Studies of the Solid - Liquid Phase Mass Transfer during the LTA type Zeolite Crystallization from Metakaolin

O. S. TravkinaB. I. KutepovM. L. Pavlov

Institute of Petrochemistry and Catalysis of RAS, Ufa, Russia

Abstract: Mass transfer between the liquid and solid phases of the reaction mix during the metakaolin crystallization into the powdered Zeolite LTA has been studied. The metakaolin crystallization has been shown to occur through the formation stage of the amorphous sodium aluminosilicate close by its composition to the Zeolite LTA (Linde type A). It has been established that during thermochemical processing of the metakaolin in aqueous sodium hydroxide solution the solid-liquid mass transfer occurred already at 30oC on account of the interaction of the metakaolin with polyhydroxy complexes of sodium. The stage rate grows as the temperature is elevated, and the sodium hydroxide concentration in the liquid phase of the reaction mix increased. All the above mentioned witnesses the crystallization centers to arise at the boundary of the solid and liquid phases of the reaction mix.

Keywords: Mass transfer, metakaolin, crystallization, Zeolite LTA

Received December 12, 2012; Revised January 15, 2013; Accepted February 28, 2013

Citation: O. S. TravkinaB. I. KutepovM. L. Pavlov, Studies of the Solid - Liquid Phase Mass Transfer during the LTA type Zeolite Crystallization from Metakaolin. Journal of Materials Physics and Chemistry, 1 (1). 1-3. January 2013. doi:10.12691/jmpc-1-1-1

1. Introduction

The highly dispersed Zeolite LTA (Zeolite A) is usually crystallized from aqueous solutions of sodium aluminate and sodium silicate [1]. In such a case, according to the existing concept [2,3,4], the feed reagents mixing at 20-30oC results in the formation of aluminosilicate hydrogels. The processes of ripening and undergrowth that occur in the course of hydrogel aging in aqueous solutions give rise to the particle size growth. During the following hydrothermal treatment stage of 70-100oC the rapid crystal growth of Zeolite A, X, or Y is in progress. The newly formed hydrogel compositions are closest to the compositions of slots being synthesized and are preset at the feed solution mixing stage.

Alternative techniques developed for the synthesis of Zeolite A aimed at the feedstock source expansion, technology simplification, and reduction of the obtained product cost make use of enriched kaolins as a single source of aluminum and silicon. The enriched kaolins contain one basic component kaolinite, the clay mineral with the layered lattice [5,6,7,8,9]. The Si/Al ratio values in the latter and in Zeolite A are close.

Due to the high stability of the kaolinite’s layered lattice in alkaline solutions, a product of the thermal kaolinite dehydration at 500-900oC, namely, the amorphous aluminum dislocate or metakaolin possessing the chemical composition of the formula Al₂Si₂O₅ is used for the zeolite crystallization. The product composition differs from that of Zeolite A, primarily by the absence of sodium; therefore the crystallization of metakaolin is performed in sodium hydroxide solutions. Thus, the major distinction between the crystallization of Zeolite A from metakaolin and the crystallization of the same structure type zeolite from solutions of sodium silicate and sodium aluminate is the presence from the very beginning of the synthesis of the solid and liquid phases significantly different in their composition. Hence, the mass transfer between those phases during the metakaolin crystallization in sodium hydroxide solutions assumes the specific importance; however, the reference data on it are not available.

In connection with the above, this work was aimed at the kinetic studies of the liquid - solid phase mass transfer during synthesis of the high dispersed Zeolite A from metakaolin in alkaline solutions of different sodium hydroxide concentrations within the temperature range of 30 thru 80oC.

2. Experimental Part

For the experiments we used the powdered metakaolin prepared from the kaolin of the Prosyanov deposit by means of thermal processing at 600-650oC in the air for 4-6 hours [9]. The prepared metakaolin contained 97.0-98% of aluminum and silicon oxides in the presence of impurities, such as iron, calcium, and titanium oxides. Thermochemical processing of the metakaolin was conducted in aqueous solutions of sodium hydroxide with the concentrations of 132, 170, and 332 g/dm³ counted on Na₂O basis, at the temperature of 30, 60, and 80oC for 4-24 hours.

The chemical composition of the solid and liquid phases was determined by weighing and Alkalimetric methods, the method of flame photometry on the device type "PFA-311" and complexometric titration [10].

The phase composition of the solid phase prior and after thermal processing was determined using automated PHILIPS PW 1800 XRD meter and the powder technique by the Debye-Sherrer. The XRD (X-ray diffraction) pattern detecting conditions was as follows: θ/2θ - scanning; the sample holder spinning of 1 RPS; the copper anode; the scattering angle range of 5-55о/2θ; the angle pitch of 0.05о; the exposure time per pitch of 2s; the anode voltage and current of 40 KV and 30 MA, respectively. XRD patterns were identified using the reference diffraction data [11].

3. Results and Discussion

In Figure 1 the data on the effect of the metakaolin holding time span at 30oC in aqueous sodium hydroxide solutions on the kinetics of the change in sodium, aluminum, and silicon concentrations in the liquid phase of the reaction mix (RM) are given.

Figure 1. Effect of the duration of the metakaolin thermal processing in aqueous sodium hydroxide solutions of different initial concentrations at 30oC on the content of the crystal-forming components in the liquid phase of the RM

It is seen that in the course of the entire holding time span the concentration of sodium in the solution slowly decreases. Simultaneously with the transfer of sodium into the solid phase the ions of aluminum and silicon move from the metakaolin into the solution. Their concentration is gradually increased and in 8 hrs reaches its maximum values of 5.1 and 7.4% of the concentration in the metakaolin, respectively. A further increase in the processing duration does not result in any variation in sodium and silicon concentrations in the liquid phase of the RM. Thus, the mass transfer between the liquid and solid phase occurs already at 30oC on account of the chemical interaction between the metakaolin and polyhydroxycomplexes of sodium. Meanwhile, the transformation of the metakaolin into the amorphous sodium aluminosilicate is started. The crystal-forming component ratios in the above mentioned phases are given in Table 1.

Table 1. The crystal-forming component ratio in the liquid and solid phase of the RM after 8 hour processing at 30oC in sodium hydroxide solutions of different concentration

It is seen that those values differ greatly from one another as well as from the proportion between sodium, aluminum, and silicon in Zeolite A.

When the crystallization temperature is elevated to 60oC (see Figure 2), the concentration of sodium oxide decreases just for the first 6 hours, and remains constant thereafter.

Figure 2. Effect of the duration of the metakaolin thermal processing in aqueous sodium hydroxide solutions of different initial concentrations at 60oC on the content of the crystal-forming components in the liquid phase of the RM

During this period the concentrations of silicon and aluminum oxides in the liquid phase of the RM grow and in 6 hrs reach the maximum of 4.2 and 6.7% of their content in the metakaolin, respectively. If the processing duration is increased further, those concentrations are gradually decreased.

Thus, at 60oC the mass transfer between the solution and metakaolin is more intense than at 30oC. The amorphous sodium aluminosilicate is formed in the solid phase (see Table 2), and after 6 hours processing its sodium oxide/aluminum oxide ratio already matches that in Zeolite A (or a little bit higher), and the crystallization actually begins.

Table 2. The crystal-forming component ratio in the liquid and solid phase of the RM after 6 hours processing at 60oC in sodium hydroxide solutions of different concentration

According to the XRD data, by that moment Zeolite A has already started to form in the solid phase and its content is about 5% by mass. If the processing duration is increased further, the concentrations of silicon and aluminum in the liquid phase of the RM are gradually decreased. The crystallization is complete in 20 hours and results in the formation of the high dispersed Zeolite A with the degree of crystallinity close to 96%.

When the crystallization temperature is elevated from 60 to 80oC (see Figure 3), the concentration of sodium counting on the sodium oxide basis decreases just for the first hour, and remains constant thereafter.

During this period the concentrations of silicon and aluminum in the liquid phase grow and reach their maximum. If the processing duration is increased further, those concentrations are decreased.

Figure 3. Effect of the duration of the metakaolin thermal processing in aqueous sodium hydroxide solutions of different initial concentrations at 80oC on the content of the crystal-forming components in the liquid phase of the RM

At 80oC the amorphous sodium aluminosilicate close by its composition to that of Zeolite A is formed already in 1 hour, and its crystallization begin (see Table 3).

Table 3. The crystal-forming component ratio in the liquid and solid phase of the RM after 1 hour processing at 80oC in sodium hydroxide solutions of different concentration

Apparently, the crystallization centers are formed on the solid-liquid phase boundary as early as the temperature is low. The nucleation process is followed by the crystal growth stage; as the temperature gets higher, the rate of growth increases. A shortage in the “construction material” for the ever growing crystals is probably made up from the solution at the expense of the aluminum and silicon cations emerged into the solution earlier. At 80oC the crystallization is complete in 12 hrs and results in the formation of the high dispersed Zeolite A with the degree of crystallinity close to 97%.

4. Conclusions

It has been established that during thermochemical processing of the metakaolin in aqueous sodium hydroxide solution the solid-liquid mass transfer occurred already at 30oC on account of the interaction of the metakaolin with polyhydroxy complexes of sodium. The amorphous sodium aluminosilicate formed in the process possessed the crystal forming component ratios greatly different from one another and from those of sodium, aluminum, and silicon in Zeolite A.

The amorphous sodium aluminosilicate has been shown to be formed in the solid phase at 60oC; the sodium/aluminum oxide ratio in it after 6 hour processing already matched that of Zeolite A and the crystallization actually began.

The transformation of the metakaolin particles into sodium aluminosilicate containing sodium cations in the equivalent quantity of the content of aluminum cations has been found to be essential for the metakaolin crystallization into Zeolite A. If the Na₂O/Al₂O₃ ratio in the aluminosilicate exceeds 1, it does not crystallize into Zeolite A.

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