1 Introduction

Vanadium is an important alloy element that is extensively used in many fields such as the steel industry, aerospace industry, and chemical industry [1, 2]. The main resources of vanadium are vanadium bearing slag, stone coal, magnetite ore, spent catalyst [3,4,5], etc. In recent years, the representative promising clean methods for vanadium recovery from vanadium bearing materials have attracted extensive attention, which mainly include calcium salts roasting, alkaline leaching such as NaHCO3 [6], (NH4)2CO3 [7], NaOH [8] and Na2CO3 [9, 10], solution purification, and V precipitation.

It is well known that solution purification is a key step for the subsequent product quality in the process of vanadium products. The commonly used methods for purifying the leaching solution are common chemical precipitation, solvent extraction, and ion exchange. Among these methods, solvent extraction is extensively used for rare metals extraction due to its high selectivity, high extraction capacity, high separation efficiency, and continuous operation. Amine extractants belong to anion extraction agents that have been employed to extract vanadium from alkaline solution. Quaternary ammonium salt is generally used for extraction of vanadium. El-Nadi et al. [11] studied and compared the extraction of vanadium from 3 M HCl and 0.1 M NaOH solutions (1.79 g/L V2O5) using 0.5 M Aliquat-336 at 25 °C. It was found that the vanadium extraction from sodium hydroxide solution was better than from hydrochloric acid medium. Qi et al. [12] reported vanadium extraction from a Na2CO3 solution (2 g/L V2O5) of stone coal oxidation roasting with carbonate-type Aliquat-336. The extraction efficiency of vanadium gradually decreased when Na2CO3 concentration was varied from 0.1 to 1.5 mol/L.

The extraction efficiency of vanadium is higher when pH of solution is between 5 and 9.5 using Aliquat-336 as extractant [13]. However, when Na2CO3 is used as leaching agent, the concentration of solution is usually 20–250 g/L and the pH of corresponding solution is also higher than 9.5 [14, 15]. The pH of solution can be adjusted by adding acid when vanadium is extracted from Na2CO3 leaching solution using Aliquat-336, which will lead to an increase in production costs. Therefore, in this work, it is attempted to adjust the pH of Na2CO3 solution by adding NaHCO3. When Na2CO3 concentration in the solution is low, the NaHCO3 can be directly added to lower the pH of solution. When Na2CO3 concentration in the solution is high, the NaHCO3 can be added after most of Na2CO3 in the solution is first separated by crystallization and the mother liquor is diluted to desired concentration with distilled water. Then the vanadium was extracted from Na2CO3-NaHCO3 solution using Aliquat-336. The influence factors, such as Na2CO3 concentration, NaHCO3 concentration, temperature and extractant concentration, on vanadium extraction were investigated, and the effect of impurity P on vanadium extraction was also studied.

2 Materials and Methods

2.1 Materials

The Aliquat-336 (99%) used as extractant was from Sinopharm Chemical Reagent CO., Ltd. The 2-octanol used as phase modifier was supplied by Tianjin Kemiou Chemical Reagent Co., Ltd. The Na2CO3 solution was prepared with analytical reagent Na2CO3 and distilled water. The feed solution containing 3.0 g/L vanadium in Na2CO3 solution was prepared with analytical reagent NaVO3 ·2H2O in water.

2.2 Methods

The sulfonated kerosene produced from common kerosene, concentrated H2SO4, and 5% Na2CO3 solution was used as diluent. The organic phase was composed of 15% extractant, 6% n-octanol modifier, and 79% dilute (except for the extractant concentration experiments). Solvent extraction experiments were carried out by pear-shape separatory funnel with a phase ratio of organic phase and aqueous phase of 1:1 for oscillating time of 1 min. Then the organic and aqueous phases were separated using separatory funnel after phase disengagement. The concentrations of vanadium and phosphorus in the aqueous solution were analyzed by ammonium ferrous sulfate titration method and phosphorus molybdenum blue photometric method, respectively. The pH of solution was determined by pH meter (PHSJ-4F). The IR spectra were recorded by IRTracer-100 (Shimadzu) at a resolution of 4 cm−1 in the range of 400–4000 cm−1.

3 Results and Discussion

3.1 Extraction Mechanism

Based on the solution chemistry of vanadium, the vanadium (V) exists in the form of V2O74− or VO3(OH)2− in solution within the pH range of 9–13 [16]. The Aliquat-336 is the anion exchange tree. The possible reaction for the vanadium extraction equilibrium in Na2CO3–NaHCO3 medium (pH ≈ 9.5) can be expressed as follows:

$$ {\mathrm{V}}_2{{\mathrm{O}}_7}^{4-}+4{\mathrm{R}}_3{\mathrm{CH}}_3\mathrm{N}\mathrm{C}1\rightleftharpoons \kern0.5em {\left({\mathrm{R}}_3{\mathrm{CH}}_3\mathrm{N}\right)}_4\kern0.5em {\mathrm{V}}_2{\mathrm{O}}_7+4\mathrm{C}{1}^{-} $$
(1)
$$ {\mathrm{VO}}_3{\left(\mathrm{OH}\right)}^{2-}+2{\mathrm{R}}_3{\mathrm{CH}}_3\mathrm{N}\mathrm{C}1\rightleftharpoons \kern0.5em {\left[{\mathrm{R}}_3{\mathrm{CH}}_3\mathrm{N}\right]}_2\kern0.5em {\mathrm{VO}}_3\left(\mathrm{OH}\right)+2\mathrm{C}{1}^{-} $$
(2)

where R stands for C8H17.

3.2 Effect of Na2CO3 Concentration

The effect of Na2CO3 concentration on vanadium extraction was studied from 10 to 160 g/L using an extractant concentration of 15% at room temperature. It can be observed from Fig. 1 that the extraction efficiency of vanadium decreased from 83.1% to 8.5% when Na2CO3 concentration was varied from 10 to 160 g/L, while pH of solution increased from 9.91 to 11.17. This indicates that the Na2CO3 concentration had a detrimental impact on vanadium extraction. For higher extraction efficiency of vanadium, the Na2CO3 concentration in solution should be as low as possible. The 20 g/L Na2CO3 solution was prepared in subsequent experiments based on the Na2CO3 concentration used in reference [14].

Fig. 1
figure 1

Effect of Na2CO3 concentration on vanadium extraction

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3.3 Effect of NaHCO3 Addition

In order to lower the pH of Na2CO3 solution, NaHCO3 was added into pre-prepared 20 g/L Na2CO3 solution to adjust the pH value. The effect of NaHCO3 addition on the extraction of vanadium was carried out under the following conditions: Na2CO3 concentration of 20 g/L, extractant concentration of 15%, and room temperature. The results are shown in Fig. 2. It was found that the pH of solution decreased from 9.68 to 9.33 when NaHCO3 concentration was varied from 10 to 22 g/L; while extraction efficiency of vanadium increased gradually with the increase in NaHCO3 concentration, and reached a maximum value of 89.6% when NaHCO3 concentration was 20 g/L, indicating addition of NaHCO3 is beneficial to improve extraction efficiency of vanadium. However, with a further increase of NaHCO3 concentration, extraction efficiency of vanadium decreased quickly. The possible reason is that the high concentration of HCO3− has a stronger inhibiting effect on solvent reaction.

Fig. 2
figure 2

Effect of NaHCO3 concentration on vanadium extraction

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3.4 Effect of Extractant Concentration

The effect of extractant concentration on extraction of vanadium was conducted from 5 to 25%, while the Na2CO3 concentration, NaHCO3 concentration, and temperature were kept constant as 20 g/L, 20 g/L, and room temperature, respectively. The results shown in Fig. 3 indicate that extraction efficiency of vanadium increased significantly and reached up to 89.2% with the extractant concentration up to 15%. When the extractant concentration was 25%, the extraction efficiency of vanadium reached 90.4%. It can be seen that the increase in vanadium extraction was rather low when extractant concentration ranged from 15% to 25%. Therefore, taking into account the economic cost, the optimal extractant concentration of 15% was selected.

Fig. 3
figure 3

Effect of extractant concentration on vanadium extraction

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3.5 Effect of Temperature

The effect of temperature on the extraction of vanadium was studied from 20 to 60 °C, while the Na2CO3 concentration, NaHCO3 concentration, and extractant concentration were kept constant as 20 g/L, 20 g/L, and 15%, respectively. As shown in Fig. 4, the extraction efficiency of vanadium indicated little change when temperature ranged from 20 to 30 °C. When the temperature was above 30 °C, extraction efficiency of vanadium decreased gradually. The possible reason is that Eq. (1) or (2) for vanadium extraction is an exothermic reaction. Therefore, the room temperature of 20–30 °C was selected.

Fig. 4
figure 4

Effect of temperature on vanadium extraction

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3.6 Effect of Impurity Phosphorus

Phosphorus was one of the main impurities in the Na2CO3 leaching solution [9, 10], which had a great influence on the subsequent vanadium precipitation due to formation of P–V heteropolyacid in the acidic solution [17]. Therefore, it was essential to investigate the effect of P on solvent extraction of vanadium. The effect of P concentration on extraction efficiency of vanadium was studied from 0.2 to 1.0 g/L, while the Na2CO3 concentration, NaHCO3 concentration, extractant concentration, and temperature were kept constant as 20 g/L, 20 g/L, 15%, and room temperature, respectively. It can be seen from Fig. 5 that the extraction efficiency of vanadium was about 91% when P concentration was 0.2 g/L, and little changed with the increase of P concentration. This indicates P had little effect on vanadium extraction in the range of the experiment.

Fig. 5
figure 5

Effect of P concentration on vanadium extraction

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In order to confirm whether vanadate and phosphate can form compounds in a Na2CO3–NaHCO3 solution, the solution was measured by infrared spectra. As shown in Fig. 6, the bands around 3400 cm−1 and 1640 cm−1 were the typical absorption peaks of OH−. The peak around 1384 cm−1 was mainly attributed to CO32−. The absorption bands at 1081 cm−1 and 989 cm−1 were ascribed to HPO42− [18, 19]. It is worth mentioning that there was an absorption peak at 970 cm−1 (V4O124−), which was shifted to 855 cm−1 (V2O74−) due to the increase of pH of solution. In summary, the above results clearly demonstrated sodium vanadate did not interact with sodium phosphate. This result is consistent with that of Zhang et al. [17].

Fig. 6
figure 6

FTIR spectra of (a) sodium vanadate, (b) sodium vanadate and sodium phosphate in 20 g/L sodium carbonate-20 g/L sodium bicarbonate solution: (a) 120 g/L NaVO3, pH = 8.6; (b) 120 g/L NaVO3 and 53 g/L Na3PO4, pH = 9.4

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4 Conclusions

A novel extraction system, i.e., Na2CO3–NaHCO3 medium, was prepared for vanadium extraction. The addition of NaHCO3 into Na2CO3 solution could lower the pH value of Na2CO3 solution, improve extraction efficiency of vanadium, and not introduce other impurities. At the same time, the pH of the Na2CO3–NaHCO3 buffer solution can be kept at a nearly constant value when vanadium was extracted, which is beneficial to extraction of vanadium. The vanadium was extracted using Aliquat-336 dissolved in sulfonated kerosene as diluent with 2-octanol as phase modifier. The single stage extraction efficiency of vanadium reached above 89% from 20 g/L Na2CO3-20 g/L NaHCO3 solution under the following conditions: extractant concentration of 15%, room temperature of 20–30 °C, 2-octanol of 6%, phase ratio (O/A) of 1:1, and oscillation time of 1 min. Sodium vanadate did not interact with sodium phosphate in the Na2CO3–NaHCO3 buffer solution. The impurity P had little influence on vanadium extraction in the Na2CO3–NaHCO3 solution.