Experimental investigation of hydrodynamic and heat transfer effects on scaling in an agitated tank

Abstract

Scale formation on process equipment is a major problem in the mineral industry because of lost production from equipment downtime required in de-scaling maintenance and its high cleaning cost. Scale formation in the Bayer process mainly occurs from crystallisation of Bayer liquor which is not fully understood. This study investigated the hydrodynamic and heat transfer effects on crystallisation scale growth and its suppression, using a novel experimental approach in a newly fabricated lab-scale agitation tank. A series of experiments were undertaken using laboratory-made potassium nitrate (KNO3) aqueous solutions.Experiments were conducted with different size impellers at varying rotational speeds using various concentrations of KNO3 solutions. It was observed that the impeller agitation speed plays a critical role in the scale growth and suppression mechanisms as the scale growth rate is enhanced at the lower and is reduced at the higher agitation rate. The effects of baffles on scale growth rate and suppression and the bottom settled scale accumulation rate were also examined. The wall scale growth rate decreased with time ranging from 58.06% to 6.79% and the corresponding bottom settled scale increased ranging from 4.19% to 80.2% depending on the agitation rate, impeller size, solution concentration and tank conditions.