Preparation of Architectural Ceramics from Coal Fly Ash: Alkali-activation and Electrochemical-detoxification of Coal Fly Ash

利用粉煤灰製備建築陶瓷:粉煤灰的堿活化與電化學解毒

Student thesis: Doctoral Thesis

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Award date7 Jul 2020

Abstract

The rapid economic development in China places a large demand for energy and as a result, thermal power plants in China are producing an enormous amount of coal fly ash (CFA) which causes severe environmental pollution. CFA, as an aluminosilicate material with small particle size, has similar chemical and phase composition with traditional ceramic raw materials. Besides, the risk of heavy metals leaching can be effectively controlled by sintering and some coupled pre-steps. Therefore, this thesis focuses on the efficient utilization of CFA as a new ceramic raw material. Environmental and economic benefits can be simultaneously achieved by preparing architectural ceramics from CFA. Aiming at the differences between CFA and traditional ceramic raw materials and the risk of arsenic and chromium in CFA, an alkali-activation pretreatment and an electrochemical detoxification method were proposed in this research, realizing the impurity removal, phase reconstruction, plasticity modification, silicate structure activation of CFA. Two kinds of high-value architectural ceramics, porcelain ceramic tiles and foamed ceramics, were successfully prepared from CFA, providing guidance for the large-scale consumption and high-value utilization of CFA. The innovative conclusions are as follows:

(1) CFA particles of different sizes and their pre-alkali-activation on ceramic products were investigated systematically. CFA particles can be divided into three classes based on composition: clay-, quartz- and feldspar-like particles, which act as clay, quartz and feldspar, respectively, in the ceramic matrix. The alkali-activation contributes to the plasticity of CFA particles, the crystal skeleton role of clay-like particles and the fluxing agent role of feldspar-like particles, improving the performance of CFA-based ceramic tiles significantly.

(2) Clay- and feldspar-like materials fabricated by alkali-activation of CFA were mixed with untreated CFA (regarded as a quartz-like material) and sintered to prepare fully CFA-based ceramic tiles. The obtained tiles exhibit excellent sintering properties, e.g., low firing temperature and a wide sintering window; further, they show better green strength (due to hydrogen bonding) and post-sintering performance (due to fluxing and mullite skeleton effects) than ceramic tiles produced exclusively from untreated CFA. The fully ash-based ceramic tiles sintered at 1100 °C exhibited optimal post-sintering properties including a bulk density of 2.5 g/cm3, a rupture modulus of 50.1 MPa, and water absorption of 0 %. The formation of glassy phases during sintering encapsulated hazardous heavy metal ions and inhibited radiation to the surrounding environment.

(3) A new foamed ceramic was successfully synthesized entirely from CFA. Before sintering, the CFA needs to be pretreated with an alkali activation that evenly coated CFA particles with a layer of hydroxysodalite crystals. The alkali-activated CFA undergoes a self-foaming reaction during sintering. When sintered at 1200 °C, the foamed ceramics exhibit optimal properties, including an apparent density of 0.41 g/cm3, the porosity of 83.60 %, a compressive strength of 8.3 MPa, and thermal conductivity of 0.0983 W/m·K. The leaching toxicity tests reveal that the hazardous heavy metals are encapsulated within the glassy phases during sintering.

(4) The exclusive alkali-activation technology was improved to a mechanochemical activation process. Mechanochemical-activated CFA was used to prepare porcelain ceramic tiles with outstanding mechanical properties. The solution after activation is the sole raw material to produce foamed ceramics with novel thermal insulation properties. During activation, some of the octahedrally coordinated Al3+ ions are converted into tetrahedrally coordinated Al3+, which can substitute for Si4+ in the tetrahedral rendering the silicate structure in activated CFA unstable. On the micro-level, solidification of the foamed ceramics can be interpreted as Si-O-Si structural transformation from two-dimensional layers to a three-dimensional network during sintering. The two products characterization shows that the good mechanical properties of the as-obtained porcelain ceramic tiles are attributed to the needle-shaped mullite derived from activated CFA and the low thermal conductivity of 0.0453 W/m·K of the as-obtained foamed ceramics arises from the unique cavity construction. The substance flow, gas emission, and heavy metal leaching behavior of the final products indicate that the improved mechanochemical activation is cleaner and more eco-friendly than the exclusive alkali-activation.

(5) A novel graphite felt (GF) cathode modified with the nanoscale MoS2/rGO heterojunction is prepared for efficient synergistic oxidative dissolution of As(III) and Cr(III) in high-arsenic CFA which has great pollution when used without efficient detoxification. By taking advantage of the p-n junction characteristics of the heterojunction and appropriate hydrophobic PTFE coating, the modified GF efficiently utilizes both dissolved O2 and gaseous O2 in the 2e- oxygen reduction reaction (ORR). DFT calculation indicates that gaseous O2 adsorbs stably on sulfur vacancies and is reduced by electrons transmitted from rGO. Experimentally, the modified GF shows excellent durability and superior ORR catalytic activity as exemplified by a high peak current density of 8.41 mA/cm2 and onset potential of 0.53 V vs. RHE. ∙OH generated by the Cr and Fe-triggered autocatalysis mechanism promotes oxidization of As(III) and Cr(III) in detoxification of high-arsenic CFA resulting in 96.1 % As removal as well as 70.74 % Cr removal in 135 min.