A Pulp Mill is a manufacturing facility that converts wood chips into a thick fibre board which can be shipped to a Paper Mill for further processing. Pulp can be manufactured using thoroughly mechanical methods, mechanical and semi-chemical methods, or fully chemical methods such as the Kraft process. The finished product may be either bleached or non-bleached depending on the customer requirements.

The process begins at the chip pile, where planer wood chips and sawmill chips are stored for one to two months for 'seasoning'. During the storage process, the chips are piled sufficiently high enough to produce heat, caused by their mass. The heat drives off some volatile materials found in their make-up in gaseous form, and the chips begin a slight decomposition.

The chips are brought into the screening section of the mill, where they are sorted and screened of sawdust. The oversize chips are rechipped or used as fuel. Once screened, the chips make their way to the Digester where they are mixed into the large vessel with sodium hydroxide and sodium sulphite liquids and heated with steam from the mill boiler.

After several hours in the digester, the chips break down into a thick porridge-like consistency and the chips are squeezed from the outlet of the digester through an airlock. The sudden change in pressure causes the chips to expand in a popcorn-like fashion, thereby separating the wood fibres further.

The wood fibres are washed clean of all chemical residue, which is recovered and recycled in the plant. The fibre, now known as 'stock' can be bleached or left unbleached. The stock is sprayed onto the pulp machine wire and the water is drained and the pulp is pressed prior to passing into the machine-room drier. The fibres at this point have been allowed to reorient themselves into non linear patterns. The pressing and drying of these fibres in this state creates a strong linking bond between them not unlike a mesh. The dried pulp is cut, stacked and bailed at the layboy, on the drier outlet.

The pulp may then be loaded into rail car, truck, or seagoing vessel.

See also

• Wood pulp
• Integrated Pulp Mill - Forest Biorefinery - pdf file