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Co-disposal of waste rock and lignite fly ash to mitigate the environmental impact of coal mining
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
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 [en]
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: urn:nbn:se:ltu:diva-60261ISBN: 978-91-7583-751-2ISBN: 978-91-7583-752-9 (pdf)OAI: oai:DiVA.org:ltu-60261DiVA: diva2:1045544
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
List of papers
1. Potential of coal mine waste rock for generating acid mine drainage
Open this publication in new window or tab >>Potential of coal mine waste rock for generating acid mine drainage
2016 (English)In: Journal of Geochemical Exploration, ISSN 0375-6742, E-ISSN 1879-1689, Vol. 160, 44-54 p.Article in journal (Refereed) Published
Abstract [en]

Acid mine drainage (AMD) due to the oxidation of sulphide bearing waste rock (WR) is a common environmental problem associated with coal extraction. Therefore, WRs from the Lakhra coal field in Pakistan, were studied to i) perform a mineralogical and chemical characterisation, ii) determine the AMD generating potential and iii) estimate the leachability of elements. The chemical and mineralogical composition was studied using ICP, XRF, XRD and SEM. Acid base accounting and weathering cell test determined the acid producing potential of WRs. Besides organic material, the WRs were composed of quartz, pyrite, kaolinite, hematite and gypsum with varying amounts of calcite, lime, malladerite, spangolite, franklinite and birnessite. 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 (19.4–113.3 g/kg). Trace elements were in the range (mg/kg) As (0.3–8), Cd (0.2–0.4), Co (15–75), Cr (67–111), Cu (25–101), Hg (0.1–0.2), Ni (50–107), Pb (8–20) and Zn (75–135). The AMD potential of WRs ranged from − 70 to − 492 kg CaCO3/tonne. During the test period of 192 days, the pH of leachates from very 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 element concentrations of Fe, SO42 − and Al from mg/L to g/L and As, B, Co, Cu, Mn, Ni and Zn from μg/L to mg/L. However, the leachates from less acidic WRs contained all major elements in mg/L and trace elements in μg/L concentrations except for B and Mn that ranged from μg/L to mg/L. The results show that the studied WRs have mild to strong acid producing potential and have the capacity to deteriorate natural water quality significantly. Therefore, necessary preventive or/and acid neutralising measures are strongly suggested.

Research subject
Applied Geology
Identifiers
urn:nbn:se:ltu:diva-15628 (URN)10.1016/j.gexplo.2015.10.014 (DOI)f29da7e5-199c-45c9-83de-1564c56d6769 (Local ID)f29da7e5-199c-45c9-83de-1564c56d6769 (Archive number)f29da7e5-199c-45c9-83de-1564c56d6769 (OAI)
Note
Validerad; 2015; Nivå 2; 20151026 (andbra)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2016-11-09Bibliographically approved
2. Potential of fly ash for neutralisation of acid mine drainage
Open this publication in new window or tab >>Potential of fly ash for neutralisation of acid mine drainage
2016 (English)In: Environmental science and pollution research international, ISSN 0944-1344, E-ISSN 1614-7499, Vol. 23, no 17, 17083-17094 p.Article in journal (Refereed) Published
Abstract [en]

Lignite (PK), bituminous (FI) and biomass (SE) fly ashes (FAs) were mineralogically and geochemically characterised, and their element leachability was studied with batch leaching tests. The potential for acid neutralisation (ANP) was quantified by their buffering capacity, reflecting their potential for neutralisation of acid mine drainage. Quartz was the common mineral in FAs detected by XRD with iron oxide, anhydrite, and magnesioferrite in PK, mullite and lime in FI, and calcite and anorthite in SE. All the FAs had high contents of major elements such as Fe, Si, Al and Ca. The Ca content in SE was six and eight times higher compared to PK and FI, respectively. Sulphur content in PK and SE was one magnitude higher than FI. Iron concentrations were higher in PK. The trace element concentrations varied between the FAs. SE had the highest ANP (corresponding to 275 kg CaCO3 tonne-1) which was 15 and 10 times higher than PK and FI, respectively. The concentrations of Ca2+, SO4 2-, Na+ and Cl- in the leachates were much higher compared to other elements from all FA samples. Iron, Cu and Hg were not detected in any of the FA leachates because of their mild to strong alkaline nature with pH ranging from 9 to 13. Potassium leached in much higher quantity from SE than from the other ashes. Arsenic, Mn and Ni leached from PK only, while Co and Pb from SE only. The concentrations of Zn were higher in the leachates from SE. The FAs used in this study have strong potential for the neutralisation of AMD due to their alkaline nature. However, on the other hand, FAs must be further investigated, with scaled-up experiments before full-scale application, because they might leach pronounced concentrations of elements of concern with decreasing pH while neutralising AMD.

National Category
Geochemistry
Research subject
Applied Geology; Tillämpad geokemi
Identifiers
urn:nbn:se:ltu:diva-13259 (URN)10.1007/s11356-016-6862-3 (DOI)000382674800019 ()c73d75aa-70a2-4066-a4b0-cc4ad0bbc97c (Local ID)c73d75aa-70a2-4066-a4b0-cc4ad0bbc97c (Archive number)c73d75aa-70a2-4066-a4b0-cc4ad0bbc97c (OAI)
Note

Validerad; 2016; Nivå 2; 2016-10-18 (andbra)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2016-11-09Bibliographically approved
3. Co-disposal of lignite fly ash and coal mine waste rock for neutralisation of AMD
Open this publication in new window or tab >>Co-disposal of lignite fly ash and coal mine waste rock for neutralisation of AMD
(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
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:nbn:se:ltu:diva-60260 (URN)
Available from: 2016-11-09 Created: 2016-11-09 Last updated: 2016-11-09
4. Effects of the co-disposal of lignite fly ash and coal mine waste rocks on AMD and leachate quality
Open this publication in new window or tab >>Effects of the co-disposal of lignite fly ash and coal mine waste rocks on AMD and leachate quality
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Lignite fly ash (FA) and waste rocks (WRs) were mixed in three different ratios (1:1, 1:3 and 1:5) and studied to compare the effects of adding FA on AMD generation from coal mining WRs, leachability of elements and the potential occurrence of secondary minerals. FA mixed with WRs showed significant differences in pH levels compared to previous research. The 1:1 mixture performed best of all the three mixtures in terms of pH and leachability of elements, mainly due to the higher proportion of FA in the mixture. The pH in the 1:1 mixtures varied between 3.3 – 5.1 compared to other mixtures (2.3­ – 3.5). Iron and SO42- leached considerably less from the 1:1 mixture compared to the others, indicating that the oxidation of sulphides was weaker in this mixture. Aluminium leached to a high degree from all mixtures, with concentrations varying from mg L-1–g L-1. The reason behind this increase is probably the addition of FA which, due to acidic conditions and the composition of the FA, increases the availability of Al. For the same reason, high concentrations of Mn and Zn were also measured. Geochemical modelling indicates that the 1:1 mixture performs better in terms of precipitation of Al3+ minerals, whereas Fe3+ minerals precipitated more in mixtures containing less FA. These results suggest that, with time, the pores could possibly be filled with these secondary minerals and sulphate salts (followed by a decrease in sulphide oxidation), improving the pore water pH and decreasing the leachability of elements. Since grain size plays a crucial role in the reactivity of sulphides, there is a risk that the results from the leaching tests may have been influenced by crushing and milling of the WR samples.

Keyword
Coal mine waste rock, acid mine drainage (AMD), fly ash mixing, weathering cells, PHREEQC, element leaching
National Category
Geochemistry Environmental Sciences Environmental Management Mineral and Mine Engineering
Research subject
Applied Geochemistry
Identifiers
urn:nbn:se:ltu:diva-60259 (URN)
Available from: 2016-11-09 Created: 2016-11-09 Last updated: 2016-11-09

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