Refractory Materials for Glass Melting Furnace: AZS, Zircon & Silica

The Glass Furnace: Unique Refractory Challenges

Glass melting furnaces operate continuously for 8–15 years before a complete rebuild, making them one of the longest-campaign refractory applications in any industry. The refractory lining must withstand molten glass at 1,450–1,600 degC, gas temperatures above 1,650 degC at the crown, and the highly corrosive nature of molten glass which dissolves most refractory materials over time.

The critical difference from other high-temperature applications is that any refractory material dissolved by the glass can become a defect (stone, knot, or blister) in the finished glass product. This means the refractory must not only survive the conditions but also maintain glass quality throughout the campaign.

Furnace Zones and Material Selection

1. Glass Contact Zone (Tank Walls and Bottom)

This is where the refractory directly contacts molten glass. The material must resist corrosion at the glass line (the most aggressive zone, where convection currents are strongest) and must not introduce defects into the glass.

AZS (Alumina-Zirconia-Silica) Fused Cast Blocks

AZS fused cast blocks are the primary glass contact refractory worldwide. They are manufactured by melting a mixture of alumina, zirconia, and silica in an electric arc furnace and casting the melt into molds.

Grade	ZrO2 (%)	Al2O3 (%)	SiO2 (%)	Application
AZS 33	33	50	15	Sidewalls (standard), bottom paving
AZS 36	36	48	14	Glass line, throat, high-wear areas
AZS 41	41	45	12	Most critical glass line areas, electrodes surroundings

Higher ZrO₂ content provides better corrosion resistance because zirconia (baddeleyite) is the most glass-resistant phase. The glass phase content in the fused cast block is also critical — lower glass phase means less exudation (sweating of residual glass phase from the block into the melt) and fewer defects.

Fused Cast Alpha-Beta Alumina

Used in superstructure areas and certain glass contact applications where AZS exudation is unacceptable. Contains > 95% Al₂O₃ with very low glass phase. Excellent for borosilicate and specialty glass furnaces.

2. Crown (Roof)

The crown operates at the highest gas temperature in the furnace (1,550–1,650 degC) and is exposed to alkali vapors from the batch and volatile components from the glass melt.

• Primary material: Silica bricks (SiO₂ > 95%) — the traditional and still dominant crown material for soda-lime glass furnaces
• Why silica: At high temperature, silica is resistant to alkali vapors (unlike alumina, which reacts with sodium to form low-melting nepheline). Silica is also light, reducing the structural load on the furnace frame.
• Key properties: Density 1.80–1.85 g/cm³, refractoriness > 1,700 degC, excellent creep resistance at temperature
• Alternative: For specialty glass (borosilicate, E-glass), AZS or fused alumina crowns may be required due to more aggressive chemical environments

3. Regenerator (Checker Chamber)

Regenerators recover heat from exhaust gases to preheat combustion air. The checker bricks see temperatures from 400 degC at the bottom to 1,350 degC at the top and are exposed to alkali-laden, sulfur-containing flue gases that condense and attack the brickwork.

• Top courses (hot zone, 1,000–1,350 degC): Fused cast AZS, bonded AZS, or high-alumina bricks with good alkali resistance
• Middle courses (600–1,000 degC): Magnesia or magnesia-zirconia bricks (basic environment) or high alumina bricks
• Bottom courses (400–600 degC): Fireclay or mullite bricks; this zone sees condensation of sodium sulfate (Na₂SO₄) which is extremely corrosive

The bottom of the regenerator is often called the “sulfate condensation zone” and is notoriously difficult to protect. Using dense, low-porosity bricks and controlling sulfur in the fuel helps extend life here.

4. Forehearth and Feeder

These are the channels that deliver conditioned glass from the furnace to the forming machines. Temperature precision is critical (typically 1,050–1,250 degC for container glass).

• Glass contact: AZS 33 or chrome-alumina-zirconia blocks for critical areas; zircon-mullite or dense alumina for less critical areas
• Superstructure: Mullite or sillimanite bricks
• Insulation: IFB and ceramic fiber board for temperature control

5. Throat and Submerged Areas

The throat connects the melting chamber to the refining/conditioning chamber. It is fully submerged in molten glass with extremely aggressive convection currents.

• Material: AZS 41 (the highest ZrO₂ grade) or fused cast high-zirconia (> 85% ZrO₂) blocks
• These are the most expensive refractories in the furnace but they are justified by the extreme corrosion conditions

Glass Type Influences Material Selection

Glass Type	Melting Temp	Corrosivity	Glass Contact Material	Crown Material
Soda-lime (container, flat)	1,450–1,550 degC	Moderate	AZS 33–36	Silica
Borosilicate	1,500–1,600 degC	High	AZS 41 or fused alumina	AZS or fused alumina
E-glass (fiberglass)	1,350–1,500 degC	Very high	Chrome-AZS or high-ZrO2	AZS or silica
Lead crystal	1,300–1,450 degC	Moderate	AZS 33	Silica

Key Performance Considerations

• Corrosion rate: Measured in mm/year. At the glass line, corrosion of 3–8 mm/year is typical for AZS 33. AZS 41 reduces this by 30–50%.
• Exudation: The residual glass phase in fused cast blocks can migrate to the hot face and drip into the melt, causing defects. Low-exudation grades (void-free cast or oxidized cast) minimize this.
• Thermal expansion: AZS blocks have a complex expansion curve due to the zirconia monoclinic-tetragonal inversion at 1,000–1,100 degC. Expansion joints must accommodate this without allowing glass leaks.
• Joints and ramming: Between fused cast blocks, a zirconia-based or AZS-based ramming compound fills the joints. Joint design and quality are critical to preventing glass penetration.

SAPL: Glass Furnace Refractory Solutions

Shanker Agencies works with glass plants across India, supplying AZS fused cast blocks, silica crown bricks, regenerator bricks, forehearth materials, and insulation products. Our experience with container glass, float glass, and specialty glass furnaces allows us to recommend the right material for each zone and glass type. Contact us for a consultation or to discuss your upcoming furnace rebuild or repair.