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Development and use of unburned magnesia calcium bricks for LF refining furnace

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Using synthetic magnesia sand and fused magnesite as raw materials, anhydrous resin as a binding agent, and adding an appropriate amount of additives, an unburned magnesia brick for LF refining furnace was developed. It was used in a 90tLF refining furnace of a steel plant and achieved a long service life and good metallurgical effects, meeting the needs of the steel plant for refining a variety of special steels.

Keywords: unburned magnesia-calcium bricks; LF refining furnace; furnace age

LF refining is one of the most commonly used out-of-furnace refining equipment in the steelmaking industry. In recent years, with the continuous increase in the production and quality of clean steel in my country, higher requirements have been put forward for refractory materials used in LF refining furnaces and other refining equipment outside the furnace. It not only requires the refractory material to have a long service life, but also has a purifying effect on the molten steel, at least without causing pollution to the molten steel. At present, the only refractory materials that can meet this requirement are magnesia-calcium refractories, including fired magnesia-calcium bricks and unburned magnesia-calcium bricks. Magnesia-calcium refractory materials have excellent high temperature resistance and slag corrosion resistance. The free CaO in them can absorb non-metallic inclusions such as [S] and [P] in steel and water, and has the function of purifying molten steel. In order to meet the needs of refining cord steel and other high-grade steel types in the LF refining furnace, unburned magnesia-calcium bricks were developed. After being used in multiple furnaces of a 90tLF refining furnace (refining rate 100%) in a steel plant, good results have been achieved.

Development of unburned magnesia calcium bricks

(1) Raw materials. The raw materials used in the development of unburned magnesia-calcium bricks are synthetic magnesia-calcium sand and fused magnesia sand produced by Shandong Magnesite Mine. The physical and chemical indicators are: synthetic magnesium calcium sand: MgO+CaO96.70%, SiO₂1.59%, Fe₂O₃0.84%, Al₂O₃0.38%, ignition loss 0.10%, particle volume density 3.30g·cm-³. Fused magnesia: MgO97.24%, CaO1.06%, SiO₂0.25%, Fe₂O₃0.42%, Al₂O₃0.21%, particle volume density 3.50g·cm-³.The binding agent uses liquid anhydrous resin produced by a certain factory. Its main physical and chemical indicators are: solid content >80%, residual carbon content >42%, viscosity 10-15Pa·s, and moisture <0.05%.

(2) Ingredients. Synthetic magnesia-calcium sand is used as granular material, and fused magnesia is used as fine powder. Three-level ingredients are used, and the material proportions are shown in Table 1.

Table 1 Material ratio of unburned magnesia calcium bricks Mass fraction w

Material particle size/mm5~33 ~0.5<0.088Compound additives (extra)
Proportion43 ~4718~2233~373 ~5

(3) Mixing. The mixing equipment adopts planetary forced mixer. Before mixing, fused magnesia powder and various additives are premixed in a premixer. When mixing, first add pellets, dry mix for 2 to 3 minutes, then add about 4% liquid anhydrous resin, wet mix for 8 to 10 minutes, until all pellets are wrapped by the binder (no white material). Then add the premixed powder of fused magnesia powder and additives, mix for 15 to 20 minutes, and then the material can be discharged. The mixed material has good plasticity and can be held in a ball.

(4) Forming. Unfired magnesia calcium bricks are pressed and formed by a 6300 kN friction brick press, and each brick is struck 9 to 10 times. The brick thickness is 118mm and the unit weight is 15.6 kg. After the formed unburned magnesia calcium bricks pass the inspection, they are stacked on a special heat treatment vehicle.

(5) Heat treatment. After the unburned magnesia calcium bricks are heat treated, the binder is solidified, which further improves the mechanical strength of the unburned magnesia calcium bricks. The heat treatment equipment adopts a tunnel-type heat treatment kiln that uses generator gas as fuel and is heated indirectly. The maximum heat treatment temperature of unburned magnesia calcium bricks is 250 ℃, the heat preservation time in the high temperature stage is not less than 8 hours, and the total heat treatment time is not less than 24 hours.

(6) Physical and chemical indicators of unburned magnesia-calcium bricks. The physical and chemical indicators of the unburned magnesia calcium brick after heat treatment are: MgO+CaO89.23%, SiO₂1.71%, Fe₂O₃0.88%, Al₂O₃0.38%, ignition loss 7.14%, volume density 2.97g/cm-3, The apparent porosity is 2.19%, and the normal temperature compressive strength is 42.25 MPa.

Use of unburned magnesia calcium bricks

The developed unburned magnesia-calcium bricks are used in a 90tLF refining furnace (refining ratio 100%) of a steel plant.

Building a furnace

The lining material of the 90tLF refining furnace is composed of: hard refractory fiberboard with a 20mm thick insulation layer. The permanent layer is high aluminum castable. The working layer of the furnace bottom is MT-10 magnesia carbon brick, and two breathable bricks and one nozzle brick are built at the bottom of the furnace. The newly developed unburned magnesium calcium bricks are laid in the non-slag line part of the working layer of the furnace wall, with a total of 19 layers built from the bottom of the furnace upward. The slag line is constructed with MT-18 magnesia carbon bricks; the furnace mouth above the slag line bricks is cast with high-aluminum castables. All working layers are constructed using dry masonry methods. The uneven areas between the lining brick layers and the gaps between the working layer and the permanent layer are leveled and filled with 0 to 0.5mm magnesia powder pads.

Oven

The newly built refining furnace must be fully baked with gas before use to remove moisture from the lining and preheat the lining to the operating temperature. When baking, place the refining furnace in an upright position and bake it over low fire for about 12 hours. When no moisture is discharged from the exhaust hole of the furnace shell, place the furnace body horizontally and bake over high fire. The total baking time is no less than 24 hours. When put into use, the ladle temperature shall not be lower than 1100℃.

Usage

During the use of the unburned magnesia-calcium brick furnace lining, the steel types refined by the LF furnace include cord steel, alloy structural steel and spring steel. The refining time for each heat of cord steel is about 90 minutes, and the refining time for other steel types is 30 to 40 minutes.

All the refined molten steel is cast into billets on a six-strand billet continuous caster.

In the early stage of use, the unburned magnesia calcium brick furnace lining has no cracks, no structural peeling, the working surface is smooth, the corrosion is uniform, and it shows good performance. As the number of uses increases, the corrosion of the furnace bottom ventilation bricks and slag line bricks becomes more and more serious. When the corrosion reaches the point where they cannot work normally, the furnace must be shut down and replaced with new ventilation bricks and slag line bricks. The primary furnace age of each refining furnace at the time of the first shutdown is shown in Table 2.

Table 2 Refining furnace usage age/time

Refining furnace number34678
One furnace age3032193238
Second furnace age2027301626
Total furnace age5059494864

After stopping the furnace, place the furnace upright to allow it to cool down slowly. Through observation, there is a thin layer of slag hanging on the surface of the unburned magnesia-calcium brick furnace lining, which protects the furnace lining. During the cooling process, no shrinkage cracks or structural spalling occurred in the unburned magnesia-calcium brick furnace lining. A 2-3mm wide annular crack occurred between the unburned magnesia-calcium bricks and the slag-line magnesia-carbon bricks, separating the two different materials of furnace lining bricks. This is due to the different thermal expansion properties of the two bricks, which is more conducive to the removal of slag line bricks. During the period of cooling down the furnace body and replacing the breathable bricks and slag line bricks, the unburned magnesia calcium bricks did not suffer from hydration or other damage, and the working surface was intact. After replacing the breathable bricks and slag line bricks, they were baked and put into use for the second time.

After reuse, as the number of uses increases, the degree of corrosion of the furnace lining becomes more and more obvious. In the later period of the furnace service, the unburned magnesia-calcium brick furnace lining showed obvious brick seams, indicating that the slag erosion rate along the brick seams was relatively fast. The six layers of bricks below the slag line are more severely eroded. The reason is that as the furnace lining is eroded, the volume of the refining furnace expands, causing the slag line to move downward to the unburned magnesia-calcium bricks, which accelerates the erosion of the unburned magnesia-calcium brick lining in this area. . There is some structural spalling in the furnace lining, but it is relatively minor. No major cracks occurred. The main reason for the final shutdown was that the ventilation bricks at the bottom of the furnace were severely corroded. After the furnace was dismantled, the length of the remaining unburned magnesia-calcium bricks was measured to be about 135 mm, and only about 27% of the bricks had been eroded (the original bricks were 185 mm long), and they can still be used. The second usage age and total furnace age of each refining furnace are shown in Table 2.

As can be seen from Table 2, the maximum furnace age of the five LF refining furnaces using unburned magnesia-calcium brick linings is 64 times, and the average furnace age is 54 times. It has exceeded the average age of the steel plant’s LF refining furnace by 50 times and achieved good results.

At present, the main factor affecting the age of the furnace is the furnace bottom ventilation brick. The main reason for the shutdown was the severe corrosion of the breathable bricks, not the failure to burn the magnesia-calcium bricks. If the service life of the breathable bricks matches the service life of the unburned magnesia calcium bricks, the furnace life can be greatly increased.

It can also be seen from Table 2 that the furnace ages of some refining furnaces differ greatly. For example, the age difference between furnace 8 and furnace 7 is 16 times. The reason is that the breathable bricks used by the steel plant are produced by multiple manufacturers, with different quality and greatly different service life. Therefore, the age of some refining furnaces is greatly different. It is recommended that the steel plant should use higher quality breathable bricks in the future and produce them from the same manufacturer to maintain the relative stability of the quality of the breathable bricks and make the service life of the breathable bricks match that of unburned magnesium calcium bricks. At the same time, it is necessary to strengthen the gunning repair of the slag line to achieve higher furnace life and better economic benefits. The use of unburned magnesia-calcium brick lining in the LF refining furnace has also achieved good metallurgical effects. The quality indicators of each refined steel type are improved compared with other types of furnace lining materials (magnesia carbon bricks, alumina-magnesia carbon bricks, etc.). More importantly, high-quality cord steel is refined. Because cord steel has very strict requirements on the content and existence form of certain non-metallic inclusions (such as Al₂O₃, etc.) in the steel. Using unburned calcium magnesium furnace lining bricks, the free CaO in the bricks can absorb non-metallic inclusions in the molten steel, making the molten steel reach a higher purity. Furnace lining bricks made of other materials do not have the function of adsorbing non-metallic inclusions in molten steel, so it is difficult to refine high-quality cord steel.

Conclusion

The developed unburned magnesia-calcium bricks have been used in a 90tLF refining furnace in a steel plant, proving that they have good performance and meet the needs of refining cord steel and other special steels. At present, the steel plant has used unburned magnesia calcium bricks in batches in the LF refining furnace according to production needs.

In addition to being used in LF refining furnaces, unburned magnesia-calcium bricks can also be used in external refining equipment such as AOD furnaces and VOD furnaces, which have high promotion value. As the production of clean steel in my country continues to increase, the use of unburned magnesia-calcium bricks and other magnesia-calcium refractory materials will continue to expand.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

Our Product have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time

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