Selection Of Raw Materials And Binders For Al2O3-SiC-C (ASC) Refractories

Al₂O₃-SiC-C (ASC) series refractories feature high strength, excellent thermal shock resistance, superior resistance to acidic and alkaline slag erosion, and good scour resistance. They are widely used in key components of iron and steel smelting, including blast furnace iron troughs (main iron troughs, runners, tapping troughs, slag troughs, etc.), linings for torpedo ladles and hot metal ladles, repair gunning mixes for torpedo ladles, tap hole clay, sidewall and bottom bricks of hot metal ladles, and continuous casting systems.

The following contents can be referred to for the selection of raw materials and binders:

1. Alumina Raw Materials

ASC refractories possess outstanding service performance, characterized by excellent resistance to chemical corrosion and scouring erosion caused by molten iron and slag. For iron trough castables, available material systems include alumina-based, magnesia-based, and magnesia-alumina-based types. Under service conditions with a carbon content of 4.5% and molten iron temperature below 1500℃, alumina-based materials can fully meet durability requirements.

Commonly used alumina raw materials include dense fused alumina, fused white corundum, brown fused alumina, tabular alumina, and sub-white fused alumina.

When used as aggregates in castables, alumina-based materials exhibit a lower degree of embrittlement after long-term repeated heating and cooling compared with other systems. In addition, alumina materials have a low expansion rate, which reduces thermal stress on the working surface during service.

Alumina oxide is generally recognized for its strong corrosion resistance against sodium carbonate, iron, phosphorus, and other harmful components. Nevertheless, pure alumina has a high thermal expansion coefficient and poor spalling resistance. Its matrix is prone to penetration and abrasion by molten slag, leading to aggregate exposure, spalling, and further damage. Therefore, single-component alumina refractories cannot fully meet the application requirements of iron trough castables and hot metal pretreatment materials.

2. Silicon Carbide (SiC) Raw Materials

Commonly known as emery, silicon carbide is generally produced by electrifying a mixture of silica rock and coke via a chemical reaction. As a covalent compound, SiC has a vitreous luster, a density of 3.17–3.47 g/cm³, and a Mohs hardness of 9.2. It begins to decompose at 2600℃ under a reducing atmosphere and exhibits a tensile strength of 71.5 MPa and a compressive strength of 1029 MPa.

SiC features strong interatomic bonding, a high melting point, high hardness, high mechanical strength, low thermal expansion, high thermal conductivity, good electrical conductivity, and excellent chemical stability, making it an ideal raw material for refractories. However, SiC is susceptible to oxidation in an oxidizing atmosphere, and its oxidation rate can only be slowed after the formation of a protective silica film.

With properties superior to ordinary oxide raw materials-such as high thermal conductivity, low thermal expansion, and low reactivity with slag-SiC has long been used as a core raw material for refractory components in areas subject to severe slag corrosion and high-temperature spalling. Its importance persists in unshaped refractories, from the era dominated by ramming mixes and plastic refractories to the current mainstream of low-cement castables.

SiC has poor oxidation resistance. During oxidation, it produces solid-phase silica and carbon, as well as gaseous CO and CO₂. When the temperature exceeds 1823 K, SiC becomes unstable and converts into SiO₂, transforming ASC refractories into an Al₂O₃-SiO₂-C system. Nevertheless, in an open system where the working lining of ASC refractories is exposed to the atmosphere and the partial pressure of CO is below 0.1 MPa, SiC can maintain its most stable state.

3. Carbon Raw Materials

Carbon does not react with molten slag and iron and has a low thermal expansion rate. Therefore, carbon-based materials provide excellent slag corrosion resistance and strong anti-sticking properties against slag. Under specific temperature conditions, silicon in the matrix reacts with carbon to form fine fibrous silicon carbide with a diameter of about 0.1–0.5 μm, achieving the reinforcing effect of SiC.

During the development of ASC refractories, carbon-containing materials with low volatile content and high fixed carbon content should be selected, and antioxidants and dispersants should be added in proper proportions.

Graphite exhibits superior thermal stability, with a sublimation temperature of 3800℃, and can easily reach saturation in molten iron, providing good resistance to molten iron erosion. Commonly used graphite types include amorphous graphite, artificial graphite, spherical graphite, and flake graphite.

Graphite contains volatile components and is prone to oxidation, making it unsuitable for applications requiring high densification of refractories. Nevertheless, graphite has high thermal conductivity, which allows it to distribute internal heat evenly within castables, reduce thermal stress, and further improve the thermal shock and spalling resistance of castables.

Both graphite and pitch can be used as carbon additives in castables. Graphite is difficult to stabilize in castable mixtures. When water is added, much of the graphite tends to migrate to the surface and be lost during construction due to its structural characteristics. Its poor wettability and dispersibility, together with the hydration tendency of alumina-based antioxidants, create numerous technical challenges, which greatly restrict the development and application of carbon-containing castables. To achieve stable incorporation of graphite in castable mixes, surface modification with appropriate additives is necessary to enhance its adhesion performance.

Using pitch as a carbon source can effectively avoid these issues. Pitch is the residue obtained from coal tar or petroleum after distillation and catalytic cracking to separate fractions with different boiling points. It is a brown-black mixture mainly composed of aromatic and aliphatic structures and is insoluble in water. Its chemical composition and physical properties vary significantly depending on the raw material source, distillation process, and subsequent treatment methods.

4. Binders

The quality of binders determines the densification of formed bodies and can affect the overall properties of refractories. Binder application technology has become one of the most critical factors influencing the performance of unshaped refractories. Two types of inorganic binders-aluminate cement and β-alumina-are introduced as follows.

(a) Aluminate Cement

The properties of aluminate cement are mainly determined by its mineral composition. Monocalcium aluminate (CA) is the dominant mineral in all high-alumina cements. It has high hydraulic activity, with moderate setting speed and rapid hardening, and serves as the main source of cement strength, especially early strength.

Dicalcium aluminate (CA₂) is abundant in high-alumina cement with low calcium oxide content. For example, in aluminate cement containing approximately 70% alumina, the mass ratio of CA₂ to CA is roughly 1:1. It hydrates and hardens slowly, contributing to low early strength but high later strength.

Tricalcium dodecaaluminate (C₁₃A₇) usually occurs in small amounts in high-alumina cement with a low Al₂O₃/CaO ratio and low silica content. It hydrates and sets rapidly but provides low mechanical strength. Excessive content of this mineral reduces later strength, and content above 10% tends to cause flash setting.

Gehlenite (C₂AS) is found in high-alumina cement with high SiO₂ content. Pure gehlenite shows no hydration activity and has no positive effect on cement hydration and hardening.

(b) β-Alumina

In the 1970s, β-Al₂O₃ was first adopted by Japanese researchers as a binder for castables. Since the 1980s, China has gradually begun research on β-Al₂O₃-bonded castables, which are now produced on a large scale.

Practical applications in high-temperature furnaces in the metallurgical and chemical industries have shown that corundum castables bonded with β-Al₂O₃ exhibit better high-temperature performance than those bonded with pure calcium aluminate cement.

The most notable feature of β-Al₂O₃ as a castable binder is its zero calcium oxide content. Because β-Al₂O₃ transforms into α-Al₂O₃ at high temperatures, β-Al₂O₃-bonded castables can be used at ultra-high temperatures above 1700℃.

After hydration, β-Al₂O₃ produces aluminum hydroxide and boehmite, which provide cementation and hardening effects. This is the core mechanism of β-Al₂O₃ as a castable binder.

With the advancement of production technology, the excellent properties of β-Al₂O₃ micropowder have been fully recognized, and its use as a binder in iron trough castables has become increasingly widespread.

Among the various crystal forms of alumina, only β-Al₂O₃ exhibits spontaneous hydration under high-temperature conditions. Its hydration reaction can be written as:β-Al2O3+2H2O→Al(OH)+AlOOH