Research on the process of preparing wollastonite powder using the gelation method

The summary describes the preparation of CaO-SiO2 dry gel powder using tetraethyl orthosilicate (TEOS) and calcium acetate as raw materials through the sol-gel method. After heat treatment at 1000°C, a wollastonite powder with a particle size of 0.2 to 1.4 μm was obtained. The sol-gel process of the CaO-SiO2 system and the effect of pre-hydrolysis of TEOS on sol-gel transformation were studied. The physical and chemical changes and phase transitions of the CaO-SiO2 gel powder during heat treatment were investigated using DTA, IR, and XRD. The morphology of the heat-treated products of the dry gel powder was observed using an optical microscope. Introduction: In the 1930s, wollastonite minerals attracted the attention of scholars worldwide due to their excellent brightness, insulation, thermal resistance, and chemical stability. After the 1960s, wollastonite mineral raw materials were widely used in various fields such as plastics and rubber, ceramics, coatings, construction materials, and steel industries, with increasing demand. However, the limitations of natural wollastonite raw materials in terms of particle size, purity, and particle morphology restrict their broader and deeper applications in various industrial fields. The sol-gel method is an effective way to prepare high-quality ceramic powders. This work attempts to prepare high-purity CaO-SiO2 gel using the sol-gel method, then ball-milling the dry gel and obtaining high-performance wollastonite powder through heat treatment. Currently, this research has not been reported domestically or internationally. Experimental: 2.1 Experimental Steps: Using TEOS, Ca(Ac)2, and water as raw materials, with alcohol and water as solvents, and hydrochloric acid as a catalyst, wollastonite powder was prepared. Results and Discussion: 3.1 Sol-Gel Process of the CaO-SiO2 System: The viscosity of the sol formed by mixing pre-hydrolyzed TEOS and calcium acetate solution initially increases slowly, and as the aging time (at room temperature) increases, the rate of viscosity increase accelerates. After 35 minutes of aging, the viscosity of the sol increases rapidly, and after 40 minutes, the viscosity no longer changes. This is because the change in sol viscosity mainly reflects the degree of cross-linking of sol particles. During the transition from sol to gel, both hydrolysis and condensation reactions occur simultaneously. In the early stage of the reaction, the hydrolysis of TEOS predominates, so the viscosity change of the sol is not significant. As the reaction progresses, the Si-OH groups produced by hydrolysis gradually increase, leading to an increase in the concentration of condensation reaction reactants, accelerating the reaction rate, and forming more cross-linked product particles, resulting in a rapid increase in viscosity. After 35 minutes of reaction, it can be explained that the condensation reaction of hydrolysis products predominates, significantly increasing the cross-linking degree of sol particles, leading to a sharp rise in sol viscosity and rapid gelation. The curve of the average volume median diameter of sol particles in Figure 2 reflects the growth process of sol particles and also confirms the change in the degree of cross-linking of gel particles during the gelation process of the CaO-SiO2 system. 3.2 Effect of TEOS Pre-Hydrolysis on CaO-SiO2 Sol-Gelation: Since TEOS is not easily hydrolyzed, it is pre-hydrolyzed before mixing with Ca(Ac)2 aqueous solution when preparing CaO-SiO2 sol. The influence of pre-hydrolysis conditions on sol-gelation is evident; the pre-hydrolysis time and water-silica ratio significantly affect the sol-gelation process of the CaO-SiO2 system. When R ≤ 4, as the amount of pre-hydrolyzed water increases, the gelation time of the sol shortens, reaching a minimum at R = 4. When R ≥ 4, the gelation time increases with the increase of pre-hydrolysis. This is because, according to the hydrolysis principle of TEOS, the water-silica ratio is 4 when TEOS is completely hydrolyzed. When R ≤ 4, a larger amount of water results in more unreacted tri-Si-OH in the system, accelerating the condensation reaction and shortening the gelation time of the sol. Conversely, when R ≥ 4, excessive water appears in the form of a solvent, reducing the concentration of -Si-OH and slowing down the condensation reaction, thus prolonging the gelation time of the sol. Table 1 also shows that the introduction of Ca(Ac)2 significantly reduces the gelation time of TEOS, as the Ca(Ac)2 aqueous solution is weakly basic. Therefore, with the introduction of Ca(Ac)2, a large number of -OH ions are also introduced, changing the hydrolysis reaction of TEOS from an electrophilic mechanism of H3O+ in the TEOS system to a nucleophilic mechanism of OH, leading to a significant reduction in gelation time. Increasing the pre-hydrolysis time allows the hydrolysis reaction of TEOS to proceed more completely, increasing the concentration of hydrolysis products like SiOH, and accelerating the condensation reaction with the solvation product of Ca(Ac)2 (HO-Ca(H2O)+), thus shortening the gelation time of the CaO-SiO2 system, as also demonstrated in Table 1. 3.3 Physical and Chemical Changes During Heat Treatment of Dry Gel Powder: The broad and flat endothermic peak on the DTA curve up to 250°C is mainly due to the removal of water, ethanol, and unbonded organic groups (-C2H5, HAc, etc.) in the dry gel. The small exothermic peak at 390°C is caused by the decomposition and oxidation of the unhydrolyzed -C2H5 groups, while the strong exothermic peak appearing after 410°C is due to the decomposition and oxidation of Ca(Ac)2. It can be seen that the CaO-SiO2 system dry gel powder shows no crystalline phase after heat treatment at 400°C, indicating it is an amorphous substance. After heat treatment at 600°C, crystalline phases appear, mainly consisting of wollastonite (α-CaSiO3) and pseudowollastonite (CaO·SiO2). Previously, it was generally believed that wollastonite could only be produced at high temperatures (≥1167°C), and the specific reasons are under further investigation. 3.4 Morphology of Heat-Treated Dry Gel Powder: The morphology of the products after ultrasonic dispersion of dry gel powder prepared under different pre-hydrolysis time conditions after heat treatment at 1000°C for 2 hours shows that when the pre-hydrolysis time is 60 minutes, the particle size of the heat-treated product is relatively uniform, with a particle size between 0.2 and 1.4 μm. If the pre-hydrolysis time is too long or too short, the particle size of the heat-treated product is uneven and larger. Therefore, the pre-hydrolysis time has a significant impact on the morphology of the heat-treated dry gel powder. Conclusion: 4.1 Using tetraethyl orthosilicate and calcium acetate as raw materials, CaO-SiO2 dry gel powder was prepared by the sol-gel method, and after heat treatment at 1000°C, wollastonite powder with an average particle size of 0.2 to 1.4 μm was obtained. 4.2 When R ≤ 4, as the amount of pre-hydrolyzed water increases, the gelation time of the CaO-SiO2 system shortens; when R ≥ 4, the gelation time increases with the amount of pre-hydrolyzed water; as the pre-hydrolysis time increases, the gelation time shortens. 4.3 When the pre-hydrolysis time is 60 minutes, the prepared wollastonite powder has a smaller particle size. 4.4 The CaO-SiO2 system dry gel powder is an amorphous substance after heat treatment at 400°C, and the product after heat treatment at 600°C is a mixture of wollastonite and pseudowollastonite.