METHOD OF TREATING FLY ASH OF A RECOVERY BOILER

A method of treating fly ash containing sodium sulfate from a recovery boiler of a chemical pulp mill. This method includes at least the following steps: a) ash is dissolved in an aqueous solution and the pH of the solution is adjusted with alkali for precipitating impurities， b) the solution is filtered for removing the impurities containing precipitate， c) sodium sulfate is crystallized from the solution and the crystals are separated from the solution by filtering or by centrifugation， and d) the crystallized sodium sulfate is used as initial material for producing sodium and sulfur containing chemicals or as process chemical.