Phase-mineral and chemical composition of coal fly ashes as a basis for their multicomponent utilization. 1. Characterization of feed coals and fly ashes

The phase-mineral and chemical composition of feed coals and their fly ashes (FAs) produced in four large Spanish thermo-electric power stations was characterized as a basis for multicomponent FA utilization. The feed fuels used are bituminous coals, semi-anthracites and anthracites with high detrital mineral abundance and mixed carbonate and sulphide-sulphate authigenic mineral tendency. Their mineral composition includes quartz, kaolinite, illite-muscovite, pyrite, chlorite, plagioclase, K-feldspar, gypsum, siderite, calcite, dolomite, marcasite, montmorillonite, jarosite, and ankerite. The FAs studied have aluminosilicate composition with higher concentrations of alkaline and alkaline-earth oxides than Fe oxide. Elements such as Ag, As, Ba, Cr, Cs, Li, P, Sb, Sc, Sn, Sr, Ti, V, Zn, and Zr are relatively enriched in these FAs in comparison with the respective mean values for bituminous coal ashes worldwide. The FAs consist basically of aluminosilicate glass, to a lesser extent of mineral matter (with high silicate abundance and dominant oxide tendency) and moderate char occurrence. The phase-mineral composition (in decreasing order of significance) of these FAs is normally glass, mullite, quartz, char, kaolinite-metakaolinite, hematite, cristobalite, plagioclase, K-feldspar, melilite, anhydrite, wollastonite, magnetite and corundum plus 42 important accessory minerals or phases. A scheme of conventional separation procedures was applied to recover sequentially six initial and potentially useful and/or hazardous products from FAs, namely: (1) a ceramic cenosphere concentrate; (2) a water-soluble salt concentrate; (3) a magnetic concentrate; (4) a char concentrate; (5) a heavy concentrate; and finally (6) an improved FA residue.     

A new approach for the classification of coal fly ashes based on their origin,composition, properties,and behaviour

The present work introduces and evaluates a new approach for the classification system of coal fly ashes (FAs) based on their origin, phase-mineral and chemical composition, properties, and behaviour. The detailed data for 41 FAs produced from various feed coals at 37 coal-fired thermo-electric power stations (TPSs) in Spain, Bulgaria, The Netherlands, Italy, Turkey, and Greece were used for that purpose. The chemical FA classification system was organized according to the contents, common geochemical associations, and significant positive or negative correlations of ash-forming elements in FAs using three end members, namely (1) sum of Si, Al, K, Ti and P oxides; (2) sum of Ca, Mg, S, Na and Mn oxides; and (3) Fe oxide. This approach divided four chemical FA types (Sialic, Calsialic, Ferrisialic, and Ferricalsialic) with three dominant tendencies (high acid, medium acid, and low acid). The most important phase-mineral FA classification system was organized according to the contents, associations, correlations, properties, and behaviours of species in FAs using also three end members, namely: (1) glass; (2) quartz + mullite; and (3) other minerals. This approach divided four phase-mineral FA types (Pozzolanic, Inert, Active, and Mixed) with three dominant tendencies (high pozzolanic, medium pozzolanic, and low pozzolanic). The specified chemical and phase-mineral FA types and subtypes were characterized and the relationships and distinctions between them were also described. It was found that characteristics such as (1) feed coal and combustion technology used in TPS; (2) water–soluble, magnetic and heavy fractions, pH, fluid ash-fusion temperature, detrital/authigenic index, and BET specific surface area of FAs; and especially (3) content, modes of occurrence, and distribution of glass, quartz, mullite, lime-portlandite, periclase-brucite, Ca sulphates, Ca and Ca–Mg silicates, magnetite-hematite, and char types in FAs; give the most valuable information for the determination of the potential utilization directions and environmental concerns of FAs.     

Mineralogy of western fly ash

Twenty-six fly ash specimens from North Dakota, Wyoming and Montana lignite and sub-bituminous source coals have been studied in detail by X-ray diffraction. Chemically, these western fly ashes are characterized by higher CaO+MgO+SO₃ contents and lower Al₂O₃+SiO₂ contents than eastern bituminous fly ashes. These western fly ashes have greater proportions of crystalline material. The characteristic phases are quartz, lime, periclase, anhydrite, ferrite spinel, tricalcium aluminate, merwinite and melilite. Alkali sulfates, a sodalite structure phase and hematite also occur in some fly ashes.     

Mineralogy and microstructure of sintered lignite coal fly ash☆

Lignite coal fly ash from the ‘Nikola Tesla’ power plant in Yugoslavia has been characterised, milled, compacted and sintered to form monolithic ceramic materials. The effect of firing at temperatures between 1130 and 1190 °C on the density, water accessible porosity, mineralogy and microstructure of sintered samples is reported. This class C fly ash has an initial average particle size of 82 μm and contains siliceous glass together with the crystalline phases quartz, anorthite, gehlenite, hematite and mullite. Milling the ash to an average particle size of 5.6 μm, compacting and firing at 1170 °C for 1 h produces materials with densities similar to clay-based ceramics that exhibit low water absorption. Sintering reduces the amount of glass, quartz, gehlenite and anhydrite, but increases formation of anorthite, mullite, hematite and cristobalite. SEM confirms the formation of a dense ceramic at 1170 °C and indicates that pyroplastic effects cause pore formation and bloating at 1190 °C.     

Study of Heat Effect on the Composition of Dunes Sand of Ouargla (Algeria) Using XRD and FTIR

The spectra of x-ray diffraction (XRD) and infrared spectroscopy (FTIR) shows that the dunes sand of Ouargla’s region consists naturally of crystalline structures of α-quartz and gypsum, as well as other uncrystallized compounds with low concentrations like kaolinite and hematite, in addition to some organic compounds. The sand heating process at temperatures between 200 and 1200 °C affects its composition. By heating at 200 °C crystalline phases of anhydrite and bassanite appear due to the continuing loss of water from the gypsum. All the gypsum transforms into anhydrite, and the kaolinite transforms into metakaolin because of the breaking of the OH bond, producing water vapor by heating in the range of 400–800 °C. The heating at 1000 °C disassembles the kaolinite into aluminium-silicon and cristobalite, and leads to the emergence of a new crystalline phase related to wollastonite resulting from the start of a reaction between the anhydride and the quartz. Heating at 1200 °C leads to the disappearance of all the anhydrite because of its interaction with the quartz, producing the wollastonite and the release of sulfur dioxide SO₂ and oxygen O₂, in addition to the increase of the cristobalite proportion because of the disintegration of all the kaolinite into mullite and cristobalite, or the transformation of quartz phase into cristobalite. Also occuring is an interaction between the hematite and the quartz producing the ferrosilite characterized by its green color.     

Features of phase formation of mullite-corundum materials in mixtures of kaolin with a fluoriding component

Processes are studied for synthesis of mullite from kaolin by an unusual method (by fluoride technology) in the temperature range from 850 to 1300°C. Depending on the amount of added fluorine agent it is possible to regulate the ratio of crystal phases that form (acicular mullite and lamellar corundum), that creates prerequisites for controlling preparation of aluminosilicate ceramic with a specific phase composition and a predictable microstructure.     

Scheme for density separation and identification of compound forms in size-fractionated fly ash

A scheme has been developed for the separation of compounds in size-fractionated fly ash by linear density gradients. Minor crystalline components such as calcium ferrite, haematite, magnetite, MgO, barite, anhydrite, rutile and mica were identified in addition to the major compounds mullite, quartz and CaO in a fly ash of pulverized subbituminous coal from western USA. The utility of the scheme in identifying minor crystalline components in fly ash has been demonstrated.     

Low cost catalytic sorbents for NOx reduction. 1. Preparation and characterization of coal char impregnated with model vanadium components and petroleum coke ash

Spanish coal, char and activated char doped with model vanadium components (V₂O₅ and NH₄VO₃) and petroleum coke ash (enriched in V, Fe, and Ni) were prepared and characterized as potential catalytic sorbents for NO_x reduction. The phase-mineral and chemical composition, content and behavior (capture, retention, distribution, and redistribution) of transition metals, as well as morphogenesis, surface area, acid-base properties, surface active sites and oxidation-reduction transformations of the catalytic sorbents were characterized. It was found that minerals and phases such as anhydrite, calcite, clay minerals, pyrite, pyrrhotite, magnetite and fusinoid-type ingredients have a leading role for the behavior of loaded transition metals. Some original (pyrite, jarosite, shcherbinaite, coulsonite, trevorite, Ni oxide) and newly formed (pyrrhotite, magnetite, wuestite, hematite, paramontroseite, karelianite) Fe, V and Ni minerals in the catalytic sorbents are perspective redox indicators for the physicochemical conditions in such complex system. The data indicate that the V-Fe-Ni containing minerals dispersed onto and into the carbon support may be the most active catalytic sites. The preparation procedure that could provide the most favorable conditions for the production of effective and low cost catalytic sorbents for NO_x reduction is also described.     

Effect of some properties of aluminosilicate refractories on their glass resistance

Conclusions The glass resistance of aluminosilicate blocks with similar porosity values depends mainly on the quantitative ratios of crystalline and glassy phases, their chemical composition, and the grain size of the mullite. The higher the crystalline phase concentration in the refractory (mullite and mullite+ corundum) the fewer the impurities (oxides of iron, titanium, calcium, sodium, potassium, etc.), and the coarser the grains of mullite, then the lower their corrosion by molten glass.An increase in the glass resistance of aluminosilicate blocks can be achieved by using pure clays and kaolins, and also by using high temperatures for firing, longer soaking periods, and also mineralizing additives.Translated from Ogneupory, No.6, pp. 40–44, June, 1970.     

High capacity cation exchanger by hydrothermal zeolitization of coal fly ash

During; hydrothermal treatment in NaOH medium coal fly ash partially transformed to zeolite P and/or hydroxysodalite, while quartz slowly dissolved and mullite remained stable. Residual coal favored the formation of zeolite P. The relative proportion of the two zeolites could be changed by seeding. During fusion of fly ash with NaOH an endothermic reaction at 170–180°C occurred, resulting in formation of an unreported Na aluminosilicate with approximate composition Na₁₅Si₄Al₃0₂₀ and major XIRD reflections at 4.793, 3.828, 3.000, 2.854, 2.578, and 2.524 Å. The product of fusion interacted with water giving aluminosilicate gel, which yielded zeolite P upon hydrothermal treatment at 100°C without aging and zeolite X after aging in water for 12 hours at room temperature. Cation exchange capacity was 420 and 400 meq/ 100 g for zeolite P and zeolite X respectively.     

A PETROGRAPHIC STUDY OF SOME SLAGS FROM BOILER FURNACES1

Observations are based on a microscopic study of the effect of slags, particularly those of an Illinois coal, on refractories. A mineralogical relation is shown between the slag and the refractory used. The principal phases present in a cooled slag are: (1) a plagioclase feldspar, (2) magnetite or hematite, (3) mullite with iron oxide in solid solution, (4) glass. The so-called “interface” between slag and refractory is largely crystalline upon cooling and consists principally of mullite which, surrounded by the slag, has grown from fine needles in the refractory to larger ones. In high alumina refractories where diaspor is present, the stained interface is much thicker and grains of corundum are formed. This zone of crystallization may be conducive to spalling. The effect of slags on various refractories is discussed.     

Changes in the properties of aluminosilicate refractories under prolonged reducing conditions

Conclusions Studies of corundum and aluminosilicate refractories of dense and granular structures in an atmosphere of hydrogen and dissociated ammonia at 1200, 1500, and 1700°C in periods of 175 and 50 h showed that the resistance of the products increases with an increase in the alumina concentration and density. The maximum resistance is exhibited by corundum products. In the aluminosilicate refractories there is some additional sintering of the material with the separation of mullite and glass. Simultaneously on the surface of the specimens we detected deeper mineralogical changes, accompanied by the decomposition of the mullite, with the formation of corundum, silicon monoxide, and glass.The changes in the phase composition are accompanied by a change in the structure, and an increase in the creep. Considering that a reduction in the temperature of 100°C causes a reduction in the creep by approximately a half [26], it can be recommended that corundum refractories should be used (under a load of 2 kg/cm²) in a reducing atmosphere at temperatures of up to 1550–1600°C, sillimanite up to 1450–1500°C, kaolin and chamotte (high-grog) up to 1300°C, with a reduction in the load and an increase in the density, the temperature of application for the products examined, especially corundum, can be increased.Translated from Ogneupory, No.5, pp.26–32, May, 1972.     

循环流化床锅炉脱硫灰和普通粉煤灰的特性研究

分析研究了循环流化床锅炉脱硫灰和普通粉煤灰的化学组成、矿物相组成和热行为等。研究结果表明,脱硫灰烧失量大,CaO含量高,其主要晶相为石英、方解石和硬石膏,还有少量莫来石,玻璃相含量较少。普通粉煤灰的主要晶相为莫来石和石英相。扫描电镜观察,普通粉煤灰主要以球形玻璃微珠为主,球形微珠表面光滑。脱硫灰则以不规则形状的颗粒为主,几乎无球形颗粒。两种粉煤灰的红外光谱分析和差热分析也表现出明显差异,主要归因于脱硫灰中大量存在的硬石膏和方解石。     

Fired Porcelain Microstructures Revisited

The microstructure of standard commercial porcelain consists of alpha-quartz grains held in a complex matrix. Quartz grains are surrounded by amorphous silica-rich solution rims while the matrix contains clay relicts of small (~200 nm × 40 nm × 40 nm) primary (2Al₂O₃:1SiO₂) mullite crystals in aluminosilicate glass and feldspar relicts of acicular (>1 µm long) secondary (3Al₂O₃:2SiO₂) mullite in potassium aluminosilicate glass. A continuous increase in mullite crystal size from the clay-feldspar relict interface to the feldspar relict center and their compositions indicate a transformation from primary to secondary mullite. In aluminous porcelain, corundum grains are observed in addition to alpha-quartz and regions of clay and feldspar relicts. Small (~50 nm × 10 nm × 10 nm) tertiary mullite crystals (formed by precipitation from Al₂O₃-rich glass) were detected adjacent to these corundum grains.     

Mineralogy and geochemistry of Greek and Chinese coal fly ash

In this paper the mineralogy and geochemistry of Greek and Chinese coal fly ash are examined. Annual production of fly ash in China is around 160 Mt while in Greece lignite fly ash accounts around 10 Mt. Even though the mineralogical and chemical composition of the fly ashes coming from these two countries differs, there are common questions on the utilization of this material. The variation of the Greek fly ash’ chemical composition, from Ca-poor to Ca-rich fly ash, has resulted to applications such as dam construction, use in cement and possibly in concrete and road construction. The Chinese fly ash, which is rich in mullite, is broadly applied for brick making.     

Degradation mechanism of high alumina refractory bricks by reaction with deposits in a rotary kiln for fluxed iron ore pellets production

The formed deposits wear out of refractory wall linings in the rotary kiln and may cause production disturbances. This study describes the chemical composition and mineralogical phase components at the deposit/refractory interface in the rotary kiln for fluxed iron ore pellets production. The main phases of refractory bricks are corundum and mullite, while the deposits mainly contain hematite and silicates. The main phases in the deposit/refractory brick contact zone are hematite, anorthite (CaAl₂Si₂O₈), mullite, corundum, and silicates. Moreover, the hematite phases in the deposit/brick interface averagely contain 6.98 wt% Al and 1.38 wt% Ti. The silicates in the contact zone contain higher aluminium content and lower iron content than the silicates in the deposits. Finally, the thermodynamic analysis indicates that the main phases in the deposits can react with the refractory to form Al₂Fe₂O₆, CaAl₂Si₂O₈, feldspar, and liquid phases lead to the degradation of bricks in the kiln during the iron ore pellets production.     

Mineralogical and elemental composition of fly ash from pilot scale fluidised bed combustion of lignite, bituminous coal, wood chips and their blends

The chemical and mineralogical composition of fly ash samples collected from different parts of a laboratory and a pilot scale CFB facility has been investigated. The fabric filter and the second cyclone of the two facilities were chosen as sampling points. The fuels used were Greek lignite (from the Florina basin), Polish coal and wood chips. Characterization of the fly ash samples was conducted by means of X-ray fluorescence (XRF), inductive coupled plasma-optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), particle size distribution (PSD) and X-ray diffraction (XRD). According to the chemical analyses the produced fly ashes are rich in CaO. Moreover, SiO₂ is the dominant oxide in fly ash with Al₂O₃ and Fe₂O₃ found in considerable quantities. Results obtained by XRD showed that the major mineral phase of fly ash is quartz, while other mineral phases that are occurred are maghemite, hematite, periclase, rutile, gehlenite and anhydrite. The ICP-OES analysis showed rather low levels of trace elements, especially for As and Cr, in many of the ashes included in this study compared to coal ash from fluidised bed combustion in general.     

不同炉型潞安煤灰理化及酸/碱溶解特性

对潞安煤经循环流化床、煤粉炉和气化炉处理后得到的煤灰进行了化学组成、矿物组成、特征基团、粒径分布、比表面积及微观形貌等理化性质的对比研究,并考察了其在酸浸和碱浸过程中Al^3+,Si^4+,Fe^3+,Ca^2+,K^+和Ti^4+等离子的溶解特性。结果表明:不同炉型潞安煤灰中铝、钙、硫等元素的含量有明显的差异;矿物组成包括晶相的鳞石英、方石英、莫来石、硬石膏及非晶相的偏高岭石、假莫来石、无定型二氧化硅等;相比较而言,气化炉灰中铝硅酸盐矿物结构更加不稳定。循环流化床灰颗粒呈具有一定孔洞结构的不规则状,而煤粉炉灰和气化炉灰均为光滑球形颗粒,循环流化床灰的粒径>煤粉炉灰的粒径>气化炉灰的粒径,循环流化床灰的比表面积>气化炉灰的比表面积>煤粉炉灰的比表面积。在HCl溶液中,Al^3+,Fe^3+,Ca^2+,K^+,Ti^4+的溶出率均较高;在NaOH溶液中,仅Si^4+和K^+的溶出率较高。不同炉型潞安煤灰中各元素的溶出率具有较大差异,主要与其矿物组成、结构稳定性、粒径和比表面积等相关。     

Crystallisation behaviour of blast furnace slag modified by adding fly ash

Crystallisation of molten blast furnace (BF) slag can increase its viscosity, which can in turn affect the quality of slag fibres. Fly ash was added to BF slag to control its crystallisation and modify its chemical composition. FactSage simulation and analyses using X-ray diffraction (XRD), scanning electron microscope-backscattered electrons (SEM-BSE) coupled to an energy dispersive spectrometer (EDS), and single hot thermocouple technique (SHTT) were performed to explore the crystallisation behaviour of the modified BF slag. The relationship between temperature, mineral precipitation, and added fly ash content was investigated. The minerals contained in the modified BF were melilite, anorthite, clinopyroxene, and spinel. Variation in the fly ash content did not change the composition of the precipitate, but changed its content and the crystallisation temperature of the minerals, which affects the initial crystallisation temperature of the modified BF slag. It decreased as fly ash content increased, and was influenced by the crystallisation of melilite when the added fly ash content was between 5% and 20%. When the added fly ash content increased to 25%, the initial crystallisation temperature was influenced by the precipitation of anorthite. The initial crystallisation temperatures obtained by FactSage simulation, XRD analysis, and SHTT experiments differed due to kinetic effects. The modified BF slag with a fly ash content of 15% is considered suitable for manufacturing of slag fibres due to its low initial crystallisation temperature and cost.     

Characterisation of the glass fraction of a selection of European coal fly ashes

Fly ash largely consists of the inorganic content of coal that remains after combustion. The crystalline phases present in fly ash may form upon cooling of a molten alumino‐silicate glass. This view is supported by the spherical shape of many fly ash particles, inferring that they have gone through a viscous fluid state. The amorphous content in fly ash is believed to dominate reactivity behaviour, under both alkaline and acid conditions, because glasses have a higher potential energy than the equivalent crystal structure and the variation of bond angles and distances in a glass makes the bond breakage easier. It is the degradation behaviour under alkaline conditions, and the subsequent release of silica from the glass phase, that is important in the use of fly ash for conversion to zeolites and for pozzolanic applications in cement. This research comprehensively studies the composition, quantity and stability of the glass phase in a series of nine fly ashes sourced from Spanish and Italian power plants. The quantitative elemental composition of the glass phase in each fly ash was determined. Samples of the ashes then underwent a series of tests to determine the internal structure of the ash particles. Heat treatment of most of the ashes results in mullite crystallising from the glass phase; this is the crystalline phase that is predicated to form by both the relevant phase diagrams and also by NMR spectroscopy. In the ashes, mullite is present as a spherical shell, tracing the outline of the particle but in some specific cases the mullite skeleton is made up of coarse crystals reach also the internal parts of the particles. The morphology and density of the mullite crystals in these shells varies greatly. This work has supported the view that some crystalline phases present in fly ashes, such as mullite, form upon cooling of the amorphous glass melt as opposed to direct conversion from existing mineral phases in the coal during the combustion process. Copyright © 2004 Society of Chemical Industry