INVESTIGATION OF THE AGITATION EFFECT ON SCALE GROWTH IN MIXING TANKS

A novel approach is proposed for the numerical evaluation of scale thickness and its distribution in a mixing tank. The methodology applied in the current study consists of two major phases: identifying an accelerated process to grow scale under controlled conditions, and physical analysis of the grown scale on the tank wall for different tank design options. For the former, the selected chemical system was based on CaO where scale forms in a reasonable time. The subsequent analysis of scale growth was achieved in a purpose-built tank that could be disassembled and measured with a coordinate measuring machine (CMM) thus giving the distribution of the scale thickness over the tank walls.

This approach was used as a qualitative and quantitative study examining the effects of the flow velocity, and baffle configuration on the pattern of the scale formed in mixing tanks. In conventional baffled tanks it has been observed that the overall mass of scale declined with an increase in the impeller speed. At the same time, results indicate when close to the liquid surface, the average scale thickness at higher impeller speeds increases, implying that the build-up of the scale at the near-surface zones becomes thicker as the speed increases.

An unbaffled tank exhibited much better scaling performance when compared to the baffled case, which can be attributed to the increased flow velocity near the walls in the absence of baffles. It is hoped that results obtained by this approach can be used to inform the design or retrofitting of mixing tanks in the alumina industry to minimise the scale growth.