Change of crystallinity and mineral composition of fly ash with mechanical and chemical activation for the improvement of phosphate uptake

A detailed investigation of the development of the mineral composition and crystallinity of fly ash (FA) activated by an integrated process, as well as the relation between that development and phosphate uptake (PU) in solution, was conducted. This process, consisting of pretreatment (mechanical milling), alkali fusion (sodium hydroxide) at 550 °C and acid attack (3 mol L-1 sulfuric acid), exhibited a remarkable activation effect. One-hour grinding could enhance PU from 0.67 to 1.66 mg PO43--P g-1 FA, and then under the optimum chemical conditions with the ratio of NaOH to FA of 0.5 g g-1 and that of H 2SO4 to FA of 3 mL g-1, the PU was further improved to 7.14 mg g-1. Results also indicated that the PU performance was closely linked with the crys-tallinity and mineral composition of FA, that is, the treated material with the lowest crystallinity and least crystal phase could achieve the highest PU. For the purpose of destroying original minerals in raw FA farthest and avoiding the production of new crystals, therefore, control on the ratio of NaOH to FA and that of acid to FA was very important during the chemical treatment. © The Author (s), 2010.