Following over 80 years of continual development and innovation, Weir Minerals’ Warman® Mill Circuit (MC) slurry pumps are the company’s global flagship product for the mining sector. Product manager for pumps, Marnus Koorts, outlines some of the modern features driving this success.
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‘Mill circuit pumps are often described as the beating heart of a mine’s minerals extraction circuit. They sit alongside every grinding mill, pumping newly ground mineral slurries up into the cyclones, which separate the coarse and the fine material, sending coarse particles back for further milling and the fines on for minerals extraction,” begins Koorts.
“Slurry pump wear rates are directly related to the volume and size of the particles. Larger pieces of rock from a primary mill impacting on the pump’s throat bush, impeller and volute liner cause much more rapid wear than the micron-sized fine abrasive particles being pumped out of the end of the mill circuit by the tailings pumps,” Koorts explains.
A typical centrifugal pump on the tailings end of a mill circuit, he suggests, might last for many months before wear components such as the liner, throat bush or impeller could need replacing, while a mill circuit pump can have a wear life of months, and possibly only weeks, if the installation is not optimised.
“The problem, of course, is if the primary mill’s circuit pump stops, the whole mill circuit must stop and minerals extraction, often for the entire mine, comes to a standstill. This immediately impacts production, with direct consequences for the income stream of the entire operation,” he points out.
“Our value offering at Weir Minerals is to produce mill circuit pumps that are not only reliable, but also have the maximum possible life between wear component changeouts,” Koorts tells MechChem Africa.
The Warman® MC: the industry benchmark
Weir Minerals reported an R&D investment of 1.3% of total global sales in its 2020 Annual Report. According to Koorts: “A high percentage of that spend goes to advancing our flagship Warman® MC pumps. We have, over the years, developed a long list of ground-breaking innovations that are unique to us. We invented the adjustable throatbush, for example, and we have been advancing that technology ahead of our competitors for many years,” he continues.
Explaining how this works, he says that material from the sump of the grinding mill goes into the pump at the throat and is propelled by centrifugal forces through the impeller to the outer volute of the pump casing. The throat bush sits around the inlet side of the pump creating a small gap between the rotating impeller and the casing. It is important to minimise this gap, as it minimises recirculation of material from the high pressure outlet side of the pump back to the inlet side.
“This gap, therefore, directly affects the pump’s efficiency: the less fluid that recirculates, the better the efficiency. As the impeller and the throatbush wear, however, the gap widens and more and more of the slurry is able to recirculate. By adjusting the throat bush gap back to the minimum specified, we are able to reduce recirculation and routinely restore pump efficiency to its best possible,” Koorts explains.
The electricity costs of driving a large multi-kilowatt motor on mill pumps far outweigh the initial investment costs, so if a couple of percentage points of the pump efficiency can be recovered by minimising recirculation – on a 1.6 kW or larger motor, for example – this quickly justifies the investment on a Warman® pump with this capability.
Pointing to a second innovation on the throat inlet, he highlights the small pre-swirl vanes that guide the material flow through the eye of the impeller, which prevents direct impacts of large particles onto the impeller blades.
Deep expelling vanes have also been added on the high pressure side of the throatbush to reduce the formation of eddies. This minimises turbulence, prevents the formation of vortices, reduces recirculation and further improves hydraulic efficiency. In addition, the expeller vanes prevent localised scouring on the throatbush face, further improving the pump’s wear life.
From a maintenance perspective, Koorts assures that Warman MC pumps are designed for easy, rapid and safe repair. He cites the quick change-out system of the pump’s wet-end as a key innovation: “The entire wet-end of our MC pumps can be unbolted and removed from the bearing and drive assembly shaft. This may sound basic, but it makes maintenance of these large pumps a far easier task, taking hours off the time needed to replace wear parts,” he explains. “Also as part of our extended safety offering, all of the assembly bolts, even on the biggest pumps, are designed to be easy to handle,” he adds.
Another core area of innovative leadership for Weir Minerals is on the materials side of pump design. “We have developed the widest selection of materials for lining our impellers and pump volutes and for our throatbushes. Most slurry pumps on the market use high chrome metal for all of these. We can offer a wide range of high chrome and various rubber compounds, along with a hybrid MCR-M option, which is a metal lined volute that can be interchanged with a rubber lined volute if required.
“In Africa, there are several copper and gold applications with quartz bearing ores, for example, and we have found that in some cases our rubber compounds offer better wear life for these specific ores. Being able to change over, however, takes away the risk of making the wrong liner choice. It gives the flexibility to exactly match the lining material to the ore being mined, which might change depending on the specific location of the ore body,” Koorts explains.
Weir Minerals has also purpose-developed several other niche metal alloys for use in key areas of its pumps. “Ultrachrome® A05 is our high-chrome alloy, which is good for resisting impact wear from large particles. But we also have an alloy called Hyperchrome® A61, which is far better for fine particle abrasive wear. We can use A05 for an impeller and volute, while using A61 for the throatbush, for example, to give the best of both. Only fine particles are able to recirculate through the narrow throatbush gap, so impact is not a problem, making A61 ideal.
“With our R55® rubber available for the volute, we have multiple optimising opportunities. No other slurry pump manufacturer can offer this level of life-optimisation flexibility,” Koorts tells MechChem Africa.
Low flow volute liners are another innovation, enabling the rated flow of a pump to be reduced by 15 to 25% depending on where the pump operates. This is an ideal solution for mine start-ups, where a lower initial capacity is needed until the mine is fully established, after which the low flow liner can be removed, and a full-sized impeller installed to deliver full run-of-mine production capacity.
“Warman® Mill circuit pumps are typically very large and involve intensive capital investment. Having flow flexibility on a big mill circuit pump makes a lot of sense for future proofing the investment.
“Speed (RPM) is a very important factor in our design philosophy. We believe that a larger pump is better for wear life, efficiency and total cost of ownership. Small impellers need to turn a lot faster than larger ones to deliver the same flow. A slower RPM translates into better wear life. It might be cheaper to buy a smaller pump, but the costs in terms of wear components and increased downtime will far outweigh the cost of a bigger pump. This means it is extremely beneficial to buy a pump bigger than needed and to then use a reduced flow liner to optimise the efficiency for low production periods, if one wants to expand in the near future,” Koorts argues.
To achieve the full benefits of these Warman® MC pump innovations, the company employs its own optimisation team to model the improvement that can be achieved by installing a best-match solution for the mine’s needs. “We make use of 3D scans of the mine environment to see where everything flows to. We do a Computational Fluid Dynamic (CFD) analysis of the slurry flow, from the sump to minimise dead spots and settling, into the pump inlet and right through the impeller and volute. We then model the pump wear for different material options and configurations.
“Once we have chosen an optimised pump for the mill circuit in question, we are happy to manage and fund an installation on a trial basis, on the understanding that if our solution delivers the savings we predict, we can then recoup the costs. To date our optimised solutions have enjoyed great success with no rejections. We have always managed to deliver measurable savings,” Koorts reveals.
“This year, we are celebrating 150 years of innovation within the Weir Group and are very proud of the legacy that has followed. We believe we now have the most efficient mill circuit pump solutions available: pumps that are more efficient, use less water, are safer, easier than ever to maintain and with the longest possible wear life of the wet-end components,” Koorts concludes.
