Synthesis of zeolite adsorbents by hydrothermal treatment of PFA wastes: A comparative study

A comparative study has been undertaken into the synthesis of zeolite adsorbents from pulverised fuel ash materials obtained from Indonesian and UK coal-fired power plants. Characterised samples of each type of raw material were pre-concentrated by magnetic separation to remove any deleterious iron contamination, before being hydrothermally treated with caustic soda over a range of experimental conditions. Cation exchange capacity levels of reaction products were determined and X-ray diffraction techniques were used to identify the types of zeolites formed. A maximum CEC value of over 280meq/100g was obtained after treatment of the UK material, representing a ten-fold increase on the raw sample, whereas over 350meg/100g was achieved with the two Indonesian PFA samples, compared with raw material values of around 18meq/100g. Zeolite HS was found to be the dominant type present, particularly in products synthesised from the Indonesian materials, and types A and X were identified in some products derived from the UK sample. In all cases, metastable zeolite R was also evident. Maximum zeolite formation and corresponding CEC values were generally achieved at high temperatures and low solid/liquid ratios, while zeolite type and degree of crystallisation were found to be dependent on the reaction conditions and the mineralogy of the raw material particularly in terms of the relative concentrations of SiO2 and Al2O3. (C) 2000 Elsevier Science Ltd. All rights reserved.