Slurry rheology of industrial minerals and its effects on wet ultra-fine grinding

Sammanfattning: This thesis investigates slurry rheology of industrial minerals (limestone and quartzite) and its effects on wet ultra-fine grinding in order to reduce energy cost, increase the throughput as well as the product fineness. The influences of solid concentration, molecular weight of a dispersant, and particle size and distribution on the rheological behaviour of original limestone slurries have been investigated. The results reveal that when the solid concentration of a limestone slurry (< 100 ¦Ìm) is increased from 60 wt.% (35.71 vol.%) to 78.5 wt.% (57.49 vol.%), the rheological behaviour of the slurry is transformed from a weakly dilatant or Newtonian characteristic to a pseudoplastic one with a yield stress, which is in combination with a thixotropic property at a higher solid concentration (i.e., ¡Ý 75 wt.% or 52.63 vol.%). At a certain shear rate, the apparent viscosity and the relative viscosity of the slurry both increase exponentially with solid concentration. The extrapolated Casson yield stress increases in a power-law form with increasing solid concentration when the solid concentration is larger than 70 wt.% (i.e., 46.36 vol.%). The maximum attainable packing solid fraction ( m) is predicted as 64.6 vol.% at the limestone-water suspension. A polymeric dispersant named Dispersant S40 with a molecular weight of 5,500 Da appears most efficient for the reduction of the apparent viscosity of limestone slurry. The smaller the particle size and the narrower the size distribution, the more evident the pseudoplastic behaviour of limestone slurry is with a higher yield stress and a larger apparent viscosity at a given shear rate in the range 12 - 1200 s-1. However, a sufficient addition dosage of Dispersant S40 significantly decreases or even eliminates the rheological differences of limestone slurries (apparent viscosity and extrapolated yield stress) resulting from differences in particle size and distribution. Wet ultra-fine grinding of a limestone material (< 100 ¦Ìm) has been investigated in a stirred media mill with respect to the effect of slurry rheology. The results obtained by varying operation parameters (i.e., molecular weight of a dispersant, solid concentration, addition dosage, addition method and bead load) are evaluated in terms of energy efficiency and the fineness. Dispersant S40 gives the superior results compared to the other dispersants (i.e., BCX-476 and BCX-552). For a certain level of bead load, an optimal solid concentration exists for more effective grinding. At 0.1 wt. % of addition dosage of Dispersant S40 or more (¡Ü 0.6 wt.%), a smaller addition amount of Dispersant S40 leads to a higher energy efficiency and a smaller median size at a lower level of specific energy input. However, the excessive amount of the dispersant causes the grinding less efficient. This can be avoided by either the multi-point addition of the dispersant or a higher bead load (¡Ý 83 vol. %). The role of slurry rheology in stirred media milling of quartzite has also been performed by varying important grindingparameters such as media bead density, addition of chemicals, solid concentration, stirrer rotational speed as well as the combined effect of these factors. Media bead density has an evident but complex effect on stirred milling performance, depending on stirrer rotational speed and solid concentration. Addition of Dispersant S40 or a lower solid concentration results in better grinding performance (i.e., a higher energy efficiency and a smaller median size) due to maintenance of lower viscosities at shear rates investigated during grinding. Stirrer rotational speed interacts with solid concentration. For a given solid concentration, an optimal stirrer speed exists. The observed phenomena can be explained by the effect of slurry rheology on the stress intensity of individual grinding bead. The flowability of ground limestone slurries during grinding has been monitored and characterized. The rheological behaviour of ground limestone slurries varies with grinding from a starting dilatant flowability to a resultant pseudoplastic one with an evident Casson yield stress in combination with a thixotropic character. The appearance of pseudoplastic flowability with a thixotropy is related to the fineness of a ground product and its specific surface area, which are both correlated to solid concentration and the addition amount of Dispersant S40. However, the rheological characterization of time-dependent fluids is dependent upon the instruments used and operating conditions. In addition, an empirical particle size-energy model provides a good fit to the wet ultra-fine grinding results for both limestone and quartzite under the experimental conditions investigated.