PAPER AND BOARD

Raw Materials
Papermaking
Paper Finishing Operations

THE PRODUCTION PROCESS

Raw Materials

As can be seen from the above diagram, there are two main fibrous raw materials used in papermaking. These are woodpulp (chemical and mechanical) and recovered paper. In addition, a quantity of additives (mainly natural mineral fillers) and dyestuffs are used together with traces of auxiliary chemicals. A further raw material is water, which is used in large quantities during the papermaking process but is then recovered and reused, or returned to the watercourse from which it is extracted.

Woodpulp 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 stock) 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.

Papermaking

THE PAPER PRODUCTION LINE

Diéter Kussani Umweltverlag

On the paper machine, yet further 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 head-box 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 on various types of endless fabric belts moving at the same speed. After drying, some papers may also undergo surface treatments e.g. sizing and calendering. The latter process consists of smoothing the surface of the paper by passing it between a series of rotating, polished, metal rollers. It is then wound into a reel.

Paper Finishing Operations

The reels from the paper machine are passed into a separate area where they are subjected to further operations. These may be either simple processes where the reel is slit into a number of more narrow reels or cut into sheets. In some cases, more complicated processes may be performed such as coating (often consisting of the application of clay-based materials for special printing finishes) or more calendaring may be performed. The final reels or sheets are then wrapped and despatched to other companies which carry out converting and printing operations.