Browsing by Author "Barnard, Eddie Gustav"

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    Paper and pulp mill waste valorisation via the production of phosphate ceramic composites
    (Stellenbosch : Stellenbosch University, 2021-03) Barnard, Eddie Gustav; Gorgens, Johann F.; Tyhoda, Luvuyo; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Paper and pulp mills in South Africa annually generate approximately 500 000 tonnes of wet paper and pulp sludge that is sent to landfill sites. Additionally, lignin is one of the most abundant biomaterials, second only to cellulose, yet due to its recalcitrance, it is mainly utilised as a low-value energy source. New legislation, that prohibits landfill disposal of solid waste that contains more than 40 wt% moisture, and an ever-increasing focus on green, sustainable production drives the paper and pulp industry towards enhanced waste valorisation. To this end, a possible solution is the production of eco-friendly bio composites that utilise paper sludge and lignin as feedstock materials. To achieve this goal, magnesium phosphate ceramic is used as binder, since it has 20 % lower carbon emissions compared to other binding agents (such as Portland cement) some of which have adverse effects on human health. The phosphate requires reinforcing fibres to ensure inexpensive production for economic viability. The paper sludge contains sufficiently sized fibres, while lignin is a bonding and “stiffening” agent in wood materials with an inherent affinity towards cellulose and hemicellulose. Previous studies have shown that these characteristics enable the utilisation of the paper sludge and lignin in the phosphate ceramic composites. However, at present there is little information on lignin/ceramic and lignin/paper sludge composites and consequently the bonding mechanisms that explain their interfacial interactions. Paper sludge and technical lignin samples from three pulping mills across South Africa were used in this study. The mills were chosen based on waste emission and pulping process. Results showed that lignin from kraft pulping black liquor must be precipitated with sulfuric acid to unlock the properties that ensure optimal bonding and mechanical performance. The precipitated kraft lignin composites performed well with moduli of rupture and elasticity respectively at 7.2 MPa and 2 793 MPa. Furthermore, the addition of pine veneer improved mechanical performance such that the moduli of rupture and elasticity becomes 22.1 MPa and 3 616 MPa, satisfying several industrial standards for composites that may be used in the construction of furniture and non-loadbearing partitioning walls. These standards, as given by the European Standards Organisation and International Organisation for Standardisation, are 9 – 18 MPa and 1 600 – 4 000 MPa respectively for moduli of rupture and elasticity. Conversely, lignosulfonates from the sulfite pulping process could only form composites with moduli of rupture and elasticity of 6.4 MPa and 1 602 MPa after lamination. The effects of lignin addition to the paper sludge reinforced phosphate composites were determined by investigating the chemical and morphological characteristics as well as the reactions to thermal changes exhibited by the individual components. The precipitated kraft lignin yielded better performing composites compared to composites that contained the kraft black liquor and lignosulfonate. Chemical and elemental analyses showed that the sulfuric acid precipitation caused alteration of lignin hydroxyl groups to form carbonyl groups (-C=O) – improving chemical bonding between the paper sludge fibres and lignin particles, while also reducing the overall hydrophilicity of the composites as well as improving the compatibility between the organic and inorganic phases. Additionally, the precipitated kraft lignin softens upon heating to mould around the fibres, resulting in improved fibre dispersion and encapsulation. These effects aid with stress transfer through the composite across phases, and ultimately increases the load bearing ability and stiffness of the composite, while reducing moisture-imposed deformation. The precipitated kraft lignin composites showcased technical viability; however valorisation also requires economic viability. Thus, a production process that include lignin precipitation, composite production, and veneering stages was developed. Mass balances were completed using paper sludge emission rates from industry as well as the experimentally determined composite ratios. All major equipment was sized and costed accordingly before total capital and annual operating expenses were predicted to determine economic viability parameters. Using a desired internal rate of return of 20 %, the minimum required selling price of R 171/m2 was calculated for kraft mills with sludge emissions of at least 13 500 dry ton/year (and a composite production rate of 800 000 panels per annum). The required selling price is competitive in the market for inexpensive construction materials, sold at wholesale prices for between R 158/m2 and R 295/m2, depending on product finishing. In conclusion, the precipitated kraft lignin composites yielded better mechanical and physical properties compared to composites that contained no additional lignin, kraft black liquor, or lignosulfonate. These precipitated lignin composites also display economic potential with competitive minimum required selling prices and strong return on investment. The properties of the precipitated kraft lignin enable the production of a bio composite that sufficiently valorises paper and pulp mill waste towards a lower carbon economy. Key words: Kraft lignin precipitation, lignin composite, paper sludge composite, phosphate ceramic, interfacial adhesion mechanisms