Nothing but rocks and sand grow in the hilly Atacama desert in northern Chile. Here the climate is dry, dry, dry. That is not so for the fertile minds of Rio Algom where ideas have blossomed into a world-beating copper producer, Compaia Minera Cerro Colorado.
The wholly-owned, US$280-million Cerro Colorado mine has expanded twice in its short life. The first copper cathodes were pulled at 8 p.m. on February 18, 1994. Output was 40,000 tonnes that year and the next. In 1996, a US$50-million expansion boosted the production rate to 60,000 tonnes. A second expansion, costing US$210 million in 1997-98, brought annual copper cathode capacity up to its current 100,000 tonnes, where it remains.
“I think we’ve reached our right size,” said vice-president of marketing and administration Francisco Costabal, “and I think Rio has more than enough other projects to spend its money on.”
Underpinning this low-cost operation are 204 million tonnes of reserves grading 1.02% total copper. Six separate geometallurgical zones have been identified, but for purposes of the mine plan, they are simply divided into oxide and sulphide ores. The difference between the ore types is easily seen: oxide ore is green and suphides are grey. About 40% of the deposit is oxide material consisting of se elimino chrysocolla and minor proportions of atacamite and antherite. A small percentage of the sulphide is altered, higher grade (1.5-2.5% Cu) material, but 95% is chalcocite.
The pit is designed with a bench height of 10 metres in both waste and ore and a 45 slope between ramps, for an overall pit wall slope of between 39 and 41. The berm width is 6 metres, and the roads are 30-metres wide with grades of 10%. At the time of CMJ’s visit, the pit was roughly one kilometre in diameter. Each year a total of 12.4 million tonnes of ore and 35 million tonnes of waste are removed; in other words, a 3:1 stripping ratio. When the pit reaches its ultimate diameter of 2.5 km and depth of 300 metres, when the deposit is exhausted, a total of 2 billion tonnes of material will have been moved.
Blastholes are bored using three Ingersoll-Rand drills. Diameters are 20 cm for presplitting and 25 cm, 22.9 cm and 20 cm for production holes. The patterns vary from 8.5 by 8.5 metres in waste, to 7.5 by 8.5 metres in oxide ore, and 7.0 by 10 metres in sulphide ore. Holes are loaded with Anfo and emulsion at 160 grams/tonne, 30% less than originally practised. The savings are made possible by stemming each hole to reduce vertical energy loss and by cast blasting. Savings are welcome because Anfo is the second largest expense, after fuel, in the pit. All loading and blasting is done under contract by Enaex, the company that supplies the explosives.
Ore and waste are hauled by a fleet of Caterpillar equipment: 25 trucks and eight front-end loaders (FEL). Originally, three Demag shovels were used for loading, but they lack the mobility of FELs, particularly when ore is to be blended. The shovels have since been retired. A combination of selective mining as well as blending at the crushers ensures more uniform metallurgical composition of the heaps, and it controls the amount of -100-micron fines. High amounts of fines adversely affect percolation and leach rates on the pads.
PREPARING ORE FOR HEAP LEACHING
Run-of-mine ore is trucked to one of two crushing circuits. The original installation, plant No.1, consists of a Fuller gyratory crusher, one Allis-Chalmers secondary crusher and three Allis-Chalmers tertiary crushers. There is also an Allis-Chalmers primary screen, three Allis-Chalmers tertiary screens, and eight fines collectors. Total capacity is 18,000 dry tonnes per day. Plant No.2, the one installed during the second expansion, has one of the largest Svedala gyratory crushers in the world at 1,370 by 1,900 mm. The secondary crusher is a Nordberg as are the two tertiary crushers. Primary screening is accomplished by a Tyler unit and tertiary screening by two Nordberg banana screens, both in the closed circuit and the open circuit. Again there are eight fines collectors. The newer crusher facility has a capacity of 28,000 dry tonnes daily. Final size is 13 mm, or 19 mm if the fines level is elevated.
The crusher control rooms are cool, quiet and dust-free. At plant No.2, for example, 750 tonnes of dust is collected daily. The dust is stored for future treatment that will recover the contained copper.
Fine ore is agglomerated in drums where sulphuric acid is added. Plant No.1 has four 2.7-metre-diameter by 8-metre-long drums. The pair at plant No.2 are 2.9 metres in diameter and 9 metres long. Pellets are stacked for 30 days for the sulphide ore and 15 days for the oxide ore. This curing period allows the latent bacteria to become active, and hardens the pellets. Pellets that crumble reduce air spaces (and leach efficiency) in the heaps.
The leaching area is divided into two parts: the older No.1 plant with 16 pads and the newer No.2 plant with 11 pads. Each pad is built on a 2.5-mm-thick waterproof liner covered with crushed barren and mine waste. A pattern of drainage pipes is laid out and covered with pellets. About one-third of the way up from the bottom of the pad, a horizontal network of 7.6-cm pipes are arrayed in a 4-metre square pattern to carry air through the heap. Air is supplied continuously by blowers and 30-cm-diameter headers to which the smaller pipes are attached. The pad height varies from 6 to 8 metres (with the taller ones having the least percentage of fines), and a total of 6-8 million tonnes is placed on each one.
Water is dripped into the heaps by emitters manually placed in short tubes. Dripping is preferable to sprinkling because the evaporation inherent in sprinkling cools the heaps enough to slow bacterial action. Sulphide heaps are often covered with plastic to retain warmth. Irrigation rates are 0.2 to 0.15 L/min/m2. Leaching time is 420 days for plant No.1 and 360 days for No.2. Copper recovery from the heaps is a respectable 79.9%.
“This is money,” says vice-president Costabal, pointing at the rich blue-green colour of the pregnant leach solution (PLS). PLS flows through ditches between the heaps to collecting basins. It is easy to note the age of each heap by the colour of the solution collected from it. The newer heaps produce PLS with a bright, deep blue-green colour. Those that are nearing the end of the leach cycle yield a solution with a thinner, paler green-blue. Pregnant solution flows by gravity to a pair of covered ponds and from them to a central distribution point at the solvent extraction plant.
Copper is recovered by conventional solvent extraction/electrowinning (SX/ EW) means. Solutions flow by gravity through five parallel trains, each with a dozen 50-metre by 50-metre tanks in the SX plant. Capacity is 800 m3 of solution per hour in each train. Copper from the PLS is loaded onto an organic component and then recovered with a solvent. The reagents are filtered out of solution and the copper-rich filtrate flows into 420 EW cells where the copper plates onto 60 stainless steel blanks over a seven-day period. About 350 grams of copper sulphate and 250 grams of guartec are added to the EW cells for each tonne of copper produced.
Cerro Colorado has a patent pending on a new type of jumper frame it created for its EW plant. This new frame allows a cell to be bypassed during cleaning or maintenance without lowering the current density. The technology is being offered for sale.
Electrolyte solution is rinsed from the cathodes when they are pulled. They are stripped in one of two Wenmec machines and strapped 60 to a bundle, about 2,500 to 3,000 kg per bundle. Each bundle is visually inspected and sampled for quality. Those that meet specifications are sold as the CMCC-registered brand on the London Metal Exchange. This recognition was received in May 1999. The largest buyers of Cerro Colorado copper are in Northern Europe (44%) and Japan (26%). Smaller shipments are made to Mediterranean countries, the United States, and the Pacific Rim.
THE HIGHEST STANDARDS: CANADIAN
Throughout the Cerro Colorado operation, Rio Algom consistently maintains the highest standards, i.e., those required of operations in Canada. Environmental protection, industrial hygiene, safety and training are as advanced as modern knowledge makes possible. In many cases these standards are more rigorous than those required by Chilean regulators. The workforce is aware of these benchmarks and proud to meet them. Cerro Colorado won the Chilean National Safety Award for mines in its class in 1993 and again in 1996. It also shared the Rio Algom President’s Safety Award in 1998.
Such an enviable safety record is possible because guidelines are clear and the workforce is committed. When CMJ visited the site in November, the maintenance department had worked five years and one month without a lost-time accident: that is over 850,000 hours.
Because this was a new mine development, the safety program was taught from the start, and that is easier than trying to promote a new system at an existing operation, according to environmental health and safety director Jorge Paredes. “All workers were taught new attitudes, especially about reporting unsafe situations. The base of the system is that people are talking. Together we can solve 98 or 99 per cent of the problems. We have worked hard to take the fear out of reporting or speaking out. We don’t want anything to disrupt our process,” he explained. Contractors on the site receive formal training and are required to follow the same safety procedures.
CERRO COLORADO PIT FLEET
|Units||Make & Model||Capacity/Size|
|4||Cat 992C/D front end loaders||10 cubic metres|
|4||Cat 994 front end loaders||18 cubic metres|
|14||Cat 777C trucks||86 tonnes|
|11||Cat 789B trucks||185 tonnes|
|2||I-R DMM2 drills||7-7/8- and 9-inch|
|1||Cat T4 drill||8-7/8- and 6-3/4-inch|
|3||Cat D10R dozers|
|3||Cat D9N dozers|
|2||Cat 824 RTDs|
|1||Tiger 690D dozer|
|2||Cat 14G graders|
|1||Cat 16H grader|
|2||Cat 769C water trucks||35 cubic metres|
|1||Cat 777 water truck||75 cubic metres|