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Dynamic Modeling for the Separation of Rare Earth Elements Using Solvent Extraction: Predicting Separation Performance Using Laboratory Equilibrium Data Kevin L Lyon, Vivek P. Utgikar, and Mitchell R. Greenhalgh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04009 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 7, 2017

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Industrial & Engineering Chemistry Research

Title Dynamic Modeling for the Separation of Rare Earth Elements Using Solvent Extraction: Predicting Separation Performance Using Laboratory Equilibrium Data Abstract Industrial rare earth element (REE) separation facilities utilize acidic cation exchange ligands such as 2ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A) for solvent extraction processes. REE separations are costly and difficult due to their chemical similarities and subsequent low separation factors. Several empirical correlations are available in the literature to predict steady state extraction equilibria for various solvent systems. However, complete solvent extraction flowsheet design for REE separations requires complex scrubbing and stripping circuits to separate and produce individual pure species. Furthermore, dynamic modeling of extraction, scrubbing, and stripping in REE separations circuits will aid in process design, optimization, and management of process fluctuations. A dynamic MATLAB/SIMULINK REE equiibrium model has been coupled with dynamic acid balances to predict REE solvent extraction processes using laboratory equilibrium data. The model was used to predict a flowsheet that produced high purity neodymium from a 25 wt% praseodymium and 75 wt% neodymium feed. Laboratory mixer-settlers were used to verify and validate model performance. Results indicated that the model reasonably predicts the dynamic behavior of a counter-current REE separation process, and accurately predicts the steady state REE concentration profiles across the cascade.

Transient

concentration predictions exhibit more deviation from experimental results due to the initial assumption of a homogeneous, well-mixed stage. The model was revised to account for variations in mixer-settler holdup volumes for future validation efforts. Current model limitations assume complete equilibrium is achieved in each stage. The model can be applied to any REE separation or solvent system provided adequate laboratory equilibrium data are available. Authors

ACS Paragon Plus Environment


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Kevin L. Lyona, Vivek P. Utgikarb, Mitchell R. Greenhalgha a

Idaho National Laboratory, Idaho Falls, ID, USA

b

Department of Chemical and Materials Engineering, University of Idaho, Moscow, ID, USA

1. Introduction Conventional separation of rare earth elements (REE) is accomplished using solvent extraction (SX). Separation and purification of individual REE requires complex SX circuits due to the chemical similarities of the REE and consequent low selectivity between adjacent REE. Traditional SX circuits selectively transfer trivalent REE cations from an aqueous phase to an organic phase containing a cation exchange ligand. The chemistry of cation exchange ligands for REE SX has been thoroughly investigated and is well-published; equilibrium behavior is highly dependent on REE concentrations and aqueous phase pH.1-5 Although the chemistry is well understood, predicting partitioning behavior throughout a counter-current SX circuit with complex extraction, scrubbing and stripping cascades remains a significant challenge. Many SX separation models only predict the steady-state concentration profiles in the organic and aqueous phases across the cascade.4,6,7 Furthermore, REE SX models utilize empirical correlations to predict extraction behavior; but do not consider more complex multicomponent scrubbing equilibrium in REE separations, particularly for adjacent REE separations.2,7-9 Scrubbing equilibria using cation exchange ligands becomes complex due to acid-metal exchange,

metal-metal exchange and

equilibrium dependencies on REE concentrations, acid concentrations, and ligand concentrations. Transient SX behavior is expected to play a significant role for troubleshooting process upsets and fluctuations from upstream ore processing as well as managing startup and the approach to steady state due to impacts on process throughput, product purities, and production schedules. Dynamic SX models have been investigated for copper SX processes19,20 and governing material balances for the approach to steady state have been developed for generic SX processes that exhibit constant partitioning behavior.10 Wichterlová and Rod developed a discretized pulse-flow dynamic model for rare earth separations to


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model the extraction of praseodymium and neodymium and the separation of cerium and praseodymium.17 However, Wichterlová and Rod’s studies were conducted using bis-(2-ethylhexyl) phosphoric acid (DHEPA) in nitric acid media; cascade arrangement was determined experimentally for a specific set of operating conditions in mixer-settlers. It would be advantageous to develop a dynamic model that can easily be combined with laboratory or literature equilibrium data to develop and optimize REE separation flowsheets and troubleshoot process fluctuations and startup conditions. The objective of this work was to develop a dynamic acid balance for acidic cation exchange ligands and incorporate it into a dynamic stage-wise REE model. This model can be coupled with equilibrium data or correlations to predict cascade separation performance for extraction, scrubbing, and stripping. The other objective of this work is to validate the dynamic model for the separation of REE using the PC88A/HCl solvent system. 2. Modeling The extraction of a trivalent REE from aqueous to organic phase mediated by an acidic cation exchange ligand (i.e. phosphonic acid) can be represented by the following equation:

REE3+ + 3HA 2 org ↔ REEHA2 3 org + 3H+ aq

(Eq. 1)

The term A represents the deprotonated phosphonic acid molecule. The extent to which the metal ion is extracted is expressed using the distribution coefficient: =

2 3 3+

(Eq. 2)

The distribution coefficient relates equilibrium metal concentrations in the organic and aqueous phases and is frequently used to evaluate the extent of extraction. The ratio of the distribution coefficients for two extracting species i and j is defined as the separation factor between the two species: &

"#/% = & '

(


(Eq. 3)


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The separation factor is frequently used as a measure of the relative selectivity the ligand has extracting one species over another. The equilibrium constant for a single equilibrium stage may be expressed as:

)*+ =

,--./0 1 234 . 5 67

1

,-- 15 67 ./ 0 234 1

=

. 5 67

1

./ 0 234 1

(Eq. 4)

Equilibrium parameters can be incorporated into a system model to determine partitioning behavior for a single solvent extraction contact. Consider the single solvent extraction stage N shown in Figure 1:

Stage N

Figure 1: Representation of a single solvent extraction equilibrium stage. REE concentrations are generically represented as C for clarity; overbars represent organic phase and no overbar represents aqueous phase. VO and VAq represent the organic and aqueous phase holdup volumes. The volumetric flow rates into stage N are expressed as νO and νAq. Generically referring to REE concentrations in the organic and aqueous phases as variable C, it has been shown that the overall single-stage material balance for stage N may be expressed as follows:10 89: ;< =9>7 ;<

8?

= @A BCDE + @/+ BC=E F @A BC F @/+ BC

(Eq. 5)

Making the following substitutions allows each phase to be represented by an ordinary differential equation (ODE): ;

= ;<
7 G>7

G: G>7

=H

(Eq. 7)

+1=J

(Eq. 8)

The ODEs represent the equilibrium species concentrations leaving stage N in the organic phase and aqueous phase are shown in Equations 9 and 10, respectively.10 8;< 8?

=

8;< 8?

: &;

;99% purity Nd; the flowsheet’s sole purpose was for model comparison and validation. The number of stages was selected to perform extraction, scrubbing, stripping, and recycling of the organic phase in a simple laboratory experiment to validate model predictions. A schematic of the flowsheet is shown in Figure 3.



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Figure 3: Preliminary flowsheet test for model validation. As previously mentioned, flow was initially established in the mixer-settlers using barren organic solvent and aqueous feeds (i.e. no REE present). The barren aqueous feeds were the same acid concentration as the actual REE-bearing feeds. Therefore, initial conditions in the model included no REE and initial acid concentrations of the respective feed for each section of the flowsheet. At time t = 0, the REE-bearing feed solutions were introduced to the mixer-settlers. The flowsheet was operated continuously for 12 hours to allow sufficient time to achieve steady state across the cascade. Samples were taken periodically throughout the test to monitor the approach to steady state. Stage 6 was chosen as a representative stage to compare model predictions to experimental data. Stage 6 aqueous and organic phase metal profiles as a function of time are reported in Figures 4-7.


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Figure 4: Stage 6 aqueous phase Nd concentration as a function of time.

Figure 5: Stage 6 aqueous phase Pr concentration as a function of time.



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Figure 6: Stage 6 organic phase Nd concentration as a function of time.


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Figure 7: Stage 6 organic phase Pr concentration as a function of time. Qualitatively, the model predictions mimic the behavior observed in the mixer-settler test. However, early time samples indicate that the model estimates concentrations up to 25% low for Nd, as observed in Figure 4. The discrepancies between the model and the laboratory data reduce to within ±10% as the system approaches steady state. Organic and aqueous steady state phase samples were collected for each stage after shutting down the mixer-settlers. The equilibrium aqueous phase and organic phase REE concentration profiles are compared to modeling predictions in Figures 8 and 9, respectively.



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Figure 8: Steady state laboratory data and model predictions for aqueous phase metal profiles.

Figure 9: Steady state laboratory data and model predictions for organic phase metal profiles.


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The model had much better agreement for the final steady state concentration profile across the cascade. The loaded organic leaving the extraction section contained 83% Nd and 17% Pr; this indicates very little selectivity during extraction of Nd and Pr when compared to the aqueous feed composition of 79% Nd and 21% Pr. Figures 8 and 9 indicate that the actual separation occurs in the scrub section. The NdCl3 scrub feed solution provides a source of metal for ion exchange, with Nd replacing Pr on the organic phase according to Equation 14.1 NdZ= + PrHA_ Z `ab ↔ NdHA_ Z `ab + Pr Z=

(Eq. 14)

The loaded organic phase becomes increasingly enriched in Nd while the Pr is transferred to the aqueous phase in the scrub section. As previously mentioned, the simple 10-stage flowsheet did not contain an adequate number of scrub stages to produce greater than 99% purity Nd. However, the dynamic model accurately predicted the final steady state product compositions in the circuit. The model was utilized to predict the number of scrub stages that would be required to produce 99% purity Nd. Predictions suggested that 12 scrub stages would produce the target purity, so another flowsheet test was performed to validate model output. Laboratory results for the two scrubbing configurations are summarized in Table 2. Table 2: Stripped product composition using 4 scrub stages vs. 12 scrub stages. Increasing the number of scrub stages increased the final Nd purity, obtaining greater than 99% Nd in 12 stages. 4 Scrub Stages

12 Scrub Stages

Species

Model

Flowsheet

Model

Flowsheet

Pr

2.8%

3.3%

0.96%

0.3%

Nd

97.2%

96.7%

99.04%

99.7%

5. Discussion



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The dynamic model predicted REE distribution behavior during the approach to steady state, but predicted the organic phase REE concentrations significantly lower in all stages within the first five hours of operation. This is most likely inherent to the physical system that the model is based upon. The governing material balance assumes ideal, homogeneous, and immiscible phases for the stage material balance with 100% stage efficiency.10 This assumption may be appropriate for the emulsion leaving the mixing chamber of a single mixer-settler, but may not accurately represent the phase compositions leaving the weir system of the settler during the approach to steady state. During laboratory testing, flow in the mixer-settler bank was initially established with REE-free organic and aqueous solutions. Upon introduction of the REE-bearing feed solutions, the emulsion exiting the mixer flows into the settling chamber and dilutes the REE due to the large volumes of organic and aqueous in the settling chamber. Samples in this study were collected directly at the weir outlets of the mixer-settler units, and consequently did not represent the model predictions at early time steps due to the assumption of homogeneous phase compositions in the model. This could potentially be improved by modeling the mixer-settler as a decoupled mixer and settler, i.e. a stirred tank with mass transfer coupled to a dilution tank with no reaction. Fluid flow patterns for solvent extraction in mixer-settlers have been characterized in literature using computational fluid dynamics.15 Several solvent extraction models in the literature treat the mixer and settler as two separate volumes to improve the accuracy of transient phase compositions.1720

Mass transfer is assumed to only take place in the mixer, and the organic and aqueous phases in the

settling chamber are each modeled as independent well-mixed dilution models with no reaction.18 Introduction of this concept to the single-stage equilibrium model presented in Equations 9-11 is represented in Figure 10.


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Figure 10: Single stage mixer-settler model. The mixer is treated as a homogeneous mixed volume with complete equilibrium phase transfer. Each phase in the settling chamber is treated as a wellmixed tank with no reaction. REE and acid concentrations are modeled in the mixer using Equations 9-11, and Equations 15-17 represent the time-dependent material balances for organic REE concentration, aqueous REE concentration, and aqueous acid concentration in the settling chamber, respectively. 8;< 8?

=c

8;< 8?

=

8. 5 < 8?

=c

E 234

OBC F BC2de Q

E OBC c>7

E >7

(Eq. 15)

F BC2de Q

(Eq. 16)

O = C F = C2de Q

(Eq. 17)

Equations 15-17 assume that each phase is a separate well-mixed volume with no further reaction taking place. The assumption of a well-mixed volume does not represent reality because the flow rates are small compared to the large holdup volume, but this assumption provides a simple approach that improves model predictions without introducing complex time and space dependent computational fluid dynamic



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models.

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The residence time of the organic and aqueous phases are determined by the settler holdup

volume and flow rate of each phase as shown in Equations 18 and 19. fgUh = f/+ =

9234,jkeelk3 G234

9>7,jkeelk3 G>7

(Eq. 18)

(Eq. 19)

The position of the interface in the settling chamber dictates the holdup volume of each phase. If a mixersettler stage does not have equal phase volumes, the residence time will change and ultimately impact the observed concentration at the outlet weir of the mixer-settler. The new mixer-settler dynamic model was used to compare predicted organic outlet compositions for a theoretical single stage extraction with constant flow rates and distribution ratios. Scenarios were modeled for various settling chamber organic phase holdup volumes with the remaining volume balance being aqueous phase. Figure 11 illustrates the mixer-settler dynamic model predictions as compared to the original model that assumed a single homogeneous stage with equal phase volumes.


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Figure 11: Single stage organic outlet concentration model predictions for varying organic holdup volumes. Modeling the mixer and settler independently has minimal impact when the organic holdup volume remains at 50%, but the transient behavior deviates from the original model by an average of 22% for different holdup organic holdup volumes.

The organic/aqueous interfaces were not measured and

recorded during Pr/Nd flowsheet testing, but the majority of the stages did not contain an equal ratio of holdup volume in the settling chamber due to differences in weir positions. This suggests that initial transient model predictions were low due to improper modeling of the residence time of each phase in the settling section of the mixer-settlers. The final steady state concentrations across the cascade were predicted accurately during flowsheet testing. Based upon fluid flow in the mixer-settler, it is much more reasonable to assume homogeneous phases once the system is operating at steady state because the weir outlet compositions for that stage represent the phase compositions in the emulsion leaving the mixing chamber.

Equilibrium laboratory

data were utilized in the dynamic model, so the final predicted compositions provide reasonable



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agreement with the laboratory data. This suggests high stage efficiency and adequate residence times for phase transfer were achieved in the mixer-settler units. The dynamic model presented specifically applied to the PC88A/HCl solvent system for the separation of Nd from a mixed Pr/Nd feed solution. However, this modeling framework can easily be applied to any group REE or adjacent REE separation simply by incorporating the appropriate laboratory data or correlation from the literature. The model can also be applied to alternative cation exchange solvent systems such as Cyanex ® 572 or DHEPA. Furthermore, the acid material balance can be deselected and allow the model to function for other solvating neutral ligand systems. Application of the model in this mode is equivalent to other solvent extraction modeling software discussed in literature.16

The

SIMULINK platform provides the user with a high degree of flexibility for flowsheet design and optimization. 6. Conclusions The capability to predict transient equilibria for REE separations will aid in the design and troubleshooting of counter-current solvent extraction processes. A dynamic model has been developed based upon literature studies and has been expanded to include acid balances for cation exchange ligands such as PC88A. The dynamic model can be coupled with simple laboratory equilibrium data to predict REE distribution behavior in a counter-current solvent extraction cascade to design separations flowsheets for any REE separation or alternative ligand solvent system. Introduction of the time-dependent acid model provides a novel method to couple empirical literature or laboratory correlations to predict stagewise REE partitioning as a function of acid concentration and REE concentration.

Initial model

evaluations coupled with laboratory equilibrium data indicates that the model reasonably predicts partitioning behavior during the approach to steady state for the PC88A/HCl solvent system, and accurately predicts REE concentration profiles at steady state operations. The model was utilized to test a scrubbing concept that produced high purity Nd from a mixed Pr/Nd feed. Deviation was observed


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between model predictions and observed laboratory results during transient operation due to the model’s initial assumption of a homogeneous, well mixed volume containing equal volumes of both phases. The model was modified to account for different holdup volumes and residence times of each phase and was demonstrated to have an impact on predicted transient REE concentrations. Future work will continue validation of the mixer-settler dynamic model. 7. Acknowledgements The authors would like to thank the Cytec Solvay Group for providing PC88A and Molycorp, Inc. for providing didymium. The authors wish to acknowledge the contributions of INL interns Jeffery B. Porter and Justin D. McAlister for their laboratory work supporting this research. The authors would also like to thank R. Scott Herbst (INL) and Amy K. Welty (INL) for their contributions during flowsheet planning and testing. The authors acknowledge the Center for Advanced Energy Studies for conducting the ICPOES analysis for this research. This research is supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office. 8. Nomenclature BCDE = Organic phase REE concentration leaving stage N F 1 and entering stage N, g/L BC = Equilibrium organic phase REE concentration leaving stage N, g/L BCg|? = Equilibrium organic phase REE concentration leaving stage N settler, g/L BC=E = Aqueous phase REE concentration leaving stage N + 1 and entering stage N, g/L BC = Equilibrium aqueous phase REE concentration leaving stage N, g/L BCg|? = Equilibrium aqueous phase REE concentration leaving stage N settler, g/L D = Distribution ratio, BC /BC



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= C=E = Equilibrium aqueous phase acid concentration leaving stage N + 1 , mol/L = C = Equilibrium aqueous phase acid concentration leaving stage N, mol/L RS# = Molecular weight of REE species , g/mol @A = Organic phase flow rate, L/min @/ = Aqueous phase flow rate, L/min A = Organic phase holdup in stage N, L / = Aqueous phase holdup in stage N, L A,#*U = Organic phase holdup in stage N mixer, L /+,#*U = Aqueous phase holdup in stage N mixer, L A,*??*U = Organic phase holdup in stage N settler, L /+,*??*U = Aqueous phase holdup in stage N settler, L βA/B = Separation factor, DA/DB τorg = Organic phase residence time in settler, min τAq =Aqueous phase residence time in settler, min References (1) Xie, F., Zhang, T.A., Dreisinger, D., and Doyle, F., “A Critical Review on Solvent Extraction of Rare Earths from Aqueous Solutions. Minerals Engineering, Vol. 56, 10-28, 2014. (2) Zhang, J., and Zhao, B., “Separation Hydrometallurgy of Rare Earth Elements,” Springer International Publishing, Switzerland, 2016. (3) Gupta, C.K., and Krishnamurthy, N., “Extractive Metallurgy of Rare Earths”, CRC Press, 2005.


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(4) Thakur, N. V., “Separation of Rare Earths by Solvent Extraction”, Mineral Processing and Extractive Metallurgy Review, Volume 21, 277-306, 2000. (5) Chunhua, Y., Jiangtao, J., Chungsheng, L., Sheng, W., and Guangxian, X., “Rare Earth Separation in China,” Tsinghua Science and Technology, Volume 11, Number 2, 241-247, April 2006. (6) Soderstrom, M.D., McCallum, T., Jakovljevic, B., and Quilodrán, A.J., “Solvent Extraction of Rare Earth Elements Using Cyanex® 572,” 2014 Conference of Metallurgists Proceedings, Vancouver B.C., Canada: Canadian Institute of Mining, Metallurgy, and Petroleum, 2014. (7) Ying, W., Huang, W., & Quan, C., “Mathematical Modelling and Simulation of Rare Earth Solvent Extraction for the System of (Gd-Tb)Cl3-HCl-HEHEHP-Shellsol D70,” Beijing General Institute for Non Ferrous Metals, Australian Nuclear Science and Technology Organization, September 1994. (8) Lee, M., Lee, J., Kim, J., and Lee, G., “Solvent Extraction of Neodymium Ions from Hydrochloric Acid Solution Using PC88A and Saponified PC88A,” Separation and Purification Technology, Volume 46, 72-78, 2005. (9) Lee, M., Lee, G., Lee, J., Kim, S., Ahn, J., and Kim, J., “Solvent Extraction of Gd from Chloride Solution with PC88A,” Materials Transactions, Vol. 46, No. 2 (2005) 259-262. (10)

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