Development of Novel and Sustainable Refractories for The Glass Industries

This research investigated the development and synthesis of modified and novel refractory materials and compositions of the Al2O3 – ZrO2 – SiO2 (AZS) systems that offer promising sustainable qualities; both from the manufacturing and service end of their application in the glass industries. In the glass industry the key challenges of production costs and glass quality are greatly affected by the performance of refractories. Therefore there is a demand for refractories that enhance furnace life and increase productivity whilst achieving fewer glass defects and downtime. Three dense AZS refractory composites, AZS-01, AZS-T1 and AZS-T2, that contain very low apparent porosities of 13%; 7% and 6% respectively were developed and synthesized by slip cast methods through reaction sintering of alumina and zircon raw materials, which met these criteria. The AZS-01 refractory is a high zirconia refractory composition, designed with a tailored matrix microstructure consisting of interlocked zirconia grains in a mullite-zircon matrix. The material was developed utilising the evaluation of compositional phase fields of ternary and binary phase diagrams and thermodynamic consideration of the phases. A hybrid of empirical particle packing models and a computational modelling approach that utilised scanned and digitised particle shapes as voxels in a 3D digital packing environment were also utilised to optimise the packing fractions of the refractory formulations. AZS-T1 and AZS-T2 are SnO2 doped variants of the AZS-01 and AZS refractories, respectively, studied in this thesis. The AZS-01, AZS-T1 and AZS-T2 refractories developed in this research, offered improved physical, thermo-mechanical and chemical properties when compared to the standard refractories, PSR-315 and PSR-333; widely used in the forehearth glass industry. PSR-315 and PSR-333, used in this research as reference refractory compositions, are well known international standard refractories widely used as forehearth and feeder glass furnace applications. Therefore, they are a benchmark to which other compositions that are used in similar forehearth and feeder application are compared to. With that being so; the developed compositions in this research, AZS-01, AZS-T1 and AZS-T2, presented enhanced thermos-mechanical properties than the benchmark refractories. They were also shown to perform better and offer improved corrosion resistant properties than the standard PSR-315 and PSR-333 in both borosilicate and soda lime silicate glass melts. The improved thermo-mechanical and corrosion resistant properties of the AZS-01, AZS-T1 and AZS-T2 provides them with long service life properties than the standard benchmark refractories, PSR-315 and PSR-333. Therefore, this research has successfully addressed the research objectives of developing and synthesising sustainable refractories for the glass industry. Results from the research have also shown successfully the effect SnO2 as a sintering aid in reaction sintering of Alumina-zircon starting mixtures above 1550 °C. Of the compositions, AZS-T1 and AZS-T2 were developed by using SnO2 as a sintering agent which brought novelty to the research. Three types of mullite crystals, MI; MII and MIII, based on their morphology and aspect ratios; were evolved across the matrix microstructure. The microstructures of the reaction sintered refractory samples were characterized by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX). Volatisation of SnO2 was mitigated by the formation of a Zr1-xSnxO2 solid solution and a pro-eutectic transient Al2O3-SnO2-ZrO2 and SnO2-SiO2 amorphous phase. Liquid phase sintering from the presence of a SnO2-SiO2 amorphous phase from the dissociated zircon grains and transient pro-eutectic Al2O3-SnO2-ZrO2 amorphous phase is believed to have enhanced densification. SEM microstructural analysis revealed coalescence and grain growth of the matrix alumina grains and the presence of an acicular tertiary MIII mullite which acts as a reinforcement and bridging network of matrix to aggregate grains. An increase in the thermo-mechanical properties of the refractories was attributed to the secondary MII and tertiary, interlocked, acicular MIII mullite crystals. This addressed the research gap where the use of SnO2 was reported to have had negligible effect on the mullitisation of the alumina-zircon composites and the thermo-mechanical properties of the sintered bodies. The chemical compatibility of the tin oxide doped compositions, AZS-T1 and AZS-T2, in this research against Soda lime Silica and borosilicate glasses, demonstrated the potential of the refractories replacing Chrome based refractories which are not sustainable environmentally. This research will contribute immensely towards industry and knowledge through increased furnace service life, a sustainable production of commercial and special glasses. Chapter 1 looks at the background to the research, the scope and the research question to the Thesis. Chapter 2 is an extensive literature survey related to the topic and research area of interest. It draws on gaps in research that this research area will cover and contribute towards.Chapter 3 and 4 are is the experimental design and methodology sections which describe the methodologies implemented in developing, synthesising and characterising the novel composition. Chapter 5 to 7 show and discuss the results obtained from the work conducted.Chapter 8 and 9 presents the thesis conclusions and highlights future work of the research.