Information

Soluble silicates - especially sodium, potassium and lithium silicates - are generally not defined stoichiometric substances (with a certain chemical formula and molecular weight). They are glasses or aqueous solutions of glasses that result from a combination of alkali-metal oxide and silicon oxide in various ratios.

The chemical composition of soluble alkali silicates is usually described with the following formula:

M₂O • nSiO₂

In this formula, M corresponds to Na, K or Li and n is the molar ratio (MR) that shows the number of moles of silicon dioxide (SiO₂) per mol of alkali-metal oxide (M₂O).

It is the usual practice in the industry to state the weight ratio (WR)  SiO₂ : M₂O. This ratio can be calculated from the MR as follows:

All alkali silicates listed above are alkaline substances; the concentrated products usually have a pH value between 10 and 13. The alkalinity of the products increases as the MR or WR decreases.

Water glass production

The raw materials needed to produce water glass are glass sand and alkali carbonate (e.g. soda for sodium carbonate / potash for potassium water glass). All raw materials must be extremely pure, since impurities that exceed certain threshold values can have negative effects on the melting process and the product properties.

Depending on the desired composition, sand and alkali carbonate are mixed in the appropriate weight ratio, and the mixture is then fed continuously into the melting furnace. The melting process takes place at temperatures between 1200°C and 1500°C. The solidified and cooled melting product (water glass) is dissolved under pressure and at high temperatures in the rotating or immobile autoclave. The resulting solutions are filtered and either used as base material for subsequent modifications or sold directly.

Furnace route

M₂CO₃ + n SiO₂ → M₂O • n SiO₂ + CO₂ (M = Na, K)

 
Another possibility for manufacturing water glass, particularly water glass with a low weight/mol ratio, is direct dissolution (hydrothermal process). In this process, glass sand is slurried with the alkali hydroxide solution and dissolved under high pressure and at high temperatures.

Hydrothermal route

2 MOH + n SiO₂ → M₂O • n SiO₂ + H₂O (M = Na, K)

Responsibility towards people and the environment

Alkali silicates were produced as early as several thousand years ago; since the beginning of the 19th century, they are manufactured and used on an industrial scale. Long experience in handling these substances and comprehensive toxicological and ecological tests have shown that alkali silicates do not represent a hazard to people and the environment if they are handled responsibly and expertly (taking into account their alkaline character).