MARKETING MATTERS CAST TIPCAST TIP
MARKETING MATTERS CAST TIP
problem is an outside crucible wall that is distorted and glaze that has an almost foam-like appearance, some- times greenish in color (Fig. 1). Most metal cleaning fluxes used in molten aluminum
production contain alkaline salts, which can be aggressive and under certain conditions can attack crucible walls. At operating temperatures, crucibles are porous, and the flux can migrate through the crucible wall toward the hottest point, the furnace combustion chamber. Tis attack can severely degrade the walls of the crucible and cause it to fail.
Eliminating Flux Attack Te key to eliminating flux attack is preventing the con-
ditions that lead to it and discouraging the migration of the flux through the crucible wall. Take the following steps to
stop flux attack before it starts: • Use the minimum amount of flux needed to clean the melt. Over-fluxing is probably the major cause of flux attack on crucibles.
• Never put flux in the bottom of an empty crucible or onto solid (un-melted) scrap in the crucible. Flux melts at a lower temperature than the aluminum and will react di- rectly with the crucible wall. Fluxing should be performed on molten metal only, preferably in a full furnace.
• Control melt and furnace temperature. Higher furnace temperatures accelerate the migration of flux through the crucible wall.
• Keep the sidewalls of the crucible free of slag build-up. This slag contains flux material that can be released when heated and quickly penetrate the crucible wall.
• Keep the furnace chamber sealed. If there are openings between the crucible and the furnace cover, during the fluxing process, molten aluminum may over flow and run down the outside walls of the crucible (inside the furnace chamber) and encourage flux attack.
Te key to eliminating flux attack is
preventing the migration of the flux through the crucible wall.
March 2011 MODERN CASTING | 63
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