Process configurations — bleaching Mill process with chlorine-containing chemicals There are several bleaching sequences and technologies in today’s pulp mills, whether modern or not so modern. The bleaching sequence is selected taking into account the investment cost, operating cost, local energy price and environmental permits as well as, in modernisations, the equipment available in the
Brightness reversion General approach Lignin-containing mechanical pulps are typically used as short-life products and bleached to a brightness level of about 80 % ISO using two alkaline hydrogen peroxide (H2O2) stages or an alkaline H2O2 stage followed by a sodium dithionite (Na2S2O4) stage.1 Therefore, many modern mechanical pulp mills can produce high-yield mechanical pulps at brightness
Bleach plant design and operation The traditional peroxide (H2O2) bleaching (stage designation P) of mechanical pulps takes place in a retention vessel or tower. Tower bleaching requires equipment for dewatering (and metal removal) ahead of the bleaching stage, mixers for mixing the chemicals into the pulp, a bleaching tower providing a proper retention time, dilution
Dithionite General approach The objective of lignin-preserving bleaching processes of chemimechanical and mechanical pulps is to selectively eliminate colour-contributing groups (i.e., chromophores) (see Lignin-preserving or lignin-retaining bleaching), while maintaining pulp yield. Additionally, bleaching chemicals should themselves be colourless and run at reasonable cost.1-3 Two main bleaching chemicals, alkaline hydrogen peroxide (H2O2/NaOH, stage designation P) (see
Peroxide General approach Hydrogen peroxide (H2O2) (referred to simply as “peroxide”, stage designation P) can oxidise the specific chromophoric groups in the lignin (see Lignin-preserving or lignin retaining bleaching) resulting in the colour removal.1 However, in this bleaching process, H2O2 alone is not enough and also alkali (NaOH) is needed for the bleaching reactions. Since H2O2
Physical parameters affecting grinding Wood in a cyclic stress field During grinding, a cyclic stress mechanism defibrates the wood. The grindstone grits pass transversally over the wood fibres, producing a cyclic sequence of pressing and shearing forces in the wood grinding zone, where the fibres are subjected to sequential compression and relaxation (Figure 1). The
Fundamental mechanisms – grinding General principle The main principle in grinding as well as in all mechanical defibration processes is to bring the wood raw material into a cyclic oscillating stress field, whereby the absorbed mechanical energy breaks down the structure of the fibrous raw material. The fibres are successively separated from the wood matrix
Process control – grinding Control requirements The ultimate grinding control requirement is that the groundwood pulp produced by one group of grinders and supplying one papermaking line fulfils the preset production and quality demands for the pulp at the lowest specific energy consumption (SEC) and low variability. Conventionally, and still in many mills, this goal
Grindstones The grinder’s production rate, the specific energy consumption and the quality of the pulp produced are dependent on several factors, such as the grinding process, the type of grinder, the wood species and wood quality and the target pulp properties. However, in meeting end-product quality requirements, choosing the right type of grindstone and conditioning
Thermo groundwood pulping (TGW) General approach Thermo groundwood (TGW) pulping is a mechanical pulping process, in which wood logs are converted into wood pulp by atmospheric grinding of logs in a chain grinder with a special system that allows shower water temperatures of 80 °C or higher.1 The yield of the pulp made by TGW
