How fluxing purifies aluminum

What are fluxes and fluxing?

Term

The term "fluxing" used to describe all the aluminum melt treatments, in which chemical compounds are used [1]. These compounds are generally inorganic and can perform several functions, such as , degassing, magnesium reduction, removal of impurities and alloying. "Fluxing" also includes the treatment of the melt with inert or reactive gases to remove inclusions or gaseous contaminants from the metal [1].

Chemical composition of fluxes

Fluxes in the solid state, as a powder, flakes or granules, usually consist of chlorides and fluorides with additional additives to impart special properties.

Most fluxes based on a mixture of KCl and NaCl salts, which form a low-temperature (665 ºС) eutectic. Another common ingredient of the flux is sodium fluoride NaF, which forms a ternary eutectic with KCl and NaCl m.p. 607 oC. Normal coating flux contains about 47,5 % NaCl, 47,5 % KCl and 5 % a fluoride salt. Low melting temperature is an important, so it increases the fluidity of flux.

The role of fluoride in fluxes

Fluoride salts of alkali metals act as surfactants, lowering the surface tension between the flux and metal, one side, and flux and oxides, with another. Chloride salts exhibit this property to a lesser extent [1].

Alkali metal fluoride salts have the ability to dissolve oxides (albeit to a very small extent), which facilitates their penetration into the oxide films in the slag and growths on the walls of the melting furnace. This leads to improved wettability, which promotes the separation of oxide inclusions from the melt and the aluminum metal from the dross.

exothermic fluxes

nitrate additives, such as KNO3, leads to the release of heat. These fluxes are exothermic. Liberated from the decomposition of the nitrate oxygen reacts with the metallic aluminum to form Al2O3 and a significant amount of heat. This locally increases turnover, facilitating separation from the metal oxides. flux, which are used for cleaning ovens, This reaction increases the penetration of flux into the furnace wall on the growths.

Fluxes for degassing

Certain compounds decompose with evolution of gases, for example, chlorine or carbon dioxide. If such fluxes enter under the melt surface, they form bubbles, which reduce the hydrogen content in the melt. The best known of these compounds is hexachloroethane (C2Cl6), which allocates chloro Cl2 and a gaseous compound of AlCl3.

As hydrogen and oxides fall in aluminum melt

The reaction of aluminum with water

Hydrogen and oxides are common impurities in aluminum melt. Their source is water from the atmosphere. The reaction between aluminum melt and water is shown in Figure 1. In this case the aluminas can be crystallized in a very hard corundum crystal structure, shown in Figure 2. This can happen, for example, on the wall of the melting furnace.


Figure 1 – The reaction of formation of oxides and hydrogen in an aluminum melt [2, 3]


Figure 2 - Crystal structure in the fund [2, 3]

How rise the hydrogen bubbles

When aluminum solidifies, the solubility of atomic hydrogen in it drops sharply (figure 3). As a result, hydrogen atoms in the molecule and are combined in the hardened metal bubbles occur. The figure 4 schematically illustrates the formation of oxides, and the occurrence of hydrogen bubbles.


Figure 3 - Solubility of hydrogen in aluminum [2, 3]


Figure 4 - The formation of oxides and atomic hydrogen in an aluminum melt. The appearance of hydrogen bubbles in solidified aluminum [3]

Atmospheric moisture reacts with molten aluminum to form an oxide film and the atomic hydrogen in between the aluminum atoms. Oxides do not remain only on the surface, some of them also fall into the melt and, when the temperature is reduced, some of these oxides having hydrogen bubbles.

In the process of casting an aluminum surface oxide film is. Part of oxide falls within the melt. These oxides may be in the form of films with a very high ratio of length and width to thickness. Some of these films may contain aluminum droplets inside [2, 3].

Isolation hydrogen blistering c, It occurs on internal defects of the melt and the crystal lattice of aluminum. In this case, oxides act as nuclei of these bubbles (Fig. 5).

Figure 5 - Formation of a hydrogen bubble around the oxide film [2, 3]

How fluorides purify aluminum melt

The oxides can be removed from molten aluminum by treating it fluxes. Fluxes based on fluorides able to bind oxides. The interfacial tension between the metal oxide and considerably higher, than the interfacial tension, which occurs between oxide and fluoride. Then, fluorides and oxides form mixed phases, because as a result of their lower energy state fluorides "stick" to oxides and cover them, and aluminum is separated from these mixed phases [2, 3].

The figure 6 schematically illustrates the process of purifying molten aluminum fluxes. Flux, containing fluorides, It is injected into the melt and mixed well in it. Flux envelops the oxides and fluorides then form a mixed oxide phase. This leads to disintegration into individual pieces oxides. These fragments oxide mixed phases have the ability to float, because their density is considerably lower, than the density of molten aluminum. This results in a slag having a low metal content. Note, that the density of pure oxides is almost equal to the density of the melt and therefore they practically do not have the ability to float [3].

Figure 6 - The principle of purification of molten aluminum oxides
the processing of flux, containing fluorides [3]

Combination treatment with the rotor flux purge

aluminum melt fluxes cleaning process can be combined with a rotary gas purging. In this processing ensures optimum degassing, removal of oxides and obtaining a slag with a low aluminum content (figure 7).

Without applying flux such rotary treatment is not as effective for the removal of oxides and, Moreover, leads to the formation of a slag with a high aluminum content (figures 8 and 9).

Figure 7 - Processing the aluminum melt flux, containing fluorides,
together with rotary blowing (degassing) [3]

Figure 8 - Aluminum melt without flux treatment and rotary degassing [3]

Figure 9 - Rotary blowing of aluminum melt without flux feed [3]

The aluminum content in slag

Slag, which is formed in the melt during processing without the use of flux, It has a high aluminum content, usually 80 to 95 %. Slag, which is obtained after melt processing fluxes, It is less dense and has an aluminum content of 15 to 35 % (drawing 11) [2, 3].

a

b
Figure 10 - Aluminum slag [2, 3]:
a - with a high aluminum content;
b - the low alumina

Shoe melter walls of oxides

The furnace without fluxes melt processing

The figure 11 It is shown as oxides can damage the walls lined melter, If the melt is not treated with fluxes. V baking coating always present microcracks. Oxides from the melt adheres to the wall of the melting furnace, and some of them penetrate the micro-cracks. In the crystallization of corundum oxides can occur depending on the time and temperature, which is capable of expanding cracks in the furnace lining (Figure 12). When it becomes too much of corundum, it also begins to get into the melt [3].


Figure 11 - The formation of oxide build-up on the wall of the melting furnace
(without crystallization of oxides into corundum) [3]


Figure 12 – Formation of oxide build-ups on the wall of the melting furnace with crystallization of oxides into corundum [3]

Oven with fluxes for melt processing

If the aluminum melt fluxes processing is applied in the oven, the situation with the oxide build-up on the stack of the furnace is completely different.

Oxides in molten flux coated, as it shown on the picture 13. Therefore there is only a small connection between them and the furnace wall lining. Moreover, It is very important, in this case there is no crystallization of corundum oxides. Fragile oxides adhering to the furnace wall is easily removed when removing slag from the melt surface.


Figure 13 - Fluxes in the aluminum melt prevent the growth of oxides on the furnace wall and their crystallization into corundum [3]

Cleaning furnace corundum build-up on its walls

To clean the walls of the smelting furnace with corundum build-ups, special fluxes are used (Figure 14).

Figure 14 - The effect of fluxes when cleaning the furnace wall from corundum [3]

Cleaning the walls of the furnace from corundum is as follows [2]. Before cleaning, the oven must be nearly empty and heated to a high temperature between 800 and 900 oC. The burners in the furnace is turned off. The flux for cleaning the furnace is sprayed on areas with corundum build-ups and around them (Figure 15). The furnace is closed on 30-40 minutes. The furnace was then opened and the purge walls conventional tool. Thus corundum must be easily separated from the walls of the furnace.


Figure 15 - The operation of applying flux to the walls of the furnace [2, 3]

Exothermic fluxes and fluxes based on fluorides

There is a special group of exothermic flux based on nitrates. Between aluminum and nitrates exothermic reaction occurs with a local increase in temperature to 2000 oC. Under the influence of this high temperature aluminum viscosity becomes very low, and therefore it can easily flow from the slag. The disadvantage here is that, that the low metal content in the slag is achieved due to the additional formation of oxides in the melt [3].

The figure 16 compares the effect on the slag exothermic fluxes and fluxes with the content of fluoride.


Figure 16 - Differences in the action of exothermic fluxes and fluxes with fluoride salts [3]

Both types of fluxes provide a low aluminum content in slag, Fluxes exothermic but not only remove oxides from the melt, but even create new and can therefore lead to the formation of corundum on the walls of the furnace.

Purification of molten aluminum from extraneous metals

Some metals are difficult to remove from the aluminum melts. This - the alkali metals and alkaline earth metals, such as lithium, sodium, magnesium,, calcium or strontium [2, 3].

metals, are not amenable to removal, They are iron, phosphorus, antimony and titanium. If the content of the alloy is too high, then the only way out is to dilute the melt with pure aluminum without these elements [2, 3].

Removal of metals may be accomplished by use of chlorine gas, which is highly toxic and is generally prohibited. An alternative to chlorine are fluxes, based or chlorides, or not ftoridax. The first group, which is based on chlorides can allocate the same chlorine and cause a strong odor. a flux, based on fluorides, do not cause strong odors [3].

Sources:

  1. The Properties and Uses of Fluxes in Molten Aluminium Processing / T.A. Utigard et al - JOM, November, 1998
  1. Mechanisms in the Cleaning of Aluminium Melts with Flux Preparations – Presentation - SCHAEFER Metallurgie GmbH
  1. Mechanisms in the Cleaning of Aluminium Melts with Flux Preparations – Industry- and foundry e.K.