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Article

Accumulation of Particles in an Annular Centrifugal Contactor Cascade and the Effect upon the Extraction of Nitric Acid

by
Alastair Baker
1,*,
Alex Fells
1,
Natalia Domenech-Garcia
1,
Chris J. Maher
2 and
Bruce C. Hanson
1
1
School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, UK
2
National Nuclear Laboratory, Central Laboratory, Seascale CA20 1PG, UK
*
Author to whom correspondence should be addressed.
Separations 2024, 11(6), 163; https://doi.org/10.3390/separations11060163
Submission received: 9 April 2024 / Revised: 8 May 2024 / Accepted: 10 May 2024 / Published: 23 May 2024
(This article belongs to the Section Separation Engineering)
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Abstract

:
Centrifugal contactors (CCs) are a technology candidate for the development of advanced reprocessing flowsheets. While they offer many advantages, such as process intensification, there are still uncertainties regarding their industrial deployment. The presence of particles in the process streams in particular may present a challenge to both performance and operability. Preliminary studies have been undertaken to evaluate the accumulation of particles in the contactors and the effect upon the extraction behaviour of nitric acid. Aluminium oxide (Al2O3) particles were suspended in the aqueous feed solution during the operation of a three-stage, 40 mm diameter CC cascade. The presence of insoluble solid particles in the aqueous feed, up to 7 g/L, were not observed to affect phase separation and entrainment under the experimental conditions investigated. The particles were centrifuged out of solution and accumulated as a thin cake/bed in the rotors of each stage. This work also illustrates that particles do entrain through the cascade. The predominant effect on the rate of accumulation was particle concentration in the aqueous feed solution, and increasing solids loading was observed to have an impact upon the extraction of nitric acid across the cascade.

Graphical Abstract

1. Introduction

A centrifugal contactor (CC), also known as an annular centrifugal contactor (ACC) or annular centrifugal extractor (ACE), is a process intensification apparatus used in counter current liquid–liquid separation [1]. Centrifugal contactors consist of a vertical centrifuge rotor (also referred to as a bowl) inside a static housing, providing both the mixing (between the rotor and static housing, also described as the annulus) and the separating of liquids (within the centrifuge rotor) in a single unit. Clear diagrams of CCs have been produced by Hamamah and Grutzner [2,3].
Several reviews provide complete descriptions of operations [3,4,5,6,7], design principles [8,9,10], and deployment in nuclear research [11,12,13]. Centrifugal contactors are also deployed in pharmaceutical industries [3] such as Lonza AG in Switzerland [14], biodiesel production [15,16], extraction of high-value biophenols from vegetable oils [17], production of cinnamyl cinnamate [18], extraction of the antibiotic spiramycin [19], extraction of the hormone hydrocortisone [20], enantioselective separation processes [21,22,23,24,25], extraction of caffeine [26], water treatment [27,28,29,30,31], and hydrometallurgy [32]/extraction of rare earth elements [33,34,35].
Notable industrial plant [35] implementation has been reported in France, where multistage CCs [1], each holding eight stages internally, were used to extract plutonium at a mass rate of 8 kg/day [36]. In addition, Baron and colleagues described the industrial application of five CCs in plutonium product purification in the UP2 recycling plant at La Hague [37,38,39], which were reported to have ‘remarkable reliability’ over 6 years. It should be noted that the contactors were deployed in sections of flowsheets with minimal entrained solids and low risk of solid forming, i.e., not the primary extract/scrub highly active (HA) feeds that are more likely to contain solids or generate solids during operations/experiments.
In the UK, the Plutonium Uranium Reduction EXtraction (PUREX) process has been implemented, using aqueous separation and solvent extraction to separate U and Pu from the unwanted fission products that arise in used nuclear fuel, also known as spent nuclear fuel [40]. The ligand and solvent used in the initial highly active (HA) extraction is tributyl phosphate in odourless kerosene (TBP/OK); this will be used in the chemical system evaluated in this paper.
In the UK, used nuclear fuel has been reprocessed by shearing the fuel assembly into a dissolver, after which the fuel is dissolved in ~8 M nitric acid in a batch process. Undissolved cladding hulls are removed in the dissolver basket, and cladding fragments are removed by sedimentation. The dissolver product solution is sent to clarification using a continuous centrifuge to remove any fine suspended particles. These are either insoluble fission products [41], or for Mixed Oxide (MOx), are likely to contain plutonium-rich oxide particles [42,43,44,45,46]. The highly active feed solution compositions vary as a result of the fuel fabrication, the burnup, the radiation history within a reactor, and the conditions used to dissolve the fuel [43].
CCs are the candidate equipment to be used in future aqueous separation and the selective partitioning of actinides in used nuclear fuel. CC technology is known to be sensitive to suspended particles in the feed solutions; these particles accumulate on the walls of a centrifugal rotor, forming a cake [47], which has been also referred to as a bed [1,48]. One key uncertainty regarding the suitability of centrifugal contactors (CCs) in future reprocessing plants is their tolerance for solids. This study aims to understand the behaviour of solids in CCs.
Very fine insoluble particulates (typically less than 10 um diameter [49]) in the HA feed may not be removed by the existing clarification process; these particulates may pass into the CCs and could affect a contactor’s performance. In situ generation of particles is also a concern for CC operability; this has been reported as arising due to precipitated solids in CC demonstrating different flowsheet chemistries and compositions [4,50,51,52,53,54,55,56,57,58,59,60,61,62,63], which are summarised in a 2022 review by Baker and colleagues [1]. Also known is the precipitation of metals with dibutyl phosphate (resulting from TBP degradation) and diluent degradation products [64,65,66,67,68]. Zirconium dibutyl phosphate and other metal complexes are often implicated.
The application of centrifugal contactors in the nuclear fuel cycle was recently consolidated in a review in Chem. Soc. Rev. [1]; the flowsheets demonstrated are PUREX [69,70], Advanced PUREX [71,72,73,74], URanium EXtraction [75], Next Extraction system for Transuranium Recovery [76,77], TRansUranium EXtraction [78,79], DIAMide EXtraction [80,81], Selective ActiNide EXtraction [53,54,56,57,58,59,82,83,84,85,86,87], UNiversal Extraction [88,89,90,91,92], StRontium EXtraction [60,93], and others [1,94]. The feeds used in these flowsheets are genuine nuclear fuel solutions or simulated compositions, several of which generate solids in situ [1,4,51,55,56,57,58,59,60,61,64,86]. The focus of a subsequent review within our research group is these generated solids. In the recent literature, there are also studies investigating the effect of solids in solvent extraction using single CCs [48,95,96,97,98]; there are no studies on a cascade of two or more CCs in which internal CC geometries may effect entrainment into subsequent CCs.
This study examines two elements: first, how suspended solids accumulate in sequential CCs in a cascade, which is currently unknown, and second, the effect on solvent extraction across a cascade under conditions relevant to the industrial PUREX process. An aqueous feed consisting of 1 M nitric acid (HNO3) and an organic feed of 30 vol% TBP/OK were examined in a cascade of three stages of 40 mm rotor diameter CCs. The nitric acid feed containing suspended solid aluminium oxide (Al2O3) was used as a simulant due to its commercial availability in two sizes in the appropriate range and its density (3.99 g/cm3), being a suitable substitute for molybdenum zirconyl hydrate (4.00 g/cm3), which is known to be a particle precipitate formed during dissolution [48].

2. Materials and Methods

2.1. Materials and Chemicals

Tributyl phosphate (TBP) (VWR, (≥99.0% pure)) was diluted with Exxsol D-80 (a refined kerosene fraction purchased from Brenntag, Essen, Germany) to obtain the desired solvent composition of 30 vol% (1.15M) TBP/OK. This was used for the contactor studies without any further pre-treatment. A total of 1 M nitric acid was prepared by diluting 65% nitric acid (VWR, min 65% AnalaR NORMAPUR analytical reagent). Two samples of Al2O3 of different size were used to evaluate the accumulation of particles in the contactor and the impact upon nitric acid extraction. The Al2O3 mean particle size distributions (PSDs) were determined using a scanning electron microscope (SEM) Hitachi TM3030 and are reported in the Supporting Information. Sample A had a mean PSD of 0.3 μm and Sample B had a mean PSD of 10.3 μm. The nature of the Al2O3 supplies was determined to be neutral, and the titration method is listed in the Supporting Information. No dispersing agents were used, as it should be considered that when Al2O3 is in an aqueous solution, agglomeration and coagulation of particles may occur.
A preliminary experiment was performed to evaluate the suspended particle behaviour in the rig and define the experimental envelope. A particle concentration of 12.5 g/L of Sample B Al2O3 powder was found to be too high for this rig’s arrangement to operate, and the experiment was stopped after 13 min; the precise reason was not recorded on the process operating instructions. As a result, lower particle concentrations were used in the subsequent experiments. We recommend that future studies investigate the sedimentation time of particles in the aqueous phase before and after contact with the solvent. This will help determine the stability of suspensions containing particles of various sizes.
Aqueous sample acidity was determined by autotitration using a potentiometric end-point determination. This involved sodium hydroxide (0.1 M) volumetric standard solution (Scientific Laboratory Supplies, Nottingham, UK) and a Mettler Toledo G10S Compact Titrator to perform the titrations.

2.2. Description of Rig

The experimental rig consisted of two 50 L feed vessels that were fed a counter-current by two diaphragm pumps (Omega PHP-803-ESC-240, Biel/Bienne, Switzerland) into a cascade of three CCs of 40 mm diameter, supplied by Rousselet Robatel, Annonay, France (BXP 040). The solvent and aqueous products were collected into two 50 L receiver vessels. The pipework consisted of stainless steel, with the exception of the pump tube connections being flexible reinforced PVC. Figure 1 shows a simplified diagram; the complete process and instrumentation diagram can be found in the Supporting Information (Figure S7).

2.3. Operation of the Rig

Below is a brief outline of the experimental ‘once-through’ method employed; the experimental matrix followed and a complete copy of the process instructions can be found in the Supporting Information.
  • The organic feed tank was charged with 30 % TBP/OK.
  • The aqueous feed tank was charged with 1 M nitric acid, which was agitated with an overhead mixer set at 15 Hz. Solid Al2O3 particles were added to the agitated aqueous feed tank via a funnel to obtain the particle suspension.
  • The CC rotors in the cascade were set at 2000 rpm.
  • Then, the suspended particle aqueous feed was pumped at 10 L/h through the cascade. Upon elution via the aqueous outlet, the organic feed began pumping at 10 L/h through the cascade to achieve a solvent–aqueous ratio of 1:1.
  • Aqueous samples were taken at 20 min intervals from sampling ports between each stage, shown in Figure 1. The sample acidity was determined by the titration stated in the materials and chemicals section.
  • Both the pumps and the CC rotors were stopped immediately at the end of the experiment. The experiment was stopped after a specified time or when the agitator approached a state of being no longer submersed in the aqueous feed solution.
  • The contactors were emptied and flushed to determine the quantity of particles held up in each stage. It was necessary to dismantle and flush particles from the separator rotor to recover the majority of the particles. The particle masses from each contactor were collected using the following process:
    • The contactors were drained via their bottom valves.
    • The contactors were then flushed with 100 mL of water via the vent, further detailed in the Supporting Information.
    • The contactors were dismantled, first by removing the motor, then by removing the rotor assembly, then by removing the centrifuge rotor from rotor assembly. The centrifuge rotor was opened, and the particle cakes that were formed were washed out and the particles were collected.
    • The mixing zone, without the contactor inserted, was flushed, and particles were collected. Any particles collected were allowed to settle, the supernatant was decanted, the particles were dried overnight in an oven at 85 °C, and the mass was measured.

2.4. Experimental Matrix

Table 1 shows the planned experimental matrix, which shows the two particle samples at three different feed concentrations.

3. Results and Discussion

The primary objective of the experiments in this paper was to determine if and how solids accumulate in CCs connected as a cascade under conditions relevant to the PUREX process. Upon the termination of the experiments, in which both pumps and all rotors were stopped, the solid particles settled in the bottom of the contactors in the absence of turbulent flows. No visible particles were observed in the receiver tank upon emptying the waste and cleaning. The observed suspended particle concentration was calculated by subtracting the mass particles found in the feed tank or settled large diameter pipework leading to the feed pump from the initial solids loading.
Figure 2 shows the solids that settled in the mixing zone of each contactor, with the largest quantity found in the first contactor (C1), which received the initial particle-laden flow from the aqueous feed pump. The second contactor shows a reduced quantity, and the third shows only trace quantities.
The contactors were drained and flushed and then dismantled. The visible observation of cake formation in the dismantled centrifuge rotor is shown in Figure 3. Figure 3a shows the phase disengagement section, with eight holes leading to the weir; half of these orifices appear to have been obscured by the cake that has been formed. Figure 3. shows the centrifuge rotor with the central shaft and vanes in place. Upon removal of the central shaft in Figure 3c, the formed cake can be clearly seen. The mixing zone (bird’s eye view, Figure 3d) shows the eight vanes after cleanout and washing; also visible are the organic and aqueous inlets.
Forces on the liquids and particles vary in different sections of a centrifugal contactor; this has resulted in different masses of particles being deposited in the four cleanout steps. Table 2 shows the recorded masses of Al2O3 particles, within each contactor, after each cleaning procedure described in the experimental section, to allow comparisons between each procedure. In the outer annulus mixing region, the solutions are exposed to high shear from the rotating rotor and high turbulence from the baffles in the mixing zone, which should maintain particle suspensions. The first cleanout procedure involved opening the valve at the bottom of each contactor and draining the liquids that were present at the end of the experiments; however, this did not yield the mass of solids that was expected. Due to the greater G-force in the rotor, particles adhere to the walls and form a cake. In this work, a flush procedure was implemented to test whether these particle cakes could be re-suspended and drained from the rotor. The procedure involved injecting water via a syringe with a needle through the vents. This procedure had a small effect but was insufficient for the re-suspension of particles, and the centrifuge required dismantling to access the rotor. The rotor was removed from the assembly, and the particle cakes were washed out into a separate container; the mixing zone (MZ) was flushed with water to remove solids still residing in the contactor housing.
A comparison of the different cleanout procedures is shown in Figure 4 and Table 3. The overall trend shows that dismantling the contactor yielded the majority of solids in all cases, irrespective of particle loading or particle sample. In the experiment performed with Sample A at 1.56 g/L (Figure 4, left), the large recovery of solids from the mixing zone may have been a result of a portion of the solid cake becoming dislodged during dismantling, which again showed our flush procedure lacked sufficient agitation to clear particle cakes. Dismantling was also shown to be the best method of solids removal from the subsequent second (C2) and third (C3) contactors; the recovered masses are shown in Table 2.
It should be noted that future work must consider the geometry of pipes/boreholes being of sufficient size that blockage will not be encountered during operations, causing malfunctions of contactors, especially in the case of the likely dislodged cake during dismantling reported in this experiment. In addition, the first entry in Table 2 shows the masses recovered from C2 and C3 are 0.02 and 0.12 g, representing small masses that are likely to suffer greater relative deviation through handing; thus, experiments should be repeated in future work.
The comparison of the percentage of mass accumulated in each contactor at different concentrations and for different particle samples is shown on the logarithmic plots in Figure 5. This clearly shows that the first contactor captures >97% of the suspended particles (>75% adhere to the C1 rotor, Table 3) as well as the subsequent entrainment of the remaining particles. The entrainment of solids in the aqueous stream through the cascade as quantities of solids were recovered from the subsequent stages; typically, these were <1.5% in C2 and <0.3% in C3.
The accumulation rate across the cascade as shown in Table 4 was calculated from each experiment aqueous solid feed duration. An accumulation rate was calculated to inform future experiments with larger volumes and longer durations, such as those reported by Kishbaugh [96] and Sakamoto et al. [48]. During the experiments, inter-stage samples were taken; these showed no gross entrainment by visual observation in either phase, even in the presence of high concentrations of suspended particles.
A plot of regression coefficients with confidence intervals is shown in the Supporting Information (Figure S4); this was produced using Modde statistical analysis software [99,100] from Umetrics. This plot shows how the factors in the experimental matrix impact the average accumulation rate. Scaling the data allows the coefficients [101] to be compared and shows that particle size does not have an effect on the average accumulation rate in C1, even when coupled with the particle concentration. The relationship between particle loading and average accumulation rate in the presence of different particle sizes in Figure 6 appears to be linear. The most significant coefficient is the particle concentration in the C1 contactor, shown in the plot of regression coefficients, as the confidence interval does not cross zero. The confidence interval crossing zero for all factors in the second contactor shows they are not significant within this dataset. The same trend is observed in the third contactor as in the second contactor, with a 10-fold lower magnitude, but this cannot be definitive as quantities measured were very low and near the detection limits of measuring the particle quantities. The data indicate that the level of particle capture in the second centrifuge increased for smaller particles, which indicates that they are more likely to entrain; however, the statistical analysis clearly shows that uncertainties are too great to validate this hypothesis, and further experiments are needed.
The duration of the experiments performed in this paper (up to 76 min) is not as long as the durations of previous experiments in the literature, where cake formation within the contactor rotor reached a maximum mass [48]. It has been previously reported that when a maximum cake depth is formed, ‘the rate of deposition is equalled by the rate of re-suspension into the aqueous phase’ [82,102] and the aqueous outlet particle concentration is the same as the feed concentration. These papers show that contactors do not block, but the cake is sheared and the particles are re-suspended; consequently, the aqueous outlet stream eventually obtains the same particle concentrations as the feed solution.

Impact of Particles on Nitric Acid Extraction

An initial assessment of the impact of particles on the extraction performance of the contactors was carried out based upon the extraction of nitric acid across the three-stage cascade. Figure 7 shows the effect of particle concentration on the percentage of acid extracted. In the absence of particles, the extraction of nitric acid in the bank of contactors is comparable with the concentration extracted at equilibrium. The results show the insoluble particles have an impact on the extraction of nitric acid, which is exacerbated with an increasing concentration of particles in the feed to the contactors. The majority of acid extraction takes place in stage 3 (C3), where the aqueous phase contacts the fresh solvent entering the contactors; given that only a small quantity of particles was recovered in stage 3, it is unexpected to observe such an effect upon the extraction of nitric acid. Typically, fine particles form denser cake, which could deteriorate the liquid extraction; this is the opposite of what has been found in other studies and will require further examination to determine the nature of this deviation from the expected outcome.
The effect of particles on extraction is substantial and counter-intuitive. The extraction performance is affected prior to the point at which contactors become blocked as a result of sufficient cake formation. The results are surprising given that the majority of particles (>97%) were observed to accumulate in the first stage of the cascade, with very few particles being entrained in the aqueous stream in subsequent stages.
Our work contrasts with earlier work by Sakamoto and colleagues [48], which reported that the average accumulation of particles in the contactor affects the hydrodynamic behaviour but does not have a detrimental impact upon the extraction performance. However, their work simulated the build-up of a cake by using metal inserts corresponding to 10%, 30%, and 50% of the volume of the separator rotor and also did not use a particle-laden flow. We propose, in the absence of the dimensions, that Sakamoto and colleagues’ [48] contactors were larger and had more separating zones than our contactors [9], so their reaction may have reached equilibrium before the separator zones.
The data set from these experiments is too small to state conclusively that larger particles have a detrimental impact on extraction. In addition, three concentrations for sample A and only two concentrations for sample B are insufficient for comparison of the effect of particle size. Further studies are required to confirm these initial observations and elucidate the effect of particle size/concentration on mass transfer behaviour in CCs and improve our understanding of solid flow regimes in pipes [103,104].
There are several plausible mechanisms that could explain the observed reduction in nitric acid stage efficiency. These include that the accumulation of solids in the CC will reduce volume and so will reduce the solvent and aqueous residence time. In addition, the presence of particles could affect the mixed solvent aqueous droplet size (surface area). It is widely reported that particulates accumulate as interface cruds (IFCs). Further work is warranted.

4. Conclusions

Suspended particles that are fed into centrifugal contactors (CCs) will result in cake/bed formation in separator rotors. Particles can travel through an entire cascade; however, when cake formation occurs, it is predominately captured in the initial contactor (C1). The average particle capture across the cascade was C1 > 97%, C2 ~3%, and C3 < 1%. The dominant factor in the rate of accumulation of particles in CCs is the weight concentration of suspended particles, irrespective of particle size (for 0.3 and 10 μm particles). The presence of solids in the aqueous feed was observed to have a detrimental impact on the extraction of nitric acid in the contactor cascade; these results indicate that this is exacerbated with increasing solids loading.
Future work is required to confirm the point at which particle accumulation leads to phase breakthrough and the time at which steady state is reached, which could be performed on a single CC. In addition, altering the geometry [9] could increase the operating window of CCs [6].

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/separations11060163/s1, Figure S1: Mass spectrum of D-80 Kerosene; Figure S2: An SEM image and log-normal size distribution (n = 314) respectively of the particles in Sample A. Results show sample A is formed by particles that range between 0.1 µm to 1.4 µm with a mean particle size of 0.3 µm; Figure S3: An SEM image and log-normal size distribution (n = 339) respectively of the particles in Sample B. Results show sample A is formed by particles that range between 3 µm to 185 µm with a mean particle size of 10 µm; Figure S4: Factor regression coefficients, with confidence intervals PS = particle size, Pconc = particle concentration, PS*Pconc = particle size couple with particle concentration; Figure S5: Coefficient plot; Figure S6: Robustness of the data; Figure S7: Process instrumentation diagram; Table S1: Approximate composition, integration of peaks normalised by n-dodecane standard; Table S2: Factors varied in this experimental campaign; Table S3: Fixed factors in this experimental campaign; Table S4: Experimental matrix of accumulation rate; Table S5: Fraction of acid extraction vs. model predictions.

Author Contributions

Conceptualization, A.B., B.C.H. and C.J.M.; methodology, A.B., B.C.H. and C.J.M.; validation, A.B., A.F. and C.J.M.; formal analysis, A.B. and A.F.; investigation, A.B., B.C.H., A.F. and N.D.-G.; data curation, A.B.; writing—original draft preparation, A.B.; writing—review and editing, A.B., A.F., B.C.H. and C.J.M.; visualization, A.B.; supervision, B.C.H. and C.J.M.; project administration, B.C.H. and C.J.M.; funding acquisition, B.C.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded under “the £46M Advanced Fuel Cycle Programme as part of the Department for Business, Energy and Industrial Strategy’s (BEIS) £505m Energy Inno-vation Programme”, and by “Engineering and Physical Sciences Research Council (EPSRC) Grant number: 2109068”, and by “Engineering and Physical Sciences Research Council (EPSRC) Grant number: EP/S022295/1”. The APC was funded by ‘EPSRC/UKRI’.

Data Availability Statement

Date are available on Supplementary Material, alongside this publication.

Acknowledgments

The authors would like to thank James Goode for initiating construction, technician Christopher Bulman for running the GM59 Leeds Nuclear Lab, technicians Bob Harris and Matthew Buckley, and Harry Newton and Alex Lowe-Bird for assisting with the rig installation. The authors greatly appreciate the comments from the anonymous reviewers.

Conflicts of Interest

The authors declare no conflicts of interest.

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Separations 11 00163 g001
Figure 1. Schematic diagram of centrifugal contactor cascade. AP1, AP2, and AP3 are aqueous sampling ports; SP1, SP2, and SP3 are solvent sampling ports.
Figure 1. Schematic diagram of centrifugal contactor cascade. AP1, AP2, and AP3 are aqueous sampling ports; SP1, SP2, and SP3 are solvent sampling ports.
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Figure 2. Images of the mixing zone at the end of experiment 6, Sample B (10 μm), at 7.09 g/L, showing the particles settling in each contactor; left = C3, middle = C2, right = C1.
Figure 2. Images of the mixing zone at the end of experiment 6, Sample B (10 μm), at 7.09 g/L, showing the particles settling in each contactor; left = C3, middle = C2, right = C1.
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Figure 3. Images of dismantled C3 centrifuge rotor after experiment 6, Sample B (10 μm), at 7.09 g/L: (a) = phase disengagement section with cake formed, (b) = centrifuge rotor with the central shaft and vanes in place with cake formed, (c) = centrifuge rotor with the central shaft and vanes removed with cake formed, (d) = bird’s eye view of the mixing zone.
Figure 3. Images of dismantled C3 centrifuge rotor after experiment 6, Sample B (10 μm), at 7.09 g/L: (a) = phase disengagement section with cake formed, (b) = centrifuge rotor with the central shaft and vanes in place with cake formed, (c) = centrifuge rotor with the central shaft and vanes removed with cake formed, (d) = bird’s eye view of the mixing zone.
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Figure 4. Percentage of particles isolated for each cleanout procedure for C1: (left) = Sample A particle concentration (g/L); (right) = Sample B particle concentration (g/L).
Figure 4. Percentage of particles isolated for each cleanout procedure for C1: (left) = Sample A particle concentration (g/L); (right) = Sample B particle concentration (g/L).
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Figure 5. Percentage of solid Al2O3 accumulated in each stage of the centrifugal contactor cascade: (left) = Sample A particle concentration (g/L); (right) = Sample B particle concentration (g/L).
Figure 5. Percentage of solid Al2O3 accumulated in each stage of the centrifugal contactor cascade: (left) = Sample A particle concentration (g/L); (right) = Sample B particle concentration (g/L).
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Figure 6. Logarithmic plot of average accumulation rate against particle concentration of different particle sizes.
Figure 6. Logarithmic plot of average accumulation rate against particle concentration of different particle sizes.
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Figure 7. Percentage of nitric acid extraction vs. equilibrium extraction against particle loading.
Figure 7. Percentage of nitric acid extraction vs. equilibrium extraction against particle loading.
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Table 1. Planned experimental matrix.
Table 1. Planned experimental matrix.
ExperimentParticle SampleSolids Added (g)Aqueous Feed Volume (L)Flow RateS/AAqueous Feed Duration (min)Solvent Feed Duration (min)
L/hrL/min
1A3535100.1717670
2A13635100.1714940
3A23635100.1717870
4B3535100.1717670
5B13635100.1717670
6B27040100.1714640
Table 2. The mass of solid Al2O3 isolated in each contactor across the cascade after each cleanout procedure. The solvent feed consisted of 30 vol% TBP/OK, the aqueous feed was 1 M nitric acid, the total flow rate was 20 L/h, the S:A ratio was 1:1, and the centrifuge speed was 2000 rpm (MZ = mixing zone). * = combined masses from the separate weir assembly (4.33 g) and rotor (23.12 g).
Table 2. The mass of solid Al2O3 isolated in each contactor across the cascade after each cleanout procedure. The solvent feed consisted of 30 vol% TBP/OK, the aqueous feed was 1 M nitric acid, the total flow rate was 20 L/h, the S:A ratio was 1:1, and the centrifuge speed was 2000 rpm (MZ = mixing zone). * = combined masses from the separate weir assembly (4.33 g) and rotor (23.12 g).
Exp.123456
Particle SampleAAABBB
Suspended Solids (g/L)0.641.564.20.232.237.09
mass solids in C1 (g)drain0.052.530.240.50.2516.93
flush0.141.030.480.120.066.16
dismantle7.784.8752.12.1627.45 *26.85
MZ flushtrace4.140.10.060.043.31
mass solids in C2 (g)drain0.02trace0.12trace0.050.31
flushtracetrace0.08trace0.020.05
dismantletrace0.191.220.090.170.23
MZ flushtracetracetracetracenonetrace
mass solids in C3 (g)drain0.12trace0.03trace0.010.02
flushnonenone0.02nonenone0.02
dismantletrace0.010.220.010.170.02
MZ flushnonetracetracenonetracetrace
Table 3. Percentage of solids recovered by contactor and percentage of mass isolated as a function of cleanout procedure.
Table 3. Percentage of solids recovered by contactor and percentage of mass isolated as a function of cleanout procedure.
Particle SampleSuspended Solids (g/L)%wt. Solids RecoveredDistribution of Solids in C1 (%)
C1C2C3DrainFlushDismantleMZ Flush
A0.6498.30.21.50.61.897.60.0
A1.5698.41.50.120.18.238.732.9
A4.2096.92.60.50.50.998.50.2
B0.2396.63.10.317.64.276.12.1
B2.2398.50.90.60.90.298.70.1
B7.0998.81.10.131.811.650.46.2
Table 4. Average accumulation of solids as a function of time.
Table 4. Average accumulation of solids as a function of time.
Particle SampleSuspended Solids (g/L)Feed Duration (min)Total Mass per Contactor (g)Average Accumulation Rate (g/h)
C1C2C3TotalC1C2C3
A0.6476.07.970.020.128.116.290.020.09
A1.5649.012.570.190.0112.779.920.150.01
A4.2078.052.921.420.2754.6141.781.120.21
B0.2376.02.840.090.012.942.240.070.01
B2.2376.027.800.240.1828.2221.950.190.14
B7.0945.653.250.590.0653.9042.040.470.05
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Baker, A.; Fells, A.; Domenech-Garcia, N.; Maher, C.J.; Hanson, B.C. Accumulation of Particles in an Annular Centrifugal Contactor Cascade and the Effect upon the Extraction of Nitric Acid. Separations 2024, 11, 163. https://doi.org/10.3390/separations11060163

AMA Style

Baker A, Fells A, Domenech-Garcia N, Maher CJ, Hanson BC. Accumulation of Particles in an Annular Centrifugal Contactor Cascade and the Effect upon the Extraction of Nitric Acid. Separations. 2024; 11(6):163. https://doi.org/10.3390/separations11060163

Chicago/Turabian Style

Baker, Alastair, Alex Fells, Natalia Domenech-Garcia, Chris J. Maher, and Bruce C. Hanson. 2024. "Accumulation of Particles in an Annular Centrifugal Contactor Cascade and the Effect upon the Extraction of Nitric Acid" Separations 11, no. 6: 163. https://doi.org/10.3390/separations11060163

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