PORE STRUCTURE OF SPECIMENS PREPARED
FROM GRAPHITE-CONTAINING REFRACTORY MIXTURES
G. M. Butyrin
Translated from Novye Ogneupory, No. 5, pp. 22 – 26, May, 2004.
The effect of technological factors (composition, temperature, and holding time) on pore structure, gas perme
ability, and specific surface of specimens molded from graphite-containing refractory mixtures is studied.
Dense refractories based on crucible and muffle mixtures can be prepared at 1100 – 1400°C using a silica sol
binder. Heat treatment of the refractories at higher temperatures (about 1600°C) is not recommended because
of the sharp increase in open pore size, which may lead to degradation of performance characteristics.
The earliest mention of the crucible melting of metals is
traceable to Aristotelian times; however, the industrial use of
crucibles dates back just to the 16th century, when the pro-
duction of metals by crucible melting had increased to a rela-
tively high level and when the natural graphite in a mixture
with clay had proved to be a suitable material for making du-
rable crucibles .
In the past 10 – 15 years, graphite (carbon) as a heat-re-
sistant material has gained increasing acceptance in crucible
melting of nonferrous metals and steel, for fabrication of
foundry ladles, pouring nozzles, components for the lining of
oxygen-blowing steel-making furnaces, etc. [2 – 7]. This is
due to the intensification of existing and newly-developed
technologies for making steels, nonferrous metals and alloys,
special steels, development of new processing techniques
(secondary steel treatment in pouring and intermediate la
dles), considering that graphite (carbon)-containing refrac
tories exhibit high stability to thermal shocks [7 – 9]. Ac
cording to predictive estimations , the share of carbon-
containing refractories may reach 30% in the near future.
A serious drawback of these refractories is oxidation of
graphite (carbon) that sets in at 450 – 500°C and which, un
less removed or neutralized, may adversely affect the eco
nomics and safety of steel-making process and refractory re
liability . These considerations have prompted a search
for new, high-performance carbon-containing materials with
high thermal, corrosion, and erosion resistance that could be
used, for example, in the preparation of high-purity steels.
So, reaction-bonded composite SiAlON – C can be used
in place of carbon-containing refractories typically employed
in continuous-casting technology, which makes it possible,
for example, to extend the service life of submersible inlet
nozzles in the neat-to-size casting technology .
At low temperatures, the oxidation of graphite is con-
trolled by graphite reactivity, whereas at higher temperature
the main factor becomes diffusion of oxygen into the porous
structure of graphite (carbon)-containing refractories [13, 14].
Consequently, decreasing gas permeability and the size of
open pores decreases the bulk diffusion rate of oxygen and,
correspondingly, the oxidation rate of graphite (carbon), which
finally results in increased service life of the refractory.
A literature survey showed that the role of structural po
rosity in the oxidation and wear of graphite-containing
refractories has been given little attention. It was shown in
works [15 – 17] concerned with the production of yellow
phosphorus that the proper orientation of pressure-molded
refractory components in the lining with respect to the direc
tion of pressure applied may be a factor to control the rate of
oxidation. This is explained by the distinction in porous
structure of specimens cut out of pressure-molded preforms
parallel and perpendicular to the direction of pressure; this
behavior was also noted in fine-grained graphite (with parti
cles finer than 90 mm across) . The present paper is con
cerned with the study of the porous structure of graphite-con
taining refractories using, as an example, crucible and muffle
mixtures shaped by semidry pressing.
To study the effect of composition and temperature on
the pore structure of graphite-containing refractories, mix
tures with a moisture content of 8 – 15% were pressed under
a molding pressure of 25 MPa into test specimens of diame
ter 36 mm and height 10 – 14 mm (Table 1). One part of
specimens were heated at 1350°C for 4 h, another part — at
900, 1100, 1400, and 1600°C; the rest were left intact. Each
Refractories and Industrial Ceramics Vol. 45, No. 4, 2004
1083-4877/04/4504-0250 © 2004 Springer Science+Business Media, Inc.
NIIgrafit Federal State Unitary Enterprise, Moscow, Russia.