Waste streams such as acidic mine drainage has been proposed as a low grade resource of critical minerals such as rare earth elements (REE) (Cravotta and Brady 2015; Cravotta III 2008; Hedin et al. 2019; Hedin et al. 2020; Stewart et al. 2017; Vass et al. 2019). These feedstocks are an attractive source of REE because of environmental benefits gained in converting wastes into valuable materials. However, major challenges exist in valorizing such wastes, including that drainage sites tend to be geographically dispersed and contain a variety of impurities that are a challenge for efficient extraction and purification of the REE (Ayora et al. 2016; Middleton et al. 2024; Mwewa et al. 2022; Park et al. 2020).

This presentation reports our previous study (Middleton et al. 2024) of supported liquid membrane (SLM) technologies for concentrating REE from acid mine drainage (AMD) of coal mine sites. As described in our prior publications (Middleton et al. 2024; Smith et al. 2019), the SLM approach is similar to solvent exchange processes except that the REE-chelating organic phase is embedded in a hydrophobic membrane (Figure 1). The feed and the acid strippant solutions are placed on opposite sides of the membrane, and the concentration gradients drive metal ion transfer from the feed to the product side of the membrane. This configuration aims to maximize the area of the liquid-liquid interface to enable increased rates of mass transfer (Chen et al. 2018). A potential advantage of the SLM process over conventional solvent extraction methods is that SLM requires much less volume of hazardous organic solvents and could be implemented in a modular system.

The objective of this study was to test the efficacy of SLM-based separations for the extraction of AMD fluids for the REE (herein defined as the 14 stable lanthanides, yttrium and scandium). We focused on the effect of AMD fluid composition for controlling SLM performance for REE extraction. Our overarching hypothesis is that REE flux during the SLM separation is dependent on a specific set of AMD feedstock variables such as pH, REE speciation, and major ion (e.g., Fe, Al, Mn, Ca) concentrations.

The need for a national supply of rare earth elements (REEs) and critical minerals (CMs) motivated the research of REE/CM feedstocks alternative to mining. Research on hard and soft rocks has demonstrated concentrations of REE/CM, yet the cost-effectiveness of processing these rocks directly for REE/CM mining remains a challenge. Therefore, the West Virginia Water Research Institute (WVWRI) began investigating recovering REE/CM from mining byproducts, including sludges generated by the treatment of acid mine drainage (AMD). This investigation was successful and is now patented; WVWRI is expanding operations to create a national REE/CM feedstock supply chain.

The first step in this supply is generating REE/CM-enriched sludge, referred to as hydraulic pre-concentrate (HPC), by treating AMD. In Appalachia, a large portion of AMD is generated at remote sites, where AMD must be treated passively with no power supply. This research investigated the feasibility of producing HPC following the Ziemkiewicz et al. patented process at a remote AMD site with a flow rate of less than 50 gallons per minute (190 L/min). This research intended to design, deploy, and operate a system capable of treating AMD in compliance with NPDES limits while generating HPC in a remote site. The process was demonstrated by constructing a system at a remote site in Clay County, WV.

The deployed system achieved steady-state flow with effective pH control. Samples were collected to assess the system's performance in terms of REE/CM recovery in the resulting HPC product. Analytical results revealed an impressive REE recovery rate of 85% and CM recovery rate of 83%, indicating substantial potential for profitability. Operations were paused for the winter season, with plans to resume in spring 2024, incorporating enhancements to system control and operation.

The demand for Rare Earth Elements (REE) and other critical elements continues to increase, to the point of exceeding their supply. Conventional deposits cannot satisfy domestic demand for the foreseeable future; intensive research on REE recovery from secondary sources is ongoing. Mining and industrial wastes are prime targets for assessing the presence of economically accessible REE and critical minerals. Montana has thousands of inactive and abandoned mines that could be an important source of REE’s. These waste sources are regulated under several different remediation actions, i.e. CERCLA and RCRA and require close coordination with various stakeholders.

The waste material and seeps generated from mine sites, as well as past ore processing facilities, may contain rich sources of rare earth elements. We have developed and implemented an extensive sampling program to collect aqueous and solid samples for REE analysis at sites located throughout Montana. For solid wastes, a suitable number of samples were collected to obtain preliminary concentrations of elements in the material. Results of reconnaissance mine sampling will guide future targeted sampling to obtain a more complete characterization of promising materials using sampling protocols developed for statistical characterization of resources.

Sample results show REE present in most of the samples collected. However, concentrations vary considerably between sites and waste sources. REE concentrations are higher in water where pH values are 4.0 or lower, while in sludge samples concentrations are highest when generated from sites treating acid mine water with lime. Solid waste samples have varied in concentrations and some possible correlations have been observed.

The large investment of time and money needed to permit a new mine is a deterrent for the exploration and development of new REEs deposits in the United States. Secondary recovery of REEs from existing waste material alleviates this time investment. Additionally, recovering REEs can contribute to environmental cleanup efforts by reducing and remediating waste piles that would otherwise be left in place. The relative composition of REE on the basis of total REE content was assessed through the use of an Outlook Coefficient. This is a ratio of high-demand REE’s to the more abundant REEs in the collected samples. The higher the coefficient, the greater the potential industrial value.