Fate of sulphate removed during the treatment of circumneutral mine water and acid mine drainage with coal fly ash: Modelling and experimental approach

The treatment of acid mine drainage (AMD) and circumneutral mine water (CMW) with South African coal fly ash (FA) provides a low cost and alternative technique for treating mine wastes waters. The sulphate concentration in AMD can be reduced significantly when AMD was treated with the FA to pH 9. On the other hand an insignificant amount of sulphate was removed when CMW (containing a very low concentration of Fe and Al) was treated using FA to pH 9. The levels of Fe and Al, and the final solution pH in the AMD–fly ash mixture played a significant role on the level of sulphate removal in contrast to CMW–fly ash mixtures. In this study, a modelling approach using PHREEQC geochemical modelling software was combined with AMD–fly ash and/or CMW–fly ash neutralization experiments in order to predict the mineral phases involved in sulphate removal. The effects of solution pH and Fe and Al concentration in mine water on sulphate were also investigated. The results obtained showed that sulphate, Fe, Al, Mg and Mn removal from AMD and/or CMW with fly ash is a function of solution pH. The presence of Fe and Al in AMD exhibited buffering characteristic leading to more lime leaching from FA into mine water, hence increasing the concentration of Ca2+. This resulted in increased removal of sulphate as CaSO4·2H2O. In addition the sulphate removal was enhanced through the precipitation as Fe and Al oxyhydroxysulphates (as shown by geochemical modelling) in AMD–fly ash system. The low concentration of Fe and Al in CMW resulted in sulphate removal depending mainly on CaSO4·2H2O. The results of this study would have implications on the design of treatment methods relevant for different mine waters.Web of Scienc

Chemical, mineralogical and morphological changes in weathered coal fly ash: A case study of a brine impacted wet ash dump

The mobility of species in coal fly ash (FA), co-disposed with brine using a wet ash handling system, from a coal fired power generating utility has been investigated. The study was conducted in order to establish if the wet ash dump could act as a salt sink. The ash was dumped as a slurry with 5:1 brine/ash ratio and the dam was in operation for 20 years. Weathered FA samples were collected along three cores at a South African power station’s wet ash dump by drilling and sampling the ash at 1.5 m depth intervals. A fresh FA sample was collected from the hoppers in the ash collection system at the power station. Characterization of both fresh FA and weathered FA obtained from the drilled cores S1, S2 and S3 was done using X-ray diffraction (XRD) for mineralogy, X-ray fluorescence (XRF) for chemical composition and scanning electron microscopy (SEM) for morphology. Analysis of extracted pore water and moisture content determination of the fresh FA and the weathered FA obtained from the drilled cores S1, S2 and S3 was done in order to evaluate the physico-chemical properties of the FA. The XRD analysis revealed changes in mineralogy along cores S1, S2 and S3 in comparison with the fresh FA. The SEM analysis revealed spherical particles with smooth outer surfaces for the fresh FA while the weathered ash samples obtained from cores S1, S2 and S3 consisted of agglomerated, irregular particles appearing to be encrusted, etched and corroded showing that weathering and leaching had occurred in the ash dump. The moisture content (MC) analysis carried out on the fresh FA (1.8%) and the weathered FA obtained from the drilled cores S1 (41.4-73.2%), S2 (30.3-94%) and S3 (21.7-76.2%)indicated that the ash dump was water logged hence creating favourable conditions for leaching of species. The fresh fly ash (n = 3) had a pH of 12.38 ± 0.15, EC value of 4.98 ± 0.03 mS/cm and TDS value of 2.68 ± 0.03 g/L, the pH of the drilled core S1 (n = 45) was 10.04 ± 0.50, the EC value was 1.08 ± 0.14 mS/cm and the TDS value was 0.64 ± 0.08 g/L. Core S2 (n = 105) had pH of 10.04 ± 0.23; EC was 1.08 ± 0.06 mS/cm and TDS was 0.64 ± 0.04 g/L, while core S3 (n = 66) had pH of 11.04 ± 0.09; EC was 0.99 ± 0.03 mS/cm and TDS was 0.57 ± 0.01 g/L. The changes in pH values can be attributed to the dissolution and flushing out of alkaline oxides like CaO and MgO from the dumped ash. The variations in pH values shows that the fly ash is acidifying over time and metal mobility can be expected under these conditions. The large decrease of EC in the drilled ash cores S1, S2 and S3 compared to the fresh ash indicated a major loss of ionic species over time in the ash dump. The XRF analysis showed the progressive dissolution of the major alumi-nosilicate ash matrix which influenced the release of minor and trace elements into the pore water enhancing their mobility as the ash dam acidified over time. Brine co-disposal on the ash may have been responsible for the slight enrichment of some species such as Na (0.27-0.56%), SO4 (0.06-0.08%), Mg (0.57-0.96 %) and K (0.02-0.34%) in the disposed weathered FA. However, there was no significant accumulation of these species in the disposed FA despite continuous addition of large volumes of highly saline brine over the 20 year period that the dump existed, indicating that the ash dam was incapable of holding salts and continually released elements to the environment over the lifetime of the dam.Web of Scienc

Comparison of CO2 capture by ex-situ accelerated carbonation and in in-situ naturally weathered coal fly ash

Natural weathering at coal power plants ash dams occurs via processes such as carbonation, dissolution, co-precipitation and fluid transport mechanisms which are responsible for the long-term chemical, physical and geochemical changes in the ash. Very little information is available on the natural carbon capture potential of wet or dry ash dams. This study investigated the extent of carbon capture in a wet-dumped ash dam and the mineralogical changes promoting CO2 capture, comparing this natural phenomenon with accelerated ex-situ mineral carbonation of fresh fly ash (FA). Significant levels of trace elements of Sr, Ba and Zr were present in both fresh and weathered ash. However Nb, Y, Sr, Th and Ba were found to be enriched in weathered ash compared to fresh ash. Mineralogically, fresh ash is made up of quartz, mullite, hematite, magnetite and lime while weathered and carbonated ashes contained additional phases such as calcite and aragonite. Up to 6.5 wt % CO2 was captured by the fresh FA with a 60% conversion of calcium to CaCO3 via accelerated carbonation (carried out at 2 h, 4Mpa, 90 o C, bulk ash and a S/L ratio of 1). On the other hand 6.8 wt % CO2 was found to have been captured by natural carbonation over a period of 20 years of wet disposed ash. Thus natural carbonation in the ash dumps is significant and may be effective in capturing CO2.Web of Scienc

Evaluation of the leachate chemistry and contaminants attenuation in acid mine drainage by fly ash and its derivatives

Philosophiae Doctor - PhDThe mining industry in South Africa has a huge potential to impact negatively on the environment. Negative impacts include generation of reactive tailings and acid mine drainage (AMD). AMD is highly acidic (pH 2-4), sulphate-rich and frequently carries a heavy metal burden. South Africa uses more than 100 million tonnes of low grade bituminous coal annually to produce cheap electricity. The associated mining operations result in millions of tonnes of polluted water and in turn coal burning power stations produce vast amounts of waste ash such as fly ash. The highly soluble CaO occurring as sub-micron fragments on the fly ash particles is highly reactive and can be utilized in the neutralization of acid mine drainage. Acid mine drainage (AMD) was reacted with two different South African fly ashes in a batch setup in an attempt to evaluate their neutralization and inorganic contaminants removal capacity. The concentrations of major constituents in the AMD were found to determine the final pH attained in the reaction mixture and the reaction time of breakthrough to circum-neutral and alkaline pH. Efficiency of elemental removal in the AMD by the FA was directly linked to the amount of FA in the reaction mixture and to the final pH attained. Most elements attained ≈ 100 % removal only when the pH of minimum solubility of their hydroxides was achieved. In the second part of the study, Acid mine drainage (AMD) was reacted with coal fly ash in a 24 hour equilibration time using 1:3 and 1:1.5 FA: AMD ratios by weight to produce neutral and alkaline process waters. The capacity of the fly ash to remove the major inorganic contaminants from AMD was examined with time. The geochemical computer software PHREEQC and WATEQ4 database were used for geochemical modeling of the process water chemistry at selected reaction times. The collected solid residues were analyzed by X-ray diffraction, scanning electron microscopy (SEM) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX). At both ratios the reaction mixture was at saturation or oversaturated with alunite, basaluminite, jurbanite, boehmite, gibbsite, diaspore, gypsum, barite, K, Na-jarosites, ettringite, amorphous Fe (OH)3 and goethite at specific contact times. The precipitation of the many inorganic contaminants was established in terms of the mineral phases at saturation or over-saturation. Sequential extraction revealed the amorphous fraction to be the most important in retention of the major and minor inorganic contaminants at pH > 6.32 which implies that the concentration of total Fe and Al in the AMD being treated has a direct effect on the clean-up efficiency of the process. In the third part of the study, a column leaching of the solid residues (SR) blended with varying amounts of fly ash (5 %, 25 %, 40 %) and 6 % Ordinary Portland Cement (OPC) was carried out to assess the contaminant attenuation with time. The columns were drained with synthetic acid mine drainage (SAMD) over a period of 165 days. In addition the solid residues were modified with 1-6% OPC and their strength development monitored over a period of 365 days. The column solid cores were observed to acidify in a stepwise fashion, exhibiting three buffer zones. The SR alone and SR blended with fly ash exhibited strong buffering capacity at pH (7.5-9) for an extended period of time (97-110 days). Encapsulation of solid residue particles by the calcium silicate hydrate gels (CSH) in OPC blended solid residues obscured the appearance of the sustained buffering at pH 7-9.5. The fly ash and OPC blend solid residues exhibited decontamination efficiencies of (82-99 %) for Al, Fe, Mn and SO4 2- over the study period. However the OPC blend SR exhibited high attenuation efficiency even as the pH dropped to below 4. SR + 6 % OPC core was observed to be the most efficient interms of retention of highly mobile elements such as B and Mo. pH was observed to be the main determining factor in contaminants attenuation. Geochemical modeling results revealed that pH and SO42- concentrations in the leachate had a significant impact on the mineral phases controlling Fe and Al concentration in the leachates. In the SR + 6 % OPC solid cores, EDX analysis revealed that CSH gels and calcium aluminate hydrate gels were being precipitated. These gels were either incorporating Fe, Mg, Mn in their matrix or encapsulating the solid residue particles that were rich in these elements. Sequential extractions of the leached solid cores revealed the amorphous fraction to be the most important in retention of the major contaminants and were most enhanced in the OPC blend solid residues. The OPC blend solid residue slurries developed unconfined compressive strength (UCS) (2-3 Mpa) comparable to paste formulated from sulphidic rich mine tailings confirming that the solid residues can be used for backfilling. Therefore the solid residues (SR) can successively be applied for a dual purpose in mined out areas namely, to remediate acid mine drainage waters and also provide support for the overburden. Keywords: Acid Mine Drainage; Fly Ash; Neutralization; Sulphates; Metal ions; Solid Residues (SR); Column Leaching; Geochemical Modeling; Sequential Extraction; Buffering.South Afric

Treatment of Acid Mine Drainage with Coal Fly Ash: Exploring the Solution Chemistry and Product Water Quality

A treatment process for Acid mine drainage (AMD) using coal fly ash (CFA) was developed. AMD was treated with CFA as the alkaline agent at different CFA: AMD ratios and pH, electrical conductivity (EC) evolution monitored over time. In a separate experiment two AMD sources with differing chemistry were treated with the same CFA to evaluate the impact of AMD chemistry on the treatment process and product water quality. Various CFA: AMD ratios were stirred in a beaker for a pre-set time and the process water chemistry determined. pH was observed to increase in a stepwise manner with buffer zones observed at 4-4.5, 4.5-7 and 6-8. AMD with low concentration of Al3+, Fe2+, Fe3+ and Mn2+ didn’t exhibit these buffer zones. Two competing processes were observed to control the evolving pH of process water: dissolution of basic oxides (CaO, MgO) from CFA led to pH increase and hydrolysis of AMD species such as Al3+, Fe2+, Fe3+ and Mn2+ led to pH decrease. These processes initiated mechanisms such as precipitation, adsorption and ion exchange that led to decrease in inorganic contaminants as the treatment progressed. Inorganic contaminants removal was directly related to amount of CFA in reaction media. Precipitation of insoluble hydroxides and Al, Fe-oxyhydroxysulphates contributed to removal of major and minor chemical species. Precipitation of gypsum contributed to removal of sulphate. Na, K and Mg remained largely in solution after initial decrease. Significant leaching of B, Sr, Ba, and Mo from CFA was observed and was directly linked to amount of CFA in the reaction media. This will be a shortcoming of the treatment process since other processes may be required to polish up the product water. The treatment of AMD with CFA was observed to depend on CFA, AMD chemistry, treatment time and might therefore be site specific

Synthesis and characterization of zeolites produced by ultrasonication of coal fly ash/NaOH slurry filtrates

Studies were conducted to determine if crystalline zeolites could be obtained from the filtrates of coal fly ash (CFA)/NaOH slurries through sonication. The CFA/NaOH slurries were obtained by aging CFA in 3 M NaOH for 24 h at 50 °C using a solid/liquid (S/L) ratio of 15 g/120 mL. FT-IR studies indicated that the ‘Lo’ regime ultrasonicated zeolite had higher intensities than that of the ‘Hi’ frequency ultrasonicated zeolite. The bands associated with the ‘Lo’ regime ultrasonicated zeolite were observed at 1066cm–1, 1400cm–1 and 3600cm–1, which were identified as Si-Oasymmetric stretch, Al-Oasymmetric stretch, O-Hbend and O-H asymmetric stretch, respectively. Based on PXRD studies, the zeolitic phases were most intense for the 90 min ultrasonicated zeolite. Powder X-ray diffraction studies showed that the phases initially in CFA, such as quartz and aluminosilicate, were converted into gismodine zeolite Na-P1. Morphological changes were seen as ultrasonication period was increased. At short ultrasonication times, irregularly shaped fused-like grain structures were observed, while at longer times separate grain-like structures became more apparent. Energy dispersive X-ray spectroscopy studies on the 90 min ultrasonicated zeolite indicated the presence of Na, O, Si and Al, which were important building-blocks for the zeolites

Optimization of Silica Extraction from Diatomaceous Earth using the Central Composite Design of Response Surface Methodology

This work evaluated the extraction efficiency of silica from diatomaceous earth (DE) by conventional solvent extraction (CSE) and ultrasound-assisted  extraction (UAE) under basic and acidic (HCl) conditions utilizing response surface methodology (RSM). A common statistical model was used to get the  best percentage yield and percentage purity using RSM. Experimental parameters such as stirring time, NaOH concentration (for CSE), sonication time,  NaOH/HCl concentration, cycle and amplitude (for UAE methods) were optimized using central composite design (CCD). The analytical responses, that is,  percentage yield and percentage purity, were analyzed using ANOVA and regression analysis. The linear coefficient of determination, R2 , was high and  precise. The overall yield and purity were highest for the UAE method under acid condition (pH 1–2, approximately), making it the most effective silica  extraction method from DE. For this method, the optimal conditions for extracting Si from DE were 2 h of sonication, 220 mL of 2.82 M HCl as a  leaching/extracting medium, 0.524 cycles and 72.6 % of amplitude. Under these conditions, 82 % of silica was yielded with a purity of about 95 %. The  experimental results indicate that high-quality silica can be produced from DE in high yield so that DE can be an alternative silica source.&nbsp

Synthesis of zeolite-P from coal fly ash derivative and its utilisation in mine-water remediation

Solid residues resulting from the active treatment of acid mine drainage with coal fly ash were successfully converted to zeolite-P under mild hydrothermal treatment conditions. Scanning electron microscopy showed that the zeolite-P product was highly crystalline. The product had a high cation exchange capacity (178.7 meq / 100 g) and surface area (69.1 m2/g) and has potential application in waste-water treatment. A mineralogical analysis of the final product identified zeolite-P, as well as mullite and quartz phases, which indicated incomplete dissolution of the fly ash feedstock during the ageing step. Further optimisation of the synthesis conditions would be required to attain complete utilisation of the feedstock. The zeolite-P was tested for decontamination potential of circumneutral mine water. High removal efficiency was observed in the first treatment, but varied for different contaminants. The synthesised zeolite-P exhibited a high efficiency for the removal of heavy metal cations, such as aluminium, iron, manganese, zinc, copper and nickel, from contaminated mine water, even with repeated use. For potassium, calcium, strontium and barium, the removal was only efficient in the first treatment and decreased rapidly with subsequent treatments, indicating preferential adsorption of the other metals. A continuous release of sodium was observed during decontamination experiments, which decreased with subsequent treatments, confirming that sodium was the main exchangeable charge-balancing cation present in the zeolite-P product