Precipitated silica

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Precipitated amorphous silica.jpg

Precipitated silicon dioxide (also: precipitated silica ) is an amorphous form of silicon dioxide (SiO 2 ); it is a white powdery material. Precipitated silicon dioxide is produced by precipitation from a solution containing silicate .

The three main classes of amorphous silica are fumed silica , precipitated silica, and silica gel . Among them, precipitated silica has the greatest economic importance. In 1999 more than a million tons were produced, half of which were used in tires and shoe soles.

Like fumed silicon dioxide, precipitated silicon dioxide is essentially non-microporous (unless it is made using the Stöber synthesis ).

Manufacturing

The processes used to make precipitated and fumed silica are compared in the following figure:

Production of pyrogenic and precipitated SiO 2 .

Precipitated silica is produced by condensation of silica from dilute alkali silicate solution, which is accessible by digesting quartz sand with sodium carbonate or potassium carbonate . This alkali silicate solution is also called water glass . Silicic acid has a strong tendency to polymerize with conversion of silanol groups (Si-OH) to siloxane bonds (Si-O-Si). The reaction kinetics (reaction speed) are strongly dependent on the pH value .

Industrial production is carried out by acidifying an aqueous alkali silicate solution (e.g. sodium silicate) with sulfuric acid . Stirring prevents gelation, otherwise silica sols or silica gels would form .

Use of precipitated silicas

  • Airgel
  • Films for battery separators.
  • Additives to tires, shoe soles and other rubber articles to increase abrasion resistance and to improve wet slip behavior and rolling resistance.
  • Additives that improve the flow behavior of feed, soup and beverage powders .
  • As powder in fire extinguishers
  • Coatings for inkjet papers that allow quick drying and better print quality.
  • Defoamers in detergents.
  • Protein adsorbers in beer production.
  • Matting agent for lacquers, especially clear lacquers in wood and automotive interiors
  • Filtration aid to improve the filterability of liquids.
  • Active powder
  • As a setting accelerator for Portland cements in building materials
  • For sealing the base of excavations and grouting in mining. Freshly precipitated, water-containing silica gels made from ortho-silicic acid are used here on site.
  • In pressed form (bonded with ceramic fibers) as a thermal insulation body (e.g. in radiant heaters for glass ceramic hobs)

Individual evidence

  1. ↑ Otto W. Flörke, et al. "Silica" in Ullmann's Encyclopedia of Industrial Chemistry, 2008, Weinheim: Wiley-VCH. doi : 10.1002 / 14356007.a23_583.pub3 .
  2. ^ Anten Chemical Co., Ltd .: Fumed silica and Precipitated silica-Anten Chemical Co., Ltd. Retrieved October 15, 2018 .
  3. ↑ C. Ozmetin, J. Schlomach, M. Kind: Polymerization kinetics of silica . In: Chemical Engineer Technology . tape 76 , no. 12 , November 17, 2004, ISSN  0009-286X , p. 1832-1836 , doi : 10.1002 / cite.200407037 .
  4. ↑ Gysau, Detlef .: Fillers: Basics and Applications . 2., revised. Vincentz Network, Hannover 2006, ISBN 978-3-87870-337-2 .
  5. ↑ silica gel | chemical compound . In: Encyclopedia Britannica . ( britannica.com [accessed October 15, 2018]).