Magnesia-chrome refractory is a refractory material mainly composed of magnesia sand and supplemented by chromite. Depending on the process and raw materials, magnesia-chrome refractory forms different degrees of secondary composite spinel and periclase-secondary spinel composite phase during the cooling process, forming a structure of magnesia sand wrapped in chromite.
Under the current production and use status, magnesia-chrome refractory will produce harmful chromium compounds (Cr6+). This compound is a strong carcinogen and has a strong corrosive effect on human skin, mucous membranes and upper respiratory system. However, due to its high strength, good volume stability and resistance to slag erosion under high temperature environment, magnesia-chrome refractory is still widely used in the copper smelting industrial system.
The magnesia-chrome refractory bricks commonly used in the copper smelting industry are: ordinary silicate-bonded magnesia-chrome bricks, directly-bonded magnesia-chrome bricks, rebonded magnesia-chrome bricks, semi-rebonded magnesia-chrome bricks, co-sintered magnesia-chrome bricks, fused-cast magnesia-chrome bricks, etc. Magnesia-chrome refractory bricks are subjected to high temperature and chemical erosion for a long time in the daily operation of high-temperature kilns in the copper smelting industry. The main damage mechanisms are as follows:
Special damage mechanism of magnesia-chrome refractory bricks for copper smelting
Although different oxides in magnesia-chrome refractory bricks for copper smelting can also affect the performance of the material to a certain extent, thereby causing the destruction of the material, such as chromium oxide, aluminum oxide, zirconium oxide, etc., the special damage mechanism of magnesia-chrome refractory bricks for copper smelting lies in the special damage of copper slag, iron silicon slag and sulfur elements to the material. First, copper slag and copper melt can gradually penetrate into the interior along the pores of magnesia-chrome bricks, and the slag and copper melt that enter the interior fill the pores and cracks, causing the furnace lining to be thermally broken down and expand and peel off.
Secondly, for the special damage of ferrosilicon slag, MgO in magnesia-chrome brick and FeO in slag form magnesia-iron spinel solid solution. With the increase of SiO2 content in the brick body, magnesia-iron solid solution is gradually replaced by low-melting-point magnesia-iron olivine, and periclase is dissolved by ferrosilicon slag, thus forming a structure of magnesia-iron olivine and forsterite encapsulating the main crystal phase magnesia-chrome spinel; and the viscosity of ferrosilicon slag is relatively low. The ferrosilicon slag that penetrates into the magnesia-chrome brick forms a continuous network, forming a metamorphic layer. The metamorphic layer causes different thermal expansion of the brick body, forming cracks in the brick body and gradually expanding, leading to the peeling of the brick body. Moreover, when SO2 migrates in the brick body, it undergoes an oxidation reaction to generate SO3, and generates low-melting-point alkaline metal salts with alkaline oxides. The density of the reaction product is small, resulting in an increase in volume, thereby exacerbating the penetration and erosion of the slag. Most experts and scholars believe that the main cause of the damage of magnesia-chrome bricks is caused by ferrosilicon slag.
For the special destruction of sulfur element
At 1500℃, when the sulfur content in the slag is high, there is a process of sulfate generation and decomposition in the migration of SO2 magnesia-chrome bricks. This process will cause the expansion of brick pores and loose structure, and deepen the corrosion of converter copper slag on magnesia-chrome refractory materials: the presence of an appropriate amount of CaO can absorb SO2 gas to reduce the generation of MgSO4, and the volume expansion caused by a small amount of MgSO4 generation when the porosity is large does not destroy the brick structure, but instead plays a role in blocking the pores and hindering the further corrosion of converter copper slag on magnesia-chrome refractory bricks.
At 1300℃, the corrosion resistance of directly bonded magnesia-chrome bricks is better than that of fused semi-rebonded magnesia-chrome bricks. At 1500℃, the corrosion resistance of fused semi-rebonded magnesia-chrome bricks is better than that of directly bonded magnesia-chrome bricks;
These results show that the selection of refractory bricks in the erection process of copper converters needs to consider many factors comprehensively. For general copper smelting converters, since the working temperature is generally 1100~1300℃, direct bonded magnesia-chrome bricks are selected at this time. Due to the presence of SO2 gas, direct bonded magnesia-chrome bricks with high CaO content and high porosity are more suitable; and for converters with higher smelting temperatures, it is advisable to use fused semi-rebonded magnesia-chrome bricks or fused rebonded magnesia-chrome bricks with better performance.
General damage mechanism of magnesia-chrome refractory bricks for copper smelting
The copper smelting industry also belongs to non-ferrous metals smelted in high-temperature kilns, so the damage mechanism of magnesia-chrome refractory bricks also follows the general damage mechanism of magnesia-chrome bricks for high-temperature kilns, which can be roughly divided into four aspects:
1) Chemical erosion caused by the reaction of material components with slag.
At high temperatures, FeO in the melt and MgO in the magnesia-chrome brick form a solid solution, and SiO2 partially corrodes MgO to generate M2S to fill between grain boundaries. Moreover, FeO and SiO2 react with MgO in the brick to generate MFS, and the low-melting products produced dissolve into the slag to cause component loss.
2) The penetration of the melt causes cracks in the material and structural peeling.
The melt in the copper smelting furnace has low viscosity and strong penetration ability, and can penetrate into the interior of the magnesia-chrome brick through capillary pores. Since the structure and properties of the original brick are very different from those of the metamorphic layer, when the temperature changes, cracks parallel to the working surface will be generated inside the magnesia-chrome brick, and in severe cases, collapse and peeling will occur.
3) The atmosphere in the furnace causes the material structure to be loose.
The copper smelting furnace contains a large amount of SO2 masonry. When SO2 gas migrates, it can undergo reoxidation reaction to generate SO3, which reacts with the alkaline oxides (MgO and CaO) in the magnesia-chrome brick to form low-melting alkaline earth metal salts, such as: MgSO4, CaSO4, etc.
4) Flue gas scouring and mechanical wear will also accelerate the erosion of magnesia-chrome bricks.
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