A silver lining

Big is generally seen as beautiful when it comes to designing plant and equipment with the capability to deal with the huge tonnages of low grade ores that characterise many mining operations. Pump manufacturers have risen to the challenge, but not without process issues and problems, as Australia’s largest gold mine in the state of Victoria discovered.

Owned and operated by Lake Resources, the Fosterville Gold mine poured its one millionth ounce of gold on January 7, 2016. It was a milestone that represented over 11 years of continuous operations with daily throughput averaging a massive 1900 tonnes each day

Originally, the mine needed the super-sized pumps that are typical to operations of this scale to handle the heavy recirculation load through the mill. This recirculating load was a result of over-expanded and worn cyclone apexes. Fosterville found itself facing a continuous – and expensive – maintenance challenge and sub-optimal operations.

With four cyclones in operation, the company was struggling to keep up with the rate at which the rubber underflow apexes were wearing out. Typically, each apex had worn from a diameter of 82mm when new to 105mm after just 672hrs, or four weeks of operations.

And after each fourth replacement, the entire cyclone had to be removed from service and rebuilt with new liners costing approximately $8000 in materials and $1000 in labour each time.

High maintenance costs were not the only problem. Fosterville found that the practice of replacing the worn apexes on a rotational basis introduced a protruding ledge where the new liner met the worn one above it, interrupting the flow of slurry through the cyclone. In particular, retained oversize material was inhibiting the dart plug operation and accelerating the wear of rotors and stators.

The worn apexes also allowed too many fine particles into the underflow which increased the recirculating load and unnecessarily accelerated the wear on mill components and discharge pumps. It also decreased the percentage of the underflow which could report to the flash flotation cell.

Fosterville thought there had to be a smarter way to manage their cyclones.

A smarter way to work

FLSmidth were invited to replace the cyclone liners with FLSmidth Krebs ceramic lined cyclones. These cyclones have underflow apex diameters that feature a lifespan up to twenty times longer than rubber versions. Maintenance takes place on an annual basis when the apex will generally have expanded to 95mm and the flow of slurry is much more consistent.

Moreover, the ceramic lined cyclones are supplied and fitted as a single complete assembly, abolishing the need for a separate stream of maintenance activity to replace just the liners.

“Once the cyclone ceramic liners were installed, the efficiency of the cyclone improved significantly because we were able to reduce recirculating load through the mill and mill pumps,” said Andrew Nash, Fosterville Gold Process Engineer.

In terms of improving the separation process, the overflow p80 sizing became more consistent and much less coarse material could be observed in the flotation cells during shut-down.

For Fosterville it represented a double hit. Maintenance costs and associated downtime were slashed with less wear to mill liners and mill process pumps.

Smaller, more efficient pumps

“The benefit we originally envisaged for Fosterville was overall reduced costs of operation and less wear to mill liners and mill process pumps due to the reduction of recirculating load. But there was another, even more interesting impact on another process. The improved separation process made it possible to install smaller and much more efficient pumps,” said Andrew Nash, Process Engineer, Fosterville Gold.

FLSmidth Krebs were invited to install and convert two competitor pumps engaged in primary and secondary mill discharge duties with the Krebs millMAX 10x8-24.

Both pumps featured patented millMAX on-line suction side wear clearance adjustment technology which was developed exclusively for mill discharge duties and then adapted for other severe abrasive slurries.

Adam Noble, Sales Engineer in FLSmidth Krebs explains: “We refer to the Krebs millMAX technology as “zero downtime” impeller gap adjustment. It has removed the down time required for impeller, bearing assembly and pulley/ belt adjustments. This helps eliminate down time while ensuring optimum pump efficiency is maintained and achieved throughout the life of the pump’s wet end.”

The new Krebs Mill Discharge pumps have achieved longer operating hours, consume less power and offer significant cost reductions to the mill pump operations.

The pumps also feature fewer parts and mill discharge pump wet end rebuilds are completed quarterly allowing maintenance planning to reduce spares inventory this in turn helps reduce onsite networking capital

The success in achieving reduced operating costs has led to additional FLSmidth Krebs pump installations throughout the mine and overall savings of more than 40% across mill pump operations.