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Silica is converted to silicon by reduction with carbon.Fluorine reacts with silicon dioxide(SiO2) to form SiF4 and O2 whereas the other halogen gases (Cl2, Br2, I2) are essentially unreactive.

SiO2 is attacked by hydrofluoric acid (HF) to produce hexafluorosilicic acid:

SiO2 + 6 HF → H2SiF6 + 2 H2O.

HF is used to remove or pattern SiO2 in the semiconductor industry.

SiO2 acts as a Lux-Flood acid, being able to react with bases under certain conditions. As it does not contain any hydrogen, it cannot act as a Br?nsted–Lowry acid. While not soluble in water, some strong bases will react with glass and have to be stored in plastic bottles as a result. SiO2 dissolves in hot concentrated alkali or fused hydroxide, as described in this idealized equation:

SiO2 + 2 NaOH → Na2SiO3 + H2O.

Silicon dioxide(SiO2) will neutralise basic metal oxides (e.g. sodium oxide, potassium oxide, lead(II) oxide, zinc oxide, or mixtures of oxides, forming silicates and glasses as the Si-O-Si bonds in silica are broken successively).As an example the reaction of sodium oxide and SiO2 can produce sodium orthosilicate, sodium silicate, and glasses, dependent on the proportions of reactants:

2 Na2O + SiO2 → Na4SiO4;

Na2O + SiO2 → Na2SiO3;

(0.25–0.8)Na2O + SiO2 → glass.

Examples of such glasses have commercial significance, e.g. soda-lime glass, borosilicate glass, lead glass. In these glasses, silica is termed the network former or lattice former. The reaction is also used in blast furnaces to remove sand impurities in the ore by neutralisation with calcium oxide, forming calcium silicate slag.

Bundle of optical fibers composed of high purity silica.Silicon dioxide(SiO2) reacts in heated reflux under dinitrogen with ethylene glycol and an alkali metal base to produce highly reactive, pentacoordinate silicates which provide access to a wide variety of new silicon compounds. The silicates are essentially insoluble in all polar solvent except methanol. SiO2 reacts with elemental silicon at high temperatures to produce SiO:

SiO2 + Si → 2 SiO

The solubility of silicon dioxide(SiO2) in water strongly depends on its crystalline form and is 3–4 times higher for silica than quartz; as a function of temperature, it peaks at about 340 °C. This property is used to grow single crystals of quartz in a hydrothermal process where natural quartz is dissolved in superheated water in a pressure vessel that is cooler at the top. Crystals of 0.5–1 kg can be grown over a period of 1–2 months. These crystals are a source of very pure quartz for use in electronic applications.



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