A method and a system for adjusting s/na -balance of a pulp mill

A method for adjusting S/Na-balance of a sulphate pulp mill, the method comprising:

• providing a pulp mill CNCG stream containing sulphur compounds into a scrubber,
• scrubbing the pulp mill CNCG stream containing sulphur compounds in the scrubber with an aqueous scrubbing solution containing an alkaline agent, whereby at least some of the sulphur compounds react with the alkaline agent, thereby producing a CNCG stream derivate and an aqueous spent scrubbing solution containing sulphides,
• introducing the aqueous spent scrubbing solution containing sulphides into a bioreactor located downstream of the scrubber,
• oxidizing the aqueous spent scrubbing solution containing sulphides in the bioreactor biologically in an oxidizing reaction by means of sulphur-oxidizing microbes, thereby obtaining an aqueous suspension containing elemental sulphur, and
• separating the elemental sulphur from the aqueous suspension in a sulphur separation unit located downstream of the bioreactor, thereby obtaining a residual solution and a precipitate containing the elemental sulphur.

A system configured to adjust S/Na-balance of a sulphate pulp mill, the system comprising:

• one or more conducts configured to feed pulp mill CNCG stream containing sulphur compounds into a scrubber,
• the scrubber configured to separate sulphur compounds from the pulp mill CNCG stream with an aqueous scrubbing solution containing an alkaline agent, the scrubber thereby configured to produce a CNCG stream derivate and an aqueous spent scrubbing solution containing sulphides,
• a bioreactor located downstream of the scrubber, the bioreactor configured to oxidize the aqueous spent scrubbing solution with sulphur-oxidizing microbes, the bioreactor thereby configured to produce an aqueous suspension containing elemental sulphur, and
• a sulphur separation unit located downstream of the bioreactor, the sulphur separation unit configured to produce a residual solution and a precipitate containing the elemental sulphur from the aqueous suspension