This is the part in the kraft pulp making process where the wood chips are cooked with chemicals (white liquor), heat and pressure to break them down into wood fibre. The chips are put into our downflow low-solids (DFLS) digester, a 20-storey high pressure cooker, where they remain for a period of six hours. The key to making this process so efficient is the extraction of spent cooking liquor and the addition of new cooking chemicals to the chips at multiple locations in the digester. This produces pulp with a very low concentration of lignin (the glue-like substance that holds the wood fibres together) but high fibre strength.
After the wood chips are digested into fibres, this ‘pulp’ is washed in the atmospheric diffusion washer where remaining black liquor and pulp is separated. The black liquor is sent to Chemical Recovery and Utilities (CRU) and the pulp is sent to be rinsed and whitened.
The pulp passes through four washing stages and is screened to remove contaminants. The screening process is followed by two rinsing stages to remove residual cooking chemical and dissolved organic materials. Next, oxygen delignification reduces the amount of bleaching chemicals required and enables Alberta-Pacific to produce a high, bright pulp in a cost effective and environmentally friendly manner. Once the pulp goes through these two stages of rinsing and oxygen delignification, the pulp is whitening using Elemental Chlorine Free (ECF) bleach before it is pumped into storage tanks and transferred to the pulp finishing area.