Escondida Copper Mine, Chile

The Escondida copper-gold-silver mine is located in the arid, northern Atacama Desert of Chile about 160km southeast the port of Antofagasta, at an elevation of 3,050m above sea level. The mine is a joint venture between BHP-Billiton (57.5%), Rio Tinto (30%), a Japanese consortium (10%) and the International Finance Corporation (2.5%). It came on-stream in late 1990 and its capacity has since been increased by phased expansions to the current level of 230,000t/d ore throughput. The mine employs around 2,200 people.

Production at the mine was cut back during the period 2003 on account of the weak world market conditions for copper. The partners in the project decided to mine lower-grade ores while maintaining the concentrator throughput, thus reducing the impact on per-ton-treated costs. Output was subsequently ramped up again, and during 2004 the mine handled 377Mt of ore and waste, and processed 82.4Mt of sulphide ore grading 1.51% copper (up from 70.3Mt at 1.43% copper in 2003).

During the year ending June 2006, the mine handled 368.3Mt of ore and waste and processed 87.7Mt of sulphide ore grading 1.61% copper. Total mill output was 1,207,100 tons. Payable copper production was 1.17Mt of copper, electrowon copper cathode output was 116,300 tons, payable gold in concentrate was 139,000oz and payable silver in concentrate was 5.9Moz.

In 2007, the copper mine boosted output by 18.2% after producing 1.245 million tons of copper contained in concrete and 238,357 tons in cathodes. However, during the first nine months of 2008, Escondida experienced a 10.4% production rate compared to the same period a year ago.

Geology and reserves

Escondida is related geologically to three porphyry bodies intruded along the major Chilean west fissure fault system. Primary hydrothermal sulphide ore grades at between 0.2% and 1% copper. Subsequent weathering and uplifting created a barren, leached cap, up to 180m thick, over a high grade supergene enriched ore, both overlying the primary sulphides. Primary sulphide mineralisation includes pyrite, chalocopyrite and bornite, with covellite and chalcocite in the enriched zone.

The combined proven and probable ore reserves of Escondida and Escondida Norte as at mid-2004 were: 2,018Mt of copper sulphide ore at 1.24% total copper; 1,701Mt of low-grade copper sulphide ore at 0.55% total copper; and 290Mt of copper oxide ore at 0.73% acid-soluble copper.

Escondida’s proved and probable reserves are currently 3,900Mt, including the addition of 1,200Mt from the Escondida Norte project. Current mine operations are projected for 34 years.

Production and expansions

1993 and 1994 expansions increased capacity from the initial level of 320,000t/y of copper-in-concentrates to 480,000t/y. Further expansion in 1996 took output capacity to 800,000t/y. In November 1998, concentrator expansion was completed, increasing capacity to 127,000t/d of ore. A new oxide ore processing plant then began operation, reaching its design rate of 125,000t/y of electrolytic copper metal in mid-1999. The $1.044bn 110,000t/d Phase 4 concentrator was completed in September 2002 using retained earnings.

The project partners are currently investing $400m in opening the new Escondida Norte pit, 5km north of the existing mine. By the end of 2004, 126Mt of material had been moved during pre-stripping operations, with the first production scheduled for late 2005. Ore will be moved by conveyor to the existing treatment facilities.

Meanwhile, the Escondida sulphide leach project, designed to produce 180,000t/y of cathode copper cathode, is costing $870m to commission. The process will use bacterially assisted leaching on low-grade run-of-mine ore from both the Escondida and Escondida Norte pits, with solvent-extraction and electrowinning to produce the copper. Production at the plant began in 2007.

Open-pit mining

Escondida is a conventional open-pit operation processing sulphide and oxide ores.

The mining fleet consists of nine Bucyrus 495-B Erie loading shovels fitted with 50yd³, 53yd³ and 55yd³ buckets, 24 Caterpillar 793B, 28 Komatsu 830-E and 23 Caterpillar 793C dump trucks, each of 218t capacity. Escondida uses eight Bucyrus 49-R and 49-RII electric drills plus an Ingersoll Rand DMM2 diesel drill. Bulk ammonium nitrate-fuel oil (ANFO) explosive is used for blasting.

Run-of-mine ore is hauled to two semi-mobile, in-pit crushers for primary reduction before being conveyed to the coarse ore stockpiles. The stripping ratio will average around 1.7:1 over the full life of the project. In-pit truck dispatching and monitoring is handled by a system from Wenco International Mining Systems, while project materials, maintenance and costing control is achieved using Mincom’s mine information management system.

Ore processing

Fluor Daniel was awarded the initial contract for designing and constructing the sulphide concentrator, and later for the expansion programmes. The sulphide concentrator employs crushing, milling and flotation circuits. 12m-high, 4m-square column cells are used for cleaning the bulk sulphide concentrate. The new concentrator uses 94 Wemco 160m³ cells for rougher and cleaner flotation. The separate oxide ore plant uses solvent extraction-electrowinning technology to produce cathode copper metal directly from leached ore. Oxide ore is crushed and sized then heap leached, the dissolved copper being recovered directly by electrolysis.

Concentrate transport and handling

Copper concentrates from the Escondida mill are pumped through a 170km-long, nine-inch-diameter pipeline, constructed by PSI-JRI Ingenieria, to the coastal port of Coloso where concentrates are dewatered and dried for export. The Canadian company, Sandwell, designed the dock facilities, including a dedicated shiploader. Concentrate dewatering employs three Larox style PF and four ceramic hyperbaric filters, which reduce the moisture content to an average of 9% after arrival at the pipeline discharge. Effluent is treated before discharge deep into the Pacific Ocean.

Recent problems

To date, Escondida has experienced an awkward 2008 following electrical faults at the Laguna Seca SAG Mill. The faults caused the total mill output at the mine to fall 208,600Mt in the three month period ending 30 September from 305,200 tons a year earlier.

Repairs at the mill are expected to take as long as nine months to repair and total copper production is expected to decline by 10 to 15% over the next two years.

Furthermore, a slump in copper prices has led to fears that the global recession will reduce demand. However, BHP Billiton president Diego Hernandez says these factors will not affect the company’s expansion or exploration projects at the mine.