Study On The Extraction Of Palladium And Rhodium By Microwave Roasting Of Automobile Waste Catalysts

With the scrapping of automobiles and the failure of automobile catalyst,a large number of spent automobile catalyst are produced,which is known as"mobile platinum group metal mines"because of high content of platinum group metals.The recovery of platinum group metals from spent automobile catalyst is of great significance to solve the shortage of mineral resources,the prominent contradiction between supply and demand and the development of green circular economy in China.Microwave metallurgy is a new green metallurgical technology,which has many unique advantages,especially microwave roasting for the recovery of platinum group metals from spent automotive catalyst.In this paper,the dielectric properties of spent automotive catalyst at 2450 MHz were measured by TM_0n0 cylindrical resonator method from room temperature to 800 ^oC,The law of microwave heating spent automobile catalyst and the regulation mechanism of NaHSO₄ on microwave roasting spent automobile catalyst was studied,the distribution law of electromagnetic field and temperature field were simulated,the heating thickness was optimized,and efficient leaching of palladium and rhodium from spent automobile catalyst was achieved.The main research contents and conclusions are as follows:(1)The dielectric properties and temperature rise characteristics of spent automotive catalyst from room temperature to 800?were studied,the results showed that the speed of microwave heating spent automobile catalyst was slow.Debye model and dielectric loss theory showed that the reason for the slow microwave heating speed is that the dipole loss and conductivity loss of spent automobile catalyst are relatively small.(2)The regulation mechanism of NaHSO₄ on the dielectric properties of spent automobile catalyst was studied.For the mixed system of spent automobile catalyst and 3?12 wt.%NaHSO₄,NaHSO₄ could effectively improve the microwave absorption characteristics of spent automobile catalyst,its micro mechanism is a double loss mechanism with conductivity loss as the main and dipole loss as the second.The dielectric properties of the mixed system of spent automobile catalyst and NaHSO₄ with mass ratio of 1:6?1:14 from room temperature to 800?were solved by using the classical mixing law,the results showed that the calculated values were in good agreement with the measured values from room temperature to 250?.(3)Using COMSOL multi physical field simulation software,a three-dimensional coupling model of electromagnetic and heat conduction in microwave heating process was established.The electromagnetic field and temperature field distribution of microwave heated spent automobile catalyst,microwave roasted spent automobile catalyst and NaHSO₄ were accurately simulated by using dynamic electrical parameters.The results showed that with the increase of NaHSO₄ addition,the uniformity of electromagnetic field distribution in the mixture increased,the coupling ability between the mixture and microwave increased,and the uniformity of temperature field distribution also increased.(4)Based on the attenuation principle of microwave propagation in materials,a physical model was constructed,and an optimization method of microwave heating thickness was proposed.The optimum heating thickness of microwave heated spent automobile catalyst,microwave roasted spent automobile catalyst and NaHSO₄ mixed system was numerically optimized,and the reliability of the optimization method was verified by Lambert's law and average penetration depth.The results showed that with the increase of NaHSO₄ addition,the microwave energy decays faster in the mixed system,and the optimal microwave heating thickness decreases gradually.When the mass ratio of spent automobile catalyst to NaHSO₄ is 1:6,1:10 and 1:14,the optimal heating thickness is 0.133 m,0.129 m and 0.125 m,respectively.The optimal heating thickness could ensure the uniform distribution of microwave energy in the mixed system.(5)The leaching of platinum group metals by microwave roasting of spent automobile catalyst was studied.The optimum calcination parameters were as follows:microwave roasting temperature 520?,holding time 60 min,and the mass ratio of spent automobile catalyst to NaHSO₄ was 1:10,the corresponding leaching ratio of Pd and Rh were 99.52%and 95.03%,respectively.Compared with conventional heating,microwave roasting could effectively improve the leaching rate of palladium and rhodium.That's because NaHSO₄ promoted the uniform distribution of temperature field,reduced the heating thickness and opened the inclusions.The results in this study showed that NaHSO₄ could not only react with precious metals Pd and Rh,but also promoted the uniform distribution of temperature field and reduced the heating thickness,the efficient leaching of Pd and Rh from spent automobile catalyst was realized.The research results not only clarified the basic theory of extracting Pd and Rh from spent automobile catalyst by microwave roasting,but also expanded the application range of dielectric properties in microwave heating.