Copper, Molybdenum and Cathodes
The Los Bronces open cut mine is located 65km north-east of Chile’s capital, Santiago. It produces copper, molybdenum and cathodes.
Los Bronces is 50.1% owned by its operator Anglo American. Mitsubishi owns 20.4% of the mine, while Chile’s state-owned Codelco and its joint venture partner Mitsui hold the remaining 29.5% stake in Los Bronces.
Although Los Bronces has been mined for more than 150 years, the current life of the mine is extendable beyond 30 years with significant exploration potential.
A $2.8bn expansion project completed in 2011 increased Los Bronces’ production capacity up to 400,000tpa with new grinding, mineral transport and concentration facilities.
Los Bronces mine, located in the Andean Mountains at an altitude of 3,500m above sea level, extracts the Porphyry cluster adjacent to Andina operations of Codelco. The ore body comprises primary sulphides such as Chalcopyrite and bornite with a thick overlaying blanket of chalcocite. The Los Bronces ore is open at deep as well as lateral extensions.
The exploration targets for future development of Los Bronces resources include the San Enrique Monolito and Los Sulfatos deposits to the south-east of the existing pit.
The existing ore reserve at Los Bronces, as of 31 December 2011, stood at 2.17 billion tons (bt) grading 0.62% Cu (11.5mt of contained copper).
Los Bronces produced 221,800t of copper in 2011 and 221,400t in 2010. Copper cathode comprised 38,400t, while copper in sulphate accounted for 4,600t. Copper concentrate production during 2011 was 178,800t. The mine also produced 948t of molybdenum concentrate during the year.
The average copper production of the mine during the next ten years is expected to rise by 200 kilo tons per annum (ktpa). Molybdenum production is expected to increase up to 5,400tpa.
Disputada, a subsidiary of Exxon Mobil, was the previous owner of Los Bronces. Anglo acquired Disputada for $1.3bn in 2002 and changed its name to Anglo American Sur.
An expansion project was implemented during the same year to attain a processing capacity of 54kt per day.
A major development project worth $2.8bn was initiated in 2007 to increase the annual production capacity of Los Bronces by 200,000t.
The Los Bronces development project involved the construction of new grinding, mineral transport and concentration facilities with an aim to increase the throughput of Los Bronces operation from 61 kilo tons per day (ktpd) to 148ktpd.
The engineering, procurement and construction (EPC) contract of the project was awarded to Bechtel.
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Construction began in December 2007 but the global recession of 2008 slowed down the project implementation. Production from the new expansion project started in November 2011.
A new primary crusher near the open pit, a new grinding plant (with one SAG mill and two ball mills) at Confluencia, a 4.4km conveyer tunnel between the mine site and grinding plant, a 52km ore slurry and water reclaim pipe line, and a new flotation plant near the existing plant at Las Tortolas were constructed as part of the project. The scope of the project also included the construction of new 220kV electric power lines.
The diameters of the ore slurry and water pipelines are 28in and 24in respectively. The elevation difference between Confluencia and Las Tortolas is around 3km. Five choke stations have been constructed to check the gravitational force of slurry and five water pumps have been constructed to lift reclaimed water from the flotation plant.
Ausenco was responsible for the basic and detailed engineering of the slurry and water pipelines.
Open pit mining is carried out at Los Bronces through drilling and blasting. The ore is extracted with the use of loaders and wheel dozers.
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The low grade ore undergoes the process of leaching, solvent extraction (SX) and electrowinning (EW) for the production of cathodes.
Higher grade ore is sent to the crushers near the mine where the ore is reduced to 1.5in pieces. The crushed ore is sent by the 4.4km conveyer to the grinding plant, which houses one 40ft SAG mill and two 26ft ball mills. The ore slurry prepared from the milled ore is sent to the Las Tórtolas flotation plant through a 52km pipeline.
The slurry undergoes froth flotation for separating the copper and molybdenum concentrates. The concentrate is dried by thickening and filter press till the water content is reduced to 9%. The concentrates are either directly sold in the market or transported to the Charges smelter for the production of copper anodes.
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