Phase 3: Furnace cleaning process (removing furnace impurities, passivating furnace lining, ensuring aluminum purity

Furnace cleaning relies on the flushing and soaking effect of molten aluminum to remove all refractory dust, broken brick slag, magnesium sand particles, and carbon slag attached to the inner wall and bottom gaps of the furnace and float and accumulate on the surface of the aluminum liquid. Subsequently, multiple slag removal operations are carried out to thoroughly remove furnace impurities and ensure that foreign inclusions are not introduced into the aluminum liquid during formal production.

The newly sintered magnesia furnace bottom has a high porosity, and the molten aluminum liquid is easily infiltrated into the pores, causing aluminum infiltration and bulging damage to the furnace bottom; During the furnace cleaning process, aluminum liquid will form a dense passive aluminum oxide film on the surface of the magnesium sand pores, sealing the surface pores of the refractory material and preventing the aluminum liquid from penetrating into the furnace lining during subsequent production processes, thereby extending the service life of the melting furnace bottom.

There are a large number of free alkaline oxides inside the new furnace lining. When directly melting the target aluminum alloy, it will adsorb alloy elements such as Si, Mg, Ti in the aluminum liquid, resulting in a low composition of the first batch of alloy and difficulty in blending; After washing the furnace, a stable inert film is formed on the surface of the lining, which will not adsorb a large amount of alloy elements during the subsequent formal melting of aluminum, ensuring precise and controllable alloy composition.

High purity remelted aluminum ingots and 1-series pure aluminum (1050, 1060 pure aluminum) are preferred for furnace cleaning. It is prohibited to use raw materials with high impurity content such as scrap aluminum, miscellaneous aluminum, and cutting edge materials for furnace cleaning to avoid introducing new impurities during the cleaning process; The amount of aluminum used for washing the furnace generally accounts for 60% to 70% of the effective volume of the melting furnace, ensuring that the aluminum liquid can completely immerse the entire magnesia sand furnace bottom and cover the entire aluminum liquid in contact with the furnace wall, achieving all-round immersion and flushing.

After gradually melting the pure aluminum ingot, heat it up to 740~760 ℃ for long-term constant temperature soaking and insulation. The insulation time is determined according to the furnace tonnage. For small furnaces below 5t, the insulation time is 12-24h, and for large melting furnaces above 10t, the insulation time is 24-48h. During the high-temperature soaking process, the slag scraper is used regularly to stir the aluminum liquid at the bottom of the furnace, strengthen the flushing effect on the gaps and dead corners of the furnace bottom, and allow impurities to fully float to the liquid level of the aluminum liquid.

During the insulation soaking period, the floating slag on the liquid surface is repeatedly removed 3-4 times, and all floating refractory dust, magnesia fine slag, and carbon slag are cleaned out of the furnace; After the insulation is completed, some of the washed aluminum liquid can be released and cast into ingots (this batch of washed aluminum ingots has high impurity content and can only be downgraded and reused, and cannot be used as finished aluminum alloy ingots). A small amount of aluminum liquid should be retained in the furnace to cover the bottom of the furnace, completing the entire washing process.

After cleaning and slag removal, open the furnace door to observe the inner wall of the furnace. If the furnace wall is clean and free of floating dust, the furnace bottom has no loose magnesium sand falling off, and there is no continuous floating slag on the surface of the aluminum liquid, it is judged that the cleaning is qualified. At this time, the melting furnace has the basic conditions for melting the target aluminum alloy.