Uticaj razlaganja pirita na rudničke vode

1. uticaj razlaganja pirita na rudnićke vode

INVESTIGATION AND ANALYSIS OF ACID MINE DRAINAGE IMPACT TO THE MINING PROCESS IN CRNAC MINE Jelena Djokic1, Gordana Milentijevic1, Blagoje Nedeljkovic1,Branko Bozovic2 1Faculty of Technical Sciences, University of Pristina, 38220 Kos. Mitrovica, Knjaza Milosa 7 2Higher Professional Technical School Zvecan, 38227, Kralja Milutina b.b. Abstract: In this paper the results of the impact analysis of mining waters of ore deposit „Crnac“ to the ore body, mining equipment and the environment were presented. The research process included hydro geology explorations about: aquifers type, ground waters occurrences (on the surface and in the adit), their yields and chemical compositions. The mineralogy and microstructure analyze of ore body, made by SEM-EDS analysis and XRD analysis before and after the mining process results proved the increased rate of pyrite decomposition due to the oxidation reaction and presence of bacteria. The chemical analyze results of the mining waters point on the certain degree of aggressive impact to the equipment, proved in practice. The particular problem emerged on the rail lines for ore and equipment transport. Key words: Pb-Zn deposit, mining waters, chemical composition, mineralogy composition, pyrite oxidation 1. INTRODUCTION The region of Northern Kosovo occupies large mineral deposits and ore bodies, which have been exploited for centuries. In the municipality of Leposavic, lead-zinc bearing ores occur in mineralized areas of Rogozna and Kopaonik. Significant mineral deposits are Belo brdo, Crnac, Koporić, Žuta Prlina and Jelakce. In 1999. The mineral exploitation in those mines is closed, except in the mines Belo Brdo and Crnac. In the area of the mine Crnac, the fissure type of aquifers is present. By the geological analysis the rock massive type is determined, as well as the porosity of the water bed. The drainage flow "Gnježdane" collects all waters from Crnac mine, as well as those from the old minings, which amounts were estimated with a maximum of 30 l / s to the minimum 0. l / s. Mine waters flow directly into Jošanička river, left tributary of the River Ibar. Ore deposit “Crnac” occupies the part of the mountain range Rogozna, covering locations Vučja Lokva, Gnježdane, Plakaonica, Duboka, Bare and Zminjak, around 15 km on the west from the municipality center of Leposavić . Some more detailed research on the area of Rogozna (localities of Crnac and Plakaonica) have started on 1957., and 1968. The mine starts the regular production. The mining reserves A+B+C1 the categories are calculated at 2 200 000 tons. The mineralization is deposited in amphibolic rocks in the form of veins. The average composition of lead and zinc in the ore is 9-11%. The range and form of the ore veins occurrence, as well as terrain configuration influenced research methodology, made by combined mining works, and exploration drilling [7]. In the opening and the development of the mineral deposit, there are ground waters occurring influencing mining works and production [5]. The chemical composition of the ground waters in the solid ore deposits, is determined by the ores composition, so different ore deposits are characterized by different ground waters compositions. The ground waters from sulfide deposits are characterized by high sulfate concentrations and low pH (acid waters) values as well as high heavy metals concentrations. The mining waters of sulfide deposits are aggressive and have impact to the environment [1]. The greatest consequence of AMD is water pollution. This in turn results in contaminated drinking water, damage to aquatic flora and fauna, and corrosion of man-made infrastructure. Microorganisms like bacteria and archaea significantly affect AMD. When metal sulfides, usually pyrite, that are contained in rock are exposed to water and air, an oxidation reaction takes place. Microbes speed up the decomposition of these metal ions. Microorganisms also play a huge part in the bioremediation of AMD. Techniques that are being researched include using metal-immobilizing bacteria, biocontrol with bacteria and archaea, and bioleaching [3]. 2.0. EXPERIMENT For the observation of the hydrogeology of the rock formations and the terrain of the Pb-Zn ore deposit „Crnac”, as well as the definition of the yield regime and flow direction and its impact to the deposit hydrology, the hydrologic mapping of the wider zone is done. The investigated area is limited on the “Gnjezdanski potok” adit and the water yield, temperatures and pH values are take n for the ground water chemistry determination [6]. The qualitative and semi-quantitative analyze of ore body were determined using JEOL JSM-6460 scanning electron microscope with energy dispersive spectrometry, EDS (Oxford Instruments). Samples are polished and coated with Carbon - thickness (nm): 20.0, density (g/cm3): 2.25. Thresholding has been selected : All quantitative results below 2 sigma have been set to zero Mineralogy compositions of ore body samples are determined by using X-ray diffractometer for powder PHILIPS PW 1710 under the following conditions: wave length CuKα = 1,54178Å, within the range of 5-70o 2θ. Chemical compositions of the waters inflow and water drainage are determined by using AAS (Atomic Absorption Spectrometry) method. 3.0. RESULTS AND DISCUSSION Geology forms and tectonic fabric conditioned hydrogeology characteristics of the ore field. There are separated the following water table types, compact spring type (sypars and slope wash), cavern water table type (scarns, hornfels, rhydacite, andensite, granodiorite, serpentined peridotite) and conditionaly waterless terrains (marled-clayed flysch sediments). From the rock massives of fissure porosity (cavern water table type, conditionaly waterless terrains), through the fissures, cracks, paraclases and paraclases areas, the mine waters penetrate in mine works of ore field. The waters from the mining works run off by gravity and now are appearing on the surface, the most often on the undermines. It is very important to determine the parameters of fissuring of rocks, for they have impact to their water bearing, and they can be defined by different methods. For this occasion it is applied photo geology method, because the fact elements can be connected logically during the interpretation so the researcher’s subject opinion is excluded. The parameters of the water bearings of rocks with fissure porosity are taken from the study done by the authors [8]. The results of the hydrology mapping have showed that all formations are extremely physically damaged, covered by decomposed deluvial deposits. On the mapped area some large number of springs was determined, but the yield was always under 1l/s. The adit“Gnježdane” drains all the waters from the “Crnac” deposit, as well from the old works. The mining waters are being directly discharged into Josanicka river, left tributary of the Ibar river [4]. The water samples previously analyzed are taken once a year, in November, at the open pit entrance, and represent the overall composition of the waters being drained from the Pb-Zn „Crnac” showing great anomalies in the heavy metals content, as they are collected from the different places, mixing with industrial waters and its aggressive reactions with the tools in the pit. For this study the following parameters of the water on the entrance are monitored: temperature, pH and yield. The temperature values were within the limits of 180C to 190C, the yield was between 0,1l/s and 31l/s, and the pH value was within the interval of 6.6-7.5. 3.3. Physical and Chemical Characteristics In order to investigate an influence of the mining process to the acidity of mine drainage some three samples of ore body from Crnac mine are taken. The first one, sample 1 was from undisturbed ore body; sample 2 was taken from mining in progress, and sample 3 is from the ore body largely influenced by acid mine drainage waters. As presented in the Fig. 1 undisturbed ore body has very coarse structure, and as shown in Fig.2 consisted mainly of pyrite, galenite and quartz. Fig. 1 Scanning Electron Micrograph of undisturbed ore Fig. 2. XRD analysis of undisturbed ore Pyrite oxidation occurs naturally at a slow rate in undisturbed rock. However, the acidity created is buffered by water. Because mining exposes more surface area of these sulfur-bearing rocks, additional acid is produced that is beyond the water’s usual buffering capabilities. When enough oxygen is available either from dissolved oxygen in the water or the atmosphere, further oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+) occurs: 4Fe2+(aq) + O2(g) + 4H+(aq) → 4Fe3+(aq) + 2H2O(l) Ferric iron (Fe3+) can either precipitate as ochre (Fe(OH)3), the reddish-orange precipitate observed in acid mine drainage waters: 2Fe3+(aq) + 6H2O(l) <→ 2Fe(OH)3(s) + 6H+(aq) or it can react directly with pyrite to make additional ferrous iron and hydrogen ions: FeS2(s) + 14Fe3+(aq) + 8H2O(l) → 15Fe2+(aq) + 2SO42-(aq) + 16H+(aq) Overall, these reactions release hydrogen ions, which decreases pH leading to an acidic environment [5]. 3.4. Oxidation of Pyrite Exposing pyrite to oxygen and water leads to an oxidation reaction, where hydrogen and sulfate ions and soluble metal cations are created: 2FeS2(s) + 7O2(g) + 2H2O(l) → 2Fe2+(aq) + 4SO42-(aq) + 4H+(aq) . The simulation run by the PHREECQ programme has given the following results: _time_ Fe(2) Fe(3) pH 0 1.0000e+02 0 7 1.1574e-03 9.8942e+01 1.0578e+00 6.0476e+00 5.7870e-03 9.8206e+01 1.7942e+00 5.8097e+00 4.1667e-02 9.6592e+01 3.4082e+00 5.5247e+00 1.6667e-01 9.4639e+01 5.3610e+00 5.3268e+00 4.1667e-01 9.2764e+01 7.2360e+00 5.1975e+00 1 9.0358e+01 9.6422e+00 5.0750e+00 2 8.7896e+01 1.2104e+01 4.9790e+00 4 8.4808e+01 1.5192e+01 4.8840e+00 6 8.2651e+01 1.7349e+01 4.8290e+00 8 8.0939e+01 1.9061e+01 4.7903e+00 10 7.9498e+01 2.0502e+01 4.7605e+00 Fig. 3. The graphic presentation of the acid mine drainage composition in time Acid mine drainage is affected by characteristics such as pore size, particle size, permeability, and mineral composition of the materials being oxidized. The size of particles directly influences the surface area of rock exposed to weathering and oxidation. Surface area and particle size are inversely related. This characteristic may let air and water penetrate further, thereby exposing more substance to oxidation and ultimately generating more acid. Conversely, fine grain substances may prohibit air and water flow, but they also have more surface area exposed to oxidation. Another important factor, air circulation is impacted by wind, barometric pressure changes, and perhaps convective gas flow due to the heat created in the oxidation reaction. Over time as substances weather, particle size is decreased, exposing more surface area and affecting the physical characteristics of the unit. After mining process is involved in the area, the structure is disturbed and deep crevices between crystals are observed, enabling air and water to penetrate deeper and exposing more surface area to oxidation as shown on Fig. 4. Fig. 4 Scanning Electron Micrograph of disturbed ore Water and oxygen availability are the most important factors though. Both materials are essential to create acid drainage. Atmospheric oxygen is needed to drive the oxidation reaction, particularly to maintain the quick bacterially catalyzed oxidation at pH values less 3.5. When the concentration of oxygen in pore spaces of mining materials is less than one or two percent, the rate of oxidation is notably reduced. The chemical composition of the seeping water compared to the chemical composition of water inflow from the ground water occurred in Crnac mine are presented in Table 1. Table 1. Chemical composition of the water and acid mine drainage Parameter Water inflow the open pit Acid mine drainage pH value 7.50 2.6 Electric conductivity (μS/cm) 880 28,400 Chlorides Cl- (mg/l) 10.63 2.7 Sulfates SO4-2 (mg/l) 0 18947.2 Calcium Ca (mg/l) 160.32 412 Potassium K (mg/l) 0 0.04 Magnesium Mg (mg/l) 187.26 3446 Manganese Mn (mg/l) 0.01 971 Iron Fe (mg/l) 0.02 701 Sodium Na (mg/l) 0.02 2.61 Chemical composition, structure and mineralogy composition are changed in the same ore body after it was exposed to acid mine drainage water. As shown in Fig. 5. and Fig. 6 the total amount of pyrite is dissolved, and remained iron is in the form of siderite in 90 days of exposure/ Fig. 5 Scanning Electron Micrograph of ore exposed to acid mine drainage Fig. 6 XRD analysis of ore exposed to acid mine drainage 4. CONCLUSION The results of investigation showed that mining waters in Crnac mine are characterized as aggresive to the mining equipment and rail lines for ore and equipment transport. For the detailed analysis of the acidity of mining waters chemical and mineralogy analyse of ore body are conducted. Based on these results it can be concluded that the total content of pyrite is dissolved by mining waters, and acidity is affected by the presence of Acidithiobacillus ferrooxidans. In udisturbed ore body, where Pyrite oxidation occurs naturally at a slow rate, the acidity created is buffered by water. In disturbed ore body the reactions lead to total dissolution of pyrite and formation of acid mine drainage with very low pH values. Acknowledgement: This research was conducted as a part of the project III 43007 which was supported by the Ministry of Education and Science of the Republic of Serbia. References: [1] Dragišić V. (2005).Hemijski sastav podzemnih voda ležišta čvrstih mineralnih sirovina. U V. Živanović (ur.), Hidrogeologija ležišta mineralnih sirovina (str. 84-87). Beograd: Rudarsko-geološki fakultet. ISBN 86-7352-145-9. [2] Milentijević, G.: Podzemne vode severnog dela Kosova i Metohije – iskorišćavanje i zaštita. Doktorska disertacija, Beograd: Rudarsko-geološki fakultet, 2005.- pp 160 [3] Milentijević G., Nedeljković B., Dutina V. (2009). Uticaj otpadnih rudničkih voda na životnu sredinu severnog dela Kosova i Metohije, projekat broj 310-02-126/09-022. Izveštaj projekta. [4] Milentijević, G., Nedeljković, B., Djokić, J.: Assessement of the mining practices effects on the water quality in the Ibar river withen the Leposavić municipality, Journal of the Geographical Institute “Jovan Cvijić’’ SASA; No 1; (2010), pp. 31-46, ISSN: 1821-2808. [5] Nedeljković B., Milentijević G., Lazić M.: Zaštita životne sredine u neaktivnim industrijskim područjima, rad po pozivu. I okrugli sto sa međunarodnim učešćem-Zaštita životne sredine u industrijskim područjima, Kosovska Mitrovica, Srbija, 19-20. April 2007., urednici Blagoje Nedeljković, Miljan Jakšić. Fakultet tehničkih nauka, 2007., pp 8-25, ISBN: 978-86-80839-13-6. [6] Nedeljković, B., Milentijević G., i dr.: Izveštaj projekta Uticaj rudarskih aktivnosti pri eksploataciji olovo-cinkane rude na promenu geološke i životne sredine kao i na zdravstveni aspekt stanovništva na području severnog dela Kosova i Metohije-evidencioni broj 14026G. [7] Stručna dokumenta RMHK „Trepča” [8] Nedeljković Blagoje, Milentijević Gordana,: Estimation of endangerment of surface and ground waters of the Ibar's middle river basin as a result of RMHK 'Trepča' activity Podzemni radovi, vol. 13, iss. 15, pp. 61-68, 2006 [9] Irrigation Water Requirements, Technical Release N.21, Soil Conservation Engineering Division,83 pp.