Never thought! The refractory lining of the smelting furnace actually suffers the most erosion!
Over the years, many researchers and factory technicians have conducted extensive research, analysis, and discussion on the damage mechanisms of aluminium melting furnace linings, achieving many results. We have studied the causes and countermeasures of corundum tumor formation in the melt pool of aluminium alloy melting furnaces. It has been found that the furnace lining damage is mainly caused by the erosion reaction between aluminium and the furnace lining. This involves the formation of corundum mineralization (commonly known as corundum tumors), which are composed primarily of Al₂O₃, MgAl₂O₄, Al, and a small amount of Si.
The formation of these corundum tumors leads to an expansion in the volume of the refractory lining, continuously pushing inward into the furnace lining, aggravating cracks and resulting in a vicious cycle. This can cause severe deformation of the furnace's steel structure, accelerating the scrapping of the furnace.
The temperature of an aluminium smelting furnace is between 700–800°C. At 750°C, the viscosity of aluminium is 0.104 Pa·s, which is very close to the viscosity of water at 20°C (0.1 Pa·s), indicating a very strong permeability. In addition, aluminium is a very active metal that easily reacts with SiO₂ and other components in the furnace lining, leading to further damage to the refractory materials.
The current situation and development of refractory materials for aluminium smelting furnaces have also been analysed. It has been found that elements such as Na, K, and Si entering the aluminium melt lower its melting point and viscosity, thereby promoting the penetration of molten aluminium into the refractory materials and causing damage to the inner lining.
The use of new refractory materials can improve the service life of aluminium melting furnaces. This paper analyses the impact of slag corrosion, drastic temperature fluctuations, gas-phase deposition, mechanical impact, abrasion, and other factors on the life of the furnace lining.
Starting from the melting process of aluminium alloy, fluxes, and additives, this paper also analyses and discusses their impact on the service life of the refractory lining in aluminium melting furnaces.
01 Additives and Fluxes
At present, in the production of aluminium alloy castings, alloy additives, covering agents, and slag removers are widely used in the aluminium melt for alloying, oxidation prevention, hydrogen absorption resistance, and slag treatment in the melt pool.
Alloying additives usually consist of fluxes, heat generators, triggering agents, and moisture-proofing agents. Among these, the heat generator is typically aluminium powder. The sole purpose of alloy additives is to adjust the chemical composition of the alloy by adding them during the aluminium alloy melting process, allowing the aluminium melt to meet the required composition specifications.
There are many types of additives, usually classified as covering agents, slagging agents, refining agents, and furnace cleaning agents. Their components mainly include chlorine salts, fluorine salts, and complex salts of alkali metals and alkaline earth metals.
According to their different purposes, fluxes can be divided into dehydrogenation and degassing fluxes, anti-oxidation covering agents, purification agents (for removing oxides and other inclusions), heat-based slagging agents, and furnace cleaning fluxes.
02 Metal Additives and Fluxes
Metal additives are composed of metal powders, co-solvents, heat generators, triggering agents, and moisture-proofing agents, mixed in specific proportions and uniformly pressed into shape. The function of the co-solvent is to increase the dissolution rate and yield of the additive in the aluminium melt.
Since the melting point of the co-solvent is relatively low, it reacts with the heat generator and various substances in the aluminium melt (including other components of the additive), resulting in a localized exothermic reaction. This reaction ignites the surrounding aluminium, producing temperatures as high as 2204 °C due to the combustion of aluminium and alumina.
In actual production, when metal additives are introduced into the melt pool, they are often placed near the furnace wall. The high localized temperature of 2204 °C can inevitably damage the refractory lining of the melt pool, as the refractory materials used in aluminium melting furnaces typically have a refractoriness lower than 1800 °C.
In addition, the fluxes used in metal additives today are mainly composed of chlorides, fluorides, carbonates, nitrates, or sulfates of alkali and alkaline earth metals. Common compounds include NaCl, KCl, MgCl₂, ZnCl₂, NaF, KF, MgF₂, Na₃AlF₆, Na₂CO₃, K₂CO₃, Na₂SO₄, K₂SO₄, NaNO₃, and others. These fluxes almost always contain alkali metals and alkaline earth metals such as Na, K, and Mg.
Because of the presence of these elements, serious erosion of the refractory lining in aluminium smelting furnaces occurs. The following section will focus on analyzing the corrosion mechanism of alkali and alkaline earth metals on the refractory lining of aluminium furnaces.
03 Erosion of the Refractory Lining by Alkaline Earth Metal Mg
The Mg component in the melt and alloy is the main source of alkaline earth metal Mg in the melt pool of the melting furnace.
Magnesium is highly chemically reactive and has a strong reducing ability toward components such as SiO₂ and Fe₂O₃ in the refractory lining. It can cause erosion of the lining through the following reactions.
In addition, Mg has a very high vapor pressure, which makes it easier than Al for the vapor to penetrate into the refractory along the microscopic channels and react with lining components such as Al₂O₃, SiO₂, and Fe₂O₃, thereby eroding the lining. Furthermore, the replacement of Fe and Si by Mg from the lining can also contaminate the alloy melt, affecting the accuracy of the alloy's chemical composition.
It can be seen that when melting aluminium alloys, the furnace lining is subject to severe erosion.
04 Erosion of refractory linings by alkali metals Na and K
The sources of Na and K are as follows: electrolytic aluminium solution, remelted aluminium ingots, and refining agents, covering agents, and slag removers used in the melting, refining, and covering processes of aluminium melts. It has been reported that the Na content in remelted aluminium ingots is about 30×10⁻⁶, while the Na content in electrolytic aluminium solution is higher, usually in the range of 40×10⁻⁶ to 50×10⁻⁶. Alloy additives in the flux, aluminium alloy refining agents, covering agents, and slagging agents are mainly composed of alkali metal and alkaline earth metal chloride salts and fluorine salts (NaCl, KCl, NaF, KF, etc.), which are also important sources of Na and K. These components have a serious impact on the service life of refractory linings.
The boiling points of Na and K are extremely low, 882.9°C and 760°C, respectively. During the aluminium alloy melting process, the furnace chamber of the aluminium melting furnace operates at temperatures ranging from 800 to 1000°C, and can reach up to 1200°C. In this temperature range, alkali metals and alkaline earth metals such as Na, K, and Mg dissolved in the aluminium melt partially evaporate to form Na, K, and Mg vapors.
As a result, the furnace chamber contains significant amounts of CO₂, CO, Na (gas), K (gas), and Mg (gas). These gases and vapors penetrate the refractory through microscopic channels and react with the refractory material, forming new expansive phases such as potassium-containing chalcocite and sodium-containing chalcocite. Both of these phases damage the structure of refractory materials, accelerating the failure of the refractory lining. The specific reactions are as follows.
By analysing the above points, we can see that alkali metals and alkaline earth metals have the greatest erosion on the refractory lining of the melting furnace, therefore, in the actual production process, we should consider applying new technology to reduce the intake of alkali metals and alkaline earth metals in the melting pool to the lowest degree.