Refractory Bricks

magnesia chrome bricks refractories in cement rotary kiln

Feb 22,2022

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

Magnesium chromium brick has the advantages of high refractoriness, high temperature strength, strong resistance to alkaline slag, and excellent thermal stability. It also has certain adaptability to acid slag. It is widely used in cement kiln firing zone and molten steel refining RH furnace Key parts such as the lower trough, circulation pipe, etc., various copper smelting furnace linings, etc. The following is an introduction to its service environment and damage, and prospects for its application in new high-temperature equipment.

 magnesia chrome brick in cement rotary kiln

1. Cement rotary kiln with magnesia chrome bricks

Rotary kiln production of cement clinker has a history of more than a century. In the early 1980s, 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 combustion gas temperature in the kiln can reach over 1700℃, even close to 2000℃. Refractories magnesia chrome bricks for the cement industry account for about 13% of all refractories. Refractories, as an indispensable lining material in the cement firing process, are also the material support conditions for the second-generation new dry cement process. Among them, the use of refractories for the firing zone is particularly important. The main mineral components of Portland cement are CaO·SiO2, 3CaO·Al2O3 and 4CaO·Al2O3·Fe2O3. The most suitable refractory material for the transition zone and firing zone of the cement kiln can only be alkaline magnesia refractories, such as As shown in Figure 1. In the past, magnesia-chrome bricks were commonly used in the firing zone. Magnesium-chrome bricks can resist the erosion of cement components. CaO from cement clinker reacts with Fe2O3 in the brick to form 4CaO·Al2O3·Fe2O3, which makes the brick easy to adhere to the clinker; brick release The Cr2O3 also has the function of stabilizing 2CaO·SiO2 in the kiln skin, so chromite plays an important role in improving the fire resistance, erosion resistance, thermal shock resistance and kiln skin resistance of alkaline refractories. The firing zone is in a high-temperature chemical atmosphere. The temperature of the charge is 1400~1500°C. With the production of molten liquid, the refractory is usually covered by the raw material. Coupled with the rotation of the kiln body, the kiln skin often peels off. The molten cement material reacts with the bricks and the molten liquid penetrates into the bricks, causing melting loss and structural spalling, and the transition zone is also roughly the same.

Austria’s Olbrich, Germany’s Bonn Refractories Company, and Refa’s Kun-neck believe that magnesia-chrome bricks will be subjected to a series of combined effects such as thermal stress, mechanical stress and chemical erosion in cement rotary kilns; the temperature in magnesia-chrome bricks Gradient will lead to local concentration of thermal stress and easily cause cracks. The erosion caused by mechanical stress includes: wear caused by the relative movement of bricks and clinker, deformation of the kiln shell caused by ovality, and spalling caused by groove formation. The erosion effects include melting erosion, liquid phase erosion, overheating load, and alkali salt penetration. , Erosion of clinker, reduction of iron cause the price of iron to peel off and so on. Qotaibi et al. analyzed the residual magnesia-chromium bricks used in the burning zone of cement rotary kilns, and found that there is no Mg-SiO4 and Mg(Al1.5Cr0.5)O4 in the interface between the hot surface and the slag layer of the magnesia-chromium bricks, and 3CaO in the cement clinker ·SiO2, 2CaO·SiO2 and 3CaO·Al2O3 have penetrated into the hot surface, and the formation of Cr6+ has been detected at the same time. Yun Sining et al. believe that structural spalling caused by cement clinker erosion mainly occurs at the hot end of magnesia-chrome bricks, while cement clinker liquid phase and alkali salt (combined from potassium, sodium, sulfur, and chlorine compounds in cement raw materials and fuels) Formation) The erosion channels at high temperature are all open pores directly bonded with magnesia-chrome bricks. Therefore, reducing the open porosity of the direct-bonded magnesia-chrome brick can effectively reduce its damage rate on the burning zone of the dry-process cement rotary kiln.

2. Magnesia chrome bricks for RH furnace

RH vacuum treatment equipment (hereinafter referred to as RH furnace) is an important refining equipment in steelmaking production. It is a circulating vacuum degassing equipment jointly developed by Ruhrstall and Heraeus in 1959. RH refining method is a kind of vacuum cycle degassing method. Due to its characteristics of fast degassing, low temperature drop, high alloy yield, wide range of smelting steel, and good refining effect, it has been widely used in the iron and steel industry in all countries in the world. application. The structure of the RH furnace body is generally divided into immersion pipe, circulation pipe, lower tank, middle tank (some RH furnaces do not include the middle tank), upper tank and other parts from bottom to top, as shown in Figure 2. Refractory materials for RH furnaces are mainly high-density fired directly combined with magnesia-chromium refractories. The dip tube, circulation tube and lower tank are directly in contact with high-speed steel, alloy and slag, so the use environment is the worst and the service life is the shortest.

Many studies have reported the damage mechanism of magnesia-chromium bricks  materials used in RH furnaces. Taikabutsu of Japan successively reported the abnormal erosion of magnesium-chromium refractories in a certain part of the RH furnace lining in 1988 and 1990. Mosser et al. described the damage mechanism of magnesia-chrome bricks in the RH degassing process, mainly in the following three processes: silicate or alumina-rich slag penetrates into the pores; between the slag and the matrix of the magnesia-chrome brick The infiltration reaction; the thermal erosion of the infiltration zone. The dense layer transitioning to the impermeable zone is easy to peel off. Spinel minerals dissolve in refractory materials and can also attack magnesia-chrome bricks. Dong et al. introduced the destruction mechanism of refractory materials in the RH-TOB refining process, and believed that Cr2O3 in bricks can react with FeO to form high melting point spinel, but FeO can form a low melting point phase with silicate in the periclase grain boundary. This results in the separation of periclase grains, and at the same time, FeO in the molten steel is reduced by CO to form iron vapor and then reacts with oxygen to generate oxidation heat, which promotes the melting of MgO and Cr2O3.

3. Magnesia chrome bricks for copper smelting furnace

Magnesium-chromium bricks refractories are widely used in high-temperature equipment in the non-ferrous smelting industry, and the copper smelting industry is the most representative. Modern copper smelting technology has a variety of processes and equipment, such as Noranda/Tiniente process, Osmet/Isa copper smelting method, flash copper smelting method, oxygen bottom-blown copper smelting method and Vanyukov /Jinfeng/Silver copper smelting method. Figure 3 shows a bottom-blowing continuous copper smelting furnace with the current advanced technology. The configuration technology of refractory materials used in this equipment is one of the key technologies that affect the production efficiency and product quality of bottom-blowing continuous copper smelting.

Magnesium chromium bricks refractories are currently the most suitable lining material for smelting furnaces in the copper industry. One of the main reasons is that magnesia chromium bricks have good corrosion resistance to slags with different alkalinity in the copper industry. Different types of magnesia-chrome bricks are selected according to different parts of the furnace lining and service environment. Generally, the magnesia-chrome bricks selected at the slag line position are electrofusion recombination, and the magnesia-chrome bricks selected at the tuyere position are semi-recombination or self-combination. Copper smelting furnace has a large amount of slag and low viscosity, which has strong wettability and permeability to magnesia-chromium refractories. Therefore, when magnesia-chromium refractories are used in copper smelting furnaces, the infiltration and metamorphic layer is thicker, and the structure is prone to appear Flaking. Cherif et al. studied the corrosion resistance of magnesia-chrome bricks prepared from sintered aggregates and fused aggregates by simulating the environment of copper converters by using the rotary anti-slag method. The Fe2O3 ratio (amount of matter ratio) has a greater correlation than the CaO/SiO2 ratio, and the larger the CaO/Fe2O3 ratio, the more severe the erosion. Wang Jibao and others analyzed the structure of the fused magnesia-chrome brick in the tuyere area ofthe Noranda furnace. Pores and microcracks, the infiltrated matte destroys the structure of the brick body, but it does not react with the oxides in the magnesia chrome brick. At the same time, under the conditions of oxygen-enriched smelting, the damage of the magnesia-chrome bricks at the working face is caused by the reaction of iron oxides and SiO2 in the copper slag with the low-melting substances in the magnesia-chrome bricks to form a liquid phase with a lower melting point. With the increase of the distance from the working surface, the damage of the magnesia-chrome brick is mainly manifested by the penetration of the matte into the interior of the brick, resulting in the structural spalling and thermal spalling of the brick body. Zou Xing et al. studied the erosion of FeO-SiO2 slag for copper blowing on magnesia-chrome bricks, and found that the corroded magnesia-chrome bricks were clearly divided into slag layer, erosion area and original brick area. Among them, the slag layer is mainly based on mafic olivine and forsterite, and spinel phase is distributed in it; the erosion zone is mainly periclase richite and spinel phase.

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