Using Alternating High-Shear and Rest-Time Environments to Examine the Build-up of Localized Yield Stress in Mineral Slurries of Nickel Laterite

Abstract

In industrial mixing applications, mineral slurries with high solids concentrations can develop macro networks during rest time and may exhibit yield stress and thixotropic behavior where the shear forces are low, causing an inactive surface zone. Nickel laterite slurries are empirically found to lie in the extremis of rheological challenges for mineral transport and processing applications. In this research, the development of yield stress in Moa Bay nickel laterite slurries was analysed by performing rest time experiments at the bench scale in the confined impeller stirred tank (CIST) using a torque transducer to continuously measure the dynamic response to shear stress. Concentric cylinder and vane rheometry were used to study the shear thinning and thixotropic behavior, and to measure the static yield stress. Different factors were investigated: the rotational impeller speed, mixing time, rest time, solids concentration, and particle size distribution. Rest time experiments showed consistently that 5 min of rest time and longer resulted in torque responses higher than the baseline level for solids mass fractions between 43 wt% and 53 wt%. For solids concentrations from 53-60 wt%, the localized or dynamic yield stress developed much faster: after 15 s of rest time at 750 rpm, and after 60 s of rest time at 1000 rpm, and above 60 wt% excess torque was recorded after 30 s of rest time at 1000 rpm. Newtonian behavior was observed for solids concentrations below 43 wt% as no torque responses above the baseline were observed at all impeller speeds. Solids above 43 wt% displayed shear thinning behavior, thixotropic behavior, and a dramatic increase in yield stresses. Vane rheometry measurements showed that adding 25 % fine particles in 3 steps (0.85 mm > n > 0.045 mm) to slurries prepared from coarse particles (2.36 mm > n > 1.18 mm) dropped the peak yield stresses, also the yield stress values of coarse particles were higher over a range of solids between 25 wt% and 70 wt%. Meanwhile, rest time experiments revealed that adding fine particles extended the time required for dynamic yield stress to develop and reduced the magnitude of the non-Newtonian behavior.