In a TMP mill, heat recovery plays an essential role in economic operating. Normally,

Around two-thirds of the refining energy can be recovered in the form of

Clean steam. The TMP steam generated in the refiners is separated from the fibers

in the cyclones (see Figs. 4.35–4.37), and then condensed in the reboiler against

Vaporizing clean steam. The main components of a typical heat recovery unit are

Illustrated in Fig. 5.15.

5 Processing of Mechanical Pulp and Reject Handling: Screening and Cleaning

Fig. 5.15 Typical thermomechanical pulp (TMP) heat recovery flow sheet (Rinheat).

Bleaching of Mechanical Pulp

Hans-Ulrich Suss

Mechanical defiberization leaves most of the lignin compounds in the fiber. A

Bleaching process using the aggressive chemicals applied in chemical pulp

Bleaching would result in the oxidation and removal of this lignin. This cannot be

The target of the process, as it would decrease the yield dramatically, require an

Enormous amount of chemical, and also destroy the optical properties of the fiber.

Bleaching of mechanical pulp has to apply chemicals with limited aggressivity, as

Well as conditions which keep the extraction of lower molecular-weight carbohydrates

And lignin compounds as moderate as possible. The high temperature of

The defiberization process causes the solubilization of polyoses and starts the

Hydrolysis of acetyl groups. Resins and lignans are partially dispersed and dissolved

[30–32]. A higher pH and a high temperature in bleaching will intensify

These effects; consequently, neutral or moderately acidic bleaching conditions are

Required in order to maintain the pulp yield.

Softwood from forest thinning or wood chips from sawmills has an unbleached

Brightness level which is sufficiently high as to allow its application in newsprint

Production, without any bleaching process. The initial brightness typically is between

55% ISO and 65% ISO. The intensity of the bark removal, the wood species,

And the storage time between wood harvest and refining has a huge impact

On the unbleached brightness level. The use of sawmill waste normally provides a

Lower unbleached brightness. Generally the use of a mechanical pulp in higher

Quality paper grades such as SC or LWC paper requires a bleaching step.

Lignin is the dominant source of chromophores in mechanical pulp, with aromatic

Ring structures with conjugated side chains, quinones, quinone methids

and metal–catechol complexes being the origin of the color. Chemically, decolorization

Or destruction of these compounds is accomplished by either reductive or

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