Sustainable industrial production
Stage 1: Raw materials
wood: hard woods are more stable than sort woods.
- 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 become popular.
- Preconsumer waste: 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 preconsumer (recycled material that has been printed but did not reach its intended end use, such as waste from printers and unsold publications).
- Postconsumer waste: 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.
Recycled papers can be made from 100% recycled materials or blended with virgin pulp, although they are (generally) not as strong nor as bright as papers made from the latter.
Stage 2: Pulping
Main Wikipedia articles: kraft process, sulfite process and soda pulping
To make pulp from wood, a chemical pulping process separates lignin from cellulose fibres. 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 fibres. 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 bleached to produce white paper, but this consumes 5% of the fibres; chemical pulping processes are not used to make paper made from cotton, which is already 90% cellulose.
The microscopic structure of paper: Micrograph of paper autofluorescing under ultraviolet illumination. The individual fibres in this sample are around 10 µm in diameter.
There are three main chemical pulping processes:
- sulfite process: dates back to the 1840s and it was the dominant method extent before the second world war.
- kraft process: invented in the 1870s and first used in the 1890s, is now the most commonly practised 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 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 fibres, thus producing weak paper. Although large amounts of electrical energy are required to produce mechanical pulp, it costs less than the chemical kind. There are two major mechanical pulps,
- thermomechanical pulp (TMP): wood is chipped and then fed into large steam heated refiners, where the chips are squeezed and converted to fibres between two steel discs.
- groundwood pulp (GWP): debarked logs are fed into grinders where they are pressed against rotating stones to be made into fibres.
Main Wikipedia article: de-inking
Paper recycling processes can use either chemically or mechanically produced pulp; by mixing it with water and applying mechanical action the hydrogen bonds in the paper can be broken and fibres 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.
Stage 3: Additives: sizing etc
Besides the fibres, pulps may contain fillers such as:
- chalk or china clay, which improve its characteristics for printing or writing.
- additives for sizing purposes may be mixed with it and/or applied to the paper web later in the manufacturing process; the purpose of such sizing is to establish the correct level of surface absorbency to suit ink or paint.
Stage 4: Producing paper: pressing and drying
Main Wikipedia articles: Paper machine and papermaking
The pulp is fed to a paper machine where it is formed as a paper web and the water is removed from it by pressing and drying.
Pressing the sheet removes the water by force; once the water is forced from the sheet, a special kind of felt, which is not to be confused with the traditional one, is used to collect the water; whereas when making paper by hand, a blotter sheet is used instead.
Drying involves using air and/or heat to remove water from the paper sheets; in the earliest days of paper making this was done by hanging the sheets like laundry; in more modern times various forms of heated drying mechanisms are used. On the paper machine the most common is the steam heated can dryer. These can reach temperatures above 200 °F (93 °C) and are used in long sequences of more than 40 cans; where the heat produced by these can easily dry the paper to less than 6% moisture.
Stage 5: Finishing: coating, reeling and cutting
The paper may then undergo sizing and coating to alter its physical properties for use in various applications.
The paper is then fed onto reels if it is to be used on web printing presses, or cut into sheets for other printing processes or other purposes.
Traditional process from Europe
Industrial paper production in India