When Fortress Specialty Cellulose (FSC) re-started the mill at the site in Thurso, Quebec, they couldn’t just jump into the dissolving pulp game. Since deriving dissolving pulp (or specialty cellulose) entails different pulping processes, equipment upgrades, and general production line changes had to be implemented first.
In the meantime, FSC has restarted the mill as a northern bleached hardwood kraft (NBHK) pulp mill, resuming operations as they were prior to the company’s purchase of the mill back in March.
The upgrades are expected to be complete within the next year, and the mill should begin production of dissolving cellulose by mid 2011.
So what is the difference between NBHK pulp and dissolving pulp?
Much of the initial pulping process remains the same. Wood chips are mixed with “cooking liquors” – black (old pulping runoff) and white (chemical compositions) – and boiled down to a sludge-like substance (brown stock).
For both kinds of pulp, this process doesn’t change.
However, whereas NBHK pulp moves on to the screening, washing and drying phase immediately after they are cooked, “regular” pulp must go through an additional process before becoming dissolved pulped. This process is called the sulfite process.
By using various salts of sulfurous acid, the sulfite process helps extract lignin – a complex chemical compound found in the cell walls of plants – from the wood pulp. Essentially, this means that the chemicals remove hemicellulose, a weaker polysaccharide inside the biological composition of individual pulp cells and ensure that the pulp has a higher cellulose content.
The fact that the pulp has a higher cellulose content means it is not only a biologically stronger pulp, but it also means that it is an entirely different product from NBHK pulp. We can see this in the products made from both kinds of pulp.
While NBHK pulp is typically distributed to make paper and paper products, dissolving pulp can be used to make stronger, more durable substances whether they be near-plastic like products such as film and cellophane, or textile products like rayon.