The wood chips are first cooked in a machine called the digester. The chips are subjected to heat, pressure, and the chemical sodium sulphate in order to separate the cellulose fibres from the lignin. Lignin is an organic compound that forms certain structural materials of the wood. It is reclaimed from this process when it is burned in the recovery boiler, producing steam and power to be fed into the National Grid.
The result of this ‘cooking’ process is a brown-coloured, raw pulp which must be bleached to produce the white-coloured paper we are all used to. This bleaching process rids the cellulose fibres that make up the pulp of any leftover lignin and other compounds. Pulp will, typically, go through this process several times, becoming whiter each time.
This white pulp is then delivered to paper mills, either as sheets of dry pulp, or a slurry that can be pumped along pipelines to the mills. The next stage of the pulp’s lifecycle will determine which of these options is chosen. If the pulp is being sent to another Group’s mill or for sale on the market, then it will go through a drying process to produce sheets of pulp. If it is to be used at the same mill in which it was produce, for example, if a mill produces both pulp and paper, then it will be pumped directly to the paper production area as slurry.
We also pay close attention to the economic efficiency of our manufacturing process, adopting processes and solutions intended to keep energy costs down and minimise environmental effects.
In the next part of this series of articles, we will look at the steps taken to manufacture paper from the white pulp produced in this stage.