Rhodium

Rhodium (Rh) - a metal named by its discoverer W. H. Wollaston after the pink colour of its compounds, is 5 times rarer than platinum (Pt) in the Earth's crust. Despite the fact that it finds great use, like Pt, in alloys, it certainly has something to offer as an element in its own right. Since we at Safina have a long and rich experience with the use of (Rh), both in alloys and in its pure form, we would like to introduce you to its properties, processing and products made from it.

Properties and uses

Rhodium belongs together with ruthenium (Ru) and palladium (Pd) to the group of light platinum metals, i.e. with a significantly lower density compared to platinum (Pt), gold (Au) or iridium (Ir). Of the platinum group metals (PGMs), Rh has the highest electrical conductivity and light reflectivity. Although it resembles very ductile Pt in crystal structure and strength in the annealed state, it exhibits extreme hardness in the deformed condition and in thin layers. It is therefore used for plating jewellery and electrical contacts for demanding applications.

It also stands out for its corrosion resistance. Evenaqua reqia, which readily dissolves Au and Pt(otherwise inert in most cases), is no match for Rh (see Figure 2). It also resists high temperatures very well – its melting point is the 3rd highest among PGMs and the 13th highest among all known elements in the periodic table.  Although it is one of the precious metals, at temperatures up to 1000 °C it is covered by a very weak layer of oxides on the surface; above this temperature they are no longer thermodynamically stable and the surface is thus shiny even under extreme heat.

In alloys with Pt, it increases its strength and corrosion resistance. Pt-Rh alloys are thus used for structural parts in glassmaking and laboratory equipment coming into direct contact with glass melt or other corrosive substances at high temperatures. It also changes its thermoelectric properties, which is used in thermocouples for measuring temperatures up to 1800 °C.

In particular, its catalytic properties are key – whether in the form of metal, alloys or compounds and complexes. Most of the world’s Rh (is used in the production of automotive catalysts for the reduction of harmful NOX in exhaust gases.

As its environmentally intensive extraction is only in the context of Ni, Cu or Pt ores, the market is not able to respond flexibly to increases in demand and therefore its price is often subject to extreme fluctuations.

Chemical refining

The high chemical resistance of Rh is a very valuable characteristic, but it greatly complicates its wet refining process. First it is necessary to dissolve all the material, which can be achieved either by diluting it with another metal to form an alloy (typically Pt-Rh) or by developing extreme conditions such as high pressure, temperature and strong oxidising environments. This is followed by chemical processes in which the rhodium is gradually separated from the other platinum metals and converted into a solution free of impurities. During refining, both classical inorganic precipitation reactions and more modern methods such as organometallic compound formation and liquid extraction are used.

The purified rhodium solution is either transformed into a metallic form commonly referred to as a sponge (the designation is based on the similarity of its structure to sponge-like formations, see Figure 1) or it is made into one of the chemicals, e.g. RhCl3, Rh(NO3)3 or Rh2(SO4)3.

Safina has the technology for refining up to high purity of 99.99% allowing its reuse for the production of PtRh thermocouples or other products where the lowest possible impurity content is required. The undeniable advantage of the process is its versatility, where we are able to process both used Rh-based products and its alloys, as well as waste in the form of solutions, sludges and other concentrates.

Metallurgical processing

The Rh sponge obtained is the starting material for products in metal form. The first step is to convert this powder into a homogeneous mass. Either by powder metallurgy or by vacuum melting and slab casting – which ensures 100% density and higher purity of the material. The ingot thus produced must then be reworked by forming into a suitable mould for the final product. Due to the extreme strain hardening and susceptibility to brittle intergranular fracture (Fig. 4), a special thermomechanical processing method must be chosen. In addition to forming, machining is also a major challenge.

Offer

In addition to standard Pt-Rh alloys, Safina offers pure Rh in powder (sponge), chemical, sheet, bar and drawing machined parts. We specialize in products used in thin film Rh deposition. Whether as a rhodium sulfate solution for electroplating, or targets or pellets for PVD processes.  The standard sizes offered start from 1 mm thickness for targets and 3 mm for pellets, however we always try to find a solution for non-standard requirements.

A responsible approach to the industry is the basis of our success. We therefore place great emphasis on recycling used products and waste to return Rh to the processing cycle. This not only reduces our environmental footprint, but also ensures the sustainability of rhodium production and processing for future generations.

We believe that our innovation, processing technology and responsible approach to the industry makes us your ideal partner for rhodium products. Thank you for your trust in SAFINA and we look forward to working with you in the future. We plan to invest in further developments and create new chapters of our successful journey together with you.

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