Recovery Boiler & Power Generation

A furnace heats the black liquor until the used chemicals form a thick liquid at the bottom and the wood waste burns-off at the top. The heat is used to generate steam to supply turbines and driers. The chemical solids are mixed with water to form green liquor - a mixture of sodium sulfide and sodium carbonate.

In scientific terms, wood is an organic composite, consisting of hollow, flexible tubes of cellulose (cellulose fibres), held in shape by a bonding agent known as lignin. In order to release the usable cellulose fibres as paper pulp, the lignin must be removed. Click on the buttons along the bottom of your screen to follow the process through the mill.

Chemical Preparation

This is where white liquor (a solution of sodium hydroxide and sodium sulfide) is produced and stored, and where calcium oxide is added to the Green Liquor waste to convert the sodium carbonate back into sodium hydroxide. Calcium carbonate is also converted back to lime for re-use.

Debarking & Chipping

The first step in breaking down the raw wood into usable fibres is to mechanically remove the bark and reduce the logs to uniformly sized wood-chips. This makes it easier to accurately control the chemical pulping process.

Pulping

The digester (or cooking plant) is a huge pressure-cooker, where heat and pressure is applied to a mixture of chips and white liquor for 2-4 hours, dissolving most of the lignin into the liquid. The mixture is then washed to separate the pulp from the liquid. At this point the pulp is known as brownstock.

Water Treatment

The used water from the pulp-washing process is treated in an evaporator, where it is concentrated to approximately 75% solids to recover the pulping chemicals and minimise effluent from the plant. The solids produced by the process, known as black liquor are then sent to the recovery boiler.

Bleaching

Bleaching is needed to brighten the brownstock pulp sufficiently for use in paper manufacturing. The process also removes the last traces of lignin from the pulp.

The paper-making process is comprised of four sections; the wet end, press section, drier section, and the calender section.

In the wet end, the bleached pulp flows from a headbox, through a slice and onto an endless fine-mesh belt. The belt runs over and through a series of rollers, drainage blades and suction boxes,

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then over the bottom roller and down and back over various guide rolls and a stretcher roll, which maintains the correct tension.

The wet web of paper is then transferred from the wire belt to a felt belt and carried through the press section,

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which removes further excess water and imparts uniform surface characteristics to each face of the sheet.

The drier section consists of tiers of steam-heated cylinders. The paper is held close to the driers by means of fabric drier felts and as the paper passes from one drier to the next, first one side,

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then the other comes in contact with the heated surfaces. The majority of the moisture content is removed in this process.

The fourth section of the machine, the calender section, consists of from one to three calender stacks, which impart the desired final finish to the paper, and a reel device for winding the paper into a roll as it leaves the machine.
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