The preparation method of silica sol has always been a hot topic of research, and the raw materials used as silicon sources are relatively simple, mainly including: water-soluble silicates, SiCl4, (RO)4Si and elemental silicon, etc. It can be divided according to the principle: one is the dispersion method, which, as the name suggests, is a method of using specific means under certain conditions to reduce the particles of SiO2 solids and disperse them in a solvent to obtain silica sol; the other method is the condensation method, which is mainly based on the polymerization principle of sol, using the chemical reaction of SiO2 ultrafine particles generated in the solution of inorganic or organic matter containing silicon to grow, and then obtain silica sol after nucleation. Usually in the process of practical application, the above two methods are used in combination. According to the preparation process of silica sol, it can be divided into the following types:
(1) Ion exchange method
This preparation method is the most mature and widely used process at present. It uses water glass as the main raw material, passes it through a strong acid cation exchange resin, removes sodium ions and other cationic impurities in the water glass, and obtains a dilute polysilicic acid solution.
Generally, in order to obtain higher purity, weakly alkaline anion exchange resin is used to remove chloride ions and other anion impurities in the dilute silica solution to obtain a higher purity active silica solution. At this time, a small amount of NaOH or other alkaline reagents need to be added as stabilizers to ensure that the pH is within the range of 8.5 to 10.5.
Then, the polysilicic acid solution is subjected to the formation of crystal nuclei and the growth of grains to obtain a mother liquor. Then, under the action of the particle growth reaction, the polysilicic acid solution is added to the mother liquor at a certain rate to obtain a monodisperse silica sol with controllable particle size. Finally, a high-purity silica sol product can be obtained by heating, concentration and centrifugal purification. The main disadvantages of this method are: the concentration of reactants needs to be strictly controlled, and the reuse of ion exchange resins will produce a large amount of wastewater.
(2) One-step dissolution of elemental silicon
A method of preparing silica sol by reacting elemental silicon with pure water under the catalytic action of an inorganic or organic base (such as pure sodium hydroxide).
The main advantage of this preparation method is that the raw materials used are of high purity and the impurity content of the silica sol product is extremely low. At the same time, the particle size, viscosity, pH value, density, purity and other indicators of SiO2 colloidal particles are easier to control than other methods. However, most of its particles are non-spherical and the morphology cannot be controlled.
(3) Electrolysis and electrodialysis method
This is a method of preparing silica sol by electrochemistry. First, electrolyte raw materials are added to the electrodialysis tank. By adjusting the pH, current density, temperature and other conditions of the electrolysis and electrodialysis reaction of the electrolyte solution, Na+ will migrate to one end of the electrode with the help of the electrolyte solution, and monosilicic acid H4SiO4 will be generated at the other end of the electrode, and a crystal nucleus will be formed under the action of the polymerization reaction. During the entire electrolysis and electrodialysis process, the newly generated monosilicic acid will be adsorbed on the surface of the crystal nucleus, and finally a silica sol product with a certain particle size will be formed.
The process flow of preparing silica sol by electrolysis and electrodialysis can be divided into: placement of electrolysis and electrodialysis cell and preparation of electrodes; addition of electrolyte raw materials; power-on for electrolysis and electrodialysis process; and preparation of silica sol product.
(4) Acid neutralization method
This method is similar to the ion exchange method. Both methods use water glass as the main raw material, and then use ion exchange resin to remove sodium ions in the solution to obtain a dilute sol and prepare a seed mother liquor. The difference is that the acid neutralization method is to add inorganic acids such as dilute sulfuric acid and dilute hydrochloric acid as acidifiers and dilute water glass directly to the previously prepared dilute sol containing crystal nuclei (i.e., acidification reaction). Under appropriate conditions, the crystal nuclei grow further, and then the silica sol product can be obtained by concentration and purification. The above process requires strict control of process conditions such as the concentration of Na+ in the mixed solution, heating temperature, solution pH and addition time.
(5) Other methods
In addition to the above four common preparation methods, there are many other silica sol preparation methods, such as gelation method, gas phase silica method, and dispersion method.