Located in southwest Bolivia, the San Cristóbal mine is among the world’s largest producers of silver, zinc and lead. With a project investment of USD 740 million, San Cristóbal is also the largest direct foreign investment in Bolivia’s mining history. FLSmidth Minerals designed and supplied the main process equipment.
Bolivia’s San Cristóbal mine is a world leader in the production of silver, zinc and lead – and has proven reserves of around 450 million ounces (12,800 tonnes) of silver, 8 billion pounds (3.6 million tonnes) of zinc and 3 billion pounds (1.4 million tonnes) of lead. FLSmidth Minerals was involved in the mine’s construction from the start, supplying the gyratory crusher, a SAG mill, two ball mills, the flotation cells and hydrocyclones, and conveyor systems.
Construction of the mine’s 40,000 tonnes per day concentrator started in early 2005. The first concentrates were shipped in mid-September 2007, and the mine was in full production by the first quarter of 2008.
Start-up
Construction in the mountains of Bolivia proved challenging. The locally-hired workforce had limited crushing and grinding experience, so the team required extensive training. FLSmidth Minerals worked with the mine every step of the way to ensure smooth construction and start-up.
The first step in the process for FLSmidth was installing the SAG mill and one of the ball mills. (The second ball mill was scheduled to come online several months later.) Some of the snags along the way included undersized pole-mounting holes for the gearless ball mill drives, undersized interconnecting lube oil lines and an undersized low voltage motor control centre. But the team from FLSmidth ensured these issues were quickly resolved so construction could stay on track.
FLSmidth’s gyratory crusher construction went smoothly and ore was being fed to the crusher by early 2007. The plant’s capacity was initially limited by the single ball mill. However, since FLSmidth’s second ball mill came online in August 2007, the plant’s capacity has steadily increased. For example, the mine’s tonnes per day throughput at 31 August 2007 was 10,058, and by 30 June 2008 it had increased to 36,000 tonnes per day.
San Cristóbal – a success story
FLSmidth’s crushing and grinding circuit at San Cristóbal was successfully commissioned, and concentrate is now being shipped to smelters all over the world. It took less than a year to reach full plant capacity. But FLSmidth’s well-organised site team was able to overcome all the hurdles – and the project is now considered a major success.
The San Cristóbal grinding flow
The San Cristóbal mine processes sulfide ore through FLSmidth Minerals’ conventional crushing, milling and flotation circuit. The gyratory crusher accepts large lumps up to a size of 1.5 m, and after crushing the product is then transported to the ore stockpile, where it stays for up to 36 hours. However, the mine is designed to handle an increase in stockpile capacity to allow for a five-day shutdown of the gyratory crusher, when required. Reclaim feeders located underneath the stockpile discharge ore onto the SAG mill feed belt. The reclaimed ore is then measured and analysed to maintain a constant feed to the SAG mill, which is controlled by variable speeds in the three reclaim feeders.
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The San Cristóbal flotation flow
The flotation circuit where the ore is processed to produce silver, zinc and lead concentrates uses FLSmidth WEMCO 130 m3 self-aspirated flotation cells. Concentrate thickeners and pressure filters then remove water from the concentrate slurries before they are loaded onto trains and transported to the port in Mejillones, Chile. Upon arrival in port, the concentrate is loaded onto ships for delivery to smelters around the world.
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