Browsing by Author "van der Watt, Liezl Carien"

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    Recovery and functionalization of cellulosic fibers from pulp and paper mill waste streams
    (Stellenbosch : Stellenbosch University, 2022-04) van der Watt, Liezl Carien; Chimphango, Annie Fabian Abel; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Pulp and paper mills lose between 1 to 3% of their production to waste effluent. The effluent contains cellulosic fibre which is normally discarded as sludge to landfills or is incinerated. This is a waste of valuable resources. The objectives of the current project were: (i) identify waste streams in Kraft pulp and paper mills, (ii) develop and validate a method to recover these fibres and (iii) to develop and validate a method to functionalize the recovered fibres to nanocellulose. The recovery method was developed based on fibre size distribution and validated based on flotation efficiency (> 55%) while the functionalization method was based on forming nanocellulose gel-like suspensions within 5 min of mechanical treatment. The waste sources were categorized into continuous and batch losses. The largest source of continuous waste is from the paper machine whitewater not treated by save-all devices (a unit installed to recover fiber internally) whereas, the greatest batch losses occur during grade changes. Sludge samples were collected from various sampling points in the Kraft mill bleaching plant as well as from paper machine broke systems and paper machine cut-offs or trimmings. The fibres from these locations are normally sprayed into paper machine, alkaline and acid effluent drains. Fibres from the effluent streams were recovered by flotation with colloidal gas aphrons (CGAs) generated using two types of non-ionic surfactants (synthetic and green-based). The average size of fibres collected were 93±3.1, 72±3.9 and 93±5.3 𝜇𝑚 for paper machine, alkaline and acid effluent streams. The synthetic surfactant used was Triton X-100 and the alternative, green-based surfactant tested was N-Dodecyl 𝛽-D-maltoside (DDM). The average recovery efficiencies achieved by both surfactants were between 50 and 78%. DDM is a promising alternative to Triton X-100 and achieved similar efficiencies. Functionalization of nanocellulose fibres was achieved by phosphorylation pre-treatment using ammonium dihydrogen orthophosphate and urea in the ratio 1:1.2:19.6, which gave nanocellulose gel-like suspensions within 5 min of blending using a high speed blender (900 W nutribullet). The suspensions obtained were highly heterogeneous by being both in the microscale (1.0 to 2.0 𝜇m) and nanoscale ranges (50 to 800 nm). Alcohol insoluble (95% ethanol) films were made from these recovered fibres. The results obtained after validating the fibre recovery and functionalization methods formed a basis for evaluating the feasibility of using soft sensors to predict the quality of nanocellulose produced. The properties of the fibres collected along the different stages in the bleaching sequence produced nanocellulose materials with varying quality as determined by particle size distribution and other measured properties of the recovered fibres like zeta potential (between -31 and -58 mV ) and crystallinity indices (between 13 to 27%). By using index quality models combined with near-infrared spectroscopy results a soft sensor can be developed that can accurately predicted the quality of the nanocellulose material produced. Based on the results of this study it was concluded that all effluent streams can be combined into a single waste source from which fibres can be recovered and functionalized to high-value products like nanocellulose.