Research Paper

Journal of the Korean Institute of Resources Recycling. December 2020. 79-87
https://doi.org/10.7844/kirr.2020.29.6.79

# MAIN

• 1. Introduction

• 2. Experimental

• 3. Results and discussion

•   3.1. Solvent extraction of HCl by commercial extractants

•   3.2. Solvent extraction of HCl by synthesized ionic liquids

• 4. Conclusions

1. Introduction

Hydrochloric acid (HCl) is widely employed in hydrometallurgical processes as a metal leaching agent for ores and secondary resources due to its excellent chemical characteristics such as strong complex formation and fast reaction kinetics1). The chloride system applied in metallurgy can be operated over a wide range of oxidizing conditions and acidity2). In order to meet the environmental regulation against the treatment of HCl and to improve process economics, it is desirable to recover HCl from the spent leaching solutions1,2).

Several methods are available to treat effluents containing inorganic acids, including neutralization, diffusion dialysis, membrane process, precipitation, solvent extraction, and ion exchange1,3,4). Among these methods, solvent extraction is considered to be one of the most effective one for extracting inorganic acids from effluents. During solvent extraction, the reaction between the extractants and the protons of the acid might be responsible for the extraction of these acids5). In general, most tertiary amines extract hydrogen ions through the protonation mechanism, while neutral extractants can extract through the solvation6,7,8,9). According to previous works, the commercial extractants such as TEHA (tris 2-ethylhexyl amine), Cyanex 923 (a mixture of four trialkylphosphine oxides), TBP (tributyl phosphate), Alamine 336 (a mixture of tri-octyl/decyl amine) have been employed for the recovery of H2SO4 and HCl10). Among them, TEHA has the best performance for extraction but stripping percentage of HCl from the loaded organic phase was low, resulting in low concentration of HCl in the recovered acid6,10). The HCl from the loaded TEHA could be completely stripped by 2 M ammonia solution11). However, it seems to be an infeasible solution in industry due to the complication of stripping liquor and the difficulty in regeneration of HCl. Although the extraction percentage of H2SO4 and HCl by Alamine 336 is higher than that by Cyanex 923 and TBP, but the stripping of the acids from the loaded Alamine 336 is also very difficult6,10,12). It has been reported that some ionic liquids (ILs) can extract hydrogen ions from weak HCl13,14). The extraction of hydrogen ion by ILs depends on solution pH. The extraction reaction of hydrogen ions from weak chloride solution by ILs synthesized from Aliquat 336 (N-Methyl- N, N, N-trioctylammonium chloride) and Cyanex 272 (bis(2,2,4 trimethylpentyl)phosphinic acid) can be described as solvation mechanism5).

The solvent extraction of inorganic acids from spent leaching solutions can be beneficial on the light of environment and process economics but most of the works have been done from weak acidic solutions12). In order to overcome some disadvantages of employing single TEHA, the influence of adding neutral extractants such as Cyanex 921 (trioctylphosphine oxide), DOS (di- n-octyl sulfide), MIBK (methyl isobutyl ketone), TBP (tributyl phosphate), and TOP (tri octyl phosphate) to TEHA has been investigated on the extraction of HCl11). LIX 63 (7-Hydroxy-5,8-diethyl-6-dodecanone oxime) is a kind of oxime extractant and many works have been reported on the synergistic effect of the extractant mixture containing LIX 6315,16). However, there are few works on the solvent extraction of HCl with the mixture of TEHA and LIX 63.

Some ionic liquids (ILs) which are synthesized by mixing Aliquat 336 and organophosphorus acids can act as bifunctional extractants and thus extract hydrogen ions. Therefore, in this work, solvent extraction of hydrochloric acid from concentrated solution was investigated by employing commercial extractants and ILs. For this purpose, the extraction and stripping behavior of HCl was compared between commercial extractants (LIX 63 and a mixture of LIX 63 and TEHA) and synthesized ILs (ALi-SCN, ALi-PC).

2. Experimental

The acidity of the solution was adjusted by adding doubly distilled water or a mixture of doubly distilled water and ethanol (C2H5OH; Daejung Co., 94.5%) to concentrated HCl (Daejung Co., 35%) and H2SO4 solution (Daejung Co., 95%). The commercial extractants such as TEHA (R3N, BASF Co., 99%), LIX 63 (HR, Is Chem. Co., 70%), Cyanex 923 (Cytec Inc., 93%), Aliquat 336 (BASF Co., 93%) were used without any purification. 1-Octanol (Duksan Pure Chemical Co., 99%) was used as a modifier. Ionic liquid (ALi-PC) was synthesized by mixing an equimolar concentration of Aliquat 336 and PC88A (2-ethylhexylphosphonic acid mono-2-ethylhexyl ester, HA, Cytec Inc., 95%) according to the method reported in the literature13). ALi-SCN was synthesized by mixing Aliquat 336 and NH4SCN (ammonium thiocyanate, Daejung Co., 99.0%) as reported17). Kerosene (Samchun Pure Chemical Co.) was used as a diluent and all the employed chemicals were of analytical grade.

The extraction and stripping experiments were conducted by shaking equal volumes of aqueous and organic phases (20 mL each) in a screwed cap bottle for 30 mins using a Burrell wrist action shaker (model 75, USA) at room temperature (22℃ ± 1). The two phases were separated using a separation funnel after holding them for 30 mins. The concentration of hydrogen ions in the aqueous phase before and after extraction was measured by volumetric titration method18). The concentration of hydrogen ions in the loaded organic phase was determined by mass balance. The experiments were repeated 3 times with an error of ± 5%.

3. Results and discussion

3.1. Solvent extraction of HCl by commercial extractants

3.1.1. Effect of nature of extractant on solvent extraction of HCl

According to the previous studies, hydrogen ions can be well extracted by amines but the stripping of hydrogen ions from the loaded amines is very difficult6,10,12). Among several tertiary amines, TEHA shows the best performance for the extraction of HCl. Addition of neutral extractants such as Cyanex 921, DOS, MIBK, TBP, and TOP to TEHA has negligible effect on the extraction of HCl11). Moreover, stripping of HCl from the loaded mixtures of TEHA and neutral extractants is still difficult6). Since stripping is the reverse reaction of extraction, it is important to find extraction systems with adequate extraction capability for HCl. It has been reported that α-hydroxyoxime like LIX 63 can interact with HCl at high concentration by using characteristic absorption of oxime19). Therefore, a mixture of TEHA and LIX 63 was first employed in this work. In order to compare the extraction behavior between HCl and H2SO4, the concentration of both acids was adjusted to 3 M. Fig. 1 shows the extraction percentage of both acids by TEHA, LIX 63 and a mixture of TEHA and LIX 63. The concentration of single TEHA and LIX 63 was 50 vol% and the total volume percentage of TEHA and LIX 63 in the mixture was 50 vol% (TEHA/LIX 63 ratio = 1) and 20 vol% of octanol was added as a modifier. The extraction performance of single TEHA for both acids was similar, while no sufuric acid was extracted by single LIX 63. Addition of TEHA to LIX 63 increased the extraction percentage of HCl from 15.7 to 25.7% and H2SO4 from zero to 17.6% compared to single LIX 63. This increase might be ascribed to the interaction between the extractants in binary system, which could lead to synergistic effect. The reactions can be represented as6,9)

##### (1)
$R3N(o)+HR(o)=R3N×HR(o)$
##### (2)
$R3N(o)+HCl(a)=R3N·HCl(o)$
##### (3)
$HCl(a)+HR(o)=HR·HCl(o)$

where R3N denotes TEHA, HR denotes LIX 63, subscript a and o represent the aqueous and organic phase, respectively.

### Effect of extractants on hydrogen ion (3 M HCl, 3 M H2SO4) extraction. Condition: 50% volume of TEHA, LIX 63 and their mixture (MIX) (25:25 vol%) in kerosene and 20% octanol as modifier; O/A = 1; shaking time = 30 mins, 22℃ ± 1.

It has been reported that temperature has a great influence on the stripping efficiency of hydrogen ions from the loaded phase by distilled water10). Therefore, the striping of HCl and H2SO4 from the loaded organic phases was carried out at 60°C. The results indicate that highest stripping percentage of both acids was obtained from the loaded mixture. Especially, H2SO4 was completely stripped with water under from the loaded mixture of TEHA and LIX 63. Besides, HCl was also easily stripped from the organic loaded phase of LIX 63 (% stripping (%S) > 81%, A/O = 1, only one stage) (see Fig. 2).

### Stripping of hydrogen ion from the loaded phase with H2O at 60℃ in 30 mins. Condition: organic phase: 50% volume of TEHA, LIX 63 and their mixture (MIX) (25:25 vol%) in kerosene and 20% octanol as modifier; O/A = 1.

In order to investigate the effect of the composition of the mixture, the volume percentage of LIX 63 in the mixture was varied from 10 to 40%, while that of TEHA was fixed at 50% v/v. A slight increase in extraction percentage of both acids was observed when the volume percentage of LIX 63 increased from 10 to 40% (see Fig. 3). Namely, the extraction percentage increased from 36.8 to 37.6% and 37.1 to 41.3% for HCl and H2SO4, respectively. However, there was little difference in the stripping percentage of both acids in the experimental ranges (see Fig. 4). The stripping percentage of H2SO4 was a little higher than that of HCl with H2O at 60℃. Fig. 4 shows that the stripping performance were less than 50%. Compared to single extractants TEHA/ LIX 63, using their mixture slightly enhance extraction performance (Figs. 1 & 3). However, stripping of HCl from the loaded phase of mixture TEHA and LIX 63 was easier than that of single TEHA (Figs. 2 & 4). Thus, the employment of LIX 63 is more efficient for extraction of HCl on the light of the stripping step.

### Stripping of hydrogen ion from the loaded mixture TEHA and LIX 63 with H2O at 60℃ in 30 mins (O/A = 1).

3.1.2. Effect of some additives on HCl extraction

In order to investigate the effect of dielectric constant of aqueous phase, C2H5OH was added to the aqueous phase to reduce the interaction between hydrogen ions and water molecules, which might enhance the extraction of hydrogen ions as well as hinder the extraction of water20). The use of polar and protic solutes shows a favorable effect on the extraction of hydrogen ions through ion pair formation. For this purpose, HCl extraction was compared between H2O and a mixture of H2O and C2H5OH. In these experiments, the volume ratio of water and C2H5OH was equal. Table 1 shows the extraction percentage of HCl by LIX 63 from the mixture was 2 times higher than that from water. Although the extraction percentage of HCl was less than 10%, this result indicates that addition of C2H5OH to water can enhance the extraction of HCl. However, addition of too much C2H5OH to water can lead to the loss of extractant owing to the increase in its solublity in water mixture. Therefore, the volume ratio of H2O and C2H5OH in the mixture was fixed at unity in further experiments.

#### Effect of C2H5OH on HCl extraction by LIX 63 50 vol% in kerosene

 Aqueous phase Extraction percentage, % HCl in H2O 4.0 HCl in H2O + C2H5OH 8.5

*Note: aqueous phases include 1 M HCl in H2O and 1 M HCl in the mixture of 50% H2O and 50% C2H5OH (v/v), O/A = 1.

Moreover, the results showed that HCl was efficiently stripped from loaded organic phases containing LIX 63. Therefore, in order to investigate the extraction and stripping behavior of HCl from the aqueous phase containing C2H5OH, three kinds of organic phases, namely, single LIX 63 (50%), a mixture of 50% LIX 63 and 20% Cyanex 923 and a mixture 50% LIX 63 and 20% Aliquat 336 were also employed. Among them, the mixture of LIX 63 and Aliquat 336 displays the best extraction capacity (see Table 2).

#### Effect of extractant on the extraction of 1 M HCl in the mixture 50% H2O and C2H5OH

 Organic phase Ratio of components, v/v Extraction percentage, % LIX63: kerosene 50:50 8.5 LIX63:Cyanex 923:kerosene 50:20:30 6.5 LIX63:Aliquat 336:kerosene 50:20:30 11.3

Modifier can affect the extraction4,7,21) and thus the addition of modifier like octanol to the extractants was tested. In these experiments, the aqueous was prepared by adding C2H5OH to water. Table 3 shows that polar modifier like octanol affects little the HCl extraction and the extraction efficiency was very low. This effect can be explained by the interaction between modifier and extractants. The interaction between extractant and modifier is proportional to the polarity of the modifier, leading to a decrease in the effective concentration of extractants22). Therefore, it can be said that addition of poplar modifier to organic phase has negative effect on HCl extraction.

#### Effect of diluent on HCl extraction by LIX 63, Aliquat 336 and their mixture

 Organic phase Ratio of components, v/v Extraction percentage, % LIX 63:octanol:kerosene 50:20:30 6.2 Aliquat 336:octanol:kerosene 50:20:30 0.0 LIX 63:Aliquat 336:octanol:kerosene 50:20:10:20 8.1 LIX 63:Aliquat 336:octanol:kerosene 50:20:20:10 6.2 LIX 63:Aliquat 336:octanol 50:20:30 7.1 LIX 63:Aliquat 336:kerosene 50:20:30 11.3

*Note: aqueous phases were 1 M HCl in the mixture of 50% H2O and 50% C2H5OH (v/v), O/A = 1.

3.2. Solvent extraction of HCl by synthesized ionic liquids

When metal ions are extracted by some kinds of ILs from weak acidic solutions, hydrogen ions are also extracted by the anion of the ILs and thus equilibrium pH of the aqueous phase increases13,14,23). Moreover, binary mixtures consisting of Aliquat 336 and Cyanex 272 can affect the extraction of hydrogen ions from inorganic acid solutions13,24). ILs which can be synthesized from Aliquat 336 and Cyanex 272 showed the excellent performance for the extraction of hydrogen ions when the initial pH range was from 0.05 to 5. These ILs can be easily prepared in the laboratory by reacting Aliquat 336 and organophosphorus acids or anion of inorganic salts through anion exchange mechanism5,25).

In this work, ILs derived from Aliquat 336 like ALi-SCN and ALi-PC were employed for the extraction of HCl. Firstly, the concentration of HCl was adjusted to 1 M in aqueous phase. For comparison, water and a mixture of water and C2H5OH were employed. The volume percentage of H2O and C2H5OH was 50% in the mixture. In these experiments, distilled water was employed as a stripping agent. As ILs, 0.5 M ALi-SCN and 0.5 M ALi-PC in kerosene were employed. Although there was little difference in the extraction percentage of HCl from water between the two ILs, the stripping percentage of HCl by ALi-SCN was much higher than that by ALi-PC (see Table 4). This may be attributed to the strong interaction of hydrogen ions with PC88A anions in the organic phase, which hindered the liberation of hydrogen ions into the aqueous phase. The addition of C2H5OH into water led to an increase in the extraction percentage of HCl by ALi-PC from 11.0 to 42.2%, whereas there was only a light raise from 10.2 to 12.8% in the case of ALi-SCN. However, it was noticed that stripping percentage of hydrogen ions from the loaded ALi-SCN was 90.8%. Meanwhile, hydrogen ions were not stripped from the loaded ALi-PC (see Table 5). These results can be ascribed to the change in the dielectric constant of aqueous phase by adding C2H5OH. Besides, the hydrophobic anions of ALi-PC may stabilize the hydrogen ions in the organic phase. This means that the hydrophobic anions of ALi-PC has stronger basicity (pKa = 4.51) than that of SCN- (pKa = 0.926) and thus these anions have stronger affinity for the hydrogen ions of HCl.

#### Comparison on extraction and stripping of 1 M HCl in distilled water by ionic liquids

 Extractants Extraction percentage, % Stripping percentage, % ALi-SCN 10.2 22.8 ALi-PC 11.0 6.3

*Note: Organic phase: 0.5 M extractant in kerosene, O/A = 1, shaking time = 30 mins, room temperature (22℃ ± 1), stripping agent: H2O.

#### Extraction and stripping of 1 M HCl in the mixture of 50% H2O and C2H5OH (v/v) by ionic liquids

 Extractants Extraction percentage, % Stripping percentage, % ALi-SCN 12.8 90.8 ALi-PC 42.2 0.9

*Note: Organic phase: 0.5 M extractant in kerosene, O/A = 1, shaking time = 30 mins, room temperature (22℃ ± 1), stripping agent: H2O.

Effect of ALi-SCN concentration on the extraction of 1 M HCl from aqueous phase was also investigated. Fig. 5 shows that as ALi-SCN concentration increased from 0.3 to 1.0 M, the extraction percentage of HCl rose from 7.6 to 13.7%. Table 6 compares the effect of stripping temperature from the loaded ALi-SCN between water and the mixture of water and C2H5OH. Compared to the nature of the stripping agent, stripping temperature shows negligible effect. Addition of C2H5OH to water greatly improves the stripping percentage of HCl from the loaded ALi-SCN. Table 7 shows the same experiments from the loaded ALi-PC. Not only stripping temperature but also the nature of stripping agents did not show any effect on the stripping of HCl. The strong affinity between hydrogen ions of HCl and anion of ALi-PC might be the cause of the difficultly in stripping. Although using mixture of C2H5OH and water as stripping agent did not improve stripping efficiency from the loaded phase of ALi-PC, addition of C2H5OH into aqueous phase was essential for enhancement of the extraction. When ALi-PC concentration increased from 0.5 to 1.0 M, the extraction percentage increased from 42.2 to 80.4% respectively (see Table 7). The solvent extraction reactions of HCl by ILs can be represented as5,20,26)

##### (4)
$2R4NA(o)+H+(a)+Cl-(a)=(R4NA)2·HCl(o)$
##### (5)
$R4N+A-(o)+HCl(a)=R4N+Cl-(o)+HA(o)$

where R4NA = ALi-PC/ ALi-SCN

### Effect of ALi-SCN concentration on extraction of 1 M HCl in the mixture of 50% H2O and C2H5OH. Condition: O/A = 1, shaking time = 30 mins, room temperature (22℃ ± 1).

#### Comparison of stripping of HCl from the loaded 1 M ALi-SCN phase

 Extraction percentage, % Stripping percentage, % 13.7 Stripping agent 22℃ 60℃ H2O 60.2 60.2 H2O + C2H5OH(*) 91 95.5

*Note: O/A = 1, shaking time = 30 mins; aqueous phase for extraction was 1M HCl in distilled water; (*): 50 vol% C2H5OH was added to water for stripping.

#### Comparison of extraction and stripping of HCl by ALi-PC phase

 Concentration of ALi-PC Extraction percentage, % Stripping percentage, % Stripping agent 22℃ 60℃ 0.5 M 42.2 H2O 0.8 0.8 1.0 M 80.4 H2O + C2H5OH (*) 3.8 3.8

*Note: O/A = 1, shaking time = 30 mins; aqueous phase was 1 M HCl in the mixture of 50% H2O and 50% C2H5OH (v/v) for extraction; (*): 50 vol% C2H5OH was added to water for stripping.

4. Conclusions

In order to investigate the recovery of HCl from aqueous solutions, solvent extraction experiments were performed by employing commercial extractants such as LIX 63 and its mixture with TEHA, Aliquat 336, and Cyanex 923 and synthesized ILs such as ALi-PC and ALi-SCN. Among these, ALi-PC showed higher extraction percentage than the others, but the stripping was negligible. In contrary, stripping of HCl from the loaded LIX 63 was effective (>81%). In the case of ILs, extraction performance of HCl by ALi-PC was higher than that by ALi-SCN. However, the stripping of HCl from the loaded ALi-PC was more difficult than from ALi-SCN. The extraction and stripping efficiency of HCl was enhanced by adding C2H5OH to the aqueous phase owing to polar modifier effect. However, addition of modifier like octanol to organic phase lowered extraction performance. Also, the effect of temperature on stripping HCl from the loaded ILs was insignificant. The difficulty in stripping of HCl from the loaded organic phase can be attributed to the excessive chemical interaction between HCl with extractant like ALi-PC, resulting in chemical conversion and formation of sustainable products in the organic phase. Further studies on the kinetics and interaction of HCl with extractants should be conducted to verify the interaction.

## Acknowledgements

This work was supported by the Technology Innovation Program (Development of Material Component Technology) (Project number: 20013071) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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