McLanahan breaks the cycle for scrubbing and screening

The panel screens can be seen through the frame of the trommel.

McLanahan’s innovative approach to scrubber and trommel optimisation is resulting in productivity gains for miners. Australian Mining writes.

Rotary scrubbers and trommels are a common sight at Australia’s mines.

They can be an important part of ore processing as they help to effectively separate and wash feed materials prior to downstream processing. Rotary scrubbers can be beneficial for operations that require large feed sizes and high tonnage, with large diameter scrubbers and trommels able to handle feeds of up to 300 millimetres and throughput up to 3000 tonnes an hour.

This makes them ideal as primary washers, separating particles of loam, clay, soft rock and other deleterious elements from the ore particles prior to further screening and crushing.

Rotary trommels, meanwhile, drain and screen the material discharged from the scrubber – typically producing a clean oversize product ready for further crushing and screening.

The trommel underflow, comprised of water, clays in suspension and ore particles up to the trommel screen cut size, is typically screened again to separate the water and fines for further wet separation processes.

Scrubbers and trommels serve as a hardy alternative to vibrating screens, which tend to be more expensive, louder and more prone to wear and fatigue failure than rotary trommels. For ore feeds containing a high component of clays, scrubbers are the only alternative for breaking down and separating clay particles at high tonnage rates.

Original equipment manufacturer (OEM) McLanahan offers a machine that combines both rotary scrubbing and screening functions into a single unit. By combining these two essential functions, the plant saves on footprint and elevation, improves productivity and reduces maintenance times.

The initial screening process can be a highly abrasive activity, causing high wear rates and impact loads. If using a vibrating screen for this duty it can lead to significant replacement costs for screen decks, side panels and exciter beams.

In a rotary trommel there are no vibrating parts with the ore subjected to a relatively low impact tumbling action, which prevents stratification and ensures all particles are presented to the screen apertures for removal of the undersize fraction. Even so, with large feed particles and high SG ores, this can be an arduous duty for trommel screen panels.

McLanahan regional manager Richard Williams and his colleagues were tasked by an Australian iron ore producer to improve its existing rubber trommel panels, which had an average life of six weeks, with superior options that could provide a reliable wear life in excess of 26 weeks. This resulted in extensive research and development to find materials and designs to attain the target wear life.

The first stage of screening, be it flat vibrating screens or rotary trommels, is an arduous duty,” Williams tells Australian Mining.

“Some operations opt to use a combined scrubber-trommel taking advantage of having washing and screening on the same level. This provides an opportunity to eliminate a floor level to discharge onto your screen — this can save a lot of money on structure. But however you choose to do this first stage of screening, it’s an area of high wear and high abrasion,” Williams explains.

“The standard supply is thick rubber panels, but you’ve got big lumps of heavy iron ore rolling around in the trommel, so those panels don’t last very long.”

Williams and the team carried out several tiers of wear testing and impact testing on eight different trommel panel designs, with two eventually selected as the most suitable for full-scale trials in the scrubber-screen unit.

The trommel panel materials and designs were selected for testing on the basis of their potential to improve wear life, screening capacity, washing capabilities, maintenance costs and operating time between shuts.

“We set up an impact test jig to simulate the in service environment within an accelerated time frame delivering a lifetime of impacts in a few hours. All materials were separately tested for abrasion and wear, as well as also impact load — we needed a material and a panel that would survive the impact and the abrasion.

The two panel options selected for full-scale trials were a composite white iron HIcast panel and a dual layer steel CCO panel. The HIcast panel consists of a hot vulcanised rubber sandwiched between a cast white iron wearing surface and a steel backing plate. The dual layer CCO panel is an ultra-high wearing fused chromium carbide overlay (UHW CCO) panel which is installed on a semi-permanent quenched and tempered (Q&T) steel backplate for increased impact resistance. Both panel types were able to satisfy the required trommel panel service life.

The HIcast panels lasted longer during trials, but were also heavier than the CCO panels which made them more difficult to install. While the CCO panels had a shorter service life they easily met the customer’s requirements and the use of the backing panel in conjunction with the lower weight made installation faster and safer than the HIcast panels.

Regardless of the circumstances, both panels offer similar benefits for miners overall, says Williams. The big advantage for operators is that they can schedule longer times between maintenance periods.”

“The HIcast is a bit more expensive but it lasts longer so in terms of cost over time there’s not much difference, except perhaps for the advantages a site might get by extending their time between shutdowns,” he explains.

The drive type used on this customer’s scubber-screen was also a factor in choosing the lighter CCO screens. Hydraulic end drives, while offering advantages of smooth start-up and easily adjustable speeds, have a disadvantage of more limited access to the inside of the machine.

This increases manual handling requirements and resulted in a preference for the lighter CCO panels. Girth gear drives allow and open ended trommel design to provide easier access to the machine, making the heavier HIcast panels more manageable with the aid of machine handling.

McLanahan’s engineering approach encouraged such flexibility, being willing to customise equipment “in ways that make sense”, according to Williams.

This keeps with the company’s mantra of ‘Safer Simpler Smarter’, which stresses collaboration with plant operators across every facet of machine performance.

“Adjustability is critical. When you design a new plant you might sample a few tonnes of ore and do some testwork, but you don’t really know the full range of operating parameters you need to run because of the inherent variability in the ore,” says Williams.

“Customers like to have a machine that has a degree of adjustability so that if there is such variability over the life of the machine it’s not too big a deal to make those changes.

“The bigger the equipment, the more value you get out of customisation. It’s not ‘one size fits all’ because each site has its own peculiar set of realities.”

The company also offers post-installation service and support options, and condition monitoring implementation to allow for predictive maintenance that ensures machines run to their optimum capacity, driving plant performance.

This article appears in the April 2019 issue of Australian Mining.

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