High-Value Recovery of the Iron via Solvent Extraction from Waste Nickel-Cadmium Battery Sulfuric Acid Leachate Using Saponified D2EHPA

Abstract

A significant amount of iron from the waste nickel-cadmium (Ni-Cd) battery sulfuric acid leachate seriously hinders the separation and recovery of nickel and cadmium. Therefore, an efficient and economical way to remove iron from this leachate is desired. This paper demonstrated the efficient iron extraction from a simulated Ni-Cd battery sulfuric acid leachate with saponified Di (2-ethylhexyl) phosphoric acid (D2EHPA). The iron-loaded D2EHPA was then stripped with oxalic acid and the iron was recovered in the form of iron oxalate. This process realizes the efficient separation and high-value recovery of iron. The results showed that the saponification of the D2EHPA greatly promoted the extraction of iron from the Ni-Cd battery sulfuric acid leachate. Under suitable conditions, the iron’s single-stage extraction rate was more than 95%, and the iron’s single-stage stripping rate was more than 85%. Moreover, the iron’s extraction rate was more than 99% after two theoretical extraction stages, and the stripping rate was 95.6% after two theoretical stripping stages. The slope analysis determines that five molecules of D2EHPA were combined with one molecule of Fe³⁺ in the extraction process. The FT-IR analysis shows that the extraction mechanism of Fe³⁺ using the saponified D2EHPA is a cation exchange. These results can help guide the industrial separation and recovery of iron from the waste Ni-Cd battery sulfuric acid leachate.

1. Introduction

Nickel-Cadmium (Ni-Cd) batteries are widely used in rail transit, aircraft, and power grid energy storage due to their stable performance, long life, and fast discharging capability [1,2]. However, the carcinogenic metals cadmium and nickel make these batteries hazardous wastes without the proper treatment and disposal, which poses a threat to human health and the environment [3]. Recently, many researchers believe that the large amounts of nickel and cadmium contained in waste Ni-Cd batteries are a high-value resource [4,5]. As such, using the appropriate methods for their separation and recycling can reduce environmental pollution and provide significant economic benefits.

The commonly used recycling technologies for nickel-cadmium batteries mainly include pyrometallurgical treatment and hydrometallurgical treatment [6]. Pyrometallurgical treatment is a mature waste battery recycling process, but it has drawbacks of high energy consumption, toxic gas emission, and low-value products [1]. The hydrometallurgical process is considered to be a more sustainable battery waste recycling route due to its good separation effect, high-value products, and lower waste emission [5].

The acid leachate produced in the hydrometallurgical process, commonly used in recycling spent Ni-Cd batteries, generally contains metals such as nickel, cadmium, cobalt, and iron [7]. The presence of iron seriously impacts the recycling of nickel, cadmium, and cobalt [8]. At present, the commonly used iron removal processes include iron-vanadium precipitation, extraction, and ion exchange [9]. Precipitation has significant advantages in removing low concentrations of iron from the acid leachate. However, it also creates difficulties in the solid-liquid separation and the loss of other metals through co-precipitation [10]. Ion exchange selectively removes most of the iron, but the expensive operating costs limit its potential application [11]. Solvent extraction for iron removal has the benefits of good separation, high product purity, and automation. These benefits have attracted the attention of many researchers [12].

Di (2-ethylhexyl) phosphoric acid (D2EHPA) [13], 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (P507) [14], tertiary amine (N235) [15], tributyl phosphate (TBP) [16], and bis (2,4,4-tri-methylpentyl) (Cyanex 272) [17] have been used as solvents to remove iron from various acidic leachates. Hu et al. studied iron’s selectivity relative to other impurities while removing it from a highly concentrated chloride solution using D2EHPA [13]. Wang et al. studied the optimal conditions for the P507 extraction of iron from coal fly ash leachate and found that P507′s iron extraction mechanism was ion exchange [14]. Sun et al. proposed a synergistic extractant consisting of D2EHPA and N235 to extract iron from an aluminum solution. The single-stage iron extraction rate was more than 97%, and iron’s stripping efficiency was greater than 99% with 1 mol/L sulfuric acid [15]. Yi et al. investigated iron’s extraction mechanism from Ni-Co-Fe chloride solutions using TBP and concluded that the iron complexes formed with TBP and 2-octanol were [H₂FeCl₄·2 TBP] and [H₂FeCl₄·5 (2-octanol)] [16]. Pav ón et al. used Cyanex 572 (a PC88A/Cyanex 272 mixture) to extract iron from phosphoric acid with an extraction rate of 98.1% and 2 mol/L HCl almost completely stripped iron [17]. Considering the economic cost and readily available raw material, D2EHPA is still the better choice for the separation of low-value metal iron via large-scale industrial extraction. In recent years, there have been several reports investigating the effects of saponification on metal extraction [18,19], but only a few studies have shown the effects of saponification on iron extraction using D2EHPA. Meanwhile, the effects of saponified D2EHPA on the extraction and separation of higher Fe³⁺ concentrations (~12 g/L) from sulfuric acid leachates that also contain high concentrations of Ni²⁺ (~56 g/L) and Cd²⁺ (~23 g/L) are also less well known.

In this work, saponified D2EHPA is used to extract iron from a simulated spent Ni-Cd battery sulfuric acid leachate. This is followed by stripping the extract with oxalic acid to obtain an iron oxalate solution as a high-value product. This method achieves the highly efficient separation of iron from the simulated Ni-Cd battery leachate and the high-value recycling of iron. The effects of saponification rate, extractant concentration, extraction time, extraction temperature, stripping agent concentration (oxalic acid), stripping time, and stripping temperature were explored. Slope analysis was used to study the mechanism of extraction. Fourier transform infrared spectra (FT-IR) analysis was used to characterize the organic phase during the saponification, extraction, and stripping process. This allows one to elucidate the mechanisms of saponification, extraction, and stripping.

2. Materials and Methods

2.1. Materials and Solutions

The extractant D2EHPA and diluent sulfonated kerosene were purchased from Zhengzhou Deyuan Fine Chemicals Co., Ltd., Zhengzhou, China. Both reagents were used without further purification. Analytical grade Fe₂(SO₄)₃·xH₂O, CdSO₄·3/8H₂O, NiSO₄·6H₂O, and CoSO₄·7H₂O were purchased from Shanghai Aladdin Chemistry Inc., Shanghai, China, and used as is.

The concentration of D2EHPA was 1 mol/L, except for the extractant concentration experiment. The D2EHPA was saponified with 10 mol/L sodium hydroxide, and the saponification rate was 70%. Deionized water used to make the solutions was obtained from a Milli-Q system. A spent Ni-Cd battery sulfuric acid leachate was simulated by dissolving Fe₂(SO₄)₃·xH₂O, CdSO₄·3/8H₂O, NiSO₄·6H₂O, and CoSO₄·7H₂O in 0.05 mol/L H₂SO₄. The metal ion content in the simulated leachate is provided in

Table 1. The major metal ion concentrations in simulated leachate.

Elements	Content (g/L)
Fe	12.25
Ni	56.30
Cd	23.28
Co	0.52

.

2.2. Experimental Procedure

The extraction experiments were conducted in a separatory funnel by shaking the extractant and simulated leachate mixtures with an oscillation rate of 200 rpm. After resting and phase separation, the aqueous raffinate layer was removed from the bottom of the separatory funnel. Stripping experiments were similar to the extraction, where the organic phase was iron-loaded D2EHPA, and the aqueous phase was the stripping agent oxalic acid.

2.3. Analytical Method

The metal ion concentration in the raffinate was determined by an atomic absorption spectrometer (AAS, AA6880F, Shimadzu, Japan). The metal ion content in the extractant was then calculated by mass balance. The pH was measured using a pH meter (PHS-3C, INESA Scientific Instrument Co., Ltd., Shanghai, China). The Fourier transform infrared spectra (ATR-FTIR, Thermo Scientific Niolet iN10) were recorded from 4000–400 cm⁻¹.

The extraction rate (E), distribution ratio (D), separation coefficient (β), and stripping rate (S) were calculated using Equations (1)–(5):

$$E=\left(\frac{{C_{a,0}-C}_{a,1}}{C_{a,0}}\right)\times \mathrm{100\%}$$

(1)

$$C_{org,1}=\frac{{C_{a,0}-C}_{a,1}}{R_1}$$

(2)

$$D=\frac{C_{org,1}}{C_{a,1}}$$

(3)

$${\beta }_{Fe/M}=\frac{D_{Fe}}{D_M}$$

(4)

$$S=\left(\frac{C_{a,2}}{{R_2\times C}_{org,1}}\right)\times \mathrm{100\%}$$

(5)

where $C_{a,0}$ is the metal ion concentration in the simulated leachate, $C_{a,1}$ is the metal ion concentration in the raffinate solution, $R_1$ is the phase ratio on the extraction, $C_{a,2}$ is the metal ion concentration in the strip liquor, and $R_2$ is the phase ratio on the stripping.

3. Results and Discussion

3.1. Extraction Performance of Fe³⁺

3.1.1. Extraction of Fe³⁺ from the Waste Ni-Cd Battery Sulfuric Acid Leachate

The major metal ions in the simulated waste Ni-Cd battery sulfuric acid leachate include Fe³⁺, Ni²⁺, Cd²⁺, and Co²⁺, with concentrations of 12.25 g/L, 56.30 g/L, 23.28 g/L, and 0.52 g/L, respectively. The concentrations of Cd²⁺ and Ni²⁺ are much higher than that in previous works [13,16], which makes it difficult to separate Fe³⁺ from the simulated leachate.

Saponified D2EHPA (saponification rate of 70%, 1 mol/L) was used to extract iron from the simulated leachate. As is shown in Figure 1, the extraction rate of Fe³⁺ was more than 95%, and the extraction rates of Cd²⁺, Ni²⁺, and Co²⁺ were all below 5%. The separation coefficients of β_Fe/Cd, β_Fe/Ni, and β_Fe/Co were 423, 476, and 394, respectively. This data indicates that the saponified D2EHPA is able to selectively extract Fe³⁺ from a sulfate solution with a high concentration of Cd²⁺ and Ni²⁺.

3.1.2. Effects of Saponification Rate of D2EHPA

When untreated D2EHPA was used to extract the metal ions, large amounts of hydrogen ions were transferred from the organic phase into the aqueous phase, inhibiting the extraction of iron [20]. To determine the optimum saponification rate, the rate was varied from 0% to 100%, while the extraction was carried out under an extraction temperature of 298 K, D2EHPA concentration of 1 mol/L, initial pH of 1.00, initial concentration of Fe³⁺ of 12.25 g/L, and extraction time of 20 min.

Figure 2 shows the iron extraction rate is only 65.12% with untreated D2EHPA. Increasing the saponification rate significantly increases the extraction rate. When the saponification rate was 70%, the iron extraction rate exceeded 95%, and the Cd, Co, and Ni extraction rates were all below 5%. As the saponification rate was further increased to 80%, 90%, and 100%, the iron extraction rate increased slightly, and the Cd and Ni extraction rates increased to over 10%, which impacted the purity of the obtained iron product. In order to maximize the iron extraction rate and reduce the co-extraction of cadmium, nickel, and cobalt, 70% saponified D2EHPA was used in subsequent experiments.

The thermodynamic equilibrium species of iron was calculated by Visual MINTEQ with a pH varying from 0.00 to 6.00. Figure 3 shows that [FeSO₄]⁺ was the main iron species at a pH from 0 to 4. The amounts of [FeSO₄]⁺ increased with pH from 0 to 1.75 and decreased with the higher pH value. Meanwhile, [Fe(OH)²⁺], [Fe₂(OH)₂]⁺⁴, and [Fe₃(OH)₄]⁺⁵ emerged at a pH of 2.50, indicating that iron began to hydrolyze [14]. The purple line in Figure 2 shows the extract pH increasing as the saponification rate increases. With 70% saponified D2EHPA, the equilibrium pH was 1.40, while [FeSO₄]⁺ was the major iron specie from Figure 3. The results show that the iron was extracted by 70% saponified D2EHPA with a high extraction rate.

3.1.3. Effects of Extraction Time

Figure 4 shows the effect of extraction time varying from 5 min to 60 min on iron extractions, where other conditions were an extraction temperature of 298 K, D2EHPA concentration of 1 mol/L, saponification rate of 70%, initial pH of 1.00, initial concentration of Fe³⁺ of 12.25 g/L, and phase ratio of 1:1. The Fe³⁺ extraction rate increased with increasing extraction times and reached a peak of ~95% at 10 min. The results suggest that the iron extraction under these conditions was completed in 10 min. In order to ensure maximum iron extraction, a 20 min extraction time was used in the other experiments.

3.1.4. Effects of Temperature on the Extraction

In this section, the effects of the extraction temperature (298 K–323 K) on the extraction of Fe³⁺ from the synthetic sulfuric acid leachate were studied under the D2EHPA concentration of 1 mol/L, saponification rate of 70%, initial concentration of Fe³⁺ of 12.25 g/L, extraction time of 20 min, initial pH of 1.00, phase ratio of 1:1. As is shown in Figure 5, the extraction rate of Fe³⁺ was basically constant with the increase of the extraction temperature. It can be inferred that extraction temperature has a slight effect on iron extraction. In order to simplify the experimental operations and reduce energy use further, extraction experiments were carried out at 298 K.

3.1.5. Effects of D2EHPA Concentration on the Extraction

Saponified D2EHPA with concentrations varying from 0.2 mol/L to 1.3 mol/L were used to extract Fe³⁺ from the simulated leachate, where saponification rate, initial pH, initial concentration of Fe³⁺, extraction time, phase ratio, and extraction temperature were 70%, 1.00, 12.25 g/L, 20 min, 1:1, and 298 K, respectively. Figure 6 shows that the iron extraction rate increased significantly with increased extractant concentration. With 0.2 mol/L D2EHPA, the Fe³⁺ extraction rate was only 26.58%. However, the Fe³⁺ extraction rate increased to 95.65% with 1.0 mol/L D2EHPA. As the extractant concentration was further increased to 1.3 mol/L and the iron extraction rate increased slightly. It has been shown that when the D2EHPA concentration exceeds 1 mol/L, difficulties in mass transfer occur due to the high viscosity of the organic extractant [21]. In order to reduce the amount of extractant used and avoid mass transfer difficulties, subsequent experiments used 1.0 mol/L D2EHPA.

3.1.6. Effects of the Phase Ratio on the Extraction

The impacts of the phase ratio on the Fe³⁺ extraction were studied by controlling the phase ratio of the saponified D2EHPA and Ni-Cd battery leachate at 0.2:1, 0.4:1, 0.6:1, 0.8:1, 1:1, and 1.3:1. The other conditions were the D2EHPA concentration of 1 mol/L, saponification rate of 70%, extraction time of 20 min, extraction temperature of 298 K, initial concentration of Fe³⁺ of 12.25 g/L, and initial pH of 1.00. From Figure 7, the Fe³⁺ extraction rate gradually increased with an increasing phase ratio. When the phase ratio was 0.2:1, the Fe³⁺ extraction was only 32.52%. With a 1:1 phase ratio, the Fe³⁺ extraction rate was 95.65%. Further increasing the phase ratio to 1.3:1, the Fe³⁺ extraction rate increased by only 3%. In order to reduce extractant consumption, the phase ratio was set to 1:1.

The McCabe-Thiele extraction equilibrium isotherms of Fe³⁺ were plotted based on the Fe³⁺ in the aqueous and organic phases under different phase ratios [22]. The concentration of Fe³⁺ in the raffinate after extraction was used as the X-axis, and the concentration of Fe³⁺ in loaded D2EHPA after extraction was used as the Y-axis. From Figure 8, with the work phase ratio of 1:1 on the extraction, the concentration of Fe³⁺ in the raffinate was less than 0.03 g/L after two theoretical extraction stages, while the iron’s extraction rate was more than 99%.

3.2. Extraction Mechanism

3.2.1. Slope Analysis of the Extraction

When extractant D2EHPA was saponified with 10 mol/L NaOH, Na⁺ was loaded into the D2EHPA and replaced the H⁺ in the organic phase. Moreover, D2EHPA in the presence of dimers was depolymerized into monomers due to saponification [19]. The extraction equilibrium of iron can be supposed as follows:

$${\mathrm{F}\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{3+}+(3+\mathrm{x})\mathrm{N}\mathrm{a}{\mathrm{A}}_{\mathrm{o}\mathrm{r}\mathrm{g}}\stackrel{\mathrm{K}}{\leftrightarrow }\mathrm{F}\mathrm{e}{\mathrm{A}}_3·\mathrm{x}\mathrm{N}\mathrm{a}{\mathrm{A}}_{\mathrm{o}\mathrm{r}\mathrm{g}}+{3\mathrm{N}\mathrm{a}}_{\mathrm{a}\mathrm{q}}^{+}$$

(6)

where ${\mathrm{F}\mathrm{e}}_{\mathrm{a}\mathrm{q}}^{3+}$ is the iron ion in the leachate, A is the D2EHPA anion, $\mathrm{N}\mathrm{a}{\mathrm{A}}_{\mathrm{o}\mathrm{r}\mathrm{g}}$ is the saponified D2EHPA, and ${\mathrm{N}\mathrm{a}}_{\mathrm{a}\mathrm{q}}^{+}$ is the Na ion in the raffinate.

The extraction equilibrium constant K can be calculated as follows:

$$\mathrm{K}=\frac{[\mathrm{F}\mathrm{e}{\mathrm{A}}_3·\mathrm{N}\mathrm{a}{\mathrm{A}]}_{\mathrm{o}\mathrm{r}\mathrm{g}}·{\left[{\mathrm{N}\mathrm{a}}^{+}\right]}_{\mathrm{a}\mathrm{q}}^3}{{\left[{\mathrm{F}\mathrm{e}}^{3+}\right]}_{\mathrm{a}\mathrm{q}}·{\left[\mathrm{N}\mathrm{a}\mathrm{A}\right]}_{\mathrm{o}\mathrm{r}\mathrm{g}}^{(3+\mathrm{x})}}$$

(7)

where $[\mathrm{F}\mathrm{e}{\mathrm{A}}_3·\mathrm{x}\mathrm{N}\mathrm{a}{\mathrm{A}]}_{\mathrm{o}\mathrm{r}\mathrm{g}}$ is the iron concentration in the extracted D2EHPA, ${\left[{\mathrm{N}\mathrm{a}}^{+}\right]}_{\mathrm{a}\mathrm{q}}$ is the Na⁺ concentration in the raffinate, ${\left[{\mathrm{F}\mathrm{e}}^{3+}\right]}_{\mathrm{a}\mathrm{q}}$ is the iron concentration in the raffinate after extraction, and ${\left[\mathrm{N}\mathrm{a}\mathrm{A}\right]}_{\mathrm{o}\mathrm{r}\mathrm{g}}^{}$ is the Na⁺ concentration in the loaded D2EHPA after extraction.

The distribution coefficient D_Fe is as follows:

$${\mathrm{D}}_{\mathrm{F}\mathrm{e}}=\frac{{\left[{\mathrm{F}\mathrm{e}}^{3+}\right]}_{\mathrm{o}\mathrm{r}\mathrm{g}}}{{\left[{\mathrm{F}\mathrm{e}}^{3+}\right]}_{\mathrm{a}\mathrm{q}}}=\frac{[\mathrm{F}\mathrm{e}{\mathrm{A}}_3·\mathrm{x}\mathrm{N}\mathrm{a}{\mathrm{A}]}_{\mathrm{o}\mathrm{r}\mathrm{g}}}{{\left[{\mathrm{F}\mathrm{e}}^{3+}\right]}_{\mathrm{a}\mathrm{q}}}$$

(8)

where ${\left[{\mathrm{F}\mathrm{e}}^{3+}\right]}_{\mathrm{o}\mathrm{r}\mathrm{g}}$ is the iron concentration in the loaded D2EHPA after extraction and ${\left[{\mathrm{F}\mathrm{e}}^{3+}\right]}_{\mathrm{a}\mathrm{q}}$ is the iron concentration in the raffinate after extraction.

Then, Equation (8) can be further derived as follows:

$$\mathrm{K}=\frac{{\mathrm{D}}_{\mathrm{F}\mathrm{e}}·{\left[{\mathrm{N}\mathrm{a}}^{+}\right]}_{\mathrm{a}\mathrm{q}}^3}{{\left[\mathrm{N}\mathrm{a}\mathrm{A}\right]}_{\mathrm{o}\mathrm{r}\mathrm{g}}^{(3+\mathrm{x})}}$$

(9)

Taking the logarithm of both sides of the Equation (9) at the same time:

$$\lg ({\mathrm{D}}_{\mathrm{F}\mathrm{e}}·{\left[{\mathrm{N}\mathrm{a}}^{+}\right]}_{\mathrm{a}\mathrm{q}}^3)=\mathrm{l}\mathrm{g}\mathrm{K}+(3+\mathrm{x})\mathrm{l}\mathrm{g}[\mathrm{N}\mathrm{a}{\mathrm{A}]}_{\mathrm{o}\mathrm{r}\mathrm{g}}$$

(10)

The saponified D2EHPA, with concentrations varying from 0.2 mol/L to 1.3 mol/L, was used to extract the Fe³⁺ from the simulated leachate. The concentration of Fe³⁺ $({[{\mathrm{F}\mathrm{e}}^{3+}]}_{\mathrm{a}\mathrm{q}}$) and Na⁺ (${[{\mathrm{N}\mathrm{a}}^{+}]}_{\mathrm{a}\mathrm{q}}$) in the raffinate can be directly determined with the instrument method. Meanwhile, the concentration of Fe³⁺ $({[{\mathrm{F}\mathrm{e}}^{3+}]}_{\mathrm{a}\mathrm{q}})$ in the loaded organic phase can be calculated by mass balance. Therefore, ${\mathrm{D}}_{\mathrm{F}\mathrm{e}}$ can be calculated by Equation (8). The relevant data was plotted in Figure 9a. The logarithm of ${\mathrm{D}}_{\mathrm{F}\mathrm{e}}·{\left[{\mathrm{N}\mathrm{a}}^{+}\right]}_{\mathrm{a}\mathrm{q}}^3$ was used as the Y-axis, and the logarithm of the concentration of the saponified D2EHPA was used as the X-axis.

The slope of the fitting line in Figure 9a is 5.07, which means the value of (3 + x) was nearly 5. The results indicate that five molecules of D2EHPA were combined with one molecule of Fe³⁺ in the extraction process.

The replacement ratio of Na⁺ loaded in saponified D2EHPA (R) before and after extraction can be calculated as follows:

$$\mathrm{R}=\frac{{[{\mathrm{N}\mathrm{a}}^{+}]}_{\mathrm{a}\mathrm{q}}}{[\mathrm{N}\mathrm{a}{\mathrm{A}]}_{\mathrm{o}\mathrm{r}\mathrm{g}}}=\frac{{[{\mathrm{N}\mathrm{a}}^{+}]}_{\mathrm{a}\mathrm{q}}}{[\mathrm{D}2\mathrm{E}\mathrm{H}\mathrm{P}\mathrm{A}]·{\mathrm{S}}_{\mathrm{A}}}$$

(11)

where ${[{\mathrm{N}\mathrm{a}}^{+}]}_{\mathrm{a}\mathrm{q}}$ is the concentration of Na⁺ in the raffinate after extraction, $[\mathrm{N}\mathrm{a}{\mathrm{A}]}_{\mathrm{o}\mathrm{r}\mathrm{g}}$ is the concentration of Na⁺ loaded in saponified D2EHPA before extraction, $[\mathrm{D}2\mathrm{E}\mathrm{H}\mathrm{P}\mathrm{A}]$ is the concentration of extractant D2EHPA, and ${\mathrm{S}}_{\mathrm{A}}$ is the saponification of the extractant.

From Figure 9b, the replacement ratio of Na⁺ was more than 80% with a D2EPHA concentration below 1.0 mol/L, which indicated most of the Na⁺ in the organic phase was replaced by Fe³⁺.

3.2.2. FT-IR Analysis of the D2EHPA after Extraction

Figure 10 shows that untreated D2EHPA, saponified D2EHPA, extracted D2EHPA, and stripped D2EHPA all display the characteristic absorption peak of -CH₃ (2956 cm⁻¹) and -CH₂ (2921 and 3853 cm⁻¹) [19]. From Figure 10a, the untreated D2EHPA shows characteristic peaks of P=O group (1225 cm⁻¹) and P-O-H group (1683 cm⁻¹ and 1030 cm⁻¹) [19,23]. After saponification, water characteristic peaks appeared at 3402 cm⁻¹ and 1642 cm⁻¹, and the characteristic P-O-H peak moves from 1030 cm⁻¹ to 1084 cm⁻¹, indicating that Na⁺ replaced the H⁺ in the P-O-H group (Figure 10b). This shift also means that the D2EHPA in the presence of dimers was depolymerized into monomers, which was similar to previous work [19]. Meanwhile, the strong P=O absorption peak at 1225 cm⁻¹ weakens and shifts to 1205 cm⁻¹. This indicates that the P=O group reacted with sodium hydroxide, thereby reducing the strength of the P=O absorbance [19].

After the extraction, the water peaks disappeared, indicating that most of the water in the organic phase was removed in the extraction process (Figure 10c). The P=O absorption peak at 1205 cm⁻¹ shifts to 1196 cm⁻¹, indicating that the Fe³⁺ was loaded into the organic phase [19]. The results mean that the extraction mechanism of Fe³⁺, using saponified D2EHPA, is also a cation exchange similar to the replacement of Na⁺ for H⁺ in the saponification process. The stripped D2EHPA spectrum was consistent with the untreated organic phase, while the P-O-H peak was observed again at 1683 cm⁻¹ (Figure 10b). This indicates that most of the Fe³⁺ was stripped from the organic phase, which also verified the stripping effect of the oxalic acid.

3.3. Stripping of Loaded D2EHPA

3.3.1. Effects of Oxalic Acid Concentration on the Stripping

Figure 11 shows the Fe³⁺ stripping rate increases with increasing the oxalic acid concentration, while under the stripping temperature of 313 K, phase ratio of 1:1, and stripping time of 40 min. When the oxalic acid concentration was 1.0 mol/L, the Fe³⁺ stripping rate was 85.59%. The solubility of the oxalic acid was 90 g/L (nearly 1 mol/L) at 293 K, thus increasing the oxalic acid concentration above 1.0 mol/L may cause precipitation problems. In order to avoid oxalic acid precipitation, the oxalic acid concentration was set to 1.0 mol/L in the other stripping experiments.

3.3.2. Effects of Stripping Time

The effects of stripping time were investigated, where the concentration of the oxalic acid, stripping temperature, and phase ratio were 1 mol/L, 313 K, and 1:1, respectively. Figure 12 shows that the Fe³⁺ stripping rate increases with increasing the stripping time. When the stripping time was 5 min, the Fe³⁺ stripping efficiency was only 36.88%. As the stripping time increased to 40 min, the stripping efficiency rose to 86.21%. Further increases in the stripping time led to a slight increase in the stripping rate. In order to maximize the iron stripping efficiency, the stripping time should be 40 min.

3.3.3. Effects of Stripping Temperature

The effects of the stripping temperature were investigated under these conditions, the concentration of oxalic acid of 1 mol/L, stripping time of 40 min, and phase ratio of 1:1. Figure 13 shows that increasing the stripping temperature facilitates Fe³⁺ stripping. At 298 K, the Fe³⁺ stripping rate was 68.37%. As the stripping temperature increased to 313 K, the Fe³⁺ stripping efficiency rose to 85.59%. Upon further increasing the stripping temperature, the Fe³⁺ stripping rate increased slowly. In order to ensure the maximum Fe³⁺ stripping rate and reduce the evaporative loss of the extractant, the optimal stripping temperature is 313 K.

3.3.4. Effects of the Phase Ratio on the Stripping

The effects of the phase ratio on the stripping were investigated, while under the stripping temperature of 313 K, the concentration of oxalic acid of 1 mol/L, and a stripping time of 40 min. Figure 14 shows that as the phase ratio for stripping decreased, the Fe³⁺ stripping efficiency increased. With a phase ratio of 2:1, the stripping rate of Fe³⁺ was 48.77%. When the phase ratio changed to 1:1, the stripping rate increased to 85.96%. Further changing the phase ratio to 0.67:1, 0.33:1, 0.25:1, the Fe stripping efficiency increased to 91%, 92%, 92%, respectively. Considering the Fe³⁺ stripping efficiency, the optimum stripping phase ratio was set to 1:1.

The McCabe-Thiele stripping equilibrium isotherms of Fe³⁺ were drawn in Figure 15 based on the Fe³⁺ in the aqueous and organic phase under different phase ratios [24]. The concentration of Fe³⁺ in the organic phase after stripping was used as the X-axis, and the concentration of Fe³⁺ in stripping aqueous after stripping was used as the Y-axis. From Figure 15, with the work phase ratio of 1:1 on the stripping, the concentration of Fe³⁺ in the stripping aqueous was less than 0.18 g/L after two theoretical stripping stages, while iron’s stripping rate was 95.6%.

3.3.5. Stripping Experiment under the Optimal Conditions

The extracted D2EHPA was prepared under the optimal extraction conditions: room temperature (298 K), [D2EHPA] = 1 mol/L, extraction time = 20 min, saponification rate = 70%, initial pH = 1.00, and phase ratio = 1:1. Then, the organic phase loaded iron and other metals were stripped with oxalic acid under the conditions: [oxalic acid] = 1 mol/L, stripping time = 40 min, and stripping temperature = 313 K. The relevant stripping data were as

Table 2. Stripping data under the optimal stripping conditions.

Elements	Metal Concentration in the Stripping Aqueous (g/L)	Metal Stripping Rate
Fe	9.951	85.51%
Ni	0.011	0.85%
Cd	0.018	3.53%
Co	0.006	28.93%

. From Table 2, the concentration of iron in the stripping aqueous was 9.951 g/L, and the stripping rate of iron was 85.51% under the optimal stripping conditions. While the concentrations of Ni, Cd, and Co were only 0.011 g/L, 0.018 g/L, and 0.006 g/L, respectively. The results showed that most of the iron in the loaded organic phase was transferred into the stripping aqueous phase, and the concentrations of other metals were very low, which guaranteed the purity of the iron product.

4. Conclusions

The difficulties in the separation and recovery of iron from the leachate of spent Ni-Cd batteries are well known. This work provides a process for the separation of iron from spent Ni-Cd battery sulfuric acid leachate and its subsequent recovery. The iron is extracted with the saponified D2EHPA and then stripped with oxalic acid to obtain the high-value product, iron oxalate. After optimization, the single-stage extraction efficiency of iron was above 95% when the concentration of D2EHPA = 1 mol/L, saponification rate = 70%, extraction time = 20 min, initial pH = 1.00, phase ratio = 1:1, and extraction temperature = 298 K. The single-stage iron stripping rate using oxalic acid was above 85% when the concentration of oxalic acid = 1 mol/L, stripping temperature = 313 K, phase ratio = 1:1, and stripping time = 40 min. Moreover, the iron’s extraction rate was more than 99% after two theoretical extraction stages, and the stripping rate was 95.6% after two theoretical stripping stages. The slope analysis indicates that five molecules of D2EHPA were combined with one molecule of Fe³⁺ in the extraction of Fe³⁺ using D2EHPA. The FT-IR analysis shows that the extraction mechanism of Fe³⁺ using the saponified D2EHPA is a cation exchange. This work provides the maximum recovery of high-value iron oxalate while allowing the continued treatment of the spent Ni-Cd battery waste stream.