Aluminum Smelting Furnace and Holding Furnace Description
Gas-combustion or oil-combustion fixed or tilting reverberators are usually used for aluminum ingots and scrap aluminum melting and alloying, sometimes, resistance reverberator or induction crucible furnace are also adopted.
Although the temperature of aluminum liquid and aluminum alloy is only 700~800℃, magnesium, silicon, manganese and other element in aluminum and aluminum alloys are very active, easy to react with some components in refractory material and cause refractory damage. The main damage mechanisms of aluminum smelting furnaces are:
Aluminum liquid has very low viscosity, is easy to permeate into refractory material.
Some alloy elements in aluminum and its alloys have very strong reduction ability, their reduction-oxidization reaction is strong heat release reaction. Some alloy elements such as Mg have very high vapor pressure, their vapor is more liable to penetrate into refractory material and then be oxidized, finally cause the metamorphism , structure looseness and damage of refractory material.
During large scale aluminum smelting furnace working, continuously added aluminum ingots and alloy will have great impaction and abrasion to furnace inlet and furnace bottom.
The addition of aluminum ingots, the discharge of aluminum liquid and temperature fluctuation in furnace can do thermal shock damage to refractory lining.
Aluminum Smelting Reverbertator Refractories
Aluminum smelting reverbertator refractories should resist aluminum liquid and magnesium vapor infiltration, have excellent abrasion resistance and thermal stress impaction, etc. Furnace lining where contact with aluminum liquid usually adopts high alumina bricks containing 80%~85% of Al2O3. For high purity aluminum smelting, corundum bricks or mullite bricks are usually adopted. At furnace bed slope and scrap aluminum feeding parts, where corrosion and abrasion is frequently happened, silicon nitride bonded silicon carbide bricks are often used. At aluminum runner and aluminum tapping hole, aluminum scouring is severe, so self bonded or silicon carbide bonded silicon carbide bricks are adopted, zircon bricks are also used for the masonry of inner lining. For furnace lining, where don’t contact with aluminum liquid, clay bricks, fireclay castable or plastic refractory are usually used. The inner lining of aluminum running trough usually adopts silicon carbide bricks or fused foam silica precast blocks.
With the enlargement aluminum smelting furnace and the strengthening of smelting, high strength anti-aluminum permeation castable which has excellent permeation resistance, abrasion resistance and thermal impaction resistance are widely used in aluminum smelting furnace.
High Strength Anti-Aluminum Permeation Castable
Item
Index
Al2O3 %
75.6
CaO %
1.25
Refractoriness ℃
1790
Bulk density g/cm3(110℃,24h)
2.85
Crushing strength MPa (1000℃,3h)
126.42
Modulus of rupture Mpa (1000℃,3h)
15.67
Linear change rate % (1000℃,3h)
+0.01
Abrasion index
4
Thermal shock resistance times (1100℃,water cooling )
The main function of tundish is keeping the temperature of steel liquid stable, making impurity float to the surface of steel liquid, promising the smooth going of the continuous casting during steel liquid shortage or steel ladle changing. The capacity of tundish is usually 15%~30% of steel ladle. Every continuous casting machine will equip 7~12 tundishes. Tundish Steel liquid temperature is 1510~1570℃. Learn more about the tundish refractory, please continue reading.
As a transition device between the ladle and the mold during the continuous casting process, the tundish is not only storage and distributor of the molten steel, but also a refining vessel of the molten steel. It can be used to stabilize the steel flow, reduce the scouring of the billet shell in the mold by the steel flow so that the molten steel has a reasonable flow field and a proper long residence time in the tundish. In order to ensure the uniform temperature of the molten steel and the separation and floating of non-metallic debris, the cleanliness of the molten steel and the continuous casting of multiple furnaces are an important part of improving the quality of steel and the efficiency of continuous casting. The structure diagram of the tundish is shown below.
Schematic Diagram of Tundish Structure
The Main Function of the Tundish
(1) Diversion of molten steel. For multi-stream and continuous casting machines, the molten steel is distributed to each mold through the tundish.
(2) Steady flow. Reduce the static pressure of the molten steel to maintain a stable liquid level in the tundish. Pour molten steel into the mold steadily.
(3) Store molten steel. When replacing the ladle with multiple furnace continuous casting, the drawing speed is not reduced. Create conditions for multiple furnace continuous pouring.
(4) Purify molten steel. In a long pouring time, the temperature of the molten steel is basically unchanged, and the inclusions in the molten steel are further raised. Avoid contact between molten steel and air, and avoid oxygen and nitrogen absorption.
Refractory Materials for Tundish
Refractory materials for tundish can be divided into three categories in terms of function.
The first category is lining materials. It is mainly composed of the thermal insulation layer, permanent layer, and working layer.
The second category is steady flow components. Including slag weir, slag baffle, impact plate, etc.
The third category is the flow control part. There are stoppers, sliding nozzles, sizing nozzles, and intrusive nozzles.
For billet continuous casting, the core of efficient continuous casting is high drawing speed. This requires that on the basis of the quality of the molten steel meeting the requirements, the continuous operating rate of the continuous caster and the casting speed should be increased as much as possible. To achieve higher billet temperature and cast billet quality. These technical indicators put forward two requirements for tundish refractories, one is a high performance, and the other is long life.
Composition of the Refractory Lining in the Tundish
The tundish is lined with refractory materials and generally includes the following parts. Refractory materials for different parts of the tundish are introduced respectively.
(1) Insulation layer (10-30mm), this layer is next to the steel shell of the tundish. Asbestos boards, insulation bricks, or lightweight castables are usually used. The best effect is aluminum silicate fiber felt, which has low thermal conductivity and is easy to build. Insulating lining. Insulating lining usually adopts clay bricks or insulating board, some steel mills may have no insulating lining.
(2) Permanent layer (100-200mm), which is in contact with the insulation layer. The material is generally clay bricks. The overall permanent lining is the most common, and the castable is generally high-alumina or mullite self-flowing castable. Permanent lining. The permanent lining is made from low cement or ultra-low cement alumina castable, which contains 60%~80% of Al2O3. The service life of permanent lining is between 200~1000 times, or even more.
(3) Working layer (20-50mm), this layer is in contact with molten steel and is the key part. Nowadays, insulation boards or coatings are commonly used. Insulation panels are generally siliceous, magnesia, or forsterite. The paint is magnesia, magnesia chromium, and magnesia calcium. The paint is sprayed mechanically or applied by hand. Corrosion lining. Corrosion lining is made from magnesia coating mix, insulating boards, or dry ramming mass.
(4) The Well Blocks are inlaid at the bottom of the tundish and used to install the tundish nozzle. The material is usually high alumina.
(5) The material of the bottom of the tundish is basically the same as that of the working layer. The bottom working layer of the tundish is easily damaged by the impact of molten steel, which requires corrosion resistance and wear-resistance. High-alumina bricks or dense high-alumina bricks and alumina-chrome bricks are generally used, and large, high-strength tar magnesia bricks are also used. The impact part of molten steel is reinforced with zircon bricks or prefabricated blocks.
(6) The tundish cover covers the tundish, which can play the role of heat preservation and molten steel splashing. The material is made of clay or high-alumina castable as the cover, usually adopts the alumina-silica castable with 60% of Al2O3.
(7) The slag retaining wall (weir), which is built in the tundish, can be a single wall or a double wall. The material of the slag retaining wall weir is usually high-alumina bricks, but can also be made into prefabricated blocks, and its purpose is to block. In order to improve the cleanliness of molten steel, a molten steel filter can also be installed on the slag retaining wall.
Impaction zone. Impaction zone can use high alumina precast blocks or high alumina bricks, magnesia precast blocks or magnesia carbon bricks can also be used.
Slag baffle. Slag baffles are mainly made from magnesia precast blocks. Magnesia calcium slag baffle which can adhere impurity in steel liquid is also under development.
Stopper. Alumina carbon integral stopper.
Metering nozzle. Zirconia metering nozzle. For small billet continuous casting, metering nozzle zirconia content can reach 94%, bulk density can reach 5g/cm3. Its service life can be longer than 20 hours.
Coating mix. Magnesia calcium coating mix can significantly decrease oxygen content and sulfur content in steel liquid, impurity index can decrease 37%.
Tundish nozzle. Corundum mullite nozzle or alumina carbon nozzle are usually used in the tundish. To avoid blockage during killed steel casting, there is also argon-blowing purging nozzle.
Heating furnace is thermal equipment used for heating steel billets or small steel ingots during steel rolling or steel forging. Heating furnace’s working temperature is usually below 1400℃. For continuous or circular heating furnaces, there are three temperature zones: low temperature zone, middle temperature zone and high temperature zone, i.e., preheating zone, heating zone and soaking zone, the temperature is 800~900℃, 1150~1200℃, 1200~1300℃, respectively.
The main damage reason of furnace lining is temperature fluctuation caused by discontinuous operation or alternative stop and open of furnace. Furnace bottom and furnace wall base part are mainly damaged by chemical reaction of molten iron oxide and firebricks. So different refractory materials should be chosen for different parts. If the working condition of heating furnace is good, the service life will be longer than 2 years.
Heating Furnace Refractory Classification
Alumina silica refractories are usually used in heating furnaces. Working lining of heating furnace is built by alumina silica firebricks, while outer lining is built by lightweight clay bricks, insulating boards and so on. For example, the roof of soaking zone and heating zone can adopt high alumina hoisting bricks. From inner layer to outer layer, furnace wall is built by high alumina bricks, clay bricks, lightweight clay bricks and insulating boards. Heating zone furnace bottom should resist iron oxide slag corrosion, so magnesia chrome bricks or magnesia bricks can be used here. At preheating zone, working layer can choose clay bricks.
With the development of castable, the ratio of casting refractory used in heating furnaces is increasing gradually. It brings great convenience for mechanization and automation. Meanwhile, the service life, heat preservation efficient and integration ratio are increased.
Sliding nozzle system are mainly used in two places, one is at the bottom of steel ladle, plays the role of steel liquid flowing rate control unit, the other is at the bottom of tundish, controlling steel liquid flowing rate from tundish to crystallizer. Sliding gate’s movement is controlled by operation unit, so the hole of up-sliding gate and down-sliding plate can be opened and closed at will, steel liquid flow rate is adjusted in this way.
Sliding nozzle system is composed of well bricks, sprue bricks and sliding gate. As to ladle sliding gate, high quality alumina zirconia carbon sliding gate and alumina carbon sliding gate’s service life is about 2~4 times. For tundhish sliding gate, it should serve a continuous casting cycle, which means the service life is usually below 8 times. For ordinary steel casting, alumina carbon sliding gate and alumina zirconia carbon sliding gate can meet production requirement. For up-long nozzle, corundum bricks, alumina carbon bricks and chrome corundum bricks are usually adopted. Their service life is between 20~30 times.
Ordinary casting sliding gate system refractory
Item
Alumina carbon up nozzle
Alumina carbon mortar
Alumina zirconia carbon sliding gate
Alumina carbon down nozzle
Alumina down nozzle
Application
Steel ladle sliding nozzle
Sliding nozzle masonry
Steel ladle
Steel ladle
Steel ladle
Al2O3 %
90
≥65
≥70
75.6
≥60
F·C %
4.7
4.7
≥7
4.87
/
Fe2O3 %
/
1.60
ZrO2≥6
/
/
Apparent porosity %
4.7
/
2
5.7
13.6
Bulk density g/cm3
3.18
/
3.07
2.86
2.40
Crushing strength MPa
112.3
/
117
105.5
65
Modulus of rupture Mpa
/
3.8(1500℃, 3h)
Refractoriness under load(T2) ℃
>1700
/
≥1700
≥1700
/
Reheating linear change rate %
/
Setting time: 60~150min
Item
Up nozzle well block
Well block
Up nozzle
AC sliding gate
AZC sliding gate
Sliding gate mortar
Material
Corundum
High alumina
Chrome corundum
Alumina carbon
Alumina zirconia carbon
Alumina chrome
Al2O3 %
97
81.5
87
93
77.7
84
Cr2O3 %
/
/
10
/
/
4.5
SiO2 %
/
11
/
6.2
/
4
CaO %
2.4
TiO2: 3.3
/
/
/
/
ZrO2 %
/
P2O5: 2.2
P2O5: 2.0
/
6.5
P2O5: 5
C %
/
/
/
9
8.2
/
Apparent porosity %
/
22
17.5
16
9
/
Bulk density g/cm3
3.0
2.75
3.25
2.79
3.07
/
Crushing strength MPa
>50
80
/
>30
200
/
Modulus of rupture Mpa(1450℃)
2.8
/
/
>7
12
/
Refractoriness under load(T2) ℃
/
>1430
>1700
≥1700
/
/
Reheating linear change rate %(1450℃,3h)
/
1.0
/
/
/
/
Main Damage Reasons of Sliding Gate
Carbon boning sliding gate can be oxidized in middle temperature oxidizing condition and lose strength, the surface of sliding gate will be abraded irregularly. It is the main damage reason of sliding gate.
Another damage reason is thermal shock. Sliding gate will endure strong thermal shock and generate great thermal stress, which can induce sliding gate crack or crushing.
The third damage reason is steel liquid scouring and abrasion. Steel liquid flowing speed is very quick, can take away oxide granule and dissolve carbon in sliding gate, so the diameter of sliding gate’s hole is enlarged.
High Oxygen Steel Casting Sliding Gate
For high oxygen steel casting, alumina carbon sliding gate and alumina zirconia carbon sliding gate’s service life can greatly decline. Sliding nozzle system must have good oxidization resistance, good iron oxide corrosion resistance, as well as thermal shock resistance and abrasion resistance. Through numerous research and application cases, alumina magnesia spinel carbon sliding gate, magnesia spinel carbon sliding gate and magnesia carbon sliding gate are tested to have good service life.
Submerged nozzle ‘s top end connects to the bottom of tundish, bottom end extends into crystallizer, so steel liquid flows from tundish won’t expose to air. The advantages of submerged nozzle are: avoiding steel liquid splashing, reducing impurity entry and protecting steel liquid from oxidization.
Submerged nozzle’s working condition is very harsh, such as very strong thermal shock, steel liquid scouring and erosion, crystallizer protection slag intense corrosion. As protection slag is low melting point, low viscosity material which contains strong corrosive fluorite, K oxide and Na oxide, its corrosion to submerged nozzle is severer than long nozzle, submerged nozzle’s technical requirement is much higher than elongated nozzle.
Submerged Nozzle Types
Silica submerged nozzle is primary used in steel mills, but the corrosion rate is very quick, can only be used in those with lower continuous casting times. Quartz submerged nozzle is not suitable for high continuous casting level or special steel continuous casting mills. Composite submerged nozzle is improved on basis of molten quartz nozzle, slag line adopts composite refractory which has better corrosion resistance to improve service life.
To meet the quick development of continuous casting technique, alumina carbon submerged nozzle is designed. Its service life is significantly prolonged, but thermal shock resistance of alumina carbon submerged nozzle is not very good. To meet thermal shock resistance, some ratio of mullite and molten quartz are added into alumina carbon submerged nozzle. Composite submerged nozzle, such as alumina zirconia carbon submerged nozzle, zirconium boride submerged nozzle are developed, which have good slag corrosion resistance and steel liquid corrosion resistance.
Submerged Nozzle Physiochemical Index
Item
Composite submerged nozzle-1
Composite submerged nozzle-2
Quartz submerged nozzle
Main body
Slag line
Purging part
Main body
Slag line
Material
Al2O3-C
ZrO2-C
Al2O3-C
Al2O3-C
ZrO2-C
Quartz
Al2O3 %≥
48
/
80
45
/
/
C %≥
30
15
15
20
12
/
ZrO2%
/
77
/
/
80
/
SiO2 %
≤15
/
/
≤20
/
≥99
Apparent porosity %≤
17
18
/
20
18
19
Cold compression strength Mpa ≥
17
/
/
16
/
40
Cold rupture strength Mpa
≥6
/
/
≥4
/
/
Thermal shock resistance/ times
≥5
/
/
≥5
/
≥5
For aluminum killed steel casting, nozzle will be blocked by clots, so argon blowing method can be used to avoid this problem, inner lining of submerged nozzle can adopt carbon free refractory and calcium zirconate to get better performance. Submerged nozzle also has another important problem, i.e., surface oxidation, which can make submerged nozzle lose structural strength and damaged. So, a layer of anti-oxidation material can be coated to the surface of submerged nozzle.
Continuous casting system stopper is installed above tapping nozzle. Through adjusting the gap between stopper and nozzle, steel liquid flowing speed can be controlled. The earliest sleeve bricks of stopper are high alumina bricks or clay bricks. Stopper end is made by alumina carbon refractory. They need to assemble with steel structural controlling rod for using. The service life of combined stopper is not very high, usually less than 6 hours. The main damage reasons of stopper are: On one hand, steel structure is softened in long term high temperature environment and loses controlling effect, on the other hand, sleeve brick is corroded too quickly. To solve this problem, steel controlling rod should have good high temperature performance, sleeve brick should have excellent corrosion resistance and stopper end should properly match with nozzle bowl part.
Another method to solve the problem of combined stopper’s low service life is adopting integral stopper, i.e., alumina carbon integral stopper. Alumina carbon integral stopper is made by isostatic pressing molding, solidifying, mechanical processing and carbonizing, there is no metal rod in alumina carbon stopper and avoids the problem of metal rod softening. The service life of alumina carbon integral stopper can reach to 10h. But stopper end and nozzle bowl part will be scoured irregular and not smooth, it will cause difficulty for flow speed control, or even can’t close. That is key factor constraining the service life of alumina carbon stopper.
Continuous Casting System Stopper Physiochemical Index
Item
Chemical composition (w %)
Cold CS/Mpa ≥
Apparent porosity % ≤
High Temp. rupture strength Mpa
Thermal shock resistance /times
Al2O3 ≥
C≥
ZrO2
SiO2
MgO
Integral stopper
60
25
/
/
/
16
19
≥5
≥5
55
23
/
/
/
15
/
≥4
≥5
Combined stopper
Sleeve brick
60
/
/
≤30
/
40
18
/
≥20
Sleeve brick
42
/
/
≤52
/
40
18
/
≥20
Stopper end
80
10
/
/
/
40
18
≥12
/
Stopper end
75
10
≥6~9
/
/
40
6
≥12
/
Composite stopper
Main body
60
25
/
/
/
16
19
≥5
≥5
Slag line
/
14
/
/
80
30
5
≥10
/
Slag line
75
10
≥6~9
/
/
25
10
≥8
/
End
/
5
≥90
/
/
/
/
/
/
End
85
5
/
/
/
/
/
/
/
Continuous Casting System Stopper applications
For steel mills using converter to produce ordinary construction steel , steel liquid supply is sufficient, it needs the long service life of stopper, the longer the better. For steel mills producing special steel, different types of steel have different corrosion to stopper, so stoppers which can resist different kinds of steel liquid corrosion is needed. For this reason, slag line part of stopper adopts magnesia carbon refractory, stopper end part adopts alumina zirconia carbon refractory.
RH is vacuum argon blowing recycling degassing equipment for steel liquid refinery, and DH is vacuum degassing refinery equipment. By blowing high speed argon in immersion tube and vacuum chamber upper part, steel liquid in ladle will flow into bottom side of vacuum chamber and be degassed, and then flow back to ladle through the other dipping tube. Through vacuum recycling degassing, harmful gases such as nitrogen, hydrogen in steel liquid can be removed. If oxygen blowing is adopted, carbon content can be decreased to a very low extent. RH and DH refinery equipments are mainly used for low carbon steel, low nitrogen steel and low hydrogen steel. Under those working conditions, furnace lining refractory materials should have good scouring resistance, vacuum resistance and thermal shock resistance.
In concern of cleaning steel production, refractory materials should better contain no carbon, nitrogen and hydrogen. Usually, vacuum chamber can use magnesia chrome bricks, inner side of immersion tube can use magnesia chrome bricks, while the outer side can adopt alumina magnesia spinel castable. The service life of magnesia chrome bricks at vacuum chamber upper part can reach to 2000 times or more, but the lower part is only about 500 times. The service life of immersion tube is about 100 times, varies with working condition and maintenance quality.
Magnesia alumina spinel brickscan be a good replacement of magnesia chrome bricks, but the corrosion resistance is not as good as magnesia chrome bricks. Another replacement of magnesia chrome bricks is low carbon magnesia carbon bricks, which has good performance price ratio compared with MgO-Cr2O3 bricks. RH immersion tube can be integrally casted by magnesia alumina spinel castable or magnesia zirconia castable. Through patching, the service life of integrally casted immersion tube can reach to or exceed magnesia chrome bricks lining.
Nozzle series refractory can be divided into long nozzle, metering nozzle, submerged nozzle and separation ring.
Long Nozzle
Long nozzle is the connection part between steel ladle and tundish. Head end of long nozzle connects to steel ladle down nozzle, bottom end embeds into tundish steel liquid. Long nozzle can keep steel liquid from contacting with air during transferring from ladle to tundish, so steel liquid won’t splash, impurity won’t enter into steel liquid. The working condition suggests that long nozzle should not only endure strong thermal shock, steel liquid scouring and corrosion, but also suffer the corrosion of tundish cover flux. With the increasing requirement on steel quality, protecting casting ratio is continuously growing, so long nozzles using ratio is surging.
Quartz long nozzle is earliest used, which has very low linear expansion rate or nearly no thermal expansion, excellent thermal shock resistance, but it’s corrosion resistance is weak, especially for the casting of special steel such as high Mn steel, the service life is even shorter. With the developing of continuous casting technique, especially the casting of special steel, quartz long nozzle can also have unfavorable effect on steel quality, so new type of alumina carbon long nozzle is developed. Alumina carbon long nozzle’s service life is significantly longer than quartz long nozzle, but alumina carbon long nozzle’s main components are corundum, mullite, graphite and molten quartz, although mullite and quartz can large scale increase thermal shock resistance, the corrosion resistance to Mn steel and other special steel is not very good.
To solve this problem, composite long nozzle is developed, i.e., slag line magnesia carbon material, zirconia carbon material, main body adopts alumina zirconia carbon material, to meet low carbon steel casting needs, inner layer adopts alumina magnesia and Sialon ceramic carbon free material.
In recent years, unshaped long nozzle is also developed. Casting molding method is used to replace isostatic pressing molding for the production of alumina carbon quartz long nozzle and alumina magnesia long nozzle.
Long Nozzle Physiochemical Index
Item
Alumina carbon long nozzle
Alumina carbon long nozzle
Quartz nozzle
Quartz nozzle
Al2O3-C long nozzle
Main body
Purging ring
Al2O3 %
43.81
≥40
≥85
/
/
≥50
F．C %
30.65
≥27
/
/
/
≥25
SiO2 %
/
/
/
≥99
≥99
/
Bulk density g/cm3
2.24
/
/
≥1.82
≥1.84
/
Apparent porosity %
17
≤20
/
≤19
≤18
12~20
Compression strength Mpa
25.2
≥20
/
≥40
≥40
≥15
Rupture strength Mpa
7.82
≥6
/
/
/
≥4
Air flow rate L/min
229
/
100
/
/
/
Thermal shock resistance/times
229
/
100
/
/
/
Item
Alumina carbon quartz long nozzle
Alumina magnesia carbon long nozzle
Alumina zirconia carbon long nozzle
Alumina carbon long nozzle
Main body
Slag line
Main body
Purging part
Material
Alumina carbon
Alumina carbon
Magnesia carbon
Alumina zirconia carbon
Alumina carbon compound
Alumina carbon compound
Al2O3 %
50
≥40
MgO, 74
≥40
50
90
F．C %
30
≥27
25
≥27
31
/
SiO2 %
14
/
/
/
/
/
ZrO2 %
/
/
/
6~10
/
/
Bulk density g/cm3
2.50
2.38
2.40
≥2.50
2.25
/
Apparent porosity %
14
≤20
16
≤18
17
/
Compression strength Mpa
/
≥20
/
≥20
24.4
/
Rupture strength Mpa
/
≥6
/
≥6
/
/
Air flow rate L/min(0.1Mpa)
/
/
/
/
/
210
Thermal shock resistance/times
/
≥5
/
≥5
5
/
Metering Nozzle
Metering nozzle is mainly used for the continuous casting of small billet. It is installed at the bottom of tundish. Metering nozzle should not have diameter enlargement or erosion under the strong scouring and corrosion of steel liquid. Inserting metering nozzle’s inner layer is made from zirconia ceramic refractory, which has excellent performance, outer layer is made from high alumina refractory. Inserting metering nozzle’s performance cost ratio is very high, so is popularly used. Integral metering nozzle is made by single type of refractory material, but performance cost ratio is not so high as inserting type metering nozzle. Worthy to be mentioned is that, a lot of steel mills insert pocket bricks and metering nozzles together and form a whole body, which is convenient for construction and safer for production.
To prolong metering nozzle’s service life, zirconia content and zirconia ceramic’s density should be improved. The content of zirconia is raised from 65% to 94%, the density is increased from 3.4g/cm3 to 5.0g/cm3. To improve thermal shock resistance, semi stable zirconia is usually adopted, CaO is used as stabilizer.
LV-VD furnace is steel liquid refinery equipment, which’s working condition is very harsh, should endure all kinds of high temperature corrosion, chemical corrosion, mechanical stress and thermal stress. So high quality refractory products should be adopted in according parts of LF-VD furnaces. Magnesia carbon bricks are mainly used at furnace bottom and working lining. Corundum precast block which has high mechanical strength is mainly used at furnace bottom impacting zone. Magnesia chrome ramming mass is mainly used at furnace wall corner. Magnesia chrome mortar can be used for the masonry of magnesia chrome bricks.
LF-VD Furnace Refractory Material Physiochemical Index
VOD is the acronym of vacuum oxygen de-carbonization, i.e., steel liquid vacuum degassing, oxygen blowing de-carbonization treatment can be carried out in VOD refinery furnace. Oxygen blowing VOD furnace’s temperature can be higher than 1700℃, desulfurization and reduction reaction are also carried out in VOD furnace, slag alkalinity will change greatly, high smelting temperature and vacuum atmosphere will also do huge damage to furnace lining refractory material. Therefore, VOD furnace should adopt high grade refractory material.
VOD Furnace Refractory Materials
VOD slag line: Slag line zone usually adopts re-bonding magnesia chrome bricks or pre-reaction magnesia chrome bricks. Ladle wall can use low grade magnesia chrome bricks. Although magnesia chrome bricks can prolong VOD furnace’s service life, but the production cost is high. So in Europe, where dolomite refractory is abundant, VOD furnace slag line zone mainly adopts ultra high temperature sintered magnesia dolomite bricks. Furnace wall can choose lower grade sintered magnesia dolomite bricks or pitch binding unburned magnesia dolomite bricks. Magnesia dolomite refractory lining’s service life is slightly shorter than magnesia chrome refractory lining, but steel smelting cost is significantly decreased, it can bring in good economic benefits.
Below slag line: With the development of magnesia carbon bricks, for furnace body, below slag line, low carbon content magnesia carbon bricks or magnesia dolomite carbon bricks become a reasonable choice, VOD molten bath’s service life can reach to 40 times or more. After proper maintenance or patching, molten bath’s service life can be higher than 60 times. Another developing trend of molten bath refractory material is alumina magnesia castable or precast block, it can ultimately eliminate carbon in furnace lining caused adverse effect.
Cr Free Refractory Bricks for VOD Furnace
Cr2O3 in magnesia chrome bricks will finally be oxidized into Cr6+ , which has great environment pollution, so magnesia chrome bricks’ application is constrained. Ultra high temperature sintered high purity magnesia dolomite brick is a good replacement of slag line magnesia chrome bricks. Molten bath can adopt low carbon magnesia carbon bricks or low carbon magnesia dolomite carbon bricks.
VOD Furnace Refractories Physiochemical Index
Item
High alumina spinel castable
High alumina castable
Direct binding dolomite brick
Direct binding magnesia dolomite brick
Resin binding dolomite carbon brick
Resin binding magnesia dolomite carbon brick
Application
Ladle cover
Vacuum cover
Molten bath
Molten bath, slag line
Slag line, molten bath
Slag line, molten bath
Al2O3%
92
70
/
/
/
/
SiO2 %
/
22
/
/
/
/
TiO2 %
/
2.8
/
/
/
/
MgO %
5
/
41
59
43
65
CaO %
2
3
57
39
55
33
C %
/
/
/
/
<3
<5
Bulk density g/cm3
3.0
2.80
2.82
3.00
2.94
2.92
Apparent porosity %
/
/
<17
<13
<5
<5
Compression strength Mpa
/
/
65
60
>60
>60
Item
Alumina chrome precast block
Corundum precast block
Magnesia chrome brick
Magnesia chrome brick
Unburned corundum spinel brick
Magnesia chrome joint material
Application
Ladle cover
Ladle cover
Slag line, molten bath
Molten bath, slag line
Molten bath
Magnesia chrome bricks lining
Al2O3%
92
94
<6
<6
>90
<7
Cr2O3 %
>3
/
18~23
12~18
/
18~23
Fe2O3%
/
/
<12
<12
/
<12
SiO2 %
/
/
<1.5
<1.5
/
<1.5
MgO %
/
/
>60
>65
6~12
>55
CaO %
2
2
/
/
/
/
Bulk density g/cm3
3.0
3.0
>3.15
>3.10
>3.05
>2.80
Apparent porosity %
<15
<15
<17
<17
<15
/
Compression strength Mpa
>30
>40
>35
>40
>40
>20
Item
Magnesia spinel bricks
Alumina magnesia castable
Alumina magnesia precast block
Low carbon magnesia carbon brick
Magnesia chrome gunning mix
Magnesia chrome gunning mix
Application
Molten bath
Molten bath
Molten bath
Molten bath, slag line
Molten bath, slag line
Molten bath, slag line
Al2O3%
8~12
>90
>90
/
<8
<8
Cr2O3 %
/
/
/
/
18~23
12~18
Fe2O3%
/
/
/
/
<12
<12
SiO2 %
/
/
/
/
<3
<3
MgO %
>80
4~8
4~8
>90
>55
>55
CaO %
/
<2
<2
/
/
/
C %
/
/
/
<5
/
/
Bulk density g/cm3
>2.90
>3.00
>3.00
>3.05
>2.50
>2.40
Apparent porosity %
<15
<15
<15
<5
/
/
Compression strength Mpa
>60
>30
>30
>80
>10
>10
Item
Magnesia calcia gunning mix
Dolomite gunning mix
Magnesia gunning mix
Magnesia calcia joint material
Application
Magnesia calcia lining
Magnesia calcia lining
Magnesia calcia and magnesia chrome lining
Magnesia calcia brick lining
MgO %
40
65
>92
43
CaO %
50
25
/
55
SiO2 %
<3
<3
<3
<3
C %
/
/
/
<5
Bulk density g/cm3
>2.40
>2.40
>2.40
>2.80
Compression strength Mpa
>20
>25
>25
>30
Methods for Prolonging VOD Furnace Lining Service Life
Apart from choosing high grade furnace lining refractory material, the following methods can be taken to prolong VOD furnace’s service life.
Improve smelting condition. Dolomite slag making method and properly decreases smelting temperature are effective measures to prolong furnace lining service life. Speeding up turnover speed and enhancing heat preservation can also significantly prolong furnace lining service life.
2. Enhancing furnace protection and furnace patching. Usually, gunning mix which has same composition with lining bricks are adopted for furnace lining patching. For example, magnesia chrome bricks lining should be patched by magnesia chrome gunning mix, magnesia dolomite bricks lining should be patched by magnesia calcia gunning mix.
