Effects of different preparation methods on the types and properties of zeolites synthesized from coal fly ash

To reasonably utilize the large emissions of solid waste coal fly ash (CFA),different methods for zeolite syn- thesis from CFA were explored,focusing on the effects on the types and properties of as-prepared zeolites. The effects of alkali content,temperature and time on the types of synthetic zeolites and ammonia nitrogen adsorption capacity were investigated. The results showed that the NaP zeolite was synthesized via direct hydrothermal method at 100 ℃for 60 h in 3. 0 mol / L NaOH solution. While NaA-X composite zeolite was synthesized via the alkali fusion hydrother- mal method without structure-directing agent,silicon or aluminum source addition at 90 ℃ for 12 h. The synthesized NaP zeolite was a granular aggregate with irregular morphology and different sizes. The morphology of the synthesized NaA-X was characterized by typical cube blocks representing the zeolite A and octahedral ones representing the zeolite X. The particles were angular,with an average particle size of about 3. 5 μm. The specific surface area of NaP and NaA-X were 55. 9 and 266. 0 m2 / g,which were 21 times and 100 times those of fly ash,respectively. The micropore specific surface area of NaP and NaA-X were 12. 0 and 211. 1 m2 / g,which were 17 times and 301 times those of fly ash,respectively. The pore volumes of the two synthetic zeolites were also much larger than that of fly ash. The ammo- nia adsorption capacity were 153. 6 mmol / 100 g and 226. 6 mmol / 100 g,which were 14 times and 20 times those of fly ash,respectively. The thermal stability of the two synthetic zeolites was in line with industrial requirement,but the NaP zeolite synthesized by direct hydrothermal method was lower than that of NaA-X zeolite. Moreover,NaP zeolite contained unreacted quartz and mullite,resulting in low utilization of raw materials. On the contrary,NaA-X zeolite synthesized by alkali fusion hydrothermal method was beneficial to the conversion of inert crystal phase into active component,with the high utilization of raw materials,high product purity,and pore structure and ammonia nitrogen ad- sorption capacity were superior to the product synthesized by direct hydrothermal method.