Metso has been selected to design and supply a 42ft x 25ft, (12.8m x 7.6m) 28MW gearless SAG mill, plus two 26ft x 42ft, (7.9m x 12.8m) 15.6MW ball mills and two VTM-1500-WB VERTIMILLS® for Newmont Mining’s Yanacocha Conga copper-gold project in Peru. The 42ft diameter SAG mill is the world’s largest and the first of its kind.

The Conga project is at its feasibility stage, and with the engineering and release for fabrication of these mills, Newmont seeks to maintain the option of proceeding with the Conga project with an accelerated schedule. Full funds approval is expected from the board of directors in the first half of 2011.

The Conga project is located approximately 560 miles (900km) north of Lima, at an altitude of 13,400ft (4100m) above sea level. It is situated near the Newmont Yanacocha gold mine, where Metso installed what was then the world’s largest single-stage SAG mill, a 32ft x 32ft (9.75m x 9.75m) EGL mill, driven by a 16.5MW ABB gearless motor, in 2008. That mill design was also the first of its kind and was customized for Newmont. It was in commercial production within one month of its start-up – another first for a Newmont project.

Reliable operation

Yanacocha has consistently seen excellent operating performance from the mill across a wide range of ore types. Plant availability has also been outstanding, with reliable operation a constant throughout the first two years of operation. Kim Hackney, project director of the Yanacocha gold mill project, noted at the time of the mill’s installation that “after just the first week of operation our incremental recovery rate was 80% – 90%, compared to 60% – 75% with leach pads.” Processing higher grade ores from various pits has consequently provided greater flexibility and revenue generation.

Metso is a frequent supplier of mills for Newmont and has a long and fruitful relationship with the company. According to Patricia Downing, project director for Conga, “Newmont is all about establishing long-term alliances with quality suppliers to ensure consistent results for its projects. The Conga project will benefit from the increased size and capacity of this Metso 42ft (12.8m) SAG mill as it will contribute to stabilizing production in periods of harder ore.”

Engineering challenge

This project also reinforces Metso’s continued good relationship with ABB, and provides the groundbreaking challenge of engineering the GMD system for the world’s first mill above 40ft (12.2m) in diameter.

The ABB gearless mill drive system will power the SAG mill with a variable speed drive that allows the mill to start smoothly with minimal mechanical stress and with automated ‘dropped-charge’ protection. This fulfills the customer’s requirements for flexibility and adjustability of the process and enables maximum control over the grinding process.

Significant development

Metso has a tradition of leading the industry in developing and producing increasingly large and powerful mills. Starting in 1959 with an 18ft (5.5m) diameter, 448kW AG mill, Metso has consistently lead the pack in anticipating mining demands for greater capacity. In successive years, Metso designed and produced the first 24ft (7.3m) AG mill, the first 32ft (9.75) AG mill, and the first 36ft (11m) AG mill, all while reducing the required energy load to operate such huge machines.

In 1986, Metso unveiled the first gearless SAG mill, which has grown through stages of 38ft (11.6m) and 40ft (12.2m) until today’s groundbreaking 42ft (12.8m) giant. According to Randy Will, VP, capital mining sales, America, Metso is looking forward to developing this new mill as the next standard range. “Concentrator plants keep getting larger and larger,” he says. “This new mill will be a significant development for the future.”

He goes on to explain that “a single line grinding circuit with a 42ft (12.8m) SAG mill allows for much higher concentrator throughputs, as compared to using a dual line grinding circuit using two SAG mills. The capital cost savings using one line versus two are significant.” Will points out, however, that the new design does not compromise the traditional aspect ratio.

Metso has also recently introduced larger cone crushers and VERTIMILLS, which allow more efficient, cost-effective options. “This will become more important as the copper grades decrease,” says Will. “More ore must be processed to meet the equivalent of current copper production. That’s where customers need the increase in mill size that Metso can now provide.”