Cement, Energy and Environment

differ significantly by their UD ratio and insignificantly by 6. 7% as to their electric power consumption. In ore BMs. there were many cases of increasing the mill length from 600 mm to1200 mm by inserting rings, thus increasing the productivity from 14 to 20 tph. On the one hand, these cases confirm the impact of the UD ratio on the grinding process, and on the other hand, they show that the grinding efficiency of coal mills fails to agree with formulas (2 and 3), and that of cement mills with (1 ). Grinding practice shows that bigger diameter mills, in contrast to small-diameter ones, yield greater productivity, but a coarse grind, and vice versa. Hence, the solution of the LID ratio problem is in a trade-off between the merits and drawbacks of big and small– diameter mills. It is optimally achieved in mills with a continuously-variable structure. 14 ~) v ~"'. The phenomenon of ball mills with a rigidly-variable structure (variable UD ratio) An attempt was made to solve the problem of effective coal grinding by using biconical mills with a rigidly-variable structure, in which a complex– shape casing comprises two opposing cones, a short inlet one with a 120° angle and long outlet one with a 60° angle. There is a cylindrical section between the cones. A feature of such mills was that for the first time in power engineering there was an attempt to implement one of the key grinding principles, i.e. separation of the multitude of balls along the mi ll casing. The mutually exclusive eclectic design of the biconical mill was an obstacle to solving this critical problem. Unfortunately, in contrast to References 1 and 2, papers 3 and 4 give no data on the character of separation of grinding bodies, this being indirect evidence of its absence. Nevertheless, the biconical mill <pi_ j_ ..( 1/ fPl 450 N,kW I I ~ ~/ (J)t 'I L! ~, ........ V' j_ l/ 'I f ~ ~ IL .1 I .-V / ~~. :,y 13 12 400 ~l_ ~ v / / It! 11 ,/ li_" 'I , 10 350 so 60 70 so 90 ncr,% Figure 1. Mill productivity Q and power consumption N {dash~d lines) vs. ball fill factor (qJ1 = 0.28; qJ2 = 0.32; qJ3 = 0.36) and mill rotational speed n. is the first, though not the sole unit with a rigid ly-variable structure. Over their length, tube-ball cement mills are separated into three chambers with two diaphragms and an outlet grid with an airlift. The involved and ineffective separation of the mill into chambers was an attempt to at least partially separate the balls by size, ensure a different fill factor for grinding bodies and a weighted average ball, and provide different ball velocities along the mill. In the ore mining industry, multistage grinding in several short mills solves the task of mill sectioning into chambers. The fill factor recommended by world practice and the ball grades (Table 2) in each chamber are different and - what is extremely important - their number is continually reduced by ~q> =3% as per chambers. Hence, though the key engineering tasks in tubular mills have not been solved in full yet, they key priorities of an optimal grinding technology were identified. Let us consider one more unconventional tubular three– chamber stage mill manufactured by the MAAG Company. It reduces chromite and magnesite at the Zaporizhia Refractory Plant in Ukraine. The diameter of the first chamber of the mill is 2.3 m, and that of the second and thi rd ones is 1.8m. The stage mill is yet another striking example of a unit with a variable UD which merits special attention. The operation of different– design ball mills can be compared provided the initial conditions be strictly met, i.e. grinding of the same material with equal mill grinding spaces and ball charges. 13