Diéter Kussani Umweltverlag There are other designs of paper machine but, in all cases, the basic principles are the same. Paper machines vary enormously in size and speed depending upon the type of product being made. For instance, a speciality filter paper might be manufactured in a machine only 1m wide and moving at less than 5km/h, whereas newsprint can be produced on machines of 10m width travelling at well over 100km.
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.
Increasingly, large volumes of used paper are recovered for recycling. Before the recovered paper can be used to manufacture new 'white' grades of paper, like the production of graphic papers, the printing inks have to be removed to increase the whiteness and purity. A chemical process using alkali and detergents is used. The recovered paper is first dissolved in water and separated from the non-fibre impurities. The fibres are then progressively cleaned in order to obtain the pulp and during this stage the ink is removed in a flotation process where air is blown into the solution. The ink adheres to bubbles of air and rises to the surface from where it is separated. After the ink is removed, the fibre may be bleached, usually with hydrogen peroxide.