The Ultrasound-Assisted Preparation of Crystal Seeds for the Hydrolysis of TiOSO₄ to H₂TiO₃

Abstract

The hydrolysis of an industrial titanyl sulfate (TiOSO₄) solution to metatitanic acid (H₂TiO₃) is the crucial step in the production of titanium dioxide (TiO₂) using the sulfuric acid process, and the extra-adding seeded route is generally adopted in industry, in which the quality of the crystal seeds plays a critical role. In this study, the optimal process conditions for preparing the crystal seeds via the NaOH neutralization method were first investigated. Then, the ultrasound-assisted preparation of crystal seeds was studied to explore the effect of the ultrasonic time and intensity on the particle size and particle size distribution of crystal seeds. The results demonstrated that ultrasonic assistance is helpful in obtaining crystal seeds with smaller particle sizes and more uniform particle size distribution, and the quality of the hydrolysis product of H₂TiO₃, i.e., the particle size and its distribution, is strictly correlated with those of the crystal seeds. Under the optimal process conditions for preparing the hydrolytic seeds, the average particle of the hydrolytic seeds prepared without ultrasonic assistance is 25.50 nm. In contrast, the introduction of ultrasonic assistance in the preservation stage could significantly decrease the particle size and narrow the particle size distribution of the hydrolytic seeds. When the ultrasonic time is 4 min and the ultrasonic intensity is 40 W, the average particle of the hydrolytic seeds is decreased to 23.48 nm. Therefore, the quality of the crystal seeds, as well as that of H₂TiO₃ products, could be significantly improved by introducing ultrasonic assistance with a suitable intensity at a suitable time in the preparation process of crystal seeds via the NaOH neutralization method.

1. Introduction

Titanium dioxide (TiO₂) is a stable and non-toxic white pigment with a high refractive index; the pigmentary applications of TiO₂ can be found in many industrial fields, including paints, plastics, paper, cosmetics, and ink [1]. Due to its excellent semiconducting properties, TiO₂ is also a widely used photocatalytic material for water splitting and photodegradation applications. Nowadays, nano-structured TiO₂ materials have raised a great deal of interest in both research and industry; owing to their several unique characteristic features, the applications can be found in the treatment of water, health and medicine, solar cells, catalysts, etc. [2]. The industrial processes for preparing TiO₂ mainly include the sulfuric acid and chlorination methods [3]. Among them, compared with the chlorination process, the sulfuric acid method has the disadvantages of a long process route and poor product quality but possesses the advantages of low production difficulty, simple equipment, and mature technology; therefore, it is still the most used industrial method for manufacturing TiO₂ in the world, especially in China.

The preparation of TiO₂ via the sulfuric acid method mainly comprises the following three steps: the reaction of titanium-containing ores with concentrated sulfuric acid to produce titanyl sulfate (TiOSO₄), the hydrolysis of TiOSO₄ solutions to metatitanic acid (H₂TiO₃), and the calcination of H₂TiO₃ and pulverization to a TiO₂ powder product. Among them, the hydrolysis of TiOSO₄ to H₂TiO₃ is the crucial step that has a significant effect on the properties of the final TiO₂ product, e.g., the particle size, particle size distribution, and coating property [4,5]. At present, the extra-adding seeded route is generally adopted in the industrial hydrolysis process for TiOSO₄, and the quality of crystal seeds, which are typically prepared using the NaOH neutralization method, has a significant effect on the hydrolysis process of TiOSO₄ [6,7]. Sathyamoorthy et al. [8,9] have demonstrated that a higher hydrolysis ratio of TiOSO₄ can be obtained using smaller crystal seeds with large specific surface area and high surface activity, while secondary nucleation has a significant effect on the formation of crystal seeds and the hydrolysis of TiOSO₄. Previous studies have also demonstrated that, by using the crystal seeds with smaller particle size and narrower distribution (2.1–3.5 nm), prepared using the microwave-assisted NaOH neutralization method, the H₂TiO₃ products exhibit a smaller particle size and narrower distribution, and more importantly, they are easier to transform into rutile crystals [10,11,12,13,14].

Ultrasonic assistance is a widely used chemical process intensification technology; previous studies [15,16,17,18] have extensively verified that ultrasound assistance can help prepare high-quality crystal seeds with a small particle size and narrow distribution by accelerating the crystallization process and regulating the surface properties of crystals. Kim et al. [15] showed that ultrasonic assistance can improve the thermal stability and particle size distribution of TiO₂ nanoparticles prepared using the hydrothermal method; the size of TiO₂ nanoparticles prepared with ultrasonic assistance ranges from 3 to 6 nm, while that of the samples prepared without ultrasonic assistance ranges from 5 to 15 nm. Xie et al. [16] demonstrated that ultrasonic treatment can not only promote the formation of crystal nuclei in the saturated CaCO₃ solution, rapidly reducing the supersaturation level of the CaCO₃ solution but also change the morphology of CaCO₃ crystals. By treating the crystallization process of CaSO₄ with ultrasound, Amara et al. [17] found that the formation of CaSO₄ crystals can be promoted due to the shortened crystallization induction period and the reduced energy barrier for nucleation.

Inspired by the previous studies verifying that the quality of crystal seeds can be improved via ultrasonic treatment, the ultrasound-assisted preparation of crystal seeds for TiOSO₄ hydrolysis from an industrial TiOSO₄ solution was investigated in this study, in which the optimum process conditions for preparing crystal seeds via the NaOH neutralization method were firstly obtained. Then, the ultrasonic field was introduced to enhance the mass transfer during the preparation process of hydrolytic seeds. The effect of the ultrasonic time and intensity on the quality of hydrolytic crystal seeds was explored to clarify the mechanism of ultrasonic treatment in regulating the quality of seeds. The as-prepared crystal seeds were then employed in the following process of TiOSO₄ hydrolysis. By comparing the H₂TiO₃ products prepared using different seeds, the correlation between the quality of H₂TiO₃ and that of the hydrolytic seeds is established.

2. Experiment

2.1. Materials

The industrial TiOSO₄ solution was obtained from Chongqing Pangang Titanium Industry Company. Its specifications are shown in

Table 1. Specifications of the industrial TiOSO₄ solution.

Total Titanium Content (g/L)	F Value	Fe/TiO2	Ti3+ Content (g/L)
175	1.88	0.30	0.59

, in which the total titanium content refers to the titanium content measured in terms of TiO₂, the F value is the mass ratio between the effective acid (the sum of the free acid and titanium-bonded acid) and the total titanium content, and Fe/TiO₂ is the mass ratio of Fe to the total titanium content in the solution [17,18].

NaOH and NH₄SCN were procured from Macklin Biochemical Technology Co., LTD.(Shanghai, China), FeH₄NO₈S₂ from Titan Technology Co., LTD., (Shanghai, China), H₂SO₄, H₃PO_4, and TiO₂ from Chuandong Chemical Co., LTD. (Chongqing, China), and ZnO from Kemiou Chemical Reagent Co., LTD. (Tianjin, China). All the reagents were analytical-grade and used without further purification.

2.2. Preparation of Hydrolytic Crystal Seeds

Using the experimental setup in Figure 1, and according to the procedure in Figure 2, the crystal seeds were prepared using the “NaOH neutralization method”, in which the TiOSO₄ solution, preheated to 85 °C, was added dropwise into the NaOH solution (10 wt%) within 3 min under stirring, and it was also preheated to 85 °C. The amount of NaOH was calculated according to the NaOH/TiO₂ ratio (the mass ratio of NaOH to the total titanium content in the industrial TiOSO₄ solutions) of 18 %. Subsequently, the solution was quickly heated to and preserved at 95–98 °C for 2 min, during which an ultrasonic field was introduced using high-frequency ultrasonography (HD3200, WIGGEN) with a frequency of 22 kHz. During the preservation process, the stability of the crystal seeds was tested using the dilution method; once the stability reached 120 mL, the preparation process of the crystal seeds was completed.

Using the dilution method for measuring the stability of crystal seeds, 10 mL of the mixed solutions for preparing the crystal seeds was sampled every 1 min, and the deionized (DI) water was gradually added until the white and turbid sediments appeared in the solution. The volume of the added DI water was then recorded to characterize the stability of the solution. The details of the method were derived from previous studies [19,20].

2.3. Hydrolysis of TiOSO₄ to H₂TiO₃

Also, as shown in Figure 2, 100 mL of the industrial TiOSO₄ solution was preheated to 96 °C, and then, 2.0 wt% of the crystal seeds prepared according to Section 2.2 were slowly added into the solution within 2 min under stirring at 300 r/min. Subsequently, the solution was heated to the boiling state, i.e., the first boiling point. When the hydrolysis solution turns turbid, i.e., the gray point, the heating was immediately stopped, and stirring was slowed (80 r/min) to start the aging process for 30 min. Then, the heating was resorted, and the stirring was accelerated to 200 r/min to raise the solution to the second boiling point. After keeping the hydrolysis process at a slight boiling state for about 90 min, a certain amount of DI water was pumped to obtain a total titanium content of 160 ± 5 g/L at the end of the hydrolysis process. The hydrolysis process was terminated 4 h after the second boiling point, and then the hydrolyzed slurry was filtered and washed using diluted H₂SO₄ solutions and DI water to obtain the final H₂TiO₃ products. The hydrolysis ratio (η) of TiOSO₄ was calculated according to Equation (1), where W₁ and W₂ are the total titanium content before and after the hydrolysis process.

$$\eta \left(\%\right)=100\times \left(1-\frac{W_2}{W_1}\right)+0.5$$

(1)

2.4. Characterization

The average particle size and particle size distribution of the crystal seeds and H₂TiO₃ samples were measured using a Malvern laser particle size analyzer (Nano ZS90), for which the samples were dispersed in diluted sulfuric acid (10 wt%) with a volume ratio of 5:1. The corresponding Span values, reflecting the particle size distribution, were calculated via Equation (2), in which D₁₀, D₅₀, and D₉₀ are the values of the particle diameter at 10%, 50%, and 90%, respectively, in the cumulative distribution.

$$\mathrm{S}\mathrm{p}\mathrm{a}\mathrm{n}=\frac{D_{90}-D_{10}}{D_{50}}$$

(2)

Scanning electron microscopy (SEM) images were collected using a ZEISS Sigma 500 to analyze the morphology of the samples, and the corresponding elemental composition was analyzed using an energy-dispersive X-ray spectrometer (EDX, OXFORD Azte). X-ray diffraction (XRD) measurements were conducted on a D/max 2500/PC diffractometer (Rigaku) using Cu Kα radiation with λ = 0.154 nm to characterize the crystalline property of the samples. Transmission electron microscopy (TEM) images were collected using a JEOL JEM-ARM200F.

3. Results and Discussion

3.1. Optimum Process Conditions for Preparing Hydrolytic Crystal Seeds

In this section, the effect of various process parameters on the quality of crystal seeds under the conditions without ultrasonic assistance is investigated to obtain the optimum process conditions for preparing hydrolytic crystal seeds.

3.1.1. NaOH/TiO₂ Ratio

During the preparation of hydrolytic crystal seeds using the NaOH neutralization method, NaOH/TiO₂ (A/T) determines the pH value of the hydrolysis system and the supersaturation level of H₂TiO₃, affecting the nucleation and growth rate of the primary crystal seeds, which ultimately determines the quality of the hydrolytic crystal seeds [4,5]. Figure 3 shows the particle size distribution, indicated via the intensity of the particles with different sizes. The average particle size (Z-Average) of hydrolytic crystal seeds prepared with different NaOH/TiO₂ ratios demonstrates that, when NaOH/TiO₂ = 18%, the particle size of the crystal seeds exhibits the narrowest distribution, and the average particle size reaches the smallest value. A deviation in the NaOH/TiO₂ ratio from the optimal value would significantly increase the particle size and distribution. With a moderate increase in NaOH concentration, the nucleation and growth rate of crystal seeds can be accelerated, leading to a decreased particle size and more uniform particle size distribution. However, when the NaOH/TiO₂ ratio is too large, the primary crystal particles become too small and easily agglomerate into larger secondary particles, resulting in a larger particle size and wider particle distribution of the crystal seeds.

3.1.2. F Value

By adding NaOH or H₂SO₄ solutions to adjust the F values of the hydrolysis system, the results in Figure 4 show that the particle size of the crystal seeds firstly decreased with an increase in the F value, reaching the smallest value (c.a. 25 nm) when F = 1.88, and then kept increasing with a further rise in the F value. The increased acidic concentration in the hydrolysis system at a more considerable F value would lead to a lower hydrolysis rate, resulting in larger particle size of the crystal seeds. Therefore, to prepare hydrolytic crystal seeds using the NaOH neutralization method, the F value should be controlled at 1.85–1.90.

3.1.3. Fe/TiO₂ Value

By freezing precipitation or adding FeSO₄ to vary the Fe²⁺ concentration in the hydrolysis system, the effect of Fe/TiO₂ values on the quality of crystal seeds was studied. The results in Figure 5 show that the particle size of the crystal seeds increased with an increase in Fe/TiO₂, and the optimal value of Fe/TiO₂ was about 0.30. The viscosity of the hydrolysis system increased with an increase in Fe/TiO₂, affecting the heat and mass transfer efficiency; consequently, the reduced hydrolysis rate resulted in larger crystal seeds.

3.1.4. Reaction Temperature

Figure 6 shows the variation in the particle size and particle size distribution of hydrolytic crystal seeds when the hydrolysis reaction temperature increased from 85, 90, 93, and 95 to 100 °C, showing that the particle size decreased with an increase in reaction temperature. However, when the reaction temperature increased from 95 to 100 °C, the particle size variation was no longer obvious. Therefore, the reaction temperature should be controlled at 95–98 °C. The hydrolysis rate of TiOSO₄ is closely related to the reaction temperature; at higher reaction temperatures, the crystal seeds grow due to the higher hydrolysis rate.

3.1.5. Hydrolysis Time

The hydrolysis time for preparing the hydrolytic seeds is determined via the nucleation and growth rate of the crystals, which is generally taken for about 3~10 min. In this study, the variation of the particle size of the hydrolytic seeds with the reaction time was tested, and the results in Figure 7 show that, with the progress of the reaction, the average particle size of the hydrolytic seeds firstly decreased, reaching the smallest value when the reaction time was 5 min, and then increased with a further increase in the reaction time. The particle size distribution results exhibited a similar trend, exhibiting the narrowest value when the reaction time = 5 min. At the same time, we carried out stability tests on the crystal seeds prepared at different reaction times, showing that the crystal seeds obtained when the reaction time = 5 min met the stability requirements, which is consistent with the testing results on the particle size and the corresponding distribution in Figure 7.

3.2. Ultrasonic-Assisted Preparation of Hydrolytic Seeds

Under the optimal process conditions for preparing hydrolytic seeds, i.e., NaOH/TiO₂ = 18%, F = 1.88, Fe/TiO₂ = 0.3, the hydrolytic temperature = 96 °C, and the hydrolytic time = 5 min, the ultrasound-assisted preparation of crystal seeds was studied to explore the effect of the ultrasonic treatment on the particle size and particle size distribution of crystal seeds. In the study, it was firstly found that the introduction of the ultrasonic field in the heating stage led to a larger particle size and broader particle size distribution of the crystal seeds. In contrast, the introduction of the ultrasonic field in the preservation stage could significantly improve the quality of hydrolytic seeds. The introduction of ultrasonic assistance can promote heat and mass transfer in the hydrolysis system due to the heat effect, cavitation effect, and mechanical effect; however, in the heating stage, when the crystal seeds are forming, the enhanced stirring impact resulting from the ultrasonic field can lead to the melting of the tiny crystal seeds [5]. Therefore, this study investigated the effect of the ultrasonic intensity and time on the preparation of the hydrolytic seeds while introducing ultrasonic assistance at the preservation stage, as shown in Figure 2.

3.2.1. Effect of Ultrasonic Time

Figure 8 shows the variation in the particle size and particle size distribution of the crystal seeds with ultrasonic time, showing that when the ultrasonic time was less than 4 min, the average particle size gradually decreased with an increase in the ultrasonic time, and the particle size distribution gradually narrowed. When the ultrasonic time was 4 min, the average particle size of the crystal seed reached the smallest values (23.5 nm), 7% lower than that without ultrasonic treatment, and the particle size distribution was also the narrowest. However, when the ultrasonic time exceeded 4 min, the average particle size gradually increased with an increase in ultrasonic time, and the particle size distribution became wider.

In order to establish the correlation between the quality of hydrolytic seeds with that of the hydrolysis products (H₂TiO₃), the crystal seeds prepared at different ultrasonic times were used for the hydrolysis of an industrial TiOSO₄ solution. The average particle size, D₅₀, and Span of the as-prepared H₂TiO₃ are shown in Figure 9, demonstrating that the H₂TiO₃ products prepared using the crystal seeds with a smaller average particle size and narrower particle size distribution also exhibited a small average particle size and narrower particle size distribution. The average particle size and D₅₀ of H₂TiO₃ products corresponding to the crystal seeds prepared when the ultrasonic time = 4 min were 420.8 and 370 nm, respectively, which were 17.5% and 18.7% lower than those corresponding to the crystal seeds prepared without the ultrasonic treatment, while the Span value was 3% lower.

Figure 10 shows that the crystal seeds with smaller particle sizes and narrower distribution led to a higher conversion ratio of TiOSO₄ and a shorter graying time, i.e., a faster hydrolysis rate. Consequently, it can be concluded that the particle size and particle size distribution of the H₂TiO₃ products were closely correlated with those of the hydrolytic crystal seeds, i.e., the high-quality crystal seeds resulted in high-quality H₂TiO₃ products.

3.2.2. Effect of Ultrasonic Intensity

By changing the ultrasonic amplitude at 20%, 30%, 40%, and 50% (corresponding to ultrasonic power of 40, 60, 80, and 100 W, respectively), the effect of the ultrasonic intensity on the preparation of hydrolytic crystal seeds was investigated. Figure 11 shows the average particle size and particle size distribution of the as-prepared crystal seeds prepared at different ultrasonic intensities. It can be seen that, when the ultrasonic intensity was small, an increase in ultrasonic intensity was able to improve the quality of the hydrolytic crystal seeds. When the ultrasonic amplitude was 40%, the crystal seeds exhibited the smallest particle size and a narrower particle size distribution; then, the particle size of the crystal seeds grew, and the distribution became broader with a further increase in the ultrasonic amplitude. Therefore, the introduction of ultrasonic assistance with a suitable intensity at the appropriate time during the preparation of hydrolytic crystal seeds is conducive to improving the quality of crystal seeds. However, the shear force induced via the ultrasonic field can also lead to the dissolution of small crystal particles, which would exert a negative effect when the ultrasonic time is too long and the ultrasonic intensity is too strong.

The crystal seeds prepared under different ultrasonic intensities were used to induce the hydrolysis of TiOSO₄ to prepare the H₂TiO₃ products, whose particle sizes are shown in

Table 2. Particle size and distribution of H₂TiO₃ products corresponding to the crystal seeds prepared with different ultrasonic amplitudes.

Ultrasonic Intensity (W)	Z-Average (nm)	D10 (nm)	D50 (nm)	D90 (nm)	Span
40	479.7	311	402	522	0.525
60	420.8	287	371	487	0.539
80	410.6	278	359	458	0.501
100	431.4	291	377	498	0.549

. It can be seen from Table 2 that the H₂TiO₃ products corresponding to the crystal seeds prepared under a 40% ultrasonic amplitude exhibited the best quality with an average particle size of 410.6 nm and a Span value of 0.501. Previous studies [21,22,23] have verified that the quality of H₂TiO₃, e.g., particle sizes, particle size distribution, and crystal form, can significantly affect that of the final TiO₂ products prepared via calcinating the H₂TiO₃ products; therefore, using the crystal seeds obtained with ultrasonic assistance is promising to improve the quality of TiO₂ products via the sulfuric acid method.

3.2.3. Morphology and Structure of Crystal Seeds

The dispersion of crystal seeds prepared under the conditions with and without ultrasonic treatment (the ultrasonic time was 4 min, and the ultrasonic amplitude was 40%) was subjected to dialysis for purification and then dried to obtain pure white H₂TiO₃ powders whose crystal structure was analyzed via XRD. The spectra in Figure 12 show that the XRD patterns of the crystal seeds prepared using the NaOH neutralization method were consistent with that of the standard anatase crystal (JCPDS 99-0008), confirming that the crystal seeds were poor-crystallinity anatase. Ultrasonic assistance did not affect the crystal type of the hydrolytic seeds, but the decreased width of the diffraction peaks indicates that the particle size of the crystal seeds prepared with ultrasonic assistance became smaller, which is consistent with the test results on the particle size.

At the end of the preparation process for the crystal seeds, a drop of suspended crystal seeds was immediately placed on a copper net; after being dried at 100 °C, the crystal seeds were characterized via TEM. It can be seen from Figure 13 that the particle size of the crystal seeds prepared with ultrasonic assistance was obviously smaller than that without ultrasonic assistance, and the particle size distribution was more uniform.

3.3. Growth Kinetics of Hydrolytic Crystal Seeds

Once the preparation process of crystal seeds was completed, i.e., the seeds met the stability requirement, the suspension of crystal seeds was sampled at regular intervals and transferred into the Malvern laser particle size analyzer to analyze the variation in the particle size. The results in Figure 13 demonstrated that the particle size of the crystal seeds first increased sharply with time, then became slow, and finally reached a plateau value.

In the preparation process of crystal seeds, H₂TiO₃ anatase crystals with poor crystallinity are rapidly formed once the industrial TiOSO₄ solution makes contact with the NaOH solution, during which the crystal nuclei are born when a certain supersaturation is generated. The crystal nuclei grow slowly under the condition of 96 °C in the preparation process of crystal seeds, while, when the heating process is stopped, the decreased temperature leads to the accelerated growth of the crystals.

Figure 14 shows that both the initial and final particle size of the crystal seeds prepared with ultrasonic assistance was smaller than that of the crystal seeds prepared without ultrasonic assistance. The thermal and cavitation effect of the external ultrasonic field can enhance the mass transfer during the preparation process of crystal seeds, resulting in a more uniform mixture of the reaction system. The ultrasonic treatment can form gas bubbles on the surface of crystal seeds due to the cavitation effect, which can effectively reduce the surface energy of the crystal particles, thus inhibiting their agglomeration and growth.

4. Conclusions

The hydrolysis of TiOSO₄ to H₂TiO₃ is the crucial step in the preparation of TiO₂ via the sulfuric acid method, while the extra-adding seeded route is the most widely used industrial process for the hydrolysis of TiOSO₄, in which hydrolytic crystal seeds play an important role in affecting the quality of H₂TiO₃ products, including the particle size, particle size distribution, and surface properties. In this study, the optimum process conditions for preparing crystal seeds for the hydrolysis of an industrial TiOSO₄ solution to H₂TiO₃ via the NaOH neutralization method were first identified by exploring the effect of the technical specifications of an industrial TiOSO₄ solution and the operating conditions on the particle size and distribution of the crystal seeds. Then, the ultrasound-assisted preparation of crystal seeds was studied, demonstrating that the introduction of ultrasound with a suitable intensity at an appropriate time during the preparation of hydrolytic crystal seeds can significantly decrease the particle size and narrow the particle size distribution of the crystal seeds. When introducing ultrasound at the heat-preservation stage with 4 min and a 40% ultrasonic amplitude (the corresponding ultrasonic power is 80W), the average particle size of the crystal seeds was 23.5 nm, 7% lower than that without ultrasonic treatment.

The crystal seeds prepared under different conditions were further used to induce the hydrolysis of an industrial TiOSO₄ solution, demonstrating that the particle size and particle size distribution of the as-obtained H₂TiO₃ products were closely correlated with those of the hydrolytic crystal seeds. The growth kinetics of the hydrolytic crystal seeds after the preparation process show that the seeds prepared with ultrasonic assistance tend to have smaller particle sizes, verifying their low surface energy. Therefore, this study demonstrated, by introducing ultrasonic assistance, that hydrolytic crystal seeds with improved properties can be prepared when producing H₂TiO₃ products with smaller and more uniform particle sizes, which is very important in industry for preparing high-quality TiO₂ products.