Magnesite: Main Types And Four Essential Indicators For Assessing Performance

Global magnesite resources are abundant. Calcined or electrofused magnesite produces excellent magnesia raw materials, with high output, low cost, and wide usage as refractory materials. In global magnesia production and application, the demand for lightly burned magnesia is far lower than that for heavily burned magnesia.

Magnesia sand has applications in building materials, automotive, aerospace, agriculture, and animal husbandry, and is an indispensable high-quality refractory material in the iron and steel metallurgy industry. Currently, global demand for magnesium sand is increasing. The production of high-grade, high-performance magnesium sand is crucial for future development. At the same time, improving and utilizing low-grade ore and magnesium sand is essential for energy conservation and sustainable development.

The main classifications of magnesium sand, depending on the production method, are as follows:

Electrofused Magnesia

Magnesium oxide obtained by refining magnesite in an electric furnace at 2750°C is called electrofused magnesia. It generally has strong high-temperature resistance, is not easily melted by alkaline substances, and is resistant to erosion by molten metal at high temperatures. It also exhibits excellent thermal conductivity at room temperature.

As a raw material for producing high-refractory magnesia-carbon bricks (composed of graphite and fused magnesia sand), it is commonly used in converter steelmaking and furnace refining. It enables shorter melting cycles, longer furnace life, and improved steel quality. Ceramics made from fused magnesia perform better at high temperatures than those made from sintered alumina.

Overall, fused magnesia has the advantages of larger grain size and higher density compared to sintered magnesia, making it a high-quality raw material for magnesia-based refractory materials.

The main crystal phase of electrofused magnesia is periclase (magnesium oxide), and the grain size is very large - the largest can reach the millimeter level. The silicate phase between crystals is distributed in the form of thin films, with a thickness ranging from 1 to 10 μm. The composition of this phase is mostly CMS or C₃MS₂. The grain boundaries are thin and relatively straight. Due to the low number of inter- and intra-crystalline pores, high bulk density, and large crystal size, electrofused magnesia exhibits excellent high-temperature performance.

Re-fired Magnesia

Also commonly referred to as low-grade magnesia, it is produced by placing magnesite blocks into vertical kilns with a capacity of around 20 to 50 m³. Coke is used as the fuel, and through one-step calcination and manual unloading, a product with an MgO content of approximately 92% and a bulk density of about 3.10 g/cm³ is obtained. This method produces magnesia at a low cost, and the resulting material is mainly used as the primary raw material for manufacturing ordinary fired magnesia bricks and unshaped refractory materials.

The main crystal phase of re-fired magnesia is periclase (MgO), with small crystal size and fine, straight grain boundaries. A silicate phase is often present between the crystals, and its composition typically fluctuates among C₃S, C₂S, C₃MS₂, and M₂S. Periclase grains are mostly bonded with silicate, and the presence of abundant silicate phases and open pores between crystals results in poor high-temperature performance.

Medium-Grade Magnesium Sand

Medium-grade magnesia refers to magnesium sand made from high-quality magnesite ore that has been lightly calcined, finely ground, formed into balls, and then further calcined to enhance its density and purity. It typically has an MgO content of ≥95% and a bulk density of ≥3.15 g/cm³. The bulk density generally ranges between 3.20 and 3.25 g/cm³. It is commonly classified into two grades: 94 and 95.

Medium-grade magnesium sand is now an important category of magnesia products. As a raw material, it is widely used in the production of medium-grade magnesia bricks, other fired refractory bricks, and certain magnesia-based unshaped refractory materials.

The main crystal phase of medium-grade magnesia is periclase (MgO). The crystals are mostly rounded and irregular in shape, with relatively large grain size. A silicate phase is present between the grains, which serves as the main bonding phase. There are also many air pores. The silicate phase composition typically includes C₃S, C₂S, C₃MS₂, and M₂S. This material offers relatively good performance characteristics.

High-Purity Magnesium Sand

To address the issue that large crystalline magnesite is difficult to sinter, China generally adopts a "two-step calcination" process to produce high-purity magnesia with MgO ≥97% and bulk density ≥3.25 g/cm³. At present, high-quality magnesite ore is commonly lightly burned in gas-fueled reflector furnaces. The lightly burned magnesia is then finely ground using a Raymond mill. After grinding, the powder is dry-pressed into balls using a high-pressure balling machine and finally calcined in a high-temperature kiln at 1500℃–2000℃. This process yields high-purity magnesia with an MgO content of 97% and a bulk density between 3.25 and 3.30 g/cm³.

Currently, high-purity magnesium sand is classified into two grades: 97 and 98. However, domestic production remains unstable. Only a few manufacturers, mainly in Liaoning Province, are capable of producing high-purity magnesium sand with MgO ≥97.5%, C/S ≥2, and bulk density ≥3.30 g/cm³.

The main crystal phase of high-purity magnesium sand is periclase (MgO). The grains are polygonal in shape, with mostly straight grain boundaries. The silicate phase and pores are typically isolated and located at grain junctions. The main crystal phase grains are directly bonded. The grain size, shape, and distribution of the periclase phase, as well as the mineral composition and distribution of the silicate phase, have a significant impact on the material's high-temperature structural strength, erosion resistance, and thermal shock resistance.

Evaluation of the Performance of Sintered Magnesium Sand: Four Key Indicators

The MgO content, particle bulk density, CaO/SiO₂ ratio, and microstructure are important indicators used to measure the performance of sintered magnesium sand for refractory materials. Generally, a comprehensive evaluation of sintered magnesium sand considers these four aspects to understand its properties.

1.MgO Content

The MgO content in sintered magnesium sand is one of the most important standards for measuring its performance. A high MgO content indicates fewer impurities in the cemented phase, which generally results in refractory materials with high resistance to high-temperature erosion.

2.Bulk Density

Bulk density is an important indicator for characterizing refractory materials. The degree of sintering and densification of sintered magnesia is directly related to its bulk density. During the decomposition of magnesite (CaCO₃ to MgO), light-burned MgO is produced, and upon high-temperature treatment, important changes occur in dead-burned magnesia. The main crystal phase of magnesite grows, volume contraction occurs, the lattice constant decreases, true density increases, and hydration resistance improves.

The densification of magnesium sand is closely related to the bulk density of its particles: the denser the particles, the better the degree of densification, and vice versa. Loose particles indicate lower densification.

3. Microstructure

Microstructure refers to the particle size, shape, and distribution of the main crystal phase (periclase) in magnesium sand, combined with the distribution of other phases, including the characteristics of the glassy phase. These features have an important impact on the material's high-temperature structural strength, thermal shock resistance, and erosion resistance.

Generally, there are two typical microstructure types:

One type is characterized by main crystal phase periclase grains that are rounded, with intergranular pores filled by silicate phases such as CMS, M₂S, and a small amount of glassy phase acting as a binder. The grain size typically ranges from 0.04 to 0.5 mm.

The other type features main crystal phase periclase grains with mostly straight grain boundaries and more regular geometric polygonal shapes. The silicate phase, including porosity, is isolated at the junctions between grains, while the main crystal grains are directly bonded to each other.

4. CaO/SiO₂ Ratio

The main component of sintered magnesium sand is MgO, along with varying relative contents of impurity oxides such as CaO, SiO₂, Fe₂O₃, and Al₂O₃. These impurity levels are fundamental factors in determining the amount of magnesite and the distribution of bonding phases within the material. Among these, the CaO/SiO₂ molar ratio is particularly important in influencing the properties of sintered magnesia.

When the CaO/SiO₂ molar ratio is between 1 and 1.5, the main bonding phases are low-melting compounds such as CMS (melting point 1498°C) and C₃MS₂ (melting point 1575°C). Magnesium sand products with this composition tend to exhibit relatively poor high-temperature structural strength, corrosion resistance, and thermal shock resistance.

In contrast, sintered magnesium sand with a CaO/SiO₂ ratio less than 1 or greater than 2 has bonding phases with higher melting points, such as M₂S (1890°C), C₂S (2130°C), and C₃S (2071°C). As a result, these products demonstrate better high-temperature structural strength and erosion resistance.

Furthermore, due to the solubility of CaO in MgO at high temperatures, low-melting phases like C₃MS₂ (1575°C) and CMS (1498°C) can appear across a wider range of CaO/SiO₂ ratios. Therefore, to avoid the formation of silicate phases with low melting points, it is often desirable for magnesium sands to have a CaO/SiO₂ ratio greater than 3.

Table 1 shows the different phase combinations of sintered magnesium sands at high temperatures and their effects on product properties.

Note: C₃S is only stable between 1249 - 1900 °C. Below or above these temperatures, it decomposes into C₂S and CaO.

Due to the varying grain sizes of magnesite in actual production, it is difficult to control the CaO/SiO₂ ratio. Consequently, low-melting-point phases are easily formed during the production of sintered magnesium sand, which significantly impairs its high-temperature performance. To improve the high-temperature properties of magnesium sand, additives are often introduced during the calcination process. These additives react with impurities to produce high-melting-point oxide–nonoxide composite materials, thereby enhancing the high-temperature performance of sintered magnesium sand.