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Co-disposal of lignite fly ash and coal mine waste rock for neutralisation of AMD
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. Quaid-e-Awam University of Engineering, Science and Technology, Nawabshah, Pakistan. (Applied Geochemistry)ORCID iD: 0000-0003-2810-8083
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. (Applied Geochemistry)
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. (Applied Geochemistry)
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The WRs from a lignite producing coal field and FA produced from a power plant incinerating lignite from this field have been investigated in this study with objectives including the effect of WR particle size, co-disposal of FA (cover and mixture) and WRs on AMD and leachate quality and potential for secondary mineralisation using PHREEQC. The effect of particle size has been clearly observed where smaller particle sizes (2mm) produced low pH (~2) leachates with elevated concentrations of the sulphide related elements (such as Fe, SO42-, Al, Zn, Co, Cr, Cu, Mn and Ni) compared to 5mm (pH~3) and 10mm (pH~3.7) particles, indicating that sulphide oxidation has been pronounced in smaller particles. Co-disposal of FA as mixture readily provides acid buffering minerals, resulting in better pH (~3–6) and leachate quality from the beginning, except initial flush-out on the first rinse. However, acidity produced by secondary mineralisation contributes towards the acidification of the system, causing stabilisation of pH at around 4.5−5. In contrast, the pH of the leachates from the FA cover scenario gradually increases from strongly acidic (pH~2) to mildly acidic (WR1: pH~5 and WR2: pH~4) and circumneutral (WR3: pH~7) along with decrease in EC and elemental leaching. Gradually increasing pH can be attributed to the cover effect, which reduces oxygen ingress, thus sulphide oxidation, causing pH to elevate. Since pH~4–5 is sufficient for the secondary Fe- and Al-mineral precipitation, the FA cover performs very well to achieve that until the conclusion of the column leaching experiment. However, due to slower reactivity of the system, leaching in the beginning cannot restricted. The co-disposal of FA as cover and/or mixture possess potential for neutralisation of AMD and improving leachate quality significantly. However, both systems need to be up-scaled and investigated for AMD neutralisation, leachate quality, kinetic modelling and geotechnical properties.

Keyword [en]
Coal mine waste rock, acid mine drainage (AMD), fly ash, co-disposal, dry cover, mixing
National Category
Environmental Management Mineral and Mine Engineering Environmental Sciences Geochemistry
Research subject
Applied Geochemistry
Identifiers
URN: urn:nbn:se:ltu:diva-60260OAI: oai:DiVA.org:ltu-60260DiVA: diva2:1045543
Available from: 2016-11-09 Created: 2016-11-09 Last updated: 2016-11-09
In thesis
1. Co-disposal of waste rock and lignite fly ash to mitigate the environmental impact of coal mining
Open this publication in new window or tab >>Co-disposal of waste rock and lignite fly ash to mitigate the environmental impact of coal mining
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Waste rocks (WRs) from coal mining and fly ash (FA) from coal combustion were studied to determine the potential of the WRs to generate AMD, FAs to neutralise it and estimate their impacts on environment. The ultimate goal was to develop a methodology based on co-disposal to mitigate the environmental problems associated to both wastes. Two methods for co-disposal were tested: i) Mixing WRs and FAs and ii) covering WRs with FAs.

WRs were sampled from the Lakhra coal mines in Pakistan (PK), which has an estimated coal reserves of 1.3 Btonne, varying from lignite to sub-bituminous quality. The FA was sampled from a power plant utilising coal from Lakhra coal mines and is situated in close vicinity (15km) of the mine site. For comparative purposes a bituminous FA from Finland (FI) and biomass FA from Sweden (SE) were also characterised.

The WRs and FA samples were characterised by mineralogical and geochemical methods. Besides organic material (coal traces), quartz, pyrite, kaolinite, hematite, gypsum and traces of calcite, lime, malladerite, spangolite, franklinite and birnessite were identified in WRs by XRD. The major elements Si, Al, Ca and Fe were in the range (wt. %) of 8 – 12, 6 – 9, 0.3 – 3 and 1 – 10, respectively, with high S concentrations (1.94 – 11.33 wt. %) in WRs. The AMD potential of WRs ranged from -70 to -492 kg CaCO3 tonne-1.

All FAs contained quartz, with iron oxide, anhydrite and magnesioferrite in PK, mullite and lime in FI and calcite and anorthite in SE. The Ca content in SE was 6 and 8 times higher compared to PK and FI, respectively. FAs were enriched in As, Cd, Co, Cr, Cu, Hg, Ni, Pb and Zn compared to continental crust. The acid neutralising potential of PK was equivalent to 20 kg CaCO3 tonne-1 compared to 275 kg CaCO3 tonne-1 (SE) and 25 kg CaCO3 tonne-1 (FI).

During the period of 192 days in weathering cell experiments (WCE), the pH of leachates from most acidic WRs was maintained from 1 to 2.5, whereas, the less acidic WRs produced leachates of mildly acidic (2.7) to neutral (7.3) pH. The leachates from very acidic WRs ranged in the concentrations of Fe, SO24− and Al from mg L-1 to g L-1.

The samples were subjected to column leaching experiments (CLE) in which mixture (FA:WR; 1:3) and cover (FA:WR; 1:5) cases were mimicked (with 10mm particle size) and effects of particle size (2, 5 and 10mm) on element leaching were studied.

Despite having the lowest acid-neutralisation potential compared to FI and SE, co-disposal of PKFA as mixture readily provides acid buffering minerals, resulting in better start-up pH conditions and leachate quality. However, acidity produced by secondary mineralisation contributes towards the acidification of the system, causing stabilisation of pH at around 4.5−5. Secondary mineralisation (especially Fe- and Al-mineral precipitation) also removes toxic elements such as As, Pb, Cu, Zn, Cd, Co, Ni and Mn, and these secondary minerals can also buffer acidity when the pH tends to be acidic.

In contrast, the pH of the leachates from the PKFA cover scenario gradually increased from strongly acidic to mildly acidic and circumneutral along with decrease in EC and elemental leaching in different WRs. Gradually increasing pH can be attributed to the cover effect, which reduces oxygen ingress, thus sulphide oxidation, causing pH to elevate. Due to the fact that pH~4–5 is sufficient for secondary Fe- and Al-mineral precipitation which also removes toxic elements (such as Cd, Co, Cu, Zn and Ni) by adsorption and/or co-precipitation, the FA cover performs well enough to achieve that pH until the conclusion of the CLE. However, due to the slower reactivity of the buffering system (additional to the initial flush-out), leaching in the beginning could not be restricted.

The co-disposal of FA as cover and/or mixture possesses potential for neutralisation of AMD and improving leachate quality significantly. Particle size of the WRs affected the leaching of the sulphide related elements (such as Fe, S, Zn, Co, Cr, Cu, Mn and Ni) in CLE and WCE. Experiments with ≤1mm particle size constantly produced acidic and metal laden leachates. Co-disposal of FA and WRs as cover and mixture need to be investigated on pilot-scales before full-scale application.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2016. 50 p.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keyword
Acid mine drainage (AMD), coal mine waste rocks, coal fly ash, co-disposal, mixing/blending, dry-cover, prevention/neutralisation
National Category
Environmental Management Mineral and Mine Engineering Environmental Sciences Geochemistry
Research subject
Applied Geochemistry
Identifiers
urn:nbn:se:ltu:diva-60261 (URN)978-91-7583-751-2 (ISBN)978-91-7583-752-9 (pdf) (ISBN)
Public defence
2016-12-15, F341, F-house, Luleå University of Technology, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2016-11-10 Created: 2016-11-09 Last updated: 2016-11-24Bibliographically approved

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