LEACHING CHARACTERISTICS OF RARE EARTH ELEMENTS FROM BITUMINOUS COAL-BASED SOURCES

The demand for rare earth elements (REEs) has increased over the last decade due to applications in high technology devices including those in the defense industry. The recovery of REEs from primary sources such as rare earth minerals are viable using physical separations followed by chemical processing. However, weak market values and environmental concerns have limited the viability of such operations. On the other hand, REE recovery from secondary sources such as apatite ore, bauxite waste, and waste recycling, provides an opportunity to take advantage of a resource that does not require mining costs as well as other associated costs given that these expenses are covered by the revenue generated from the production of the primary material. Coal-based materials represent a potential source for REEs which may be extracted and concentrated by the use of physical and/or chemical processes. The current study focused on developing a leaching process to extract REEs from the pre-combustion coal sources including coarse and fine refuse and low-valued material obtained from coal preparation plants. Materials collected for leaching characteristic studies were found to have average total REE concentrations in the range of 200-350 ppm on a whole sample basis. Mineralogy studies performed on Fire Clay seam coal refuse using SEM-EDS detected micro-dispersed rare earth phosphate mineral particles which are generally difficult to dissolve in strong acid solutions. On the other hand, XRD analysis results from a high REE content segment of the West Kentucky No. 13 coal seam indicated the presence of fluorapatite which is soluble in weak acid solutions. The mineral associations of REEs were studied by extracting REEs using different types of acids under various pH conditions. Differential extraction of the REEs was examined along with the associated impurity elements such as iron, aluminum, and calcium among others. The findings showed that the light REEs were primarily associated in a phosphate mineral form, whereas the heavy REEs were mostly present in an ion substitution form associated with clay minerals. Relatively high concentrations of REEs were discovered in mixed-phase particles consisting of both coal and mineral matter. By reducing the particle size, more leachable forms of REEs were liberated and recovered along with the associated mineral matter embedded in the coal structure. The type of lixiviant played an important role during the initial stage of leaching but was found to be insignificant as the system reached equilibrium. Solids concentration in the leaching medium has an important role in establishing the throughput capacity of the leaching system. Test results found that an increase in solids concentration had a significant negative effect on rare earth recovery. This finding may be explained by higher concentrations of soluble calcium-based minerals such as calcite which provided localized pH increases near and within the pores of the solids. The result was precipitation of CaSO4 within the pores which blocked access for the lixiviants. This hypothesis was supported by the findings from BET and XPS analyses which found lower pore volume in high solid concentration systems and the existence of CaSO4 on the surface of the solids. Leaching test results obtained using sulfuric acid over a range of temperatures showed that the leaching process was mainly driven by a diffusion control process. The activation energy determined for an Illinois No. 6 coal source was 14.6 kJ/mol at the beginning of the reaction and 35.9 kJ/mol for the rest of the leaching process up to 2 hours. For material collected from the Fire Clay coal seam, the apparent activation energy was 36 kJ/mol at the start of the leaching reaction and decreased to 27 kJ/mol over the remaining period of the test. The activation energy values were nearly equivalent to the upper-level values that generally define a diffusion control process and the lower values of a chemical reaction control process. The lack of clarity in defining a clear control mechanism is likely associated with the variability in associated mineralogy, various modes of occurrence of the REEs and the interfacial transfer of product through the porous structure of the coal-based particles which requires relatively high activation energy. As such, both diffusion control and chemical reaction control mechanisms are likely occurring simultaneously during the leaching process with diffusion control being more dominant

High efficiency n-type silicon solar cells featuring passivated contact to laser doped regions

Minimizing carrier recombination at cell contacts becomes increasingly important for reaching high efficiency. In this work, the passivated contact concept is implemented into n-type silicon solar cells with laser-processed local back surface fields. The passivation and contact characteristics of the SiO2/amorphous silicon (a-Si:H) stack on localized laser doped n+ regions are investigated. We find that the SiO2/a-Si:H stack provides not only good passivation to laser doped n+ regions but also allows a low contact resistivity after thermal annealing. With the implementation of the SiO2/a-Si:H passivated contact, an absolute efficiency gain of up to 1.5% is achieved for n-type solar cells

Leaching Kinetics of Rare Earth Elements from Fire Clay Seam Coal

Recovery of rare earth elements (REEs) from coal samples collected from the Fire Clay coal seam using diluted mineral acid solutions was investigated. The initial processing step was coal recovery using conventional froth flotation which concentrated the REEs in tailing material resulting in an upgrade to values around 700 ppm on a dry whole mass basis. Leaching experiments were performed on the flotation tailings material using a 1.2 M sulfuric acid solution adjusted to a temperature of 75 °C to study the extractability of REEs from coal material. The effect of particle size, leaching time, leaching temperature, and solid concentration on REE leaching recovery were evaluated. The kinetic data obtained from leaching over a range of temperatures suggested that the leaching process follows the shrinking core model with possibly a mixed control mechanism that may be a result of several heterogenous materials leaching simultaneously. Leaching recovery increased rapidly at the beginning of the reaction then slowed as the system reached equilibrium. The apparent activation energy determined from test data obtained over a range of temperatures using 1 M sulfuric acid was 36 kJ/mol for the first 20 min of reaction time and 27 kJ/mol for the leaching period between 20 and 120 min. The leaching of light REEs during the initial stage was determined to be driven by a chemical reaction, followed by the formation of a product layer, which required lower activation energy in the later stage of leaching. In regards to the heavy REEs, the major mechanism for leaching is desorption and the product layer formation does not affect the heavy REEs significantly

Structural Basis for Clonal Diversity of the Public T Cell Response to Dominant Epitopes from Cytomegalovirus and Influenza

A diverse T cell receptor (TCR) repertoire is a prerequisite for effective viral clearance. However, knowledge of human TCR repertoire to defined viral antigens is limited. Recent advances in high-throughput sequencing (HTS) and single-cell sorting have revolutionized the study of human TCR repertoires to different types of viruses. In collaboration with the laboratory of Dr. Nan-ping Weng (National Institute on Aging, NIH), we applied unique molecular identifier (UMI)-labelled HTS, single-cell paired TCR analysis, surface plasmon resonance, and X-ray crystallography to exhaustively interrogate CD8+ TCR repertoires specific for cytomegalovirus (CMV) and influenza A (Flu) in HLA-A2+ humans. Our two CMV-specific TCR-pMHC structures and two Flu-specific TCR-pMHC structures provide a plausible explanation for the much higher diversity of CMV-specific than Flu-specific TCR repertoires in humans. Our comprehensive biochemical and structural portrait of two different anti-viral T cell responses may contribute to the future development of predictors of immunity or disease at the individual level

Evaluation of Frother Types for Improved Flotation Recovery and Selectivity

A laboratory study was conducted to evaluate and compare the effectiveness of nine different frother types when used in a three-phase, continuously operating froth flotation system. The frothers included several that are commonly used in the industry (e.g., MIBC, 2EH, and F-1) as well as unique frother types (e.g., F-3). The tests were conducted in a 5-cm diameter laboratory flotation column that provided near plug-flow mixing conditions due to a length-to-diameter ratio of around 50:1. Test results indicate that F-1, MIBC, and MPC (in order of decreasing effectiveness) provided the weakest performance in terms of combustible recovery while F-2, MAC, and 2EH were the top three generating the highest separation efficiencies. When processing ultrafine coal, the ash content of the flotation concentrate ranged from 10% to 15% while recovering over 80% of the combustible material. F-3, F-4, and DIBC provided over 80% recovery of combustibles at the expense in the amount of hydraulic entrainment. The flotation performances were also closely examined in accordance with the fundamental properties of the nine tested frothers, and their correlations were addressed in detail

An acid baking approach to enhance heavy rare earth recovery from bituminous coal-based sources

The recovery of rare earth elements (REEs) from secondary resources, particularly coal-based materials, has recently received attention due to supply and demand imbalance. Research reported to date indicates that a high REE recovery can be realized when treating bituminous coal sources using strong acid solutions of approximately100 g/L or higher. This study introduces an approach to enhance the total rare earth recovery (TREE), especially for heavy rare earth elements (HREEs), from these coal sources at significantly lower acid concentrations. Based on the presence of REE minerals like monazite and xenotime, a detailed investigation was undertaken to quantify three pre-leach treatment methods, i.e., 1) roasting, 2) direct acid baking, and 3) acid baking after roasting. Roasting tests at 600 ◦C revealed that the recovery of light REEs (LREEs) was enhanced while the recovery of HREEs remained relatively unaffected. LREE and HREE recovery values of 38.3% and 21.3%, respectively, were achieved using a 50 g/L (0.5 M) sulfuric acid solution at 5% solid concentration and a solution temperature of 75 ◦C for 2 h. Comparatively, direct acid baking at 250 ◦C provided substantial increases in LREE and HREE recovery values to approximately 49.4% and 53.0%, respectively, using an equivalent acid dosage. Recoveries were maximized to 77.0% and 79.6% for LREE and HREE, respectively, by roasting followed by acid baking. Similar results were obtained from the treatment of a second bituminous coal source. Due to strong correlations between REE and Al recovery values, tests were performed on kaolinite and illite, which were prominent clay minerals within the source coals. These experiments revealed that the REE recovery improvements were likely a result of dehydroxylation of clays and subsequent release and decomposition of REE-bearing minerals such as monazite, xenotime and zircon