Although natural leaching is as old as the earth, leaching as a mining process is a relatively new procedure and involves fewer steps than concentrating and smelting. Leaching is a hydrometallurgical process because it uses liquid to separate the copper from the other minerals in a rock. The leaching process used by the copper mining industry is known as Solution Extraction/Electrowinning or Solvent Extraction/Electrowinning, (SX/EW).
In this process the ore may be blasted (or fragmented) and sometimes is crushed a little to give the rock more exposed surfaces over which the leaching fluid will flow. The rock is either left in place or is transported to a leaching pad or pile. This is a structure that often has a floor made of a tough plastic to ensure that liquid will not escape from the ore to the ground water. The pad is constructed in such a way that it provides a controlled environment so gravity allows the liquid to drain out of it naturally. This is where the solution extraction (SX) part of the process actually begins.
When the oxide ore has been placed on the leaching pad or pile, a weak solution of sulfuric acid is sprayed or dripped onto it. This liquid percolates down through the layers of ore. As the liquid seeps through the ore, it gathers copper from the rock much like water in a drip coffee maker picks up coffee as it passes through the ground up coffee beans. The result of the percolation at the mine, is a mixture called pregnant leach solution (PLS) that contains copper, sulfuric acid, water, and other chemicals.
The PLS flows into holding ponds and then is fed to a solution extraction tank house. While in the tank house, the PLS goes through several steps to transfer the copper into a solution suitable for the electrowinning process. First, an organic substance is added to the PLS, and the two are mixed together. The copper is attracted chemically to the organic and clings to it. Once mixing is stopped, the organic, now loaded with copper, floats on top of the PLS (just like oil floats on top of water). The leach solution, stripped of its copper, is piped back to the leach pads as raffinate. Here it is again sprayed or dripped over ore to pick up another load of copper. |
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Meanwhile, back at the tank house the loaded organic is stripped of its copper to make a solution used in the electrowinning process. A liquid called lean electrolyte, which contains a lot of sulfuric acid, is added to the tank with the loaded organic, and the two liquids are mixed together. The copper, attracted by the strong sulfuric acid, is forced to change locations, leaves the organic, and becomes part of a solution of sulfuric acid and copper known as rich electrolyte. The stripped organic, like raffinate, is returned to the system to be used again and again to strip PLS of its copper.
Now, the rich electrolyte is ready for the electrowinning (EW) part of the process. The rich electrolyte is placed in tanks in which lead anodes are alternated with stainless steel starter sheets. Electric current removes the copper from the rich electrolyte and plates it onto the starter sheets. In 5 to 10 days 250 pounds of copper have been electrically “won” from the electrolyte and plated onto the stainless steel starter sheets making 99.9999% pure cathode copper.
The depleted, lean electrolyte is sent back through the system to gather more copper from the loaded organic. The 250-lb. cathodes are pried from the starter sheets and sent to rod plants or other manufacturers.
Cathodes from the pyrometallurgical and hydrometallurgical processes may be cast as pure copper but are often alloyed with other metals. Cathodes may be melted to form wire rod, which is drawn down by wire plants to make pure copper wire of all gauges; billet copper, 30' logs about 8” in diameter that are sawed into shorter lengths, extruded, and drawn into tube, rod, and bar stock of various sizes and shapes; cake copper, slabs of pure copper about 8” thick and up to 28' long, that may be rolled into plate, sheet, strip, and foil; and ingot copper, bricks of pure copper used by mills for alloying with other metals or used by foundries for casting.
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