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Infrared study of grinding kaolinite with alkali metal chlorides
131
Technology, 12 (1975) 131-138 0 ElsevierSequoia S-A., Lausanne - Printed in The Netherlands
Powder
Infrared
Study
of Grinding
Kaolinite
with Alkali
Metal
Chlorides
SHMUEL YARZV Department
of Geology,
The Hebrew
University, Jerusalem
(Israel)
(ReceivedSeptember3,1974; in revisedform February19,197s)
SUMMARY Kaolinite was mechanically ground in the presence of NaCl, KCI, RbCl and CsCl and in their absence, and i.r. spectra were recorded after various periods of grinding. Changes in the intensity and sharpness of the absorption bands were interpreted using accepted assignments. The following effects were observed: (1) Destruction of the kaolinite structure and formation of amorphous material. (2) Solid state diffusion of atoms, particularly protons, leading to the formation of lattice defects. (3) Delamination of the tactoid of kaolinite. (4) Enhanced hydration. The relative weight and significance of these reactions depend on the alkali halide present. Thus, after grinding, different spectra are obtained with different alkali halides. In the presence of CsCl hydrogen bonds were formed between basal hydroxyls and sorbed water molecules.
therefore expected that infrared studies may provide information on the mechanisms of the reactions which occur during grinding of kaolinite in the presence of various diluents. In the present study the effects of grinding kaolinite together with alkali chlorides on the infrared spectra were investigated. The effects of grinding kaolinite in the absence of any alkali halide were also investigated.
EXPERIMENTAL
Materials The kaolinite used in this study was from Georgia (Oneal Pit, Macon) supplied by Ward’s Natural Science Establishment, Inc. It contained less than 5% montmorillonite (estimated by X-rays). The clay powder was obtained by carefully abrading the rock. The halides were analytical grade reagents. In most of the experiments they were used after being ground for 5 minutes and dried for 1 hour at 110°C. Parallel experiments were carried out with unground crystalline chlorides.
INTRODUCTION Grinding of kaolinite is an important process in industry. The effects of grinding on pure kaolinite have been studied by many investigators (see e.g. Miller and Oulton [l] for a list of references). Surprisingly little is known about the effects of grinding kaolinite admixed with diluents [2]. Recently, during a study of the infrared spectrum of kaolinite in KBr disks, Miller and O&on [ 33 and Yariv [4] showed that if grinding is carried out during the preparation of the KBr disk, significant changes occur in the intensities of the absorption bands. It is
Methods
(1) Disks of unground kaolinite were prepared from 0.4 mg of the clay and 135 mg of the suitable ground chloride. To avoid grinding effects in the preparation of the disks, the mixture was gently mixed manually before pressing the disk. (2) A mixture of 5 mg kaolinite and 500 mg of alkali chloride (either powder or crystalline) was ground in a Fisher automatic porcelain mortar, for various time periods. For the preparation of the disks, 15 mg of the mixture were gently mixed manually with 120 mg of the suitable ground chloride.
132
Preliminary experiments with increasing clay concentration (up to 200 mg clay) showed essentially the same results, but the reaction rates were much slower. With increasing the chloride content (up to 1000 mg) the same results were also obtained but, again, the reaction rates were much slower. (3) 500 mg of kaolinite were ground in the same mortar for various time periods. The disks for infrared spectra were prepared from 0.4 mg of the clay and 140 mg KBr. (4) Infrared spectra were recorded on a Perkin-Elmer grating spectrophotometer, model 237.
RESULTS
Some of the spectra of disks of unground kaolinite and kaolinite ground in the presence of potassium chloride, together with extrapolated base-lines, are given in Fig. 1. The following positions for the different bands of the unground samples are (in cm-‘): A, 3700; B, 3674 (shoulder); C, 3657 (weak); D, 3627; E, 1115 (very weak); F, 1031; G, 1007; H, 937 (weak); I, 912. The symbols used are those of Miller and Oulton [ 11. No significant differences were detected for the different salts. As will be discussed later, it is difficult to estimate the position of band P for unground samples. This band appears as a broad shoulder if the mixture of kaolinite and alkali halide is carefully mixed. Values as low as 1080 cm-’ were sometimes obtained. All bands become so broad after prolonged grinding that with the exception of band F and the bands representing water they can hardly be identified. This indicates that the kaolinite structure is destroyed and the material becomes amorphous_ Water content can be estimated from both the Hz0 absorption bands, the stretching vibration at about 3400 cm-’ and the bending vibration at ‘about 1630 cm-‘. With unground sample the water content of the disks decreased in the order NaCl > KC1 > RbCl > CsCl. Water content increased with increasing the time of grinding. The disks of the unground mixtures lost most of their water by being heated overnight at 110°C. The longer the mixture was ground, the more firmly the disk held its water. Even after three weeks at 110°C almost no water
MICRONS 30
w ,“,/ f
10
3600
3200
Fig. 1. The ix. absorption spectra of kaolinite in KC1 disks and extrapolated base-lines. (a) Unground, (b) ground for 1 min, (c) ground for 5 min, (d) ground for 30 min, (e) ground for 60 min, (f) ground for 90 min in the presence of KCI.
was lost from disks made of mixtures which were ground for 90 minutes. The positions of the maxima of the Hz0 stretching bands depend on the salt. They are 3510 - 3440,3430,3400,3390 and 3375 cm-l for NaCl, KBr, KCl, RbCl and CsCl, respectively.
133
INTERPRETATION
OF THE
SPECTRA
By choosing band F as a reference standard, one can show the individual effects of the grinding on the intensity of any band i. The choice of band F for this purpose is justified due to the persistence of its peak even after pralonged grinding when X-ray and DTA show that the kaolinite is completely transformed into an amorphous material (Fig. 1). Thus, if a suitable base-line is drawn for band F, it may be used as a reference for the amount of total silica in the system. The percentage changes in absorbance (P( %) values) due to the grinding were calculated using the following equation, which was previously derived by Miller and Oulton [l] _
Aio -Abio -A
P(S)
= Aio
-_A
FO
bFo
-Abio
A FO -AbFo where Ai is the absorbance value of the peak of band i in the given spectrum of the sample; Abi is the absorbance value of the extrapolated base-line at the waveIength of the peak of band i in the spectrum. AF and A,, are the absorbance values of the peak of the reference band F and of the extrapolated base-line at the same frequency. Subscript g represents the ground sample (g is the time of grinding) and the subscript o represents the unground sample (zero time of grinding). Extrapolated base-lines are drawn for each peak as shown in Fig. 1. Thus the sharpness of the bands may be measured and interferences of water bands or bands of amorphous material are practically avoided. Percentage changes in absorbance as a function of the time of grinding are plotted in Figs. 2 - 6. A scattering of points in many of these curves is to be expected in view of the difficulty of obtaining reproducible absorption measurements with fine solid particles. Despite this difficulty, trends in the P(%) values of the bands are significant. Band P
Among the Si-0 stretching bands, band P is the most sensitive to grinding. This band reveals two contradictory tendencies. In the first stage, on brief grinding it becomes sharp and shifts to higher frequencies. In the second stage, on prolonged grinding it becomes broad
and shifts to lower frequencies, until it disappears. Changes occurring in the spectrum during the first stage can be interpreted from the work of Farmer and Russell [ 51 on the connection between the sharpness and definition of this band and the physical properties of the clay particle. For delaminated kaoiinite, an infrared absorption occurs when the radiation is perpendicular to the crystal plates. Due to a field which opposes the dipole vibration, the force constant increases causing a rise in the vibration frequency vl. With laminated kaolinite (tactoid), absorbance and frequency of absorption depend on the angle between the direction of the propagation of the beam and the normal to the lamella of the kaolinite. Thus a broad shoulder having a range of values is obtained which corresponds to absorption by randomly oriented particles. The frequencies of these absorptions are always lower than that of the single layer kaolinite. The sharpness and position of the band can therefore give information on the delamination of kaolinite during grinding. Changes occurring in the spectrum during the second stage are considered to be due to the destruction of the clay crystal. As will be shown in the paragraph which deals with bands A and D, “prototropy” occurs during grinding; protons migrate from their original position within the kaolinite. Some of the migrating protons may approach the fields of those oxygens which are common to the tetrahedral and octahedral layers, perturbing their vibrations. This means that the changes occurring in the spectrum during the second stage may essentially be due to_prototropy and not necessarily to the destruction of the crystal. The relative weight and significance of each of the two stages depends on whether the kaolinite is ground in the presence or absence of alkali halide. In the absence of alkali halide the first stage is only minor. On grinding for more than 7 minutes band P becomes increasingly broad and disappears before the disappearance of bands A, D, G and 1. This means that this band disappears due to prototropy before the crystal structure of kaolinite is destroyed. When grinding is performed in the presence of alkali chloride, the first stage is much more pronounced. The baud shifts to higher frequencies on grinding and becomes sharp, in-
-IQ0
I
I
I IO
20
30
40
50
4
I
I
/
60
70
80
00
I
100
Min
Fig. 4. Percentage Fig. 2).
dicating
change in absorbance
delamination
of the
P (%) for band D as a function
tactoid.
With NaCl,
KC1 and RbCl it reaches 1105 cm-’
after 5 min grinding. After 8 min grinding with CsCl this band reaches the frequency of 1114 cm-‘. This indicates that a higher proportion of single sheets is obtained with CsCl. The effect of grinding on the absorbance percentage of bt-nd P is given in Fig. 2. Increase in absorbance of band P shows that kaolinite continues to be delaminated at grinding times up to lo,15 and 8 min with NaCl, KC1 and CsCl, respectively.
of grinding time (for details, see legend to
On prolonged grinding band P becomes broad and its maximum is shifted to lower frequencies. It also diminishes, but at a slower rate than bands A, D, G and I. This proves that in the presence of the salts mentioned the destruction of the kaolinite structure as well as prototropy causes the diminution of the band P.
A, C, D, Hand I Bands A and D are due to the stretching vibrations of the basal and inner hydroxyls
Bands
-100
Min Fig_
5. percentage
Fig. 2).
change
in absorbance
P (70)
for band I as a function of grindingtime(for details,see legendto
133 INTERPRETATION
OF THE
SPECTRA
By choosing band F as a reference standard, one can show the individual effects of the grinding on the intensity of any band i. The choice of band F for this purpose is justified due to the persistence of its peak even after prolonged grinding when X-ray and DTA show that the kaolinite is completely transformed into an amorphous material (Fig. 1). Thus, if a suitable base-line is drawn for band F, it may be used as a reference for the amount of total silica in the system. The percentage changes in absorbance (P( ‘%) values) due to the grinding were calculated using the following equation, which was previously derived by Miller and Oulton [l] _ 100 P(B)
=
Aig -Abig
Aio -Abio
AFg-&Fg -&o-Aio - Abio A FO - AbFo
AbFo
where Ai is the absorbance value of the peak of band i in the given spectrum of the sample; Abi is the absorbance value of the extrapolated base-line at the wavelength of the peak of band i in the spectrum. AF and AbF arethe absorbance values of the peak of the reference band F and of the extrapolated base-line at the same frequency. Subscript g represents the ground sample (g is the time of grinding) and the subscript o represents the unground sample (zero time of grinding). Extrapolated base-lines are drawn for each peak as shown in Fig. 1. Thu, the sharpness of the bands may be measured and interferences of water bands or bands of amorphous material are practically avoided. Percentage changes in absorbance as a function of the time of grinding are plotted in Figs. 2 - 6. A scattering of points in many of these curves is to be expected in view of the difficulty of obtaining reproducible absorption measurements with fine solid particles. Despite this difficulty, trends in t.he P(B) values of the bands are significant. Band P Among the Si-0 stretching bands, band P is the most sensitive to grinding. This band reveals two contradictory tendencies. In the first stage, on brief grinding it becomes sharp and shifts to higher frequencies. In the second
stage, on prolonged grinding it becomes broad
and shifts to lower frequencies, until it disap-
pears. Changes occurring in the spectrum during the first stage can be interpreted from the work of Farmer and Russell [ 5 J on the connection between the sharpness and definition of this band and the physical properties of the clay particle. For delaminated lraolinite, an infrared absorption occurs when the radiation is perpendicular to the crystal plates. Due to a field which opposes the dipole vibration, the force constant increases causing a rise in the vibration frequency z+. With laminated kaolinite (tactoid), absorbance and frequency of absorption depend on t.he angle between the direction of the propagation of the beam and the normal to the lamella of the kaolinite. Thus a broad shoulder having a range of values is obtained which corresponds to absorption by randomly oriented particles. The frequencies of these absorptions are always lower than that of the single layer kaolinite. The sharpness and position of the band can therefore give information on the delamination of kaolinite during grinding. Changes occurring in the spectrum during the second stage are considered to be due to the destruction of the clzy crystal. As will be shown in the paragraph which deals with bands A and D, “prototropy” occurs during grinding; protons migrate from their original position within the kaolinite. Some of the migrating protons may approach the fields of those oxygens which are common to the tetrahedral and octahedral layers, perturbing their vibrations. This means that the changes occurring in the spectrum during the second stage may essentially be due to_prototropy and not necessarily to the destruction of the crystal. The relative weight and significance of each of the two stages depends on whether the kaolinite is ground in the presence or absence of alkali halide. In the absence of alkali halide the first stage is only
minor.
On grinding for
more than 7 minutes band P becomes increasingly broad and disappears before the disappearance of bands A, D, G and I. This means that this band disappears due to prototropy before the crystal structure of kaolinite is destroyed. When grinding is performed in the presence of alkali chloride, the first stage is much more pronounced. The band shifts to higher frequencies on grinding and becomes sharp, in-
---A-_
\
;\.
\
-.-_..-..-.___.
---
1
-.--..-o-..,__.
-..__
u -_. 1
20
3b
..__
I
40
50
‘Y\. 60
I 70
I
00
I
90
100
Min Fig. 2. Percentage change in absorbance P(S) for band P as a function of grinding time. 0 Grinding in the absence of alkali halide, * grinding in the presence of NaCI, * grinding in the presence of KCl, 8 grinding in the presence of CsCI, X grinding in the presence of crystalline NaCl.
-lOOl
I
IO
I
20
I
30
I
1
40
Min
I
I
1
I
60
70
00
90
I 100
5o
Fig. 3. Percentage change in absorbance P (9%) for band A as a function Fig. 2).
of grinding time
(for details, see legend to
13’7
alkali halides examined. This tail persists if the kaolinite is ground without the halide. It diminishes on grinding kaolinite with alkali chlorides. Its diminution depends on the time of grinding and the salt used. It is rapid with KCI, RbCl and CsCl, slower and to a less extent with NaCl. It has been previously suggested [4] that the increase of band A and the diminution of its tail are the result of movements of hydroxyls to their ideal positions_ In the absence of any salt or in the presence of NaCl, KC1 and RbCl, diminution of band A is mainly due to prototropy. On the other hand, in the presence of CsCI, not only prototropy, but also sorption of the salt together with water molecules, leading to hydrogen bonding between the basal hydroxyls and the sorbed molecules, causes a diminution in the absorbance of band A. At the same time the band at 3606 cm-’ increases_
Grinding
with uystalline
sclts
Drastic differences between the spectra of kaolinite ground with powder and crystalline chlorides were obtained only for NaCl. When ground with crystalline NaCl the kaolinite destruction is much more rapid than when ground with powdered NaCl. From the percentage changes in absorbance (Figs. 2 - 6) it can be seen that prototropy in inner hydroxyls as well as in basal hydroxyls is so rapid that delamination can hardly be followed. When kaolinite is ground with crystalline KC1 and CsCl, the absorption spectra obtained are similar to those obtained by using powdered salts, except that the destruction of the kaolinite is more rapid. In the case of crystalline CsCl the delamination and the water interaction with the basal hydroxyls are also more rapid.
DISCUSSION
Bands F and G The paired bands F and G are S-0 in plane vibrations, the oxygens being basal. On prolonged grinding with NaCl, KC1 or RbCl, as well as without any salt, these bands are slightly shifted to higher frequencies, 1035 and 1010 cm-’ respectively. On grinding with CsCl they are shifted to lower frequencies, 1026 and 1005 cm-’ respectively. The sharpness of band G may give information as to whether the hexagonal sheet is more or less uniform. The method used here for base-line extrapolation enables one to measure the sharpness of this band. The effect of grinding on the percentage of absorbance of band G is given in Fig. 6. The small increment observed for NaCl may be due to errors in the estimation of A,, , 1o08. The results obtained with KC1 are significant. In the tactoid, disturbing effects existing in the layers may cause some basal oxygens to move from their ideal positions. Increase in sharpness of band G for up to 5 minutes grinding may be the result of the movement of more and more oxygens to their ideal positions due to the release of the tension in the layer, as a result of delamination. On further grinding, both the cutting of the edges of the lattice and the thermal diffusion of silicons and oxygens from their ideal position, creating lattice defects, are sufficiently rapid to give a broadening of this band and a decrease in its absorption.
The effects of mechanical grinding on kaolinite as were interpreted from the infrared spectra may be summarized as follows: (1) Destruction of kaolinite structure and formation of amorphous material is very slow if the mineral is diluted with ground alkali chlorides. The rate of the process is only slightly affected by the kind of alkali chloride present. Rate of formation of amorphous material seems to decrease in the order NaCl > KC1 > CsCl. (2) Diffusion of atoms and the formation of lattice defects. Although this effect occurs with all kinds of atoms, it is best illustrated with the migration of the small proton (prototropy). This process begins at the start of grinding in the absence or presence of alkali halides. A problem arises as to where these protons move. Bands which may be ascribed to protonated water molecules [9] were not detected in the spectra. Decrease in absorbance as well as broadening of band P is partly due to the approach of protons to the inner oxygens. According to Miller and Oulton [I], some of the protons interact with other hydroxyls to give water molecules which may be evaporated at low temperatues. The broad band in the 3370 - 3600 cm-’ region may also be in part due to hydroxyls of wideiy varying nat,ure. (3) Separation of the lamellae (delamination). This process begins as soon as the grinding starts with the salts studied here. It occ’urs
13s
only to a small extent in the absence of salts. Delamination leads to two important observati.ons, the hydration of the layer, which will be discussed later, and a release of tension existing in the layer when it is part of a tactoid, which may perhaps be from flexure. In the tactoid this tension may result in that some of the atoms will move from their ideal position. By delamination and release of tension the sheets become more uniform. With KC1 and KBr [4] this was observed for the basal hydroxyls as well as for the basal oxygens. With CsCl, due to the effects following the hydration of the layer, the release of tension is observed at the beginning of grinding only. With NaCl, only the hydroxyls seem to move to their ideal positions. The hexagonal sheet does not seem to become more uniform. It might perhaps be due to penetration of small sodium ions into the hexagonal holes of the layer. In the absence of any of the salts this phenomenon is not observed either for the hydroxyls or for the hexagonal sheets. The water used by the kaolinite during grinding is sorbed from the atmosphere. The reaction with water may be divided into three stages. In the first stage, when, as a result of the mechanical disintegration of the layer, more atoms become exposed, protons are attracted by exposed oxygens and hydroxyls by exposed aluminiums and silicons. In the second stage excess protons are sorbed by the kaolinite. This may be enhanced by the lattice defects caused during the grinding process. In this stage the surface of the kaolinite gains a positive charge. In the third stage the positively charged kaolinite sorbs free water molecules. At the same time the water molecules are polarized by the alkali cation. Models for the dual interaction are given in Fig. ‘7. The higher the Hi’ Hi‘ Hi’
H
H/‘.__O~~H___&_H Hi-
M
0
HIi HI’ b
M = No.
K. Rb.Cs
Fig. 7. Models of the interactionof water on kaolinite surface.
alkalicationsand
polarity of the cation, the lower will be the sorption energy of the water by the kaolinite. Thus electron donor properties of water decrease in the environment of the alkali halides in the following order: CsCl, RbCl, KCl, NaCl. This is shown by the perturbation of the stretching frequencies of water. The basal hydroxyls are very poor proton donors. Hydrogen bonding with water may occur in the presence of CsCl only and not in the presence of RbCl, KC1 or NaCl (Fig. 7a).
ACKNOWLEDGEMENTS
The author wishes to thank Professor L. Heller-Kallai and Dr. H. Cross of Jerusalem for the critical reading of the manuscript and for the helpful discussions and remarks. He wishes also to thank Eli and Yehuda Yariv for their technical assistance.
REFERENCES J-G. Miller and J.D. Oulton, Prototropy in kaolinite during percussive grinding, Clays Clay Miner., 18 (1970) 313 - 323. S.A. Mishirky, S. Yariv and W.I. Siniansky, Some
effects of grindingkaolinitewith calcinedkaolinite,
Clay Sci., 4 (1974) 213 - 224. J-G. Miller and J-D. Oultcn, Some effects of grinding kaolinite with potassium bromide, Clays Clay Miner., 20 (1972) 389 - 390. S. Yariv, Some effects of grinding kaolinite with potassium bromide, Clays Clay Miner., 23 (1975) in press. V.C. Farmer and J.D. Russell, Effects of particle size and structure on the vibrational frequencies of layer silicates, Spectrocbim. Acta, 22 (1966) 389 - 398. R-L. Ledoux and J.L. White, Infrared studies of the hydroxyl groups in intercalated kaolinite complexes, Clays Clay Miner., Proc. 13th Nat. Conf., Madison, Wisconsin, 1964 (W-F. Bradley and SW. Bailey, eds.), (1965) 289 - 315. R.L. Ledoux and J.L. White, Infraredstudiesof hydrogen bonding of organic compounds on oxygen and hydroxyl surfaces layer lattice silicates, hoc. Int. Clay Conf., Jerusalem, 1966 (A. Weiss and L. Heller, eds.), 1 (1966) 361 - 374. M. Cruz, A_ Laycock and J.L. White, Perturbation of OH groups in kaolinite dono2--acceptor complexes. I. Formamid~methylformamide and dimethylformamide-kaolinite complexes, Proc. Int. Clay Conf., Tokyo, 1969 (L. Heller, ea.), 1 (1969) 775 - 789. R.D. Gillard and G. Wilkinson, Adducts of protonic acids with coordination compounds, J. Chem. Sot., (1964) 1640 - 1646.
Technology, 12 (1975) 131-138 0 ElsevierSequoia S-A., Lausanne - Printed in The Netherlands
Powder
Infrared
Study
of Grinding
Kaolinite
with Alkali
Metal
Chlorides
SHMUEL YARZV Department
of Geology,
The Hebrew
University, Jerusalem
(Israel)
(ReceivedSeptember3,1974; in revisedform February19,197s)
SUMMARY Kaolinite was mechanically ground in the presence of NaCl, KCI, RbCl and CsCl and in their absence, and i.r. spectra were recorded after various periods of grinding. Changes in the intensity and sharpness of the absorption bands were interpreted using accepted assignments. The following effects were observed: (1) Destruction of the kaolinite structure and formation of amorphous material. (2) Solid state diffusion of atoms, particularly protons, leading to the formation of lattice defects. (3) Delamination of the tactoid of kaolinite. (4) Enhanced hydration. The relative weight and significance of these reactions depend on the alkali halide present. Thus, after grinding, different spectra are obtained with different alkali halides. In the presence of CsCl hydrogen bonds were formed between basal hydroxyls and sorbed water molecules.
therefore expected that infrared studies may provide information on the mechanisms of the reactions which occur during grinding of kaolinite in the presence of various diluents. In the present study the effects of grinding kaolinite together with alkali chlorides on the infrared spectra were investigated. The effects of grinding kaolinite in the absence of any alkali halide were also investigated.
EXPERIMENTAL
Materials The kaolinite used in this study was from Georgia (Oneal Pit, Macon) supplied by Ward’s Natural Science Establishment, Inc. It contained less than 5% montmorillonite (estimated by X-rays). The clay powder was obtained by carefully abrading the rock. The halides were analytical grade reagents. In most of the experiments they were used after being ground for 5 minutes and dried for 1 hour at 110°C. Parallel experiments were carried out with unground crystalline chlorides.
INTRODUCTION Grinding of kaolinite is an important process in industry. The effects of grinding on pure kaolinite have been studied by many investigators (see e.g. Miller and Oulton [l] for a list of references). Surprisingly little is known about the effects of grinding kaolinite admixed with diluents [2]. Recently, during a study of the infrared spectrum of kaolinite in KBr disks, Miller and O&on [ 33 and Yariv [4] showed that if grinding is carried out during the preparation of the KBr disk, significant changes occur in the intensities of the absorption bands. It is
Methods
(1) Disks of unground kaolinite were prepared from 0.4 mg of the clay and 135 mg of the suitable ground chloride. To avoid grinding effects in the preparation of the disks, the mixture was gently mixed manually before pressing the disk. (2) A mixture of 5 mg kaolinite and 500 mg of alkali chloride (either powder or crystalline) was ground in a Fisher automatic porcelain mortar, for various time periods. For the preparation of the disks, 15 mg of the mixture were gently mixed manually with 120 mg of the suitable ground chloride.
132
Preliminary experiments with increasing clay concentration (up to 200 mg clay) showed essentially the same results, but the reaction rates were much slower. With increasing the chloride content (up to 1000 mg) the same results were also obtained but, again, the reaction rates were much slower. (3) 500 mg of kaolinite were ground in the same mortar for various time periods. The disks for infrared spectra were prepared from 0.4 mg of the clay and 140 mg KBr. (4) Infrared spectra were recorded on a Perkin-Elmer grating spectrophotometer, model 237.
RESULTS
Some of the spectra of disks of unground kaolinite and kaolinite ground in the presence of potassium chloride, together with extrapolated base-lines, are given in Fig. 1. The following positions for the different bands of the unground samples are (in cm-‘): A, 3700; B, 3674 (shoulder); C, 3657 (weak); D, 3627; E, 1115 (very weak); F, 1031; G, 1007; H, 937 (weak); I, 912. The symbols used are those of Miller and Oulton [ 11. No significant differences were detected for the different salts. As will be discussed later, it is difficult to estimate the position of band P for unground samples. This band appears as a broad shoulder if the mixture of kaolinite and alkali halide is carefully mixed. Values as low as 1080 cm-’ were sometimes obtained. All bands become so broad after prolonged grinding that with the exception of band F and the bands representing water they can hardly be identified. This indicates that the kaolinite structure is destroyed and the material becomes amorphous_ Water content can be estimated from both the Hz0 absorption bands, the stretching vibration at about 3400 cm-’ and the bending vibration at ‘about 1630 cm-‘. With unground sample the water content of the disks decreased in the order NaCl > KC1 > RbCl > CsCl. Water content increased with increasing the time of grinding. The disks of the unground mixtures lost most of their water by being heated overnight at 110°C. The longer the mixture was ground, the more firmly the disk held its water. Even after three weeks at 110°C almost no water
MICRONS 30
w ,“,/ f
10
3600
3200
Fig. 1. The ix. absorption spectra of kaolinite in KC1 disks and extrapolated base-lines. (a) Unground, (b) ground for 1 min, (c) ground for 5 min, (d) ground for 30 min, (e) ground for 60 min, (f) ground for 90 min in the presence of KCI.
was lost from disks made of mixtures which were ground for 90 minutes. The positions of the maxima of the Hz0 stretching bands depend on the salt. They are 3510 - 3440,3430,3400,3390 and 3375 cm-l for NaCl, KBr, KCl, RbCl and CsCl, respectively.
133
INTERPRETATION
OF THE
SPECTRA
By choosing band F as a reference standard, one can show the individual effects of the grinding on the intensity of any band i. The choice of band F for this purpose is justified due to the persistence of its peak even after pralonged grinding when X-ray and DTA show that the kaolinite is completely transformed into an amorphous material (Fig. 1). Thus, if a suitable base-line is drawn for band F, it may be used as a reference for the amount of total silica in the system. The percentage changes in absorbance (P( %) values) due to the grinding were calculated using the following equation, which was previously derived by Miller and Oulton [l] _
Aio -Abio -A
P(S)
= Aio
-_A
FO
bFo
-Abio
A FO -AbFo where Ai is the absorbance value of the peak of band i in the given spectrum of the sample; Abi is the absorbance value of the extrapolated base-line at the waveIength of the peak of band i in the spectrum. AF and A,, are the absorbance values of the peak of the reference band F and of the extrapolated base-line at the same frequency. Subscript g represents the ground sample (g is the time of grinding) and the subscript o represents the unground sample (zero time of grinding). Extrapolated base-lines are drawn for each peak as shown in Fig. 1. Thus the sharpness of the bands may be measured and interferences of water bands or bands of amorphous material are practically avoided. Percentage changes in absorbance as a function of the time of grinding are plotted in Figs. 2 - 6. A scattering of points in many of these curves is to be expected in view of the difficulty of obtaining reproducible absorption measurements with fine solid particles. Despite this difficulty, trends in the P(%) values of the bands are significant. Band P
Among the Si-0 stretching bands, band P is the most sensitive to grinding. This band reveals two contradictory tendencies. In the first stage, on brief grinding it becomes sharp and shifts to higher frequencies. In the second stage, on prolonged grinding it becomes broad
and shifts to lower frequencies, until it disappears. Changes occurring in the spectrum during the first stage can be interpreted from the work of Farmer and Russell [ 51 on the connection between the sharpness and definition of this band and the physical properties of the clay particle. For delaminated kaoiinite, an infrared absorption occurs when the radiation is perpendicular to the crystal plates. Due to a field which opposes the dipole vibration, the force constant increases causing a rise in the vibration frequency vl. With laminated kaolinite (tactoid), absorbance and frequency of absorption depend on the angle between the direction of the propagation of the beam and the normal to the lamella of the kaolinite. Thus a broad shoulder having a range of values is obtained which corresponds to absorption by randomly oriented particles. The frequencies of these absorptions are always lower than that of the single layer kaolinite. The sharpness and position of the band can therefore give information on the delamination of kaolinite during grinding. Changes occurring in the spectrum during the second stage are considered to be due to the destruction of the clay crystal. As will be shown in the paragraph which deals with bands A and D, “prototropy” occurs during grinding; protons migrate from their original position within the kaolinite. Some of the migrating protons may approach the fields of those oxygens which are common to the tetrahedral and octahedral layers, perturbing their vibrations. This means that the changes occurring in the spectrum during the second stage may essentially be due to_prototropy and not necessarily to the destruction of the crystal. The relative weight and significance of each of the two stages depends on whether the kaolinite is ground in the presence or absence of alkali halide. In the absence of alkali halide the first stage is only minor. On grinding for more than 7 minutes band P becomes increasingly broad and disappears before the disappearance of bands A, D, G and 1. This means that this band disappears due to prototropy before the crystal structure of kaolinite is destroyed. When grinding is performed in the presence of alkali chloride, the first stage is much more pronounced. The baud shifts to higher frequencies on grinding and becomes sharp, in-
-IQ0
I
I
I IO
20
30
40
50
4
I
I
/
60
70
80
00
I
100
Min
Fig. 4. Percentage Fig. 2).
dicating
change in absorbance
delamination
of the
P (%) for band D as a function
tactoid.
With NaCl,
KC1 and RbCl it reaches 1105 cm-’
after 5 min grinding. After 8 min grinding with CsCl this band reaches the frequency of 1114 cm-‘. This indicates that a higher proportion of single sheets is obtained with CsCl. The effect of grinding on the absorbance percentage of bt-nd P is given in Fig. 2. Increase in absorbance of band P shows that kaolinite continues to be delaminated at grinding times up to lo,15 and 8 min with NaCl, KC1 and CsCl, respectively.
of grinding time (for details, see legend to
On prolonged grinding band P becomes broad and its maximum is shifted to lower frequencies. It also diminishes, but at a slower rate than bands A, D, G and I. This proves that in the presence of the salts mentioned the destruction of the kaolinite structure as well as prototropy causes the diminution of the band P.
A, C, D, Hand I Bands A and D are due to the stretching vibrations of the basal and inner hydroxyls
Bands
-100
Min Fig_
5. percentage
Fig. 2).
change
in absorbance
P (70)
for band I as a function of grindingtime(for details,see legendto
133 INTERPRETATION
OF THE
SPECTRA
By choosing band F as a reference standard, one can show the individual effects of the grinding on the intensity of any band i. The choice of band F for this purpose is justified due to the persistence of its peak even after prolonged grinding when X-ray and DTA show that the kaolinite is completely transformed into an amorphous material (Fig. 1). Thus, if a suitable base-line is drawn for band F, it may be used as a reference for the amount of total silica in the system. The percentage changes in absorbance (P( ‘%) values) due to the grinding were calculated using the following equation, which was previously derived by Miller and Oulton [l] _ 100 P(B)
=
Aig -Abig
Aio -Abio
AFg-&Fg -&o-Aio - Abio A FO - AbFo
AbFo
where Ai is the absorbance value of the peak of band i in the given spectrum of the sample; Abi is the absorbance value of the extrapolated base-line at the wavelength of the peak of band i in the spectrum. AF and AbF arethe absorbance values of the peak of the reference band F and of the extrapolated base-line at the same frequency. Subscript g represents the ground sample (g is the time of grinding) and the subscript o represents the unground sample (zero time of grinding). Extrapolated base-lines are drawn for each peak as shown in Fig. 1. Thu, the sharpness of the bands may be measured and interferences of water bands or bands of amorphous material are practically avoided. Percentage changes in absorbance as a function of the time of grinding are plotted in Figs. 2 - 6. A scattering of points in many of these curves is to be expected in view of the difficulty of obtaining reproducible absorption measurements with fine solid particles. Despite this difficulty, trends in t.he P(B) values of the bands are significant. Band P Among the Si-0 stretching bands, band P is the most sensitive to grinding. This band reveals two contradictory tendencies. In the first stage, on brief grinding it becomes sharp and shifts to higher frequencies. In the second
stage, on prolonged grinding it becomes broad
and shifts to lower frequencies, until it disap-
pears. Changes occurring in the spectrum during the first stage can be interpreted from the work of Farmer and Russell [ 5 J on the connection between the sharpness and definition of this band and the physical properties of the clay particle. For delaminated lraolinite, an infrared absorption occurs when the radiation is perpendicular to the crystal plates. Due to a field which opposes the dipole vibration, the force constant increases causing a rise in the vibration frequency z+. With laminated kaolinite (tactoid), absorbance and frequency of absorption depend on t.he angle between the direction of the propagation of the beam and the normal to the lamella of the kaolinite. Thus a broad shoulder having a range of values is obtained which corresponds to absorption by randomly oriented particles. The frequencies of these absorptions are always lower than that of the single layer kaolinite. The sharpness and position of the band can therefore give information on the delamination of kaolinite during grinding. Changes occurring in the spectrum during the second stage are considered to be due to the destruction of the clzy crystal. As will be shown in the paragraph which deals with bands A and D, “prototropy” occurs during grinding; protons migrate from their original position within the kaolinite. Some of the migrating protons may approach the fields of those oxygens which are common to the tetrahedral and octahedral layers, perturbing their vibrations. This means that the changes occurring in the spectrum during the second stage may essentially be due to_prototropy and not necessarily to the destruction of the crystal. The relative weight and significance of each of the two stages depends on whether the kaolinite is ground in the presence or absence of alkali halide. In the absence of alkali halide the first stage is only
minor.
On grinding for
more than 7 minutes band P becomes increasingly broad and disappears before the disappearance of bands A, D, G and I. This means that this band disappears due to prototropy before the crystal structure of kaolinite is destroyed. When grinding is performed in the presence of alkali chloride, the first stage is much more pronounced. The band shifts to higher frequencies on grinding and becomes sharp, in-
---A-_
\
;\.
\
-.-_..-..-.___.
---
1
-.--..-o-..,__.
-..__
u -_. 1
20
3b
..__
I
40
50
‘Y\. 60
I 70
I
00
I
90
100
Min Fig. 2. Percentage change in absorbance P(S) for band P as a function of grinding time. 0 Grinding in the absence of alkali halide, * grinding in the presence of NaCI, * grinding in the presence of KCl, 8 grinding in the presence of CsCI, X grinding in the presence of crystalline NaCl.
-lOOl
I
IO
I
20
I
30
I
1
40
Min
I
I
1
I
60
70
00
90
I 100
5o
Fig. 3. Percentage change in absorbance P (9%) for band A as a function Fig. 2).
of grinding time
(for details, see legend to
13’7
alkali halides examined. This tail persists if the kaolinite is ground without the halide. It diminishes on grinding kaolinite with alkali chlorides. Its diminution depends on the time of grinding and the salt used. It is rapid with KCI, RbCl and CsCl, slower and to a less extent with NaCl. It has been previously suggested [4] that the increase of band A and the diminution of its tail are the result of movements of hydroxyls to their ideal positions_ In the absence of any salt or in the presence of NaCl, KC1 and RbCl, diminution of band A is mainly due to prototropy. On the other hand, in the presence of CsCI, not only prototropy, but also sorption of the salt together with water molecules, leading to hydrogen bonding between the basal hydroxyls and the sorbed molecules, causes a diminution in the absorbance of band A. At the same time the band at 3606 cm-’ increases_
Grinding
with uystalline
sclts
Drastic differences between the spectra of kaolinite ground with powder and crystalline chlorides were obtained only for NaCl. When ground with crystalline NaCl the kaolinite destruction is much more rapid than when ground with powdered NaCl. From the percentage changes in absorbance (Figs. 2 - 6) it can be seen that prototropy in inner hydroxyls as well as in basal hydroxyls is so rapid that delamination can hardly be followed. When kaolinite is ground with crystalline KC1 and CsCl, the absorption spectra obtained are similar to those obtained by using powdered salts, except that the destruction of the kaolinite is more rapid. In the case of crystalline CsCl the delamination and the water interaction with the basal hydroxyls are also more rapid.
DISCUSSION
Bands F and G The paired bands F and G are S-0 in plane vibrations, the oxygens being basal. On prolonged grinding with NaCl, KC1 or RbCl, as well as without any salt, these bands are slightly shifted to higher frequencies, 1035 and 1010 cm-’ respectively. On grinding with CsCl they are shifted to lower frequencies, 1026 and 1005 cm-’ respectively. The sharpness of band G may give information as to whether the hexagonal sheet is more or less uniform. The method used here for base-line extrapolation enables one to measure the sharpness of this band. The effect of grinding on the percentage of absorbance of band G is given in Fig. 6. The small increment observed for NaCl may be due to errors in the estimation of A,, , 1o08. The results obtained with KC1 are significant. In the tactoid, disturbing effects existing in the layers may cause some basal oxygens to move from their ideal positions. Increase in sharpness of band G for up to 5 minutes grinding may be the result of the movement of more and more oxygens to their ideal positions due to the release of the tension in the layer, as a result of delamination. On further grinding, both the cutting of the edges of the lattice and the thermal diffusion of silicons and oxygens from their ideal position, creating lattice defects, are sufficiently rapid to give a broadening of this band and a decrease in its absorption.
The effects of mechanical grinding on kaolinite as were interpreted from the infrared spectra may be summarized as follows: (1) Destruction of kaolinite structure and formation of amorphous material is very slow if the mineral is diluted with ground alkali chlorides. The rate of the process is only slightly affected by the kind of alkali chloride present. Rate of formation of amorphous material seems to decrease in the order NaCl > KC1 > CsCl. (2) Diffusion of atoms and the formation of lattice defects. Although this effect occurs with all kinds of atoms, it is best illustrated with the migration of the small proton (prototropy). This process begins at the start of grinding in the absence or presence of alkali halides. A problem arises as to where these protons move. Bands which may be ascribed to protonated water molecules [9] were not detected in the spectra. Decrease in absorbance as well as broadening of band P is partly due to the approach of protons to the inner oxygens. According to Miller and Oulton [I], some of the protons interact with other hydroxyls to give water molecules which may be evaporated at low temperatues. The broad band in the 3370 - 3600 cm-’ region may also be in part due to hydroxyls of wideiy varying nat,ure. (3) Separation of the lamellae (delamination). This process begins as soon as the grinding starts with the salts studied here. It occ’urs
13s
only to a small extent in the absence of salts. Delamination leads to two important observati.ons, the hydration of the layer, which will be discussed later, and a release of tension existing in the layer when it is part of a tactoid, which may perhaps be from flexure. In the tactoid this tension may result in that some of the atoms will move from their ideal position. By delamination and release of tension the sheets become more uniform. With KC1 and KBr [4] this was observed for the basal hydroxyls as well as for the basal oxygens. With CsCl, due to the effects following the hydration of the layer, the release of tension is observed at the beginning of grinding only. With NaCl, only the hydroxyls seem to move to their ideal positions. The hexagonal sheet does not seem to become more uniform. It might perhaps be due to penetration of small sodium ions into the hexagonal holes of the layer. In the absence of any of the salts this phenomenon is not observed either for the hydroxyls or for the hexagonal sheets. The water used by the kaolinite during grinding is sorbed from the atmosphere. The reaction with water may be divided into three stages. In the first stage, when, as a result of the mechanical disintegration of the layer, more atoms become exposed, protons are attracted by exposed oxygens and hydroxyls by exposed aluminiums and silicons. In the second stage excess protons are sorbed by the kaolinite. This may be enhanced by the lattice defects caused during the grinding process. In this stage the surface of the kaolinite gains a positive charge. In the third stage the positively charged kaolinite sorbs free water molecules. At the same time the water molecules are polarized by the alkali cation. Models for the dual interaction are given in Fig. ‘7. The higher the Hi’ Hi‘ Hi’
H
H/‘.__O~~H___&_H Hi-
M
0
HIi HI’ b
M = No.
K. Rb.Cs
Fig. 7. Models of the interactionof water on kaolinite surface.
alkalicationsand
polarity of the cation, the lower will be the sorption energy of the water by the kaolinite. Thus electron donor properties of water decrease in the environment of the alkali halides in the following order: CsCl, RbCl, KCl, NaCl. This is shown by the perturbation of the stretching frequencies of water. The basal hydroxyls are very poor proton donors. Hydrogen bonding with water may occur in the presence of CsCl only and not in the presence of RbCl, KC1 or NaCl (Fig. 7a).
ACKNOWLEDGEMENTS
The author wishes to thank Professor L. Heller-Kallai and Dr. H. Cross of Jerusalem for the critical reading of the manuscript and for the helpful discussions and remarks. He wishes also to thank Eli and Yehuda Yariv for their technical assistance.
REFERENCES J-G. Miller and J.D. Oulton, Prototropy in kaolinite during percussive grinding, Clays Clay Miner., 18 (1970) 313 - 323. S.A. Mishirky, S. Yariv and W.I. Siniansky, Some
effects of grindingkaolinitewith calcinedkaolinite,
Clay Sci., 4 (1974) 213 - 224. J-G. Miller and J-D. Oultcn, Some effects of grinding kaolinite with potassium bromide, Clays Clay Miner., 20 (1972) 389 - 390. S. Yariv, Some effects of grinding kaolinite with potassium bromide, Clays Clay Miner., 23 (1975) in press. V.C. Farmer and J.D. Russell, Effects of particle size and structure on the vibrational frequencies of layer silicates, Spectrocbim. Acta, 22 (1966) 389 - 398. R-L. Ledoux and J.L. White, Infrared studies of the hydroxyl groups in intercalated kaolinite complexes, Clays Clay Miner., Proc. 13th Nat. Conf., Madison, Wisconsin, 1964 (W-F. Bradley and SW. Bailey, eds.), (1965) 289 - 315. R.L. Ledoux and J.L. White, Infraredstudiesof hydrogen bonding of organic compounds on oxygen and hydroxyl surfaces layer lattice silicates, hoc. Int. Clay Conf., Jerusalem, 1966 (A. Weiss and L. Heller, eds.), 1 (1966) 361 - 374. M. Cruz, A_ Laycock and J.L. White, Perturbation of OH groups in kaolinite dono2--acceptor complexes. I. Formamid~methylformamide and dimethylformamide-kaolinite complexes, Proc. Int. Clay Conf., Tokyo, 1969 (L. Heller, ea.), 1 (1969) 775 - 789. R.D. Gillard and G. Wilkinson, Adducts of protonic acids with coordination compounds, J. Chem. Sot., (1964) 1640 - 1646.