A multi-torque model for the effects of dispersants and slurry viscosity on ball milling

Abstract

Diverse ball milling conditions were employed by changing ball size, media charge, mill speed and slurry hold-up in order to study the effect of polymeric dispersant aids on the grinding of dense slurries. In the absence of a dispersant, as the pulp thickens, there occurs an increasingly disproportionate split of the grinding charge between cascading and cataracting-centrifuging masses, resulting in a complex relationship between the instantaneous viscosity and mill torque. In the presence of dispersants, the partitioning of charge in a relatively fluid suspension occurs to a much lesser extent. A multi-torque mill model has been derived which tracks the overall mill torque as a function of changing viscosity of the pulp, permits estimation of the charge split between cascading and cataracting-centrifuging regimes and explains the occurrence of a peak torque value at some intermediate stage. The size distributions of the ground product are a function of the cumulative energy input only, even when the power drawn by the mill varies appreciably with milling conditions and/or grinding time. Implication is that the grinding rates are proportional to the mill torque and the latter can be used directly to monitor and analyze the mill performance and role of grinding aids, notwithstanding the fact that the slurry may undergo major changes in its rheological character.