The refractory lining of the ladle has a significant impact on energy consumption during the production process. The design of the refractory lining helps to reduce the heat loss of the molten steel pool and the temperature of the ladle shell. The development of a new generation of non-sintered zero-carbon refractory has two specific advantages:
(i) replacing sintered refractory bricks to reduce the CO2 footprint;
(ii) replacing carbon-containing refractory materials while improving performance. The refractory brick has a unique microstructure, has better thermal properties and similar thermodynamic properties, and can be used for both the working lining and permanent lining of the ladle.
Zero Carbon Refractory Brick Technology
The configuration of ladle refractory is mostly determined by the customer’s operating cycle and the type of steelmaking plant. As shown in Figure 1, the common problems of refractory lining can be distinguished:
Slag line:
carbon-containing basic refractory bricks, such as magnesia carbon bricks and dolomite carbon refractory bricks;
Molten steel bath refractory bricks:
Refractory specifications can vary greatly: basic refractory bricks, similar to slag line refractory bricks, or aluminum-based refractory materials, such as fired high-alumina bricks, tempered bricks (Al2O3.MgO.C—AMC or MgO.Al2O3.C—MAC) or castables;
Permanent lining:
also has a wide spectrum. For slag line bricks, basic refractory bricks are required, and for the ladle slag line lower package refractory bricks and ladle bottom refractory bricks, alumina and alumina silica bricks are usually used, and castables can also be used. All these bricks are fired between 1300 and 1500℃. The purpose of this paper is to provide original solutions for energy-saving products to replace conventional steel bath refractory bricks and permanent lining refractory bricks to minimize ladle shell temperature.
Zero carbon unburned refractory brick technology is based on the development of a new binder system, which surface treats the fine particles of the ingredients to give the raw materials a binder effect. No resin or asphalt is added, achieving zero carbon absorption in the production of ultra-low carbon ULC steel.
This new binder can be used for most refractory materials, such as basic bricks and alumina bricks. Basic bricks include dead burned magnesia (DMB), fused magnesia (FM) and magnesia chromium (MgO-Cr) products. Alumina bricks cover a range of products from 45% to 95% alumina content. The factory is very flexible in manufacturing this binder refractory bricks. All refractory bricks are delivered to customers immediately after tempering. The tempering temperature range is 170~250℃, the same as carbon binder bricks.
Two main goals can be achieved by adopting this new refractory route:
(1) Replace fired refractory products such as ladle bricks, electric furnace (EAF) refractory bricks and most of the permanent lining bricks of the converter (BOF);
(2) Replace carbon-containing refractory brick products, mainly used for ladle refractory bricks below the slag line of the ladle. Some advantages compared to standard technology can be summarized as follows:
- Non-polluting binder.
- Only used for tempering this refractory brick in heat treatment furnaces.
- Faster production preparation time compared to bricks that need to be baked.
- Dimensional tolerances of refractory bricks are not a concern.
- Refractory bricks do not oxidize during ladle preheating or during ladle storage and steel treatment.
- In-situ spinel formation is used to close the brick joints to ensure safe use of ladle bricks and permanent linings.
- No carbon absorption by the molten steel in ultra-low carbon ULC steel production.
- Energy savings: Reduced ladle shell temperature and temperature losses in the ladle steel pool due to lower thermal conductivity compared to conventional carbon bonded bricks.
- Easy recycling due to the absence of carbon or antioxidants in the composition.
- Improved performance in most cases when this technology replaces carbon bricks.
Ladle brick refractory or working lining in contact with molten steel
To meet the quality requirements of ultra-low carbon ULC steel with strong demand for high-strength and high-toughness plates, some steel customers require zero-carbon refractory bricks for ladle contact with molten steel. Ladle refractory bricks are affected by high temperature, erosion by refining slag, long-term contact with molten steel and many refining factors, which require high-quality refractory products.
Typically, the current refractory solution is a fired brick based on aluminum-magnesium composition, which is fired at a temperature of more than 1500°C before delivery to form magnesium-aluminum (MA) spinel (MgO.Al2O3). This new phase has the following unique properties: melting point 2135℃, high mechanical properties at high temperatures, better corrosion resistance to alkaline slag than alumina aggregates, low thermal expansion value, thermal shock resistance similar to alumina, high vacuum stability, environmentally friendly, and no risk of chromium-containing products. Although natural graphite and C from phenolic resin have been identified as potential sources of C.
The development of unfired aluminum-magnesium bricks brings the benefit of no carbon increase for ultra-low carbon steel production, and also replaces the original carbon-containing refractory bricks for other carbon-containing steel grades in the ladle. Due to the low thermal conductivity of the refractory bricks, the shell temperature of the ladle is reduced, and all customers can benefit from this positive factor. During the operation of the ladle, the spinel MgO.Al2O3 will still react in the joints, expand and close the brick joints, and prevent the infiltration of molten steel and slag, thus having better corrosion resistance and operational safety.
At all temperatures, AM-K has a higher bulk density than MAC bricks due to a higher proportion of raw materials and fewer defects and impurities. The apparent porosity of AM-K is similar to that of MAC, which mainly occurs after firing at 1400°C. The combustion of volatiles and the formation of spinel phase are the main reasons for the increase in MAC porosity. For the C-free product (AM-K), the main factor is the formation of expanded spinel phase. Due to the difference in binder system, AM-K refractory bricks show higher cold compressive strength (CCS) after firing at 1400°C, while MAC bricks have slightly higher CCS after tempering and 1000°C.
In terms of elastic modulus, AM-K showed better performance after exposure above 1400°C due to its higher sintering capacity and lower number of internal defects compared to MAC bricks. This rapid sintering also produced a better effect on the corrosion index due to the formation of a physical barrier on the hot face and the formation of spinel.
AM-K bricks show positive volume expansion at 1000℃ and 1400℃. During the ladle production operation, the brick joint closure starts earlier than MAC products, preventing the penetration of liquid metal. In addition, at 1400℃, the expansion value of AM-K is lower than that of MAC products, so the stress generated in the refractory bricks in contact with the ladle steel liquid is smaller.
Use Cases
Customer A – 350 ton ladle
The figure shows the average performance results after the introduction of this novel zero-carbon refractory brick concept over the operating cycle of a smelting shop. The average performance of ladles containing AM-K linings increased by 23% compared to MAC linings. Significant remaining thickness was also noted during the removal of the refractory work lining off the ladle. Over the years, the average life of MAC’s ladles has been approximately 96 heats, with a remaining safety thickness of 40mm. AM-K refractory bricks reach 130 furnaces and the remaining thickness is 70mm.
One of the most significant advantages of AM-K lining is the reduction of temperature loss in the molten steel pool of the ladle. Temperature detection data of more than 220 pieces of steel were collected when the ladle arrived at the refining furnace and started temperature detection before leaving the refining furnace. Comparison of the working linings built with two refractory materials, the results are shown in the figure. The temperature loss of the molten steel bath was reduced by 8%, which reduced the raw materials consumed by the customer to maintain the required temperature and also had a significant impact on energy savings.
The image below shows an example of a visual inspection of an AM-K lining at the end of a cycle of ladle refractory. No defects were found during the entire cycle.
Customer B – 320 ton ladle
The evaluation was performed by directly comparing the MAC and AM-K linings on the same ladle to ensure that both products had the same operating conditions throughout the production process. Figure 5 shows the refractory brickwork below the slag line. The zero carbon refractory brick lining is easily identifiable. The figure shows the two products after 180 heats, with the AM-K having better coating and thermal insulation protection.
During the field comparison, the ladle shell temperature was monitored using an infrared camera to measure the benefits of using carbon-free refractory bricks, as shown in Figure 7. A total of 49 measurements were taken, and the average ladle shell temperature was reduced by 28°C (82.4°F), which also directly saved energy. Figure 8 is a laser scan of the ladle shell at high temperature at 180 furnace times. The minimum remaining refractory brick thickness for the two products is as follows: MAC refractory brick is 47mm, while AM-K refractory brick is 87mm.
Ladle permanent lining or working lining in contact with molten steel
The safety lining of the ladle is designed to withstand a small amount of heat in the event of a failure of the working lining. As a backup lining, a lower refractory grade is used. The production of these refractory bricks requires high-temperature firing, ranging from 1300°C to 1500°C, and the carbon dioxide emissions per ton of refractory bricks fired are about 600 kg. Some customers use castables as safety linings, but for most customers, refractory bricks are still the first choice. The general rule is that the slag line part of the ladle is to use basic high-quality slag line bricks, and the refractory bricks below the slag line bricks and the refractory bricks at the bottom of the ladle are 60-85% alumina refractory bricks and 40-45% alumina-silica refractory bricks. The new unfired zero carbon technology binder can be used to replace sintered bricks for all grades of permanent lining.
Table 2 gives the physical and mechanical properties of conventional fired 65% alumina bricks (referred to as 65-F) and zero carbon unfired 65% alumina bricks (referred to as 65-K) as permanent linings for ladle. The chemical analysis of both refractory bricks is very similar, as the aim is to maintain the same fired product specification characteristics, but replace the fired refractory brick with a tempered refractory brick product.
The transport and transfer characteristics of 65-K refractory bricks are very similar to those of fired bricks. These requirements are necessary for transport and ladle masonry. After firing, the porosity of 65-K is smaller than that of 65-F, which mainly occurs on the hot face of the permanent lining bricks. Field trials were also carried out on the working lining below the slag line of a 285-ton ladle in a different ladle at the same steel plant. During the test, the ladle shell temperature was evaluated by infrared, as shown in Figure 9. Even taking into account the fluctuations in the measurements, it is possible to see clear differences from the material design. The final production process of both products is the same, but due to the changes in the pore shape after heat treatment during the cycle of the ladle (the pore size is smaller, but well distributed in the matrix), the unfired zero-carbon refractory brick technology has lower thermal conductivity.
Summary and Conclusion
The new generation of unfired zero carbon refractories is used to replace fired refractory bricks and carbon-containing refractories, thus reducing the CO2 footprint during manufacturing, being easily recycled, and achieving the required performance using an on-site rapid sintering process at the steel mill customer site.
This new binder can be applied to most basic and alumina refractory products, making the technology flexible to meet plant production and customer needs.
Customer production field trials have shown a significant reduction in ladle shell temperature. This advantage has also been shown for the use of this refractory brick on permanent linings due to the better heat dissipation associated with the porous distribution, highlighting the potential of this zero carbon technology to save energy in practical applications in steel workshops.
