Paper is actually a multi-functional biomass product, which can be made of renewable woods, grass, rags, waste paper, and other materials. No matter which material is used for papermaking, the first step is always to dissolve the material into pulp.
Modern pulp production is based on the best available technologies and concepts, creating more products with less raw materials, water, energy, and environmental impact. Today, the most commonly used pulp making methods are chemical pulping, chemi-mechanical pulping, and mechanical pulping.
No matter which pulping method is used, the material must be broken down into its constituent elements in order to separate the fibers. The pulping process produces a large number of individual fibers. The fibers are then washed and screened to remove any remaining fiber bundles. The water is then pressed out and the residue is dried.
The paper pulp is then ready to be used directly for natural paper manufacturing or it can be bleached for bleached white paper manufacturing. The paper pulp will be fed directly to paper pulper machines in integrated paper mills. Alternatively, it will be dried and pressed into bales ready for use as a raw material in paper mills worldwide. The paper pulp production process is more than energy self-sufficient generating wood-based bioenergy for the national electricity grids. Moreover, paper pulp manufactured from softwood can generate valuable by-products and production residues such as lignin, turpentine, and tall oil which can be further processed to innovative products.
Paper Pulp Manufacturing Process
The paper pulp production process can be divided roughly into three stages: wood handling, fiber processing, and drying and packaging.
1. Wood handling
Wood is first debarked and cut into small chips for more efficient pulping. The bark of the trees is used for bioenergy production at the mill.
2. Fiber processing
While the raw material affects significantly the properties of the end-product, the process for both hardwood and softwood is nearly identical, the only difference being the structure of the wood. Softwood consists mostly of cellulose and lignin, and it contains less hemicellulose than hardwood. Softwood fibers are longer than hardwood fibers and thus softwood pulp is called long fiber pulp, whereas hardwood pulp is short fiber pulp. Softwood’s long fibers contribute to the strength of the material it is used for. Cooking the wood chips in the presence of sodium hydroxide and sulfide liquor under high pressure removes the lignin and separates the wood into cellulose fibers. During the cooking process, approximately half of the wood dissolves. The pulp is then washed, screened for quality, and bleached. The spent cooking chemicals and dissolved wood material is called black liquor. This substance is recovered and burned in a recovery boiler to produce energy that keeps the process running. It is renewable, wood-based, pure energy – perfect for replacing fossil fuels. In causticising, even the cooking chemicals are processed for re-use.
3. Drying and baling
The pulp is then dried for easier handling and transportation, cut into sheets, and baled. Now the pulp is ready to travel the world.
Paper pulp is graded and classified according to the method of the production (e.g. chemical or mechanical pulp); the species of tree used (e.g. softwood or hardwood); the levels of processing (e.g. bleached or unbleached). The recycled paper pulp is similarly graded.
In the papermaking industry of Europe, half of the paper and board produced in Europe is generated from recycled paper. Of the pulp that is sourced from wood, 26% comes from wood residues leftover from other industries, such as saw mills, construction and furniture making, and around 50% of the new wood used comes from commercial thinning, i.e. trees which have been felled in order to keep Europe’s forests healthy.