Performance characteristics of chrome magnesia bricks and magnesia chrome bricks

At present, the most commonly used method for preparing magnesium aluminum spinel is solid state reaction synthesis, that is, using oxides, chlorine oxides or carbonates as raw materials, mixing and compacting raw materials, and reacting to prepare spinel at high temperature (more than 1400 ℃).

Electrofusion is another commonly used method to synthesize spinel in industry, which also has the disadvantage of high energy consumption.

Although wet chemical methods such as sol-gel method and hydrothermal method can synthesize spinel at low temperature, their operation process is complex, equipment required is expensive, and cost is too high to meet the needs of large-scale industrial production.

Magnesia alumina spinel bricks produced with magnesia and magnesia alumina spinel sand as raw materials are usually called magnesia alumina (periclase spinel bricks). The magnesia raw materials used for the production of periclase spinel bricks should have the lowest impurity content (especially CaO).

The domestic sintered magnesia MS95, MS97, MS97.5 and DMS97 are widely used. Using spinel sand as granular material, magnesia sand as fine powder and part of granular material, according to the mixing, forming and firing process of high-grade magnesia alumina bricks, products with good high-temperature performance and high thermal shock stability can be manufactured.

Magnesia alumina spinel bricks are commonly used in cement rotary kilns, glass kilns, iron mixers and refractory kilns where the temperature changes greatly.

2. Magnesia alumina chrome spinel brick:

The magnesia alumina chrome spinel brick system is the periclase/magnesia alumina chrome spinel system: MgO-R2O3 (Cr2O3, Al2O3) system.

In order to compare the production of magnesium aluminum chromium products, the introduction of Cr2O3 into MgO is mostly in the form of chromite, and the manufacturing process is often accompanied by Cr2O3, Al2O3, Fe2O3 and other sesquioxides.

First of all, from the perspective of high temperature resistance and slag resistance, MgO-MgO · Cr2O3 system has advantages in MgO-R2O3 system. In addition to its high eutectic temperature (2350 ℃), it is also due to the low solubility of Cr2O3 in silicate liquid phase.

As we all know, low solubility should have strong crystallization ability, which can effectively reduce the interface energy between lattices, make the silicate liquid phase tend to move towards the constant grain gap in an isolated form, which is easy to realize the direct combination between periclase grains or through the bridge of magnesia chrome spinel, which can effectively improve the high temperature strength, inhibit the penetration of slag, and improve the slag resistance.

However, Cr2O3 has a high volatility ratio, and its stability is poor at high temperatures, especially under vacuum conditions.

Secondly, from the perspective of thermal shock resistance, MgO-MgO · Al2O3 system has more advantages. Because the solid solution colloidal effect of MgO · Al2O3 or Al2O3 in MgO at high temperature is much weaker than that of MgO · Cr2O3 or Cr2O3, especially MgO · Fe2O3 or Fe2O3, and the vapor pressure of MgO · Al2O3 at high temperature above 1600 ℃ is also lower than that of MgO · Cr2O3.

Therefore, MgO-MgO · Al2O3 system materials are relatively stable when the temperature fluctuates, and have good thermal shock resistance.

Compared with MgO-MgO · Al2O3 and MgO-MgO · Cr2O3 systems, when the MgO/R2O3 ratio is certain, it can be inferred that molten MgO-MgO · Al2O3 system materials have less precipitation spinels in periclase grains, and more intercrystalline spinels, while MgO-MgO · Cr2O3 system materials have more intracrystalline precipitation spinels and less intercrystalline spinels.

Obviously, changing the proportion of each component in R2O3 can adjust the distribution of spinel phase and change the microstructure.

3. Magnesium iron aluminum spinel brick:

Magnesia iron aluminum spinel bricks are mainly used in the cement kiln burning belt. The traditional refractory materials used in the cement kiln burning belt are mainly magnesia chrome refractories. As the highly toxic hexavalent chromium in the used magnesia chrome bricks that can be dissolved in water will cause serious environmental pollution.

It is recognized by experts to look for chromium free alternative products for cement kilns. Magnesia ferroalumina spinel brick was proposed by RHI Company in the 1990s. It is formed by mixing magnesia and pre synthesized ferroalumina spinel and fired at high temperature under a certain process.

Now it has been widely used in the lining of cement rotary kiln to replace magnesia chrome bricks.

The microstructure of magnesia alumina spinel is relatively complex, mainly composed of magnesia microstructure and ferruginous alumina spinel microstructure. The interface between magnesia and ferruginous alumina spinel is closely combined through the mutual diffusion of Mg2+, Al3+, Fe2+ions to achieve the direct combination of materials. Like magnesia chrome bricks, the main mineral composition of magnesia chrome bricks is spinel and periclase. It is also a composite alkaline refractory product made of sintered magnesia and chromite in proper proportion. The proportion of raw materials is not clearly specified. Generally, the ones with less than 50% of the addition amount of pyrite are called magnesia chrome bricks, and those with 50% or more are called chrome magnesia bricks. Its production process, performance characteristics and uses are basically similar.