CHEMICAL PULPING Nordic Pulp & Paper Research Journal Vol 29 no (3) 2014 383 Bleaching of kraft pulps produced from green liquor pre- hydrolyzed South African Eucalyptus grandis wood chips Jerome E. Andrew, Jonas Johakimu and Bruce, B. Sitholé KEYWORDS: Eucalyptus grandis, Biorefinery, Green liquor, Pre-hydrolyzed kraft pulp, Bleaching, Chlorine dioxide, Ozone, Hexenuronic acids SUMMARY: The effect of hemicellulose pre-extraction of South African Eucalyptus grandis wood chips using green liquor, on subsequent kraft pulping and bleaching processes was studied. This was done in the context of a biorefinery mill producing both ethanol and bleached Eucalyptus kraft pulp. The pre-extraction (hydrolysis) of hemicelluloses prior to kraft pulping reduced the demand of pulping chemicals by as much as 20% and still resulted in pulps with similar kappa numbers and yields as pulps produced from unhydrolyzed wood chips (control pulps). In addition, the hexenuronic acid (HexA) content of the brownstock prehydrolyzed kraft (PHK) pulps were about 30% lower compared to the control pulps. This led to improvements in the subsequent pulp bleach-ability. Savings in chlorine dioxide for PHK pulps ranged between 2-18% for sequences that used chlorine dioxide as the primary oxidative bleaching chemical. When ozone was used in combination with chlorine dioxide, mixed results were obtained – PHK pulps showed better bleach- abilities when ozone followed immediately after oxygen delignification, i.e. OZDED or OZD(EP)D, but not when ozonation followed the acid hydrolysis (A) stage. ADDRESSES OF THE AUTHORS: Jerome E. Andrew (jandrew@csir.co.za) and Johannes Johakimu (jjohakimu@csir.co.za): CSIR, Natural Resources and the Environment, PO Box 17001, Congella, Durban, 4013, South Africa, and Bruce B. Sitholé (bsithole@csir.co.za): CSIR, Natural Resources and the Environment/University of KwaZulu-Natal, Department of Chemical Engineering, Durban, South Africa. Corresponding author: Jerome Andrew The pulp and paper industry, like many other manufacturing industries in the world is facing significant challenges related to soaring energy costs and water shortages. Some of these challenges have a direct influence on production costs and hence on the international competitiveness of pulp and paper producers. In addition, there is ever-increasing pressure on the industry to make changes, improvements and/or adaptations to their processes in order to achieve cleaner production technologies that are more environmentally friendly. This has resulted in declining revenues in recent years which have been further exacerbated by a strong decline in the demand for pulp and paper products, leading to some companies closing or downsizing their mill operations. Maximising the capital effectiveness of pulp and paper operations has thus become a focus area of the industry as a way to increase revenues. One promising approach to achieve this could be by adaptation of the biorefinery concept (van Heiningen 2006). This means that traditional tree growing and liberation of fibres, whilst inefficiently burning spent liquors, becomes old technologies. In its place is forest stewardship and the processing of wood in a way to extract fibre, fuel, chemicals and power streams that are valued by society and the marketplace. Producing bioenergy and new biomaterials, in addition to traditional wood, pulp and paper products, may thus lead to competitive synergies, new markets and increased product flexibility for the pulp and paper industry, whilst at the same time, mitigating some of its environmental impacts (van Heiningen 2006; Chambost et al. 2008; Carvalheiro et al. 2008; Al-Dajani, Tschirner 2008; Helmerius et al. 2010; Jin et al. 2010). One option to convert a pulp mill into a pulp and biofuels biorefinery involves the extraction of hemicelluloses from wood prior to pulp and paper manufacture. Pre-treatment of wood chips prior to pulping using dilute acids, bases, pressurized hot water, or steam, enables extraction and recovery of hemi- celluloses traditionally wasted in the spent liquor (Kautto et al. 2010; Vila et al. 2012). Further treatment of the extracted hemicelluloses through evaporation, acidic precipitation and/or ultrafiltration can provide feedstocks for chemicals and biofuels production (van Heiningen et al. 2003; Périn-Levasseur et al. 2010). However, at the same time, it is important to understand the impact of the hemicelluloses pre-extraction processes on traditional paper making properties during the subsequent pulping and bleaching stages. It has been found, for example, that during bleaching, pulp yield and viscosity, a primary focus for pulp and paper manufacture, is severely affected when the hemicelluloses are extracted prior to kraft pulping (Chirat et al. 2011). The removal of hemi- celluloses prior to pulping has also been found to decrease the pulp beatability/refinability, tensile and burst strengths (Kautto et al. 2010; Goyal et al. 2007; Yoon et al. 2008; Duarte et al. 2011). In some cases, reduced tear and zero span tensile strengths have also been reported (Santos et al. 2008). These drawbacks may have serious implications on the economic feasibility and commercial integration of the approach within the pulp and paper industry. On the other hand, the partial removal of the hemicelluloses prior to pulping and modification of the subsequent pulping process may also offer certain advantages. These may include enhanced removal of lignin during kraft pulping (Kautto et al. 2010; Duarte et al. 2011; Olm et al. 1996; Ban et al. 2004; Johakimu et al. 2011; Johakimu, Andrew 2013), reduction of solids content in the black liquor leading to increased pulp production due to improved recovery boiler capacity (Carvalheiro et al. 2008; Al-Dajani et al. 2008; Helmerius et al. 2010; Johakimu, Andrew 2013), increased reduction in kappa number after oxygen delignification (Kautto et al. 2010; Chirat et al. 2011), better pulp bleachability