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 article is the second in a series explaining how the kraft process works.
Part 2: Recovery
Once the pulp has been cooked, the next step is to wash it and dry it in order to ship it out to buyers. However, during the washing process (which we will read about in the next part of this series) manufacturers of kraft pulp go through a very important chemical process called recovery.
Invented some 50 years after the kraft process, the recovery boiler became a revolutionizing step in the pulping industry. Recovery allows the chemicals and “liquors” used in the cooking process to be reused in future cooking processes, therefore making the kraft pulp more affordable and efficient than other pulping processes such as the sulfite process.
As you may have read in the first part of this series, there are two types of cooking liquors used in the kraft process – black liquor and white liquor. The black liquor is runoff created when the pulp is being cooked and is a combination of lignin residues, hemicellulose, and the inorganic chemicals used in the cooking process. White liquor consists mainly of chemicals such as sodium hydroxide and sodium sulfite, and is an active ingredient in dissolving the wood chips.
In the recovery process, the two liquors are separated in the recovery boiler. In this boiler, the white liquor is retained for use in the next pulping process and the black liquor is burned off which generates heat, electricity and/or steam for the pulp mill.
As mentioned, the recovery process ensures that several elements for pulping are re-used for future pulping, thus making it not only a viable process, but a near necessary process for any pulp mill.