Abstract:

Like most chemical processes, the refining of PGMs involves several specific process safety hazards. The noble character of PGMs often necessitates highly aggressive conditions to achieve dissolution and chemical separation. The harsh chemicals used in traditional PGM refining processes (e.g. HCl, HNO₃, Cl₂,…) - and particularly their combinations - can give rise to dangerous situations or unintended by-products. Additional hazards arise from the chemical properties of the PGM compounds themselves and may be either acute (e.g. RuO₄ formation) or long‑term (e.g. sensitization to platinum salts).

These process hazards are generally well understood when the generation or use of hazardous species is intentional and properly controlled. However, the same chemistry can create significant issues when it occurs unintentionally or under uncontrolled conditions. Several incidents and near misses illustrate how such situations can escalate into safety events. Through a series of case studies, this presentation aims to share these learnings more broadly with the industry and raise awareness of less obvious but critical process safety hazards.

A first group of process hazards is linked to the presence of hydrogen. Hydrogen evolution is a common concern in hydrometallurgical processes, particularly when non‑noble metals are contacted with concentrated acids. A less widely known issue is that certain noble metals, and their mixtures, can also cause H₂ evolution even though this is not immediately apparent when considering the standard electrochemical series alone.

This behaviour is explained by the fact that industrial-scale PGM processing occurs almost exclusively in hydrochloric acid media. Strong chloride complexation can significantly alter the redox behaviour of metals under these conditions. A notable example is copper, which may evolve hydrogen under specific combinations of acidity and temperature. More detailed thermodynamic analysis shows that high activities in concentrated HCl solutions, together with the Cu(I)/Cu(II) stability domain, play a key role in this phenomenon.

Hydrogen‑related hazards also include hydrogen uptake by PGMs through adsorption or absorption. Finely dispersed PGM materials may exhibit pyrophoric behaviour associated with hydrogen uptake.

A second category of hazards is associated with the formation of nitrogen trichloride (NCl₃) during the refining of Pt/Pd through ammonium salt precipitations. NCl₃ is a shock-sensitive and highly unstable liquid that decomposes violently. Its formation can occur via reaction between ammonium species and chlorine and has been described in the literature. Adequate venting is therefore required to prevent accumulation of NCl₃ during PGM processing.

Finally, the formation of organic species from inorganic carbon—typically present after pyrometallurgical operations—may also pose safety hazards. Identification of these species and quantification of their formation rates are important to ensure safe operation.

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Mr. Bert Smits