Various Paper Pulping Process


Throughout the history of paper production, the pulp making process is arguably the most important process. This process involves the formation of pulp, a lignin-free product. As the quality of pulp formed directly affects the quality of the paper product produced, the pulp making process has to constantly evolve to suit the market demand for different quality products.

Mechanical pulping, the oldest form of pulping, removes unwanted lignins from fibres via the grinding of wood. Some forms of mechanical pulping include stone groundwood pulping (SGW) and refiner mechanical pulping (RMP), which uses artificial bonded stones and grooved discs respectively to separate lignins from fibres.

Paper_MachinesSGW (left), RMP (right)

These two processes give somewhat shorter fibre lengths, resulting in low-grade papers with weaker strength. To fulfil the higher expectations of the market, thermomechanical pulping (TMP) and chemi-thermomechanical pulping (CTMP) was used to generate longer fibres, thereby providing stronger paper. This improved process involves the use of steam to soften wood chips before grinding to generate better quality fibres. However, TMP and CTMP require larger energy usage due to the steaming process.

Due to the relatively lower paper quality produced from mechanical pulping, chemical pulping processes are developed to compensate for the lower strength and quality. The two most prominent chemical pulping process currently is the Kraft’s process and the Sulfite process. The Sulfite process makes use of Sulfurous acid along with limestone as the chemical make-up during the cooking process to produce longer fibres and stronger papers. Likewise, the Kraft’s process is able to produce papers with better quality. However, this process uses a chemical make-up comprising of strong bases such as Sodium Hydroxide along with Sodium Sulfate during the cooking process.

Comparing these two different pulping methods, chemical pulping is more prevalent despite it giving a lower yield and higher cost. This is because the chemical pulping process is able to give longer and higher quality fibres that are suitable for a wider range of paper products. In addition, mechanical pulping require a large amount of energy. This factor is a major concern in an energy-intensive society and is unfavourable in comparison to chemical pulping. On the other hand, chemical pulping is a chemical intensive process, resulting in the paper industry being one of the “dirtiest” industries around.

Although chemical pulping is the major pulp making process today, some companies still prefer the use of mechanical pulping in the production of lower grade paper products such as newsprints, magazines, and catalogues due to the cheaper cost for production.

Over the years, many alternative methods of pulping have been developed to provide both a cost and chemical efficient paper making process. American Process Inc.’s patented hot-water extraction process enhanced with their Nanocellulose technology is one example of such innovation whereby hot water is used for pulping rather than a chemical make-up. Organosolv pulping process is another environmentally benign alternative to the popular Kraft’s chemical pulping as the solvents are easily recovered via the distillation process, resulting in more effective chemical usage. In addition, this process produces relatively high quality and high yield pulp, adding value to this process. It is reported that the organosolv pulping process using an ethanol-water solution gives a yield 4-4.5% higher than that of the Kraft’s process.

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