Aluminum fluxing

What is fluxing

Term

The term "fluxing" is used to describe all chemical compounds or mixtures thereof, which are used to process molten aluminum and not only aluminum. These compounds are usually inorganic. In most cases, metal salts are used in powder form, granules or tablets. When applied, they usually melt and work in a liquid state..

Fluxes are injected manually or using special equipment. They can perform one or more functions., including degassing, cleaning, alloying, oxidation, deoxidation or refining. The term "fluxing" also includes the treatment of an aluminum melt with inert or reactive gases to remove solid or gaseous impurities.

The term "fluxing" broadly refers to a method of processing a metal melt, which contains such impurities and inclusions. Fluxing of the melt ensures collection and removal of such unwanted components from it.

Oxides and non-metallic inclusions in aluminum

When melting aluminum, especially when melting foundry waste and other aluminum scrap, usually in one or another amount - small or large - oxides of aluminum and other metals are formed, and non-metallic pollution. These contaminants are in the form of liquid or solid inclusions., which can get into aluminum products. These inclusions arise in the aluminum melt from contaminated tools, mold materials, slag, furnace linings, aluminum oxides and alloying elements of alloys.

The influence of temperature on the fluxing process

The fluxing process is temperature dependent. The processing temperature of the melt must be high enough to achieve good physical separation of contaminants or impurities from the melt or to efficiently undergo the necessary chemical reactions. At a high enough temperature, the fluidity increases like aluminum, and flux, which ensures good contact between them and better chemical interaction.

Chemical composition of fluxes

Special compounds or chemicals, which are used in fluxes, depend on the assigned flux. Most fluxing compounds consist of mixtures of inorganic salts. The various constituents of these salts or other materials in the flux serve to, to [1]:

• Form low-melting compounds with high fluidity at a given operating temperature. This is exactly what happens with the NaCl-KCl mixture..
• Decompose at operating temperature to form anions, such as nitrates, sarbonate and sulfate, capable of reacting with contaminants in the melt. As a result, various oxides or other compounds are formed with a density from the density of liquid aluminum, which makes it possible to physically remove harmful impurities.
• Act like fillers, to reduce the cost per kilogram, to also be a carrier for the active ingredients and, Moreover, provide complete coverage of the melt.
• Absorb or agglomerate the reaction products of the fluxing process.

Not all such components are required or required to be contained in every flux..

• Table 1 lists the properties of various materials, which are commonly used in industrial fluxes for aluminum [1].
• Table 2 functional properties of various components of fluxes are presented [2].

The choice of flux components depends on:

• operating temperature,
• this flux is used for melt or solid charge,
• the required chemical activity of the flux,
• chemical composition of aluminum melt.

for instance, sodium-containing fluxes are not used with aluminum-magnesium alloys, to avoid contamination of the melt with sodium.

Table 1 – Characteristics of some materials used as fluxes for aluminium alloys [1]

Table 2 – Typical fluxing compounds employed [2]

Flux types for aluminum

For aluminum, the following main types of fluxes are used:

• cover fluxes
• cleaning fluxes
• drossing fluxes
• refining fluxes
• wall-cleaning fluxes.

Cover fluxes

These fluxes are mainly used:

• as a physical barrier to melt oxidation in small furnaces,
• as a cleaner for loaded aluminum charge.

Cleaning fluxes

These fluxes usually have a high content of chloride salts., and also contain fluorides, to promote wetting of oxide inclusions for easier removal of them from the melt.

Drossing fluxes

These fluxes help separate the aluminum oxide (Al2O3) slag layer., which forms on the surface of the melt, from liquid metal. This layer of slag contains a large amount of liquid and solid aluminum..

Not so long ago, the standard composition of such a flux was a mixture in equal proportions by weight of NaCl and KCl salts. The equimolar composition of the flux was also often used - 56 % KCl and 44 % NaCl. However, although this flux is an effective cover flux, and also well dissolves aluminum oxide, it is not very effective for aluminum coalescence. To do this, small amounts of fluoride salts are added to the flux.. The most popular such salt is cryolite (Na₃AlF₆), as well as sodium and potassium fluorides.

Slag fluxes are widely used in the aluminum industry to reduce the metal content in the slag.. Unfluxed slag may contain from 60 to 85 % free metal [1, 2]. If such slag is allowed to burn, then this metal will turn into irreversible Al2O3.

When the slag is still on the surface of the melt, it can react with the exothermic slagging flux., as a result of which a significant part of the metal can be separated from the slag, while the metal content in the slag will decrease to 30 %. Thus approximately 50 % metal is returned directly to the melt.

The figure 1 two types of slag are compared: non-fluxed slag with high metal content (a) and fluxed slag with low metal content (b).

Fig. 1 – Compraison of metal-rich (a) and powdery, low-metallic dross obtained by proper fluxing (b) [1]

Refining fluxes

These fluxes contain compounds, which decompose at operating temperature and react with certain metallic elements in the aluminum melt. for instance, some chlorine-containing compounds can react in molten aluminum, containing magnesium, calcium, lithium, sodium and potassium, with the formation of insoluble chlorides of these metals. these chlorides can then be removed from the melt.

Wall-cleaning fluxes

These fluxes contain compounds, that soften oxide buildup, that form on the walls of the furnace. These fluxes are often applied to the walls of the furnace using special spraying devices (“guns”).

Proper use of fluxes

When applying fluxes, it is important to follow the following recommendations [1]:

• Each flux must be applied at a temperature, which is specified by its manufacturer, to ensure its maximum ability to chemical reactions.
• Flux components are usually somewhat hygroscopic and should therefore be stored in a dry place.. They cannot be applied, if they are wet, because it can lead to an explosive chemical reaction. Moreover, wet flux will introduce hydrogen into the aluminum melt, which has a high solubility in liquid aluminum.
• Tools (shovels, scrapers, picm, perforated scoops), which are used in fluxing, should always be clean and dry. They should always be warmed up before use. 95 0From, to avoid the formation of bubbles when they come into contact with the melt.
• The flux must be applied in full accordance with the manufacturer's recommendations..
• Coating fluxes can be dispersed over the melt by hand or with a shovel.
• Slag fluxes usually need to be mixed into the slag layer.. The flux must be thoroughly mixed with this layer of slag using a scraper.. It needs to be done like this, not to introduce too much new liquid metal into the slag.
• After applying the flux, it must be carefully separated from the melt before, how to start pouring or draining metal. It is necessary, to prevent flux inclusions from entering the cast metal.
• After metal processing with fluxes, it is always recommended to apply metal settling. It is necessary, to allow heavy inclusions to sink to the bottom or lighter flux salts or oxide inclusions to float, flux trapped. The optimal settling time may vary from 5-10 minutes for small chill melt up to 1-2 hours for large 50-ton furnaces.

Sources:

1. Molten Aluminum Processing and Casting // Aluminum and Aluminium Alloys (ASM Speciality Handbook) – 1996
2. Aluminum Fluxes and Fluxing Practice // ASM Handbook Volume 15: Casting – 2008