Paper and pulp mill waste valorisation via the production of phosphate ceramic composites

Barnard, Eddie Gustav (2021-03)

Thesis (MEng)--Stellenbosch University, 2021.

Thesis

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

AFRIKAANSE OPSOMMING: Die Suid-Afrikaanse papier en pulp industrie genereer jaarliks ongeveer 500 000 ton papier en pulp slyk wat na stortingsterreine geneem word. So ook is lignien, naas sellulose, die volopste biomateriaal, maar word slegs as ‘n goedkoop brandstof gebruik. Nuwe wetgewing en die toenemende fokus op groen, volhoubare produksie dryf die papier en pulp industrie om hul afval te omskep in waardevolle produkte. Een so ‘n oplossing is die produksie van ‘n saamgestelde materiaal deur die papier slyk, lignien, en magnesium fosfaat keramiek as roumateriaal te gebruik. Die fosfaat veroorsaak 20 % laer koolstof vrystelling as ander binders, waarvan sommige binders newe effekte op menslike gesondheid het. Die fosfaat keramiek moet vir ekonomiese vatbaarheid deur vesels aangevul word en die papier slyk bestaan grootliks uit vesels wat voldoen aan grootte vereistes, terwyl lignien in sy natuurlike vorm as binder en verstywingsmiddel optree en inherent ‘n affiniteit het tot die sellulose. Huidiglik bestaan beperkte inligting omtrent die lignien/keramiek mengsels asook die bindingsmeganismes wat die koppelvlak interaksies verduidelik. Hierdie studie maak gebruik van papier slyk en tegniese lignien monsters van drie pulp meule regoor Suid-Afrika, gebaseer op totale afval vrystelling en tipe pulp proses. Eksperimentele werk het bewys dat kraft lignien gepresiteer moet word ten einde die gesogte eienskappe te ontsluit wat die samestellings se meganiese vermoë verhoog. Die gepresiteerde kraft lignien samestellings kon onderskeidelik modulusse vir skeuring en elastisiteit van 7.2 MPa en 2 793 MPa behaal, wat dit moontlik maak om die materiale te gebruik vir die produsering van meubels en nie-lasdraende strukture. Hierdie standaarde, soos uiteengesit deur die Europese Standaarde Organisasie en die Internationale Organisasie for Standardisering, is respektiewelik 9 – 18 MPa en 1 600 – 4 000 MPa vir die modulusse van skeuring en elastisiteit. In teenstelling het die lignosulfonaat samestellings swakker gevaar met modulusse vir skeuring en elastisiteit van 6.4 MPa en 1 602 MPa na laminasie. Die utwerking wat lignien op die papier slyk samestellings het is bepaal deur die chemise en morfologiese eienskappe, asook termiese invloede op die komponente te ondersoek. Die gevolgtrekking is dat die gepresiteerde lignien samestellings die beste resultate toon omdat die lignien presipitasie plaasvind deur die hidroksiel groepe te verander in vry suurstof radikale wat gelyktydig die chemise bindings tussen die lignien en papier slyk verhoog, die algehele hidrofilisiteit van die samestelling verlaag, en die versoenbaarheid tussen die organiese en anorganiese fases verbeter. Verder versag die gewysigde lignien met ‘n toename in temperatuur om sodoende om die papier slyk vesels te vorm, wat lei tot verbeterde vesel verspreiding en omhulling. Hierdie meganismes bevorder stres oordrag tussen fases om sodoende die lasdraende vermoë van die samestelling te verhoog terwyl vog veroorsaakde vervorming verlaag. Die gewysigde lignien samestellings het uitstekende tegniese vermoë getoon, maar om ‘n produk met toegevoegde waarde te produseer, is ekonomiese lewensvatbaarheid belangrik. Dus, ‘n produksie proses wat die lignien wysiging, samestelling vervaardiging, en finerings fases insluit is ontwerp. Massa balanse, gebaseer op industriële papier slyk vrystellingstempo’s en eksperimenteel bepaalde komponent verhoudings, is voltooi ten einde die grootte en koste van die hoof toerusting te bepaal. Verder is totale kapitaal- en jaarlikse vervaardigingskoste bereken voordat die ekonomiese vatbaarheidsparameters voorspel is. Deur ‘n begeerde interne verdieningskoers op belegging van 20 % aan te neem, is ‘n minimum verkoopsprys van R 171/m2 bereken vir kraft meule wat ten minste 13 500 ton slyk per jaar vrystel – gelykstaande aan 800 000 standaard panele per jaar. Hierdie verkoopsprys is kompeterend in ‘n mark vir goedkoop boumateriaal wat verkoop word teen groothandel pryse tussen R 158/m2 en R 295/m2, afhangend van produk afwerking. Ter samevatting, die gewysigde lignien samestellings het die beste meganiese en fisiese eienskappe in vergelyking met samestellings wat geen addisionele lignien bevat, samestellings met ongewysigde kraft lignien, en samestellings met lignosulfonaat. Die samestellings blyk ook ekonomies vatbaar te wees met kompeterende verkoopspryse en ‘n stewige koers op belegging. Dit is die uiteindelike eienskappe van die gewysigde kraft lignien wat die produksie van die biomateriaal moontlik maak om sodoende waarde te heg aan die papier en pulp afval in die strewe na ‘n laer koolstof ekonomie.

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