In the morning, the UBC mining group got on a short bus ride from Arequipa and arrived at the copper-molybdenum Cerro Verde Mine primarily owned by Freeport McMoran. At the mine site, the group stopped at various locations and was given an explanation on the different aspects of the mine. Upon arrival at each stop, the group was greeted by an employee who was an expert of that specific process.
The first stop was at the view point for the water treatment plant. Here the group learned about the special case of Cerro Verde and how it neighbors to the major city of Arequipa, located just 20 miles away from the mine. Apart from the technical aspects of building a mine, Cerro Verde had the challenge of facing the social aspects of being so close to a major city. Cooperation between the mine and city was possible by having a good working relationship with the people and government. One of the major factors to this relationship was the water treatment plant at Cerro Verde. The water treatment plant treats wastewater from the city and uses 1 m3/s of the treated water for its mining operations. The rest of the treated water is then returned to the river. 17 km of pipe is required to get the sewage from the city to the plant and back to the river. The water treatment plant has 2 phases which uses trickling filters. The primary clarifiers remove 30% of the organic matter and 70% of suspended solids in the sewage. The secondary clarifiers use trickling filters with the help of biological mass to remove a minimum of 90% of the total organic matter. The efficiency of the treatment should be 95% but at the time of our visit, the guide was pleased to tell the group that it was working at a high rate of 97%. Finally, chlorine is added to the water at the last stage of the water treatment where the water is now of good quality according to both Peruvian and North American standards.
The second stop was at the C1 (Concentrator 1) viewpoint where UBC mining group’s very own chaperone AJ explained the process plant, who was involved with the design and commissioning of the plant. Here the group learned about how Cerro Verde was the first major base metal mine to utilize high pressure grinding rolls (HPGRs). The plant was built in 2006 with a capacity of 108,000 tpd, then expanded to 120,000 tpd. The process begins with a primary crusher discharging product onto a coarse ore stockpile. The coarse ore is screened: the oversize is sent to 4 MP1000 crushers and the undersize is sent to 4 HPGRs. The HPGR product is then sorted by trippers into four fine ore bins. The fine ore is wet screened, with the oversize reporting back to the HPGRs. The undersize is then pumped to cyclones in closed circuit with four 13 MW ball mills. The cyclone overflow is sent to 4 rows of 10 rougher flotation cells combined with a regrind circuit to obtain a bulk copper-molybdenum concentrate. The bulk concentrate is then sent to the molybdenum plant for further separation. The copper bulk concentrate primarily contains chalcopyrite with an initial head grade of 0.4% copper. The copper concentrate is trucked to Lo Joya, transferred to rail, and then sent to port to be exported internationally, while the molybdenum concentrate is shipped to mainly the United States. An extra detail that AJ mentioned was that the plant was built to withstand major earthquakes of up to 9 on the moment magnitude scale because of its location in an earthquake zone. AJ joked that C1 was the perfect pilot plant project for C2 as it provided all the information that the C2 design needed.
The group then had a quick stop at the leaching pads where the solvent extraction process was explained. The leaching pads are built with oxidized ore as well as ore from the newly developed Cerro Negro pit which has a head grade of up to 0.8% copper. The system includes clay underliners, liner, and ore with a cutoff grade of 0.08% copper. The entire system moves 20,000 gal/min of solvent with a total recovery of 80% in 900 days.
After seeing the leach pads, the bus drove the group to see the Cerro Verde and Santa Rosa open pits which have a combined production of 360,000 to 400,000 tpd. The Santa Rosa and Cerro Verde pits contain sulfides and sends its oxidized or low grade ore to the leach pad ore. The higher grade sulfide ore is sent to the two concentrators. Here the guide gave a summary of the specifications of the open pits. 12 active shovels were required to maintain the high production at Cerro Verde with a fleet of 93 haul trucks at 240 ton to 300 ton capacities to transport the ore. Two of the shovels were hydraulic while the rest were electric cable shovels. The guide explained that the hydraulic shovels offered more flexible operation as they did not have to deal with moving any electric cables. However, electric cable shovels were needed because they provided higher capacities. Outside the pit, the guide drew our attention to another P&H 4100AC electric shovel was in the process of being built. It would be ready for operation in late May or June. The guide also mentioned that the commissioning of yet another shovel would commence 3 months after that. According to the guide, the employees get to decide the names of the shovels. This introduces a bit of fun to the process and improves the relationship between supervisors and operators. The target bench angle for both pits are 68 degrees, however, the actual bench angles depend on various specific factors. One factor mentioned was the dryness of the weather which allows for steeper bench angles. The open pits use a double bench method with 50 m high and 30 m wide benches. Blasts at the two pits occur daily with the main explosive used being ANFO. During every blast, a couple shovels need to be stopped for safety reasons. To maintain production, Cerro Verde relocates trucks to the other shovels during this time. In addition, they have a specific group of employees that switch on during lunch time and breaks which increases the utilization time. Currently, the utilization is at 85% while the plan is to reach 86%. The ideal utilization target is to reach 90% in the future, which is extremely high.
After seeing the different viewpoints, the group was treated by the company to a delicious lunch. After lunch, personnel protective equipment was distributed to the group before heading to the C2 mill tour.
At first thought, the group was extremely impressed at how clean and large the C2 mill was. In addition, all the equipment was somehow neatly compressed into an organized and efficient mill. The tour of C2 began by walking up stairs to see the surprisingly quiet HPGRs. The group also visited the ball mills, cyclones, and control room. In the control room where it was less noisy, the entire process was described to the group. One advantage that C2 has over many other mills is that there is more instrumentation installed to allow for detailed and accurate analysis of the process such as multi stream XRF analyzers. In total, the mill has 8 HPGRs and 8 MP 1250 crushers. The ore is then grinded in 22 MW balls mills using 2-2.5-inch steel balls as the grinding media. The comminution product is then sent to the flotation cells where a 22 to 23% copper final concentrate grade is obtained. The nominal capacity of C2 is at 240,000 tpd. However, Cerro Verde is increasing its target to 280,000 tpd. Just a few weeks ago, one of the managers stated that C2 had a new record of 326,000 tpd! Altogether with the SX plant, C1, and C2, the production can reach over 450,000 tpd.
After the C2 mill tour, the UBC mining group returned the personal protective equipment and got back on the bus in order to make the next tour at Moly Cop Smelter in the afternoon. As aspiring mining engineers and metallurgists, it was a once in a lifetime experience to visit one of the largest copper producing mines in the world. Thank you, Cerro Verde, for taking the time to share such an amazing technical and socially challenging project with the UBC mining group.