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Ball Mill Speed, Influence of Mill Speed

Ball Mill Speed

Grinding Plant Description

The milling pilot plant consists of two containers mounted on top of each other. The bottom container holds the mill, the drive train, rollers and the high-tension cabinet. The feed boxes (fine and coarse feed) are built into the top container. The sizes of the available mills are:

• 0.82 x 1.0 m (used for the testwork described below) • 1.3 x 2.2 m.

The following parameters are measured online:

• Mill inlet water (l/h) • Mill outlet water (l/min) • Speed (rpm) • Mill torque on the driving shaft (nm) • Electric power (kw) • Mill weight (kg) • Throughput (kg/h).

The absorbed power is calculated by multiplying the torque on the driving shaft with the shaft speed (rad/sec) and hence excludes any losses associated with the gearbox and motor. The pulp weight in the mill is obtained by subtracting the mill weight during the test from the empty mill weight, including the ball charge, at the start of the test. The pulp volume of the pulp in the mill is then calculated using the mill discharge density (before dilution) obtained during the sampling. The ball charge, and hence the volume of the voids between the balls in the mill, is known and we are then able to calculate the factor ‘volume pulp/volume voids between the balls’.


Mill Speed Test Repeatability

The repeatability of the pilot plant was investigated in the past with 2 repeat test runs with the 0.82 x 1 m grate discharge mill in open circuit with a charge in equilibrium with a 60 mm ball top-up and a 32.4% ball filling degree.

Overall, the repeatability is good as the variations between tests 8 and 9 are small. There is only a 10 μm difference in the P80, and a 1% difference in the <%75μm, and the particle size tendencies are correct (the finest feed gives the finest discharge).

In addition, the Wi and Wio are almost identical. It was felt that two repeats were adequate to illustrate the repeatability, as the changes described further in this paper are so large that the variation of the investigated parameters would be substantially greater than the repeatability error.

Mill Speed Test Summary

These tests were done with the ∅0.82 x 1 m overflow mill with a 30 mm graded ball charge and a 30% filling degree. The speed was increased from 65% to 90% Vcrit. The data is summarized below (Table-1). Contrary to common belief, it can be seen that the extra power input obtained by increasing the speed from 65% Vcrit to 90% Vcrit does not result in a finer grind in this case. The best grinding efficiency measured in terms of Operating Work Index (Wio) or ‘Kwh/T passing a certain screen’ is obtained with the lowest speed. The extra power input achieved by increasing the speed does not seem to have any effect and does not result in a finer grind. The particles size distribution slope is not affected by the mill speed (Table 2) as the curves are mostly overlapping.


Grinding Efficiency for Secondary Stage Grinding

These tests were done with a ∅0.82 x 1 m grate discharge mill with a 30 mm graded ball charge and a 30% filling degree. The mill discharge pulp density was increased from 68.8% to 75.9% solids. The data is summarized in Table-2. The example in Table-2 illustrates that the density has a large influence on the grinding efficiency. In this case, the optimum %<75 μ is in the region of 73.4% or 1.989 kg/l (Graph 3).

If one goes below that, the higher dilution will ‘flush’ the fines out of the mill and reduce the overall residence time in mill. This will result in a coarse grind with a slightly steeper particle size distribution curve (Graph 4).

If one increases the density, the pulp will become too sticky and the ball charge expands. The balls become coated and the grinding efficiency decreases. The pilot mill is also equipped with a sensor measuring the ‘ball and pulp toe and shoulder angles’ (Figure 3). These data are then used to calculate the total pulp and ball charge angle ( Graph 5).

Graph 5 clearly illustrates how the ball charge stays quite compact till about 73% solids as the total media angle does not change. However, once one goes above this, the media charge starts expanding due to the high pulp viscosity and ball coating.


The Influence of Mill Speed

Increasing the speed from 65% Vcrit to 90% Vcrit does not result in a finer grind in this case. The best grinding efficiency measured in terms of Operating Work Index (Wio) or ‘Kwh/T passing a certain screen’ is obtained with the lowest speed. The extra power input obtained by increasing the speed does not have any effect and does not result in a finer grind. However, one should bear in mind that this testwork was done on a small pilot plant mill. The tendencies on a large mill might not be the same. It would be advisable to conduct plant surveys on a large scale mill equipped with a variable speed drive to validate this work.

Pulp density has a large influence on the grinding efficiency. In this case, the optimum is in the region of 73.4% or 1.989 kg/l. If one goes below that, the higher dilution will ‘flush’ the fines out of the mill and reduce the overall residence time in the mill. This will result in a coarser grind with a slightly steeper particle size distribution curve. Increasing the density will make the pulp more sticky and the ball charge expands. The balls media are being coated and the grinding efficiency decreases.

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