In the melting of commercial non-ferrous metals and alloys (aluminum, magnesium, copper, zinc, and lead), various auxiliary molten metal processing steps are necessary other than melting and alloying.
Generally, the historic practices of fluxing, metal refining, deoxidation, degassing, and grain refining have been used, and they apply to virtually all nonferrous metal systems. In addition, molten metal pumping and filtration are two somewhat newer but now commonly practiced technologies in nonferrous molten metal processing. This article will describe the molten metal processing methodologies currently used in conventional nonferrous molten metal operations. These process methodologies pertain not only to foundry melting and casting but also to smelting, refining, and in certain cases mill product operations.
In the melting of commercial non-ferrous metals and alloys (aluminum, magnesium,
copper, zinc, and lead), various auxiliary molten metal processing steps are necessary
other than melting and alloying.
Generally, the historic practices of fluxing, metal refining, deoxidation, degassing,
and grain refining have been used, and they apply to virtually all nonferrous metal
systems. In addition, molten metal pumping and filtration are two somewhat newer but
now commonly practiced technologies in nonferrous molten metal processing. This article
will describe the molten metal processing methodologies currently used in conventional
nonferrous molten metal operations. These process methodologies pertain not only to
foundry melting and casting but also to smelting, refining, and in certain cases mill
The term fluxing is used in this article to represent all additives to, and treatments
of, molten metal in which chemical compounds or mixtures of such compounds are
employed. These compounds are usually inorganic. In some cases, metallic salts are
used in powder, granulated, or solid tablet form and may often melt to form a liquid
when used. They can be added manually or can be automatically injected, and they can
perform single or, in combination, various functions, including degassing, cleaning,
alloying, oxidation, deoxidation, or refining.
The term fluxing also includes the treatment of nonferrous melts by inert or reactive
gases to remove solid or gaseous impurities. Fluxes are commonly used to some extent
with virtually all nonferrous molten metal operations in both the foundry and in the
production of mill products.
Fluxing of Aluminum Alloys
In aluminum melting, and especially in the remelting of foundry returns or other
scrap, oxide formation and nonmetallic impurities are common. Impurities appear in the
form of liquid and solid inclusions that persist through melt solidification into the
casting. Inclusions can originate from dirty tools, sand and other molding debris,
sludge, metalworking lubricant residues, and the oxidation of alloying elements and/or
the base metal.
The term fluxing, in the broadest sense, applies to a treatment technique to the melt
containing such impurities and inclusions as those mentioned above. Fluxing of the melt
facilitates the agglomeration and separation of such undesirable constituents from the
Some materials that are being used as fluxes for aluminum are listed below:
- Aluminum chloride, AlCl3
- Aluminum fluoride, AlF3
- Borax, Na2B4O7
- Calcium chloride, CaCl2
- Calcium fluoride, CaF2
- Carnalite, MgCl2KCl
- Zinc chloride, ZnCl2
- Cryolite, 3NaFAlF3
- Lithium chloride, LiCl
- Magnesium chloride, MgCl2
- Potassium chloride, KCl
Four principal types of fluxes are used for aluminum alloys. They are: cover fluxes,
cleaning fluxes, drossing fluxes, and refining fluxes. Wall-cleaning fluxes are also
employed, but these are usually sprayed onto furnace walls rather than added to the
Cover fluxes are designed to be used primarily with smaller (pot,
crucible) furnaces to provide a physical barrier to oxidation of the melt or to serve
as a cleanser for alloys, scrap foundry returns, or fresh ingot being charged.
Cleaning fluxes are usually higher in chloride salt compound content
and usually contain fluorides to facilitate wetting of the oxide inclusions for easier
separation from the melt.
Drossing fluxes are designed to promote separation of the aluminum oxide
(Al2O3) dross layer that forms on the surface of the melt
from the molten metal. Drosses and liquid or solid metal are usually intermingled
in the dross layer. The drossing fluxes are designed to react with Al2O3 in the slag
or dross layer and to recover metal. The fluorides wet and dissolve thin oxide
films according to the general reaction.
6Na2SiF6 + 2Al2O3 → 4Na3AlF6 + 3SiO2 + 3SiF4
With sufficient mechanical agitation through rabbling with a rake, these films will
be broken long enough to release entrapped metal. Drossing fluxes are used to great
advantage in the aluminum industry to reduce the rich metallic content of the dross.
Untreated dross may contain 60 to 85% free metal, which, if allowed to burn or thermite,
will convert to unrecoverable Al2O3.
Wall-cleaning fluxes contain compounds that help soften the oxide
buildup that occurs on furnace walls. These fluxes can often be applied with a typical
refractory gunning device.
Fluxing of Magnesium Alloys
Magnesium and its alloys are exceptionally susceptible to oxidation, melt loss, and
fires because of the extreme reactivity between magnesium and oxygen. Protection is
therefore always required when melting this alloy family. Historically, this has
involved the so-called flux process, which uses salt fluxes as a cover, or more recently
the fluxless process, which uses inert gas.
Molten magnesium oxidizes readily to form a magnesium oxide (MgO) film. This film is
easily disturbed, and discontinuous MgO liquid film inclusions readily wet and coat
solid charge materials and can also entrain liquid metal. Fluxes are therefore used
to protect the melt from oxidation, to agglomerate nonmetallic inclusions originating
with the charge, and to break up and collect the oxide inclusions and skins that may
form during melting. These fluxes are usually low-melting mixtures of halide salts
capable of wetting both solid and liquid metal surfaces.
A typical flux composition includes approximately 49% MgCl2, 27% KCl, 20%
BaCl2, and 4% CaF2. The magnesium and potassium chloride salts
provide the low-melting eutectic; the fluoride, the surface wettability and chemical
reactivity with magnesium oxide; and the heavy barium chloride salt constituent, the
density component to effect mixing and sludging capability for separation. Other
useful cover fluxes include a simple mixture of sulfurous compounds with fluoborate
salts or boric acid.
All flux materials should be kept clean and dry and should be stored in their original
containers. All tools used with fluxes should be clean, dry, and preheated to drive off
any surface moisture and to minimize thermal shock when placed into the melt.