Effects of wet ball milling on lead stabilization and particle size variation in municipal solid waste incinerator fly ash

Water-extracted municipal solid waste incinerator (MSWI) fly ash was treated by a process of wet ball milling, using desalinated water as the milling solution. We investigated the influence of the milling process on the partitioning and leaching characteristics of lead (Pb) and the particle size distribution. The results show that 93.11% of the Pb was partitioned into the milled ash, 2.60% to the milling balls, and 0.17% to the inner surface of the milling jar, while amounts lower than the detection limit remained in the milled solution. As tested by the toxicity characteristic leaching procedure (TCLP), the leaching of Pb was inhibited after short-term grinding (from 5.2 to 1.2 mg/L after 1 h of milling), and further reduced by about 96% after 96 h of ball milling. The mobility of the heavy metal was analyzed after a sequential extraction procedure. The results also show that Pb tended to become more stable after milling. The size distribution of particles was analyzed by a laser particle diameter analyzer and their morphology during grinding was observed using scanning electron microscopy. The median size of the fly ash decreased significantly from 36 to 5 μm after 0.5 h of milling, but then only slightly, from 5 to 2 μm, with further milling from 0.5 to 96 h, due to the concurrent actions of fragmentation and/or agglomeration. The reason for the stabilization of Pb by ball milling was probably that Pb was sealed in the milled fly ash during the fragmentation and agglomeration of particles.