Selective Extraction of Zirconium from Sulfuric Acid Solutions at High Concentration with Trioctylamine (TOA)

Abstract

Zirconium (Zr) and hafnium (Hf) are very important in nuclear and high-temperature applications, but their similar physical and chemical properties bring great challenges to separation. The current extraction methods have defects, such as low efficiency at high metal concentration. In this article, a zirconium (Zr)/hafnium (Hf) solvent extractive separation from sulfuric acid solutions using trioctylamine (TOA) as the extractant was researched at room temperature. The aqueous solution is prepared using zirconium sulfate (containing Hf), and the concentration of metal ions (Zr⁴⁺ and Hf⁴⁺) was about 1.096 mol·L⁻¹. The effects of the aqueous acidity, the concentration of TOA, the contacting time, and the organic to aqueous O/A ratio on the separation of Zr and Hf were investigated. It is observed that the Zr can be extracted in the organic phase selectively, and the optimal conditions were: TOA concentration of 40 vol%, organic to aqueous O/A ratio of 3, contacting time of 5 min. Under these conditions, the single-stage extraction rate of Zr is 61.23%, while the Hf is almost not extracted. The mechanism of Zr extraction by TOA was studied through the saturation capacity and slope methods. Based on the results, it is believed that the structure of the extracted complex may be [R₃NH]₂[Zr(SO₄)₃]. This study provides a new approach for the development of industrialized Zr-Hf separation.

1. Introduction

Zirconium (Zr) and hafnium (Hf) belong to the same main group of elements in the periodic table and have very similar ionic radii (74 pm for Zr⁴⁺, 75 pm for Hf⁴⁺) and physical and chemical properties. In nature, Zr and Hf are usually found together; therefore, the smelting process often faces challenges in separating the two elements [1]. Zirconium and hafnium show excellent corrosion resistance, good mechanical properties, and thermal conductivity, and have a wide range of applications in many fields. But in the nuclear industry, zirconium metal, which contains less than 500 ppm hafnium, can be used as an ideal coating material for nuclear power reactors because of its small thermal neutron absorption cross-section (0.18 × 10⁻²⁸ m²) [2,3,4]. Some high-temperature-resistant zirconium alloys would limit their hafnium content. Therefore, it is necessary to separate the hafnium from the zirconium.

Solvent extraction was the main method for the separation of Zr and Hf in industrial production. The processes based on extractants of methyl isobutyl ketone (MIBK) or tributyl phosphate (TBP) are employed on a commercial scale. In the MIBK process, hafnium is preferentially extracted, and the separation factor (β_Hf/Zr) can reach 12.61, which has the characteristics of a simple process and fast extraction kinetics. However, there are toxic byproducts (amines/cyanides), high water solubility leading to water pollution, and a lengthy process [5,6,7]. The TBP process is effective in nitric acid solution, selectively extracting Zr with a separation factor of up to 10, and has good separation and stability. However, it can lead to equipment corrosion and complex phase separation [8,9,10]. In addition, the separation processes based on acidic phosphine extractants or neutral extractants, such as di (2-ethylhexyl) phosphate (D2EHPA), bis (2,4,4-trimethylpentyl) phosphonic acid (Cyanex 272), and 2-ethylhexylphosphonic acid mono (2-ethylhexyl) ester (PC 88A), were reported [11,12], as well as diisobutyl ketone (DIBK) and trioctyl phosphine oxide (TOPO) [13]. However, the low separation coefficients (<2) restrict the application [14,15,16].

Another important method for Zr and Hf (IV) separation is the use of amine-based extractants. Liquid−liquid extraction studies have been performed to investigate several amine-based extractants, including the selective extraction of zirconium (Zr) and hafnium (Hf) in a hydrochloric acid medium [17]. Sato and Watanabe [18,19] found that some long-chain aliphatic amines can selectively extract Zr from a low-concentration Zr sulfate solution. Wang compared TOA with other amines and found that when the initial concentrations of 0.5 mol·L⁻¹ H₂SO₄, Zr, and Hf were all 0.2 g·L⁻¹ and O/A = 1, the TOA concentration was 0.44 vol%. In a low concentration feed solution, TOA has higher extraction efficiency but lower selectivity [20]. However, systematic research on TOA has been scarce, especially in the few studies examining the separation of Zr from high-concentration Zr sulfate solutions. This is driven by the practical demands of industrial-grade zirconium/hafnium feed solutions that typically exhibit elevated metal ion concentrations. In high-concentration systems, the competitive coordination mechanisms between Zr and Hf differ markedly from those in low-concentration systems. Furthermore, high-concentration feed solutions not only improve extraction efficiency per unit volume but also demonstrate enhanced viability for industrial-scale applications. In this work, a Zr/Hf solvent extraction separation from sulfuric acid solutions with TOA as the extractant was re-searched, and the experiments were carried out with a high metal concentration solution. The optimized conditions of selective zirconium extraction were researched, and the mechanism of Zr extraction was also discussed.

2. Experimental

2.1. Experimental Materials and Instruments

The raw experimental material was primarily Zr sulfate tetrahydrate. TOA, triisooctylamine (TIOA), trioctyl/decyl tertiary amine (N235), kerosene, n-octanol, and phosphoric acid were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Sulfuric acid and perchloric acid were purchased from Modern Oriental (Beijing) Technology Development Co., Ltd. (Beijing, China). Kerosene was used as the diluent. In the experiments, the initial concentrations of Zr (IV) and Hf (IV) were about 1.096 mol·L⁻¹ and 0.0112 mol·L⁻¹, respectively. Reagent H₂SO₄ was used to adjust the acidity of the solution.

Instruments: ZNCL-BS intelligent digital display magnetic stirrer (Shanghai Sensitive Instrument Equipment Co. Ltd., Shanghai, China), electronic digital display PH meter (METTLER TOLEDO Technology (China) Co. Ltd., Changzhou, China), HD-100 B desktop oscillator (Changzhou Yineng Experimental Instrument Factory, Changzhou, China), LC-OES-200 cantilever electric stirrer (Lichen Technology, Shanghai, China), and inductively coupled plasma emission spectroscopy (ICP-OES, Optima5300V, Perkin-Elmer, Waltham, MA, USA).

2.2. Experimental Methods

2.2.1. Solvent Extraction Procedure

In the extraction experiment, the contacting operation was carried out in a 100 mL threaded cap six-edged bottle. A bench-top oscillator was used to fully mix and oscillate the aqueous phase and the organic phase. The contacting time was set to 0.5, 1, 2, 3, 4, 5, and 7 min. The concentration of sulfuric acid solution is set to 0, 0.5, 1, 1.5, 2.0, and 2.5 mol·L⁻¹. The TOA concentration is set to 10 vol%, 20 vol%, 30 vol%, 40 vol%, and 50 vol%. The extraction O/A ratio is set to 1, 2, 3, 4, and 5. After the extraction was completed, the aqueous phase was separated using a liquid separation funnel. The extraction experiment was conducted at room temperature. ICP-OES was used to accurately measure the concentrations of metals in the aqueous phase, and the concentrations of metals in the organic phase were calculated based on the principle of mass balance. Freshly prepared aqueous solutions were used in the experimental process to ensure the accuracy and reliability of the experimental data [21].

2.2.2. Mechanism Study

• Acidification

The ratio of the organic phase of TOA and the 2 mol·L⁻¹ sulfuric acid solutions O/A ratio was 1, the extraction time was 5 min, the organic phase of TOA was in full contact with sulfuric acid solutions, and acidification was used in the liquid separation funnel static phase separation.

2.

Saturation capacity method

Solutions of 4.37 vol%, 8.73 vol%, and 13.10 vol% were prepared with TOA as the extractant, and the extraction experiments were carried out with a 1.096 mol·L⁻¹ Zr solution. The O/A ratio was 1, and the mixture was oscillated at room temperature for 5 min and then placed in a liquid separation funnel. After phase separation was complete, the Zr in the raffinate was sampled and the Zr concentration was analyzed. After the first extraction, a fresh Zr solution was added and a re-extraction experiment was carried out. This process was repeated until the concentration of Zr in the raffinate was consistent with that in the extraction solution. It was considered that Zr reached saturation concentration in the organic phase. The organic phase was then digested, and 1 mL of the organic phase was placed in a conical flask with deionized water (20 mL), concentrated sulfuric acid (10 mL), phosphoric acid (5 mL), and perchloric acid (5 mL). The organic phase was heated, boiled, cooled to room temperature, and diluted to determine the Zr concentration of the solution after digestion.

3.

Slope method

To study the effects of TOA concentration on Zr extraction, a series of organic solutions with TOA concentrations of 30 vol%, 35 vol%, 40 vol%, 45 vol%, and 50 vol% in an appropriate kerosene diluent were prepared. These organic solutions were mixed with 0.05 mol·L⁻¹ Zr aqueous solution at the O/A ratio of 1:3. The mixture was shaken at room temperature for 5 min to ensure full contact with the reaction and then transferred to a separation funnel for phase separation. After phase separation, the Zr in the raffinate was sampled and analyzed, and the distribution ratio (D) of Zr was calculated. The relationship between the TOA concentration and the distribution ratio (D) of Zr was studied by varying the TOA concentration.

2.3. Calculation

The distribution ratio (D) and extraction rate (E) were calculated as follows [22]:

$$D={\displaystyle \frac{{\left[M\right]}_0-{\left[M\right]}_{aq}}{{\left[M\right]}_{aq}}}$$

(1)

$$E={\displaystyle \frac{{\left[M\right]}_0-{\left[M\right]}_{aq}}{{\left[M\right]}_0}}\times 100\%$$

(2)

where M is the ion concentration (M = Zr or Hf), subscript 0 represents the original solution, and subscript aq represents the aqueous phase after extraction.

3. Results

3.1. Extractant Type Influence on Zr/Hf Separation Efficiency

We used the conditions of an O/A ratio of 3, sulfuric acid solutions of 2 mol·L⁻¹, a Zr concentration of 1.096 mol·L⁻¹, no acidification, an extraction time of 5 min, TIOA, N235, and TOA. The effects of different extractants on the extraction efficiencies of Zr and Hf were investigated. As shown in Figure 1, according to the experimental results, TIOA did not extract either Zr or Hf. By contrast, the extraction rate of Zr by N235 was 50.8%, and the extraction rate of Hf was 8.94%. Although N235 can extract these two metals, it cannot achieve effective selective separation. The extraction rate of Zr by TOA was 61.23%, and almost no Hf was extracted, and its separation effect of TOA is better than those of TIOA and N235.

3.2. Impact of Sulfuric Acid Concentration

Under the conditions of TOA concentration of 40 vol%, O/A ratio of 3, extraction time of 5 min, and sulfuric acid solutions concentration range of 0–2.5 mol·L⁻¹, the influence of the sulfuric acid solutions concentration on the extraction rate was investigated. The results are presented in Figure 2. It is observed from the figure that the same extraction efficiencies of Zr and Hf (100%) were obtained for the sulfuric acid solutions concentrations of 0 and 0.5 mol·L⁻¹, and with a further increase in the sulfuric acid solutions concentration, the extraction rate of Zr and Hf decreases sharply. For the sulfuric acid solutions concentration of 1.5 mol·L⁻¹, the extraction rate of Zr decreased to 48.47%, and almost no Hf was extracted. As the concentration of sulfuric acid solutions increased, the extraction of Zr first increased and then decreased, and Hf remained almost unextracted. When the concentration of sulfuric acid solutions was 2 mol·L⁻¹, the extraction rate of Zr reached 61.23%. With a further increase in the sulfuric acid solutions concentration to 2.5 mol·L⁻¹, the Zr extraction rate decreased to 52.2%. When the concentration of sulfuric acid solution decreases, the ion activity in the solution is higher, and the interaction between the extractant and the ion is stronger. The competitive relationship among Zr and Hf and hydrogen sulfate is weakened, and TOA can be fully complexed with sulfuric acid solutions of Zr and Hf, improving the Zr and Hf extraction efficiencies.

The extraction efficiencies of Zr and Hf decreased with increasing sulfuric acid solution concentration because of the competition between the metal complexes and hydrogen sulfate and the reaction with amines. The maximum extraction rate of Zr and Hf by TOA is 100%, which is achieved in 0~0.5 mol·L⁻¹ sulfuric acid solutions, and cannot effectively separate Zr and Hf. Rather, Zr can be selectively obtained by one extraction only when Hf is almost unextracted and the extraction rate of Zr reaches the maximum value. When the concentration of sulfuric acid solutions is 0.5–2.5 mol·L⁻¹, sulfuric acid solutions are protonated by tertiary amine extraction, and the metal complex competes with the hydrogen sulfate salt to react with the amine, leading to a decrease in the extraction rate of Zr and Hf. When the concentration of sulfuric acid solutions is 2 mol·L⁻¹, the acidity of the solution may enable the TOA to achieve the optimal protonation state that not only ensures that sufficient R₃NH⁺ combines with Zr(SO₄)₃²⁻, but also avoids excessive H⁺ leading to the supersaturation of the extractant, making the reaction more effective. This also creates the conditions for obtaining Zr by extraction.

3.3. Effect of the Concentration of TOA

Under the conditions of the O/A ratio of 3, sulfuric acid solutions of 2 mol·L⁻¹, Zr concentration of 1.096 mol·L⁻¹, no acidification, and extraction time of 5 min, the TOA concentrations of 10 vol%, 20 vol%, 30 vol%, 40 vol%, and 50 vol% were selected. The influence of the TOA concentration on the Zr and Hf extraction efficiencies was investigated, and the optimal concentration of the TOA extractant was determined. Figure 3 shows the variation in the Zr and Hf extraction efficiencies with the TOA concentration. The extraction rate of Zr increased with increasing extractant concentration. At a TOA concentration of 10 vol%, the Zr extraction rate was 11.68%, while at a TOA concentration of 50 vol%, the Zr extraction rate was 79%, showing an almost linear relationship between the extraction rate and the TOA concentration. When the TOA concentration is low (≤40 vol%), almost no Hf is extracted. Hf was extracted only when the concentration of the TOA extractant reached 50 vol%, with an extraction rate of 18%.

With increasing TOA concentration, more TOA molecules around the Zr ions reacted with each other to form complexes, promoting the formation of extraction complexes. The distribution ratio (D) is shown in

Table 1. Distribution ratio of Zr and Hf (D) in a mixed solution with different concentrations of TOA extractant at a sulfuric acid solution of 2 mol·L⁻¹ and Zr concentration of 1.096 mol·L⁻¹.

TOA Concentration	Distribution Ratio of Zr and Hf D
	DZr	DHf
10 vol%	0.044	0
20 vol%	0.11	0
30 vol%	0.23	0
40 vol%	0.59	0
50 vol%	1.25	0.073

. Because the distribution coefficient (D) is positively correlated with the concentration of TOA within a certain range, as the concentration of TOA increases, the reaction shifts in the direction of generating more organic phase extracts; that is, D_Zr increases such that more Zr is extracted into the organic phase and the extraction rate increases linearly. When the TOA concentration is low (≤40 vol%), it is difficult for the TOA molecules to effectively bind to Hf ions, and Hf is not extracted because of the competitive relationship among Zr, Hf, and hydrosulfate and the high mass transfer resistance of the organic phase. As the TOA concentration increased to 50 vol%, the number of TOA molecules gradually increased, and the likelihood of TOA binding to Hf ions was greatly increased by coordination with Hf ions to form a stable extraction complex. Therefore, the optimal TOA concentration for separating Zr and Hf was determined to be 40 vol%. Under these conditions, the extraction rate of Zr reached the maximum value of 61.23%, and Hf was almost unextracted, achieving efficient selective extraction of zirconium.

3.4. O/A Ratio Effects on Extraction

Under the conditions of TOA concentration of 40 vol%, sulfuric acid solutions of 2 mol·L⁻¹, Zr concentration of 1.096 mol·L⁻¹, non-acidification, and extraction time of 5 min, the effect of the O/A ratio on the extraction efficiencies of Hf and Zr was investigated to determine the optimal O/A ratio and optimize the extraction process. The results are shown in Figure 4. The extraction rate of Zr gradually increased with an increasing O/A ratio. When O/A < 4, increasing the ratio significantly improved the extraction rate of Zr. When the O/A ratio was 4, the Zr extraction rate was close to 100%. With an increase in TOA concentration, more of the organic phase contacts a given volume of the aqueous phase, reducing the interfacial resistance between the aqueous and organic phases, and thus increasing the Zr extraction rate.

For Hf extraction, for O/A ratio values of 1, 2, or 3, Hf was hardly extracted. For O/A > 3, Hf extraction increased sharply and then reached close to 100% for an O/A ratio of 5. It is indicated that Hf extraction depends strongly on the O/A ratio, with O/A ratio values of 1, 2, or 3, with 3 as the critical threshold. When the O/A ratio exceeds this value, the Hf extraction efficiency increases rapidly. The underlying reason for these results may be that under low O/A ratio conditions, the extractant lacks sufficient capacity to coordinate with the Hf ions and form complexes, resulting in negligible Hf extraction. As the O/A ratio increased, the volume of the organic phase increased, thereby enhancing the availability of the extractant and providing more binding sites for the Hf ions. Consequently, the Hf distribution ratio increased, directly contributing to the improved extraction efficiency. According to the experimental results, at a TOA extractant concentration of 40 vol%, 3:1 was the best O/A ratio. Under these conditions, Hf was almost unextracted, and the Zr extraction rate was maximized.

3.5. Effect of Extraction Time on Metal Extraction

As shown in Figure 5, the solution was a mixture with H₂SO₄ and Zr concentrations of 2 mol·L⁻¹ and 1.096 mol·L⁻¹, respectively. The variation in the extraction efficiencies of Hf and Zr was probed between 0.5 min and 7 min for a TOA concentration of 40 vol% and an O/A ratio of 3. Figure 5 shows that TOA has a high extraction capacity, and the extraction process is almost at equilibrium after approximately 3 min of mixing well. Considering the volume of the extracted solution and the influence of the extraction speed, the extraction time was set to 5 min to ensure that the extraction reached complete equilibrium. Therefore, the extraction time for the subsequent experiments was maintained at 5 min.

3.6. Effect of TOA Acidification

In the TOA extraction of metal anions, to make the extractant form an active group that can effectively participate in the extraction reaction, or to adjust the extractant to a suitable pH value for extraction, it is necessary to first contact the configured organic phase with the acid solution for acidification treatment. This will lead to the TOA generating the corresponding salt, creating suitable conditions for the subsequent metal anion extraction. However, in this experiment, the tertiary amines were not in contact with the sulfuric acid solutions used for pretreatment prior to extraction. Because the extraction was carried out in sulfuric acid solutions, sulfuric acid solutions were extracted to protonate TOA at the beginning of the extraction, and the complex of Zr and Hf was then extracted from the sulfuric acid solution [23]. Prior to extraction, acidification was performed, and the other group was not acidified, and Zr and Hf extraction were performed for a comparative experiment.

The experimental results are presented in Figure 6. Large differences in the extraction of Zr and Hf were observed between acidified and non-acidified organic phases of TOA. Under the condition that TOA is not acidified, the extraction rate of Zr was high (61.23%), while Hf was not extracted. However, after acidification, the extraction efficiency of Zr decreased to 38.15%, while the extraction rate of Hf increased to 3.93%. This is believed to be because the active group of TOA is occupied when it is loaded with sulfuric acid solutions. This prevents the TOA from effectively forming a complex with Zr, reducing the extraction selectivity for Zr, and leading to a decrease in the Zr extraction rate. Hf may exist in the form of a certain cation or complex cation in sulfuric acid solutions. This study offers a theoretical foundation and experimental guidance for the efficient separation of zirconium from sulfuric acid solutions. The superiority of this method is further confirmed through comparisons with other extractants (as shown in

Table 2. Comparative analysis of Zr/Hf extraction methods: current results and published literature.

Extractant	Extraction Type	Concentration Range (Zr/Hf)	Selectivity	Extraction Rate (Zr/Hf)	Separation Factor (β)	References
Trioctylamine (TOA)	H2SO4	1.096 mol·L−1 Zr/0.0112 mol·L−1 Hf	Zr	Zr: 61.23%; Hf: 0%	βZr/Hf > 100	This work
MIPK	HSCN	0.0221 mol·L−1 Zr/0.000477 mol·L−1 Hf	Hf	-	βHf/Zr = 12.61	[6,7]
TBP + Cyanex 923	HNO3	0.285 mol·L−1 Zr/0.0028 mol·L−1 Hf	Zr	Zr: 53%; Hf: <1%	βZr/Hf = 186	[8,9]
Alamine 308 (TOA)	H2SO4	0.00219 mol·L−1 Zr/0.00112 mol·L−1 Hf	Zr	Zr:65%; Hf: 15%	βZr/Hf = 12.4	[20]
Alamine 300	H2SO4	0.00219 mol·L−1 Zr/0.00112 mol·L−1 Hf	Zr	Zr: 75%; Hf: 28%	βZr/Hf = 10.4	[20]
D2EHPA	H2SO4	0.00219 mol·L−1 Zr/0.00112 mol·L−1 Hf	Hf	Hf: 90%	βHf/Zr = 66	[14]
Cyanex 272	H2SO4		Hf	Hf: 50–80%	βHf/Zr up to 23.7	[14,15]
Aliquat 336	HSCN	1.052 mol·L−1 Zr/0.0126 mol·L−1 Hf	Hf	Hf: 94.5%	βZr/Hf = 19.4	[22]

).

The effects of the acidification carried out in 2 mol·L⁻¹ sulfuric acid solutions were examined by infrared spectroscopy, with the spectrum of the TOA extractant determined before and after acidification. Through the analysis of the appearance of the characteristic peaks and the wavenumber shift of the peaks of the functional groups, the reaction bonding mechanism of the extraction before and after the loading of sulfuric acid solutions was preliminarily explored.

As shown in Figure 7, the absorption peak of the H-N stretching vibration is a broad peak centered at 3424 cm⁻¹. An absorption peak with enhanced sulfuric acid solutions acidity appeared at 1167 cm⁻¹, which is a typical characteristic absorption peak of (-C=O). At the same time, in the range of 2700–3000 cm⁻¹, a broad peak of R₃NH⁺ and a TOA characteristic frequency relative to low-wave velocity motion appeared [24,25]. This can be considered as indicating a mechanism of synthetic ion salt production by the TOA mixed solvent in the extraction of dilute sulfuric acid solutions. The absorption peak of the C=O group shifted from that obtained in pure sulfuric acid solutions to 1167 cm⁻¹, reflecting a redshift in the characteristic absorption peak due to the hydrogen bond (C=O) between the tertiary amine extractant and sulfuric acid solutions. That is, the mixed extraction solvent undergoes hydrogen bond association solvation while extracting sulfuric acid [26,27]. Therefore, it can be concluded that the reaction mechanism involved in the complexation extraction process of TOA is the ionic bond salt formation mechanism. The specific reaction is as follows.

[CH₃(CH₂)₇]₃N_(org) + H₂SO_4(aq) = [CH₃(CH₂)₇]₃NH⁺·HSO₄⁻ _(org)

(3)

3.7. Extraction Mechanism of Zr by TOA

In sulfuric acid solutions, the complexation extraction mechanism of polar organic compounds such as TOA is uniquely complex. Investigating the complexation extraction mechanism of Zr in sulfuric acid solutions not only elucidates separation dynamics to enhance Zr recovery and Zr/Hf selectivity but also resolves phase separation/emulsification challenges, reducing production costs and environmental impacts. The optimization of the complexation extraction and separation of Zr in sulfuric acid solutions is of great practical significance [28].

3.7.1. Saturation Capacity Method Analysis

The experimental results show that when the concentration of the organic phase is 0.1 mol·L⁻¹, the Zr concentration in the saturated organic phase is 0.0522 mol·L⁻¹. Therefore, the extraction product is characterized by [Zr]/[TOA] = 0.522. For the organic phase concentration of 0.2 mol·L⁻¹, the Zr concentration in the saturated organic phase is 0.097 mol·L⁻¹. Therefore, the extraction product is characterized by [Zr]/[TOA] = 0.488. For the organic phase concentration of 0.3 mol·L⁻¹, the Zr concentration in the saturated organic phase is 0.166 mol·L⁻¹. Therefore, the extraction product is characterized by [Zr]/[TOA] = 0.555. In summary, for the organic phase molar concentrations of 0.1, 0.2, and 0.3 mol·L⁻¹, the obtained ratios were approximately equal to 0.5 (As shown in

Table 3. Saturated zirconium concentration and [Zr]/[TOA] ratio under different organic phase concentrations (about 0.5).

Organic Phase Concentration (mol·L−1)	Saturated Zr Concentration (mol·L−¹)	[Zr]/[TOA] Ratio
0.1	0.0522	0.522
0.2	0.097	0.488
0.3	0.166	0.555
Summary	-	All ratios are approximately 0.5

).

According to the solution chemistry of Zr, under the condition of sulfuric acid solutions, Zr exists in the form of Zr⁴⁺ in the solution; in particular, it is found in the form of Zr(SO₄)₂ in the solution at low acidities, and as Zr(SO₄)₃²⁻in higher acidity solutions. Combined with the results of the Zr present mainly in the form of Zr⁴⁺ in the sulfuric acid solutions and the saturated capacity experiment, the Zr complex extracted by TOA in the 2 mol·L⁻¹ sulfuric acid solutions can be identified as [R₃NH]₂[Zr(SO₄)₃] [29].

3.7.2. Infrared Spectroscopy Analysis of the Extraction Process

The TOA extractant and the organic phase before and after extraction were analyzed by infrared spectroscopy, with the results presented in Figure 8. After Zr extraction, vibration peaks were observed in the range of 662 cm⁻¹ to 1248 cm⁻¹. These peaks are typically related to the symmetric and asymmetric stretching vibrations of the sulfonic acid group. It can be speculated that the bands at 1248, 1153, 1001, and 662 cm⁻¹ are related to the sulfonic acid group. The characteristic peak band centered at 1153 cm⁻¹ is attributed to the coordination reaction between the C=O bond and Zr⁴⁺ in the extraction process of the TOA mixed solvent. This coordination reaction usually leads to a change in the stretching vibration frequency of the C=O bond; thus, the characteristic absorption peak appears in the infrared spectrum [30,31].

3.7.3. Slope Analysis for Determining TOA/Zr Molar Ratio in Extraction Complexes

In the experiment, the distribution ratio (LgD) of Zr corresponding to the TOA concentration of 30 vol% was 1.466 (Figure 9). Similarly, the Zr distribution ratios corresponding to the TOA concentrations of 35 vol%, 40 vol%, 45 vol%, and 50 vol% were 1.547, 1.675, 1.818, and 1.882, respectively, as shown in Figure 9. These Lg [TOA] and LgD data were fitted, and a linear relationship between Lg [TOA] and LgD was obtained by fitting as given by:

$$LgD=1.9848 \left[R_{3}N{H}^{+}\right]+2.4812$$

(4)

with the R² correlation coefficient of 0.9818.

The linear slope of the fitting equation is 1.9848, which is close to 2, indicating that the molar number of TOA in the extracted complex is 2. Based on the Zr forms present in the sulfuric acid solutions, the extraction reaction equation can be derived as follows:

Zr(SO₄)₃²⁻_(aq) + 2R₃NH₂SO_4(org) = (R₃NH)₂Zr(SO₄)_3(org) + 2HSO₄⁻ _(aq)

(5)

4. Conclusions

This study provides a theoretical basis and experimental guidance for the efficient separation of Zr from sulfuric acid solutions under laboratory conditions at room temperature. Such separation is highly significant for the application of Hf-free Zr in the nuclear, chemical, and medical industries.

For the 1.096 mol·L⁻¹ Zr solution, the optimal process conditions for the selective extraction of Zr were obtained as follows: sulfuric acid solutions of 2 mol·L⁻¹, O/A ratio of 3, TOA concentration of 40 vol%, extraction time of 5 min, and no acidification. Under these conditions, Hf was hardly extracted.

The selective extraction mechanism of Zr by TOA in the sulfuric acid solutions was further explored by infrared spectroscopy analysis, the saturated capacity method, and the slope method. This yielded a Zr to TOA molar ratio of 0.5, indicating that the extraction complex was [R₃NH]₂[Zr(SO₄)₃]. A linear relationship was obtained by fitting Lg [TOA concentration] and LgD to further elucidate the extraction mechanism as follows:

R₃N_(org) + H₂SO_4(aq) = R₃NH⁺·HSO⁴⁻_(org)

(6)

Zr(SO₄)_3(aq)²⁻ + 2R₃NH₂SO_4(org) = (R₃NH)₂Zr(SO₄)_3(org) + 2HSO⁴⁻_(aq)

(7)

The TOA extraction method is an efficient and highly selective zirconium extraction method and can achieve the enrichment of low concentration Hf in the raffinate phase, thereby optimizing the hafnium resource recovery process.