Everything about Electrowinning totally explained
Electrowinning, also called
electroextraction, is the
electrodeposition of
metals from their
ores that have been put in solution or liquefied.
Electrorefining uses a similar process to remove impurities from a metal. Both processes use
electroplating on a large scale and are important techniques for the economical and straightforward purification of
non-ferrous metals. The resulting metals are said to be
electrowon.
In electrowinning, a current is passed from an inert anode through a liquid
leach solution containing the metal so that the metal is extracted as it's deposited in an electroplating process onto the cathode. In electrorefining, the anodes consist of unrefined impure metal, and as the current passes through the acidic electrolyte the anodes are corroded into the solution so that the electroplating process deposits refined pure metal onto the cathodes.
History
Electrowinning is the oldest industrial
electrolytic process. It was first demonstrated experimentally by
von Leuchtenberg in
1747. Later the English chemist
Humphrey Davy obtained
sodium metal in
elemental form for the first time in
1807 by the
electrolysis of molten
sodium hydroxide.
James Elkington patented the commercial process in
1865 and opened the first successful plant in
Pembrey, Wales in
1869. The first commercial plant in the United States was the
Balbach and Sons Refining and Smelting Company in Newark, New Jersey in
1883.
Applications
The most common electrowon metals are
lead,
copper,
gold,
silver,
zinc,
aluminium,
chromium,
cobalt,
manganese, and the
rare-earth and
alkali metals. For aluminium, this is the only production process employed. Several industrially important active metals (which react strongly with water) are produced commercially by electrolysis of their pyrochemical molten salts. Experiments using electrorefining to process spent nuclear fuel have been carried out. Electrorefining may be able to separate heavy metals such as
plutonium,
caesium, and
strontium from the less-toxic bulk of
uranium. Many electroextraction systems are also available to remove toxic (and sometimes valuable) metals from industrial waste streams.
Process
Most metals occur in nature in their oxidized form (
ores) and thus must be reduced to their metallic forms. The ore is dissolved following some preprocessing in an
aqueous electrolyte or in a molten
salt and the resulting solution is electrolyzed. The metal is deposited on the
cathode (either in solid or in liquid form), while the
anodic reaction is usually
oxygen evolution. Several metals are naturally present as metal
sulfides; these include copper, lead,
molybdenum,
cadmium,
nickel,
silver, cobalt and zinc. In addition,
gold and
platinum group metals are associated with sulfidic base metal ores. Most metal sulfides or their salts, are electrically conductive and this allows electrochemical
redox reactions to efficiently occur in the molten state or in aqueous solutions.
Some metals, including
arsenic and nickel don't electrolyze out but remain in the electrolyte solution. These are then reduced by chemical reactions to refine the metal. Other metals, which during the processing of the target metal have been reduced but not deposited at the cathode, sink to the bottom of the electrolytic cell, where they form a substance referred to as anode sludge or anode slime. The metals in this sludge can be removed by standard
pyrorefining methods.
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're not reusable and must be sent off for recycling. Alternatively, starter cathodes of pre-refined metal can be used, which become an integral part of the finished metal ready for rolling or further processing.
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