Sustainable Magnesium Production and Processing

By J.P. Weiler, Meridian Lightweight Technologies, Inc.

Editor’s Note: This is the sixth in a series of articles highlighting developments in the magnesium industry, with the aim of addressing common misconceptions. The first five articles focused on primary production of magnesium, automotive and other applications, magnesium’s effect on aluminum alloys, and magnesium myths. This sixth article focuses on magnesium sustainability.

Introduction

The topic of sustainable production has become increasingly critical to society. A product’s life cycle, assessed from cradle to grave, encompasses all stages including raw material extraction, material production, product manufacturing, usage, and end-of-life management. The International Magnesium Association (IMA) conducted and published a comprehensive life cycle assessment (LCA) of magnesium alloys,¹ later releasing an updated version of the LCA, incorporating more recent data and inputs.²

The environmental impact of magnesium and magnesium alloy components during the use phase varies significantly depending on the application. For instance, in the automotive sector, this phase is heavily influenced by the vehicle’s powertrain—whether it uses an internal combustion engine or is an electric vehicle (EV). In EVs, the environmental impact is further affected by the energy source used to charge the batteries. Further, according to the information in the IMA’s LCAs, the raw material production, product manufacturing, and recycling and end-of-life processing are key contributors to the life cycle greenhouse gas emissions of magnesium. For this reason, these topics will be the focus of this article.

Magnesium Primary Production

The primary production of magnesium involves extracting the metal from its natural form, either from magnesium-rich minerals, such as dolomite or magnesite (Figure 1), or from magnesium chloride, found in seawater or brine. The primary production of magnesium can be categorized into two main methods: electrolytic processes and thermal reduction processes.

The electrolytic process uses purified dehydrated magnesium chloride (MgCl₂) primarily extracted from sea water or brine. Magnesium metal is produced from electrolysis of the MgCl₂ in a molten salt bath producing chlorine gas as an output.

Thermal reduction processes extract the magnesium from minerals, such as magnesite (MgCO₃) or dolomite (CaMg(CO₃)₂), through reduction and calcination processes. Magnesite or dolomite is calcined at high temperatures to produce magnesium oxide, which in turn is reduced to magnesium using a reducing agent (such as ferrosilicon) in a high-temperature furnace in an inert atmosphere or under vacuum.

The greenhouse gas emissions from the electrolytic process for magnesium production primarily result from the significant energy consumption required for the process.¹ As such, the electrolytic process holds the potential for achieving near-carbon-neutral primary magnesium production.

A change in energy supply from oil to natural gas for the electrolytic process reduces greenhouse gas emissions by 34%,¹ while incorporating a portion of renewable energy further decreases emissions by an additional 38%.^2,5 Utilizing the lowest-carbon energy sources available globally—such as those with an emission factor of approximately 30 g CO₂/kWh found in regions like Quebec, Norway, Sweden, France, and Iceland—could reduce greenhouse gas emissions to below 1 kg CO₂/kg Mg, based on the total energy consumption required.⁵

The greenhouse gas emissions from the thermal reduction processes are largely due to the production of the reductant, as well as the calcination and reduction processes.¹ In the case of the Pidgeon process, where the reductant is ferrosilicon (FeSi), these three process steps account for nearly 90% of the total emissions.^1,2 The majority of Pidgeon processes reported in the IMA’s LCAs utilize either natural gas or coke oven gases to fuel the calcination and reduction processes, resulting in increased greenhouse gas emissions.

Similar to the electrolytic process, thermal reduction processes could be converted to low-carbon electric energy sources to significantly reduce the greenhouse gas emissions of the Pidgeon process. Notwithstanding the reductant production, the greenhouse gas emissions could be reduced to below 1 kg CO₂/kg Mg for these processes.² However, the production of the FeSi reductant results in emissions reported to be over 10 kg CO₂/kg Mg.² Several researchers have reported that the FeSi can be substituted with aluminum,^6,7 reducing both the greenhouse gas emissions of the reductant and the energy required during the thermic reduction processes.

Magnesium Component Manufacturing

The most common application of magnesium products is in high-pressure die castings for the automotive industry due to their advantageous physical properties including low density, good castability, and excellent strength-to-weight ratio. The high-pressure die casting process involves injecting molten magnesium into a steel mold at high pressure and high velocities. Rapid filling and solidification enable fast cycle times and high production rates for complex thin-walled components.

Molten magnesium requires an inert atmosphere to prevent oxidation and contamination, typically by using a protective gas, such as sulfur hexafluoride (SF₆), sulfur dioxide (SO₂), or fluorinated ketones.⁸ Following die casting, the component can be processed through various secondary processing including machining, assembly, and finishing, dependent upon the application.

The greenhouse gas emissions from magnesium die casting primarily arise from the energy required for the melting and holding of molten magnesium and powering the die casting machine, as well as from the use of protective cover gases.¹ The furnace for melting magnesium can be powered by either electricity or natural gas, while the die casting machine is powered electrically. The energy required to produce a magnesium die casting, including a simple deburring secondary process, ranges from 1.8 to 2 kWh/kg of casting, depending on the method used to power the furnace.¹

Again, if these processes were converted to low-carbon electric energy sources, greenhouse gas emissions from the die casting process could range from 0.05 to 0.16 kg CO₂/kg Mg cast. This represents a significant reduction compared to the 0.6 kg CO₂/kg Mg cast calculated in the IMA’s first LCA,¹ which was based on the greenhouse warming potential of the electrical grid at the time of publication.

Greenhouse gas emissions from protective cover gases depend on the flow rate, concentration, and type of cover gas, as well as the carrier gas used to deliver it. The concentration of the cover gas is typically below 1%, with the carrier gas consisting of some mixture of CO₂, air, argon, or nitrogen.⁹ The global warming potential of cover gases ranges significantly, with SF₆ having one of the highest CO₂ equivalents of any greenhouse gas. The usage rate of cover gases ranges from 1 to 2 g/kg Mg processed, while the usage rate of the carrier gas CO₂ is approximately 6.4 g/kg Mg processed. Most cover gases are applied using a mixture of dry air and CO₂ (Figure 2).⁹

Based on estimated usage rates and global warming potentials, greenhouse gas emissions for magnesium processing range from approximately 10–20 g CO₂/kg Mg processed for low global warming potential cover gases to several kilograms of CO₂ per kilogram of magnesium cast when using SF₆.^1,9

Magnesium Recycling

Magnesium recycling can be considered as either process scrap or end-of-life recycling. Process scrap refers to material generated during manufacture of magnesium components, primarily at die casting facilities. This includes scrap parts, gate and runner systems, and off-cuts, among others, which are typically processed on-site at die casting facilities or at dedicated recycling plants (Figure 3). The remelting and re-alloying of sorted process scrap to meet specifications requires low energy input, with energy consumption of approximately 2–3 kWh/kg Mg processed. This results in greenhouse gas emissions of less than 1 kg/kg of recycled magnesium produced. The use of low carbon energy could even further reduce these greenhouse gas emissions.²

The recycling rate of post-consumer scrap is low due to contamination and market conditions. The automotive sector is the largest user of magnesium components, where post-consumer recycling involves separating metallic scrap into ferrous and non-ferrous categories. Typically, non-ferrous scrap is recycled into secondary aluminum alloys. According to a recent study on critical materials, over 50% of functional magnesium is lost to non-functional recycling into aluminum alloys or is disposed of.¹⁰

Decarbonization and circular economy initiatives can accelerate post-consumer scrap recycling for magnesium. Achieving this will require advancements in scrap sorting technologies to enable more efficient separation and recovery processes. These improvements could reduce the reliance on primary magnesium production and drive further growth of magnesium applications.

Future Production

The potential for low carbon magnesium production and processing can be achieved through several key improvements. These include improving primary production through adopting low carbon energy sources, development of electrolytic processes or alternative thermal reductants, eliminating SF₆ cover gas, and enhancing post-consumer scrap recycling initiatives. With these improvements, it is conceivable to achieve production and processing of magnesium die castings with less than 1 kg CO₂/kg of magnesium product.

References

1. Ehrenberger, S., “Life Cycle Assessment of Magnesium Components in Vehicle Construction,” IMA, May 8, 2013.
2. Ehrenberger, S., “Carbon Footprint of Magnesium Production and its Use in Transport Applications,” IMA, October 30, 2020.
3. Descouens, Didier, “Dolomite, Azcárate Quarry, Navarre, Spain,” Wikimedia Commons.
4. St. John, James, “Magnesite piece,” Wikimedia Commons.
5. “Life Cycle Environmental Data for Production of Magnesium Diecastings,” Hydro Magnesium, 1998.
6. Palumbo, A., “Evolution of Primary Magnesium Metal Production Leads to the Aluminothermic Reduction Process,” Light Metal Age, October 2023, pp. 54–59.
7. Bugdayci, M., et al., “Effect of Reductant Type on the Metallothermic Magnesium Production Process,” High Temperature Materials and Processes, Vol. 37, No. 1, 2018, pp. 1–8.
8. Milbrath, D.S., “Development of 3M Novec 612 Magnesium Protection Fluid as a Substitute for SF6 over Molten Magnesium,” Proceedings of the 2^nd International Conference on SF₆ and the Environment, San Diego, CA, November 21–22, 2002.
9. Trannell, G., et al., “Alternatives to SF6/SO2 for Magnesium Melt Protection – Final Report of the IMA-SINTEF Collaboration Project,” SINTEF, 2004.
10.  T.E. Graedel, et al., “Alloy information helps prioritize material criticality lists,” Nature Communications, Vol. 13, Article 150, January 10, 2022.

Editor’s Note: This article first appeared in the February 2025 issue of Light Metal Age. To receive the current issue, please subscribe.