Research progress on the production of silica gel from silicate minerals

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The preparation methods of silica aerogel are divided into three categories: the vapor phase method, the precipitation method, and the decomposition of silicate minerals. The industrial production of silica aerogel involves a combination or modification of these three methods. The preparation of silica gel by the vapor phase method involves the hydrolysis of silicon tetrachloride gas at high temperature in a hydrogen-oxygen stream. The product has extremely high purity and excellent performance, but it requires sophisticated equipment, its raw materials are scarce, the cost is high, technical control is complex, and the investment required is substantial. The precipitation method for producing silica gel involves reacting water glass with acid to first form silicic acid, which is then decomposed to obtain the product. This method uses readily available raw materials, has a simple process, and requires modest investment, but it consumes a lot of energy and imposes high environmental demands. The silicate mineral decomposition method uses silicate minerals such as bentonite, kaolin, sepiolite, etc. as raw materials; upon heating and in the presence of an activator, crystalline silica is converted into active amorphous silica. This method is technically feasible and offers good economic benefits, opening up a new path for the deep processing and comprehensive utilization of silicate minerals. China is rich in silicate mineral resources. In the development and utilization of various silicate minerals, a significant amount of amorphous silica remains underutilized. Fully exploiting these siliceous resources is an effective way to improve the utilization rate and economic benefits of silicate mineral resources, as well as to reduce environmental pollution. Converting crystalline silicon in crystalline silica and silicates into active amorphous silicon is the key technique for simplifying the production process of silica gel, as well as reducing energy consumption and costs. Therefore, the silicate mineral decomposition method is particularly suitable for preparing silica gel using silicate minerals. 1 Preparation of silica from bentonite: Bentonite is a clay mineral whose main mineral component is montmorillonite, and its chemical composition is quite stable. Montmorillonite is a 2:1 layered silicate mineral containing crystalline water, composed of two layers of silicon-oxygen tetrahedra connected at their apices, with a layer of aluminum (magnesium) oxygen (hydroxide) octahedra connected along their edges; it is one of the clay minerals with the most varied crystal structures. The results of slow scanning experiments using a diffractometer show that the particle size of montmorillonite is close to the nanoscale, making it a natural nanomaterial. When montmorillonite is heated in the presence of an activator, its layered structure is easily disrupted, converting crystalline silica into active amorphous silica. Through a series of processing steps such as activation treatment, bentonite can be directly converted into silica with high purity, and its key technical parameters meet the **standards** of silica produced by precipitation methods. The energy consumption required to produce silica gel using bentonite as raw material is only 1/5 of that in the precipitation method (acid neutralization of water glass), and the cost of raw materials is only 1/3 of that in the precipitation method; thus, the cost per ton is approximately 1,000–2,000 yuan lower compared to silica gel produced by the precipitation method. The production process for preparing silica using bentonite is innovative and advanced, features a rational flow, low energy consumption, inexpensive and readily available raw materials, high added value for the product, and by-products that can be used as water purifiers. Its cost is much lower than that of traditional processes, making it a viable new method. China’s technology for producing silica using bentonite lags behind that of foreign countries and is still in its initial stages. 2 Preparation of silica from kaolin: Kaolin is a type of clay or clay rock composed primarily of clays belonging to the kaolinite group; it is made up mainly of tiny flaky or tubular crystals of kaolinite minerals with a size of less than 2 μm. The minerals in the kaolinite group include kaolinite, halloysite, nacre, 7 nm illite, and 10 nm illite. These belong to the 1:1 type of layered silicates and have a monoclinic crystal structure. China has abundant kaolin resources, ranking fourth in the world. In terms of ore quality ； Most kaolin ores in China are used for ceramics, with an Al2O3 content of around 20%, ranging up to over 38% at its highest and not falling below 10% at its lowest. The method for preparing silica gel from kaolin involves preparing a suspension of kaolin ore with water, drying it and then grinding it; followed by calcination. Sulfuric acid with a mass fraction of about 50% is then added for a reaction. After the reaction is complete, the mixture is diluted with water and filtered. The filter cake is placed in a reactor, dilute nitric acid solution is added to it, and the mixture is heated to 60–80°C while stirring. The reaction proceeds for about 1 hour, after which the mixture is filtered again. The filter cake is washed with water until it reaches a neutral pH, and after drying, calcination, and cooling, it is ground to obtain silica gel. Its product quality can meet the standards of dry-process production, while the technology and equipment involved are simpler than those in dry-process production, the costs are lower, and there is virtually no emission of waste gases, wastewater, or solid waste, which is beneficial for environmental protection. To increase the yield of silica, the kaolin raw material can be calcined before use. 3 Preparation of silica gel from wollastonite: Wollastonite is a chain-shaped metasilicate mineral that exists in 3 polymorphic variants; two of these are high-temperature variants, commonly referred to as pseudowollastonite ； Two low-temperature variants, namely triclinic wollastonite and monoclinic wollastonite. The common form in nature is low-temperature triclinic aragonite. Wollastonite crystals develop along the (001) plane; their aggregates are fibrous or acicular in shape, and appear bright white or white with slight gray, green, or red tones, exhibiting a glassy or pearly luster. The fibrous variety has a silky luster ； It features a low oil absorption rate, high electrical resistance, low dielectric constant, non-magnetism, a low coefficient of thermal expansion, as well as excellent heat and corrosion resistance. Wollastonite is chemically inert at low temperatures, but it readily undergoes solid-state reactions with kaolinite, pyrophyllite, talc, and others at high temperatures. Wollastonite contains calcium metasilicate with a purity of over 94%, has few impurities, and is insoluble in hydrochloric acid, making it an ideal raw material for the production of silica gel. The key technique in producing silica gel from mineral silicon sources is to convert crystalline silica and silicates into amorphous silica. The process for preparing silica gel using wollastonite as raw material involves acidulating wollastonite with dilute sulfuric acid, followed by precipitation, washing, drying, and crushing; appropriate amounts of inorganic and organic additives also need to be added during this preparation process. Its reaction equation is: CaSiO3 + H2SO4 = CaSO4 + H2O + SiO2. The silica produced by this method is in powder form; it is a porous silicon dioxide with a high specific surface area. It has the advantages of a high silicon dioxide content, low loss on ignition, and no waste gas emissions. 4 Preparation of silica gel from diatomite: Diatomite is a porous biogenic siliceous rock formed through preliminary lithification of the remains of ancient diatoms, with SiO2 being its main chemical component. It is composed of diatom frustules, which have multiple layers of numerous micro-pores arranged in an orderly manner. This unique structure endows it with many excellent properties: it has stable performance, acid resistance, a large pore volume, large pore sizes, and a large surface area. White carbon black is produced from diatomite by reacting diatomite powder with caustic soda under heating conditions to form sodium silicate. The reaction products are filtered to remove residues, sodium salts are added, water is used to adjust the concentration, the mixture is neutralized with acid, and the resulting substance is aged, filtered, washed, and dried to yield the white carbon black product. Compared with the traditional water glass method, this approach has advantages such as readily available raw materials, a simple process, as well as lower energy consumption and production costs per unit of product. 5 Preparation of silica gel from other silicate minerals – Silica gel can also be produced using other silicate minerals such as serpentine, zircon, sepiolite, coal cinder, and attapulgite. White carbon black and lightweight magnesium oxide can be produced using serpentine. The preparation method is as follows: Serpentine is soaked and filtered; the resulting solution is neutralized and oxidized to remove impurities, after which carbonization and calcination are carried out to produce lightweight magnesium oxide ； Water glass is produced by reacting solid silica with an alkali solution having a mass fraction of 20%-40%, and silica gel can be obtained by adjusting the pH of the water glass in two steps. This method features reliable processing, excellent product quality, and low production costs. The method for producing silica gel from zircon is as follows: zircon is decomposed using an alkali; the decomposition products are treated with water to isolate a sodium silicate solution with high alkalinity, and then the precipitate obtained through this process is treated with acid to yield silicic acid ； By mixing a high-alkalinity sodium silicate solution with silicic acid, and then undergoing processes of neutralization, precipitation, filtration, washing, drying, and crushing, white carbon emulsion can be obtained. This method has advantages such as simple processing, low equipment investment, low raw material consumption, and low costs. Silica gel can also be produced from coal gangue. The preparation process is as follows: First, coal gangue is crushed, and then mixed with soda ash in a ratio of 1:50 (mass ratio of coal gangue to soda ash). Sodium silicate is obtained through high-temperature melting, water extraction, leaching, and concentration ； Sodium silicate is then used to prepare a water glass solution; through reaction and aging, and finally via filtration, washing, and drying, the finished product, activated silica, is obtained. This method can be used to produce high-value activated silica from waste coal gangue, offering advantages such as low investment, low costs, and good product quality. 6 Conclusion How to increase the degree of conversion of crystalline silica and silicates into amorphous silica is the technical key to simplifying the production process of silica gel, as well as to reducing energy consumption and production costs. Therefore, if China increases its investment in research and development for the technology of producing silica from silicate minerals, it will surely make significant progress.