Removal of iron from aluminium

Iron and silicon in aluminium

Iron

Iron is the most common element in aluminum alloys, which stems from the bauxite and steel tools used during both primary and secondary production. Iron has a high solubility in molten aluminium and is therefore easily dissolved at all molten stages of production. The solubility of iron in the solid state I very low (max. 0,05%) [1] (Fig. 1).

Silicon

Silicon, like iron, is the most common impurity in aluminium alloys. Silicon is also the main alloying additive in casting aluminium alloys [2]. Therefore, iron usually forms second phases in the aluminum alloys, such as Al₃Fe, α-AlFeSi, and β-AlFeSi. These Fe-rich phases have an appreciable harmful effect on the mechanical properties of the alloy. The complexity of the chemistry of the aluminum-iron-silicon system is shown in Fig. 2.

Fig. 1 – The Al-Fe phase diagram in Al-rich region [2]

Fig. 2 – Al corner of ternary Al-Fe-Si system diagram [3]

Why iron is so harm?

The mechanical properties of cast aluminum alloy are generally imperiled by the presence of iron. Fe-rich intermetallic phases have much more complex morphologies, with fragile and brittle appearance than what is shown in two-dimensional observation.

Among all the Fe-rich phases, β-AlFeSi is thought to be the most harmful, and most efforts have been devoted on how to avoid the formation of β-AlFeSi. β-AlFeSi has an undesirable platelet morphology as shown in Fig. 3, which is brittle and generally assumed to act as stress raisers and points of weak coherence [3].

Figure 3 – Three-dimensional reconstruction of β-phase:
(a) original two-dimensional phases;
(b) three-dimensional β-phase [3]

How to remove iron from aluminum?

There are currently no effective practical methods to directly remove iron from aluminum alloys by conventional refining [1].

Here is an overview of existing techniques and theories on direct iron removal from aluminum or reduce its content in aluminium [1-3]:

• Modification Fe-rich phases
• Thermal interactions
• Iron removal technologies.

Mofification of platelet iron phases

Addition of other metals

Common Fe-rich phase have following forms, which tends to embrittle the metal:

• A platelet Fe-rich phase is mainly the brittle β-Al₅FeSi phase (Al-Si cast alloys)
• A long needles shaped Cu₂FeAl₇ phase (Al-Cu alloys).

The addition of a suitable neutralizer like Mn, Cr, Be, or the some rare earth elements could sever to control the deleterious effect of Fe in aluminum alloys and prp. Modification/neutralization of the Fe-rich phases can be obtained by modifying the platelet morphology and promoting

• A compact morphology such as Chinese script, polyhedral or star like shape.

Manganese

Manganese has low cost and is readily available. Although Mn by itself is also harmful to the mechanical properties of aluminum alloys, it is widely used to neutralize Fe in Al-Si cast alloys.

With the addition of sufficient manganese the platelet Fe-rich phases transformed from

the brittle platelet to the compact morphology such as:

• Chinese script
• globule
• polyhedral.

Thermal interactions

Molten aluminum superheating

The size of Fe intermetallic compounds decreased as the superheating temperature

increased up to 500°C above the melting point. That Chinese script α-phases formed instead of platelet β phases when the melt temperature was >800°C, the subsequent thermal cycling of the melt slightly affected the Chinese script morphology.

However, the high superheating temperature in aluminum alloys is not recommended owing to the increasing of gas pick-up, oxidation loss and the formation of oxide inclusions as well as higher energy costs and furnace wearing.

 Cooling rate

According to the Al-Fe-Si phase diagram the β-Fe rich phase is a stable phase under the equilibrium cooling rate. When casting is conducted under a very high cooling rate and/ or the melt is superheated to a high temperature, however, the iron rich phase crystallizes into the a-phase in metastable forms. The cooling rate increases as the critical content level of Fe to form β–Fe increases.

Heat treatment

Fe-rich phases can be dissolved, fragmented and globularized during the solution heat treatment at the solidus temperature. High treatment temperature is required because of the low diffusivity of Fe in the solid aluminum.

This method is limited by the possible damage caused by incipient melting.

Iron removal technologies

Precipitation of high Fe-rich phases

Iron is removed by the formation of primary Fe-rich intermetallics, generally, primary α-phases, called “sludge”. The crystallization of the sludge under an appropriate holding temperature and an initial Fe and Mn content has proved to be an effective method to remove Fe from Al-Si alloys.

This method is a well-established technique for Al-Si cast alloys. The process consists of two steps: formation of primary Fe-rich intermetallic particles, followed by the removal of the particles:

• The alloy is melted and held at a high temperature (750~800°C) for some time in order to be well homogenized.
• Then, the melt is cooled to a holding temperature in the range of 600–650°C for the formation and growth of the sludge.

 Gravity separation

 Gravity separation is a method employed to remove the sludge. The melt is held at the sludge formation temperature for a relative long time. Longer settling time and lower holding temperature encourages higher iron removal efficiency. Figure 4 shows a microstructure of an aluminum alloy after gravity separation for a long time. Most of the sludge settled to the bottom and thus the upper part of the alloy was purified.

Fig. 4 – Microstructure of an aluminum alloy
after Fe-rich phase precipitation and
subsequent gravity separation [2]

Filtration

Primary, Fe-rich intermetallic particles can also be removed by porous filters similar to the removal of nonmetallic inclusions by filtration. Figure 5 shows the schematic steps of the filtration operation. After a short time (10–20 min) holding at sludge formation temperature, the melt is decanted through a preheated filter.

Fig. 5 – Schematic of the filtration process
(T1: melting temperature, T2: holding temperature) [4]

 Centrifugal separation

The centrifugal separation technique was applied to directly remove iron-rich phases from the partially solidified aluminum alloy melts without any other elements addition, such as Mn. The centrifugal separation apparatus that was used is shown schematically in Fig. 6. The iron-rich phases moved to the edge side of the melt and the central part was purified as shown in Fig. 7.

Fig. 6 – Schematic of vertical centrifugal separation apparatus [2]

Fig. 7 –  Microstructure of the transverse cross section of the centrifugal separated melt [2]

 Electromagnetic (EM) separation

Figure 8 shows the principle of EM separation. When a uniform EM force is applied to a liquid metal, the metal is compressed by the EM force (Lorentz force) and a pressure gradient is generated in the metal. The non- or less-conductive particle suspended in the liquid metal receives only the pressure force because it does not experience the EM body force. As a result, the particle is forced to move in the opposite direction of EM force.

Fig. 8 – Principle of electromagnetic separation of inclusions from the molten metal [2]

Electrolysis

The process of three-layer-cell electrolysis is the most successful technique for the removal of iron and silicon from the molten aluminum so far. Figure 9 shows a sketch of a three-layer refining cell. The impure molten aluminum anode, the barium-sodium-aluminum halide electrolyte and the pure molten aluminum form three layers in the cell. Only aluminum in the anode can be dissolved during the electrolysis process and deposits on the cathode. Because the pure molten aluminum is the lightest, it will stay on the top of the three layers. Thus, the purified aluminum is obtained.

This process is expensive. The energy consumption for this process is relatively high, 13–14 kWh for 1 kg of metal.

Fig. 9 – Sketch of three layers electrolysis process [2]

Electroslag refining

Electroslag refining process (ESR) is a secondary refining process in which the slag or flux is used both as a heating source and as a refining medium, as shown in Fig. 10. The process is already well established for ferrous metals but has not been used for aluminum refining.

Fig. 10 – Sketch of the electroslag refining apparatus [3]

Direct centrifugal separation

During the method of centrifugal separation, the melt is cooled to a temperature at which solid phases form, that is, the melt is partially solidified. The first solidified phases include mainly silicon, iron, or titanium. Then, the centrifuge is started and cooling is continued. Liquid is expelled from the solid framework and collected. Temperature is an important factor controlling the removal efficiency of impurities because the precipitation of solid crystals contributes to the success of extraction (Fig. 10).

Fig. 11 – Centrifugal apparatus:
(a) Components of the removable-wall centrifuge bowl;
(b) The centrifuge bowl assembled [3]

Sources:

1. Physical Metallurgy /Aluminum and Aluminum Alloys – ASM Specialty Handbook // Ed. J.R. Davis – 1993
2. Iron in Aluminium Alloys: Impurity and Alloying Element // N.A. Belov, A.A. Aksenov, Dmitry G. Eskin – 2002
3. Iron: Removal from Aluminum / Encyclopedia of Aluminum and Its Alloys // Ed. G.E. Totten, M. Tiryakioğlu, O. Kessler – 2019
4. Removal of Iron from Molten Recycled Aluminum through Intermediate Phase Filtration  – Materials Transactions, Vol. 47, No. 7 (2006)