Introduction

Three important issues will be on the agenda of the aluminium industry today and in the coming period: flexibility, independence and sustainability.

However, in order to follow this agenda, especially sustainability, or to achieve the goals on this agenda set for us, we need more metallic materials, especially aluminium.

It is possible to define this contradictory situation in economic terminology as a rebound effect or take-back effect. For example, we want to build more solar power plants to replace fossil fuels, but for this, we need to produce more metals. 1 MW solar power plant requires 35-45 tonnes of steel, 4.5 tonnes of copper and 3.5-8 tonnes of aluminium (framed vs frameless panels)¹.

At this point, the most important tool we have is recycling. But as we always say, recycling is more than recycling. With recycling, it’s possible to both produce secondary aluminium and recover some of the alloying elements from scrap.

Why do we need to alloy pure aluminium?

Aluminium, which we can define as a modern material in terms of its usage areas, varies according to its pure and alloyed forms.

While pure aluminium finds use with its properties such as low melting temperature, high thermal and electrical conductivity, high reflectivity and elevated ductility, its mechanical properties are not particularly good.

A number of technologically important characteristics of aluminium can be changed by an order of magnitude or more by suitable means such as alloying additions, plastic deformations or heat treatment².

Pure aluminium doesn’t have good mechanical properties. For example, its tensile strength is about 90 MPa (13 ksi). However, with the addition of alloying elements and tempering, aluminium’s strength is much higher. One of the strongest aluminium alloys is AA7068-T6, which has an ultimate tensile strength of 710 MPa (103 ksi).

Wheels are installed on NASA’s Mars Perseverance rover inside Kennedy Space Centre’s Payload Hazardous Servicing Facility on March 30, 2020. Perseverance will liftoff aboard a United Launch Alliance Atlas V 541 rocket from Cape Canaveral Air Force Station in July 2020.

Credits: NASA/JPL-Caltech (https://www.nasa.gov/solar-system/nasas-perseverance-mars-rover-gets-its-wheels-and-air-brakes/)

This means that, as a result of alloying, tailor-made solutions can be produced for almost any industry in terms of material selection.

What is the advantage of recovering alloying elements from scrap?

Resource Efficiency

Let’s give an example of UBC smelting due to its short life time and easy availability. Although it varies depending on the melting technology, based on our experience, let’s assume that UBC scrap has the following chemical composition after melting, with a pessimistic estimate:

Table 1: Molten UBC composition (experimental)

Si%	Cu%	Mn%	Fe%	Zn%	Mg%
0.30	0.20	1.00	0.50	0.20	0.90

This table shows us that when approximately 1 tonne of UBC scrap is recycled, 9 kg of magnesium and 10 kg of manganese can be recovered in addition to aluminium.

Energy Saving

On average, producing 1 tonne of metallic magnesium from dolomite ore requires 18,500 kWh of electricity by electrolysis, 2,200 Nm³ natural gas and 2,200 kWh by the Pidgeon process and 2,200 Nm³ natural gas and 1,700 kWh electricity by the modified Pidgeon process. ³

By recycling 1 tonne of UBC, we recover 9 kg of magnesium, which also saves the energy of approximately 20 Nm³ natural gas and 15,3 kWh electricity by the Pidgeon process.

GHG Saving

Assuming that the magnesium used to alloy aluminium is produced from primary ore, the average carbon footprint of the process per kg of magnesium is:

China’s average, which produces more than 85% of global magnesium, 21.8 kg, has the lowest production, 5.76 kg, modified Pidgeon process and 7.07 kg electrolytic process.4

Again, let’s take an optimistic approach and note that magnesium, used as an alloying element, is produced by the modified Pidgeon method. By recycling 1 tonne of UBC, we recover 9 kg of magnesium, which also prevents the emission of approximately 52 kg of carbon equivalent.

This calculation also shows how high the damaging process has not only monetary but also environmental costs.

Minimisation of Alloying Costs and Difficulties

In aluminium alloying, if the alloying additives’ (hardeners’) melting point is lower than the liquid aluminium temperature, melting occurs (for example, magnesium alloying); if the alloying additives’ melting point is higher than the liquid aluminium temperature, then dissolution occurs (for example, silicon alloying). Dissolution rate is controlled by temperature and increases with temperature.

• Oxidation

Magnesium is the most problematic alloying element because of its higher oxygen affinity than aluminium. Magnesium acquires into the dross due to selective oxidation during alloying, causing some aluminium loss.

Selectively oxidising elements (e.g. Mg, Sr and Ca) have a higher affinity for oxygen than aluminium. They tend to oxidise out of the melt at a high rate and form separate phases, where the oxidation rate increases with the increasing temperature of the melt and with increasing content. In non-selective oxidation (of e.g. Cu, Fe, Zn), which takes place at a low rate, the oxides that form are incorporated into the Al₂O₃ lattice depending on the atomic or ionic size of the element and generate a change in the oxidation behaviour by altering the oxide layer’s structure and density.⁵

• Enthalpy Change

The enthalpy changes due to melting and dilution (that is, not only the sensible heat) can have a large effect on the temperature of the aluminium bath.

Table 2: Calculated enthalpy change (ΔH) and temperature change ΔT when adding 1 mass % of an alloying element⁶

Alloying additives	ΔH (kJ/kg)	ΔT (⁰C)
Si(s)	1.43	-12.1
Zn(l)	0.16	-1.4
Cu(s)	-0.07	+0.6
Mg(l)	-0.36	+3.1
Mn(s)	-1.16	+9.9
Fe(s)	-1.50	+12.7
Li(l)	-3.89	+33.1

As an example, aluminium alloying with 1 mass% Silicon treated. Assuming an adiabatic system (no heat transport in or out), the temperature will drop by around 12.1 ⁰C of the melt due to the enthalpy change.⁶

• Stirring

The main target in alloying practice is to achieve the target metal composition quickly and with minimal cost. In most cases, it is necessary to use a stirrer to shorten the alloying time and to ensure homogeneous distribution of the alloying elements in the liquid aluminium bath.

Conclusion

Sustainability is a top priority in modern metallurgy. In addition to recycling aluminium, it’s possible to reduce the carbon footprint and energy saving of alloy production by recovering alloying elements from scrap. Now, the next step is to design and use new, recycle-friendly aluminium alloys.

References

• Mark Davies, Rio Tinto, Bank of America SmartMines 4.0 Conference, June 2023, www.riotinto.com
• Dietrich G. Altenpohl, “ Aluminium: Technology, Applications, and Environment-A Profile of Modern Metal”, TMS,1998
• İlhan Goknel, “ Comparision and Development of Processes for Primary Magnesium Production”, IMA Conference, Barcelona, 2022
• Martin Tauber, International Magnesium Institute, “A Global Decarbonization Strategic Roadmap for the Magnesium Industry”, www.intlmag.org
• Dierk Raabe, Dirk Ponge, Peter J. Uggowitzer, Moris Roscher, Mario Paolantonio, Chunlai Liu, Helmuy Antrekowitsch, Ernst Kozechnik, David Seidmann, BaptisteGault, Frederic De Geuser, Alexis Deschamps, Christopher Hutchinsan, Chunhui Liu, Zhiming Li, Phillip Prangnell, Joseph Robson, Pratheek Shantraj, Samad Vakili, chad Sicnlair, Laure Bourgeois, Stefan Pogatscher, “Making Sustainable Aluminum by Recycling Scrap: The Science of “Dirty”
• Alloys, Progress in Materials Science 128, 2022
• Thorvald Abel Engh, Geoffrey K. Sigworth, “Principles of Metal Refining and Recycling”, Oxford University Press, 2021