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PROCESSING SOLUTIONS 78 E&MJ; • APRIL 2016 www.e-mj.com Gain Improved Tank Slurry Agitation via Swirl Flow Technology By Jie Wu, Steven Wang, Bon Nguyen, Tom Connor, Marjavaara Daniel and Eriksson Ola Large-scale tanks with working volumes in the range of 1,000-5,000 m 3 per tank are used in the minerals processing in- dustry to provide feed storage and various continuous hydrometallurgical processes such as leaching (digestion), precipita- tion, adsorption, oxidation, tailings wash- ing and neutralization. Typically, single or multiple impellers with vertical baffl es inside these tanks are used to provide sol- ids suspension and mixing. In some ap- plications, draft tube agitator or air-lifting pipes are used. It is not uncommon that slurry tanks experience reliability issues, which lead to maintenance shutdowns. Sedimenta- tion accumulation due to solids settling on the tank bottom can result in agitator bogging; and scale formation often caus- es increased sedimentation buildup, due to solids being "glued" together into large lumps by precipitates. Cleanup of sedi- mentation and scale lumps requires con- siderable tank offl ine time. Other issues, including erosive wear and mechanical failure, also reduce operational reliability of slurry tanks. These reliability problems lead to lost production and increased la- bor cost. The minerals industry is facing the dual challenges of prolonged lower com- modity prices and a long-term trend of decreasing ore grades. It is imperative to improve plant productivity and reduce operating costs. The authors have found there are signifi cant improvement op- portunities available through innovation in the area of slurry tank agitation tech- nology—typically requiring only simple design changes at a relatively low cost. This article introduces our experience in improving off-bottom solids suspension and mixing to dramatically increase plant productivity by applying CSIRO's swirl fl ow agitation technology. The Basics To illustrate the approach, it is useful to introduce an empirical coeffi cient S to quantify "status of suspension" at the tank bottom: where N is the agitator (rev/s), ρ L is the liquid density (kg/m 3 ), Δρ is the density difference between solid and liquid (kg/ m 3 ), d is the solid particle diameter (m), D is the impeller diameter (m), ν is the kinematic viscosity of liquid (m 2 /s), and X is the solids loading (weight of solids/ weight of liquid × 100). The S coeffi cient Figure 1 (a-d)—Sedimentation and suspension in stirred tank (a-top left) defi nition; (b-top right) sedimentation depth (normalized) vs. S, extracted from data published in references 2 and 3 ; (c-bottom left) effect of swirl fl ow by removing baffl es; (d-bottom right) mixing research tank facilities in CSIRO, Melbourne.