Chips from the storage bins are fed into a digester to which chemicals have been added. The woodchips are then 'cooked' to remove lignin. Lignin is the binding material which holds the cellulose fibres together.
The chips are 'cooked' by heat and pressure in caustic soda and sulphur.
The chemical process is energy self-sufficient as nearly all by-products can be used to fire the pulp mill power plant. The chemical pulping process produces lower fibre yield than mechanical pulping, typically 50-60%.