We have been investigating the zeolite synthesis from the paper sludge (PS) ash. The development of the zeolite synthesis technique is essential from the viewpoint of recycling of wasting materials, since a large amount of the PS ash, that is 160, 000 t/year, is discharged from the paper manufactory in Fuji area of Shizuoka Prefecture. The PS ash mainly consists of calcite (CaCO3), talc (Mg3Si4O10 (OH) 2) and kaolinite (Al2Si2O5 (OH) 4), which were originally present in papers. In addition, it contains gehlenite, which were formed by the reaction between kaolinite and calcite via the formation of metakaolinite and lime (CaO) in the incineration process at the temperature higher than 850°C. There is little amount of Si and Al for the utilization as a starting material of zeolite crystallization, since the gehlenite formation requires Si and Al from metakaolinite. Also, in some kinds of the ashes, the excess amount of lime remains that was not consumed for the gehlenite formation in the incination process. In this study, the influence of the excess amount of Ca on the zeolite synthesis was studied.
The mixtures of Ca (OH) 2 and metakaolinite were dipped into diluted alkali (NaOH) solution, and then the interaction between Ca and aluminosilicate derived from metakaolinite was examined in terms of dissolution characteristics of Si and Al from amorphous Ca-silicate hydrate. The results of molecular orbital calculation showed that co-existing Ca would suppress the formation of Si-O-Al bonds, supporting the possibility that amorphous Ca-silicate hydrate is more stable than Si-OAl bond. These samples were characterized in terms of the solubility in diluted alkali solution, the mineralogical composition and the LTA conversion.
Mineralogically characterization of the above mixtures by means of a thermogravimetry and an X-ray diffraction measurement led us the supposition that the chemical formula of the amorphous Ca-silicate hydrate is Ca3Al2 (SiO4) 3-x (OH) 4x, which gave the conversion rate of LTA-type zeolite. Further, the mixtures of metakaolinite, Ca (OH) 2 and calcite were treated with hydrothermal reaction in 3.0 M NaOH solution, and the conversion rates were estimated by the same way. It became clear that the LTA-type zeolite occupies 84wt% and 64wt% ideally in the cases that Ca (OH) 2 are added 12wt% and 31wt%, respectively, to the metakaolinite heated at 600°C. On the basis of these results, the ideal mineral composition range of PS for the zeolite synthesis was obtained.
