Abstract
The worldwide supply of rare earth element (REE’s) is restricted due to the scarcity of sources for these elements. This has led to a need to broaden extraction from both previously productive reserves, such as China, and to expand extraction to new sources, possibly including coal and its ash products. The extraction of REE’s from coal and/ or coal ash depends on a number of factors, one of the most important being the concentration of the elements in the feed coal and derived ash.
The aim of the study was to determine the levels of REE’s in three South African coals and their associated ash products (8 samples), with the intention of identifying the viability of commercial extraction of REE’s from these base materials. The REE’s considered in this study are Sc, Y and the lanthanides: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Th, Yb, and Lu which together are broadly known as the lanthanoids. The coal and ash samples were analysed using a variety of routine and advanced techniques, including Malvern Particle Size analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), inductively coupled plasma-mass spectrometry (ICP-MS/OES), and instrumental neutron activation analysis (INAA).
The XRD analysis did not find typical REE bearing minerals in these samples. SEM examination indicated that there is a correlation between REE’s and the presence of aluminium and silicon as well as coal organic matter. Different digestion techniques, as well as different acids, were applied in order to identify the best method that leads to the highest recovery of REE’s. Sodium peroxide fusion proved to be the best technique compared to multi-acid digestion for digesting coal and ash samples for REE determination. The analytical results obtained were compared with certified values of SARM 18, 19 and 20 for validation. The ICP-MS data revealed that, as expected the ash samples have a higher concentration of REE’s than the coal samples. The ash samples have total REE concentration ranging from 402 to 599 ppm, and the coal samples have total REE concentration ranging from 95 to 149 ppm. There was an excellent correlation of REE ICP-MS data obtained through sodium peroxide fusion with the INAA results. The comparison of particle size distribution with REE concentration showed that there is no clear correlation between particle size and REE concentration of the ash samples, contradictory to literature.
M.Sc. (Geology)