Potential prediction and coupling relationship revealing for recovery of platinum group metals from spent auto-exhaust catalysts based on machine learning

As hazardous waste, the massive generation of spent auto-exhaust catalysts (SACs) puts enormous pressure on environmental management, but provides a rare opportunity for platinum group metals (PGMs) recycling. In this study, machine learning (ML) method was firstly applied to accurately predict regional SACs generation in China for 2025-2050 under five shared socio-economic pathways (SSPs) scenarios, based on which economic and carbon emission reduction potential of PGMs recycling were estimated. Population-GDP-GDPII-GDPIII and Random Forest were determined as key variables and the predictive model. Results indicate that SACs will reach 28.15 million sets (1.7 times that of 2020) and PGMs have economic potential of $890 million by 2050 (SSP1). Furthermore, based on environmental impact assessment, the capture enrichment-electrodeposition purification process is proposed as the best low-carbon recycling solution for SACs. And the integrated recovery process based on copper capture can realize 1.51 million tons of carbon emission reduction in China in 2050 (SSP1). This study can provide decision-making guidance for PGMs recovery and environmental management, as well as technical references for SACs recovery program selection.

A New Impregnated Adsorbent for Noble Metal Ion Sorption

Noble metals (NM) such as gold, platinum, palladium, and rhodium are widely applied in the electronics and automotive industries. Thus, the search for cheap and selective sorbents for noble metals is economically justified. Nitrolite does not sorb noble metal ions. A new impregnated sorbent was prepared. The natural sorbent Nitrolite was impregnated with Aliquat 336 using a new warm impregnation method. After the impregnation process, Nitrolite adsorbed platinum(IV), palladium(II), and gold(III) ions from the chloride solutions. The values of the sorption capacity for palladium(II) and platinum(IV) ions were 47.63 mg/g and 51.39 mg/g, respectively, from the 0.1 M HCl model solution. The sorption capacity for gold(III) ions was estimated to be 73.43 mg/g from the 0.1 M HCl model solution. An exhausted catalytic converter was leached, and platinum(IV), palladium(II), and rhodium(III) were transferred to the chloride solution. The impregnated sorbent Nitrolite-Aliquat 336 was used in the investigations of the platinum(IV), palladium(II), and rhodium(III) ions' sorption from a real solution. The impregnated sorbent Nitrolite-Aliquat 336 proved to be suitable for the recovery of platinum(IV) and palladium(II) ions, whereas rhodium ions were not sorbed from the leached solutions. Notably, 1 M thiourea in the 1 M HCl solution desorbed platinum(IV), palladium(II), and gold(III) above 94%.

Recovery of palladium from a low grade palladium solution by anionic-ion exchange: kinetics, equilibrium, and metal competition

Petroleum spent catalysts may contain a significant amount of palladium (Pd) together with other major [aluminum (Al), nickel (Ni), and molybdenum (Mo)] and minor [iron (Fe), lead (Pb), and vanadium (V)] elements. Due to the high intrinsic value of Pd and its scarcity in natural ores, its recovery is highly desired. For this purpose, the ability of a strong basic anionic- resin, Purogold™ A194 resin, to remove Pd from the solution was assessed. Data from kinetic and equilibrium studies, performed under batch mode in 1 mol/L of NaCl and 1 mol/L of HNO₃ at (21 ± 1) °C, revealed that the removal of Pd fits well a pseudo-second-order kinetic model [constant rate value, k₂, of (0.062 ± 0.010) g/(mmol.min)] and a Langmuir isotherm [maximum sorption capacity of (0.80 ± 0.02) mmol/g with an affinity of resin binding sites towards Pd, K_L, of (0.18 ± 0.02) L/mmol], respectively. The sorption of other metals (Al, Fe, Pb, Mo, Ni, and V) that may be present in spent catalyst leachates was tested under similar experimental conditions [C_M = 2.5 mmol/L, 1 mol/L of NaCl and 1 mol/L of HNO₃ at (21 ± 1) °C)] and the resin showed little affinity towards each one of these metals. Also, simultaneous multi-element batch experiments with Pd and the major components (M = Al, Ni, and Mo ions) ([M]/[Pd] molar ratios between 3.4 and 52 were used) pointed out that the resin is highly selective towards Pd suggesting that the resin can be used in the selective recovery of Pd from petroleum spent catalyst leachates.

Platinum Recovered from Automotive Heavy-Duty Diesel Engine Exhaust Systems in Hydrometallurgical Operation

The current study is focused on platinum recovery from the secondary sources of end-of-life heavy-duty diesel oxidation catalysts (DOCs) and heavy-duty catalyzed diesel particulate filters (c-DPFs) in order to reduce the supply–demand gap within the European Union. The extraction of platinum was based on a hydrometallurgical single-step low acidity leaching system (HCl-H2O2-NaCl) and a calcination step that takes place before the leaching process. The parameters of calcination and leaching process were thoroughly investigated in order to optimize recovery efficiency. The optimized results proved that a calcination step (at 800 °C for 2 h) improves the recovery efficiency by a factor of 40%. In addition, optimal Pt recovery yield was achieved after 3 h of leaching at 70 °C, with a solid-to-liquid (S/L) ratio of 70 g/100 mL (70%) and 3 M HCl-1% vol H2O2-4.5 M NaCl as leaching conditions. The optimum Pt recovery yield was 95% and 75% for DOC and c-DPF, respectively. Since the secondary feedstock investigated is highly concentrated in platinum, the pregnant solution can be used as a precursor for developing new Pt-based catalytic systems.

Application of Ion Exchange for Recovery of Noble Metals

The production of noble metals has started to shift towards using different types of wastes. The leaching solutions collected during the processing of waste have low concentrations of noble metals; therefore, it forces the use of sorption methods in recovery technology. This work focused on recovering noble metals with a technological solution, obtained during the processing of waste from refining processes. The research consisted of a set of experiments using a batch method that enabled determination of the parameters of the process and selection of the leading resins. Sorption isotherms were determined and kinetic studies were conducted, along with the preliminary elution tests with the use of different eluents. Cementation experiments were the final part of the research. During the experiments, it was concluded that the leading resins for the sorption of noble metals were Puromet MTS9200, Puromet MTS9850, and Lewatit K 6362. The volume ratio Vr:Vs = 1:10 and the reaction time 15–30 min could be used as basic conditions to conduct the experiments in the column; the solution of thiourea in hydrochloric acid can be used as an eluting agent from which noble metals could be cemented using powder zinc.

Status of Recovery of Strategic Metals from Spent Secondary Products

The need to drive towards sustainable metal resource recovery from end-of-cycle products cannot be overstated. This review attempts to investigate progress in the development of recycling strategies for the recovery of strategic metals, such as precious metals and base metals, from catalytic converters, e-waste, and batteries. Several methods for the recovery of metal resources have been explored for these waste streams, such as pyrometallurgy, hydrometallurgy, and biohydrometallurgy. The results are discussed, and the efficiency of the processes and the chemistry involved are detailed. The conversion of metal waste to high-value nanomaterials is also presented. Process flow diagrams are also presented, where possible, to represent simplified process steps. Despite concerns about environmental effects from processing the metal waste streams, the gains for driving towards a circular economy of these waste streams are enormous. Therefore, the development of greener processes is recommended. In addition, countries need to manage their metal waste streams appropriately and ensure that this becomes part of the formal economic activity and, therefore, becomes regulated.