If you have an aluminum furnace, you’re familiar with corundum. When molten aluminum oxidizes with oxygen in the atmosphere, corundum forms. And it’s dangerous, whether it’s forming in a melt furnace or in a holding furnace:
- When corundum forms inside of the furnace refractory, it expands and destroys the refractory lining and eventually the joints of the furnace.
- When corundum forms outside of the furnace refractory, it decreases the capacity of the furnace and can lead to a complete stoppage.
Many of you already know that daily hot cleaning is important and do an excellent job maintaining your furnaces.
But not everyone in the aluminum or die casting industry does this well; many of the repairs could have been prevented.
Hot cleaning should be performed every day to remove any oxidation that might be adhering to the wall. For heavy production, it may require cleaning up to three times per shift (depending on the charge frequency).
The Hidden Cost of Corundum Growth
Everyone is familiar with the costs associated with downtime. Daily hot cleaning can go a long way to preventing equipment failure, preventing the stop-gap repairs.
There are additional ways the corundum reduces your bottom line that may not be as visible:
Fast corundum growth causes reduced capacity.
Corundum buildup causes heating elements to work harder and longer, shortening their lifespan and increasing heating costs.
When the heating elements work harder, you end up overheating the hood and shortening its lifespan.
Corundum buildup can compromise your refractory.
Corundum buildup contaminates the metal. This increases rejection rate and reduces sales.
The inside of the Aluminium melting or holding furnace shown in the titular image has been subject to severe corundum growth in the wetted bath area and moderate corundum growth in the surrounding upper walls and burner quarls. According to site visits and reports, the corundum attack began in the burner quarl materials, and spread to the surrounding areas. The titular image also shows occasional corundum growth between the liquid metal line and the furnace roof, which is more than likely caused by liquid metal splashes from charging.
External corundum growth
Corundum growth found on the inside surface of aluminium reverberatory furnace refractory linings, sometimes referred to as “external corundum growth”, can occur due to a combination of solid refractory, liquid aluminium and atmospheric oxygen at the liquid metal line, also known as the “bellyband area”. Due to the oxygen in the furnace atmosphere, the corundum formation is more severe with penetration extending above the liquid metal line. Molten aluminium penetrates the surface of the refractory lining, reacting with the available oxygen to form corundum mushrooms. If not removed regularly by way of cleaning the furnace, the subsequent build-up of surface corundum leads to a reduction in furnace capacity, which can subsequently lead to other significant problems .
Internal corundum growth
The formation of corundum inside the refractory lining is usually caused due to molten aluminium penetration, which reacts with gaseous oxygen also present within the refractory pores . Another possible cause of “internal corundum growth” is when the molten aluminium reacts with the oxides contained within the refractory material, such as silica and iron oxide.
Potential causes of internal corundum growth can be attributed to the extensive thermal gradient at the liquid metal line. It is possible that the refractory hot face temperature below the metal line may be as low as 720C, whilst the hot face temperature above metal may be in the region of 1100C.
Effect of corundum growth
Upon formation of corundum within reverberatory furnace refractory linings, the energy efficiency of the furnace can be reduced due to a reduction in thickness of the refractory lining, which in turn can lead to hot spots on the outer casing.
Internal corundum growth at the bellyband area can lead to significant degradation of the refractory materials which in turn can lead to hotspots on the outer casing, with a loss in performance and efficiency.
Corundum growth in reverberatory furnaces requires regular intense cleaning of the refractory surface, using mechanical tools and cleaning fluxes (1). If left without regular cleaning, as is often the case, the affected refractory materials must be removed and a complete furnace reline completed.
Hot face refractory material containing less silica normally exhibit a superior resistance to aluminium penetration and therefore corundum growth (1).
The inclusion of a non-wetting additive into the refractory mix protects the wetted portion of the refractory from molten aluminium penetration. However, if the wetted portion of the refractory lining is subjected to roof setpoint temperatures (1000C-1100C) then the non-wetting additive, such as barium oxide, is degraded/eliminated leading to molten aluminium penetration.
The use of phosphate bonded refractory materials should lead to a reduction of corundum growth due to greater flexibility of the refractory grain structure when compared to conventional course refractory aggregates .
Numerous professional bodies have undertaken laboratory testing of refractory materials under simulated real-world conditions to determine when corundum forms. Their findings can be summarised as follows:
- Aluminium alloys containing approximately 5% magnesium are more prone to corundum attack, especially if a small quantity of cyrolite is left over in the melt from the electrolysis process .
- During testing it was proved that external corundum growth requires the presence of oxygen to occur. Two methods were used to prove this: firstly the hot face refractory material was coated in a 10mm layer of 50% magnesium chloride / 50% potassium chlorideprotective mixture, preventing the gaseous atmosphere penetrating into the refractory; secondly the same or similar results were achieved under a nitrogen atmosphere.
Additionally, the results obtained of a 60% alumina non-wetting castable under the aforementioned conditions proved the build-up of corundum due to the following mechanism:
- Corundum forms at the surface of the molten metal where oxygen is available from the gaseous atmosphere;
- As the corundum grows, it may come into contact with the hot face refractory material above the liquid metal line. As the structure of the newly formed corundum promotes a wicking action, liquid metal is brought into contact with the refractory material above the liquid metal line, where the temperature is significantly higher than below;
- The high temperature combined with the wicking action the newly formed corundum offers leads to penetration of the refractory materials by liquid aluminium alloys, which in turn lead to internal corundum growth.
Following testing, some professionals have determined that the increase in microsilica, which uses a significantly smaller grain size to conventional refractories, leads to a decrease in corrosion resistance, and is more likely to be penetrated by liquid aluminium .
The lifecycle and successful operation of a melting furnace is dependent on the proper selection of refractory materials to suit the process.
Unfortunately, the process design and charging procedure of the melting furnace means the wetted portion of the refractory lining, though only 600mm deep, will be subject to extreme temperatures throughout its lifecycle, which may lead to the degradation and elimination of any non-wetting additives in the hearth refractories.
In summary, the corundum growth that occurred in the melting furnace can be attributed to a combination of the following:
- Degradation/elimination of non-wetting additive in "below metal" refractory materials due to being exposed to elevated temperatures;
- Inclusion of magnesium in the alloy;
- Refractory material selection: high silica content of the "below metal" refractory materials; "above metal" refractory materials subjected to severe temperature conditions; high silica content of the above metal refractory materials where occasional metal splashes may occur.
Refractory lining of an aluminum melting and holding furnace are engineered with a primary design objective to keep the furnace condition stable throughout the life of the furnace. In consideration of energy saving, ideally all the heat added to the furnace should be used to heat the load or stock but in practice, a lot of heat is lost in several ways. Energy input to metal output depends on factors apart from energy losses through the furnace wall i.e. through flue gas, moisture in fuel, hydrogen in fuel, opening of furnace door. These heat loses can be differentiate as, wall losses at steady state operating condition and heat storage loss during transient condition. Practically apart from flue gas loss, majority of the heat loss took place during the transient condition. Heat loss through refractory wall during steady state condition depends on thermal conductivity, resistance against thermo chemical attack from aggressive liquid aluminium and its alloy and resistance against mechanical wear. High porosity, low thermal conductive materials due to lower strength and lower resistance towards chemical attack reduce life of the furnace and in contrast high density refractory materials needs multilayer backup to save potential energy loss through the refractory wall.