Based on the experimental data obtained in the present study, the following conclusions can be drawn:
Of the xylan pre-extraction processes investigated i.e. dilute acid and mild alkaline, the latter demonstrated favourable results. It was possible to solubilise high molecular mass xylan of 53,400 g/mol from E. grandis (paper II), 32,793 g/mol from SCB (paper III) or 42,500 g/mol from giant bamboo (paper IV) prior to modified kraft or sodaAQ pulping whilst subsequent kraft or sodaAQ pulping of the extracted materials retained or improved final pulp yields and pulp viscosities at comparable kappa number.
Futhermore, the demand of the pulping chemicals could be reduced although a decrease of <20% in some handsheet strength properties was observed.
Among the three different lignocellulosic materials studied, SCB required milder processing conditions compared to E. grandis and giant bamboo. As expected, attractive xylan yield of 69.1% was easily recovered from herbaceous SCB under moderate alkaline conditions while E. grandis and giant bamboo had lower xylan solubility with only 12.4% and 13.6% recovered respectively. The combination of high xylan content together with lower lignin content in the non-extracted SCB explains the reasonable yields of xylan extracted from SCB prior to pulping and low processing chemical demand when the SCB was well depithed.
Nonetheless, although prepared under different pulping process, pulp fibres produced from SCB are generally of lower quality in terms of handsheet strength properties compared to E. grandis while giant bamboo was better or comparable to those of E. grandis. Alkali xylan extraction combined with mild sodaAQ pulping was the preferred option for SCB. This can be contrasted with the alkaline extraction process which integrated well with kraft pulping for both E. grandis and giant bamboo. The difference of chemical composition and morphological properties among the three feedstocks studied was evident in both xylan solubilisation and subsequent pulping performance.
The overall screened yield of kraft pulps produced from alkaline pre-extracted E. grandis or giant bamboo was maintained at desirable industrial levels of 50% and bleachable kappa number range without reduction in viscosity. These conditions improved tensile index and breaking length for pulps produced from extracted E. grandis and a slight increase in burst index was observed for pulps produced from extracted giant bamboo. On the other hand, although the pulping process used for sugarcane bagasse was sodaAQ, alkaline extraction improved the pulp yield (45%) with no reduction in pulp viscosity.
These conditions provided brighter pulps with superior tear index whilst breaking length and burst index was retained in the same level as those of pulps produced from non-extracted SCB.
Another important feature of alkaline pre-extraction of SCB was that, the higher pulp yields were produced at lower kappa number unlike high kappa number observed from giant bamboo and E. grandis pulps. In fact, pulps produced from giant bamboo presented higher kappa number compared to pulps produced from E. grandis treated under similar pulping conditions. This could be ascribed to higher extractives content observed for giant bamboo and more condensed lignin structures as documented in other works (Ribas Batalha et al., 2012). Therefore, a larger demand of bleaching chemicals to remove residual lignin can be expected for bamboo pulps.
Handsheet strength properties produced from beaten non-extracted or extracted giant bamboo pulps presented high tensile and breaking length as E. grandis pulps similarly treated, that are currently used commercially as fibre source in SA pulp industry. Particularly, the tear and burst indexes of giant bamboo pulps were much higher than in E. grandis pulps. Morphological properties of bamboo such as longer fibre length and thicker cell walls than that of hardwoods, positively influenced the tear and burst indexes of bamboo pulps.
Contrary to mild alkaline, dilute sulphuric acid pre-extraction of xylan did not favour the subsequent modified kraft pulping of the pre-extracted E. grandis, SCB or giant bamboo. The screened pulp yield and the overall handsheet paper strength properties were compromised. This could be attributed to the formation of low molecular mass carbohydrates with high reducing end groups under acidic condition that becomes more hydrolysed in the subsequent high alkaline conditions of the kraft pulping process.
When selecting the best pulping process for integration of dilute sulphuric acid hemicelluloses pre-extraction and pulping process, sodaAQ pulping is recommended depending on the amount of pre-extracted xylan. Moderate alkali charge, comparable pulp yield and handsheets strength properties observed for giant bamboo in which 11.3% xylose was extracted under dilute acid conditions made sodaAQ pulping more attractive compared to kraft pulping. AQ has an effect on stabilisation of carbohydrates against peeling reaction in pulping process.
Combination of hot water pre-extraction of SCB together with sodaAQ pulping may be the more attractive method as pulp yield was increased and the tear and burst indexes were improved. However, the hot water method was disadvantaged by liquid fractions containing lower xylan concentrations compared to acid pre-extractions.
Pre-extraction step of SCB can serve the purpose to decrease the scaling problems influenced by the silica present in most non-wood materials during pulping process, thereby improving the efficiency in chemical recovery of black liquor. The silica will solubilise from the fibres during the initial xylan extraction stage, although some purification of the solubilised xylan may be required.
The dissolution of xylan and other components into the extraction media decreased the mass of the extracted material and can increase the pulping capacity without additional investment in pulping digester. Moreover, xylan can be removed before pulping without degradation in molecular mass under alkaine conditions and the extracted xylan represents a valuable product.
With regards to the investigation of the suitability of giant bamboo as a potential source of fibre for South African pulp and paper industry, the present study showed that giant bamboo can produce pulp of sufficient quality for papermaking, especially so for the alkaline extracted giant bamboo. By utilisation of giant bamboo as a raw material for pulp and paper products, future prospects both for giant bamboo and pulp and papermaking industry will emerge. The pulp produced for giant bamboo revealed characteristics suitable for the production of paperboards.