Aluminum reduction cell is one of the high-temperature equipment which is easy to be damaged in aluminum smelting industry. It is very important to study its service life for aluminum electrolysis industry. The following is the analysis of refractory fire bricks used in various parts of aluminum reduction cell collected and sorted out by refractory brick manufacturers in combination with more than 10 years of production experience.
- Anode inert material:
Due to the precipitation of oxygen on the anode during electrolysis, using carbon as anode material will not only oxidize to produce CO and CO2, pollute the environment, but also consume a lot of carbon anode. About 300kg carbon anode material is required to produce 1tal. Therefore, seeking an inert anode material with good conductivity, no reaction with the oxygen precipitated from the anode, oxidation resistance, no pollution to the aluminum liquid, and resistance to the corrosion of cryolite melt and Al liquid has become a major research topic in the aluminum industry.
There are many reports on inert anode materials, but industrialization and commercialization have not been seen so far. Most of the existing studies focus on:
(1) Ceramic type: there are ceramic materials composed of SnO2, NiO, Fe2O3, Sb2O3 and CuO with low solubility in cryolite melt and good conductivity at 900 ℃, such as 96% SnO2 + 2% Sb2O3 + 2% CuO;
(2) Alloy type, such as Cu Ni Fe base;
(3) Cermet type: such as nife2o4-18% nio-17% Cu.
The ceramic anode material composed of oxide has low coefficient of thermal expansion, low solubility in cryolite-al2o3 melt, stable below 1500 ℃, and good oxidation resistance and corrosion resistance; However, its conductivity and strength are not easy to meet the requirements, and it is very brittle. The alloy anode material composed of metal has good conductivity and strength, but it does not meet the requirements in oxidation resistance, corrosion resistance and pollution to molten aluminum. Cermet anode materials have good metal conductivity and oxidation resistance. The combination of the two into composite materials is good for strength and overcoming brittleness, but there are still problems in corrosion resistance. The longest service time of cermet inert anode studied by Alcoa is 12.5 days (300h); They hope the inert anode will last up to six months.
- Cathode material:
The cathode of aluminum reduction cell is made of carbon brick. Al liquid will react with carbon to form Al4C3, and Al liquid has poor wetting with the cathode at the bottom of the carbonaceous tank; As a result, some poor conductivity substances will be deposited at the bottom of the tank, resulting in an increase in voltage drop and power consumption. TiB2 has good conductivity, corrosion resistance to Al liquid and cryolite, and good wetting between TiB2 and Al liquid. It is suitable for cathode material of aluminum electrolytic cell. Some test reports have shown that after covering a layer of TiB2 coating on the carbon block at the bottom of the cell, due to the good wettability between Al liquid and TiB2, the surface of TiB2 coating is closely adhered by Al liquid, there is no sediment accumulation at the bottom of the cell, the electrode spacing is reduced, the power consumption is reduced, and the service life of the electrolytic cell is prolonged.
- Side wall material:
In the past, carbon bricks have been used for the side wall of aluminum reduction cell. The damage of side wall carbon brick affects the normal operation of the electrolytic cell and reduces the service life of the electrolytic cell. In order to prevent the side wall from oxidation, have high resistance and resist the erosion of Al liquid and cryolite, the side wall of aluminum reduction cell has been built with Si2N4 combined with silicon carbide brick. After the brick is built, the service life of the electrolytic cell is prolonged, the leakage is reduced and the power consumption is reduced; It can also reduce the thickness of the original side wall and increase the volume of the electrolytic cell.
- Barrier against electrolyte penetration under the bottom of carbon tank:
NaF and other liquids and vapors in the electrolyte can penetrate into the lower insulation layer through the carbonaceous cathode at the bottom of the tank. After the insulation layer infiltrates NaF, the thermal conductivity increases and the temperature of the carbon cathode will decrease. If the temperature drops below 850 ℃, NaF will crystallize in the carbon cathode and swell and destroy the carbon brick. In order to ensure that the temperature in the carbon cathode is above 850 ℃, the method is to lay a layer of impervious material between the carbon cathode and the thermal insulation material to keep the temperature of the carbon cathode above 850 ℃, and NaF can not penetrate into the thermal insulation layer. Allaire C’s research shows that when the mass ratio of Al2O3 to SiO2 in Al2O3-SiO2 impervious material is greater than 0.9, NaF will react with Al2O3-SiO2 material to produce nepheline (naalsio4 melting point 1520 ℃) and block its pores, thus preventing the penetration of NaF. Dry impervious material is often used as lining here.
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