Trees provide the primary raw material for the paper and board industry. Wood is made from cellulose fibres that are bound together by a material called lignin. In a pulp mill, the fibres are separated from one another into a mass of individual fibres and after separation, the fibres are washed and screened to remove any remaining fibre bundles. The pulp may then be used directly to make unbleached paper, or be bleached for white paper. Pulp may be fed directly to a paper machine in an 'integrated paper mill' or dried and pressed into bales to be used as a raw material by paper mills worldwide. Chipping is not necessary when mechanical pulp is produced as it can be made of round wood.
The pulp-making process
In chemical pulping, the wood chips are cooked in a digester with chemicals. Cooking removes lignin, breaking up the wood into cellulose fibres. The process results in a slurry in which fibres are loose but intact and have maintained their strength.
Most chemical pulp is made by the alkaline kraft or sulphate process, which uses caustic soda and sodium sulphate to 'cook' the woodchips. In the unbleached stage, a dark brown but very strong pulp results and this can be bleached to a high brightness if required. The cooked pulp is then washed and screened to achieve a more uniform quality. The cooking chemicals are recovered through evaporation and burning plants and reused.
An alternative chemical method
The alternative method is the sulphite pulping process. This method is based on an acid-cooking liquor process, and is best suited for speciality pulp. The sulphite mills produce pulps that are easily bleached, generally with hydrogen peroxide. These pulps fulfil today's demand for 'chlorine-free' products in the hygiene paper sector and also in printing and writing papers.
The yield in both chemical processes is much lower than in the manufacture of mechanical pulp, as the lignin is completely dissolved and separated from the fibres. However, the lignin from the sulphate and some sulphite processes, can be burnt as a fuel oil substitute. In modern mills, recovery boiler operations and the controlled burning of bark and other residues makes the chemical pulp mill a net energy producer which can often supply power to the grid, or steam to local domestic heating plants
Woodpulp was first produced in the mid-19th century by grinding logs against a water-lubricated, rotating, stone-faced drum. The heat generated by grinding softens the lignin and the mechanised forces separate the fibres to form groundwood. This process is still used today, especially for newsprint. During the second half of the 20th century, newer mechanical techniques using 'refiners' were developed. In a refiner, woodchips are subjected to intensive shearing forces between a rotating steel disc and a fixed plate. In subsequent modifications to this process, the woodchips are pre-softened by heat (thermo-mechanical pulp - TMP) to make the fibrillation more effective, or by heat and a mild chemical treatment with sodium sulphite (chemi-thermo mechanical pulp CTMP). After grinding, the pulp is sorted by screening to suitable grades. It can then be bleached with peroxide for use in higher, value-added products.
Mechanical pulping provides a good yield from the pulpwood because it uses the whole of the log except for the bark, but the energy requirement for refining is high and can only be partly compensated by using the bark as fuel. The investment costs for mechanical pulp mills are relatively low in comparison with other types of pulp mills. The investment costs for mechanical pulp mills are relatively low in comparison with other types of pulp mills.
Mechanical pulp is well suited for 'bulk' grades of paper. Strong softwood fibres (from pine and spruce) are abundantly available in Europe and these fibres, as well as recovered fibres, are good in the production of publication papers (for example newsprint, super-calendered paper, lightweight coated paper). Softwood kraft pulp is also used as a component in fine paper production.
However, mechanical pulp has lower strength characteristics than softwood chemical pulps and, because it retains most of the lignin that reacts with ultra-violet light, can turn 'yellow' when exposed to bright light.
Finally the pulp may be bleached (in four to seven stages). Quality papers require a pulp which does not discolour during storage or go yellow when exposed to sunlight, and which retains its strength. Bleaching achieves all three requirements and has the additional advantage of improving absorption capacity, removing any small pieces of bark or wood left behind as well as giving a high level of purity.
In Europe, ECF-bleaching (elemental chlorine-free) and TCF-bleaching (total chlorine-free) methods are widely used for environmental reasons.
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.