To turn freshly
cooked, brown-colored wood fiber stock into white pulp, the St. Helens mill uses
chemical bleaching. The bleach process uses two different processes, both of which
use sodium hydroxide and peroxide.
brown stock begins a journey through a series of washers that produce the white
paper pulp by the end.
Inside a washer the pulp,
in this example it is hardwood pulp, is constantly mixed and rinsed, removing
wood compounds that give wood its structure and color.
the end of the bleaching process, this hardwood pulp is bright white!
Before pulp is introduced to the paper machines,
it is subject to a series of cleaning and refining stages. This is done to improve
the strength and appearance of the paper. At right is a pulp cleaner machine.
agents are also added to the pulp to improve certain paper qualities. Sizing agents,
for example, improve the paper's ability to resist water penetration.
If colored paper is being produced, coloring agents can
also be added before the pulp flows to the paper