Method for purifying sodium sulfate residue

A method is provided for purifying a sodium sulfate residue comprising sodium sulfate, sodium carbonate, and sodium chloride, and optionally insoluble substances. The method comprises: (a) dissolving the residue with a first aqueous solution to obtain a first aqueous suspension comprising at least 5 wt% and at most 32 wt% sodium sulfate; (b) optionally separating the insoluble substance from the first aqueous suspension to obtain separated insoluble substance and a second aqueous solution; (c1) acidifying the second aqueous solution with sulfuric acid to a pH of less than 6 in order to convert at least a portion or all of the dissolved sodium carbonate to sodium sulfate; (c2) optionally aerating the acidified second aqueous solution with oxygen or air to convert the sulfite to sulfate; (c3) alkalifying the acidified second aqueous solution with a base to a pH of about 7 or greater; (d) removing a portion of the water from the second aqueous solution to obtain a mother liquor and crystallizing the sodium sulfa