Precious metal mainly refers to the 8 kinds of metal elements of gold, silver and platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, platinum). Most of these metals have beautiful colors, and quite strong in resistance to chemicals, which means under the normal conditions, chemical reaction is unlikely to be caused. Today, the precious metal is no longer limited to the manufacture of ornaments and play a role in the economic and financial, but widely used in microelectronics, aerospace, missiles, rockets, light industry, petroleum, environmental protection, military and medicine, and many other aspects, much precious metals and their alloys, compounds have shown important technical features in high-tech fields, such as catalysis, electronics, carcinogenic, fine chemicals and so on. Accordingly, the precious metal is also known as vitamin of modern industry.

 

Currently, the mainly industrial precious metal refining methods are pyrometallurgical, hydrometallurgical and biotechnology metallurgy. Among them, the pyrometallurgical is an important precious metal recovery technology to extract precious metals from the discarded appliances, which is stripping non-metallic materials by heating to high temperatures in a metallurgical furnace, precious metals melt in the other molten metal or molten salt melting materials, and then separate. Hydrometallurgical process is using the properties of precious metals can be dissolved in nitric acid, aqua regia or other caustic acid, to remove them from discarded appliances and then recovered from the liquid phase.

 

Photocatalytic technology can be used to achieve the extracting of noble metal by the deposition of metal ions. In the experimental study of photocatalytic reduction of Hg²⁺ of titanium oxide and tungsten trioxide photocatalyst showed that, Hg²⁺ showed a strong adsorption on the surface of tungsten trioxide, and the adsorption amount increased as the pH increases. Furthermore, after the light intensity increased, the photocatalyst absorbed more photons per unit time, which can significantly speeded up the precious metal deposition rate; In addition, the photocatalyst filed elemental silver experiments showed that the recovery ratio of the amount of silver and the amount of catalyst can up to 3 :1.

 

The prominent advantage of photocatalytic reaction (Like tungsten trioxide photocatalyst, etc.) to extract precious metals is that it can apply to a very dilute solution which the conventional method is not applicable, it is possible to enrich the precious metal on the catalyst surface in a more simple way, and then collected by other means recovery processing. More importantly, photocatalyst can even allow the separation of mixed ions, because of the oxidation-reduction potential of different kinds of metal, when conditions controlled properly, they will be precipitated sequentially and selectively.