Research On The Mechanism And Application Of Platinum Group Metals Enrichment By Low-Temperature Iron Capture-Electrolysis Process

PGMs(platinum group metals)are regarded as strategic metals due to indispensable application value to petrochemical,automobile,defense and military industries.China contributes more than 30%of the global PGMs consumption,while the reserves of mineral resources are less than 0.4%.The external dependency is more than 90%.Thus,there is a serious imbalance between supply and demand of PGMs.Catalysts are the most important application area of PGMs.About 30,000 tons of PGMs catalysts were scrapped in 2021,which is expected to reach 50,000 tons in 2025.In light of this,the green recovery of PGMs from waste catalysts should attract widespread attention,which is in line with the national resource security,as well as the carbon peaking and carbon neutrality goals.Currently,there are two major challenges for the green and efficient enrichment of PGMs from spent catalysts:the related mechanism is still unclear;key technologies have not been broken through.Therefore,problems such as heavy metals pollution,low recovery rate,and high material/water consumption have emerged.In order to solve above problems,this paper focuses on the mechanism of low-temperature iron capture of PGMs and the design regulation of slag phase.An electrolytic enrichment-leaching-electrodeposition technology of PGMs was proposed for the extraction of PGMs from Fe-PGMs alloys.Furthermore,we conducted the pilot test and evaluated the economic benefits and environmental impact.The main research contents and conclusions are as follows:(1)Based on the experimental analysis,the "solid solution" mechanism for the low-temperature iron capture of PGMs was proposed.In thermodynamics,PGMs can be reduced by C,CO and Fe to meet the alloying conditions.Due to the huge differences in physical and chemical properties such as chemical bond,viscosity,density and surface tension with slag,PGMs do not melt into the slag phase.Based on the second law of thermodynamics and the principle of minimum energy,PGMs spontaneously melt into the iron phase and form δFe-PGMs solid solution after contacting the iron phase.Meanwhile,the surface free energy of the system is reduced.When reaching the critical diameter,the alloy particles settle to the bottom of the crucible.Finally,the alloy transforms into aFe-PGMs replacement solid solution with the cooling process.(2)The slag-design mechanism was studied for low-temperature iron capture of PGMs.Based on the motion model of metal particles in melt,the slag design principle of low melting point,low viscosity and low density was proposed.According to the composition characteristics of raw materials,a CaO-Al2O3-SiO2Na2O slag system was selected to reveal the influence of composition on the physical and chemical properties of slag phase.It was found that increasing the basicity,m(Na2O)/m(CaO)and m(B2O3)were beneficial to reduce the formation of refractory phases such as CaZrSi2O7 and Ca3Si2O7,which promotes the aggregation and settlement of metals.The optimized conditions are as follows:basicity 1.1,m(Na2O)/m(CaO)=1/3,5 wt.%Na2B4O7,5 wt.%CaF2,15 wt.%Fe.and 5 wt.%C.The residual concentrations of Pt,Pd and Rh in the slag were 0.53 g/t、2.79 g/t and 0.87 g/t,respectively.The enrichment ratio of PGMs was 7.6.(3)The electrochemical behavior of Fe-PGMs alloy was revealed.Iron passivation and hydrogen evolution hindered the separation of iron.The adjustion of voltage,temperature,Fe2+ concentration and initial pH value could effectively inhibit the side reaction.The optimized electrolytic conditions were as follows:voltage of 1.0V,initial pH of 2,Fe2+ concentration of 0.7 mol/L,SO42-concentration of 1.0 mol/L and temperature of 60℃.The enrichment factor of PGMs was 15 after 2 h of electrolysis.(4)An electrolysis-leaching-electrodeposition process was developed to extract PGMs from Fe-PGMs alloy.97.25%Pt,99.29%Pd and 98.70%Rh were recovered by 0.65V electrolysis at 60℃ for 15h.The contents of PGMs in the cathode sediment reached more than 99%.The activation energy of the cathode reaction for Pd is 14.57 kJ/mol.The diffusion coefficient of Pd(Ⅱ)and Rh(Ⅲ)are 9.45×10-6 cm/s and 6.53×10-5 cm/s at room temperature,respectively.The electrode processes of Pd and Rh are quasi-reversible reactions controlled by diffusion.(5)The feasibility of PGMs enrichment for industrial application was evaluated.The estimated profit is about 10507 yuan for recovering lt leaching residue of spent automobile exhaust catalyst.The reagent and energy costs mainly come from the smelting stage.Life cycle assessment analysis indicates that the main environmental impact comes from primary energy consumption.