Swirl Flow Agitation for Scale Suppression

Topics: Velocity, Shear stress, Reaction rate Pages: 24 (7381 words) Published: March 18, 2013
International Journal of Mineral Processing 112–113 (2012) 19–29

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International Journal of Mineral Processing
journal homepage: www.elsevier.com/locate/ijminpro

Swirl flow agitation for scale suppression
J. Wu a,⁎, G. Lane a, I. Livk a, B. Nguyen a, L. Graham a, D. Stegink b, T. Davis b a b

CSIRO Process Science and Engineering, Minerals Down Under Flagship, Graham Rd, Highett, Victoria 3190, Australia Queensland Alumina Ltd, Parsons Point, Gladstone, QLD 4680, Australia

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Scale formation is a serious problem in the mineral processing industry. To better understand the options available for mitigating this problem, a novel scale–velocity model is proposed in this paper for slurry systems commonly found in mineral processing plants. The new qualitative scale growth model predicts that at very low fluid velocities the scale growth rate is enhanced by an increase in fluid velocity due to the mass transfer-controlled scale growth. At higher fluid velocities, the scale growth rate decreases with increasing fluid velocity due to the increased flow erosion effect, for slurry systems. This suggests the potential of particulate erosion as a scale suppression mechanism. The model predicts the existence of an optimal slurry flow velocity, where scaling rate and equipment erosion rate are both zero. The optimal fluid velocity value is proposed to be used as the main parameter to improve engineering design of mineral processes in terms of scale suppression. A novel agitator design, swirl flow technology (SFT), developed and patented by CSIRO and Queensland Alumina Ltd (QAL) in Australia was introduced as an agitator design that better meets requirements for scale suppression than a widely used conventional draft-tube agitator system. SFT agitation has been installed in gibbsite precipitators at QAL's alumina plant for a decade. As shown by CFD simulations and laboratory measurements, swirled flow agitation generates more uniform velocity distribution and higher velocity values at the wall than conventional agitators for the same power input; this reduces the maximum growth rate of scaling in the tank leading to significantly prolonged precipitator's service life. Based on the full-scale operational experience at QAL, it can be suggested that SFT agitation roughly halves the scale growth rate as compared to that measured in the conventional draft tube agitator systems. © 2012 Elsevier B.V. All rights reserved.

Article history: Received 21 November 2011 Received in revised form 8 July 2012 Accepted 14 July 2012 Available online 22 July 2012 Keywords: Scale Scale suppression Erosion Swirl flow Agitator

1. Introduction Scale formation in slurry equipment has been a serious on-going problem for the mineral processing industry. Its enormous cost for the mineral industry is manifested through increased capital expenditure, reduced capacity and throughput, and continuous requirement for human intervention. Scale growth is a major cause of lost production through tank downtime required for de-scaling cleaning operations. Scale is probably a more serious problem in the minerals industry than any other process industries. Slurry tanks are used extensively for hydrometallurgical processing such as leaching, digestion and precipitation. Often a large number of slurry tanks are installed for continuous chemical reactions. In low viscosity Newtonian slurry mixing tank operations, agitators are often designed on the basis of achieving off-bottom solids suspension, as other mixing processes often satisfactorily follow once the solids are suspended. It is useful to mention that, in a fully suspended Newtonian slurry tanks the times required to mix liquid and solids or ⁎ Corresponding author. E-mail address: Jie.Wu@csiro.au (J. Wu). 0301-7516/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2012.07.007

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