2.5.1 Environmental Effects of the Extractive Metallurgy Industry
Beginning from the point where the ore is mined from the earth down to the stage when the gold isfinally purified by smelting, the processes involved impacting the environment in ways that are not always favorable. The effect of the activities leading to extraction of gold may be grouped into two, namely, the impact of mechanical activities like removal of earth during the extraction of ore-bearing rocks and the impact of chemicals and tailings during the extraction of gold from the ore. Specifically, the effects of these activities include:
i. Erosion of exposed hillsides, mine dumps, tailings dams and resultant siltation of drainages, creeks and rivers significantly impact surrounding areas and communities.
ii. Destruction of wilderness and attendant disturbance of ecosystems and loss of biodiversity.
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iii. The dumping of the runoff in surface waters or in forests poses an equal danger to the environment.
iv. In farming areas, activities of mining may disturb or destroy productive grazing and croplands.
v. Mining may produce noise pollution, dust pollution and visual pollution, which are nuisance and harmful to nearby settlements and workers.
vi. Contamination of the area surrounding mines due to the potentially damaging compounds and metals removed from the ground with the ore is always a threat.
vii. Contamination of soil, groundwater and surface water by chemicals employed or exposed during the extraction of gold. The contamination arising from the leakage of chemicals affects the environment and health of the local popula- tion. The result can be unnaturally high concentrations of some chemicals such as arsenic, sulfuric acid and mercury over a significant area of surface or subsurface.
In view of the foregoing, after the ore has been mined and the gold extracted, a number of outstanding issues remain that must be dealt with before closure of the mine. Among these are how to treat the degraded land, how to treat the cyanide-bearing wastewater before discharge into the environment or recycled (where necessary), how to treat the ore tailings (spent ore) before discharge and in the case of heap leach processes what to do with the residual heap. Necessarily, these issues must be addressed because: (i) a degraded land if not reclaimed could mean a lost land or a long period of fallow before land can be reused. Also, unclaimed land can pose danger to those using the land as huge manholes could become traps to those trespassing the land; (ii) cyanide contaminated soils and water post danger to flora, fauna and residents of affected communities because cyanide is highly poisonous if taken in; (iii) Government regulation requires that standards be met in land state and cyanide remnants in soils and water before mines can be closed.
2.5.1.1 Reclaiming Degraded Land
Land rehabilitation attempts to return land affected by mining to some degree of its former state. Efforts employed to restore land degraded by mining activities include:
i. Waste dumps areflattened and stabilized against erosion.
ii. If the ore contains sulfides, it is usually covered with a layer of clay to prevent access of rain and oxygen from the air, which can oxidize the sulfides to produce sulfuric acid.
iii. Top soil is used to cover landfills and later vegetation is planted on it.
iv. Vegetation cover of dumps is protected from denudation by fencing them against intrusion by livestock.
v. Dams and tailings arefirst left to dry up. They are thenfilled with waste rock, followed by clay if necessary, and then soil. Finally, it is planted to stabilize it with vegetation.
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2.5.1.2 Treating Cyanide Contamination
Cyanide contamination of soil and water is another key environmental problem that the gold extraction industry must deal with. Treating cyanide contamination requires a consideration of many issues, like what to use to perform the treatment.
This depends on the form of cyanide, the nature of the medium in which the cyanide is, availability of the requisite materials to treat the cyanide, the economy of the process to be adopted to do the treatment andfinally government regulation.
Other considerations are the method of treatment to be adopted, how long the treatment will last, the precaution to be adopted during treatment and the means of assessment of the efficiency of treatment. An overview of methods for treating cyanide is presented in Table2.7.
Measurement of cyanide concentration in a medium depends on the form of the cyanide. Cyanide is usually measured in three forms. These are free cyanide, which is the cyanide anion (CN–) present in solutions and usually bonded to cations such as Na, K, Ca or Mg or HCN. In general, free cyanide is difficult to measure. The second is in the form of weak acid cyanide (WAD), which are cyanide complexes that upon treatment with a weak acid (pH = 4.5) gives off HCN. It includes free cyanide, simple cyanide and weak cyanide complexes of zinc, cadmium, silver, copper and nickel. The third form is total cyanide, which includes all forms of cyanide (free and complex) including iron, gold, cobalt and platinum complexes.
Not all methods can measure each of these forms of the cyanide. Indeed, most measurement techniques are unable to accurately determine the cyanide concen- tration in a medium.
2.5.2 Safety
Subsurface mining has always had issues to contend with. Prominent safety con- cerns in the mining industry include:
i. Exposure to harmful gases, heat and dust inside subsurface mines, which are the result of poor ventilation, can lead to harmful physiological effects, including death.
ii. Gases in mines can also poison the workers or displace the oxygen in the mine, causing asphyxiation. For this reason, some countries require that mine workers use gas detection equipment in groups of miners. The equipment must be able to detect common gases such as CO, O2, H2S and the per- centage Lower Explosive Limit of these gases.
iii. High temperatures and humidity may result in heat-related illnesses, including heat stroke, which can be fatal.
iv. Dusts in mines can cause lung problems such as silicosis, asbestosis and pneumoconiosis.
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Table 2.7 Methods of treatment of cyanide
Method Description of process
Rinsing Rinsing is applied to cyanide found in heap leaches. Freshwater or recycled (treated) water is sprinkled over the heap over a time to dissolve or extract cyanide in the heap. The rinsate is subsequently treated. Chemicals may be dissolved in the water to oxidize and facilitate the extraction of cyanide from the heap. The water may contain dissolved chemicals that could oxidize the cyanide and facilitate its extraction from the heap. In this case, further treatment of the rinsate may not be necessary.
Sulfur process Cyanide (CN–) in solution is oxidized into cyanate (CNO–) using sulfur dioxide (INCO Method) or ferrous sulfate (Noranda Method) in the presence of copper ions. This later method is suited for ore with a significant concentration of arsenic and antimony. The former method is slow at low temperatures and does not remove cyanate, ammonia or thiocyanate and does not sufficiently remove toxic metals.
Alkaline chlorination Chlorine or a hypochlorite solution is used to oxidize the cyanide to cyanate. The method is applicable to clear wastewaters and slurries.
The pH is maintained in the alkaline range using lime. Precipitated metals are removed in a clarifier before discharging the wastewater.
The method does not remove iron cyanides. Also, it leaves chloramines and free chlorine in the solution that is toxic tofish.
Hydrogen Peroxide process
Hydrogen peroxide is used to oxidize cyanide to cyanate in the presence of copper ions. The method is applicable to wastewater.
When applied to slurries, reagent requirements increase. Ammonia, which is a by-product of the process, is toxic tofish. The method requires specially designed equipment as hydrogen peroxide is a strong oxidizer and gives rise to explosion andfire in the presence of combustible materials. Also, it is expensive and, therefore, has cost implications.
AVR Cyanide recovery process
The method is applicable to barren solutions. Sulfuric acid is added to the cyanide-containing solution to generate HCN, which is then absorbed in NaOH to generate NaCN. The NaCN can then be recycled.
Natural degradation When left unattended natural processes combine to degrade cyanide.
The processes include microbial generation of ammonia and cyanate in soil; volatilization of cyanide in solution after absorbing CO2and SO2from atmosphere and consequent formation of acid; cyanide hydrolysis in soils; anaerobic degradation of cyanide; complexation with stable metal ions. Efficiency of cyanide degradation may be slower at the bottom of ponds and in the interior of heaps.
Biological treatment Microbial agents, e.g.pseudomonas pseudoalcaligenes,are applied to treat the cyanide. The microbes transform cyanide,first, to ammonia and then to harmless nitrates. The method can be used to treat barren solutions, cyanide wastewater and heap leach pad solutions.
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v. Since mining entails removing dirt and rock from its natural location creating large empty pits, rooms and tunnels, cave-ins are a major concern within mines as they result in accidents.
vi. The presence of heavy equipment in confined spaces also poses a risk to miners, in spite of modern improvements to safety practices.
vii. Without sufficient knowledge and proper training anyone who attempts to explore abandoned mines face the threat of danger. Stagnant water in abandoned mines may hide deep pits and trap gases. These pose a significant hazard to intruders.
viii. Entrance to an old mine can be very dangerous as surrounding rock and earth may have been weakened by the weather.
ix. Old mine workings and caves often lack oxygen in the air making them hazardous. This condition in mines is called blackdamp. Again, old mines can contain deadly gases making them very dangerous.
x. The noise made by the heavy equipment used in the mines combined with the confined workspace of underground mines can cause hearing loss in miners.
In Ghana, the Mineral Commission through its Mining Inspectorate Division has the responsibility to seeing that players in the industry comply with safety regu- lations. Among others, the Commission is responsible for:
i. Enforcing compliance with occupational health and safety regulations.
ii. Inspection of areas of mining operations to ascertain whether any nuisance is created in the area by the mineral operation.
iii. Control of mines, mills and other mineral treatment plants, ensuring that wasteful mining or ore treatment is not used in these operations.
iv. Examination and enquiries into mining-related accidents and incidents.
v. Compilation of mine accidents statistics and dissemination of information thereon.
vi. Technical inspection, control and enforcement of technical regulations for mining.
vii. Inspection of distribution, storage and handling of explosives, enforcement of Explosives Regulations both within and outside the mining sector.
viii. Training of personnel handling explosives.
ix. Certification of mine management, certification for blasting and operations of winding engines.
x. Technical and Health and Safety Supervision of small-scale diamond mining.
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