White carbon black, also known as silica, is scientifically termed precipitated hydrated silica. It is a high‑tech inorganic fine chemical product whose main constituent is SiO₂. Its physical properties resemble those of carbon black, appearing as a highly dispersed white amorphous powder or flocculent powder; it is also processed into granular forms for commercial use. Lightweight, with an original particle size below 0.3 μm and a relative density ranging from 2.319 to 2.653, it has a melting point of 1750°C. It is insoluble in water and most acids, but absorbs moisture from the air, forming hygroscopic fine particles. It is soluble in caustic soda and hydrofluoric acid, while remaining inert to other chemicals; it does not decompose at high temperatures nor does it burn. It is chemically inert toward substrates, active ingredients, and additives, and exhibits excellent compatibility with many active components commonly used in vitamins, hormones, fluorides, antibiotics, enzyme preparations, and cosmetics. It possesses high electrical insulation properties. Owing to its porosity and large specific surface area, it demonstrates strong dispersibility in raw rubber and imparts superior reinforcing performance in rubber compounds. Surface‑modified hydrophobic silica readily disperses in oils, rubbers, and plastics as a reinforcing filler, significantly enhancing mechanical strength and tear resistance. Due to variations in manufacturing processes, the physical and chemical characteristics, as well as the microstructure, of silica may differ, leading to distinct application areas and performance outcomes.

　　Precipitated silica and fumed silica are broadly classified according to their production methods.

　　White carbon black is a collective term for white, powdery X-ray‑amorphous silicic acid and silicate products, primarily referring to precipitated silica, fumed silica, and ultrafine silica gel, and also encompassing powdered synthetic aluminosilicates and calcium silicates, among others.

　　White carbon black is broadly classified, according to its production method, into precipitated white carbon black and pyrogenic white carbon black. Pyrogenic white carbon black is, under normal conditions, a white, amorphous, flocculent, semi‑transparent solid—essentially a colloidal nanoparticle with a particle size of less than 100 nm; it is non‑toxic and possesses an exceptionally large specific surface area. All pyrogenic white carbon black consists of nano‑sized silicon dioxide, with product purity reaching up to 99% and particle sizes ranging from 10 to 20 nm; however, its preparation process is complex and the material is relatively expensive. Precipitated white carbon black, in turn, is divided into conventional precipitated white carbon black and special‑process precipitated white carbon black. The former refers to silica produced using sulfuric acid, hydrochloric acid, CO₂, and sodium silicate as the primary raw materials, while the latter denotes silica manufactured via specialized techniques such as ultra‑high‑gravity processing, the sol–gel method, chemical crystallization, secondary crystallization, or the reverse‑micelle microemulsion method. Precipitated white carbon black is primarily used as a reinforcing agent for both natural and synthetic rubbers, as well as a polishing abrasive in toothpaste. Pyrogenic white carbon black is mainly employed as a reinforcing filler in silicone rubber, as a thickening agent in coatings and unsaturated resins, and ultrafine silica gels and aerogels are chiefly utilized as matting agents and thickeners in paints, as well as release agents for plastic films.

　　　　Driven by growing demand in sectors such as tire manufacturing, the silicone rubber industry, the coatings industry, and new energy, China’s precipitated silica (silicon dioxide) market is expected to expand at an annual average rate of approximately 10% over the coming years. By 2015, Asia will emerge as a major market for highly dispersible precipitated silica, with demand growth far outpacing the global average of 9%, reaching 15% or higher. With China accounting for 40% of the Asian market, it will become the world’s largest single market.

　　　　What are the traditional methods for preparing precipitated silica?

　　The conventional method for preparing precipitated silica uses sodium silicate, silicon tetrachloride, and tetraethyl orthosilicate as silicon sources; with the exception of sodium silicate, the other materials are quite expensive. The new approach employs inexpensive non‑metallic minerals as the silicon source, significantly reducing the production cost of precipitated silica.

　　Traditional methods

　　(1) Gas-phase method

　　The primary method is chemical vapor deposition (CVD), also known as pyrolysis, the dry process, or the combustion method. The typical feedstocks are silicon tetrachloride, oxygen (or air), and hydrogen, which react at high temperatures. The reaction equation is:

　　SiCl4 + 2H2 + O2 → SiO2 + 4HCl

　　Air and hydrogen are separately pressurized, separated, cooled to remove moisture, dried over silica gel, and filtered to remove particulates before being fed into the synthesis hydrolysis furnace. The silicon tetrachloride feed is sent to a distillation column for rectification, then heated and evaporated in an evaporator; the resulting vapor is carried by dry, filtered air and introduced into the synthesis hydrolysis furnace. At high temperatures (flame temperature 1000–1800°C), silicon tetrachloride is vaporized and undergoes gas-phase hydrolysis with a controlled amount of hydrogen and oxygen (or air) at approximately 1800°C. The gaseous silica produced is fine and forms an aerosol that is difficult to capture; therefore, it is first agglomerated into larger particles in an agglomerator, then collected in a cyclone separator, and finally conveyed to an acid‑removal furnace, where the gaseous silica is washed with nitrogen‑enriched air until its pH reaches 4–6, yielding the final product.

　　(2) Precipitation method

　　The precipitation method, also known as the sodium silicate acidification method, involves reacting a water glass solution with an acid, followed by precipitation, filtration, washing, drying, and calcination to obtain precipitated silica. The reaction equation is:

　　Na2SiO3 + 2H+ —> White carbon black + 2Na+ + H2O

　　Most domestic manufacturers employ the precipitation method.

　　Precipitated silica and fumed silica are broadly classified according to their production methods.

　　White carbon black is a collective term for white, powdery X-ray‑amorphous silicic acid and silicate products, primarily referring to precipitated silica, fumed silica, and ultrafine silica gel, and also encompassing powdered synthetic aluminosilicates and calcium silicates, among others.

　　Precipitated silica and pyrogenic silica are broadly classified according to their production methods. Pyrogenic silica is, under normal conditions, a white, amorphous, flocculent semi‑

　　Transparent solid‑gel‑like nanoparticles (with particle sizes below 100 nm), non‑toxic and possessing an exceptionally large specific surface area. Fumed silica is entirely composed of nano‑sized silicon dioxide, with a purity that can reach 99% and particle sizes ranging from 10 to 20 nm; however, its preparation process is complex and costly. Precipitated silica, in turn, is categorized into conventional precipitated silica and specialty precipitated silica: the former refers to silica produced using sulfuric acid, hydrochloric acid, CO₂, and sodium silicate as raw materials, while the latter encompasses silica synthesized via advanced methods such as supergravity technology, the sol–gel process, chemical crystallization, secondary crystallization, or the reverse micelle microemulsion method. Precipitated silica is primarily employed as a reinforcing agent for both natural and synthetic rubbers, as well as a polishing abrasive in toothpaste. Fumed silica is mainly used as a reinforcing filler for silicone rubber, as a thickening agent for coatings and unsaturated resins, whereas ultrafine silica gels and aerogels are predominantly utilized as matting agents and thickeners in coatings, as well as release agents for plastic films.