Saint Petersburg Mining University, Saint Petersburg, Russia:

R. A. Pakhomov, Associate Professor, Candidate of Technical Sciences, e-mail: pakhomovra@gmail.com
S. B. Fokina, Associate Professor, Candidate of Technical Sciences, e-mail: fokina_sb@mail.ru
E. A. Mushikhin, Undergraduate Student, e-mail: s190666@stud.spmi.ru

Kola MMC, Monchegorsk, Russia:
S. V. Shcherbakov, Chief Metallurgist at the Engineering Office

One of the largest reserves of precious metals includes metal-bearing waste, in particular spent automotive catalytic converters. They contain up to 0.3 wt % of platinum group metals (PGMs), which by dozens of times exceeds their concentration in mineral raw materials. Metallurgical processing of such type of materials helps tackle the issue of disposing of spent secondary raw materials by regenerating and recycling a part of PGMs. The paper considers a collector smelting process designed for primary metallurgical processing of spent automotive catalysts aimed at raising the concentration of PGMs in the catalytic mass by 1–2 orders. This technique helps consolidate PGMs into the melt. The process performance and efficiency are largely dictated by the composition and physical and chemical properties of the slag. This research focused on a spent ceramic catalytic converter containing a considerable amount of aluminium and silicon oxides. Indialite Mg₂Al₄Si₅O₁₈ is the basic constituent of the sample, while zirconium and cerium oxides (ZrO₂, Ce₂O₃), as well as silicates BaMg₈Al₁₈Si₁₈O₇₂ and KMg₄Al₉Si₉O₃₆ are also present. The authors conducted a series of thermodynamic calculations to determine the solidus and liquidus of the material, and they established that even when an automotive catalyst is smelted in the temperature range of 1.700 to 2.000 oC, it won’t reach a completely molten state. This confirms that the sample in view contains some refractory components. A theoretical analysis of multicomponent phase diagrams of the Si – Al – Mg – Ca – Na – B – Zr – Ce – Ba – Fe – F – O type helped define molten slag systems that would be most suitable for testing, the theoretical liquidus temperature of which varies in the range of 700 to 1.400 ^oC. Having conducted a thermodynamic analysis that looked at the formation of liquid and solid phases depending on the process temperature, the authors established that when the smelting of the initial material takes place in the presence of fluxes, the melting point of the burden drops from 1.700 to 1.075–1.100 ^oC. The conducted experimental study showed that the practical melting point of an automotive catalyst is, on average, 100 to 300 ^oC higher than the predicted one, which can be attributed to the presence of hightemperature zirconium and cerium oxides in the initial material. The authors defined the compositions of slag systems for recycling ceramic catalysts with an optimum combination of such parameters as melting point and amount of fluxing agents: FeO – Al₂O₃ – SiO₂; CaO – Al₂O₃ – SiO₂; Na₂O – B₂O₃ – Al₂O₃ – SiO₂.

This research was funded through a subsidy for fulfilling Governmental Assignment No. FSRW-2020-0014 for research in 2021.

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