Do You Know The Causes Of Corrosion Of Magnesium-chromium Bricks?

Magnesium-chromium refractories are a type of refractory material with magnesia sand as the primary component and chromite as a supplementary material. Depending on the process and raw materials used, magnesium-chromium refractories form different composite phases during cooling, such as secondary composite phlogopite and magnesite-secondary spinel. This results in a structure where magnesia sand is wrapped around chromite ore.

Although magnesium-chromium refractories produce harmful chromium-containing compounds during use, they are still widely used in the copper refining industry due to their high strength, excellent volumetric stability, and resistance to slag erosion at high temperatures.

Commonly used magnesium-chromium refractories in the copper refining industry include: common silicate-bonded magnesium-chromium bricks, direct-bonded magnesium-chromium bricks, magnesium-chromium bricks with secondary bonding, semi-bonded magnesium-chromium bricks, common sintered magnesium-chromium bricks, and fusion-cast magnesium-chromium bricks, among others.

01. General Destruction Mechanism of Magnesium-Chromium Refractories

Steelmaking industrial kilns also fall under the category of high-temperature kilns for non-ferrous metal smelting. Therefore, the destruction mechanism of magnesium-chromium refractories follows the general destruction pattern of high-temperature kilns. This can be roughly divided into four main aspects:

1.Chemical fusion caused by material components and slag reactions. High-temperature slag, which contains FeO, reacts with the MgO in magnesium-chromium bricks to form a solid solution. SiO2 may also fuse with MgO, producing M2S, which fills the grain boundaries. Additionally, FeO and SiO2, in combination with magnesium oxide in the bricks, generate MFS, resulting in a low-melting phase that dissolves into the slag, leading to component loss.

2.Penetration of the melt, causing cracks and structural spalling. The low viscosity and strong infiltration ability of the melt in the steelmaking furnace allow it to invade the interior of magnesium-chromium bricks through capillary pores. Because of the structural differences between the original brick and the metamorphic layer, temperature fluctuations cause cracks to form parallel to the working surface inside the bricks. In severe cases, this can lead to chipping and spalling.

3.Atmospheric effects causing the material structure to loosen. The copper refining furnace contains a large amount of sulfur dioxide gas. When gas migration occurs, re-oxidation reactions can generate sulfur trioxide. The sulfur trioxide then reacts with the alkaline oxides (such as magnesium oxide and calcium oxide) in magnesium-chromium bricks to form low-melting-point alkaline earth metal salts, such as MgSO4 and CaSO4.

4.Flue gas scouring and mechanical abrasion, which also accelerate the erosion of magnesium-chromium bricks.

02. Special Destruction Mechanism of Magnesium-Chromium Refractories

Although various oxides in MgCr refractories for copper refining can affect the material properties to some extent, leading to the destruction of the material (e.g., chromium oxide, aluminum oxide, zirconium oxide), the special destruction mechanism of MgCr refractories in copper refining is primarily caused by copper slag, iron-silicon slag, and sulfur elements.

Copper Slag and Copper Melt: Copper slag and copper melt fill the pores and cracks in the refractories, causing thermal penetration of the furnace lining, which leads to expansion and spalling.

Iron-Silicon Slag: Research has shown that the magnesium oxide in magnesium-chromium bricks reacts with iron oxide in the slag to form magnesium-iron spinel. As the SiO2 content in the brick increases, the magnesium-iron solid solution is gradually replaced by low-melting magnesium-iron peridotite. This causes the magnesia in the brick to fuse with the iron-silica slag, forming magnesium-iron peridotite, with magnesium peridotite wrapping the main crystalline phase of magnesium-chromium spinel. Additionally, because iron-silica slag has a relatively low viscosity, the slag infiltrates the magnesium-chromium bricks, forming a continuous network that leads to the creation of a metamorphic layer. The formation of this metamorphic layer causes thermal expansion differences within the brick, resulting in cracks and, eventually, spalling. The migration of SO2 in the brick body leads to oxidation reactions, generating SO3, which reacts with alkaline oxides to form low-melting-point alkaline earth metal salts. These reaction products are less dense and cause an increase in volume, exacerbating the slag penetration and erosion. Several industry researchers believe that the primary cause of the destruction of magnesium-chromium bricks is iron-silicon slag.

Sulfur Elements:

At 1500°C: When the sulfur content in the slag is high, SO2 migrates into the magnesium-chromium bricks, causing a sulfate reaction and decomposition. This process causes expansion of the brick body's pores and loosens the structure, which deepens the erosion of the magnesium-chromium refractories by copper slag. However, the presence of an appropriate amount of CaO can absorb SO2 and reduce the formation of MgSO4. If only a small amount of MgSO4 is formed, and the brick has high porosity, the volume expansion is not sufficient to destroy the structure of the brick. Instead, it can help block the pores, thereby hindering further erosion of the refractory material by copper slag.

At 1300°C: Direct-bonded magnesium-chromium bricks exhibit better erosion resistance compared to electrofused semi-recombined magnesium-chromium bricks. However, at 1500°C, electrofused semi-recombined magnesium-chromium bricks show better erosion resistance than direct-bonded bricks. These results suggest that various factors need to be considered when selecting refractory materials for copper converters. For typical copper refining converters, where the working temperature is generally between 1100°C and 1300°C, direct-bonded magnesium-chromium bricks are more appropriate. Due to the presence of SO2 gas, higher CaO content is needed, and direct-bonded bricks with higher porosity are better suited. For higher melting temperature converters, fusion-cast or semi-recombined magnesium-chromium bricks with superior performance should be used.

03. Conclusion

The development of the refractory industry is closely tied to resources, energy, and the environment. Magnesium-chromium refractories, which are excellent materials, are widely used in the copper refining industry. However, they generally have a low service life due to damage caused by various mechanisms. The damage mechanisms of MgCr refractories in copper refining can be categorized into general and special mechanisms. The general damage mechanism follows the damage evolution pattern of MgCr bricks used in high-temperature kilns, while the special damage mechanism focuses on the erosion caused by different types of slag produced in the copper refining process.