Annex 24 Complementary information to source category 2i Magnesium Production
Overview of recent revisions
All PCDD/PCDF emission factors are confirmed for this source category. Some new emission factors are proposed for PCBs and HCB with a medium level of confidence.
Table III.24.1 PCB emission factors for source category 2i Magnesium Production
2i Magnesium Production Emission Factors (µg TEQ/t magnesium) Classification Air Water Land Product Residue 1 Production using MgO/C thermal treatment in Cl2 – no treatment on effluent, limited gas treatment 2 Production using MgO/C thermal treatment in Cl2 – comprehensive pollution control 0.7 3 Thermal reduction process 0.02
Table III.24.2 HCB emission factors for source category 2i Magnesium Production
2i Magnesium Production Emission Factors (µg/t magnesium) Classification Air Water Land Product Residue 1 Production using MgO/C thermal treatment in Cl2 – no treatment on effluent, limited gas treatment 2 Production using MgO/C thermal treatment in Cl2 – comprehensive pollution control 3 Thermal reduction process 800
Derivation of emission factors
Release to Air
Emission factors to air from the production of magnesium by using the chlorination electrolytic process are quite uncertain. PCDD/PCDF are formed and released from the chlorination furnace where magnesium oxide is converted into magnesium chloride. The following data are reported in the EU BREF document: 0.8 ng TEQ/Nm³ were found from chlorination off-gas treatment (EF = 12 μg TEQ/t); for the vent gases from chlorination, an emission factor of 28 μg TEQ/t was determined and concentrations in the hall from electrolysis and chlorination gave an emission factor of 13 μg TEQ/t (BREF 2009).
An emission factor of 250 μg TEQ/t of production is estimated for electrolytic processes, which do not have afterburners but use wet scrubbers (class 1). For processes with multi-stage wet scrubbers and afterburners an emission factor of 50 μg TEQ/t of production (class 2). This emission factor is confirmed by data from China where PCDD/PCDF concentrations ranging from 6.5 to 13.2 pg TEQ/Nm³ were measured on two plants. Using production data and gas flows reported in the paper, emission factors can be derived (6 and 23 µg TEQ/t). In 2010, an emission factor of 0.41 µg TEQ/t was reported by the same team (Nie et al. 2010).
For plants applying the thermal reduction process (class 3), an emission factor of 3 μg TEQ/t will be used (BREF 2009).
Emissions could be much higher if the gas treatment is limited or where a high PCDD/PCDF producing carbon source is used. Indeed, one of the Chinese sources mentioned above reported concentrations of 10.1 pg TEQ/Nm³ which, combined with an uncommon low production flow (1 kg/h), corresponds to a high emission factor (assumed to be higher than 500 µg TEQ/t).
Regarding air emissions of PCBs and HCB, emission factors were assessed in China and are available for installations falling under both classes 2 and 3 (Nie et al. 2010).
Release to Water
Releases to water will depend on the amount of PCDD/PCDF formed in the process, the efficiency of the scrubbing systems to remove PCDD/PCDF in gas streams and crucially on the treatment applied to the effluents.
There is insufficient information to estimate releases from processes other than those including a thermal treatment of MgO/coke in Cl2.
For processes fitted with comprehensive water treatment (including high efficiency solids removal), an emission factor is estimated based on releases reported from the Norwegian plant in the late 1990s or under 1 g TEQ per year. Prior to the installation of the water treatment system releases to water were estimated at 500 g TEQ per year and this is used to estimate an emission factor to be used where no treatment occurs.
An emission factor of 9,000 μg TEQ/t of Mg is used where direct discharge of the untreated effluent occurs. From European plants, an emission factor of 33 μg TEQ/t of Mg metal was reported (BREF 2009).
Release to Land
A release to land may occur where part of the water treatment involves release to a lagoon. Quantities are estimated in the residue section.
Release in Products
PCDD/PCDF levels in magnesium produced are expected to be negligible.
Release in Residues
Residues from scrubbing processes may be expected to contain PCDD/PCDF. A stage in the water treatment may include settling in a lagoon, which would constitute a release of the residue to land. To estimate the release from the electrolytic process, it can be assumed that 0.01 ton of PCDD/PCDF-containing sludge is generated in the water treatment plant (BREF 2009).
Very little information is available on the concentrations of PCDD/PCDF in residues from this process or the amounts of residue produced. Initial estimates only may be made.
It is assumed that where no water treatment is used, no PCDD/PCDF is found in residues (although some may arise from other parts of the process). So the emission factor is zero. Where comprehensive water treatment is applied it is assumed that the difference in the release to water will approximately equal the PCDD/PCDF captured and therefore be present in the residues. An emission factor of about 9,000 μg TEQ/t of production is given to make an initial estimate.