Digesting. First, the chips are “cooked” in a “digester” to free up the cellulose fibers. The wood chip digester at the mill works in batch mode. Chips are mixed with an aqueous solution of sodium hydroxide (NaOH) and sodium sulfate (Na2S) called “white liquor” and heated with steam, under pressure, at 340-350oF and a pH of 12-14. The chemical solution dissolves the lignin in the wood that holds the cellulose fibers together. The end result, after cooking for a few hours, is a slurry of freed cellulose fibers (pulp) in a solution of lignin, sodium carbonate (Na2CO3), sodium sulfate (Na2SO4), carbohydrates and miscellaneous inorganics. This slurry is called “brown stock”. The organic components of brown stock account for about half of the weight of the original wood chips, and is eventually burned as fuel in the recovery furnace.
Most of the brown stock is diverted at this point to the sawdust pulping plant, where it is washed together with the brown stock emerging from the continuous digester. The rest is used here to produce pulp bales for market.
Washing. Pulp fibers are washed clean of the cooking slurry in a multi-stage, countercurrent washer. Fresh water is used to rinse the cleanest stage of pulp, then that used water enters the next rinsing stage, until finally the dirtiest water is used for the brown stock coming straight out of the digester. At that point the spent wash water, full of lignin and cooking chemicals, is sent to the power plant’s recovery furnace as dilute “black liquor”.
It is important to wash the pulp fibers well, for the sake of both pulp quality and chemical recovery. But the wash water serves to dilute the black liquor greatly. Typically the black liquor leaving the washer is about ten times the weight of the pulp fibers produced. This creates quite a load for the evaporators in the power plant.
Finishing. After washing, the pulp is screened to remove oversized debris, formed into a continuous sheet, pressed and heated to remove as much water as possible, reeled, cut into bales and packaged for market. The process of drying the pulp is one of the ways that the mill uses process steam from the boilers.