Precipitated silica is a relatively pure form of the mineral silica, which is produced by the chemical process known as precipitation. In chemical precipitation, a solid forms from a liquid due to a chemical reaction. Precipitated silica is formed by mixing solutions of sodium silicate with sulfuric acid in water. A chemical reaction occurs, and pellets of pure silica form.
Sodium silicate and sulfuric acid are the raw materials used to produce precipitated silica. Large quantities of these materials are mixed with water simultaneously in measured amounts while under constant agitation. The two chemicals react with each other as the sodium silicate is alkaline and the sulfuric acid is a strong oxidizer, being a very strong acid. The overall pH of the mixture is kept alkaline, as the precipitation reaction is not as vigorous under neutral or acidic conditions. While other acids would also work to create the conditions necessary to precipitate silica, sulfuric acid is used almost exclusively.
The physical properties of the resulting silica can vary slightly depending on the exact conditions under which the precipitation reaction takes place. Changes in temperature, concentration of the solutions used, and the ratio of the ingredients in the mixture can all alter the physical properties of the silica particles. Manufacturers manipulate these conditions to change the size and porosity of the silica grains.
After the silica has formed, it is separated from the solution, cleaned, and dried. This involves washing the silica to remove all traces of chemical salts and by-products of the precipitation process. The washing and drying process can also affect the character of the particles. Particle size, porosity, and density can all be manipulated this way to some extent. Manufacturers can prepare precipitated silica in a number of ways depending on its intended use by the customer.
Precipitated silica products are used in a number of applications, including the food industry, pharmaceuticals, manufacturing, and other industries. Pure silica is used as an anti-caking agent in some foods and pharmaceutical applications, as an abrasive in toothpastes, and as a thickening agent in some liquids. Industrial uses for precipitated silica abound as well. It is used as an additive to some types of rubber and plastic to improve characteristics such as durability and flexibility. Among its many other uses, precipitated silica is sometimes used for sandblasting.