Life Cycle Assessment of Metallurgical Grade Silicon Comparing Charge Mixtures and Yields

Metallurgical-grade silicon is considered a critical raw material in Europe due to its supply risk and wide range of applications in technologies needed to transition to a low-carbon society. It is produced by reducing SiO2 in quartz using carbon, traditionally a mix of fossil and biocarbon. As part of the transition to a low-carbon future, fossil carbon is being replaced by biocarbon. While several studies have investigated the environmental impact of silicon production concerning the conventional carbon mix, little information is found on the consequences of switching to a purely biobased mix. A parametric life cycle assessment has been conducted to investigate the impact of going from a traditional reductant charge mix to a purely biobased carbon mix, and through mass and energy balance, the consequences for the cradle-to-gate life cycle inventory. The inventory analysis was performed as a mass and energy balance of metallurgical grade silicon production. The study was conducted for two charge mixes: one representing the traditional charge mix, and one based on biocarbon only. Three different yields for each mix were examined to investigate how silicon yield affects performance, giving six scenarios in total. The results showed that a completely biobased carbon mix reduced the impact for 10 of 18 midpoint impact indicators and all three endpoint indicators, but only when the biocarbon considered is sustainably sourced. The contribution analysis showed that in addition to carefully sourcing biocarbon raw materials of sustainable origin, reducing direct emissions through off-gas treatment is another option to decrease the environmental impact of biobased production further.