Abstract
Rare earth element recovery in molten salt electrolysis is approximately between 91 and 93%, whereof 8% is lost in waste molten salt slag. Presently, minimal research has been conducted on the technology for recycling waste rare earth molten salt slag, which is either discarded as industrial garbage or mixed with waste slag into qualified molten salt. The development of a new approach toward the effective treatment of rare earth fluoride molten salt electrolytic slag, which can recycle the remaining rare earth and improve the utilization rate, is essential. Herein, weak magnetic iron separation, sulfuric acid leaching transformation, water leaching, hydrogen fluoride water absorption, and cycle precipitation of rare earth are used to recover rare earth from their fluoride molten salt electrolytic slag, wherein the thermodynamic and kinetic processes of sulfuric acid leaching transformation are emphatically studied. Thermodynamic results show that temperature has a great influence on sulfuric acid leaching. With rising temperature, the equilibrium constant of the reaction gradually increases, and the stable interval of NdF3 decreases, while that of Nd3+ increases, indicating that high temperature is conducive to the sulfuric acid leaching process, whereof the kinetic results reveal that the activation energy E of Nd transformation is 41.57 kJ/mol, which indicates that the sulfuric acid leaching process is controlled by interfacial chemical reaction. According to the Nd transformation rate equation in the sulfuric acid leaching process of rare earth fluoride molten salt electrolytic slag under different particle size conditions, it is determinable that with the decrease of particle size, the reaction rate increases accordingly, while strengthening the leaching kinetic process. According to the equation of Nd transformation rate in the sulfuric acid leaching process under different sulfuric acid concentration conditions, the reaction series of sulfuric acid concentration K = 6.4, which is greater than 1, indicating that increasing sulfuric acid concentration can change the kinetic-control region and strengthen the kinetic process.
Highlights
The focus of world research gradually tends to functional materials (Gao et al, 2020; Kong et al, 2020; Zhang et al, 2020; Zhang et al, 2021), rare earth and its compounds have excellent physical and chemical properties, such as electricity, magnetism, light, and catalysis, which make them widely used in metallurgy, chemical industry, electronics, machinery, new energy, new materials, aerospace, and other fields (Huang et al, 2007; Biedermann, 2014; Shen et al, 2017)
Lanthanum, praseodymium, neodymium, dysprosium, and other single rare earth metals, as well as Pr-Nd, Nd-Fe, Dy-Fe, and other alloys are all produced by the molten salt electrolysis process of the fluoride system (Li, 1990)
The results show that when the abrasive particle size is −200 meshes, the temperature is 50°C, hydrochloric acid is added to the final system to keep the pH value at 0.5, and the reaction time is 4 h, the removal rate of main non-rare earth metal impurities in the waste slag is over 94%, and the total recovery rate of rare earth is 97.56%
Summary
The focus of world research gradually tends to functional materials (Gao et al, 2020; Kong et al, 2020; Zhang et al, 2020; Zhang et al, 2021), rare earth and its compounds have excellent physical and chemical properties, such as electricity, magnetism, light, and catalysis, which make them widely used in metallurgy, chemical industry, electronics, machinery, new energy, new materials, aerospace, and other fields (Huang et al, 2007; Biedermann, 2014; Shen et al, 2017). The utilization rate of rare earth resources in the world is only approximately 10%, which is not directly proportional to the value of rare earth. For these kinds of nonrenewable, scarce, and strategic resources, it is of great significance to recycle and reuse the rare earth in them (Chen, 2011; Huang et al, 2015; Ferron and Henry, 2016)[8–10]. Lanthanum, praseodymium, neodymium, dysprosium, and other single rare earth metals, as well as Pr-Nd, Nd-Fe, Dy-Fe, and other alloys are all produced by the molten salt electrolysis process of the fluoride system (Li, 1990). An increasing number of experts and scholars, locally and globally, are studying the recovery and utilization technology of rare earth molten salt electrolytic slag because of its high rare earth content and great reuse value (Chen et al, 2005; Xiao et al, 2015; Federica et al, 2019; Onal and Binnemans, 2019; Yurramendi et al, 2019)
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