Sintered vs Electrofused AZS Refractories: What’s the Key Differences

Sintered AZS Bricks

Sintered AZS bricks are manufactured using zircon and alumina as the main raw materials, with pre-synthesized batches as aggregates. Since the non-plastic powders limit density improvements from higher molding pressures alone, multiple compactions of large bricks can cause lamination and uneven density.

To achieve sufficient densification, measures are taken including:

  • Optimizing critical particle size distribution
  • Adding blended binders
  • Incorporating minor additions of TiO2, MgO sintering aids
  • Employing enhanced sintering techniques

These modifications facilitate dense, quality sintering of the unfavorable AZS powder system. The production process is tailored to overcome the inherent challenges of refractory oxide compaction and firing.

This in situ reaction between zircon and alumina is irreversible, also called reaction sintering. The substantial volume effects during reaction bonding can deform and crack the unfired body.

To mitigate this, pre-synthesized zircon mullite aggregates are used with sintered or electrofused AZS raw materials to create a three phase material of mullite, zirconia, and corundum. The resulting microstructure has fine zirconia particles uniformly dispersed between the primary mullite and corundum phases.

The three phase sintered AZS microstructure provides superior strength and thermal shock resistance compared to two phase alternatives. But the prereaction step is essential to prevent flaws from the exothermic reaction during firing. Tailoring of the starting particles and process is crucial to obtain high performance sintered AZS refractories.

Electrofused Recombined AZS Bricks

Electrofused recombined AZS bricks utilize melted AZS clinker, crumbs, or reclaimed brick fragments as the raw material aggregate, with small amounts of kaolin or alumina as binder. When heated to high temperatures, the glassy phase of the electrofused AZS aggregates reacts with the binder to form mullite, promoting sintering.

The encapsulating reaction between the vitreous and crystalline phases in the electrofused AZS also produces mullite, with the mullite and zirconia in an enclosing relationship. This aids sintering of the varied particle sizes of molten AZS grains.

Leveraging the “green sintering” behavior of electrofused aggregates is the fundamental principle for manufacturing recombined AZS bricks. The pre-fused particles enable dense refractory fabrication utilizing lower energy processes compared to conventional sintering or fusion.

The key microstructural transformations during high temperature firing of electrofused recombined AZS bricks are:

  • The glassy phase of the coarse aggregates diffuses outwards to react with Al2O3, forming a mullite shell that seals off channels, limiting the encapsulating reaction to interior regions.
  • The vitreous content of the electrofused AZS powders reacts with the reactive Al2O3 binder, mullitizing the matrix.

The resulting microstructure of recombined AZS refractories displays dense interlocking crystals of mullite, corundum, and zirconia.

This fusion-based fabrication route enables electrofused grains to be securely bonded through the formation of secondary mullite phase. The process promotes sintering while retaining the high quality premixed AZS clinker composition.

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