1) The wood
The harvested logs are delivered to the pulp and paper mill, cut according to their intended use and their bark is removed in a debarking drum.
For the production of pulp, the debarked wood is cut into small chips. The so-called wood chips are stacked in piles. The storage process causes an oxidation of the resin, which thus becomes innocuous already prior to the beginning of the cooking process.
The production process begins with conveying the wood chips into the boiler. Depending on the procedure applied, the cooking with alkaline or acid solution lasts between two and five hours at 160 to 180°C and at a pressure of eight to nine bar. The alkaline process generates sulphate pulp (kraft pulp), the acidic process leads to the production of sulphite pulp.
The aim of cooking, also called digestion process, is to release lignin (delignification), which causes inter alia yellowing of paper, and thus to separate the cellulose fibres. Also resins and hemi-celluloses are partly released. When cooking the wood in acid or alkaline bath, it is important to stop the digestion process in time, as overcooking could damage the cellulose fibres. Lignin is the lignifying substance of wood and, if not removed, does not only cause yellowing, but also an early decomposition of the paper.
The sulphite pulp process uses magnesium bisulphite as pulping liquor (cooking acid), while for the sulphate process sodium hydroxide and sodium sulphate are used. Compared to sulphite pulp, sulphate pulp fibres show a higher tensile strength. Sulphate pulp is mainly used to produce soft hygienic paper, while sulphate pulp is used for the production of super white writing and printing paper.
After the digestion process, the pulp and the cooking liquor (waste liquor) need to be separated. In addition to the chemicals, the waste liquor contains the separated lignin. During the digestion process, extremely odorous sulphur compounds develop.
Today the sulphate process is the most commonly used pulping process. It is suited for most types of wood, even for pine, which is rich in resins. The yield rate for pulp production is around 50%.
3) The brown pulp
The finished pulp is then pumped to the bleachery for bleaching or dried in a drainage machine and cut as unbleached pulp (brown) in a sheet cutter and baled for shipment.
After cooking and removal of the waste liquor, the pulp is washed and dirt and low quality material are removed during sorting.
In order to produce white paper, it is necessary for the still brown pulp to undergo a bleaching process, which aims at removing residual lignin. During bleaching the chemical digestion that has been initiated during cooking is continued by using more selective chemicals. Bleaching processes are either elementary chlorine free (ECF) or totally chlorine free (TCF).
The ECF process uses chlorine dioxide, liquid oxygen and hydrogen peroxide as bleaching agents. For TCF bleaching, oxygen, hydrogen peroxide and potentially also ozone are used to reach maximum brightness. There are, however, limits to bleaching: The higher the degree of brightness, the lower the strengths of the paper.
5) White pulp and shipment
In general, the paper quality is determined by the fibre length, the digesting process (cooking) and the various types of refining.
The pulp is either transferred to the refining unit if there is an integrated paper mill (pulp and paper production) or is conveyed to a drainage machine, dehydrated and dried (90% dry content), cut into pulp sheets and baled for sale.