Note: Descriptions are shown in the official language in which they were submitted.
~64974
The invention relates to a process for the manufacture
of fired refractory bric~s from a mi~ture of sin-ter magnesia and
chrome ore and in particular is concerned with improving the
high-temperature stability of such bricks, while at the same time
obtain a good resistance to changes o~ temperature.
In order to obtain a high thermal stability in magne-
sia-chrome ore bricks, it is known to add the chrome ore to the
magnesia material prior to the sinter ~iring and subject the
mixture to a joint sinter firing at high temperature above 1700C,
preferably above 2100C. With such a firing, extensive dissolu-
tion of the chromite in the periclase matrix occurs and during
the subsequent cooling newly formed chromite spinels are separated
out.
With a magnesia to chrome ore ratio of 60 : 40, the
proportion o-f chromite which remains, i.e. of residual constitu-
ents of the chrome ore originally used, in a sintered material
produced in this way is 10% by volume at the most, but is gene-
rally less than 5% by volume. Bricks which are made from such
a pre-reacted sintered material by moulding and repeated firing
show a high degree of direct bonding between the periclase crys-
tals and the newly formed chromite spinels and are distinguished
b~ high stren~th at room temperature and in particular by high
stxength at high temperature and b~ good resistance to acid and
basic sla~s. A disadvantage of this process of manufacture is,
of course, the high cost which is incurred by the joint high-
temperature sinter firing.
The invention provides a manufacturing process wherein
the chrome ore is first added to the sinter magnesia and wherein
the bricks obtained show high-temperature stability and durabi-
lity values which correspond to those of bricks produced from
pre~reacted granular material without purer base materials (sin-
ter magnesia and chrome ore) having to be employed.
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~(~64974
There is known from U.S. Patent Specification 3,535,134
Jacques ~. Martinet et al issued October 20, 1970, a process for
the manufacture of direct-bonded magnesia-chromite bricks which
are said to have high high-temperature stability and good slag
resistance. In this process, magnesia (periclase) which contains
10 to 20% by weight of fine component below 0.147 mm, prefera~ly
at least one half of this fine component having a grain size
below 0.044 mm, is mixed with 15 to 25% by weight of finely divided
chromite, which is present entirely in the grain size below 0.147
mm with at least one half having a grain size below 0.044 mm, and
with l to 5% by weight of a cold binder, and moulded and the
moulded bodies obtained are fired at a temperature of at least
1650C and preferably at least 1700C. Such bric~s/ however, have
an inadequate resistance to changes of temperature and the propor-
tion o~ chromite is too small for achieving the best results for
slag resistance. If, however, in accordance with the known process,
the proportion of chromite is increased, an appreciable decline in
the high-temperature stability properties,occurs. Moreover, an
MgO content of at least 95% by weight is required for the magnesia
componenk.
According to the process known from U.S, Patent Specifi-
cation 3,321,32~ ~arr~ ~ Mikami, issued May ~3, 1967, a mixture
of chromike and magnesia grains is formed into bricks and these
are fired at a temperature which is so high that the chromite grains
dissolve in the periclase in the form of a solid solution and newly
formed chromite spinel separates out on cooling. Since the disso-
lution of the chromite begins only at temperatures above about
1870C, brick firing temperatures of 1930 to 2090C are required
for this process. The make-up of the grains of the magnesia~chro-
mite starting mixture is so adjusted that an optimum packing den-
sity is achieved, no distinction being made between the make-up of
the grains of magnesia and chromite and the mixture containing, for
.
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~10~4~7~
example, about 38% by weight of fine component below 0.15 mm. In
the case of the magnesia component, an MgO content of at least 96%
by weight i~ preferred.
Moreover, there is known from Austrian Patent Specifica-
tion 202,506 a process for the manufacture of shaped magnesia-chro-
mite bodies which are manufactured from an initial mixture consist-
ing of 70 to 80% by weight of sinter magnesia and at least 20% by
weight of chrome ore, the chrome ore being present entirely in a
grain size below 0.2 mm and containing more than 40% by weight,
advantageously more than 70% by weight, of very fine component
below OoOG mm. The sinter magnesia used consists of 10 to 50% by
weight of coarse grains with a size of 1.7 to ~.5 mm; the remain-
der of the sinter magnesia is present in a grain size of O to 1.5
mm. The shaped bodies may be fired at temperature not indicated in
detail. The resistance to changes of temperature is intended to be
obtained by means of the coarse grains of magnesia, but this is
achieved only to an inadequake degree. The known bricks are also
unsatisfactory as regards their high~temperature stability pro-
perties.
Having regard to the foregoing, the present invention
seeks to improve the high-temperature stability and resistance to
changes of temperature of magnesia-chromite bricks without the
nec~ssity o~ extremel~ high firing temperatures as required in
known processes.
According to the invention there is provided a process
for the manu~acture of fired refractory bric~s from a mixture of
sinter magnesia and chrome ore, in which said magnesia and said
ore contain fine grains with a size below 0.15 mm, which comprises
mouIding the mixture into bricks and firing the bricks at a tem-
perature of at least 1700C, wherein said mixture contains 2 to
10% by weight, based on the weight of the mixture, of sinter
magnesia having a grain size of up to 0.1 mm, at least 70% by
.
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~6~
weight of this sinter magnesia being present in a grain size below
0.06 mm, the chrome ore is present in the mixture in an amount of
26 to 56% by wei.ght/ based on the weight of the ~ixture, comprising
coarse grains of 0.5 to 5 mrn, in ~n amount of 10 to 40% by weight
of the chrome ore, and fine grains up to 0.1 mm in an arnount of
90 to 60% by weight of the chrome ore, the proportion of these
fine chrome ore grains below 0.06 mm amounting to at least 70%
by weight, and the firing of the bricks is carried out at a
temperature of 1700 to 1900C.
Preferably the ooarse grains of chrome ore have a grain
size of 0.7 to 2 mm' and preferably the firing is conducted at a
temperature of 1700 to 1850C.
According to another aspect o~ the invention there is
provided a cornposition for producing ~ired refractory bricks havlng
the composition described in the proceeding paragraph.
According to another aspect of the invention there is
provided fired refractory bricks which comprises a fired composi-
tion of the invention.
By the use of a major proportion of the chromite in -the
form of inely ground grains with a size of 0 to 0.1 mm, at least
70% by weight of the fine component o~ ~he cllromite being present
in a grain si~e below 0.06 ~m, a cons.iderabl~ incr~se in -khe sur~
~aces ~or r~action betw~en th~ chron~t~ and magn~sia is obtained.
This results in extensive dissolution of the fine component o-f the
chrornite in the periclase already at brick firing temperatures of
1700 to 1850 C or to 1900C, as the case may be. Extremely high
firing temperatures above 1900C, which can only be reached with
high cost, for example in consecluence of the need to use oxygen,
are not necessary.
Moreover, owing to the fact that a proportion of 2 to
10% by weight of the sintex magnesia used is also present in finely
ground grains with a size of 0 to 0.1 mm, again at least 70% by
weight of this fine component having a grain size below 0.06 mm, a
1~64974
further intensification of the reacti.on between the chrome ore and
magnesia is produced, whereby a high degree of dir~ct bonding bet-
ween the chromite and periclase and between the periclase grains
with one another is obtained and the high~temperature stability
properties of the bricks are improved.
It is true that a brick in which the entire chrome ore
component is present in the indicated fineness would have a very
high hiah-temperature stability, but the resistance to changes of
temperature would be quite inadequate. m erefore, according to
the invention, a proportion of lO to 40% by weight of the chrome
ore is used in coarse grains with a size of 0.5 to 5 mm, prefera-
bly O.7 to 2 mm, whereby a good resistance to changes of tempera-
ture is achieved. The reduction in high-temperature stability
produced thereby in comparison with a brick without such coarse
chrome ore grains is only slight and can be accepted.
For the purpose of achieving optimum values for the high-
temperature stability, resistance to changes of temperature and
slag resistance, the chrome ore is advantageously used in the
. starting mixture in an amount o~ 35 to 56% by weight.
The conventional good refractory hrome ores may be used
in the process according to the invention, i.e. chrome ores with
a Cr203 content of ~5 to S7% by weig~nt. ~his means that sinter
magnesia and chrome ores are advantageously used in the starting
mixture in such a ratio that the mixture has a Cr203 content of
around 15 to 25% by weight, preferably around 20 to 25% by weight.
For the magnesia component, there may be employed sinter
magnesia derived Prom natural magnesite ores poor in iron and rich
in iron, sea water magnesia poor in iron and synthetic varieties of
sinter magnesia, as well as mixtures of various kinds of sinter
magnesia. It is an advantage of the invention, however, that no
special demands need to be made on the purity of the sinter magne-
sia. Thus, the sinter magnesia may suitably be used with an MgO
: - 5 -
~16~974
content of 88 to 94% by weight and with an Fe203 content of at
least 3% by weight. To achieve an excellent high-temperature sta-
bility, it is necessary, however, to keep the CaO and ~iO2 contents
in the starting materials low in order to obtain a CaO + SiO2 con-
tent of at the most 6% by weight, pre~erably at the most 4% by
.weight, in the brick.
In the firing of the bricks, they are advantageously
subjected to the above-mentioned firing temperature for about 4
hours, heating-up and cooling-down times not being included.
The following examples o~ carrying into effect and com-
parison tests are intended to illustrate the invention more fully.
Example 1:
Starting materials with the following chemical analyses
were used:
Sinter magnesia Chrome ore
SiO2 0.3% by weight3.0% by weight
2 3 13.5% " "
Fe23 6.5% 15.0% " "
CaO 1.7% " " 0.3% " "
MnO 0.7% ~l _
MgO 90.4% 1I ll }5~0% 1I ll
Cr23 ~ 53.2% 1l "
From these materials, brick mixtures, each with a ratio
by weight of sinter magnesia to chrome ore o~ 60 to 40, were pre-
pared wikh the ~ollowing grain make-ups (in % by weight~:
Variant A~ B C D
: Sinter magnesia 1 - 3 mm 35 35 35 35
Sinter magnesia 0.1 - 1 mrn 20 20 25
Sinter magnesia O -- 0.1 mm 5 5 - 25
Chrome ore 0.7 - 1.5 mm 10 - - 10
Chrome ore O - 0.7 mm '- - - 30
Chrome ore O - Ool mm 30 40 40
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4~317~
In the case of the two O - 0.1 mm ~ractions the propor-
tion below 0.06 mm was in each case in the range of 75 to 80% by
weight.
The components were mixed in conventional manner with the
addition of 4% by weight of magnesium sulp~ate solution o 26 Bé /
as binder, moulded into bricks in a die press under an applied pres-.
sure of about 1100 kp/cm2 and these bricks were fired for 4 hours
at a temperature of 1800C. The following test values were found
in the fired bricks:
Variant A B C D
Compressive strength at ro~m
temperature k~/cm 555 675 670 435
CompOessive strength at
~ 1600 C kp/cm 93 135 105 39
: Gross density g/cm3 3~08 3.12 3.11 3.12
Porosity % by Vol. 19.3 18.3 18~4 18.6
: Resistance to changes of
temperature Quenchings
until 58 :44 4 :1 1 :0~ 100: >100
fracture
It will be seen that contrary to the comparison variants
B, C, D, the variant A according to the invention shows optimum
values both ~or the high-temperature stability and for the resis-
tance to changes of temperature.
Example 2:
There served as starting materials a sinter magnesia of
the composition given in Example 1 and a chrome ore with the fol-
lowing chemical analysis:
SiO2 1.0% by weight
A123 13.0% " "
23 ~ 23.0%
CaO 0.1% " "
MgO 12.~% " "
Cr23 50.5%
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4974
Bri:ck mixtures with the following grain make-ups ~in %
by weight) were prepared:
Variant A B C
Sinter magnesia 1 - 3 mm 35 35 35
Sinter magnesia 0.1 - 1 mm 17 17
Sinter magnesia 0 - O.1 mm 8 8 25
Chrome ore 0.7 - 1.5 mm 8 - -
Chrome ore 0.1 - 1 mm - 10 40
.
Chrome ore 0 - 0.1 mm 32 3p
Fired bricks were produced from these components in the
same way as in Example 1 and the following test values were found
in these bricks:
Variant A B C
Compressive strength at room
temperature kp/cm2 S10 580 330
CompOessive strength at 2
1600 C kp/cm 97 105 61
Gross density g/cm3 3.18 3~21 3.15
Poroslty % by Vol. 17.7 17~6 18.9
Resistance to changes of
temperature Quenchings> 100 : 58 1: 5 72 : 79
until
~racture
The optimizat.ion o~ the propertie9 in the case o~ the
variant A according to the invention in contrast to the comparison
variants B, C will again be recognized.
In general, the bricks produced by the process according
to the invention show a high-temperature compressive strength at
1600C of at least 60 kp/cm and in some cases considerably more
than this.
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