(399d) Process Operability Analysis of the Recovery of Rare Earth Elements from Coal Fly Ash

Rare Earth Elements (REEs) have been drawing the attention of government and industry sectors due to their applicability in diverse fields such as green energy, environmental protection technologies, military and defense industries. REEs are a group of seventeen metallic elements composed of scandium, yttrium, and lanthanides, characterized as critical materials due to uncertainties in their supply and prices. REE world resources are divided into primary (e.g., bastnaesite and monazite deposits) and secondary (e.g., coal and acid mine drainage) [1]. According to the United States Department of Energy [2], several investigations have identified coal containing a potentially significant amount of REEs. Additionally, REEs in coal ash and coal mine refuse/rejection, if released, can have significant adverse environmental impacts.

In this work, the modeling of a recovery process of REEs from coal fly ash (CFA) is developed in Aspen Plus^®. This REE feedstock is considered to tackle a recurring issue of the utilization of post-combustion coal residues, acknowledging that this represents a promising alternative source for REE recovery [3]. Carbon mineralization is selected as the studied pathway due to the large availability of the mineral feedstock and its thermodynamically favored process. Moreover, this process represents a promising route for Carbon Capture and Utilization (CCU), converting captured CO₂ into carbonates. Among the several branches of carbon mineralization, this work considers the two-step carbonation process with a dissolution stage. In this process, the leaching section enables the release of metal ions by the acid, and the carbonation section occurs with the addition of CO₂ and the adjustment of pH with a base solution [4].

Systems analysis studies are performed to identify bounds for the desired region of process operation considering physical and chemical constraints. In particular, process operability analysis is carried out to assess process feasibility in early design stages [5]. The multimodel formulation adopted results in multiple linearized models that represent the process input-output mapping as a set of connected polytopes. This process mapping enables the subsequent evaluation of the inverse model to map the desired output region into feasible operational inputs. Furthermore, the Operability Index (OI) is evaluated to rank the designs given the operational inputs. The comparison for each scenario will be discussed considering the rescue of REEs, acid and base solutions usage, and the production of carbonates.

References

[1] Talan, D., & Huang, Q. (2022). A review of environmental aspect of rare earth element extraction processes and solution purification techniques. Minerals Engineering (Vol. 179). Elsevier Ltd. https://doi.org/10.1016/j.mineng.2022.107430

[2] United States Department of Energy. (2017). Report on Rare Earth Elements from Coal and Coal Byproducts.

[3] Huang, Z., Fan, M., & Tian, H. (2020). Rare earth elements of fly ash from Wyoming’s Powder River Basin coal. Journal of Rare Earths, 38(2), 219–226. https://doi.org/10.1016/j.jre.2019.05.004

[4] Zhang, N., Chai, Y. E., Santos, R. M., & Å iller, L. (2020). Advances in process development of aqueous CO2 mineralisation towards scalability. Journal of Environmental Chemical Engineering (Vol. 8, Issue 6). Elsevier Ltd. https://doi.org/10.1016/j.jece.2020.104453

[5] Gazzaneo, V., Carrasco, J. C., Vinson, D. R., & Lima, F. V. (2020). Process Operability Algorithms: Past, Present, and Future Developments. Industrial and Engineering Chemistry Research, 59(6), 2457–2470. https://doi.org/10.1021/acs.iecr.9b05181