Electrowinning is a process used to remove metal ions from liquid solutions such as rinse water, plating baths, and used process solutions. Most often used to recover precious metals, it relies on an electronic current to plate the metal ions onto a cathode. Recovered metals can then be sold for scrap or processed for use in plating. The solution can also be recycled with less stress on water treatment systems.
An electrowinning unit consists of a large tank known as a reaction chamber. This chamber houses at least one set of cathodes, which are negatively charged electrodes, and anodes, which are positively charged electrodes. When the unit is turned on, direct current (DC) flows from the cathode to the anode through the liquid solution. The flow of electricity causes metals in the solution to plate onto the cathode in a process similar to electroplating.
The electrolysis process used in electrowinning allows the elemental metal to be recovered. This sets electrowinning apart from other recovery technologies, such as evaporation and ion exchange, which result in little more than a more concentrated metal-bearing solution. Despite this, the metal recovered is usually still not pure enough to be used in plating if left unprocessed. As a result, it is often sold as scrap. Some companies use this recovery technique as a way of creating extra income by selling the scrap metals.
The efficiency of the electrowinning process depends on the concentration of metal in the solution. It becomes progressively less efficient as more metal is removed. The surface area of the cathode also affects efficiency. The greater the surface area, the greater the efficiency, which is why electrowinning units have been designed that use large rectangular plates or mesh grids as cathodes.
The final variable that affects efficiency is the type of metal being recovered. Metals most commonly recovered by electrowinning include copper, silver, and gold. Cadmium, zinc, and even nickel can also be recovered using this process. Nickel is least common because the solution must be carefully ph-controlled throughout the process in order for it to be effective.
Although electrowinning is a useful process, it does have some drawbacks. It becomes very inefficient if the solution has less than 1,000 mg of metal ions per liter. Also, it cannot be used in solutions that contain chlorine ions, such as hydrochloric acid, because electrolysis of chlorine ions can result in chlorine gas, which is extremely hazardous.