Removal of Iron Impurity from Aluminum by Electroslag Refining

Chong Chen, Jun Wang*, Da Shu and Baode Sun

The State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, P. R. China

The effect of electroslag refining on removal of iron from commercial purity aluminum using KCl-NaCl-Na3AlF6 slag containingNa2B4O7 was studied. The iron content decreases with the decrement of the remelting speed and the iron content can decrease from 0.42% to0.20 mass% after electroslag refining. The chemical reaction between melt and slag to form Fe2B in the electroslag refining process is the mainreason for the reduction of iron content. Thermodynamic calculation of the chemical reaction theoretically accounts for the formation of Fe2Bspontaneously in the electroslag refining process. The ultimate strength and elongation of commercial aluminum are improved obviously afterelectroslag refining. [doi:10.2320/matertrans.M2010435]

(Received December 27, 2010; Accepted February 24, 2011; Published April 20, 2011)

Keywords: electroslag refining, aluminum, iron, thermodynamic, mechanical properties

1. Introduction

Electroslag refining (ESR) is a secondary refining processalready well established for a variety of metals and alloys.1–3)

In the refining process, a slag or flux is used both as a heatsource and as a refining medium. The molten droplets formedat the tip of the electrode fall through the molten slag, andcollect in a pool on the baseplate to solidify. Refining takesplace because of the reaction between the metal and the slagin the process. By suitable choice of slags, removal ofimpurity elements can be achieved by chemical extractionwith the slag. J. C. Stoephasius et al.3) have investigated theremovability of the main impurties out of titanium andtitanium-aluminum alloys by electroslag refining using aCaF2-based active slag.

Iron, as one of the most harmful impurities in aluminum, isalways present in alloys made from commercially purity basematerial.4,5) The maximum equilibrium solid solubility ofiron in solid aluminum is very low (at 0.05 mass%), andtherefore iron is usually present therefore in the form of Fe-rich intermetallic compounds together with other alloyingelements, such as silicon, manganese and copper.6,7) Thesecompounds, which are very hard and brittle and have arelatively low bond strength with the matrix, act as stressraisers which deteriorate the mechanical properties ofaluminum alloys.8–10)

Iron removal from aluminum alloy can be achieved by theformation and subsequent sedimentation and/or filtration ofintermetallic phases containing iron.11–14) However, suchmethods generally involve the addition of other harmfulelements, such as Mn, to facilitate the formation ofintermetallics and the elimination efficiency is easily influ-enced by the chemistry, processing temperature and otherparameters.

In view of its potential for removal of impurity elements,an attempt has been made to study the possibility of usingelectroslag refining for removal of iron from commercialaluminum. In this paper, the effects of electroslag refining oniron reduction and the mechanical properties of commercial

aluminum after electroslag refining were investigated.Furthermore, the mechanism of iron reduction was discussedthermodynamically.

2. Experimental

The electroslag refining experiments were carried out in a60 KVA single phase AC unit. Figure 1 is the schematicillustration of the electroslag refining unit. Before theelectroslag refining experiments, the commercial purityaluminum was cast to electrodes of 40 mm diameter and80 cm length, and the slag was prefused at 250�C in an oven.The composition of the material is given in Table 1. Theslag used was a mixed chloride-fluoride flux, containing47 mass%KCl, 30 mass%NaCl and 23 mass%Na3AlF6. Inaddition, the Na2B4O7 was added to the slag for the removalof iron and the addition of Na2B4O7 was 10 mass% of theslag. The quenching curve of the KCl-NaCl-Na3AlF6 slagcontaining 10 mass%Na2B4O7 is shown in Fig. 2. Theprecipitating temperature of Na2B4O7 in the slag is possiblybetween 650�C and 690�C as there appear several small

Fig. 1 Schematic illustration of the electroslag remelting unit.

*Corresponding author, E-mail: junwang@sjtu.edu.cn, chenchongsjtu@

sjtu.edu.cn

Materials Transactions, Vol. 52, No. 6 (2011) pp. 1320 to 1323#2011 The Japan Institute of Metals RAPID PUBLICATION

fluctuations in the curve, and the melting point of the KCl-NaCl-Na3AlF6 slag is �630�C. The density of the slag is1.68 g�cm�3 at 740�C.

In the refining process, the solid starting technique wasused through the arc striking agent. A 0.5 kg mass of slag wasmelted to form the slag bath and the molten slag was super-heated to 740�C in the refining process. The electrodes wereimmerged to obtain ingots of 80 mm diameter and 25–30 cmlength under a voltage of 10–15 V and a current of 600–700 A. Finally, the sludge was collected for X-ray diffraction(XRD) analysis, and aluminum samples for metallographicobservation and mechanical properties testing were takenfrom the ingots.

The physical properties of slag were measured withcomprehensive measuring instrument of melt’s physicalproperties (RTW-10). The chemical compositions of thealuminum sample were analysed with an ICP AES machine(ICP, Iris Advangtage 1000). Metallographs were observedby scanning electron microscopy (SEM, JSM-6460). Theoriginal NaCl and KCl in the sludge were removed through adeionized water filter process and phases in the sludge weredetected with an X-ray diffractometer (XRD, D/MAX2550VL/PC). Tensile properties were tested at room temper-ature by Zwick/Roell materials testing machine at a tensilespeed of 1 mm�min�1. Each value of the tensile propertiesreported was the average of four tests at the same condition.The sizes of the specimens for mechanical properties testingwere shown in Fig. 3.

3. Results and Discussion

The dependency of iron content under different remeltingspeeds is shown in Fig. 4. The iron content of the aluminumsample is reduced obviously after electroslag refining, andthe iron content decreases with the decrement of theremelting speed. Under the remelting speed of 180 g�min�1,the iron content can decrease to about 0.20% from0.42 mass% of the unpurified sample and the rate of ironreduction is more than 50% through electroslag refining. The

iron content changes unconspicuously when the remeltingspeed is less than 180 g�min�1. It indicates that reactionsbetween melt and molten slag have nearly taken placecompletely under the remelting speed of 180 g�min�1.Therefore, it is unnecessary to remelt under a smaller speedbecause too small remelting speed is adverse to efficiency.

The microstructure of the aluminum sample before andafter electroslag refining is shown in Fig. 5. The plateletsAl-Fe binary phase at grain boundaries becomes less andthinner after electroslag refining, as shown in Fig. 5(b).

Figure 6 shows the X-ray diffraction analysis of thecollected sludge is shown in. The elpasolite (K2NaAlF6)phase, Al2O3 and intermetallic compound Fe2B are found inthe collected sludge. The elpasolite phase is a resultant ofAlF6

�3 combined with Naþ1 and Kþ1, because Na3AlF6,KCl, NaCl is resolved into Naþ1, AlF6

�3, Kþ1 and Cl�1 in themolten slag. Al2O3 is the common phase in the experiment.The iron boride phase may form as a result of chemicalreaction between the iron and Na2B4O7 in the molten slagand subsequently be captured by the molten slag during theelectroslag refining process. The melting point of Fe2B is1389�C higher than the temperature of this molten slag.15)

Therefore, the formed particulate iron boride can be capturedby the molten slag and is finally removed with the sludge,which accounts for the reason why the electroslag refiningcan reduce the iron content in the aluminum.

In this chemically complicated slag-melt system, thefollowing reaction may take place to produce Fe2B amongNa2B4O7, Fe and Al. Here, s, l and g represent solid, liquidand gas, respectively.

Na2B4O7(l)þ 14/3Al(l)þ 8Fe(l)

¼ 7/3Al2O3(s)þ 4Fe2B(s)þ 2Na(l) ð1ÞUnder practical conditions, the Gibbs free energy of

reaction (1) should be calculated by eq. (2). where �Al2O3,

Table 1 The composition of the commercial purity aluminum (mass%).

Impurity

elementsFe Si Ca Cu Na B Zn Al

Content 0.4218 0.0425 0.0270 0.0059 0.0312 0.0048 0.0113 Balance

Fig. 2 The quenching curve of KCl-NaCl-Na3AlF6 slag containing

10 mass%Na2B4O7.

Fig. 3 Size of the specimens for mechanical properties testing.

Fig. 4 Dependency of iron content under different remelting speeds.

Removal of Iron Impurity from Aluminum by Electroslag Refining 1321

�Fe2B, �Na2B4O7, �Al, �Fe and �Na are activities of Al2O3, Fe2B,

Na2B4O7, Al, Fe and Na in the molten droplets, respectively.�Al2O3

and �Fe2B can be considered as 1 for solid state matter.For simplification, �Al, �Fe, �Na and �Na2B4O7

are replaced bytheir mole atomic concentrations, i.e. [Al]mole, [Fe]mole,[Na]mole and [Na2B4O7]mole respectively.

�G1013K ¼ �G�1013K þ RT ln�7=3

Al2O3�4

Fe2B�2Na

�14=3Al �8

Fe�Na2B4O7

¼ �G�1013K þ RT ln½Na�2mole

½Al�14=3mole½Fe�8mole½Na2B4O7�mole

ð2Þ

In this experiment, the weight concentration of Al and Fein the aluminum are 99.4555 mass% and 0.4218 mass%, andthe addition of Na2B4O7 was 10 mass% of the slag. Thus,[Al]mole, [Fe]mole, [Na]mole and [Na2B4O7]mole can be calcu-lated as:

½Al�mole ¼0:994555=27

0:994555=27þ 0:004218=56¼ 0:997959 ð3Þ

½Fe�mole ¼0:004218=56

0:994555=27þ 0:004218=56¼ 0:002041 ð4Þ

½Na�mole ¼

�10%�

23� 2

23� 2þ 11� 4þ 16� 7

� �23

0:994555=27þ 0:004218=56¼ 0:026824 ð5Þ

½Na2B4O7�mole ¼

10%

23� 2þ 11� 4þ 16� 7

47%

39þ 35:5þ

30%

23þ 35:5þ

23%

23� 3þ 27þ 19� 6þ

10%

23� 2þ 11� 4þ 16� 7

¼ 0:038001 ð6Þ

In eqs. (3)–(6), 27, 56, 23, 11, 16, 39, 35.5 and 19 are theatomic weights of Al, Fe, Na, B, O, K, Cl and F respectively.

The standard Gibbs free energy at the temperature of1013 K, �G�1013K , can be calculated by eq. (7), based onGibbs free energy function (�T) Method. Using the standard�01013K and H�298K data of the substance shown in Table 2,16)

�G�1013K of the reaction (1) can be calculated to be�839614 J�mol�1.

�G�1013K ¼ �H�298K � T��01013K ð7Þ

Substituting T with 1013 K, R with 8.314 J�mol�1�K�1,[Al]mole with 0.997959, [Fe]mole with 0.002041, [Na]mole with0.026824, [Na2B4O7]mole with 0.038001 and �G�1013K with�839614 J�mol�1 in eq. (2), the Gibbs free energy of thereaction (1) should be

Fig. 5 SEM micrographs of the aluminum sample: (a) before ESR (b) after

ESR.

Fig. 6 X-ray diffraction spectra of the collected sludge.

Table 2 Thermodynamic data of the reaction (1).

Substance Al2O3 Fe2B Na Na2B4O7 Al Fe

�01013K/J�mol�1�K�1 103.136 96.990 71.182 305.135 43.108 42.632

H�298K/J�mol�1 �1675274 �71128 0 �3276490 0 0

1322 C. Chen, J. Wang, D. Shu and B. Sun

�G1013K ¼ �455591 J�mol�1 ð8ÞThe negative value of �G1013K indicates that the reaction

(1) can spontaneously take place in the slag-melt system.Therefore, thermodynamic calculation theoretically accountsfor the formation of Fe2B in the electroslag refining process.In addition, as the remelting current enters the molten slag inthe electroslag refining process, it interacts with its inducedmagnetism to create the electromagnetic force. The electro-magnetic force has a function of stirring to the molten slag.Therefore, when the metal pass through the slag drop by dropin the electroslag refining process, the reaction interfacebetween melt and slag is renewed continuously under thefunction of electromagnetic force, and the reaction (1) isimproved to great degrees.

Figure 7 shows the mechanical properties of the aluminumsamples with iron content after electroslag refining. Appa-rently, the iron reduction of the aluminum samples brings animprovement of these mechanical properties. The ultimatestrength and elongation increases with the decrement of ironcontent after electroslag refining. The ultimate strengths ofthe specimens can be improved to 65 MPa from 60 MPa,increased by 8%. The tensile elongation of the specimenscan be improved to 46% from 34%, increased by 32%. Theimprovement of tensile elongation and ultimate strengthscan be attributed to reduction of iron.8,9) After electroslagrefining, the iron content of the aluminum sample decreasesgreatly and the hard and brittle Al-Fe binary phase at grainboundaries becomes less and thinner. The probability of thesebrittle Fe-rich phases acting as stress raisers decreasesdistinctly under the pulling force function.9) Therefore, themechanical properties of commercial aluminum are im-proved after electroslag refining.

4. Conclusions

Electroslag refining can decrease the iron content incommercial aluminum from 0.42% to 0.20 mass% (more thana 50% reduction in iron content). The iron content decreaseswith the decrement of the remelting speed and the optimalremelting speed is 180 g�min�1. The chemical reactionbetween melt and slag to form Fe2B in the electroslagrefining is the main reason for the reduction in iron.Thermodynamic calculation of the chemical reaction ac-counts for the formation of Fe2B spontaneously in theelectroslag refining process. The mechanical properties ofcommercial aluminum are improved after electroslag refin-ing. The ultimate strength and elongation of commercialaluminum increase by 8% and 32%, respectively.

Acknowledgement

This work was financially supported by the NationalNatural Science Foundation of China (Nos. 50825401 and50821003). The authors are grateful to the researchers in theInstrumental Analysis Center of Shanghai Jiao Tong Uni-versity for their help in the ICP analysis.

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Fig. 7 Mechanical properties of the aluminum samples with iron content

after ESR.

Removal of Iron Impurity from Aluminum by Electroslag Refining 1323