Application and damage of magnesia chrome brick in cement rotary kiln, RH furnace and copper smelting furnace

With the functions of natural decarburization, fine adjustment of composition, temperature rise of molten steel, forced decarburization by oxygen blowing and desulfurization by powder spraying, the RH furnace has become an indispensable external refining equipment for smelting high-end steel such as electrical steel, pipeline steel, ship plate steel, galvanized steel, tinplate, etc. During the operation of RH furnace, the furnace is in a negative pressure state, and the molten steel and argon are in a fountain shape and circulate at a high speed in a closed pipe composed of a rising insertion pipe, a lower groove and a falling insertion pipe. At the same time, a large number of alloys or desulfurizers are added to adjust the composition of the molten steel, which has strict requirements on the vacuum stability, high temperature performance, erosion resistance and slag resistance of the refractory; RH furnace is a kind of metallurgical equipment with intermittent operation. The inserted tube and the lower groove are frequently subjected to the thermal shock of alternating cold and hot in the whole furnace service. Therefore, the refractory materials are required to have good thermal shock resistance.

1. Cement rotary kiln

Cement clinker production in rotary kiln has a history of more than a century. In the early 1980s, the cement rotary kiln technology was basically finalized. At present, the new dry process kiln is the mainstream production technology. Its firing temperature is about 1450 ℃, and the temperature of combustion gas in the kiln can reach more than 1700 ℃, even close to 2000 ℃.

Refractories used in cement industry account for about 13% of all refractories. Refractory, as an indispensable lining material in the cement burning process, is also the material support condition of the second generation of new dry cement process, in which the refractory for burning belt is particularly important. The main mineral components of Portland cement are CaO · SiO2, 3CaO · Al2O3 and 4CaO · Al2O3 · Fe2O3. The most suitable refractory for the transition zone and firing zone of key parts of the cement kiln can only be alkaline magnesia refractory, as shown in Figure 1.

In the past, magnesia chrome bricks were widely used in the firing zone. The magnesia chrome bricks can resist the corrosion of cement components. CaO from cement clinker reacts with Fe2O3 in the bricks to form 4CaO · Al2O3 · Fe2O3, which makes the bricks easy to adhere to the clinker; Cr2O3 released from bricks can also stabilize 2CaO · SiO2 in the kiln skin, so chromite plays an important role in improving the refractoriness, corrosion resistance, thermal shock resistance and hanging kiln skin of basic refractories. The firing zone is in a high temperature chemical atmosphere. The furnace charge temperature is 1400~1500 ℃. With the production of molten liquid, the refractory is usually covered by raw materials. In addition to the rotation of the kiln body, the kiln skin often peels off. The reaction between the cement raw materials in the molten state and the bricks and the penetration of molten liquid into the bricks cause melting loss and structural peeling, and the transition zone is roughly the same.

In other words, when adding alumina to magnesia chrome refractories, it is necessary to consider the CaO and SiO2 components in the raw materials and add alumina appropriately; If most Al2O3 is added, it can react with the silica calcium component in the brick to form a low melting phase, and it can show a continuous distribution in the brick. At this time, as the amount of liquid phase increases in the sintering process, it can promote the material transmission in the sintering process, promote the sintering of the brick, and improve the density of the products; On the contrary, if the content of CaO and SiO2 is too low, it is not enough to meet the conditions for consuming Al2O3 to generate liquid phase. At this time, Al2O3 will react with the MgO component in the brick to generate spinel. At this time, the volume expansion caused by the formation of spinel cannot be well alleviated, the density of magnesium chromium products will be reduced, and the normal temperature compressive strength will be affected.

3. Effect of zirconia on magnesia chrome refractories

The addition of zirconia can improve the performance of magnesia chrome refractories to a certain extent: (1) ZrO2 has strong chemical stability, is chemically inert to general glass melts and acids and alkalis, and is not easy to be wetted by metal solutions; (2) ZrO2 can change the aggregation state and crystal shape of crystal boundary phase in magnesia chrome refractories, reduce the dihedral angle between crystals, and promote the combination between crystals.

However, excessive ZrO2 addition is unfavorable to magnesia chrome refractories, because the solid solubility of ZrO2 in magnesia is small. If excessive ZrO2 is added, the residual ZrO2 will remain between grains, hindering mass transfer during sintering and not conducive to densification of bricks. a