This invention is related to a process for the secondary obtention of sodium carbonate from a waste liquor (known as FLP waste liquor) containing principally sodium chloride, ammonium bicarbonate, ammonium chloride and carbon dioxide, which is obtained in the filtering section within the solvay process for the primary obtention of sodium carbonate, by adding a compound or a mixture of compounds sodium ions to the FLP waste liquor in the presence of carbon dioxide, in order to precipitate sodium bicarbonate therefrom, and treating the precipitation and sodium bicarbonate to obtain the secondary production of sodium carbonate.