Chemical pulping
To make pulp from wood, a chemical pulping process separates lignin from cellulose fibre.. This is accomplished by dissolving lignin in a cooking liquor, so that it may be washed from the cellulose; this preserves the length of the cellulose fibre. Paper made from chemical pulps are also known as wood-free papers–not to be confused with tree-free paper; this is because they do not contain lignin, which deteriorates over time. The pulp can also be bleachedto produce white paper, but this consumes 5% of the fibre; chemical pulping processes are not used to make paper made from cotton, which is already 90% cellulose.
There are three main chemical pulping processes: the sulfite process dates back to the 1840s and it was the dominant method extent before the second world war. The kraft process, invented in the 1870s and first used in the 1890s, is now the most commonly practiced strategy, one of its advantages is the chemical reaction with lignin, that produces heat, which can be used to run a generator. Most pulping operations using the Kraft process are net contributors to the electricity grid or use the electricity to run an adjacent paper mill. Another advantage is that this process recovers and reuses all inorganic chemical reagents. Soda pulping is another specialty process used to pulp straws, bagasse and hardwoods with high silicate content.
Mechanical pulping
There are two major mechanical pulps: thermomechanical pulp (TMP) and ground-wood pulp (GW). In the TMP process, wood is chipped and then fed into steam heated refiners, where the chips are squeezed and converted to fibre between two steel discs. In the ground-wood process, debarked logs are fed into grinders where they are pressed against rotating stones to be made into fibre. Mechanical pulping does not remove the lignin, so the yield is very high, >95%, however it causes the paper thus produced to turn yellow and become brittle over time. Mechanical pulps have rather short fibre, thus producing weak paper. Although large amounts of electrical energy are required to produce mechanical pulp, it costs less than the chemical kind.
DE-inked pulp
Paper recycling processes can use either chemically or mechanically produced pulp; by mixing it with water and applying mechanical action the hydrogenbonds in the paper can be broken and fibre separated again. Most recycled paper contains a proportion of virgin fibre for the sake of quality; generally speaking, DE-inked pulp is of the same quality or lower than the collected paper it was made from.
There are three main classifications of recycled fibre:.
? Mill broke or internal mill waste – This incorporates any substandard or grade-change paper made within the paper mill itself, which then goes back into the manufacturing system to be re-pulped back into paper. Such out-of-specification paper is not sold and is therefore often not classified as genuine reclaimed recycled fibre, however most paper mills have been reusing their own waste fibre for many years, long before recycling became popular.
? Pre-consumer waste – This is offcut and processing waste, such as guillotine trims and envelope blank waste; it is generated outside the paper mill and could potentially go to landfill, and is a genuine recycled fibre source; it includes DE-inked pre-consumer (recycled material that has been printed but did not reach its intended end use, such as waste from printers and unsold publications).
? Post-consumer waste – This is fibre from paper that has been used for its intended end use and includes office waste, magazine papers and newsprint. As the vast majority of this material has been printed – either digitally or by more conventional means such as lithography or rotogravure – it will either be recycled as printed paper or go through a de-inking process first.