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Teknisk Tidskrift
Fig. 5.
rite, i. e. the impurities present in kaolin VA, do not
fundamentally change the reaction conditions.29 The
amount of reacted CaO is however considerably larger
at all reaction temperatures, which may be ascribed
both to impurities and to individual properties of this
kaolin.
The great reactivity of this substance is also shown
by the maximum, which is not only much higher than
with pholerite but also displaced towards somewhat
lower temperatures. A faint trace of it evidently
appears even ön the curve corresponding to a
pre-paration preheated at 1 000° for 5 hours. lf kaolin
VA is preheated at 1 200°, however, the oxides have
lost their capacity to react with CaO in that special
way, which implies an intermediately occurring high
CaO-consumption.
29 The röle played by the impurities shall be discussed
låter in this paper.
From the results of experiments conducted at
temperatures below 850° using preparations preheated at
600° and 800°, it is possible to draw the conclusion
that preheating at a higher temperature or for a
longer period will decrease the reactivity of the
meta-kaolin. Preheating at 600° for 5 hours (VA600/5h)
seems to be equivalent to preheating at 800° for 1
hour (VA800/1 h.), but the reactivity of VA600/1 is in
general considerably greater than that of VAgoo^
At reaction temperatures above 850° the relative
position of the yield curves is reversed. This can be
easily explained, if we assume that small quantities
of volatile impurities still present in preparations
preheated below 800° are rapidly removed at higher
temperatures, thus to some extent expanding the
powder and consequently reducing the contact surface
between the grains. The higher the temperatures or
the longer the time of preheating the smaller is the
amount of such impurities and their influence ön the
18
12 april 1941
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