How is wood turned into papermaking pulp?

There are two main processes, depending on the end use of the pulp.

To make mechanical pulp, the wood is first chipped (a wood chip is typically about 50mm x 30mm x 5mm in size) and then the chips are fed to a refiner which disintegrates the chips into individual fibres, by forcing them between two large ribbed disks which counter-rotate at high speed, very close together. Often the chips are first softened by heating, or soaking in caustic soda or other chemicals. Mechanical pulp is typically used in newsprint, magazine paper and paper towels. In mechanical pulping, the lignin remains with the fibres.

To make chemical pulp, the woodchips are cooked with chemicals (e.g. caustic soda) to dissolve the lignin, leaving the cellulose fibres intact. The dissolved lignin is burnt to (a) provide energy, and (b) recover the chemicals. Chemical pulp makes stronger paper than mechanical pulp (which is why the most common process is called "kraft" - the German word for "strong"). Chemical pulp is thus important for packaging grades like bag and sack kraft and linerboard (for corrugated cardboard boxes).

Chemical pulp fibres are much more flexible and conformable than mechanical pulp fibres, so they are ideal for making fine papers (such as office paper).