The Peruvian mine, which is winding down operations after 18 years, has also built a cyanide detoxification plant to treat cyanide contained in the site’s heap leach pad.

“Cyanide is often used to leach gold contained inside ore, and this was the case at Pierina,” said Jorge Lobato, environmental and closure manager at Pierina. “Even when operations at Pierina come to an end, cyanide will be present in the solutions from the heap leach pad and must be treated. The cyanide detoxification plant will operate until all cyanide has been consumed or destroyed on site.”

Pierina is located about 300km north of Lima in a high precipitation region of Peru. Average annual rainfall is 1,200mm which, combined with natural conditions of the area, make conditions ripe for acid rock drainage. Acid rock drainage refers to the acidification of water that occurs when sulphide-based ore is exposed to air and water. It occurs naturally in the area due to the sulphur-based rock in the region and can be exacerbated by large open pits and waste rock produced during the mining process, if left exposed.

Barrick said the new treatment plants will replace existing treatment facilities on site and underscore its’ commitment to proper mine closure.

“We made a substantial investment to build these plants and we are committed to the responsible closure of this mine,” Lobato said. “This is one of the first experiences in Peru of a large mine closure, and we want to set the standard for responsible, safe and sustainable mine closure.”

All water that comes into contact with the mine site is funnelled to the water treatment plants before being discharged off site. Discharged water must comply with new regulations that recently went into effect in Peru. “There are limits for content of various metals, salts and the acidity level of the water,” explained Wesley Ubillus, process and water treatment manager at Pierina.

Most of the water treated at the plants is not used by local communities, but some of it is channelled into several communities in the nearby Pucaurán and Pacchac valleys for irrigation use. Both treatment plants at Pierina contain reverse osmosis technology - sophisticated water purification technology that removes sulphate, carbonate and other salts from water.

Mine closure plan

The water treatment plants are an important part of the Pierina closure plan, which will unfold over a period of decades. In addition to the treatment plants, the site’s heap leach pad and existing waste dump will be covered with a layer of clay, topsoil and vegetation native to the area. This will restore the natural landscape and significantly reduce the amount of acid rock drainage.

“We can’t control the amount of rain that falls at Pierina, but we can limit the amount of exposed, potentially acid-generating rock, as well as the amount of water that comes into contact with the mine site,” Lobato said.

Another way the operation is reducing the amount of potentially acid-generating rock is by backfilling the mine’s open pit. Almost half of the open pit will be filled in non-sulphur material taken from the pit itself. According to Barrick, this will not only minimise the potential for acid rock drainage, it will also help ensure the long-term stability of the open pit walls. Work on the pit infill is expected to be complete in 2018.

Before work began on the closure plan, Barrick engaged in consultations with local governments, communities and other stakeholders. Brochures explaining the key elements of the plan were developed and town hall meetings were held to help people better understand the process.

“We continuously talked to people and informed them of what we were doing,” Lobato said. “This is something we continue to do. People understandably have concerns, and only by implementing best practices - as we are doing at Pierina - and supporting those practices with open dialogue and transparency, will we fully earn the community’s trust.”

The water treatment plants conduct daily water sampling and on-site analysis, measuring various metrics including water acidity, or pH levels, and turbidity. The environment team also regularly collects water samples off site and sends the samples to an independent, certified laboratory for analysis. Results from the analyses are reported to authorities on a quarterly basis to ensure Pierina is in compliance with its permit and Peruvian regulatory standards.

“We take great pride in living up to our responsibility to leave a positive legacy,” added Rodolfo Najar, general manager of Pierina.

This article was first published in Barrick Beyond Borders