Preparation of the fibres in the refiner
The type of refining which takes place in the refiner has a decisive influence on the properties of the paper to be produced. A refiner is a refining aggregate with rotating and
stationary cutters, the so-called rotors and stators. The variable positioning of these rotors and stators in relation to each other determines whether the fibres are being cut (free stock refining) or fibrillated (wet refining). Fibrillating is a fine bleeding of the fibre ends, resulting in a close-knit connection between the individual fibres. In the final paper this, in turn, results in greater strength.
Additional raw materials for the base paper
Process materials include water, fillers, sizing substances, dyes and additives.
Fillers serve multiple purposes: they make the paper more opaque, more closed in its surface, brighter in shade as well as softer and more flexible depending on the requirement. Besides minerals, such as kaolin and china clay, the modern production process of paper makes extensive use of calcium
carbonate (chalk), which has the additional advantage of making the paper more resistant to ageing. The total percentage of fillers used can be as high as 30% of the stock. In industrial paper production, the respective quantities and density ratios are regulated by computer controlled proportioning systems. This is the only way to guarantee a uniform quality standard in the production of high-quality brand papers.
But by far the most important process material is water. For each kilo of paper approximately 100 litres of water are required. In our time, the only justification – economically as well as ecologically – for the use of such enormous quantitiesof water, is closed circulation and effective waste water treatment.
The Sappi paper mills have the highest expenditures for environmental protection, even when compared to the high national standards.
In the proportioning system water, stuffs and fillers are brought together in mix tubs. The so-called constant part of a paper machine constitutes the transition from pulp preparation to the headbox of the paper machine. Another element of this constant part is the sorting unit, where impurities, foreign substances and patches are removed.