Thermomechanical Pulping Analysis

iii. Thermomechanical Pulping (TMP): the wood chips are preheated to soften them before grinding. This type gives the highest grade of mechanical pulp with long fibers, but it uses high energy due to its use of steam (Bajpai, 2012). The pulp gotten is dark-coloured therefore increasing the cost to bleach (Martin et al, 2000). It is the mechanical process that is commonly used. iv. Chemi- Thermomechanical Pulping (CTMP): sodium sulphite is added to the wood chips before pulping to allow for better strength of the fibers and high fiber content. The fibers are then preheated at 120-130°c and ground in the refiner. The fibres produced are very flexible with lighter pulp. Its use of a lot of energy is its major disadvantage. (Martin et al, 2000).

Sulphite pulping: The remaining 2% of the chemical pulping industries in U.S make use of this process (AF&PA, 2002). It is mostly used in Europe because the spent liquor is used for ethyl alcohol production. The solvents used in this process are sulfurous acid (H2SO3) and Bisulfate ion (HSO3) which are produced from burnt sulfur mixed with a basic solution (U.S. DOE 2005a;Martin et al, 2000). They are used to produce specialty products like rayon and acetate (Wang et al, 2008) Due to the fact that chemical pulping uses a lot organic chemicals and have a lot of waste product (lignin), the waste products are burnt to substitute fuel oil used to run paper mills and the inorganic chemicals are recovered and reused. The two processes produce similar quality of paper
However, the Kraft process is commonly used because it can be used for different types of tree species. The fiber strength is higher as opposed to that of the sulfite process which gives fiber with shorter length. The Kraft process removes higher amount of lignin of about 90% and it has a higher tolerance rate of wood contaminants. Compared to the sulphite process, the chemical recovery is very efficient (U.S. EPA, 2002; U.S. DOE 2005a)
Table 2.1: Pulping Processes and Their

Light contaminants float to the top of the slurry and are removed using ragger mechanism while heavy contaminants are removed from the bottom of the tank (Martin et al, 2000;Biermann, 2005). The ink from the recycled paper is removed by applying chemical surfactants.This method uses small amount of energy compared to both mechanical and chemical pulping which varies with the type of contaminants present and the resulting pulp. Modern contaminant removal methods are being applied now which make the paper produced from recycled pulp to be almost as good as other types of paper except those of the highest quality gotten from long fibers (Martin et al,