The predominant platinum group

Summary

Platinum group elements include platinum, palladium, iridium, osmium, rhodium and ruthenium. The predominant platinum group deposits which are mined occur in the Bushveld in South Africa and Noril'sk in Russia. All other mines are generally nickel-copper with by products or co-products of platinum group elements and cobalt. In most cases the ore is mined using conventional underground mining techniques. Conventional crushing is used with gravity and flotation concentration.

Platinum group elements are primarily used for autocatalysts, jewellery, medicine, chemical industry, electrical (capacitors and computer hard discs), glass and petroleum refining. The main uses for palladium are in autocatalysts, electronics, dental use, chemical industry and jewellery. Rhodium is also used in autocatalysts, chemical industry, electrical and glass industries

Introduction

This essay primarily focuses on platinum group elements (PGEs) deposits. Topics covered include mineralogy, location, processing, economics and political aspects. Platinum-group elements (PGEs) include platinum (Pt), palladium (Pd), iridium (Ir), osmium (Os), rhodium (Rh) and ruthenium (Ru).

The predominant commercial PGEs deposits occur in the Bushveld and Noril'sk Complexes which together account for around 93% of world production. Platinum grades at the Bushveld average around 5 g/t, producing by products of nickel, copper and cobalt. All other mines are generally nickel-copper mines with by-products or co-products of PGE and cobalt. Minerals such as chalcopyrite, pyrrhotite and pentlandite, or chromite are associated with these copper and nickel sulphides.

Platinum Group Element Formation

Platinum-group elements prefer to form metallic bonds over ionic bonds. This causes them to form readily with Fe and Ni. A second property is that they prefer to form covalent bonds with sulphur over ionised bonds with oxygen. This means they tend to also bond with Cu,Ag and Au. These PGEs are broken up into two subgroups which are Os, Ir, Ru as the Iridium Platinum Group Elements (IPGE) and Pt, Pd and sometimes Rh as the Platinum Platinum Group Elements (PPGE). Each subgroup behaves differently in magmatic processes. The PGEs behaviour and distribution within the Earths layers are controlled by the presence or absence of metallic phases or sulphide phases. The distribution has been strongly affected by the sequestration of sulphide liquids in the mantle and crust. A high-grade PGE ore deposit generally contains several grams per tonne of Pt and Pd combined, with cut-off grades typically 2 g/t. Although these values can change dramatically depending on a number of factors such as depth of mine and mill costs.

Basaltic magmatism is a key part in PGEs formation. These basaltic magmas occur in the upper mantle and travel to the crust. To form these PGE deposits, these magmas must initially be rich in PGEs. When these large volumes of magma have high concentrations, PGEs are deposited into small volumes of rock. This is done by two collector phases which are a liquid sulphide and chromite. These act as a mechanical collector when the large volumes of magma pass through a conduit, where the collected material is deposited. Some of these conduits include dyke (the Great Dyke, Zimbabwe), cylindrical bodies of ultramafic cumulate (Alaskan-Uralian complexes), sills (Noril'sk in Russia, Bushveld Intrusion in South Africa and Stillwater Intrusion in USA) and large layered intrusions. These large layered intrusions can be a trough, embayment, local depression or a widening of a magma chamber.

Mineralisation zones formed by these collector phases may also be zones of disseminated or massive sulfides. They form pools along the bases of magmatic conduits or chambers forming stratiform bodies enriched in sulfide or chromite. They can also form layered or tubular concentrations of chromite, which is hosted by ultramafic rocks.

Platinum group elements have tend not to form naturally. Although there is one case were PGEs are able to precipitate directly from silicate magmas phenocryst (large crystal) phases. A mineral that this process occurs in is laurite (RuS2). For this to occur specific conditions are required which involve temperature, pressure, oxygen and sulphur. With the correct conditions it is possible for laurite to precipitate from basaltic liquids. Laurite can be found generally as inclusions in natural chromite crystals in layered intrusions and ophiolites. Most other PGEs are generally formed during a process of cooling and recrystallisation of primary minerals. This is where a constant inflow of magma occurs and crystallisation would start from the bottom of a chamber forming layers of minerals i.e reefs.

www.bgs.ac.uk (British Geological Survey)

The predominant commercial PGE deposits occur in the Bushveld and Noril'sk Complexes which together account for around 93% of world production. Platinum grades at the Bushveld average around 5 g/t, producing by products of nickel, copper and cobalt. Other mines such as Stillwater in Montana USA have 20g/t platinum plus palladium. All other mines are generally nickel-copper mines with by-product or co-product of PGEs and cobalt.

Processing

In most cases the PGE ore is mined using conventional underground mining techniques, although there are a couple of open-pit operations such as Lac des lles. The ore is then crushed and ground where it is then concentrated by either gravity or flotation. The concentrate is then sent to directly to either a refinery or to a converter where both separate copper and nickel sulphides are produced.

These sulphides are crushed, roasted and smelted to produce impure cooper and nickel. These are then refined using electrolytic cells. During the conversion process most of the PGE goes into the nickel sulphide. The PGEs are found in an insoluble residue that accumulates in the electrolytic cells. This residue is roasted and leached to remove copper, nickel iron and sulphur. Wet chemical treatment is then used to recover gold and PGEs. Calcination is then used to yield purer platinum and or palladium with a purity of 99.99%.

The primary objective of conventional flotation is to increase the recovery of metals. Problems arise from conventional processes when processing PGEs. At the mill chromite tends to be over-ground due to its high density and brittle nature. Since the chromite is over-ground, fine chromite is produced. This fine chromite is then found in the final concentrate. This causes implication when the concentrate is smelted due to excessive levels of chromium oxide. This implies that it is desirable to have low levels of chromite in ore to be processed.

Other problems which arise from conventional flotation are separating the PGEs. It is difficult to maintain an economic recovery during separation. The flotation rates of the sulphide minerals were these PGEs are found is quite slow. To achieve an economic recovery a number of stages of cleaning and re-cleaning circuits are put in to the conventional flotation circuit.

The degree of alteration of the ore also effects how much talc is in the ore to be processed. Up to a certain point talc can be handled by the addition of a depressant such as carboxymethyl cellulose (CMC), but large quantities create serious difficulties.

Uses for these commodities include jewellery, autocatalysts, medicine, chemical industry, electrical uses (capacitors and computer hard discs), glass and petroleum refining. The main uses for palladium are in autocatalysts, electronics, dental use, chemical industry and jewellery. Rhodium is also used in autocatalysts, chemical industry, electrical and glass industries.

Properties that make these elements useful are that platinum and palladium are the most corrosion resistant and the most malleable of all the PGEs. Ruthenium and osmium have the strongest abrasion resistance. Ruthenium is used as an electrical contact and as a titanium coating alloy. Rhodium and iridium are the least abrasion resistant and are used as alloying elements for platinum. (www.EPA.gov)

Generally platinum is used in autocatalysts located in exhaust systems. Catalytic converters are used to limit the smog-producing chemicals that come from combustion, such as nitrogen oxides. The exhaust passes through the catalytic converter that contains platinum and iridium. When the gas passes through the converter 75% of the nitrogen oxide is converted into nitrogen and oxygen. Also more than 95% of carbon monoxide and other hydrocarbons are oxidised. The platinum works by lowering the energy needed to cause these chemical changes, causing reduction in pollution.

Some PGEs are used in chemotherapy, particularly to fight leukemia. Platinum-iridium compounds are used to make biomedical devices. An alloy of platinum and osmium is used in pacemakers to regulate heart function and in heart replacement valves.

Discussion

The majority of production of these PGEs comes from two countries South Africa and Russia. Essentially these two companies control the market of PGEs. They control the amount of PGEs that are available on the market which goes hand in hand with the price. Having such an important group of commodities being produced by mainly two countries has some risk involved globally. If a country becomes politically unstable and the production of PGEs is halted or changed, the global market and availability of these elements could be affected. Important products such as pacemakers and autocatalysts depend on the availability of these products.

Conclusion

The predominant commercial PGEs deposits occur in the Bushveld and Noril'sk Complexes with many others located around the world. Basaltic magmatism is a key part in PGEs deposit formation. These basaltic magmas occur in the upper mantle and travel to the crust. To form these PGE deposits, these magmas are initially be rich in PGEs. With large volumes of magma depositing high concentrations of PGEs must be into small volumes of rock. They form pools along the bases of magmatic conduits or chambers forming stratiform bodies enriched in sulfide or chromite. They can also form layered or tubular concentrations of chromite hosted by ultramafic rocks.

In most cases the PGE ore is mined using conventional underground mining techniques. The ore is crushed and ground and then concentrated by either gravity or flotation. Once concentrated the ore is smelted with its end products being used around the world. These PGEs are used for various implication such as jewellery, autocatalysts, medicine, chemical industry, electrical uses (capacitors and computer hard discs), glass and petroleum refining.

References

  • Mungall J.E. EXPLORATION FOR PLATINUM-GROUP ELEMENTS DEPOSITS, Mineralogical Association of Canada Short Course Series Volume 35, 2005 pg 1-9

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