The Ultrasound-Assisted Preparation of Crystal Seeds for the Hydrolysis of TiOSO 4 to H 2 TiO 3

: The hydrolysis of an industrial titanyl sulfate (TiOSO 4 ) solution to metatitanic acid (H 2 TiO 3 ) is the crucial step in the production of titanium dioxide (TiO 2 ) 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 H 2 TiO 3 , 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 H 2 TiO 3 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.


Introduction
Titanium dioxide (TiO 2 ) is a stable and non-toxic white pigment with a high refractive index; the pigmentary applications of TiO 2 can be found in many industrial fields, including paints, plastics, paper, cosmetics, and ink [1].Due to its excellent semiconducting properties, TiO 2 is also a widely used photocatalytic material for water splitting and photodegradation applications.Nowadays, nano-structured TiO 2 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 TiO 2 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 TiO 2 in the world, especially in China.
The preparation of TiO 2 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 (TiOSO 4 ), the hydrolysis of TiOSO 4 solutions to metatitanic acid (H 2 TiO 3 ), and the calcination of H 2 TiO 3 and pulverization to a TiO 2 powder product.Among them, the hydrolysis of TiOSO 4 to H 2 TiO 3 is the crucial step that has a significant effect on the properties of the final TiO 2 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 TiOSO 4 , and the quality of crystal seeds, which are typically prepared using the NaOH neutralization method, has a significant effect on the hydrolysis process of TiOSO 4 [6,7].Sathyamoorthy et al. [8,9] have demonstrated that a higher hydrolysis ratio of TiOSO 4 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 TiOSO 4 .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 H 2 TiO 3 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 -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 TiO 2 nanoparticles prepared using the hydrothermal method; the size of TiO 2 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 CaCO 3 solution, rapidly reducing the supersaturation level of the CaCO 3 solution but also change the morphology of CaCO 3 crystals.By treating the crystallization process of CaSO 4 with ultrasound, Amara et al. [17] found that the formation of CaSO 4 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 TiOSO 4 hydrolysis from an industrial TiOSO 4 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 asprepared crystal seeds were then employed in the following process of TiOSO 4 hydrolysis.By comparing the H 2 TiO 3 products prepared using different seeds, the correlation between the quality of H 2 TiO 3 and that of the hydrolytic seeds is established.

Materials
The industrial TiOSO 4 solution was obtained from Chongqing Pangang Titanium Industry Company.Its specifications are shown in Table 1, in which the total titanium content refers to the titanium content measured in terms of TiO 2 , 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/TiO 2 is the mass ratio of Fe to the total titanium content in the solution [17,18].
NaOH and NH

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 TiOSO 4 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/TiO 2 ratio (the mass ratio of NaOH to the total titanium content in the industrial TiOSO 4 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.

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  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].

Hydrolysis of TiOSO 4 to H 2 TiO 3
Also, as shown in Figure 2, 100 mL of the industrial TiOSO 4 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 H 2 SO 4 solutions and DI water to obtain the final H 2 TiO 3 products.The hydrolysis ratio (η) of TiOSO 4 was calculated according to Equation (1), where W 1 and W 2 are the total titanium content before and after the hydrolysis process.

Characterization
The average particle size and particle size distribution of the crystal seeds and H 2 TiO 3 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 D 10 , D 50 , and D 90 are the values of the particle diameter at 10%, 50%, and 90%, respectively, in the cumulative distribution.
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.

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.

NaOH/TiO 2 Ratio
During the preparation of hydrolytic crystal seeds using the NaOH neutralization method, NaOH/TiO 2 (A/T) determines the pH value of the hydrolysis system and the supersaturation level of H 2 TiO 3 , affecting the nucleation and growth rate of the primary crystal seeds, which ultimately determines the quality of the hydrolytic crystal seeds [4,5].
Crystals 2023, 13, 1553 5 of 13 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/TiO 2 ratios demonstrates that, when NaOH/TiO 2 = 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/TiO 2 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/TiO 2 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.
process conditions for preparing hydrolytic crystal seeds.

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.

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

F Value
By adding NaOH or H 2 SO 4 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.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.

Fe/TiO2 Value
By freezing precipitation or adding FeSO4 to vary the Fe 2+ 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.

Fe/TiO 2 Value
By freezing precipitation or adding FeSO 4 to vary the Fe 2+ concentration in the hydrolysis system, the effect of Fe/TiO 2 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/TiO 2 , and the optimal value of Fe/TiO 2 was about 0.30.The viscosity of Crystals 2023, 13, 1553 6 of 13 the hydrolysis system increased with an increase in Fe/TiO 2 , affecting the heat and mass transfer efficiency; consequently, the reduced hydrolysis rate resulted in larger crystal seeds.3.1.3.Fe/TiO2 Value By freezing precipitation or adding FeSO4 to vary the Fe 2+ 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.

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.

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 TiOSO 4 is closely related to the reaction temperature; at higher reaction temperatures, the crystal seeds grow due to the higher hydrolysis rate.

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.

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.
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.

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

Ultrasonic-Assisted Preparation of Hydrolytic Seeds
Under the optimal process conditions for preparing hydrolytic seeds, i.e., NaOH/TiO 2 = 18%, F = 1.88,Fe/TiO 2 = 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.

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.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.

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 In order to establish the correlation between the quality of hydrolytic seeds with that of the hydrolysis products (H 2 TiO 3 ), the crystal seeds prepared at different ultrasonic times were used for the hydrolysis of an industrial TiOSO 4 solution.The average particle size, D 50 , and Span of the as-prepared H 2 TiO 3 are shown in Figure 9, demonstrating that the H 2 TiO 3 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 D 50 of H 2 TiO 3 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 tribution led to a higher conversion ratio of TiOSO4 and a shorter graying time, i.e., a fa hydrolysis rate.Consequently, it can be concluded that the particle size and particle distribution of the H2TiO3 products were closely correlated with those of the hydro crystal seeds, i.e., the high-quality crystal seeds resulted in high-quality H2TiO3 produ  Figure 10 shows that the crystal seeds with smaller particle sizes and narrower distribution led to a higher conversion ratio of TiOSO 4 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 H 2 TiO 3 products were closely correlated with those of the hydrolytic crystal seeds, i.e., the high-quality crystal seeds resulted in high-quality H 2 TiO 3 products.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.

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

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 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.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.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 The crystal seeds prepared under different ultrasonic intensities were used to induce the hydrolysis of TiOSO 4 to prepare the H 2 TiO 3 products, whose particle sizes are shown in Table 2.It can be seen from Table 2 that the H 2 TiO 3 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 H 2 TiO 3 , e.g., particle sizes, particle size distribution, and crystal form, can significantly affect that of the final TiO 2 products prepared via calcinating the H 2 TiO 3 products; therefore, using the crystal seeds obtained with ultrasonic assistance is promising to improve the quality of TiO 2 products via the sulfuric acid method.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 H 2 TiO 3 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.

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 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.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.

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

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, H 2 TiO 3 anatase crystals with poor crystallinity are rapidly formed once the industrial TiOSO 4 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.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.

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

Conclusions
The hydrolysis of TiOSO 4 to H 2 TiO 3 is the crucial step in the preparation of TiO 2 via the sulfuric acid method, while the extra-adding seeded route is the most widely used industrial process for the hydrolysis of TiOSO 4 , in which hydrolytic crystal seeds play an important role in affecting the quality of H 2 TiO 3 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 TiOSO 4 solution to H 2 TiO 3 via the NaOH neutralization method were first identified by exploring the effect of the technical specifications of an industrial TiOSO 4 solution and the operating conditions on the particle size and distribution of the crystal seeds.Then, the ultrasoundassisted 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 TiOSO 4 solution, demonstrating that the particle size and particle size distribution of the as-obtained H 2 TiO 3 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

Figure 2 .
Figure 2. Process flowchart of the preparation of crystal seeds and hydrolysis.

Figure 3 .
Figure 3. Particle size distribution (a) and average particle size (Z-Average) (b) of hydrolytic crystal seeds prepared using different NaOH/TiO2 ratios.

Figure 3 .
Figure 3. Particle size distribution (a) and average particle size (Z-Average) (b) of hydrolytic crystal seeds prepared using different NaOH/TiO 2 ratios.

Figure 4 .
Figure 4. Particle size distribution (a) and average particle size (b) of hydrolyzed crystal seeds prepared at different F values.

Figure 4 .
Figure 4. Particle size distribution (a) and average particle size (b) of hydrolyzed crystal seeds prepared at different F values.

Figure 4 .
Figure 4. Particle size distribution (a) and average particle size (b) of hydrolyzed crystal seeds prepared at different F values.

Figure 5 .
Figure 5. Particle size distribution (a) and average particle size (b) of hydrolytic crystal seeds prepared under different Fe/TiO2.

Figure 5 .
Figure 5. Particle size distribution (a) and average particle size (b) of hydrolytic crystal seeds prepared under different Fe/TiO 2 .

Figure 6 .
Figure 6.Particle size distribution (a) and average particle size (b) of hydrolytic crystal seeds prepared under different reaction temperatures.

Figure 6 .
Figure 6.Particle size distribution (a) and average particle size (b) of hydrolytic crystal seeds prepared under different reaction temperatures.

Figure 7 .
Figure 7. Particle size distribution (a) and average particle size (b) of hydrolytic crystal seeds with different reaction times.

Figure 7 .
Figure 7. Particle size distribution (a) and average particle size (b) of hydrolytic crystal seeds with different reaction times.

Figure 8 .
Figure 8. Particle size distribution (a) and average particle size (b) of hydrolytic crystal seeds prepared at different ultrasonic treatment times.

Figure 8 .
Figure 8. Particle size distribution (a) and average particle size (b) of hydrolytic crystal seeds prepared at different ultrasonic treatment times.

Crystals 2023 , 9 Figure 9 .
Figure 9. Particle size and particle size distribution of H2TiO3 products prepared using diffe crystal seeds.

Figure 10 .
Figure10.Hydrolysis ratio of TiOSO4 and graying time of the hydrolysis reaction using di ent crystal seeds.

Figure 9 .
Figure 9. Particle size and particle size distribution of H 2 TiO 3 products prepared using different crystal seeds.

Crystals 2023 , 13 Figure 9 .
Figure 9. Particle size and particle size distribution of H2TiO3 products prepared using different crystal seeds.

Figure 10 .
Figure 10.Hydrolysis ratio of TiOSO4 and graying time of the hydrolysis reaction using different crystal seeds.

Figure 10 .
Figure 10.Hydrolysis ratio of TiOSO 4 and graying time of the hydrolysis reaction using different crystal seeds.

Figure 11 .
Figure 11.Particle size distribution (a) and average particle size (b) of hydrolyzed crystal seeds prepared at different ultrasonic amplitudes.

Figure 11 .
Figure 11.Particle size distribution (a) and average particle size (b) of hydrolyzed crystal seeds prepared at different ultrasonic amplitudes.

Crystals 2023 , 13 Figure 12 .
Figure 12.XRD spectra of the hydrolyzed crystal seed.The green lines refer to the characteristic peaks of the standard anatase crystal (JCPDS 99-0008).

Figure 13 .
Figure 13.TEM images of hydrolyzed crystal seeds: (a) without ultrasonic treatment and (b) with ultrasonic treatment.

Figure 12 .
Figure 12.XRD spectra of the hydrolyzed crystal seed.The green lines refer to the characteristic peaks of the standard anatase crystal (JCPDS 99-0008).

Crystals 2023 , 13 Figure 12 .
Figure 12.XRD spectra of the hydrolyzed crystal seed.The green lines refer to the characteristic peaks of the standard anatase crystal (JCPDS 99-0008).

Figure 13 .
Figure 13.TEM images of hydrolyzed crystal seeds: (a) without ultrasonic treatment and (b) with ultrasonic treatment.

Figure 13 .
Figure 13.TEM images of hydrolyzed crystal seeds: (a) without ultrasonic treatment and (b) with ultrasonic treatment.

Figure 14 .
Figure 14.The mean particle size of seeds varies with time.

Figure 14 .
Figure 14.The mean particle size of seeds varies with time.

Table 1 .
Specifications of the industrial TiOSO 4 solution.

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

Table 2 .
Particle size and distribution of H 2 TiO 3 products corresponding to the crystal seeds prepared with different ultrasonic amplitudes.