Method for optimising material recovery in a chemical pulping process

A method for optimising material recovery in a chemical pulping process, the method comprising

• treating of cellulosic raw material with a liquid phase comprising cooking chemicals in a cooking step, where lignin and optionally other substances are at least partly dissolved into cooking liquid phase and separated from fibres,
• separating fibres from the said cooking liquid phase, which comprises used cooking chemicals and dissolved substances from wood, such as lignin,
• transferring fibres to a bleaching step, where the fibres are brought into contact with bleaching chemical(s), and separating the bleached fibres from the bleaching liquid phase,
• separating lignin from the cooking liquid phase,
• recovering the cooking chemicals from the cooking liquid phase in a chemical recovery step and circulating the recovered cooking chemicals back to the cooking step,
• characterised in
• using separated lignin for formation of a treatment composition comprising at the most 1 weight-% of aluminium, calculated on basis of the weight of dry lignin in the composition,
• treating the bleaching liquid phase with the treatment composition and removing of organic humic substances, preferably recalcitrant organic substances, from the bleaching liquid and forming organic sludge,
• using the organic sludge for energy production in the chemical recovery step.