Selective precipitation of rare earth and critical elements from acid mine drainage - Part II: Mechanistic effect of ligands in staged precipitation process
Graphical abstract
Introduction
Recent developments in high-tech and green energy initiatives have led to the use of a greater number of mineral commodities in larger quantities. U.S. Department of Interior has identified 50 mineral commodities critical to the U.S. economy and national security (U.S. Department of the Interior (DOI), USGS 2021). For many of these elements, such as aluminum (Al), rare earth elements (REEs), cobalt (Co), and manganese (Mn), with applications in the high-tech industry, energy storage, and green energy initiatives, the U.S. is heavily reliant on foreign sources (U.S. Geological Survey 2022; Vaziri Hassas et al., 2020; Haxel et al., 2002; Rozelle et al., 2021; Rozelle et al., 2020). To address the supply risk of these elements, it is crucial to explore and extract all viable primary and secondary sources, including electronic waste and mining and processing waste streams such as acid mine drainage (AMD) (Zhang et al., 2015; Li et al., 2019; Rozelle et al., 2021; Rozelle et al., 2020). AMD has been of environmental concern for decades but recently, it has been viewed as a viable source of multiple critical elements (Rezaee et al., 2013; Rezaee and Honaker, 2020; Vass et al., 2019; Vass et al., 2019). The traditional AMD treatment processes focus on meeting the environmental regulations for the discharge rather than recovering valuable elements.
Selective recovery of critical elements from AMD during the remediation to address the environmental concerns enhances the sustainability of the treatment processes and provides revenue through waste-to-value conversion. Numerous approaches have been suggested for the recovery of REEs from AMD and mine refuse natural leachates, which demonstrate an enormous opportunity to achieve a circular economy by recovering and recycling these elements from secondary resources (Ayora et al., 2016; Royer-Lavallée et al., 2020; Costis et al., 2021; Sun et al., 2012; Nogueira et al., 2019).
A patented novel AMD precipitation process was previously developed to selectively precipitate Al and REEs from AMD through staged carbonate precipitation (using CO2 mineralization or utilizing Na2CO3) while neutralizing AMD, followed by precipitation of Co-Mn at the third Stage through either ozone oxidative or ammoniacal precipitation (Vaziri Hassas et al., 2020; Vaziri Hassas et al., 2021; Rezaee et al., 2021, Shekarian et al., 2022). The products of this process can be further purified through a closed circuit precipitation process or other hydrometallurgical separation processes (Vaziri Hassas et al., 2022; Zhang et al., 2016). A thorough understanding of the mechanism, optimizing the process and determining the scale-up parameters require the study of the kinetic and formation of the products. This two-part study examines the formation and rate of precipitation of the products in the three-step staged precipitation process. In Part I of this study, the kinetics and thermodynamics of the precipitation reactions and fundamentals of nucleation and precipitation were studied (Vaziri Hassas et al., 2022). The current study (Part II) elucidates additional mechanistic details on the formation and precipitation of the elements at each stage as well as the surface charge and coagulation of the precipitates. Precipitation behavior of the elements, especially for stages I and II along with their surface charge were studied to address the concerns regarding the selective precipitation of Al and REEs as previous works have reported possible surface adsorption and co-precipitation of REEs with Al precipitates in these stages.
As previously reported, the precipitation of the REEs does not happen at just one specific pH point. Instead, their precipitation is distributed over a range of pH values, and the precipitation yield at each pH depends on a number of parameters (Vaziri Hassas et al., 2020; Zhang and Honaker, 2018). Furthermore, in sampling the sites, it was observed that the color and characteristics of precipitates change as a function of distance from the treatment point, where the treatment chemical is added to neutralize the pH. Figure 1 shows an AMD treatment site in central Pennsylvania from which the AMD sample was collected for this study. The gradient of various colors (especially in the treatment pond) implies the changes in the concentration of a specific group of elements at each point of the pond. It should be noted that the local pH also varies from the treatment point to the discharge point. Further evaluation of elemental content and details of the sampled site are provided in the results and discussion section.
From the above observations, it is clear that the reaction rate plays a key role in the precipitation of metals during AMD treatment. The importance of the kinetics of precipitation in the recovery of REEs from AMDs is indisputable for the design of the treatment pond and the residence times needed for the process, which are related to the formations and the structure of the precipitates. Moreover, the effect and significance of aging time on the precipitation of metal salts have also been reported previously (Philippini et al., 2008; Rodriguez-Blanco et al., 2014). The precipitation process is basically the formation of a solid phase from a saturated solution, which consists of several sub-processes, including nucleation, growth, ripening, and agglomeration (Myerson, 2002; Erdemir et al., 2009). However, these sub-processes often are not fully distinguishable in practice. In order to fully understand the mechanisms controlling the precipitation process, both thermodynamics and the kinetics of the process should be studied. The precipitation kinetics for the elements of interest in this study (i.e., Al, REEs, Co, Mn) was reported and discussed in a companion paper (Part I) of this two-part study (Vaziri Hassas et al., 2022). In addition to the kinetics, the formation, surface and bulk properties, and interparticle interactions of precipitates also play significant roles in the observed segregation of the precipitates in the ponds. Properties and the behavior of the solids in the solution are governed by a number of interactions known as “colloidal stability.” These interactions can be either repulsive or attractive, resulting in a stable colloidal system or aggregation of the solid particles. These interactions have been summarized in DLVO theory after Derjaguin, Landau, Verwey, and Overbeek, which is the combination of van der Waals attraction and double layer repulsion forces (Israelachvili, 2011). This phenomenon controls the coalescence of the nucleates and can be a significant parameter in the precipitation process. Using flocculants during the ripening of the crystals or even after the precipitation can enhance the selective recovery of certain precipitates (Roca, 2020).
Another significant factor in controlling the behavior of the precipitates is the precipitant used in the treatment, as it determines the formation of the precipitates. Surface complexation and adsorption of REEs on the Fe and Al precipitates have been reported during the AMD treatment (Lozano et al., 2020; Lozano et al., 2019). However, such interference was not observed in the staged precipitation process using Na2CO3 for the selective recovery of Al and REEs. Such peculiarity in the precipitation mechanism points to the difference between the precipitants such as carbonates (e.g., Na2CO3, or CO2 mineralization) and hydroxides (e.g., NaOH, Ca(OH)2) used for REE precipitation and neutralization of AMD. This mechanistic difference is further studied through the formation and surface charge analyses of precipitates.
The formation of the precipitates under various conditions was studied to draw a relative conclusion on which forms of REEs precipitated in AMD treatment when Na2CO3 was used instead of NaOH. Such information on the formation of the precipitates is useful when the kinetics of precipitation is discussed. Due to the presence of a multitude of anions in the AMD, the complexation of REEs with any of these ligands is possible. Mathias Wickleder (Wickleder, 2002) has reported the possible formation of REEs as silicates, phosphates, sulfates, chlorites, carbonates, and many other anions. In their study of Nd-carbonate equilibria, Meinrath and Takeishi (Meinrath and Takeishi, 1993) reported the precipitation of REE-carbonates in very low carbonate concentrations in the system (CO2 partial pressure as low as natural aquatic systems in the range of 0.03%). Moreover, Verplnk et al. (Verplanck et al., 2004) reported that at the pH range of 5.1 to 6.6, REEs start to fractionate and heavy REEs adsorb on the precipitated Fe particles. They concluded that this fractionation is probably due to sulfate ions in the AMD as a dominant complex. Lee et al. (Lee et al., 2002) also reported the adsorption of trace elements on the Fe, Al, and Mn precipitates at pH 4, 5, and 8, respectively. Confirming the formation of the precipitates can provide insight into their surface properties and behavior. It may also provide an answer to the long-standing question of whether the use of Na2CO3 in AMD treatment influences the complexation of REEs with Al precipitates. Surface characterization (e.g., surface charge analysis) of the precipitates can also address the mechanisms of the reported co-precipitation or adsorption behavior of the REEs on the other precipitates during the treatment process.
Section snippets
Materials and methods
A 1200 L sample of the AMD stream (Site A) originating from the Lower Kittanning coal seam was collected to conduct the staged precipitation experiments, the details of which were provided elsewhere (Vaziri Hassas et al., 2021). The 3-stage precipitation process (Fig. 2) described in Part I of this study (Vaziri Hassas et al., 2022), was used to produce precipitates at pH 5 (Stage I), pH 7 (Stage II), and pH 9 (Stage III) using Na2CO3 (at Stages I and II) and NH4(OH) (at Stage III). These
Precipitation patterns in AMD treatment ponds
Two AMD treatment facilities (Sites A and B described in Section 2.1) in Pennsylvania were sampled for elemental content variation to establish and validate the patterns of precipitation and co-precipitation during the treatment. The samples were collected from various locations of the treatment ponds, and the variation in concentration of elements of interest was calculated as a relative concentration to the feed (Fig. 3).
It should be noted that in both treatment facilities, the chemical
Conclusions
Recovery of critical elements from secondary resources, such as AMD, is viewed as a potential path to meet the nation's need for critical minerals due to the recent high-tech and green energy developments. This study reported precipitation patterns in AMD treatment ponds when NaOH and Na2CO3 were used for AMD treatment. NaOH was found to cause a significant spike in pH of the AMD at the mixing (feed) point, which seems to be the main reason for the immediate and simultaneous precipitation 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, Formal analysis, Investigation, Visualization, Validation, Writing – original draft. Mohammad Rezaee: Supervision, Project administration, Funding acquisition, Conceptualization, Methodology, Data curation, Resources, Investigation, Validation, Writing – review & editing.
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.
Acknowledgment
The authors are grateful to Penn State EMS Energy Institute (EI), Institutes of Energy and the Environment (PSIEE), Energy and Environmental Sustainability Laboratories (EESL), and Material Research Institute (MRI) for providing partial funding and technical facilities. The authors thank Mr. Aaron Pontzer and the Pennsylvania Department of Environmental Protection (PADEP) for for valuable contributions to this research project through providing samples, process data, and access to the treatment
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Cited by (2)
Selective precipitation of rare earth and critical elements from acid mine drainage - Part I: Kinetics and thermodynamics of staged precipitation process
2023, Resources, Conservation and RecyclingCitation Excerpt :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.