(744b) Process Design and Economics of a Novel Process for the Extraction of Rare Earth Elements from Coal Ash

Rare earth elements are used extensively in military
hardware, high-end aircraft equipment, refinery catalysts and the clean energy
industry. However, the rare earth mineral resources in the U.S. are limited,
with 95% of their supply coming from foreign imports.^[1] In this
context, coal ash is an abundant waste material from coal power plants with
high content in rare earth elements (REE), particularly in heavier REE (e.g.,
yttrium, dysprosium, and others) important in high performance magnets, flat
panel displays, compact fluorescent lighting, LEDs, and lasers. Coal ash can be
a promising, low-cost resource of heavy and light rare earths.^[2] Moreover,
coal ash contains other valuable components such as carbon and magnetite, and
is widely utilized as cement substitute in concrete and construction materials.^[3]
Therefore, it is useful but also challenging to design a chemical plant to process
coal ash for recovering the REE, as well as other valuable byproducts.

Figure 1:
Simplified diagram of physical and chemical enrichment of REE from coal ash.

Our process for REE production from coal ash involves two
main stages: a physical enrichment step and a chemical enrichment/extraction step.
The process comprises several key steps, shown in Figure 1, including screening,
flotation, magnetic separation, size separation, aqueous washing, nitric acid
digestion, and solvent extraction. Batch laboratory-scale tests of each of
these steps, as well as continuous flow test for key steps, were first carried
out. Then, a steady state model of a continuous ash processing plant was developed
in ASPEN Plus. The model was used for the simulation of the production capacity
and efficiency of REE extraction, as well as the estimation of the streams of coproducts
produced, such as carbon, magnetite, and the ashcake as cement substitute. The model
was first matched with the laboratory results and then scaled up to a high
throughput plant. Subsequently, economic analysis was performed to examine the –commercial
feasibility of the process and identify the most cost-sensitive and challenging
steps that need to be researched further. The analysis showed that the process is
profitable with sufficient recycling of the acid and organic solutions. Several
case studies were explored for the process economics, by varying the price of
the REE concentrate and the cost of recycling the reagents to assess the product/coproduct
pricing and profitability space.

Acknowledgment

This work was supported by the Office of the Secretary of
Defense (OSD) with Dr. David Shifler of the Office of Naval Research (ONR) as
the Program Manager under Contract N00014-15-C-0039.

References

[1] Critical Materials Strategy, U.S. Department of Energy,
December 2010 and December 2011.

[2] Seredin, V. V.; Dai, S. Coal deposits as potential
alternative sources for lanthanides and yttrium. International Journal of
Coal Geology, 2012, 94, 67.

[3] Blissett, R.S.; Rowson, N.A. A review of the
multi-component utilisation of coal fly ash. Fuel, 2012, 97, 1.