The mechanical pulping process refers to a method of producing pulp for papermaking that involves the mechanical separation of wood fibers through the use of mechanical force. This process is different from chemical pulping, which involves the use of chemicals to dissolve lignin and separate wood fibers.
In the mechanical pulping process, the wood fibers are separated from each other by mechanical energy applied to the wood matrix causing the bonds between the fibers to break gradually and fiber bundles, single fibers and fiber fragments to be released. It is the mixture of fibers and fiber fragments that gives mechanical pulp the favorable printing properties. In the mechanical pulping the objective is to maintain the main part of the lignin in order to achieve high yield with acceptable strength properties and brightness. Mechanical pulps have a low resistance to aging which results in a tendency to discolor.
The mechanical pulping process has been in use for centuries, with evidence of its use dating back to the 16th century. However, it was not until the 19th century that it became a widespread method of producing pulp for papermaking. The introduction of the Fourdrinier machine in the early 1800s made it possible to produce paper at a faster rate, and the mechanical pulping process was an important part of this development.
The mechanical pulping process is an important method of producing pulp for papermaking, particularly for newsprint and other low-cost papers. It is a cost-effective method of producing pulp, as it requires less energy and fewer chemicals than other pulping methods. The resulting pulp is also of high yield, meaning that more pulp can be produced from a given amount of wood. However, the process has some drawbacks, such as high energy consumption and potential environmental impacts, which we will discuss in more detail later in this guide.
How the Mechanical Pulping Process Works
The mechanical pulping process involves several steps that are designed to separate the wood fibers and produce pulp for papermaking. These steps may vary depending on the type of mechanical pulping process used, but generally include the following:
- Preparation: The logs or wood chips are debarked, cleaned, and screened to remove impurities.
- Grinding: The wood chips are ground down into small particles using a grinder or chipper.
- Refining: The ground wood particles are refined using a refiner, which is a machine that uses pressure to separate the wood fibers and produce pulp.
- Screening: The pulp is screened to remove any remaining impurities and to ensure that the fibers are the right size for papermaking.
- Bleaching (optional): The pulp may be bleached to achieve the desired color or brightness.
The main processes and techniques are Stone Groundwood Pulping (SGW), Pressure Groundwood Pulping (PGW), Thermo-Mechanical Pulping (TMP) or Chemi-Thermo-Mechanical Pulping (CTMP). The main raw materials, yields on wood and end-uses of pulps are summarised in the table below:
| Pulping process | Raw material | Yield on wood | Typical end-uses |
| Groundwood pulp | Spruce and fir(softwood) | 95-97% | Printing & writing papers and newsprint |
| TMP | Spruce and fir(softwood) | 93-95% | Printing & writing papers and newsprint |
| CTMP | Dominantly spruce, but also aspen and beech, NaOH, SO2 and H2O2 | 90-94% | Printing & writing papers, tissue and packaging boards |
Overview over the mains steps in mechanical pulping:
There are two processes used mainly for the manufacturing of mechanical pulping. In the stone ground wood process (SGW) or in the pressurised ground wood process (PGW) logs are pressed against a rotating grinder stone with simultaneous addition of water. Refiner Mechanical Pulps (RMP, Thermo-Mechanical Pulps = TMP) are produced by defiberizing wood chips between disc refiners. The elements causing the mechanical action – grits on a pulp stone in the grinder and bar edges on a steel disc in the refiner – will give the resulting pulps a typical blend of fibers and fiber fragments. Groundwood pulp has a higher proportion of fine material and damaged fibers giving the pulp good optical and paper-surface properties. The more gentle treatment in the refiners produces a higher yield of intact long fibers which gives the pulp higher strength, which is valuable in furnishes for products with a high requirement on runnability.
The characteristics of the pulp can be affected by increasing the processing temperature and, in the case of refining, by the chemical treatment of the chips. Both steps will increase the energy consumption as well as the pollutant level because of a lower pulping yield. The chemi-thermo-mechanical-pulping process (CTMP), in which the wood is pre-softened with chemicals, is generally considered to be a mechanical pulping technique since the chemicals principally soften the lignin prior to the mechanical stage rather than fully dissolve it out as in true chemical pulping processes.
Most mechanical pulping is integrated with paper manufacture. Mechanical pulp is typically included in a paper furnish to increase the opacity of the paper product.
Equipment and Machinery Used in the Mechanical Pulping Process
The equipment and machinery used in the mechanical pulping process may vary depending on the type of process used, but generally includes the following:
Grinders or chippers: These machines are used to grind or chip the wood into small particles.
Refiners: These machines use pressure to separate the wood fibers and produce pulp.
Screens: These machines are used to screen the pulp and remove impurities.
Bleaching equipment (optional): If the pulp is to be bleached, equipment such as hydrogen peroxide or chlorine may be used.
Factors That Affect the Efficiency of the Mechanical Pulping Process
Several factors can affect the efficiency of the mechanical pulping process, including:
Wood species: Different wood species have different properties that can affect the efficiency of the process.
Wood moisture content: The moisture content of the wood can affect the grinding and refining process.
Refining conditions: The pressure and temperature used in the refining process can affect the quality and yield of the pulp.
Equipment maintenance: Regular maintenance of the equipment is important to ensure that it is functioning properly and efficiently.
Screening efficiency: The efficiency of the screening process can affect the quality of the pulp and the yield of the process.
Advantages of the Mechanical Pulping Process
Cost-effectiveness
One of the main advantages of the mechanical pulping process is its cost-effectiveness. Compared to other pulping methods, such as chemical pulping, the mechanical pulping process requires less energy and fewer chemicals, making it a more affordable option for producing pulp for papermaking. This cost-effectiveness makes it an attractive option for producing newsprint and other low-cost papers.
High Yield
Another advantage of the mechanical pulping process is its high yield. The process is able to extract a high percentage of wood fibers from the raw material, meaning that more pulp can be produced from a given amount of wood. This high yield also reduces the amount of waste generated during the production process, making it a more sustainable option for producing pulp.
High-quality Pulp
Despite being a cost-effective method, the mechanical pulping process is still capable of producing high-quality pulp. The resulting pulp has good strength properties, making it suitable for use in newsprint and other low-cost papers. The pulp can also be bleached to achieve a desired level of brightness, making it suitable for use in other paper products.
Sustainable Resource Utilization
The mechanical pulping process is a more sustainable option for producing pulp compared to other pulping methods. This is because it uses less energy and fewer chemicals, which reduces the environmental impact of the process. Additionally, the process is able to utilize a wider range of wood species, including those that are not suitable for other pulping methods, which helps to maximize the utilization of forest resources.
Disadvantages of the Mechanical Pulping Process
Environmental Impacts
One of the main disadvantages of the mechanical pulping process is its environmental impact. While it uses fewer chemicals than other pulping methods, it still generates significant amounts of waste that must be disposed of properly. Additionally, the process can lead to the emission of greenhouse gases and other pollutants, which can contribute to climate change and air pollution.
Quality Concerns
Another potential disadvantage of the mechanical pulping process is the quality of the resulting pulp. The process can result in shorter wood fibers, which may not be suitable for certain types of paper products, such as fine paper or specialty paper. Additionally, the mechanical process can result in a higher percentage of shives or other impurities in the pulp, which can affect its quality.
Energy Consumption
While the mechanical pulping process is generally considered to be a more energy-efficient option compared to other pulping methods, it still requires significant amounts of energy to operate the equipment and machinery involved in the process. This energy consumption can contribute to the overall carbon footprint of the papermaking industry.
Health and Safety Risks
Finally, there are potential health and safety risks associated with the mechanical pulping process. The equipment used in the process can be dangerous if not operated properly, and workers may be exposed to harmful dust or other pollutants generated during the process. Proper safety measures and training are necessary to ensure the health and safety of workers involved in the process.
Conclusion
In summary, the mechanical pulping process is a cost-effective and high-yield method for producing pulp for papermaking. It has been used in the paper industry for over a century and is still widely used today, particularly for producing newsprint and other low-cost papers. However, the process also has its disadvantages, including its environmental impact, potential quality concerns, energy consumption, and health and safety risks.
Overall, the mechanical pulping process plays an important role in the paper-making industry by providing a more sustainable and cost-effective option for producing pulp. While there are some concerns regarding its environmental impact and potential quality issues, it remains a valuable option for producing newsprint and other low-cost papers. Additionally, proper safety measures and training can help to mitigate the health and safety risks associated with the process.
In the future, the mechanical pulping process may continue to be improved and optimized to further reduce its environmental impact and increase its efficiency. For example, advancements in equipment and technology may allow for the more efficient utilization of raw materials and the reduction of waste generated during the process. Additionally, efforts to develop more sustainable and renewable sources of raw materials may help to further increase the sustainability of the process.