Recovery of Lithium Using Tributyl Phosphate in Methyl Isobutyl

Sep 11, 2012 - ABSTRACT: Lithium recovery from salt-lake brines was explored using the ... NH4Cl was a suitable washing agent for magnesium ions but n...
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Recovery of Lithium Using Tributyl Phosphate in Methyl Isobutyl Ketone and FeCl3 Zhiyong Zhou, Wei Qin,* Shengke Liang, Yuanzhong Tan, and Weiyang Fei State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, P.R. China ABSTRACT: Lithium recovery from salt-lake brines was explored using the extraction equilibria of lithium with tributyl phosphate (TBP) in methyl isobutyl ketone (MIBK), with FeCl3 coextractant, for various volume concentrations of TBP, molar ratios of Fe to Li, and volume ratios of the organic to aqueous phases. Washing and stripping equilibria of magnesium and lithium ions with HCl, NH4Cl, and LiCl/HCl and NH4Cl/HCl combinations were investigated. The extraction of lithium ions from saltlake brines was successful. NH4Cl was a suitable washing agent for magnesium ions but not for stripping lithium ions into the aqueous phase. HCl can wash magnesium ions and strip lithium ions but corrodes equipment. The LiCl/HCl and NH4Cl/HCl combinations reduced equipment corrosion and washed and stripped magnesium and lithium ions, respectively, at appropriate volume ratios. MIBK loss was reduced using high-salinity solutions and large volume ratios during extraction and adjusting the volume ratio and overall chloride-ion concentration during washing/stripping.

1. INTRODUCTION Lithium is well-known as an energy metal: Lithium and lithium compounds are used in lithium-ion batteries1−3 and in nuclear energy systems,4 and these applications are attracting increasing attention. The demand for lithium and lithium compounds has increased in recent years.5−8 Currently, 80% of the lithium produced globally is obtained from salt-lake brines using sedimentation,9 calcination,10 and solar evaporation.11 Salt-lake brines in China have high Mg/Li concentration ratios, unlike brines in other countries, and it is very difficult to recover lithium from salt-lake brines in China using sedimentation and calcinations methods because of preferential magnesium extraction. In the treatment of salt-lake brines in China, sodium, potassium, and boron have always been removed first, by solar or heat evaporation, to obtain NaCl and KCl, and the main metal ions in the treated salt-lake brines are Li+ and Mg2+. For example, treated Da Qaidam salt-lake brine contains the following species concentrations (mol/L):12 Li+, 0.4; K+, 0.0167; Na+, 0.0492; Mg2+, 4.568; SO42−, 0.196; and B2O3, 0.099. Treated salt-lake brine supplied by China International Trust and Investment Corporation Guoan contains the following species concentrations (mol/L):13 Li+, 0.297; K+, 0.01; Na+, 0.139; Mg2+, 4.83; Cl−, 9.108; and B2O3, 0.0825. Therefore, the recovery of lithium from treated salt-lake brines in China with high concentrations of Mg2+ is a problem that urgently needs to be solved. Recent research has focused on lithium recovery from salt-lake brines or seawater using methods such as precipitation,14 adsorption on nanocrystalline MnO2,15−18 adsorption by acid and sodium Amberlite,19 and extraction using supported liquid membranes,20 but few industrial applications can be found in China. The liquid− liquid extraction of lithium from salt-lake brines is also attracting increasing attention.21,22 The most widely used extractant is tributyl phosphate (TBP)/kerosene−FeCl3.23−25 TBP is a popular neutral organophosphorus extractant, and kerosene is a typical diluent. In this system, FeCl3 solution acts as a coextractant and contributes © 2012 American Chemical Society

greatly to lithium extraction. The extraction reaction equations for TBP and FeCl3 are as follows:26 When the molar concentration of HCl is less than 2 mol/L FeCl3(a) + 3TBP(o) ⇄ FeCl3·3TBP(o)

(1)

When the molar concentration of HCl is greater than 6 mol/L FeCl3(a) + Cl−(a) ⇄ FeCl4 −(a)

(2)

H3O+(a) + FeCl4 −(a) + H 2O(a) + 2TBP(o) ⇄ [H3O(TBP)2 (H 2O)]+ [FeCl−4 ](o)

(3)

where the aqueous and organic phases are denoted by “(a)” and “(o)”, respectively. Obviously, to achieve ion-association extraction of lithium ions when the cation is a lithium ion, the molar concentration of chloride ions must be greater than 6 mol/L. MgCl2 is used as the chloride source because of the characteristic high Mg/Li ratios in salt-lake brines in China. The extraction reaction equations for TBP and lithium ions and magnesium ions are as follows: Li+(a) + FeCl4 −(a) + nTBP(o) ⇄ LiFeCl4 ·nTBP(o) (4)

Mg 2 +(a) + 2FeCl4 −(a) + nTBP(o) ⇄ Mg(FeCl4)2 ·nTBP(o)

(5)

For extraction processes to give high-purity products, fractional extraction processes must be used (i.e., extraction, washing, and stripping). It should be noted that the stoichiometric ratio of chloride ions in the washing and Received: Revised: Accepted: Published: 12926

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phase sample was taken from the bottom layer (aqueous phase) for solute concentration analysis at 25 ± 2 °C. 2.3. Analysis. The cation concentrations of the aqueous samples were analyzed using an optical atomic absorption spectrometer (Z-5000-AAS, Hitachi, Tokyo, Japan). The concentration of cations in the organic phase was calculated from the material balance. Preliminary experiments on stripping of the organic phase indicated that the deviations of the calculated values of lithium-ion concentrations were within ±3%.

stripping reagents should be set so that ferric ions remain in the organic phase by adjusting the volume ratios of the two phases. Compounds with chloride ion must be considered in the washing and stripping processes. Because a third phase could be formed between the aqueous and organic phases with TBP/ kerosene as the solvent, in this study, methyl isobutyl ketone (MIBK) was used as the diluent instead of kerosene, to avoid formation of a third phase in the extraction process.22 HCl is a typical univalent acid and is a potential washing and stripping agent. NH4Cl is a typical inorganic chloride salt and is also a potential washing and stripping agent. The extraction equilibria for H2O−Mg/Li−FeCl3/TBP/MIBK for various TBP volume concentrations, Fe/Li molar concentration ratios, and R(O/A) values (volume ratio of the two phases) were investigated. The washing and stripping equilibria were studied using HCl and NH4Cl in various molar concentrations and with various R(O/ A) values. Finally, mixtures of LiCl + HCl and NH4Cl + HCl were selected as washing and stripping agents, respectively, and the optimum operating conditions for this combined system were studied.

3. RESULTS AND DISCUSSION 3.1. Extraction Behavior at Various TBP Volume Concentrations. The extraction behaviors of lithium solutions at various volume concentrations of TBP are shown in Figure 1.

2. EXPERIMENTAL SECTION22 2.1. Materials. The analytical reagents were as follows: LiCl from Beijing Yili Fine Chemical Co., Ltd., purity > 97%; FeCl3 from Tianjin Yongda Chemical Reagent Co., Ltd., purity > 99%; MgCl2 and NH4Cl from Beijing Modern Eastern Finechemical Co., Ltd., purity > 98%; HCl from Beijing Modern Eastern Finechemical Co., Ltd., 36−38 wt %; TBP and MIBK from Beijing Yili Fine Chemical Co., Ltd., purity > 99 wt %. The characteristics of the salts and the acid are listed in Table 1, and the physical properties of the solvent, provided by the manufacturers, are listed in Table 2. Table 1. Physical Properties of Salts and Acids chemical

formula

average molecular weight

solubility (g/100 g of H2O, 20 °C)

lithium chloride ferric chloride magnesium chloride hydrochloric acid ammonium chloride

LiCl FeCl3 MgCl2

42.39 162.21 95.22

78.5 55.1 74.0

HCl

36.46

36.5

NH4Cl

53.49

37.2

Figure 1. Variations in separation factor and partition coefficient with respect to TBP volume concentration.

It can be seen that the partition coefficient (D = Co/Ca, where Co is the equilibrium concentration of lithium ion in the organic phase and Ca is the equilibrium concentration of lithium ion in the aqueous phase) of the lithium ions first increased and then decreased with increasing volume concentration of TBP, which indicates that the TBP/MIBK−FeCl3 system is a typical synergistic extraction system.27 The mean value of the partition coefficient was larger than 3, which was much larger than that in other lithium-extraction processes. The separation factors (β = DLi/DMg, where DLi is the partition coefficient of lithium ion and DMg is the partition coefficient of magnesium ion) for Li/ Mg were all larger than 200. A volume concentration of TBP of 50−60% gave the best partition coefficient (3.504) and separation factor (433) for Li/Mg. 3.2. Extraction Behavior at Various Fe/Li Molar Ratios. The effects of the Fe/Li molar ratio on the extraction behavior were investigated using TBP/MIBK as the extractant, FeCl3 as the coextraction agent, and MgCl2 as the chloride source; the results are shown in Figure 2. It can be seen that the partition coefficient of the lithium ions increased with increasing Fe/Li molar ratio, which indicates that the partition coefficient of the lithium ions increased with increasing concentration of ferric ions in a certain range. The mean value of the partition coefficient of the lithium ions was more than 2.5. The separation factor for Li/Mg decreased with increasing Fe/Li molar ratio, because the loading of lithium ions in the organic

Table 2. Physical Properties of Extractants and Diluents chemical

formula

average molecular weight

ρ (g·cm−3)

TBP MIBK

OP[O(CH2)3CH3]3 CH3COCH2CH(CH3)2

266.32 100.16

0.980 0.796

2.2. Methods. All extraction experiments were conducted using 50-mL flasks at 25 ± 2 °C. TBP/MIBK (volume concentration of TBP = 30%, 40%, 50%, 60%, or 70%) and a mixed solution of LiCl (0.05 mol/L), FeCl3 (Fe/Li molar ratio = 1.0, 1.2, 1.5, 1.8, or 2.0), and MgCl2 (3.5 mol/L) were added to each flask. All washing and stripping experiments were conducted with 100-mL flasks at 25 ± 2 °C. The flask containing the mixture was shaken for about 10 min by hand and then left to equilibrate and settle for 30 min, during which the two phases separated. Preliminary experiments were carried out, and it was found that equilibrium was reached in all experimental solution systems within 10 min. An aqueous12927

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Generally speaking, the extractant TBP/MIBK with FeCl3 as the coextraction agent can effectively extract lithium ions from salt-lake brines. Considering the high viscosity of TBP, the volume concentration of TBP was usually between 20% and 80%, and it is proposed that a TBP volume concentration of 50% or 60% and appropriate Fe/Li molar concentration ratios and R(O/A) values are suitable for extracting lithium ions from different salt-lake brines. 3.4. Washing and Stripping Behaviors with HCl and NH4Cl. In previous work,23 the extraction equilibrium of lithium was studied using TBP in kerosene, with FeCl3 as a coextraction agent, and three salts, namely, MgCl2, CaCl2, and NH4Cl, as chloride sources at different Fe/Li molar ratios. The results showed that the extraction capacities for lithium ions followed the sequence MgCl2 > CaCl2 > NH4Cl. There is competition between Li+ and NH4+, Ca2+, and Mg2+ when they are combined with TBP and FeCl3, and MgCl2 benefits from weaker competition. The binding capacities of cations with TBP + FeCl3 followed the sequence Li+ > NH4+ > Ca2+ > Mg2+. As HCl has been used as a washing and stripping agent for lithium ions,28 the sequence should be as follows: H+ > Li+ > NH4+ > Ca2+ > Mg2+. Therefore, any of agents HCl, LiCl, NH4Cl, and CaCl2 can be used to wash magnesium ions from the organic phase to the aqueous phase to purify the lithium ions loaded in the organic phase, and HCl is the only agent that can strip lithium ions into the aqueous phase. In this study, HCl and NH4Cl were selected as the washing and stripping agents, and the effects of different molar concentrations of HCl and NH4Cl and different R(O/A) values on the washing and stripping behaviors were investigated. As MgCl2 was selected as the chloride source, the magnesium ions in the organic phase should first be washed down to the aqueous phase as much as possible to obtain a higher-purity lithium product, with as large amounts as possible of lithium and ferric ions remaining in the organic phase. Then the lithium ions should be stripped for enrichment and product preparation with as large amounts as possible of ferric ions remaining in the organic phase for circulation. As shown in Figures 4 and 5, larger molar concentrations of Cl− and R(O/A) values promote retention of ferric ions in the organic phase. Larger R(O/A) values and smaller molar concentrations of HCl and NH4Cl promote retention of lithium ions in the organic phase. According to Figures 4b and 5, both HCl and NH4Cl are suitable washing agents for magnesium ions. As shown in Figure 4a,c, a larger molar concentrations of HCl promotes ferric ion retention in the organic phase and increases lithium-ion stripping into the aqueous phase. Adjustment of R(O/A) to an appropriate value using HCl as the stripping agent strips lithium ions well. As shown in Figure 5a,c, the molar concentration of NH4Cl was not the main factor affecting lithium-ion stripping, and the conditions of maximum retention of ferric ions in the organic phase and maximum lithium-ion stripping into the aqueous phase cannot be simultaneously satisfied by adjusting R(O/A). Also, ferric hydroxide could easily precipitate as a result of the high pH values of the NH4Cl solutions. NH4Cl is not suitable for stripping lithium ions from the organic phase into the aqueous phase, but HCl is suitable, which confirms the above inference regarding the sequence H+ > Li+ > NH4+ > Ca2+ > Mg2+. 3.5. Washing and Stripping Behaviors with Mixtures of LiCl + HCl and NH4Cl + HCl. The results reported in the previous section show that HCl is suitable for washing out magnesium ions or stripping lithium ions into the aqueous

Figure 2. Variations in separation factor and partition coefficient with respect to Fe/Li molar ratio.

phase gradually became saturated and the partition coefficient of the magnesium ions increased with increasing Fe/Li molar ratio. Fe/Li = 1.2 gave the best partition coefficient (2.468) and separation factor (340) for Li/Mg. 3.3. Extraction Behavior at Various R(O/A) Values. The volume ratio between the organic and aqueous phases, R(O/ A), is another typical factor that influences the extraction equilibrium of lithium ions. As shown in Figure 3, the partition

Figure 3. Variations in separation factor, extraction efficiency (%), and partition coefficient with respect to R(O/A) value.

coefficient of the lithium ions first increased and then decreased with increasing R(O/A) value, which is similar to the influence of the TBP volume ratio on the extraction equilibrium. This is because the loading of lithium ions in the organic phase first increased; then became almost saturated with increasing R(O/ A) value; and when the value of R(O/A) was larger than 1, the molar concentration of lithium ions gradually decreased. The separation factor of Li/Mg first increased and then decreased slightly with increasing R(O/A) value. R(O/A) = 1 gave the best partition coefficient (7.588) and separation factor (452) for Li/Mg. 12928

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Figure 4. Variations in washing or stripping efficiency (%) for cations with respect to HCl molar concentration and R(O/A) value.

Figure 5. Variations in washing or stripping efficiency (%) for cations with respect to NH4Cl molar concentration and R(O/A) value.

phase. However, HCl corrodes the equipment. It is necessary to find a washing or stripping agent that can not only reduce the effects of acid corrosion but also achieve good washing or stripping efficiencies and avoid precipitation of ferric hydroxide.

Based on the sequence H+ > Li+ > NH4+ > Ca2+ > Mg2+, mixtures of LiCl + HCl and NH4Cl + HCl were selected as washing and stripping agents, respectively. The dependence of 12929

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the operating conditions on the equilibrium was studied; the results are shown in Figures 6 and 7. Larger R(O/A) values

Figure 7. Variations in stripping efficiency (%) for cations with respect to R(O/A) value using HCl + NH4Cl as the stripping reagent.

adjusting R(O/A), which can be removed in producing the raw materials Li2CO3 for Li-ion batteries from LiCl. 3.6. MIBK Losses during Extraction, Washing, and Stripping Processes. Because MIBK is a strongly polar diluent, the loss of MIBK during the extraction, washing, and stripping processes should be considered. The percentage losses of MIBK during extraction using TBP/MIBK−FeCl3 as the extractant are shown in Figure 8, from which it can be seen

Figure 6. Variations in washing efficiency (%) for cations with respect to R(O/A) value using HCl + LiCl as the washing reagent.

promote the washing out of magnesium ions and the retention of ferric and lithium ions in the organic phase. Smaller molar concentrations of HCl and larger molar concentrations of LiCl promote the retention of lithium ions in the organic phase and reduce the acid corrosion of the equipment. In addition, the lithium ions in the washing agent can exchange with magnesium ions in the organic phase by cation-exchange reactions as the value of R(O/A) increases, so the washing efficiency for lithium ions becomes negative. The stripping behavior of lithium ions for enrichment and product preparation using a mixture of NH4Cl and HCl is shown in Figure 7. It can be seen that the maximum retention of ferric ions in the organic phase and the maximum stripping of lithium ions into the aqueous phase can be simultaneously satisfied by adjusting R(O/A). Smaller molar concentrations of HCl and larger molar concentrations of NH4Cl also reduce acid corrosion of the equipment. The concentration of ferric ions in the stripping solutions can be reduced to 0.1 mg/L by

Figure 8. Variations in percentage loss of MIBK during the extraction process using TBP/MIBK−FeCl3 as the extractant.

that the loss of MIBK decreased for high-salinity solutions and larger R(O/A) values. Figure 9 shows that the loss of MIBK decreased sharply with increasing R(O/A), and larger overall chloride-ion concentrations reduced the loss of MIBK during the washing and stripping processes. Mixtures of LiCl + HCl and NH4Cl + HCl with the same overall chloride-ion concentrations resulted in similar MIBK losses. Adjustment of the R(O/A) value and overall chloride-ion concentration can decrease MIBK losses. Finally, a possible process for the recovery of lithium from salt-lake brines in China is shown in Figure 10. 12930

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achieves good stripping of lithium ions. Because HCl corrodes the equipment, mixtures of LiCl + HCl and NH4Cl + HCl were selected as washing and stripping agents, respectively. Larger R(O/A) values promoted the washing out of magnesium ions and the retention of ferric and lithium ions in the organic phase. The maximum retention of ferric ions in the organic phase and stripping of lithium ions into the aqueous phase, using a mixture of NH4Cl and HCl as the stripping agent, can be simultaneously satisfied by adjusting the R(O/A) value. Smaller molar concentrations of HCl and larger molar concentrations of LiCl or NH4Cl reduce acid corrosion of the equipment. MIBK losses can be decreased efficiently using high-salinity solutions and larger R(O/A) values during the extraction process. MIBK losses decreased sharply with increasing R(O/ A) values, and large overall chloride-ion concentrations during the washing and stripping processes reduced MIBK losses. Adjustment of the R(O/A) value and the overall chloride-ion concentration reduce MIBK losses.

■

Figure 9. Variations in percentage loss of MIBK during washing or stripping processes with LiCl + HCl and NH4Cl + HCl as the washing or stripping agents.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 10 62782748. Tel.: +86 10 62782748. Notes

4. CONCLUSIONS In this work, the equilibrium data for lithium-ion extraction using TBP in MIBK with FeCl3 as a coextraction agent and MgCl2 as a chloride source for various TBP volume concentrations, Fe/Li molar ratios and R(O/A) values were investigated. The extraction system TBP/MIBK with FeCl3 as the coextraction agent can effectively extract lithium ions from salt-lake brines. A volume concentration of TBP of 50% or 60% and appropriate Fe/Li molar ratios and R(O/A) values for different salt-lake brines are suitable for the extraction processes. The binding capacities of cations with TBP + FeCl3 followed the sequence H+ > Li+ > NH4+ > Ca2+ > Mg2+. Both HCl and NH4Cl are suitable washing agents for magnesium ions. The precipitation of ferric hydroxide could occur easily because of the high pH values of the NH4Cl solutions. NH4Cl is not suitable for stripping lithium ions from the organic phase into the aqueous phase. Adjustment of R(O/ A) to an appropriate value with HCl as the stripping agent

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National High Technology Research and Development Program of China (2008AA06Z111) and the QingHai Key Technology R&D Program (2011-J-154).

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NOTATION C0 = initial concentration of lithium ion in the aqueous phase Ca = equilibrium concentration of lithium ion in the aqueous phase Co = equilibrium concentration of lithium ion in the organic phase Co′ = initial concentration of lithium ion in the organic phase D = partition coefficient = Co/Ca Di = partition coefficient of component i

Figure 10. Process for lithium recovery from salt-lake brines in China. 12931

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E = extraction efficiency = (C0 − Ca)/C0 R = phase ratio R(O/A) = volume ratio = VO/VA VA = volume of the aqueous phase VO = volume of the organic phase β = separation factor, β1/2 = D1/D2

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