Abstract
Rare earth elements (REEs) include the lanthanide series, yttrium, and scandium, described as the cornerstone of modern technologies. Due to their unique properties, they are in demand in developing the latest technologies, leading to exploration and research of alternative sources to meet the demand. Coal combustion by-products such as coal ash have been identified as potential alternative sources, with world coal ashes having an average REE concentration of 445 parts per million (ppm). However, coal ash as an alternative source for critical materials such as REEs has not been fully exploited, and there is a need to assess potential ways to extract REEs from coal ash.
This study aimed to extract REEs from the power station and laboratory-derived coal ash. Samples were characterised using X-Ray Fluorescence, X-Ray Diffraction, Inductively Coupled Plasma-Mass Spectrometry, and Mineral Liberation Analyser (MLA). The extraction technique was carried out in two phases. Phase 1 was conducted on power station ash to optimise the extraction process. The pre-treatment methods include alkali leaching with sodium hydroxide (NaOH) solution and thermal roasting with sodium carbonate additive. The parameters evaluated were NaOH and hydrochloric (HCl) acid concentration, reaction temperature and time, solid-to-liquid ratio, and roasting temperature and time. Phase 2 was conducted on laboratory-derived coal ashes following the optimised conditions developed in Phase 1.
The samples were characterised by high ash, low moisture, low volatility, and low fixed carbon. The most abundant oxides determined in all the samples were silica, aluminium oxides, calcium oxide, and ferric oxide. The mineral compositions of the ash samples commonly consisted of quartz, mullite, amorphous materials, kaolinite, hematite, and magnetite. The total concentration of REEs ranged from 542 to 997 ppm in the ash samples. The ash samples had high concentrations of critical REEs (203-406 ppm) compared to excessive REEs (179-348 ppm) and uncritical REEs (129-256 ppm). MLA imaging determined four rare earth-bearing mineral phases: monazite-REE, xenotime-REE, perrierite, and cerium-oxide grouped as REE-cerium and REE-silicate.
The optimum conditions in the alkali leaching method were 3 Molar (M) NaOH, 80 ⁰C reaction temperature, 180 minutes (min) reaction time, and 1:12 solid-to-liquid (S/L) ratio. The overall leaching efficiency was improved from 66% to 79% when the effects of NaOH solution conditions were evaluated. After alkali pre-treat, the optimum HCl leaching conditions were 3 M at 70 ⁰C for 120 min at a 1:10 S/L ratio. The overall leaching efficiency was improved from 76% to 79% when the effects of HCl leaching conditions were tested. The optimum conditions in the thermal roasting method were 851 ⁰C for 30 min roasting temperature and time. After
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roasting, the optimum HCl leaching conditions were 3 M HCl at 70 ⁰C for 120 min at 1:20 S/L ratio. When the effects of HCl leaching conditions were evaluated, the overall leaching efficiency enhanced from 82% to 91%.
Numerous observations were made in the optimisation process for REEs extraction. The alkali leaching method favoured CFA-K and SML-20, while UNI2-M was relatively challenging. Excessive REEs were more abundant than critical and uncritical REEs in all the samples except for SML-20. SML-20 had the highest concentration levels of critical REEs, followed by excessive then uncritical REEs. The leaching efficiency varied by ash type within this method. The thermal roasting process favoured CFA-K and CFA-M, while SML-20 was relatively challenging to leach. The approach favoured the power station-generated ash samples over laboratory-derived ash samples. Critical REEs were more abundant than excessive and uncritical REEs in all the samples except for SARM 18 ash. The optimised extraction process was deemed viable for SML-20 and indicates this sample has potential as an alternative source for REEs due to the high leaching efficiency and high values for critical REEs.