History and Description

Primary Feed

The "ore" - actually tailings from a flotation plant - contain about 30% iron pyrite in a rhyolite gangue. Silver occurs as complex sulfides of silver with other metals - minerals such as jamesonite, boulangerite, silver-bearing tetrahedrite, and franckeite (lead antimony tin sulfide). These sulfides occur as coatings and fracture fillings in the pyrite.

The tailings are "mined" with a front end loader and screened to discard oversize material. All 6 mesh undersize (600 tonnes per day) is fed to an attritioning tank where it is mixed with barren process solution. Acid (100 tonnes/month) and salt (900 tonnes/month) are added along with the ore. The process solution has the following active components:

• Sodium chloride (salt) 200 grams/liter
• Iron chlorides 30 grams/liter
• Hydrochloric acid 10 grams/liter

Leaching / Solution Heating

The ore is leached at a temperature of 75C in three agitated leach tanks in series for a total residence time of 8 hours. Chlorine is fed into the leach tanks at the rate of 10 kg per tonne of ore in order to oxidize the iron chloride. The iron chloride then dissolves the silver minerals. Less than 3% of the feed weight is dissolved during leaching.

Filtration & Tailings Disposal

Leached slurry, at an average flowrate of 30 cu m per hour, is filtered in a bank of three recessed plate filters. The filter cake discharges into a large, plastic-lined "basement", where it is re-slurried and pumped to a new tailings dam which is fully lined with 40 mil PVC. At the end of the operation the new tailings pond will be sealed with a PVC and earth cover. The feed material - the "Itos Tailings" - exist as an exposed mound of oxidizing pyrite. The new tailings pond will result in a significant, permanent improvement of the environment in the southern part of the city of Oruro.

The water for slurrying the tailings makes a continuous loop from the new tailings dam. The system is designed for zero discharge. Minor soluble impurities - primarily zinc, aluminum, magnesium, and sulfate - are controlled by inclusion within the normal moisture contained in settled tailings.

Solution Processing

Pregnant solution from the filters, nominally at 75C after filtration, is cooled in a heat exchanger to precipitate about 6 tonnes per day of lead chloride. Lead chloride crystals are collected and stored for future processing. Solutions are then warmed in a second heat exchanger and proceed to metals precipitation.

Silver, antimony, arsenic, copper and residual lead are precipitated in a rotating drum using two tonnes per day of scrap iron. Following iron precipitation, the metals exit the drum as a fine suspension in the process solution. The metal powder is thickened and filtered. Two tonnes of filter cake are produced per day, which represents the final product of the hydrometallurgical section. This cake contains about 6% silver, 20% antimony, 20% lead, 10% copper, small quantities of various other metals, and about 40% mineral fines which escape the filtration circuit.

Pyrometallurgy

The metal precipitate which is in the form of a damp, alkaline filter cake is dried in a gas-fired tunnel oven. It is then briquetted using a roll-type briquetting press to form pellets approximately 6mm diameter and 25mm long. The briquettes are melted and partially refined in a reverberatory furnace using sodium carbonate (trona) and borax to remove arsenic, iron and miscellaneous impurities.

The "reverb metal" consisting of antimony, silver and lead is transferred to a cupel furnace where it is blown with air. Crude antimony oxide is produced and collected in bag filters. During a campaign lasting 2-4 days, the silver content of the metal in the cupel increases from 10% silver to 95% silver. Copper and lead form a slag which is re-dissolved for copper recovery; the lead residue is recycled.

Acid Plant

In September 1999 a Hargreaves Process plant was commissioned to make hydrochloric acid on site. In this plant, briquettes of salt (sodium chloride) are reacted at 600°C with sulfur dioxide gas (made by burning sulfur), air and water vapor to produce HCl vapor and sodium sulfate. The hydrochloric acid vapor is absorbed in the barren plant solution to raise the level of hydrochloric acid to operating levels. Acid costs have dropped from $90,000 per month using imported acid, to $30,000 per month.