Estimating direct CO2 and CO emission factors for industrial rare earth metal electrolysis

Rare earth (RE) metals are now being widely applied in new material industries with rapidly increasing production. However, there is limited research on greenhouse gas emission factors from RE metals production by electrolysis and currently no method exists to account for this industry's CO2 emissions in the '2006 IPCC Guidelines for National Greenhouse Gas Inventories'. This study employed two independent methods to determine direct carbon emission factors for industrial rare earth metals production by molten-salt electrolysis: continuous emissions monitoring by FTIR and time-integrated sampling with offline lab analysis, both measuring CO2 and CO concentrations in the exhaust gases from Pr-Nd, Dy-Fe and La potlines in three companies in China. The study confirmed that CO2 contributes >97% of the total carbon emission factor. Direct carbon emissions per tonne RE metal electrolysis is equivalent to one tenth to a half of the emission factor for aluminum production (due to much lighter molar mass of Al) but is similar on a mole basis (carbon emissions per mole metal). Emission factors vary with the type of rare earth metal or alloy produced and from one facility to another, ranging from 165.0 to 672.4 kg/t-RE metal for CO2, 3.00 to 8.23 kg/t-RE metal for CO and 46.4 to 186.8 kg/t-RE metal for total carbon. Direct CO2 and CO emission factors from the two exhaust gas monitoring methods agreed well considering the uncertainties.