The valorisation of paper sludge for green composite material

Chimphango, Anderson (2020-03)

Thesis (MEng)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: The South African pulp and paper industry generates about 500 000 wet tons of paper sludge (PS) per annum with the traditional means of waste disposal being landfilling. Increased efforts have been put forward to develop alternative use for paper sludge. Paper sludge (PS) contains short cellulose fibres that could be developed into construction material and limit the need for virgin fibres. This study investigates the feasibility of utilising PS as feedstock in the production of composite boards that are lightweight, durable, and more environmentally friendly and of comparable quality with industry wood based composites. In order to account for inevitable variability of PS, samples were collected from recycled fibre (RN-PS), corrugated recycle fibre (CR-PS) and virgin fibre (VP-PS) pulping mills in South Africa. In the study, the boards were produced by a combination of PS, magnesium-based phosphate cement, prepared with heavy magnesium oxide (MgO) and monopotassium phosphate (KH2PO4) as binder, and binder replacement filler, which included silica fume, fly ash and calcium carbonate. Subsequently, the physical and mechanical properties including modulus of elasticity (MOE), modulus of rupture (MOR), water absorption (WA), thickness swelling (TS) and volume swelling (VS) were determined. A response surface methodology (RSM) was used to establish a relationship between the responses and variables, from which optimum conditions for improving board properties were predicted. For the RN-PS, the optimum process conditions were fibre: Inorganic binder ratio, 1.94; binder ratio (KH2PO4: MgO), 5.07; filler (% of binder), 20%; temperature 180°C. For CR-PS, fibre: Inorganic binder ratio, 1.94; binder ratio (KH2PO4: MgO), 5.07; filler (% of binder), 22.5%; temperature, 90°C. For VP-PS, fibre: Inorganic binder, 1.94; binder ratio (KH2PO4: MgO), 5.05, filler (% of binder), 15%, temperature, 25°C. Experimental testing revealed that the composite boards only met the minimum requirements for physical properties for cement bonded particleboard (EN 634-2:2007) and particle board according to the international standard (ISO 16893: (ISO 106893:2016, 2016)). The composite boards produce had medium to high density (0.98-1.01 g/cm3) that could be used for non-structural interior finishes with no load bearing capabilities. The optimum conditions and experimental procedure was then used to develop an economic model to simulate the manufacturing of the composite boards. Key economic indicators such as payback period (PBP), internal rate of return (IRR), and net-present value (NPV) were used to evaluate economic viability for each of the PS process, a combined scenario with all PS feeding to a central location (Combined scenario). The results show that Combined scenario proved to be the more profitable scenario’s with minimum required selling price (MRSP)’s R 157.10 /board approximately less than the average wholesale selling price of R155/board with an IRR of 20%.. Furthermore, the RN-PS scenario had the worst profitability among the PS scenarios with an MRSP of R248.4/board above the average market selling price of R155/board making it unfeasible. High OPEX cost combined with low volume throughput (9 093 m3/year) made RN-PS process economically unattractive. Sensitivy analysis showed that decreasing feedstock cost resulted in MRSP below average market except for the RN-PS scenario. It was concluded that the only the Combined scenarios was deemed to be economically viable option.

AFRIKAANSE OPSOMMING: Die Suid-Afrikaanse pulp-en-papier industrie genereer omtrent 500 000 nat ton papierslyk (PS) per jaar met die tradisionele metode van afvalverwydering, wat grondopvulling is. Verhoogde pogings is voorgestel om alternatiewe gebruike van papierslyk te ontwikkel. PS bevat kort sellulose vesels wat ontwikkel kan word in konstruksiemateriaal en die noodsaaklikheid vir rou vesels beperk. Hierdie studie ondersoek die uitvoerbaarheid daarvan om PS as voermateriaal te gebruik in die produksie van saamgestelde planke wat liggewig, duursaam, meer omgewingsvriendelik is en van vergelykbare kwaliteit met industrie hout-gebaseerde samestellings. Om rekening te hou van onvermydelike veranderlikheid van PS, is steekproewe gekollekteer van herwinde vesel (RN-PS), geriffelde herwinde vesel (CR-PS) en rou vesel (VP-PS) papiermeule in Suid-Afrika. In die studie, is die planke geproduseer deur ’n kombinasie van PS, magnesium-gebaseerde fosfaatsement, voorberei met swaar magnesiumoksied (MgO) en monokaliumfosfaat (KH2PO40) as binder, en binder vervanging vuller, wat silikadamp, stofas en kalsiumkarbonaat insluit. Vervolgens is die fisiese en meganiese eienskappe insluitend modulus van elastisiteit (MOE), modulus van barsting (MOR), waterabsorpsie (WA), dikte swelling (TS) en volume swelling (VS) bepaal. ’n Respons oppervlak metodologie (RSM) is gebruik om ’n verhouding tussen die respons en veranderlikes te vestig, waarvan optimum kondisies vir verbetering van plankeienskappe voorspel is. Vir die RN-PS is die optimum kondisies vesel:anorganiese binderratio, 1.94; binderratio (KH2PO4): MgO), 5.07; vuller (% van binder) 20%; temperatuur 180 °C. Vir CR-PS: vesel:anorganiese binderratio, 1.94; binderratio, 5.07; vuller 22.5% van binder; temperatuur, 90 °C. Vir VP-PS: vesel:anorganiese binderratio, 1.94; binderratio, 5.05; vuller, 15% van binder, en temperatuur, 25 °C. Eksperimentele toetse het aangetoon dat die saamgestelde planke slegs die minimum vereistes behaal vir fisiese eienskappe van sement-verbinde-partikelplank (EN 634-2:2007) en partikelplank volgens die internasionale standaard (ISO 16893: (ISO 106893:2016, 2016)) . Die saamgestelde planke geproduseer het medium tot hoë digtheid (0.98–1.01 g/cm3) gehad wat gebruik kan word vir nie-strukturele interieure afrondings met geen dramuurvermoë nie. Die optimum kondisies en eksperimentele prosedure is toe gebruik om ’n ekonomiese model te onwikkel om die vervaardiging van die saamgestelde planke te simuleer. Sleutel ekonomiese indikators soos terugverdientyd (PBP), interne opbrengskoers (IRR), en netto huidige waarde (NPV) is gebruik om ekonomiese lewensvatbaarheid vir elk van die PS-prosesse te evalueer, asook ’n gekombineerde scenario met al die PS wat na ’n sentrale plek voer (Gekombineerde scenario). Die resultate het aangetoon dat die gekombineerde scenario bewys is om die mees winsgewende scenario’s te wees met minimum vereiste verkoopspryse (MRSP) ongeveer minder as die gemiddelde netto huidige verkoopsprys van R155/plank met IRR van 20%. Verder, die RN-PS-scenario’s het die swakste winsgewendheid gehad van al die PS scenarios met ’n MRSP meer as dubbeld die gemiddelde markverkoopsprys van R155/plank, wat dit onuitvoerbaar maak. Hoë OPEX-koste gekombineerd met lae volume deursit (9 093 m3/jaar) maak RN-PS-proses ekonomies onaantreklik.Daar is tot die gevolgtrekking gekom dat die gekombineerde scenario’s beskou is as ekonomies lewensvatbaar.

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