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

Abstract: This thesis investigates rheology of limestone slurries and its effects on wet ultra-fine grinding in order to reduce energy cost and increase the throughput and the product fineness. The influences of solids concentration, molecular weight of a dispersant, particle size and distribution, and temperature on the rheological behaviours of limestone slurries have been investigated with rotational and cone-plate viscometers. The results reveal that when the solids concentration of a limestone slurry (< 100 um) is increased from 60 wt.% (35.71 vol.%) to 78.5 wt.% (57.49 vol.%), the rheological behaviour of the slurry is transformed from the weakly dilatant characteristic to the pseudoplastic with a yield stress, which is in combination with the thixotropic at a higher solids concentration (i.e., > 75 wt.% or 52.63 vol.%). At a certain shear rate, the apparent viscosity and the relative viscosity of the slurry increase exponentially with the solids concentration. The extrapolated Bingham yield stress increases in a power- law form with increasing solids concentration when the solids concentration of the slurry is larger than 70 wt.% (i.e., 46.36 vol.%). The maximum packing solids fraction attainable is predicted as 64.6 vol.% at the certain limestone-water suspension system. Besides, the smaller the particle size, the more evident the pseudoplastic property of limestone slurry are with a larger yield stress and a larger apparent viscosity at a given shear rate. Furthermore, the apparent viscosity and the extrapolated Bingham decrease with increasing temperature in a range of 20 to 40 oC. Also, wet ultra-fine grinding of a limestone powder (< 100 um) has been investigated based on the effect of slurry rheology. The grinding results through various parameters (i.e., molecular weight of a dispersant, solids concentration, additive dosage, additive method, feed temperature and bead load) are evaluated in terms of energy efficiency (increment of the specific surface area of a product per unit of specific energy consumption) and the product fineness. Sodium polyacrylate with a molecular weight of 5,500 (i.e., Dispersant S40) appears to be most effective grinding aid, i.e., for the reduction of the apparent viscosity. A sufficient addition dosage significantly decreases or even eliminates the effect of particle size and distribution on the flowability of limestone slurries. However, excessive amount of the dispersant is detrimental to wet ultra-fine grinding of limestone. This is avoided by the multi-point addition of the dispersant or by a higher bead load (> 83 vol.%). For a certain level of bead load, an optimal solids concentration exists for more effective grinding. The influence of feed temperature studied on wet ultra-fine grinding of the limestone is insignificant. Besides, the higher the bead load, the better the cumulative energy efficiency, the higher the milling rate, the larger the specific surface area of the product and the smaller the median size at the same specific energy consumption. In wet ultra-fine grinding of the limestone slurry with a given additive amount of a dispersant (i.e., Dispersant S40), the slurry from an initial feed to a final FP slurry (a discharge slurry where the grinding operation is automatically terminated by a safety control device due to poor slurry flowability) is transformed through different rheological behaviours due to the size reduction with a prolonged grinding time. For a given solids concentration of a limestone slurry (< 100 um), the characteristic sizes and the specific surface area of a final FP product are only related to the additive amount of Dispersant S40. In addition, an empirical particle size-energy model provides a good fit (R2 > 0.991) to the experimental results regardless of whatever grinding conditions.