PHYSICAL MODELLING OF IMPELLER EROSION IN ALUMINA PRECIPITATORS

Impellers in draft tube precipitators used at alumina refineries are prone to erosion damage, with consequent costs associated with downtime and impeller replacement. Whilst one option to manage the erosion issue is to replace the draft tube system with another such as SWIRLFLOW®, many draft tube systems are still in service and will remain so until they reach end-of-life. Thus, there is still interest in managing draft tube impeller erosion in a low-cost manner.
Several design options were considered for reducing erosion on draft tube impellers, including application of erosion resistant coatings. A test program was developed at CSIRO to evaluate the erosion performance of model impellers including a ceramic coating and a hardened tip section. Aluminium substrates were used for these impellers to give accelerated erosion and comparisons were made to an impeller blade made completely from aluminium. A large test rig was developed using a 1.5 m diameter tank and associated accessories mimicking the full-scale flow system such that realistic erosion distributions were formed. Three impeller blades were tested simultaneously on the same hub so that direct comparisons could be made between a control, a ceramic coating and a blade with hardened tips. Quantitative erosion measurements were made using a coordinate measurement machine.
The erosion patterns observed on the aluminium impeller blade in the tests corresponded with the full-scale experience. The ceramic coated blade showed significant erosion in the region immediately downstream of the step between the coating and the blade substrate. Its erosion was also higher than expected. The blade with the flush hardened tip showed the lowest erosion and was recommended for full-scale trial. It has subsequently been placed in service at Queensland Alumina with good results so far in terms of reduced erosion damage. The general approach of modelling the impeller’s erosion presented here is applicable for many erosion applications both in the alumina industry and the wider minerals processing industry).