Das, and three co-authors reported in 2018 that given increasing demand and importance of rare earth elements (REE), exploration is underway to find alternatives to ore-extracted product. With REE concentrations varying between 270 and 1480 ppm, coal ash has been deemed as one such potential source. A number of research groups are exploring technologies to separate REEs from coal ash and supercritical extraction has emerged as a high yield contender. Estimating the economic viability of this lab-scale process at the industrial scale is both important and challenging.
In this study the authors estimated the industrial scale cost and revenues of production of REEs from coal ash by combining prior laboratory results, scaling models, combinatorial scenarios and sensitivity analysis. The processing cost of extracting REEs from one ton of coal ash using supercritical CO2 and tributylphosphate (TBP) is found to vary between $380 and $1200 for 550 g of REE. The value of REE oxides that may be obtained per ton of ash is estimated to vary between $6 and $557, with a median of $250. Scandium is the most expensive REE and can account for up to 90% of the value of the yield. The results suggest that factors critical to the economic viability of the process include scandium content & yield, reagent use, and processing time.
In recent years there have been studies to establish the technical feasibility of coal ash as a source of REEs (Franus et al., 2015) and assess potential environmental impacts (Mayfield and Lewis, 2013). This is the first study to test economic feasibility. This work serves not only as an estimate of the profitability of the nascent technology, but also rank priorities for future research in this field. Our analysis suggests that supercritical extraction could be an economically viable method to extract rare earth metals from coal ash, but only under certain conditions. These conditions include:
(i) The ash will have to have a high concentration of high value REE. The value of the REE content has to be close to maximum of what is found in coal ash globally. Scandium and neodymium are the two most expensive REEs and they heavily impact the value of the content. In the literature review we found that the ash sample with the highest value of REE came from a plant in Poland. There was however no way to ascertain the origin of the coal used to produce the ash. A preliminary analysis should therefore be done to ascertain of value of the REE content before engaging in this process. Currently, U.S. coal ash has far lower REE content and is therefore unlikely to be suitable for the process.
(ii) The molar ratio of REE oxides to TBP reagent should be close to 1:20. TBP is expensive and increase in TBP use can quickly diminish profitability. The TBP-HNO3 complex is used to dissolve REEs, the level of solvation of various REEs depends on the ratio TBP: HNO3. Empirical work is thus needed to achieve high yields of the most valuable REEs while at the same time managing reagent use, i.e. not exceeding a REE oxide:TBP ratio of 1:20.
(iii) Profitability is heavily dependent on scandium recovery, currently the most expensive REE. Processes must therefore have high scandium yield. Neodymium is the second most expensive REE, at about half the value. As prices change in the future, this will impact economic feasibility. REE ores do not contain some of the more valuable REEs such as scandium, which are currently obtained from other sources like red mud (Wang et al., 2013; Zhou et al., 2008). This rarity lends to their high price. Ores do however contain more of elements like cerium and lanthanum, the output of which far exceeds their current demand. This imbalance between supply and demand and the ratios in ores of individual elements leads to stockpiling of the more abundant elements and further drives down their prices (Binnemans et al., 2018). Since initially it is unlikely that supercritical extraction will occur at a large enough scale to impact REE prices, profitability from the process will be determined by prices set by ore based production.
In addition to being a case-study, this work also contributes to TEA methodology. As is typical with TEA, data availability was a challenge due to lack of prior industrial development of the process of interest, in this case, supercritical extraction on mineral processing. Also, much of the data in the mining industry is proprietary and not easily accessed. In this work we used available data from similar industries and results from laboratory experiments, leveraging both to create scenarios for plausible industrial scale-up. The exercise here, as in all TEA, is beset with uncertainty. While there is no way to dispense the uncertainty, the combinatorial scenarios proposed here is a useful approach to clarify how outcomes depend on combinations of uncertain parameters.
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