Study On Leaching And Leaching Kinetics Of Platinum Group Metals From Spent Auto-catalysts

The installation of auto-catalysts which contain platinum group metals?PGMs?as the active component is the most effective method to reduce automobile exhaust pollution for the environment.After automatic vehicle running a certain time or mileage,the auto-catalysts will lose its catalytic activity which results waste becoming the most important secondary resourcescontaining of PGMs.The latest report of recycling process from spent auto-catalysts?SAC?mainly were the pyrometallurgy,hydrometallurgy and pyrometallurgy-hydrometallurgy.These methods either exist higher requirement for equipment and higher cost of investment or lower recovery efficiency of PGMs.To solve the above problems,the existing form of PGMs in solution were analysed by referring to the related thermodynamic date and combining Pt-Cl-H₂O,Pd-Cl-H₂O and Rh-Cl-H₂O system.As seeing from the potential pH diagram,Pt,Pd and Rh were feasible to be dissolved in solution containing [Cl-].The thermodynamic analysis results show that in the solution acidity pH of about-1,700 mv of the electrode potential could dissolve platinum,500 mv to palladium and only 420 mv to rhodium.Increasing the acidity of solution,the concentration of chlorine ion were good for leaching of PGMs.Theoretically,the HCl-H₂SO₄-NaClO₃ system could realize leaching of PGMs.Recovery of PGMs from SAC by leaching were studied.The single factor experimental results show that initial acidity [H+]9 mol·L-1,HCl-H₂SO₄ ratio 4:1,5% sodium chlorate of the amount of the raw material,leaching temperature 105?,stirring speed 250 rmp,leaching time90 min,liquid-solid ratio 5:1 are the the best conditions for leaching.In the leaching process under the best condition of continuous quadratic leaching the leaching rate for platinum,palladium and rhodium were 90%,96.3% and 81.2% respectively.Orthogonal experiments were studied to further optimize the leaching conditions by means of to designorthogonal experiments using L9?34?orthogonal table.The orthogonal experiment results show the sequence of influence factors for platinum leaching rate isHCl-H₂SO₄ ratio>pH>extraction time> temperature;as for palladium,acidity> temperature>HCl-H₂SO₄ ratio>leaching time;as for rhodium,acidity>HCl-H₂SO₄ ratio>temperature>leaching time.And the best technologicalconditions are the initial acidity [H+]11 mol·L-1,leaching temperature 105?,HCl-H₂SO₄ ratio3:1,leaching time 90 min.In the leaching process under the best condition of continuous quadratic leaching the leaching rate for platinum,palladium and rhodium were 92.27%,96.69% and 82.98%,respectively.The reason for the low leaching rate were analysised through XRD,SEM and other modern characterization methods.It shows that the carrier composition of magnesia cordierite structure forms a parcel which block the leaching of PGMs.Leaching kinetics were studied combing with the characteristics of the liquid-solid reaction.The effects of the liquid-solid ratio,oxidant dosage of sodium chlorate,temperature,initial concentration of hydrogen ions,chloride ion concentration on the leaching of platinum and rhodium were studied in HCl-H₂SO₄-NaClO₃ system.The results show that the leaching of platinum and rhodium in SAC follow“the nuclear reaction to cut”model,controlled by chemical reaction.Enhancing the leaching temperature,initial [H+]and initial [Cl-]could accelerate the leaching speed of platinum and rhodium and raise their leaching rate.The others have little influence on the leaching rate of platinum and rhodium.As for platinum,the leaching reaction activation energy is 45.34 kJ·mol-1,hydrogen ion reaction series is 1.712,the chloride ion reaction series is 0.613.As for Rhodium,the leaching reaction activation energy is 66.719 kJ·mol-1,hydrogen ion reaction series is 0.779,the chloride ion reaction series is0.296.In view of the low concentration leaching liquor,hydrazine hydrate and NaBH₄ were used to enrich PGMs.The result shows that the reduction rate for hydrazine hydrate can reach Pt98.41%,Pd 99.84% and Rh 95.55% respectively from 8% of the amount of hydrazine hydrate as raw materials,raw material liquid acidity control at around 1.2 mol·L-1,reaction temperature at 70?,the reaction time for 10 min.The reduction rate for NaBH4 can reach Pt99.47%,Pd 99.67% and Rh99.32% respectively from 5% sodium borohydride,raw material liquid acidity of [H+] close to 0 mol·L-1,reaction temperature of 40?,reaction time for 10 min.The reduction effect of NaBH₄ is better than that of hydrazine hydrate and producing slag of higher grade.