Effect of Mechanical–Chemical Modification Process on Mercury Removal of Bromine Modified Fly Ash

As a new modification method, mechanochemistry features the remarkable advantages of simple operation process, low energy consumption, easy chemical modification, and suitability for industrialization. In this work, the coal-fired byproduct fly ash was modified by a mechanical–chemical method through omnidirectional planetary ball mill. The effect of the mechanical–chemical modification process parameters on the performance of mercury removal and physicochemical properties of the fly ash and the relationship between the mercury removal efficiency and the physicochemical properties were studied. The experimental results showed that, under the condition of single mechanical ball milling, the mercury removal efficiency of fly ash (FA) was slightly higher than that of raw FA. After being modified by NaBr, the mercury removal efficiency of FA increased considerably with the increase of ball milling time and speed and decreased with the increase of the size of the grinding ball. However, it was no longer significantly improved with further increasing the ball milling time and speed owing to the limited unburned carbon content in FA. The best modification process parameters were determined from the ball milling time of 1 h, the ball milling speed of 400 rpm, and the ball size of 5 mm. The characterization results showed that there was no big difference in physical properties of FA between various mechanical–chemical modification processes. However, the content of carbonyl and carboxyl/ester groups and C–Br covalent groups on modified FA demonstrated a key role in promoting mercury removal performance. The contents of carbonyl and carboxyl/ester groups and C–Br covalent groups were positively proportional to the mercury removal rate, and they were consumed during mercury adsorption. The results confirmed that the improvement of mercury removal efficiency of modified FA was dominated mainly by the surface chemical properties. Compared with the carbonyl and carboxyl/ester groups, the C–Br covalent group was the major chemisorption site of Hg⁰.