Located in an area of historical copper mining, Collahuasi consists of three major copper deposits: Ujina, Rosario
and Capella. Lying at an altitude of 4,100 metres, Ujina contains second- ary sulphides and oxides. This deposit (plus a smaller oxide deposit called Huinquintipa) was the first to be mined; representing the first stage of Collahuasi. Currently a total of 50,000 tonnes per day of rock is extracted from Ujina and a total of 45,000 tonnes per day is extracted from Capella, which contains oxide copper ore.
Still higher, at an altitude of 4,600 metres is Rosario, a deposit that con- tains secondary enriched sulphides and oxides with a relatively high copper grade of 1.1%. Stripping at Rosario began in 2002 and this has now become the principal source of ore at Collahuasi; currently over 600,000 tonnes per day of rock are mined. Rosario Sur 1, a new area where work has only just started, is contributing another 10,000 tonnes per day for a grand total of 705,000 tonnes per day of extracted rock.
Most of the mining equipment is deployed at Rosario, including the
boasts over 40 ultra class haul trucks from different suppliers, with a rated capacity of between 240 and 370 ton- nes. These are loaded by a fleet of elec- tric rope shovels, supplemented by a few hydraulic shovels.
After drilling and blasting, the Run- of-Mine ore is hauled to the processing area. Because of the presence of both, sulphide and oxide ores, Collahuasi employs two processing methods. Sulphide ore is crushed, ground and treated by flotation and oxide to yield a concentrate whereas oxide and mixed ores are treated by heap-leaching, sol- vent extraction and electro-winning (SX-EW).
The sulphide ore enters a primary gyratory crusher prior to transport to two parallel grinding circuits, each consisting of a SAG mill and a ball mill. Secondary closed circuit grind- ing includes cyclone classifiers, from which the overflow feeds the flotation circuit. Flotation concentrate is thick- ened to 50% solids for slurry transport in a 200 km-long pipeline to the port of Punta Patache, 60 km south of Iquique, where it is thickened and filtered before shipment to smelters worldwide.
The oxide ore is reduced to -10mm by three-stage crushing. The fine ore is agglomerated before being conveyed to the leach pads where it is leached using sulphuric acid. Copper is recov- ered from the pregnant leach solution in the electro - winning plant, where the copper is stripped using organic solutions; the stripped liquor is then returned to the leach pads for re-use.
The strip solution is cleaned using dilute acid and flotation and is filtered prior to the electro-winning phase, where the copper is plated onto cath- odes. These are stripped by the Kidd process on a seven-day cycle and the cathode copper is then transported by road to Iquique for export.
Hard Rock Geologically speaking, Collahuasi is a porphyry copper deposit. Approxi-mately 50-60% of the world’s copper and 95% of molyb- denum comes from porphyry deposits and indeed, the greatest concentration of large copper porphyrys is in Chile. The major ore minerals at Collahuasi
are chalcocite, chalcopyrite and bor- nite. Oxide mineralisation occurs mainly as chrysocolla with minor brochantite, native copper, and copper- iron-manganese oxides and hydroxides.
“The rock here has special charac- teristics; it’s altered with quartz so it’s harder,” says Correa, explaining that the rock at Collahuasi has an average strength of 100 megaPascal (MPa), with some sectors reaching even 250 MPa. For comparison, the rock at other local mines has a typical strength of 60 MPa.
Nevertheless, there are some geo- logical faults. “Some areas can be un- stable with the risk of wall collapse,” says Correa, explaining that this, how- ever, does not happen abruptly. A radar system is used to scan the walls so the risk of collapse can be controlled.
The stripping ratio at Collahuasi varies depending on pushback develop- ment but the average is 4:1 (waste: ore). Bench heights are a standard 15 met- res with 1 and 2 metres of subdrilling depending on the drilling pattern. There are no double benches due to the
low quality of the rock mass. Blasting at Collahuasi is carried out once a day and four different drilling patterns are used depending on the area. “Some areas contain ground water so we have to take this into consideration. Depending on the presence or not of water, we use heavy ANFO or ANFO & emulsion,” says Correa, adding that they employ a remote electronic initiation system.
Drilling & Vipers
On the drilling front Collahuasi currently employs a fleet of 19 rigs of which 11 are used specifically for blast- hole production drilling including the two Pit Viper units. They are the only diesel drilling rigs working in produc- tion as the other nine are electric rigs.
There are three other rotary diesel machines from another supplier, which are being used to drill 7 ⅞” buffer holes. These are the holes positioned on the last two or three rows close to the walls. “These holes must be drilled with a smaller diameter than production
The Collahuasi mine, one of the world’s largest copper resources, is located high in the Andean plateau and operations take place at altitudes of between 4,000 and 4,800 metres above sea level.
holes as they require a smaller explo- sive charge in order to minimize damage to the wall or the slopes,” ex- plains Correa, adding that normally, these holes are drilled to the height of the benches, without subdrill.
For pre-splitting, Collahuasi em- ploys four smaller diesel rigs, which are drilling 4 ½” diameter holes to a depth of 16 metres and at an angle of 70°.
The Pit Vipers were delivered in April and May 2011 and both went straight to Rosario to work on produc- tion drilling. The PV-271 unit is drill- ing 10 ⅝” diameter blast holes and the PV-351 is drilling 12 ¼” diameter blast holes. Both rotary machines are drill- ing vertically and to a maximum depth of 18 metres.
According to Correa, the diesel machines have proved ideal for work at this deposit: “We’ve hit a phase in the Rosario deposit where the working area is narrow and there is water,” he explains. “It is very convenient for us not to have to deal with the cables of the electric machines in that area.”
Also important is the machines’ versatility: “Moving electrical cables about results in lost time, and that’s why we decided to go for a diesel ver- sion. We like the versatility of the Pit Vipers as we can move them around quickly and without problems.”
However, the high altitude and con- sequent reduction in oxygen, means a loss of power in any diesel machine. “This makes the rigs less efficient and so we had to compensate for this by fitting them with higher-powered en- gines and compressors,” says Paulyn Espindola, Product Manager, Atlas Copco Drilling Solutions (ADS), ex- plaining that the Pit Vipers at Colla- huasi are fitted with a compres- sor giving a low pressure rotary of 3,800 cfm @ 110psi instead of 3,000, which would have normally been enough. Equally, the rigs feature a more powerful engine, in this case one delivering 1,650hp @ 1,800rpm.
The high altitude also means extreme low temperatures, especially in the South American winter (July to
September), when temperatures can reach -40°C. “That means that the rigs also have to be fitted with a heater and an on-board generator,” continues Espindola, explaining that elements such as the water pump, fuel, battery and oil can all be affected by the very low temperatures.