To recover effectively salt and potassium salt by a method wherein a collected ash collected from an exhaust gas of a soda recovery boiler is mixed with water to make slurry, salt in the collected ash is dissolved in water, and when a solid content is separated from the liquid after depositing the solid content, temperature and pH of the slurry are respectively specified.

When salt and potassium salt are removed from a collected ash collected from an exhaust gas in a pulp manufacturing plant, at first, the collected ash of an electric static precipitator and water are fed to a first agitating bath 1 to make slurry. In this case, temperature of the slurry is kept at 20°C or above with a heat exchanger 4, and pH of the slurry is adjusted at 10 or above, by feeding diluted sulfuric acid 12 controlled with a pH adjustor 15. after keeping this state for 1-2h, the slurry is fed to a second agitating bath 6, water or ice is fed therein to keep temperature at 20°C or below, and crystallization of Na2SO4 is accelerated. Then, after separating a solid content in the slurry with a decanter 10, sludge is dissolved again in black liquor before concentration, and the solid content is recovered by returning it to a front courrent of a black liquor concentrator.