Abstract
Many studies have been published in recent years focusing on the recovery of rare earth elements (REEs) from coal-related materials, including coal, coal refuse, coal mine drainage, and coal combustion byproducts particularly fly ash. The scientific basis and technology development have been supported by coal geologists and extractive metallurgists, and through these efforts, the concept has progressed from feasibility assessment to pilot-scale production over the last five years. Physical beneficiation, acid leaching, ion-exchange leaching, bio-leaching, thermal treatment, alkali treatment, solvent extraction, and other recovery technologies have been evaluated with varying degrees of success depending on the feedstock properties. In general, physical beneficiation can be a suitable low-cost option for preliminary upgrading; however, most studies showed exceedingly low recovery values unless ultrafine grinding was first performed. This finding is largely attributed to the combination of small RE-bearing mineral particle size and complex REE mineralogy in coal-based resources. Alternatively, direct chemical extraction by acid was able to produce moderate recovery values, and the inclusion of leaching additives, alkaline pretreatment, and/or thermal pretreatment considerably improved the process performance. The studies reviewed in this article revealed two major pilot plants where these processes have been successfully deployed along with suitable solution purification technologies to continuously produce high-grade mixed rare earth products (as high as +95%) from coal-based resources. This article presents a systematic review of the recovery methods, testing outcomes, and separation mechanisms that are involved in REE extraction from coal-related materials. The most recent findings regarding the modes of occurrence of REEs in coal-related materials are also included.
Highlights
Rare earth elements (REEs) including the 15 lanthanides plus yttrium and scandium have been identified as critical commodities by several international agencies and national governments due to their crucial roles in clean energy, high tech, and national defense industries [1,2,3]
The geological aspects such as accumulation mechanisms and modes of occurrence of REEs in coal and coal refuse have been extensively investigated by coal geologists, and these topics have been systematically reviewed in several prior publications [5,14,15,28,29]
Given the low recovery and subsequent high production costs associated with physical beneficiation, direct chemical extraction of REEs from coal refuse has been evaluated by a number of researchers [11,20,21,25,41,42,43]
Summary
Rare earth elements (REEs) including the 15 lanthanides plus yttrium and scandium have been identified as critical commodities by several international agencies and national governments due to their crucial roles in clean energy, high tech, and national defense industries [1,2,3]. The concept has been discussed in several review articles [5,14,15,16]; these articles have primarily addressed geologic considerations, such as depositional settings and resource characteristics To this end, the U.S Department of Energy initiated research and development efforts in 2014 to evaluate the technical and economicviability of extracting REEs from coal-related materials with a particular focus on technology maturation and process development [17]. During the last five years, the extraction of REEs from coal-related materials has progressed through feasibility assessment, field sampling and characterization, bench-scale REE separation, and pilot-scale REE production stages. Recent findings regarding the modes of occurrence of REEs in coal-related materials and how these findings inform process design are covered
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