A proven approach for reduced downtime and increased throughput when compared with conventional agitation systems
Mixing and agitation systems traditionally employed in slurry tanks used at mineral processing operations generally utilize single or multiple impellers in baffled tanks, an impeller in a long draft tube or an airlift agitation system such as a Pachuca. These slurry tanks often experience reliability issues that result in extended maintenance shut-downs, causing lost production and increased operating cost.
Common factors contributing to these reliability issues include scale formation, sedimentation buildup, erosive wear and mechanical failure. Scale formation due to chemical precipitation leads to lost production from tank downtime during de-scaling operations. Sedimentation accumulation due to solids settling at the tank bottom may cause an agitator to bog down or have difficulty restarting. Cleanup of sedimentation during scheduled or unscheduled shutdowns leads to production loss and increased operating cost. Mechanical failure may occur, for example, when a long cantilever shaft breaks due to highly fluctuating slurry-flow forces in unstable gas sparging operations; or a draft tube steel structure can fail from increased weight caused by scale material. Wear damage is caused by abrasive solids, particularly when coarse solids are present and impellers operate at high tip velocities.
Processing operations often must cope with lower ore grades, and may choose to maintain solids suspensionin tanks (e.g., leach tanks) at higher concentration to maintain production throughput. However, problems associated with this approach include po-tential issues of solids settling out, increased agitator bog risk and significantly higher agitation power requirements.
A number of years ago, in partnership with Queensland Alumina Ltd. (QAL) in Australia, CSIRO developed and patented Swirl Flow Technology (SFT) to exploit the relatively low-energy swirling motion (Figure 1) that can be created and sustained for a wide range of slurries by using a short-shaft impeller positioned near the top of a vessel. The effect created by the impeller picks up solids from the base of the tank and imparts energy to them to ensure good mixing as they continue to swirl around the outer portion of the tank en route to the base of the tank again. The flow pattern is very similar to that generated by a tornado in nature. CSIRO obtained rights from QAL in 2013 to commercialize SFT.
|Figure 1—Swirl Flow Technology, as shown in the left-hand diagram, generates a tornado-like flow pattern in suspended solids. A laboratory-scale visualization of the suspension process is shown here from agitator startup (a), followed by initiation of the pattern (b) and establishment of a full swirl pattern (c).|
QAL has been replacing broken draft tube agitators in their precipitation tanks with SFT systems since 1997; currently there are 22 precipitation tanks operating with SFT at QAL (Figure 2). Because of the short shaft design, SFT equipment installed at QAL does not experience similar mechanical failures-and because SFT impellers operate at a lower tip speed than conventional agitation systems, they have much lower wear rates and cause much less attrition to particles.
|Figure 2—Swirl Flow Technology components installed in a precipitation tank at QAL (left), replacing a failed draft tube in a tank (right) measuring 11 m in diameter x 28 m tall.|
SFT agitator systems typically cost only a fraction of the conventional draft tube system originally used at QAL. Due to the short shaft design, a SFT agitator system experiences significantly less force loading than a baffled agitator system or draft tube structure used in traditional mixing systems. This results not only in substantial capital cost savings but also in high mechanical reliability.
A distinct advantage offered by SFT's swirling motion is increased "self-cleaning" of the reactor wall, providing an opportunity to suppress scale growth and reduce de-scaling maintenance downtime. CSIRO can model scaling formation in its laboratory to enable development and optimization of SFT designs for demonstration on a smaller scale before being validated at commercial scale. Figure 3 shows reduced scale formation on tank walls achieved by replacing a conventional agitator design (axial flow impeller with baffles, (a)) with SFT (b). The tests were conducted in the laboratory using a supersaturated potassium nitrate solution to simulate precipitation in a full-scale plant.
|Figure 3—Demonstration of reduced scale formation in laboratory, using precipitation from a supersaturated potassium nitrate solution: (a) scale formation using a traditional impeller with baffles; (b) reduced scale formation using SFT impeller.|
Since the first commercial scale installation of SFT in QAL in 1998, the operation has experienced significant scale reduction and subsequently, roughly 50% less tank de-scaling downtime (Figure 4).
|Figure 4—Full-scale installation of Swirl Flow Technology at QAL has resulted in an approximate 50% reduction in scale formation and a subsequent decrease in descaling maintenance downtime.|
SFT can be used to provide more reliable agitation when a tank is operating at high-concentration levels. This can be effective in maintaining production from lower grade ores while using a plant's existing tank infrastructure. On the other hand, conventional agitation technology can be problematic when operating at high concentration; for example, at mineral plant operations in remote locations, unplanned power outages can occur. A conventional agitator is likely to experience difficulty restarting when the sedimentation bed depth is high, as illustrated in Figure 5 (a), as the bottom impeller could be buried and require excessively high startup power. The upper limit of solids loading at which a conventional agitator system could restart directly can be around ~40-50% w/w, unless time-consuming pump-out of excess slurry is carried out.
|Figure 5—SFT provides an advantage when restarting agitation after a power outage when compared with conventional systems, because its higher position in the tank allows the impeller to stay clear of bedded sediment.|
Due to its tornado flow pattern, an SFT system with its short shaft design can restart from an unplanned shut-down at a significantly higher solids con-centration, because the impeller is not buried in sediment. Figures 6 (a) and (b) illustrate a laboratory demonstration of restarting an SFT system in a suspension of water glass mixture (~74% w/w). Figure 6 (a) shows solids settled out to form sedimentation with supernatant water (dyed with red color for visual contrast). The sedimentation depth was approximately 2 m out of a total liquid height of 2.5 m in the laboratory test tank. Figure 6 (b) shows full suspension after the impeller was started.
|Figure 6—Shown here is a laboratory-scale restart of a SFT system from a dead stop when operating at a high solids concentration of 1,300 gpl (74% w/w), solids SG 2.5, d50~0.10 mm. Figure 6 (a) shows a settled sedimentation bed in water (dyed red) with the impeller stopped. Figure 6 (b) illustrates the tank contents in full suspension 10 minutes after restart. The test tank is 1 m diameter x 2.5 m tall.|
With its lower capital cost and potential capability to reduce operating costs having been demonstrated, SFT is attracting attention from a number of operating companies processing bauxite/alumina, gold and base metals, and with prospects for an even broader range of applications.
Jie Wu, Bon Nguyen, Lachlan Graham and John Farrow are associated with the CSIRO Minerals Resources Flagship in Advanced Processing Technologies. For additional information on Swirl Flow Technology, contact Wu at [email protected].