Paper is made from the cellulose fibres that are present in hardwood and softwood trees. Whether using wood or recovered paper, the first step is to dissolve the material into pulp. Regardless of the type of pulping process used, the wood or recovered paper is broken down into its component elements so that the fibres can be separated. The pulping results in a mass of individual fibres being produced.
The fibres are then washed and screened to remove any remaining fibre bundles. The water is then pressed out and the residue is dried. The pulp is then ready to be used directly or it can be bleached and made into white paper.
In an 'integrated paper mill' the pulp will be fed directly to a paper machine. Alternatively, it will be dried and pressed into bales ready for use as a raw material in paper mills worldwide.
The pulp-making process
1. Timber and debarking
2. Chipping machine and pulping process
3. Cleaning and bleaching
4. Washing and drying
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); and by level of processing (e.g. bleached or unbleached). Pulp generated from recovered paper is similarly graded.
Half of the paper and board produced in Europe is generated from recovered paper. Of the pulp that is sourced from wood, 26% comes from wood residues left over from other industries, such as saw mills, construction and furniture making, and around 50% of the new wood used comes from commercial thinnings, i.e. trees which have been felled in order to keep Europe's forests healthy.
Making pulp from recovered paper
Recycled paper is a type of paper that completely or partially consists of recycled fibres. These fibres can have very different origins and therefore also very different characteristics when it comes to being a component in new paper. Newsprint, tissue and paperboard are the products primarily produced using recycled paper as raw material.
Collected paper must first be sorted into different categories. The sorting can take place either directly in the paper mill or at special sorting stations. How the sorting takes place depends to a large extent on how the collection of the paper takes place, which varies from country to country. In most cases bales or loose paper waste is transported to the pulper using conveyor belts.
Before printed paper, such as office waste and newspapers, can be processed into graphical paper grades, the ink needs to be removed. There are two main processes for de-inking waste paper - washing and flotation.
In the washing process the waste paper is placed in a pulper - a huge tank that liberates the paper fibres from the paperweb by agitation with large quantities of water - and broken down to slurry. Staples and other undesirable material are removed by using centrifugal screens, thereby diminishing the risk of damage in the processes that follow. Most of the water containing the dispersed ink is drained through slots or screens that allow ink particles through. The pulp does not pass through. Adhesive particles, known as ‘stickies', are removed by fine screening.
In the flotation process the waste is made into slurry and contaminants are removed. Special surfactant chemicals are added to the slurry, which produces froth on the top of the pulp. Air is then blown into the slurry. The ink adheres to the bubbles of air and rises to the surface. As the bubbles reach the top, a foam layer is formed that traps the ink. The foam is removed before the bubbles break so the ink does not go back into the pulp.
When completed, the clean, useful fibre is piped to a storage chest and consequently to the papermaking machine, while the excess materials are skimmed off or dropped through centrifugal force into a sludge that is then burned for fuel, otherwise used or landfilled.