*Magnesium Alloys for Sustainable Weight-Saving Approach: A Brief Market Overview, New… DOI: http://dx.doi.org/10.5772/intechopen.102777*

temperatures from 680 to 750°C to develop the basic reaction: MgCl2 → Mg (liquid metal) + Cl2 (gas). While the Dow process was starting and ramping up US national production, Germans continued investigating carbo-chlorination of magnesite to produce liquid anhydrous magnesium chloride. During those years, when the second peak of magnesium demand rose, Canadian scientist Lloyd Montgomery Pidgeon developed the thermal process for reducing magnesium oxide with silicon in externally heated retorts. Silicon is generally obtained by ferrosilicon ores, and it is produced in an arc furnace, mixed with calcined dolomite, and then briquetted. The briquettes are placed in a retort and heated to extract magnesium vapors condensed at the cold end of the retort with a relatively small diameter. The process is a batch process. It requires metal to be removed from the condensers, slag to be evacuated as a solid, and finally, it is possible to recharge the retort. Thus, the Pidgeon process has reduced the productivity of magnesium per day compared to big electrolytic cells plants. We would simplify the basic reaction as: 2CaO + 2MgO + Si → 2Mg + Ca2SiO4. It is an endothermic reaction, and a large amount of heat must be applied to initiate it and continue.

Therefore, the Pidgeon main environmental problem is the combustible used for firing furnace; oil or gas are commonly used for the scope. Former literature ascribed to oil firing the high environmental impact of polluting emissions, ranging from 37 to 47 kgCO2eq/kg of Mg extracted [22].

Another thermal process, the Bolzano process, like the Pidgeon process, employs the dolomite-ferrosilicon briquettes. Briquettes are stacked on a unique charge support system through which internal electric heating is conducted to the charge. In that case, most carbon emissions are drawn by the indirect carbon emissions of the energy mix used. Depending on the electricity carbon footprint on a local base, the Bolzano process ranges from 13 to 33 kgCO2eq/kg of Mg depending on the local electricity share produced by hydropower [29].

In brief, we can summarize. On the one hand, the Pidgeon process advantage consists of low investments to recoup, fewer constraints on the minimum size to be profitable, short time for facility construction, equipment installation, and plant ramp-up, flexible production. On the other hand, it suffers from low productivity, high labor requirement, and high energy consumption.

But to reply to the big question: Are the environmental concerns about magnesium extractive processes still valid today? It is necessary to ponder data from the most recent life cycle assessment studies committed to an in-depth investigation of magnesium green ability. In 2013, the International Magnesium Association (IMA) published the study "Life Cycle Assessment (LCA) of Magnesium Components in Vehicle Construction" which analyzed the entire life cycle of magnesium components for transport applications [30]. The study addressed primary magnesium production, alloying, component production, use phase, and the end-of-life of magnesium components, particularly for passenger vehicles comparing differences in emissions among Pidgeon process employed in small factories during the first decade of the 2000s with the most recent Pidgeon process practiced in larger plants. The worst numbers in former LCA studies before 2011 addressed massive emissions from small Pidgeon process plants developed in China from the 1990s to 2011. Pollution emissions from small plants significantly decreased following the imposed shutdown of several high pollutant firms, including small magnesium plants, ordered by the Chinese Government 1 month before starting the Beijing Olympic Games to improve air quality. Small factories in the primary magnesium business would have targeted more stringent environmental prescriptions before they could resume production,

but several small factories had not restarted production. Survived small plants restarted, at higher operating costs, improving the energy Efficiency with substitution of coal by gaseous fuels, with more efficient re-use of waste heat, and installing additional air treatment equipment.

Therefore, the 2013 LCA analysis published by the International Magnesium Association downscaled the overall average emissions from the Pidgeon process under the improvements mentioned above to 28 kg CO2eq, including all upstream processes [30]. It is worth noticing the magnesium production plant located in Brazil uses a silicothermic process, a modified type of the Bolzano Process. It targeted an excellent result of 10.1 kgCO2eq/kg magnesium.

Alternatively to the Pidgeon process, the big electrolytic plants could have a meager environmental impact, depending on the primary energy source. The Dead Sea Magnesium plant, which produces magnesium from the Dead Sea evaporite deposits in Israel, uses natural gas as an energy supply. The global warming potential of this process is accounted for 17.8 kg CO2eq/kg Mg [30]. As in this electrolysis plant, two main by-products are produced, liquefied chlorine (Cl2) and KCl-rich salt. They can have a wide range of potential uses; thus, they are used as raw materials for other sectors. Credits for their re-use, therefore, contribute to decreasing the global warming potential to 14.0 kg CO2eq/kg magnesium [30].

Since 2017 a new electrolysis plant with a capacity of 100,000 metric tons per annum has been operating in China by the Qinghai Salt Lake Magnesium Co. (QSLM). The QSLM electrolytic magnesium smelter is located at Golmud in Qinghai Province. This process produces pure magnesium from magnesium chloride (MgCl2) brine, an adjacent potash production waste product. The smelter produces low CO2 embedded magnesium metal thanks to energy power used for the complex supplied from regional hydro facilities (75%), solar (9%), and wind, as well as a local thermal power station. With support from the Qinghai Provincial Government and the national Government in Beijing, QSLM has plans to expand the production of pure magnesium alloys from current levels to 150,000 mtpa soon and then to 450,000 mtpa. Adjacent to the electrolytic magnesium smelter, Magontec has constructed a new primary magnesium alloy cast house facility with an output capacity of 60,000 metric tons per annum that will take pure liquid magnesium from the adjacent smelter. Magontec's plant benefits the QSLM's energy supply of 75% hydro and nearly 10% solar. The overall greenhouse gas emissions of the electrolysis amount to 8.5 kg CO2eq/kg magnesium. Apart from pure magnesium, the electrolysis of magnesium chloride produces gaseous chlorine. The amount of chlorine produced cannot finally be predicted at this stage of the project, but a chlorine yield of around 2.5 kg per kg of magnesium can be assumed. This by-product is used as feedstock for the nearby PVC plant. Producing 2.5 kg of chlorine usually leads to greenhouse gas emissions of about 3.2 kg CO2eq [30]. Thus, crediting these emissions, which the magnesium electrolysis has saved, leads to overall emissions of 5.3 kg CO2eq/kg of magnesium ingot [30]. The Qinghai plant has not reached its total capacity but is still ramp-up.

It is a fact that government policies of the country in which plants operate play an essential role in the environmental impact of magnesium. The national electricity mix used for plant operations, disposal, and recycling routes and the grade of technical solution development drastically reduce electrolytic routes' effects. Under the updated LCA data, the following **Table 4** recalculates the GWP for the body panel case study (refer to **Table 3**). GWP for the three options refers to average updated data published in [30]. Finally, since GWP are expressed per unit mass of material used, it is necessary to consider the actual usage of light material for the specific application.

*Magnesium Alloys for Sustainable Weight-Saving Approach: A Brief Market Overview, New… DOI: http://dx.doi.org/10.5772/intechopen.102777*


*2 Average value for electrolytic process powered by high share renewable energy (QSLM plant) [30].*

*3 Average value for Pidgeon process revised [30].*

### **Table 4.**

*Recalculated GWP data for comparative scenarios in manufacturing a lightweight outdoor body panel with light metal alloys.*

**Table 4** estimates the kgCO2eq emitted by aluminum and magnesium solution for substituting each kg of steel in the inner door panel for the same function, at equivalent (or higher) stiffness, and equivalent (or higher) denting capability. The calculation method follows:

(1)

The last line of **Table 2** shows the recalculated GWP for aluminum and magnesium light solutions to the "steel parity" calculated as:

(2)

The (2) represents the GWP of metal alloys give parity to body panel made of steel at equal (or higher, as for magnesium solution) stiffness and load capability.
