What to Look for When Purchasing Corundum Bricks

Corundum bricks, which include sintered and fused-cast types, have the highest alumina content among refractory bricks. What should you watch for when buying them? See the details below.

Corundum brick is a refractory with an alumina content above 90% and corundum as the main crystalline phase. It has high cold crushing strength (up to 340 MPa), a high refractoriness-under-load onset temperature (above 1700°C) and good chemical stability, with good resistance to acidic and alkaline slags, metals and molten glass. Its microstructure strongly affects thermal shock resistance: the products have good corrosion resistance but relatively poor thermal shock resistance.

It can be divided into sintered corundum brick and fused corundum brick. Raw materials can be sintered alumina and fused corundum, or bauxite clinker with a high Al2O3/SiO2 ratio plus sintered alumina. Unfired corundum bricks can also be made with phosphoric acid or other binders. They are mainly used in ironmaking blast furnaces, blast furnace hot stoves, secondary refining furnaces, slide gates, glass melting furnaces and petrochemical furnaces.

They are used in high-temperature furnaces across metallurgical blast furnaces, hot stoves, petrochemicals, gasification, gas generators, cracking furnaces, ceramics, building materials, defense and research. Corundum bricks in these kilns are constrained by process conditions and are also attacked by high temperature, high pressure, slag and strongly reducing gases. This chemical attack and structural damage shortens their service life.

High-temperature chemical reactions: Alumina melts at 2050°C; at high temperatures it reacts with other oxides, lowering the melting temperature along the melting curve. Only chromium oxide and alumina form a continuous solid solution, with a melting point between those of the two oxides.

The main impurities in corundum brick are silica and iron oxide. In the Al2O3–SiO2–FeO ternary phase diagram, the lowest eutectic is only 1205°C, meaning a glass phase can form above about 1200°C.

Titanium oxide and niobium oxide lower the melting point of alumina products and also act as sintering aids for sintered alumina, forming aluminum titanate or aluminum niobate during sintering.

Vanadium pentoxide is a low-melting oxide. It causes grain-boundary melting of alumina crystals at low temperatures, freeing the grains. In addition, V2O5 and CaO form low-melting calcium vanadate (618°C) in an oxidizing atmosphere and volatile calcium vanadate in a weakly reducing atmosphere. V2O5 damages corundum brick because it crystallizes between corundum grains, causing cracks, structural spalling and large-area peeling.

Sodium and sulfide attack on corundum brick comes from sodium oxide condensed from process gases, which can attack corundum brick at 1000°C

converting α-Al2O3 in the corundum to β-Al2O3, causing large volume expansion (about 26%) and bursting and spalling of the brick surface.

Below 1000°C, Na2O and SO3 react with sillimanite and mullite to form nosean and β-Al2O3, increasing volume by 86% and causing spalling and cracking.

Applications: Fused corundum bricks are mainly used in critical wear- and heat-resistant areas of industrial furnaces, such as skid rail bricks in pusher-type furnaces, steel troughs in walking-beam reheating furnaces, waste incinerator linings, petrochemical furnace linings, mill bottoms and secondary reformers in ammonia synthesis.

Are corundum bricks expensive? Answer: Corundum brick is a type of refractory brick priced somewhat higher than other materials. What does corundum brick cost?

Classification of corundum bricks:

1. By process:

1.1 Fired corundum brick: sintered corundum refractory brick made mainly from corundum with refractory powder and binder added, fired at 1600–1800°C. Different firing schedules are applied depending on the brick's intended use, performance requirements and degree of dispersion.

1.2 Bonded sintered corundum brick: made mainly from sintered corundum with corundum powder added to form a mix, then formed, dried and fired.

1.3 Fused corundum brick: made mainly from corundum; unlike fired corundum brick, it is produced by melting in an electric furnace.

1.4 Composite fused corundum brick: made from fused corundum clinker with fused or sintered corundum powder, formed, dried and fired.

2. By material:

2.1 Brown fused alumina: brown corundum brick is made mainly from bauxite and coke (anthracite), smelted at about 2000°C in an electric arc furnace.

2.2 White fused alumina: white corundum brick is made from alumina powder melted at high temperature.

2.3 High-alumina corundum: made mainly from bauxite, smelted by a special process in an electric furnace at 2500°C.

2.4 Zirconia corundum: made mainly from bauxite, smelted by a special process in an electric furnace at 2500°C [as stated in the original].

2.5 Monocrystalline corundum: monocrystalline fused alumina (also called 32A) abrasive is made mainly from bauxite with a suitable amount of sulfide added and melted at high temperature.

2.6 Microcrystalline corundum: made mainly from bauxite, melted at high temperature and crystallized by rapid quenching.

With this basic introduction, you should now know the main types of corundum brick. As is well known, when buying corundum or other refractory bricks, different materials mean different performance and different prices.

What to watch for when buying corundum bricks

Different refractory materials perform differently, and corundum bricks are no exception: different raw materials—brown, white, black corundum and so on—give different properties. When purchasing, customers should specify their property requirements and ask the manufacturer to provide a matching test report after production, ensuring all indicators meet requirements.

Points to note when using fused-cast AZS bricks

Fused-cast AZS (alumina-zirconia-silica) brick is a type of zirconia-corundum brick used mainly in glass melting furnaces. Also known as "white iron brick" or corundum-baddeleyite brick, it contains 50%–70% Al2O3, 20%–40% ZrO2, with the balance SiO2. Its main minerals are baddeleyite (ZrO2), corundum (α-Al2O3) and a glass phase. Baddeleyite crystals form the skeleton of the brick; ZrO2 has a high melting point (2715°C) and good chemical stability, giving strong corrosion resistance to acidic and alkaline media, especially molten glass.

In glass melting furnaces and other industrial furnaces, fused AZS bricks are used for the upper tank walls, port sills, port piers, tongues and breast walls. The following issues should be noted in use:

① Irregular thermal expansion. The expansion curve of fused AZS brick shows an anomaly near 1000°C, where ZrO2 undergoes a reversible crystal transformation with a large volume change. During furnace heat-up, temperature should not fluctuate too much in the 900–1150°C range; the heating rate should generally not exceed 15°C/h and must be steady. Some areas must be protected from cold air with other bricks to prevent cracking.

② Shrinkage cavities.

In casting, shrinkage cavities often occur at the pouring end, where the brick is more porous and less dense. When building glass tank walls, the cavity end is therefore placed facing into the furnace; if it faced outward, molten glass could leak once the brick wears thin. In the upper combustion space of the tank, however, service life is long and glass leakage is not an issue, so the cast end faces outward to extend service life.

③ Eutectic melting.

When fused AZS brick is laid in contact with fireclay brick, eutectic melting occurs at 1300°C. Refractories that form severe eutectics should therefore not be laid in contact. For example, silica brick placed beneath fused AZS brick is easily corroded by it.

Corundum brick sizes and specifications

Customers buy corundum bricks in standard and special shapes; some require the manufacturer to design, produce and machine them. To ensure brick dimensions match the drawings and meet on-site installation requirements, bricks that are pre-assembled should be machined and marked during pre-assembly, and marked again at shipment to facilitate installation on site.