Metso solve pegging issues at Ghanaian gold mine

Chronic pegging problem eliminated with Trellex LS modular rubber screening media

WHEN Gold Fields Ghana’s Tarkwa gold mine faced the challenge of chronic pegging of their screening media, causing daily downtime, inefficiency and wear, they solved it by installing Metso’s modular, rubber LS screening media, which is anti-pegging and offers a long wear life.

The switch led to increased uptime of 90h, corresponding to 108,000 tons more material being processed annually, as well as a reduction in screen replacement costs and a safer working environment.

The Tarkwa gold mine, located in the Western Region of Ghana, is owned and operated by Gold Fields Ghana Ltd. Tarkwa is a centre of gold and manganese mining, and the open-pit mine is one of the largest gold mines in southern Ghana, with four pits currently in operation.

Approximately 100 million tonnes is excavated and processed each year to yield around 24 tonnes of gold. The processing involves an extensive system of conveyors and comminution equipment to ensure the right product is produced at the right capacity.

The Tarkwa mine currently has two processing plants: a heap leach facility and a carbon-in-leach (CIL) facility. The heap leach facility is a three-stage crushing circuit with a 1,200 tonnes/h capacity. The tertiary screen is a two-deck unit which utilizes rubber and polyurethane on the top and bottom decks respectively. These media were previously sourced from a competing supplier.

Problems arose when the near-mesh rocks clogged the screening media openings, impeding the passage of the undersize material and leading to a higher recirculation load. The mine operators had to stop the line every day for about 15 minutes to knock the trapped material out of the screening media. Moreover, the tertiary bins became overfilled due to the bottleneck downstream.

Gold Fields asked their supplier for screening media that would not peg, but the solution they came up with proved inadequate. The new media were shaped to prevent pegging but they wore out and broke rapidly during operation, allowing large rocks to pass through and creating further problems downstream.

The mining company, therefore, evaluated several alternative suppliers, with Metso coming out on top for meeting the mine’s primary selection criteria. As a result, in January 2011, Trellex LS modular, rubber screening media was installed. These lasted 10 months compared with an estimated three months for the previous supplier’s screens, and did not collapse under heavy loads.

With Metso’s screening media in place, the mine has eliminated the need to stop production on a daily basis to unblock the clogged screens. According to the most recent reports, annual operational downtime at the mine has been reduced by 90h, meaning that an extra 108,000 tonnes of ore can be processed annually. Gold Fields are now implementing the new media in other parts of the mine, including the CIL facility.

The most significant benefits come from the elimination of pegging; now there is no daily downtime for knocking screens. The fact that material no longer has to be removed from pegged screening media also makes the mine a safer, cleaner place to work. Metso’s media also lasts longer, further enhancing efficiency, safety, and profits – and also reducing waste.

Annual cost savings from the new Metso screening media are said to be at least US$315,000, and possibly more. Moreover, the benefit to managers in removing a chronic problem and reducing headaches is ‘priceless’, according to Louis Baanuo, ag unit met manager NHL, at Tarkwa mine. ‘The installation of the Metso panels has brought great relief: our operational downtimes have been drastically reduced, increasing the plant’s throughput,’ he said.