Electrorefining and Electrowinning
In the field of electrometallurgy, electrorefining and electrowinning (different parts of the same procedure) are important techniques that allow purification of nonferrous (noniron-based) metals to an extreme degree. Economical and straightforward, they use electrical current to separate metals from their accompanying impurities. The overall process is known as electrolysis, which literally means to break down using electricity. Electrorefining is the least expensive way to purify metals because it is so selective in terms of what it produces. In other words, it produces a pure metal in just one step, saving time and money. There are commonly used alternate names for both techniques: electrorefining is also called electrolytic refining, while electrowinning is also called electroextraction.
Electrorefining works by running a current through an electrolyte solution via an electrolytic cell. An anode is the positive electrode (conductor of electricity) through which electrons leave an electrolytic cell, and a cathode is the negative electrode through which electrons enter the cell. An electrolyte solution is a conductive medium containing the particular metal's salts; when current moves through the medium, ions (electrically charged atoms) move with it.
To purify a metal by electrorefining, it must be made the anode in the electrolytic cell. For instance, to purify a big chunk of copper, the chunk would be put into the electrolytic cell as the anode. There would also be a small piece of very pure copper in the cell to act as the cathode. As the electricity passed through the cell, electrons would be removed from the impure copper anode. Copper ions would form and dissolve into the electrolyte. Those same ions would then flow to the cathode, and after electrons were added, would end up as pure copper.
Some metals, including arsenic and nickel, do not collect at the cathode after leaving the anode, but remain in the electrolyte solution. They require a different method of collection involving reduction and purification of the electrolyte solution.
The collection process itself is called electrowinning. The amount of metal that is electrowon varies greatly according to the size of the original piece of ore at the anode, but large electrolytic cells can electrowin hundreds or thousands of pounds of a metal in one day.
Another big benefit of electrorefining is that after the process for the original target metal is complete, there might be valuable "leftovers." For example, after electrorefining copper, there are usually tiny amounts of silver and gold that are released from the anode as the copper is oxidized. but which are not deposited at the cathode. These metals sink to the bottom of the electrolytic cell, where they form a substance called "anode sludge" or "anode slime." Just like the copper, the gold and silver can be collected and purified, although they need to go through more steps to reach the same state. Otherwise, the ore's original impurities remain at the anode.
The most common electrowon metals are lead, copper, gold, silver, zinc, aluminum, chromium, cobalt, manganese, and the rare-earth, alkali, and alkaline-earth metals. All of these metals are essential ingredients in many industrial manufacturing processes, including those in the communications, transportation, and electronic fields. In the mid-1990s, the U.S. Department of Energy even began experimenting with using electrorefining to help the nation deal with its huge amounts of nuclear waste. Electrorefining could separate such dangerous heavy metals as plutonium, cesium, and strontium from the less-toxic bulk of the nuclear waste, not only leaving less to dispose of, but removing many materials that could harm the environment or potentially be made into weapons.
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