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 cm2/g according to Blaine. The product finenesses were 3000 and 3800 cm2/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.