Research On The Mechanism And Application Of Platinum Group Metals Enrichment From Spent Catalysts

Platinum group metals(PGMs)are strategic metals.They are widely used in the fields of automotive,petrochemical,electronic and electrical,aerospace,energy and environmental protection due to their excellent physical and chemical properties such as corrosion resistance,high temperature resistance and good catalytic activity.However,the reserves of PGMs are unbalanced.The reserves in China only accounted for 0.4%of PGMs,but China consumed about 30%of total PGMs.Therefore,it is of strategic significance to realize the recycling of PGMs,alleviating the supply risk and meeting the shortage of strategic reserve metal demand.Spent catalysts are the most important secondary source of PGMs since more than 90%of PGMs are used in catalysts industrially.In general,there are two maj or methods to enrich PGMs from spent catalysts,but they have following problems:high concentration of H+ and some toxic oxidizing agents such as aqua regia and cyanide are usually needed by hydrometallurgical methods,consequently causing large amounts of carrier dissolution,serious acid mist and toxic chlorine emissions;copper/lead/nickel pyrometallurgical trapping cause heavy metals pollution,while plasma iron trapping will form ferrosilicon alloy with poor solubility due to the high melting temperature.To solve these issues,the mechanism and technology of green enrichment of PGMs are studied in this paper.We focus on the research on Pt enrichment by hydrogen peroxide oxidation-chlorine ion complexation,PGMs oxidation by Fe3+ and principles of slag design in iron-collection system,achieving Pt extraction with low acid and chlorine-free gas,high leaching efficiency of Pd and reusing of Fe3+,and industrial application of low temperature iron collection of PGMs.We found that higher concentration of chloride ion can significantly improve the stability of PtC162-and reduce the oxidation potential of Pt,which provided scientific basis for the replacement of hydrochloric acid with sodium chloride.Using spent petrochemical Pt-containing catalyst as raw materials,the leaching behavior and kinetics of Pt were studied by controlling the concentration of chloride ion and hydrogen ion in the leaching agent.The Pt leaching efficiency was 98.08%under the leaching conditions that optimized by response surface methodology:HC1 of 1.45 mol/L,liquid to solid ratio of 4.85,10%H2O2/spent catalysts of 0.66 ml/g.Fe3+ is utilized for leaching through oxidation of Pd in a mild condition.This process is environmentally friendly which avoids the emission of toxic gases.According to the ε-pH diagram of Pd-Cl-H2O system,the Fe3+/Fe2+potential is in the PdCl42-stabilization zone,which provides a theoretical basis for the efficient leaching of Pd by Fe3+.In this paper,Pd was concentrated in mixed solution of HCl,NaCl and FeCl3.Then PdCl42-was selectively adsorbed by R410 anion exchange resin,and the tail liquid was reused after composition adjustment.The leaching efficiency was 99.53%for Pd under the following conditions:FeCl3 of 0.67 mol/L,HCl of 2.0 mol/L,NaCl of 4.0 mol/L,solid-liquid ratio of 1:5,at 80℃ for 90 min.The activation energy of Pd leaching was 47.62 kJ/mol,which fit best to the shrinking-core model of surface chemical reaction.The enrichment of PGMs by iron collection at low temperature(<1400 ℃)was developed based on the formation of a continuous solid solution alloy between Fe and Pt,Pd,Rh.We clarified the relationship between the carriers of spent catalysts and slag design,and optimized the composition of slag,fluxing agent.The influences including basicity,slag compositions and ferrous dosage,on the behavior and distribution of PGMs were studied.The occurrence state of Pt,Pd and Rh in ferroalloys and the mechanism of PGMs reduction,collection and setting were explained.The formation of FeSi2,FeSi was avoided at low melting temperature(<1400℃).The recovery efficiency of PGMs was 99.25%under the optimized conditions:mass ratio of CaO/Na2O=35:20,CaF2 5%,Na2B4O7 8.5%,collector Fe 15%,where the concentration of Pt,Pd and Rh in slag phase were 2.398,3.879 and 0.976 g/t,respectively.This technology have achieved industrial application.