## 3 Results
### Particles Size
#### 3.5.2 Effect of Coagulation Time  
Coagulation time is an important parameter that should be taken into consideration during a precipitation and filtration process. Figure 9 shows that particles size increased as a function of coagulation time. This is in perfect coherence with the results obtained above (Section 3.1.2). In fact, when a long coagulation time is used, molecules of lignin had enough time to agglomerate and make particles with large sizes. This explains the higher filtration flux obtained when 180 min of coagulation time was applied. However, using a short coagulation time led to the formation of small particles due to insufficient time for lignin particles to agglomerate with each other and to form particles big enough to facilitate the filtration. These results are also consistent with those obtained for molecular weight (Section 3.4.2). The molecular weight of lignin decreased when a higher coagulation time was used. This is logical since the contribution of molecules with low molecular weight became higher, which led to a lower average molecular weight and, at the same time, contributed to an increase in the size of lignin particles. Based on the results and discussion above, it appears that the optimal conditions to recover lignin from the pre-hydrolysate are 8.5 kg\(\cdot\)m\({}^{-3}\), 180 min, and 40 \({}^{\circ}\)C for sulfuric acid concentration, coagulation time, and filtration temperature, respectively. These conditionswere the most advantageous to use because they provided the highest lignin recovery with the highest purity even though no washing step was used. The incorporation of a washing step in the proposed process might enhance the quality and the purity of the lignin. The lignin obtained is also characterized by a low molecular weight and polydispersity. For the purpose of comparison, lignins extracted from kraft hardwood (KHW) and softwood (KSW) black liquors using a commercial process (LignoForce systemTM) were analyzed with respect to the molecular weight, FTIR, and 13P NMR analysis and compared with the pre-hydrolysate lignin (PHL) that was obtained using the conditions mentioned above. The molecular weight distribution of KSW, KHW, and PHL lignins are shown in Figure 10. It can be seen from this figure that PHL lignin has a significantly lower molecular weight and polydispersity index than KSW and KHW lignins. This was confirmed when both UV (A) and MALLS (B) detectors were used. This can be attributed to a higher fraction of small lignin molecules in PHL lignin compared to KSW and KHW lignins. This is clearly highlighted in the elution volumes shown in Figure 10A: 21, 19, and 18 mL for PHL, KHW, and KSW, respectively. The same trend was observed when the MALLS detector was used. This demonstrates the advantage of pre-hydrolysis treatment (which can be considered a simultaneous depolymerization step) that led to the formation of a lignin with a lower molecular weight and narrower distribution than kraft lignins. As shown in Figure 11, FTIR spectrums demonstrated that KSW, KHW, and PHL lignins are rich in hydroxyl groups. The big band around 3400 cm\({}^{-1}\) is due to the stretching vibration of phenolic and/or aliphatic hydroxyl groups [29]. The intensity of this band was clearly higher in the case of PHL lignin, indicating the presence of higher hydroxyl group contents compared to KSW and KHW lignins. The absorption peak around 1700 cm\({}^{-1}\) is associated to C=O stretching from the aldehyde, ketone, and ester groups [30]. The absorption peak at around 1114 cm\({}^{-1}\) which is associated to the syringl units [31] was also higher for PHL and KHW than KSW lignin. This result was expected since it is well known that lignin from hardwood species contains more syringl groups than lignin from softwood species.