Nodularisation off-gas collection and dedusting

The type of exhaust collection applied depends on the technique used for nodularisation (see 2.4.12.4). Depending on the technique, significant amounts of MgO can be released as a white smoke. Properties of the various nodularisation techniques are given in Table 3.20. In general techniques with a higher take-up efficiency result in lower emissions. This is the case for the in-mould process, where the nodularisation takes place during pouring. 

For in-crucible nodularisation, techniques using a lid or cover result in a significant reduction of the emission. 

If the nodularisation off-gas is not captured, this could result in the melting shop filling-up with a white smoke, and MgO dust drop out. Exhausting without filtration can result in a visible emission from the foundry. A large volume of visible particulate can be generated in a relatively short period of time, but usually only lasts for a short duration (5 to 10 minutes for each batch treated). 

In-crucible nodularisation may be performed at a specific stand or location in the melting shop. The crucible with the molten metal is brought to this point after pouring, but before taking it to the casting furnace or station. This allows the installation of a fixed hood for exhaust capture. 

The collection of the MgO fume is hampered by the fact that the gases are very hot and that the intensive Mg reaction causes sparks. Due to the high upward velocity and temperature large quantities of surrounding air need to be extracted as well. This requires a large sized, and high cost, installation. 

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