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Pulp mill

 Milling process

The process begins at the chip pile, where planer wood chips and sawmill chips may be stored for one to two months for 'seasoning' for mechanical pulping.

The wood from the trees contains three main components, in addition to water. The components are cellulose polymer fibres, which are the desired material for papermaking, lignin, which is a three-dimensional polymer that cements the cellulose fibres together to produce the strength inherent in a tree or in lumber, and so-called hemicelluloses, which are shorter polymer chains of sugars. The aim of chemical pulping is to separate the useful cellulose fibres from the lignin and hemicelluloses. The balance is to remove all of the lignin without reducing the strength of the cellulose fibres by cutting them.

Chemical pulping processes such as the Kraft (or sulphate) process and the sulphite process reduce much of the hemicelluloses and lignin. The kraft process is more successful than the sulphite at allowing the strength of the cellulose fibres to remain. The chemical pulping processes use a combination of high temperature and alkaline (kraft) or acidic (sulphite) chemicals to break the chemical bonds of the lignin.

The various mechanical pulping methods, such as groundwood (GW) and refiner mechanical (RMP) pulping, physically tear the cellulose fibres one from another. Much of the lignin remains adhering to the fibres. Strength is impaired because the fibres may be cut.

There are a number of related hybrid pulping methods that use a combination of chemical and thermal treatment to begin an abbreviated chemical pulping process, followed immediately by a mechanical treatment to separate the fibres. These hybrid methods include thermomechanical pulping (TMP) and chemithermomechanical pulping (CTMP). The chemical and thermal treatments reduce the amount of energy subsequently required by the mechanical treatment, and also reduce the amount of strength loss suffered by the fibres.

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 sulphide 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 resulting fibre suspension in water solution is termed pulp or stock.

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.


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