Beneficiation of sawdust waste material within the context of an integrated forest biorefinery.
Andrew, Jerome Edward.
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Wood waste streams such as sawdust are composed of potentially high value products and finding alternative and innovative uses for them may transform the face of Forestry, Timber, Pulp and Paper (FTPP) industry, both economically and environmentally. Currently, a large portion of these waste streams are either stockpiled on site or landfilled, and these are not sustainable options for waste disposal going into the future. The diversion of all organic waste from landfill has now become a priority in South Africa. Within the context of the biorefinery concept, and in direct response to the afore-mentioned priority, the key focus of this study was to explore options for the beneficiation of sawdust waste streams produced primarily, but not exclusively, from the sawmilling industry. Two options were investigated, with the primary objective being to investigate the production of cellulose nanocrystals (CNC) from sawdust waste material. Cellulose nanocrystals are nanoparticles (<100 nm) that have impressive mechanical, optical and rheological properties. As a result, they have potential applications in several industries such as the automotive, construction, paper, medical, food, environmental and industrial sectors. Typically, CNC are produced from high purity cellulose products via one of several processes that degrade the amorphous regions of cellulose leaving behind the crystalline CNC. This dissertation presents a novel technology that combines ammonium persulphate (APS) oxidation with acid hydrolysis (AH) to produce CNC directly from wood sawdust, without the need for pre-treatment stages such as pulping and bleaching that are conventionally used for the isolation of cellulose from wood. In the novel (APS+AH) process, the sawdust was treated with 3 M APS solution at 60°C for 6 h, thereafter followed by AH of the residual material with 64% sulphuric acid at 50°C for 1-2 h. Cellulose nanocrystals with dimensions ranging between 122-213 nm in length and 4-6 nm in width were produced. Their characterisation by Fourier transform infrared (FTIR) spectroscopy, and comparison to model compounds for CNC produced by acid hydrolysis of microcrystalline cellulose (MCC), confirmed that the structural and compositional integrity of cellulose was not compromised following the (APS+AH) process. The resulting CNC also exhibited high crystallinity (81%), as revealed by X-ray diffraction (XRD) analysis, and high thermal stability (320°C), as revealed by thermogravimetric (TG) analysis. These are all important pre-requisites for most thermoplastic composite applications. In addition, a CNC yield of 40% was achieved, which when expressed as a percentage of the original wood, represented a four-fold increase in yield compared to conventional processes that are used for CNC production and which make use of acid hydrolysis and wood derived cellulose products as the starting material. As part of the process to delignify and isolate cellulose from sawdust prior to the production of CNC, the second objective of this study was to investigate conventional industrial processes such as the kraft and pre-hydrolysis kraft (PHK) pulping processes for delignification of the sawdust. It was found that sawdust kraft pulps with acceptable yields (48%) and fibre morphologies that were comparable to conventional kraft pulps were produced using typical kraft pulping conditions. As expected, the exception was pulp strength properties such as burst, tear and tensile strengths which were 50-70% lower than conventional kraft pulps produced from wood chips. During the pre-hydrolysis stage of the PHK process, up to 24 g l-1 xylose could be removed from sawdust with minimal removal of lignin (0.1 g l-1) and cellulose (2.5 g l-1). Pulping of the pre-hydrolysed sawdust resulted in pulp yields of around 35%, expressed as a percentage of the original wood. Characteristics of the unbleached PHK sawdust pulp such as pentosan content (3-4%), brightness (41%) and viscosity (760-850 ml g-1) alluded to its potential for the production of dissolving pulp.