Newmont Mining Corporation

Waihi, New Zealand

Due to an average annual rainfall in excess of 7 feet (2,200 mm), and a restriction on discharging treated water, Newmont Waihi Gold – which operates the Martha open pit and Favona underground mines – had to control a surplus (considered a positive water balance) of roughly 260 million gallons of water per year.

The mine has captured on-site stormwater in an extensive network of collection ponds since its mine license was granted in 1987. From there, it treats the water to comply with strict environmental standards prior to being discharged from the site to a river.

In recent years, Waihi has been developing new and exciting opportunities to extend its operational life. These developments expanded the site's footprint, further increasing the volume of water that required capturing, treatment and discharge. As a result, the site was in danger of consuming all available storage space for water.

At the same time, on-site monitoring revealed in 2003 that the quality of stored water was deteriorating. It contained levels of selenium, a mineral found in ore. The mine controlled these levels to maintain compliance, but this compromised the efficiencies of the existing water treatment plant.

As a result, the site was forced to reduce water discharges until other treatment options for selenium could be evaluated. This put water volumes out of balance because the reduced discharges led to more water accumulating in on-site storage areas.

As the Favona underground mine was being developed in 2004, a new problem emerged: the presence of antimony, a naturally occurring metal. Its concentration in the tailings storage facility was increasing at an alarming rate. The site's senior metallurgist, Jake Croall and his team, guided by mill manager Kirsty Hollis, soon realized they could not continuously scramble to find treatment options for individual metals.

"The amount of work we had to do to understand selenium and antimony treatment was enormous," Croall said. "If we didn't find a way to remove these metals, it would have been a different landscape."

To fully understand the problem, the site team enlisted modeling expertise from Newmont's Denver office and from others in Tucson and Brazil to develop a customized, stateof- the-art "dynamic water balance" model. The model took into account 100 years of historical climate data, the chemical makeup of both the ore and processing solutions, as well as the efficiency of the water treatment operations.

The model found the only reasonable solution would be to increase the volume of water permitted to be discharged, a factor beyond the control of the site. Regulators would allow the mine to increase the amount of water it discharged, but only if there was a corresponding improvement in the water quality.

The only way to accomplish this was to install a more technologically advanced water treatment system robust enough to treat a variety of compounds – more efficiently. This would reduce the mass load of water containing metals being released into the environment.

Waihi undertook an exhaustive investigation into potential water treatment technologies. It found an ideal but unusual solution for a mine site: a reverse-osmosis plant. This technology contains an indiscriminant molecular filter that is effective for treating nearly anything. The filter treats water to an extremely pure level – far better than what is required by regulators – which makes it extremely expensive to install ($NZ11 million) and operate.

The site commissioned the reverse-osmosis plant in 2008, and regulators increased the discharge consent later that same year. Slowly but surely the stored water volume was reduced and were once again in balance. The plant also has the capacity to treat more water. Problem solved.

Jake Croall contributed to this story. He was Waihi's senior metallurgist until 2010, but recently transferred to the Malozemoff Technical Facility in Denver. Today, he serves as an environmental process development scientist.

April 18, 2011