Synthesis Of Zeolites From Coal Fly Ash By Alkali Fusion-supercritical Hydrothermal Method For Mercury Removal From Coal Derived Fuel Gas

Fly ash is a kind of solid waste which is collected by the flue dust collector in the coal-fired power plant after coal combustion. At present, China emits the largest amount of fly ash in the world, and the fly ash emissions amounted to 620 million tons in 2015. Fly ash takes up a lot of land when piled up in the open air. In addition, fly ash contains more than 20 kinds of harmful substances to the human body and environment, including heavy metal elements which can cause disease, and ultimately endanger human health through the food chain. Recycling of fly ash is mainly concentrated in areas such as agriculture, building materials, road engineering, but the overall utilization rate of fly ash is only about 36%. Therefore, it is time to develop a way to reuse fly ash more efficiently and environmental-friendly. The main chemical composition of fly ash are SiO2 and Al2O3, which are suitable to be used as the raw materials for the preparation of zeolite materials, obtaining both environmental protection and economic benefits. Supercritical hydrothermal method is a new kind of materials synthesis method in recent years, it is not only environmentalfriendly, but also easy to operation. It has been used in the preparation of ultrafine particle materials such as metal oxide, silicate and other particulate materials.The purpose of this article is to synthesize fly ash zeolite using alkali fusion- supercritical hydrothermal method, based on the fly ash as raw materials, and then the zeolite is used for removing the elemental mercury from simulated coal derived fuel gas. The effects of the alkali species, calcination temperature, and alkali ash ratio on the fly ash activation effect in the alkali fusion process of fly ash were examined; The influence of synthesis conditions such as crystallization temperature, alkali concentration and crystallization time, liquid-solid ratio and silica alumina ratio on the formation of the fly ash zeolite were studied; And the mercury adsorption properties of fly ash zeolite in simulated coal derived fuel gas was discussed preliminarily.The experimental conclusions of this study is shown as following 3 aspects:（1） In the stage of alkali activation of fly ash, when the alkali fusion temperature of 550°C and 750°C, the fly ash could not be fully activated. With NaOH as activator, the pre-treated fly ash with the highest activation degree can be achieved at 850°C for 3 h with alkali ash ratio of 0.8:1. With Na₂CO₃ as activator, the pre-treated fly ash with the highest activation degree can be achieved at 850°C for 3 h with alkali ash ratio of 1.2:1. After comprehensive consideration, selected（850 °C, Na₂CO₃, 1.2, 3 h） as the optimal activation conditions.（2） In the process of supercritical hydrothermal synthesis of fly ash zeolite, cancrinite could be obtained within 5 min, and the crystallinity is good. On the basis of alkali residue after the molten alkali pre-treatment of fly ash, applying additional NaOH solution will inhibit growth of cancrinite and even prompt the transformation from cancrinite to grossular. The optimal preparation conditions for synthesis of cancrinite: crystallization temperature of 400°C, distilled water added, liquid to solid ratio of 5:1, SiO2/Al2O3 ratio of 2.5:1, crystallization time of 5 min.（3） The transformation to zeolite from fly ash can improve its removal property of mercury from coal gas. Fly ash zeolite has a high activity of mercury removal from coal gas at low temperature, and Fe₂O₃ plays a major role. The modification of Fly ash zeolite by Pd can improve the mercury removal efficiency of fly ash zeolite at high temperature, and the modification effect of supercritical water impregnation method is better than that of equal volume impregnation method.