CPC D21C 11/066 (2013.01) [B01D 9/005 (2013.01); B01D 9/0018 (2013.01); B01D 15/361 (2013.01); C01D 5/16 (2013.01); D21C 11/0007 (2013.01); D21C 11/0035 (2013.01)] | 17 Claims |
1. A method of treating fly ash containing sodium sulfate from a recovery boiler of a chemical pulp mill, wherein the fly ash is removed from a chemical cycle of the chemical pulp mill for adjusting the chemical balance, said method comprising:
a) dissolving the fly ash in a first aqueous solution, and adjusting the pH of the first aqueous solution with alkali to precipitate impurities from the first aqueous solution with the fly ash,
b) filtering the first aqueous solution with the fly ash to remove the precipitated impurities,
c) crystallizing sodium sulfate in the filtered first aqueous solution with the fly ash to form sodium sulfate crystals,
d) separating the sodium sulfate crystals from the filtered first aqueous solution with the fly ash by filtering or centrifugation,
e) dissolving the sodium sulfate crystals in a second aqueous solution to form dissolved sodium sulfate crystals,
f) decreasing with acid the pH of the second aqueous solution with the dissolved sodium sulfate crystals to precipitate calcium from the second aqueous solution, and
g) after step f), using the dissolved sodium sulfate crystals as an initial material for producing sodium and sulfur containing chemicals and/or as a process chemical.
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