Today, we closed our Chilean Research Trip with a tour of Centinela Mine. The property is 70% owned by Antofagasta Minerals, the only private Chilean mining company, and 30% by Marubeni Corporation. Like most mines in the Calama region, Centinela produces copper as its primary product. We began our trip with an introduction to the property and a safety briefing. The site consists of four active pits, primary crushing and grinding, separate oxide and sulphide processing lines (which produce 29 ktpd and 89 ktpd respectively) and a thickened tailings storage facility.
After receiving our PPE, we began our tour with a visit to Esperanza, Centinela’s sulphide pit. Esperanza moves 420,000 tpd to send 100,000 tpd to the mill and 20,000 tpd to a stockpile. The pit is currently 450 m deep and when finished, with be 2 km in diameter and 650 m deep. In addition to Esperanza, the site has three pits producting oxide ore (Central and Northeast Tesoro and Mirador) that move 150,000 tpd to send 30,000 tpd to a leach pad. To facilitate this, the site has an impressive fleet of equipment: eighty trucks, five cable shovels, two hydraulic shovels, six front end loaders and sixteen drills.
The current mine plan will develop eight pits within an area spanning 40 km distance. To manage the challenges of material transport, Centinela plans to heap leach oxides near their farthest pits and transport pregnant solution by pipeline to the SXEW plant. By the time operations is ready to mine sulphides, another flotation plant will be built closer to the pits. Currently, ore from Esperanza is crushed at the pit and transported via a 3.5 km conveyor from the primary crusher to the ore stockpile.
Out next stop was to the plant, were we were excited to find the Mill Manager was a former UBC graduate. Francisco Melo worked on his Masters of Applied Science in 2005 and helped explain to us Centinela’s sulphide processing line. We began at the control room, which has stations to control the primary crusher, grinding circuit, pebble crushers and flotation circuit. After that, we were able to view the entire mill from a viewpoint behind the control room. The mill processes 110 to 114 thousand tpd with a head grade typically between 0.48 to 0.50% copper and 0.25 g/t gold. After identifying that their SAG mill was at capacity and that their two ball mills were underutilized, Centinela saw the opportunity to increase throughput by adding a secondary and tertiary crushing plant. The plant can crush up to 20ktpd of material fed from the main stockpile. This is crushed to a P80 of 9mm and fed straight into the ball mills, bypassing the SAG.
Once reduced to a P80 of 180 microns, the ore is sent to flotation. Like its neighbour, Sierra Gorda, Centinela uses seawater for its flotation process. Fifty to sixty thousand cubic meters of saline water is transported by a 145 km pipeline from the coast. Processing with sea water adds challenges to the mill. Instead of depressing pyrite with lime, as with freshwater flotation, sodium methyl bisulphate is used to increase the pH because of the buffering caused by seawater. Additionally, the mill has to take extra precautions to mitigate corrosion, such as using liners and using special paint.
The plants two rougher lines recover 94% copper and sends the product to a regrind to reduce the P80 to 40 microns. The ore then goes to a two stage cleaner line which uses conventional and then column flotation. The ultimate recovery is 87% copper to produce a concentrate with a 24% copper grade. The final product also contains gold at 6 to 8 ppm and molybdenum at 0.5%.
Like every operation, Centinela manages operating challenges. Specific to the mill is the management of high pyrite grades and the need to meet and improve throughput demands. Because Centinela is located in the driest desert in the world, water is a definite concern. The mine aims to achieve to 67% solids in its tailings storage facility and does so by using three thickeners and two paste thickeners. Centinela operates at a USD 1.80 per pound production cost and is targeting USD 1.30 per pound by using economies of scale and adapting innovate technologies. This includes a thermal solar plant to heat SXEW process water and the use of seawater in its sulphide circuit. In the future, Centinela plans to add a separate molybdenum circuit to improve the recognized value of its products.
The mine is an impressive example of innovation and technology and it was great to visit and learn from their mining and milling expertise. Material movement from several small pits is very similar to mining diamonds from several small pipes located over a large areas and it was interesting to see different solutions to moving ore across a large site. Because of our location in British Columbia, we are familiar with sulphide flotation, however, the ability to see a plant in open air was a great opportunity to appreciate the scale of the operation and the separate circuits involved. Antofagasta Minerals was a great host and we are grateful for the tour!
– Grace Ma