Of many physical parameters critical to design of grinding processes, impact of grinding media is among the most difficult to measure or predict. Yet impact of falling grinding balls, pebbles, or rods accomplishes the fine grinding essential to metallurgical recovery of most important minerals. Unfortunately, the same impacts that break ore fragments deform or crack grinding mill liners.

Design of an instrumented impact test ball for in-mill testing involved several considerations; survival of ball and instruments, retrieval of ball, maximizing generation of impact data, and fabrication of a ball capable of these ends.

Six Protect-A-Pak accelerometers were chosen to optimize data recovery per test, covering the expected impact range in reasonably small increments. Clusters of 3 units arranged in equilateral triangles in each ball half require a 72mm diameter cavity 51mm long. Geometry of the cavity requires a minimum sphere of 89mm to contain it and 10-15mm additional metal surrounding the compartment to provide strength. Final ball diameter was 140mm. The test ball must be substantially larger than standard 75mm balls in the Climax mills to be easily recovered and minimize chance of loss in the ball charge.

The ball was machined from alloy steel shafting and weighs 7.14kg with 6 accelerometers. Large grinding balls 89-127mm are common in larger mills and weight 2.9 8.4kg, a range which includes weight of the impact test ball.

Series I drop tests used a 76mm alloy steel grinding ball at 63HRC surface hardness with Protect-A-Pak accelerometers of several ratings cemented to a flat ground on the ball. The test ball, suspended in a light wire harness, was dropped from gradually increased heights until the accelerometers responded in hitting a worn 100kg mill end liner of 20%Cr 2%Mo 1%Cu martensitic iron alloy.

Test Series II with same equipment but with the liner plate covered with a single-particle layer of sized Climax ball mill feed material (high-silica granite molybdenum sulfide ore, Bond Work Index 10.1-10.8) in Tyler screen sizes +100-65 mesh, +65-10 mesh, +10-3 mesh, +3 mesh-9.5mm, and greater than 9.5mm, were tested. A fresh layer of ore material was used for each drop. Larger, resettable Protect-A-Pak Omni-G accelerometers measured decelerations.

Data from drop ball tests seem to be fairly well confirmed by in-mill tests with the hollow instrumented ball which, although heavier, is larger in diameter, less convex, softer steel, and strikes either the tumbling mass of ore and balls or descending mill liners. The ball was placed and recovered in the feed end of the mill where impact is known to be less severe.

Ball mills are extensively used in the size reduction process of different ores and minerals. The fill level inside a ball mill is a crucial parameter which needs to be monitored regularly for optimal operation of the ball mill. In this paper, a vibration monitoring-based method is proposed and tested for estimating the fill level inside a laboratory-scale ball mill. A vibration signal is captured from the base of a laboratory-scale ball mill by using a5g accelerometer. Features are extracted from the vibration signal by using different transforms such as fast Fourier transform, discrete wavelet transform, wavelet packet decomposition, and empirical mode decomposition. These features are given as input to an artificial neural network which is used to predict the percentage fill level inside the ball mill. In this paper, the predicted fill level obtained by using different features are compared. It is found that the predicted fill level due to features obtained after fast Fourier transform outperforms other transforms.

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Nayak, D.K., Das, D.P., Behera, S.K. et al. Monitoring the fill level of a ball mill using vibration sensing and artificial neural network. Neural Comput & Applic 32, 15011511 (2020). https://doi.org/10.1007/s00521-019-04555-5

KIMA Echtzeitsystemes fill-level measuring device, used for ball mills in the cement industry for over seven years, has now been adapted and developed for use in coal mills. A new fill-level sensor enables reliable fill-level measurements, even with fluctuations in coal quality or moisture levels. Tested over a period of several months, the SmartFill for Coal has proven itself able to take precise fill-level measurements reliably and without errors. The technology helps to achieve homogenous degrees of fineness, thereby improving the combustion properties of the coal. This, in turn, leads to a reduction in unburned coal and, consequently, better energy efficiency, the company says. (http://www.kimae.de)

We can calculate the steelcharge volume of a ball or rod mill and express it as the %of the volume within the liners that is filled with grinding media. While the mill is stopped, the charge volume can begottenby measuring the diameter inside the liners and the distance from the top of the charge to the top of the mill. The %loading or change volume can then be read off the graphbelowor approximated from the equation and calculation:

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