Supercritical hydrothermal synthesis of zeolites from coal fly ash for mercury removal from coal derived gas

Highlight
• CFAs zeolites were prepared by using supercritical hydrothermal method in 5 min.
• The CFAs zeolites have good activity for Hg removal.
• Iron can be activated during the supercritical hydrothermal processing.

The supercritical hydrothermal method was proposed as a new approach to synthesize zeolite from Coal fly ashes (CFAs). Eight kinds of CFAs from different power plants were used as raw material. The effects of the types of CFAs, the concentration of NaOH, liquid-solid volume ratio, reaction time and SCW temperature on the formation of zeolites and their properties were investigated. All the CFAs can form zeolite in supercritical conditions in 5 min at 400 °C, especially for the coal fly ash with medium mass ratio of SiO2/Al2O3 and low content of Fe2O3 and CaO. The different type of zeolites (cancrinite and sodalite) could be formed by changing the solution concentration of NaOH. The best crystallinity can be obtained under the conditions of the SCW temperature of 400 °C, liquid-solid volume ratio of 15:1 and NaOH concentration of 1 mol/L. The potential application of the products synthesized from CFAs was studied and the results suggested that CFA zeolites had a good activity for Hg removal from the simulation coal gas.