Here we describe how copper is produced in Chile. In Chile, rocks containing copper sulfate are mined and then smelted (melted). If the copper were in almost-pure form, like with gold or iron mining, then it would be separated using acid.
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Our goal is to explain how copper is smelted so that we can then explain where the sulfur dioxide pollution comes from during this process. Sulfur dioxide is a byproduct of copper smelting. This pollution can be reduced by using filters, which are easier to install or build into the plans of a new facility than an old one, like the old Codelco plant in Quintero. Chile has among the dirtiest copper extraction plants in the world with regard to sulfur dioxide pollution. We will show how Chile compares to other countries with regards to that in our reporting. Chile also has had many accidents with sulfur where clouds of gas have blown off making the people nearby sick for a time.
Crush, Grind, and Separate with Water and Air
Copper is mined in Chile as copper sulfates. These are rocks whose copper concentration is more than 1%, which is the point at which extracting copper from the rock is profitable. That price point changes with the world price for copper and input costs like electricity. Some copper mines in Chile, like Salvador, are no longer profitable, because the cost of extraction exceeds the price at which copper can be sold. Copper production costs are going up in Chile as the percentage of copper in the rocks dug from the existing mines is going down, meaning the easily mined copper has already been mined. Of course, Chile continues to dig new mines all the time and would only do so if those mining pits would be profitable. In Chile, most copper is pit mined, meaning scooped up from a hole in the ground, albeit a very large hole, covering hectares. Giant trucks drive down into these pits and carry out the rock that is scooped up by giant scoopers.
The first step in copper production is to carry the giant rocks to where they can be crushed and ground . The ground-up copper is mixed with air and water and a reactive agent. Air bubbles separate the copper from the crushed rock creating a gooey mix that is between 25-35% copper (cu). The copper sticks to the bubbles and are pushed off the top and then sent to dry.
At the Andinas mine above Santiago, the rocks are first brought down in trucks from the Andes to less steep terrain to be processed. After it is ground and separated, it is carried by train to be processed at the Codelco copper smelter at Las Ventas on the Quintero Bay north of Viña del Mar.
The next step is to separate the sulfur (S) and iron (Fe) and turn the solid copper rocks into what is called white metal, which is 70-75% copper. The copper smelter used to do that in Chile is mainly the Teniente oven, which is somewhat more energy efficient than other models. The Teniente oven is heated with electricity. Copper smelters use enormous amounts of electricity.
(The oven is named for the Teniente mine above Rancagua, where copper is mined and extracted high in the Andes, all in one place. Here is an excellent film from the 1960’s showing the copper smelting process at the Teniente copper smelter. It is interesting to note that at that time, the smelter used huge eucalyptus tree trunks to speed up the fire and burn off the sulfur, now they use pure oxygen. Wood, of course, is too limited and expensive to use it for that anymore.)
The Teniente oven operates at 1,200 C degrees to convert the solid copper to liquid form and expel the sulfur dioxide gas and iron. The sulfur dioxide is captured and converted to sulfuric acid (H2SO), which can be sold to gold and iron mining operations and for other extraction methods that use acid instead of heat. Sulfuric acid is extremely dangerous, but in Chile it is properly handed and there have been no or few accidents with that.
Oxygen and silicone dioxide (SiO2) are added to the furnace to raise the temperature and blow off the iron and sulfur. Think of blowing or fanning a campfire or cookout fire and you can see that air speeds up the burning. Since copper sulfate is copper + sulfate, you can see that melting the copper would release the sulfur part of that.
A by production of the fusion process is tailings. That is just waste with no commercial value that piles up with no good place to store that. It is highly toxic.
In the conversion step, the white metal is converted to copper blister, which is 96% copper, in a Pierce-Smith smelter, and then turned into anodes, which are 99.7% copper. The sludge that is a byproduct of this is cleaned and sent back into the smelter to extract more copper from that and then the remaining sludge is moved to long-term storage as a waste byproduct.
The next process is to purify the 99.7% anodes to 99.9% copper. If the diagram looks familiar, the electrowinning process is the same design as a battery, where a copper cathode (which has a negative electrical charge) and an anode (which has a positive electrical charge) is covered with sulfuric acid and water to create an electrical charge. This electrolysis process takes 6 to 7 days for the anodes to to flow onto the cathodes. Copper with no electric charge falls out of the bottom. The 0.01% remaining part of the copper is sulfur.
The cathodes are then loaded onto ships and shipped to international markets. The sulfur acid is sold as well plus traces of gold, silver, and iron recovered during the smelting process.
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March 21, 2017
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I really enjoyed this story. It made me think about my own predisposit
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