| Itos Bolivia Photos |
Orientation Photos
Itos is located about half way between La Paz and Potosi on the Bolivian altiplano.
Oruro is a town of about 150,000 people at an elevation of 12,300 ft (3700 meters) above sea level. It was established in the late 1500's to service the San Jose and Itos mines, which are located in the ridge below the photographer. The Itos chloride leach plant is on the other side of the ridge (behind the photographer).
Looking north along the ridge, the veins of the Itos mine can be seen as they outcrop. A large open pit occurs on the other side of the far ridge. The mine is currently 500 meters deep with unmined ore reserves from the 360 m level on down. The mine contains about 480,000 tonnes of ore in unmined but delineated veins, with more ore to be found on vein extensions. Veins are about one meter wide, and assay 480 grams silver per tonne (15 ounces/short ton).
The Itos tailings are the result of 35 years of processing the Itos and San Jose ores by flotation. Flotation could only recover about 60% of the silver in the ore. We built the Baremsa Itos plant in 1995 to process these tailings, which contained originally 1.8 million tonnes of material with 220 grams silver/tonne (7 ounces/ton) plus 25% additional values in by product metals: lead, antimony, copper and tin. Approximately 1.2 million tonnes remain to be processed.
This scanning electron photomicrograph shows a mineral grain on the right with composite colors, and the same grain at smaller scale in four veiws on the left. For clarity the blue arsenic color has been left off the composite photo. This photos shows that the grain is mostly iron and arsenic (i.e. arsenopyrite) with a "veinlet" of a silver-antimony mineral (Stephanite?) running through it. This is fairly typical of the Itos tailings - the Stephanite is too small to be separated effectively from the arsenopyrite by flotation, so a leaching process (the Itos chloride leach is required. Ferric chloride dissolves the complex silver sulfide minerals by oxidizing sulfide to sulfur. The ferric iron is reduced to ferrous. The metals don't change their valence state, but enter solution in combination with chloride. The reaction on the second line shows precipitation of the metals by iron, liberating ferrous chloride.
This ore micrograph in reflected light shows a pyrite grain which looks like a piece of swiss cheese, after it has undergone leaching. Prior to leaching, the holes were filled with the silver sulfide minerals. Itos Chloride Leach Plant Photos
An aerial view of the Itos plant shows the feed tailings on the extreme left. The large building in the center is the leach/precipitation plant. The refinery building is located to the left of the leach plant. The white-colored installation to the extreme left is the hydrochloric acid plant. The new tailings pond is located about 750 meters to the right.
The process begins by feeding the Itos tailings through a screen and into a tank where they are slurried to 50% solids with process solution. From here the slurry is pumped about 500 meters to the leach tanks.
The slurry is leached for 8 hours in agitated tanks at 75 degrees C.
The leached slurry is filtered on three very large plate and frame filters, and the clear "pregnant solution" is pumped to the tanks in the background. The filter cake drops to the basement, where it is re-slurried and pumped to the new tailings pond.
Hot solution is cooled with cold water, and lead chloride is crystallized out. This material is stored - we currently have about 1900 tonnes of lead worth $800,000 in storage.
The lead-free solution is sent to an 8-ft diameter, 30 ft long precipitation drum where it is contacted with scrap iron. A metal precipitate is formed containing the silver and other metals. The precipitate is then collected and filtered in the filter shown in the center of the photo.
About two tonnes of precipitate are made each day, assaying 8% silver, 10% copper, 20% lead, 20% antimony, 2% iron, 2% arsenic. The balance is fine particles of ore that escaped the filters.
Approximately 3 tonnes per day of chlorine gas is used to provide the oxidant for leaching the ore. It is received in one ton tanks, imported into Bolivia from Peru. One of the new plant improvements underway at the time of the recent shutdown was to replace chlorine oxidation with air oxidation resulting in a savings of $30,000 per month. A pile of scrap iron - used in the precipitation drum - is shown in the foreground. Refinery Photos
The process building is in the background. To complete the process, the carts of precipitate are transferred to the refinery building in the foreground. Baghouses collect and recycle dust from the furnaces.
One of the carts of precipitate can be seen in the tunnel drying furnace above gas burner. After drying, the precip is mixed with about 30% of its weight in fluxes (sodium bicarbonate, charcoal and borax) and smelted.
The ore is smelted in one tonne batches in a gas-fired reverberatory furnace. Arsenic, iron and much of the lead and antimony are slagged off. The metal from this furnace is enriched to 20% silver.
The metal is then placed in cupel furnaces where it is blown with air for about 3 days. Antimony oxide is collected in baghouses. Lead and copper go into slag. The final product from these furnaces in silver bullion.
Kappes, Cassiday & Associates are operators of the Itos plant on behalf of Baremsa. Ing. Alberto Quinones, Operations Manager for KCA, is shown in the white hat. Business services, governmental relations, and metal sales are handled by Hans Tordoir, La Paz-based principal of Barex. This photo shows a typical weekly production of 500 to 600 kg silver - each bar weighs about 25 kg and contains 95% silver, 5% copper. |
