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The electrowinning of metals, or the production of metals from their ores once put in solution or liquefied, is the oldest industrial electrolytic process. Sodium metal was first prepared in 1807 by the English chemist Humphrey Davy, who obtained it using electrolysis of molten sodium hydroxide. Several industrially important metals (the active metals which react with water) are produced commercially today by electrolysis of molten salts.
Metal deposition rates are key parameters in developing an effective electrowinning unit. Units are most effective when installed in rinse tanks immediately following drag-out tanks. Electrowinning is not a viable recovery technology for all metals. While the process works well for metals with high electro-potential, such as gold, silver, copper, cadmium and zinc, it does not work as well on others, such as chromium. Nickel can be recaptured also. However, the process is very pH sensitive and must be rigorously maintained for any deposition to occur. For metals which electrowinning works, the following criteria are important in the operating performance of electrowinning units:
Cathode surface area: Because metal deposition rates are related to available surface area, maintaining properly working cathodes is important. Two cathode types exist, flat-plate and reticulated cathodes, each with its own advantages. Flat-plate cathodes can be cleaned and reused, and plated metals recovered. Reticulated cathodes have a much higher deposition rate, compared to flat-plate cathodes. However, they are not reusable and must be sent off for recycling.
Current density: The deposition rate of metals onto cathodes increases with higher current densities. However, "excess" current is wasted on converting water to hydrogen and oxygen gas, instead of plating out desired metal contaminants.
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