Date of Graduation
Statler College of Engineering and Mineral Resources
Civil and Environmental Engineering
Research performed by the West Virginia Water Research Institute (WVWRI) in partnership with the Department of Energy (DOE) has identified that coal mining by-products contain significant amounts of Rare Earth Elements (REEs). REEs currently are in high demand, and extraction from coal mining reject presents a potential for commercialization if extraction is feasible. The coal mining reject include Acid Mine Drainage (AMD) in the aqueous phase and sludge from AMD treatment. A novel two split selective precipitation method was implemented to remove gangue metals, increasing the concentration of REEs in the sludge.
This process presents a two-fold benefit: 1) treatment and maintenance of AMD producing sites, and 2) creation of an REE precipitate for further metal extraction and separation. Exploring AMD in the aqueous form is beneficial because of its long-term future availability. Underground mines, refuse piles, valley fills are examples of sites providing aqueous sources. This research was performed to produce, collect, and dewater the REE sludge that will feed the extraction and separation phases. The research consists of three phases: 1) augmenting the sludge precipitation with polymer aided flocculants and determining the optimum dosage for the two-split treatment sludge settling and filtration, 2) testing the defined chemical dosages in a bench-scale production, and 3) performing tests to filter and dewater the sludge.
The first phase was performed by testing six polymer-aided flocculants and one coagulant to identify the more suitable floc production. The six polymers vary in their charge and density but are equal in structure (linear chain), and the coagulant has a neutral charge. A series of jar tests were performed according to ASTM requirements to evaluate formed floc size, precipitation velocity, supernatant turbidity for each flocculant, and coagulant under different dosages. The selected flocculant candidate was a cationic flocculant with a medium charge. The produced floc sizes ranged from 2 mm to 20 mm, and turbidities ranged from 0.6 to 1.1 NTU. Next, the optimum dosage was defined, envisioning floc dewaterability capability identified by the Specific Resistance to Filtration (SRF) test, producing a floc size of 9mm.
The second phase was the proof-of-concept of a continuous operation. A simulation of an AMD treatment station with selective precipitation was designed, built, and commissioned in a 20L clarifier. The operation showed it is possible to maintain the targeted pH of 4.5 and 8.5 +/- 0.2 and precipitate large flocs (5 mm to 8 mm size range) that can be filtered and dewatered, maintaining the REE concentration in the precipitate (now called preconcentrate or HPC).
The third phase researched geotextile fabrics for optimum REE precipitate filtration and dewatering. The filtration testing consisted of pouring the REE precipitate into a cylinder tube with the tested geotextile attached to the bottom of the cylinder. The flow rates, the mass of solids in the slurry, filter cake, and filtrate were measured to determine hydraulic conductivity and filtration efficiency. Testing showed that a non-woven geotextile with a 0.150 mm apparent opening size (AOS) obtained >88% filtration efficiency in first contact with optimized flocculant dosage. After filter cake formation, filtration efficiency increased to >99%. Different flocculant dosages tested showed that the hydraulic conductivity was not affected at low concentrations.
The fourth phase studied the consolidation properties of the material for dewatering continuation after removal of free water. Two main consolidation processes are self-weight (overburden) and water evaporation (desiccation). Consolidation tests showed that the compression index of the material is similar to soft clays and the major influence in consolidation is the initial moisture content. Desiccation proved efficient when following the optimum SRF point reaching 90% solids, indicating rapid dewatering in the field.
Capillary Channel Fibers (CCFs) were investigated for dewatering augmentation. CCFs are fibers with microgrooves that can wick water out from a porous media through capillary action. These fibers are a novel material and have proved successful in a few geotechnical applications to control moisture. The use of CCFs in flocculated materials presents a potential to improve dewatering in several industries, including wastewater treatment, mine tailings, dredging, and others.
The relevant outcomes of this research were:
1) The achievement of a steady-state two-split precipitation REE novel precipitate production with good settling velocity producing low-turbidity supernatant and filtration characteristics (floc size) for industrial production;
2) Solids characterization and filtration analysis of the preconcentrate determining a geotextile filter to obtain >88% filtration efficiency,
3) Determination of an optimal flocculant dosage for dewatering during saturated and unsaturated conditions.
4) Development of a preliminary mathematical model for applying capillary channel fibers to dewater soils.
Lira Santos, Iuri, "Methodology for enhanced filtration and dewatering of fine-grain AMD precipitate using geocomposites for rare earth elements recovery" (2022). Graduate Theses, Dissertations, and Problem Reports. 11444.
Available for download on Thursday, June 06, 2024