Optimization of continuous ball mills used for finish-grinding
of cement by varying the L/D ratio, ball charge filling ratio,
ball size and residence time
Polysius AG, Beckum, Germany
During the last decade, semi-finish-grinding plants have been used more and more for the energy efficient grinding of high-
quality cement. In 1999, it was found that by decreasing the ball charge filling ratio it was possible to lower the specific energy
demand for grinding significantly.
It was obvious, too, that the L/D ratio influences the specific energy demand and the mill throughput as well. Therefore, a
huge test program was carried with a semi-industrial ball mill, which was operated in closed circuit. The mass-specific surface
area of the two feed materials (intermediate product) used were quite typical for industrial semi-finish grinding plants. The
values were 2200 and 3000 cm
/g according to Blaine. The product finenesses were 3000 and 3800 cm
/g, respectively. The L/
D ratio of the ball mill was varied in four steps of 1.75, 2.1, 2.79 and 3.49, and the ball charge filling ratio was varied in three
steps of 15%, 20% and 25%. The experiments clearly indicated that the optimal L/D ratio and the optimal ball charge filling
ratio are different for each feed fineness.
The influence of the ball charge grading on the specific energy demand, characterised by the average ball diameter, was
tested by means of a discontinuous laboratory ball mill. The results showed that by using a finer ball grading the specific energy
demand could be lowered considerably.
The obtained results can be explained well by theoretical considerations regarding the ruling stress intensity and the number
of stress events. The stress intensity expressed as the power input per ball is dependent on the ball diameter to the third power
and only slightly dependent on the inner diameter of the mill. The number of stresses can be characterised by the average
retention time of the ground material inside the mill if the ball charge grading remains unchanged. The optimal retention time
depends not only on the feed material and the desired comminution result but also on the ball charge filling ratio and particularly
on the L/D ratio. On the basis of the present results and considerations, a specific optimisation of ball mills in semi-finish-
grinding plants can be done.
D 2004 Elsevier B.V. All rights reserved.
Keywords: ball mills; L/D ratio; finish-grinding
0301-7516/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
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Int. J. Miner. Process. 74S (2004) S55– S63