Thermal and acid treatment on natural raw diatomite influencing in synthesis of sodium zeolites

Abstract

A natural raw diatomite and four modified diatomites by calcination and acidification were used as starting materials for the sodium zeolitization process. Iron impurity content present in the natural raw diatomite which was expressed as Fe2O3 is about 5.8% and Q4 ([Si(OSi)4]) unit 64%, whereas the modified diatomite which was treated with hot 6 M H2SO4 (refluxed at 100 °C) for 24 h and followed with calcination at 1100 °C for 5 h, contains about 0.5% of Fe2O3 and 97% of Q4. The results showed that under the studied conditions (NaOH 10–30%w/v, 100–180 °C, solid:liquid 1:10 and 1:30, reaction time 24–180 h) only Na-P1, analcime, cancrinite and hydroxysodalite were obtained and the highest yield of each sodium zeolite product was obtained from the modified diatomite treated with hot H2SO4 and followed with calcinations at 1100 °C. This results from the reduction of iron content by acid leaching and the elimination of silanol groups after calcination to form more siloxane groups in the structure of starting materials. In addition, a role of iron content was found to be significant in zeolite formation compared to a role of Q4 sites. It was found that a percentage of crystallinity is very high in the case of all starting materials, which first underwent iron impurity elimination before further thermal treatments.