Properties of Fluxes used in Molten Aluminium Processing

T. A. Utigard1, R.R. Roy1 and K. Friesen2

1 Department of Metallurgy, University of Toronto, Toronto, Ontario, Canada M5S 3E4 2Hatch Associates, Montreal, Quebec, Canada

(Received August 21,2000)

ABSTRACT combustion of hydrogen containing fuels. Another group

of cover fluxes are based on MgCl2-KCl which forms a Molten salt fluxes play an important role in the low melting eutectic at 424°C. These cover fluxes have processing of molten aluminum. Most salt fluxes are high fluidity and form a thin layer on the melt surface. based on either the KCl-NaCl or the KCI-MgCl2 binary Since, MgCl2 is expensive it is primarily used when systems. Additives include other chlorides, fluorides, treating aluminum alloys with very low limits of Na and nitrates, carbonates or sulphates. They are used either in Ca and with more than 2 wt% Mg. a passive role to cover the metal from oxidation or NaF and KF based salts decrease the interfacial sometimes in an active role to remove inclusions, alkali tension between the flux and the metal and between the metals and magnesium. Salt fluxes can also be used to flux and oxides l\l. The reason is that with fluoride salts, imporve metal recovery from drosses as well as during the aluminum easily picks up some sodium or potassium melting of recycled scrap. The physical and chemical due to reactions such as properties of these various molten salts are reviewed and analysed in terms of applications to the aluminum 3NaF + AI A1F3 + 3Na (1) industry.

3KF + Al —» A1F3 + 3K (2)

COMPOSITIONS AND PROPERTIES OF Since both Na and Κ are surface active elements they COMMON SALT FLUXES will concentrate at the surface of the aluminum. Therefore, when fluoride salts are brought in contact with molten Fluxes may perform several functions such as aluminum rapid exchange reactions take place. NaF and degassing, removal of Li, Na, Ca and Mg, inclusions KF improve also the wettability favouring separation of removal, cleaning and alloying. In addition, fluxes are also oxide inclusions from the aluminum and metallic used to minimize oxidation and to promote metal/oxide aluminum from the dross. In addition to the pickup separation, improving recoveries. Several common fluxes problem, their disposal is subject to stricter environmental are based on mixtures of KCl and NaCl which form a low regulations than for pure chloride salts. Some specific temperature (665 °C) eutectic, improving the fluidity of compounds may decompose into chlorine, C02 or metal the flux. Cover fluxes prevent oxidation of the molten halide gases(AICl3). The most notable gas-releasing com- aluminum by providing a physical barrier to oxidation. pound is hexachloroethane(C2Cl6) which generates Cl2

This is particularly important under highly oxidizing and gaseous A1C13. Many secondary aluminum alloy pro- conditions (T > 775 °C), melting of fines and chips, or ducers use a NaCl-KCI based cover flux and depend on making alloys containing more than 2 wt% Mg. A cover Cl2 or C2C16 for degassing /2-4/ and magnesium removal. flux will also decrease the rate of hydrogen pickup from Drossing fluxes are used to separate molten aluminum reactions between aluminum and H20 formed by the from the dross layer by reaction with metallic aluminum

303 Vol. 20, Nos. 3-4, 2001 Properties of Fluxes Used In Molten Aluminium Processing

to generate heat and improve fluidity. Too little exothermic combustion reduces fluxing efficiency while too much flux burns excessively, creating fume and loss of metallic aluminum. To clean furnace walls an exothermic wall cleaning flux is typically applied when the walls are as hot as possible to aid heating and softening of oxide buildup 151.

THERMODYNAMICS OF ALUMINUM - MOLTEN SALT REACTIONS

The elements to control in molten aluminum are lithium, sodium and calcium in small concentrations (< 20 ppm) and magnesium in large concentrations (0.2 to Fig. I: Standard Gibbs energy of formation of several 10%). The standard Gibbs energy of formation of several sulphides, oxides, chlorides and fluorides. The species is given in Figure 1 161. With a few exceptions, the data are given at 723 °C per mole of S, O, Cl2, thermodynamic stability decreases from the fluorides and F2, respectively /13/. down to the sulphides in the following order:

fluorides > chlorides > oxides > sulphides

Metal chlorides with a AG0 value more negative than that for A1C13 are more stable than A1C13. This means that when Cl2 is injected into aluminum containing various metallic elements, the chlorine will preferentially react with these metallic impurities. The same also applies to fluorides. Li, Na, K, Ca, Mg and Ba all form chlorides and fluorides more stable than aluminum and can therefore be removed by Cl2, F2, C2C16 or SF6 injection. The reaction in the case of Mg is

0 Mg(in Al) + Cl2 = MgCl2, AG = -481 kJ/mol (3) Metal 1 i The equilibrium constants for reactions such as Fig. 2: Exchange equilibrium between aluminum and different metal chlorides and metal fluorides at Al + 3MeX = 3Me + AIX3, X = 723 °C. CI or F and Me = Li, Na, Κ (4)

A1+1.5MeX2= 1.5Me + AlX3, X = earth chloride electrolyte has no tendency to react with CI or F and Me = Ca, Mg, Ba, Sr (5) aluminum while metal fluoride electrolytes are more reactive. Chloride salts are therefore suitable as a cover are shown in Figure 2. Equilibrium constants much greater flux during Cl2 injection since they will promote a high than unity imply that the reaction proceed as written, purity aluminum product. The removal of impurities such while a value much less than one indicates that the as Zn, Si, Fe, and Cu by chlorine or fluorine treatment is reaction goes in reverse. Therefore, an alkali or alkali- basically impossible and when fluxes contain these

304 Τ. A, Utigard et al. High Temperature Materials and Processes

metals, they will contaminate the aluminum. To remove the metal falls. This may lead to emissions of A1C13 and sodium or lithium from primary aluminum, the TAC HCl due to reactions with moisture in the air. In such process that employs the injection of A1F3 powder into the cases, it has been found that a thin salt flux cover will trap metal, may be used. the AICI3 gas before it is emitted into the atmosphere. The removal of Mg from the aluminum scrap is further s A1F3 + 3Na(in metal) = Al + 3NaF, Keq = 4· 10 (6) enhanced by adding NaF and/or KF to the chloride flux. However, one disadvantage is that this leads to 16 AIF3+ 3Li(in metal) = Al+ 3LiF, Keq = 210 (7) contamination of the aluminum with Na and/or Κ as given by the following exchange reaction: These reactions are highly favourable and sodium and/or lithium are removed from the aluminum. 2NaF + Mg = 2Na + MgF2, Keq = 2.6 (8)

This is made even worse by the fact that any fluorides CONTROL OF MAGNESIUM DURING will decrease the activity coefficient of magnesium in the TREATMENT OF MOLTEN ALUMINUM salt flux, driving this reaction further to the right. This shows that as long as there are fluorides present in the flux Many useful aluminum alloys contain large amounts and magnesium in the metal, the removal of Na is very of magnesium and during recycling there is often a need difficult and higher concentrations of MgCl2 in the flux to demag the aluminum scrap. Based on the thermo- are required. dynamic data given in Figure 3, it is clear that magnesium In the case of magnesium alloys in contact with cannot be removed based on exchange reactions with calcium compounds, the aluminum may pick up some NaCl and KCl based fluxes alone. Either a reactive gas or calcium due to reactions such as another type of flux has to be used. In general there are three types of demagging processes; i) chlorination, ii) use Mg(in Al) + CaCl2 = MgCl2 + Ca(in Al), of solid chlorine-containing fluxes (C2C16), and iii) the 9 Keq = 6.2-10" (9) injection of A1F3 or NaAlF4 Π-10/. One serious problem when using chlorine gas to remove magnesium is that the Although the equilibrium constant is small, since the demagging efficiency drops as the magnesium content in activity coefficient of Ca in aluminum is very small (« 0.005), calcium is easily picked up by the aluminum. Figure 4 shows how the sodium and calcium contents

in aluminum vary with the MgCl2 content of the cover flux used. These results are based on experimental tests with an Al-4.5% Mg alloy doped with sodium and

calcium before the metal was treated with a 10% Cl2 -

90% N2 gas mixture for 30 minutes. It is only after the

MgCl2 content increases to 50 wt% that it is possible selectively to remove the Na and Ca while keeping the magnesium in the alloy. When calcium carbonate is used(as flux or as caulking material), the following two reactions may cause calcium pick-up:

Mg(in Al) + CaC03 = 4 Fig. 3: Exchange equilibrium between magnesium MgO + C02(g) + Ca(in Al), Keq = 7.610" (10) impurities in aluminum and different metal

oxides, metal chlorides and metal fluorides at 2A1 + 3CaC03 = 16 723 °C. A120, + 3C02(g) + 3Ca(in Al), Keq = 3.7· 10" (11)

305 Vol. 20. Νos. 3-4, 2001 Properties of Fluxes Used In Molten Aluminium Processing

Cl2-N2 injection into Al(4.5% Mg) with KCI-MgClj salt cover at 740 °C

35 40 45 50 55 60 65 Wt% MgCI, in KCI-MgCI, 10 15 20 Duration(Minutes)

Fig. 4: PPM sodium and calcium in aluminum versus Fig. 5: Percent A1203 inclusions (in AA6061) during MgCl2 content in the flux. « treatment with NaCl-KCl with additions of KF.

cancelling the effect of KF. INCLUSION REMOVAL BY THE USE OF SALT FLUXES

RECOVERY OF MOLTEN ALUMINUM Among common inclusions we find various carbide FROM A COVER FLUX particles, oxide inclusions such as A1203, MgO or MgAl 0 films, clusters or dispersions formed during 2 4 With the use of salt fluxes we run the risk of loosing melting, alloying or metal transfer, and salt particles. metallic aluminum droplets entrained in the cover flux. Although chlorine gas is effective in removing inclusions, For salt fluxes containing substantial amounts of MgCl2, it has become a focus of strict environmental regulations fluoride salt additions become much less potent /13/. As by government agencies /ll/. Experimental work over the an example, for a melt with 45% MgCl2 in KCl, a last few years has shown that salt fluxes may successfully minimum of 10 wt% NaF is required to promote replace chlorine for this purpose. The injected salt flux coalescence, as opposed to less than 1 wt% NaF in pure may coat the inclusion particles, leading to i) coalescence NaCl-KCl. The reason for this is that MgCl2 will of individual particles and ii) de-wetting of the inclusions neutralise the alkali fluoride salts by reactions such as from the aluminum. The result is that inclusions can be separated from the melt much more easily. As seen in Fig. 8 MgCl2 + 2NaF = MgF2 + 2NaCl, Keq = 2.3-10 (13) 5 increasing amounts of NaF and KF increase the rate of inclusion removal. Beland et al. (12) have shown on a plant scale that salt flux injection indeed has the potential CONCLUSIONS to completely replace chlorine for the purpose or removing inclusions. The choice of which components to use in a flux

In the case of MgCl2-KCl based fluxes, MgCl2 seemed depends on the objective (alkali removal, cleanliness, to inhibit as well as to delay the effect of the NaF and KF dross separation). For example, sodium-bearing fluoride

additions. For 50% or more MgCl2 in the base flux, no containing fluxes should not be used with aluminum- inclusion removal was observed even with up to 10% KF magnesium alloys in order to avoid sodium contamination additions. This can be explained by the following of the metal. When removing calcium from high exchange reaction magnesium alloys it is recommended to use a flux with

around 50 wt% MgCl2. NaF, KF and Na3AlF5 additives

MgCl2 + 2KF-> MgF2 + 2KC1, Keq= 1.410" (12) are useful for the purposes of coalescence of small

306 T.A, Uligarcl et al. High Temperature Materials and Processes

aluminum particles, recovery of aluminum from a dross 5. A. Flores, M.H. Hinojosa, A.H. Castillejos, E. flux and removal inclusions from the metal. Macias and F.A. Acosta, Light Metals, TMS Annual From a thermodynamic point of view, metal fluorides Meeting, Orlando, 1997, pp. 879-884. are more stable than corresponding chlorides, oxides and 6. T.A. Utigard, Proceedings of the international sulphides. MgCl2 is not a very stable salt as compared to symposium on extraction, refining, and fabrication alkali and other alkali earth chlorides. However, MgCl2 of light metals, Canadian Institute of Mining, 2 forms MgCl4 " complexes, effectively stabilizing the Metallurgy and Petroleum, Ottawa, 1991, pp. 353- magnesium chloride. MgF2 is a very stable compound and 365. when a fluoride salt is added to a chloride mixture 7. D.V. Neff and B. P. Cochran, Light Metals, TMS containing magnesium, it will stabilize the magnesium in Annual Meeting, Denver, 1993, pp. 1053-1060. the salt. 8. B.L. Tiwari, Journal of the Institute of Metals, 34(7), 54-58(1982). 9. M.C. Mangalick, Journal of the Institute of Metals, REFERENCES 27(6), 6-10(1975). 10. P.N. Crepeau, M.L. Fenyes, J.L. Jeanneret, Modern 1. T.A. Utigard, K. Friesen, R R. Roy, J. Lim, A. Silny Casting, 82(7), 28-30 (1992). and C. Dupuis, Journal of Metals, pp. 38-43 (1998). 11. G. Beland, C. Dupuis and J.-P. Martin, Light Metals, 2. R.D. Peterson, Second International Symposium - Ed. J. Evans, TMS, Las Vegas, 1995, pp. 1,189- Recycling of Metals and Engineering Materials, 1,195. TMS, 1990, pp. 69-84. 12. G. Beland, C. Dupuis, G. Riverin, R. Desmeules and 3. F.K. Ho and Y. Sahai, Second International L. Rose, Light Metals, Ed. B. Welch, TMS, San Symposium - Recycling of Metals and Engineering Antonio, 1998, pp. 843-847. Materials, TMS, 1990, pp. 85-103. 13. K. Friesen, T.A. Utigard, C. Dupuis and J.P. Martin, 4. S. Lavoie, C. Dube, and G. Dube, Light Metals, Light Metals, Ed. R. Huglen, Orlando, TMS, 1991, pp. 981-985. Orlando, 1997, pp. 857-864.

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