Characterization of Overburden and Fly ash Materials for Back filling in Opencast Coal Mines vis-a-vis Hydrogeochemical Analysis – A Case Study

Abstract

Coal is likely to remain the key energy source for India, for at least the next few decades, as India has significant domestic coal resources compared to other natural resources. There is an increased demand for coal production to meet the industrial as well as domestic requirements. Opencast mining is preferred to get the desired production since the production from underground mines is limited and most of the coal seams in our country occur at shallow depth.However, opencast mining creates large voids due to the extraction of overburden, partings and coal. These voids are usually filled with overburden (OB) and waste materials as part of the reclamation procedure. In many mines, the OB and mine waste available for filling the mine voids are not adequate due to the low stripping ratio. In order to meet the additional requirement of suitable waste materials as well as to enhance the utilization, fly ash (FA) is also being used for filling the voids. However, these operations inevitably require excessive planning and control to minimize the environmental impact of mining.While choosing a particular material for backfilling in opencast mines, one should examine the material’s short and long term mechanical properties and expected behaviour following placement.This will allow an assessment of the fill’s ability to act as a ground support material. Moreover, the geochemical and mineralogical properties are required to be ascertained for assessing its leaching potential.An analysis of these properties will help to determine whether the design objectives of the fill will be met.
In order to evaluate the impact of backfilling the voids with coal mine waste and fly ash, samples of overburden material from various opencast mines of MCL, and fly ash from Balanda opencast project were collected. The geotechnical, geochemical (soil quality and leachability potential), and mineralogical characterization of overburden (OB) sample and OB+30% FA samples were carried out to assess their suitability for backfilling. OB samples from Ib valley and Talcher coalfields were named as IOB1, IOB2, IOB3, IOB4; and TOB1, TOB2, TOB3, TOB4 respectively. Similarly, the OB sample with 30% fly ash were named as IOBF1, IOBF2, IOBF3, IOBF4; and TOBF1, TOBF2, TOBF3, TOBF4. Additionally, water samples from mine sumps, upstream and downstream of nallahs and rivers nearby 15 opencast mines of MCL were collected; and their physical, chemical and heavy metal contents were analysed. This was done to assess the impact of backfilling on the water quality of surface water bodies.
The geotechnical studies were carried out using slake durability, standard proctor compaction, plasticity, permeability and direct shear test, and particle size analysis following standard procedure. The study for Ib valley and Talcher coalfield establishes that samples IOB1, IOB3, TOB1, TOB2 and TOB4 are of medium-high durability. So they can be used for backfilling. After addition of fly ash, the permeability increases for some OB samples, while liquid limit, plastic limit, and plasticity index (PI) decreased after addition of 30% fly ash; which is good from backfilling point of view.
Both the OB and OB+30% FA samples were found to be mostly acidic, low in organic carbon content and slightly high in conductivity. They contain high amount of Fe, Al, K, and Mg; mild quantity of Na and Mn and low concentrations of Cr, Zn, Cu, B, Co, Ni, As, Se, Cd, and Pb.
The leachability potential study confirms that when the waste material is subjected to leaching with different leaching agents, various leachable ions and metals such as nitrate, sulfate, chloride, fluoride, As, Ni, Zn, Co, Fe, Mn, Cu, Se, Cd and Cr are released. It is also observed that the leaching potential of acid is more than both rainwater and distilled water. After further analysis, we also observed that the leaching potential of rainwater is more than distilled water.
The details of mineral phases of the OB and OB+30% FA mixture were confirmed by the XRD, FTIR, TEM maps, TEM-EDX spectra, and HAADF-STEM elemental mapping. The mineral phases include essential minerals quartz, muscovite surrounded by a matrix such as kaolinite, dickite, zinnwaldite, illite, hematite, and siderite. The OB+30% FA samples were observed to have micro-cracks and pores, which confirms that increasing of permeability after addition of fly ash. Reasonable pozzolanic activity of fly ash with OB has been confirmed in some of the SEM images as glassy fibrous particles were observed, though no peaks for calcium silicate hydrate and calcium aluminium hydrate was observed in XRD analysis.
Water quality analysis studies were carried out using standard procedures. The analysis included detection of a few parameters at the site using Horibal multiparameter water quality analyzer. In the laboratory, water samples were analyzed using Atomic absorption spectrophotometer, Inductively coupled plasma mass spectrophotometer, Double-beam UV-VIS spectrophotometer, Flame photometer, Ion meter etc.
The results indicated that the water in most part of the Ib valley and Talcher coalfields are neutral to slightly alkaline in nature. However, highly acidic water have been noticed in the mine sumps of Lajkura, Samleswari, Bharatpur opencast projects. The water from these sumps needs proper treatment before it is discharged outside the mine premises. The major water type is found to be calcium-magnesium-sulfate-chloride in both the coalfields. The ions, heavy metals, and other elements were found in low concentrations in the monsoon season than premonsoon due to the dilution effect caused by rain and surface runoff. Water quality analysis of the samples reveals low pH, and high concentration of EC, Turbidity, TDS, TSS, fluoride, sulfate, magnesium, Fe, Se, Cu, Cd, Ni, Si, Al, and, oil and grease in a number of samples in Ib valley coalfield. Talcher coalfield water samples are mostly contaminated with TSS, conductivity, turbidity, Mg, Iron, selenium. Mildly contaminated with BOD5, sulfate, hardness, fluoride, silica, boron. Similarly, among the heavy metals, selenium has been found above the permissible limit in most of the water samples of both the coalfield in premonsoon and monsoon season. In case of Ib valley coalfield, Z- Test result shows significant difference for TDS, turbidity, acidity, total hardness, chloride, sulfate, copper and DO% between the premonsoon and monsoon seasons. Similarly, for Talcher coalfield, significant difference for various parameters such as EC, temperature, TDS, turbidity, acidity, total hardness, chloride, sulfate copper, DO% has been observed between premonsoon and monsoon seasons.
WQI has been found to be good mostly in different sumps and upstream of nallahs of both the coalfield. However, at all other places such as ETP, MDTP, STP and downstream of nallahs, the WQI values are found to be poor to very poor to even unfit for drinking. Similarly, HPI value has been found above the tolerable limit at certain locations such as STP, ETP, railway siding, and downstream in both the coalfields. As far as Cd, and HEI is concerned, majority of the area comes under low to medium category of Cd, and HEI apart from some locations which are affected mostly by surface mining and related transportation. The various indices such as SAR, RSC, PI, and Na% except for MH of most sample locations shows excellent to good water quality for use in irrigation.
The results of geochemical, mineralogical analyses, and water quality studies; and the subsequent derivation of different Indices confirm that mine discharge is responsible for the degradation of water quality of nearby nallahs and rivers. The mining activity, surface runoff, coal burning, atmospheric precipitation, weathering, leaching, industrial activity and anthropogenic activity may be mostly controlling the water chemistry of the area.