Chemistry

15. Solvay Process. It is used for manufacture of washing soda. It is also called Ammonia Soda process.
Raw materials. Sodium chloride (NaCl), ammonia (NH₃) and limestone (CaCO₃)
Process.
(i) In this process a cold and concentrated solution of sodium chloride (called brine) is saturated with ammonia.
(ii) The ammonical brine is fed from the top of the carbonating tower packed with perforated plates.
(iii) Carbon dioxide (CO₂), is introduced from the base of the tower which reacts with NH₃ and H₂O to form ammonium bicarbonate (ammonium hydrogen carbonate).
NH₃ + H₂O + CO₂ → (NH₄)HCO₃ . . . . . (i)
(iv) Ammonia hydrogen carbonate reacts with sodium chloride (NaCl) to form sodium hydrogen carbonate and ammonia chloride.
. . . . . . . (ii)
(v) CO₂ used in first reaction is produced by heating limestone in lime kiln (furnace).

(vi) Quiklime reacts with H₂O to form slaked lime.

(vii) Slaked lime reacts with ammonium chloride produced in reaction (ii) to generate ammonia which can be used again in reaction (i).
Thus, most of ammonia can be recovered and reused, therefore, this process is economical. Secondly, calcium chloride is obtained as a by-product.
(viii)Sodium hydrogen carbonate, formed in reaction (ii) is sparingly (partially) soluble in water and can be separated by filtration.
(ix) Sodium hydrogen carbonate is heated to form sodium carbonate.

CO₂ formed is recirculated., i.e. used again in reaction (i).
(x) Sodium carbonate is recrystallized by dissolving in water to get washing soda.