ISSN ………… - International Network for Bamboo and Rattan

The most important industrial refiner mechanical pulping process today is the thermomechanical
pulping (TMP). The process involves an impregnation and preheating step of washed wood chips
with saturated steam under pressure. The preheated chips are fed to the disk refiner for defibration at
approximately the same temperature and pressure as in the preheating stage. The secondary refining
stage is generally carried out at atmospheric pressure. Therefore, the defibred material is expanded
into a cyclone where the steam is removed, and refined in one or two stages to the desired freeness.
The rejects from screening and cleaning are thickened and recycled to the refining step or separately
refined. The TMP process yields 91-98 per cent pulp which has a lower brightness than groundwood
pulp.
In the chemimechanical process (CMP), mechanically destructed chips are impregnated with
alkaline peroxide liquor (NaOH/H2O2) at 40-60 0 C at atmospheric pressure for 1.5–2 hrs before lowconsistency
(5%) refining.
Air pollution problems in mechanical pulping are less significant than in chemical pulping. However,
RMP, TMP and particularly CRMP and CTMP processes cause mill effluents with considerable
amounts of extractives.
The world-wide increase in the application of mechanical, thermomechanical and chemimechanical
pulping processes to produce pulps from non-wood fibre sources including bamboo is described
extensively by Misra (1980) 521 .
4.2. Chemical pulping
Chemical pulping employs chemical reagents to effect a separation of the cellulose fibres from other
wood components. Wood chips are cooked with suitable chemicals in aqueous solution, usually at
elevated temperatures and pressures. The objective is to dissolve the lignin and other extraneous
compounds, leaving the cellulose intact and in fibrous form. This objective can be realised to a
commercially satisfactory degree through the use of chemical reagents, although there is an
appreciable dissolution of carbohydrate material and degradation of cellulose. Pulp yields are usually
about 50 per cent of the wood weight. Because the chemical processes consume relatively large
quantities of inorganic chemicals such as alkalies, paper makers devised methods for reagent chemical
recovery from the spent cooking liquor; recovery has remained an integral part of chemical pulping.
Environmental and economical concerns necessitated chemical recovery as a very important part of
chemical pulping.
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