Aluminum smelting dross: problems and solutions

  • Dross is a surface layer on a metal melt, which has a high oxide content.
  • Slag is formed on the surface of melts as a result of interaction with air gases, primarily, with oxygen.
  • Slag is a physical mixture of oxides and other solid compounds, as well as molten metal, which is inside them.
  • Dross, which forms on the surface of the aluminum melt, is an inevitable by-product.
  • Slag is "foam on molten metal" [1].

What is aluminum slag

Aluminum slag is a source of problems in all industries, who are engaged in melting aluminum and aluminum alloys. In solid form, the slag is all the more undesirable., as it is a valuable by-product, which greatly depends on the profitability of production operations.

This is because, that the slag, in addition to oxides, often contains a large amount of usable metallic aluminum, which is captured by these oxides (figure 1). If proper measures are not taken to prevent excessive slag formation, as well as maximum extraction of metal aluminum from it, then this can lead to significant losses of potentially usable metal.


Figure 1 - Kind of aluminum slag with high aluminum content
under an electron microscope [2]

Influence of melting technology on slag formation

The overwhelming proportion of aluminum is melted in gas reflecting furnaces (Figure 2). These furnaces are characterized by the following technological factors, which have a great influence on the technological losses of metal in the furnace.

Figure 2 - Cross section of a reflective melting furnace:
1) transfer of heat to the surface of the melt by radiation;
2) heat transfer deep into the melt by convection [3]

Gas / air ratio on gas burners

The first factor is the gas / air ratio on gas burners. Excessive air supply leads to an increased rate of slag formation, if a fresh aluminum surface appears as a result of any movement of the melt surface. The situation is getting worse, if the burners "hit" directly on the surface of the melt or on the aluminum charge.

Holding temperature of the melt

Second, and also an important factor, is the holding temperature of the melt after melting. Temperature rise only by 25-55 ºС can have a very large impact (figure 3).

Figure 3 Logarithmic increase in the rate of aluminum oxidation
with increasing holding temperature (in percent) [1]

Need to keep in mind, that thermocouples in a bath of molten aluminum actually measure the temperature in the depth of the bath. It is impossible to measure the temperature directly on the uppermost surface, which can be significantly hotter, than the temperature in the depth of the melt. Consequently, heat, shown in Figure 1, can be achieved at the surface of the melt. This will give a sharp increase in aluminum oxidation., what, as a result, will lead to an increase in the thickness of the slag layer.

Slag layer thickness

Excessive thickness of the slag layer above the melt can have a thermal insulating effect and cause increased burner operation., to maintain the desired melt temperature. In this case, the temperature in the slag layer rises and, respectively, the rate of metal oxidation increases, which enters the slag from the surface of the melt. As a result, there is a logarithmic increase in the loss of metal during melting.. The effect of increasing the slag thickness on the melting rate is shown in the figure. 4, which is confirmed by most aluminum smelters.

  • For this reason, in smelting furnaces, the thickness of the slag on the surface of the melt is usually not allowed to exceed 40 mm.

Figure 4 - Influence of the thickness of the slag layer on the reduction of the melting rate of the reflective smelting furnace (in percent) [1]

Usually the temperature of the slag layer is about 50-85 ºС above, than the melt temperature. Therefore, the high temperature of the melt causes the risk of ignition of the slag and spontaneous combustion of metallic aluminum.. This thermite reaction can have serious consequences.. If we allow the development of this reaction, the temperature in the oven may exceed 1650 oC, which can lead to melting of the refractory lining and complete failure of the furnace.

Loading lightweight scrap

Loading of lightweight scrap directly into the melt of reverberatory furnaces of the bath type can be a source of an additional increased increase in melting losses of metal.. It happens, if slag, which formed on the surface of the melt, then exposed directly to the flame of the burners. This slag is a "liquid" mixture of oxides and metal entrained in the slag.. It can contain up to 80 or even 90 % good metal. If this slag is not quickly removed from the surface or treated with a flux to separate the metal from oxides and contaminants, then this metal, will quickly oxidize and this will lead to additional loss of usable metal.

melt Stirring

To reduce the formation of slag on the surface of the melt, it is important not only to carefully control the operations of loading the charge and control the temperature., but also creating conditions for better heat transfer to the melt. With a decrease in the surface temperature of the melt, the temperature difference between the heat source and the melt increases, which provides a higher rate of heat transfer and, Consequently, faster melting. This is facilitated by: 1) the correct gas / air ratio on the burners and 2) stirring the melt. This makes it possible for the heat from the radiant energy of the vault and walls to more efficiently get to the surface of the melt and spread into its depth..

Slag removal

Dry hearth furnaces are free of some of these problems., since surface oxides from the charge remain on the inclined hearth, and the molten aluminum flows into the furnace. If wet or oiled aluminum scrap is loaded onto an inclined bed, then it doesn't create those problems, which arise when it is directly loaded into the melt. However, oxides build up quickly on the inclined hearth, which can impede the flow of molten aluminum. Therefore, these oxide accumulations need to be cleaned off regularly..

Slag removal should be done if necessary., usually once per shift, if so-called "wet slag" is not formed [contains 60-80 % aluminum].

Recommended practice for the removal of "wet" slag from the melt [1]:

  • at least once every four hours
  • when the slag reaches its thickness 40 mm (cm. Figure 2).

Furnace fluxing of aluminum slag

Slag treatment with flux

When slag forms on the surface of the melt, then it must be removed, to:

  • ensure normal melting rates
  • avoid further metal loss.

methods, which are used in industry, usually a combination of slag treatment in a furnace or a charging shaft, as well as regular cleaning of the walls and bottom of the ovens.

For this, several types of solid fluxes (powder or granular) are usually used., which are mixtures of chemical compounds, having different functions. These mixtures may include reactive (reactive) components, as well as fillers, which provide a convenient volume or cost reduction.

Slagging fluxes

It is known, that a large percentage of usable metal may be lost, when the "wet" slag is discharged from the furnace [containing metal 60-80 %]. Many industries are trying to reduce the metal content in such slag by treating the slag in a furnace using exothermic fluxes.. These fluxes contain oxidants and fluorides, to raise the temperature and help separate the oxides from the metal. This exothermic reaction consumes aluminum to generate heat and can cause up to 20 % metal, which is contained in the slag.

Special slag fluxes are designed for, to help separate metal aluminum, which is trapped in the slag. This slag consists of droplets of metallic aluminum., which is located inside the shell made of aluminum oxide (figure 5).

Figure 5 – Schematic view of a slag shell
with aluminum trapped inside it [1]

The composition and structure of the slag depends on the type of aluminum charge and the type of melting furnace.. The fluxes used are usually reactive, which provides good contact with the slag to separate it from the aluminum. Flux can also contain exothermic components, so that the generated heat accelerates the separation process. The use of thermal and mechanical stirring results in an increase in local surface temperature and fluidity., which helps to soften the oxide shell, release of aluminum metal droplets, their coalescence and return to the melt.

Typical Aluminum Slag Fluxing Procedure

Application methods for both types of flux can vary greatly from production to production., but they all require tight mixing of the flux and slag, to achieve good separation of aluminum from oxides [1].

  • The initial stage is the uniform distribution of the flux over the slag surface.. Flux consumption can be different, but some flux manufacturers recommend starting around 1 kg per square meter of melt area.
  • Then the flux and slag are mixed with a scraper, to start the process of separation of oxides and aluminum.
  • After that, the furnace window is closed and the burners are turned on for a short time at normal power.. Raising the temperature promotes the reaction of the flux with the slag and accelerates the separation of aluminum and oxide.
  • The oven window is opened slightly and the operator mixes gently, rakes and breaks the slag layer with a scraper, what to even more mix flux with slag.
  • This mixture is then removed from the oven so quickly, how is it possible, to reduce the heat loss of the furnace, which can be quite significant in cleaning and fluxing operations.
  • You need to try, so that the aluminum freed from the slag flows as much as possible from the slag-flux mixture into the melt.

A typical such operation is shown in the figure. 6.

Figure 6 - The kiln operator cleans the side walls of the kiln and
kneads flux into slag on the surface of the melt [1]

Not chemistry, but physics

Gotta understand, that during the fluxing of the slag, there is no reduction of aluminum from oxide. This can only be done electrolytically.. As a result of the treatment of the slag with a flux, a purely physical separation or fracture of the fragile oxide layer occurs and the release of aluminum, which was contained inside it (figure 7).


Figure 7 - Scheme of the release of aluminum from the slag shell [1]

The purpose of fluxing slag is to turn white, lump slag with a high aluminum content, into a dark powdery slag with a low aluminum content. In figures 8 and 9 shows the appearance of these two types of slag - untreated and treated with flux, respectively. Slag with a high aluminum content looks light and shiny, that is, obviously saturated with aluminum, and the treated slag looks dull and powdery.


Figure 8 - Untreated white slag
with high aluminum content [1]


Figure 9 - Flux treated dark powdery slag
with high aluminum content [1]

Slag economics

Typically, the aluminum content in the untreated slag ranges from 85 to 90 %. Furnace fluxing can return about half of this amount., if you act actively and energetically.

The flux treated and skimmed slag is still suitable for additional processing on auxiliary equipment, for example, on a special crushing press. After this treatment, the amount of remaining aluminum in the slag can be about 25 % (drawing 10).

It should be noted, that all these numerical indicators can only be achieved by a very laborious, careful and accurate work. The share of reduced aluminum may decrease by 50 % and more, if the operation of processing and removal of slag is carried out unprofessionally.


Figure 10 with high aluminum content [1] 1with high aluminum content [1],
2with high aluminum content [1]
3) slag after additional mechanical treatment [1]

The practice of handling aluminum slag varies greatly from production to production.. Some industries perform fluxing and refining operations very carefully.. Others simply skim the slag into containers and send it to local slag recyclers.. These slags account for a large proportion of metal losses, that occur in the metallurgical smelting industry. These losses can amount to 1-2 % from the weight of a clean and dry aluminum mixture, but can go up to 6-10 % and even higher for contaminated aluminum scrap.

Most efficient aluminum smelters ship slag for recycling containing aluminum 40-50 %, whereas those enterprises, which do not produce slag fluxing in the furnace or produce it insufficiently, can ship slag containing aluminum 75 and even 85 %.

How to reduce the amount of slag

To reduce the amount of slag, which is formed in smelting production operations, it is recommended to adhere to, as far as the type and design of the furnace allow, following rules [1].

  • Apply clean, dry aluminum charge.
  • Use charge materials with a high mass-to-surface ratio, to minimize the amount of oxides, which are introduced into the furnace.
  • Apply coating fluxes to protect the melt from oxidation.
  • Minimize the melting cycle by stirring the melt.
  • Hold, as much as possible, closed ovens to reduce oxidation, especially when holding the melt.
  • Avoid direct contact of the burner flame on molten metal.
  • Ensure the correct fuel / air ratio on the burners, to minimize the conditions for oxidation.
  • Withstand molten metal at the lowest possible temperature.
  • Minimize the transfer of molten metal through pipelines and ladles. Minimize turbulence and cascading when handling liquid metal.
  • Apply a system for the recovery and extraction of aluminum from slag.
  • Cover oven tools with an appropriate protective coating, which is not wetted in liquid aluminum.
  • Follow, so that the loading window of the furnace is hermetically closed.
  • Provide overpressure in the reverberatory smelting furnace.
  • Follow, that the thermocouples are operational and correctly calibrated, to reliably control the temperature of the aluminum melt.

Sources:

1. Dross, Melt Loss, and Fluxing of Light Alloy Metals // ASM Handbook, Volume 15: Casting – 2008
2. Reduction of Oxidative Melt Loss of Aluminum and Its Alloys – Final Report DE-FC36-00ID13898 – DasSecat, Inc, 2006
3. Aluminum Recycling – Second Edition / Mark E. Schlesinger – CRC Press, Francis Group, 2017