Process development for selective separation of critical elements from secondary resources
Open Access
- Author:
- Vaziri Hassas, Behzad
- Graduate Program:
- Energy and Mineral Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 26, 2022
- Committee Members:
- William Burgos, Outside Unit & Field Member
Sarma Pisupati, Major Field Member
Mohammad Rezaee, Chair & Dissertation Advisor
Athanasios Karamalidis, Major Field Member
Mort Webster, Professor in Charge/Director of Graduate Studies - Keywords:
- Critical Elements
Rare Earth Elements
Secondary Resources
Acid Mine Drainage
Staged Precipitation - Abstract:
- Recent high-tech developments and green energy transition have become possible largely thanks to the unique and superior properties of the rare earth elements (REEs) and other metals used in these industries. Alloys and complexes of these elements have been a crucial part of these developments. Although abundant in the earth's crust, the scarcity of feasible resources to mine REEs has rendered these elements critical and strategic commodities. It is imperative to establish reliable resources for such critical elements to reduce the reliance on foreign resources. As a result, there have been rising interests in recovering and recycling critical elements from secondary resources (e.g., acid mine drainage, mine tailings, and e-waste) to supply the increasing demands for these elements. Acid mine drainage (or acid rock drainage) is an effluent stream of water seeping through host rocks containing sulfide minerals (especially pyrite), generating a considerable amount of acidity through a series of reactions that produce protons in the solution. The low pH of the solution with a continuous flow results in the dissolution of a wide variety of elements in the acid mine drainage streams. Both elevated mental contents and acidity of AMDs are of environmental concern. In accordance with Clean Water Act, these streams are required to be treated prior to discharge to the environment. Depending on the acidity and flow rate of AMD streams, their treatments can impose significant costs. During the AMD treatment and neutralization process, the dissolved metals in the solution precipitate due to a drop in solubility of metals in circumneutral solutions pH. The precipitates (sludge material) are then stored in sludge ponds. Both AMD and associated sludge materials have been found to be viable sources of critical elements. These waste streams offer significant advantages over primary and other critical elements resources as they do not carry any mining costs. Additionally, both sources contain a number of critical elements, selective recovery of which will make the whole enterprise financially attractive. However, to date, the treatment systems mainly aim to achieve specific discharge limits and remain compliant with environmental regulations. This research aims to develop an efficient, low-cost, and environmentally friendly process for the selective recovery of critical elements while treating AMDs to address environmental issues. The precipitation behavior of elements including critical elements, REEs, and other major elements was studied through a staged precipitation process in presence of various ligands, namely hydroxide, carbonate, phosphate, sulfate, and ammonia. Carbonate was found to effectively decrease the precipitation pH of the REEs while eliminating the need for increasing the pH to above 7 to achieve better recovery of REEs. Carbonate was also found to effectively precipitate Al at pH 5, which provides a window of separation through staged precipitation for precipitation of Al at pH 5 and precipitation of REEs at pH 7. In the proposed two-staged precipitation process using carbonate ligand (Na2CO3), 94% of Al was precipitated and recovered from the solution at pH 5 (Stage I), while 85% of the total REEs was selectively precipitated at pH 7 (Stage II). Furthermore, it was established that the best ligand to recover Co-Mn from the solution is ammonia at pH 9.5. Based on these findings, CO2 was also studied as an alternative source for carbonate ions in the solution to be used for the precipitation of REEs and other critical elements, while providing an opportunity for CO2 sequestration. A CO2 mineralization process was proposed for the recovery of Al and REEs at two stages. The precipitation behavior of the REEs was also studied, and it was established that the precipitation of the elements strongly depends on their interaction with the ligands in the solution, which is a function of the coordination chemistry of the elements. It was found that, when NaOH was used, the heavy REEs tend to precipitate at lower pH compared to the light REEs. Furthermore, the effect of carbonate ligand on the precipitation behavior of the REEs showed that both light and heavy REEs tend to precipitate at much lower pH. Kinetics, thermodynamics, and mechanism of reaction were studied to understand the fundamentals of the process and to acquire the required parameters for process scaleup. Kinetics and thermodynamics data showed that supersaturation is the main driving force in the precipitation of the elements from AMDs in the carbonate precipitation process. It was also revealed that the partial charge of the formations plays a significant role in the precipitation and coalescence of the REE-carbonates during the treatment process. Calculated activation energy values for the elements using the Avrami kinetics model indicate that the precipitation of the elements in the proposed process is a diffusion-controlled reaction. Furthermore, the mechanistic effects of the ligands on the formation and surface charge properties of the REE precipitates were studied. These findings indicated that the presence of carbonate ions in the solution governs the formation of the precipitates and the REEs tend to precipitate as REE-carbonate in such conditions. It was also established that when Na2CO3 was used in the stage precipitation process the REEs and critical elements neither adsorb on the other precipitates nor coprecipitate with other elements, but they tend to precipitate as separate formations in the carbonate precipitation process. Based on the findings, a three-stage precipitation process was designed and tested to produce high-purity REE and critical element products from AMDs and pregnant leaching solutions obtained by leaching the sludge materials. Three AMD samples were collected in central Pennsylvania with Lower Kittanning coal seam origin and evaluated for their elemental content. One of the samples with a higher REE concentration was selected to be used in three-stage precipitation process tests. In this process, the pH of the feed solution was first increased to 4 and Fe was precipitated with oxidation through the addition of H2O2 and aeration (Fe removal pretreatment). After Fe removal, the pH of the solution was increased to 5 to precipitate Al (Stage I). Once the Al precipitates were removed from the system, the pH of the solution was then increased to 7 to precipitate REEs (Stage II). In both Stage I and Stage II, Na2CO3 was used for pH adjustment. Once the REE precipitates were collected, the pH of the solution was increased to 9.5 using NH4OH to precipitate Co and Mn from the solution. The overall recovery of >99% for Al, REEs, Co, and Mn was achieved in this staged precipitation process. This selective precipitation process yielded an Al product consisting of 55.7% boehmite (AlO(OH)) and 37.1% dawsonite (NaAl(OH)2(CO3)) in the first stage. The REE product of the second stage contained 88.5% adamsite (Na(REE)(CO3)2). The Co-Mn product in the third stage contained 68% ramsdellite (MnO2) and 6.9% cobalt(II,III) oxide.