Cerro Lindo is a polymetallic (zinc, copper, lead, silver) mine originally owned by Milpo, then was purchased by Votoratim, and is currently owned by the Brazilian company Nexa Resources. In 2007, the mine’s daily production rate was 5,000 tonnes which has since been increased to 20,000 tonnes per day, making it the largest underground mine in Peru by throughput. 25 years ago Cerro Lindo installed a desalinization plant to provide the mine with water while still preserving the local stream and river levels. Currently, based at an altitude of 1820 meters, the plant processes and provides the mine with 1000 m3 which is sourced from the Pacific Ocean utilizing 60 km of pipe and three major pumping stations. The mine is also a major source of jobs in Peru. There are currently 2900 employees and 26 different contractors working at Cerro Lindo.
Cerro Lindo’s vertical footprint extends approximately 300 meters. The mining method is sublevel stoping with paste backfill. Backfill composition varies between the first pour and the subsequent pours in a stope. The first pour typically has 5% cement until the back height of the drift is reached. Subsequent pours reduce the cement content to 3% for the remaining height of the stope. Additionally, stopes are supported extensively with cable bolts and plastic fibre shotcrete to minimize dilution potential. Drifts throughout the mine have a standard width and height of 5mW x 4.5mH. Since Cerro Lindo relies on desalinization to provide water for the site, water conservation is very important on site. The mine collects all its process water and water from the mine workings on the lowest levels. The water is then pumped back to surface and treated to be reused in either mining or processing activities.
Production drilling is done using Simba ITH drills. These drills use 4” bits for the production blast holes. The burden and spacing follows a square 1.5 m x 1.5 m pattern in the slot and the production rings. Each stope is started with a 2.1 m diameter raisebore to provide the initial void required for blasting the slot. The explosives used in the mine vary depending on the location, but typically ANFO and non-electronic detonators are used. Emulsion is used in areas with more water and electronic detonators are used for larger blasts and for situations where timing is more crucial.
Typical blasted stopes provide the mine with approximately 60,000 tonnes of ore. This ore is handled with 9 or 11 yard LHDs. The LHDs move the broken muck to ore passes where an additional LHD loads 50 tonne haul trucks. Trucks move the ore to the underground crusher where it is then conveyed via five conveyors to the mill. When the mine is producing more ore than the underground crusher can handle, some material is trucked to a surface stockpile where it is fed to a mobile crusher and rejoins the run of mine ore on the conveyor system. The conveyor system consists of five separate belts. The mobile crusher is positioned such that it feeds the fifth conveyor. At very low throughputs, all material can be trucked to the mobile crusher which allows the underground crusher and four lower conveyors to be turned off.
Cerro Lindo is an operation that will be around for a while. The current mine life is approximately 8 years. The mine is heavily investing in exploration and logged 40,000 m of exploration drilling last year.
Like the mine, Cerro Lindo’s plant has grown significantly in capacity to match the mine’s production. The mine utilizes three stages of crushing, starting with an underground primary crusher, followed by two secondary cone crushers, and lastly three ball mills. Typical feed grade at Cerro Lindo is around 0.8% Cu, 3-4% Zn, and can have Ag grades up to 2 oz / tonne.
In the flotation circuit, copper and lead are separated first in a standard bulk flotation and then copper is suppressed and lead is floated to fully separate the two metals. After the copper and lead are separated out, zinc is floated using copper sulphate. The material leaving the plant is split into three streams: 2 tonnes of concentrate, 10 tonnes of dry stack tailings, and 10 tonnes of paste fill tailings. All of the streams are filtered to reduce the water content to 7%.
In Cerro Lindo’s plant, high frequency screens are used in place of hydrocyclones due to high recirculating loads that were causing problems in the past. Unlike many other lead zinc processing plants, Cerro Lindo reuses all of its process water. The plant uses activated carbon to clean the process water intermittently, improving the flotation results. The plant’s concentrate grades are typical for polymetallic mines (27% Cu, 65% Pb, 60% Zn). Recoveries for the three principle metals are 90% Zn, 85% Pb, and 60% Zn.