This page contains orientation photos. Process photos are included on separate pages.
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.
To see actual photos of the process, please go to the page "Plant Photos".