The electrolytic cell is an important part in the process of electrolytic aluminum, and the two main factors that affect the life of the aluminum electrolytic cell are the carbon cathode and the lining refractory material. This article briefly describes the application of silicon carbide refractory bricks in aluminum electrolytic cells, as well as the current problems and research status:
The lining of the aluminum electrolytic cell can be divided into the bottom lining and the side lining according to the area. From the functional point of view, the bottom lining supports the cathode structure and thermal insulation. The side lining is mainly used to protect the steel metal shell surface from the corrosion of the electrolyte melt.
Aluminum electrolytic cell
The side lining of the aluminum electrolytic cell is an important structural part of the electrolytic cell. According to modern concepts, it is proposed that the side wall material of the aluminum electrolytic cell should have the following important properties at high temperature: high resistivity, good thermal conductivity, and not The molten cryolite reacts chemically, has a small porosity, does not penetrate the electrolyte and aluminum, and is not oxidized by air.
The valence bond structure of silicon carbide determines its series of excellent properties, such as high strength, high hardness, high temperature resistance, oxidation resistance, high thermal conductivity, low thermal expansion rate, excellent thermal shock resistance, and good chemical stability , It is not wetted by non-ferrous metals, and has good resistance to high temperature chemical corrosion, especially suitable for lining refractory materials of aluminum electrolytic cells.
With the development of material technology and the increase of the capacity of the electrolytic cell, the material structure of the inner lining of the side of the electrolytic cell has gradually evolved from the early double-carbon block plus the insulation brick structure to no insulation brick, single-layer carbon block, and even Today's separate silicon carbide is combined with silicon nitride materials.
Existing problems and current research status
Silicon carbide refractory brick is a new type of furnace building material recently promoted and used in China's non-ferrous industry. It was first applied to the 320kA large pre-baked aluminum electrolytic cell of Pingguo Aluminum in China. In recent years, the use of the company has shown that no matter whether pure silicon carbide refractory brick side blocks or composite side blocks are used, there are generally varying degrees of fracture and shedding during the production process, and the silicon carbide layer of the composite side blocks still has Lifting phenomenon.
Silicon carbide brick
The temperature difference (with the upper edge of the artificial extension leg as the boundary) is the main reason for the fracture of the silicon carbide brick. Although silicon carbide bricks have good thermal conductivity and a low coefficient of thermal expansion, because the products are fired at a high temperature above 1450 ° C to form a hexagonal structure ceramic body, the thermal shock resistance and temperature difference resistance are poor. In the environment where the temperature difference between the upper and lower parts of the silicon carbide refractory brick is repeated, it is easy to break.
When the silicon carbide composite layer is broken, the electrolyte is immersed in the crack. When the electrolytic cell returns from the effect temperature to the normal temperature, the electrolyte in the crack solidifies and shrinks, and the new electrolyte enters the crack and solidifies again. When the next effect occurs, the solid electrolyte immersed in the crack will be heated and expanded to lift the upper fracture block. When the tank temperature returns to normal, the electrolyte will solidify and shrink to form a gap, and then enter the new electrolyte and solidify again. When the effect comes again, it expands again to push the fractured block upwards, so the repeated action gradually raises the upper part of the fracture.
In the process of electrolytic aluminum, the temperature is close to 1000 ° C. At this time, the erosion of the lining of the electrolytic cell is mainly divided into 3 parts, the metal aluminum melt near the bottom, the melt electrolyte in the middle part, and various aggressive gases (such as HF, AINaF4, etc.). Normally, in electrolyte. Alumina melt, the cation penetration is mainly Na + ore, and the anion penetration is mainly F-.
Various factors such as porosity, matrix phase, wettability, etc. can affect the erosion resistance of the refractory. Materials with high porosity and large pore diameters must have poor erosion resistance, because cryolite or aluminum liquid can penetrate directly into the material. Xiao Yaming pointed out that the limit pore diameter of molten metal for refractory materials is 30μm for molten steel, 5μm for molten iron, and 0.5μm for aluminum melt, so the penetration of aluminum liquid into the material is quite strong. The matrix phase is thin and the corrosion resistance is good, but the strength will be directly affected, and the matrix phase is thick, the corrosion resistance is poor. If the material has a large wetting angle with the aluminum liquid, its corrosion resistance is superior.
The anti-electrolyte corrosion performance of various bonded phases of SiC materials was studied. The results show that in addition to self-bonded SiC, Si3N4 bonded SiC materials have the best electrolyte resistance performance. Although the Si3N4 bonded phase is wetted by the melt, the penetration is very shallow. There is no decomposition.
The results of Geshan et al. Showed that the damage of Si3N4 combined with SiC products in the air is mainly due to the oxidation of Si3N4 and SiC: and at the interface between cryolite electrolyte and air, the vicious cycle of oxidation-erosion-infiltration caused by chemical reaction makes the corrosion The most serious, the dissolution of electrolyte in the aluminum liquid and the pores in the structure of the sample itself may be the main reasons for the erosion of Si3N4 combined with SiC products in the aluminum liquid.
Silicon nitride bonded silicon carbide brick
Zhang Lipeng and others studied the erosion behavior of silicon nitride combined with silicon carbide refractory with different silicon nitride content in cryolite molten salt, and found that corrosion mainly occurred before 25h, and the weight gain rate of Si3N4 / SiC material remained basically unchanged after 25h. , Si3NgSiC material with low Si3N4 content (13%) has good resistance to cryolite melt corrosion.
Etzion et al. Found that in the aluminum electrolysis process, the porosity and β-Si3N4 content have an important effect on the corrosion resistance of Si3N4 combined with SiC refractories. The higher the porosity or the higher the content of β-Si3N4, the more severe the erosion. When the amount of SiC aggregate is between 80% and 85%, the anti-erosion effect is good.