Colour control in fly ash as a combined function of particle size and chemical composition

This paper presents the results of an investigation on the combined effect of particle size and chemical composition on the colour of fly ash - a property that determines whether fly ash polymer composites can be engineered to have very light appearance satisfying the need for a wide range of commodity applications, particularly in the building material and computer housing industry. Four fly ash samples were collected from Tarong power plant Queensland, Australia, namely fly ash from first hopper (T59), classified fly ash from first hopper (T60), grinded and then classified fly ash from first hopper (T63) and fly ash from fourth hopper (T64). It was found that the particle size of T64 is smaller but still in the same order as T63. Colour measurement and chemical composition analysis of the different FA samples showed that there is a correlation between the particle size, chemical composition and colour of the fly ash. This information could be effectively used in fly ash recycling industry.     

Experimental study of chemical treatment of coal fly ash to reduce the mobility of priority trace elements

Coal fired electric power plants produce large volumes of fly ash and other coal combustion by-products (CCBs) every year. Although almost 50% of the fly ash produced in the US is recycled for beneficial use, most of the ash material is disposed in dry landfills and ash lagoon impoundments. Fly ash may contain hazardous leachable trace elements such as As, B, Cr, Mo, Ni, Se, Sr and V which have a negative impact on the environment due to potential leaching by acid rain and groundwater with time. Many of the older CCB disposal facilities are unlined and unmonitored and as a result the EPA is currently developing national standards for monitoring CCB disposal sites. The cost to the US electric power industry could exceed one billion dollars if existing and closed CCB disposal facilities come under regulation. Thus simple, low-cost and effective in situ chemical treatment techniques are needed to stabilize hazardous leachable trace elements in the coal combustion by-product (CCB) materials. This paper reports the results of experiments designed to chemically treat fly ash with ferrous sulfate solutions to immobilize hazardous leachable trace elements after disposal.The current study is focused on three acidic and one alkaline fly ash samples collected from electric power plants located in the southeastern United States that were treated with two ferrous sulfate treatment solutions. The first treatment solution contained ferrous sulfate (FS) to give 322 mg/L of dissolved iron, while the second treatment solution contained the same concentration of ferrous sulfate along with excess calcium carbonate (FS + CC) to buffer the pH. Fly ash treatment experiments were carried out at solid:liquid (S:L) ratios of 1:3 and 1:30. The effectiveness of the treatment methods was evaluated by sequentially leaching the treated and the untreated fly ash samples using a synthetic acid rain (SAR) solution (US EPA Method 1312B SPLP fluid) as the leachate. The best overall treatment result was shown by the unbuffered ferrous sulfate solution at the 1:30 S:L ratio, which substantially reduced the mobility of the oxyanion trace elements. The overall mobility reduction achieved for As was 23-72%, B mobility was reduced by 43-80%, Cr by 45-77%, Mo by 21-90%, Se by 41-85% and V by 41-53% The unbuffered ferrous sulfate treatment was not effective for immobilization of the cationic trace elements Ni and Sr.     

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.     

Determination of glass content and estimation of glass composition in fly ash using quantitative X-ray diffractometry

The proportion of amorphous or glassy material in a series of fly ashes has been evaluated by X-ray powder diffraction (XRD) using the Rietveld-based SIROQUANT software package. Several different sample preparation and processing methods were investigated, including XRD analysis of samples spiked with known masses of synthetic corundum and zinc oxide as well as techniques based on analysing the raw or unspiked fly ash directly using the SIROQUANT process. In the latter case, two different poorly crystallised silicate mineral patterns, metakaolin and tridymite, were used in the SIROQUANT processing of the raw ash XRD data to represent the amorphous constituents. The results of the different methods based on XRD of spiked samples were found to be mutually consistent, and also consistent with other published data for an international reference fly ash sample. SIROQUANT analysis of the unspiked fly ashes gave similar results, although different poorly crystallised silicate reference patterns seem to be more suited for ashes from Australian and North American sources.The mineralogy of the ashes, including the proportions of quartz, iron oxide and glassy constituents, appears to be related to the nature of the mineral matter in the relevant feed coals. Calculations based on subtracting the inferred chemistry of the crystalline minerals in the fly ashes from the total fly ash chemistry were also used to estimate the overall chemical composition of the glass fraction in each ash. The results indicate that ashes derived from lower-rank coals in the samples studied have different glass compositions to those derived from higher-rank (bituminous) materials. These different glass compositions appear to be related to several other ash properties, including particle density and particle surface area. Evaluation of glass content and composition may be significant in different aspects of ash utilisation, and also in evaluating interactions with water at ash disposal sites.     

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.     

Electron microscopy and phase analysis of fly ash from pressurized fluidized bed combustion

The characterization of the typical fly ashes from pressurized fluidized bed combustion system (PFBC) in Japan and Europe was carried out by electron microscopy and phase analysis using energy-dispersive X-ray spectroscopy (EDX). The purity of limestone as in-bed sulfur removal sorbent influences the desulfurization reaction. The high-purity limestone yielded both hydroxyl ellestadite and anhydrite in Japanese PFBC ashes, while dolomite-rich limestone yielded anhydrite in European PFBC ashes. When the high-purity limestone was used, hydroxyl ellestadite particles were observed as the independent particles or the rim around limestone particles. The Al₂O₃ content in the glassy phase was inversely proportional to the CaO content in the glassy phase, suggesting that the glassy phases were formed from metakaoline and calcite as end members. Since hydroxyl ellestadite, glassy phase and metakaoline are reactive under hydrothermal conditions, PFBC ashes are expected to be used as raw materials for autoclaved products.     

Use of zeolite, coal bottom ash and fly ash as replacement materials in cement production

In this research, the effects of zeolite, coal bottom ash and fly ash as Portland cement replacement materials on the properties of cement are investigated through three different combinations of tests. These materials are substituted for Portland cement in different proportions, and physical properties such as setting time, volume expansion, compressive strength and water consistency of the mortar are determined. Then, these physical properties are compared with those of PC 42.5. The results showed that replacement materials have some effects on the mechanical properties of the cement. The inclusion of zeolite up to the level of 15% resulted in an increase in compressive strength at early ages, but resulted in a decrease in compressive strength when used in combination with fly ash. Also, setting time was decreased when zeolite was substituted. The results obtained were compared with Turkish Standards (TS), and it was found that they are above the minimum requirements.     

Development of rapid method for the estimation of reactive silica in fly ash

Reactive silica (SiO₂) is an important component of fly ash controlling its use in cement and building materials. Literature search shows that the methods available for the estimation of reactive silica are very time consuming and tedious. It requires a minimum of four days by the conventional gravimetric method described in the standards. In the current paper a rapid volumetric method has been developed where it is possible to estimate reactive silica in fly ash in 4 h. Besides this a gravimetric method has been developed which takes two and half days.     

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.     

Characterization of the coal fly ash for the purpose of improvement of industrial on-line measurement of unburned carbon content

The influence of selected properties of fly ash on the measurements of an on-line analyser was described. Fly ash studied in the research originated from the pulverized coal fired boiler. The samples were taken using an inspection method within a period of 3 months. Systematic observation of the properties of the ash allowed monitoring of the work of the industrial analyzer during a relatively long period of the power plant work. Samples of coal fly ash were examined for their chemical and physical properties. Morphology was analysed by scanning electron microscopy. Unburned carbon content in fly ash was determined by using loss-on-ignition (LOI) and thermogravimetry analysis (TGA). The particle size distribution of fly ash was examined. Correlation between laboratory and on-line industrial measurements of the unburned carbon content of ash was discussed.     

Evaluation of the radiological safety aspects of utilization of Turkish coal combustion fly ash in concrete production

The aim of this study is to evaluate radiological safety aspects of the utilization of fly ash in concrete manufacturing in the construction industry. The specific activities of ²²⁶Ra, ²³²Th and ⁴⁰K in one hundred 55 concrete mixture samples incorporating 10, 20 and 30 wt.% of fly ash collected from the 11 coal-fired thermal power plants were measured by means of gamma-ray spectrometry with HPGe detector. The results of the measurement were used to evaluate the radiological safety aspects of utilization of the fly ash as cement replacement in concrete by assessing the radium equivalent activity, the gamma index, the absorbed gamma dose rate and the corresponding annual effective dose due to the external exposure in indoor. The results of evaluation show that all concrete mixture samples are within the recommended safety limits except for concrete mixture samples incorporating 30 wt.% fly ash of Kangal coal-fired thermal power plant.     

A simplified model for prediction of pozzolanic characteristics of fly ash, based on chemical composition

The correlation between type and quantity of glassy phase and chemical composition of fly ash has been reviewed. A simplified model based on above has been proposed for assessment of pozzolanic reactivity of fly ash in terms of compressive strength of fly ash cement mortar. The model is fitted for 10%, 20%, 35% and 50% of fly ash replacement and for 28, 91 and 365 days of curing period using a least squares technique. The model is found to predict well for more than 20% fly ash replacement. The correlation coefficient (R²) between predicted and experimental values is maximum for 50% replacement. The model fit for 10% replacement of fly ash is poor.     

Enhanced mechanical properties of wood ash and fly ash as supplementary cementitious materials

The incorporation of fly ash (FA) and wood ash (WA) in concrete as supplementary cementitious materials (SCM) was studied. The chemical composition of ordinary Portland cement, FA and WA was determined according to ASTM C-114. SEM and optical microscopy were used for the analysis of concrete. Setting time, compressive strength, water absorption and acid resistance of the concrete with different percentages of SCM ranging from 0 to 60% were evaluated. The results obtained showed that setting time and rate of water absorption increased with the increase in percentage of SCM. After 7 and 28 days, the compressive strength of concrete with 20% FA as SCM was higher than that with substitution with 20% WA. Resistance of concrete against sulphate attack increased with an increase in the percentage of FA. It was found that incorporating more than 20% WA resulted in a decrease in sulphate attack resistance.     

Factors affecting the suitability of fly ash as source material for geopolymers

The suitability of fly ash stock piles for geopolymer manufacturing was studied. The results of chemical analyses, X-ray diffraction (XRD) and particle size distribution (PSD) of five sources of fly ash obtained from coal-fired power generating plants in the US are presented. Geopolymer paste and concrete specimens were prepared from each stock pile. The specimens were subjected to an array of chemical and mechanical tests including XRD, RAMAN spectroscopy, setting time and compressive strength. A correlation study was undertaken comparing the fly ash precursor chemical and crystallographic compositions as well as particle size distribution, with the mechanical and chemical characteristics of the resulting geopolymer. Factors inherent to the fly ash stockpile such as particle size distribution, degree of vitrification and location of the glass diffraction maximum were found to play an important role in the fresh and hardened properties of the resulting geopolymer.     

Analysis and design of a stabilized fly ash as pavement base material

The main objective of this study is to utilize a class F fly ash as base material in road pavements. Since class F fly ashes do not manifest desirable engineering properties for this purpose, it was decided to stabilize the material with cement. Fly ash may be utilized with or without aggregate as a pavement layer. It should be noted that, in this research only aggregate free stabilized mixtures (fly ash and cement only) were used since the aim was to utilize high volumes of this waste material. Cement content in the stabilized, laboratory prepared samples were between 2%, 4%, 8%, and 10% by total weight. Initially, Texas triaxial test was carried out to justify the suitability of the fly ash as pavement material. Then, mechanical tests were performed to obtain the fundamental properties of the cement stabilized material in order to analyze the pavement structure. Under repeated wheel loading, fatigue cracking is the primary mode of failure of stabilized materials in which cracks initiate due to the repeated tensile stresses. Utilizing an accelerated full scale road test data for the fatigue performance of cement stabilized fly ash and performing a mechanistic-empirical design procedure, required layer thickness for different lives were obtained for different amount of cement content.     

Comparison of chars in slag and fly ash as formed in pf boilers from B?dzin Power Station (Poland)

The amount of unburned organic matter in solid residues from coal combustion process is one of the indicators of the process and may influence their usage as by products. During the examination of slag and fly ash from B?dzin Power Station (Poland) the following forms of unburned coal were distinguished: crassispheres, tenuispheres, isotropic and anisotropic networks, tenuinetworks, honeycombs, inertinite and detritus. Slag characterises higher crassispheres and networks contents while high detritus contents are typical of fly ash. Spheres content in both slag and fly ash is always lower than vitrinite content in the feed coal. Fusinite content in the feed coal is lower than inertinite content in the solid residues. Other inertinite macerals probably participated in the formation of honeycombs.     

A new methodology for removal of boron from water by coal and fly ash

High levels of boron concentrations in water present a serious problem for domestic and agriculture utilizations.The recent EU drinking water directive defines an upper limit of 1 mgB/l. In addition, most crops are sensitive to boron levels >0.75 mg/1 in irrigation water. The boron problem is magnified by the partial (∼60%) removal of boron in reverse osmosis (RO) desalination due to the poor ionization of boric acid and the accumulation of boron in domestic sewage effluents. Moreover, high levels of boron are found in regional groundwater in some Mediterranean countries, which requires special treatment in order to meet the EU drinking water regulations. Previous attempts to remove boron employed boron-specific ion-exchange resin and several cycles of RO desalination under high pH conditions. Here, we present an alternative methodology for boron removal by using coal and fly ash as adsorbents. We conducted various column and batch experiments that explored the efficiency of boron removal from seawater and desalinated seawater using several types of coal and fly ash materials under controlled conditions (pH, liquid/solid ratio, time of reaction, pre-treatment, regeneration). We examined the effect of these factors on the boron removal capacity and the overall chemical composition of the residual seawater. The results show that the selected coal and fly ash materials are very effective in removing boron such that the rejection ratio of boron can reach 95% of the initial boron content under certain optimal conditions (e.g., pH = 9, L/S = , reaction time > 6 h). Our experiments demonstrated that use of glycerin enables regeneration of boron uptake into coal, but the boron uptake capacity of fly ash reduces after several cycles of treatment-reaction. The boron removal is associated with Mg depletion and Ca enrichment in the residual seawater and conversely with relative Mg enrichment and Ca depletion in the residual fly ash We propose that the reaction of Ca-rich fly ash with Mg-rich seawater causes co-precipitation of magnesium hydroxide in which boron is co-precipitated. The new methodology might provide an alternative technique for boron removal in areas where coal and fly ash are abundant.     

Synthesis of pure zeolites from supersaturated silicon and aluminum alkali extracts from fused coal fly ash

In this study, the silicon and aluminum in fly ash was activated by fusion with sodium carbonate. The fused product was treated by a method involving a desilicification and hydrothermal process. From the supersaturated silicon and aluminum extracted from fly ash, different pure zeolites were synthesized. The structure-directing agents and crystal seeds made a promoting effect on the type of crystallization achieved. The zeolites were characterized in terms of mineralogical composition and cation exchange capacity (CEC). The results showed that the pure zeolites exhibited high CEC ranged from 3.2 to 4.6 meq/g, and the yield ratio reached to 70%.     

Effect of fly ash on the physico-chemical and mechanical properties of a porcelain composition

The ceramic industry is one of the largest consumers of natural raw materials but has also the capacity and potential to make significant contributions in solving environmental problems by consuming solid rejects of various industries.     

The effect of combustion conditions in a full-scale low-NOx coal fired unit on fly ash properties for its application in concrete mixtures

The wide implementation of low-NO_x combustion technologies in pulverized coal combustion can lead to higher levels of carbon in fly ash and increase the adsorptivity toward surfactants of the carbon. Consequently, the air entraining agent (AEA) requirements of the fly ash used for concrete production increases, which can complicate the stabilization of entrained air. In this study, a low-NO_x tangential fired 875 MW_th power plant burning bituminous coal have been operated under extreme conditions in order to test the impact of the operating conditions on fly ash adsorption behavior and NO_x formation. It was found that the AEA adsorption of the fly ash was reduced up to five times compared to reference operation, when the plant was operated with minimum furnace air staging, three levels of burners instead of four and without recycled flue gas. The lower AEA requirements of the fly ash at these conditions were primarily caused by a reduction in total carbon content, while the AEA adsorptivity of the residual carbon was lowered to about 60% of reference value. The tested operation mode, however, increased the NO_x level in the flue gas before the DeNO_x plant by 60% compared to reference operation.