Iron(III) extraction from sulfate solutions with D2EHPA in the presence of organic proton-donor additives

doi:10.21203/rs.3.rs-2995318/v1

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Iron(III) extraction from sulfate solutions with D2EHPA in the presence of organic proton-donor additives

https://doi.org/10.21203/rs.3.rs-2995318/v1

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The effect of organic proton-donor additives (HA) on the extraction of iron(III) from sulfuric acid solutions with di(2-ethylhexyl)phosphoric acid (HR) in decane and toluene was studied. It was shown that adding the additives into the organic phase causes an antagonistic effect. Iron(III) extraction in the presence of additives decreases in the series octyl alcohol ≈ 4-tert-butyl phenol > octanoic acid. The low activity of octanoic acid is attributed to self-association and the formation of dimers in the organic phase, which significantly prevents the formation of intermolecular associates between D2EHPA and monocarboxylic acid. Octanol and 4-tert-butyl phenol form stable associates with D2EHPA leading to a decrease in the ferric iron extraction efficiency thus facilitating efficient iron stripping from the loaded organic phase. In the D2EHPA and octyl alcohol system [FeR₂·HR₂] is formed. The stripping of iron from the HR and octyl alcohol mixtures was shown to be much more efficient than that from D2EHPA alone and can be successfully achieved using ~4.5 mol/L H₂SO₄ or H₂SO₄ (~3.5 mol/L) and NaCl (1.0 mol/L) mixture. Extraction systems containing D2EHPA and aliphatic alcohols can be used to recover iron from various industrial solutions, in particular, from those derived from zinc, nickel production, and others.

Sulfate solutions

iron

solvent extraction

di(2-ethylhexyl)phosphoric acid

proton donor additives

octyl alcohol

Di(2-ethylhexyl)phosphoric acid (D2EHPA, HR) is one of the most effective extractants and is widely used in the processing of complex technological solutions, in particular, the solutions obtained in zinc (Kozlov et al. 2008; Tahmasebizadeh and Javanshir 2021) and rare earth elements (Cheremisina et al. 2019) production, in lithium ion (Pavon et al. 2021) and nickel-metal hydride (Pospiech and Gega 2019) batteries waste.

D2EHPA is a very selective extractant for iron due to which its stripping is very difficult and can only be achieved using high HCl (~ 6 mol/L) (Nogueira et al. 1980) or H₃PO₄ (6–12 mol/L) (Levin et al. 1979) concentrations. More efficient stripping can be achieved using oxalic acid solutions(Cheremisina et al. 2019; Yuan 2019) or sulfuric acid containing reducing agents (H₂SO₃ or Na₂SO₃) (Liu et al. 2014). Unfortunately, these methods have certain drawbacks; i.e., oxalic acid is quite expensive, while the reductive stripping takes 2–3 hours to be completed.

The extraction mechanism of iron(III) with D2EHPA in inert diluents (kerosene, octane) has been studied quite well. It has been shown that the composition of the extracted complex with an excess of the extractant is Fe(HR₂)₃ (Sato et al. 1985; Baes and Bacer 1960). It was also shown that the extraction of iron from these solutions is very slow (Sato et al. 1985). Extraction kinetics of Fe(III) with D2EHPA has been studied by a number of authors (Karpacheva and Ilozheva 1969; Ciceri et al. 2014; Li et al. 202). However, the data obtained in these studies are quite contradictory. Thus, Karpacheva and Ilozheva (1969) reported that the extraction proceeds in the kinetic region, Ciceri et al. (2014) showed that Fe(III) extraction occurred under a mixed reaction-diffusion resistance regime, while Li et. al. (2020) deduced that the extraction regimes are mixed controlled with diffusion and chemical reaction for Fe(III).

Apart from D2EHPA, two new extractants, such as N, N-n-octylamine di(methylene phenylphosphonic acid) (OADMPPA) and N, N-n-hexadetamine di(methylene phenylphosphonic acid) (HADMPPA), can be used to selectively recover iron(III) from a mixture of sulfuric and phosphoric acids (Xu et al. 2022; Tran et al. 2019). The results showed that both OADMPPA and HADMPPA have good extraction ability toward iron(III) even at higher sulfuric acid concentration. However, effective stripping of iron(III) from the loaded organic phase is obviously very difficult but it can be achieved using oxalic acid (Xu et al. 2022. Tran et al. (2019) used mixtures of extractants, in particular, Aliquat 336 and D2EHPA to extract iron(III). These mixtures may be of some interest for the extraction of iron(III) from chloride solutions, while its extraction from sulfate solutions is significantly less efficient. It can be added that such work (Xu et al. 2022; Tran et al. 2019) have not yet gone beyond the scope of laboratory studies.

Earlier, it was shown (Grigorieva et al. 2022) that the introduction of proton-donor additives (HA), in particular, monocarboxylic acids, aliphatic alcohols, and phenol derivatives, into the organic phase containing D2EHPA significantly improves the indium stripping efficiency. Therefore, it is of great interest to study the extraction of iron from sulfuric acid solutions with D2EHPA in the presence of the aforementioned organic additives (HA) and the effect of adding HA on its stripping efficiency from the loaded organic phase.

The aim of the present study was to explore the main features of the extraction of iron(III) by D2EHPA in the presence of organic proton-donor additives and, based on the data obtained, to develop an extraction system comprising both efficient extraction and efficient stripping of iron(III).

Reagents

Di(2-ethylhexyl)phosphoric acid (D2EHPA, HR) containing 98% of the main substance and purified according to the published procedure (Stoyanov et al., 1984b), produced by OOO Volgogradpromproekt (Russia), was used as the extractant. Octanoic acid of 98% purity produced by Sigma Aldrich (USA), octyl alcohol of 99.5% purity produced by Fluka Analytical (Germany), 4-tert.butylphenol of > 97% purity produced by Acros Organics (Belgium) were used as proton-donor additives (HA).

Decane and toluene of pure grade were used as diluents. The reagents and diluents were used without further purification and were products of domestic production. The mineral salts and acids used were of chemical or analytical grade.

Procedure

The liquid–liquid extraction experiments were carried out by mechanical mixing of the phases in tubes or separating funnels at a temperature of 25 and 50^оС. The aqueous phase acidity was varied by adding appropriate amounts of sulfuric acid to the solution. The phase contact time during extraction and stripping was 2 h. According to preliminary data, this contact time was enough to reach equilibrium in the system. Any changes to these parameters are noted in the text.

The aqueous phases were analyzed for iron while its content in the organic phases was calculated from the mass balances. High concentrations of iron in the aqueous phases were determined by titration with EDTA in the presence of sulfosalicylic acid used as an indicator. Precision of this method does not exceed 1%. Low concentrations of iron in the aqueous phases were determined by atomic absorption spectrophotometry using an AAnalist 400 spectrophotometer (Perkin Elmer). The relative error of the method does not exceed 1%.

The least squares method was used to find the slope values of various dependencies. The accuracy of the calculated values was characterized by a 95% confidence limit.

Electronic spectra of the organic phases in decane were recorded using a highly sensitive fiber-optic spectrometer AvaSpec-ULS2048L using a 1.0 cm cell.

The effect of phase contact time on the extraction of iron(III) with D2EHPA in decane is shown in Tables 1 and 2. As seen, the extraction of iron from acidic solutions is slow. Thus, it takes at least 120 min to reach equilibrium at 25°С. As reported by Karpacheva and Ilozheva (1969), the slow extraction is due to the slow rate of formation in the aqueous phase of the iron(III) complex with the D2EHPA anion (FeR²⁺) caused by the low concentrations of the initial reagents (Fe³⁺ and R^–) in the aqueous phase. However, a decrease in acidity results in an increase in the R^– concentration in the aqueous phase and in an increase in the extraction rate. Thus, when extracting Fe³⁺ from slightly acidic sulfate solutions (pH = 1.7), the system reaches equilibrium after 3–6 min (Kozlov et al. 2008).

Table 1

Effect of phase contact time (τ) on the extraction of iron(III) with D2EHPA in decane. Aqueous phase: 0.93 g/L Fe(III) + 100 g/L H₂SO₄. Organic phase: 0.3 mol/L D2EHPA in decane. O:A = 1:1, Т = 25°С.
No.	1	2	3	4	5	6	7	8	9
τ, min	2.5	5	10	15	30	60	120	180	240
Fe_aq, g/L	0.616	0.42	0.21	0.115	0.042	0.036	0.029	0.029	0.029
Fe_org, g/L	0.314	0.51	0.72	0.815	0.888	0.894	0.901	0.901	0.901
D_Fe	0.51	1.21	3.43	7.09	21.14	24.83	31.07	31.07	31.07
ε, %	33.8	54.8	77.4	87.6	95.,5	96.12	96.9	96.9	96.9

Table 2

Effect of phase contact time (τ) on the extraction of iron(III) with D2EHPA in decane. Aqueous phase: 0.93 g/L Fe(III) + 100 g/L H₂SO₄. Organic phase: 0.3 mol/L D2EHPA in decane. O:A = 1:1, Т = 50°С.
No.	1	2	3	4	5	6
τ, min	2.5	5	10	15	30	60
Fe_aq, g/L	0.012	0.010	0.009	0.009	0.007	0.007
Fe_org, g/L	0.948	0.950	0.951	0.951	0.953	0.953
D_Fe	79.0	95.0	105.7	105.7	136.14	136.14
ε, %	98.80	98.96	99.06	99.06	99.93	99.93

If necessary, the rate of iron extraction can be increased by increasing the temperature. As seen from Table 2, at a temperature of 50°C, almost complete extraction of iron can be achieved in this system after 5–10 minutes of phase contact. It should be noted that iron extraction can also be accelerated by the introduction of various catalysts. Thus, it has been reported Sinegribova and Muravyova 2000) that, when recovering iron(III) by D2EHPA in the presence of ethoxylated sodium lauryl sulfate, the system reached equilibrium after 1.5 min.

The effect of sulfuric acid concentration on the extraction of iron(III) with D2EHPA alone and in the presence of octyl alcohol was investigated and the results are shown in Fig. 1. As seen from the figure, the extraction efficiency of iron with the extractant (curve 1) is very high and the recovery of iron into the organic phase is close to 100% (97%) even at a sulfuric acid concentration of 1.0 mol/L. Therefore, stripping of iron from the loaded organic phase is difficult and requires a high concentration of H₂SO₄ (> 8.0 mol/L).

Figure 2 shows the effect of various proton-donor additives (HA) on iron extraction with D2EHPA. As seen, in the presence of all these additives, the iron extraction efficiency decreases which is due to the antagonistic effect caused by the interaction of D2EHPA and the additives resulting in the formation of stable associates. The formation of associates between HR and proton-donor additives (HA), such as АH···O = P(OR)₂OH···OAH, with the coordination of HA both to the oxygen atoms of the POO group of HR and to the acidic proton of HR have been reported by Grigorieva et al. (2022). It is obvious that the strength and type of the interactions for different additives are different. Thus, according to ³¹P NMR data, in the case of octyl alcohol, a preferential interaction between the extractant and the alcohol through the oxygen of the octanol alcohol group and the hydroxy group of the phosphoric acid occurs, whereas, in the case of octanoic acid, its hydroxyl group preferentially interacts with the POO group of D2EHPA. In the D2EHPA and 4-tert-butylphenol mixtures, both types of interactions take place.

Figure 2 shows that, in the presence of octanoic acid (curve 1), the iron extraction efficiency decreases to a much lesser extent than in the presence of octyl alcohol and 4-tert-butylphenol (curves 2 and 3). This can be explained by the tendency of octanoic acid to self-associate and form dimers in the organic phase. It is the formation of stable dimers that makes these acids "inert" and to a significant extent prevents the formation of intermolecular associates. Octyl alcohol (curve 2) and 4-tert-butylphenol (curve 3), like octanoic acid, decreases iron extraction, however, unlike monocarboxylic acid, the effect of these additives on iron extraction is basically the same, despite the difference in their structure. The D2EHPA + octyl alcohol system is of great practical interest, therefore, this system has been studied in more detail.

In the presence of octyl alcohol, iron(III) extraction by the monomeric form of D2EHPA with a large excess of the extractant can be described by Eq. (1) (Stoyanov et al. 1984b):

Fe³⁺_(aq) + (3 + s)HR_(org) (FeR₃·sHR)_(org) + 3Н⁺_(aq) (1)

with the extraction constant defined as follows:

K _Fe−H = C_Fe(org)·C³_H+(aq)·γ_e.c.(org)·γ³_Н+(aq) /C_Fe(aq)·С^3+s_HR(org)·γ _Fe(aq)·γ ^3+s_HR(org) (2)

where C_Fe(org), C_Fe(aq) and C_H+(aq) are the analytical concentrations of iron in the organic and aqueous phases and hydrogen ions in the aqueous phase, C_HR(org) is the concentration of D2EHPA monomers in the organic phase, s is the solvation number, i.e. the number of extractant molecules in the extracted iron complex, γ_e.c.(org) is the activity coefficient of the extracted compound, γ_HR(org) is the activity coefficient of monomeric D2EHPA, γ_H+(aq) and γ_Fe(aq) are the activity coefficients of hydrogen and iron(III) ions in the aqueous phase respectively. At constant ionic strength, γ_Fe(aq) and γ_Н+(aq) are constant. Since the concentration of iron in the initial aqueous solution is low (5.0·10^− 3 mol/L), the equilibrium concentrations of C_HR(org) and C_H+(aq) can be equated to their initial concentrations. In this case, the distribution ratio of iron (D_Fe) can be written as follows (Eq. 3):

D _Fe = K^I_Fe−H·С^3+s_HR(org)·γ^3+s_HR(org) /C³_H+(aq)·γ_e,c,(org) (3)

where K^I_Fe−H = K_Fe−H·γ_Fe(aq)·γ³_Н+(aq)

When octyl alcohol is the diluent, in which D2EHPA exists in the monomeric form, γ_HR(org) and γ_e.c.(org) are constant. Therefore, assuming that the activity of octyl alcohol is constant, we can write Eq. (3) as follows (Eq. 4):

logD _Fe = logK^II_Fe−H + (3 + s)logС_HR(org)) – 3logC_H+(aq) (4)

According to Eq. (4), the values of the slope of the logD_Fe = f(logС_HR(org)) and logD_Fe = f(logC_H+(aq)) dependencies should be equal to 3 + s and − 3 respectively. As shown in Figs. 3 and 4, the slope of the plot of logD_Fe against pH (Fig. 3) is close to − 3 (–2.7 ± 0.2), while the slope of the plot of logD_Fe against logС_HR(org) (Fig. 4) is equal to 4 (3.9 ± 0.2). The latter suggests that the solvation number (s) is equal to one and therefore, the extracted iron species in the D2EHPA and octyl alcohol mixture in excess extractant is [FeR₃·HR] or [FeR₂·HR₂].

It should be added that, in the extraction systems with D2EHPA and three-charged elements, including iron (Baes and Bacer 1960), the solvation number is usually equal to 3 (Baes and Bacer 1960; Levin et al. 1972). Nevertheless, lower solvation numbers, 2, 1, and 0, can also be achieved in these systems for bismuth, thallium, and gallium respectively (Levin et al. 1972).

The electronic spectra of the extracted species of iron with D2EHPA in decane and octyl alcohol are given in Fig. 5. As seen, in decane, the position of the maximum is at 281 nm (curve 1) and this peak arises due to the electronic d–d transitions of Fe³⁺ in the octahedral environment (Cotton and Wilkinson 1969). The spectrum of the extracted species in octanol shows a maximum shifted to the long wavelength region (~ 300 nm), which indicates different structures of the complexes in decane and octanol.

The structures of the extracted iron species with an excess of the extractant in decane and in octyl alcohol are shown in Fig. 6. Obviously, the compositions of the extracted iron species in decane and octyl alcohol are different. Thus, the major extracted species are Fe(HR₂)₃ (Baes and Bacer 1960) and [FeR₂·HR₂] in decane and octyl alcohol respectively.

The antagonistic effect of alcohol on iron extraction is clearly shown when comparing the iron extraction with D2EHPA alone (Fig. 1, curve 1) and its mixture with octanol (Fig. 1, curves 2 and 3). As seen, in the presence of alcohol, the extraction efficiency of iron significantly decreases compared with using D2EHPA alone, which allows efficient stripping of iron from the loaded organic phase using 3.5–4.0 mol/L H₂SO₄. Figure 7 shows iron stripping isotherms for two stripping agents. It can be seen that, in both cases, quite efficient iron stripping is achieved. Also, stripping of the loaded Fe(III) with H₂SO₄ in the presence of HCl proceeds much more efficiently (curve 2). Thus, for the loaded organic phase containing 2.0 g/L Fe(III), almost complete stripping of iron can be achieved in 5–6 steps at an О:A ratio of 6:1, i.e. when concentrating the iron six times.

Iron stripping, like iron extraction, is slow, however an increase in temperature significantly accelerates the stripping process (Table 3). It should be noted that the temperature affects the extraction efficiency to a much greater extent than the stripping efficiency.

Table 3

Effect of phase contact time (τ) on the stripping of iron(III) from D2EHPA and octyl alcohol in decane at different temperatures. Organic phase: 0.3 mol/L D2EHPA + 1.2 mol/L octyl alcohol in decane, C_Fe(initial) = 2.80 g/L. Aqueous phase: 350 g/L H₂SO₄ + 36.5 g/L (1.0 mol/L) HCl. τ is variable, O:A = 5:1.
No.	1	2	3	4	5	6
τ, min	2,5	5	10	15	30	60
25°С
Fe_aq, g/L	8.61	8.87	9.12	9.37	9.49	9.58
Fe_org, g/L	1.06	1.02	0.97	0.93	0.89	0.87
D_Fe	0.123	0.115	0.106	0.099	0.094	0.091
50°С
Fe_aq, g/L	10.60	10.71	11.02	11.24	11.20	11.41
Fe_org, g/L	0.66	0.63	0.61	0.56	0.56	0.53
D_Fe	0.062	0.059	0.055	0.050	0.050	0.046

Figure 8 shows an iron extraction isotherm with a mixture of D2EHPA and octyl alcohol in decane together with an operating line (McCabe-Thiele diagram). It is seen that, in a five-stage extraction system, at an aqueous to organic phase ratio of 6:1, iron is recovered from the aqueous phase almost completely. In this case, iron is significantly concentrated in the organic phase and its further concentrating can be achieved at the stripping stage. Non-ferrous metals (in particular, Zn, Co, Ni) are almost not extracted under these conditions. Thus, during the extraction of zinc from sulfate solution containing 80 g/L Zn, the concentration of loaded Zn was 70–80 mg/L, i.e. ~0.1% of Zn is recovered with the mixture of D2EHPA and octyl alcohol. It can be added that, when producing sufficiently pure iron, it can be considered as a target component to be used for the production of ferrites, which are widely used as magnetic materials in radio engineering, electronics, etc.

It should also be added that, when the systems with D2EHPA are employed as extractants, the use of a rather expensive alkaline reagent (NaOH) is not required to neutralize the aqueous solution, which is necessary when iron is extracted with monocarboxylic acids (Van der Zeeuw 1976).

It was shown that the extraction of iron(III) from sulfate solutions (≥ 1.0 mol/l H₂SO₄) with D2EHPA is slow and the extraction equilibrium is only reached after 2.0 hours at room temperature. An increase in temperature significantly increases the rate of iron(III) extraction; thus, at 50°C, the extraction equilibrium is reached after 10–15 min. The extraction efficiency of iron with individual D2EHPA is very high. The introduction of proton-donor additives (octyl alcohol, 4-tert-butyl phenol, octanoic acid) into the organic phase containing D2EHPA leads to antagonistic effects, which are due to the interaction between D2EHPA and the additives with the formation of stable associates АH···O = P(OR)₂OH···OAH. The iron(III) extraction efficiency decreases in the order octanol ≈ 4- tert-butyl phenol > octanoic acid.

Based on the data obtained, it can be concluded that the extraction of iron(III) by D2EHPA in octyl alcohol proceeds with the formation of the extracted species [FeR₃·HR] or [FeR₂·HR₂]. Owing to the antagonistic effect, stripping of iron from D2EHPA in octyl alcohol can be successfully achieved with a solution of sulfuric acid. However, in the presence of sodium chloride, iron stripping is much more efficient.

Extraction systems containing D2EHPA and aliphatic alcohols can be used to recover iron(III) from various industrial solutions, in particular from the solutions derived from zinc, nickel, and vanadium production. Also, in case of producing sufficiently pure iron, it can be considered as a target component to be used for the production of ferrites, which are widely used as magnetic materials in radio engineering, electronics, etc.

Acknowledgments

This research was funded within the state assignment to the Institute of Chemistry and Chemical Technology SB RAS (Project No. 0287-2021-0014) and conducted using the equipment of the Krasnoyarsk Regional Research Equipment Centre SB RAS. Appreciation for valuable remarks is extended to Dr. D.S. Flett.

Conflict of Interest

All authors declare that they have no conflicts of interest to disclose.

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Iron(III) extraction from sulfate solutions with D2EHPA in the presence of organic proton-donor additives

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