Effects of pool volume on wet milling efficiency
Date
2013-07-19
Authors
Katubilwa, Franҫois Mulenga
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Abstract
The volume of slurry in a rotary mill has a bearing on the presence of a pool of
slurry and therefore on milling efficiency. Load behaviour was investigated at
different volumes of slurry. The insight gained was then used to evaluate the
implications of slurry pooling on milling.
First, the effects of viscosity on mill charge behaviour were measured using
photographic techniques applied to a Perspex mill. A model of the angular
location of the free surface of the slurry pool, as affected by slurry filling was
proposed. Next, a real ore was used and the load behaviour was measured using
non-invasive sensors fitted to a pilot mill. At this point, the angular position of
the pool and the net power draw were correlated to the volume of slurry for mill
speeds ranging from 65 to 85 % of critical. An additional series of tests was
carried out on a mill filled with grinding media only, for speeds spanning from
approximately 24 to 110 % of critical. The aim here was to isolate and study the
media charge. Lastly, a laboratory mill was used to run batch grinding tests on a
Platinum ore for slurry fillings U between 1.0 and 3.0 and at 65 % solids content.
Two ball fillings were considered for identical slurry volumes: J = 20 % and 30 %.
Results showed that not only did the proposed pool model work well using an
artificial slurry in the Perspex mill, but it also worked for the Platinum ore tested
in the Wits pilot mill. The behaviour of the media charge was not substantially
affected by slurry viscosity and slurry filling. The net power drawn by the ‘dry
mill’ compared well with DEM prediction for non-centrifuging speeds. The effect
of slurry pooling on net power draw, on the other hand, was best accounted for
using a Torque-arm model and an empirical model developed to this end. As for
milling kinetics, results suggested that the slurry pool should be avoided because
milling efficiency deteriorated as a result. However, the production of fines was
not largely altered.