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Correct valve installations double the operational lifespan at Tarkwa mine

Seemingly minor details can make a major difference in unplanned costs. Valves are small, yet essential part of mining industry processes, and if problems with the valves should occur, it can lead to unplanned downtime and the equivalent losses in output. The downgrading of valve specifications as a cost-saving measure at the Tarkwa mine, the largest gold producer in Ghana, proved to be counterproductive as the premature failure of replacement products resulted in costly stoppages and maintenance expenditures in the gold processing plant. But with the help of Metso’s valves business service experts, the mine was able to achieve consistent reliability that ultimately led to improvements in the plant’s overall efficiency.

Ghana is consistently ranked among the world’s top 10 gold producers. One of the key contributors to this status is the Tarkwa mine operated by Gold Fields Ghana.

Tarkwa is situated in the southwest part of the country, some 300 kilometers west of the capital Accra. It began opencast mining operations in 1998. Rated as a world-class, low-cost surface mine, the operation is the country’s largest producer of gold, extracting over 14,000 kg (500,000 oz) annually. This is well over 12% of the national total.

Refining this volume of precious metal requires the operation to annually process an average of around 13,000,000 metric tons of ore-bearing material, which, until 2004, was processed using the Heap Leach method of refinement.

With a switch to the use of Carbon in Leach (CIL) technology that same year, a newly commissioned CIL plant was introduced and subsequently expanded in 2009. This resulted in some unexpected consequences: an unprecedented reduction in the lifespan of valves.

Quality issues leading to unplanned downtime

These valves, both in terms of their numbers and functionality, play a critical role in the CIL process, and premature failure has the two-fold outcome of unplanned downtime accompanied by equivalent losses in output.

Metso’s Service Sales Engineer for Valves business, Christian Appiah, explains that valves in the mining environment work under extremely demanding conditions.

“Valve and actuator failure can arise for a number of reasons. For example, high levels of pollution owing to dust and other contaminants results in very poor air quality. Added to this are the problems of very high ambient temperatures and pressures, not to mention the challenges posed by vibration. An erratic and low-voltage power supply also compounds potential issues,” he says.

Christian goes on to explain that the valves fitted as original equipment to the second-generation plant performed as expected in terms of life expectancy, however it was after routine replacements that problems began to emerge.

“There seemed to be a problem with quality, even in instances where the valves were replaced by seemingly identical products from the same brand. This is something that could be attributed to the use of lower grade materials.”

“One of the challenges in this situation is that supply chains regard valves and similar products as commodities. Price therefore, becomes the main motivating factor behind the buying decision, irrespective of potential consequences down the line.”

Metso has enjoyed a longstanding relationship with Gold Fields Ghana through its supply of heavy duty equipment, including crushers, pumps and screens. It was from this association that Tarkwa was approached with regard to the possibility of sourcing alternative valve requirements from Metso’s Valves business services.

Specialist analysis yields step changes in efficiency and reliability

That was the beginning of an intensive process involving a great deal of groundwork and technical analysis over a five-year period. Christian was deeply involved in the process that ultimately provided the solution to the customer’s needs.

To cover the demanding application requirements, the best fitting valves were selected from Metso’s large portfolio of products for different mining and mineral processing plants.  For the Tarkwa mine, the solution was control and on/off valves, including ball, butterfly and V-port control valves that had been supplied in 2015 as replacements for items that had reached the end of their working lives.

“This was very much a team effort between technology and sales, and this level of cooperation was essential as CIL is a massive process in a plant of this magnitude.”

“A key component in the planning phase prior to the adoption of Metso products for the CIL process was Nelprof, an advanced software solution that helped us to select the best fitting control and on-off valves for the specific application,” says Christian.

The dual advantages of the scientific selection and installation of valves to the correct specification and of a durable quality were soon recognized by Tarkwa Engineering management. “There has been significant reduction in downtime and gain in productivity matched by reduced operational expenses,” notes Solomon Boateng, the Plant Engineer for Tarkwa mine - CIL.

The Tarkwa mine engineering management has been satisfied with the performance of the installed Metso valves as they have yielded step changes in their consistent reliability and the plant’s overall efficiency.

Solomon Boateng puts the positive transition from unreliability to constant productivity into context:

“These valves have now given us peace of mind concerning the operation of the critical elution circuit in the CIL process. The use has been extended to other essential segments of our recovery processes.”

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