Solvent extraction is often used as an intermediate processing stage that receives an aqueous phase containing both REEs and impurities from an upstream process, such as leaching, and provides a concentrated solution of REEs to a downstream process, such as precipitation. It is thus desirable for the process to reject disturbances in the feed from the upstream unit in order to provide a product with consistent purity to the downstream unit. Current industrial-scale solvent extraction processes are generally operated with a feed flowrate and composition that are tightly controlled due to the process’s challenging dynamics: the response of system pH to acid dosage is highly nonlinear, there are multiple recycle streams used for enhancing REE recovery, the system has a long settling time, and it is difficult to measure REE concentrations in real-time. These challenging dynamics make the overall REE production process inadequate for handling changes in feedstock composition.
In this work, we develop a dynamic model for the mixer-settler type of solvent extraction process to study and develop control strategies to make this process more agile. Several stages are modeled with a counter-current flow configuration. This model development work is based on a pilot plant developed at the University of Kentucky1 for the recovery of rare earth elements from fire clay and coal tailings. The solvent extraction process involves two distinct steps. The first step is the extraction of the rare earth metals from the aqueous feed acidic solution to the organic solvent, also known as loading2. The second step is the back extraction stage, in which elements are transferred from the loaded organic solvent into a barren aqueous solvent. The loaded organic solvent is first scrubbed of impurities with a high affinity for the aqueous phase, then the REEs are stripped from the loaded solvent. The dynamic model developed in this work can be applied for both the steps. In literature, there have been various dynamic models developed for solvent extraction where some mass transfer coefficients have been quantified3–5. However, due to lack of time-series experimental data, it is difficult to identify the mass transfer coefficient associated with each REE. In this work, distribution coefficients of each element have been expressed as a function of the aqueous phase pH and the amount of extractant in the organic phase. Model parameters have been estimated by using data from the open literature. Regulatory and supervisory controllers are developed to achieve the desired recovery and concentration of REEs by considering manipulated variables such as the pH and extractant dosage in the organic feed solution as degrees of freedom for both the loading and stripping steps. Various disturbances are simulated by changing the inlet feed flowrate and composition from the upstream leaching unit.
Acknowledgements
This effort was funded by the U.S. Department of Energy’s Process Optimization and Modeling for Minerals Sustainability (PrOMMiS) Initiative, supported by the Office of Fossil Energy and Carbon Management’s Office of Resource Sustainability.
Disclaimer
This project was funded by the United States Department of Energy, National Energy Technology Laboratory an agency of the United States Government, in part, through a support contract. Neither the United States Government nor any agency thereof, nor any of its employees, nor the support contractor, nor any of their employees, makes any warranty, expressor implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof, or any of their contractors.
References
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