Recycling of waste ceramic rollers for the production of corundum-mullite as a high- alumina synthetic refractory material

Technical Summary: Recycling of Waste Ceramic Rollers for Corundum-Mullite Production

Problem Statement
The tile manufacturing industry, particularly in Iran, relies heavily on roller-type tunnel kilns. Over time, the ceramic rollers used in these kilns fracture and are discarded as waste. While uncontaminated rollers (from cutting or kiln ends) have been studied for reuse, the majority of waste rollers are "contaminated" with glaze coatings. At high kiln temperatures, molten glaze penetrates the roller surface, creating a bonded layer that cannot be removed by physical means. Consequently, these high-alumina waste materials are currently discarded, representing both an environmental burden and a loss of valuable raw materials. Furthermore, while synthetic corundum-mullite refractories are typically produced from fresh raw materials, there is no published research on synthesizing these materials from glaze-coated waste rollers or utilizing acid-leaching to remove the glaze impurities.

Methodology
The study proposes a chemical recycling process to transform glaze-coated waste rollers into a high-alumina synthetic refractory material. The methodology involved the following steps:

1. Preparation: Glaze-coated rollers (20 mm internal diameter, 1–2 mm glaze layer) were crushed to particles smaller than 10 mm. Particles were not pulverized to powder prior to leaching, as this yielded poor results.
2. Acid Leaching: Hydrofluoric (HF) acid was used to dissolve the silicate-based glaze and associated pigments. Four concentrations (10%, 20%, 30%, and 40%) were tested at room temperature (25°C) and elevated temperature (40°C). The solid-to-liquid ratio was maintained at 1:3.
3. Processing: After leaching, solids were separated, washed, and wet-ground via planetary milling to achieve a particle size of -20 µm. The slurry was dried to a semi-dry state (~5 wt.% humidity).
4. Sintering: Powders were pressed into briquettes at 100 MPa, dried, and sintered in a shuttle furnace at 1550°C for 2 hours.
5. Characterization: The chemical composition was analyzed via X-ray fluorescence (XRF). Phase identification and quantification were performed using X-ray diffraction (XRD) with Rietveld refinement. Microstructure was examined using Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS). Physical properties (bulk density, porosity, water absorption, permanent linear change) were measured according to ASTM standards.

Key Results

• Leaching Efficiency: The dissolution time of the glaze layer was inversely proportional to HF concentration and temperature. At 25°C, complete dissolution required 17 hours for 10% HF, reducing to 2 hours for 40% HF. Increasing the temperature to 40°C further reduced the time; for instance, 10% HF required only 8 hours.
• Chemical Composition: The leaching process effectively removed silicate impurities. The resulting leached particles contained 80.13 wt.% Al₂O₃ and 3.08 wt.% ZrO₂, with significantly reduced SiO₂ (10.01 wt.%) compared to the fresh rollers (22.70 wt.%).
• Phase Composition: XRD and SEM analysis of the sintered material identified Corundum (71.9 wt.%) and Mullite (25.2 wt.%) as the major phases, with Baddeleyite (2.9 wt.%) as a minor phase.
• Physical Properties: The sintered corundum-mullite material exhibited superior physical properties compared to refractory-grade sintered bauxite:
  • Bulk Density: 3.35 g/cm³ (vs. 3.12 g/cm³ for bauxite).
  • Apparent Porosity: 0.18% (vs. 7.88% for bauxite).
  • Water Absorption: 0.05% (vs. 2.53% for bauxite).
  • Permanent Linear Change (PLC) @ 1650°C: < 0.1% (vs. 0.2% for bauxite).
  • Thermal Expansion: Lower reversible thermal expansion (0.59% at 1200°C) compared to bauxite (0.71%), attributed to the higher mullite content which has a lower coefficient of thermal expansion.

Significance and Claims
The authors claim that this work presents a novel solution to the environmental issue of glaze-coated waste rollers by converting them into a valuable synthetic refractory raw material. The study demonstrates that acid-leaching with HF is an effective method to remove glaze-based impurities, allowing the recovery of high-alumina content.

The resulting synthetic corundum-mullite material is claimed to be a viable substitute for refractory-grade sintered bauxite. The authors note that while the Al₂O₃ content is slightly lower than some bauxite sources, the material possesses a higher combined content of Al₂O₃, SiO₂, and ZrO₂ (95.44 wt.% vs. 93.03 wt.% for bauxite) and significantly lower impurity levels. The presence of ZrO₂ (baddeleyite) is highlighted as beneficial for refractory properties. The paper concludes that this recycling method not only addresses an environmental waste problem but also preserves a valuable high-alumina resource, offering a material with comparable or superior physical and thermal properties to traditional refractory raw materials.