Solid aluminum fluxing issues.

Economics, safety, quality and environmental factors need to be addressed when beginning your fluxing program.

The first part of this series provided a basic understanding of the science of fluxing, focusing on applications, handling and delivery, and the composition of solid fluxes. In this part, the economics of drossing flux is covered as well as some concerns foundries should keep in mind when using flux, including safety, casting quality and legal disposal.

Economics of Recovery

In addition to maintaining metal cleanliness and furnace life, fluxes economically recover metallic aluminum contained in dross. Untreated dross may contain 80 wt.% or more metallic aluminum, much of which can be returned to the melt with a drossing flux. The added advantages lent by furnace cleaning fluxes--reducing long-term refractory maintenance cost, extending refractory life and removing a source of inclusions--are more subtle. The economic value of using drossing fluxes, however, is easily calculated.

Three scenarios in which no flux, correct flux and poor flux practices are employed in a 100,000 lb aluminum melt. This data is based on an initial melt loss of 4%, initial dross composition of 67% TABULAR DATA OMITTED metallics, and final dross compositions of 40% metallics after correct flux practice and 30% metallics after poor flux practice. Some metallics in the dross are consumed by an exothermic reaction with the flux--5% for correct flux practice and 20% for poor practice of adding excessive amounts of an overly exothermic flux.

Because the oxides are about half oxygen and half metallics, the melt loss initially forms 4800 lb of relatively dry dross. A correct flux practice of an appropriate 100 lb addition of flux combusts 160 lb of metallics and returns 1693 lb aluminum alloy to the melt, leaving 3367 lb of salable dross.

In contrast, a poor flux practice of a 150 lb addition of an inappropriate flux can combust 640 lb of metallics in the dross and recovers only 1261 lb of aluminum alloy, resulting in 4329 lb of dross. The final melt loss rates are reduced to 2.3% if flux practice is correct and 2.7% if it is incorrect.

Although dross is salable, the dross recycler pays for metallic content at a sharp discount. Table 2 shows that when no flux is used, $800 ( @ $0.25/lb of contained metallic) is earned from the sale of the dross. However, $2800 worth of aluminum formed the dross, so the resulting net melt loss remains at 4% and costs the foundry $2000.

In comparison, the correctly chosen and applied drossing flux returned $1185 worth of aluminum to the melt. Although the dross can only be sold for $135, the total melt loss (including the cost of the flux) is reduced to $1505.

Poor flux practice, on the other hand, reduces the values of the dross and the metal returned to the melt. Although the metal content of the dross is low, additional metallics burn during excessively hot exothermic reactions. Only $883 is credited to aluminum recovery and $130 to dross sale. The resulting melt loss cost (including the cost of the flux) is $1827.

This example demonstrates that melt loss reduction can more than offset the cost of dross treatment. Correct flux practice saved the foundry $495 every time it melted 100,000 lb aluminum compared with no fluxing. Most drosses will initially contain more metallics, so even greater savings are possible.

Although substantial amounts of aluminum were also recovered by a poor flux practice, aluminum is wasted when an excessive exothermic reaction occurs. The general rule is to choose a reputable flux supplier and strictly follow recommendations on product selection and quantity.

Handling & Fumes 

Since most foundry fluxes are a mixture of metal halide salt powders, they are stable at room temperature and don't present much of a hazard, although they may produce dusts that TABULAR DATA OMITTED can irritate mucous membranes. Certain fluxes contain poisonous ingredients and require special handling.

On the other hand, many varied and complex reactions occur when fluxes are introduced to the high temperatures of the furnace. Smoke is commonly produced by reactive fluxes. Some flux components are vaporized by direct flame impingement; others may release Cl-, F- or P-containing fumes, some of which are noxious.

Fuming of skimmed flux can be minimized by rapid cooling. Because each situation is different, foundries need to determine their own fume-control procedures with the aid of advice and material safety data sheets from their flux suppliers.

Released |Cl.sub.2~ is not only a health hazard; it can also corrode furnace and fume collection equipment. In addition, flux fines can settle on surfaces and accelerate corrosion. Therefore, many fluxes are not recommended in electric furnaces heated with glowbars.

Explosions & Porosity

Almost all of the ingredients in fluxes are hygroscopic, meaning they will absorb moisture from the atmosphere. Some components may have water molecules bonded chemically, forming hydrates. Flux should remain packaged prior to use and be resealed after opening. One popular method is to deliver flux in small packages, which, once opened, are used in their entirety.

Moisture-laden flux that is thrown into the aluminum bath may steam and pop. Rabbling damp fluxes into the metal or plunging them deep beneath the surface could potentially cause a violent steam explosion. (This isn't a concern with flux injection because damp flux won't flow through the lance).

More often, however, damp flux will release hydrogen into the melt by reaction of the water molecule, |H.sub.2~O, with Al. Accordingly, degassing fluxes should be kept particularly dry. Gases used for sparging and flux injection, such as |N.sub.2~ and Ar, should be specified at 99.9% purity or better (also known as prepurified grade). This will minimize the amount of moisture added to the melt.

Flux Inclusions

Prior to casting, spent flux and its residuals must be completely skimmed off the melt. The fluidity of the cover flux is an important factor, so it should be chosen according to anticipated melt temperatures. If the bath is too cold for the flux, the salt-dross mixture may not separate completely from the melt. If the bath is too hot, the flux will be too fluid to be skimmed off.

Failure to remove all of the flux can lead to entrainment of flux inclusions in resulting castings. A flux component commonly seen in cover fluxes, Mg|Cl.sub.2~, is especially prone to the formation of inclusions that remain entrained in the melt. For this reason, cover fluxes should never be rabbled. In addition to causing porosity and embrittlement, salt inclusions accelerate corrosion due to their hygroscopic nature.

Alloy Problems

Many degassing and cleaning fluxes contain chlorides and fluorides that unintentionally remove Mg, Na and Sr. Even partial loss of these elements can lower mechanical properties. In addition to reducing ductility, loss of Sr changes shrinkage characteristics of the alloy, which may cause porosity- and leaker-related defects.

Elements in flux can interfere with eutectic silicon modification or primary silicon refinement. Fluxes containing P should not be used in combination with Sr or Na additions (or in any hypoeutectic Al-Si alloy). Fluxes containing Na should be avoided when P is added to hypereutectic alloys such as 390. These combinations are disruptive because P forms metallurgically inactive compounds with Sr and Na.

Alloys containing more than 2 wt.% Mg, such as 5XX series alloys, are especially sensitive to Na. Even trace amounts of this element drastically reduce the alloy's mechanical properties. With these alloys, only Na-free fluxes can be employed.

Environmental Questions

Environmental regulations at all levels change daily and are continuing to become even more stringent. It is no longer legal to take skimmed flux to the city dump, which leads to stiff fines. Proper waste disposal of dross and spent flux requires expensive testing to deposit it in regulated landfills.

Most foundry dross, which also contains spent flux, is salable to a recycler, eliminating the need for disposal. Yet in light of current cradle-to-grave regulations, foundries should not only consider the sales price of their dross, but also whether their recycling company can (or will) comply with testing and disposal requirements.

Legal and environmentally friendly alternatives exist for flux disposal, especially if it can be classified as a by-product. In addition, fluxes that don't contain fluorides have been developed to comply with federal and state fluoride emission laws. While these fluxes still contain chloride salts, which present disposal problems, they address the stack emissions issue and improve foundry environments by reducing smoke and acrid fumes.

References

I.A. Abreu, "Products Used in Treating Metallic Aluminum Baths," AFS International Metals Journal, vol 2, pp 57-71 (Sept 1977).

R.W. Bruner, "Basics of Fluxing Aluminum," Die Casting Engineer, vol 30, pp 42-43 (Nov-Dec 1986).

J.E. Gruzleski, B.M. Closset, "The Treatment of Liquid Aluminum-Silicon Alloys," American Foundryman's Society (1990).

R.J. Harris, "Aluminum Treatment Technology of the Future," BNF 6th International Conference, Birmingham, England (Sept 1986).

R.J. Kissling, J.F. Wallace, "Fluxing to Remove Oxide from Aluminum Alloys," Foundry, vol 91, pp 76-81 (March 1963).

M.H. Kogan, "Design and Development of Fluxing Agents for Aluminum Foundry Alloys," Proceedings of the 2nd Molten Aluminum Processing Conference, Orlando, Florida, AFS; p 25 (Nov 1989).

T. Utigard, "Thermodynamic Considerations of Aluminum Melting and Fluxing," Proceedings CIM, Ottawa, Canada, vol 24, p 353 (Aug 1991).

Editor's Note: This information was presented during a panel discussion at the 96th AFS Casting Congress in Milwaukee, May 3-6, 1992.