Fused Fly Ash Research Articles

Influence of aluminium source on the crystal structure and framework coordination of Al and Si in fly ash-based zeolite NaA

In this study zeolite NaA with different crystal sizes and % crystallinity was prepared from a clear solution extract of fused fly ash. Sodium aluminate or aluminium hydroxide was used to adjust the aluminium content in the fused fly ash extract and hence adjust the Si/Al ratio of the various synthesis molar regimes for the synthesis of zeolite NaA. Each molar regime was hydrothermally treated for 3h at 100°C. Chemical, mineralogical, morphological and structural composition of the synthesized products was determined using XRF, laser ablation ICP-MS, XRD, SEM and 29Si and 27Al NMR. This study showed that the variation of Si/Al ratio of the synthesis mixture (0.27 to 0.80) allowed control of zeolite NaA crystal/sizes that ranged from 0.67 to 3.18μm. The decrease in Si/Al ratio of the synthesis mixture was proportional to the crystal size (μm) and the 29Si NMR peak width and area of the synthesized zeolite NaA. The increase in crystallinity and decreases in crystal size caused the 29Si NMR peak width to be narrower as the peak area reduces. Interestingly, the Si/Al ratio of the synthesis mixture was inversely proportional to the crystallinity of zeolite NaA when Na-aluminate was used as Al source. The effect of Al source was also noticed by the appearance of a small additional distorted tetrahedral Al peak between 15 and 17ppm (27Al NMR) when Al-hydroxide was added to fused fly ash extract.

Synthesis of zeolite NaA membrane from fused fly ash extract

ABSTRACTZeolite-NaA membranes were synthesized from an extract of fused South African fly ash on a porous titanium support by a secondary growth method. The influence of the synthesis molar regime on the formation of zeolite NaA membrane layer was investigated. Two synthesis mixtures were generated by adding either aluminium hydroxide or sodium aluminate to the fused fly ash extract. The feedstock material and the synthesized membranes were characterized by X-diffraction (XRD), scanning electron microscopy (SEM) and X-ray fluorescence spectroscopy (XRF). It was found by XRD and SEM that the cubic crystals of a typical zeolite NaA with a dense intergrown layer was formed on the porous Ti support. The study shows that the source of Al used had an effect on the membrane integrity as sodium aluminate provided the appropriate amount of Na+ to form a coherent membrane of zeolite NaA, whereas aluminium hydroxide did not. Morphological, the single hydrothermal stage seeded support formed an interlocked array of zeolite NaA particles with neighbouring crystals. Also, a robust, continuous and well-intergrown zeolite NaA membrane was formed with neighbouring crystals of zeolite fused to each other after the multiple stage synthesis. The synthesized membrane was permeable to He (6.0 × 106 L m−2h−1 atm−1) and CO2 (5.6 × 106 L m−2h−1 atm−1), which indicate that the layer of the membrane was firmly attached to the porous Ti support. Membrane selectivity was maintained showing membrane integrity with permselectivity of 1.1, showing that a waste feedstock, fly ash, could be utilized for preparing robust zeolite NaA membranes on Ti support.

Synthesis of zeolites at low temperatures in fly ash-kaolinite mixtures

Coal fly ash from an Italian coal-fired power plant, kaolinite from the Source Clay Repository of The Clay Minerals Society, and four derived mixtures were used to synthesize zeolite using hydrothermal treatment at 45 °C in distilled water after alkaline fusion. The results documented that geopolymers, A-type and X-type zeolites were formed in different quantities, depending on the starting material and the duration of the experiment. Zeolite-X was the prevailing phase synthesized using pure fly ash, zeolite-A formed in higher amounts from kaolinite, and comparable amounts of A- and X-type zeolites crystallized, thereby adding 20 and 40% kaolinite to the fly ash, respectively. Zeolite-A as main phase was synthesized already adding 60% or even up to 80% kaolinite to the fly ash. Sodalite occasionally formed from the source materials, whereas zeolite ZK-5 was synthesized from only fused fly ash (100FA). The data indicated that, in addition to the Si/Al ratio of solid source materials, zeolite formation was controlled by the time and chemistry of the solution. The polymerization of alumina-silicate gels changed during the experiments, likely due to the amorphization of metastable zeolites.

Formation of NaP zeolite from fused fly ash for the removal of Cu(ii) by an improved hydrothermal method

Fly ash with a Si : Al ratio of 3.63 has been converted to a zeolite of high Cu2+ sorption capacity using a fusion conversion process and an improved hydrothermal method.

In-situ ultrasound study of the kinetics of formation of zeolites Na–A and Na–X from coal fly ash

The kinetics of synthesis of zeolites Na–A and Na–X from fused South African class F coal fly ash were studied by using an ultrasound device as a real-time, in-situ diagnostic tool. Ex-situ techniques, for example XRD, ICP, and SEM, were used to complement the results of the kinetic study. Reaction rate, reaction order, and activation energy of crystallization processes in clear solution extracted from fused fly ash were calculated on the basis of ultrasound signal data recorded at different crystallization temperatures. Zeolite Na–X and zeolite Na–A crystals were both obtained without ageing. The zeolite Na–X sample showed contaminations of zeolite P and sodalite depending on the synthesis temperature. For zeolite Na–A the impact of ageing on the process of formation was also studied.

In situ ultrasonic diagnostic of zeolite X crystallization with novel (hierarchical) morphology from coal fly ash

In this paper the applicability of an in situ ultrasonic diagnostic technique in understanding the formation process of zeolite X with a novel morphology was demonstrated. The complexity of the starting fly ash feedstock demands independent studies of the formation process for each type of zeolite since it is not known whether the crystallization mechanism will always follow the expected reaction pathway. The hierarchical zeolite X was noted to follow a solution phase-mediated crystallization mechanism which differs from earlier studies of the zeolite A formation process from unaged, clear solution extracted from fused fly ash. The use of the in situ ultrasonic monitoring system provided sufficient data points which enabled closer estimation of the time of transition from the nucleation to the crystal growth step. In order to evaluate the effect of temperature on the resulting in situ attenuation signal, synthesis at three higher temperatures (80, 90 and 94°C) was investigated. It was shown, by the shift of the US-attenuation signal, that faster crystallization occurred when higher temperatures were applied. The novel hierarchical zeolite X was comprised of intergrown disc-like platelets. It was further observed that there was preferential growth of the disc-shaped platelets of zeolite X crystals in one dimension as the synthesis temperature was increased, allowing tailoring of the hierarchical morphology.

Characterization of MCM-41 mesoporous materials derived from polish fly ashes

The synthesis of mesoporous molecular sieves from fly ashes is an interesting direction of use of the values of fly ash, and specifically its chemical composition (Si, Al), for obtaining high-quality porous materials offering wide possibilities of utilization. An attempt was made in the present work to obtain the MCM-41 mesoporous materials from 10 different fly ashes originating from Polish Power Stations. The synthesis was carried out by the hydrothermal method using the supernatants of coal fly ashes (fused fly ash solutions) and cationic cetyltrimethylammonium bromide (CTAB) surfactants as the structure-directing agents. Fly ashes were chemically analyzed and used to prepare mesoporous materials (MCM-41). The current work uses powder X-ray diffraction (XRD), TG/DTG, FTIR, SEM, TEM, nitrogen adsorption and 27Al MAS NMR, 29Si MAS NMR techniques to characterize physico-chemical properties of fly ash-based MCM-41 materials. The properties of the synthesized mesoporous materials were compared with those prepared from pure silica sources. Studies have shown that extracts suitable for the synthesis of the MCM-41 mesoporous material cannot be obtained from all fly ash types (even those with a very high Si content and a very high Si/Al ratio).

Synthese of Cancrinite from Alkali-Fused Fly Ash by “Dry Method”

In this study, zeolite was prepared from fused fly ash with very little water treatment. The as-prepared zeolite was characterized by X-ray diffraction (XRD) and scanning electron microscope. XRD results showed that cancrinite was the major phase in the product. The influence of preparation conditions on the yield of zeolite was described. Experiments showed that the crystallization time of zeolite became shorter and the crystallinity of cancrinite became higher with the increase of NaOH dose in fusion stage. Based on the fact that the water was not enough to be as the solvent which was different from conventional method, it can be concluded that the synthesis mechanism of zeolite from fused fly ash is the Solid-state Conversion Theory.

Mesoporous materials prepared using coal fly ash as the silicon and aluminium source

Coal fly ash was converted into two types of porous materials, MCM-41 and SBA-15 (both of hexagonal structure), using the supernatant of the fly ash. It was found that most of the Si and Al components in the fly ash could be effectively transformed into mesoporous materials, depending on the hydrothermal conditions, and that fusion is essential. Investigation by 29Si and 27Al MAS NMR demonstrated that fusion plays an important role in enhancing the hydrothermal conditions for synthesis of these materials. A high concentration of Na ions in the supernatant of the fused fly ash was not found to be critical in the formation of Al-MCM-41 when prepared under controlled pH conditions. Pyridine adsorption experiments on Al-MCM-41 prepared from coal fly ash revealed the presence of Bronsted and Lewis acid sites. It was also found that the catalytic activity in the cumene cracking reaction is linked only to the accessible aluminium, and not to the total incorporated aluminium present in the Al-MCM-41.

Characterization and Catalytic Activities of Faujasites Synthesized by Using Coal Fly Ash.

Coal combustion by-product fly ash was converted selectively into faujasite (Y type) zeolite and was used as a catalytic material. The conversion process tried to alleviate the disposal problem of large amounts of fly ash, and turned fly ash into useful materials. Fused fly ash powder and supernatant were used in the synthesis of Y type zeolite. The prepared Na-Y zeolites were characterized and ion-exchanged followed by calcinations to obtain H-Y. The catalytic properties of the solid obtained were evaluated using cumene cracking and compared with those of commercially available (standard) zeolites. It was found that most of the Si and Al components in the fly ash could be effectively transformed into Y type zeolite in the presence of seeds but not the mineral phase, such as mullite. Moreover, the supernatant of the fused powder solution can produce purer faujasites and the sediment can be reused to generate solution for the further preparation of zeolites. Investigation by NMR demonstrated that fusion plays an important role in enhancing the hydrothermal conditions for zeolite synthesis. The H-Y zeolite derived from the supernatant of fly ash solution shows excellent cracking activity compared to that of standard.

Synthesis of Zeolites A and X from Fly Ashes and Their Ion-Exchange Behavior with Cobalt Ions

A modified fusion process was studied to synthesize zeolites A and X from fly ash. It was found that the addition of aluminum hydroxide to the fused fly ash solution followed by hydrothermal treatment at 60 °C produced single-phase zeolite A or a mixture of zeolites A and X depending on the source of the as-received fly ash. The result confirms that the quantity of dissolved Al species is critical for the type of zeolite formed in precursor solutions prepared from fused fly ashes. The ion-exchange behaviors of treated fly ashes containing zeolites A and X were tested with Co2+ ions. It was found that the ion-exchange capability (IEC) of treated fly ash with Co2+ ions was much larger than that with Cs+ ions. In addition, the Co2+ ion-exchange rate of zeolite X was faster than that of zeolite A. This is probably related to the bigger pore channels in zeolite X. The Co2+ IEC of treated fly ash containing zeolite A reached 67.6% of commercial zeolite A and that containing zeolite X 83.1% of commercial zeolite X. Our study demonstrated that fly ashes can be converted to zeolites A and X, and these treated fly ashes have great potential for removing heavy metals.

Conversion of Fly Ash into Mesoporous Aluminosilicate

Mesoporous aluminosilicate in the hexagonal phase (MCM-41) has been synthesized from fused fly ash solutions and cationic cetyltrimethylammonium bromide (CTAB) surfactants. We provide direct evidence that an MCM-41 aluminosilicate with a homogeneous chemical composition of Si/Al = 13.4 can be prepared with cationic surfactant. Our results indicate that coal combustion byproducts can be utilized for producing mesoporous molecular sieves even though they contain significant amounts of impurities.