Aluminium remelting physics
Learn about the physics of aluminium remelting and the principles behind working with an aluminium remelting furnace.
Aluminium remelting furnaces
Each aluminium extrusion plant usually has an aluminium remelting shop to remelt its own processing scrap, as well as purchased aluminium scrap. The resulting aluminium melt is then poured into extrusion ingots. Typical melting furnaces for this production are gas reverberatory furnaces with direct charge loading, stationary or tiltable (Figure 1).
Figure 1 – A Typical Reverberatory Aluminium Remelting Furnace [1]
1 – charging and service door, 2 – preheating ramp, 3 – burners, 4 – steel structure,
5 – pressure control damper, 7 -metal bath, 8 – refractory lining
The following are aluminium remelting physics, the basic physical principles, regularities and phenomena, that must be considered when working with an aluminium remelting furnace.
Four heat transfer mechanisms
Aluminium remelting physics says that in a reverberatory melting furnace, ther are four main mechanisms for transferring heat to an aluminium charge [2]:
- Heat radiation from the lining (vault and walls)
- Thermal radiation from the volume of combustion gases over the metal
- A direct thermal radiation from the flame to the metal burner
- Convective heat transfer from the hot gases, which extend along the metal surface.
For reverberatory furnaces, heating by radiation is generally considered the main source of heat for melting aluminium charge. However, at some stages of the melting cycle, this mechanism of heat transfer to the charge can be very insignificant [1, 2]. Efficient operation of any melting furnace requires maximum use of all the heat transfer mechanisms due to their optimization on various stages of the melting cycle.
Thermal conductivity of aluminum: solid and liquid
The solid aluminium is a very good heat conductor. For this reason a furnace with direct loading at the beginning of the melting cycle has very high speed of heating.
In the liquid state, the thermal conductivity of aluminium falls by about half of its value in the solid state (Figure 2). This property of the liquid aluminium can significantly reduce the efficiency of the melting furnace charge loaded directly into the melt. To avoid this, typical reverberatory furnaces have an inclined entrance (see Figure 1). In this oblique inlet preliminary drying occurs batch, but it may also be heated up to the melting temperature.
Figure 2 – Coefficient of thermal conductivity of
unalloyed aluminium and aluminium alloys
depending on temperature [2]
Aluminium heating physics
The heat energy required for remelting and casting is the sum of:
- the heat to raise the temperature to unalloyed aluminium melting point or over a temperature range between a solidus and liquidus of aluminium alloys,
- the heat of fusion to convert it from solid to liquid, and
- the heat to raise the molten aluminium to the desired temperature for casting.
The figure 3 shows the amount of heat, which is required for melting, and casting to adjust the temperature of one kilogram of aluminium. Ninety-three percent of this heat is absorbed by the aluminium, while it is in a solid state. Therefore, the efficiency of a direct loading melting furnace depends on, how much heat the solid charge has time to absorb before its still unmelted part is immersed below the surface of the melt.
Figure 3 – Specific heat for aluminium melting and casting temperatures [3]
Melting cycle of a aluminium remelting furnace
The changes in temperature and power parameters of the burners in a reverberatory aluminium melter are shown in Figure 4.
Figure 4 – Temperature and energy input during a aluminium remelting furnace cycle [1]:
1 – metal temperature, 2 – furnace temperature, 3 – flue gas temperature, 4 – energy input
At the beginning, the cold metal is loaded into a hot furnace. As a result, the temperature of the lining is significantly reduced. Cold aluminium scrap is loaded into the furnace and, very quickly absorbs heat from the combustion products gas stream. The stream of hot gases in many cases strikes directly into the aluminium charge (Figure 5). At this stage, the total surface area of the charge is very large and so there is an effective transfer of heat from hot combustion gases to the batch. For this reason, the furnace off-gases are at a relatively low temperature (see Figure 4).
Figure 5 – Heat input of the hot stream of combustion products of the burner
into aluminium charge: left – full; right – partial [3]
As heating solid shields intensity of its heat exchange with the hot gases of combustion products is reduced. Power consumption is also reduced burners. The charge begins to melt and take a flat shape (figure 6). At this stage, the melting temperature of the cycle gas leaving the furnace rises sharply due to reduced temperature difference between the hot gases and metal, and to reduce the contact area of their interaction.
Figure 6 – Immersed aluminium solids are hard to heat [2].
Temperature profile (right ) shows ΔT s through:
(1) furnace gas, (2) boundary resistance, (3) dross,
(4) liquid, (5) sediment, and (6)
Solid charge inside the melt
Aluminum in the solid state has more higher density, than in liquid state (Figure 7). Therefore, usually a solid blend easily sinks to the bottom of the bath of molten aluminum. If the surface charge, for example, aluminum chips, is too large compared to its mass, it can float on the melt surface due to surface tension.
Figure 7 – Dependence of density of pure aluminium on temperature [4]:
a – solid aluminium, b – liquid aluminium
As soon as the solid charge submerged into the molten aluminium, its heat exchange with the furnace limited thermal conductivity of the metal, in which it is. The main mechanism of heat transfer to the flat surface of the melt is by radiation heat transfer from the lining, flame and combustion products. It is therefore important, that at this stage of furnace operation, it had the highest possible operating temperature.
Oxidation of liquid aluminium
It could seem, that at this stage the most efficient way to complete the cycle is to increase the melting temperature of the melt. But, Unfortunately, aluminum in liquid state exhibits very high chemical activity.
Figure 8 shows the effect of an increase in the temperature of an aluminum melt on the formation of dross. When the temperature exceeds aluminum 760 oC, the rate of formation of dross increases dramatically. The more dross is formed, the more the metal is lost.
Figure 8 – Increase of oxidation with temperature [1]
Effect dross thickness
A thin layer of slag is even useful, as it reduces the reflection properties of the aluminum melt. This contributes to a better absorption of heat radiation from the lining, gas flame and combustion products. If the slag layer becomes too thick, it acts as a heat insulator. In this case, to transfer heat into the melt must further increase the temperature on its surface.
The depth of the melt in the furnace
The density of the liquid aluminum with substantially no increase in temperature, but decreases (see Figure 7). It means, that for heating the melt from above, its lower layers will always be “heavier” the top. The melt will be in a state of hydrostatic equilibrium and without external influence any internal motion it will not happen. Heat for heating the lower layers of the melt can be transferred only from the upper hot layer due to the heat conduction mechanism [1]. therefore, the deeper the bath with liquid aluminum, which is immersed solid charge, the harder it is to bring it to the required melting heat.
In general, the deeper the furnace require more energy for their work and have a higher waste. It is believed, that for the reflective aluminum melting furnaces optimum depth of the melt is 500-600 mm. But in this case the temperature difference between the top and bottom of the melt 23-25 ºS [1].
Melt stirring
To increase the heating rate of the melt using various methods of mixing. Most often this is done with the help of mechanical tools, such as hand-held scrapers or large scrapers, mounted on the forklift. However, within a few minutes after this operation, the molten bath returns to its previous steady state [1]. Moreover, for such mixing is necessary to open the loading box furnace, resulting in further formation of slag. Therefore, large ovens and large industries employ sophisticated mixing systems using different melt pumps – centrifugal, electromagnetic and other, which can stir the melt in the continuous mode.
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