Hydrometallurgy is a process for recovering valuable metals dissolved in a water-based chemical mix containing metallic salt compounds. Such metal recovery usually is based upon sulfuric acid solutions and the metals that are targeted for recovery are either precious metals such as gold, platinum, and palladium, or more common industrial metals like copper, nickel, and zinc. The chemical processes are typically carried out in a titanium reactor that, though susceptible to corrosion by sulfuric acid, is protected by the interaction of the dissolved metals in the solution itself. The reactors are run at high temperatures and pressures, and can employ various methods of removing the metals, though leaching is the most common.
While hydrometallurgy has been practiced as far back as the 16th century, only in recent years has it become widespread due to the increasing value of precious metals like gold. This has made it economically viable to extract such metals from very low concentrations in ore. It is estimated that, as of the year 2011, over 70 different metallic elements were being extracted through hydrometallurgy processes, including such widely varying metals as uranium, mercury, and cobalt.
Another term for hydrometallurgy is solution mining, and its often an important secondary process in industries that regularly use acids such as sulfuric acid and hydrochloric acid. The process has a dual benefit. It both purifies the acids, which are often used as metal cleaning or electrolyte agents, and it separates out the trace metals for resale.
Where electrolyte processes are used that involve copper and zinc, hydrometallurgy allows for a combination of metals to be recovered from the solution. For copper electrolyte impurities, this includes nickel, arsenic, and tin, and, for zinc electrolytes, the metals of magnesium and manganese can also be recovered. The recovery process involves introducing a resin to which the acid binds, separating it from the metals so that it can be siphoned off. The downstroke or purification process to the recovery cycle takes only five minutes to complete, with the primary benefits being that most of the acid is recovered for further industrial use and that the resin has a long life cycle for repeated purification steps.
The leaching process in a reactor built specifically for hydrometallurgy is more demanding. The temperature in the reactor has to rise to above 392° Fahrenheit (200° Celsius). The recovery levels for metals are also not uniform, with recovery working best for gold and only leaching out part of the solution concentration of other metals. This fact requires that many different types of solutions be used to maximize recovery, from chlorides and halides to compounds based on the dangerous element cyanide such as thiocyanate, a form of thiocyanic acid.