Though Fortress Paper Ltd. originally purchased the pulp mill in Thurso, Quebec for the purpose of creating specialty cellulose, a number of transitions have to take place before the mill can start churning out the product that will eventually be used to make commodities like rayon clothing and cellophane.
While waiting for these transitions, Fortress Specialty Cellulose (FSC) has re-started the mill as a northern bleached hardwood kraft (NBHK) mill, which was its previous role prior to the FSC purchase.
This article is the first in a series detailed the kraft process.
Part 1: Cooking
Invented in the late 19th century by a German scientist by the name of Carl F. Dahl, the kraft process (from the German for “strength” or “power”) is one of the most popular ways of producing wood pulp.
The process starts with wood chips. The chips are first saturated with a mixture of chemicals and liquids – known in the industry as “cooking liquors.” There are two types of liquors – black liquor and white liquor – the black created in the boiling process, the white a chemical concoction used to dissolve the wood.
Once the chips are saturated, they are then “cooked” in a machine called a digester, and boiled for several hours at a temperature of 130 to 180 °C (266 to 356 °F). Under these high temperatures, the wood begins to break down to a hemicellulose core, producing a sludge-like substance called solid pulp.
The solid pulp – called brown stock because of its colour – is collected and washed and moves on to a whole new process called “the recovery process”