Paper Machine Water Chemistry - Technical Papermaking

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 of process water

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
Increase in pH	Decrease in pH
Level of bacteria in water increases	Deposit precipitation increases – deposits on the machines
Higher amount of anionic trash in the system
Surface charge of fibres increase

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.

Biocides are used to control the bacterial problem. The main goal of the biocides is to limit the growth of sessile bacteria, i.e. those that are attached to surfaces. These are the bacteria that tend to build up, cause slime deposits and holes, hurt productivity, and hurt product quality.