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Article

Phase Separation in a Novel Selective Lithium Extraction from Citrate Media with D2EHPA

by
Tiaan Punt
*,
Steven M. Bradshaw
,
Petrie Van Wyk
and
Guven Akdogan
Department of Process Engineering, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa
*
Author to whom correspondence should be addressed.
Metals 2022, 12(9), 1400; https://doi.org/10.3390/met12091400
Submission received: 25 July 2022 / Revised: 19 August 2022 / Accepted: 20 August 2022 / Published: 24 August 2022
(This article belongs to the Special Issue Extractive Metallurgy for the Sustainable Supply of Metals in Lithium-Ion Batteries)
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Abstract

:
Lithium-ion battery (LIB) recycling has received continued interest in recent years due to its benefits, which include reducing the environmental impact of spent LIBs and providing a secondary source of valuable metals, such as Li, Co, and Ni. This paper characterized the Li separation with D2EHPA from citrate media as a function of pH and identified the optimal overall Li separation at a pH of 5.5. The Li separation was optimized at a pH of 5.5, with which it was concluded that 23 vol.% D2EHPA and an O/A ratio of 4 provided the best Li separation, for which 66.1% Li was extracted with 26.9% residual Mn, 6.8% Co, and 7.7% Ni in a single stage. The formation of a reversible hydrophobic third phase was identified during Li extraction at a pH of 5.5 or greater. Investigation of the third phase revealed that more than 99% of the Li, Co, and Ni were extracted to the third phase, while more than 69% of the Mn was extracted to the organic phase. STEM images of the third phase revealed a honeycomb-like structure, which was hypothesized to be a 2D mesoporous film caused by the insolubility of the organometallic complexes in the aqueous and organic phase.

1. Introduction

The prominence of lithium-ion battery (LIB) recycling has risen in recent years due to these batteries’ widespread application in portable electronics and, more notably, the adoption of LIBs in stationary energy storage systems and electric vehicles (EVs), which require large volumes of LIBs [1]. As the production of LIBs accelerates to meet the demand for energy storage, the global storage capacity of LIBs is set to increase to over 12.7 million tons in 2030 [2,3]. The associated increase in spent LIB waste streams may be delayed by up to 10 years due to the long life-cycle of LIBs [4], with the LIB recycling market recovering up to EUR 555 million in valuable materials by 2030 [5]. The superior performance of LIBs, such as their high energy density and low weight-to-volume ratios [6], is due to the unique properties of Li, which include low density, high charge-to-weight ratio, high redox potential, and high specific heat capacity [7].
LIBs contain a copper anode coated with carbonaceous material, an aluminium cathode coated with active cathodic material, an electrolyte, and a plastic separator. The disposal of LIBs poses significant environmental hazards due to the toxic components, such as heavy metals [8], LiFP6 [9], poly-tetra-fluor-ethylene (PTFE) binders [10], and electrolytes [11], which are contained and treated during recycling. Preventing pollution with these toxic components contained within spent LIBs through pathways such as landfills enables recycling to significantly reduce the environmental impact of spent LIBs [12]. LIB recycling also provides a secondary source of valuable metals, such as Li, Co, and Ni, the demand for which has risen sharply due to their application in LIB production [13]. The increased demand for LIBs and recent global socio-political issues has led to a sharp rise in the commodity prices of Li, Co, and Ni [14]. This significantly influences the economic feasibility of LIB recycling, which could support the supply of metals and enable a circular economy for LIBs. In conjunction with recent reports indicating the poor end-of-life recycling rates for Li, Co, and Ni at <1%, 37%, and 60%, respectively [1], the importance of an LIB recycling process that can separate the metals to achieve higher metal recoveries of reusable quality has been highlighted.
Multiple pyrometallurgical LIB recycling processes have been developed over the past 20 years [15,16]; however, these processes have large energy requirements to reach the operating temperatures and produce harmful gases that need to be trapped/treated [17]. The major drawback of pyrometallurgical processes to date has, however, been the inability to recover the Li, as light metals such as Li and Al respond to the slag phase [18], which is of particular concern due to the high Li content in LIBs and the low end-of-life recycling rate of Li to date. Hydrometallurgical processes for LIB recycling have increased in popularity, most notably due their relatively low energy requirements, reduced gas emissions, and high metal recovery enabled by the high solubility of cathode metals in acidic solutions [19,20]. One of the concerns with hydrometallurgical processes are the toxic waste effluents produced by strong chemicals, such as acids required for leaching, which need to be reduced to minimise the health and environmental risks of these processes.
Recent research on hydrometallurgical LIB recycling processes has evaluated the potential of organic acids as lixiviants, specifically citric acid due to its reduced toxic gas emissions [21], low ecotoxicity [22,23], and limited toxicological hazards [24,25,26]. Citric acid has proven to be a promising lixiviant due to its relatively low cost and good leaching performance [27,28,29], with studies like that of Li et al., illustrating that the leaching performance of citric acid may exceed that of conventional lixiviants, such as HCl and H2SO4, when using ultrasonic-assisted leaching under select conditions [30]. Citric acid’s capability to achieve high metal leaching performance, notwithstanding its weak acidic properties, has largely contributed to its strong chelating abilities, which enables citric acid to complex with metal ions when paired with a reductant, such as hydrogen peroxide (H2O2) [27,29,31].
Citric acid (C6H8O7) contains three carboxylic functional groups and one hydroxyl functional group, as illustrated in Figure 1 for HCitH3. Citric acid is often written as HCitH3, where the first proton refers to the proton of the hydroxyl group and the last three refer to the three carboxylic groups. The α-carboxylic group (illustrated in red in Figure 1) is deprotonated first due to its higher acidity, after which the two β-carboxylic groups (illustrated in blue in Figure 1) are deprotonated, and the α-hydroxyl group is deprotonated last due to its strong bond, which requires extreme alkaline conditions for deprotonation [31,32]. The deprotonation of HCitH3 to Cit4− is illustrated below with the respective pKa values at 20 °C averaged from various studies [31,33,34]. Due to the sequential deprotonation of citric acid and its various acidic functional groups, a wide variety of citrate ions may be present at a defined pH, leading to a wide variety of metal citrate complexes and greatly complicating the understanding of a citrate leach solution with numerous metals, such as with LIB recycling.
The metal separation and recovery from citrate media have, however, proven to be difficult, with many studies separating Mn from the citrate leach solution first using di-(2-ethyl-hexyl)phosphoric acid (D2EHPA) [35,36,37], a phosphorus-based organic extractant. D2EHPA is known to arrange into dimeric structures and has an eight-membered ring structure, as illustrated in Figure 2. The eight-membered rings have a proton that can be exchanged in a cation exchange reaction to substitute a metal cation to form a hydrophobic organometallic complex, which is highly soluble in the organic diluent and extracted to the organic phase as a result. Due to the four oxygen atoms, and their free electron pairs, which surround the central phosphorus atom, D2EHPA is selective for tetrahedral coordination geometries.
Select studies have investigated the separation of Co, Ni, and Li from the complex metal citrate leach solutions using precipitation with Mn-D2EHPA solvent extraction [37,39] or complete precipitation of all the metals [40]. These processes, however, require chemical-intensive and complex operations, which lead to significant dilution of the metal content through undesired precipitant addition. Recent research on metal separation from citrate LIB leach solutions has identified a novel method for Li separation through solvent extraction with D2EHPA [41]. The novel separation of Li not only enables the near complete separation of Li without precipitant addition but also enables the separation of Li prior to Co and Ni, which has not been demonstrated from any lixiviant media to date. The recovery of Li typically occurs last due to the strong alkaline conditions required for the precipitation of LiOH [42,43], Li2CO3 [44], or Li3PO4 [39,40].
The selective separation of Li from citrate media with D2EHPA using solvent extraction provides a unique metal separation process for LIB recycling where the recovery of Li can be improved through separation prior to Co and Ni. This is of significant importance, as the molar ratio of Li remains high in cathode chemistries, such as LiNi1/3Mn1/3Co1/3O2, LiCoO2, LiFePO4, and LiNi0.8Co0.15Al0.05O2, due its importance for LIB functional characteristics, while the contents of other metals, such as Co, Ni, Mn, Fe, and Al, vary greatly [45]. It would therefore be useful to better understand Li separation from citrate media with D2EHPA as this may enable future processes to improve the end-of-life Li recycling rate by prioritising the extraction of Li before Co and Ni under weak acidic conditions. Improving the end-of-life Li recycling rate, which currently lies at under 1% [1], is of great interest due to the increasing demand for Li among battery manufacturers, as this could provide a rich secondary source of Li and improve the economic feasibility of the LIB recycling processes.
To better understand the metal separation from citrate media, it is necessary to understand the metal citrate chemistry and how this influences metal separation using solvent extraction or precipitation. The metal citrate complexes vary greatly among the different metals present in the LIB recycling streams due to the unique properties of each metal, as well as the various dissociated citrate ions present in the leach system as the pH is changed. This, paired with the metal citrate and organic extractant complexation chemistry, the phase transfer equilibria, and the potential of citrate-extractant moieties formed prior to metal extraction, leads to a highly complex system.
Studies on Co-citrate complexes have concluded that the coordination is octahedral [46,47] and, even though the hydration of the complexes is low, citric acid acts as a bridging ligand between complexes [48]. This results in large complex structures with a relatively high stability and a coordination that is not favoured by phosphorus-based extractants. The Ni-citrate complex has been reported to have a coordination number of 4 or 5 [49], with superior stability [50,51,52] and relatively high hydration [47,53]. The separation of Ni with organic extractants is expected to be poor, regardless of its compatible coordination, due to its high stability with citrate, which acts as a competing ligand, and its high hydration, which favours hydrophilic conditions.
The coordination of Mn-citrate has been reported as tetrahedral [54], which phosphorus-based organic extractants, such as D2EHPA, have a high affinity for [38], and with limited hydration and a notably lower stability compared to Co or Ni citrate [47,50]. These properties can greatly enhance the extraction capability of Mn with phosphorus-based extractants, such as D2EHPA, which has been illustrated by various studies [35,36,37,39,41].
The coordination number of Li has been determined to be 4 or 6 [55], with the structure of Li-citrate often in the form of tri-lithium citrate complexes [56,57]. The Li complexes with four or six oxygen atoms and complexes with citrate ions and water molecules, which offer critically important hydrogen bonds for stability, leading to high hydration in many complexes [58,59,60]. The hard acid property of Li indicates its preference to complex with oxygen, which is a hard base [61], and this has been exploited to separate Li with crown ethers that have four oxygen atoms [62,63,64]. The strong hydration of Li due to its charge density and affinity for oxygen significantly hinders its extraction and, therefore, extractants with oxygen atoms that can successfully complex with Li are desired.
D2EHPA has oxygen atoms that complex with metal ions, with four oxygen atoms surrounding divalent metals during extraction (see Figure 2); therefore, D2EHPA is an ideal candidate for Li extraction. It is, however, unclear what the structure of Li-D2EHPA may look like. The Li may be extracted from the citrate complex, with the coordination saturated by hydration as illustrated in Figure 3a, or the citrate may be extracted with the Li, as illustrated in Figure 3b. Further studies on the stability of Li-citrate, as well as the extraction mechanism and stoichiometry of Li-D2EHPA from citrate media, are required to better understand this process and are topics for future investigation. Both structures depicted in Figure 3 have hydrophilic ligands (water and citrate), as well as hydrophobic ligands (D2EHPA), which may complicate the complex solubility.
This study investigated the performance of Li extraction with D2EHPA from citrate media, with a focus on the phase separation, as summarised below in Figure 4. The Li separation from Mn, Co, and Ni was evaluated as a function of pH, which plays a key role in the structure of the metal citrate complexes, as well as the cation exchange mechanism of the organic extractant. The Li separation from a stream with Co, Ni, and residual Mn was optimised to ensure maximum Li separation over all other metals. The formation, composition, and structure of a reversible third phase observed during the Li extraction were also analysed and quantified to better understand the phase equilibria. The results of this study significantly improve the understanding of the phase separation challenges for a novel method of Li solvent extraction from citrate media, which is of notable interest for LIB recycling processes.

2. Materials and Methods

2.1. Materials

The black mass produced from waste laptop LIBs through discharging and alkaline leaching, as illustrated in Figure 4, contained 84.2 wt.% LiNi0.45Mn0.40Co0.15O2 and 15.8 wt.% LiCoO2, with trace amounts of LiMn2O4 and Al2O3, as detailed in a previous study [9]. The average metal content of the black mass used in this study is further summarised in Table 1 [9]. The black mass was leached with 99.8 wt.% anhydrous citric acid powder and 50 wt.% H2O2 pellets supplied by KIMIX Chemical and Lab Supplies cc. The black mass was effectively leached with 1.5 M citric acid and 2 vol.% H2 at 95 °C and 20 g/L for the optimal time, which was determined to be 20 min [41].
All the solvent extraction tests were performed with kerosene supplied by KIMIX as diluent and 95 wt.% D2EHPA supplied by Industrial Analytical as extractant. The Mn and Al in the citrate leach solution were significantly reduced by a single-stage extraction with 12 vol.% D2EHPA, O/A ratio of 2, pH of 3, and temperature of 20 °C to produce the feed for the Li solvent extraction [41]. Figure 4 illustrates the flow of the sequential metal separation, and the feed stream is summarised in Table 2. Negligible amounts of Al were present in the lithium solvent extraction feed stream, with dilute amounts of Mn. The pH control during extractions used a 10 M NaOH solution prepared with 98 wt.% NaOH pellets and a 1.5 M citric acid solution prepared with 99.8 wt.% anhydrous citric acid, both supplied by KIMIX. A 1.5 M citric acid solution, prepared with the 99.8 wt.% anhydrous citric acid supplied by KIMIX, was also used as a stripping agent. All aqueous dilutions were made with demineralised water.

2.2. Equipment

The solvent extraction experiments were performed in a covered beaker containing a magnetic stirrer bar placed on a magnetic stirrer. The pH was measured with a Hanna H11310 pH probe during all extraction experiments. The contents of the beaker were transferred to a separating funnel and the volume of each phase was drawn with a graduated cylinder, with the aqueous phase stored in falcon tubes for analysis. All aqueous samples were analysed for Al, Co, Cu, Fe, Li, Mn, and Ni using a Perkin Elmer Avio 500 ICP-OES, and the composition of the organic phase was determined through a mass balance. The third phase was analysed with a Carl Zeiss Merlin Electron Microscope with a scanning transmission electron detector for scanning transmission electron microscopy (STEM) analysis. Due to the high viscosity and hydrophobic nature of the third phase, a copper transmission electron microscopy (TEM) grid was placed in a drop of the third phase for 10 min, after which the sample was briefly rinsed with distilled water. The TEM grid was subsequently placed in STEM grid holder and dried for 2 h at 35 °C before analysis.

2.3. Experimental Conditions

In this section, the experimental conditions and parameters are summarised. The solvent extraction and stripping time were set as fixed parameters of 30 min and 15 min, respectively, for all experiments due to the fast kinetics, which could reach equilibrium as quickly as 3 min [65]. The temperature was maintained at 20 °C for all extraction and stripping tests as is commonly done due to the fast kinetics at room temperature and the limited benefit of elevated temperatures [66]. The metal extraction from the feed stream, summarised in Table 2, was first characterised by investigating the influence of pH between 2.5 and 7.0, at intervals of 0.5, with 0.9 M D2EHPA and an O/A ratio of 1. The D2EHPA concentration and O/A ratio were chosen to provide sufficient extractant for complete metal extraction. The metal separation performance of D2EHPA was evaluated by quantifying the metal extraction performance, as well as the metal separation factor, as described by Equations (1) and (3), respectively. The mass of metal M is referred to with m , while D M describes the distribution coefficient of metal M and β M 1 / M 2 describes the separation factor of metal M1 from metal M2:
m e t a l   M   e x t r a c t i o n   % = m M P L S   b e f o r e − m M P L S   a f t e r m M P L S   a f t e r × 100
D M = m o r g , a f t e r m a q , a f t e r
β M 1 M 2 = D M 1 D M 2
The phase equilibria between the aqueous, organic, and third phases were then investigated at a pH of 5.5 for D2EHPA concentrations of 5 vol.% to 23 vol.% (intervals of 6 vol.%) and volumetric O/A phase ratios of 1 to 5 (intervals of 1). After the optimal lithium extraction was determined, the compositions of the aqueous, organic, and third phases were determined. The compositions of the organic and third phases were determined by performing the optimal lithium extraction and separating the three phases. The organic and third phases were stripped separately thereafter and the stripped liquor from each phase was analysed. A previous study indicated that stripping with 1.5 M citric acid at an A/O ratio of 2 provides nearly 80% Li stripping in a single stage [41]. The compositions of the strip liquors from the organic and third phases enabled the determination of the metal extraction for each phase of the ternary system through a mass balance. The stripped organic and third phases were each put into contact with fresh feed stream to determine the presence of the extractant. The extraction performance of each stripped phase indicated the presence of the extractant and to which phase the extractant responded in the ternary system.

2.4. Experimental Procedure

The extractions were started by transferring the aqueous feed to a beaker containing a magnetic stirrer bar. The pH probe was placed in the beaker and the beaker was covered with a watch glass before turning on the stirring to 200 rpm. The pH was adjusted to the desired value and, once the desired pH was reached, the organic extractant was added to the beaker and the experimental time was started. The stirring was subsequently increased to 600 rpm to ensure maximum contact between the phases. The pH was continuously measured throughout the extraction and the desired pH was maintained with dropwise addition of 10 M NaOH or 1.5 M citric acid. After the experimental time was completed, the contents of the beaker were transferred to a separating funnel to allow the phases to separate, after which the volume of each phase was measured and the aqueous phase was stored for analysis.
The stripping experiments were started by adding the stripping agent to a beaker with a magnetic stirrer bar, after which the loaded organic or third phase was added. The stirring was increased to 600 rpm and the beaker covered with a watch glass as the experimental time was started. After the stripping experiments were completed, the phases were drawn and their volumes measured, after which the aqueous strip liquor was stored for analysis.

3. Results

3.1. Influence of pH

Due to the significant influence of pH on both the metal-citrate chemistry and the metal extraction performance, the metal extraction as a function of pH was investigated first. Figure 5a illustrates the metal extraction from the feed stream with 0.9 M D2EHPA and an O/A of 1 at 20 °C. It can be observed in Figure 5a that the Mn extraction was greater than 90% for a pH of 4 or less, after which the Mn extraction rapidly declined as the pH was increased to 6. The Co and Ni extraction from the citrate media with D2EHPA was observed to be poor for all the pH conditions investigated, with a slight increase for weak acidic conditions when the pH was increased from 5.5 to 7, for which a maximum Co and Ni extraction of 16.5% was achieved.
Figure 5a illustrates the poor Li extraction under strong acidic conditions, which increased steadily from a pH of 2.5 to 4.5, for which a maximum Li extraction of 42% was achieved. The Li extraction remained roughly 40% as the pH was further increased to 7 and was favoured over all other metals at a pH of 5.5 or higher. The maximum separation factors for Li from Ni ( β L i / N i = 657 ) and Co ( β L i / C o = 13.4 ) at pH levels of 3 and 4, respectively, are illustrated in Figure 5b and are largely attributed to the poor cobalt and nickel extraction. The maximum Li separation from Mn ( β L i / M n = 3.8 ) was achieved at a pH of 6 or greater due to the poor Mn extraction with D2EHPA under weak acidic conditions. Figure 5b further illustrates a rapid decline in the separation efficiency of Li from Co and Ni as the pH was increased from 5.5 to 7, which was attributed to the slight increase in Co and Ni extraction under weak acidic conditions as the Li extraction remained unchanged.
The formation of a reversible third phase was observed for the experiments performed at a pH of 5.5 or greater, as illustrated by the phase volumes in Figure 6. The phase disengagement was observed to be longer when the third phase was present, and this was attributed to its physical properties, such as its high viscosity and hydrophobic nature. The third phase was observed to be insoluble in both the aqueous and organic phases, and it was noted that the organic phase volume decreased notably as the third phase formed. It was therefore hypothesised that the third phase contained organometallic complexes that were insoluble in the citrate aqueous and kerosene organic phases. The third phase is pictured in Figure A1 in the ternary system, located between the aqueous (bottom phase) and organic phase (top phase). The third phase disappeared readily when the pH was decreased to below 5.5, enabling the recovery of the organic phase during stripping. The reversible nature of the third phase enabled easy operation with the ternary system; however, it was unclear if the metals were selectively extracted to either of the different hydrophobic phases (third phase and organic phase). If the Li selectively responded to one of the hydrophobic phases as the other metals responded to the other hydrophobic phase, the Li separation could be enhanced further by separately stripping the third phase and organic phase.
If no manganese were present in the feed stream, the lithium extraction would be optimal with a pH of 3 to 4, depending on the cobalt and nickel content, as the optimal Li separation from cobalt and nickel occurs at pH levels of 4 and 3, respectively. The feed to the Li solvent extraction for this study, however, contained roughly 274 mg/L Mn, as summarised in Table 2, and it is further known from Figure 5 that Mn was extracted preferentially over Li with D2EHPA at a pH of 5 of less. The preferential extraction of Mn reduced the extraction efficiency of Li with D2EHPA, which was not desired. It was thus decided that, due to the presence of Mn in the feed stream, the Li solvent extraction should be undertaken at a pH of 5.5 or more to ensure the best selectivity of the extractant for Li, as this would enable the best Li separation. Furthermore, Figure 5b illustrates that the highest average Li separation factor at a pH greater than 5 was 9.7, which was achieved at a pH of 5.5. The optimal pH for Li separation from the feed with D2EHPA was therefore chosen as 5.5 to ensure the extractants’ selectivity for Li was maximised for optimal Li extraction performance, which is the focus of this study.

3.2. Optimisation of O/A Ratio and D2EHPA Concentration

The extractant concentration and O/A ratio were optimised simultaneously, as they both limit the extraction mechanism through the stoichiometric requirements of metals due to their direct impact on the extractant quantity introduced into the experiments. Figure 7a,b illustrates the Li and Mn extractions as functions of the O/A ratio and D2EHPA concentration at the chosen optimal pH of 5.5, respectively. The formation of a third phase was observed for all experiments performed in Figure 7 and was attributed to the extraction pH of 5.5, as concluded previously in Section 3.1 and illustrated in Figure 6.
Figure 7a shows that both the O/A ratio and the D2EHPA concentration had significant influences on the Li extraction, as the Li extraction increased sharply as the O/A ratio or D2EHPA concentration was increased. Figure 7a further illustrates that up to 70% Li extraction could be achieved with 23 vol.% D2EHPA and an O/A ratio of 5 at a pH of 5.5 within a single stage. It can be observed from Figure 7b that the Mn extraction was limited to below 35% for all the extraction conditions evaluated at a pH of 5.5. The poor Mn extraction was desired to enable the maximum Li separation from the feed, and it was enabled by the choice of the extraction pH, which was chosen as 5.5, as seen in Figure 5 and discussed in Section 3.1. The superior selectivity for Li over Mn observed for the O/A ratio and extractant concentration conditions evaluated in Figure 7 thus confirms that the pH plays a key role in D2EHPA’s selectivity for Li over the other metals present in the feed.
It was desired to have an O/A ratio that was as low as possible to ensure that the metals were not diluted to the organic phase, as an O/A ratio greater than 1 would lead to a reduction in the metal concentration to the organic phase. Figure 7a illustrates that an O/A ratio of 1 or less would not enable efficient Li extraction and, therefore, O/A ratios of up to 5 were investigated, with O/A ratios of 4 and 5 enabling >65% Li extraction in a single stage. Furthermore, high organic extractant concentrations are typically avoided, as excessively high concentrations in the organic phase increase the organic phase properties, such as the viscosity, which may limit industrial applications due to the pumping requirements. This study investigated D2EHPA concentrations of up to 23 vol.%, as shown in Figure 7a, for which the viscosity of the organic phase was not increased significantly. It can be observed in Figure 7a that D2EHPA concentrations of 17 vol.% or 23 vol.% were required to achieve Li extraction of up to 61% and 70%, respectively. Figure 7b illustrates that, with 17 vol.% and 23 vol.% D2EHPA, up to 21% and 31% of the Mn was extracted.
The influences of the O/A ratio and D2EHPA concentration on the Co and Ni extraction are illustrated in Figure 8a,b, respectively. It can be observed that the extractions of both Co and Ni were limited to below 15% for all conditions evaluated, and the poor extraction performance was attributed to the poor selectivity of D2EHPA for both Co and Ni at the extraction pH of 5.5, as shown in Figure 5 and discussed in Section 3.1. The maximum Co and Ni extractions of 13% and 11% were achieved with 23 vol.% D2EHPA and an O/A ratio of 5. The maximum extraction under the aforementioned conditions was attributed to the high extractant concentration and O/A ratio, producing a higher extractant-to-metal stoichiometric ratio and better contact between the extractant and the metals, Co and Ni.
The optimum O/A ratio and D2EHPA concentration for Li separation from the feed at a pH of 5.5 were determined to be 4 and 23 vol.%, respectively. Using an O/A ratio of 4 and 23 vol.% D2EHPA at a pH of 5.5 enabled the extraction of 66.1% Li, 26.9% Mn, 6.8% Co, and 7.7% Ni. Decreasing either the O/A ratio or the D2EHPA concentration led to significant decreases in the Li extraction, while the co-extraction of the other metals only decreased marginally, producing poorer Li separation. Increasing the O/A ratio or D2EHPA concentration beyond 4 and 23 vol.% nearly doubled the co-extraction of Co and Ni, while the Li extraction only increased marginally, also producing poorer Li separation. The Li extraction with 23 vol.% D2EHPA, an O/A ratio of 4, and a pH of 5.5 could be further enhanced by using counter-current stages, and a previous study has achieved a Li extraction of up to 93.6% with three stages, projecting a 99.5% Li extraction for six stages [41].

3.3. Organic and Third Phase Composition

The formation of a reversible third phase when extracting Li with D2EHPA from citrate media at a pH of 5.5 or greater was observed in each experiment through the optimisation of pH, D2EHPA concentration, and O/A ratio. The formation of the third phase could not be suppressed with the addition of modifiers such as TBP or by operating at elevated temperatures of up to 45 °C. The analysis of the metal extraction performance remained unaffected by the formation of the third phase and its reversible nature enabled the removal of the third phase during stripping when the pH was reduced to below 5.5.
The organic and third phases were stripped separately with 1.5 M citric acid and an A/O ratio of 2, as illustrated in Figure 9a. Stripping of the loaded organic phase only led to the recovery of 3.4% Mn with <1% of the Co, Li, and Ni extracted during the loading of the organic and third phases. The stripping of the loaded third phase led to the recovery of 76% Li, 32% Ni, and 37% Co for the extracted metals with 1.5% Mn. More than 69% of the Mn was therefore recovered from the organic phase during stripping, while ≥99% Co, Li, and Ni were recovered from the third phase. The increase in Co and Ni extraction as the pH increased from 5.5 to 7 is shown in Figure 5. The slight increase in third phase volume as the pH increased from 5.5 to 7 is shown in Figure 6. It was therefore hypothesised that D2EHPA or citrate-D2EHPA moieties formed organometallic complexes with Li, Co, and Ni, which were insoluble in the organic phase at a pH of 5.5, leading to the formation of a reversible third phase.
After the organic and third phases were stripped, their individual extraction performance was evaluated by putting each phase in contact with the feed stream at an O/A ratio of 4 and pH of 5.5. Figure 9b shows that the stripped organic phase only extracted 4.6% Li, 2.2% Ni, 1.3% Mn, and 0.6% Co from the feed stream and indicates that the organic phase contains minimal amounts of D2EHPA. The extraction efficiency of the third phase in Figure 9b, however, indicates a highly concentrated presence of D2EHPA, as 86% Li, 46% Ni, 44% Co, and 37% Mn were extracted. The large concentration of D2EHPA in the third phase was indicated by the large metal loading capacity of the third phase, demonstrated by the 86% Li extraction, notably higher than the 69% Li extraction at 23 vol.% D2EHPA and O/A 4 described in Section 3.1. The 44% Co and 46% Ni co-extraction was also significantly larger with the third phase compared to 23 vol.% D2EHPA and O/A 4.
This provides supporting evidence for the hypothesis that D2EHPA formed organometallic complexes with Li, Co, and Ni, which were insoluble in the organic phase. The formation of the third phase therefore enabled the selective extraction of Li with traces of Co and Ni to the third phase, while the Mn co-extraction responded to the organic phase. The formation of the third phase also led to the separation of D2EHPA from the kerosene organic phase into the reversible third phase. Stripping the third phase with an acid to reduce the pH to below 5.5 thus reversed the extraction mechanism, returning the D2EHPA to the kerosene organic phase and stripping the metals to the aqueous stripping agent.

3.4. STEM Analysis of the Third Phase

The analysis of the reversible third phase was difficult due to it its disappearance at pH levels under 5.5 and its properties, such as high viscosity and hydrophobicity. The third phase could therefore not be dried or mounted onto grids, but studies on similar mediums have used cryogenic transmission electron microscopy (Cryo-TEM) to produce insightful images of micellular [67] and vesicle structures [68] from liquid samples. The third phase in this study was analysed using STEM, which works similarly but does not change the properties of the liquid third phase sample during sample preparation, as discussed in the Materials and Methods section. The STEM analysis thus produced visual images of the third phase on a microscopic level, without changing any of the properties of the third phase, giving an accurate visualisation of the third phase characteristics.
Figure 10 illustrates a STEM image of the third phase at 5870× magnification (5.87 kx), where a distinct honeycomb-like structure can be observed. The structure of the third phase contained thin, grey films surrounding light and dark vesicles with varying shapes and sizes, as illustrated in Figure 10. The distinct structure observed within the third phase may be attributed to amphiphilic complexes produced by the extraction of metal citrate complexes, which have varying degrees of stability depending on the metal, hydration, coordination, pH, and temperature [47,48,50,51,52,53,54,58]. If citrate ions are extracted with metals using D2EHPA to the third phase, the phosphorus-based D2EHPA will be hydrophobic, while the citrate ligand will be hydrophilic.
These theoretical amphiphilic complexes therefore aligned themselves in 2D micelle layers so that the polar (citrate) site faced the aqueous phase and the non-polar (D2EHPA) site faced the organic phase. This alignment of the organometallic complexes satisfied the solubilities of both the citrate and D2EHPA ligands in the complex, leading to the production of a reversible third phase. These amphiphilic complexes could further also assembled themselves in bi-layer sheets, in which the complexes arranged their hydrophobic (D2EHPA) and hydrophilic (citrate) sites towards those of the other complexes. The size, structure, and composition of the complexes formed greatly influenced the specific nature of the third phase, leading to countless potential structures, such as spherical micelles, inverse micelles, vesicles, planar bilayers, and 2-D mesoporous films; therefore, further analysis was required to determine their exact nature.
The STEM samples were further mapped with high-angle annular dark-field imaging (HAADF) during analysis, in which elements with larger atomic numbers scattered more electrons at higher angles and showed up whiter in the STEM images. This enabled the distinction between larger atoms, which showed up as whiter, from smaller atoms, which show up as darker.
Figure 11 illustrates a STEM image with HAADF mapping with 11,310× magnification (11.31 kx), where a honeycomb-like structure similar to that of Figure 10 was observed. It is important to consider that the third phase samples analyzed with STEM for this study were in liquid phase and, therefore, the STEM images illustrate the samples in a three-dimensional space. As the samples were analyzed in a 3D space, the “honeycomb” structure observed in Figure 10 and Figure 11 suggests that the third phase structure resembled the form of a typical 2D mesoporous film. It was therefore considered that the “pores” within the 2D film were distinct, encapsulated fluid regions, referred to as vesicles from here onwards, surrounded by thin films. The 2D mesoporous film structure of the third phase was supported by the slow phase segregation observed for the third phase, as the third phase had to self-assemble into the honeycomb structure observed in the STEM images.
Due to the three-dimensional nature of the liquid third phase in the STEM images, it was difficult to draw conclusions about the structures observed and the size of the vesicles, as it was unclear what was present at the front or back of the sample. Furthermore, the three-dimensional nature of the third phase could lead to very blurry STEM images with structures of different depths overlapping, resulting in poor clarity for the third phase.
Figure 11 shows that the vesicles within the honeycomb structure were mostly dark black vesicles surrounded by grey films. It was therefore concluded that small atoms were concentrated within the dark vesicles of the third phase honeycomb structure. Li and H were the smallest atoms present in the system and, therefore, it was suspected that the dark regions could contain both these elements, as it is known that Li is extracted to the third phase from the citrate leach. It is, however, important to note that the STEM images had relatively low magnifications and did not show atoms or even organometallic complexes but different regions within the third phase that contained different molecules, ligands, or complexes.
Carbon, oxygen, and phosphorus are medium-sized atoms that were used during the extraction and, therefore, the grey films within the third phase could have been the organic extractant (D2EHPA), which has long carbon chains, a central phosphorus atom, and oxygen atoms coordinating with the metals. The grey films could also have been kerosene, used as diluent, which surrounded the extracted complex in the dark vesicles, as kerosene consists of aliphatic hydrocarbons. If the films consisted of kerosene, this would limit the surface contact between the extracted organometallic complexes in the third phase and, thus, influence the metal stripping from the third phase.
The white regions observed in Figure 11 are suggested to be the transition metals, such as Mn, Co, and Ni, as they were the largest atoms present in this study. These white vesicles could therefore illustrate the co-extracted transition metals, which were observed to also be surrounded by grey films. Due to the poor depth sensitivity of the STEM analysis, another potential explanation for the white vesicles is that they could simply have been a grey film surrounding a dark vesicle that was bulging upwards towards the viewer or a combination of various layers of grey films overlapping.
The grey films surrounding the white vesicles suggest that the washing of the third phase with demineralised water during the sample preparation did not remove any entrapped transition metals. This supports the hypothesis that the grey films were hydrocarbons, such as the carbon chains of D2EHPA or kerosene, as the washing with water during sample preparation could not make contact with these regions due to the hydrophobic nature of the hydrocarbons. This hypothesis is further supported by the extremely hydrophobic and viscous nature of the third phase, which greatly limited mixing with aqueous media, such as water.
It is unclear what the black wormlike structures within some of the white vesicles in Figure 11 were, as their straight alignment suggests that this phenomenon could have been electron scattering through two slits and could, therefore, have been due to the analytical technique. The black wormlike structures could also have been small elements, such as Li, which were concentrated in that region; however, due to the precise, straight nature of the structure, this would be unlikely, as the images represent the sample in three dimensions and the black wormlike structures were always observed in a straight vertical or horizontal alignment (see Figure A2 and Figure A3).
Some of the concerns with the operation of the third phase included its high viscosity and prolonged phase disengagement, which required equipment capable of operation with high-viscosity liquids. The phase disengagement could further be improved using enhanced gravity separation equipment, such as a centrifuge, increasing the phase disengagement. These considerations must be taken into account during the design of the process and can significantly influence the capital costs of the process. If the feed to the Li separation process can be produced with negligible amounts of Mn, the Li separation can be performed between a pH of 3 and 4 with the Li separation from Ni and Co, respectively, maximised. The performance of multiple counter-current stages for Mn extraction prior to Li separation, as illustrated in a previous study [41], is therefore of critical importance for the selection of the optimal Li separation conditions.

4. Conclusions

The influence of the pH of the metal separation with 0.9 M D2EHPA and O/A 1 was evaluated for pH levels from 2.5 to 7.0 for a feed stream with reduced Mn content. The Li extraction increased steadily from a pH of 2.5 to a pH of 4.5, for which the maximum Li extraction of roughly 40% was observed between pH levels of 4 and 7. The extraction of the residual Mn was observed to be favoured, with more than 90% Mn extraction at a pH of 4 or less and a rapid decline in the Mn extraction from pH levels of 4 to 6. Due to the residual Mn in the feed stream, a pH of 5.5 or greater was required to ensure the preferential extraction of Li with D2EHPA over all the other metals. A gradual increase in the co-extraction of both Co and Ni was observed as the pH increased from 5.5 to 7.
The maximum Li separation from Ni and Co was observed at pH levels of 3 and 4, for which the Li separation factors for Ni and Co were 657 and 13.4, respectively. The superior separation factors for Ni and Co within the pH range from 3 to 4 was attributed to their poor extraction performance and is recommended as the optimal separation pH for feed streams without Mn. The formation of a reversible third phase was observed and measured for the Li extraction at a pH of 5.5 and greater. The third phase disappeared readily when the pH was decreased to below 5.5, allowing for the regeneration of the organic phase during stripping. Due to the preferential extraction of Mn over Li at a pH of 5 or less and the presence of 274 mg/L Mn in the feed stream, the Li separation was optimised at a pH of 5.5, for which the average Li separation factor is 9.7. Concurrent optimisation of the D2EHPA concentration and O/A ratio for Li separation was performed at a pH of 5.5 and 20 °C. It was concluded from the optimisation that the optimal Li extraction with minimal co-extraction was achieved with 23 vol.% D2EHPA and an O/A ratio of 4, where 66.1% Li was extracted with 26.9% residual Mn, 6.8% Co, and 7.7% Ni.
The organic phase and third phase compositions were investigated by stripping each phase separately with 1.5 M citric acid at an A/O ratio of 2. It was observed that more than 69% of the Mn was recovered from the organic phase, whereas more than 99% of the Li, Co, and Ni were recovered from the third phase. The stripped organic and third phases were put into contact with the feed stream at an O/A ratio of 4 and a pH of 5.5 to evaluate the extraction capabilities of each phase. The stripped organic phase only extracted 4.6% Li, 2.2% Ni, 1.3% Mn, and 0.6% Co, while the stripped third phase extracted 86% Li, 46% Ni, 44% Co, and 37% Mn from the feed stream. The high metal extraction in the third phase indicated that the third phase had a high extractant concentration and, therefore, a much larger metal loading capacity, which also notably increased the Co and Ni co-extraction. The transfer of D2EHPA to the third phase from the organic phase supports the hypothesis that insoluble organometallic complexes were formed in the organic phase and produced the reversible third phase observed at a pH of 5.5 or greater.
The third phase was analysed using STEM and the microscopic images revealed a honeycomb-like structure within the third phase, where thin films surrounded vesicles of varying shapes and sizes. It is proposed that amphiphilic organometallic complexes formed, which further aligned into vesicles, planar bilayers, or 2D mesoporous films. These structures were most likely responsible for the prolonged phase segregation observed. The STEM images were also mapped with HAADF, where it was observed that the vesicles within the third phase were mostly dark vesicles that contained small atoms, such as Li and H. The thin, grey films are proposed to be carbon-, oxygen-, and phosphorous-rich fluid, such as the hydrocarbons from the diluent or the D2EHPA. The light vesicles contained transition metals, such as Mn, Co, and Ni.
Operation with the third phase requires various considerations due to its high viscosity and prolonged phase disengagement. Therefore, it is recommended that the Li separation be performed with a feed with no residual Mn between a pH of 3 and 4, for which the Li separation from Ni and Co will be maximised and no third phase will occur.

Author Contributions

Conceptualization, T.P.; Data curation, P.V.W. and G.A.; Formal analysis, T.P.; Funding acquisition, S.M.B. and G.A.; Investigation, T.P.; Methodology, T.P.; Project administration, G.A.; Supervision, S.M.B. and P.V.W.; Validation, S.M.B., P.V.W., and G.A.; Writing—original draft, T.P.; Writing—review and editing, S.M.B., P.V.W., and G.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

Funding from the Wilhelm Frank Trust and the Stellenbosch University Postgraduate Scholarship Programme is gratefully acknowledged.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Metals 12 01400 g0a1 550
Figure A1. Ternary solvent extraction system with the organic phase (indicated by a black arrow), third phase (indicated by an orange arrow), and aqueous phase (indicated by a white arrow).
Figure A1. Ternary solvent extraction system with the organic phase (indicated by a black arrow), third phase (indicated by an orange arrow), and aqueous phase (indicated by a white arrow).
Metals 12 01400 g0a1
Metals 12 01400 g0a2 550
Figure A2. STEM image 2 with HAADF mapping of third phase. Vesicles: bright and dark amorphous areas with distinct boundaries indicated by black and white arrows, respectively; films: filamentous grey regions surrounding vesicles indicated by yellow arrows; wormlike structures: black linear regions within the light vesicles indicated by blue arrows.
Figure A2. STEM image 2 with HAADF mapping of third phase. Vesicles: bright and dark amorphous areas with distinct boundaries indicated by black and white arrows, respectively; films: filamentous grey regions surrounding vesicles indicated by yellow arrows; wormlike structures: black linear regions within the light vesicles indicated by blue arrows.
Metals 12 01400 g0a2
Metals 12 01400 g0a3 550
Figure A3. STEM image 3 with HAADF mapping of third phase. Vesicles: bright and dark amorphous areas with distinct boundaries indicated by black and white arrows, respectively; films: filamentous grey regions surrounding vesicles indicated by yellow arrows; wormlike structures: black linear regions within the light vesicles indicated by blue arrows).
Figure A3. STEM image 3 with HAADF mapping of third phase. Vesicles: bright and dark amorphous areas with distinct boundaries indicated by black and white arrows, respectively; films: filamentous grey regions surrounding vesicles indicated by yellow arrows; wormlike structures: black linear regions within the light vesicles indicated by blue arrows).
Metals 12 01400 g0a3

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Figure 1. Citric acid deprotonation, adapted from Heller et al. [32], and average pKa values at 20 °C [31,33,34].
Figure 1. Citric acid deprotonation, adapted from Heller et al. [32], and average pKa values at 20 °C [31,33,34].
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Figure 2. Extraction mechanism for D2EHPA with R = C8H17, adapted from Wilson et al. [38].
Figure 2. Extraction mechanism for D2EHPA with R = C8H17, adapted from Wilson et al. [38].
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Figure 3. Hypothetical structures for Li extraction with D2EHPA (R = C8H17), where the Li coordination is saturated with (a) H2O molecules and (b) a citrate ion.
Figure 3. Hypothetical structures for Li extraction with D2EHPA (R = C8H17), where the Li coordination is saturated with (a) H2O molecules and (b) a citrate ion.
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Figure 4. Block flow diagram for the Li solvent extraction from citrate media summarizing the black mass preparation by Punt et al. [9] (orange striped box) and the sequential metal separation by Punt et al. [41] (blue striped box).
Figure 4. Block flow diagram for the Li solvent extraction from citrate media summarizing the black mass preparation by Punt et al. [9] (orange striped box) and the sequential metal separation by Punt et al. [41] (blue striped box).
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Figure 5. (a) Metal extraction and (b) Li separation factor with 0.9 M D2EHPA and O/A 1.
Figure 5. (a) Metal extraction and (b) Li separation factor with 0.9 M D2EHPA and O/A 1.
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Figure 6. Phase volume distribution as a function of pH.
Figure 6. Phase volume distribution as a function of pH.
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Figure 7. (a) Li and (b) Mn extractions as functions of O/A ratio and D2EHPA concentration at pH 5.5.
Figure 7. (a) Li and (b) Mn extractions as functions of O/A ratio and D2EHPA concentration at pH 5.5.
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Figure 8. (a) Co and (b) Ni co-extraction as a function of O/A ratio and D2EHPA concentration at pH 5.5.
Figure 8. (a) Co and (b) Ni co-extraction as a function of O/A ratio and D2EHPA concentration at pH 5.5.
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Figure 9. (a) Metals stripped from the organic and third phases and (b) extraction efficiency of each phase.
Figure 9. (a) Metals stripped from the organic and third phases and (b) extraction efficiency of each phase.
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Figure 10. STEM image of third phase. Vesicles: bright/dark amorphous areas with distinct boundaries indicated by white arrows; films: filamentous grey regions surrounding vesicles indicated by yellow arrows.
Figure 10. STEM image of third phase. Vesicles: bright/dark amorphous areas with distinct boundaries indicated by white arrows; films: filamentous grey regions surrounding vesicles indicated by yellow arrows.
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Figure 11. STEM image with HAADF mapping of third phase. Vesicles: bright and dark amorphous areas with distinct boundaries indicated by black and white arrows, respectively; films: filamentous grey regions surrounding vesicles indicated by yellow arrows.
Figure 11. STEM image with HAADF mapping of third phase. Vesicles: bright and dark amorphous areas with distinct boundaries indicated by black and white arrows, respectively; films: filamentous grey regions surrounding vesicles indicated by yellow arrows.
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Table
Table 1. Average metal content and standard error of the black mass powder by weight %, adapted from Punt et al. [9].
Table 1. Average metal content and standard error of the black mass powder by weight %, adapted from Punt et al. [9].
AlCoLiMnNi
Weight %1.2%±0.0%35.4%±1.4%10.2%±0.4%23.4%±0.9%29.7%±1.2%
Table
Table 2. Lithium solvent extraction feed stream composition.
Table 2. Lithium solvent extraction feed stream composition.
MetalCoLiMnNi
Concentration (mg/L)4026±221165±9273.8±6.13951±36
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Punt, T.; Bradshaw, S.M.; Van Wyk, P.; Akdogan, G. Phase Separation in a Novel Selective Lithium Extraction from Citrate Media with D2EHPA. Metals 2022, 12, 1400. https://doi.org/10.3390/met12091400

AMA Style

Punt T, Bradshaw SM, Van Wyk P, Akdogan G. Phase Separation in a Novel Selective Lithium Extraction from Citrate Media with D2EHPA. Metals. 2022; 12(9):1400. https://doi.org/10.3390/met12091400

Chicago/Turabian Style

Punt, Tiaan, Steven M. Bradshaw, Petrie Van Wyk, and Guven Akdogan. 2022. "Phase Separation in a Novel Selective Lithium Extraction from Citrate Media with D2EHPA" Metals 12, no. 9: 1400. https://doi.org/10.3390/met12091400

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