Use of natural products for zeolite synthesis. V. Self-bonded zeolite pellets from rhyolitic pumice

Useof natural products for zeolite synthesis. V. Self-bonded zeolite pellets from rhyolitic pumice R. Aiello, A. Nastro and F. Crea

Dipartimento di Chimica, Facolt~ d'Ingegneria, Universith della Calabria, Cosenza, Italy and C. Colella Istituto di Chimica Applicata, Facolt~ d'lngegneria, Universith di Napoli, Italy (Received 21 October 1981) A method for preparing self-bonded zeolite pellets by hydrothermal treatment of preformed pumice-alkali-water mixtures is described. Zeolite crystallization in this particular system is discussed. Formation of these crystals, together with a gel-like phase, is shown to be responsible for the hardening of the pellets. Basic characterization of self-bonded pellets, including attrition resistance and water vapour diffusion, has been performed in comparison with commercial zeolite pellets.

Keywords: Zeolite synthesis; self-bonded zeolite pellets; pelletization mechanism; zeolite L; pumice

INTRODUCTION Rhyolitic pumice has already been shown to be extremely interesting as a starting material for zeolite synthesis. It is a low cost product with constant chemical composition, easily available on a commercial scale, practically inexhaustible - the island ccf Lipari is entirely made of rhyolitic pumice. In previous studies 1-4, numerous zeolites were produced in good yields from rhyolitic pumicealkali-water mixtures, by hydrothermal treatment in which the temperature, the cation type and concentration in the alkaline contact solution, and the solid/liquid ratio were varied over wide ranges. The products included zeolites A, X, L, F, P1, P2, chabazite, phillipsite, gmelinite, basic sodalite, K-I and Li-A. Experimental results of previous researches on the production of alternative building materials, through hydrothermal treatment of preformed pozzolana-alkali-water mixtures, have also shown that, in systems characterized by very high solid/ liquid ratios, the neo-formation in situ of zeolite species promotes a marked hardening of the samples s. On this ground a systematic research has been started, devoted to study of the zeolite crystallization process in preformed mixtures of natural glasses and alkaline solutions, with the aim of investigating the possibility of obtaining selfbonded zeolite pellets directly during the synthesis process. 0144-2449/82/020290-05503.00 © 1982 Butterworth & Co. (Publishers) Ltd. 290 ZEOLITES, 1982, Vol 2, October

The preliminary results of this research, mainly concerning the mechanism of the pelletization process and the basic characterization of the products obtained, are reported in this paper. EXP,E RIME NTAL

The sample of rhyolitic pumice came from the island of Lipari. It was X-ray amorphous and had, after oven-drying at 105°C, the following chemical composition: SiO2 70.85%, A1203 12.83%, MnO 0.11%, TiO2 0.15%, Fe203 1.02%, FeO 1.35%, CaO 0.83%, MgO 0.55%, Na20 4.46%, K20 4.70%, H20 3.71%. Reagent grade NaOH and KOH were used for preparing alkaline solutions. Samples of pumice (2 g), ground to a fineness of 400 mesh, were carefully mixed with 0.4 ml of NaOH, KOH or NaOH-KOH solutions of total concentration ranging between 1 and 16 modal. The mixtures were then quantitatively transferred to a tempered steel cylindrical form, internal diameter 10 mm, and compressed by a hydraulic press, with compaction pressures ranging between 20 and 400 kg cm -2. Cylindrical compacts prepared in this way were then hydrothermally treated in sealed Teflon containers, where they were placed on Teflon supports partially immersed in distilled water, to ensure a saturated water vapour pressure in the system during the treatment. Runs were carried out at temperatures between 80 ° and 160°C, for programmed times 1-14 days. At the end of the treatment the pellets were dried at 110°C and successively equilibrated over Ca(NO3)2 saturated solution for a week.

Use o f natural products for zeolite synthesis" R, Aiello et at.

Newly-formed crystalline phases were identified by X-ray analysis, with a Guinier-de Wolf Camera, on powdered samples. Quantitative X-ray analysis was performed with conventional methods using a Philips diffractometer. The fracture surface of the pellets, previously coated with Au-Pd alloy, was examined by a Leitz AMR-1200 scanning electron microscope. Attrition tests were performed on 2 g samples, previously ground up to an 8-14 mesh grain size, utilizing a fluidized bed operated by an air current, allowing the fine powder generated by attrition to leave the top of the bed. The weight loss of the pellets was measured as a function of the attrition time. Samples of 13-X and 5-A pellets from Union Carbide, ground up to the same grain size, were used as reference. Water adsorption and desorption of the pellets were investigated with a Netzsch STA Mod. 429 Thermoanalyzer. A complete desorption-adsorption cycle was recorded for each sample, 8-14 mesh grain size, activating at 350°C to constant weight, cooling successively to room temperature under dry nitrogen flow and finally allowing it to readsorb water vapour from the air to saturation. RESULTS AND DISCUSSION

Crystallization fields The crystallization fields of the zeolites grown in the pellets, prepared as described above, during 6 days of reaction at 120 ° and 140°C are reported in Figure 1, as a function of the total alkalinity and of the KOH/(KOH + NaOH) molar ratio of the solution employed in the mixture. Below 120°C, only very moderate zeolite yields have been observed, while above 140°C, cocrystallization of undesirable phases such as sanidine or analcite appears to be favoured. Conversion of the original glass into zeolite is never complete. After reaction for 6 days or more, scanning electron micrographs of fracture surfaces of zeolite L pellets (Figure 2) showed unreacted glass shards encrusted with zeolite L microcrystals (B), much smaller than the zeolite X crystals in commercial pellets (D). This incrustation apparently retards diffusion of the solution to the glass shards, inhibiting the reaction 6. A possible joint cause for the incomplete zeolite crystallization in these compact systems may be differential dissolution of the original glass, due to its microheterogeneity7. X-ray diffractometer traces of pellets at different reaction times showed that intensity ratios of the diffraction bands of the residual glass differ from those of the pumice, suggesting that areas richest in silica react more quickly, favouring formation of less soluble residual phases.

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Figure 1 shows that numerous zeolites can be obtained in these synthesis systems by varying the reaction conditions. Some phases, particularly silica-rich zeolites such as mordenite and erionite, have not been obtained previously 1,2'4 from the same material, when solid/liquid ratios ranged between 1/100 and 1/5. This suggests that the present hydrothermal system, which is compact and permits only tow mobility, favours the growth of silica-rich zeolites with Si/A1 ratio probably close to that of the starting glass. The presence of sodium in the system evidently promotes cocrystallization of different zeolites, while potassium markedly favours the growth of zeolite L. This paper pays particular attention to the formation of zeolite L pellets, as they represent a simple model. Only a rough estimate of zeolite L content in the pellets can be given, due to the lack of reference standards with similar composition. After 14 days of reaction, for example, water adsorption indicates about 50% of zeolite L, a value indirectly confirmed by X-ray quantitative analysis of the residual amorphous fraction of the samples. Pelletization mechanism Figure 3 reports, as a function of time of hydrothermal reaction, water lost at 350°C to constant weight (upper curve) and water successively

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Use of natural products for zeolite synthesis: R. Aiello et al.

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Figure 2 Scanning electron micrographs of fracture surfaces of the pellets. A, B and C: zeolite L pellets. Synthesis conditions: OHconcentration in the starting mixture: 14 molal; KOH/(KOH + NaOH): 1 ; compaction pressure: 250 kg cm-2; treatment temperature: 140°C; reaction time: 6 days. D: commercial 13-X pellets

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coefficient of the samples 9, are very close to each other.

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The above results confirm the versatility of rhyolitic pumice as a starting material for zeolite synthesis, and also its potential use for production of self-bonded pellets of commercially interesting zeolites.

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readsorbed at room temperature (lower curve), in zeolite L pellets. The lower curve refers to 'zeolitic' water, which can be lost and regained by zeolite L in successive runs. The remaining 'non-zeolitic' water in the samples exceeds that associated with the residual glass in the pellets; some water must therefore be related to the presence of a gel-like phase which may be an intermediate stage in the crystallization process of zeolite L from the original glass. 8 The formation of this gel, together with the crystallization of the zeolitic matrix in the pellets, is probably responsible, as previously suggested s, for the mechanical properties of the pellets. Pellet characterization Pellet characterization in this preliminary stage of the research has been essentially devoted to the control of their mechanical and diffusional properties, as they are the most important parameters for practical applications.

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Figure 4 summarizes the results of the attrition tests on zeolite L pellets in comparison with commercial pellets. The degree of cementation of the pellets obtained b y hydrothermal treatment of preformed glass-alkali-water systems obviously depends on the initial compaction pressure of the original mixture and on the length of the hydrothermal treatment. Excluding the case of very low compaction pressures, e.g. 20 kg cm -2, zeolite L pellets show an attrition resistance quite close to that of commercial zeolite pellets. The diffusion coefficient of zeolite L pellets also appears to be similar to that of commercial 13-X pellets. Figure 5 compares rates of readsorption of water at room temperature on dehydrated pellets. The slopes of the straight central part of these curves, which are proportional to the diffusion

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the incomplete crystallization, researches are in progress utilizing more soluble glasses or starting systems containing both amorphous and crystalline phases.

ACKNOWLEDGEMENTS This work has been carried out with the financial support of the National Research Council (CNR Progetto Finalizzato Chimica Fine Secondaria).

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REFERENCES 1 Aiello, R. and Colella, C. Ann. Chim. 1 9 7 1 , 6 1 , 1 2 2 2 Colella, C. and Aiello, R. Rend. Accad. ScL Fis. e Mat. (Napofi), 1971,38, 243 3 Colella, C. and Aiello, R. Rend. Accad. ScL Fis. e Mat. (Napoli), 1972, 39, 103 4 Colella, C. and Aiello, R. Rend. Soc. ItaL Miner. ePetroL 1975, 31,641 5 Colella, C., Aiello, R. and Bevilacqua, L. Ann. Chim. 1975, 65, 9 / 6 Aiello, R., Colella, C., Casey, D. G. and Sand, L. B. 'Prec. 5th Int. Conf. on Zeolites' (Ed. L. V. C. Rees), Heyden, London, 1980, p. 49 7 Kingery, W. D., Bowen, H. K. and Uhlmann, D. R. 'Introduction to Ceramics' 2nd Edn. John Wiley & Sons, New York, 1976, p. 110 8 Aiello, R., Colella, C. and Sersale, R.Adv. Chem. Series 1971, 191, 51 9 Barrer, R. M. and Fender, B. E. F. J. Phys. Chem. Solids 1961, 21, 12