling per chambers, and various ball velocity conditions along the mill. The effect of the mill rotational speed and the ball fill factor on grinding efficiency The basic drawback of ball mills with a constant rotational speed is that the optimal ball lifting height changes continuously (drifts) with variable internal factors, such as intensive attrition of the ball charge and the lining profile. The widely changing external factors (coarseness, moisture content, grinding ability, and components ratio) require adjustment of the balls' lifting height depending on the character of the grinding process. This is impossible to ensure, given the lining profile and the degree of filling of grinding bodies. The classical system for automatic control of the source materi al charge does not compensate for internal and external factors. A transition to grinding cement of other grades requires readjustment of mill operating conditions. According to process requirements, the operating conditions can be selected effectively and instantaneously (compensate for external and internal perturbations) by adjusting the grinding process automatically with the mill rotational speed . From equation (2) it follows that parameters n°· 8 and cp 0 · 6 have no critical impact on mill productivity. By the same reasoning, these parameters are absent in equations (1) and (3). The outcome of the authors' long-term studies in BM with a variable-speed electric drive in different industries was not only providing a 50% productivity boost and accurately stabilizing (±0.5%) grinding fineness, but also proving that the grinding efficiency at specified LID ratios depends mainly on the mill rotational speed strictly in compliance with the grinding bodies fill factor. The variable-speed electric drive provides the following functionality: • To perform smooth mill starting, thereby ensuring extended reducing gear life. • To choose an optimal ball dropping path onto the "ball impact spot" with account of continuously changing characteristics of the material being milled and the condition of the balls, as well as to adjust the static characteristic drift with time in the automatic mode. • To implement an optimal combined control system based on two channels, i.e. the inertia-less mill rotational speed control channel and the inertial (adjustment) channel for controlling the source material charge . 14