Synthesis Of Porous And Fibrous Materials Based On Acid-extraction Residues Of Fly Ash

Fly ash is a major solid industrial waste created by combustion of pulverized coal in thermal power plants. Many researches have been carried out to utilize this by-product. Aluminum is usually affluent in fly ash. The alumina content of fly ash produced by some power plants can be as high as 50%. For this reason, using fly ash as raw material in alumina production can alleviate the strain on diminishing bauxite resources. The acid residue remaining after extracting the alumina presents a new problem: that of solid waste disposal. Fly ash residue extracted-Al2O3 is viewed as a hazardous industrial waste and it has bad effect on the agriculture land and groundwater if this slag is consigned to landfill directly. So it is important for utilization of fly ash to study the fly ash residue, whereas very little research about the utilization about fly ash residue extracted- Al2O3. The different utilization methods are carried out based on the study of characteristic of fly ash residue extracted-Al2O3.Fly ash acid residue has irregular particles of different sizes about several microns. The chemical composition of acid residues is silica, unburned carbon, aluminum oxide and other minor metal oxides. The original structure of fly ash is destroyed in the process of extracting Al2O3 and thus Si O2 has amorphous structure. The significant silica and alumina content of acid residue makes it a suitable raw material in the synthesis of value-added zeolitic products for various environmental applications. Zeolite P and X is synthesized from acid residues by one step hydrothermal method. The effect of Na/Si molar ratio, hydrothermal temperature, reaction time and mechanism was studied. The result showed the BET surface and pore volume was significantly increased after synthesizing zeolite. The BET surface of zeolite X and P was 314.8 m2/g and 78.1 m2/g, respectively.The adsorption behaviour of Cr3+ and methylene blue(MB) on zeolites was also analyzed, in order to reveal the relationship between the adsorption capability and structural properties of zeolite P and X. The results obtained from batch sorption experiments indicate that zeolites prepared from fly ash acid residues is an efficient sorbent for the removal of Cr3+ and MB from solutions. The effect of contact time, p H values of initial solution and mechanism was studied. The removal of MB and Cr3+ by zeolites adequately described by the Langmuir isotherm, from which, maximum uptake capacity of zeolite P and X for MB was estimated to be 13.7 mg/g and 14.7mg/g, and the capacity of zeolite P and X for Cr3+ was estimated to be 1.0 mmol/g and 1.2 mmol/g, respectively.In this study, tobermorite was successfully synthesized from these two kinds of industry wastes by single step hydrothermal method. The adsorption behaviour of Cr3+ on tobermorite was also analyzed, in order to reveal the relationship between the adsorption capability and structural properties of tobermorite. The results show that well-crystallized tobermorite can be obtained within 6 h at 180 oC with the Ca/Si in the range of 0.8-1.1, in spite of large amount of impurities introduced by raw materials. The Al3+ impurities speed up the crystallization process of tobermorite significantly. While, the carbon impurities display slight opposite effect, leading to longer synthesis time and lower crystallinity of tobermorite. When the initial concentration of Cr3+ is relatively low, the resultant tobermorite displays attractive adsorption capacity for Cr3+(4.0 mmol/g), which is 2-3 times higher than common clay minerals. The tobermorite has the good performance to removed the Cr3+ when the initial concentration in the range of 0-35mmol/L. Meanwhile, high initial concentration of Cr3+ causes the exsolution of structural Ca2+ in tobermorite channels, leading to the damage of crystallinity and decrease in sorption capacity of adsorbent. The mechanism involves ion exchange with Ca2+ from surface and edge as well as complexation with Ca-OH and isolated Si-OH species on the surface. Then the Cr3+ exchanged with the interlayer space and framework layer, and results in the amorphisation of the lattice. The unburned carbon in the acid residues has little influence on the tobermorite formation processing. But unburned carbon in the raw material could reduce the intensity of tobermorite. The crystallinity directly impacts the maximum uptake capacity and the scope of application. However,uncalcined raw material could reduce energy consumption, and production costs. It also has a good application foreground.The pure xonotlite has been synthesized at 240 °C for 72 h, the silica concentration is 34.13 g/L and the Ca/Si molar ratio is 1.0. The xonotlite fibers with good single crystalline structure have the diameter of 50–300 nm and length of 10–15 μm. From the experiment results, we considere the formation mechanism of the xonotlite synthesized from fly ash as follows: First, the calcium silicate hydrates gel(C-S-H gel) is produced from siliceous material and Ca(OH)2. Then the C–H–S gel transforms into 11 ? tobermorite. Third, the tobermorite was converted to xonotlite. At last, the xonotlite fiber was growing up.The xonotlite fiber was synthesized from calcium hydroxide and carbide slag respectively. The effect of Ca/Si molar ratio, hydrothermal temperature, reaction time and mechanism were studied. Meanwhile, the effect of carbon and alumina in carbide slag was discussed in this research. The result showed the alumina in the carbide slag could accelerate the process of xonotlite, and the carbon in the carbide hinders the convertion of tobermorite to xonotlite. Then the xonotlite fiber was synthesized from silica powder and carbide slag. The results showed the alumina in the acid residues could accelerate the formation of xonotlite. Add small doses of alumina could reduce the hydrothermal hours.