Bauxite, the
principal Alumina containing
ore, is usually an earthy rock
mainly composed of hydrated
alumina materials, including
gibbsite, boehmite and diaspore.
It is medium soft to hard rock,
has a cellular, porous or fine
grained compact structure, with
conchoidal or uneven fracture
surface. The specific gravity
varies from 2.6 to 3.5. The
usual colour of bauxite is pink.
The colour changes from whitish
to raddish brown with increase
in iron content of the rock.
Bauxite
of chinese origin is rated as
the best bauxite deposit of the
globe. Because of nature and
lower degree of impurities, it
exhibits better thermal and
thermo-mechanical behaviour at
high temperature. The bauxite
rocks are usually contaminated
by mineral impurities such as
anatase, hematite, quartzite
etc. The Fe2O3
rarely falls below 2.5% and
usually found more than 5%. A
very low Fe2O3
level in chinese bauxite makes
it most suited for refractory
formulations.
Refractory
grade bauxites potentially have
a high refractoriness, as it can
be deduced from the system SiO2-Al2O3,
where these materials are
represented on the basis of
their two main constituents,
alumina and silica.
However, although in practice
their refractoriness is high,
but RUL is low (1450-1500)
degree C compared to other
refractory products of lower
alumina content. The behavior is
attributed to the formation of
liquid phase at the elevated
temperature due to presence of
impurities in bauxite, mainly
TiO2, CaO, Fe2O3
etc.
The
mineralogical composition of
bauxite is extremely complex.
They may contain all the
different modifications of
alumina hydrate. During
calcination, gibbsite (Al2O3.3H2O)
undergoes dehydration reaction
at a temperature range of 290 -
340 degree C and transforms to
boehmite (Al2O3.H2O).
Boehmite turns to gama
alumina between 490-550 degree
C. Gama alumina gradually
changes to alfa alumina, which
is stable form, above 900 - 1200
degree C.
It
is important to note here that
all the impurities in bauxite
enter the glassy phase, for
instance Fe2O3
may form solid solutions with
the main crystalline phases -
corundum and mullite, enters
into glassy phase. In case of
CaO and MgO, a large part of
them form glass during
sintering. If alkaline oxides
are present, then practically
the whole amount of it enters
into the glassy phase. The
amount and nature of the glassy
phase affects considerably the
sintering characteristics of the
material.
With
raising the sintering
temperature the content of
liquid phase resulted from
impurities increases, so that
for each bauxite there is a
special sintering temperature.
The temperature depends on the
content of impurities, i.e.
lesser the impurity content
higher the sintering
temperature.
..................................................................................................................
