## 3 Results
### Particles Size
#### 3.5.2 Effect of Coagulation Time
Analyses of the different functional groups of KSW, KHW, and PHL lignins are shown in Figure 12. The 31P NMR spectra show that the number of carboxylic hydroxyl units is almost the same for all fractions of lignin recovered from softwood and hardwood black liquors and from the pre-hydrolysate. However, it appears that the pre-hydrolysate lignin is more reactive than KSW and KHW lignins since there are more phenolic, aliphatic, and total hydroxyl groups in lignin obtained from the pre-hydrolysate than in lignins extracted from softwood and hardwood black liquors. Thus, the pre-hydrolysate lignin could be an excellent candidate for applications within phenol-formaldehyde resins and polyurethane-based materials. Figure 10: Molecular weight distribution of kraft softwood (KSW), kraft hardwood (KHW), and pre-hydrolysate (PHL) lignins as determined by (**A**) UV and (**B**) MALLS. With the optimal conditions determined and the PHL lignin quality demonstrated, it is time to investigate the mass balance for the proposed process. Figure 13 shows the mass distribution of different pre-hydrolysate components in the feed, retentate, permeate, and filtrate obtained using the nanofiltration and precipitation of lignin from the pre-hydrolysate. It can be seen that, while most of acetic acid and formic acid ended up in the  
Figure 11: The FTIR of KSW, KHW, and PHL lignin. Figure 12: Quantitative \({}^{31}\)P NMR spectra for KSW, KHW, and PHL lignins. permeable during the nanofiltration step, most of the lignin and sugars were maintained by the membrane in the retentate. However, it seems that small amounts of lignin and sugars were lost in this step. This is due to the losses of retentate in the system tubing (an amount of the retentate remained in the piping and tubing of the nanofiltration system). This problem can be easily overcome at the industrial level by adopting a continuous membrane filtration system. The filtrate obtained after precipitating and recovering lignin (52% of the initial lignin) contains most of chemical products present in the retentate. A valorization step is encouraged to diversify the portfolio of the pulp mill. Since it already contains sulfuric acid (resulting from the lignin precipitation step), a chemical conversion of sugars into a wide range of bioproducts such as furfural and hydroxymethylfurfural could be the most adapted for this case. The best way to valorize the resulting solution (residue) is by sending it to the recovery cycle because it contains the remaining organic bioproducts and lignin. The latter has a high calorific value and could be burned in the recovery boiler to produce energy. For the permeate, the use of ion-exchange resin technology might remove the acetic acid and then the water obtained can be reused in the pre-hydrolysis step. Lignin is a complex and three-dimensional macromolecule. This usually leads to a nonuniform reaction with other reagents. In fact, only functional groups at the surface of lignin can be easily reached. In addition to the problems related to the high diversity of its structure, lignin also has another issue of high importance. This issue is the large distribution of its molecular weight. Lignin is a very heterogeneous molecule [32; 33; 34; 35]. In the same sample of lignin, the molecular weight of molecules of lignin can change significantly [30]. This makes its marketing a difficult task [32; 35] and its use in some industrial applications a problematic issue [32; 34; 36]. Thus, the structure, the reactivity, the polydispersity, and thermal properties of lignin change according to its molecular weight. [PERSON] et al. [37] and [PERSON] et al. [38] reported that lignin fractions with a low molecular weight have good homogeneity and high guaiacyl content compared to lignin fractions with high molecular weight. [PERSON] et al. [35] found that the glass transition temperature of lignin increases with its molecular weight. The same observations were reported by [PERSON] et al. [39] and [PERSON] et al. [37].