Although the melting point of aluminum is not high, its activity is good and the viscosity of aluminum liquid is extremely low. The part of the molten pool below the slag line in the aluminum melting furnace is the most vulnerable part of the aluminum melting furnace to be damaged by infiltration. These characteristics plus the operation mode of the aluminum melting furnace determine the damage of the furnace molten pool lining in the following ways: (1) Physical wetting and penetration of molten aluminum or aluminum alloy melt; (2) Chemical melting of molten aluminum and slag Erosion; (3) mechanical damage caused by adding aluminum, scraping slag, and electromagnetic stirring; (4) thermal spalling and cracking caused by sudden temperature changes.
1. Chemical attack
The chemical attack of refractory materials by aluminum melt is one of the important causes of damage. The melting point of metal aluminum is about 660°C, and its own fluidity is excellent, but the maximum smelting temperature in the aluminum smelting furnace can reach 1200°C or even higher, and the viscosity of aluminum liquid at 750°C is only 0.104Pa s, which is the same as that of 20 The viscosity of water at ℃ (0.1Pa·s) is quite close, and the aluminum liquid has excellent wettability to it, so that at high temperature, the aluminum liquid can easily penetrate into it through the pores and penetrate into the furnace lining. At the same time, the Mg, Zn, Si, and Mn added in the melting of aluminum are all very active elements during alloy smelting, which have strong reducibility, and the alloy Mg and Zn elements have a high vapor pressure and are more permeable than molten aluminum. To the inside of the furnace lining, these reducing substances react chemically with SiO2 and Al2O3. The possible reactions are shown in the following five formulas. These reactions form a metamorphic layer different from the original structure and properties and produce volume changes. Stress is generated inside, and the thermal expansion coefficient of the metamorphic layer is different from that of the matrix itself, and the temperature in the furnace is not constant. The furnace is constantly repeating the operation process of “feeding-melting-out of the furnace”, and the lining is subjected to great thermal stress. If the furnace lining is not strong enough, it is easy to cause cracking and peeling at the metamorphic layer, exposing a new surface, and this new surface has larger cracks than before, which makes the aluminum melt penetrate deeper into the refractory material, and the reaction metamorphic layer is further thickened. Flaking is also more severe, thus creating a vicious cycle that accelerates damage.
2. Physical damage
(1) Mechanical damage caused by feeding, slag removal and magnetic stirring
The aluminum melting furnace has a simple charging sequence during operation: small pieces and flakes that are easy to burn are loaded on the bottom of the furnace to reduce the burning of materials and protect the bottom of the furnace from the impact of large metals. Large pieces of waste and primary aluminum ingots are installed in the middle layer, that is, on the waste that is easy to burn. The master alloy is evenly installed in the middle and upper layers. Even so, the furnace door still cannot avoid mechanical damage to the lining of the aluminum melting furnace during the process of feeding or charging at the top of the furnace. In addition, in the process of melting aluminum, it is necessary to remove slag and make slag several times in order to improve the purity of molten aluminum. Since slag removal is manual or mechanical operation, it is difficult to accurately control the slag removal equipment, so it is easy to collide with the furnace and cause damage .
In the process of aluminum smelting, after the charge is leveled, in order to accelerate the uniform melting and avoid local overheating of the melt, it is generally necessary to stir the aluminum melt. Most of the technology used now is to install an electromagnetic stirring device at the bottom of the furnace, which has many advantages such as: increasing the melting rate, shortening the melting time, increasing the productivity of the furnace, reducing energy consumption, improving the refining effect, reducing metal oxidation damage, and reducing the labor intensity of workers, etc. . However, because the electromagnetic stirring drives the aluminum liquid and the aluminum slag to rotate, it causes the lining of the aluminum melting furnace to be scoured. In addition, high-temperature electrolytic aluminum liquid will be added during the aluminum melting process, which will cause damage to it and cause the refractory material to peel off.
(2) Stress damage caused by rapid cooling and rapid heating
The process of aluminum smelting is a working process of “feeding – melting – furnace” continuous cycle. When charging, the temperature difference between the inside and outside of the furnace, the lining of the furnace and the aluminum material can reach more than 700°C. In addition to operations such as slag removal, material quality inspection, and fire viewing, the furnace needs to be switched on and off within a day. door dozens of times. The lining material is repeatedly cooled and heated by thermal shock, and thermal stress is generated inside the material. When the thermal stress is higher than the strength of the material, its structure will be damaged. The result of the damage is cracks inside and on the surface of the lining material. Due to the thermal stress There are many factors for the formation of cracks, and the direction of cracks will be very complicated, and will continue to expand with the increase of service time, which will eventually lead to peeling and falling of the lining material. At the same time, the generation and development of cracks will inevitably reduce the quality of refractories. The strength and resistance to chemical attack will also aggravate its damage.
