In case you missed last week’s information on the paper making process, you can view it here. We discussed the different ways to refine the pulp and then the process of screening and cleaning it. Today we will divulge into the final steps of the pulping process.
The refined, screen and cleaned pulp is now ready for the final step in the pulping process, the bleach plant. A wide variety of bleach chemicals are used in pulp bleaching. The main chemical used in the bleaching of mechanical pulp is hydrogen peroxide (H2O2.) Peroxide is usually used in conjunction with sodium hydroxide (NaOH) commonly referred to as caustic. These two chemicals make up the bulk of the bleach recipe in most modern mechanical pulp mills. There are a whole host of other chemicals that are used in various processes to achieve higher brightness pulp, such as silicate, magnesium sulphate and on and on, but we will ignore them for simplicity sake.
The chemical dosages in the bleach plant will depend on the brightness targets set out for the specific grade of paper to be made on the paper machine. Generally a higher brightness requires higher dosages. Brightness is a measure of how much light the paper sheet reflects.
The unbleached pulp is typically thickened by the use of either a screw press or a twin wire press to raise the consistency of the pulp before bleaching. Twin wire presses are machines which use two fabrics, or “wires” which travel around electrically powered rolls to push the pulp through a narrow area (wedge zone) as well as pinch points between rolls which forces the pressate (fluid) out of the pulp, raising the consistency.
The bleach chemicals are then introduced to the thickened pulp in a mixer which can be thought of as a mini refiner. The high consistency pulp is conveyed via a screw on one end of the mixer into the mixing zone between the mixer plates while simultaneously being injected with a specifically controlled flow of bleach chemicals. The pulp is then conveyed to a bleach tower where it resides for a controlled period of time (retention time) in a high temperature environment during which the peroxide “oxidizes the chromophores which are the pigment carrying components in the pulp” or as I like to say, bleaches it. Once the pulp has been bleached it is now ready to be diluted once again and pumped to a storage tank. This is where the pulp process ends and the paper making process begins… stay tuned!