Step 1 - Wood preparation
The bark is removed from in-coming logs, and these are then chipped. Sometimes, the
wood arrives at the plant already chipped, meaning that this step is unnecessary.
Step 2 - Cooking
The wood chips are heated in a solution of NaOH and Na2S in a pressure cooker, during
which time a lot of the lignin (the reinforcing susbstance that make tree cells wood hard
and 'woody' rather than soft like those of other plants) is removed from the wood. The
pressure is then released suddenly, causing the chips to fly apart into fibres.
Step 3 - Pulp washing
The pulp is washed with water to wash out the cooking chemicals and lignin from the fibre
so that they will not interfere with later process steps.
Step 4 - Pulp screening
A sieve is used to remove knots and clumped-together uncooked fibres from the pulp.
Step 5 - Bleaching
This is done in two stages. Firstly the pulp is treated with NaOH in the presence of O2.
The NaOH removes hydrogen ions from the lignin and then the O2 breaks down the
polymer. Then, the pulp is treated with ClO2 then a mixture of NaOH, O2 and peroxide
and finally with ClO2 again to remove the remaining lignin.
Step 6 - Paper making
The fibres are mechanincally treated to make them bond better to each other
(strengthening the paper), chemicals added to provide special properties such as colour or
water resistance, and then the water is squeezed out and the pulp is rolled smooth and
dried.
Various ancilliary processes result in the recovery of CaO, NaOH and Na2S, the major
chemicals used in the process. Various utilities ensure that such conditions as sufficient
reaction times and adequate mixing are met.
On site processing removes the lignin from the liquid wastes, and solid wastes are
generally taken to a landfill. Efforts continue to be made to reduce water consumption by
recycling, as smaller volumes are easier to process. The most obvious environmental
problem continues to be the sulfurous emissions that give Kraft pulping plants their
characteristic smell. These are decreased by gas incineration, but are not able to be wholly
eliminated.