One of extraction processes of mining, the pyrometallurgical
processes, causes emission of sulfur dioxide into the atmosphere. If it is not controlled, it will lead to acid deposition. Acid deposition has a destructive and unhealthy impact on the environment. For example, in water, acid deposition kills fish, with acid that can be directly deposited into bodies of water, or the acid can leach heavy metals in the water and release harmful metals into the aquatic environment, and after marine life absorb these metals, they die, or become sterile, or produce mutated offspring. Another impact of acid deposition is the destruction of artifacts, where limestone or marble are extremely vulnerable to corrosion from acid.However, this can be controlled. Emission of particulates and sulfur dioxide can be a small percentage of the uncontrolled values. The energy requirements of pyrometallurgy are largely supplied by the oxidation of the sulphur content of the ore and are much less than those necessary for hydrometallurgy. There is an evidence that innovative design can reduce process-gas volumes and fugitive emissions and can lead not only to less environmental stress but also to more efficient operation.
Four practices that would achieve these ends were recommended by Kellogg (1981) and Environment Canada, which are:
1. Maximum use of oxygen in place of air, for, more complete oxidation
2. Use of intensive reactor design, such as flash, injection and cyclone smelting
3. Full use of fuel value of sulphide concentrates as an energy supply for the process
4. Designs of continuous processes to replace batch processes
The large amount of heat generated by pyrometallurgical processes is often vented to the atmosphere; however, some operations are able to recover much of the heat from smelting (Snelgrove and Taylor 1989), as well as from sulphuric acid plants (Bond 1989). The heat is usually used to make steam and to generate electricity.
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Another problem caused by mining is the emission to air of dust and metals/metal compounds and of sulfur dioxide if there are roasting and smelting sulfide concentrates or using sulfur fuels or other materials. The pyrometallurgical processes are potential sources of dust and metals from furnaces, reactors and the transfer of molten metal.
Primary measures which may assist in reducing the formation and release of pollutant emissions include:
1. Use of Hydrometallurgical Processes:
Use of hydrometallurgical processes rather than pyrometallurgical processes where possible, as a significant means to preventing emissions. Closed-loop electrolysis plants will contribute to prevention of pollution.
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2. Quality Control of (Scrap) Feed Material:
The presence of oils, plastics and chlorine compounds in scrap feed materials should be avoided to reduce the generation of PCDD/PCDF during incomplete combustion or by de-novo synthesis. Feed material should be classified according to composition and possible contaminants. Selection and sorting to prevent the addition of material that is contaminated with organic matter or precursors can reduce the potential for PCDD/PCDF formation. Storage, handling and pre-treatment techniques will be determined by feed size distribution and contamination.
Methods to be considered are:
o Oil removal from feed (e.g. thermal de-coating and de-oiling processes followed by afterburning to destroy any organic material in the off-gas)
o Use of milling and grinding techniques with good dust extraction and abatement. The resulting particles can be treated to recover valuable metals using density or pneumatic separation.
o Elimination of plastic by stripping cable insulation (e.g. possible cryogenic techniques to make plastics friable and easily separable)
o Sufficient blending of material to provide a homogenous feed in order to promote steady-state conditions.
3. Effective Process Control:
Process control systems should be utilized to maintain process stability and operate at parameter levels that will contribute to the minimization of PCDD/PCDF generation, such as maintaining furnace temperature above 850 °C to destroy PCDD/PCDF. Ideally, PCDD/DF emissions would be monitored continuously to ensure reduced releases. Continuous emissions sampling of PCDD/PCDF has been demonstrated for some sectors (e.g. waste incineration), but research is still developing in this field. In the absence of continuous PCDD/PCDF monitoring, other variables such as temperature, residence time, gas components and fume collection damper controls should be continuously monitored and maintained to establish optimum operating conditions for the reduction of PCDD/PCDF.
4. Use flash smelting technology
The most effective pollution prevention option is to choose a process that entails lower energy usage and lower emissions. Where pyrometallurgical techniques are used, use of flash smelting technology rather than older technologies (e.g., roasters, blast furnace, etc.) is a significant means to reducing energy use and to reducing emissions. Flash smelting will also result in high concentration of sulphur dioxide in the off-gas stream, which would permit the efficient fixation or recovery of sulphur dioxide prior to off-gas venting.
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Secondary measures which may assist in reducing the formation and release of pollutant emissions include:
1. High Efficiency Gas Cleaning and Conversion of SO2 to Sulphuric Acid
For SO2 rich off-gases (typically 5% or greater) generated by pyrometallurgical processing of sulphide ores or concentrates, high efficiency pre-cleaning of off-gases followed by conversion of SO2 to sulphuric acid are together considered BAT for this type of source. Emission concentrations of PCDD/PCDF with use of this combination of techniques are use of high efficiency gas cleaning and conversion of SO2 to sulphuric acid are < 5 pg TEQ/m3.
2. Fume and Gas Collection
Air emissions should be controlled at all stages of the process, from material handling, smelting and material transfer points, to control the emission of PCDD/PCDF.
3. High Efficiency Dust Removal
Very high efficiency dust removal techniques should be employed to remove dust and metal compounds so as to reduce PCDD/PCDF emissions, for example, ceramic filters, high efficiency fabric filters or the gas cleaning train prior to a sulphuric acid plant.
Returned/collected dust from dust control equipment should be treated in high temperature furnaces to destroy PCDD/PCDF and recover metals.
Fabric filter operations should be constantly monitored by devices to detect bag failure.
Preference should be given to fabric filters over wet scrubbers, wet electrostatic precipitators (ESPs), or hot ESPs for dust control.
Dust that is captured but not recycled will need to be disposed of in a secure landfill or other acceptable manner.
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