This study focuses on the synthesis of zeolites from Spanish ¯y ashes at laboratory (g and kg sample size) and pilot plant (metric ton sample size) scales, and on the potential application of the synthesized zeolitic material. To this end, eleven Spanish ¯y ashes were selected and characterized in detail. A matrix of experimental parameters was built to optimize the synthesis of mono-mineral zeolite products from the different ¯y ashes. This was achieved by using Parr digestion bombs, KOH and NaOH as activation agents, under experimental conditions ranging from T 150 to 2008C; t 3 to 48 h, C 0:5 to 5 M, P 4:7 to 15.3 atm, and solution/sample ratio from 2 to 18 ml g 21 . Subsequently, experiments to synthesize NaP1 zeolite were scaled up to kg sample and pilot plant scale. The zeolitic material obtained at these two scales was very similar. The NaP1 zeolite content was estimated as 40%. The major potential applications of the zeolites obtained were tested by determining the cation exchange capacity (CEC) with ammonium and different heavy metals. The results showed CEC values from 160 to 260 meq. 100 g 21 for NaP1, herschelite, KM, linde F and K-chabazite for the majority of the cations investigated. The zeolitic material synthesized at pilot-plant scale yielded the following CECs for these heavy metals: