The 1.8 million tonne Itos tailings deposit is the result of several years of processing by Comibol of the San Jose and Itos Mine ores at Oruro, Bolivia. The historic flowsheet consisted of a flotation circuit in which silver-bearing galena and stibnite were concentrated. Approximately half the silver in the ore was not recovered, and the existing tailings contain 220 grams silver per tonne in the form of complex sulfides of silver, antimony, lead, and other metals coating grains of pyrite.

An extensive laboratory program was conducted to examine conventional processing methods such as cyanide leaching and regrinding followed by flotation. However, the silver-bearing sulfides are in intimate contact with the pyrite and the pyrite comprises 30% of the total tailings weight. Cyanide leaching recovers only 40% of the silver and the cyanide consumption is uneconomically high. Flotation produces an upgraded, but still uneconomic, concentrate.

For this reason, chloride leaching had been evaluated by Comibol, and in the mid-1980's Comibol undertook an innovative test consisting of two 300 tonne field heap leaches using a cold chloride solution. The tests were partially successful, but could not be applied to the bulk of the tailings which are already ground to 50% minus 200 mesh.

In 1992, Kappes, Cassiday & Associates undertook to develop a hot agitated chloride leach flowsheet. In 1995 KCA constructed a 600 tonne/day (15,000 tonne per month) treatment plant under a turnkey contract from a Bolivian company, Baremsa, S.A. Using this process, 81% of the remaining silver can be recovered, with significant by-product credits from antimony, lead, copper and tin.

The project features hot, agitated acidic chloride leaching. Silver is recovered as 95% bullion metal by furnace refining. Lead and tin are recoverable from the process solutions, and copper and antimony from refinery products. At this time copper is the only by-product metal being commercially recovered. Lead and antimony are being stockpiled as intermediate products, and tin is lost to tailings.

Economics

Total capital cost of the physical installations including civil works, infrastructure and construction management services, was about US$9,000,000. Startup was longer than expected and startup costs were much higher than expected. This resulted in a chronic shortage of startup and operating capital which prolonged the startup period. Total capital invested through the end of 2000 including startup capital and financing costs has been $17 million.

Operating costs during 2000 were about $25.00 per tonne of ore treated, or $6.00 per ounce recovered. Once by-product facilities and other plant improvements are in place, operating cost (with by-product credits) will be below $2.40 per ounce.

Time Schedule

Laboratory testing to develop the process (based on an extensive history of chloride process evaluations) was begun in early 1992 and continued until mid 1993. The feasibility study was completed in October 1993. Financing was completed in early 1994, and detailed engineering design was begun at that time. Groundbreaking for the civil works and buildings took place in December 1994. Installation of equipment began in mid April 1995. The plant reached full startup in March 1996. It reached full tonnage throughput in September 1998.

Total capital cost of the physical installations including civil works, infrastructure and construction management services, was about US$9,000,000. Startup was longer than expected and startup costs were much higher than expected. This resulted in a chronic shortage of startup and operating capital which prolonged the startup period. Total capital invested through the end of 2000 including startup capital and financing costs has been $17 million.

Operating costs during 2000 were about $25.00 per tonne of ore treated, or $6.00 per ounce recovered. Once by-product facilities and other plant improvements are in place, operating cost (with by-product credits) will be below $2.40 per ounce.

Staffing Levels and Costs

Total staffing including the acid plant is 200 personnel including 140 operators and operating foremen (35 per shift), 40 maintenance personnel, 10 support (office and lab) personnel, and 10 Bolivian engineers/senior managers. Skilled operators and mechanics earn Bs1100/month including fringe benefits, equal to US$160 per month.