Upgrading pyrolytic tyre char through acid-alkali demineralisation

Henry, Kirsty Louise (2015-12)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Waste tyres are proving to be a critical environmental and social problem not only in South Africa, but worldwide; with significant hazards associated with illegal dumping, stockpiling and burning. Efforts are currently directed towards the management of waste tyres, and pyrolysis is proving to be a promising form of waste tyre recycling, with the production of potentially valuable products. Pyrolytic tyre char, in particular, is a carbonaceous solid product originating from the carbon black filler within tyres, and constituting 40% of the total product mass produced by pyrolysis. Pyrolytic tyre char, however, has an undesirable structure and is plagued by contaminants, thus considerably decreasing it value and market potential. Currently, South African waste tyre pyrolysis plants are stockpiling great quantities of char, which reduces revenue and decreases investments. The problem statement was thus that the current char produced by waste tyre pyrolysis is crude and has minimal market value or use which, in turn, reduces waste tyre pyrolysis sustainability and waste tyre recycling efforts. A systematic approach was utilised, with a combination of literature research, experimentation, engineering design and economic evaluation methods employed to (i) investigate pyrolytic tyre char upgrading methods and product opportunities; (ii) develop and propose a process for crude pyrolytic tyre char upgrading; (iii) analyse the economic viability and appeal of the proposed crude pyrolytic tyre char upgrading process. A thorough literature review revealed that, utilising an acid-alkali leaching process, crude pyrolytic tyre char could potentially be upgraded to a carbon black product. Industrial char and carbon black N330 were thus characterised to establish benchmark targets and process requirements, including ash content reduction from 16.24 to 1.95wt.%; BET surface area increase from 58.9m².g-¹ to 80.22m².g-¹; and average particle size reduction from 153.2 to 38.5μm. Furthermore, a mineralogical study revealed the ash contained approximately half silicon/silicon dioxide and half metallic components; thus substantiating the use of both an acid and an alkali. Acids and alkalis mentioned in literature, with regards to pyrolytic tyre char demineralisation, were screened as per demineralisation ability and raw material costs. Hydrochloric acid and sodium hydroxide proved to be superior. Subsequently, significant process factors, including extraction time, extraction temperature and lixiviant concentration, were analysed through kinetic trials and sequential extractions. A final acid-alkali demineralisation process was thus established; and pyrolytic tyre char was upgraded to a carbon black product using 3x 5.0M NaOH washes at 88°C for 60minutes followed by 1x 1.0M hydrochloric wash at 80°C for 20minutes, including water washes prior to each lixiviant wash. Additionally, 100g lixiviant was utilised per wash; with the char: lixiviant ratio decreasing from an initial 20:100 (g/g). The upgraded pyrolytic tyre char was comparable to commercial carbon black N330 in terms of ash content (1.83wt.%) and BET surface area (84.7m².g-¹); however, it had much coarser particles (130.6μm). Finally, using the proposed acid-alkali process, economic analyses were applied to an industrial process design. An industrial upgrading project cannot be eliminated as unprofitable; however project investment is not highly appealing, since the upgraded pyrolytic tyre char has to sell for a high price for the process to be economically feasible, and may not be able to compete in the relevant markets. Two key recommendations for further research include (i) acid-alkali process optimisation aiming to reduce raw material and utility expenditures; (ii) analysing the influence of the upgraded carbon black product on tyre characteristics with the aim of suitability assessment and the requirement of a particle size reduction process.

AFRIKAANSE OPSOMMING: Afvalmotorvoertuigbande blyk tans 'n kritiese omgewings- en sosiale probleem te wees, nie slegs in Suid-Afrika nie, maar wêreldwyd; met noemenswaardige gevare geassosieer met die onwettige storting, opgaring en brand daarvan. Pogings is tans gerig op die bestuur van afvalmotorvoertuigbande, en pirolise blyk tans ’n belowende vorm van herwinning van afvalmotorvoertuigbande te wees, met die produksie van potensiële waardevolle produkte. Pirolitiese motorvoertuigbandsteenkool, in besonder, is 'n koolstofryke soliede produk, afkomstig van die koolstof swart substraat in motorvoertuigbande, en wat 40% uitmaak van die totale produkmassa wat deur pirolise geproduseer is. Pirolitiese motorvoertuigbandsteenkool het egter ’n ongewenste struktuur en word geteister deur kontaminante, wat dus die waarde en markpotensiaal daarvan aansienlik verminder. Tans gaar Suid-Afrikaanse afvalmotorvoertuigbandpirolise-aanlegte aansienlike hoeveelhede steenkool op, wat inkomste verminder en beleggings laat afneem. Die navorsingsverklaring was dus dat die huidige steenkool wat deur afvalmotorvoertuigbandpirolise geproduseer is, ru is en minimale markwaarde of gebruik besit, wat op sy beurt, die volhoubaarheid van afvalmotorvoertuigbandpirolise- en herwinningspogings verminder. 'n Sistematiese benadering is gebruik, met 'n kombinasie van literatuurnavorsing, eksperimentering, ingenieursontwerpe en ekonomiese evalueringsmetodes wat gebruik is om (i) pirolitiese motorvoertuigband opgraderingsmetodes- en produkgeleenthede te ondersoek; (ii) 'n proses te ontwikkel en voor te stel vir die opgradering van ru pirolitiese motorvoertuigbandsteenkool; (iii) die ekonomiese lewensvatbaarheid en aantreklikheid van die voorgestelde ru pirolitiese motorvoertuigbandsteenkool opgraderingsproses te analiseer. 'n Deeglike literatuuroorsig het getoon dat, met behulp van 'n suur-alkali logingsproses, ru pirolitiese motorvoertuigbandsteenkool potensieel opgegradeer kan word na ’n swart koolstof produk. Industriële steenkool en koolstof swart N330 is, dus, gekenmerk om maatstafteikens en prosesvereistes te vestig, insluitend vermindering van asinhoud vanaf 16.24 - 1.95wt.%; BET oppervlakte toename van 58.9m².g-¹-80.22m².g-¹; en gemiddelde deeltjie-grootte vermindering van 153.2 - 38.5μm. Verder, 'n mineralogiese studie het getoon dat die as ongeveer ’n helfte silikon/silikondioksied en ’n helfte metalliese komponente bevat het; dus ter stawing van die gebruik van beide 'n suur en ’n alkali. Sure en alkalië in die literatuur genoem, met betrekking tot pirolitiese motorvoertuigbandsteenkool demineralisasie, is gekeur volgens demineralisasievermoë en roumateriaalkostes. Soutsuur en natriumhidroksied het bewys om superior te wees. Daarna beduidende proses faktore, insluitende ekstraksietyd, ekstraksietemperatuur en lixiviantkonsentrasie, is ontleed deur kinetiese proewe en sekwensiële ekstraksies. ’n Finale suur-alkali demineralisatieproses is, dus, gestig; en pirolitiese motorvoertuigbandsteenkool is opgegradeer na 'n koolstof swart produk met behulp van 3x 5.0m NaOH was teen 88 °C vir 60 minute, gevolg deur 1x 1.0m soutsuur was teen 80 °C vir 20 minute, insluitend waterwasse voor elke lixiviant was. Addisioneel, is 100g lixiviant benut per was; met ’n afname in die pirolitiese motorvoertuigbandsteenkool: lixiviant verhouding van 20:100 (g/g) per was. Die opgegradeerde pirolitiese motorvoertuigbandsteenkool was vergelykbaar met kommersiële koolstof swart N330 in terme van asinhoud (1.83wt.%) en BET oppervlakte (84.7m².g-¹); egter, het dit baie growwer deeltjies (130.6μm). Ten slotte, deur die gebruik van die voorgestelde suur-alkali proses, is ekonomiese ontledings toegepas op 'n industriële prosesontwerp. 'n Industriële opgraderingsprojek kan nie uitgeskakel word as nie-winsgewend nie; maar projekbelegging is egter nie hoogs aanloklik nie, met hoë terugbetalingstydperke en lae beleggingsopbrengstariewe. Twee belangrikste aanbevelings vir verdere navorsing sluit in (i) suur-alkali proses optimalisering gemik op die vermindering van roumateriaal- en nutuitgawes; (ii) ontleding van die invloed van die opgegradeerde koolstof swart produk op motorvoertuigbandeienskappe, met die doel op geskiktheidsevaluering en die vereiste van 'n deeltjie-grootte verminderingsproses.

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