How to Refine Molten Aluminum with Solid Flux

How Should Flux Refining Be Carried Out?

Before refining, select a suitable flux according to the aluminum alloy grade. For alloys with a high magnesium content, use sodium-salt-free No. 2 flux instead of an ordinary flux.

Solid fluxes contain substantial amounts of chloride salts, which readily absorb moisture from the air. Deliquescent flux must not be used. Before refining, dry the flux thoroughly to prevent moisture from contaminating the molten aluminum. Moisture can also cause an explosion, resulting in molten metal splashes and burns. Tools used for refining must also be dried. Maintain the flux drying temperature at 250–300°C for at least four hours.

In aluminum processing plants, the typical flux dosages for different alloys in melting and holding furnaces are shown in Table 6-9.

Table 6-9. Flux Dosages

Increase the flux dosage appropriately in humid regions and during humid seasons. A higher dosage may also be needed when producing large-diameter round ingots.

In primary aluminum smelters, a flux dosage of 10–12 kg per tonne of aluminum may be used. Flux pieces measuring approximately 50–80 mm are suitable. Remove the solid refining flux from the drying furnace and place it in a dry refining bell. This bell is a cage-shaped steel container with a movable lid. Once it is filled with flux, secure the lid firmly.

When hexachloroethane (C₂Cl₆) is used as the refining agent, its dosage depends on the alloy composition. For example, magnesium-free 413Z.1 (ZL-102) aluminum–silicon alloy requires a C₂Cl₆ dosage of 0.2%–0.4%. For 356Z.1 (ZL-101) aluminum–silicon alloy containing 0.2%–0.4% magnesium, the dosage is 0.5%–0.75%. This is because C₂Cl₆ reacts with magnesium to form MgCl₂. Although MgCl₂ can contribute to degassing, its effectiveness is much lower than that of AlCl₃.

To achieve good refining results, select an appropriate refining temperature, generally 30–50°C above the alloy’s melting point. Before starting, disconnect the mixing furnace from its power supply and place a “Work in Progress” warning sign on the electrical control panel. To minimize heat loss during refining, open only the furnace door at the working position and keep all other doors closed.

During refining, slowly lower the flux-filled refining bell into the molten aluminum near the furnace bottom. Move it gently along the bottom to stir the melt evenly, allowing the flux pieces to melt gradually and disperse throughout the molten aluminum.

Do not allow the refining bell to float in the upper layer of the melt. All the flux must melt and disperse uniformly throughout the bath to avoid untreated areas. Achieving this distribution is particularly difficult in large mixing furnaces and is a limitation of refining with solid flux pieces. Operators should develop practical experience with the process to reduce operating errors.

Use smooth, controlled movements inside the furnace. Prevent molten aluminum from splashing onto the heating elements, and avoid touching them with tools carrying molten aluminum, as this could cause a short circuit when the elements are energized.

After refining, remove the refining bell, skim the floating dross from the entire melt surface, and close all furnace doors.

When removing the refining bell and dross rake, allow any molten aluminum remaining on the tools to drain off thoroughly.