Pulping is the production of a fibre suspension to use for paper-making. The raw material is pulped by chemical or mechanical means (or a combination of the two), which allows for the separation of individual fibres. Pulps for the manufacture of paper come from a variety of fibre sources, such as wood, cotton, hemp, recovered fibre etc., although wood is the main source. Bark is a contaminant of the paper-making process and must be removed before any other operation is carried out. The presence of bark causes dark specks in the final product, consumes pulping and bleaching chemicals and contains substances such as silica and calcium which can interfere with chemical recovery operations. Debarking operations are therefore necessary, with the removed bark generally used as a fuel.
There are four categories of pulping operation:
- Chemical (e.g. kraft, bisulphite, organosolv)
- Semi-chemical (e.g. neutral sulphite semi-chemical)
- Chemi-mechanical (e.g. hot sulphite, cold-soda, alkaline peroxide, chemi-thermomechanical)
- Mechanical (e.g. stone-ground wood, pressurised ground wood, refiner mechanical, thermo mechanical)
These are listed in order of the amount of energy required to separate the fibres (lowest energy at the top), and decreasing reliance on chemical reactions. The more that chemicals are used, the lower the ultimate yield and lignin content. Chemical pulping produces fibres that are not damaged and gives stronger papers. Yields from mechanical pulps are typically of the order of 92-96%, and from chemical processes (45-50%). Chemical pulping involves the impregnation of wood chips with a pulping chemical solution (liquor), and involves no mechanical processes (apart from digester blowing at the end of the cooking cycle). The chemicals in the liquor degrade the lignin and solubilise the fragments, allowing for fibre separation. Pulping takes place in large pressure vessels (digesters), which can operate in the batch or continuous mode. In a chemical pulping operation, it is desirable to remove as much lignin as possible (known as delignification selectivity), whilst retaining the holocellulose intact (holocellulose is cellulose plus hemicellulose). Acid sulphite pulping is more selective than kraft pulping, but the paper is weaker due to acid degradation of the cellulose. Also known as the sulphate process and is the most commonly used chemical pulping process. Use of paper from mechanical pulps is restricted to low cost, short life products, such as newspapers or magazines. Compared to the kraft process, the costs of constructing a mill are relatively low.
Contributor: Dr Callum Hill FIMMM