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Triphenylphosphine acetylacetone carbonyl rhodium (ROPAC) is an important catalyst in the petrochemical industry, and its deactivated waste catalyst holds significant value for recovery. This study focuses on the existing forms of rhodium (Rh) in waste catalysts and the current status of traditional processes. A green, efficient, and continuous recovery technique was developed using a sealed stainless steel microchannel reactor. The influence of reaction temperature, reaction time, and phase ratio on the Rh recovery rate was investigated, and the process parameters were optimized using response surface methodology (RSM). The results indicate that the magnitude of the impact on the Rh recovery rate follows the order: reaction temperature > reaction time > phase ratio. The optimized process parameters were determined as follows: a reaction time of 29 min, a reaction temperature of 110 °C, and a phase ratio of 1:1, with a corresponding maximum recovery rate of Rh of 66.06%. Furthermore, secondary treatment was performed on the organic phase after primary recovery using the same process conditions, resulting in an overall Rh recovery rate of 95.6%, indicating satisfactory recovery efficiency. Moreover, the application of FTIR and ICP-OES analysis provided definitive evidence that the oxidative dissociation of the rhodium-phosphine chemical bond by H2O2 within ROPAC leads to the conversion of Rh+ into Rh3+. Subsequently, Rh forms chloroaquorhodium (III) complexes that enter the aqueous phase, enabling effective recovery of Rh. [ABSTRACT FROM AUTHOR]

Silver, with excellent physicochemical properties, has been widely used in modern industry. Silver recovery from end-of-life product is economically significant for sustainability. However, silver leaching solution (SLS) containing diverse heavy metal ions as well as dissolved silica is a challenging waste stream for treatment and silver recovery. In this work we explored treatment of a real SLS via direct contact membrane distillation (DCMD). Results showed that silica scaling in DCMD process was closely related to the coexisting aluminum ions (Al3+) in SLS. The amount of Al(OH) 3 formed was dependent on the initial pH of SLS and was effective to reduce concentration of the total dissolved silica in SLS, which was the key factor that influenced particle size distribution of SLS and degree of silica scaling. The highest silica removal rate of 83.7% was achieved at pH of 11. Moreover, it was found that the Al(OH) 3 with positive charge accelerated mono-silicic acids aggregation, leading to prompt silica polymerization. This hypothesis was underpinned by evidence that amorphous yet compact aggregates formed on membrane surface and trimer silicates formed in the concentrated SLS with initial pH of 11. As a result, the most severe silica scaling occurred during DCMD treatment of SLS, resulting in the lowest initial permeate flux and water recovery. Our results shed light on prevention and management of silica scaling in treatment of waste stream. Image 1 • The coexisting metal ions Al3+ participated in silica scaling was proved. • The amount of Al(OH) 3 formed was the key factor that influenced the degree of silica scaling. • Al(OH) 3 as absorbent with positive charged influenced the particle size distribution of SLS. • Silicic acids aggregated and polymerized fast in SLS in the presence of Al(OH) 3. • The most severe silica scaling on DCMD performance occurred at initial pH of 11. [ABSTRACT FROM AUTHOR]

Precious metals, such as platinum group metals (PGMs) with distinct catalytic activity, are widely used as active components in various industrial catalysts. It is, therefore, highly desirable to recover these valuable components from the end-of-life products. We explored treatment of refining wastewater from precious metals recovery using direct contact membrane distillation (DCMD). The role of various initial pH of refining wastewater on DCMD performance was assessed. Results suggested that hydrochloride acid (HCl) and high-quality water can be reclaimed from the real refining wastewater by adjusting initial pH. Furthermore, DCMD water flux decline was mainly caused by silica and chromium (III) scaling, which was dependent on initial pH of refining wastewater. Silica scaling was responsible for the decrease of DCMD performance when the initial pH of refining wastewater increased from original 0.03 to 5 and 7. Silica oligomers in the concentrated feed with various initial pH were identified using mass spectra. Dichlorotetraaquochromiun was identified by X-ray photoelectron spectroscopy and ultraviolet and visible absorbance spectra as the main species contributing to the green colour and scaling on the PTFE membrane surface. Our results suggest that DCMD can be used as a promising and feasible solution for resource recovery from acidic refining waste stream. Image 1 • DCMD process was first applied to the treatment of real refining waste stream. • High-quality hydrochloride acid (HCl) was recovered. • System performance was negatively influenced by silica and chromium (III) scaling. • Chromium (III) scaling was dependent on initial pH of feed. • Chemistry of silica and chromium (III) in membrane scaling was elucidated. [ABSTRACT FROM AUTHOR]