A TEAM of CSIRO researchers using a scaled-down model of an alumina slurry mixing tank have succeeded in improving tank productivity without increasing power consumption.
The innovation has been tested in a mixing tank operated by Queensland Alumina Limited (QAL). The result: on the same operating cycle, less scale forms on the tank walls, making the de-scaling task easier and reducing the risk of damage to agitators.
During alumina extraction, finely ground bauxite is mixed with recycled caustic soda solution and steam to dissolve the alumina content. At one stage of this process, the slurry viscosity is high and mixing the slurry becomes very energy-intensive. Even a power capacity of 100 amperes still sees cement-like scales forming at the walls of the slurry mixing tanks. Increasing motor power is cost-prohibitive, leaving QAL with no option but to shut down its tanks and undertake elaborate de-scaling operations.
“To reduce scaling we needed to increase velocity at the walls without increasing power or introducing bigger motors,” explains Dr Jie Wu, fluid dynamicist with CSIRO Materials Science and Engineering. “To achieve that goal we applied science relating to the slurry’s non-Newtonian flow.
“The science says that the slurry’s problematic viscosity can be reduced by increasing the shear rate. Technically, that meant we needed to increase the impeller-blade rotating speed. We achieved that by lowering blade pitch angle so that power consumption is maintained.”
With their model tank fully instrumented for torque and speed measurements, the research team experimented with altering the pitch angle of the rotating blades to increase operating speeds and obtain different fluid behaviours.
“With a lower-than-standard impeller-blade pitch angle, we were able to increase slurry flow velocity by 30 per cent,” Dr Wu says. “An increase of that magnitude is equivalent to doubling the power, but we achieved it with the same electrical power consumption.”
The increased slurry flow velocity reduces scale build-up, making the de-scaling task easier. The new design has been successfully tested at one of QAL’s real-world tanks for two years.
Dr Wu believes the strategy of increasing the shear rate could be useful for highly viscous slurries found in many other mineral processing plants.
This article first appeared in Process (October 2007) – a publication of CSIRO’s Minerals Resources sector.
Dr Jie Wu
CSIRO Materials Science and Engineering