Production process of traditional precipitation silica

Precipitated silica is also known as hydrated silica, active silica, precipitated silica and precipitated hydrated silica. Precipitated silica is divided into traditional precipitation silica and special precipitation silica. The former refers to white carbon black produced with sulfuric acid, hydrochloric acid, CO2 and water glass as the basic raw materials, and the latter refers to the use of hypergravity technology, sol-gel method, chemical crystallization method, secondary crystallization method or reversed-phase micellar microarray. Silica produced by special methods such as emulsion method.

This paper focuses on the production process of traditional precipitation silica.

Precipitated silica is usually formed by the chemical reaction of sodium silicate (ie, water glass) and an acid (in most cases, sulfuric acid) to produce precipitation. The product of this reaction between sodium silicate and sulfuric acid is precipitated silica (scientific name hydrated silicic acid or hydrated silica), and the by-products of the reaction are sodium sulfate and water. In order to control this process, it must be considered that the formation of precipitated silica is a reversible process. Under special conditions, such as when the pH value or temperature is high, the reaction will proceed towards the raw material side. This chemical reaction is an equilibrium state that can be influenced by process parameters.

1. Precipitation

An aqueous solution of water glass and sulfuric acid was added simultaneously to a reactor with a stirrer under the specified concentration and reaction conditions. During the reaction, elementary particles are first generated in the reactor; along with dehydration, these particles react with each other to form aggregates. Within the aggregates, the elementary particles are held together by silanol bonds. The aggregates then collide with each other and combine to form agglomerates. In these agglomerates, the aggregates are held together by hydrogen bonding or van der Waals interactions, which are much weaker than silanol bonds, so the bonding is reversible. The agglomerates are easily broken and dispersed into aggregates during the mixing process. In the process of precipitation reaction, the important process parameters are pH value, reaction temperature (usually 50-90° C.), concentration of each component, reaction time, and the metering and mixing of water glass and sulfuric acid. These process parameters will affect the surface area, structure, dispersion and concentration of silanol groups of the resulting silica, among others.

2. Filtration

The obtained suspension is filtered in a plate and frame filter press or other filter equipment, and the filter cake is washed to remove sodium sulfate. In the filtration process of silica, the important process parameters are the filling and washing time, the solid content in the filter cake and the residual sodium sulfate content, etc.

3. Drying

The aqueous filter cake is stirred to become a silica suspension in water, and then spray-dried or dried by other drying methods. A simple classification of the different types of dryers is to divide them into long-term dryers and short-term dryers. The residence time of silica in long-term dryers is measured in hours, and the residence time in short-term dryers is measured in seconds. In the drying process of silica, important process parameters are the type of dryer, drying temperature, solids content before drying and residence time in the dryer, etc. These process parameters will affect the moisture content, surface area, structure, dispersibility and particle size distribution of the resulting silica.

4. Granulation

In order to reduce or eliminate the dust when the product is conveyed, the powdered product is granulated into flakes or granules. During granulation, important process parameters are feed rate and granulation pressure. The granulation process will affect the structure, dispersion, particle size distribution and dust content of silica.

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