Optimum Operating Conditions and Suspension Formulation for Comminution of Submicron Mineral Particles in Stirred Media Mills

Abstract

Mineral particles in the submicron size range are widely used in the pharmaceutical, chemical and paper industries mainly due to their high solubility and homogeneity. Generally, particles with this size characteristics are produced by comminution in stirred media mills. However, comminution is an energy intensive process consuming up to 40% of the total energy cost of the mineral processing. Furthermore, particles in this size range are subject to strong attractive inter-particle interactions that may lead to the appearance of agglomerates. For that reason, determining both, the optimum operating conditions and suspension formulation, are critical elements to achieve price competitiveness. To this end, these parameters were studied experimentally and throughout computer simulations. Calcite, quartz and pyrophyllite were used as the mineral samples for the experiments. In case of the operating conditions, the stirrer tip speed and the grinding media size were studied. Their effect in the grinding process were judged based on the specific energy required to achieve particular product sizes. Additionally, the experimental results were analysed and explained based on Kwades energy model. For the suspension formulation, different solid mass concentrations and four different surfactants were studied. Similarly to the case of the operating conditions, the effect of the solid mass concentration was analysed based on the specific energy requirements. The studied surfactants were sodium polyacrylate, ammonium polyacrylate, SND Dispersant and CO10 Dispersant. The optimum concentration of each additive was determined experimentally and their performance was judged based on their particle stabilization capacity and effect over the rheological properties of the slurries. Next, throughout computer simulations, the comminution process was characterized and the effect of the recirculation in continuous grinding was studied. Finally, an equation to estimate the power input of the mill based on the operating conditions and suspension formulation was proposed. Results regarding the operating conditions showed that the stirrer tip speed as only a significant effect over the energy consumption when the feed particles approach their grinding limit

and grinding media sizes which provide the minimum required stress intensities for particle breakage and maximizes the stress number are the most efficient. In case of the suspension formulation, solid mass concentrations between 30 and 40 wt.% proved to be the most efficient for calcite slurries. Additionally, with the addition of surfactants, the viscosity of slurries reduced, and shifted their behaviour from pseudo-plastics to Newtonian fluids. Among the studied surfactants, sodium and ammonium polyacrylate proved to be the most efficient. Finally, the simulation results showed that the general breakage rate reduces for the longest grinding times due to the accumulation of fines. Also, it was found that a recirculation grinding process can be optimized based on their flow rate and breakage rate.