Chemical Pulping and Mechanical Pulping Process

The pulping process is an important link in the papermaking industry. To learn why it is important, we have to start from the pulp.

chemical paper pulp

Pulp can be classified mainly into two types by the pulping methods: chemical pulp and mechanical pulp. The chemical pulp is produced through the chemical pulping process, while the mechanical pulp is produced through mechanical pulping.

There are some differences between the two types of pulp.

Mechanical Pulp

In the production of mechanical pulp, woods are ground against a water-lubricated rotating stone. During this process, the heat generated by grinding softens the lignin-containing fibers, and the mechanized forces separate the fibers to form the groundwood we want.

mechanical pulping machine

After 1950s, a new mechanical technology of using refiners was developed out. In the refiner, woodchips are subjected to intensive shearing forces between a rotating steel disc and a fixed plate.

In the followed 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 pulp produced is light-colored and has longer fibers.

Through 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 mixing whole fibers and fiber fragments with different sizes.

Paper that contains a high grade of mechanical pulp and a low 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 high, which can only be partially compensated by using the bark as fuel.

Chemical Pulp

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.

chemical pulping digester

During the process, about 50% of the wood will dissolve into black liquor. The cooked pulp will be then washed and screened to achieve more uniform quality. The black liquor will be separated out from the pulp before the bleaching process. Most of chemical pulp are 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, which can be further bleached to get a high whiteness according to the requirements.

chemical pulping principle

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 pulping processes is relatively lower than that in the mechanical pulping processes. The reason is 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 for 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 helps us get the high-quality paper with better properties of strength and brightness.

Technical parameters of mechanical pulp and chemical pulp:

Mechanical pulpChemical pulp
CostLowHigh
YieldHigh:approx.90%Low:approx.50%
ChemicalsFewMany(re-usable)>99%
EnergyIntensiveEnergy generation
BrightnessLowHigh
YellowingHighLow
OpacityHighLow
StiffnessLowHigh

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. Pinewood 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.

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