Recovery of TiO<sub>2</sub> from spent SCR catalyst by acidolysis and additional crystal seeds-hydrolysis method

Bo Wang; Zhongnan Li; Hangkai Liu; Libin Tian; Qiaowen Yang; Ruoyan Bai; Yue Chen; Xu Liu

doi:10.4491/eer.2023.004

Environ Eng Res > Volume 28(6); 2023 > Article

Wang, Li, Liu, Tian, Yang, Bai, Chen, and Liu: Recovery of TiO2 from spent SCR catalyst by acidolysis and additional crystal seeds-hydrolysis method

Research

Environmental Engineering Research 2023; 28(6): 230004.

Published online: February 22, 2023

DOI: https://doi.org/10.4491/eer.2023.004

Recovery of TiO₂ from spent SCR catalyst by acidolysis and additional crystal seeds-hydrolysis method

Bo Wang^1,^†

, Zhongnan Li¹, Hangkai Liu¹, Libin Tian¹, Qiaowen Yang¹, Ruoyan Bai¹, Yue Chen¹, Xu Liu²

¹School of Chemical and Environmental Engineering, China University of Mining and Technology-Beijing, No. Ding-11 Xueyuan Road, Haidian District, 100083, Beijing, China

²College of Physical Education, Taiyuan University of Technology, No. 79 Yingze West Street, 030024, Taiyuan, Shanxi Province, China

^†Corresponding author: E-mail: xsywb2020@126.com, Tel: +86 18241339476, Fax: +86-010-62346931

Received December 31, 2022 Revised February 12, 2023 Accepted February 20, 2023

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In this study, a rapid and efficient method of recovering TiO₂ from spent SCR (selective catalytic reduction) catalyst supplied by a coal-fired power plant in Shanghai was developed. The acid leaching and sodium roasting-water leaching residues (titanium slag) from spent SCR catalyst were employed as the raw material. First, titanium slag was dissolved into H₂SO₄ solution under different conditions. The optimal acidolysis conditions were 85 wt% H₂SO₄, 5.0 g/g acid-residues ratio, 90°C acidolysis temperature and 2.0 h acidolysis time. Simultaneously, the highest acidolysis efficiency can reach 92.02%. The acidolysis solution was collected and concentrated to 200 g/L (measured by TiO₂ concentration). Then, acidolysis solution was adjusted to pH 0.52–9.15, followed by adding 0–40 mL additional crystal seeds and reacting under certain conditions to obtain the hydrolysis product. The optimal hydrolysis conditions were 25 mL additional crystal seeds, pH 7.46, 120°C hydrolysis temperature and 3.0 h hydrolysis time. Finally, the recovered product was obtained by roasting hydrolysis product at 700°C for 2.0 h. The results of hydrolysis stability test indicated that this method can make the hydrolysis efficiency of acidolysis solution stabilize at approximately 90% under optimal conditions. The main component of recovered product was anatase TiO₂ with the purity of 86.88%.

Keywords: Acidolysis, Additional crystal seeds-hydrolysis, Recovery of TiO₂, Spent SCR catalyst, Titanium slag

Graphical Abstract

Keywords: Acidolysis, Additional crystal seeds-hydrolysis, Recovery of TiO₂, Spent SCR catalyst, Titanium slag

1. Introduction

China is a large country with various energy, while the distribution of energy production structure is extremely unreasonable, among which the coal accounts for more than 80%, petroleum accounts for 10% and natural gas accounts for only 4.4% [1]. According to the statistics, 63% of energy in China derives from the coal combustion in coal-fired power plant [2]. However, the exhaust gas released from coal-fired power plant contain large amounts of NO_x pollutants, such as NO, NO₂, N₂O, N₂O₄ and N₂O₅, which will destroy the ozonosphere, further produce photochemical smog and finally cause extensive damage to animals and plants [3]. Therefore, it is necessary to carry on the denitrification to the flue gas.

Nowadays, the denitrification technologies include SNCR (selective non-catalytic reduction) and SCR (selective catalytic reduction) [4]. SNCR method is mainly used in the case of high dust (30–120 g/Nm³), high alkali metals content (>8 wt%) and high catalyst consumption (3–6 layers catalyst, 8–10 m³), such as the denitrification process of cement plant, while the denitrification efficiency is only 50%–80% [5–8]. Owing to the usage of catalyst, SCR method can efficiently convert NO_x into harmless N₂ and H₂O so that the denitrification efficiency can reach over 90%, as well as low reaction temperature and stable operation [9]. Therefore, SCR method has become the mainstream denitrification technology in industry.

Catalyst is the core part of SCR method, among which V₂O₅-WO₃/TiO₂ has been the commercial SCR catalyst [10]. In the flue gas environment containing sulfur and heavy metals, catalyst is prone to cause rapid deactivation. After using 2–3 years, most of SCR catalyst loses their activity so that are difficult to regenerate and further need to be treated [11]. Traditional landfill and burning methods fail to thoroughly solve the environmental pollution, as well as causing waste sources [12]. Thus, recovering valuable metals from spent SCR catalyst is significant for environment and economy.

TiO₂ is the carrier and main component of vanadium-titanium based catalyst, and simultaneously V₂O₅ and WO₃ are added into catalyst as the active components. TiO₂ accounts for over 80 wt% in catalyst and approximately 70 wt%–90 wt% of TiO₂ can be recovered in SCR catalyst [13]. Thus, considering the perspective of resource recycle and the production of TiO₂, employing spent SCR catalyst as a titanium source is a promising option. Among many processes, acid leaching can simplify the recovery process of vanadium so that improve the recovery efficiency of vanadium, while sodium additives roasting shows the highest leaching efficiencies of vanadium and tungsten. For the recovery of titanium, acidolysis-hydrolysis is the traditional technology. Wu et al. [14] adopted alkali leaching method to extract vanadium and tungsten from spent SCR catalyst, followed by dissolving titanium slag into dilute HCl and changing the conditions of hydrolyzing TiOCl₂ to obtain H₂TiO₃. Finally, anatase TiO₂ with the purity of 98% through roasting H₂TiO₃ was obtained. However, the hydrolysis efficiency of TiOCl₂ solution failed to be studied. Ma et al. [15] developed a novel method to recycle nano-TiO₂ from spent SCR catalyst. They first used Na₂CO₃ roasting-water leaching to separate titanium slag from other components. Then, titanium slag was dissolved into the dilute H₂SO₄ to obtain TiOSO₄ solution. Finally, H₂TiO₃ precipitation was obtained by adding Na₂CO₃ to adjust solution to neutral. The hydrolysis efficiency of TiOSO₄ solution fluctuated from 84.4% to 99.3%. Similarly, Chen et al. [16] employed the process of acidolysis and hydrolysis to recover TiO₂ from spent SCR catalyst. The hydrolysis efficiency of acidolysis solution and purity of TiO₂ can reach 92.15% and 96.28%, respectively. Tian et al. [17] investigated the influence of some factors, such as F value (acidity coefficient), volume ratio of pre-adding water to TiOSO₄ solution, heating rate and pH of pre-adding water on the hydrolysis efficiency of TiOSO₄ solution, and particle sizes and pigment properties of hydrolysis product. The results showed that higher F value can diminish the hydrolysis rate and further form less crystal nucleus, as well as producing the precipitation with smaller particle sizes, resulting lower hydrolysis efficiency. The obtained product presented higher whiteness under higher volume ratio of pre-adding water to TiOSO₄. Increasing heating rate and pH of pre-adding water was helpful to improve the nucleation rate and crystal growth rate, namely enhancing the hydrolysis efficiency. The product was anatase TiO₂ after hydrolysis precipitation was roasted at high temperature.

Based on many reports, it can be seen that although both the acidolysis efficiency of titanium slag and purity of recovered TiO₂ are high, the hydrolysis efficiency of TiOSO₄ solution still fails to keep stable [15, 18, 19]. This is due to the fact that one of the most important hydrolysis technologies for the acidolysis solution of titanium slag is self-generated crystal seeds hydrolysis. This method is complex to operate during hydrolysis process, as well as existing many uncertain influencing factors, which leads to the unstable hydrolysis efficiency of titanium solution [20, 21]. Therefore, in order to develop a novel route with stable indexes for recovery of TiO₂, titanium slag obtained from spent SCR catalyst after acid leaching and roasting-water leaching was used as the raw material in this study, and then a process of acidolysis and rapid hydrolysis to recover TiO₂ was researched.

2. Materials and Methods

2.1. Materials

The spent SCR catalyst was supplied by a coal-fired power plant in Shanghai, China. In order to diminish experimental errors, the spent SCR catalyst should keep the same batch during experiment. The H₂SO₄ (98 wt%, AR), NaOH (96 wt%, AR), Na₂CO₃ (99 wt%, AR), and TiOSO₄·2H₂O (93 wt%, LR) used in this study were purchased from Aladdin Reagent (Shanghai) Company, China.

2.2. Characteristics

The oxides composition analysis of the spent SCR catalyst and other solids was performed by X-ray fluorescence spectrum (XRF, SPECTRO XEPOS, SPECTRO Analytical Instruments Company, Germany). The mineral phases of the spent SCR catalyst and other solids were identified by X-ray diffraction (XRD, D8 Advance, Bruker Company, Germany). The scanning electron microscopy (SEM, Tescan Mira Lms, Tescan Orsay Holding, Czech Republic) was employed to study the surface topography of the spent SCR catalyst and other solids. The valency states of elements in spent SCR catalyst were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Thermo Fisher Scientific Company, America).

2.3. Experimental Procedure

The spent SCR catalyst was pretreated through being leached in hot water for 1.0–2.0 h and filtrated. The filter residues were dried at 100°C for 3.0 h, ground and directly sieved into sizes of 100–200 meshes. The treated spent catalyst was first leached by H₂SO₄ solution and ascorbic acid. The filtrate was used to recover vanadium. Then, the filter residues were roasted at 650°C for 2.0 h by adding NaOH and Na₂CO₃ additives and the roasted clinkers were leached in water at 90°C for 2.0 h. The filtrate was collected to recover tungsten, while the filter residues were employed to recover titanium. In this paper, the recovery process of TiO₂ was researched with the filter residues of acid leaching and roasting-water leaching as the raw material. The titanium slag first dissolved in 40 wt%–95 wt% H₂SO₄ solution with 2.0–7.0 g/g acid-residue ratio at 30–140°C for 0.5–5.0 h and then the deionized water with 4.0 mL/g liquid-solid ratio was added into the slurry-state mixture at 120°C to leach the reaction products for 2.0 h, as well as being continuously stirred during the whole acidolysis process. After filtration, acidolysis solution with 200 g/L (measured by TiO₂ concentration) was adjusted to pH 0.52–9.15 by adding 40 wt% NaOH solution, followed by adding 0–40 mL additional crystal seeds and further reacting at 30–140°C for 0.5–4.0 h. Then, the liquid-solid mixture was cooled to the room temperature and filtrated to obtain the titanate precipitation. Finally, anatase TiO₂ was obtained by roasting the titanate at 700°C for 2.0 h. The expression of acidolysis efficiency of titanium slag, hydrolysis efficiency of titanium element and TiO₂ purity of recovered product are shown as Eq. (1)–(3). The brief flow sheet of recovering titanium from spent SCR catalyst is presented in Fig. S1.

(1)

η_{1} = \frac{m_{1} - m_{2}}{m_{1}} \times 100 %

(2)

η_{2} = \frac{C_{1} \times V_{1} - C_{2} \times V_{2}}{C_{1} \times V_{1}} \times 100 %

(3)

η_{3} = \frac{m_{3}}{m_{4}} \times 100 %

where η₁ is the acidolysis efficiency of titanium slag, %; m₁ is the mass of titanium slag after acid leaching and roasting-water leaching, g; m₂ is the mass of acidolysis residues, g; η₂ is the hydrolysis efficiency of titanium element, %; C₁ and C₂ are the titanium element concentration of solution before and after hydrolysis, g/L; V₁ and V₂ are the volume of solution before and after hydrolysis, L; η₃ is the purity of recovered product; m₃ is the mass of TiO₂ in recovered product; m₄ is the mass of recovered product.

3. Results and Discussion

3.1. Characterization of Fresh SCR Catalyst, Spent SCR Catalyst and Titanium Slag

Table 1 presented the main components of spent SCR catalyst were TiO₂ with a small amount of SiO₂, WO₃ and V₂O₅, while the main ingredients of titanium slag were Na₂TiO₃ and TiO₂ with some impurities of SiO₂ and Na₂SO₄. Compared with fresh catalyst, the spent SCR catalyst contained more Ca, S, Na and K elements, as well as less Ti, W and V elements, indicating that the loss of active components and the existence of alkali or alkaline-earth metals were the important reasons to lead to the sharp reduction of catalyst activity. In addition, there were large amounts of impurities in titanium slag, such as Si, W, Ca, Al, Fe, Na and K. Therefore, in order to remove impurities and further recover TiO₂ in titanium slag, the steps of acidolysis and hydrolysis were necessary to be conducted.

Fig. S2(a) and S2(b) showed the active components was evenly and densely distributed on the surface of fresh catalyst, as well as failing to occur the agglomeration and sintering phenomenon. Fig. S2(c) and S2(d) showed the white crystal was uniformly distributed on the surface of titanium slag without obvious reunion phenomenon, indicating that acid leaching treatment can dissolve partial agglomerate. Fig. S3 showed that the active components evenly and loosely distributed on the surface of spent SCR catalyst, indicating that the loss of active components may also be one of the reasons for catalyst deactivation, while the caking phenomenon was occurred in some local positions, attributing to the generation of some salts on the surface of catalyst. Combined with Table 1, it can be also inferred that the active components reacted with SO₃ in the exhaust gas to produce sulfate and covered the catalyst surface, as well as adsorbing some alkali metals so that occupied the active site, thus causing the inactivation of catalyst.

Fig. S4(a) showed that the main phase composition of spent SCR catalyst was anatase TiO₂, while the characteristic peaks of other components failed to appear in the picture, indicating that the content of other components was quite small and evenly distributed on the TiO₂ carrier. In addition, Fig. S4(b) showed that the main phase composition of the titanium slag was Na₄Ti₅O₁₂ and TiO₂ with a small amount of SiO₂, indicating only a part of TiO₂ reacted with NaOH or Na₂CO₃ to generate titanate and the impurity of titanium slag was mainly SiO₂. Fig. S4(c) and S4(d) showed that the valence state of titanium element still kept +4 valence after spent SCR catalyst was treated by acid leaching and roasting-water leaching, indicating TiO₂ only occurred the chemical combination reactions instead of redox reactions during the process of acid leaching and roasting-water leaching.

3.2. Acidolysis of Titanium Slag

To selectively recover TiO₂ from titanium slag and simultaneously separate other impurities, titanium slag was dissolved into the dilute H₂SO₄. The main components in titanium slag, such as Na₄Ti₅O₁₂ and TiO₂, reacted with H₂SO₄ to produce TiOSO₄, followed by adding water to leach the solid products for 2.0 h at 4.0 mL/g water-residues ratio and 120°C, and filtrating to obtain the acidolysis solution of titanium slag.

3.2.1. Effect of acid concentration on the acidolysis efficiency

The effect of acid concentration on the acidolysis efficiency of titanium slag was investigated in Fig. 1(a). Based on the composition of titanium slag, the dissolution reactions of titanium slag in dilute H₂SO₄ are shown as follows in Eqs. (4)–(7). When acid concentration was 40wt%, on one hand, H₂TiO₃ generated from Eqs. (4) failed to completely dissolve into the acid; on the other hand, TiO₂ partially dissolved into the acid (Eqs. (6)), so the acidolysis efficiency of titanium slag was only 55.11%. After that, within a certain range, increasing acid concentration promoted Eqs. (4)–(7) to proceed towards the positive direction, thus causing a rapid increase of acidolysis efficiency of titanium slag. When acid concentration reached 85 wt%, the acidolysis efficiency was the highest with 84.85%. However, when acid concentration exceeded 85 wt%, the acidolysis efficiency of titanium slag decreased instead, attributing to the fact that excess H₂SO₄ tremendously increased SO₄²⁻ concentration in solution, further inhibited the dissociation of TiOSO₄ in Eqs. (7) and reduced the concentration of TiO₂₊ in solution, namely the concentration of TiOSO₄ increased, resulting to make Eqs. (5) and (6) reversely proceed. Therefore, 85 wt% acid concentration can be used as the optimal condition.

(4)

{N a}_{4} {T i}_{5} O_{12} + 2 H_{2} {S O}_{4} \to 2 H_{2} {T i O}_{3} + 3 {T i O}_{2} + 2 {N a}_{2} {S O}_{4}

(5)

H_{2} {T i O}_{3} + H_{2} {S O}_{4} \to {T i O S O}_{4} + 2 H_{2} O

(6)

{T i O}_{2} + H_{2} {S O}_{4} \to {T i O S O}_{4} + H_{2} O

(7)

{T i O S O}_{4} ⇌ {T i O}^{2 +} + {S O}_{4}^{2 -}

3.2.2. Effect of acid-residues ratio on the acidolysis efficiency

The effect of acid-residues ratio on the acidolysis efficiency of titanium slag was researched in Fig. 1(b). As shown in Fig. 1(b), when acid-residues ratio was 2.0 g/g, owing to the fact that only a part of acidolysis reactions proceeded, the acidolysis efficiency was only 68.55%. Thereafter, an increase dilute H₂SO₄ allowed more Na₄Ti₅O₁₂, H₂TiO₃ and TiO₂ to dissolve [15]. As acid-residues ratio increased to 5.0 g/g, the acidolysis efficiency reached 85.08%. However, as acid-residues ratio continued to increase, the acidolysis efficiency began to decrease, attributing to the fact that excess sulfate ions in solution inhibited the dissolution of H₂TiO₃ and TiO₂ and the dissociation of TiOSO₄. Therefore, 5.0 g/g acid-residues ratio was selected as the optimal condition.

3.2.3. Effect of acidolysis temperature on the acidolysis efficiency

In general, improvement of reaction temperature was helpful to increase the reaction rate, thus accelerating the acidolysis reactions [22, 23]. However, the dissolution reactions of Na₂TiO₃ and TiO₂ were exothermic reactions (Eqs. (4)–(6)). Excess high temperature had an adverse effect on generating TiOSO₄, on the contrary it promoted the hydrolysis of TiOSO₄ [15]. As shown in Fig. 1(c), as temperature improved from 30°C to 90°C, the acidolysis efficiency increased from 72.57% to 87.25%, while as temperature increased from 90°C to 140°C, the acidolysis efficiency rapidly decreased from 87.25% to 80.57%. Therefore, 90°C acidolysis temperature was the optimal condition.

3.2.4. Effect of acidolysis time on the acidolysis efficiency

The effect of acidolysis time on the acidolysis efficiency of titanium slag was studied in Fig. 1(d). As shown in Fig. 1(d), when time was 0.5 h, owing to the inadequate reactions among Na₄Ti₅O₁₂, H₂TiO₃, TiO₂ and H₂SO₄, only a part of titanium slag was dissolved into H₂SO₄ and the acidolysis efficiency was only 65.5%. As time lengthened from 0.5 h to 2.0 h, the acidolysis efficiency increased from 65.5% to 83.58%. When time increased from 2.0 h to 5.0 h, owing to the fact that the acidolysis reactions nearly completed, the acidolysis efficiency changed little. Therefore, 2.0 h acidolysis time was selected as the optimal condition.

3.2.5. Replication experiment of acidolysis

Based on the results of single-factor experiments, the acidolysis conditions were optimized. The optimal acidolysis conditions were 85 wt%, 5.0 g/g acid-residues ratio, 90°C and 2.0 h. Table 2 showed the acidolysis efficiencies at different experiment times under optimal acidolysis conditions. As presented in Table 2, the highest acidolysis efficiency can reach 92.02%, indicating that most of titanium slag can be dissolved into H₂SO₄ solution to obtain TiOSO₄ under optimal acidolysis conditions, which was helpful to further recover titanium.

3.3. Hydrolysis of Titanium Acidolysis Solution

In this part of experiments, acidolysis solution of titanium slag kept 200 g/L (measured by TiO₂ concentration) through mixing and concentrating a certain amount of acidolysis solution. First, acidolysis solution was adjusted to a certain pH through slowly adding 40 wt% NaOH solution, followed by adding additional crystal seeds and reacting under certain temperature and time. During experiments, the solution was stirred. A series of hydrolysis conditions were discussed in this part. The acidolysis and leaching conditions kept unchanged and were shown as follows: (1) acidolysis conditions: 90 wt% H₂SO₄, 4.0 g/g acid-residues ratio, 120°C, 2.0 h and 200 r/min; (2) leaching conditions: 4.0 mL/g water-residues ratio, 120°C, 2.0 h and 200 r/min.

3.3.1. Effect of pH on the hydrolysis efficiency

Fan [20] thought the acidity had an important impact on the hydrolysis efficiency of titanium solution and further confirmed that the high acidity can inhibit the hydrolysis of TiOSO₄. Other studies also considered that excess H₂SO₄ solution can dissolve titanate generated from hydrolysis process and prevented TiOSO₄ from further hydrolysis [24–26]. The effect of pH on the hydrolysis efficiency was investigated in Fig. 2(a). As NaOH solution began to be added into acidolysis solution, the white flocculent precipitation was first generated and then quickly disappeared. When pH of acidolysis solution was less than zero, precipitation failed to appear in the solution, attributing to the fact that large amounts of H₂SO₄ existed in solution can inhibit TiOSO₄ hydrolysis and further make the hydrolysis reactions of TiOSO₄ proceeded in reverse direction. As shown in Fig. 2(a), when pH increased from 0.52 to 7.46, the hydrolysis reactions sharply occurred and hydrolysis efficiency rapidly increased from 52.17% to 83.68%, attributing to the fact that decreasing H₂SO₄ concentration in the system can accelerate hydrolysis reactions towards the positive direction. When pH continued to increase from 7.46 to 9.15, the hydrolysis efficiency almost remained unchanged. pH 7.46 was selected as the optimal condition.

3.3.2. Effect of hydrolysis temperature on the hydrolysis efficiency

Fa et al. [27] considered that when TiOSO₄ was hydrolyzed at low temperature, the surface binding energy of primary particles was great, which was easy to form the larger aggregates with stable structure and failed to generate the secondary aggregates, thus failing to accelerate further hydrolysis of TiOSO₄. However, when temperature rose, additional crystal seeds with increasing activity obviously promoted the hydrolysis process and the formed primary particles presented tiny crystals, which was easy to generate the secondary aggregates, thus promoting the hydrolysis of TiOSO₄. As presented in Fig. 2(b), when temperature was 30°C, the hydrolysis efficiency was only 60.56%. As temperature increased from 30°C to 120°C, hydrolysis reactions constantly proceeded and the hydrolysis efficiency increased from 60.56% to 70.55%. However, when temperature was over 120°C, the hydrolysis efficiency largely remained the same. Therefore, 120°C hydrolysis temperature can be selected as the optimal condition.

3.3.3. Effect of hydrolysis time on the hydrolysis efficiency

Some studies showed that the hydrolysis of TiOSO₄ mainly included crystal nuclei formation, crystal nuclei growth and precipitation of hydrolysis products [28, 29]. The effect of time had a significant influence on the hydrolysis efficiency of titanium solution, which was investigated in Fig. 2(c). As presented in Fig. 2(c), when time was 0.5 h, the hydrolysis efficiency was only 55.28%, attributing that although there were a certain number of additional crystal nuclei in the solution, some crystal nuclei failed to grow into primary aggregates and precipitate from the solution at the initial stage of hydrolysis. When time increased from 0.5 h to 3.0 h, owing to the fact that more TiOSO₄ was hydrolyzed and precipitated on the surface of crystal nuclei to gather and further form the aggregates, the hydrolysis efficiency rapidly increased from 55.28% to 75.60%. However, when time was over 3.0 h, the hydrolysis efficiency almost kept unchanged. Therefore, 3.0 h hydrolysis time can be used as the optimal condition.

3.3.4. Effect of additional crystal seeds dosage on the hydrolysis efficiency

Additional crystal seeds technology can effectively promote the hydrolysis of titanium solution, as well as decreasing operational requirements and hydrolysis time [30]. In this study, additional crystal seeds were prepared through adding pure TiOSO₄ solution into NaOH solution in the proportion [31]. During hydrolysis experiments, different volume additional crystal seeds were added into the acidolysis solution of titanium slag under certain conditions.

The effect of additional crystal seeds dosage on the acidolysis efficiency of titanium slag was investigated in Fig. 2(d). As presented in Fig. 2(d), when additional crystal seeds failed to be added into the solution, owing to the fact that the titanium solution only occurred partial hydrolysis, the hydrolysis efficiency was only 54.20%. As additional crystal seeds dosage increased from 5 mL to 25 mL, the additional crystal seeds can be used as the crystal center, further TiOSO₄ in solution adhered to the crystal center and simultaneously rapid hydrolysis reactions occurred, so the hydrolysis efficiency increased from 66.58% to 85.89% [32]. When additional crystal seeds dosage was over 25 mL, the hydrolysis efficiency almost kept unchanged. Therefore, 25 mL additional crystal seeds dosage can be selected as the optimal condition.

3.3.5. Stability test of hydrolysis efficiency

The optimal conditions of hydrolysis for the acidolysis solution of titanium slag were 25 mL additional crystal seeds, pH 7.46, 120°C and 3.0 h. In order to verify the stability of hydrolysis efficiency under optimal conditions, the replication experiment was carried out. As presented in Table 3, the hydrolysis efficiency still kept approximately 90% after 6 times experiments, indicating that this technology can realize the rapid hydrolysis of titanium solution and hydrolysis efficiency showed an excellent stability as well.

3.4. Characterizations of Hydrolysis Product and Roasting Product

After 200 g/L acidolysis solution of titanium slag reacted at 120°C for 2.0 h with being continuously stirred through adjusting pH to 2.85, the mixture was filtered to obtain the hydrolysis product. Then, the final product was obtained through roasting hydrolysis product at 700°C for 2.0 h. Fig. 3 showed the characterizations of hydrolysis product and roasting product. As presented in Fig. 3(a) and 3(b), the main phase composition of hydrolysis product was H₂Ti₃O₇, TiO₂ and SiO₂, while the main phase composition of roasting product was anatase TiO₂ and SiO₂. Fig. 3(c) presented that isothermal adsorption and desorption curves of roasting product were the isotherms of H₃-type hysteresis loops and failed to appear the obvious saturated adsorption platform, which belonged to the characteristic curves of flaky or granular materials, indicating that the pore structure of roasting product was very irregular. As shown in Fig. 3(d) and 3(e), the surface of hydrolysis product presented a smooth dense leaf like structure, indicating that additional crystal seeds hydrolysis method can obtain the precipitation with small particle sizes and uniform distribution, while the surface of roasting product showed a clear layered structure and almost failed to occur the agglomeration phenomenon. Therefore, it can be inferred that the hydrolysis and roasting reactions were shown as follows:

(8)

3 {T i O S O}_{4} + 4 H_{2} O_{4} \to H_{2} {T i}_{3} O_{7} + 3 H_{2} {S O}_{4}

(9)

{T i O S O}_{4} + H_{2} O \to {T i O}_{2} + H_{2} {S O}_{4}

(10)

H_{2} {T i}_{3} O_{7} \to 3 {T i O}_{2} + H_{2} O

As shown in Table 4, the roasting product mainly contained Ti element with a small amount of other impurity elements, such as Si, W, S, Al, Ca, Fe, Na and K. The purity of recovered product can reach 86.88%. As presented in Table 5, compared with commercial TiO₂, the recovered product had larger specific surface area, approximate pore volume and smaller pore diameter, indicating that the recycled product in this study can be employed as the catalyst carrier. However, owing to the fact that the surface of hydrolysis product and recovered product presented white with a bit of yellow (Fig. 3(a) and 3(b)), the recovered product failed to be applied in coating field.

3.5. Comparision of TiO₂ Recovery Methods

The traditional process of recovering TiO₂ can be generally described as follows [20, 33]: (1) the titanium slag containing impurities was first dissolved into 70 wt%–90 wt% sulfuric acid solution and then concentrated the acidolysis solution to 150–180 g/L, as well as keeping F value of solution at 1.5–2.3 by adding NaOH or sulphuric acid. (2) the acidolysis solution was first added into bottom water in the reactor at a certain proportion and dropping speed. The solution was heated to the boiling point at a certain rate. Then, in order to make generated crystal seeds grow up in mild conditions, the solution needed to be aged for 10–60 min by stop heating and stirring. After aging, dilution water was added into the solution and solution was heated to the boiling point, as well as reacting for a period of time.

In summary, the technology of recovering TiO₂ included acidolysis and hydrolysis process. Hydrolysis process was the key link for recovery of TiO₂. The industrial hydrolysis process of titanium solution was self-generated crystal seeds. During hydrolysis process, many parameters needed to be controlled, such as F value of titanium solution, concentration of titanium solution, addition amount of bottom water, dropping speed of titanium solution, boiling time and so on.

In this study, the acidolysis process of titanium slag was improved and a novel process of titanium solution hydrolysis was developed. First, 200 g/L titanium solution was directly adjusted to pH 0.52–9.15, which was more convenient and accurate than adjusting the F value of solution. Then, titanium solution and additional crystal seeds were added into the reactor at one time without adding bottom water or dilute water and controlling the feeding speed. Finally, after feeding, the solution directly conducted the hydrolysis operation at a certain temperature. Based on the experimental results, the hydrolysis efficiency and the purity of final product can reach over 90%. Compared with the self-generated crystal seeds method, this route was convenient and easy to operate, as well as coming true the rapid hydrolysis of titanium solution. However, the purity of product was a little low and need to be further improved.

Conclusions

This study used titanium slag as the raw material, and then the acidolysis and hydrolysis conditions for recovering TiO₂ were investigated. In the acidolysis experiments, acid concentration, acid-residues ratio, acidolysis temperature and acidolysis time had an important impact on the acidolysis efficiency of titanium slag. The highest acidolysis efficiency of titanium slag can reach 92.02% by single-factor experiments.
In the hydrolysis experiments, the hydrolysis of titanium solution was greatly promoted through adding additional crystal seeds. Hydrolysis temperature, hydrolysis time and pH had a remarkable impact on the hydrolysis efficiency of titanium solution. The highest hydrolysis efficiency of titanium solution can reach 91.22%. The hydrolysis product was H2Ti₃O₇ with other impurities. After roasting hydrolysis product at 700°C for 2.0 h, the final product with the TiO₂ purity of 86.88% was obtained. The final product was mainly TiO₂ with some impurities, such as SiO₂, WO₃, Al₂O₃ and CaSO₄. This technology simplified the hydrolysis steps of titanium solution and reduced operational requirements, as well as keeping a better hydrolysis efficiency. However, although the recovered product possessed relatively large specific surface, the recovered product was white crystal with slight yellow and only used as the catalyst carrier.

Nomenclature

C₁

Titanium element concentration of solution before hydrolysis (g/L)

C₂

Titanium element concentration of solution after hydrolysis (g/L)

m₁

Mass of titanium slag after acid leaching and roasting-water leaching (g)

m₂

Mass of acidolysis residues (g)

m₃

Mass of TiO₂ in recovered product (g)

m₄

Mass of recovered product (g)

V₁

Volume of solution before hydrolysis (L)

V₂

Volume of solution after hydrolysis (L)

η₁

Acidolysis efficiency of titanium slag (%)

η₂

Hydrolysis efficiency of titanium element (%)

η₃

Purity of recovered product (%)

Supplementary Information

eer-2023-004-Supplmentary.pdf

Acknowledgments

Declared none.

Notes

Conflict-of-interest Statement

The authors declare that they have no clear financial interests or personal relationships that could have emerged to affect the work reported in this paper.

Author Contributions

W.B. (Ph.D. student) designed the experiments and wrote the original draft. L.Z.N (M.S. student) conducted experimental operation. L.H.K. (M.S. student) and T.L.B. (M.S. student) employed software. Y.Q.W. (Professor) wrote the review and edited the manuscript. B.R.Y. (M.S. student), C.Y. (M.S. student) and L.X. (Assistant professor) prepared experimental materials. All authors have read and agreed to the published version of the manuscript.

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Fig. 1

Effect of Acid Concentration on the Acidolysis Efficiency of Titanium Slag Under the Same Leaching.

Fig. 2

Effect of (a) pH, (b) Hydrolysis Temperature, (c) Hydrolysis Time and (d) Additional Crystal Seeds Dosage.

Fig. 3

Characterizations of Products (a) XRD Pattern of Hydrolysis Product; (b) XRD Pattern of Roasting.

Table 1

Oxides Composition of Fresh SCR Catalyst, Spent SCR Catalyst and Titanium Slag Determined by XRF Analysis

Sample	Oxides Composition (wt%)

	TiO₂	SiO₂	WO₃	CaO	Al₂O₃	V₂O₅	Fe₂O₃	SO₃	Na₂O	K₂O	Others
Fresh	81.78	6.83	4.20	2.06	1.75	1.19	0.29	0.18	0.01	0.06	1.65
Spent	81.46	7.37	4.05	2.75	1.98	0.82	0.34	0.67	0.11	0.07	0.38
Ti slag	82.21	3.06	3.16	2.06	0.93	—	0.27	0.06	7.87	0.04	0.34

Table 2

Replication Experiment of Acidolysis for the Titanium Slag

Experiment times	1	2	3	4	5	6
Acidolysis efficiency/%	90.15	91.75	92.02	90.70	91.28	91.99

Table 3

Replication Experiment of Hydrolysis for the Acidolysis Solution of Titanium Slag

Experiment times	1	2	3	4	5	6
Hydrolysis efficiency/%	90.55	91.22	89.95	89.98	90.37	91.10

Table 4

Oxides Composition of Roasting Product Determined by XRF Analysis

Ingredient	TiO₂	SiO₂	WO₃	SO₃	Al₂O₃	CaO	Fe₂O₃	Na₂O	K₂O	Others
Composition (wt%)	86.88	5.50	2.02	1.98	1.70	1.22	0.21	0.20	0.13	0.16

Table 5

Comparison of Recovered Product and Commercial TiO₂ on Specific Surface Area, Pore Volume and Pore Diameter

Sample	Specific surface area (m²/g)	Pore volume (mL/g)	Pore diameter (nm)
Commercial TiO₂	11.19	0.18	19.46
Recovered product	15.07	0.16	11.00

Recovery of TiO2 from spent SCR catalyst by acidolysis and additional crystal seeds-hydrolysis method