REACTIONS OF SALTS WITH KAOLINITE AT ELEVATED TEMPERATURES. I · PDF fileClay Minerals (1978) 13, 221. REACTIONS OF SALTS WITH KAOLINITE AT ELEVATED TEMPERATURES. I L. HELLER-KALLAI

Clay Minerals (1978) 13, 221.

R E A C T I O N S OF SALTS WITH K A O L I N I T E AT E L E V A T E D T E M P E R A T U R E S . I

L. H E L L E R - K A L L A I

Department of Geology, The Hebrew University, Jerusalem, Israel

(Received 20 October 1977)

A B S T R A C T : On heating at temperatures below 600~ kaolinite reacts with salts of alkali metals according to the equation

2SIO2 �9 A1203 �9 2H20 + 2nMX ~ 2SIO2 �9 Al2Oa - nM20 + 2nHX + (2- n)H20

The reaction commences with the dehydroxylation of the clay and is favoured by high solubility of the salt and small size of the alkali ion.

It appears that on dehydroxylation the clay becomes reactive and, concurrently, the water liberated dissolves adjacent salt particles and catalyses the reaction.

Infrared spectra indicate that alkali ions are incorporated directly into the aluminosilicate structure, without the formation of metakaolinite as a detectable intermediate. The initial product is disordered, but as n approaches 1, crystalline phases tend to be formed.

I N T R O D U C T I O N

The reactions o f montmori l loni te heated with alkali halides at temperatures below their melting points have been previously investigated (Heller-Kallai, 1975). I t was shown that excess cations were taken up by the clay at relatively low temperatures by deprotonat ion of structural hydroxyls and the suggestion was made that reactions of montmoril lonite with proton acceptors may play a part in clay diagenesis. The question then arose whether kaolinite, with its different crystal structure, would undergo similar reactions. These are o f particular interest because o f their possible relevance not only to clay diagenesis but also to the science o f ceramics.

This study deals with the reactions o f kaolinite with salts of alkali metals. Preliminary experiments showed that the effects obtained with salts o f di- or polyvalent metals differ and require separate treatment. The investigation was largely confined to short-term reactions carried out at temperatures below 600~ i.e. a thermal regime under which partial or extensive dehydroxylat ion of kaolinite occurs. Reactions at higher temperatures will be treated in a separate paper.

E X P E R I M E N T A L

Materials

The kaolinite used was a sample f rom Oneal Pit supplied by Wards. Aliquots of a ground and homogenized sample were used without further pretreatment. A few experiments were carried out with a sample f rom Zettlitz, Czechoslovakia. The salts used were of Analar quality.

0009-8558/78/0600-0221 $02.00 �9 The Mineralogical Society

2 2 2 L. Heller-Kallai

0

0

T

I

I

o ~ e~

~ ' ~

e-,

e-,

<

<

o~

r ~

o

o ~ ~D

~,~ t ~ ~,.~ ~ ~ t ~ ~ ~-~ ~ ~ ~ ~'~ ~ ~ ~-~ ~ ~ ~ I ~'~ ~. ~

+ + 4 - + + + 4 - 4 - + + +

Reactions of salts with kaolinite 2 2 3

o

I

6 ~ 6 ~

4- _]_ § 4-

,,...:,

>.

r,~ e-,

LL :__... -6 =

-6 + -

IL , 5 ~

8

<

224 L. Heller-Kallai

Procedure

Aliquots of the kaolinite samples were heated with various salts in open crucibles in a furnace controlled at _5~ The salt was used in large excess in most experiments (kaolinite:salt ratio 1:17 by weight) but experiments were also carried out with lower kaolinite: salt ratios.

The samples were washed with distilled water until the washings were clear of the appropriate anions. The solids were checked for the absence of adhering salts by X-ray diffraction and, where applicable, for the absence of carbonate or bicarbonate groups by infra-red spectroscopy.

Six washings sufficed for soluble salts but less soluble ones, e.g. sodium and potassium carbonate, sometimes required more. Some salts, e.g. NaF, could not be entirely removed by washing.

Infra-red patterns of both washed and unwashed specimens in the form of KBr disks were recorded on a Perkin-Elmer 237 spectrometer. X-ray diffraction patterns of the washed samples were obtained on a Nonius focussing camera. Thermal analysis of selected samples was performed on a Mettler thermoanalytical apparatus, which records DTA, TG and D T G curves simultaneously. The samples were heated under a stream of pure nitrogen (99.997~) at a uniform heating rate of 10~ rain-1. Scanning electron micrographs were taken of some of the washed and unwashed samples. The samples, after exhaustive washing, were finally dissolved by HC1/HC104/HF treatment and analysed by atomic absorption.

R E S U L T S A N D I N T E R P R E T A T I O N

Chemical analyses

Analyses showed that, after heating the kaolinite-salt mixtures, various amounts of alkali ions were incorporated into the alumino-silicate. Table 1 presents the amount of alkali ions taken up per formula unit of the kaolinite initially present. Complete analyses of selected samples (marked with an asterisk in the table) showed that the SIO2/A1203 ratio did not change significantly on heating for 1 h at temperatures below 600~ The initial ratio is 1.23, slightly higher than the theoretical 1.18 for pure kaolinite, due to the presence of minor amounts of quartz. After heating, the ratio ranged from 1.23 to 1.37. Although the increase may in part be due to experimental error, it is probably genuine, since a considerable decrease in A1 content was observed in some samples after more drastic heat treatment. These reactions will be considered separately. For the present purpose it may be assumed that in the samples under consideration the SIO2/A1203 ratio did not change significantly in the course of the reaction. The amount of alkali ions taken up by most of the samples was therefore determined relative to the amount of A1 present. The error introduced by this assumption is small and does not affect the conclusions.

X-ray data

The products obtained on heating kaolinite or most of the kaolinite-salt mixtures at temperatures ranging from 480 to 590~ for short periods of time are amorphous to X-rays. On more prolonged treatment at 590~ pure kaolinite remained amorphous but some of the salt-kaolinite mixtures gave rise to weak X-ray patterns of phases of the type 2SiO 2 �9 A120 a �9 M20, where M is the appropriate alkali ion, as shown in Table 2.

Reactions of salts with kaolinite

TABL~ 2. X-ray identification of product obtained after heating kaolinite with various salts at 590~ (a) for

30 min; (b) for 24 h

Salt added (a) For 30 min (b) For 24 h

- - Amorphous Amorphous KBr Amorphous Amorphous KF Kaliophilite Kaliophilite KHCO3 Amorphous Kaliophilite K2COa Amorphous Kaliophilite NaHCI3 Amorphous Carnegieite* LiCl Amorphous Amorphous

* The X-ray pattern is similar to that of the 'L phase' described by Kubo et al. (1966).

225

Some of the samples were washed after heating at 590~ for 30 min and were subsequently ignited at 1000~ for 11 h. After this treatment the sample of pure kaolinite gave rise to a weak X-ray pattern of mullite. Similarly, samples which had been heated with K z S O 4 or LizSO 4, neither of which reacted appreciably with kaolinite at 590~ (Table 1) also showed a weak pattern of mullite. In contrast, the sample which had been heated with KBr at 590~ and washed remained amorphous after heating at 1000~ while a similarly treated KzCOa-kaolinite mixture gave rise to the X-ray pattern of kaliophilite and corundum. With KF the pattern of kaliophilite appeared after heating the mixture at 590~ for only 30 rain. A 1 : 3 mixture of kaolinite with KBr, which had been heated up to 910~ under a stream of nitrogen and washed to remove excess KBr, was amorphous to X-rays. A similarly treated kaolinite-KzCO 3 mixture gave rise to the X-ray pattern of kaliophilite.

Infra-red spectra

There is general agreement in the literature on the assignment of the absorption bands of kaolinite and metakaolinite. Percival & Duncan (1974) listed the frequencies of Si-O and AI-O vibrations of kaolinite and metakaolinite and their assignments, based on a survey of the literature. The assignments of the OH stretching and bending vibrations are given by Farmer & Russell (1964).

In the present study the changes produced in kaolinite by various heat treatments were followed by studying the bands listed in Table 3. Some of the results are included in Table 1.

No differences were observed in the IR patterns of heated kaolinite-KBr mixtures before and after washing six times with distilled water. The presence of some of the other salts used renders the patterns unclear and minor changes occurring on washing may be obscured. For convenience, the investigation was therefore initially carried out with KBr and was later extended to other salts.

Some kaolinite-salt mixtures were equilibrated at relative humidities ranging from very low to 100~. The spectra of the heated specimens were not affected by the initial moisture content.


TABLE 3. IR bands studied (cm-1) and their assignments, following Farmer & Russell (1964) and Percival

& Duncan (1974)

Kaolinite Metakaolinite

* 3697 "] * 3669 [ . * 3652 ~ OH stretching

t 3620J

* 936 "~ OH bending ~- 915 f

1035 t 1012 794 Si-O 752 693 .J 540 AIVI-O

1071 Si-O 807 Ailv-o

* Outer hydroxyls. I" Inner hydroxyls.

Si-O and Al-O vibrations. When kaolinite was converted to metakaolinite the strong absorptions at 1035 and 1012 cm -1 were replaced by a broad band centred at 1060- 1070 cm -1. In addition, the Si-O bands at 794, 752 and the A1-O absorptions at 539 cm-1 disappeared and a new band was observed at about 800 cm-1 (Fig. l d).

When kaolinite was heated with K Br, changes occurred in the spectrum at temperatures at which the kaolinite spectrum was still preserved in the absence of salt (Figs. la and b). The main Si-O absorption was broadened considerably and the maximum at 1005 c m - was relatively strong. On further heating with KBr, broad maxima appeared in the vicinity of 1010 and 720 cm-1, i.e. at considerably lower frequencies than in metakaolinite (Fig. l c). Tarte (1967) showed that isolated AIO4 tetrahedra absorb at lower frequencies than condensed A104 tetrahedra. Day & Rindone (1962)demonstrated that the presence of non-bridging oxygen atoms reduced the wavenumber of the Si-O absorption of soda-aluminosilicate glasses. Incorporation of K into the metakaolinite structure could thus account for the observed reductions in wavenumbers of both the Si-O and A1-O vibrations.

The products obtained on heating kaolinite with other salts gave rise to various spectra (Fig. 1 and Table 1). Some resemble the spectrum of unmodified metakaolinite, others show absorption bands at even lower frequencies than those observed on heating with KBr (Figs. I f and g). It appears from Table l that for any particular alkali ion there is a qualitative correlation between the amount of alkali ion sorbed and the position of the absorption maxima after any specific heat treatment. Thus, after heating kaolinite with various K salts at 590~ for 30 min, the maxima of the Si-O and A1-O absorptions range from 1050 to 990 c m - 1 and from 790 to 695 c m - i respectively and the corresponding K content from 0.3 to 2.1 K per formula unit. KF treated samples gave rise to relatively sharp absorptions (Fig. I g) and the spectrum resembles that of nepheline or kaliophilite. Heating metakaolinite with KBr at 590~ for 30 min led to a very small reduction in frequency of the Si-O absorption at 1060 c m - 1.

Reactions of salts with kaolinite 227

Microns

I0 12 14 16 i I ~ I I

i I J ) i ] 200 I000 800

Wavenumber (cm - j )

FIG. 1. 1200-650 cm -1 region of spectra of samples heated for 30 min. (a) Kaolinite, 490~ (b) kaolinite+LiCl, 460~ (c) kaolinite+KBr, 490~ (d) kaolinite, 590~ (e)

kaolinite+ KBr, 590~ (f) kaolinite+ K2CO3, 590~ (g) kaolinite+ KF, 590~

After heating for 24 h at 590~ or heating for shorter periods at higher temperatures, further changes occurred. The absorptions in the 1200-600 cm- ~ range of the spectra of kaolinite heated with K2CO3 and KHCO 3 became relatively sharp and like those of the KF treated sample, resembled the spectrum of kaliophilite. Similarly the spectrum of NaHCO 3 treated kaolinite resembled that of carnegieite or of the 'L' phase described by Kubo et al. (1966) and of LiCI treated kaolinite that of fl-eucryptite (Kolesova, 1963). In contrast, more prolonged treatment with KBr caused small shifts of the absorption bands to lower wavenumbers, but the absorptions remained broad and did not resemble those of kaliophilite. This is in agreement with X-ray data which showed that the KBr treated kaolinite remained amorphous even after prolonged heating, while the KzCO 3 and


Microns I0 12 14 16

i I I

i [ i [ i 1200 1000 800

Waven~mber (cm -I)

FIG. 2. 1200-650 cm ~ region of spectra of: (a) 50~o kao l in i te+50~ mixture M ; (b) 50~ metakaol in i te + 5 0 ~ mix ture M ; (c) 25~o me takao l i n i t e+ 75~o mix ture M. Mixture M = kaolinite+K2COa, heated at 590~ for 24 h, washed (contains 2.3 K per formula unit).

NaHCO3 treated samples gave rise to weak patterns compatible with kaliophilite and carnegieite respectively. The X-ray pattern of fl-eucryptite was, however, not obtained from the samples studied.

The question arises whether the products formed in the early stages of the reaction were mixtures of metakaolinite and a modified phase or whether the composition of the entire material was affected. Figs. 2(a-c) show the spectra of artificial mixtures of kaolinite or metakaolinite with material designated 'Mixture M', which was obtained by heating kaolinite with K2CO 3 and contained 2.3 K atoms per formula unit. It is evident that both the kaolinite and metakaolinite bands predominate and would have been detectable in less reacted samples. It must therefore be concluded that when kaolinite reacts with salts, the composition of the entire alumino-silicate phase is modified and metakaolinite is not formed, at least not as a persistent intermediate. The absorption bands are broad, indicating that the material may not be homogeneous.

Hydroxyl vibrations. On heating kaolinite both with and without the addition of a salt, all the hydroxyl absorptions were progressively weakened. The ratio of the absorbance of the two strong stretching vibrations at 3696 and 3620 cm -~ remained approximately unchanged, until both disappeared entirely. DeKeyser et al. (1963) found that on heating kaolinite the four OH stretching vibrations preserved their relative intensities. This may also be inferred from the spectra presented by Percival & Duncan (1974), but is in striking contrast to the results reported by Pampuch (1966), who studied the dehydroxylation of


two samples of kaolinite, one from Zettlitz, the other from Colombia. He showed that on heating up to 600~ corresponding to 80~ dehydroxylation, the band at 3620 era-1 persisted, while the other hydroxyl vibrations disappeared. No explanation can be offered for the discrepancy between these results. It can only be stressed that in the present study the relative absorbance of the two strong hydroxyl stretching bands was preserved with all the samples studied, including one from Zettlitz. It can therefore be inferred that in these experiments all the hydroxyl groups of any particular sample were lost at the same rate.

The hydroxyl bending absorptions of kaolinite were progressively weakened on heating, in accordance with the changes observed in the OH stretching region. On heating with KBr and other salts, the Si-O absorptions were shifted to lower frequenc|es, as described. The broad absorptions overlap the OH bending region and partially obscure these bands (Fig. lb). In the temperature range in which dehydroxylation was only partial and X-ray diffraction patterns showed some unchanged kaolinite, the spectrum is, in fact, a super- position of that of unchanged kaolinite and of the modified metakaolinite phase. Since the OH bending vibrations are partially obscured, they cannot be used to indicate the extent of the dehydroxylation reaction.

Fig. l(c) shows the spectrum of kaolinite reacted with LiC1 at 460~ The spectrum resembles that of kaolinite, but there is a change in slope of the ascending branch of the Si-O absorption, at about 980 era- 1. Although the relative intensities of the absorptions at 936 and 915 cm- 1 may, on mere inspection of the spectrum, appear to have undergone some changes, measurements show that they are, in fact, unaltered. The special feature of this spectrum remains to be elucidated. It is uncertain whether a similar transition stage also occurs with other kaolinite-salt mixtures. It was detected with some kaolinite- K2CO3 mixtures at lower temperatures, but has not yet been found with other mixtures.

Thermal analysis

The DTA, TG and DTG curves of kaolinite and of a mixture of kaolinite with KBr are shown in Fig. 3. It is evident that kaolinite did not react with KBr below the dehydroxylation temperature of kaolinite. The weight loss in this region was greater than with pure kaolinite, indicating that an additional reaction occurred. Weight loss of the mixture continued up to and beyond the melting point of KBr (730~ Pure KBr showed no weight loss below its melting point.

Scanning electron mierographs

Previous investigators have shown that the morphology of kaolinite was not appreciably affected by heating at temperatures below 900~ (Bates, 1971). This was confirmed by scanning electron micrographs of the present samples. On heating kaolinite for 30 min at 590~ with KBr or K2CO 3 the hexagonal outlines were preserved but the edges became rugged and the surfaces appeared to be corroded (Fig. 4a). In contrast, a sample of kaolinite similarly heated with KF was strongly corroded (Fig. 4b).

A sample of kaolinite which was first converted into metakaolinite and subsequently heated with KBr for 30 min at 590~ preserved the sharp hexagonal outlines of the original material (Fig. 4c). Heating the various samples for 24 h at 590~ did not seem to cause any further changes in the morphology of the particles.


~ o

_o IO

20

:::::::::::::::::::::::::::::::::::::: 'o' IO

30

30

[ I I I I I I I

(b)

I

\ I I I I I I I I

0 I00 200 500 400 500 600 700 800 900

~

FIG. 3. DTA ( - - ) , TO (- ) and D T G ( . . . . ) curves of (a) kaolinite (50 mg) and (b) kaolinite (50 m g ) + KBr (150 mg) (weight loss calculated with respect to pure kaolinite).

Effect of experimental conditions

It is evident from Table 1 that an increase in temperature enhanced the uptake of alkali ions under the experimental conditions studied. More prolonged heating at any particular temperature achieved the same effect.

As expected, grinding the specimens also promoted the reaction. Thus, when kaolinite was ground with excess KBr for 60 min at room temperature, 0.04 K atoms were retained and after heating this mixture for 30 min at 490~ 0.60 K atoms per formula unit were taken up, compared with 0.30 for the similarly treated stock sample.

Most of the experiments were carried out with the salt in large excess. Reduction of the salt:kaolinite ratio decreased the rate of the reaction. It appears from Table 1 that the


FIG. 4. Scanning electron micrographs of: (a) kaolinite+KBr, (b) kaolinite+KF, (c) metakaolinite+KBr. The mixtures were heated at 590~ for 30 rain and washed. Original

magnification : 20,000.

magni tude of this effect differs with different sal ts: it is relatively small for the more highly reacted L iCl - or K B r - k a o l i n i t e mixtures and is greater for NaC1- or KC1- kaol ini te mixtures. The reac t ion of K2CO3-kao l in i t e mixtures are par t icular ly sensitive to the compos i t ion of the react ing mixtures.


D I S C U S S I O N

Kaolinite heated with various salts incorporates some cations into the alumino-silicate framework. The reactions are not affected by the initial moisture content of the reagents, but the rate is considerably reduced by dehydroxylating the clay before use. The reactions commence with the dehydroxylation of the clay, i.e. at temperatures considerably below the melting point of some of the salts examined, the rate depending both upon the cationic and anionic component of the salt. The same cation combined with different anions reacts with kaolinite at very different rates, e.g. the various potassium salts listed in Table 1. Similarly, no systematic differences could be detected for a series of cations combined with the same anion, e.g. the chlorides of the alkali metals. At the reaction temperatures investigated there is no correlation between the melting points of the salts and the amounts of alkali ions taken up. Some of the salts pass through phase changes in the course of heating, which might be expected to render them more reactive, but no effect on the reaction rate could be detected. A striking correlation was, however, observed between the solubility of the salts and the extent of the reactions studied. The data quoted in Table 1 refer to solubilities in hot water and are therefore not strictly applicable to the present systems. Keevil (1942) showed that the solubilities of alkali halides in closed systems at elevated temperatures increase with temperature. The solubilities of alkali sulphates and carbonates, on the other hand, decrease with increasing temperature.

The solubilities shown in Table 1 may be regarded as a basis for comparison of the relative solubilities of the alkali halides and the sparingly soluble sulphates at elevated temperatures. It is particularly significant that KF, which is very soluble, reacted readily with kaolinite, whereas infra-red spectra showed that practically no reaction occurred with NaF, which is very sparingly soluble. Chemical analyses of the kaolini te-NaF mixtures were not carried out, because the samples could not be freed of excess salt.

The reactions of carbonates and bicarbonates with kaolinite pose a problem. Not only does sodium and particularly potassium carbonate react more readily than would have been expected from their reduced solubility at higher temperatures, but, in addition, although bicarbonates are converted into carbonates at temperatures far below those at which the reactions were carried out and are therefore expected to react similarly, this is contrary to the results obtained. Preliminary experiments showed that potassium carbonate reacts with kaolinite at relatively low temperatures, possibly by proton transfer, which activates the kaolinite and facilitates further reaction. The amount of cation sorbed is very sensitive to the salt: kaolinite ratio. For the purpose of the present discussion the reactions of carbonates must be regarded as special cases. These reactions are under further investigation. The bicarbonates, which do not undergo low temperature reactions with the clay, reacted with the alumino-silicate at elevated temperature according to the solubility of the corresponding carbonate.

Comparison of the reactions of different alkali chlorides demonstrates that solubility is not the only factor governing the reactions: the solubility of LiCI is similar to that of RbC] and much lower than that of CsCI, yet the amount of ki taken up by kaolinite under corresponding conditions is much larger. It appears that smaller cations are more easily incorporated into the structure than larger ones.

The reactions of kaolinite with salts may then be interpreted as follows: water liberated


on dehydroxylation of the clay acts as a solvent and the salts react according to the equation:

2SIO2 �9 A1203 �9 2H20 +2nMX--*2SiO2 . AI203 . n M 2 0 + 2 n H X + ( 2 - n ) H 2 0 (1)

The amount of any particular cation taken up at this stage of the reaction is determined by the solubility of the salt. The reaction proceeds more readily the smaller the cation, suggesting that it occurs by a diffusion mechanism. Verduch & Corral (1972), who studied the reaction of Li2CO 3 with kaolinite, concluded that it is diffusion controlled. They postulated a counter diffusion mechanism of protons derived from the clay against the Li ions. Although such a mechanism alone cannot explain the data presented in Table t, it seems probable that the second stage of the reaction is, indeed, diffusion controlled. The primary step, however, is partial dehydroxylation of kaolinite which liberates water and renders the aluminosilicate reactive. This was confirmed by thermal analysis of kaolinite-KBr mixtures, which showed that no reaction other than loss of adsorbed water occurs below the temperature at which dehydroxylation of kaolinite commences, and that the weight loss at this stage is greater than that corresponding to dehydroxylation alone.

All the mixtures examined except that with KF gave rise to X-ray amorphous products after short periods of heating. Kaolini te-KF mixtures showed the X-ray pattern of kaoliophilite even after short periods of heating at 590~ KF is very soluble and it seems that the reaction with kaolinite is completed in the course of dehydroxylation of the clay. Scanning electron micrographs showed that, of all the samples examined, this was the only one which was disintegrated. It seems probable that the HF liberated according to equation (1) attacks the aluminosilicate and accelerates the reaction. The kaliophilite formed appears to be stable and does not react with KF.

All the mixtures continue to react at elevated temperatures. Moreover, metakaolin containing little or no structural water also reacts with salts on heating. The mechanism of these reactions is different from that under consideration and will be discussed in Part II of this paper.

Little information is available on the properties of water expelled from minerals at elevated temperatures, though it is generally accepted that such water plays an important part in metamorphic processes. It seems probable that water liberated on dehydroxylation is adsorbed on the clay platelets, forming films which dissolve neighbouring salt particles. The critical temperature of water is thereby reduced. The reaction then proceeds by diffusion of cations into the alumino-silicate framework, preserving the morphology of the clay, unless the acids formed destroy the crystallites.

Kubo & Yamabe (1969), who studied the reactions of LiECO3, Na2CO 3 and K2CO 3 with kaolinite by selected area electron diffraction, postulated that metakaolinite is formed as an intermediate phase. This was contradicted by Verduch & Corral (1972), who showed that the reactions of kaolinite and metakaolinite with LiECO 3 differed. Our data indicate that kaolinite reacts directly with soluble salts, incorporating various amounts of alkali cations and generally forming an X-ray amorphous phase, without the intermediate formation of metakaolinite. The composition of the intermediate phase is variable, depending on the salt and the experimental conditions. If the reaction is very rapid, as with KF, a crystalline phase is formed even under a relatively mild thermal regime. On heating with very sparingly soluble salts, unmodified metakaolinite is produced. In general, mixtures of kaolinite with potassium or sodium salts led to crystal-

234 L. H e l l e r - K a l l a i

line products when the value of n in the reaction (see equation (i)) approached 1. This does not apply to kaolinite-LiCl mixtures, which did not give rise to the X-ray pattern of eucryptite on heating at 590~ although n reached a value close to 1 after heating for only 30 min, and the infra-red pattern of the products resembled that of eucryptite.

C O N C L U S I O N S

On heating kaolinite with salts of alkali metals a modified metakaolinite phase is formed, incorporating alkali ions. The reaction commences with the dehydroxylation of kaolinite. It appears that the water liberated acts catalytically, accelerating the reactions between the salt and the clay. The reaction is favoured by higher solubility of the salt and smaller radius of the cation.

Catalysis by water liberated on dehydroxylation of minerals is a concept frequently invoked in the interpretation of metamorphic processes in nature, yet few, if any, systems seem to have been experimentally studied. The present experiments were carried out in an open system, which renders quantitative thermodynamic treatment difficult. Systems of this kind are encountered in the manufacture of ceramics, where the effect of impurities is of considerable importance. It is well known that some impurities are more detrimental than others, but the data in the literature seems to be largely empirical and partly contra- dictory. Kromer & Schtiller (1974) pointed out that chemical composition alone is not a sufficient criterion for the phase composition of ceramics after firing, due to the fact that equilibrium is rarely reached. The present results show that with the salts of alkali metals, at least, the composition of the first transition phase formed on firing is determined by the solubility of the salt and the radius of the cation. The preliminary X-ray study suggests that the composition of this intermediate phase may have a decisive effect on the mineralogical composition of the final product.

ACKNOWLEDGMENTS

I wish to thank Dr M. Frenkel of the Casali Institute of the Hebrew University for the thermal analyses and Miss Yael Blottner for technical assistance. Dr A. Matthews kindly read the manuscript. Financial support from the Israel Commission for Basic Research, National Academy of Sciences, is gratefully acknowledged.

REFERENCES

BATES T.F. (1971 ) The kaolin minerals. The Electron-Optical Investigation o f Clays (J. A. Card, editor), pp. 107-158. Mineralogical Society, London.

DAY D.E. & RINDONE G.E. (1962) J. Am. Ceram. Soc. 45, 489. FARMER V.C. & RUSSELL J.D. (1964) Spectrochim. Acta 20, 1149. HELLER-KALLAI L. (1975) Proc. Int. Clay Conf., Mexico, 361. Clays Clay Min. 23, 462. KEEVm N.B. (1942) Am. Chem. Soc. J. 64, 841. DE KEYSER W.L., WOLLAST R. & DE LAET L. (1963) Proc. Intern. Clay Conf. Stockholm, 75. KOL~SOVA V.A. (1963) Izv. Akad. Nauk. SSSR, ord. Khim. Nauk. 187. KROMER H. & SCHi3LLER K.H. (1974) N. Jb. Min. Abh. 122, 145. KtJBO Y. & YAMABE Y. (1969) Proe. Intern. Clay Conf. Tokyo 1, 915. KVBO Y., YAMACUCHI G. & KASAHARA K. (1966) Amer. Mitt. 51,516. PAMPUCH R. (1966) Polska Akademia Nauk, Komis[a Nauk Mineralogicznya 53. PERCIVAL H.J. & DUNCAN J.F. (1974) J. Amer. Ceram. Soc. 57, 57. TAR~E P. (1967) Spectroehim. Aeta 23 A, 2127. VERDUCH A.G. & CORRAL J.S.M. (t972) Proc. Intern. Clay Conf. Madrid t31.

Reactions of salts with kaolinite

RI~SUMI~: Lors du chauffage ~ des temp6ratures inf6rieures h 600~ la kaolinite r6agit avec les sels des m6taux alcalins selon la relation :

2SIO2 �9 A1203 �9 2H20 + 2nMX~2SiO2 �9 A1203 �9 nM20 + 2nHX + (2 --n)H20

La r6action commence par la d6shydroxylation de l'argile, et est favoris6e par la solubilit6 61ev6e du sel et par la dimension r6duite de l'ion alcalin.

On constate que lors de la d6shydroxylation l'argile devient r6active et que dans le mSme temps l'eau lib6r6e dissout les particules de sel voisines et catalyse la r6action.

Les spectres infrarouges indiquent que les ions alcalins sont directement incorpor6s dans la structure de l'aluminosilicate, sans formation de m6takaolinite comme interm6diaire d6tectable. Le produit d'origine est d6sordonn6, mais lorsque n approche de 1 il y a tendance

la formation de phases cristallines.

K U R Z R E F E R A T : Kaolinit reagiert beim Erhitzen auf Temperaturen unter 600 ~ C mit Alkalimetallsalzen nach der Gleichung

2SIO2 �9 A1203 �9 2H2Oq 2nMX-2SiO2 �9 A1203 �9 nM20~ 2nHX~ ( 2 - n ) H 2 0

Die Reaktion beginnt mit der Dehydroxylierung des Tons und wird dutch hohe L6slichkeit des Salzes und durch geringe Gr6sse des Alkali-ions begiinstigt. Es scheint, dass der Ton bei der Dehydroxylierung besonders reaktionsf~ihig wird und gleichzeitig das freigesetzte Wasser benachbarter Salzkristalle 16st und somit die Reaktion beschleunigt. Infrarot- Spektren zeigen an, dass Atkali-ionen direkt in die Alumosilikatsstruktur eingebaut werden, ohne dass eine Metakaolinitbildung als Zwischenphase beobachtet wird. Das anfangliche Produkt ist ungeordnet, doch bilden sich mit Ann~therung yon n gegen I kristalline Phasen.

R E S U M E N : Calent~indola a temperaturas inferiores a 600~ la caolinita reacciona con sales de metales alcalinos segfm la ecuaci6n

2SIO2 " AI202 " 2H20+2nMX~2SiOz �9 AIzO3 " nM20+2nHX+(2--n)H20

La reacci6n comienza con la deshidroxilaci6n de la arcilla y se ve favorecida por la gran solubilidad de la sal y el tamafio pequefio del ion alcalino.

Parece que al producirse la deshidroxilaci6n, la arcilla se vuelve reactiva y, al mismo tiempo, el agua liberada disuelve particulas adyacentes de la sal y cataliza la reacci6n.

Los espectros infrarrojos indican que los Jones alcalinos se incorporan directamente a la estructura del aluminosilicato, sin la formaci6n de metacaolinita como fase intermedia detectable. E1 producto inicial es desordenado, pero al aproximarse n a 1, tienden a formarse fases cristalinas.

235

Documents

REACTIONS OF SALTS WITH KAOLINITE AT ELEVATED TEMPERATURES. I · PDF fileClay Minerals (1978) 13, 221. REACTIONS OF SALTS WITH KAOLINITE AT ELEVATED TEMPERATURES. I L. HELLER-KALLAI