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Teknisk Tidskrift
that a liquid is formed and becomes the medium
through which reaction occurs."
In recent years much research work has been
carried out ön the reactivity of oxides used in the
applied chemistry of silicates or in other materials
composed of oxides3. The reaction conditions in
powder mixtures containing kaolin and CaO have
also been studied. Tammann and Pape4 heated
mixtures of CaO with kaolin or A1203 in order to compare
puré A1203, and such A1203 which is formed when
kaolin is heated above its decomposition temperature,
with respect to their capacity to form calcium
alu-minate. In agreement with the results of earlier
experiments (Hedvall5), showing that the reactivity
of solids varies with previous thermal treatment and
crystallographic or topochemical development, the
reactivity of the A1203, formed by decomposition of
kaolin, was found to be considerably greater. Rieke
and Völker6 have studied the chemical action of
CaO or MgO ön different clays from a ceramic
view-point. It was observed that in this reaction with
CaO there is tendency to form a Ca-Al-silicate of
anortliite type, if the temperature is high and the
heating time long enough. It is also interesting to
note that the reactions with MgO occur in another
way than with CaO.
The following investigations may also be referred
to here. Taylor and Pole7 found that when heating
mixtures of A1203, Si02 or mullite (3 A1203 • 2 Si02)
with CaC03 or CaO the mullite reacts möre intensely
than Si02 or A1203 and that A1203 is attacked by
CaO easier than Si02. A number of other
investiga-tors have studied the reactions in mixtures of CaO
and Si02 or A1203 in varying proportions and the
thermal stability of the reaction products8.
There are especially two series of investigations,
carried out by Weyer and by Jander and Petri,
which must be considered möre seriously, because of
their close relation to the reactions described låter
in this paper. Before we begin to discuss their
results further it seems appropriate to make a short
survey of the changes winch take place when heating
kaolin to high temperatures.
These reactions have been —- and are still to some
extent — subject for a discussion betwéen a great
number of investigators. This is caiised by the fact
that crystallographically imperfectly developed phases
of the component oxides (A120, and Si02) or their
compounds are produced in the interval between the
breaking down of the hydrated kaolin lattice and the
3 cf. ,T. A. Hedvall: Reaktionsfähigkeit fester Stoffe
(Leipzig 1938), 188—220 (survey).
4 G. Tamhan a. W. Pape: Z. anorg. u. allg. Chem. 127
(1923), 43.
5 J. A. Hedvall: Z. anorg. u. allg. Chem. 96 (1916), 64,
71; 08 (1916), 57; 120 (1922), 327; 121 (1922), 217.
6 R. Rike a. B. Völker, Ber. Deutseh. Keram. Ges. 11
(1930), 608.
i N. W. Taylor a. G. R. Pole: J. Amer. Ceram. Soc. 18
(1935), 325.
s W. Jander a. B. Hoffman: Z. anorg. u. allg. Chem. »18
(1934), 211; B. T. Carlsson: Röck Products SI, (1931), 52;
Bur. Standards J. Res. 7 (T931), 893 ; H. Ehrenberg : Z.
phy-sik. Chem. B 11, (1931), 421 ; J. Konarzewski : Roczniki Chem.
11 (1931), 607; S. Nagai : Z. anorg. u. allg. Chem. 206 (1932),
177; 297 (1932), 321; K. Hild a. G. Trömel: Z. anorg. u.
allg. Chem. 215 (1933), 333; B. Garre: Z. anorg. u. allg.
Chem. 164 (1927), 205; R. Jagitsch : Sitz. Ber. Akad. Wiss.
Wien, mathem. phys. H5 (1936), 226.
formation of stable mullite (3 A1203 • 2 Si02). Naturally
the chemical or physical properties of compounds
in such transition states vary, möre or less, with the
thermal conditions and the purity and
crystallo-graphic character of the kaolin. In rather broad lines
these reactions may be summèd up as follows.
The lattice of kaolin (A1203 • 2 SiO, • 2 H,0) breaks
down with loss of the water at about 550° at which
temperature meta-kaolin is formed representing a
quasi-homogenous phase of molecularly mixed oxide
components, which either in a peculiar way is still
keeping the structure of the mother substance, or
perhaps is a kind of a "faint chemical compund".9
It can be safely assumed that the heating of this
phase at higher temperatures produces topochemical
aging. In the interval 800—900°, finally, it cannot
longer exist as the highly disperse mixture of
crystallographically imperfect oxides. The
meta-kaolin is brolien up and exothermic processes
con-sisting- in the slow formation of y-Al203 and
cristo-balite set in10. There are, however, many experiences
which show that the y-Al203 represents a very active
form of A1203 both with respect to catalytic activity
and to chemical reactivity in general of kaolin
preparations which are kept between about 800—
900011. Thus it seems probable that the reaction
which takes place above about 900° between A1203
and Si02 already begins with 7-Al:203 and continues
during its transition into the stable a-Al203
(corund-um)12. In this connection attention shoidd be drawn
to the fact that the reactivity of such active transition
phases may differ from that of the stable
modifica-tions not only qiiantitatively but even quälitatively.
It follows from the abnormally high movability of the
particles at exteriör or interiör $urfaces of such
phases that compounds can be formed at temperatures
which differ möre or less from those corresponding
to the normal formation of the same products in
mixtures of stable modifications. Such phenomena are
still very little studied.
In earlier investigations concerning the
recombina-tion of A1203 and Si02 it was assumed that sillimanite
(A1203 • SiO,) was formed when heating kaolin at or
above about 1 000°. We know now, however, that
only mullite (3 A1203 • 2 Si02) is the stable silicate
above about 1000°13. The great similarity between
the structures of sillimanite and mullite makes it
very difficult to decide, if, during the recombination
process at lower temperatures in presence of faulty
lattices, we might have an intermediate formation of
9 O. Krause a. H. Wöhner: Ber. Deutseh. Keram. Ges. IS
(1932), 485; W. Eitel: Z. angew. Chem. 1,9 (1936), 896, 900;
N. L. Bowen a. J. W. Greig: J. Amer. Ceram. Soc. 7 (1924),
238; W. Büssem a. W. Dawihl: Ber. Deutseh. Keram. Ges.
15 (1934), 459; L. W. Mellor a. A. Scott: Träns. Ceram.
Soc. 23 (1924) 322; L. Tscheischwili, W. Büssem a. W.
W!eyl: Ber. Deutseh. Keram. Ges. 20 (1939), 249; S. T.
Hendricks : Z. Krist. A. 95 (1936), 247 ; G. Hüttig a. E.
Herr-mann: Kolloid. Z. 92 (1940), 9.
i° W. C. Hansen a. L. T. Brownmiller: Amer. J. Sci. 15
(1928), 225, 239.
11 R. Jagitsch : Sitz. Ber. Akad. Wiss. Wien, mathem.
phy-sik. 11,5 (1936), 226; R. Fricke : Ber. Deutseh. Chem. Ges.
72 (1939), 1568; K.-E. Zimens : Svensk Kem. Tidskr. 52
(1940), 295.
12 J. F. Hyslop a. H. P. Rooksby : Träns. Ceram. Soc. 27
(1927—28), 93, 299.
is R. W. G. Wyckoff, N. L. Bowen a. J. W. Greig : Amer.
J. Sci. 11 (1926)„ 459.
2
11 jan. 1941
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