On Wednesday morning we awoke to the sweet sounds of our alarm clocks in order to catch our 7:00am bus out to the Sierra Gorda mine, the second to last tour of our trip.
Sporting a 0.39% copper head grade, Sierra Gorda is a relatively new operation having commissioned the mill in late 2014. Due to this, the class was treated to the finest facilities that we’ve experienced so far on the trip. Every aspect of this 110,000 tonne per day operation reflected state of the art design considerations.
The tour began, as per usual, with a safety & introduction presentation, after which we were served breakfast sandwiches and coffee (to the delight of everyone). As we ate breakfast and chatted with the office staff, small groups were rotated through the sleekest, most modern control room that we had ever seen. Even the mill rats (myself included) were impressed! Various operators explained to us their respective unit operations and happily fielded any questions we had. The guide had to drag each group out to ensure that the others would get a chance to see it.
Once everyone had rotated through the control room and breakfast was done, we loaded onto a pit bus and headed out to the lookout point of Sierra Gorda’s primary pit. Another advantage of a new operation is that the pit is not too deep, especially compared to Chuquicamata. The shallow pit allowed us to see five of the six Bucyrus-Eerie shovels loading the 53 Komatsu 930E trucks at any given time and actually wrap our heads around the production that was taking place. There was a blast scheduled for 1:00pm that the pit shifter invited us to stay and watch but unfortunately there was an issue with moving the drill off the pattern and the blast was delayed. We thanked the shifter for answering our questions and hopped back on the bus for the final leg of the tour.
Sierra Gorda has a very unique milling process; instead of conventional crushing and primary grinding with a SAG mill, the operation utilizes a three-stage crushing circuit. The tertiary step consists of four parallel high-pressure grinding rolls. HPGR technology is a relatively new approach to comminution. It typically has an energy savings of 10 – 15% over the conventional approach, as well as several other operational benefits downstream. So why isn’t it more common across other operations? The mining industry is typically reluctant to embrace novel technologies, given the large capital expense necessary to build concentrator facilities. This conservative nature results in the selection of methods that have been proven time and time again (regardless of how old they might be).
The image below shows an outside view of the HPGR’S.
Following the secondary crushing (4×1250 Metso cone crushers) and tertiary HPGR step, the banana-screen undersize is fed to three gearless-drive ball mills. The closed ball mill circuit reduces material to a float feed P80 of 160 microns.
Bulk flotation follows, recovering a copper and molybdenum concentrate. Selective floatation then separates the copper and moly, ultimately producing two concentrates. Currently, overall plant recovery of copper is around 75% with intentions of improving that to ~85% in the future. Sierra Gorda has one of the highest moly head grades in the world: 15% for the first five years of the mine life. This compares to other copper-primary operations averaging closer to ~0.04%. Special considerations have been taken in designing the moly circuit. It can handle the large production that comes with the astronomical head grade at the start of mine life and can then be easily retrofitted to accommodate the much lower head grade from year 5 onward.
Click here for a simplified animation of the entire Sierra Gorda Milling process:
It’s also important to note that being in the middle of a desert, there isn’t an abundant amount of water readily available for use in the process. Sierra Gorda has gotten around this by constructing a 143 km pipeline to the coast to obtain used cooling water from a coastal power plant. The 42-inch pipe feeds ~2,000 liters per minute over the distance and elevation gain of ~1,700 m with the help of three pump houses.
Sierra Gorda serves as an example of the potential of HPGR technology. In our world of falling head grades and ever-rising costs, it takes innovations and a willingness to step away from the status quo to maintain and increase global metal production.
The class thanks all of the staff at Sierra Gorda for organizing such a well-run tour. None of us had ever seen a circuit fully committed to HPGR technology resulting in an exciting and truly engaging experience. Tomorrow holds the final mine tour of our trip, Minera Centinela.
– Jim Mackay