Metakaolin

A gel is defined as a hydrous metal aluminosihcate prepared from either aqueous solutions, reactive soflds, colloidal sols, or reactive aluminosihcates such as the residue stmcture of metakaolin and glasses. 

The 2eohtes are prepared as essentially bindedess preformed particles. The kaolin is shaped in the desired form of the finished product and is converted in situ in the pellet by treatment with suitable alkaU hydroxide solutions. Preformed pellets of 2eohte A are prepared by this method. These pellets may be converted by ion exchange to other forms such as molecular sieve Type 5A (1). ZeoHtes of higher Si02/Al202 ratios, eg, 2eohte Y, can be obtained by the same method, when sodium metasiUcate is incorporated in the preshaped pellets, or when acid-leached metakaolin is used. 

Engelhard s in-situ FCC catalyst technology is mainly based on growing zeolite within the kaolin-based particles as shown in Figure 3-9A. The aqueous solution of various kaolins is spray dried to form micR)spheres. The microspheres are hardened in a high-temperature l,3f)(TF/704°C) calcination process. The NaY zeolite is produced by digestion of the microspheres, which contain metakaolin, and mullite with caustic or sodium silicate. Simultaneously, an active matrix is formed with the microspheres. The crystallized microspheres are filtered and washed prior to ion exchange and any final treatment. 

Subsequently, the dried ore is reduced in an electric furnace to ferronickel. Drying of the ore ensures smooth operation in the reduction furnace. As another example, reference may be drawn to the processing of kaolinite, Al2(Si05)(0H)4, for the recovery of alumina. The mineral is leached with dilute sulfuric acid. In the hydrated form, the mineral is insoluble in dilute mineral acids, and on drying at 400 to 800 °C, kaolinite is converted to the amorphous form, metakaolin, A1203 2 Si02 ... 

As discussed in Chapter 10, a wide variety of additives is used in the polymer industry. Stabilizers, waxes, and processing aids reduce degradation of the polymer during processing and use. Dyes and pigments provide the many hues that we observe in synthetic fabrics and molded articles, such as household containers and toys. Functional additives, such as glass fibers, carbon black, and metakaolins can improve dimensional stability, modulus, conductivity, or electrical resistivity of the polymer. Fillers can reduce the cost of the final part by replacing expensive resins with inexpensive materials such as wood flour and calcium carbonate. The additives chosen will depend on the properties desired. 

The relative ease with which VpOr can be reduced to V(III) in aluminosilicates indicate the exiirence of weak metal-surface interactions and the inability of the surface to effectively passivate vanadium. Similarly, V on Kaolin (and metakaolin) exist mostly as the "free oxide and can (in part) be reduced to V(III) species. Therefore, DFCC systems containing metakaolin microspheres (or amorphous aluminosilicates (15)) should not be as effective as sepiolite in passivating metals TTke Ni and V. In fact, DCC mixtures loaded with 5000 ppm Ni-equivalents (that is 0.6% V + 0.38% Ni) are not metals resistant when metakaolin is used as a metals scavenger (1) ... 

DFCC mixtures containing 40% sepiolite and 60% GRZ-1 are equally effective in passivating high (10,000 wtppm) levels of vanadium impurities (1 ). In both cases, metakaolin microspheres do not... 

The first is metakaolin. This is a partially calcined product that forms above about 500 °C. Only about 10% of the original hydroxyl groups of the kaohnite are retained and much of the crystalline nature of the structure is destroyed. Metakaolin is considerably more reactive than the original kaolin and appears to have an especially reactive surface. It is generally used uncoated and finds most use in plasticised PVC cable insulation, where it is reported as giving uniquely useful electrical properties [86]. 

Barrer and Mainwaring (20) report the use of metakaolin as the aluminosilicate raw material for reaction with the hydroxides of K and Ba as well as the binary base systems Ba-K and Ba-TMA to form zeolites. Zeolite phases previously synthesized in the analogous hydrous aluminosilicate gel systems were crystallized with KOH, including phillipsite-, chabazite-, K-F-, and L-type structures. The barium system yielded two unidentified zeolite phases (Ba-T and Ba-N) and a species Ba-G,L with a structural resemblance to Linde zeolite L. Ba-G,L was reported previously by Barrer and Marshall (21) as Ba-G. Similar phases were formed in the Ba-K system and in the TMA-Ba system where, in addition, erionite-type phases were formed. The L-type structures are said to represent aluminous analogs of the zeolite L previously reported (22). 

Vanadium is much more mobile than nickel and its mobility is controlled by the nature of the matrix or passivating agent present in the FCC. Sepiolite, attapulgite, MgO and AI2O3 appear to be more active for trapping vanadium than other matrices such as metakaolin and AAA-alumina (1,5,10,23,24,25), because of the ability that these supports have to form heat stable V-compounds (5,23). 

Geopolymers are another type of intermediate products that lie between cements and ceramics [7]. A geopolymer is made by pyroprocessing naturally occurring kaolin (alumina-rich clay) into metakaolin. This metakaolin is then reacted with an alkali hydroxide or sodium silicate to yield a rock-Uke hard mass. Thus, a chemical reaction, which is not fully understood, is employed to produce a hard ceramic-Uke product. Though this product is produced like cement, its properties are more like a sintered ceramic. It is dense and hard like a rock. 

The above procedure of incorporating sodium to fresh catalyst has an inherent shortcoming. Sodium from FCC feedstock accumulate on catalysts which have been hydrothermally aged. During hydrothermal aging, the zeolite unit cell size decreases from above 24.50 A to typically lower than 24.30 A, the surf ace area of both zeolite and matrix decreases and transformation of kaolin clay to metakaolin occurs. 

This evidence suggests that not all Na species are mobile. Some Na species must in fact have reacted irreversibly with components on the catalyst, leaving it unavailable to poison the acid sites. It is likely that these reactions occur during the early stages of hydrothermal deactivation. The exact mechanism is unclear, but may involve reactions with extraffamework alumina. As the zeolite dealuminates from 24.55 to 24.25A unit cell size, approximately 65% of the initial framework alumina (about 15 wt% of the zeolite) comes out of the zeolite structure. Sodium, which also must leave the exchange sites as the zeolite dealuminates may react with this very reactive form of alumina. The other possibility is that as kaolin undergoes its transition to metakaolin at 800K... 

Preparation of Porous Silica by Acid Activation of Metakaolins... 

Different metakaolins were prepared from a Spanish natural kaolin, by calcination at various temperatures. These solids were submitted to acid activation under different conditions. The behaviour of these samples was studied by different techniques paying special attention to the study of their porosity. This treatment produces the modification of the starting materials and the result was the synthesis of porous silica. Optimal conditions of activation were found. 

However, different studies have revealed that the amorphization of kaolin produces solids more reactive under chemical treatments. This amorphization can be performed by calcination, obtaining metakaolin by heating from 550 C to 900 C. The metakaolin is an amorphous phase formed by the dehydroxylation of the octahedral layers and the deformation of the tetrahedral sheets [5-7]. Also the grinding produces the amorphization of kaolin [8], by means of delaminating [9,10]. 

Acid activation has long been known as a preparation method for porous materials. In general, the acid solution dissolves the octahedral sheets of the clay and produces a significant modification over the tetrahedral sheets [11]. The solids obtained have higher surface area and better adsorptive and catalytic properties than the original clays, depending on the starting material and the conditions of the treatment [12-15]. The activation of metakaolin has been reported only in a few articles, and the application of the solids obtained to different catalytic reactions has been described [3,7,16-18]. 

This study focuses on a systematic analysis of the chemical activation of kaolin with HCl solutions under different conditions of time and temperature, with a complete discussion of the phenomena governing this process and a wide characterization of the solids obtained. Such a systematic study lacks in the literature and the interest in looking for new applications of natural clays justifies it. By this reason, we have carried out the acid activation of the metakaolins obtained by calcination of a kaolin between 600-900°C and a complete study of the solids prepared, where the porous properties of the prepared materials receive special attention, due to the high importance that these properties have over the industrial applications of the final solids. 

The metakaolins were prepared by calcination of the < 2pm fraction at 600, 700, 800 and 900°C with a heating rate of 10°C min. Calcination was maintained for 10 h, under air atmosphere. The solids obtained were named MK-T, where T indicates the calcination temperature. The metakaolins were treated with 6M HCl solutions using the following conditions 6 hours at room temperature and 6, 12, 18 and 24 hours at 90 C, under reflux conditions. 6.0 g of the metakaolin were suspended in 180 mL of the acid solution and stirred during the times indicated. The leached samples were separated by centrifugation, washed until free of chloride anions and dried at 50"C. The nomenclature employed for these samples is as follows MK-T-HCl-C-T -t, C being the acid concentration, 7 the treatment temperature and t the time of the treatment. 

The XRD patterns of kaolin and metakaolins (Fig. la) clearly show the amorphization of kaolin produced during its thermal activation. The characteristic peaks from kaolin [20] disappear and instead a halo at 20 from 15 to 30° due to amorphous silica appears. These XRD patterns also reveal the thermal stability of the mineralogical impurities. 

Figure 1. X Ray diffraction patterns of a) kaolin, metakaolins and the solids treated at 90°C for 6 h b) MK-600 and the leached solids obtained from this metakaolin.

The formation of silica strongly depends on the parent metakaolin. The XRD patterns (Fig la) show how the MK-900 sample is the least activated. The high temperature of the calcination employed causes the sinterization of the particles and complicates the attack of the acid. The behaviour of the other metakaolins is quite similar for all of them. 

Figure 2 shows the FT-IR spectra of kaolin and MK-600 metakaolin and the samples treated at 90 C (the treatment at room temperature did not modify the metakaolin). The spectrum of the metakaolin shows the structural changes produced during the calcination. The four Al-OH stretching bands, from 3700 to 3600 cm [21-24], disappear due to the dehydroxylation, the water bands being observed at 3470 and 1630 cm. ... 

Figure 2. Infrared spectra of kaolin, MK-600 metakaolin and some leached solids.

The SEM micrographs of the kaolin and metakaolins show plate shape particles for both types of solids (Figure 3a). Calcination of the kaolin produces its dehydroxylation but does not destroy its platy shape. For samples treated under reflux conditions the plates are dissolved and the micrographs (Fig 3b,c) show amorphous particles that coexist with the metakaolin plates. There are not many differences between the samples activated at different times, although the chemical composition indicated that the amount of Al removed by activation for 24 h was higher than during activation for 6 h. 

The nitrogen gas adsorption-desorption isotherms of the metakaolins are classified as type II (BDDT classification [27]). They are almost reversible with a closed hysteresis cycle, indicating the absence of micropores. The samples obtained at room temperature show N2 isotherms similar to those from metakaolins. This treatment did not significantly modify the structure of the metakaolin and the porous properties of these samples are very close to those of the parent metakaolin. The samples obtained under reflux conditions for 6h show nitrogen gas adsorption-desorption isotherms different from those of the parent metakaolins. They show an increase of adsorption at low relative pressures and reach a plateau at intermediate values of P/Po. This kind of isotherm is classified as type I (BDDT classification, [27]) and it is characteristic of microporous materials. For treatment times higher than 6 h, the isotherms are analogous to those of metakaolins, classified as type II [27], which indicates the loss of the microporosity formed at lower times. 

The BET specific surface area values are listed in Table 2. Acid treatment at room temperature did not significantly modify the surface area of the metakaolin, thus confirming the low effectiveness of this treatment. Activation at 90°C during 6 h produces solids with relatively high surface area, 200 m g, when the kaolin is calcined from 600 to 800°C. These high values are due to the formation of an amorphous silica phase during this acid treatment. However, the solid obtained from MK-900 has a surface area of only 59 m g", due to the above mentioned sinterization of the kaolin at this temperature. 

Table 2. Porous properties of the kaolin, metakaolins and the acid leached samples BET surface area (from BET method), external surface area, micropore surface area and micropore volume (from t-method).

The pore size distribution is displayed in Fig. 5 for the samples activated under reflux conditions during 6 h, calculated by the Dubinin-Astakhov method [28]. These samples have a similar pore diameter, with a value between 17 and 19 A but the intensity of the curves is different. This difference shows the influence of the starting metakaolin over the formation of porosity, where MK-900 shows again the worse properties. The samples activated at longer time did not show internal surface. 

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