Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 65

Originally known as natron, sodium carbonate hydrates have been the main flux used ever since the beginnings of glassmaking. As exemplified by the celebrated Wadi el Natroun deposits in Egypt (Chapter 10.3), very pure Na₂CO₃ hydrates did form naturally but such deposits currently satisfy only about 30 % of 50 million tons used annually in industry, half of which for glassmaking. The cheapest source of sodium would be NaCl, but this salt is quite unfit for production of oxidized glasses in view of chlorinated emissions that would in particular dramatically degrade the wearing resistance of the furnace refractories. Soda [NaOH] would be a much better alternative, but its price on an Na₂O basis is twice that of synthetic Na₂CO₃ as manufactured with the Solvay process [15], which currently supplies 59% of the market.

Sodium is most conveniently added to the batch as Na₂CO₃, whereas CaCO₃ is the most common source of carbonate ions. Hence, the goal of the Solvay process is to achieve the overall reaction:

(3)

Now, this reaction could not proceed directly in the solid state even if its Gibbs free energy of about 100 kJ/mol were not positive. But Na₂CO₃ is readily obtained through heating of NaHCO₃ precipitates at around 200 °C:

(4)

The trick of the Solvay process thus is to produce sodium bicarbonate in an aqueous solution from an NaCl brine with the reaction

(5)

which, as a by‐product, yields calcium chloride, a valuable compound:

(6)

The first step then consists in producing a solution of ammonium bicarbonate with

(7)

such that NH₃ and CO₂ are in the end “totally” recycled within the process.