Environmental Code of Practice for base metals smelters and refineries: chapter 3

3: Environmental Concerns

This section presents an overview of environmental concerns related to the major activities and processes used in the smelting and refining of base metals. Typical preventative and control measures taken in modern systems are indicated.

3.1 Roasting

Sulphur dioxide and particulate matter are the principal air contaminants generated during roasting of the concentrates. The sulphur dioxide can be recovered at on-site sulphuric acid plants if the type of roaster used generates a high enough concentration of sulphur dioxide in the off-gas. Otherwise, the roaster off-gases are cleaned in electrostatic precipitators and then released to the atmosphere via a stack. Fluid bed roasters, waste heat boilers, cyclones, and scrubbers may also be used to treat off-gases. Metals that may be present in the particulate matter include copper and iron oxides, arsenic, cadmium, lead, mercury, and zinc.

3.2 Smelting

The major environmental concerns associated with smelting are energy consumption, releases of sulphur dioxide and particulate matter to air, and the generation of residues, such as slag and captured dust.

Bath smelting consumes more energy than alternative processes such as flash smelting. Flash smelting makes use of the autogenous reaction between sulphur and oxygen to fuel the smelting process, thus requiring less fuel and energy than bath smelting. Taking primary copper production as an example, bath smelting has energy requirements ranging from 35 to 47 gigajoules per tonne (GJ/tonne) of cathode copper produced.31 Flash smelting has been reported to have energy requirements of approximately 23 GJ/tonne of cathode copper produced, which is about half of the energy required for bath smelting.32

Releases of sulphur oxides, including sulphur dioxide, are a significant environmental concern for primary smelters. Sulphur in the concentrate feed that does not remain in the slag, matte, or bullion is oxidized to form sulphur dioxide. Smelter off-gases containing a sulphur dioxide concentration of 5-7% or higher can be used for the manufacture of sulphuric acid. Flash smelting generates higher percentages of sulphur dioxide in the off-gases compared with bath smelting, and both flash smelting and continuous processes allow for better collection of the off-gases, producing a consistent concentration of sulphur dioxide in the off-gas, which is ideal for the production of sulphuric acid.

Smelter off-gases also contain particulate matter, organics, and volatile metals, such as mercury.

Slag from the smelting process is also an environmental concern. In general, smelter slags may not contain a high enough concentration of the metal of value to be returned to the smelter. Slags are typically cleaned to recover the remaining metal of value and are then disposed of in landfills or tailings ponds. Cleaned slags have been used as aggregate in the construction industry or as an abrasive for sandblasting.

3.3 Converting

Off-gases from the converter require treatment to remove sulphur dioxide, dust or particulate matter, and fume prior to discharge to ambient air. Off-gases from batch converters are high in volume and low in sulphur dioxide. With these characteristics, the off-gas may be unsuitable as a feed for acid plants. Therefore, it is often conditioned to remove particulate matter and then vented to ambient air. However, continuous converting produces a more consistent and higher concentration of sulphur dioxide in the off-gases than batch converting, and the off-gases are suitable for acid production/sulphur fixation.33

Slag generated during the converting process is often returned to the smelter to recover metals.

3.4 Fire Refining

Air releases of nitrogen oxides, particulate matter, and metals arise from the fire-refining process.

Fugitive emissions are generated during the charging and discharging of the anode furnaces. These may be collected with a secondary hood or an enclosure around the furnace with a small opening.

Slag produced from the anode furnace is minor in amount and can be recycled within the plant.

3.5 Electrorefining

Electrolytic refining does not produce releases to the 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 anode slime. The slimes are collected and processed to extract precious metals, such as silver, gold, and tellurium.

3.6 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 may be dried and recovered for recycling.

3.7 Leaching

A significant environmental issue arising from the leaching process is the generation of ferrite residues. These iron-based residues contain various concentrations of heavy metals and present a risk to the environment by the gradual leaching of heavy metals from the residue material. Residues generated during the leaching process may be landfilled or stored in a secure site, stabilized to immobilize the metals, or sent to another process for recovery of remaining metals of value.

3.8 Electrowinning

As electrowinning takes place in tank houses open to the atmosphere, oxygen or other gases generated during the electrowinning process can entrain the acid or other solvent into the air.

3.9 Casting

Air releases of particulate matter and metals arise from the transfer of molten metal to the mould and from the cutting to length of the product with torches. Wastewater effluents are generated during the cooling and cleaning of the hot metal and can contain scale particles and oil. Wastewater is typically treated and reused or recycled. Solid waste is generated from the cutting of the metal but is minor in amount and is recycled within the plant.

3.10 Process Off-Gas Conditioning

Process off-gas conditioning generates collected dusts and sludges, which are either returned to production processes for recovery of metals or disposed of. The type of off-gas conditioning technology may also be of environmental concern. In some cases, there is a potential that the use of wet electrostatic precipitators and wet scrubbers could result in the cross-media transfer of pollutants.

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