Research progress on leaching technology and theory of weathered crust elution-deposited rare earth ore

Rare earth (RE) with specific application value has become the relatively important strategic resource for all countries. Weathered crust elution-deposited rare earth ore (WCED-REO) are rich in middle-heavy RE, and its exploitation restrict the global supply of the middle-heavy RE. The leaching technology directly determines the leaching efficiency of RE in WCED-REO. The selection and use of the leaching agents are the most important parts of the leaching technology. The leaching mechanism is at the core of the leaching technology and also affects the effective utilization of the leaching agents. The research and development of leaching technologies, leaching agents, and leaching mechanisms of WCED-REO are discussed in this article. The development processes of exiting leaching technologies are introduced. The advantages and disadvantages of barrel leaching, pool leaching, heap leaching, in-situ leaching, and other leaching technologies are compared. And the new and practical leaching technologies are put forward. The research progresses of ammonium salt leaching agents, composite ammonium salt leaching agents, and novel non-ammonium leaching agents in recent years are reviewed. The leaching effects of different leaching agents are compared comprehensively to provide guidance for industrial application and research direction of the best leaching agent in the later period. The leaching basic theory, as well as the mass transfer process and leaching kinetics of rare earth are introduced. The mass transfer processes in leaching mechanism are explained from the macroscopic and microscopic perspectives. The further research directions of the leaching technologies, leaching agents, and leaching mechanism of WCED-REO are put forward, which provide the theoretical guidance and technical supports for the green and efficient exploitation of WCED-REO.