Comminution processes are used in the metallurgical industry to reduce the particle size of mined ores in order to liberate valuable minerals for downstream separation and extraction. Comminution is a very energy intensive process and an incentive therefore exists for metallurgical operations to optimise the efficiency of the comminution processes employed. Stirred media mills are more efficient for regrinding and fine grinding duties as compared to ball mills that have traditionally been used in these applications. The efficiency of stirred media mills are influenced by both the operating conditions and physical design of the mill. Using the shear based power model, Radziszewski, 2013, hypothesised that the power draw of a vertical stirred media mill operating with pin or disc type stirrers could be increased by adding stationary liners to the mill shell. If this hypothesis holds true this approach might be used to improve the design of stirred media mills for new applications or it might be used to optimise existing mills by modifying the mill internals to improve on the stirrer and mill shell designs. The aim of this investigation was to test the hypothesis and to evaluate the effect of liner addition on the productivity and energy efficiency of the mill.
An experimental test work programme was developed to measure the mill power draw of four different mill geometries under comparative conditions. Two different stirrer designs were evaluated, the first stirrer consisted of a pin type agitator and the second stirrer consisted of a ring type agitator design. Testing was conducted with the two different stirrers operating in both a smooth mill vessel and in a mill vessel fitted with a stationary disc liner. The test work programme also included grinding tests to compare the productivity and energy efficiency of the four different mill configurations. Milling tests were conducted on mono-sized quartz feeds in particle size ranges of around -150 + 106 μm and -106 + 75 μm. Results of these tests were used to calculate both the time based specific rate of breakage (Si) and the energy normalised specific rate of breakage (SiE). The breakage rates were used to compare the four different mill geometries in terms of productivity (Si) and energy efficiency (SiE). Further tests were conducted on quartz feed material with a natural feed size distribution. The specific energy requirement (kWh/t) and milling times (minutes) to reach a given target product grind were used to compare the performance of the four different mill geometries. Discrete Element Modelling (DEM) was used to qualitatively study and compare the charge conditions in each of the four geometries.
The experimental results supported the hypothesis and showed that the power draw of the pin and ring stirrer mills increased with the addition of a stationary disc liner to the mill shell. The additional mill power draw resulted in an increase in the productivity of the mill without any negative effect on the energy efficiency. The results of this work shows that there could be a potential to improve the milling performance of stirred media mills by adding stationary discs to the mill shell. Specifically a higher mill power draw could lead to either a smaller equipment footprint in the case of new mills or to a larger throughput capacity or finer product grind in existing mills that have been retrofitted with an improved internal design. Further work would be required to confirm the results of this study on a larger scale and in a continuous milling configuration.
Some general observations were made on the shear based power model proposed by Radziszewski, 2013. When applied to this current investigation the shear based power model correctly predicted that the addition of the disc liner to the mill vessel will result in an increase in the mill power draw. However the model did not correctly predict the relative power draw of the pin versus ring stirrer designs. The shear based power model predicted that the ring stirrer will draw more power than the pin stirrer, but the experimental work showed the opposite result. The pin stirrer had a higher power draw than the ring stirrer. DEM data showed that the pin stirrer resulted in a higher average bead velocity in the mill as compared to the ring stirrer. It is therefore postulated that the reason for the higher power draw with the pin stirrer was that this design provided a better transfer of movement from the mill shaft to the mill charge.
Dissertation (MEng)--University of Pretoria, 2020.