Fusing fly ash

A modified fusion process to synthesise zeolites A and X from fly ash was studied by Chang et al. (2000). It was found that the addition of aluminium hydroxide to the fused fly ash solution followed by hydrothermal treatment at 60°C produced single-phase zeolite... 

A and X depending on the source of the fly ash. The result confirms that the quantity of dissolved aluminium specie is critical for the type of zeolite formed from fused fly ash. Sutamo and Arryanto (2007) s5mthesised faujasite from fly ash and its application for hydrocracking catalyst of heavy petroleum distillates has been studied. Faujasite was synthesised from fly ash by hydrothermal reaction in alkaline solution via a combination of reflux treatment of fly ash with HCl and fusion with NaOH. 

Fig. 5.14 Thermal response of the raw fly ash, fused fly ashes and the standard zeolite RZP by a TGA and b DTA analyses...

Mishra and Tiwari [35] have studied the performance of zeolite 13X, synthesized by employing hydrothermal activation of NaOH for 20 h, and fused fly ash at 600 °C. The authors have demonstrated that the BET surface area of the original fly ash ( 6 m /g) gets increased up to 430 m /g because of the alkali activation, which can be attributed to the formation of the zeolites at par with their commercial grades. It has been reported that these zeolites, can exhibit selectivity of cations corresponding to the trend as Cu " > Co " > Ni " under different pH and temperature conditions. 

The principal limitations of ESCA include the inability to detect elements present at trace concentrations within the analytical volume, and insufficient lateral resolution to characterize single micrometer-sized particles. The inability to characterize trace species is illustrated in Figure 10 for a sample of coal fly ash particles (11). The fly ash results from the noncombustible mineral components of the coal and consists largely of fused iron oxides and aluminosilicates (42). In addition, most elements are present in at least trace concentrations (22, 42), and many of these elements are highly enriched in the surface region of the particles (evidence for this will be discussed in the next section). However, the ESCA spectrum acquired over several hours of counting time indicates only the presence of detectable surface S and Ca in addition to the fly ash matrix constituents. 

Aqueous washing of the fly ash provides a sorbent with the capacity to remove the elements in amounts in excess of that originally leached. The sorbent characteristic of fly ash is favored by combustion temperatures that lead to the fusion of the fly ash during its formation and the time it remains in the fused state. No correlation could be established between the sorbent characteristic of the fly ashes and their bulk major, minor, trace elemental compositions and the particle size of the fly ash particles. Only the carbon content of the fly ash could be related to its organic removal properties. 

The sorbent and leaching characteristics of fly ash can be related to operating temperatures in the boiler and to coal ash compositions that provide low ash fusion temperatures. Boiler temperatures that favor the fusion of the ash and maintain the ash in the fused state reduce the amount of trace elements leached from the fly ash and improve the sorbent characteristics of the fly ash for removal of these elements from ash pond effluents. In addition, the leachable amounts of each of the elements analyzed in this study can be correlated with the fly ash particle area and with their bulk compositions in the original coal. No correlation could be identified between the sorbent characteristics of fly ashes and their particle size and bulk, major, minor and trace elemental compositions, with the exception of the carbon content. Only organic removals, as measured by COD from ash pond effluent could be correlated with the carbon content of the fly ash particles. 

SEM microphotogaphs and EDAX scans of the cross section and outer surface of the slag deposit, illustrated in Figure 10, indicate the chemistry of the deposit is not uniform. The bulk of the fused material is rich in silica, low in iron, and virtually depleted of potassium. The outermost layers, no more than 2 to 3p thick, are very rich in iron and frequently also rich in calcium. On occasion, the outer surface is covered with small particulate, several microns in diameter, or undissolved cubic or octahedral crystals whose origin is pyrites. Similar formations have been observed in full-scale operation. The evidence indicates deposits form under axial symmetric flow conditions in the furnace by the fluxing action at the heat transfer surface of small particles, <8p in diameter, of decidedly different chemical composition and mineral source. Migration of the fly ash to the surface is by means of eddy diffusion, thermophoresis, or Brownian motion. 

Fly ash starts out as impurities in coal, mostly clay, shales, limestone, and dolomite, which ends up as ash, and fuse at high temperature becoming glass. Two U.S. classifications of fly ash are produced. Class C and Class F, according to the type of coal used. Class C fly ash, typically obtained from subbituminous and lignite coals, must have more than 50% total of silica, alumina, and iron oxide. Class F fly ash, typically obtained from bituminous and anthracite coals, has more than 70% of these oxides. 

Inorganic ash spheres (IAS) Microscopic spheres of inorganic material formed by the fusing of mineral components during fossil-fuel combustion. A component of fly-ash. 

In the laboratory simulations, carried out on sample 3 from the New Vaal mine, it was clearly observed by Mossbauer spectroscopy how the amount of pyrite diminished as the combustion temperatures increased and how it altered until finally to the point where the iron was taken up in the glass and the hematite. At 200 °C, the pyrite started to change, at 400 ° C al ready 60% had been altered to hematite, and at 600 °C only 20% of the pyrite remained. At 800 °C, most of the pyrite had been transformed and above a temperature of 1000 °C the pyrite had fused completely into the glass and the oxide and thus similar products were formed in the fly ash from the power plant and the laboratory-produced ash as shown in Fig. 30.12, with Mossbauer parameters, given in Table 30.2, consistent with those found in the literature [8]. 

In order to augment the quantity and quality of final synthesis yield as obtained from conventional hydrothermal activation, another modified method has been introduced which utihzes two different steps, an initial high temperature fusion of fly ash-alkah mixture, prior to employing the final stage of hydrothermal activation of the fused product. The main variables have been fusion temperature and time, alkali type and its concentration and crystallization time in hydrothermal synthesis process, which can affect the quahty and yield of final product. As such, it has been confirmed that the final yield can be quantified to exhibit zeolitic conversion up to 62 % together with by production of alkaline waste solution which can become a threat to the environment after disposal. A flowchart of the synthesis process is depicted in Fig. 3.3 [1, 2, 9, 10, 12, 43, 44]. 

Shigemoto et al. [9] have reported about selective formation of Na-X zeolite from a mixture of coal fly ash procured from two different sources. It has been reported that most of the fly ash particles got converted into silicates and aluminates of sodium during 1 h of fusion at 773 K, whereas the hydrothermal activation for 6 h of the fused product, crystallized into Na-X, Na-A, Na-Pl and Hydroxy-sodalite zeolites in different proportions based on Na/Fly ash ratio (i.e., 1.2-1.8) employed and Al content of fly ash. It has been opined that fly ash enriched with aluminium, can be crystallized to zeolite Na-A in place of Na-X. 

Kumar et al. [2] have synthesized meso-porous, MCM-41 type zeolitic material from supernatant obtained after filtration of aqueous solution (L/S = 4) of fused mixture of NaOH and fly ash, which has Brunauer-Emmett-Teller (BET) siuface area (SSAbet) and CEC around 4.5 m /100 g and 0.8 meq./lOO g, respectively. They have concluded that most of the Si and Al components in fly ash can be... 

Several researchers have established fusion of fly ash-NaOH mixture, at 450-900 °C. Accordingly, the fly ash gets partially converted to zeolite X when the fused ash is further treated in a hydrothermal system. However, such a solid residue (the end product) exhibits low cation exchange capacity. Such deficiency in the zeolite can be attribute to improper contact between the NaOH and the fly ash particles, during their... 

As a result of hardening of these layers, further fusion of the residual fly ash with the molten NaOH would get restricted. This drawback of the conventional fusion process still remains unattended. In this situation, grinding of a fused residue manually is warranted to break the SSL and SAL and ascertain their mixing with the residual fly ash, before the second and third steps of the fusion. To investigate the extent of fusion of the RFA, the characterization (viz., physico-chemico-mineralogical, morphological) of aU the residues was also essentially conducted. 

Table 6.15 Pore size distribution for fly ash, fused ash based on BET analysis...

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