Production of Sintered spinels
Their properties include a high melting point (above 2100 °C), excellent insulation, hardness (8 on the Mohs scale), abrasion and corrosion resistance, low thermal expansion and specific thermal conductivity. Due to these properties, the spinel materials SP67, SP78 and SP90 are mainly used in the refractory industry and to a lesser extent as abrasives and column packing agents.
Spinels are produced through mechanochemical activation of magnesia and alumina and heat treatment at around 1800 °C. The heat treatment, also known as sintering, produces the characteristic AB2O4 face-centered cubic (FCC) crystal structure, in which Al3+ ions occupy tetrahedral positions and Mg2+ ions occupy octahedral positions. The structure is extremely stable, which forms the basis for the spinel’s distinctly expressed properties, such as its thermal and chemical stability.
Silkem offers a wide range of sintered spinels, which are produced by crushing, grinding and sieving, and secondary mechanical and magnetic separation.
Properties of Calcined alumina
- High purity: the Al₂O₃ content typically exceeds 99%.
- High hardness: comparable to corundum (9 on the Mohs hardness scale), which makes it ideal for abrasive applications.
- Thermal stability: withstands extremely high temperatures, making it ideal for refractory materials.
- Chemical inertness: resistant to acids and bases, ensuring a long lifetime in harsh environments.
- Controlled particle size: available in different sizes for specific applications.
- Controlled specific surface area: can be adjusted to improve reactivity or inertness.
Refractory Materials
Spinels have properties that make them ideal for use in the refractory industry. The melting point of a stoichiometric spinel is around 2100 °C. Adding spinel to the refractory matrix improves its thermal shock and corrosion resistance, which is particularly useful in refractory products intended for the steel industry.
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| Chemical Composition (%) | Spinel SP67 (Typical) | Spinel SP78 (Typical) | Spinel SP90 (Typical) |
|---|---|---|---|
| Al2O3 | 67.0 | 78.0 | 90.0 |
| MgO | 32.0 | 21.4 | 9.4 |
| Na2O | 0.17 | 0.12 | 0.12 |
| SiO2 | 0.13 | 0.10 | 0.10 |
| Fe2O3 (Total) | 0.25 | 0.15 | 0.15 |
| CaO | 0.45 | 0.20 | 0.20 |
| Physical Characteristics1 | Min | Max | |
|---|---|---|---|
| Bulk Specific Gravity | (g/cm3) | 3.2 | |
| Apparent Porosity | (%) | 1.2 | 2.6 |
| Water Absorption | (%) | / | 1.0 |
1 For all three types of spinels
Standard grain size2
| (mm) | (mesh) | (mm) | (mesh) |
|---|---|---|---|
| 3–6 | 3–6 | 0–0.5/0.6 | –28 |
| 1–3 | 6–14 | 0–0.090 | –170 |
| 0–1 | –14 | 0–0.045 | –325 |
| 0,5/0,6–1 | 14–28 | 0–0.020 | –635 |
2 Other grain sizes are also possible