Passive remediation of acid mine drainage using cryptocrystalline magnesite: A batch experimental and geochemical modelling approach

Acid mine drainage is generated when mining activities expose sulphidic rock to water and oxygen leading to generation of sulphuric acid effluents rich in Fe, Al, SO4 and Mn with minor concentrations of Zn, Cu, Mg, Ca, Pb depending on the geology of the rock hosting the minerals. These effluents must be collected and treated before release into surface water bodies. Mining companies are in constant search for cheaper, effective and efficient mine water treatment technologies. This study assessed the potential of applying magnesite as an initial remediation step in an integrated acid mine drainage (AMD) management system. Neutralization and metal attenuation was evaluated using batch laboratory experiments and simulations using geochemical modelling. Contact of AMD with cryptocrystalline magnesite for 60 min at 1 g: 100 mℓ S/L ratio led to an increase in pH, and a significant increase in metals attenuation. Sulphate concentration was reduced to ≈1 910 mg/ℓ. PH redox equilibrium (in C language) (PHREEQC) geochemical modelling results showed that metals precipitated out of solution to form complex mineral phases of oxy-hydroxysulphates, hydroxides, gypsum and dolomite. The results of this study showed that magnesite has potential to neutralize AMD, leading to the reduction of sulphate and precipitation of metals.SP201

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 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

Hydrothermal Conversion of South African Coal Fly Ash into Pure Phase Zeolite Na‐P1

South African coal combustion power utilities generate huge amounts of coal fly ash that can be beneficiated into zeolitic products. This chapter reports on the optimization of the presynthesis and synthesis conditions for a pure‐phase zeolite Na‐P1 from selected South African coal fly ashes. The hydrothermal treatment time, temperature, and molar quantities of water during the hydrothermal treatment step were successfully optimized. The optimum hydrothermal treatment time and temperature were 48 h and 140°C, respectively. Pure‐phase zeolite Na‐P1 was obtained with a molar regime of 1 SiO2:0.36 Al2O3:0.59 NaOH:0.49 H2O at an aging temperature of 47°C for 48 h. The optimized conditions were applied to two fly ashes from two coal‐fired power utilities, and high‐purity zeolite Na‐P1 was obtained. The third coal fly ash with a different chemical composition gave a low‐quality Na‐P1 under the optimized conditions. The cation exchange capacity for the high‐purity zeolite phase was 4.11 mEq/g, indicating that the adjustment of reactant composition and presynthesis or synthesis parameters leads to yields of high‐quality zeolite Na‐P1. The results also show that conversion of the coal fly ash into high‐purity zeolite also depends on the chemical and mineralogical composition of the coal fly ash

A Geochemical Analytical Scheme for the Appraisal of Partitioning and Mobility of Major elements in Weathered Dry Disposed Coal Fly Ash

South Africa is endowed with significant deposits of coal which is utilized in electricity generation to meet the nation’s energy demand. A large volume of waste solid residue from the combustion of pulverized feed coal in power stations is dry disposed in stock piles or dumps. Chemical interactions of dry disposed fly ash with ingressed CO2 from the atmosphere and infiltrating rain water would cause dissolution of the soluble components in the fly ash matrix. Chemical partitioning and mobility of major elements in samples from cores drilled into serially stacked weathered dry disposed fly ash were investigated using a modified five steps sequential extraction scheme. A total acid digestion was carried out on the original ash core samples prior to extraction to validate the extraction procedure. The geochemical distribution of the investigated major elements in 59 drilled core samples was determined by x-ray fluorescence and inductively coupled plasma mass spectrometry. The relationship between SiO2 and chemical index of alteration (CIA) showed 8 year and 20-year-old core samples have a moderate to high degree of weathering. Conversely, 1-year-old cores samples showed characteristics between low and moderate-high degrees of weathering. A cluster and discriminant analysis of the major elements was also able to reveal the subtle chemical alteration differences of the core samples. Functional analysis revealed the disparities in the dissolution patterns of major soluble components in the matrix of the drilled core samples. Modified sequential extractions revealed high concentration of the major species in the leachates for every mineralogical fraction; although the bulk of the major elements are locked up in the insoluble phase of the core samples (i.e. residual fraction) which would not be released under normal environmental conditions. It is noteworthy that the concentration of major elements in the labile fractions (water soluble + exchangeable + carbonate) was high and this has implications for the long-term durability of residual mineral phases. Relative enrichment and depletion trends of major elements are promoted by heterogeneity in the ash dump (i.e. moisture content), gradual reduction of pore water pH and continuous brine and water irrigation.Key words: Coal fly ash; Weathering; Sequential extraction scheme; Cluster analysis; Factor Analysis

Mineralogy and Geochemical Appraisal of Paleo-Redox Indicators in Maastrichtian Outcrop Shales of Mamu Formation, Anambra Basin, Nigeria

The Mamu Formation exhibits two types of shales, viz. grey and dark shales. The geochemical and mineralogical compositions of these shales were investigated using X-ray fluorescence (XRF) and Laser Ablation- Inductively Coupled Plasma Spectrometry (LA-ICPMS) and X-ray diffraction techniques. The basal part of the section is characterized by presence of quartz and kaolinite as the major crystalline minerals with minor quantity of hematite. The presence of hematite in the basal part of the shale sequence suggests oxidizing diagenetic environment of deposition. The second geochemically specific interval (upper part) is characterised by quartz and kaolinite as major crystalline minerals with traces of halloysite and grossite. The ternary plot of these major elements indicates the majority of shale samples examined are variably enriched with SiO2 relative to Al2O3 and CaO. The positive correlations of K2O, TiO2, and Na2O, with Al2O3 indicate that these elements are associated entirely with detrital phases. Some trace elements such as Cr, Ni, and V are positively correlated with Al2O3 which suggest that these elements may be bound in clay minerals and concentrated during weathering. The K2O/Al2O3 ratio is close to the lower limit of clay mineral range, which suggests that kaolinite is the dominant clay minerals. The Al2O3/TiO2 and low Cr/Ni ratios suggest that felsic components were the main components among the basement complex source rocks. The geochemical indices such as Th/Cr, Cr/Th, Th/Co and Th/Cr ratios suggest that these shales were derived from felsic source rocks. The chemical index of alteration values indicates that these shales have experienced strong chemical weathering at the source area. In addition, the depletion of Na and Ca also illustrates an intense chemical weathering of the source rocks. The mineralogical index of alteration values of the studied shale samples indicates an intense to extreme weathering of mineralogical components. The shale units exhibits different degrees of trace-element enrichment, with the approximate order of enrichment relative to an average shale being Co > Pb > Ni > Zr > Cu > Rb > V > Cr > Ba > V > Sr > U. The inverse correlation between Eh, pH, EC and TDS in outcrop Maastrichtian shale samples suggests well oxygenated environment of deposition. In addition, based on previously established thresholds, V/Cr, Ni/Co, Cu/Zn and U/Th ratios support that these shales were deposited under oxidizing diagenetic environment. Keywords: mineralogy, geochemistry, paleo-redox conditions, trace element enrichments, shales, Mamu Formation, Anambra basin, Nigeria

An Investigative Study on the Chemical, Morphological and Mineralogical Alterations of Dry Disposed Fly Ash During Sequential Chemical Extraction

The hazardous elements associated with various physicochemical forms in coal fly ash are of environmental concern due to their leaching potential and subsequent contamination of surface and groundwater in the vicinity of the ash dump. Selective sequential extraction was performed on dry disposed fly ash samples from a coal-fired power station in Mpumalanga province, South Africa. The alteration of the chemical, morphological and mineralogical species of weathered fly ash during the selective sequential extraction was investigated using X-ray fluorescence (XRF), Nano-scan electron microscopy (NANOSEM) and X-ray powder diffraction (XRD). Insoluble residue from the water-soluble fraction is composed of amorphous alumino-silicate. The residues from exchangeable carbonate and Fe and Mn fractions consisted mostly of amorphous alumino-silicate spheres with a lesser quantity of iron-rich spheres. The iron-rich spheres are surrounded by amorphous alumino-silicate spheres.  The leaching behavior of trace metals (such as Ce, Y, Nb, Rb, U, and Tl) in weathered dry disposed fly ash was considered to have a dependency relationship with the components of SiO2, CaO, MgO, P2O5, and amount of unburned carbon. The decrease in the quantities of calcite with successive extraction could be considered as a marker of progress of sequential extraction technique. At the same time, the increase in the quantities of quartz could be also considered as an indicator of progress of the sequential extraction scheme. Trace elements bound to exchangeable or carbonate fraction during sequential chemical extraction were found associated with calcite. The elemental concentrations, as determined by electron dispersive x-ray spectroscopy (EDS), were consistent with XRF and XRD data. Therefore, the chemical extractant used in this study proved efficient for extraction of inorganic metals associated with various physicochemical forms in weathered fly ash.Key words: Coal fly ash; Selective sequential extraction; Major components; Trace elements; Morphology; Mineralogical compositio

Geochemistry and mineralogy of the Campanian Sandstone of Lokoja-Basange Formation, Middle Niger Basin (Lokoja sub-basin), Nigeria: Implications for provenance, weathering, tectonic setting and paleo-redox condition

Twenty six road-cut sandstone samples from two lithological sections of Lokoja-Basange Formation in Middle Niger Basin (Lokoja sub-basin) situated by the side of Auchi-Igarra road (07º 07.201'N, 006º 13.011'E) were investigated using integrated mineralogical, geochemical and pore water chemistry studies. The medium to coarse grained sandstone bodies are poorly sorted suggestive of deposition in a low energy setting, probably in a shelf or floodplain. The observed variations in the sandstone colourations are attributed to the nature of the cementing materials. Based on the mineralogical composition, two specific geochemical intervals were established; the first interval revealed quartz and kaolinite as major crystalline minerals with traces of hematite. The second geochemical interval showed quartz and kaolinite as the major crystalline minerals with minor quantities of grossite and halloysite. The geochemical datasets obtained revealed mature, lithic arenites including sub-greywacke and protoquartzites. The inverse correlation between redox potential (Eh) and electrical conductivity (EC), total dissolved solids (TDS) and Mg (at 0.05 significant levels) suggest well oxygenated environment of deposition. The high chemical index of alteration (CIA), plagioclase index of alteration (PIA), and chemical index of weathering (CIW) indices revealed high detrital input dominated by intense chemical weathering. This process eventually led to the formation of clay minerals by hydration and leaching of all major cations, such as Ca+2, K+, and Na+, present in feldspar minerals. The average mineralogical index of alteration (MIA) values are indicative of intense to extreme weathering of mineralogical component of the detrital materials from the source areas. The studied sandstones samples are plotted in the field of the active and passive continental margin settings. The mean ratio of Ti/Zr also corroborates active and passive continental margin settings. The higher ratios of La/Y and La/Th and corresponding lower ratios of La/Co and Th/Co indicates felsic source rock. Moreover, the lower ratios of Ba/Sr, Cr/Zr, Ti/Zr and higher ratio of Zr/Y probably suggest felsic source rock. Based on the previously established thresholds, the low Cu/Zn ratios in the studied sandstone samples suggest deposition under oxidizing conditions. Keywords: Lokoja-Basange Formation, Middle Niger Basin, Mineralogy, Geochemistry, Pore water chemistry, provenance, tectonic setting, weathering, Redox proxy