Melting aluminum: physical principles

 

aluminum melting furnace

Each production of aluminum extruded profiles usually has its own site or shop on their own smelting process waste, as well as suitable aluminum scrap purchased. The resulting aluminum melt is then poured into ingots-pillars for pressing (see. more here). Typical melting furnaces for this production are gas reflex ovens with direct charge loading, stationary or tiltable (figure 1).


Figure 1 - Typical Reflective Aluminum Melting Furnace [1].

The following are the basic physical principles, regularities and phenomena, that must be considered when working with such ovens.

Four heat transfer mechanism

In smelters with direct heating, reverberatory furnaces such as, the heat source is one or more gas burners. In such a melting furnace, the main mechanisms for transferring heat to an aluminum charge are [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 lining by radiation is generally considered the main source of heat for melting aluminum 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 smelting cycle.

Thermal conductivity of aluminum: solid and liquid

The solid aluminum is a very good heat conductor. For this reason a furnace with direct loading at the beginning of the melting cycle can be loaded batch heat transfer at very high speed.

In the liquid state, the thermal conductivity of aluminum falls by about half of its value in the solid state (Figure 2). This property of the liquid aluminum 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 aluminum alloy and 6061
depending on temperature [2]

Heat for melting batch

The figure 3 shows the amount of heat, which is required for melting, and casting to adjust the temperature of one kilogram of aluminum. Ninety-three percent of this heat is absorbed by the aluminum, while it is in a solid state. Therefore, the melting efficiency of the furnace with direct heating depends, how much heat the solid charge has time to absorb before its still unmelted part is immersed below the surface of the melt [1].

Figure 3 - Specific heat for aluminum melting and
heating it to the casting temperature [2]

melting cycle reverberatory furnace

Changes in temperature and power parameters of the burners in the reflectance melter direct loading shown in Figure 4.

Figure 4 - Change of temperature parameters and
power consumption of burners in the melting cycle of a reverberatory furnace [1]

At the beginning of the cold metal of the melting cycle is loaded into a hot oven. As a result, the temperature of the lining is significantly reduced. adamant, is loaded into the furnace, very quickly it absorbs heat from the combustion products gas stream. The stream of hot gases in many cases strikes directly into the aluminum 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 - Passage of the hot stream of combustion products of the burner
through an aluminum charge: a) full; b) partial [2]

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 - The effect of hot gases of the burner on the flat melt in the furnace [2]

The solid melt blend

Aluminum in the solid state has a higher density, than in liquid (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 the density of pure aluminum on temperature [3]:
and - a solid aluminum, b - the liquid aluminum

As soon as the solid charge submerged into the molten aluminum, 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 aluminum

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.

The figure 8 the effect of an increase in the temperature of an aluminum melt on the formation of slag (Al2O3). When the temperature exceeds aluminum 760 oC, the rate of formation of slag increases dramatically. The more slag is formed, the more the metal is lost.

To form other than a high temperature slag is mandatory presence of metal in contact with oxygen. The main source of oxygen in the furnace are air volume, which penetrates from the outside, and air, which has not had time to burn in the burner. Good burner must operate without supplying excess air to the furnace volume.

Figure 8 - Dependence of the rate of aluminum oxidation on temperature [2]

Effect slag 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.

Sources:

  1. Handbook of Aluminium Recycling / Сh. Schmitz, 2006
  2. Direct Charged Melters / Donald F. Whipple – Bloomengineering, 2004
  3. Handbook of Aluminium: Vol. 1 / ed. Toten @ McKenzie, 2003