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Investigation of pyrite oxidation and acid mine drainage characterization associated with Razi active coal mine and coal washing waste dumps in the Azad shahr–Ramian region, northeast Iran

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Acid mine drainage (AMD) pollution is considered to be the most serious water pollution problem in mining areas. AMD containing iron sulfates and other components can affect the receiving water bodies. Pyrite oxidation and AMD generation can be
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  ORIGINAL ARTICLE Investigation of pyrite oxidation and acid mine drainagecharacterization associated with Razi active coal mine and coalwashing waste dumps in the Azad shahr–Ramian region,northeast Iran F. Doulati Ardejani  • B. Jodeiri Shokri  • M. Bagheri  • E. Soleimani Received: 9 July 2009/Accepted: 8 January 2010/Published online: 3 February 2010   Springer-Verlag 2010 Abstract  Acid mine drainage (AMD) pollution is con-sidered to be the most serious water pollution problem inmining areas. AMD containing iron sulfates and othercomponents can affect the receiving water bodies. Pyriteoxidation and AMD generation can be considered asimportant processes that may take place in the wastesproduced by coal mining and coal washing operations inthe Golestan province (northeast Iran). The study area ischaracterized by appropriate atmospheric conditions thatfavor pyrite oxidation and the presence of a large amountof water bodies. This study attempts to consider pyriteoxidation and AMD generation in the Azad shahr–Ramianregion. The impact of AMD on the quality of the surfacewater bodies was investigated by taking samples and ana-lyzing them for hydro-geochemical parameters. Stiff andPiper diagrams were used to represent chemical analyses of water samples. The coal samples taken from differentdepths at four points on two different coal waste dumpswere analyzed to find the fraction of pyrite that remained inthe waste particles to investigate the pyrite oxidation pro-cess. A computational fluid dynamic package calledPHOENICS was used to model pyrite oxidation processnumerically. The results obtained from the geochemicalanalyses of water and coal samples and numerical simu-lation show pyrite oxidation and acid generation in theregion. However, the presence of carbonate rocks raisedthe pH of the water samples. The drainages of theRazi mine may be recognized as natural alkaline minedrainages. Keywords  Coal waste    Acid mine drainage   Pyrite oxidation    Numerical model    Azad shahr–Ramian Introduction The Golestan province with 11 active mines in the Azadshahr–Ramian and the Gheshlagh region is noted to be amajor coal producer in Iran. Razi, Vatan, Olang and Kalatare the most important coal mines in the Gheshlagh region.Mining operations in the Gheshlagh region often leave landin a condition unsafe for local people and wild life. Fur-thermore, coal mining may have a long-term environmentalimpact. The post-mining mine drainage condition is iden-tified as one of the pollutants. The quality of drainage waterdepends on various factors such as geology of the strata,hydro-geological characteristics and mining parameters thatcan vary considerably from one mine site to another.Pollution problem in coal mines is generally due to theoxidation of pyrite. In coal mines, pyrite is commonly foundeither in coal seams or mudstone interbeds of marine srcin.When pyretic and iron-bearing minerals are exposed toatmosphere and/or water, they undergo rapid oxidation thatproduces acid waters (Ricca and Schultz 1979; Atkins andPooley 1982; Adam et al. 1997; Canovas et al. 2007; Zhao et al. 2007). This process has detrimental effect on surfacewaters, groundwater aquifers and soils (Atkins and Pooley1982; Rubio and Del Olmo 1995; Dinelli et al. 2001). In coal mining operations, huge amounts of coal wasteare normally produced. The drainage that srcinates fromsuch waste dumps has harmful effects on water quality.Moreover, the wastes produced by coal washing plants F. Doulati Ardejani    B. Jodeiri Shokri ( & )    M. BagheriFaculty of Mining, Petroleum and Geophysics,Shahrood University of Technology, Shahrood, Irane-mail: behshad_jodeiri@yahoo.comE. SoleimaniFaculty of Chemistry, Shahrood University of Technology,Shahrood, Iran  1 3 Environ Earth Sci (2010) 61:1547–1560DOI 10.1007/s12665-010-0469-7  often pose many environmental problems due to the oxi-dation of pyrite and acid water generation.The acid mine drainage (AMD) is considered to be amajor cause of water pollution, contributing to high con-centrations of iron, SO 2  4  , low pH and variable concen-tration of toxic metals in waters (Williams 1975; Ericksonet al. 1982; Banks and Banks 2001; Kim and Chon 2001; Moncur et al. 2005; Lee and Chon 2006; Canovas et al. 2007).Pyrite oxidation and AMD generation are importantprocesses that may take place within the wastes producedby coal mining and coal washing operations in the Golestanprovince (northeast Iran). The study area is characterizedby appropriate atmospheric conditions that favor pyriteoxidation and the presence of a large amount of waterbodies.A basic study of the hydro-geochemistry of minedrainage, a detailed investigation of AMD generationwithin the wastes produced by mining and coal washingactivities in the study area and development of a numericalmodel to simulate oxygen diffusion through pore spaces of waste dumps and pyrite oxidation within the wastes arenecessary for any researcher who deals with pollutionproblems related to mining wastes. Furthermore, knowl-edge of mine drainage quality helps in the design of a minerehabilitation program.Computational fluid dynamics (CFD) has been usedrecently in many applications related to environmentalstudies as a way of predicting the impact and developingcontrol strategies for the long-term impact of mining onenvironment and many other activities (Singh and DoulatiArdejani 2004).Many studies have been carried out in the past to modelpyrite oxidation and pollution generation. The studiespresented by Cathles and Apps (1975), Jaynes et al. (1984), Davis and Ritchie (1986), Davis et al. (1986), Elberling et al. (1994), Walter et al. (1994a, b), Lefebvre and Gelinas (1995), Wunderly et al. (1996), Gerke et al. (2001), Doulati Ardejani et al. (2004) and Singh and Doulati Ardejani(2004) are all noteworthy.Cathles and Apps (1975) presented a one-dimensionalmodel for oxidation and leaching processes, which con-siders temperature dependence, oxygen balance and airconvection effects. This model assumes air convection to bethe main mechanism of oxygen transport through the dumpto oxidize ferrous ion in the presence of iron oxidizingbacteria. The leaching process takes place through theaction of both chemical and diffusion controlled processesin which the sulfides are oxidized primarily by ferric ions.Such a model may present a reasonable assumption for themechanism occurring in coarse copper waste dumps.Davis and Ritchie (1986) and Davis et al. (1986) developed a one-dimensional mathematical model forpyrite oxidation within the White’s overburden dump atRum Jungle, Australia, where diffusion of oxygen to thereaction sites is the rate-limiting factor. The modeling wascarried out in a two-stage approach with: first, the diffusionof oxygen into the pore space of the dump, and second, ashrinking core model to describe the diffusion of oxygeninto the individual particles through the oxidized coatingthat forms around the unreacted core of the particles. Themodel is widely accepted and used in overburdened dumps.However, the role of bacteria in the oxidation of pyrite andsubsequent transport of the oxidation products were notincluded.Jaynes et al. (1984) developed a one-dimensional finitedifference model called POLS for simulation of AMD in areclaimed strip mine. Direct molecular oxygen oxidationand ferric iron oxidation were included in the model and itwas assumed that pyrite oxidation is controlled by first-order kinetics and the rate of diffusion of oxidant into thereaction particle. The model further assumes that the oxi-dation products are removed from the reclaimed open cutmine by percolating surface water, and the role of groundwater in transporting the oxidation products wasneglected. Furthermore, the interaction between the solidphase and dissolved species was not included. The modelwas further developed by Doulati Ardejani et al. (2004) andSingh and Doulati Ardejani (2004) with different numeri-cal technique and considering many physical, chemical andbiological processes involved in AMD. The main feature of the model is the simulation of the interaction between solidphase and solute species.The numerical model developed by Elberling et al.(1994) can be used to simulate pyrite oxidation in tailings.The model was formulated by combining the effect of oxygen diffusion to a depth where oxidation takes placewith first-order kinetics with respect to oxygen. The rate of pyrite oxidation in the model is based on the continuityrelationship for oxygen with both oxygen transport throughthe tailings and the consumption of oxygen at the surface of the sulfide minerals grains. One of the important features of the model is the sensitivity of the model to physicalparameters related to mine tailings, such as pyrite fraction,particle size and effective diffusion coefficient using aseries of simulation runs.Walter et al. (1994a, b) developed a multi-componentreactive transport model MINTRAN to simulate themobility of potentially toxic dissolved metals from a minetailings source into an aquifer. The oxidation of metalsulfide minerals released metals such as Pb, Zn, Cu, As andNi into the tailings pore water. The initial concentrationsfor the MINTRAN simulations have been determined byequilibrating the observed water chemistry with theobserved mineral phases, using the geochemical speciationmodel MINTEQA2. The main physical and chemical 1548 Environ Earth Sci (2010) 61:1547–1560  1 3  processes involved in the simulations were advection,convection, pH-buffering, mineral dissolution and precip-itation reactions. The results show that the bufferingreactions in the carbonate aquifer lowered the dissolvedmetal concentrations by several orders of magnitude. In thecase of a 12-year source duration, the precipitation reactionimmobilized most metals. This model is suitable for theprediction of sulfide minerals oxidation, AMD generationand the transportation of the oxidation products associatedwith metallic mine tailings.Lefebvre and Gelinas (1995) presented a numericalmodel for the simulation of AMD generation in waste rock dumps. The main processes included in the model arehydrology, gas and heat transfer, geochemistry and masstransport process. The important features of the model werethat it was multi-component, multiphase and non-isother-mal. A reaction core model was used to take into consid-eration the rate of pyrite oxidation within the waste rock dumps. The model takes into consideration the effects of both the surface reaction kinetics and the rate of oxygendiffusion. According to the authors, high temperatureslimited the bacterial activity. Furthermore, the model pre-sumes that oxygen is the only effective pyrite oxidant. Inaddition, since the model does not directly take intoaccount ferric iron as an oxidant, the leachate speciationwas avoided. It was concluded that the air convectionprocess controlled the rate of contaminant generation.Wunderly et al. (1996) developed a numerical modelcalled PYROX that couples one-dimensional oxygen dif-fusion and sulfide mineral oxidation to simulate pyriteoxidation in the vadose zone of mine tailings. For thesimulation, a shrinking core model and a finite elementnumerical approach were used to simulate the transport of oxygen and the oxidation of pyrite particles. The PYROXmodel has been coupled with a two-dimensional finiteelement reactive transport model, MINTRAN, for con-taminant transport and MINTEQA2 to solve the equilib-rium geochemistry. The chemical reactions considered inthe model are based on a local equilibrium assumption. Theresulting model has been called MINTOX. A two-stepequilibrium approach has been used for the simulation.Gerke et al. (2001) modeled the effects of the spatialdistribution of pre-oxidized zones and pyrite and buffermineral contents on the movement of acidity and oxidationproducts in overburdened mine spoils. The major processesconsidered consist of variably saturated water flow, oxygendiffusion, shrinking core kinetics of pyrite oxidation, multi-component reactive solute transport and geochemicalequilibrium reactions between aqueous and mineral com-ponents. The results show that differences in the spatialdistributions in a physically heterogeneous pyritic minespoil heap can lead to differences in the temporal evolutionof pyrite oxidation and acid mine generation.Literature review shows that despite much researchrelated to waste rock dumps, open cut mines and minetailings, the environmental problem associated with coalwashing operations has not been completely solved (Dou-lati Ardejani et al. 2008). In the present work, attention hasbeen focused on numerical simulation of oxygen transportand pyrite oxidation in the wastes produced by coalwashing operation. To achieve the goal the PHOENICS(Spalding 1981) as a CFD package was used to model theproblem at hand. Furthermore, the impacts of AMD on thequality of surface water bodies were investigated by takingsamples and analyzing them for hydro-geochemicalparameters.Localization and geologic setting of the study areaThe study area was located 40 km from Shahrood and7 km of the Azadshahr-shahrood Road, northeast Iran(Fig. 1). Gheshlagh is the main coal mine in the area. Thecoal washing plant is located near the main tunnel of the mine. The topography of the mining area is rough. Theweather is very cold in winter and moderate and humid insummer. The average annual precipitation at the site hasbeen 702 mm for a 10-year period (Rainfall data of Golestan province 2006). The study area is located inKhoush Yeylagh 1:100,000 geological sheet. Coal seams inthe Gheshlagh mine are interbedded with Upper Triassic–Lower Jurassic argillites, siltstones and sandstones of theShemshak formation. The coal seam is bounded by dolo-mite limestone on the lower part (Elica formation) and bythick layers of limestone (Lar formation) on the upper part(Geological survey of Iran 2002). Figure 1 shows the geographical situation of the Gheshlagh coal mine.LocalizationCoal in the study area is mined by the Alborz Sharghi coalcompany. Development of mining activities in the regionhas resulted in many low grade waste dumps and posedmany environmental problems. The biggest waste dump islocated at the corner of the river flowing through the minesite. This may increase the risk of AMD generation andtransportation of the oxidation products. Figure 2 shows ageneral view of the low grade coal waste dumps.The wastes produced by mining activities are mostlydumped at the foothills in the northern part of the region.The wastes are also transported to the flat areas in thenearby lands where they are dumped and degraded to leavespace for more wastes.Field investigation showed that the mining activitiesproduced many environmental problems, including lack of soil vegetation around the waste dumps, discharge of mineeffluents and water drainage, formation of waste disposal Environ Earth Sci (2010) 61:1547–1560 1549  1 3  sites near rivers, water courses and agricultural and forestareas, and deterioration of the natural landscape. Materials and methods SamplingDue to appropriate atmospheric conditions and the pres-ence of moisture and oxygen, pyrite from the coal wastes isoxidized and produces acid waters. To investigate suchprocesses, 28 coal waste samples, 1 kg each in weight,were taken from different depths at four points from twowaste dumps.Simple mechanical tools were used for collecting sam-ples from the waste dumps. To avoid mixing the samples,they were placed into separate cloth bags. The sampleswere then sent to the mineral processing lab of ShahroodUniversity of Technology for preparation and performingfurther processes required before mineralogical and Fig. 1  Geographical situation of the Gheshlagh mine (modified from Encyclopaedia of roads in Iran, 2008) Fig. 2  Low-grade waste dumps in the Gheshlagh coal region1550 Environ Earth Sci (2010) 61:1547–1560  1 3  quantitative analyses for the determination of the fractionof pyrite that remained in the waste samples.Moreover, four water samples were taken from the studyarea to carry out a hydro-geochemical analysis to investi-gate the impact of mining and coal washing operations onthe quality of the surface water bodies. One control samplewas not affected by the mining activity. The second onewas taken from the effluent of the coal washing plant andtwo others were taken from the Gheshlagh mine and Razimine drainages. Water samples were collected in cleanplastic bottles 250 mL each in capacity. Water sampleswere directly stored under cool conditions at about 5  Cwithout acidification. Physical properties of the watersamples including Eh, total dissolved solid (TDS) andelectric conductivity (EC) were measured in the waterlaboratory of the Semnan Science and Technology Park,Iran. Furthermore, a pH meter model, ORION 420A, wasused to measure pH. The chemical analysis of watersamples was performed using Palintest-Photometer 7000for cations and anions. Concentrations of heavy metals inthe Razi mine drainage were determined using an induc-tively coupled plasma mass spectrometer (ICP-MS, ACMEAnalytical Laboratories Ltd., Canada).Mineralogical analysisThe mineralogical analysis was conducted to qualitativelyinvestigate the pyrite oxidation process at various depths of the coal waste dump. The study indicates the presence of pyrite in the unoxidized waste and pyrite depletion in theoxidized zone. At the dump surface, oxygen is readilyavailable. So, the rate of pyrite oxidation is very high. Thepolished section from the dump surface (Fig. 3, top) indi-cates the lack of pyrite. Due to the reduction in oxygendiffusion with depth, the pyrite content decreased from thesurface to a depth of 0.5 m (Fig. 3, middle). However, theparticles of pyrite can be easily seen in the polished sectionfrom a depth of 1 m (Fig. 3, bottom), where no oxygen aspotential oxidant is available. Figure 3 also shows thesimulation results of modeling for the fraction of pyrite thatremained within the waste particles and oxygen diffusionas a function of waste depth.Quantitative analysisQuantitative analysis was carried out to investigate theconcentration of pyrite that was not oxidized and remainedin the waste particles. According to Table 1, seven coalsamples were taken at different depths from waste dumpsat each location. A total of 28 coal samples were takenfrom two coal waste dumps. A method presented by ASTM(Gladfelter and Dickerhoof  1976) was employed to deter-mine organic pyrite from the non-organic pyrite. Thesamples were first sieved in the mineral processing labo-ratory at Shahrood University of Technology to obtain aparticle size of  \ 75  l . HCl was used to dissolve sulfates.Pyrite was extracted from the coal using HNO 3 . An AA-670 Shimadzu atomic absorption was used to measure ironin the solution. It was then employed to determine theconcentration of pyrite that remained in the waste particles.Table 2 gives the concentration of pyrite that remainedin the waste particles versus depth at four different Fig. 3  Polished section of coalsamples ( left  ), showing pyritedepletion in surface layers of coal wastes. The sample wastaken at ( a ) the dump surface,( b ) at a depth of 0.5 m, ( c ) at adepth of 1 m. Modelingpredictions for oxygenconcentration ( middle ) andpyrite concentration thatremained in the waste particles( right  ) versus depthEnviron Earth Sci (2010) 61:1547–1560 1551  1 3
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