Swirl flow agitation for scale suppression

Abstract

Scale formation is a serious problem in the mineral processing industry. To better understand the options available for mitigating this problem, a novel scale–velocity model is proposed in this paper for slurry systems commonly found in mineral processing plants. The new qualitative scale growth model predicts that at very low fluid velocities the scale growth rate is enhanced by an increase in fluid velocity due to the mass transfer-controlled scale growth. At higher fluid velocities, the scale growth rate decreases with increasing fluid velocity due to the increased flow erosion effect, for slurry systems. This suggests the potential of particulate erosion as a scale suppression mechanism. The model predicts the existence of an optimal slurry flow velocity, where scaling rate and equipment erosion rate are both zero. The optimal fluid velocity value is proposed to be used as the main parameter to improve engineering design of mineral processes in terms of scale suppression.A novel agitator design, swirl flow technology (SFT), developed and patented by CSIRO and Queensland Alumina Ltd (QAL) in Australia was introduced as an agitator design that better meets requirements for scale suppression than a widely used conventional draft-tube agitator system. SFT agitation has been installed in gibbsite precipitators at QAL's alumina plant for a decade.As shown by CFD simulations and laboratory measurements, swirled flow agitation generates more uniform velocity distribution and higher velocity values at the wall than conventional agitators for the same power input; this reduces the maximum growth rate of scaling in the tank leading to significantly prolonged precipitator's service life. Based on the full-scale operational experience at QAL, it can be suggested that SFT agitation roughly halves the scale growth rate as compared to that measured in the conventional draft tube agitator systems.