The primary goals of pulping are to free fibers in wood from the lignin that binds these fibers together, and then to suspend the fibers in water into a slurry suitable for paper making. As the oldest form of pulping, mechanical pulping uses mechanical energy to weaken and separate fibers from wood via a grinding action. The advantage to mechanical pulping is that it produces much higher yields than chemical pulping processes (up to 97% - BREF, 2010. p. 422). However, because this process does not dissolve lignin and because mechanical grinding produces shorter fibers, the fiber strength and age resistance of the resulting pulp are low. Consequently, most mechanical pulp is used for lower grade papers such as newsprint, magazines, and catalogues. Mechanical pulping also requires more raw materials screening to remove contaminants such as dirt, and knots than chemical pulping processes. There are four main types of mechanical pulping (Kramer, et al., 2009. p. 15):

• Stone groundwood (SGW) pulping: In the SGW process, small logs are ground against artificial bonded stones made of silicon carbide or aluminum oxide grits. The process gives a high yield, but the fibers produced can be very short and often must be combined with expensive chemical fibers to be strong enough to pass through the paper machine and subsequent coating and printing processes.
• Refiner mechanical pulping (RMP): In RMP wood feedstock is ground between two grooved discs. The process keeps the high yield advantages of the SGW process, while producing somewhat longer fibers with greater strength. This permits lighter weight paper to be used for printing and result in more print media per ton of feedstock. The RMP process can use wood feedstock other than logs, such as wood scraps and sawdust from lumber mills.
• Thermomechanical pulping (TMP): In TMP wood chips are first steamed to soften them before being ground in the same manner as the RMP process. The TMP process generates the highest grade mechanical pulp but is also a high energy intensity process due to its steam use. This process can also produce a darker pulp that is more costly to bleach. Despite these drawbacks, TMP is the most common mechanical process in use today.
• Chemi-thermomechanical pulping (CTMP): CTMP involves the application of chemicals to wood chips prior to refiner pulping. The chemical pre-treatment of wood chips allows for less destructive separation of fibers from the feedstock, resulting in longer fibers, higher fiber content, and far fewer shives. The CTMP process also produces more flexible fibers (which provide higher sheet density, burst strength, and tensile strength) and higher pulp brightness than the TMP process. Its primary drawback, like TMP, is that it is a high energy intensity process (Kramer et al., 2009. p.15).

The specific energy consumption in mechanical pulping is dependent on the particular pulping process, the properties of the raw material and, to a large extent, the quality demands on the pulp set by the end product (the freeness value). A substantial part (>90%) of the energy input to the mechanical pulping processes is converted into heat, some of which can be recovered (BREF, 2010. p.423).