Advances in Understanding of the Application of Unit Operations in Metallurgy of Rare Earth Elements

Ecological and Health Risks Attributed to Rare Earth Elements in Coal Fly Ash

The occurrence and distribution of yttrium and rare earth elements (REYs), along with major elements and heavy metal(loid)s (HMs) in coal fly ash (CFA) from five coal-fired power plants (CFPPs), were analyzed, and the REY-associated ecological and health risks were assessed. The individual REYs in CFA were abundant in the following order: Ce > La > Nd > Y > Pr > Gd > Sm > Dy > Er > Yb > Eu > Ho > Tb > Tm > Lu. The total REY content ranged from 135 to 362 mg/kg, averaging 302 mg/kg. The mean light-to-heavy REY ratio was 4.1, indicating prevalent light REY enrichment in CFA. Significantly positive correlations between the REYs suggested that they coexist and share similar origins in CFA. REYs were estimated to pose low to moderate ecological risks, with risk index (RI) values ranging from 66 to 245. The hazard index (HI) and target cancer risk (TCR) of REYs from CFA, estimated to be higher for children (HIc = 0.15, TCRc = 8.4 × 10-16) than for adults (HIa = 0.017, TCRa = 3.6 × 10-16), were well below the safety limits (HI = 1, TCR = 1.0 × 10-6). However, the danger to human health posed by HMs in the same CFA samples (HIc = 5.74, TCRc = 2.6 × 10-4, TCRa = 1.1 × 10-4) exceeded the safe thresholds (excl. HIa = 0.63). The mean RI and HI attributed to REYs in CFA were 14% and 2.6%, respectively, of the total risks that include HMs.

Spatial Variability of Rare Earth Elements in Groundwater in the Vicinity of a Coal-Fired Power Plant and Associated Health Risk

This study investigated the occurrence and distribution of rare earth elements (REEs), including 14 lanthanoids, scandium (Sc), and yttrium (Y), in groundwater around a large coal-fired thermal power plant (TPP). The ICP-MS technique was used to analyze 16 REEs in groundwater samples collected from monitoring wells. REE concentrations ranged from 59.9 to 758 ng/L, with an average of 290 ng/L. The most abundant was Sc, followed by La, accounting for 54.2% and 21.4% of the total REE concentration, respectively. Geospatial analysis revealed the REE enrichment at several hotspots near the TPP. The highest REE concentrations were observed near the TPP and ash landfill, decreasing with the distance from the plant and the landfill. REE fractionation ratios and anomalies suggested the Light REE dominance, comprising over 78% of the total REEs. Correlation and principal component analyses indicated similar behavior and sources for most REEs. Health risk assessment found hazard indices (HI) of 1.36 × 10-3 and 1.98 × 10-3 for adults and children, respectively, which are far below the permissible limit (HI = 1). Likewise, incremental lifetime cancer risks (ILCR) were all below 1 × 10-6. Nevertheless, ongoing ash disposal and potential accumulation in the environment could elevate the REE exposure over time.

Dispersive Solid–Liquid Microextraction Based on the Poly(HDDA)/Graphene Sorbent Followed by ICP-MS for the Determination of Rare Earth Elements in Coal Fly Ash Leachate

A dispersive solid-phase microextraction (DSPME) sorbent consisting of poly(1,6-hexanediol diacrylate)-based polymer microspheres, with embedded graphene microparticles (poly(HDDA)/graphene), was synthesized by microfluidic emulsification/photopolymerization and characterized by optical microscopy and X-ray fluorescence spectrometry. This sorbent was applied for simple, fast, and sensitive vortex-assisted DSPME of rare earth elements (RREs) in coal fly ash (CFA) leachate, prior to their quantification by inductively coupled plasma mass spectrometry (ICP-MS). Among nine DSPME variables, the Plackett–Burman screening design (PBD), followed by the central composite optimization design (CCD) using the Derringer desirability function (D), identified the eluent type as the most influencing DSPME variable. The optimum conditions with maximum D (0.65) for the chelating agent di-(2-ethylhexyl) phosphoric acid (D2EHPA) amount, the sorbent amount, the eluting solvent, the extraction temperature, the centrifuge speed, the vortexing time, the elution time, the centrifugation time, and pH, were set to 60 μL, 30 mg, 2 M HNO3, 25 °C, 6000 rpm, 1 min, 1 min, 5 min, and 4.2, respectively. Analytical validation of the DSPME method for 16 REEs (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) in CFA leachate samples estimated the detection limits at the low ppt level, the recovery range 43–112%, and relative standard deviation within ± 22%. This method was applied to a water extraction procedure (EP) and acetic acid toxicity characteristic leaching procedure (TCLP) for leachate of CFA, from five different coal-fired thermoelectric power plants. The most abundant REEs in leachate (20 ÷ 1 solid-to-liquid ratio) are Ce, Y, and La, which were found in the range of 22–194 ng/L, 35–105 ng/L, 48–95 ng/L, and 9.6–51 μg/L, 7.3–22 μg/L, 2.4–17 μg/L, for EP and TCLP leachate, respectively. The least present REE in TCLP leachate was Lu (42–125 ng/L), which was not detected in EP leachate.

A holistic approach for the recovery of rare earth elements and scandium from secondary sources under a circular economy framework - A review

Limited natural resources and a continuous increase in the demand for modern technological products, is creating a demand and supply gap for rare earth elements (REEs) and Sc. There is therefore a need to adopt the sustainable approach of the circular economy system (CE). In this review, we defined six steps required to close the loop and recover REEs, using a holistic approach. Recent statistics on REEs and Sc demand and the number of waste generations are reported and studies on more environmentally friendly, economic, and/or efficient recovery processes are summarized. Pilot-scale recovery facilities are described for several types of secondary sources. Finally, we identify obstacles to closing the REE loop in a circular economy and the reasons why secondary sources are not preferred over primary sources. Briefly, recovery from secondary sources should be environmentally and economically friendly and of an acceptable standard concerning final product quality. However, current technologies for recovery from for secondary sources are limiting and technology needs will vary depending on the source type. The quality/purity of the recovered metals should be proven so that they do not result in any adverse effects on the product quality, when they are being used as secondary raw material. In addition, for industrial-scale facilities, process improvements are required that consider environmental conditions.

Advances in Understanding of Unit Operations in Non-Ferrous Extractive Metallurgy 2021

The high demand for critical materials, such as rare earth elements, indium, gallium, and scandium, raises the need for an advance in understanding the unit operations in non-ferrous extractive metallurgy [...]

Electrochemical Study of Nd and Pr Co-Deposition onto Mo and W from Molten Oxyfluorides

Electrodeposition processes of neodymium and praseodymium in molten NdF3 + PrF3 + LiF + 1 wt.%Pr6O11 + 1 wt.%Nd2O3 and NdF3 + PrF3 + LiF + 2 wt.%Pr6O11 + 2 wt.%Nd2O3 electrolytes at 1323 K were investigated. Cyclic voltammetry, square wave voltammetry, and open circuit potentiometry were applied to study the electrochemical reduction of Nd(III) and Pr(III) ions on Mo and W cathodes. It was established that a critical condition for Nd and Pr co-deposition in oxyfluoride electrolytes was a constant praseodymium deposition overpotential of ≈−0.100 V, which was shown to result in co-deposition current densities approaching 6 mAcm−2. Analysis of the results obtained by applied electrochemical techniques showed that praseodymium deposition proceeds as a one-step process involving exchange of three electrons (Pr(III)→Pr(0)) and that neodymium deposition is a two-step process: the first involves one electron exchange (Nd(III)→Nd(II)), and the second involves an exchange of two electrons (Nd(II)→Nd(0)). X-ray diffraction analyses confirmed the formation of metallic Nd and Pr on the working substrate. Keeping the anodic potential to the glassy carbon working anode low results in very low levels of carbon oxides, fluorine and fluorocarbon gas emissions, which should qualify the studied system as an environmentally friendly option for rare earth metal deposition. The newly reported data for Nd and Pr metals co-deposition provide valuable information for the recycling of neodymium-iron-boron magnets.