There are two methods of sheet formation:
1/ Dipping method (for fine/thin paper). The pulp is diluted with water and put into a masonry trough or vat. The lifting mould (mesh on a wooden frame) is then dipped into the trough, shaken evenly and lifted out with the pulp on it. The consistency of the pulp in the tank should be kept constant.
2/ Lifting method (all paper and card). A fixed measure of the pulp is poured evenly onto a mould, which is clamped between two wooden deckles (frames) in a water tank and dipped. The mould is then raised, using a lever mechanism, to drain the excess water.
Couching. After the sheet formation is completed, the wet paper is transferred onto a cloth/felt sheet and a stack of interleaved sheets is built up.
Pressing. A hydraulic press is used to remove the excess water from the sheets. Pressing reduces the bulkiness of the paper i.e. the sheets become more compact. This improves the physical properties of the paper and facilitates drying.
Drying. After pressing, between 50 and 65% of moisture remains in the sheets. The sheets are dried by hanging them in open areas of sunlight to remove the rest of the moisture. Solar dryers can speed up this process and reduce the amount of space needed. Colored paper is sometimes dried in the shade to avoid the bleaching effect of the sun.
Cleaning and sizing. Small dirt particles and other foreign matter are removed manually with a sharp instrument. The cleaned sheets are given a coating with starch to improve the physical properties of the paper and prevent feathering. This is called sizing and can be done manually
with a brush or by the dipping method, where the sheets are immersed in a tub of sizing chemicals.
Calendering. The sheets are placed between metallic plates and passed through spring loaded rollers in a calendering machine. This smoothes the paper and enhances the gloss. Cutting. The sheets are neatly cut to the required size using a cutting machine.
Paper making equipment – Technical considerations
When considering purchasing or renting premises for setting up a paper-making facility, care should be taken to ensure that there are adequate services provided at the premises, or that these services can be easily accessed.
The paper industry consumes enormous quantities of water. In 1988 the average North
American paper mill used 72 cubic meters of water for every tonne of paper produced (m3/t). This figure can be much higher where water efficiency measures have not been introduced. Although efficiencies in water consumption are improving, there will always be a need for relatively large quantities of water during the paper making process. Most of this water is either reused or returned to the environment after treatment (see later in this section).
The next consideration is adequate power at the site. Papermaking is also an energy intensive industry, especially when the process is mechanized. This energy can comes from a variety of source such as electricity, wood, oil, gas or coal. This energy is used for providing much of the heat energy required. Other residues, such as bagasse or rice husks can also be used as fuel. In large-scale plants, reclamation and incineration of spent chemicals can also provide a significant amount of energy. An electrical supply of sufficient capacity is required in most circumstances for powering motors, pumps, lighting, etc.
This can be supplied from the mains where the mains is accessible, from a diesel generator set, or from a renewable energy source (such as hydro-power or wind power), where such power sources are available. Steam raising for the drying process can be carried out using a variety of technologies; oil-fired boiler, steam engine, combined heat and power plants. Careful costing of the available options can bring considerable savings. The energy demand for processing of reclaimed paper is much less that for virgin wood.
Effluent treatment and disposal is another topic which needs careful attention. The effluent from a paper mill can contain thousands of different chemical species, which, if discharged directly into the environment, would cause untold damage. In medium and large-scale plants specialized recovery equipment is used to reclaim chemicals for reuse or for incineration to provide energy. This is not cost effective in smaller plants and so some form of treatment and/or disposal is required.
Biological treatment plants, such as the anaerobic digester, are sometimes used to treat the effluent. This method has the added benefit of producing methane through digestion of the organic matter in the effluent, which can be used to provide as much as 30 % of the mills energy requirement. The remaining sludge can then be disposed of on the land.