Introduction
Platinum group metals (PGMs) are a group of six transition metals—platinum, palladium, rhodium, iridium, osmium, and ruthenium—which have similar physical and chemical characteristics. They have high melting points, neutral oxidation states, good catalytic properties and resistance to corrosion. PGMs are used in a wide range of applications including automobile catalytic converters, electronics, fuel cells, jewelry, and biomedical uses. The extraction and separation of PGMs requires complex processes, as they are typically found combined with other metals in ore deposits.
Solvent extraction
Solvent extraction is one of the most widely used processes to extract and separate PGMs from ore, and involves immersing the ore in an organic solvent and determining the PGMs to the organic phase. This produces a mixture of the desired PGMs, which are subsequently separated from each other by further processing.
The first step of solvent extraction process involves the dissolution of PGMs in an aqueous feed solution. The crude aqueous feed solution is then transferred to a solvent extraction vessel, where an organic phase is added. The organic phase contains an extractant, which is a chemical species that has an affinity for the metal ions. The extractant attaches to the metal ions, forming a metal-extractant complex that is soluble in the organic phase. The two phases, organic and aqueous, are then separated, and the metal-extractant complex is transferred to the organic phase. After the metal-extractant complex has been extracted, it is then passed through a series of separative steps to separate the PGMs from each other.
The most commonly used extractant for the separation of PGMs is di-(2-ethylhexyl) phosphoric acid (D2EHPA). D2EHPA is preferred since it is relatively non-toxic and has better detergent extraction than other phosphoric acid based schemes. Other extractants such as Cyanex 272 and Cyanex 301 are also used.
Ion exchange
Ion exchange is another process used to extract and separate PGMs, and it is based on the principles of ion exchange chromatography. In this process, metal ions in solution are exchanged with metal ions bound to the ion-exchange resin. The metal-ion complex is then transferred to a resin bed, which acts as an exchanger and binds the metal ions in exchange for others present in the solution.
The metal ions are then released from the resin bed by passing an eluent solution (i.e. a solution containing a different metal ion) through the system. This process can be used to separate and extract PGMs from a crude solution containing a variety of metals and other contaminants.
Conclusion
PGMs are a group of metals with similar chemical and physical properties that are used in many industries. PGMs are typically found in ore deposits combined with other metals and require extraction and separation processes. Two of the most widely used processes are solvent extraction and ion exchange.
Solvent extraction involves immersing the ore in an organic solvent and transferring the desired metals to the organic phase. The metals are then separated from each other by further processing. The most commonly used extractant is di-(2-ethylhexyl) phosphoric acid (D2EHPA).
Ion exchange is based on the principles of ion exchange chromatography, and involves the exchange of metal ions in solution with metal ions bound to an ion-exchange resin. The metal ions are then released from the resin bed by passing an eluent solution through the system. This process can be used to separate and extract PGMs from a variety of contaminants.
