PURPOSE: To manufacture the titled sodium carbonate useful as a starting material for a synthetic detergent by calcining a mixture of sodium hydrogencarbonate with sodium carbonate and water, and adding water to the resulting granules to bring the surface layers of the granules into a hydration reaction.

CONSTITUTION: 1pt.wt. of sodium hydrogencarbonate is well kneaded with 0.2W 5pts. (expressed in terms of anhydride) sodium carbonate and water at 40W105°C, and the kneaded material is calcined at 100W300°C to decompose the sodium hydrogencarbonate and to remove the water. Water is sprayed on the resulting granules by an amount (weight) about 0.02W0.2 time the amount of sodium carbonate (expressed in terms of anhydride), and the surface layers of the granules are hardened by a hydration reaction to manufacture sodium carbonate having 0.4W0.9 apparent specific gravity and dissolving in water in ≤40sec. An aqueous soln. having caking action such as an aqueous soln. of CMC or sodium silicate may be used in place of said water.