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PROCESSING SOLUTIONS 80 E&MJ; • APRIL 2016 www.e-mj.com these tanks have, for many years, expe- rienced inadequate stirring that reduced surge capacity and resulted in production loss due to stoppages at the concentration and pelletizing plants. LKAB commis- sioned CSIRO to conduct an experimen- tal research using laboratory modeling. It was concluded from small-scale tests that the original four-bladed, axial-fl ow impeller located near the bottom could be made to provide improved off-bottom sus- pension and dramatically better mixing at approximately the same power input by using swirl fl ow after the tank baffl es were removed. A larger impeller diameter was required to maintain the same power input after baffl e removal. This research result was implemented in one of LKAB's full-scale slurry tanks (Figure 2), achiev- ing a successful outcome of more con- sistent blending, which allowed ~50% increase in tank surge capacity. Queensland Alumina, Australia Essentially the same swirl fl ow agitation approach applied at LKAB Kiruna was also used in bauxite slurry holding tanks at Queensland Alumina (QAL) to solve a sedimentation-related agitator bogging problem that caused frequent shutdowns and reduced tank-online campaign time. Swirl fl ow in QAL's nine slurry holding tanks (Figure 3) was achieved by remov- ing all four vertical baffl es while retain- ing the original Lightning A310 impellers (three or four impellers). Additional motor power capacity became available after baffl es were removed, allowing an exten- sion of the bottom impeller diameter to improve off-bottom solids suspension and resulting reduced build-up of sedimenta- tion solids. This modifi cation, carried out in 2013, improved tank reliability with online time for HT2 extended from ~3-4 months to ~13 months. Swirl fl ow can also be generated by a single radial impeller located at the tank top, as schematically shown in Figure 4 (a). In this design (patented), a swirling fl ow pattern picks up solids from the base of the tank, lifts them and mixes them well as they swirl around the outside of the tank and return to the base of the tank. The fl ow pattern is very similar to that of a tornado in the natural environ- ment. Due to the short shaft design, this swirl design experiences signifi cantly less force loading than in baffl ed agitator systems or draft tube structures used in traditional mixing systems. This results in substantial capital cost savings and high mechanical reliability. QAL has been replacing broken draft tube agitators with swirl fl ow designs in their precipitation tanks since 1997; cur- rently, there are 21 precipitation tanks operating with swirl fl ow at QAL (Figure 4(b) and (c)). Swirl fl ow agitator systems typically cost a small fraction of conven- tional draft tube systems at QAL, and due to their short shaft design swirl rotors do not experience mechanical failure. Swirl rotors operate at a lower tip speed than conventional agitation systems and therefore have much lower wear rates and cause less attrition to particles. One dis- tinct advantage of the swirling motion is higher wall velocity, which acts to "self- clean" the walls of the reactor; this sup- presses scale growth, reduces downtime and the need for de-scaling maintenance. Successful application of swirl fl ow at QAL has improved the operating factor of the converted tanks, increasing both plant yield and capacity. With its demonstrated advantages of improved mixing, reduced capital cost and improved reliability in lowering op- erating costs, the CSIRO swirl fl ow tech- nology is attracting attention from min- eral producers and is now being rolled out across several application sectors, including bauxite/alumina, iron ore, zinc and uranium. For more details, contact jie.wu@csiro.au. Dr. Jie Wu and Bon Nguyen are affi liated with the Fluids Engineering Laboratory, Mineral Resources, CSIRO, Australia. Steven Wang is research fellow, ETH, Zu- rich, Switzerland. Tom Connor is produc- tion analyst, Queensland Alumina Ltd., Queensland, Australia. Marjavaara Daniel and Erikson Ola are senior research engi- neers, LKAB Technology & Business De- velopment, Kiruna, Switzerland. References 1. Zwietering TN., 1958. Suspension of Solids in Liquid by Agitators. Chem. Eng. Sci. 8, 244–253. 2. Wu, J., et al., 2011. AIChE Journal 57(9), 2316-2324. 3. Wu, J., et al. 2016 (in press), Chem- ical Engineering Technology. Figure 4 (a-c)—Swirl fl ow application at QAL, (a-left) tornado-like swirl fl ow schematic, (b-middle) swirl rotor, (c-right) precipitation tanks operating with swirl fl ow. Figure 3—Bauxite holding tanks at QAL employ swirl fl ow agitation.