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