New insight into oxidative roasting and leaching for NdFeB waste

NdFeB waste is an essential rare earth (RE) resource, which is the unique solid waste to realize large-scale RE recovery via HCl preferential dissolution method. However, the challenges of low oxidation efficiency and high acid consumption are seriously hindering RE efficient recovery. Herein, the mechanism between the phase structure transformation of roasting RE solid waste and RE elements' leaching behavior was first explored. The results showed the oxidation efficiency of NdFeB oil sludge (NOS) was improved from 2.5% to 99.2% with the temperature increasing. And a leaching model was proposed: Dense Layer Structure (≤400 ℃), Loose Layer Structure (400-600 ℃), and RE Insoluble Structure (≥600 ℃). The analysis of the leaching model indicates the dense oxide layer and the RE insoluble structure (RE insoluble NdFeO 3 phase and particle adhesion) will affect the leaching efficiency of RE elements. Based on the oxidation efficiency, XPS, SEM, Raman, FTIR, and EDS-mapping analysis, an external oxide layer was verified on the microparticle of roasted NOS. Furthermore, the corresponding mechanism is discussed, and the improvement strategies are put forward. This study will supply a new insight into the theoretical basis for efficient and low HCl consumption recycling of NdFeB waste. During the decomposition and oxidation roasting of NdFeB waste, the formation of NdFeO 3 , adhesion of particles, and external dense oxide layers can inhibit the leaching efficiency of RE element, in addition, the leaching model is established based on different oxide layer phase structure. • The interaction mechanism between the roasting and leaching was first explored. • The dense external oxide layer was confirmed, and leaching model was established. • The NdFeO 3 can inhibit the leaching of RE and increase HCl consumption. • The test method of oxidation rate of NdFeB waste is proposed. • The improvement strategies for efficient and low HCl consumption are put forward.