FIELD OF THE INVENTION
The present invention relates to pitch bonded magnesia bricks for metal zone of steel
ladle and a process for its manufacture. More particularly, the present invention relates
to pitch bonded magnesia bricks for metal zone of steel ladles with improved quality
adapted to enhance ladle life, higher ladle availability and improved productivity.
Importantly, the pitch bonded magnesia carbon refractory bricks according to the present
invention is developed by a process using a selective combination of sea water magnesia
and fused magnesia in selective fractions of brick granulometry and mixed with specified
amount of graphite, aluminium powder and molten pitch at desired specified
temperature, which is mixed , hot pressed and heat treated to form bricks for
application in metal zone of steel ladles. The pitch bonded magnesia carbon bricks of the
present invention is adapted to ensure 29-30% enhancement of ladle life with about 19-
20% reduction in cost per heat thus increasing ladle availability with reduced specific
consumption of refractory and hence reduced cost of steel produced.
BACKGROUND ART
In an integrated steel plant, steel ladles are the major consumer of refractory. It is also
experienced from the operational data that the average lining life of steel ladles in
continuous casting shop in steel plants varies from 60 to 90 heats depending upon the
secondary processing units and ladle capacity. Higher steel ladle life is essential to
increase ladle availability for higher production.
Magnesia carbon bricks are already being manufactured in-house to fulfil partial
requirement of ladle bricks. It has been observed that the average lining life of 150T
steel ladles in steel melting shop of Rourkela Steel Plant is about 90 heats.
Developmental work has been continuing in the past to enhance lining life of steel ladles
through in-house manufactured bricks. It is further experienced in the art that lining
wear is not uniform in all areas of the ladle. Wear is more in slag zone and bottom impact
area and for this reason intermediate repairs are carried out to replace the refractory
lining in these zones. But, Metal zone of the ladle remains for the full campaign and
finally ladle life is decided by the refractory used in this area. There has been therefore a
need to improve the quality of magnesia carbon used in metal zone to enhance the
overall ladle life.
Magnesia is the main raw material used in magnesia carbon bricks. Magnesia content,
impurities, bulk density and crystal size are important properties of magnesia. Graphite
is the next important constituent of these bricks. Graphite purity, size and its amount
plays great role in increasing corrosion resistance, non wetting characteristics, thermal
conductivity and thermal shock resistance. Both magnesia and graphite are quite inert
materials and therefore a hydrocarbon liquid (pitch/resin) is used for providing strength
at low temperature as well as working temperature. At high temperature, carbon network
is formed due to pyrolyses reaction of hydrocarbon binder and this provides high
temperature binding as well as strength of the brick. Graphite/carbon has the problem of
oxidation and therefore certain antioxidant metal powders are used as additives for
minimising oxidation and improving hot strength. However, these additives form carbides
at high temperature during operation of steel ladle. Some of these carbides may react
with atmospheric moisture during repair in cold condition leading to structural spalling in
subsequent operation. Therefore, selection of an appropriate type and quantity of
antioxidant is very important to achieve desired quality and life of campaign.
A number of prior arts in the related field dealt with the limitation of enhancing the
refractory lining life for the steel ladies but could not achieve lining life extension
substantially, particularly for application in metal zone of steel ladles.
Granted Indian patent no. 221579 of the applicants disclosed an invention entitled 'A
process of manufacturing pitch bonded tempered magnesia carbon refractories' wherein
magnesia carbon bricks using molten pitch as bonding material has been used. The
bricks were tempered at selective temperature range to achieve desired properties in
finished bricks. These bricks however could not provide required properties to be used
satisfactorily under the operating conditions of metal zone of steel ladles.
US 4306030 disclosed an unburned carbon-containing refractory bricks consisting
essentially of 5-75 weight % of a carbonaceous material, 1-10 weight % of a metal
powder selected from the group consisting of aluminum powder, magnesium powder or
mixtures thereof, 0.5-6.0 weight % silicon powder, and wherein the residual portion of
the brick consists essentially of an inorganic refractory material, wherein the
carbonaceous material is selected from the group consisting of natural graphite, artificial
graphite, electrode waste, petroleum coke, foundry coke, carbon black or mixtures
thereof and wherein the inorganic refractory material is selected from the group
consisting of magnesia, chrome, spinel, dolomite, alumina, zircon, silicon carbide,
titanium carbide, silicon nitride, and boron nitride. Aluminum and/or magnesium is added
to and mixed with the material of the aforesaid unburned carbon-containing refractory
bricks to increase the rate of residual carbon in the bonded part remarkably and excess
metal powders are turned into carbides favouring compacting the brick structure.
Addition of silicon powder can satisfactorily prevent the hydration of carbides. The bricks
produced ensure improving the rate of residual carbon, increase in oxidation resistance,
reduced apparent porosity and increased strength. Thus, the unburned carbon-containing
refractory bricks according to the invention enables to greatly prolong the useful life of
furnace when used as wear lining.
US 4210453 is a prior patent that states about a Carbon bonded refractory brick
composition. This invention relates to a moldable refractory composition for making
refractory brick which includes about 50 to 90 parts by weight periclase and between 3 to
27 parts by weight carbon. A portion of the carbon prior to subjecting the composition to
curing at elevated temperatures is In the form of a liquid carbonaceous bonding resin. For
example, the liquid carbonaceous bonding resin may be a phenolic liquid resin such as a
phenol formaldehyde bonding resin. The liquid carbonaceous bonding resin may be
present in an amount of about 8 percent by weight of the uncured refractory
composition. To provide dispersion of the resin, it is preferable that about 25 percent of
the liquid bonding resin be present as a phenolic resin powder. A preferred composition
comprises about 80 percent periclase refractory material, 12 percent graphite and about
8 percent bonding resin.
US 4431745 disclosed an invention titled 'Carbon-bonded magnesia carbon bricks' that
relates to carbon-bonded magnesia carbon bricks for use in converters, ladles, ladle
refinery, mixing cars, mixing furnaces, etc., and the method for obtaining such carbon-
bonded magnesia carbon bricks having high properties in respect of resistance to thermal
and structural spalling, slag resistance, thermal strength, shock resistance, etc. thereby
enabling to conform to the gradually intensified operational conditions of said converters
and the like by means of 60-97 weight % sintered magnesia clinker of bulk specific
gravity above 3.40, 3-40 weight % carbonaceous material and a carbon-bonding
formative agent wherein shock resistance is improved with less deterioration of the
strength due to improved affinity between the carbon bond and the sintered magnesia
clinker. The carbonaceous material comprises plumbago, synthetic graphite, electrode
waste, petroleum coke, foundry coke, carbon black, etc., among which plumbago is
preferable in view of its oxidization resistance, high packing density. The carbon-bonding
formative agent comprises materials capable of forming carbon bond when heated, for
example, thermosetting resins, thermoplastic resins, resinous pitch, tar, etc. The carbon-
bonded magnesia carbon bricks according to the invention which use the sintered
magnesia clinker having a compact structure and improved surface properties have a
greater affinity between carbon bond and magnesia clinker, whereby not only the
thermal strength and shock resistance are highly improved but also the strength is less
deteriorated even when used for a long period of time or subjected to thermal cycles of
heating and cooling.
The above cited prior patents thus could not provide cost effective solution for improving
strength and thermal properties commensurate with high temperature application of
magnesia carbon bricks ensuring longer service life while they used expensive resin
based bonding material for the refractory composition.
There has been therefore a need to improve the quality of magnesia carbon bricks
adapted to be used in metal zone to enhance the overall ladle life by using low cost pitch
as bonding material, while ensuring desired oxidation resistance and structural stability
avoiding hydration of carbides making them suitable for high temperature application.
OBJECTS OF THE INVENTION
The basic object of the present invention is thus directed to providing an improved pitch
bonded magnesia carbon bricks for metal zone of steel ladles adapted to ensure
enhanced service life involving pitch as bonding material and a process for its
manufacture.
A further object of the present Invention is directed to an improved pitch bonded
magnesia carbon bricks for metal zone of steel ladles which would provide higher steel
ladle availability and thereby increase in production and reduce cost of steel.
A further object of the present invention is directed to an improved pitch bonded
magnesia carbon bricks for metal zone of steel ladles wherein provision of
antioxidant/aluminium powder in selective proportion would minimize the oxidation of
carbon improving hot strength.
A still further object of the present invention is directed to an improved quality pitch
bonded magnesia carbon bricks for metal zone of steel ladles wherein use of combination
of sea water magnesia and fused magnesia in selective proportions and particles sizes,
along with other constituents, favour improving the average ladle lining life substantially
in terms of number of heats.
A still further object of the present invention is directed to an improved quality pitch
bonded magnesia carbon bricks for metal zone of steel ladles wherein the enhancement
of average lining life reduces the specific consumption of refractories.
A still further object of the present invention is directed to an improved quality pitch
bonded magnesia carbon bricks for metal zone of steel ladles wherein due to increase in
lining life of ladles by 30% using such bricks, the cost per heat is reduced by about
20%, in spite of marginal 1-2% increase in cost of manufacture of such bricks.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is thus directed to a pitch bonded magnesia
carbon brick composition for metal zone of steel ladle adapted to enhance lining life
comprising
Magnesia comprising a combination of sea water magnesia in the range of 40 to 60 wt%
and fused magnesia 60 to 40 wt %;
Graphite in the range of 6 to 8 wt%;
Pitch in the range of 3 to 5 wt %; and
Aluminium metal powder in the range of 0.5 to 1.5 wt%.
A further aspect of the present invention is directed to said pitch bonded magnesia
carbon brick composition, wherein said magnesia comprising a combination of sea water
magnesia and fused magnesia wherein
(i) said sea water magnesia having the chemical composition (wt percent) comprising
MgO(min) :97.0
CaO(max) : 2.0
SiO2(max) :0.35
Fe2O3(max) :0,2
Al2O3(max) :0.12
B2O3(max) :0.03 and
physical characteristics comprising
Bulk Density gm/cm3 : 3.4
Average crystal size(micron): 100
(ii) said fused magnesia having the chemical composition (wt percent) comprising
MgO(min) :98
SiO2(max) :0.6
Fe2O3(max) :0.5
Al2O3(max) :0.3
CaO/SiO2(min):2.0
and physical characteristics comprising
Bulk Density gm/cm3 : 3.5
Average crystal size(micron): 400
A still further aspect of the present invention is directed to said pitch bonded magnesia
carbon brick composition, wherein said graphite comprises
Fixed Carbon (%) 96-98 min
Ash (%) 1.5-3.5 max
Volatile matter (%) 0.5 max
Moisture (%) 0.5 max
Size range (%)
+0.295mm 20 max
-0.295 + 0.211 mm 60 min
-0.211mm 20 max
A still further aspect of the present invention is directed to said pitch bonded magnesia
carbon brick composition, wherein said pitch comprising
A still further aspect of the present invention is directed to said pitch bonded magnesia
carbon brick composition, wherein said Aluminium metal powder comprising
Al (min) (wt %) : 99.0
Particle size :-200mesh
According to yet another aspect of the present invention is directed to said pitch bonded
magnesia carbon brick composition, wherein micro fine fraction is from sea water
magnesia and coarse fractions are from both the types of magnesia (sea water magnesia
: fused magnesia) grains in certain ratio ranging from 30:70 to 70:30 so as to achieve an
overall ratio of in the range of 40:60 to 60:40.
A still further aspect of the present invention is directed to said pitch bonded magnesia
carbon brick composition, wherein said graphite is used in the range of 6-8 wt% and is
added along with fine fractions of the brick mix.
A still further aspect of the present invention is directed to said pitch bonded magnesia
carbon brick composition, wherein Aluminium metal powder is used in the range of 0.5 -
1.5 wt% and is premixed with 15-20% of the fine magnesia and added in fine fractions.
According to yet another aspect of the present invention is directed to a process for
manufacturing the above said pitch bonded magnesia carbon bricl< composition,
comprising the steps of
providing said magnesia comprising said selective combination of sea water magnesia
and fused magnesia wherein preferably micro fine fraction is from sea water magnesia
and coarse fractions are from both the types of magnesia (sea water magnesia : fused
magnesia) grains in certain the ratio ranging from 30:70 to 70:30 so as to achieve an
overall ratio of in the range of 40:60 to 60:40;
providing said graphite in the range of 6-8 wt% and adding along with fine fractions of
the brick mix;
providing said aluminium metal powder in the range of 0.5-1.5% and premixed with 15-
20% of the fine magnesia , added in fine fractions;
mixing said coarse fractions with 3-5 wt % molten pitch and is heated in a preheated
mixer for 10-15 minutes;
adding the said fine fractions directly to the rotating mixer and further mixed for
additional 30-45 minutes to get uniform mix;
hot pressing the magnesia carbon mix thus produced to form bricks using standard
equipments and tools; and
finally heat treating said bricks at temperatures 200 to 300°C for 20 to 40 hrs to obtain
magnesia carbon bricks for use in metal zone of steel ladles ensuring enhanced ladle life.
The present invention and its objects and advantages are described in greater details
with reference to the following accompanying non limiting illustrative drawing.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1: is the schematic illustration of the lining pattern of the present invention
involving the magnesia carbon bricks according to the present invention as compared to
the existing lining pattern used in metal zone of steel ladles.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING DRAWING
The present invention relates to an improved quality pitch bonded magnesia carbon
refractory bricks and a process for its manufacture. The magnesia carbon bricks
according to the invention have been developed involving a mix comprising combination
of sea water magnesia and fused magnesia in certain fractions of brick granulometry, a
specified amount of graphite of desired purity and particle size distribution, a specified
amount of a particular metal powder of specified size, mixed with a specified amount of a
molten pitch of specified quality at certain specified temperature, hot pressed and heat
treated to form bricks for application in metal zone of steel ladles.
According to an embodiment of the present invention, the input raw material comprised
sintered and fused magnesia, graphite, pitch and aluminium powder wherein
a)Magnesia comprising
(i) Sea water magnesia having the chemical composition (wt percent) comprising
MgO(min) :97.0
CaO(max) : 2.0
SiO2(max) :0.35
FeaO3(max) :0.2
Al2O3(max) :0.12
B2O3(max) :0.03
and physical characteristics comprising
Bulk Density gm/cm3 : 3.4
Average crystal size(micron): 100
(ii) Fused magnesia having the chemical composition (wt percent) comprising
MgO(min) :98
SiO2(max) :0.6

Fe2O3(max) :0.5
AlzO3(max) :0.3
CaO/SiO2(min):2.0
and physical characteristics comprising
Bulk Density gm/cm3 : 3.5
Average crystal size(micron): 400
b) Graphite comprising
Properties Values
Fixed Carbon (%) 96-98 min
Ash (%) 1.5-3.5 max
Volatile matter (%) 0.5 max
Moisture (%) 0.5 max
Size range (%)
+0.295mm 20 max
-0.295 + 0.211 mm 60 min
-0.211mm 20 max
c) Pitch comprising
Properties Values
Softening point (R&B), °C 90-95
Coking Residue (%) min 50
Benzene insoluble (%), min 20
Quinoline insoluble (%), min 8
ß- resin (%) min 12
Moisture (%) Trace
Distillation fractions (%)
0-300°C, max 0.5
300-360°C, max 5
d) Al metal powder comprising
Al (min) (wt %) : 99.0
Particle size :-200mesh
According to the process of the present invention for producing the improved pitch
bonded magnesia carbon bricl