Paper derives from the word “papyrus”. Today, paper includes a wide range of products with very different applications: communication, cultural, educational, artistic, hygienic, and sanitary as well as storage and transport of all kinds of goods. It’s almost impossible to imagine a life without paper.
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. After separation, the fibres are washed and screened to remove any remaining fibre bundles. The pulp may then be used directly to make unbleached papers, or bleached for white papers. 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.
Pulping, the wood chips are cooked in a digester with chemicals. Cooking removes lignin, breaking up the wood into fibres. The process results in slurry where fibres are loose but intact and have maintained their strength.
Generally, chemical pulp is based on one of two processes: sulphate or sulphite.
Most chemical pulp is made by the alkaline kraft or sulphate process which uses caustic soda and sodium sulphate to “cook” the wood chips. In the unbleached stage, a dark brown, but very strong pulp results which can be bleached to a high brightness if required. The cooking chemicals are recovered back to the process through evaporation and burning plants. Cooked pulp is washed and screened to achieve more uniform quality.
Finally the pulp is bleached (in 4-7 stages). Quality papers require a pulp which does not discolor 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.
Paper and board production involves two steps. First, the fibres need to be produced. This is done in a pulp mill where pulp is produced using chemical or mechanical processes. Pulp production can be integrated with paper production, or produced in a separate pulp mill. The paper itself is then produced on a paper machine from a mixture of fibres (which can be virgin or recovered fibres), chemicals and additives.
Each paper or board grade is produced on equipment tailored for this particular grade and mill. Tissue machines producing toilet or napkin paper, or paperboard production differ considerably from the newsprint production process and newsprint paper machine. Production processes are optimized for each grade. There are many variables: raw material composition (mixture of chemical softwood and hardwood pulp, mechanical pulp, recovered paper, fillers, pigments, additives, etc.), machine size (width, speed), type of production equipment, and automation level.