Paper Making Process
Pulpwood normally arrives at the paper mill in the form of very thick sheets and recovered paper normally arrives in the form of large, compressed bales. Both these materials have to be broken down so that the individual fibres they contain are completely separated from each other. This process is performed in large vessels, known as ‘pulpers', where the raw materials are diluted with up to 100 times their weight of water and then subjected to violent mechanical action using steel rotor blades.
The resulting slurry (known as papermaking pulp) is then passed to holding tanks. During this preliminary stage, auxiliary chemicals and additives may be added. The auxiliary chemicals are usually combined with the fibrous raw materials at levels from below 1% to 2% and can be sizing agents, which reduce ink and water penetration, and process anti-foaming agents. Common additives consist of clay, chalk or titanium dioxide that are added to modify the optical properties of the paper and board or as a fibre substitute. The stock is then pumped through various types of mechanical cleaning equipment to the paper machine.
In the pulp and papermaking industry 95% of the water used is cleaned and reused on-site and all waste water is treated in accordance with European legislation and standards. Paper mills have state of the art wastewater treatment installations to the extent that in some cases water in lakes close to paper mills is now cleaner than it was in the past. Since 1994, the paper industry has reduced annual water consumption by over 14% per year.
On the paper machine, more water is added to produce a fibre suspension of as little as 1-to-10 parts fibre to 1000 parts water and the resulting mixture is passed into a headbox which squirts it through a thin, horizontal slit across the full machine width (typically 2 - 6 m) on to a moving, endless wire mesh.
The water is then removed on this wire section by a mixture of gravity and suction in a process known as sheet formation where the fibres start to spread and consolidate into a thin mat, which is almost recognisable as a layer of paper on top of the wire mesh.
This web of wet paper is then lifted from the wire mesh and squeezed between a series of presses where its water content is lowered to about 50%. It then passes around a series of cast-iron cylinders, heated to temperatures in excess of 100ºC, where drying takes place. Here the water content is lowered to between 5% and 8%, its final level. Throughout its passage from the wire mesh to the drying operation, the paper web is supported by various types of endless fabric belts moving at the same speed. After drying, some papers may also undergo surface treatments e.g. sizing and calendaring - a process consists of smoothing the surface of the paper by passing it between a series of rotating, polished, metal rollers to produce a glazed or glossy appearance. The paper is then wound into a reel.
