Selective precipitation of rare earth and critical elements from acid mine drainage - Part I: Kinetics and thermodynamics of staged precipitation process
Graphical abstract
Introduction
U.S. Department of Interior has identified over 50 mineral commodities that are critical to the U.S. economy and national security, which clearly demonstrates the importance of these elements (U.S. Department of the Interior (DOI) 2021). In this list, aluminum (Al), rare earth elements (REEs), cobalt (Co), and manganese (Mn) are of exceptional prominence due to their significant role in the industry, advanced technology, and green and sustainable energy initiatives. Al is widely used in transportation and packing (U.S. Geological Survey 2022), and Co is used in aircraft engines and superalloys (U.S. Geological Survey 2022). However, the main importance of Co-Mn, along with lithium (Li), is their use in battery production (Rozelle et al., 2021). Recently, REEs have been in the spotlight recently, due to their crucial role in sustainable technological developments (Vaziri Hassas et al., 2021). REEs have been widely used in catalysts (He et al., 2013), magnets (Chakhmouradian and Wall, 2012, Buschow et al., 1995), LEDs (Armelao et al., 2010), electronics (Zhang et al., 2016), and many other high-tech industries to improve the properties of crucial end products. REEs include 15 lanthanide group elements, Yttrium (Y), and Scandium (Sc). The unique properties of this group of elements originate from their distinctive behavior due to their electron configuration, where their 4f shell is being filled (except for Y and Sc). Electrons added to the 4f shell are shielded by the 5s and 5p orbitals and result in beneficial phenomena and practical concepts such as lanthanide contraction and tetrad classification described elsewhere (Vaziri Hassas et al., 2020). As a result of contraction and relatively constant valance shell, these elements demonstrate very similar physicochemical properties. Y and Sc are generally considered with lanthanides as REEs due to their similarities in physicochemical properties. The overall similarity in properties of REEs poses a challenge in the separation of these elements. Although called “rare”, REEs are more abundant than platinum group elements and are in the same order of occurrence as the other transition metals. However, viable and feasible resources to mine these elements are not as copious (Haxel et al., 2002). Hence, to meet the rising demand and address the supply risk of these elements, secondary resources (e.g., acid mine drainage (AMD), coal and coal by-products, recycled materials, e-waste) are being investigated as viable resources for the recovery of these critical elements (Zhang et al., 2015, Li et al., 2019). REEs in mining and processing waste streams including AMD have been studied extensively, and various processes have been suggested to recover these elements from both AMD and mine refuse leachate (Ayora et al., 2016, Royer-Lavallée et al., 2020, Costis et al., 2021, Sun et al., 2012, Nogueira et al., 2019). A modified acid mine drainage treatment process to selectively precipitate Al and REEs through staged precipitation while neutralizing the pH of the stream and a potential third stage for the recovery of Co-Mn has been previously reported (Vaziri Hassas et al., 2021, Vaziri Hassas et al., 2020). It is noteworthy that the REEs precipitate as a mixed REE carbonate product and further purification and separation processes (Zhang et al., 2016, Krishnamurthy and Gupta, 2016, Vaziri Hassas et al., 2022) are needed to produce individual REEs, which is not in the scope of the current work. In order to scale up the proposed treatment process, it is necessary to study the fundamentals of the primary reactions in the system. Further optimization through a detailed reaction kinetic study is also needed to determine the scale-up parameters for the process engineering, design, costing, and economic analysis. Moreover, the formation of the final products of the process needs to be fully characterized for the effective design of the downstream processes. Therefore, Part I of this two-part study investigates the kinetics and thermodynamics of the main reactions governing the precipitation of CEs and REEs in the proposed precipitation process. Furthermore, in Part II, the mechanistic details of the formation and precipitation behavior of these elements along with their surface charge and coagulation potential at each stage of the precipitation process were studied (Vaziri Hassas and Rezaee, 2022). The current rate of precipitation study provides an insight into the necessary parameters needed for the process scale-up.
Section snippets
Theory
Precipitation of elements from aqueous solutions involves three main steps: nucleation, growth, and ripening, which are described below.
Materials and methods
A sample of 1200 L was collected from an AMD stream that originated from the Lower Kittanning coal seam in central Pennsylvania. The treatment facility is operated by the Pennsylvania Department of Environmental Protection (PADEP) with controlled caustic soda (NaOH) addition. The pH of the AMD sample was measured as 3.7 with a total REEs (TREEs) concentration of 500 µg/L. More details about the sample and its elemental content and the staged precipitation and filtration process were reported
Solution chemistry study
The saturation index for each element at various stages with the corresponding precipitant was calculated. The solution chemistry study was conducted based on the solution equilibrium calculations using the Visual MINTEQ software. The saturation index (SI), as defined in Eq. (9), was used to predict the degree of supersaturation.
Where IAP and ksp are the ion activity product and solubility product of the possible solid components. Fig. 2 shows the supersaturation index as a function
Conclusion
Selective recovery of critical elements such as Al, REEs, Co, and Mn from AMD through a precisely controlled precipitation and crystallization process is deemed viable. It was determined that supersaturation is the main driving force in the precipitation of elements during the AMD treatment. The considerably high nucleation followed by Ostwald ripening and growth can be suggested for all precipitation reactions. At the same time, supersaturation controls the nucleation and the initial rate of
Data Availability
The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.
CRediT authorship contribution statement
Behzad Vaziri Hassas: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Visualization, Writing – original draft. Younes Shekarian: Conceptualization, Methodology, Validation, Formal analysis, Writing – review & editing. Mohammad Rezaee: Supervision, Project administration, Funding acquisition, Conceptualization, Methodology, Data curation, Resources, Investigation, Validation, Writing – review & editing.
Declaration of Competing Interest
The authors declare no conflicts of interest.
Acknowledgment
Behzad V.H. is grateful to Dr. Allan Myerson for the advice and comments on the crystallization and kinetics study. The authors are grateful to Penn State Energy Institute (EI), Institutes of Energy and the Environment (PSIEE), Energy and Environmental Sustainability Laboratories (EESL), and Material Research Institute (MRI) for providing funding and technical facilities. Appreciation is also extended to Mr. Aaron Pontzer and the Pennsylvania Department of Environmental Protection (PADEP) for
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