Characterization of low-temperature coal ash behaviors at high temperatures under reducing atmosphere

The coal ash obtained at 815 °C under oxidizing atmosphere was further treated at 1300 °C and 1400 °C under reducing atmosphere. The resultant ashes were examined by XRD, SEM/EDX and FTIR. The results show that the residence time of coal ash at high temperatures has considerable influences on the compositions of coal ash and little effect on the amounts of unburned carbon. The amorphous phase of mineral matters increases with the increasing temperature. The FTIR peaks due to presence of different functional groups of minerals support the findings of XRD, and supply additional information of amorphous phase which cannot be detected in XRD. The ash samples generated from a fixed bed reactor during char gasification were also studied with FTIR. The temperatures of char preparation are responsible for the different transformation of minerals during high temperature gasification.     

Synthesis of zeolite from coal fly ashes with different silica–alumina composition

Coal fly ashes can be converted into zeolites by hydrothermal alkaline treatment. This study focuses on the effect of Si/Al molar ratio of the fly ash source on the type of formed zeolite, which also is affected by the alkaline condition. The fly ashes were mixed with an aqueous NaOH solution and hydrothermally treated at about 100 °C. Zeolite Na–P1 and/or hydroxy-sodalite appeared after the treatment. Zeolite Na–P1 predominantly formed from silica-rich fly ash at a low-NaOH concentration. The cation exchange capacity of the product with a large content of zeolite Na–P1 reached a value of 300 meq/100 g. The type of the product was controlled by addition of aerosil silica or alumina. It was found that silica addition effectively enhances the formation of zeolite Na–P1, even at a high-NaOH concentration. These results were discussed on the basis of a formation mechanism of zeolite from coal fly ash through dissolution–precipitation process.     

Effect of size fraction and glass structure of siliceous fly ashes on fly ash cement hydration

Paper presents effect of size fraction and glass structure of fly ashes on cement hydration. Fly ashes below 16 μm and 16–32 μm, both from the 1st and 3rd section of electro-filter, were applied. Hydration heat, content of Ca(OH)₂ and unreacted C₃S were studied and compressive strength and microstructure were analysed. Results show that finer ashes have higher depolymerization degree of SiO₄ units in glass what increases pozzolanic reactivity. Incorporation of fly ashes below 16 μm from the 3rd section gives cement class 52.5 N. At 180 day, Ca(OH)₂ content decreases by 67% and C₃S hydration degree increases by 50% relative to control sample.     

Combustion reactivity of different oil-fired fly ashes as received and leached

The combustion of heavy fuel oil for power generation leads to a great production of fly ashes, usually disposed of in controlled expensive landfills. As possible alternatives to their conventional disposal, hydrometallurgical processes are used for the recovery of valuable metals such as vanadium and nickel and the carbonaceous residues, so obtained, could be burned to recover energy.In this study, the combustion of oil-fired fly ashes, as received and leached, was investigated by thermo-analysis in atmospheres at different oxygen concentration (6, 12 and 21 vol%). Two commercial coals were used as reference samples. The ignition temperature and the thermo-kinetic analysis were used to compare the reactivity of the different samples.Besides, in order to value the feasibility of co-combustion of leached fly ashes with coals in pre-existent coal boilers, blends of leached fly ash with coal were prepared and their combustion behaviour was investigated.     

Adsorption of an acid dye onto coal fly ash

In this study, we found the raw coal fly ash (CFA) that had not been subjected to any pretreatment process had superior adsorbing ability for the anionic dye Acid Red 1 (AR1) than did two modified coal fly ashes (CFA-600 and CFA-NaOH). The adsorption capacities followed the order CFA > CFA-600 > CFA-NaOH, and they each increased upon increasing the temperature (60 °C > 45 °C > 30 °C). The adsorptions of AR1 onto CFA, CFA-600, and CFA-NaOH all followed pseudo-second-order kinetics. The isotherms for the adsorption of AR1 onto the raw and modified coal fly ashes fit the Langmuir isotherm quite well; the adsorption capacities of CFA, CFA-600, and CFA-NaOH for AR1 were 92.59–103.09, 32.79–52.63, and 12.66–25.12 mg g^?1, respectively. According to the positive values of ΔH° and ΔS°, these adsorptions were endothermic processes. The ARE and EABS error function methods provided the best parameters for the Langmuir isotherms and pseudo-second-order equations, respectively, in the AR1–CFA adsorption system.     

Hydrothermal alkaline treatment of oil shale ash for synthesis of tobermorites

The hydrothermal alkaline activation of the oil shale fly ash was studied using SEM/EDX, XRD and ²⁹Si and ²⁷Al high-resolution MAS-NMR spectra. The silicon in the original fly ashes was completely converted into calcium-alumino-silicate hydrates, mainly into 1.1 nm tobermorite structure during 24 h treatment under hydrothermal conditions at 160 °C in the presence of NaOH. The local structure of synthesized tobermorite samples implies long silicate chains with small number of bridging sites. The results obtained in the study prove that the oil shale fly ash can be used for production of Al-substituted tobermorites.     

The effect of grinding process on mechanical properties and alkali–silica reaction resistance of fly ash incorporated cement mortars

The effect of fineness of fly ash on mechanical properties and alkali–silica reaction resistance of cement mortar mixtures incorporating fly ash has been investigated within the scope of this study. Blaine fineness of fly ash has been increased to 907 m²/kg from its original 290 m²/kg value by a ball mill. Test samples were prepared by replacing cement 20, 40 and 60%, with finer and coarser fly ashes and kept under standard and steam curing conditions until testing. Test results showed that grinding process improved the mechanical properties of all samples significantly. The beneficial effect of grinding fly ash, may increase utilization of this by-product in precast and ready-mix concrete industries. Incorporation of fly ash with different fineness values and ratios also decreased the expansions to harmless levels of cement mortars due to alkali–silica reaction.     

Synthesis of glass-ceramics using glass cullet and vitrified industrial by-products

This study concerns the recycling of inorganic waste materials for the production of glass-ceramics and the evaluation of the developed physical properties. Four industrial by-products were selected due to their mass production: (i) two high calcium lignite fly ashes, (ii) slag derived from the production of Fe–Ni and, (iii) steel slag. In order to examine the role of the SiO₂ in the crystallization process, glass cullet and Egyptian sand were added. Thermal treatment, at 1450 °C, enables the production of glasses using mixtures of these materials at appropriate proportions. The crystallization was achieved by heating at 900, 950 and 1000 °C. The produced materials were examined concerning their structure by X-ray diffraction and scanning electron microscopy (SEM-EDS). The results showed that the crystalline phase is greatly depending on the structure of the raw material and the thermal process, influencing accordingly the hardness of the final products.     

Suitability of Sarawak and Gladstone fly ash to produce geopolymers: A physical,chemical, mechanical,mineralogical and microstructural analysis

Two types of fly ash sourced from Sarawak, Malaysia and Gladstone, Australia reflect differences in chemical compositions, mineral phase and particle size distributions. In this paper, the Sarawak fly ash was used to produce geopolymer in comparison to the well-developed Gladstone fly ash-based geopolymer. Characteristics of fly ash and mixtures proportions affecting compressive strength of the geopolymers were investigated. It is found that the variations of both fly ash types on particle size distributions, chemical compositions, morphology properties and amorphous phase correspond to the compressive strength. The results obtained show that after 7 days, geopolymer using Sarawak fly ash has lower compressive strength of about 55 MPa than geopolymer using Gladstone fly ash with strength of about 62 MPa. In comparison with Gladstone fly ash-based geopolymer, it showed that Sarawak fly ash-based geopolymer can be a potential construction material. Moreover, the production of Sarawak fly ash-based geopolymer aids to widen the application of Sarawak fly ash from being treated as industrial waste consequently discharging into the ash pond.     

Effect of Temperature on Phase and Alumina Extraction Efficiency of the Product from Sintering Coal Fly Ash with Ammonium Sulfate

A new developed technology for extracting alumina from coal fly ash was studied in this paper. In this technology, coal fly ash is first sintered with ammonium sulfate, forming ammonium aluminum sulfate in the resultant product, where alumina can be easily leached without using any strong acid or alkali. The products obtained under different sintering conditions were characterized by X-ray diffractometry. Alumina extraction efficiency of these products was also investigated. The results show that the sintering temperature and time substantially influence the phase composition and alumina extraction efficiency of sintered products, while the heating rate has little influence. The optimal sintering condition is 400 °C for 3 h in air with a heating rate of 6 °C·min^− 1. Under the optimal sintering condition, the alumina extraction efficiency from as-sintered coal fly ash can reach 85% or more.     

Effect of carbonaceous matter contents on the fire resistance and mechanical properties of coal fly ash enriched mortars

In this paper, the thermal and mechanical behaviors of mortars mainly composed of coal fly ash (80 wt%) are studied with the aim of analyzing the influence of the unburned matter of the ash on the fire resistance and mechanical strength characteristics of mortars potentially used for passive protection against fire. All the properties have been evaluated after 28 days of setting time, comparing the properties of mortars containing ashes with different loss on ignition (LOI) contents and studying the behavior of mortars in which fly ash has been submitted to a pre-treatment aimed at removing the unburned matter. The insulating properties of the mortars have been analyzed by means of thermal analysis techniques; also the standard fire resistance test has been reproduced on 200 mm-height, 50 mm-diameter cylindrical test probes. Different mechanical properties such as compressive and flexural strength of the probes (before and after the fire resistance test), superficial hardness and dynamic elasticity modulus have been measured. The results obtained show a slight influence of the ash unburned matter content on the insulating capacity and mechanical properties of the mortars. On the other hand, the thermally-treated ash mortars showed better mechanical and insulating properties, although presumably the cost of the thermal pre-treatment does not justify the improvement achieved.     

Hydration of Portland cement with high replacement by siliceous fly ash

The effects of two different low calcium fly ashes on the hydration of ordinary Portland cement (OPC) pastes containing 50 wt.% of fly ash were investigated over a hydration time of 550 days. The results were compared with a reference blend of OPC containing 50 wt.% of inert quartz powder allowing the distinction between "filler effect" and pozzolanic reaction.Until 2 days, no evidence of fly ash reaction was measured and its influence on the hydration is mainly related to the “filler effect”. From 7 days on, the effects of the pozzolanic reaction were observed by the consumption of portlandite, the change of the pore solution chemistry, the formation of a presumably water-rich inner hydration product and the change of the C–S–H composition towards higher Al/Si ratio compared to the C–S–H of neat OPC. Additional strength due to the pozzolanic reaction developed after 28 days of hydration.     

The effect of alkalinity of Class F PC fly ash on metal release

Thirty-five samples of Class F fly ash from pulverized coal (PC) combustion boilers were leached with deionized water, acetic acid, and sulfuric acid. The release of metal ions from fly ash was related to the pH of the leachant solution and to the alkalinity of the ash. The maximum soluble concentration of the metallic elements was measured when the pH of the leachate was less than 5. Twenty-three of the samples were strongly alkaline and buffered the leachate until the ash was neutralized. The leachate from the other 13 samples became acidic with the addition of less than 1 L of 0.1 N acid. The alkalinity of the ash and the volume of acid required to neutralize it were a function of the Ca concentration.     

High temperature elemental losses and mineralogical changes in common biomass ashes

The elemental losses from ashes of common biomass fuels (rice straw, wheat straw, and wood) were determined as a function of temperature from 525 °C to below 1525 °C, within the respective melting intervals. The experimental procedure was chosen to approach equilibrium conditions in an oxidizing atmosphere for the specific ash and temperature conditions. All experiments were conducted in air and used the ashes produced initially at temperatures of 525 °C as reactants. Losses during the initial ashing at 525 °C were negligible, except for a K₂O loss of 26% for wood and a Cl loss of 20% for wheat straw. Potassium losses are positively correlated with temperature for all fuel ashes. The K₂O loss for wood ash commences at 900–1000 °C. Carbonate is detected in the wood ashes to about 700–800 °C and thus cannot explain the retention of K₂O in the ashes to 1000 °C. Other crystalline phases detected in the wood ashes (pericline and larnite) contain little or no potassium. Petrographic examinations of high temperature, wood ash products have failed to reveal potassium bearing carbonates, sulfates, or silicates. The release of potassium, thus, appears to be unrelated to the breakdown of potassium-bearing crystalline phases. The straw ashes show restricted potassium loss compared to wood ash. The potassium content declines for both straw ashes from about 750 °C. Cristobalite appears in the straw ashes at about 700–750 °C and is replaced by tridymite in the rice straw ash from about 1100 °C. Sylvite (KCl) disappears completely above 1000 °C. The Cl content starts to decline at about 700 °C, approximately at the same temperature as potassium, suggesting that the breakdown of sylvite is responsible for the losses. The K–Cl relations demonstrate that about 50% of K (atomic basis) released from breakdown of sylvite is retained in the ash. The presence of chlorine in the ash is, therefore, best attributed to the presence of sylvite. Potassium is easily accommodated in the silicate melt formed at temperatures perhaps as low as 700–800 °C from dehydration, recrystallization, and partial melting of amorphous components. Loss of potassium persists for ashes without remaining sylvite and points to the importance of release of potassium from partial melt at temperatures within the melting interval for the fuel ashes.     

Crystallization behavior,microstructure and dielectric properties of lead titanate glass ceramics in the presence of Bi2O3 as a nucleating agent

The crystallization behavior of PbO–TiO₂–B₂O₃–SiO₂ glasses in the presence of Bi₂O₃ as a nucleating agent were studied utilizing XRD, DTA, SEM. The glass samples heat treated in the range of 557–630 °C for different soaking times, all developed PbTiO₃ (PT) with perovskite structure. It was found that the addition of 0.5–1.0 mol% Bi₂O₃ resulted in the formation of homogenous, nano-structured glass ceramics with a mean crystallite size of 20–25 nm and PbTiO₃ as the major crystalline phase. The dielectric constant and dissipation factors for the prepared glass ceramics were in ～140–400 and ～0.04–0.4 ranges respectively.     

T–T–T behaviour of bioactive glasses 1–98 and 13–93

In this work crystallization kinetics of bioactive glasses 1–98 and 13–93 are discussed. Within a certain temperature–time window these glasses can be hot worked into various products without interfering with crystallization. The crystallization was studied isothermally by heating glass plates at different temperatures for different times. Phases in the samples were studied through XRD and SEM analyses. The nucleation-like curves and crystallization characteristics were measured with DTA. The temperature of maximum nucleation was measured for glass 1–98 at 725 °C and for 13–93 at 700 °C. The activation energy of crystallization of both glasses was 280 kJ/mol. The Johnson–Mehl–Avrami exponent and the SEM micrographs of the samples suggested surface crystallization. The primary crystalline phase was wollastonite. The growth rate of the crystallized surface layer was 1 order of magnitude higher in the plates of 1–98 than in 13–93. The results can be utilized to optimize the parameters in hot-working of the glasses.     

Preparation and characterization of crednerite glass–ceramics in the MnO·CuO·SiO2 system

Nanoscale crednerite (CuMnO₂) was prepared in the system MnO·CuO·SiO₂, using glass–ceramics technique for the first time. Based on obtained data from differential thermal analysis (DTA), the prepared samples were heat-treated at 700 and 800 °C for 2 h. The presence of crystalline phases after and before heat treatment was investigated by X-ray diffraction analysis. Crystallization of crednerite (CuMnO₂), manganese silicate (Mn₂SiO₄) and traces of cuprite (Cu₂O) and cristobalite (SiO₂) phases were recognized. Transmission electron microscopy showed nanoscale crystals in the range 5–10 nm. The prepared glass–ceramics showed ferrimagnetic properties with wide range coercivity from 53 to 2217 Hci and magnetization saturation from 0.21708 to 1.2 emu/g. From IR reflection data; the reflection intensity of the light is high in the range of orange–red color and violet–blue colors and low in the range of green color.     

Dry pressed ceramic tiles based on fly ash–clay body: Influence of fly ash granulometry and pentasodium triphosphate addition

Fly ash from brown coal (70 wt.%) and stoneware clay (30 wt.%) were used for the dry pressed ceramic tiles (according to EN 14411) raw materials mixture. The effects of fly ash milling and pentasodium triphosphate addition as a deflocculant and fluxing agent on the properties of green body (flexural strength, bulk density) and fired body (EN ISO 10545—water absorption, bulk density, true density, apparent porosity, flexural strength, frost resistance) were studied and explained as a function of the firing temperature (1000–1150 °C). Fly ash milling (corresponding to 5 wt.% residue of fly ash grains on 0.063 mm sieve) increased the sintering abilities of the fly ash–clay body. A similar effect was achieved by 1.3 wt.% pentasodium triphosphate (PST) addition with an increase in green body flexural strength and a decrease in water content of the granulate. Fly ash–clay bodies can be frost resistant with water absorption above 10% due to positive pore size distribution, which were examined using the high-pressure mercury porosimetry method.     

Adsorption of heavy metal ions from aqueous solution by fly ash

The removal characteristics of lead and copper ions from aqueous solution by fly ash were investigated under various conditions of contact time, pH and temperature. The influence of pH of the metal ion solutions on the uptake levels of the metal ions by fly ash were carried out between pH 4 and 12. The level of uptake of Pb²⁺ and Cu²⁺ ions by the fly ash generally increased, but not in a progressive manner, at higher pH values. The effect of temperature on the uptake of Pb²⁺ and Cu²⁺ ions was investigated between 30 °C and 60 °C, the adsorption of being enhanced at the lowest temperature. Rate constants were evaluated in terms of a first-order kinetics. The rate constant, k for uptake of Pb²⁺ and Cu²⁺ ions were 1.77 × 10⁻² s⁻¹ and 2.11 × 10⁻² s⁻¹, respectively. The experimental results underline the potential of coal fly ash for the recovery of metal ions from waste water. The main mechanisms involved in the removal of heavy metal ions from solution were adsorption at the surface of the fly ash and precipitation.     

Microwave-assisted zeolite synthesis from coal fly ash in hydrothermal process

Coal fly ashes, which include much amount of silica and alumina, can be converted into zeolite by hydrothermal alkaline treatment. In the present work, the effect of microwave irradiation on the zeolite formation was investigated with emphasis on the change in yield of zeolite during the reaction. The fly ash was mixed with 2 M NaOH solution and heated by oil bath or microwave for 2 h. Zeolite Na-P1 formed after the conventional treatment using oil bath, but no zeolitic product was obtained by microwave heating. When microwave was applied in the course of hydrothermal treatment, zeolitization was promoted by the early-stage irradiation. This is due to the stimulated dissolution of SiO₂ and Al₂O₃ from coal fly ash. On the other hand, the microwave irradiation in the middle to later stage retarded the crystallization of zeolite. The microwave is effective to produce the zeolite from coal fly ash in a short period by control of irradiation schedule in the early stage.