Co-Removal of Fe/V Impurity in H₂TiO₃ Synthesized from Ti-Bearing Blast Furnace Slag

Abstract

Ti-bearing blast furnace slag (TBFS) can be converted to impurity bearing TiOSO₄ solution for TiO₂ pigment production. However, the H₂TiO₃ (MTA) hydrolyzed from the solution has too high Fe/V impurity to meet the standard for TiO₂ pigment. In this study, we found that Fe³⁺ and V³⁺ were easily hydrolyzed and entered the MTA lattice, and hence could not be removed by washing. Furthermore, Fe/V was hard to co-remove by the traditional reduction method. Therefore, the Fe/V non-hydrolysis condition (Ti³⁺ = 0.01 M, F = 3.0, T = 130 °C; Ti³⁺ = 0.01 M, F = 3.5, T = 150 °C) was determined by thermodynamic calculations. However, at these conditions, the Ti hydrolysis ratio was low or the reaction time was long. Therefore, a new two-step hydrothermal hydrolysis process was proposed. Step 1 (130 °C, 2 h) ensured the non-hydrolysis of V³⁺, and Ti was partially hydrolyzed to increase the H₂SO₄ concentration. Step 2 (150 °C, 2 h) ensured a high Ti hydrolysis ratio (>0.95) and short total reaction time (4–6 h). Finally, a high-purity MTA was obtained (Fe = 21 ppm, V = 145 ppm). These results provide new insights into the control of the hydrolysis of impurity ions in solutions and help to optimize the process of TiO₂ pigment preparation from TBFS.

1. Introduction

Titanium-bearing blast furnace slag (TBFS) is a byproduct during the smelting of vanadium-bearing titanomagnetite, which contains up to 23 wt.% TiO₂ and various impurities (Ca, Si, Al, Mg, Fe, V, Mn) [1,2]. In China, TBFS production is more than 5 million t/a. The high content of TiO₂ reduces the CaO activity, which inhibits applications in cement production, resulting in a low utilization rate (<3%) and significant slag accumulation (80–100 million tons in total) [2,3].

Extraction of valuable elements by hydrometallurgy has received widespread attention due to its low cost [4,5,6]. Recently, the extraction of Ti from TBFS by the sulfuric acid roasting method has received wide attention because of its energy saving, low cost, and high Ti recovery rate [7,8,9,10]. In this method, TBFS is roasted with concentrated H₂SO₄ (80–95%) at 100–300 °C, then leached and filtered to form an impurities-bearing TiOSO₄ solution, where the extraction of Ti is ~88% [7,11]. Like the sulfate process for TiO₂ pigment production, the impurities-bearing TiOSO₄ solution hydrolyzes to form H₂TiO₃ (MTA). Then MTA is calcined to form TiO₂ pigment [11,12]. However, due to the low concentration of TiOSO₄ (0.6–0.8 mol·L⁻¹, vs. 2.4–2.8 mol·L⁻¹ in the industry) and the various impurities (Al, Mg, Fe, V, Mn), the production of pigment-grade TiO₂ has not at this time been achieved [9,11,12].

One important reason is that some impurity ions hydrolyze at high temperatures and co-precipitate with MTA, which cannot be removed by washing [13,14]. The color-developing impurity elements in the impurities-bearing TiOSO₄ solution include Mn, Fe, and V, which all lower the whiteness of pigment [15]. The occurrence of these elements is as the dissolved species Mn²⁺, Fe³⁺/Fe²⁺, and VO²⁺/V³⁺, respectively. Among them, Fe³⁺ and V³⁺ are easily hydrolyzed [12,16,17]. However, there are strict requirements for the content of both Fe (30 ppm) and V (7 ppm) in MTA [12,18].

For the removal of Fe and V from MTA, the method used in the sulfate process is to reduce Fe³⁺ to Fe²⁺ and use high concentrations of H₂SO₄ to inhibit the hydrolysis of V³⁺ [12,18]. However, the impurities-bearing TiOSO₄ solution from TBFS has a low concentration of H₂SO₄ and more V³⁺ [11], so the V³⁺ can more readily hydrolyze. If V³⁺ is oxidized to VO²⁺, this will lead to the formation and possible hydrolysis of Fe³⁺, so it is difficult to achieve acceptable co-removal of Fe/V. Therefore, it is necessary to increase the H₂SO₄ concentration by vacuum evaporation, which significantly increases the energy consumption [19,20]. Another viable option is to separate the Fe/V from the original solution, such as via adsorption, extraction, etc. Elnagar et al. [21] extracted trace Fe³⁺ (0.03–80 mg/L) from titanium concentrates with organic complexation and solvent extraction (efficiency > 98.50%). Middlemas et al. [22] proposed a new method for the production of titanium dioxide pigment, which used solvent extraction to remove Fe³⁺ and finally produce good quality anatase pigment (Fe < 20 ppm). Peng et al. [23] adsorbed vanadium(V) with melamine, resulting in 99.89% removal. However, these methods have high agent consumption, are long processes, and have increased costs.

In this study, for the MTA made from impurity bearing TiOSO₄ solution, we found the Fe/V impurity could not be co-removed by the acid neutralization followed by solution reduction method traditionally. Therefore, we determined the conditions for non-hydrolysis of Fe/V by thermodynamic calculation. However, at these conditions, the Ti hydrolysis ratio was low and the reaction time was long. Therefore, we provided a new simplified two-step hydrolysis process. Without acid neutralization, Ti³⁺ in the original solution inhibited the hydrolysis of Fe. Step 1 (130 °C, 2 h) ensured the non-hydrolysis of V and generated enough free H₂SO₄. Step 2 (150 °C, 2 h) promoted the hydrolysis of TiOSO₄. Finally, high purity MTA was obtained (Fe = 21 ppm, V = 145 ppm) with a high Ti hydrolysis ratio (>0.95) and reasonable reaction time (4–6 h).

2. Materials and Methods

2.1. Materials and Reagents

TBFS was obtained from the Panzhihua Iron & Steel (Group) Co., Ltd. (Panzhihua, China). The TBFS composition determined by X-ray fluorescence spectroscopy (XRF) is shown in

Table 1. The composition of TBFS determined by XRF.

Elements	TiO2	CaO	SiO2	MgO	Al2O3	TFe	MnO	V2O5
wt.%	21.4	27.2	27.0	8.8	12.8	1.5	0.7	0.2

. Commercial chemicals of analytical grade and deionized water with a resistivity >18 MΩ·cm⁻¹ were used in the experiments. Titanyl sulfate-sulfuric acid hydrate (TiOSO₄·xH₂O·yH₂SO₄, 93%), aluminum sulfate octa decahydrate (Al₂(SO₄)₃·18H₂O, 99%), and anhydrous magnesium sulfate (MgSO₄, 99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Ammonium iron (III) sulfate dodecahydrate (NH₄FeSO₄·12H₂O, 99%), calcium oxide (CaO, 99%), methyl orange (indicator grade), and ammonium thiocyanate (NH₄SCN, 99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Sulfuric acid (H₂SO₄, 98%), aluminum powder (Al, 99%), and sodium hydroxide (NaOH, 99%) were purchased from Beijing Tong Guang Fine Chemicals Company (Beijing, China). All of the chemicals were used without any further purification.

2.2. Experimental Method

The process of making impurity bearing TiOSO₄ solution from TBFS was according to our previous research [7,11,24]: TBFS was decomposed by 90% H₂SO₄ at 150 °C for 1 h to obtain solid mixtures. The mixtures were leached with water at 65 °C for 1 h and filtered to separate the solid (mainly SiO₂ and CaSO₄) to obtain the impurity bearing TiOSO₄ solution. The impurities were sulfates of various impurity ions. The properties of TBFS and the reactions of the TBFS decomposition were studied previously [2,7].

The composition of the solution was determined by using inductively coupled plasma atomic emission spectroscopy (ICP-AES), as shown in

Table 2. The composition of impurity bearing TiOSO₄ solution determined by ICP-AES and titration.

Component	Ti	Mg	Al	Fe	Mn	V	F-Value	Ti3+
mol·L−1	0.719	0.520	0.722	0.027	0.013	0.006	3.02	0.011

. The F-value is the acidity factor determined as given by Equation (1)

$$F=\frac{m\left(H_{2}S{O}_4 in TiOS{O}_4\right)+m\left(Free H_{2}S{O}_4\right)}{m\left(Ti{O}_2\right)}$$

(1)

where m is the mass concentration, g/L. The H₂SO₄ in TiOSO₄ refers to the H₂SO₄ generated by the hydrolysis of TiOSO₄. The sum of the two is referred to as the effective H₂SO₄ in the sulfate process [25]. The effective H₂SO₄ is determined by 0.5000 mol·L⁻¹ NaOH neutralization titration (methyl orange as the indicator) [11]. The concentration of Ti³⁺ was determined by titration with 0.1000 mol·L⁻¹ NH₄FeSO₄ (NH₄SCN as the indicator).

The impurity bearing TiOSO₄ solution was treated with two different processes. Following the traditional sulfate process, the solution was neutralized by CaO to adjust the F-value = 1.50, then the Ti³⁺ concentration increased by reacting with Al powder for 1–5 h, and finally thermally hydrolyzed at 110 °C for 4 h at atmospheric pressure. For the new two-step hydrothermal hydrolysis, without any pre-treatment, the solution was sealed in a Teflon-lined autoclave, and firstly hydrothermally hydrolyzed at 110–130 °C for 2–4 h, and then hydrothermally hydrolyzed at 150 °C for 2 h. After hydrolysis, MTA was obtained by filtering. The MTA was washed twice with 5% H₂SO₄ and twice with deionized water and then dried in an oven at 105 °C for 4 h. Then, the MTA was dissolved in 98% H₂SO₄ at 250 °C for 0.3–0.5 h, and the clarified solution was diluted by deionized water and used to determine the Fe/V content in the MTA by using ICP-AES.

2.3. Characterization

The element analysis of the power sample was carried out by X-ray fluorescence spectroscopy (XRF, ARL PERFORM X, Thermo Fisher, Waltham, MA, USA). The solution ion concentration was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, ARCOS II, Spectro, Kleve, Germany). The phase was identified from measurements using an X-ray diffractometer (Bruker-AXS D8 Advance, Karlsruhe, Germany) with Cu Kα (0.154178 nm) radiation. The surface composition of the samples was examined with an X-ray photoelectron spectrometer (XPS, PHI-5300, PHI, Lafayette, LA, USA) using an Al Kα (1486.6 eV) X-ray source. All spectra were calibrated to the binding energy of the adventitious C 1 s peak at 284.8 eV. The morphology and microstructure of the samples were characterized with a field emission scanning electron microscope (FESEM, JSM 7401F, JEOL, Hitachi, Tokyo, Japan).

3. Results and Discussion

3.1. The Hydrolysis Behavior and Occurrence form of Fe/V in MTA

The main impurity elements of TiOSO₄ solution obtained from TBFS were Mg, Al, Fe, V, and Mn, where both Fe and V possibly existed in two states (Fe²⁺/Fe³⁺ and V³⁺/VO²⁺).

Table 3. The theoretical pH at the beginning of the impurity ions hydrolysis.

Ions	Mg2+	Al3+	Fe2+	Fe3+	V3+	VO2+	Mn2+
mol·L−1	0.520	0.722	0.029	0.029	0.005	0.005	0.013
pKsp(25 °C) [16]	10.74	32.88	15.1	37.4	34.4	22.13	12.73
Precipitation pH	8.74	3.17	7.22	2.05	3.30	7.39	10.39

shows the theoretical pH at the beginning of the hydrolysis of these impurity ions. The precipitation pH was calculated by the ion concentrations and K_sp of the corresponding hydroxides. Al³⁺, Fe³⁺, and V³⁺ had low precipitation pH, which meant they were easy to hydrolysis.

Figure 1a summaries the two processes for the synthesis of MTA from the impurity bearing TiOSO₄ solution. Firstly, in order to ensure the hydrolysis ratio would be >95%, CaO slurry was added to neutralize the H₂SO₄ with a decrease in acidity from F = 3.02 to F = 1.50 (F is the acidity factor defined in Equation (1)). The reduction of the solution acidity promoted the hydrolysis of Fe/V. If the solution was directly hydrolyzed at 110 °C, Fe-MTA was obtained consistent with the hydrolysis of Fe³⁺. In the sulfate process, Ti³⁺ was used to reduce the Fe³⁺ to Fe²⁺ to inhibit the hydrolysis of Fe and the adsorption of Fe³⁺ [12]. In order to generate Ti³⁺, we applied Al reduction ($Al+Ti{O}^{2+}+2H^{+}=A{l}^{3+}+T{i}^{3+}+H_{2}O$). The added Al did not significantly affect the concentration of Al in the solution. However, if the solution was reduced, V-MTA was formed. The composition of Fe-MTA and V-MTA is shown in

Table 4. The impurity content (TiO₂ wt.%) of Fe-MTA and V-MTA.

	TiO2 (%)	SO3 (%)	Mg (ppm)	Al (ppm)	Fe (ppm)	V (ppm)	Mn (ppm)
Fe-MTA	79.2	3.16	<5	1138	3854	<4	<4
V-MTA	78.4	3.31	<5	1202	8	1205	<4

; all the impurity contents are in terms of TiO₂ wt.%. Though Al was hydrolyzed, it was colorless and therefore did not affect the pigment quality.

Figure 1b shows the Fe/V hydrolysis behavior in the unreduced solution. The concentration of V was unchanged, while the concentration of Fe was decreasing exponentially. This means the only Fe was hydrolyzed and precipitated with MTA. With the hydrolysis of TiOSO₄, the concentration of H₂SO₄ was increased ($TiOS{O}_4+{2H}_{2}O=H_{2}Ti{O}_3\downarrow +H_{2}S{O}_4$), so finally the hydrolysis of Fe was inhibited and the concentration of Fe was unchanged. Figure 1c shows the Fe/V hydrolysis behavior in the Al reduced solution. Though Fe was unhydrolyzed due to Al reduction with an unchanged concentration, V was hydrolyzed and the concentration was decreasing exponentially like Fe. The reason for this was the formation of V³⁺, as shown in Equations (2)–(4), where E⁰ is the standard electrode potential of the reactions at 25 °C.

$$F{e}^{3+}+e^{-}=F{e}^{2+} E^0=0.771\mathrm{V}$$

(2)

$$V{O}^{2+}+2H^{+}+e^{-}=V^{3+}+H_{2}O E^0=0.337\mathrm{V}$$

(3)

$$Ti{O}^{2+}+2H^{+}+e^{-}=T{i}^{3+}+H_{2}O E^0=0.100\mathrm{V}$$

(4)

When the solution was unreduced, no Ti³⁺ was formed, so the solution was in the oxidized state. Fe existed as Fe³⁺, and V existed as VO²⁺. Fe³⁺ was easy to hydrolyze and formed Fe-MTA. When the solution was reduced by Al, Ti³⁺ was generated, Fe³⁺ converted to Fe²⁺, and VO²⁺ converted to V³⁺. V³⁺ was easy to hydrolyze and formed V-MTA. According to the Nernst equation, the concentration of Ti³⁺ affects the redox potential of the solution and finally determines the concentration of Fe²⁺/Fe³⁺ and V³⁺/VO²⁺. Therefore, the aim was to find a suitable Ti³⁺ concentration in the solution for minimal Fe/V hydrolysis, as shown in Figure 1d. The results show that Fe or V in MTA was always >500 ppm at different Ti³⁺ concentrations, so the Fe/V could not be removed simultaneously by Al reduction.

Figure 2a shows the XRD patterns of Fe/V-MTA and blank MTA. The blank MTA was synthesized from the Mg/Al-bearing TiOSO₄ solution at the same conditions with Fe-MTA. The solution was prepared from reagents (TiOSO₄, MgSO₄, Al₂(SO₄)₃) without Fe/V impurities. The crystalline product in these samples was anatase (PDF 21-1272).

Table 5. The analysis of the anatase (101) peak in XRD patterns.

	2θ (°)	Peak Height	FWHM	Crystal Size (Å)
Blank	25.223	5799	0.617	134
Fe-MTA	25.186	4377	0.821	100
V-MTA	25.352	4825	0.805	102

shows the analysis of the (101) peak. Fe/V-MTA had lower diffraction intensity and smaller crystal size than blank MTA, which means the Fe/V in the solution affected the hydrolysis of TiO²⁺ and inhibited the growth of the anatase nanocrystals in MTA. The changes of 2θ for Fe/V-MTA indicate that Fe/V may have entered the anatase nanocrystals and changed the lattice spacing. Figure 2b shows the XPS Ti 2p spectra of Fe/V-MTA and blank MTA. The peaks at ~465.1 and 459.4 eV of blank MTA were assigned to Ti 2p_1/2 and Ti 2p_3/2 of the Ti⁴⁺ state [26]. For Fe/V-MTA, the negative shift of binding energy ~1.8 eV indicated that some Ti⁴⁺ ions were replaced by Fe/V, and the Ti-O-Ti in anatase changed to Ti-O-Fe/V. Figure 2c shows the XPS Fe 2p spectra of Fe-MTA. The peaks centered at ~723.9 and 712.6 eV were assigned to Fe 2p_1/2 and Fe 2p_3/2 of the Fe (III) species, and the spin–orbit splitting characteristic of Fe³⁺ corresponded to typical features of Fe(OH)₃ [27]. Figure 2d shows the XPS V 2p spectra of V-MTA. The peaks centered at ~521.8 and 514.4 eV were assigned to V 2p_1/2 and V 2p_3/2 of V³⁺, which was close to V(OH)₃ [28,29]. Therefore, the results indicate that some of the Fe³⁺/V³⁺ hydrolyzed with TiO²⁺ at high temperature and entered the lattice of anatase nanocrystals so these impurities could not be removed by dilute H₂SO₄ or water.

3.2. Thermodynamic Equilibrium Calculation of the Hydrolysis Process

It has been proposed that Fe/V enters the MTA lattice by hydrolysis with TiO²⁺, so the key to controlling the impurity content is to inhibit the hydrolysis of Fe³⁺/V³⁺. It is known that if a reaction cannot proceed thermodynamically, then kinetically the reaction cannot proceed. Therefore, assessing if the Fe³⁺/V³⁺ hydrolysis reaction will happen thermodynamically at different reaction conditions will help to identify the Fe³⁺/V³⁺ non-hydrolysis condition. Therefore, a thermodynamic calculation for the hydrolysis process of impurity bearing TiOSO₄ solution was carried out. The equilibria of the impurities-bearing TiOSO₄ solution for the thermodynamic calculation are given in Equations (5)–(16).

$$S{O}_4^{2-}+H^{+}=HS{O}_4^{-}$$

(5)

$$HS{O}_4^{-}+H^{+}=H_{2}S{O}_4$$

(6)

$$M{g}^{2+}+S{O}_4^{2-}=MgS{O}_{4\left(aq\right)}$$

(7)

$$A{l}^{3+}+S{O}_4^{2-}=AlS{O_4}^{+}$$

(8)

$$AlS{O_4}^{+}+S{O}_4^{2-}=Al(S{O}_4{)}_2^{-}$$

(9)

$$Ti{O}^{2+}+S{O}_4^{2-}=TiOS{O_4}_{\left(aq\right)}$$

(10)

$$F{e}^{3+}+S{O}_4^{2-}=FeS{O}_4^{+}$$

(11)

$$V{O}^{2+}+S{O}_4^{2-}=VOS{O_4}_{\left(aq\right)}$$

(12)

$$F{e}^{3+}+V^{3+}+H_{2}O=F{e}^{2+}+2H^{+}+V{O}^{2+}$$

(13)

$$V{O}^{2+}+T{i}^{3+}=V^{3+}+Ti{O}^{2+}$$

(14)

$$2V^{3+}+3S{O}_4^{2-}=V_2(S{O}_4{{)}_3}_{\left(aq\right)}$$

(15)

$$H^{+}+O{H}^{-}=H_{2}O$$

(16)

The standard equilibrium constant or relevant electrode potential was obtained from various handbooks and articles [16,17,30,31,32]. By using Van’t Hoff, Kirchhoff’s, and Nernst equations, the equilibrium constants for Equations (5)–(16) at 110–150 °C were obtained, as shown in Figure 3a. By using the Newton–Raphson method, based on the original composition of the solution in Table 2 and equilibrium constants, the equilibrium composition was calculated, as shown in Figure 3b,c. Here, α was referred to as the hydrolysis ratio, which was determined by Equation (17).

$$\alpha =\frac{{\left[Ti{O}^{2+}\right]}_0-\left[Ti{O}^{2+}\right]}{{\left[Ti{O}^{2+}\right]}_0}$$

(17)

The temperature (110 °C), F-value (1.50), and [Ti³⁺] (0.01 mol·L⁻¹) used in the calculation were the experimental conditions. With increasing hydrolysis ratio, [SO₄²⁻], [TiOSO₄], [TiO²⁺], and [OH⁻] decreased, while [HSO₄⁻], [H⁺], and [H₂SO₄] increased, which was because H₂SO₄ was formed during the hydrolysis of TiOSO₄ ($TiOS{O}_4+2H_{2}O=H_{2}Ti{O}_3\downarrow +H_{2}S{O}_4$). The concentration of impurity ions changed a little, but it is worth noting that the concentration of H⁺ was relatively low (0.001–0.01 mol·L⁻¹). As shown in

Table 6. The conc. of free H₂SO₄ for different F-value and TiOSO₄ conc.

Conc. (mol·L−1)	Impurities-Bearing TiOSO4 Solution				Sulfate Process
TiOSO4	0.719				2.375
F-value	1.5	2.0	2.5	3.0	1.8
Free H2SO4	0.163	0.459	0.755	1.051	1.115

, the lower TiOSO₄ concentration compared to that for the sulfate process contributed to the lower free H₂SO₄, corresponding to the lower concentration of H⁺. Even at higher F-value, the free H₂SO₄ of the impurities-bearing TiOSO₄ solution was much lower than for the industrial sulfate process in the industry. Furthermore, a large quantity of SO₄^2- introduced by the high concentration of impurity ions from TBFS promoted a positive proceeding of Equation (5), so lots of H⁺ was consumed.

Based on the equilibrium composition, the concentration quotient (Q_c) for Fe(OH)₃ or V(OH)₃ can be calculated, and whether Fe/V will be precipitated can be judged by comparing it with the corresponding solubility product (K_sp). If Q_c > K_sp, precipitation is expected, and vice versa. To validate the results of the equilibrium composition, a comparison was performed for the condition that is used to synthesize Fe-MTA and V-MTA, as shown in Figure 4. For the Fe-MTA condition (Figure 4a,b), at any hydrolysis ratio, Fe(OH)₃ formation was expected as Q_c > K_sp and V(OH)₃ formation was expected as Q_c < K_sp. For the V-MTA condition (Figure 4c,d), V hydrolyzed while Fe did not at a low hydrolysis ratio, and neither Fe nor V was hydrolyzed at a high hydrolysis ratio. The calculations were consistent with the experimental results. Furthermore, the Q_c values for Fe(OH)₃/V(OH)₃ significantly decreased with the increasing of α, so increasing the concentration of free H₂SO₄ (F-value) was expected to inhibit the hydrolysis of Fe/V.

To identify the conditions for Fe/V co-removal, a simulation for different hydrolysis conditions (temperature, F-value, and Ti³⁺ concentration) was performed and the results are shown in Figure 5. As discussed before, increasing the concentration of free H₂SO₄ close to that of the industry standard is an effective way to inhibit the hydrolysis of Fe/V, so a suitable F-value was 2.0–3.5. However, increasing the F-value lowered the hydrolysis ratio, so a higher temperature was necessary (T = 130–150 °C). There were two ways for the co-removal: (1) inhibit Fe hydrolysis under oxidizing conditions (No Ti³⁺) or (2) inhibit V hydrolysis under reducing conditions (0.01 M Ti³⁺). The results show that Fe³⁺ was always hydrolyzed at a low hydrolysis ratio (Figure 5a,b), while V³⁺ had unhydrolyzed conditions (Figure 5c,d). Therefore, method (2) was optimal for Fe/V co-removal. The possible conditions were (130 °C, F = 3.0) and (150 °C, F = 3.5). However, for the condition (130 °C, F = 3.0), the Ti hydrolysis ratio was only 0.638 after 4 h; likewise, for the condition (150 °C, F = 3.5), the hydrolysis ratio was 0.842 after 4 h. Therefore, though the two possible conditions may ensure the Fe/V co-removal, they cannot ensure the Ti hydrolysis ratio >0.95 within a reasonable reaction time (4–6 h), which will lower the production efficiency.

3.3. Two-Step Hydrothermal Hydrolysis for Fe and V Co-Removal

To ensure a high hydrolysis ratio (experimental α > 0.95) within a reasonable reaction time (4–6 h), the hydrolysis properties of impurities-bearing TiOSO₄ solution were investigated, as shown in Figure 6a. For the solution formed by dissolution of TBFS, the F-value was 3.0. If the F-value was higher (F = 3.5), additional concentrated H₂SO₄ was required, and the temperature or reaction time for hydrolysis needed to be higher or longer. If the F-value was lower (F = 2.5), V³⁺ would hydrolyze during the initial stages of Ti hydrolysis as shown in Figure 5c,d. Thus, F = 3.0 was selected as optimal. For F = 3.0, the conditions whether V³⁺ hydrolyzed at different temperatures and hydrolysis ratios were investigated, so that several hydrolysis processes were provided as shown in Figure 6a,b. If the solution was hydrolyzed at 150 °C for 4 h, V³⁺ may have hydrolyzed at the initial stages of Ti hydrolysis (Q_c > K_sp when α = 0), as shown in Figure 6b. If the solution was hydrolyzed at 130 °C for 4 h, though the V³⁺ was not hydrolyzed, the hydrolysis ratio was very low (0.32, 2 h), as shown in Figure 6a. Therefore, we provided a new two-step hydrothermal hydrolysis method, as shown in Figure 6c. For Step 1, the impurities-bearing TiOSO₄ solution was hydrolyzed at low temperature to inhibit the V hydrolysis. According to Figure 6b, the suitable condition for hydrolysis was at 110–130 °C for 2–4 h to allow Ti to partially hydrolyze. As hydrolysis proceeded, [H₂SO₄] was increased, and the Ti hydrolysis ratio > 0.25, V³⁺ was not hydrolyzed at higher temperatures. Therefore, for Step 2, the solution was hydrolyzed at a higher temperature to ensure α > 0.95 within 6 h. According to Figure 6c, to reduce the total hydrolysis time, the hydrolysis at 150 °C was conducted for 2 h.

Figure 7 shows the experimental validation of the two-step hydrothermal hydrolysis method, where only the conditions of Step 1 were varied. Figure 7a shows the hydrolysis ratio was increased by the temperature and reaction time of Step 1; α > 0.95 could be achieved only if the reaction time was longer than 4 h or the temperature was 130 °C. Figure 7b shows the Fe content in MTA was only 21 ppm for the reaction time of 2 h, which met the requirement of TiO₂ pigment (<30 ppm) and was much lower than Fe-MTA (3854 ppm), but it increased rapidly with hydrolysis time. This may have been because the oxygen in the autoclave gradually oxidized Ti³⁺ to TiO²⁺ as time went on, while Fe²⁺ was converted to Fe³⁺, resulting in an increase of the Fe content in MTA. Figure 7c shows that the V content in MTA at the different conditions was almost the same, which was in the range of 140–160 ppm. Despite not meeting the requirement of TiO₂ pigment (>7 ppm), it was much lower than V-MTA (1205 ppm). Considering both the extent of hydrolysis and the Fe/V content, the optimal condition for Step 1 was hydrolysis at 130 °C for 2 h. The sample obtained with high purity is shown in Figure 7d. Compared with the traditional method (Figure 1a), the process was simpler. More importantly, Fe and V were both excluded by the two-step method (Fe = 21 ppm, V = 145 ppm). Compared with the optimal Fe/V content in Figure 1d, the impurities content of the two-step method was notably decreased and the color was white. Compared with previous relevant works [13,14,20], Fe was decreased from ~10,000 ppm to 21 ppm.

Though the two-step method reduced the Fe/V content of MTA, the V impurity was still high. Thus, we studied the possible reason for how V³⁺ entered the MTA from which the results are shown in Figure 8. As shown in Figure 8a, the reduction in V concentration up to 4 h was significantly less than before (Figure 1c). Furthermore, Figure 8b shows that the two-step MTA and blank MTA had the same XRD (101) peak position of anatase, which means the V³⁺ did not enter the lattice of MTA. This indicates that V³⁺ did not enter the MTA by hydrolysis but was present due to other mechanisms. Figure 8c compares the Ti 2p XPS spectra of the two-step MTA and blank MTA. The peaks at ~465.1 and 459.4 eV of blank MTA were assigned to Ti 2p_1/2 and Ti 2p_3/2 of the Ti⁴⁺ state. For the two-step MTA, the negative shift of binding energy ~0.2 eV indicated that some V³⁺ was located on the MTA surface and affected the electronic state of Ti⁴⁺, as previous studies have shown [33]. Therefore, V impurities in MTA may have been due to the adsorption of V³⁺. Furthermore, the V cannot be washed off, so the adsorption was not a simple physical adsorption. According to the formation and structure of MTA, we provided a possible mechanism for how V³⁺ enters the MTA in two-step hydrolysis, as shown in Figure 8d. The SEM image shows that the two-step MTA formed from impurity bearing TiOSO₄ solution had hierarchical structures [34]; the aggregation of 30–100 nm primary anatase TiO₂ particles to form final 1–3 μm MTA aggregates. Previous studies show that SO₄²⁻ (4–6 wt.% SO₃ of TiO₂) is strongly adsorbed at the surface of the primary particles [34,35]. Furthermore, V³⁺ can readily complex with SO₄²⁻, so the V³⁺-SO₄²⁻ complex may be formed on the primary particles surface of MTA, like the interaction between Al³⁺ and the hydrated TiO₂ clusters [36,37]. As hydrolysis proceeds, primary particles agglomerate and therefore V³⁺ is trapped in MTA, which is difficult to wash off.

For future studies of Fe/V co-reduced, the key to reduce the V content in MTA is to inhibit the formation of the V³⁺-SO₄²⁻ complex. Specifically, there are two ways: (1) selective complexation of V³⁺ with complexing agents, and (2) converting V³⁺ to VO²⁺ with oxidizing agents to facilitate the dissolution of V in solution.

4. Conclusions

In this study, the formation of Fe/V bearing MTA from the impurities-bearing TiOSO₄ solution was investigated, with the following conclusions:

(1)

Fe³⁺ and V³⁺ easily hydrolyzed with TiO²⁺ and entered the MTA lattice, resulting in these impurities being difficult to remove by washing. Traditionally manipulating Ti³⁺ could only remove one of Fe or V.

(2)

Based on thermodynamic calculations, the conditions for neither Fe nor V hydrolysis were determined: (a) Ti³⁺ = 0.01 M, F = 3.0, T = 130 °C. (b) Ti³⁺ = 0.01 M, F = 3.5, T = 150 °C.

(3)

To improve the Ti hydrolysis ratio (>0.95) and reduce the reaction time (4–6 h), two-step hydrolysis was provided (130 °C, 2 h + 150 °C, 2 h, Ti³⁺ = 0.01 M, F = 3.0), and impurity levels of Fe/V were notably reduced (Fe = 21 ppm, V = 145 ppm).

(4)

The residual V impurity may have been due to the adsorption of the V³⁺-SO₄^2- complex on the surface of the MTA particles.

These findings will provide new insights into the control of the hydrolysis of impurity ions in solutions and help to optimize the process of making TiO₂ pigment from TBFS.