Molten Light Metal Processing: Part One

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.

Fluxing

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 melt.

Some materials that are being used as fluxes for aluminum are listed below:

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 melt.

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.