Lignosulfonate of a certain quality and method of preparation of lignosulfonate of a certain quality

(canceled)

8-10. (canceled)

A lignosulfonate-containing product comprising less than 4 wt. % sulfate (SO42−) based on dry material obtained by a process comprising:

• one step in which a lignosulfonate-containing fraction at least partly derived from spent sulfite cooking liquor is subjected to filtration using a membrane having a cut-off of 40-150 kD (high cut-off) at the pressure, flow rate and temperature of the step and the permeate is recovered;
• and
• another step in which the permeate is subjected to filtration using a membrane having a cut-off of 1-20 kD (low cut-off) at the pressure, flow and temperature of the step and the retentate is recovered.

A lignosulfonate-containing product derived from spent sulfite cooking liquor comprising less than 4 wt. % sulfate (SO42−) based on dry material obtained, wherein less than 4 wt. % of the lignosulfonate molecules in the product have a molecular weight of more than 60 kD.

(canceled)

(canceled)

A system for improving the quality of spent sulfite cooking liquor comprising at least two membrane filtration units, wherein one of the membrane filtration units comprises at least one membrane designed to have a cut-off of 40-150 kD (high cut-off) during operation and another one of the membrane filtration units comprises at least one membrane designed to have a cut-off of 1-20 kD (low cut-off) during operation, said membrane filtration units being connected to a digester of a sulfite pulping plant such that spent sulfite cooking liquor from the digester can be routed to the membrane filtration units.