Various supported catalysts are widely used in the chemical and petrochemical industries, such as in the anthraquinone process for hydrogen peroxide production, urea synthesis, CO removal from ammonia synthesis gas, benzene hydrogenation to cyclohexane synthesis, catalytic combustion of low-concentration volatile organic compounds, and reforming, isomerization, cracking, hydrogenation, and dehydrogenation reactions in petroleum refining.
Al₂O₃ is the most commonly used support, with smaller amounts used in SiO₂ and CaCO₃. Platinum and palladium are the primary metals used, forming two series: Pt/Al₂O₃ and Pd/Al₂O₃. Depending on the intended use, the platinum and palladium content in the catalyst is 0.01-0.5%, classifying it as low-grade waste. Al₂O₃ supported catalyst waste is mostly in the form of 3-5 mm spherical or cylindrical particles. The crystalline structure of the support is divided into α-type and γ-type, with significant differences in acid solubility; α-type is insoluble in acid, while γ-type is readily soluble. The composition of various waste catalysts is simple, involving only the recovery of individual platinum or palladium particles.
The key to recycling technology is the effective separation of platinum or palladium from the carrier to extract platinum-palladium concentrates or refined metals. One method involves directly leaching platinum and palladium with acid or other chemical reagents (or percolation, soaking), followed by enrichment from the solution using methods such as anion exchange, zinc (or iron, aluminum) displacement, activated carbon adsorption, sodium sulfide precipitation, and ethyl xanthate precipitation. The opposite method involves leaching the carrier to obtain platinum-palladium-enriched residues. After extracting the platinum-palladium concentrates or crude metals using either method, further purification to pure metals presents no technical difficulties.
