In the papermaking industry the pulping process is an important technology. When talking about it, we have to start from pulp.
Generally, pulp can be classified into two main types: chemical pulp and mechanical pulp. Surely, still there are other types of pulp actually. Here we want to stress that the pulp made from recovered paper faces a more different challenge for the paper making enterprise, that is, it will have to do a de-inked procedure and remove other contaminants.
Next, we will further explain the differences of two types of pulps.
For the production of mechanical pulp, wood shall be ground against a water lubricated rotating stone. During this process, the heat generated by grinding can soften the lignin containing the fibers and then the mechanized forces can separate the fibers to form ground wood we want to get.
After 1950s, a newer mechanical technology of using refiners was developed out. In a refiner woodchips are subjected to intensive shearing forces between a rotating steel disc and a fixed plate.
In the following modifications to this process, the wood chips are pre-softened by heat (TMP stands for thermo-mechanical pulp) to make the fibrillation more effective. The resulting pulp is light-colored and has longer fibers.
Through a further development, TMP pulp has become CTMP pulp. To be specific, the wood chips will be impregnated with a chemical treatment with sodium Sulphite before the grinding. The final result is a lighter-colored pulp with better strength characteristics.
After the grinding, this type of pulp will be sorted by screening into suitable grades. For higher value addition of products, it can be bleached with peroxide for using mechanical pulp that consists of a mixing whole fibers and fiber fragments with different sizes.
Paper that contains a high grade of mechanical pulp and a smaller grade of chemical pulp is called wood-containing paper. Mechanical pulp can bring the paper with a yellowish/grey tone with high opacity and a very smooth surface. Mechanical pulping can provide us with a good yield from the pulpwood because its full use of the whole log except the bark. But the energy consumption for refining is relatively higher, which can only be partially compensated by using the bark as a fuel.
For the production of chemical pulp, logs are chopped into wood chips at first and then are well cooked with relevant chemicals under high pressure. The cooking can remove lignin away and separate the wood into cellulose fibers. The produced slurry contains loose but intact fibers maintaining their strength.
During the process, about 50% of the wood will dissolve into black liquor. The cooked pulp will be then washed and screened to get a more uniform quality. The black liquor will be separated out from the pulp in ahead of the bleaching process. Most of chemical pulp will be produced by Sulphate (or Kraft) process, in which caustic soda and sodium Sulphate can help cook the woodchips.
In the unbleached stage, people can get a dark and brown but very strong pulp that can be further bleached to get a high brightness according to the requirements.
The Sulphite pulping process is an alternative method that is suited for special pulp very well, which can be easily bleached with hydrogen peroxide generally. The pulp can reach the demands of Chlorine-free products in the hygiene paper, printing paper and writing paper.
The pulp yield in the chemical processes is relatively lower than that in the mechanical processes. It is the reason that the lignin is completely dissolved and separated from the fibers. However, the lignin from the Sulphate and some Sulphite processes can be burnt as a replacement of fuel oil. In modern workshops, the operation of recovery boiler and the controlled burning of bark and the other residues can make the chemical pulp producer be a net energy producer, which can often supply power to the power grid, or steam to local domestic heating plants.
Comparison of mechanical pulp and chemical pulp
Adopting chemical pulp to make paper is more expensive than mechanical pulp or recovery paper for us, but it can help us to get higher quality of paper with better properties of strength and brightness.
Technical parameters of mechanical pulp and chemical pulp:
|Mechanical pulp||Chemical pulp|
Softwood Kraft pulp, as one of the main chemical pulp, is popularly applied to provide the required strength of lightweight publication papers. Fine papers (copy papers or writing papers) are produced mainly from hardwood pulp, which can be reinforced by a small amount of stronger and more expensive softwood Kraft pulp. Pine wood and spruce wood can provide the strongest pulp (for example softwood Kraft), but hardwood Kraft is made from birch, eucalyptus, aspen, acacia and many other mixed tropical species. Fast growing trees, such as planted eucalyptus and acacia, are the most rapidly emerging pulp used as raw material.