# Paper-Mill Wastes for Bioethanol Production in Relation to Circular Economy Concepts: A Review
## 2 Paper and Cardboard Manufacturing Process
### Pulping Process  
The pulping process refines raw material into fibers to form paper or paperboard. During pulping, cellulose fibers are broken apart by mechanical and/or chemical digestion to optimize paper strength. This includes digestion and shearing at elevated temperature and pressure, resulting in fibers swelling and straightening out and creating fines. This process can cause the fiber's internal area to collapse, leading to an increased bonding area, although the bulk (density) of the paper is decreased [30]. Pulping can be done by mechanical, chemical, or semi-chemical pulping methods. The selected pulping method is based on the properties required in the final paper and considers the fiber's morphology and characteristics.  
Mechanical pulping grinds and shreds the wood chips and can convert up to 95% of wood's dry weight [31], yet requires a large amount of energy. Fibers produced by mechanical pulping are lower in strength than chemically pulped and are mainly used for newspaper and non-permanent paper products [32]. Chemical pulping is mainly done by the kraft pulping method [33]. In this method, alkaline solutions (NaOH and Na\({}_{2}\)S) at 155-175 \({}^{\circ}\)C are used to produce writing paper, paper bags, and specialties. During this process, lignin is degraded into smaller alkali-soluble fragments, mainly through the cleavage of \(\alpha\)- and \(\beta\)-aryl ether bonds, which leaves the cellulose fibers intact. The delignification rate decreases at around 90% lignin removal, probably due to the remaining alkali-stable bonds between lignin and polysaccharides/hemicelluloses and within lignin. Since the selectivity of delignification is reduced at that stage, the process is stopped at a maximum of 95% delignification, and the remaining lignin is removed by bleaching agents [34]. Other chemical pulping methods use sulfuric acid (sulfite pulping). These processes allow easier bleaching due to the oxidative nature of the acids. However, the resulting pulp is less strong than kraft pulping [32]. Semi-chemical pulping combines mechanical processes with chemicals. An example of semi-chemical pulping is utilizing natural substances, such as Na\({}_{2}\)CO\({}_{3}\), followed by the mechanical separation of the fibers. As the generated pulp in such a process is rather stiff, it is mainly used for corrugated carton [32; 35]. Since printing and writing paper have higher demands while newspapers have a negative growth rate, chemical pulp has become dominant, while mechanical pulp has been reduced in volumes [36]. Additionally, the environmental impact of the kraft  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Compound**} & \multicolumn{2}{c}{**Chemical Composition (\%)**} \\ \cline{2-3}  & **Softwoods** & **Hardwoods** \\ \hline Cellulose & 40–44 & 45–50 \\ \hline Hemicellulose & 25–29 & 25–35 \\ \hline Lignin & 26–31 & 18–24 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Composition of softwoods and hardwoods, adapted from [22].  
process is smaller, as the chemicals in use are simple, and the energy recovery system is efficient [37].  
Delignification degree is determined by a redox reaction. The result is presented by the kappa number, which is an important mill control parameter. The higher the kappa number, the higher the lignin content in the pulp. If the kappa number is too low, the pulp yield and physical properties will decrease considerably [38]. Recent studies show that cellulose pulp with a kappa number of 64 is considered to be optimal, based on pulp properties and expected costs. Both the tear index and the produced paper area of paper made from that pulp exhibited the highest score of 6.13 mN \(\times\) m\({}^{2}\)/g and 5596 m\({}^{2}\) from 1 metric ton of wood. The total paper production yield was 45.7%, only slightly less than for pulp, with a high kappa number of 89.7, with 47.2% [39].