History
Almost 2000 years ago the Chinese manufactured paper from the bark of the mulberry tree. For several centuries the process was kept secret, but eventually it slipped out via Arabian traders to Europe. By the mid 1600s paper was being made there although it was very labour-intensive and was largely based upon the use of straw, and linen rags. These materials, when broken down into a fine slurry in water, were settled out in thin layers. When the water was removed, the sheet of fibres was pressed flat and dried into a rough sheet of paper.
About 1799 two English brothers, the Fourdriniers, met an inventor, Roberts, who had designed a machine able to produce continuous/endless sheets of paper on a revolving wire mesh belt. They proceeded to develop the concept, and eventually the Fourdrinier's machine came to revolutionise paper manufacture, lifting production rates and reducing costs dramatically.
The other major step forward was the development of techniques to manufacture pulp for paper making from wood, involving various combinations of heat and mechanical and chemical breakdown of wood into its component fibres.
Within a relatively short period, wood became, as it is now, the major raw material used in the manufacture of paper. Other materials such as cotton, linen, straw, bamboo, esparto grass and hemp also produce suitable fibre.
Pulp is the product of the mechanical or chemical breakdown of fibrous cellulose materials into fibres. When mixed with water the mass of fibres can be spread as thin layers of matted strands. When the water is removed the layer of fibres remaining is essentially paper, although other materials may be added to give the paper a better surface for printing, greater density or extra strength as is the case for cardboard used in packaging, etc.
Production of pulp and paper may occur on the same site (as at Kinleith, near Tokoroa and at Kawerau, in New Zealand) in plants that are known as integrated mills. Or factories may be 'dedicated' to either pulp or paper manufacture, using raw materials delivered to the site (both the Pan Pac Mill near Napier and the WPI Mill near Ohakune are dedicated pulp mills).
Chemical Pulps
The main purpose of chemical pulping is to remove lignin and other materials binding individual cells together, and so make fibres directly available for papermaking. In the process, fibres are less likely to be damaged than in other pulping processes.
In the past, chemical pulping was done in large batch digesters (tanks that carried a single charge at a time) which held 40-50 tonnes of wood chips. The chips were mixed with the appropriate chemicals and cooked at around 150oC and about 1.0 MPa pressure. The resulting pulp slurry and cooking liquor were then separated and the delignified pulp further defibred, washed and screened for use.
Advances on this technique include the development of continuous digesters, where fresh chips and cooking liquor are regularly added to the batch, producing 1000-1500 tonnes of pulp per day. This process permits good control as rates of chemical and fresh chip addition are carefully managed and cooking liquor temperature is also controlled by circulation into and out of the digester via heat exchangers.
The digesting liquid, which at the end of the process contains lignin, polysaccharides and other sugars, resin acids and so on, is usually recycled to recover as much of the digestion chemical as possible.
Other chemicals are recovered during the recovery process, including turpentine, obtained by flashing steam-bearing volatile chemicals from the digester and condensing them. Tall oil is recovered from the condensed black cooking liquor, and the soap skimmed from the surface of the cooking liquor contains other resin acids. The success of all recovery operations is dependent on the scale of the operation as the economics of extraction are often quite marginal.
Chemical pulping requires significant quantities of energy, mostly for process heat but uses less electrical energy than mechanical processes. However, many modern kraft pulp mills are totally self-sufficient in energy, with combustion of residues and waste products meeting all heat and electrical energy needs.
Soda Pulp
Soda pulp is the original chemical pulp and is produced by cooking chips of (usually) deciduous woods in a solution of caustic soda under pressure. This leaves a relatively pure cellulose pulp which is then washed and bleached. Soda pulp produces relatively soft, bulky papers (as a filler with other pulps) used in books, magazines and envelopes. Caustic soda dissolves most of the lignin in wood while having little effect on the cellulose. Cooking liquor is recovered during the washing process.
Sulphite Pulp
This is often derived from less resinous softwood chips, cooked in magnesium, calcium or ammonium bisulphite with excess sulphur dioxide present. The process yields pulps with relatively high cellulose content and good bleaching properties. This process (often also referred to as the 'acid-sulphite process') involves high-acid cooking liquor and a substantial chemical plant and recovery process. The pulp produced is made up of longer, stronger and more pliable fibres and is favoured where strength properties are particularly important.
Sulphate (kraft) Pulp
This process is an advance on the soda process, where chips are cooked in a mix of more or less equal parts of caustic soda and sodium sulphide. The 'sulphate' name relates to the fact that chemical losses in the process are made up by adding sodium sulphate (sulphur) which is reduced to sulphide in the recovery process.
The kraft process is applicable to almost any wood and produces a pulp with strong fibres, but which also takes more bleaching that other chemical pulps. It is suitable for even quite resinous pine species.
Kraft pulp is used where strength, wear and tear resistance and colour are less important. the most obvious examples are brown paper bags, cement sacks and similar sorts of wrapping paper.
Semi-Chemical Pulps
Semi-chemical pulps are essentially mechanical pulps that have been pre-treated with a sulphite liquor to improve breakdown and reduce energy requirements during processing. Pulps tend to retain some of the properties of mechanical pulp, including good yields of fibre, but are also suitable for better classes of paper manufacture.
Mechanical Pulps
This type of pulp is made by grinding logs (often called 'billets' or 'bolts') against a revolving abrasive stone. The process uses large quantities of electrical energy and is mostly used with low density softwood species, although some of the softer hardwoods are also processed in this way.
The pattern on the grinding face of the stone and density and structure of the wood being processed have a large bearing on the amount of energy consumed and also the quality of the pulp mass produced.
Mechanically produced pulp has a higher proportion of broken cell fragments (called 'fines') among the fibres. Thus, when used to make paper, the long fibres form the matrix of the sheet within which the fines are trapped. Paper derived from mechanical pulps, therefore, tend to be denser and is often a component of newsprint and other printing papers.
However, because mechanical pulps are not chemically processed they still contain lignin and other natural wood substances, and paper with a high component of mechanical pulp tends to yellow quickly in sunlight. Mechanical pulps have been improved over the years by altering or adding extra steps to the pulping process.
Stone Groundwood
Is produced by grinding short billets of softwood against a roughened stone. Grinding produces heat, softening the fibres at the surface being ground and helping them to break free. Many are broken away and the pulp is, therefore, mostly blended with other (usually chemical) pulps where unbroken, longer fibres predominate. These form the matrix around which the packaging of shorter fibres and fines fits, giving both the strength and density needed for high speed printing presses, etc.
Pressurised Groundwood
Involves grinding at higher pressures, creating increased temperatures and improving pulp quality while reducing energy consumption. This pulp is blended with other pulps where good printing paper is needed.
Refiner Mechanical Pulp
Is a two-stage process involving the high speed grinding and refining of chips, and further refining of the fibre bundles created during the first pass. Refiner pulps are significantly better in terms of their paper making qualities.
Thermomechanical Pulp
Is the most recent replacement for refiner mechanical pulp, involving pre-steaming of chips for a short period before they are refined in a pressurised container. Further improvements come from treating steamed chips with sodium sulphate chemical - chemi-thermomechanical pulping. Like other chemical pulps, fibre recovery is reduced and pulp has lower opacity, but fibre quality is higher and may be used as a substitute for chemical pulps in the best situations. Bleaching is also far easier with pulps produced in this manner.
Mechanical pulping processes all use a lot of electrical energy and water. However, they also provide 80-90% recovery of total fibre. Mechanical pulp processes are cheaper to operate than more sophisticated chemical based systems. There are also fewer environmental issues, such as chemical contamination of sites and unpleasant smells.