Acid Mine Drainage (AMD) problems are the most important environmental problems in sulphide mines especially mines which contain pyrite. Oxidation of sulphides in contact with air and water produces sulphuric acid which reduces the pH of water. The result of these reactions is increase in the solubility of heavy metals and consequently pollution of water resources. The Sarcheshmeh copper mine is one of the largest porphyry copper mines in the world which is located in south-eastern Iran. Chemical analysis of over than 600 water samples taken from streams and springs in the mining region and surrounding area of pit showed extensive changes in water quality and composition due to the passage of water through waste dumps and mining region. In this research the quality and composition of water in these two regions were analysed and discussed and the methods of water resources protection were also proposed. Key words: Sarcheshmeh, AMD, Copper, Mine, Water quality, Heavy metals
The Sarcheshmeh porphyry copper-molybdenum deposit with a reserve about 1200 Mt. is located in a mountainous region with 2600 m height above sea level in south-eastern Iran. The average grade of this ore body is about 0.7% copper and approximately 0.03% molybdenum. A large scale open pit mining was started up in 1974 by the National Iranian Copper Industries Company (NICICO). Presently Sarcheshmeh pit has an oval shape with 2300m long diameter and 1200m short diameter. The size of pit will be increased in next few years by executing the expansion plan.
GEOLOGY AND HYDROLOGY
The Sarcheshmeh copper deposit is located in central part of an elongated NW-SE mountain belt, which is principally composed of folded volcano-sedimentary complex. The geology of Sarcheshmeh deposit is complex, with widely varying rock types. Mineralization in this deposit is associated with a Late Tertiary granodiorite porphyry stock. The whole complex is criss-crossed by a series of post-mineral dikes. The original sub-circular Sarcheshmeh porphyry stock exhibits an east-west elongation due to dilation by the dike swarm, which strike is predominantly NNW (Waterman and Hamilton, 1975).
The main sulphide minerals of this deposit are Chalcocite, Chalcopyrite, Covellite, Bornite and Molybdenite. The oxide zone of deposit consists mainly of Cuprite, Tenorite, Malachite and Azurite. Pyrite is the main gangue mineral which has the principal role in AMD production.
The Sarcheshmeh pit has a catchment area of about 21 squared kilometres and average annual precipitation of about 440 mm which mostly falls as snow during November to March. The average annual evapotranspiration of this basin is about 1170 millimetres (Karimi Nasab and Sahraei Parizi, 2000).
The outlet of catchment area is in the northwest part, where the pit is placed, and the general slope of the basin is toward the pit (Fig. 1).
REASERCH METHOD AND RESULTS
There are many valleys in the catchment area of Sarcheshmeh pit in which water flows toward the pit. In order to prevent the pit from entrance of floods and surface water, these valleys has been filled by waste materials which are rich in pyrite.
In order to study the role of these waste dumps and other mining activities in the production of AMD we take more than 600 water samples from upstream and downstream of waste dumps and from in-pit channels, sumps and bores during about 3 years. We measured the pH and electrical conductivity (Ec) of these samples in the field by portable instruments and then water samples carried to NICICO laboratory for chemical analysis. Figure 2 shows the position of the waste dumps and water sampling stations in the catchment area of Sarcheshmeh copper mine and table 1 shows the average concentration of different anions and cations and other properties of these samples.
According to our studies the composition of water in the pit channels and sumps changes extensively with time and is a function of resident time while the time changes of water composition in upstream and downstream of waste dumps is negligible (Fig. 3). One of the most useful graphs for representing and comparing water quality analyses is the pattern diagram which first suggested by Stiff and presently known as Stiff diagram. In this diagram concentrations of cations will be plotted to the left of vertical zero axis and anions to the right; all values are in milliequivalents per litter. The resulting points, when connected, form an irregular polygonal pattern; waters of a similar quality define a distinctive shape (Todd, 1980).
Another method for plotting chemical quality analyses is the radial diagram. In this diagram concentrations of cations and anions are plotted on the vectors; all values are in milligrams per litter. The connection of the resulting points form an irregular polygon; waters of different quality have different polygonal shapes. In order to study the changes of water quality due to its passage through waste dumps and mining area we prepare Stiff and radial diagrams for all of the sampling stations according to table 1. Figures 4 and 5 show some of these diagrams.
DISCUSSION AND CONCLUSIONS
A) CHANGE OF WATER QUALITY AND COMPOSITION
According to table 1 and figures 4 and 5 the passage of water through waste dumps makes extensive changes in quality and composition of water, so that bicarbonatic water with pH>7 at the upstream part of waste dumps changes to a water with sulphatic composition and pH< 4 which is rich in heavy metals such as Cu, Fe, Mn, Pb, … in downstream part of these dumps.
In the mining area the change of quality and composition of water is a function of resident time. Figure 4 and table 1 show these changes from best conditions (short resident time) to the worst conditions (long resident time) in in-pit channels and sumps and in-pit bores.
B) MECHANISM OF AMD FORMATION
When surface mining activities expose waste materials to an oxidizing environment a complex series of chemical weathering reactions will be initiated. The mineral assemblages contained in the waste materials are not in equilibrium with the oxidizing environment and almost immediately begin weathering and mineral transformations. From these reactions the overall pyrite reaction series is among the most acid producing of all the weathering processes in nature which takes place in four steps as below (Growitz, 2005(a)).
In the first step pyrite reacts with oxygen and water to produce ferrous iron, sulphate and acidity:
2FeS2 + 7O2 + 2H2O 2Fe2+ + 4SO42- + 4H+
The second step involves the conversion of ferrous iron to ferric iron:
4Fe2+ + O2 + 4H+ 4Fe3+ + 2H2O
This second reaction has been termed the “rate determining” step of the overall pyrite reaction sequence which can be greatly accelerated by some species of bacteria such as Thiobacillus ferrooxidans. This bacterium and several other species thought to be involved in pyrite weathering are widespread in the environment. T. ferroxidans has been shown to increase the iron conversion reaction rate by a factor of hundreds to as much as one million times (Singer and Strumm, 1970).
The third step involves the hydrolysis of ferric iron with water to form the solid ferric hydroxide and the release of additional acidity:
4Fe3+ + 12H2O 4Fe (OH)3 + 12H+
This reaction is pH dependent; under very acid conditions (pH<3.5) ferric iron remains in solution and solid hydroxide does not form. At higher pH values a precipitate forms commonly referred to as “yellow boy” (Growitz, 2005(a)).
The fourth step involves the oxidation of additional pyrite by ferric iron:
FeS2 + 14Fe3+ + 8H2O 15Fe2+ + 2SO42- + 16H+
This cyclic propagation of acid generation by iron takes place very rapidly and continues until the supply of ferric iron or pyrite is exhausted.
The ferric iron can also oxidise other sulphide minerals such as chalcopyrite and chalcocite which by themselves do not contribute to the formation of acid waters (Sahraei Parizi and Karimi Nasab, 2001):
CuFeS2 + 4Fe3+ + 2H2O + 3O2 Cu2+ + 5Fe2+ + 2SO42- + 4H+
Cu2S + 4Fe3+ + 3/2 O2 + H2O 2 Cu2+ + 4Fe2+ + SO42- + 2H+
The production of acidity in these reactions lowers the pH of water and increases the solubility of many heavy metals such as Mn, Zn, Pb, Ni and etc. C) AMD PREVENTION AND MITIGATION
Acid production in mining areas is governed by the fallowing variables:
a) Availability of metallic sulphides specially pyrite
So that exclusion of one of these variables can prevent or at least mitigate the formation of AMD at the pollution source. This can be take place by two general approaches:
a) Controlled placement of waste materials
b) Water management
Both approaches have been in use for at least 20 years with reported results ranging from no improvement to complete success in prevention or mitigation of AMD (Growitz, 2005(b)).
Controlled placement is a preventive measure involving the placement of waste materials during mining to minimize the formation of AMD. Placement of pyritic materials encompasses either an attempt to exclude oxygen, usually by complete submergence below the water table; or an attempt to isolate the materials from water contact to avoid leaching of acid salts (Growitz, 2005(b)).
Water management strategies both during and after mining are another option for reducing acid generation. Water management can include the following
a) Routeing surface drainages away from pyritic materials. b) Prompt removal of pit water to lessen the amount and severity of acid generated. c) Isolation of polluted water from non contaminated sources to reduce the quantity of water requiring treatment. d) Construction of closed conduit drains to prevent the contact of water with acid forming materials.
With regard to the above mentioned materials, our recommendations for treatment of AMD at Sarcheshmeh copper mine are as follows:
a) Placement of new dumps in places where the probability of water ponding and infiltration is low (places other than valleys). b) Planning and construction a peripheral draining system in upstream part of the dumps in order to drain non-contaminated water before ponding behind the dumps. c) Planning and construction a peripheral draining system in downstream part of the dumps in order to isolate the polluted water from non-contaminated sources. d) Prompt executing the underground dewatering plan of pit in order to reduce the amount of water which is in contact with air and acid forming minerals in the pit channels and sumps. e) Using closed conduit drains such Polyethylene pipes instead of open channels for draining the in-pit water. f) Finally treatment of contaminated water by hydrated lime. g)
The authors like to thank NICICO for supporting this research and permission to publish this paper. The collaboration of mine manager and our colleagues at geology and dewatering department of Sarcheshmeh copper mine is greatly appreciated.
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