Making Pulp from Wood
Most chemical woodpulp is made by the sulphate
process. Chips from de-barked logs are dissolved in caustic soda and sulphur by
heat and pressure leaving a strong brown pulp, coloured by the effect of the
chemicals on the lignin and wood sap. The pulp is commonly known as kraft - the
German word for strong. About 20% of chemical pulp is made by the sulphite
process. Chemical pulping uses less energy than mechanical pulping. In modern
mills, recovery boiler operations and the controlled burning of bark and other
residues make the chemical pulp mill a net energy producer which can often
supply power to the grid, or steam to local domestic heating plants.
Chemical pulp, however, produces a strong liquid
effluent that needs to be treated. The term "woodfree" is often used in the
trade to signify that the chemical pulping process has been used. it is not
meant to be misleading and has been used for decades to mean "free from
mechanical wood" (or "ground wood free").