What is fluxing
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.
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.
Aluminum melt fluxing
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.
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.
There are different types of fluxes - depending on the function, which they perform: for the formation of an optimal slag – integumentary, for cleaning the oven, for degassing, for alloy modification and so on.
Slag on molten aluminum
When aluminum melts, aluminum oxide Al appears and floats on its surface2O3, which forms the secondary phase, known as slag. Under the influence of surface tension, liquid aluminum also enters the slag. The aluminum content in fresh slag can reach from 15 to 80 % depending on melting conditions. Therefore, the task of reducing the loss of aluminum with slag is very urgent., especially with remelting aluminum scrap.
Slagging flux for aluminum
To reduce aluminum losses, which leaves with slag, use one of the most important types of fluxes - fluxes for slag processing – so-called drossing fluxes. These fluxes destroy the oxide film on the melt and release the trapped aluminum.
Slag flux requirements
A good slag flux has the following properties:
- covers molten aluminum and limits further oxidation;
- dissolves or absorbs dirt, oxides and other non-metallic substances;
- provides coalescence of aluminum droplets in the slag;
- has a melting point below, than aluminum (660 ºС);
- has a density less, than liquid aluminum (2.3 g / cm3);
- does not react or contaminate molten aluminum;
- does not damage the furnace lining;
- is not toxic;
- has a low vapor pressure;
- does not absorb water;
- is not expensive and easy to dispose of;
- has a low viscosity;
- easily separates from aluminum.
The chemical composition of slag fluxes
This rather long list of requirements reduces the list of acceptable materials for slag fluxes to a short list of chloride and fluoride salts..
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 % KClи 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.
Источник: Aluminum and Aluminium Alloys