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Kemi
sillimänite, especially when heating impure substances
and taking into accoimt the röle played by the
topo-chemical factors just mentioned14. When heating for
longer times or at temperatures above about 1 000°
the formation of mullite is unequivocally shown15.
Thus if kaolin is kept above the temperatiire of
recombination the product consists of mullite and
cristobalite, the latter substance being formed instead
of tridymite if no flux is present.
It has also been shown that if sillimänite is heated
at high temperatures it is transformed into the stable
mullite16. The process proceeds slowly below about
1 200—1 300°, if the compound is perfectly puré. In
the presence of fused substances or even some solid
impurities, e.g. CaO, the reaction is considerably
facilitated.
We now return to the investigations carried out by
Weyer and by Jander and Petri mentioned above.
Weyer17 heated mixtures of CaC03 and kaolin at
temperatures between about 600° and 1 400°, the
proportion CaO : (A1„03 • 2 Si02) generally being 16 :1.
Instead of puré kaolin the mineral pholerite from
Neurode was used. The analysis of this mineral
corresponds very closely to the kaolin formula:
A1203 • 2 Si02 • 2 H20. The heating times were as a
rule very long, only at the highest temperatures (at
or above 1 400°) such short times as 2—3 hours were
used. The heating mixtures generally consisted of
CaC03 and of pholerite which had not previously been
dehydrated; the laminae of pholerite having a
diameter of about 0,03 mm. The following principal
results were obtained. Meta-kaolin (A1203. 2 Si02),
which is formed at temperatures above about 500—
550°, reacts möre intensely with CaC03 or CaO than
the free oxides A1203 and Si02. This is to be
ex-pected because of the changes in the kaolin phase
described above. The formation of the
mono-alu-minate CaO • Al2Os is setting in with noticeable speed
already in the temperature range between 550° and
600°. lf the proportions of the mixture make it
possible 5 CaO • 3 A1203 and 3 CaO • A1203 are also
produced. The formation of the latter slowly starts
at about 1000°, the yield becoming large only at
temperatures above 1 250—1 300°.
In agreement with the results of other investigators
mentioned above, Weyer has found that the reactions
between CaO and Si02 occur with less intensity than
those between CaO ’and A1203. In his experiments the
di-calcium silicate, 2 CaO ■ Si02, was not formed to
any larger extent until between 700° and 750°.
3 CaO • Si02 was only formed at still higher
temperatures if a sufficient amount of CaO was present
and very slowly below about 1 300°. The formation
of the mono-silicate CaO • Si02 is normally slow and
is generally a secondary reaction between the
dical-cium silicate, which is easier produced, and free SiO,2
In some cases Weyer also worked with mixtures
of CaO and dehydrated kaolin. Considering his long
heating times it is natural that the results obtained
n cf. W. C. Hansen a. L. t. Brownmiller : loc. cit. a. J. F.
Hyslop a. H. P. Rooksby: loc. cit.
i» j. f. Hyslop a. H. P. Rooksby : loc. cit.; l. Navais :
J. Amer. Ceram. Soe. 8 (1925), 296; E. Posnjak a. J. W.
Greig: J. Amer. Ceram. Soc. 16 (1933), 569.
is N. 1.. Bowen, J. W. Greig a. B. G. Zies : J. Acad. Sci.
Wash. U (1924), 183.
it I. "Weyer: Dissert. Kiel 1930.
in such experiments did not differ from those of the
former ones.
Weyer seems to assume that no liquid phase
appears in his reaction mixtures below about 1 400°
(loc. cit. p. 12). In reaction systems of this
composition, where different silicates and aluminates are
formed, the lowest eutectical point, however, lies much
lower, at 1 165018. It is hard to compute the influence
of a liquid phase ön the reaction processes in his
experiments because such an influence depends ön
the quantity of this phase and consequently also ön
the reaction times, and it must be remembered that
very small quantities of a eutectic do not necessarily
increase the reaction yield19. It is often possible to
get a rough estimate of this influence by the ’degree
of cohesion of the reaction mixture, but it must be
emphasized, that only for perfectly loose powders
may it be assumed that liquid phases have been
with-out influence, because of the fact that reactions in
completely solid systems also have a tendency to make
the powder coherent, often very strongly20.
Another investigation has been carried out
re-cently by W. Jander and J. Petri21. They have
möre closely studied the formation of anorthite
(CaO • A1203 • 2 Si02) and other reaction products in
powder mixtures of CaO, A1203, Si02 (or synthetically
produced compounds of these oxides), and kaolin,
having in mind their importance for problems of
mineral chemistry and cement production. Because
of the melting conditions, just mentioned, the reaction
temperatures were always kept below 1165°. The
molecular proportions of the mixtures were made to
correspond to the composition of the desired reaction
product. Those results having a eonnection with the
problems treated in this paper were briefly as follows.
In mixtures of CaO, a-Al,03 and crystallized SiO,,
heated between 1 050° and 1100°, the reaction
products CaO • A1203 and 2 CaO ■ Si02 are prevalent
after heating for about 3 hours. The gehlenite
(2 CaO • A1203 • Si02) seems not to form in noticeable
quantities until after about 4 honrs and anorthite
(cf. above) after still longer times. When using
mixtures of CaO (or CaC03) and kaolin or CaO (or
CaCO,), Si02 and y-Al203 the result of the reaction
processes is somewhat changed, anorthite being easier
formed in both cases if the reaction conditions are
kept as mentioned. At these temperatures the
formation of anorthite seems to be considerably favoured
by the great reactivity of the unstable meta-kaolin
phase, and even the presence of y-Al203 facilitates
this reaction to some extent. This was shown by
comparative experiments with mixtures containing
either «-Al203 or y-ALOg, this being a good example
of the influence of topochemical factors even
quali-tatively. Anorthite seemed to be the end product in
all mixtures with suitable proportions between the
oxides, but the time necessary for its formation
increases considerably if kaolin is exchanged for
y-Al203, and still möre so if a-Al203 is used. Only
18 W. Jander a, J. Petri: Z. f. Elektrochem. H (1938), 748.
10 J. A. Hedvall a. I. Bergstrand : Z. anorg. u. allg. Chem.
205 (1932), 251.
20 J. A. Hedvall : Reaktionsfähigkeit fester Stoffe (Leipzig
1938), pp. 81—85.
21 W. Jander a. J. Petri: Z. angew. Chem. H (1938),
747—753.
11 jan. 1941
3
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