| Coal fly ash (CFA) is formed by combustion of coal in coal-fired power station as a waste product. The amount of CFA in China is significantly increasing every year in the electric power plants, and it is over 0.57 billion tons in 2013. Generally, power plants consuming 4 tons of coal will produce one ton of CFA. Efficient disposal of CFA, which has amount of deposit as an energy source, has been a worldwide issue because of its massive production and harmful effects on the environment. Due to the shortage of landfill sites and tighter environmental regulation, new ways of utilizing CFA are needed. The aluminosilicate glass in CFA is a readily available source of Si and Al for zeolite synthesis. As a technique for recycling CFA, as well as other mineral wastes, zeolitization has attracted a great deal of attention. Zeolite synthesis is one of main potential applications for obtaining high-value industrial technology using CFA, and CFA zeolitic materials have been used for a wide range of purposes, including agricultural, environmental, and industrial applications. It is reported that most of CFA zeolites have been used in the field of gas purification, separation and water purification for its high adsorption and ion exchange capacity.The following methods were used:classic hydrothermal thermal (one step process method and two step process method), alkaline fusion method, salt-thermal method. However, the zeolite products still had significant amounts of disadvantages, for example, the long reaction time, the cumbersome experimental steps and the high energy consumption. Microwave-assisted and adding crystal method can ease the weakness of the long reaction time, but they have other some above problems. In recent years, supercritical water (SCW) synthesis has received a great deal of attention as a generalized crystallization method for the production of metal oxide particles, load type adsorbents and nanometer compounds. The gas-liquid transport properties, high nucleation rate and very low surface tension of supercritical water have received a great deal of attention as a generalized crystallization method. So, supercritical water synthesis has been proposed as a new approach to zeolitization of CFA by our research group.The eight kinds of CFAs obtained from different power plants, the scale is from 200 MW to 600 MW and the location is mainly in Shanxi, were used as raw material. A matrix of experimental parameters was built to optimize the synthesis of CFA, this was achieved by using NaOH as activation agent, under experimental conditions ranging from NaOH concentration of 0.1-10 mol/L, liquid-solid volume ratio of 5:1-20:1, reaction time of 1-120 min, temperature of 350-420 ℃. The experimental products were characterized by means of X-ray diffraction (XRD), XPS and energy dispersive X-ray coupled-scanning electron microscope (EDS/SEM). Combined with our research work, the major potential applications of the CFA zeolites obtained were tested by determining Hg removal efficiency from coal derived fuel gas in a fixed bed configuration. As a kind of noble metal, Pd has quite high mercury removal efficiency at middle temperature, so Pd/CFA zeolites prepared using supercritical water synthesis were also tested by determining Hg removal efficiency from coal derived fuel gas. The conclusions are mainly summarized as follows:(1) CFA zeolites can be formed using eight kinds of CFA in supercritical conditions. The low or medium SiO2/Al2O3 mass ratio and low content of Fe2O3 and CaO in CFA raw materials were beneficial to the synthesis zeolite from CFA in supercritical water.(2) CFA can transform to CFA zeolites using the supercritical hydrothermal synthesis in only below 10 min. With increasing NaOH usage, CFA can transform to two zeolites:cancrinite and sodalite. With increasing liquid-solid ratio, the crystallinity is better.(3) The sodalite and cancrinite formed by CFA during the supercritical water synthesis have good activity for Hg removal at low temperature of 100 ℃. The CFA zeolites modified by Pd have a good activity for Hg removal at medium temperature of 200-300 ℃. The addition of H2S in reaction gases is bad for the activity of Hg removal. |
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