The erosion of aluminum silicate refractories by molten aluminum, such as in aluminum melting furnaces and holding furnaces, usually results in the formation of aluminum oxide deposits on the refractory. The degree of erosion is increased in the presence of alkali and in a reducing atmosphere. This is related to the conversion of alumina bricks into sodium aluminate, which increases the kinetics of formation in a reducing atmosphere, promoting the formation of aluminum nitride. In melting furnaces and holding furnaces, the alkali may come from the metal charge.
In aluminum melting furnaces and holding furnaces, aluminum silicate refractories come into contact with liquid aluminum, usually resulting in the formation of bonded interfacial deposits containing mainly aluminum oxide, most of which is the result of the reaction of aluminum with oxides in the refractory, especially with silicon dioxide, with the following reaction formula:
4AL+3SiO2→2AL2O3+3Si
The kinetics of the reaction decrease rapidly after the deposit appears. It is concluded that this deposit becomes a barrier to aluminum penetration into the refractory material, and the presence of alkali in a reducing atmosphere will increase the kinetics of this deposit. There are two sources of alkaline sodium oxides; in the aluminum ingots produced by the electrolytic cell or in the refractory material. In the latter case, the active presence of Na2O cannot be confirmed. On the other hand, the relevant aspects of the reducing atmosphere have not yet been explained.
The main experiment of this time is the reaction of the aluminum silicate refractory material to aluminum corrosion caused by alkali and reducing atmosphere, and the determination of the possible effects of high aluminum refractory castables (70% Al2O3) with aluminum fluoride (ALF3) as an infiltrant on this corrosion when the alkali is present in the refractory material. This product is a representative of amorphous refractory materials without infiltration additives, and is used in industry as aluminum insulation furnaces and aluminum smelting furnace linings.
The test temperature is determined according to the operating temperature of the aluminum holding furnace and the aluminum smelting furnace. The temperature at the place in contact with the metal reaches 850℃, and the temperature at the flame radiation part reaches 1200℃~1500℃. The erosion test of industrial high-aluminum refractory castables with aluminum fluoride (ALF3) as a non-wetting agent is explained, and the corrosion caused by aluminum and the role of basic oxides contained in the refractory are inferred.
It seems that, especially in the presence of a reducing atmosphere, beta alumina is the active phase in the refractory in contact with liquid aluminum. From a thermodynamic point of view, the action of β-alumina on liquid aluminum leads to the formation of metallic sodium, and the reaction equation is as follows:
6NaAL11O17+2AL→6Na+34AL2O3 (2)
When the oxygen partial pressure is higher than 10-19atm, the metallic sodium produced in the aluminum solution (aNa~0.1) will be oxidized, and the reaction is as follows:
2Na+1/2O2→Na2O (3)
When the oxygen partial pressure is lower than 10-19atm, the presence of Na2O must be related to the action of refractory oxides (especially in silica) on metallic sodium, and the reaction equation is as follows:
4Na+SiO2→2Na2O+Si (4)
On the other hand, in the presence of ALF3, metallic sodium may also be produced, and the reaction equation is as follows:
6NaAL11O17+2ALF3→6NaF+34AL2O3 (5)
3NaF+AL→3Na+ALF3 (6)
Through analysis, when the liquid electrolyte exists at a temperature higher than 888℃, reaction equations (5) and (6) can produce metallic sodium, which is conducive to the exchange between the reactants. Under such conditions, refractory castables is expected to obtain a higher Na2O generation kinetics, so if the latter contains ALF3 as a wetting agent, aluminum will corrode the refractory material faster. The interface deposit mainly contains corundum (a-AL2O3) and also contains aluminum (AL) and aluminum nitride (ALN). It is believed that the aluminum nitride present in the deposit may participate in this corrosion process. The participation of aluminum nitride is consistent with the analysis of the reaction products obtained between aluminum and sodium carbonate in air and nitrogen. It was identified that the main reaction product obtained at 900℃ is sodium aluminate (NaALO2), which exists in the form of hydrate (NaALO2·3H2O). The specific gravity of sodium aluminate is 2.69g/cm³, while that of aluminum oxide is 3.96g/cm³. Therefore, the protective aluminum deposit should be accompanied by an increase in volume when it is converted into sodium aluminate. The increase in volume facilitates the formation of cracks, which facilitates the penetration of aluminum and makes the refractory material susceptible to erosion.
