Sorry dislexic - should have read sx-ew plantSolvent Extraction...

Sorry dislexic - should have read sx-ew plant

Solvent Extraction Electrowinning (SXEW)

A good read for anyone with a few hours to kill lol

The SX/EW Process

The SX/EW Process is a hydrometallurgical process since it operates at ambient temperatures and the copper is in either an aqueous environment or an organic environment during its processing until it is reduced to the metal. Because of its dependence on sulfuric acid, the SX/EW Process is at present not a substitute for, but rather an adjunct to conventional smelting. However, it is also applicable in locations where smelter acid is not available by the purchase of sulfuric acid or the manufacture of sulfuric acid from sulfur or pyrite. In addition, it offers the opportunity to recover copper from an entirely different set of ores and mining byproducts than is possible by smelting; namely, oxidized materials. These may be mined copper minerals that are in oxidized form - minerals such as Azurite (2CuCO3 · Cu(OH)3), Brochantite (CuSO4), Chrysocolla (CuSiO3 · 2H2O) and Cuprite (Cu2O), residual copper in old mine waste dumps whose sulfide minerals have been oxidized by exposure to the air or sulfididic copper minerals that have been oxidized by another new technology - bacterial leaching. In addition, the process can be used to extract copper in situ. That is, without removing the material from the waste pile or from the ground. The net result of the use of this process is that copper can be produced from sources that in the past would have gone untouched, thus reducing the reliance on conventional ore bodies. Further, the process is capable of removing copper from waste materials where otherwise it would have been considered a contaminant to the environment. In the United States, for example, copper is considered to be a toxic material released to the environment once it is mined under Emergency Planning and Community Right-to-Know Act (EPCRA) and the Environmental Protection Agency's Toxic Release Inventory (TRI). Copper mine dumps and flotation tailings constitute a significant inventory of copper that is considered to be a contaminant to the environment under TRI.

The SX/EW process, itself, has very little environmental impact because its liquid streams are very easily contained, There is no effluent inasmuch as all impurities are returned to the site where they originated and the sulfuric acid is eventually neutralized by the limestone in the ore body or waste dump where it is deposited as calcium sulfate (gypsum) - a very insoluble substance.

The SX/EW Process has its roots in analytical chemistry where it is used to separate one metallic ion from another. It was first used as a large-scale process during World War II for the recovery of uranium from its ores. The key to the process is the development of organic extractants that are specific to the metal to be extracted. The first extractant that was specific for copper and used on a commercial scale was developed by General Mills Corporation and sold under the name LIX 64® (LIX for Liquid Ion Exchange and Roman for 1959 - the year of the first patent.) Ranchers Exploration and Development Corporation at its Bluebird Mine in Arizona in 1968 first demonstrated it on a large scale.

The process involves leaching the material with a weak acid solution. This solution, known as pregnant liquor, is recovered and then contacted with an organic solvent, referred to as the extractant, in the solvent extraction stage (SX). Here the copper is extracted away from the aqueous phase leaving behind most of the impurities that were in the leach solution. Since the copper ion is exchanged for hydrogen ion, the aqueous phase is returned to its original acidity and recycled to the leaching step of the process. Meanwhile, the copper-bearing organic phase is stripped of its copper by contacting it with a strongly acidified aqueous solution at which time the copper is moved to the aqueous phase while the organic phase is reconstituted in its hydrogen form. The copper-bearing aqueous phase is advanced to the electrowinning (EW) stage of the process while the barren organic phase is returned to the extraction stage of the process. In the electrowinning stage of the process the copper is reduced electrochemically from copper sulfate in solution to a metallic copper cathode. Electrowon copper cathodes are as pure as or purer than electrorefined cathodes from the smelting process. Thus they are well received by the market.

Whereas the conventional process requires an estimated 65 MJ/kg of energy the SX/EW process requires an estimated 15 MJ/kg (from heap or dump leaching) to 36 MJ/kg. (from mined and crushed ore). In addition to variations in energy consumption based on the origin of the material leached, there are also variations based on the particular circumstances of the installation. In the United States, for example, the energy requirement for SX/EW processing of heap or dump leaching ranges from an estimated 10 MJ/kg to 25 MJ/kg - the difference being largely the pumping costs. In the case of the higher energy consumer, the pregnant liquor is pumped five miles from the leach site to the SX/EW plant.

The electrowinning of copper requires considerably more electrical energy than does the electrorefining process - an average of about 8 MJ/kg for electrowinning vs about 1.5 MJ/kg for electrorefining. In electrowinning the copper must be reduced from the cupric form to metal; whereas, in electrorefining the copper is already in metallic form and is merely transported from the anode to the cathode to purify it. Steps are under development to reduce the energy requirement of electrowinning by about 40% by a modification of the anode reaction.

Another advantage of the process is its low capital investment requirement relative to the smelting process and its ability to be operated economically in a small scale. In China, for example, where copper deposits are not plentiful and tend to be small, there are 40 to 50