Electrolytic manganese metal (EMM) is an important raw material for industrial production that is widely used in various industrial fields and occupies an important position in the national economy (Zhang et al., 2020). China is a major producer of electrolytic manganese metal (Shu et al., 2019). In 2020, 96.5% of the world's electrolytic manganese metal was produced in China. The production process for EMM mainly includes leaching, impurity removal, electrolysis, and product posttreatment (Tao et al., 2018). Electrolytic manganese anode slime (EMAS) is a solid waste found in the anode chamber during the production of electrolytic manganese metal (Tran et al., 2020). At present, the global annual output of EMM is approximately 1.5 million tons, and approximately 75,000 to 225,000 tons of EMAS is generated each year. In China, most EMAS is dumped as hazardous waste or sold for small additional value. Therefore, EMAS has become a bottleneck hindering the development of the electrolytic manganese industry.
Many scholars have proposed the method of resource-based treatment of EMAS, but the main methods are reduction leaching and acid leaching roasting activation (Zhang et al., 2018). For example, Guo et al. used the roasting leaching method to remove Pd from EMAS and then prepared lithium manganate material from Pd-removed EMAS (Guo et al, 2018). However, the process for this method is more complicated, and the removal efficiency of Pd is low (Chen et al, 2019). Many researchers recycle manganese and Pd from EMAS by the use of different reducing agents (Chen et al., 2018; Cheng et al., 2009; Li et al., 2017; Niu et al., 2012; Gui et al., 2014; Wei et al., 2017). However, these methods have certain limitations, such as high energy consumption, complex operation, low efficiency and added value (Cheng et al., 2009; Ye et al., 2015; Xie et al., 2021). The removal of Pd and the regulation of the crystal form of manganese oxides are the key factors limiting the application of the above techniques. Regulation of the manganese oxide crystal form and Pb removal have become urgent problems to be solved in the electrolytic manganese metal industry.
The vacuum carbothermal reduction method has always been a research hotspot in the field of vacuum metallurgy, and great research progress has been made (Zhang et al., 2019). The vacuum carbothermal reduction method has the characteristics of both reduction roasting and vacuum smelting and has the characteristics of low energy consumption, simple processing, and environmental friendliness; it is widely used in the fields of chemical engineering and metallurgy (Brkic et al., 2017; Lv et al., 2019; Tian et al., 2020; Xu et al., 2021). Recovery of Pd in the form of pure metallic Pd with a purity of 99.3% from CRT funnel glass by the vacuum carbothermal reduction method has been reported(Qi et al., 2019). Ji et al. and Tang et al. used the vacuum carbothermal reduction method to recover Ga from yellow phosphorus fume and Si from diamond wire-saw silicon powder (Ji et al., 2021; Yang et al., 2019). Tang et al. studied the vacuum carbothermal reduction method to recover lithium and cobalt in waste lithium-ion batteries. The recovery efficiency for Co reaches over 93%, and the recovery efficiency for Li reaches 99% (Tang et al., 2019).
EMAS mainly consists of Mn (52.25 wt.%.), and its phase mainly consists of γ-MnO2, β-MnO2 and ε-MnO2. MnOOH, PbSO4 and Pb2−xMn6O8. Pb (6.22 wt.%) and other impurities exist in EMAS, which is disadvantageous to manganese dioxide discharge. It is necessary to control the crystal form of manganese oxide to improve the resource utilization for EMAS (Shu et al., 2017). Manganese oxide undergoes a phase change during the heat treatment process as follows: amorphous manganese oxide is transformed into β-MnO2 when the temperature is 286.8°C (Liu et al., 2012). MnO2 is reduced to Mn2O3 at 577.3°C, and the content of Mn2O3 increases with increasing temperature. Mn2O3 is then reduced to Mn3O4 at a temperature of 761.3°C (Jacob et al., 2011).
In this study, the vacuum carbothermal reduction method was first used for Pb-removal in EMAS. The effects of process parameters such as reduction temperature, holding time and mass ratio of carbon on the Pd-removal process were investigated, and the removal mechanism for Pd was analyzed. In addition, the preparation process for chemical MnO2 by the “roasting-acid washing disproportionation” process was studied. This study provided a new idea for the high-value resource utilization of EMAS.