A method for preparing spherular silica gel particles

Abstract: In the present invention, minute spherular silica gel particles suitable as polymer catalyst carriers can easily be obtained without using organic substances simply by controlling the moisture content in a slurry of silica hydrogel particles and water. The obtained spherular silica gel particles have physical properties, such as specific surface area, average pore diameter, and pore volume, that may be easily controlled. The spherular silica gel particles also have sufficient particle strength.

Description: This invention relates to a method for preparing spherular silica gel particles, especially to a method for preparing spherular silica gel particles suitable as a carrier of a catalyst for initiating the polymerising of olefin. Spherular silica gel particles have been widely used as carriers for a catalyst used to initiate the polymerization of olefin. Since the spherular silica gel particles are inactive porous particles, they are desirable carriers of the polymerization-initiating catalyst. Furthermore, spherular silica gel particles have desirable fluidity for vapor-phase polymerization. The physical properties, such as surface area, average pore diameter, pore volume, particle strength, and average pore diameter, of the spherular silica gel particles used as carriers should be strictly controlled. These physical properties of the spherular silica gel particles should be generally kept within the following ranges: surface area - 50 to 700m²/g; average pore diameter - 20 to 1000Å; pore volume - 0.3 to 2.0ml/g; average particle diameter - 5µm to 5mm; and particle strength - 1 to 4. However, spherular silica gel particles with physical properties satisfying the above ranges have been difficult and expensive to prepare. One known method for preparing spherular silica gel particles is disclosed in Japan Published Examined Patent Application No. S54-9588. By the method disclosed therein, colloidal silica is formed into spherular particles with an average particle diameter of 0.5µm to 50µm using hydrophilic organic substances as a binding agent. However, this method requires an intricate step where the mixture of prepared silica sol particles and the organic substances are sintered at 500°C minimum for four hours or longer to remove polymerized organic substances. This method has another problem caused by the fact that the physical properties of the spherular silica gel product are hard to control because the physical properties of the original colloidal silica determine those of the silica gel product. On the other hand, a method for preparing spherular silica gel particles without using organic substances is proposed in Japan Published Unexamined Patent Application No. S53-65295. In the method described therein, silica sol is spray-dried to obtain spherular silica gel particles. However, irregular silica gel particles such as hollow or indented gel particles result from this method. These silica gel particles have less fluidity and have varying particle strength. Another known method is proposed in Japan Published Examined Patent Application No. S47-35676. The method described therein comprises the steps of mixing sodium silicate and sulfuric acid at 23°C to 50°C, preparing a mixture solution of pH9.8 to pH10.4 to form silica hydrogel slurry, aging the silica hydrogel slurry, decreasing the pH to 3 or below, increasing the pH between 8 and 10.2, aging, and spray-drying. However, since this method requires many intricate steps, it is an industrially disadvantageous method. Further, the particle strength of the resulting silica gel particles is hard to control. Another known method is proposed in Japan Published Unexamined Patent Application No. S62-275014. The method described therein comprises the steps of reacting sodium silicate solution and mineral acid to prepare silica hydrogel slurry, mixing aqueous ammonia with the silica hydrogel slurry, wet-grinding and spray-drying. However, by this method, the particle strength of the silica gel product is hard to control. One object of this invention is to provide a method for preparing spherular silica gel particles, without using organic substances, while controlling surface area, average pore diameter, pore volume, average particle diameter, and particle strength of the spherular silica gel particles. Through research it has been discovered that when the silica hydrogel slurry resulting from the reaction of alkali metal silicate and mineral acid is ground and spray-dried, the concentration of the components of the silica hydrogel slurry determines the particle strength of the spherular silica gel products. As a result, to attain the above object, the present invention provides a method for preparing spherular silica gel particles comprising the steps of reacting alkali metal silicate and mineral acid to prepare silica hydrogel particles, wet-grinding the silica hydrogel particles, and spray-drying the slurry of the silica hydrogel particles and water to produce spherular silica gel particles. In the method the weight of water in the slurry is adjusted to between 0.2 and 1.5 times the weight of the silica hydrogel particles in the slurry. Before the step of wet-grinding the pH of the slurry is adjusted to a value of 1-3. First, in this invention an aqueous solution of alkali metal silicate and an aqueous solution of mineral acid are reacted to prepare silica hydrosol. As alkali metal silicate, sodium silicate and potassium silicate are employed. Sodium silicate is best. As mineral acid, sulfuric acid, hydrochloric acid, and nitric acid are used. Alkali metal silicate and mineral acid can be reacted by batch process or continuous process. The reaction conditions are not limited, but the concentration of SiO₂ in the silica hydrosol resulting from this reaction should range from 10% to 22% by weight. When the concentration of SiO₂ in the silica hydrosol falls below this range, it takes a longer time to congeal the silica hydrosol. Such a low concentration is thus industrially disadvantageous. When the concentration exceeds this range, the silica hydrosol is congealed so quickly that uniform silica hydrogel particles cannot be produced. To adjust the concentration of SiO₂ in the silica hydrosol, the concentration and the usage of the alkali metal silicate and the mineral acid can be varied. Usually, an alkali metal silicate solution contains from 15% to 27% by weight of SiO₂, and the concentration of the mineral acid solution is between 3N and 18N. The silica hydrosol congeals into silica hydrogel particles. Subsequently, the silica hydrogel particles are separated from the mixture and rinsed. In this invention, hydrothermal treatment prior to wet-grinding of silica hydrogel particles is desirable. During the hydrothermal treatment, the silica hydrogel particles are suspended in demineralised water, aqueous ammonia is added to adjust the aqueous suspension to a range of pH of 1-10. The aqueous suspension is stored for one to fifty hours at 50°C to 200°C. The pH should be in the range between one and ten, because a pH outside this range increases the solubility of silica. The silica hydrogel particles are separated from the hydrothermally treated mixture. The separated silica hydrogel particles are coarsely ground using a machine, such as an impact mill, a jet mill, a ball mill, a stone mill or the like, until the particle diameter of 100 µm to 200 µm is obtained. During the grinding, moisture is removed from the surface of the silica hydrogel particles. In this invention, a slurry of silica hydrogel particles and water is formed, ajusted to a pH of 1-3 and subsequently wet-ground. The wet-ground slurry is spray-dried to obtain spherular silica gel particles. The weight of water used to form the slurry is 0.2 to 1.5 times, preferably 0.3 to 1.4 times, that of the silica hydrogel particles in the slurry. When too little water is used, the slurry cannot be formed, spherular silica gel particles cannot be obtained by spray-drying, or sufficient particle strength of the silica gel particles cannot be obtained. When the slurry contains too much moisture, the particle strength of the silica gel particles is insufficient. For silica hydrogel particles with specific surface areas equaling or exceeding 400m²/g, when the concentration of the slurry is adjusted such that sufficient particle strength is obtained, the slurry becomes too viscous to mix and wet-grind. To facilitate the mixing and wet-grinding, the slurry should be adjusted to between pH1 and pH3. The wet-grinding influences the surface condition and configuration of the end product of the silica gel particles. During the wet-grinding, a mill such as a ball mill, a vibrating mill, a mixing ball mill, a rod mill, a disc attrition mill, or a stone colloid mill, is used. The wet-grinding should be repeated until the particle size of the silica hydrogel particles in the slurry becomes 1 µm to 50 µm, preferably 10 µm to µ30 m. In this invention, by spray-drying the slurry of silica hydrogel particles containing a specific rate of moisture, spherular silica gel particles are obtained. For a spray-drying method, a pressure nozzle method, a rotating disc method, and a binary fluid nozzle method are available. The sprayed slurry is dried by hot air of 100°C to 500°C. Through the spray-drying, the silica gel particles can have desired particle diameters about between 1µm to 200µm. The silica gel particles as carriers of a catalyst for initiating polymerizing olefin should have particle diameters of 30µm to 100µm. EMBODIMENTS 1-3 By mixing and reacting a sodium silicate solution and a sulfuric acid solution, silica hydrosol with the SiO₂ concentration of 18.5% by weight was generated. The silica hydrosol was stored for one hour and congealed to form silica hydrogel particles. Subsequently, the silica hydrogel particles were filtered, rinsed and mixed with demineralized water. To the mixture of silica hydrogel particles and demineralized water, 28%-by-weight aqueous ammonia was added to prepare a slurry of silica hydrogel particles in water and to adjust the pH of the slurry to 8.5. Then, the temperature of the slurry was raised to 95°C, and at 95°C the slurry was hydrothermally treated for thirty-eight hours. Subsequently, the silica hydrogel particles were again filtered from the slurry, and were coarsely ground using an impact type mill (e.g. an M-4A type free crushing mill manufactured by Nara Kikai Seisaku-sho). The ground gel particles were mixed with demineralized water to prepare a slurry of a predetermined percentage by weight as shown in Table 1. The silica hydrogel particles in the prepared slurry were wet-ground using a stone type mill (e.g. Super Mass Colloider, manufactured by Masukou Sangyo Ltd.). After wet-grinding, micro spherular silica gel particles were collected by spray-drying the slurry using a disc-type spray drier (e.g. CS-100, manufactured by Ohkawara Kakouki Ltd.), in which the peripheral speed of the disc was approximately 45m/sec. and the temperature of hot air was 370°C at the inlet of the drier and 150°C at the outlet of the drier. The average particle diameter, surface area, and particle strength of the collected spherular silica gel particles were measured as shown in Table 1. To obtain comparison data, when the moisture rate of the slurry was reduced to less than 0.2 times by weight, the slurry could not be formed. On the other hand, when the rate was increased to over 1.5 times by weight, the particle strength was so strong that the particles were cracked during the preparation for the measurement. An ultrasonic cleaner of VO150FS-S type manufactured by Sinmei Kogyo Ltd. and a grain-size measuring apparatus of type CAPA-700 manufactured by Horiba Ltd. were used for the embodiments. The method for measuring the grain-size distribution used the phenomenon of particles' precipitation. EMBODIMENTS 4-6 These embodiments use almost the same method as that used for the embodiments 1-3. The embodiments 4-6 are different from the embodiments 1-3 in that the slurry is hydrothermally treated at pH6-7 and 72°C for seven hours, the pH of the slurry prior to spray-drying is 2.0, and the moisture rate in the slurry is as shown in Table 2. The physical properties of the collected spherular silica gel particles are also shown in Table 2. EMBODIMENTS 7-9 These embodiments follow method of embodiments 4-6 for preparing silica hydrogel particles and spherular silica gel particles. The physical properties of the collected spherular silica gel particles are shown in Table 3.

Claims: A method for preparing spherular silica gel particles comprising the steps of: preparing a hydrosol slurry containing silica hydrogel particles from alkali metal silicate and mineral acid in which the weight of water in the slurry is from 0,2 to 1,5 times the weight of the silica hydrogel particles in the slurry; and the pH-value of the slurry is from 1 to 3; wet-grinding the silica hydrogel particles; and spray-drying the wet-ground silica hydrogel particles to produce silica gel particles. The method of claim 1, in which the alkali metal silicate is chosen from the group consisting of sodium silicate and potassium silicate. The method of claim 2, in which the alkali metal silicate is sodium silicate. The method of claim 1, in which the mineral acid is chosen from the group consisting of sulfuric acid, hydrochloric acid, and nitric acid. The method of claim 1, in which the alkali metal silicate and mineral acid are contained in aqueous solutions. The method of claim 1, in which the slurry contains from 10% to 22% by weight of SiO₂. The method of claim 5 or 6, in which the alkali metal silicate solution contains from 15% to 27% by weight of SiO₂ and the concentration of the mineral acid solution is from 3N to 18N. The method of claim 1, in which the silica hydrogel particles are wet-ground until the silica hydrogel particles have a diameter of from 1 µm to 50 µm. The method of claim 1, in which the weight of the moisture in the slurry is from 0,3 to 1,5 times the weight of the silica hydrogel particles in the slurry. The method of claim 1, in which the slurry is wet-ground by using a grinder chosen from the group consisting of a ball mill, a vibrating mill, a mixing ball mill, a rod mill, a disc attrition mill, and a stone colloid mill. The method of claim 1, in which the wet-ground slurry is spray-dried using a method chosen from the group consisting of the pressure nozzle method, the rotating disc method, and the binary fluid nozzle method. The method of claim 1 further comprising the step of drying the wet-ground, spray-dried slurry with air having a temperature from 100°C to 500°C. The method of claim 1 further comprising the steps of: separating the silica hydrogel particles from the slurry; and rinsing the silica hydrogel particles before the step of wet-grinding the silica hydrogel particles. The method of claim 1 further comprising the step of hydrothermally treating the silica hydrogel particles before the step of wet-grinding the silica hydrogel particles. The method of claim 14, in which the step of hydrothermally treating the silica hydrogel particles comprises the steps of: suspending the silica hydrogel particles in demineralized water; adjusting the pH of the aqueous suspension to from pH1 to pH10 by adding aqueous ammonia; and storing the pH-adjusted aqueous suspension for from one to fifty hours at from 50°C to 200°C. The method of claim 1, further comprising the step of coarse-grinding the silica hydrogel particles to a diameter of from 100 µm to 200 µm before the step of wet-grinding the silica hydrogel particles. The method of claim 1, in which the concentration of the components of the hydrogel slurry determines the strength of the spherular silica gel particles. A method for preparing spherular silica gel particles comprising the steps of: preparing a hydrosol slurry containing silica hydrogel particles from alkali metal silicate solution and mineral acid solution in which the weight of the water in the slurry is from 0,3 to 1.4 times the weight of the silica hydrogel particles in the slurry and the pH-value of the slurry is from 1 to 3; separating the silica hydrogel particles from the slurry; rinsing the silica hydrogel particles; suspending the silica hydrogel particles in demineralized water; adjusting the pH-value of the aqueous suspension to from 1 to 10 by adding aqueous ammonia; storing the pH-adjusted aqueous suspension for from one to fifty hours at from 50°C to 200°C; coarse-grinding the silica hydrogel particles until the silica hydrogel particles have a diameter of from 100 µm to 200 µm; wet-grinding the silica hydrogel particles until the silica hydrogel particles have a diameter of from 1 µm to 50 µm; spray-drying the wet-ground silica hydrogel particles to produce silica gel particles; and drying the wet-ground, spray-dried slurry with air having a temperature from 100°C to 500°C. The method of claim 18, in which the alkali metal silicate is sodium silicate. The method of claim 19, in which the alkali metal silicate solution contains from 15% to 27% by weight of SiO₂ and the concentration of the mineral acid solution is from 3N to 18N.

Inventors: HORINOUCHI YUZO, ITO MUTSUHIRO, KANO JIRO, ORII RYUJI, HORINOUCHI, YUZO, ITO, MUTSUHIRO, KANO, JIRO, ORII, RYUJI
Applicants: FUJI SILYSIA CHEMICAL LTD, FUJI SILYSIA CHEMICAL LTD.
Application year: 1990
Main classification: C01B33
IPC codes: C08F10, C08F4, B01J21, C01B33, B01J37