Speciation of rare earth elements in acid mine drainage precipitates by sequential extraction
Introduction
Rare earth elements (REEs) are critical raw materials for numerous technology and defense applications. They are considered essential for modern society; however, the current supply chain includes relatively few primary producers, with the vast majority of ore production originating in China (Gambogi, 2020). The risks associated with REE supply have been reported in numerous studies over the last decade (Bauer et al., 2011, Goodenough et al., 2018). These challenges have in turn prompted considerable public and private investment to identify alternative REE sources (Cox and Kynicky, 2018). In one example, researchers working under a US Department of Energy funded study have identified acid mine drainage (AMD) as well as the associated treatment byproducts (i.e. acid mine drainage precipitate, AMDp) as potentially promising sources of REEs. Results from these studies show that AMD from the Northern Appalachian (NAPP) and Central Appalachian (CAPP) coal basins contain 282 μg/L of REEs, and conventional AMD treatment concentrates the REEs in the raw water by a factor of >2000, resulting in AMDp with a REE concentration exceeding 700 g/t on average. These results were confirmed through a field study that evaluated 141 AMD treatment sites and 623 individual AMDp samples (Vass et al., 2019a, Vass et al., 2019b).
In Vass et al. (2019a), the researchers propose a conceptual framework for the recovery of REEs from AMDp whereby the REEs are extracted and refined using a traditional hydrometallurgical processing route. After recovering the AMDp from storage cells and drying ponds, the initial processing step involves the redissolution of this material through acid leaching. The resultant pregnant leach solution is then purified by solvent extraction or selective precipitation prior to oxide production and downstream refining. In later studies, the same researchers have shown that this overall flowsheet can be used to produce high-grade mixed rare earth oxides (REO) exceeding 80% purity.
As in any hydrometallurgical process, the efficacy of the leaching/redissolution step is highly dependent upon the chemical and morphological characteristics of the target ore deposit - AMDp in this case. Unlike typical virgin mineral ores, AMDp is a product of both autogenous and anthropogenic processes, and thus, the chemical and physical properties of the material can vary considerably with respect to: (1) the initial chemistry of the host rock and the raw AMD, (2) the specific AMD treatment method, (3) the chemicals and reagents used in the AMD treatment process, (4) the AMDp disposal method, and (5) the extent of environmental exposure to the disposed material. With respect to the source AMD, the acid generation process is complex and can involve chemical, biological, and electrochemical reactions. Reactants including pyrite, oxygen, and water combine to generate ferrous hydroxide, and sulfuric acid. AMD often contains not only iron but other acid soluble major metals such as aluminum, manganese, and several dominant anions, including sulfate and chloride (Ackman, 1982, Johnson and Hallberg, 2005, Skousen et al., 1993). Typical active AMD treatment methods incorporate alkaline addition (e.g. lime, caustic, hydrated lime) as well as mechanical oxidation to increase pH and remove metal ions from solution. The addition of alkaline reagents forces the dissolved metal ions to precipitate as hydroxides, and the resultant precipitate (AMDp) is considered a nuisance waste product (Akcil and Koldas, 2006, Kalin et al., 2006). As such, AMDp is normally stored in surface drying cells prior to landfilling or injected underground into old mining works. Each step in this process can introduce variability that ultimately influences the final chemical and physical makeup of the AMDp. For example, the consistency of AMDp can vary from moisture-rich amorphous gel to hardened granular powder, depending on the relative iron content as well as the degree of passive dehydration (Ackman, 1982, Vass et al., 2019a).
Detailed systematic studies on the specific leaching characteristics of AMDp are rare, and to the authors’ knowledge, no such studies have been conducted with a specific focus on the extraction of REEs in AMDp. Nevertheless, the development and deployment of cost-effective and environment friendly REE recovery processes relies on fundamental information regarding the speciation of REEs in AMDp, including the chemical species and physical distribution and/or association with other phases. Given this knowledge gap, the objective of the current study is to characterize the mineralogical and chemical speciation behavior of REEs of three distinct AMDp specimens to support overall process development and optimization.
In addition to standard bulk chemical, particle, and crystalline structure analyses (i.e. ICP-MS, SEM-EDS, and XRD, respectively), the current study heavily leverages data from staged sequential extraction to identify the REE mode of occurrence in AMDp. Sequential chemical extraction procedures have been widely used to study the chemical activity and speciation of many materials, particularly soil and aqueous sediments (Feng and Hong, 1999, Filgueiras et al., 2002, He et al., 2013, Long et al., 2009, Rao et al., 2008, Su and Wong, 2004, Wali et al., 2015, Zhang and Shan, 2001). Filgueiras et al. has provided a review of more than 400 documents containing sequential extraction procedures, which delineates those references into 15 different extraction schemes, each containing a different mix of reagents at different sequential extraction stages (Filgueiras et al., 2002). One common method by the Community Bureau of Reference (BCR) is used to classify the extraction of different trace metals (e.g. Zn, Pb, Cu, Cr, Co, Ni, Mn, and Fe) in soil into four fractions, namely as exchangeable and soluble fractions, reducible fraction, oxidizable fraction and residues (Wali et al., 2015). Alternatively, Dold presented a seven-step sequential extraction procedure which was adapted to specific mineralogy to increase the selectivity and the accuracy of geochemical data interpretation for copper sulfide ores (Dold, 2003). In a separate study, iron and aluminum precipitates generated from passive treatment systems were studied by sequential extraction procedure together with x-ray diffraction (XRD) and scanning electron microscopy (SEM) (Caraballo et al., 2009).
Of the many sequential extraction methods found in the literature, only a few studies have specifically addressed the mobilization of REEs rather than other heavy metals. Mittmuller et al. described a modified sequential extraction procedure for REEs in mine tailings and notes better results than the BCR method, particularly with regard to REE-phosphates (Mittermüller et al., 2016). In recent years, several studies have specifically addressed sequential extraction of REEs from coal byproducts, including refuse and fly ash. Lin et al. used a seven step sequential extraction to characterize the occurrence of REEs in coal fly ash (Lin et al., 2018). Similar results were shown by Pan et al. who used a four step method and developed a mathematical model to show the correlation between REE and major metal extraction (Pan et al., 2019). A group of related studies utilized sequential extraction to show the association of REEs and other critical elements in various coal refuse samples (Zhang et al., 2020, Zhang and Honaker, 2020, Zhang and Noble, 2020). Follow on studies showed how those association characteristics were altered through calcination pretreatment (Zhang and Honaker, 2019).
Given the information above as well the scientific objectives described in Section 1.1, the sequential extraction method used in this study was slightly modified based on the widely-applied procedures of Tessier et al. (Tessier et al., 1979) and Novikov et al. (Novikov et al., 2009), which partitioned the REEs into five fractions, namely: exchangeable; carbonates; Fe and Mn oxides; organic matter; and residual. The ion-exchangeable form includes physically-sorbed REE cations, which can be easily released in the presence of an inorganic salt solution. The carbonate phases are dissolved by acetic acid. The metal oxides include Fe and/or Mn oxy-hydroxides. The oxyhydroxide matrix decomposes as the metals are reduced and acidified by the mixture of hydroxylamine hydrochloride (NH2OH∙HCl) and acetic acid. The organic/sulfide form represents carbonaceous matter and sulfides which are dissolved by NaOH and H2O2. Lastly, the residues contain silicate and aluminosilicate, which are predominantly comprised quartz, glass, and mullite. This information on chemical speciation will bolster broader process design efforts by providing fundamental information on the anticipated leaching performance using different lixiviant schemes. This data in turn can be used for techno-economic analyses and process optimization.
Section snippets
Sample location, acquisition, and preparation
The samples acquired for the current study were recovered from AMD treatment sites located in the Northern (NAPP) and Central Appalachian (CAPP) coal basins in West Virginia as shown in Fig. 1. Table 1 lists the unique characteristics of each site. The authors note that these sites were included in regional REE survey reported by Vass et al., (Vass et al, 2019b), and the selection of these three sites from the population of 141 was based on the desire to simultaneously assess promising
REE and major metal concentrations
The moisture content, individual REE content, and grouped REE totals (i.e. LREE, HREE, CREE, and TREE) were determined for the three AMDp samples evaluated in this study. For each sample, up to six representative splits were recovered from the same sample lot and independently analyzed to ensure experimental repeatability. The averages and confidence intervals at the confidence level of 95% from these samples are shown in Table 3.
The TREE content between the three sites varies considerably,
Properties of AMDp and REEs
The AMDp samples are generally taken from the mine site with a bit moisture. The mineralogy of AMDp studied by XRD, SEM-EDS as well as ICP-MS demonstrate the presence of metal hydroxides, carbonates, organic matters, and silicate minerals such as quartz, calcium aluminum silicate, and magnesium silicate. The exact makeup for mineralogy depends on the raw mine site and the treatment strategies on the acid mine drainage. During the precipitation process with the pH neutralization to 7, rare earth
Conclusions
AMDp has been identified as a potential source of REEs. This study evaluated AMDp from three different sites using detailed particulate characterization techniques along with a standard sequential extraction procedure to better understand the mineralogy and leaching characteristics of this material. Key conclusions from this study include:
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The total REE content of AMDp samples evaluated in this study varied from 440 g/t for Site-2 to 1391 g/t for Site 3. This range includes materials
CRediT authorship contribution statement
Yan Wang: Conceptualization, Methodology, Investigation, Data curation, Analysis, Writing - Original draft, Writing - Review & Editing. Aaron Noble: Methodology, Data Analysis, Writing - Review & Editing, Fuding acquisition, Supervision. Christopher Vass: Project administration. Paul Ziemkiewicz: Methodology, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This material is based upon work supported by the U.S. Department of Energy under Award Number DE-FE0026927.
Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the US Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed,
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2023, Science of the Total EnvironmentCitation Excerpt :Studies have shown that AMD easily dissolves surrounding rocks such as limestone, thereby releasing REEs from bedrock into AMD (Chen et al., 2015; Li et al., 2021b; Lozano et al., 2020; Mwewa et al., 2022; Sun et al., 2011). Therefore, in addition to containing toxic and harmful metal elements and sulfates, AMD can also contain considerable amounts of soluble REEs, which makes AMD a potential REE resource that has attracted much attention (Li et al., 2021b; Wang et al., 2021). Generally, AMD is enriched of the middle REEs (MREEs), which is related to the preferential dissolution of MREEs-enriched minerals in the parent rock, to colloidal fractionation, and to the solid-liquid exchange of MREEs with surface coatings and/or clays (Grawunder et al., 2014; Li et al., 2021b; Soyol-Erdene et al., 2018; Wallrich et al., 2020).