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Types of Pulping Processes

Pulp can be divided into two principal types: chemical or mechanical pulp. In fact there are more than two types of pulp . Pulp from recovered paper poses a different challenge for the papermaker as it often has to be de-inked and other contaminants removed.

Type

Mechanical Pulp

Chemical Pulp

Process

For the production of mechanical pulp, wood is ground against a water lubricated rotating stone. The heat generated by grinding softens the lignin binding the fibres and the mechanised forces separate the fibres to form groundwood.

During the second half of the 20th century, newer mechanical techniques using ‘refiners' were developed. In a refiner, woodchips are subjected to intensive shearing forces between a rotating steel disc and a fixed plate.

In subsequent modifications to this process, the woodchips are pre-softened by heat (thermo-mechanical pulp - TMP) to make the fibrillation more effective. The resulting pulp is light-coloured and has longer fibres.

A further development of the thermo-mechanical pulp is CTMP pulp, in which the wood chips are impregnated with a chemicals treatment with sodium sulphite before the grinding. The end result is an even lighter-coloured pulp with better strength characteristics.

After grinding, the pulp is sorted by screening to suitable grades. It can then be bleached with peroxide for use in higher value-added products.

Mechanical pulp consists of a mix of whole fibres and fibre fragments of different sizes.

Paper containing a high level of mechanical pulp and a smaller level of chemical pulp is called ‘wood containing paper'.

Mechanical pulp gives the paper a yellowish/grey tone with high opacity and a very smooth surface.

Mechanical pulping provides a good yield from the pulpwood because it uses the whole of the log except for the bark, but the energy requirement for refining is high and can only be partly compensated by using the bark as fuel.

For chemical pulp, logs are first chopped into wood chips which are then cooked with chemicals under high pressure.

Cooking removes lignin and separates the wood into cellulose fibres. The resulting slurry contains loose but intact fibres which maintain their strength.

During the process, approximately half of the wood dissolves into what is called black liquor. The cooked pulp is then washed and screened to achieve a more uniform quality. The black liquor is separated out from the pulp before the bleaching process.
(graphic from paperonline:
http://www.paperonline.org/cycle/pulpprod/chempulp_frame.html)
Most chemical pulp is made by the sulphate (or Kraft) process, in which caustic soda and sodium sulphate ‘cook' the woodchips.

In the unbleached stage, a dark brown but very strong pulp results and this can be bleached to a high brightness if required.

The sulphite pulping process is an alternative method best suited for speciality pulp which can be easily bleached, generally with hydrogen peroxide. These pulps fulfil the demand for ‘chlorine-free' products in the hygiene paper sector and also in printing and writing papers.

The yield in both chemical processes is much lower than in the manufacture of mechanical pulp, as the lignin is completely dissolved and separated from the fibres. However, the lignin from the sulphate and some sulphite processes can be burnt as a fuel oil substitute. In modern mills, recovery boiler operations and the controlled burning of bark and other residues makes the chemical pulp mill a net energy producer which can often supply power to the grid, or steam to local domestic heating plants.

A chemical pulp or paper is called woodfree, although in practice a small percentage of mechanical fibre is usually accepted.

Energy consumption

1000 KW/tonne of pulp

Self-sufficient

Yield (from wood material)

95%

45%

Fibre length

Fibre fragments of different sizes

Mainly longer fibres

Paper strength

Low

High

Pulp production in Europe

32%

66%

Production cost

Lower

Higher than in case of mechanical pulp

Comparison

Using chemical pulp to produce paper is more expensive than using mechanical pulp or recovered paper, but it has better strength and brightness properties.

Softwood Kraft pulp, one of the main chemical pulp grades, is used to provide the required strength when producing lightweight publication papers. Fine papers (e.g. copy papers or writing papers) are an example of the type of paper produced mainly from hardwood pulp, which is reinforced by a small amount of stronger and more expensive softwood kraft pulp.

Pine and spruce provide the strongest pulp (for example softwood kraft), while hardwood kraft is produced from birch, eucalyptus, aspen, acacia and many other mixed tropical species. Fast growing species of tree, such as planted eucalyptus and acacia, are the most rapidly emerging pulp raw material.

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