Where does the slag come from?
Aluminum ingots, pillars for pressing, ingots or finished products, It comprises the steps of, which are causes slag formation. Such operations include loading the scrap and aluminum waste, and various types of melt processing, for example, his movement intrashop, and mixing, fluxing and alloying of the melt.
Aluminum dross is a mixture of aluminum metal and aluminum oxide. The increased amount of ash is a problem, since it leads to an increased loss of aluminum, It requires its processing and recycling. Therefore, reducing the aluminum slag is one of the most important tasks of any foundry.
Melting and casting aluminum
Main process steps aluminum foundry shown schematically in Figure 1. Some manufacturing operations are directly related to the formation of slag. Any manufacturing operation, which causes metal flow turbulence, metal splashes or melt capture air contributes to the formation of slag.
In melting aluminum scrap and waste occurs an increased oxidation of metal. The extent of oxidation depends on the proportion of scrap in the feed furnace, type and pollution, as well as a method of melting. The old rule of the aluminum industry is well known: every one percent of pollution, loaded into the furnace, It gives a percentage loss of metal at the furnace exit. It is important to understand, that the degree of contamination of the scrap (water, oil, paint, plastic, mud, etc.. slows down the melting process of aluminum and reduces the yield of suitable metal.
It is always useful to be able to immerse lightweight aluminum scrap directly into the melt. Lightweight aluminum scrap should never be exposed to direct heat torch burner.
When loading a "dry" oven light scrap is charged first, and then - a massive scrap. Direct exposure to flames on the batch should always be avoided. In direct contact with the flames on the batch melting of the surface occurs and ejection of droplets of aluminum into the air, which leads to their immediate oxidation. This also leads to penetration of the molten metal on the furnace wall and door.
Modern loading machines are loaded into the furnace batch quickly and with uniform distribution of it on the bottom of the molten bath. This provides faster cycle melting and reducing aluminum intoxication.
Moving liquid aluminum
Moving liquid aluminum - inside the shop or between shops - other operation, which is associated with increased formation of slag. This is because, that when moving the melt increases considerably the turbulence of its movement with the capture of air into the melt, that it is directly related to increased formation of slag.
Practice operation removing slag from the melt surface can affect the amount of generated slag. There may be many nuances. How to achieve removal from the melt only aluminum oxide, and not aluminum itself? How does the design of scraper tools help separate oxides from metal? How often is the slag removed?
It is necessary to determine an optimum balance between the rate of removal of slag and laborious cleaning operation walls and hearth of the furnace slag. Too frequent removal of slag resulting in an increased loss of metal, whereas rare slag removal will increase the buildup of slag on the walls of the melting furnace, which can then be difficult to remove when it is clean.
Aluminum Melt Temperature
The metal temperature is the most important factor to determine a controlled level of the slag in the furnace. If the metal temperature exceeds 780 oC, the rate of formation of slag increases dramatically, as shown in Figure 2.
Stirring aluminum melt
Proper mixing can give melt reduction to slag formation 25 %. If the melt in the melting furnace mixed insufficiently, in it there is an excessive temperature gradient from the bottom to the surface of the melt. The magnitude of this gradient depends on the power of heat supplied to the melt. The elevated temperature of the melt surface may provide a significant contribution to the slag formation.
The same problems may arise when the wrong type of gas burner or a wrong direction of its axis relative to the bath surface.
At all, any burner promote slag formation. This happens for two reasons:
- flame away from direct contact with the charge or melt and
- creation of hot spots on the bath surface directly below the torch flame. In both cases there is an increased oxidation of aluminum.
Metallic aluminum from slag
Reducing the amount of slag in aluminum smelting is a top priority for foundry workers. Maximum recovery of aluminum metal from slag, which has already been removed from the melt and is outside the furnace is another important task. Special drainage devices for separating aluminum from slag are only effective in combination with slag cooling technology.
Cooling of aluminum slag
The old method of slag cooling is cooling it directly on the floor of the workshop. Modern methods of slag cooling include the use of inert gases and special presses.
Reactivity of aluminum slag
The efficiency of slag cooling depends on the degree of slag reactivity. for instance, slags of aluminum alloys with a high magnesium content are characterized by increased reactivity. Another factor is the amount and chemical composition of the fluxes., which are contained in the slag. Without the use of cooling, the increased reactivity of the slag and its combustion leads to a rapid loss of metal - it turns into oxides.
To flux or not to flux?
There is disagreement about, whether it is necessary to flux the melt in a reverberatory furnace. Conventional melting and holding furnaces – reflection. Flux is commonly used to separate oxides and dirt from free metal. Previously, exothermic fluxes were widely used. With their help “warmed up” slag, what, as believed, made it possible for aluminum to drain from the slag into the melt. It is now recognized by all, what happens is exactly the opposite: such a slag sucks aluminum from the melt. Therefore, now in industry they mainly use endothermic fluxes., although exothermic fluxes are also used, especially in Asia.
A source: Aluminum Extrusion Technology Seminar, 2004.