Pulping process
Pulping of wood can be done in
two ways: mechanically or chemically.
Mechanical pulp
In the case of mechanical pulp,
the wood is processed into fibre form by grinding it against a quickly rotating
stone under addition of water. The yield* of this pulp amounts to approx. 95%.
The result is called wood pulp or MP – mechanical pulp.
The disadvantage of this type
of pulp is that the fibre is strongly damaged and that there are all sorts of
impurities in the pulp mass. Mechanical wood pulp yields a high opacity, but it
is not very strong. It has a yellowish colour and low light resistance.
Chemical pulp
For the production of wood
pulp, the pure fibre has to be set free, which means that the lignin has to be
removed as well. To achieve this, the wood chips are cooked in a chemical solution.
In case of wood pulp obtained
by means of chemical pulping, we differentiate between sulphate and sulphite pulp,
depending on the chemicals used. The yield of chemical pulping amounts to approximately
50%. The fibres in the resulting pulp are very clean
and undamaged. The wood pulp produced by this process is called woodfree. It is
this type of pulp which is used for all Sappi fine papers.
The sulphate process is
an alkaline process. It allows for the processing of strongly resinous wood
types, but this requires expensive installations and intensive use of
chemicals.
The sulphite process utilises
a cooking acid consisting of a combination of free sulphur acid and sulphur
acid bound as magnesium bi-sulphite (magnesium bi-sulphite process).
In the sulphite process, the
cooking liquid penetrates the wood in the longitudinal direction of the fibres,
which are aligned in this same longitudinal direction in the chips. When the
cooking liquid penetrates the wood, it decomposes the lignin, which, during the
actual cooking process, is converted into a water-soluble substance that can be
washed out. The decomposition products of the carbohydrates are included in the
cooking liquid as sugar.
When the waste fluids are
concentrated in order to recycle the chemicals, these sugars are processed to
alcohol and ethanoic acid. In this stage, the sulphite pulp is slightly brown
and therefore has to be bleached to obtain a base colour suitable for white papers.
This bleaching process, in which no chlorine or chlorine compounds are used,
also takes place in the pulp mill as an integrated part of the overall operation.
The strength of sulphite pulps
is less than that of sulphate pulps. Sappi uses only the magnesium bi-sulphite
process in its own pulp mills.
Pulp
bleaching
Initially,
wood pulp has a brown or brownish colour. To obtain the brightness required for
white papers, it has to be bleached. During this process of bleaching, the
remaining lignin is removed as well. In practical terms, bleaching is a
continuation of the chemical cooking process, taking place directly afterward in
the pulp mill as an integrated next step of the overall procedure. Bleaching is
a complex process, consisting of several chemical process steps, with washing
taking place between the various chemical treatments.
The wood
pulp can be bleached with chlorine / chlorine compounds, ozone / oxygen in
different forms as well as hydrogen peroxide.
Based on
the negative impact of some chlorine containing decomposition products, there
are, however, environmental objections against the use of chlorine and chlorine
products.
For this
reason, Sappi has long ago switched to chlorinefree processes.
These
processes are referred to as Totally Chlorine Free (TCF).
From the
unbleached to the bleached pulp
Source : The Paper Making Process. SAPPI (South African Pulp and Paper Industries)
