Paper Machine Water Chemistry
Process water is the name given
to the back water on the paper machine, water that is reused within the
papermaking process.
The backwater is a mixture of
chemicals and fines that were not used the first time around (one pass
retention). The back water can have detrimental effect ts to the process if the
parameters of the water fall outside of specific ranges/ conditions. For example
if the conductivity of the back water became too low or outside of normal
operating conditions it would mean the retention on the wire is reduced.
Low conductivity (-18mV) shows
there is a high volume of anionic trash within the system which when mixes with
the cationic starch or polymer, hydrocol will bond with the trash rather than
the fibres leading to poor formation on the wire, pick outs and deposits
forming around the machinery and a reduction in run ability.
pH can be described as the single most important aspect of
wet end chemistry, this is because most/ all aspects of the chemistry relies on
in some aspect the pH.
When the pH of the water increases (becomes more alkaline), the surface charge of
the fibres also increases. This will affect the attraction of the retention
aids and other cat-ionic substances to the fibres. The most undesired effect
would be the substantial increase of bacteria in the system.
Another effect of high pH ius fiber swelling, fiber swelling is useful during refining because the higher surface area and increased flexability of the fiber leads to high de-fibrilation (versus cutting of the fiber). Caustic acid is added to create this effect.
fiber swelling is also used within de-inking plants. swelling of the fibers pre floatation allows the inks and binder to split and break off when the fiber swells this aids in the bleaching and deinking process leading to high brightness of the finished pulp.
pH can affect quite strongly the dissolving ability of wood
components and to changes in the dissolved substances. Increase in pH improves
wood components dissolving ability in the water system and thus the amount of
anionic particles dissolved and colloidal substances.
Because
of the undesirable fractions within water, the water needs to be cleaned at
some stage with different kind of methods like for example with disc-filtering and or chemicals. The wet-end of the paper machine contains the
highest amount of water and its chemistry has to be controlled by a variety of
chemicals like, retention chemicals, fixatives, de-foaming agents and biocides.
A decrease of the pH value leads to deposits on the machine
as the precipitation of non-wood materials increases. A fine balance has to be made; typically a pH
of 7 is achieved within the water loop.
Machine Issues caused by change in pH
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Increase in pH
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Decrease in pH
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Level of bacteria
in water increases
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Deposit
precipitation increases – deposits on the machines
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Higher amount of
anionic trash in the system
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Surface charge of
fibres increase
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Bugs/ Bacteria - Biocides
Paper machines run well when the operating conditions remain
constant. Water plays a large part in paper production and any slight
disturbances in the water can cause negative effects on the machine.
Bugs, bacteria thrive in water and damp conditions. Bacteria
build up within the water system will cause a variety of issues. For example;
Slime build up, lowering PH within the water (acidification) and upsetting the
chemical balance within the water loops.
