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FLUXES FOR MOLTEN ALUMINUM


Fluxing is a reliable means of obtaining clean metal by preventing excessive oxide formation, removing non-metallic inclusions from the aluminum melt, and preventing and remov- ing oxide build up from furnace walls. In general, fluxes may be grouped in two classes: gaseous or solids. Gaseous fluxes may be a blend of an inert and a chemically active gas that is injected into the molten bath. Solid fluxes are blends of salts. Solid fluxes can broadly be categorized as passive or active fluxes. Passive fluxes protect the surface of the molten alumi- num from oxidation and prevent hydrogen pick up by the melt. Active fluxes strip away the aluminum oxide layer from molten metal promoting coalescence of metallic drops, and help in reducing unwanted chemical impurities.


Solid fluxes are basically blends of sodium chloride and potassium chloride salts, with or without addition of fluorides. In addition, small quantities of oxidizing compounds, such as carbonates, sulphates, and nitrates, are added to promote exothermic chemical reactions. Such reactions are important because they prompt the coalescence of the trapped liquid aluminum particles in the dross.


The mechanism of how salt fluxes work has been attributed to thermodynamic chemical reactions and surface tension effects between the aluminum oxide and the flux, the aluminum oxide and the molten metal, and the molten metal and the flux. In addition, the effect of the flux on the liquid metal will depend on the chemistry of the flux used, morphology of the flux, total amount of flux added, molten metal temperature, flux contact time, rabbling (stirring) technique, etc. Different combinations and proportions of ingredients will impart different flux properties, such as flux density, flux fluidity, flux wettability, and flux reactivity.


FLUX INGREDIENTS


CHLORIDES: Chlorides are mainly used because of their fluidizing effects and because they are used as fillers. Fluxes based only on chloride salts should not react with molten aluminum, or at least the reaction should be negligible. In addi- tion, these salts provide negligible effects on surface tension as


compared to fluorides. FLUORIDES: Fluorides salts act as surfactants affect- ing surface tension forces between flux, liquid metal, and aluminum oxides. As the flux wets the interface between the aluminum oxide particles and the liquid metal, the adhesion force between the liquid aluminum and the oxides decreases, promoting oxide separation and metal coalescence. Fluorides


attention to lower scrap grades because of the greater probability of poorly defined composition and content of


deleterious contaminants. Inclusions: Te source of inclu-


sions derives from the type of metal charge, alloying additions, melting practices, and liquid metal treatments and handling practices. Inclusions can be broadly classified as intermetallic and non-metallic. Intermetallic inclusions are primary


compounds that result because of the precipitation and growth phenomena from the liquid state.


are the most effective compounds being used in fluxes to improve


aluminum recovery from wet dross. OXIDIZING COMPOUNDS: Oxidizing compounds are used to promote exothermic chemical reactions. They react with the smallest molten aluminum particles that are present in the dross, yielding aluminum oxides as well as considerable heat. The purpose of the exothermic reaction is twofold: it allows larger pockets of entrapped aluminum to coalesce and fall back into the molten bath, and it facilitates reactions between alumi- num and fluorides. The exothermic reaction continues until all the


fine aluminum particles are burned. SOLVENTS OF ALUMINUM OXIDES: The Hall and Heroult patents both covered the electrolysis of aluminum oxide in a bath of molten halide salts. Since then, dissolution of aluminum oxides (Al2


O3 ) by cryolite has been documented. CLASSIFICATION OF FLUXES


Historically, solid fluxes have been classified in four categories depending on their use and function at the foundry operation. These categories are: cover fluxes, drossing fluxes, melt cleaning


fluxes, and furnace wall cleaner fluxes. COVER FLUXES: Cover fluxes are designed to be liquid at operating metal temperatures in such a way that the flux will form a molten barrier “blanket” on the surface of the metal to protect it


against oxidation and hydrogen gas absorption. DROSSING FLUXES: Drossing fluxes may be based on salt blends of chlorides, simple and or double fluorides, and oxidiz- ing compounds. Therefore, they are able to react exothermically, generating heat and improving flux wettability. The wetting action of the flux promotes coalescence, which brings the fine aluminum


drops tighter to form larger drops that are much easier to recover. CLEANING FLUXES: Melt cleaning fluxes are designed to remove aluminum oxides from the melt and/or reduce unwanted chemical alkali impurities from the molten bath. Melt cleaning fluxes usually are higher in chloride salt compound content. They may use similar chlorides and oxidizing compounds as the ones used in the drossing fluxes, but in different proportions. The main purpose of the cleaning flux is to facilitate wetting of the oxide


inclusions for easier separation of the melt. FURNACE WALL CLEANING FLUXES: Wall cleaning fluxes are spe- cifically designed for the softening and removal of excessive aluminum oxide build up that occurs on melting furnaces walls, especially along the melt line. This type of flux helps keep crucible and furnace walls above and below the melt line free of oxide build up.


Non-metallic inclusions can be


present in the form of films, fragments, particles, and clusters. Te inclu- sions can have different composition, texture, morphology, and appearance. Common types of non-metallic inclu- sions are: borides, carbides, nitrides, oxides, and salts. Non-metallic inclusions are


typically grouped as exogenous, or as in-situ. Inclusions that are imported to the molten metal from external sources are referred to as exogenous while inclusions that arise from either chemical reactions within the melt


or as a result of a melt treatment are considered indigenous. Sources for exogenous inclusions


include refractory particles, usually from degradation of furnace walls, transfer ladles, launders, riser tubes, filling funnels, and in some instances from pieces of the sand mold. In addi- tion, inclusions derived from charging materials are also considered exog- enous. Sources for in-situ inclusions are oxides, fluxing products, alloying elements, and intermetallic com- pounds. Figure 1 depicts the sources of non-metallic inclusions.


August 2017 MODERN CASTING | 33


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