While waiting for the full transition to dissolving pulp, Fortress Specialty Cellulose (FSC) has re-started the Thurso mill as a northern bleached hardwood kraft (NBHK) mill, which was its previous role prior to the FSC purchase.
This is the last article in a three-part series explaining how the kraft process works.
Part 3: The Finished Product
After the pulp has been cooked and the chemicals have been recovered for the next pulping process, the sludge-like product (which is now called brown stock) moves on to the next process called blowing. To do this, the atmospheric pressure is reduced, which releases steam (which can also be captured to power the mill) and reduces some of the liquid contents of the pulp.
Once it has been dried out a bit, the brown stock is then screened on multi-tiered cascading sieves to eliminate any remaining shives, knots, dirt or debris. Often the rejected material can be reprocessed in the digester to maximize pulp output.
From the screens, the brown stock goes to the washing stage, and if you’ll recall, this is where the recovery process happens. During washing, the cooking liquors are separated from the cellulose fibers leaving chemical-free pulp.
In many mills, the pulp then undergoes a near-final step in a long line of complex processes. Depending on what the pulp will be used for, bleaching is the last stage. Here, the brightness of the wood is increased for products like paper, textiles, etc. However, if the pulp is going to be used to make things such as brown paper bags or linerboard for cardboard boxes, it doesn’t have to be bleached.
At the end of all these processes, the finished pulp is steam-dried, packaged in the form of sheets or bales, and sold to buyers.