China is currently the largest producer of magnesium worldwide (85% of the total) with an output of 0.64 Mtons in 2012; the Chinese produce their magnesium in batch using the Pidgeon process. The Pidgeon process heats briquettes containing dolomite ore (dolomite: MgO, CaO) and Ferrosilicon (FeSi) to high temperatures in a silicothermic reduction and then cools the evaporated magnesium to obtain magnesium metal.

The hot reaction zone is either gas-fired, coal-fired, or electrically heated in a furnace; the condensing section is water-cooled. The very high purity magnesium crowns produced by distillation are remelted and cast into ingots.

The US Department of Energy (DOE) Advanced Research Projects Agency - Energy (ARPA-E) noted, as part of its rationale for its Modern Electro/Thermochemical Advancements for Light-metal Systems (METALS) program (earlier post), that the energy requirements and CO₂ emissions of the Pidgeon process for magnesium are both very high; 102 kWh/kg and 37 kgCO₂/kg, respectively.

(US magnesium production uses an electrolytic process that requires approximately 43.6 kWh/kg and has emissions of 6.9 kgCO₂/kg, according to ARPA-E. Total capacity is 0.045 Mtons/year. The cost of producing Chinese-Pidgeon magnesium is $2.50/kg. The US Magnesium LLC cost, using a more environmentally sound approach, is approximately $3.31/kg.)

In their paper on the new microwave-driven Pidgeon process, the researchers observed that:

The lightweight nature of Mg, combined with its high energy density, suggests that the global consumption of this metal will increase drastically in the near future. … the impact of Mg metal production on climate change needs to be understood at a global scale and a new, energy-saving process for fabricating Mg ingots should be considered.

Chemical reactions performed under microwave irradiation often demonstrate high reaction rates and high selectivity, which allows for a more compact reactor and a more energy-efficient process than conventional heating. … Some researchers have also proposed smelting metal using microwave heating in order to achieve a decrease in CO₂ emissions. … In this study, we have investigated a new Pidgeon process, using microwaves instead of coal as the heat source, for producing Mg with lower energy inputs and a decrease in greenhouse gas emissions.

—Wada et al.

Normally, dolomite is a poor absorber of microwave energy and does not generate heat. A conventional Pidgeon briquette consisting of a homogeneous mixture of 80 wt.% calcined dolomite and 20 wt.% ferrosilicon thus cannot be heated well by microwave radiation.

To get around this, the researchers created briquettes with a microwave resonance structure to confine microwave energy within—i.e., an antenna effect. In other words, the briquettes themselves acted as antennae with a quarter length of the wavelength of microwaves and/or an integral multiple of the quarter length to capture electromagnetic waves. They achieved this by using an antenna made of ferrosilicon placed at the center of the briquette, with the ferrosilicon particles acting as a continuous conductor.

Schematic of the different antenna types and a plot of the measured temperatures as a function of microwave irradiation time. Samples (A), (B), and (C) contain a stack of five briquettes, using briquettes (a), (b), and (c), respectively (height of one-wavelength (66 mm). Sample (D) consists of five briquettes (b), stacked with zirconia beads between each two briquettes to a total height of 78 mm. Sample (E) contains five briquettes (b), stacked with four quartz glass wool separators to a total height of 86 mm. The measured temperature for each antenna type as a function of microwave irradiation time is shown on the right. Wada et al. Click to enlarge.

In a small-scale experimental reactor, 1g of magnesium metal was smelted successfully. To estimate the energy more accurately, a demonstration furnace about 5 times larger than the experimental furnace was produced and experiments were conducted, resulting in the successful smelting of about 7g of magnesium metal. This can reduce energy by 68.6% compared with the conventional method.

This success in saving energy for smelting magnesium metal has led to the possibility of this technique being developed and applied to the high temperature reduction process of oxides. In the future, through further development of this research, it will be applied to the smelting of other metal materials to save energy with steel, metals, materials, and chemistry, which have not advanced, and help reduce carbon dioxide emissions.

Resources

• Yuji Wada, Satoshi Fujii, Eiichi Suzuki, Masato M. Maitani, Shuntaro Tsubaki, Satoshi Chonan, Miho Fukui & Naomi Inazu (2017) “Smelting Magnesium Metal using a Microwave Pidgeon Method” Scientific Reports 7, Article number: 46512 doi: 10.1038/srep46512