(MERAF) for the Base Metals Smelting Sector - CCME

sulphur dioxide in the off-gases than batch converting and the off-gases are
suitable for acid production/sulphur fixation 32 .
Slag generated during the converting process is often returned to the smelter to
recover metals.
2.4.5. Fire Refining
Air emissions of nitrogen oxides, sulphur dioxide, particulate matter and metals
arise from the fire-refining process.
The sulphur dioxide exiting the anode furnace is filtered to remove any dust. The
dust is recycled back into the smelter and the gas is fed into the sulphuric acid
plant where it is converted into sulphuric acid.
Fugitive emissions are generated during the charging and discharging of the
anode furnaces. These should be collected with a secondary hood or an
enclosure around the furnace with the minimum possible opening.
Slag produced from the anode furnace is minor in amount and can be recycled
within the plant.
2.4.6. Electrorefining
Electrolytic refining does not produce emissions to atmosphere unless the
associated sulphuric acid tanks are open to the atmosphere. However, spent
electrolyte and wash water contain significant quantities of metal compounds in
solution and are treated before discharge to water. The metal compounds that
are deposited at the bottom of the electrolytic cell during the electrorefining
process (i.e., the impurities), form what is known as an anode slime. The slimes
are collected and processed to extract precious metals such as silver, gold and
tellurium.
2.4.7. Carbonyl Refining
Carbonyl refining produces gas bleed streams that contain waste nickel carbonyl,
a highly toxic substance. Incinerators should be used to convert the nickel
carbonyl to nickel oxide and carbon dioxide. Particulate matter may be released
from the transfer of nickel oxide concentrate, from the drying of solids recovered
from the aqueous effluent and from local exhaust ventilation gases. Electrostatic
precipitators are typically used for dust abatement since inlet temperatures are
too high for fabric filters. The collected dust may be sluiced with water on
discharge and should be dried and recovered for recycling.
2.4.8. Leaching
A significant environmental issue arising from the leaching process is the
generation of ferrite residues. These iron-based residues contain various
32 European Integrated Pollution Prevention and Control Bureau (EIPPCB), Reference Document on Best
Available Techniques in the Non Ferrous Metals Industries, Spain, May 2000. http://eippcb.jrc.es
19