Ore Raw and process materials

Raw materials can be liquids, sludges or solids, and include scrap jewellery, substandard ingots, industrial catalysts, discarded electronic circuitry, sludges and wastewaters, for example from photographic processing and silver plating. In general, precious metal recovery works receive a mixture of waste materials from a wide range of suppliers that may include petrochemical, pharmaceutical and chemical plants.

The substances used in the recovery processes can be in either solid, liquid or gaseous forms. They may include acids, fluxes, sand, cooling water, modifying chemicals, liquid chlorine, and a variety of organic and inorganic solvents. Some works use liquefied flammable gases as burner fuels for smelting, and nitrogen to provide an inert atmosphere when working with flammable materials.

Method of delivery

Raw and process materials are delivered to site either in bulk quantities (eg large volumes of metal waste and scrap) or in smaller quantities (eg gold scrap or sweepings). Fuel oil or gas for burning, smelting and similar operations are delivered by bulk road tanker or by direct pipeline. Acids, alkalis and solvents are typically delivered by bulk road tanker. Chemicals which are used in smaller quantities are delivered in drums, kegs and intermediate bulk containers. Coke used for pyrometallurgical processes and fluxes are received in bulk, primarily by road transport, and usually stored in uncovered areas, often directly on unprotected soil. Large recovery plants may also incorporate research and laboratory facilities and therefore may receive and store a wide variety of chemicals, typically in small quantities.

Transfer of materials

Raw materials are transferred within the recovery plant using a variety of methods depending on their physical state and quantity. Solids (which include resins, computers, gold jewellery and ingots) are transferred using a combination of mechanical means, eg fork-lift trucks, conveyors, cranes and manual systems. Pumped systems are typically used for molten metal, sludges/suspensions, acids and alkalis, and solvents that are in a liquid or semi-liquid state. Kegs, bags and bottles of chemicals are often handled manually by site personnel.

Operations at the many works involved with the casting of ingots may entail the use of degreasing agents, eg chlorinated hydrocarbons. These are generally stored in bulk storage tanks or drums and either pumped from the tanks or transferred in drums to the place of use.

Recovery processes and products

Generally, the types of processes used in precious metal recovery depend on the quality of the received materials and the interests of the company. Depending on their quality and state prior to refining, the raw materials may be first processed to produce a metal concentrate by chemical treatment, burning or drying, and melting. The metals most commonly recovered by the above processes are silver, gold and PGMs, for sale or for conversion into special weight bars of high purity. They are usually delivered from the works as ingots. Alternatively, they can be returned to the raw material supplier as precious metal compounds, catalysts or fabricated products.

Metals such as iron, tantalum, iridium, zinc, copper and aluminium, sometimes present as impurities in the raw materials brought onto a recovery site, may also be processed.

Most recovery works incorporate casting operations and possibly surface finishing operations to suit customer requirements.

Chemical and melting processes

One chemical treatment is the precipitation of metals from solution by substitution using a more active metal such as iron, zinc, copper or aluminium at optimal pHs. Typical compounds used in silver precipitation processes include sodium sulphide, and acids and alkalis for pH control. Another treatment involves ion exchange resins which are widely used for the removal of precious metals (particularly silver) from wastewaters. The resins can be regenerated using a variety of chemicals, including acids, brine and ammonium chloride or thiosulphate, producing solutions containing the precious metals, which are generally sent for electrolytic refining. However, for the highest value metals such as gold, the spent resins may be burnt in a crucible type unit, leaving a residue of molten gold metal and other metal impurities. The use of organic solvents to extract metals from waste solutions is another chemical treatment.

Metal scrap, such as low grade bullion and jewellery, is usually melted and then subjected to pyrometallurgical processing. In smelting operations using a blast furnace, the scrap is mixed and processed along with coke and fluxes (eg litharge and borax). The resulting molten metal concentrate is then typically subjected to the cupellation process: oxygen is blown across the metal to volatilise some of the minor metals present and to remove any iron and residual copper and lead oxides as dross. The remaining molten metals can then be cast as electrodes and subjected to electrolytic processing to produce a refined product. Alternatively they can be cast into pellets and treated with nitric acid to remove base metals. Sulphuric acid is often used if the lead content of the pellets is low.

Very low grade scrap may be melted, cast into anodes and electrolysed in a sulphate salt solution. The gold, which remains in the electrolytic vessel as a mud, is recovered by converting it to gold chloride (possibly by bubbling chlorine gas through the bath) and then precipitating the gold using ferrous sulphate and pH control. If further purification is required, the gold precipitate can be removed, dried, melted and cast into anodes for electrolytic refining.

Electrolytic refining

Refining of gold concentrates can be achieved using the Wohlwill process in which pure gold is recovered from a chloride solution using alternative use of DC and AC currents and collecting the gold on the cathode. The gold is flaked off the cathodes which are usually made from stainless steel.

The solution remaining after electrolysis may contain other PGMs which can be separated using a variety of techniques. For example, platinum is separated from the solution by dibutyl carbinol solvent extraction, followed by a series of treatments using ammonium chloride, sodium chloride, aqua regia and bromate solutions. A combination of processes involving formic acid, ammonia and hydrochloric acid can be used to separate out palladium after platinum removal.