Beginning late in the 1800s and continuing into the early years of this century, small copper, gold and alum (kalinite) mines sprang up in the rugged rocky desert of northwestern Argentina. But it took the knowledge and determination of partners Rio Algom Ltd. (25%), and the Australian firms MIM Holdings Ltd. (50%) and North Ltd. (25%) to create Argentina’s first large, modern mining operation among the hills and cactuses. Bajo de la Alumbrera has become a most successful copper and gold producer, operated by Minera Alumbrera Ltd.
The mine and mill site lie about 2,600 metres above sea level. Construction began in 1995, and the open pit mine and mill reached commercial rates of production two years ago, in February 1998. Output is 178,000 tonnes of copper contained in concentrate and 590,000 ounces of gold annually. Concentrate slurry is piped 316 km to Tucamn, where it is dewatered. The dry concentrate moves by rail another 830 km to the port of Rosario, on the Rio Parana, northwest of Buenos Aires.
The Alumbrera reserves lie within a classic porphyry copper-gold deposit. Mineralization is associated with hydrothermal alteration zones in andesite volcanic rocks and dacite plutons. The primary ore, which is unweathered, contains chalcopyrite, native gold and pyrite. Gold values correlate closely with copper values in the chalcopyrite and are consistent throughout the deposit. Proven and probable reserves as of Jan.1, 2000, are pegged at 422 million tonnes averaging 0.53% Cu and 0.65 g/t Au.
The ultimate pit size, after 20 years of mining, will be 1,900 metres in diameter and 450 metres deep. Mining is carried out using four Bucyrus 41-m3 shovels and 12 Caterpillar 793B and 21 Caterpillar 793C 218-tonne trucks as well as two large Letourneau loaders. The current stripping ratio for waste-to-ore is 3-to-1. As of last December, 340,000 tonnes of ore and waste was removed daily, and that is the expected rate for the next eight years. Shift changes are made on-the-fly, and spare drivers fill in during lunch breaks.
The challenge of mining in the bajo is its topography. It is a shallow valley filled with small, closely-spaced, conical hills, perhaps 75 to 100 metres tall, that resemble pails of beach sand upended at regular intervals by a giant child. Surrounding the area are larger, rockier hills. Winding through the bajo is a small stream that must be preserved. The upshot is that there is only one haulage road to and from the pit. Efficient equipment dispatch is made possible using a Modular Mining Systems dispatch system, which uses a global positioning system. A Caterpillar VIMS (vital information monitoring system) provides data on individual truckloads to maximize productivity, minimize maintenance, and improve operator skills.
The waste rock is assessed for its net acid-generating potential and segregated as necessary.
Ore is crushed using a Svedala 1,520-mm by 2,770-mm gyratory crusher, capable of operating at 8,500 tonnes/ hour. The ore passes over an apron feeder onto a 1.7-km-long conveyor that leads to the stockpile. The 500,000-tonne pile is drawn down equally into the two grinding lines, each of which handles 45,000 tonnes/day.
MINE AND MILL FULLY INTEGRATED
The mine and mill operate as an integrated system. Using information from the mine dispatch system, the VIMS, and data from the area of the pit, the characteristics of the fine ore are known before milling. Knowing the size distribution of ore, thanks to a split engineering image analysis system, the mill optimizes feed size and grinding efficiency.
Each grinding line has an 11-m diameter by 5.14-m variable-speed Svedala SAG mill. Slurry is discharged to a cyclone feed sump and pumped to a pair of Krebs Cyclopaks. Each of the 12 cyclones per unit is 600-mm diameter. Cyclone overflow goes to the flotation circuit. Underflow is passed across a screen, with the oversized being reground in a pair of 6.1-m diameter by 9.34-m Allis-Chalmers ball mills. Discharge from the ball mills also flows to the cyclone feed sump.
Undersize from the screen passes through the gravity circuits, which consist of two 12,600-mm Knelson concentrators per cyclone underflow line. About 10% of the gold is recovered in the gravity concentrate and refined weekly into 80% Au dor bullion.
At the head of the flotation circuit are four banks of eight 100-m3 Outokumpu OK16 rougher cells. Tails are pulled from the last of these banks, and overflow from the roughers is reground in the two ball mills to a nominal 85% -400 mesh. Regrind slurry is hydrocycloned, with the oversize returned to the ball mills and the undersize pumped to the primary cleaning cells. The primary cleaners consist of two lines of four Jameson cells each, and the concentrate from the fourth cells feeds the fifth cells, or secondary cleaners. Tails from the secondary cleaners pass through the cleaner-scavengers, consisting of two more Jameson cells in each line. The tails from the cleaner-scavengers and the roughers are combined as the total plant tailings.
Reagent use is simple due to the uncomplicated metallurgy of the ore. Lime is added at the SAG and ball mills and the primary cyclone feed pump sump to adjust the pH. Xanthate collector is added mid-way through the rougher circuit. Alcohol and Oreprep frother are also added to the cyclone sump.
The final concentrate product is pulled from the second cleaner circuit. It averages 27% to 30% Cu with a targeted copper recovery of 92% from unweathered ore. The concentrate also contains 25 to 30 g/t Au. When oxidized and enriched ores are milled, the recovery rate drops, but the copper grade rises. Concentrate is dewatered in 30-m diameter Superflow thickeners to 65% solids, and is stored in preparation for being piped to Tucamn. There it is dewatered to 9% moisture using three 120-m2 Larox pressure filters.
Process control including crushing, grinding and flotation is fully automated. Everything from fresh water addition to tailings assays are monitored from the control room using Foxboro software. An expert system is under development for the primary grinding, flotation and regrind circuits. Operation of the concentrate pipeline is monitored from the mill control room, as well. Automatic sampling of water, reagents and regrind size is practised; other streams are sampled manually.
THE ENVIRONMENTAL EDGE
Alumbrera prides itself on being a zero-discharge operation. Tailings are stored behind a leaky dam, which will have a finished height of 140 metres. The tailings dam was designed to Canadian standards. Water is deliberately allowed to collect below the dam and is recycled to make up 85% of the process water. The remainder comes from six of seven deep wells located about 30 km from the mill. The mill consumes about 9,000 m3 of water hourly. No effluent is discharged from the site.
By integrating production and environmental matters, Alumbrera has created a win/win situation. The provincial and federal regulators are on-side, and red tape is minimal. Minera Alumbrera filed its first environmental impact statement in 1995, with the EIS updated every two years as is required by law. The final closure plan will be submitted shortly, in March 2000.
There are no “endangered” species in Argentina; rather, all animals must be protected. During construction of the project, including the power lines and pipelines, steps were taken to preserve the natural habitat. All work was documented and shared with provincial authorities. Potential creation of acid drainage is controlled to international standards. Company officials gave a two-day workshop on their precautions to provincial regulators. Quarterly environmental reports are made to regulators face-to-face to further enhance goodwill.
During construction of the tailings area, Alumbrera relocated several hundred cactuses estimated to be between 300 and 400 years old, something that the company was not obliged to do. Extensive environmental monitoring of water, air quality, and acid drainage is carried out at all sites and nearby villages.CMJ