Selenium pollution has been a topic of extensive research dating back further than the last decade and has attracted significant attention from several environmental and regulatory agencies in order to monitor and control its discharge from myriad industrial sources. The mining industry is a prime contributor of hazardous selenium release in the aquatic systems and is responsible for both acute and chronic impacts on living organisms. Herein we provide an overview of selenium contamination issues, with a specific focus on selenium release from mining industries, including a discussion of various technologies commonly employed to treat selenium-impacted waters from mining discharge. Different cases pertaining to selenium release from Alaskan mines (during years 2000–2015) are also presented, along with measures taken to mitigate high concentration releases. For continued resource exploration and economic development activities, as well as environmental preservation, it is important to fundamentally understand such emerging and pressing issues as selenium contamination and investigate efficient technological approaches to counter these challenges.

Rare earth elements (REEs) have been identified as valuable and critical raw materials, vital for numerous technologies and applications. With the increasing demand for and supply gap in REEs, many research studies have focused on alternative sources of REEs. This study involved an elemental and mineralogical characterisation of discarded coal from a coal plant and coal fly ash (CFA) from a power station in South Africa for REE presence. XRD results revealed that the discard coal sample consisted mainly of kaolinite, pyrite, siderite, quartz, calcite, gypsum, and muscovite, whereas CFA contained abundant glassy amorphous phases, alumina silicates, quartz, gypsum, calcite, and minute levels of muscovite and hematite. SEM-EDAX showed REE-carrying grains containing phosphorus in both discard coal and CFA samples. This was followed by investigating the leaching potential of REEs using hydrochloric acid from discard coal and CFA. This research’s potential impact is possibly providing a new and sustainable source of REEs. For that purpose, multiple batch leaching tests were performed to investigate the effects of temperature and acid concentration on the leaching efficiencies of REEs from discard coal and CFA. The experimental results indicated that temperature strongly influences REE leaching efficiency, while acid concentration has a lesser impact. This study identifies the best leaching conditions for the total REE recovery as 1 M HCl and 80 °C for discard coal and CFA.

South Africa is an important participant in the global coal market. The four Upper coal seam situated in Springs-Witbank Coalfield was subjected to XRT sorting and subsequent mineralogical studies to characterize REE content. The average total REE content (LREE and HREE) of the coal were 280 ppm on ash basis. The surrounding elemental composition besides carbon primarily consisted of O, Si, S and Fe with minor quantities of Al, P and Ti. Major minerals, such as quartz, dolomite, illite, gypsum, siderite, pyrite correlated positively with ash content (r > 0.9) except kaolinite, and microcline, indicating incongruent source and formation mechanisms. Additionally, the HREEs Lu, Tm, Dy, Ho, Gd and Tb displayed strong correlations (r > 0.95) with ash content but were negatively associated with fixed carbon. Sc is negatively associated with coal ash. Fixed carbon is weakly associated with La but negatively associated with HREEs. La showed a weak association with Ce. Y is strongly correlated with HREE elements especially with Dy, Tm, Lu and Ho while at the same time LREE are strongly associated with Ce, Pr, Nd, La, Eu but negatively with Sc and fixed carbon levels.

The modern air-based density separator achieves density-based separation for particle sizes greater than 6 mm. Parametric studies, conducted with a modified laboratory-scale unit, operating at an optimal feed rate of 200 kg/hr to upgrade low-rank coal, have demonstrated the applicability to clean finer size fractions up to 1 mm. However, there is a lack of fundamental understanding of the separation process that can be provided utilizing numerical modeling techniques. The separation process was numerically modeled using K-Epsilon and RSM turbulence formulations and validated using experimental dataset. The results prove that the effect of fine coal vortices forming around the riffles act as a transport mechanism for higher density particle movement across the table deck resulting in 43% displacement of the high-density particles to the product side. The velocity and vector plots show high local variances of air speeds and pressure near the feed end and an increase in feed rate results in a drop in the deshaling capability of the table.