Tailings Management – Dewatering Flume Site Trials

The safety, environmental and social concerns regarding tailings dams are at the forefront of the industry, and reductions in the volume of water stored in dams have a direct impact on alleviating these concerns. Moreover, more efficient separation of liquid and solids provides an opportunity for utilising coal that otherwise would have been waste. This project examined the application of a novel passive settling rig, relying only on geometry and the Boycott effect, to more efficiently separate water and solids.

The objectives were:

1. Develop a test program to probe the complex relationships between slurry concentration, flow rate, flocculation and rig geometry on the resulting flow and performance.
2. Design and build a scalable settling rig for site applications.
3. Provide refinements and potential optimisation for future work.

Given that the only major design constraint was for the settling rig to be passive, this simultaneously provided a challenge and a high degree of freedom in the conceptual design stage. Due to the complex physics involved, Objective 1 was examined through a combination of individually simplified studies, which, in combination, would result in richer findings than the sum of their individual parts. This approach involved a combination of a detailed literature review, Computational Fluid Dynamics (CFD) and bench and pilot-scale testing.

Objective 1 provided a series of outcomes and findings that assisted in the design and operation of the site test rig. These findings include:

• The importance of minimising the recirculating currents and exposing the flow to the inclined plates as much as possible.
• CFD was concluded to be most suited for flow optimisation and proved a valuable tool to rapidly conduct a large number of simulation studies comparing different geometries and orientations.
• Simple bench scale settling tests are a quick means for obtaining an indication of the concentration that maximises the settling mass flow rate.
• Flocculant increases performance but has a higher sensitivity to flow rate.
• Solids concentration was found to be the most influential parameter, with bench-scale testing of site samples indicating concentrations in the vicinity of 10% being the most optimal for maximising the mass of solids settled.

Objective 2 was successfully completed with the development and testing of a site test rig that demonstrated it can effectively reduce the solids concentration over the range of system variables tested. These results confirmed that the settling velocity increases with decreasing concentration, and it is possible to optimise the feed C_w to achieve the maximum reduction in solids. Findings from the site testing include:

• Both the C_w and d₅₀ at the outlet were reduced when compared with the inlet feed for flow rates up to 10 m₃ h⁻¹ and inlet feeds of up to 18% C_w.
• As the flow rate reduced, there was a greater reduction in the outlet C_w when compared to the inlet C_w.
• Reducing the C_w of the feed resulted in a greater reduction in the outlet C_w and, similarly, a greater reduction in the PSD d₅₀.

This project demonstrates the efficiency of using passive settling to effectively reduce wastewater disposal in tailings dams, and the report describes potential future work.