Development, evalation and application of solid fluxes.

Inside This Story:

* Powder and granular fluxing techniques are presented, providing details on flux chemistry and composition.

* Present day fluxing practices are discussed, including the use of flux in one or more stages within the treatment and processing of molten aluminum.

When melting and handling molten aluminum alloys, foundries must combat the formation of dross, non-metallic inclusions and oxide build-up. Improper burner adjustments, metal temperatures, flame type, furnace door conditions, furnace refractory conditions and melting practices all negatively affect molten aluminum and cause oxide build-up.

To overcome these problems, foundrymen rely on fluxes. A flux is a material added to molten metal that unites with impurities to form dross or slag, which rises to the surface of the metal and can be removed by skimming. Fluxing is the best means of obtaining clean metal, preventing excessive oxide formation, removing nonmetallic inclusions from the aluminum melt, and preventing and removing oxide build-up from furnace walls.

This article will discuss present foundry practices in the use of powder and granular fluxes in crucible furnaces, transfer ladles and rever-beratory furnaces.

Understanding the Problem

Molten aluminum alloys have two inherent characteristics: the tendency to absorb hydrogen gas and the ability to readily oxidize. When an aluminum alloy melt reacts with the atmosphere or moisture, it forms an amorphous continuous alumina [Al.sub.2][0.sub.3] film on the surface of the bath.

Alumina films are an important part of the melting process because they protect the metal underneath from further oxidation. In foundry operations, however, the surface of the molten bath is always moving due to charging, skimming, cleaning, degassing, transferring and ladling. Any of these melting practices cause the thin film to break and re-oxidize, causing rapid alumina film thickening. The movement and breaking of these alumina films cause the films to crumble, to thicken and to encapsulate unoxidized molten aluminum, generating wet dross. Dross is considered to be a main contributor to total metal loss during melting. Depending upon the efficiency of the furnace and melting practices, the amount of dross generated may be 5-10% of the total metal melted.

In addition to the elimination of dross, aluminum foundries must deal with the impurities that are suspended and floating in the bath. These impurities can be introduced into the melt during charging, molten metal treatment and/or handling.

The solution to these problems can be achieved by fluxing. Fluxing is the addition of chemical compounds to clean molten aluminum baths. It is the first step in obtaining clean molten metal by preventing excessive oxide formation, removing non-metallic inclusions from the melt and/or removing oxide build-up on furnace walls.

In general, fluxes can be grouped into two classes: gaseous and solids, Gaseous fluxes are a blend of an inert and a chemically active gas that is injected into the molten bath. Solid fluxes, which are preferred in most foundries, are a blend of salts. This article will focus on solid fluxes.

Solid fluxes can be broadly categorized as passive or active. Passive Fluxes protect the surface of the molten aluminum from oxidation and prevent hydrogen pick-up by the melt. Active fluxes react chemically with the aluminum oxides and clean the melt more effectively.

Solid Fluxes

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

Salts are used in solid fluxes because they are:

* cost effective;

* easy to use (combine easily with other ingredients);

* serve as fillers for active ingredients;

* have a lower density than aluminum;

* have the ability to cover molten surface;

* allow a low-melting point, high-fluidity product;

* have the capability to absorb oxides and reaction products from the fluxing action.

Solid fluxes have been classified in four categories, depending on their use and function in the foundry operation.

Cover Fluxes--Cover fluxes are designed to be liquid at operating metal temperatures. The flux forms a molten barrier on the surface of the metal to protect it against oxidation and hydrogen gas adsorption. The basis of any cover flux is a blend of sodium chloride (NaCI) and potassium chloride (KCl) salts with small quantities of fluorides. Fluxes having these three compounds are suitable for almost all aluminum alloys, excluding hypereutectic aluminum alloys and aluminum magnesium alloys with more than 7% magnesium.

Cover fluxes are used in foundries and smelting operations. Foundries use these fluxes during the melting of heavily oxidized foundry returns and machining chips, as well as when metal holding temperature exceeds the 1420-1450F range.

Drossing Fluxes--Drossing fluxes, which are based on salt blends of chlorides, simple and double fluorides, and oxidizing compounds, are able to react exothermically, generating heat and improving flux wettability. This wetting action of the flux promotes coalescence, which allows the fine aluminum drops to form larger drops that are easier to recover.

The reactivity in a drossing flux is due to the combination of oxidizers and double fluoride compounds. The flux may appear too reactive due to an excess of oxygen-bearing compounds that may be burning excessive amounts of good metallic aluminum without dissolving aluminum oxides in the melt.

A good drossing flux must be designed to reduce the rich metallic aluminum content of the dross. As the dross is treated with the flux, it changes from a wet dross appearance (bright, shiny metallic color) to a dry dross appearance (dark, powdery). Proper flux treatment can reduce the amount of metallic aluminum content of the dross up to 30%.

Cleaning Fluxes--Melt cleaning fluxes are designed to remove aluminum oxides from the melt. They have similar chlorides and oxidizing compounds as drossing fluxes, but in different proportions. In addition, the composition of a melt cleaning flux typically includes only simple fluorides as compared to both the simple and double fluorides present in drossing fluxes. Since the melt cleaning flux is less reactive than a drossing flux, it yields less dry dross.

Furnace Wall Cleaning Fluxes-Wall cleaning fluxes are specifically designed for the softening and removal of excessive aluminum oxide build-up that occurs on melting furnace walls, especially along the melt line. This flux helps keep crucible and furnace walls above and below the melt line free of oxide build-up.

Wall cleaning fluxes contain the highest amount of double fluoride compounds. The exothermic reactions that occur because of the oxidizing compounds and the double fluorides enhance penetration of the flux into the oxide build-up. This facilitates the removal of the oxide build-up at the furnace wall. Fluoride free fluxes cannot be used effectively as a wall cleaning flux.

Granular vs. Powder Fluxes

For nearly 60 years, most foundries have utilized powder fluxes. However, the basic formulations and compositions of the fluxes have not changed dramatically during this period.

In the last ten years, research and development for possible new formulations and variations of traditional mixes have evaluated the use of fluorides more closely because of environmental emission concerns. While emissions from powder fluxes are within standard environmental limits, it has been reported that emissions can be reduced by at least 50% if the morphology of the flux changes from powder to granular in fluxes containing fluorides.

A granular flux may or may not have the same formulation as a powder flux. Because of the different grain morphology, granular fluxes offer other operational and product advantages over powder fluxes. In addition to being less polluting to the atmosphere, granular fluxes reduce smell and smoke, allowing operators to work closer to the furnaces during drossing operations.

Granular fluxes also are easier to apply and to spread over the molten surface since they are dust free, thereby reducing application rates. As the furnace tender throws the powder flux, the flux falls in a big cloud after leaving the cup. The granular flux travels farther before falling into the melt. The difference in traveling distance facilitates spreading and directing the granular flux inside a furnace.

Because powder fluxes are blends of salt, each grain represents only one of the different chemical compounds from which the flux is made (Fig. 1). Granular fluxes have 100% grain uniformity in chemistry, so each grain represents the same chemistry as the flux recipe (Fig. 2).

Foundry Practices

Powder and granular fluxes can be added either by weight percentage of the total metal melted or by weight (lb) based on the molten surface area to cover. Manufacturer recommendations are the starting point, with typical recommendations shown in Table 1. However, the exact quantity of flux will depend on a Variety of operating conditions such as furnace type, charging and melting practices, initial melt cleanliness, molten metal temperature and final metal cleanliness required.

Present day fluxing practices include the use of flux in one or more stages within the treatment and processing of molten aluminum. While a majority of foundries flux their molten metal in the melting furnace, the metal may be fluxed in several different locations.

Crucible Furnaces--To take advantage of the mixing capabilities and oxide removal of the rotary equipment, the preferred time to flux a crucible is to apply the flux on the metal surface while melting and degassing. With this procedure, however, it is important to pay close attention to the degassing technology being used.

Assessment and evaluations of fluxes and degassing equipment have shown that the reactivity and molten surface coverage by the powder or granular fluxes is greatly affected by the design of the rotor, as well as the efficiency of the equipment. The effects of different rotor designs and degassing equipment on the stirring and activation of fluxes can lead to a powdery and dry dross that has little metallic aluminum (approximately 30%).

Transfer Ladles--Molten metal fluxing performed on the metal in the ladle may include the same fluxing techniques used in crucible furnaces, However, there are two possible variations when simultaneously degassing and fluxing into a ladle. One variation is to add the flux at the bottom of the ladle before tapping metal into it.

When this method is used, the only flux that reacts is that which gets trapped below the initial surge of the molten metal cascading on top of it. The remaining flux stays un-reacted, and tends to float on the surface of the bath. Once the ladle is full, the flux must be stirred and rabbled to activate before the degassing occurs.

The other variation is to add the flux by directing it at the molten metal stream during the tapping of the furnace into the ladle. This technique has the same negative effects with respect to activating flux. Only flux that becomes trapped in the molten stream gets activated, while the flux falling down inside the ladle does not. In addition, this process of adding flux is less safe since splashing molten aluminum may burn the furnace tender more easily.

Reverberatory Furnaces--While reverberatory furnaces are generally used as breakdown or holding furnaces, melting and metal ladling can be performed from the same furnace. Operational procedures for the skimming and drossing of the molten metal and the cleaning of the furnace sidewalls are normally divided in exclusive sections according to the design of the furnace. Furnace sidewall cleaning should be done once a week, but it is normal to do it up to three times per week.

Table 1

Typical Addition Rates for Powder and Granular Fluxes.

Application for:      Addition based on:   Powder flux   Granular flux

Crucibles and Ladles  % of total metal     0.25-0.50%    0.20%
                      charged or melted
Reverberatory         lb of flux/sq ft of  0.35-0.50 lb  0.30 lb
                      molten surface area

For More In formation

"Understanding Aluminum Degassing," D. Neff, MODERN CASTING, May 2002, p.24-26 "Degassing Effect of RE Flux on Aluminum Melt," H. Ni, 8. Sun, H. Jiang, D. Shu and W Ding, 2002 AFS Transactions, p.267-270.

RELATED ARTICLE: Flux chemistry 101

The effect of fluxes on the molten aluminum depends on the chemistry of the flux used, morphology of the flux, total amount added, molten metal temperature, flux contact time and stirring technique. Different combinations and proportions of ingredients will impart different flux properties, such as density, fluidity, wettability and reactivity.

To understand how these flux properties are accomplished, it is necessary to recognize that each different ingredient in a flux provides different effects that directly influence the final property of a flux. Ingredients can be classified in three major groups based on their primary influence over the mixture.

Chlorides--Chlorides are mainly used because of their fluidizing effects and because they are used as fillers. Fluxes based on chloride salts should have a negligible reaction with molten aluminum. These salts also provide negligible effects on surface tension as compared to fluorides.

Fluorides--While fluoride salts have limited solubility for oxides, they can not dissolve massive aluminum oxides. They act as surfactants affecting 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 oxide decreases, promoting oxide separation and metal coalescence. Fluorides are the most effective compounds 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 present in the dross, yielding aluminum oxides as well as considerable heat.

About the Author

Rafael Gallo is the production application manager of aluminum foundries at Foseco Metallurgical, Inc., Cleveland, Ohio. He has occupied technical and senior management positons in Ohio Aluminum Ind., Doehler Jarvis, Ravenna Aluminum and Superior Ind.