Haile et al. Bioresour. Bioprocess.
(2021) 8:35
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white papers. Nearly half of the Kraft production is in bleached grades. The bleaching of pulp is carried out to increase the whiteness and brightness of the pulp (Bajpai 2013; Hammett et al. 2001). This is important for mak- ing the paper suitable for selected products such as those produced in tissue and printing enterprises. Enhance- ment of physical and optical qualities of the pulp is achieved by removing impurities such as entangles of fib- ers, barky fragments, or decolorizing the lignin making it a potential benefit in the chemical pulping process (Al- Dajani and Tschirner 2008, Chakar and Ragauskas 2004). Kraft bleaching has been refined into a stepwise pro- gression of chemical reaction, evolving from a single- stage hypochlorite treatment to a multi-stage process, involving chlorine), chlorine dioxide, hydrogen peroxide, and ozone. The optimized and controlled condition must be used in the bleaching process. If bleaching conditions are too severe there will be fiber damage, leading to a lower strength of the paper (Brännvall 2009; Angevine, 1998). After bleaching stock preparation is conducted to convert raw stock into finished stock to be used in the paper machine. Raw stock can be available in different forms. In batch processes, the delivery can be in loose pulp or bale and the integrated pulp to paper conversion suspensions can be used as well (Dimmel and Gellerstedt 2009, 2009). In the chemical pulping process, it is necessary to recover the used cooking chemicals, and a proper chemi- cal recovery system is needed (Gellerstedt 2009; Walker 2004). At the pulping mills, the weak black liquor also called spent cooking liquor, from the different rinsing stages is transferred to the chemical recovery which as well is situated alongside the mills (Santos et al. 2011; Al-Dajani and Tschirner 2008). The chemical recov- ery process is conducted at different stages to ensure proper reconstitution of the cooking liquor. Initially, the weak black liquor is concentrated followed by combus- tion of organic matters and reduction of inorganic mat- ters. Finally, the cooking liquor is reconstituted in the required concentration. All the stages need critical con- trol on the parametric allowances. The quality of paper produced from the pulp is affected by the quality of water and different companies have incorporated water treat- ment schemes along with the pulping process. To prevent deposits on pulp, water treatment chemicals such as anti- scale agents and pitch control agents are incorporated during the route of manufacturing. As part of pulping process dregs, lime mud, and grits are generated in the chemical recovery process. Green liquor contains sodium sulfide and sodium carbonate; and insoluble unburned carbon and inorganic impurities called dregs, which are removed in a series of clarifica- tion tanks. The green liquor is a residue of dissolution of
the molten inorganic salts (smelt) from the black liquor concentration process (Angevine 1998, Brännvall 2009). The lime mud is generated in the causticization process of chemical recovery in the decantation of the green liq- uor using calcium oxide (lime) (Ai et al. 2007). The grits are originated from the white liquor slurry in the slacker by gravity and classified as unreacted lime (Kinnarinen et al. 2016). Grits are insoluble compounds, most of which come from purchased lime, which collect in the slacker and need to be removed and disposed of. Final pulping operations are involved in the boiler unit which causes the generation of large content of ashes. In particular, fly ash is produced as by-products of the biomass combustion process which is generated during the high-temperature combustion of hog fuel in power boiler units (Cherian and Siddiqua 2019). As the flue gas approaches the low-temperature zones, the fused substances solidify to form fly ash. The fly ash consists of fine particulates and precipitated volatiles, typically with a high specific surface area, whereas bottom ashes tend to be coarser in texture (Cherian and Siddiqua 2019; Scheepers and du Toit 2016). Most of the pulping operations so far especially those involving chemicals are not ecofriendly. Nowadays new pulping methods are used for manufacturing differ- ent grade pulps that utilize enzymes (Lin et al. 2018). These enzymatic pulping techniques are being used for the preparation of dissolving pulp for the production of fibrous materials. In the enzymatically assisted pulping process xylanase, cellulase, and hemicellulase enzymes are utilized for segregation of the pulp from the rest of lignocellulosic mass and after-treatment (Lin et al. 2018; Rashmi and Bhardwaj Nishi 2010; Yang et al. 2019b). It is reported that the purity of enzymatically produced pulp is better than conventional kraft pulping processes. Papermaking process The process adopted for the making of paper is identi- cal for all pulp types (Bajpai 2018b; Woiciechowski et al. 2020). The pulp from the chest compartment is screened which when required is refined to the required level. The pulp slurry is formed by mixing the refined pulp in water in a wet end operation. This slurry at the headbox is put through a paper machine and pressed by the press com- partment. In the press section, the sheet forming process is commenced through the draining of the water. The next operation is passing of the formed sheet in a dryer which involved hierarchically arranged compartments of the dry end for coating and drying. The final dried and finished product is transferred to calendaring operation for reducing the thickness of paper as required and sheet surface smoothing before winding on to the take-up reels.
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