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 newer but now commonly practiced technologies in nonferrous molten metal processing. We 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.

Fluxing

The term fluxing represents all additives to, and treatments of, molten metal in which chemical compounds or mixtures of such compounds are employed.

These compounds are usually inorganic. Sometimes, 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 automatically injected, and they can perform single or, in combination, various functions, including degassing, cleaning, alloying, oxidation, deoxidation, or refining.

It 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

Oxide formation and nonmetallic impurities are common, in aluminum melting, and especially in the remelting of foundry returns or other scrap. 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 the base metal.

In the broadest sense, the term fluxing, applies to a treatment technique to the melt containing such impurities and inclusions those mentioned. Fluxing of the melt facilitates the agglomeration and separation of such undesirable constituents from the melt.

Some materials that are being used as fluxes for aluminum:

• 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:

•   Refining fluxes
•   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. Also to great advantage in the aluminum industry to reduce the rich metallic content of the dross

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. Untreated dross may contain 60 to 85% free metal, which, if allowed to burn or termite, will convert to unrecoverable Al2O3.

Wall-cleaning fluxes are also employed, but these are usually sprayed onto furnace walls rather than added to the melt. It contains 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

Because of the extreme reactivity between magnesium and oxygen, Magnesium and its alloys are exceptionally susceptible to oxidation, melt loss, and fires. When melting this alloy family Protection is always required. It 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 also entrain liquid metal. So Fluxes are 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 wet ability and chemical reactivity with magnesium oxide; and the heavy barium chloride salt constituent, the density component to effect mixing and sledging capability for separation. Other useful cover fluxes include a simple mixture of sulfurous compounds with flu borate 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.