While the raw material affects significantly the properties of the end-product, the process for both hardwood and softwood is nearly identical, the only difference being the structure of the wood. Softwood consists mostly of cellulose and lignin, and it contains less hemicellulose than hardwood. Softwood fibres are longer than hardwood fibres and thus softwood pulp is called long fibre pulp, whereas hardwood pulp is short fibre pulp. Softwood's long fibres contribute to strength of the material it is used for.
Cooking the wood chips in the presence of sodium hydroxide and sulfide liquor under high pressure removes the lignin and separates the wood into cellulose fibres. During the cooking process, approximately half of the wood dissolves.
The pulp is then washed, screened for quality and bleached.
The spent cooking chemicals and dissolved wood material is called black liquor. This substance is recovered and burned in a recovery boiler to produce energy that keeps the process running. It is renewable, wood-based, pure energy - perfect for replacing fossil fuels. In causticising, even the cooking chemicals are processed for re-use.