The effect of impeller speed on scale formation within a swirling flow tank

Abstract: Scale is a major issue in a wide range of applications, including within the minerals industry. A common feature of many mineral processing operations is neutralisation, which leads to the precipitation of mineral compounds and scale formation. De-scaling processes within the minerals industry involve significant maintenance costs, production losses and safety risks. Hence, there is a need to explore potential fluid dynamics strategies to minimise or avoid the need for descaling in unit operations. The present study investigated the dependence of mineral scale formation on impeller speed in a mixing tank, under conditions relevant to the minerals industry., Experimental investigations were carried out using a series of laboratory-scale mixing tanks, together with a CSIRO-designed SWIRLFLOW impeller. A scale system was developed in-house to mimic the formation of gypsum scale. Impeller speeds were varied from 250 to 750 RPM for an aspect ratio of Dimp/Dtank = 0.28. Laser scanning was employed to measure scale thickness and distribution within the tank. The results revealed a dependence of gypsum scale formation on the impeller speed for the mixing tanks investigated here, with higher speeds leading to a significant reduction in scale thickness on the tank walls, while the majority of the scale then formed as discrete particles in suspension. Higher impeller speed may therefore offer the potential to reduce scaling issues for mineral processing plants. Keywords: Scale mitigation, Swirling flow, Mineral processing, Industrial flows