Geochemistry and Mineralogy of Coal Spoils: A Case Study of Takht Coal Mine, Golestan Province

This study investigates the geochemical and mineralogical characteristics of Takht Coal Spoils in Minoodasht, Iran. For this purpose, six representative spoil samples were collected. The geochemistry of the samples were studied using XRF and ICP-OES analyses, microscopic studies, XRD, and SEM-EDX spectra. To examine the potential of Acid Mine Drainage (AMD) by the spoil samples, different static tests including pH and EC measurement of saturated paste, Acid-Base Accounting tests (ABA), and Net Acid Generation (NAG) / Net Acid Potential (NAP) test were applied. Based on the geochemical data, the studied samples are not enriched with Fe, Mn, Ni, Zn, low enriched with Cu and Cd, moderately enriched with Mo, significantly enriched with Sb and Pb, and strongly enriched with As. Quartz, muscovite, clinochlore and kaolinite are the major mineral phases in the studied samples. Static tests indicate that most of the samples are characterized by saturated paste pH<5.5, pH NAG<4.5, negative net neutralization potential (NNP), and positive NAPP values. NNP and NAG values as well as the position of the samples on the geochemical diagrams show that the formation of acid drainage is likely through the oxidation of spoils discarded around the mine. Regarding the environmental hazards imposed by acid mine drainage, the proper management of the spoils in the studied area deems necessary.

Coal is one of the most important fossil fuels that has been used by humans since ancient times due to its unique properties. However, coal extraction typically results in environmental problems such as land destruction, mine collapse, mine explosions, land subsidence, surface and groundwater pollution, soil and air pollution, and the production of Acid Mine Drainage (AMD). AMD is produced through the oxidation of metal sulfides (especially pyrite) present in waste rock dumps, processing wastes, underground mine tunnels, and open-pit mines. Oxygen, moisture, and ferric iron resulting from the oxidation of iron-bearing sulfides (especially pyrite) are the most important factors in sulfide oxidation (Munksgaard et al., 2012). AMD is one of the most important environmental problems in mining industry and one of the main causes of water pollution (Mohanty et al., 2018; Rezaie and Anderson, 2020), which reduces the pH and enhances the electrical conductivity (EC) of water and increases the concentration of potentially toxic elements (especially copper, zinc, arsenic, manganese, cobalt, nickel, lead, cadmium, barium, and mercury) in the surrounding areas (Pan et al., 2021; Kavehei et al., 2021). Moreover, the detrimental effects of AMD persist for decades and even centuries after mine closure. Coal spoils are among the most important sources of AMD production, because they usually contain high amounts of sulfidic minerals (e.g., pyrite, chalcopyrite, and arsenopyrite). Considering the negative impacts of AMD on the whole ecosystem, especially in coal mines where very large amounts of spoils are produced during extraction of coal, the prediction of AMD formation is of great importance that is useful in proper managing of spoil’s disposal and taking actions for preventing the formation of AMD (Kavehei et al., 2021). The aim of this study is investigating the geochemistry, mineralogy, and potential of AMD production by the coal spoils of Takht Coal Mine, located in Minoodasht County, Golestan Province, Iran. Coal extraction produces a large amount of waste materials, which are disposed around the mine tunnels. Since coal spoils is one of the most important sources for AMD production, examining their potential for acid generation is of great importance. Moreover, the geochemistry and mineralogy of the studied spoils were investigated by XRF and ICP analyses, microscopic studies, XRD, and SEM-EDX spectra of the representative samples.

Five representative samples were taken from the waste rock (spoil) dumps disposed near the extraction tunnels, and one representative sample was taken from the spoils discarded around Takht Coal Mine. To obtain a representative sample at each station, at least 30 sub-samples were collected and mixed together. The weight of each sub-sample was approximately 3 kg. To identify the minerals present in the samples, polished sections were prepared and studied, and X-ray diffraction (XRD) and SEM-EDX analyses were implemented. The geochemical compositions of the studied samples were studied by X-ray fluorescence (XRF) (for major oxides and sulfur) and ICP-OES (for major and trace elements) analyses. To predict the potential of AMD production by the studied samples, the most common statistic tests were conducted using standard methods 1) measuring pH and electrical conductivity (EC) of saturation pastes of the samples; 2) the modified Acid-Base Accounting (ABA) tests; and 3) Net Acid Generation (NAG) and Net Acid Potential (NAP) tests.
 

The results showed that the spoils of Takht Coal Mine is not enriched with Fe, Mn, Ni, and Zr, significantly enriched with Sb, Pb, Mo, and extremely enriched with As. The high enrichment of toxic elements (i.e., As, Sb, and Pb) in the studied samples is likely due to the presence of pyrite and chalcopyrite in the samples, as confirmed by the mineralogical studies in which pyrite presents as framboidal aggregates and semi-idiomorphic particles. Moreover, the high concentration of potentially toxic elements in the studied samples must be considered a potential risk which may result in serious environmental impacts on the surrounding areas. The statistic tests showed that the pH values of samples 1, 2, 3, and 4 in the net Acid Generation (NAG) test are <4.5. On the other hand, these samples have a positive Net Acid Production Potential (NAPP) and a negative Net Neutralization Potential (NNP). On the other hand, in samples 5 and 6, NAG pH is greater than 4.5, and the pH value of the saturation paste is approximately neutral. The two samples have the lowest sulfur among all samples. Moreover, XRD spectra showed that muscovite and quartz are present in theses samples, which can prevent acid production or neutralize the produced acid. The results of the static tests showed that samples 5 and 6 have a negative NNP and a positive NAPP. Therefore, the possibility of acid production in the waste materials is uncertain.

Considering the high concentrations of potentially toxic elements in the spoil samples of Takht Coal Mine, in the event of acid mine drainage formation, the surface and groundwater resources and soil of the studied area will be seriously affected. To reduce the environmental impacts of Takht Coal Mine’s spoils, taking proper measures is very relevant. Some possible measures include adding lime to the waste materials to neutralize the spoils, adding soil to the waste material, providing drainage channels under the waste dump bed to transfer produced acid into ponds containing lime materials, depositing waste material away from surface water paths, and separating pyrite from the waste material can be taken into account.