When it comes to pumps, there are many challenges associated with precious-metal mining applications. For instance, there is a high degree of difficulty in handling abrasive paste with varying density, over long distances and without frequent breakdowns. This difficulty is compounded when chemicals are added to the paste. However, the latest peristaltic hose pumps, such as Bredel models available from Watson-Marlow Fluid Technology Group, are proving themselves highly adept at overcoming these obstacles.
In the first stage of filtration-based mining, ore is crushed and ground, after which the fine ore is mixed with water in froth flotation cells, which serve to extract precious metals. The resulting tail froth is thickened in a sedimentation tank, where the overflow is water and the underflow contains concentrated ore paste. The solids content of the paste is typically 40-75%, reaching the higher end of this range at the bottom of the tank. It is the pumps – which are installed beneath the thickener – that determine the maximum allowable dry solid content. As a result, pump technology is the limiting factor.
The application
Mines will typically feature at least two pumps per tank, for suction and to feed the filter press, which can sometimes be located up to 300m away. Importantly, flow is independent of factors such as changes in paste density, fluctuations in viscosity, and changes in suction and delivery head. Aside from peristaltic hose pumps, no other pump types can withstand such variations.
Traditionally, centrifugal pumps have been adopted widely in precious metal mining, especially for thickener underflow applications, but they have many notable shortcomings. By way of example, the quantity of dry solids that can be accommodated by centrifugal pumps is limited. In numerous applications, sludge pump rotors and impellers last just a few weeks due to factors such as strong acidity and/or abrasive content. Additionally, centrifugal pumps slow down and even stop as paste density increases towards the bottom of the underflow thickener.
In contrast, hose pumps keep pumping at the same flow rate, irrespective of sludge density. Put simply, the performance of peristaltic hose pumps is not affected by the abrasive/aggressive nature of sludge and chemicals. As a further benefit, the use of hose pumps can reduce filter capacity by as much as 75%. With disc filters costing around US$100,000 each, the savings are substantial.
Peristaltic hose pumps like those in the Bredel range are virtually maintenance-free as there are no impellers, liners or mechanical seals to replace, no check valves to clog, and no rotors or stators to wear out. The only wear part is the hose, which can be replaced in a matter of minutes without any requirement for special tools.
The results
Among mines benefitting from Bredel hose pumps is the CMOC International mine in Ouvidor, Brazil, which is mining and processing niobium, an element found increasingly in the superalloys used to manufacture heat-resistant jet engine components. Paste at Ouvidor is made up of 44% niobium with water, sulphuric acid, isopropyl and a flocculant. The solids content rises to 75% as the pulp becomes more concentrated at the bottom of the underflow thickener tank.
Previously, the mine discovered that centrifugal pumps could not manage such a high concentration, and suffered badly due to abrasive wear and chemical attack. Instead, the Ouvidor mine today has several Bredel hose pumps, some of which have been providing reliable operation for nearly a decade. Bredel 100, 80, 65 and 50 units are all pumping niobium pulp in tank-transfer duties, running 18 to 24 hours per day.
And there are many more examples. For instance, at the Waterval plant of Sibanye Platinum in South Africa, six Bredel 100 hose pumps are being deployed to transfer concentrated platinum paste with specific gravities of between 1.4 and 1.7. Elsewhere, in a thickener underflow application at the Queiroz mine of AngloGold Ashanti in Brazil, hose pumps from WMFTG have replaced centrifugal models due to extremely high maintenance costs and high water consumption. Two units (four pumps) serve two tanks handling gold paste at 70% solids content with a viscosity of 2150 mPa.s.
In summary, to help maximise uptime, and thus profitability, as well as reduce maintenance costs and boost site safety, increasing numbers of mine operators are today leveraging the benefits of peristaltic hose pumps. The market costs of precious metals and minerals, in tandem with remote locations and challenging conditions, means that optimised pump selection has never been more important, not only for mines, but also for the environment. The application of peristaltic hose pumps can help ensure the environment is better protected through safe and responsible transfer.
Wilfried Staijen is with Watson Marlow Fluid Technology Group
