The San Cristobal mine in Bolivia is the country’s largest mining operation and one of the world’s biggest producers of zinc, lead, and silver.
Producing since 2007, the San Cristobal zinc, lead and silver mine is owned and operated by Minera San Cristobal, a subsidiary of Japan-based Sumitomo Corporation.
The project was previously owned by Apex Silver Mines, a mining company based in the US. Sumitomo acquired a 35% stake in the mine in 2006 and purchased the remaining 65% interest from Apex Silver Mines for a cash purchase price of approximately £20m ($27.5m) in March 2009.
The total investment on the San Cristobal mining project is estimated to be approximately £1.36bn ($1.8bn).
The San Cristobal mine commenced partial production in August 2007 and achieved full-scale production by November 2008.
Mining operations were temporarily suspended in response to Bolivia’s imposed curfew to prevent the spread of COVID-19 in March 2020.
Project location and geology
The San Cristobal zinc, lead, and silver open-pit mine is located in the San Cristóbal mining district of the Potosi Department in southwestern Bolivia, approximately 500km south of the city of La Paz.
The San Cristobal mine consists of multiple mining concessions covering a total area of approximately 500,000 acres.
San Cristobal is a low-grade but a bulk mineable polymetallic deposit. It comprises volcaniclastic rocks (tuff and tuffites), sedimentary and igneous (intrusive) rocks of the Miocene age rocks which lie within a circular volcanic crater of approximately 4km diameter. The two major mineralised zones are Jayula and Tesorera.
Mineralisation and reserves
Mineralisation occurs in volcaniclastic units as disseminated stratabound mineralisation, restricted to breccias, veins and as disseminations in the interior of the intrusive bodies. The dominant sulphide minerals hosting the zinc, lead, and silver is sphalerite, galena, and tetrahedrite, respectively.
The San Cristobal project was estimated to hold 223.9 million tonnes (Mt) of recoverable ore reserves containing 406 million ounces (Moz) of silver, 322.4Moz of zinc, and 119.5Moz of lead in 2007, the year the mine commenced production.
Zinc, lead and silver production at San Cristobal
The San Cristobal mine produces approximately 1,300 tonnes (t) zinc-silver concentrates and 300t of lead-silver concentrates a day, with an estimated annual output of approximately 600,000t of total concentrates.
Mining method and equipment details
San Cristobal is an open-pit mining operation employing traditional drill and blast methods. The mine produces a daily average of approximately 150,000t of rock of which approximately 52,000t go into the concentration plant for treatment.
The mining fleet comprises six units of Atlas Copco drilling rigs, seven dozer and front loader units from Komatsu and Caterpillar, and 30 haul and dump trucks. The additional auxiliary equipment includes eight Caterpillar D10R and D10T crawler tractors, five Caterpillar 16H motor graders, one Caterpillar CS563D compactor, and a Parker JQ 1165 mobile crusher unit.
Ore processing for the San Cristobal mine
The concentration plant for the mine houses a primary crusher, a 1.7km-long conveyor belt, a giant dome for dust suppression, a milling circuit comprising a semi-autogenous (SAG) mill and two ball mills, cyclone banks for the re-circulation of the load, a differential flotation circuit to separate zinc-silver and lead-silver concentrate, and a filtration and drying plant. The plant has a dry mineral processing capacity of 52,000 tonnes per day (tpd).
The ore from the stockpile is infused with lime to regulate the pH level and passed on to the semi-autogenous grinding (SAG) mill. The pulp from the SAG mill is then introduced into a cylindrical sieve known as a trommel and onto the ball mills.
The mill oversize is recirculated for milling while the undersize is sent on to the flotation cells. The concentrated froth is then moved to the thickening circuit to remove the process water and produce thick zinc-silver and lead-silver concentrates.
The two concentrates are separately filtered in the filtration circuit with horizontal press filters. The final concentrates are stored separately and transported in closed cylindrical metallic containers by rail to the Port of Mejillones.
The San Cristóbal mine is accessible via an improved gravel road from the town of Uyuni, and from the Chilean border town of Ollagüe. The mine site is connected via a spur to a railroad between Bolivia and Chile which is located approximately 50km south of the project area.
The project receives power from the national interconnected system, while the industrial water for the processing operations is received from the northern and southern well fields of the Jaukihua aquifer, located 10 km southeast of the mine.
Potable water supply is received from a well in northern Jaukihua aquifer which is treated in a reverse osmosis plant.
Carlos Caballero SRL was the main contractor for the installation of a stockpile containment dome, one of the largest of its kind in South America, for the mine, while Geometrica was contracted for the engineering, manufacturing, and supply of the dome.