Magnesium-carbon bricks are unfired carbon-based refractory materials made from magnesium oxide (melting point 2800°C), a high-melting-point alkaline oxide, and high-melting-point carbon materials that are resistant to slag infiltration, with various non-oxide additives incorporated. They are bonded using a carbon-based binder.
Magnesium-carbon bricks are unfired carbon-based composite refractory materials made from magnesium oxide (a high-melting-point alkaline oxide with a melting point of 2800°C) and high-melting-point carbon materials that are resistant to slag penetration, with various non-oxide additives incorporated. They are bonded using a carbon-based binder. Magnesium-carbon bricks are primarily used for the linings of converters, AC arc furnaces, and DC arc furnaces, as well as for the slag lines of steel ladles.
As a composite refractory material, magnesia-carbon bricks effectively leverage the strong resistance to slag erosion of magnesia and the high thermal conductivity and low thermal expansion of carbon, thereby compensating for magnesia’s major drawback of poor resistance to spalling.
Features:
1. Excellent high-temperature resistance
2. High resistance to slag
3. Good thermal shock resistance
4. Low high-temperature creep
Uses of Magnesium-Carbon Bricks
Magnesium-carbon bricks are primarily used for the linings of converters, AC arc furnaces, and DC arc furnaces, as well as in areas such as the slag line of ladles. As a composite refractory material, magnesium-carbon bricks effectively leverage the high resistance to slag erosion of magnesia and the high thermal conductivity and low thermal expansion of carbon, thereby compensating for magnesia’s major drawback—its poor resistance to spalling.
In the early days, the refractory materials used for the slag line of steel ladles were high-quality alkaline bricks such as direct-bonded magnesium-chromium bricks and electrically fused, re-bonded magnesium-chromium bricks. Following the successful application of MgO-C bricks in converters, they began to be used in the slag line of refining ladles as well, yielding excellent results. Currently, both China and Japan generally use resin-bonded MgO-C bricks with a carbon content of 12% to 20%, while Europe predominantly employs asphalt-bonded MgO-C bricks, typically containing around 10% carbon.
At Sumitomo Metal Industries’ Kokura Steel Works in Japan, MgO-C bricks with an MgO content of 83% and a carbon content of 14–17% were used in the slag line of the VAD process to replace directly bonded magnesium-chromium bricks, extending the service life of the slag line from 20 runs to 30–32 runs [9]. At the Sendai Steel Works in Japan, MgO-C bricks were used to replace magnesium-chromium bricks in LF refining ladles, extending the service life of the slag line from 20–25 runs to 40 runs, yielding favorable results. Osaka Ceramics Refractories Co., Ltd. investigated the effects of carbon content and types of oxidant inhibitors on the oxidation resistance, slag resistance, and high-temperature flexural strength of MgO-C bricks. The study concluded that MgO-C bricks produced from a mixture of fused magnesia and sintered magnesia, with the addition of 15% flake graphite and a small amount of magnesium-aluminum alloy as oxidant inhibitors, demonstrated excellent performance. when used in a 100-metric-ton LF ladle slag line, the rate of brick damage was reduced by 20–30% compared to MgO-C bricks with 18% carbon content that did not contain antioxidants, with an average erosion rate of 1.2–1.3 mm per furnace.
Since MgO-C bricks were adopted to replace magnesium-chromium bricks in China’s steel refining ladle slag lines, the overall performance has improved significantly. Baosteel Group’s 300-metric-ton ladle slag line has been using MT-14A magnesium-carbon bricks since July 1989, maintaining a slag line service life of over 100 passes; the 150-metric-ton electric furnace ladle slag line uses low-carbon magnesium-carbon bricks to smelt wire rod steel at tapping temperatures of 1,600°C to 1,670°C, achieving notable results.
Erosion of Magnesium-Carbon Bricks and Its Control:
Composite refractory materials are widely used as important materials or critical components in the steelmaking process; however, their service life is often affected by localized erosion that occurs when they are used at the slag-metal interface.
For example, modern ladles typically employ MgO-C bricks (magnesium-carbon bricks) to reinforce the slag line area, while the general walls are lined with a zoned arrangement of oxide-based refractories. However, localized melting damage often occurs at the boundary between these two materials, resulting in reduced service life and eventual scrapping. This localized melting damage occurs at the boundaries between different types of refractories, with significant damage primarily occurring at the slag-metal interface.
Another example is the localized melting damage in blast furnace iron-tapping channels, which occurs not only on the slag surface but also significantly at the slag-molten iron interface. Localized melting occurs because a thin, liquid slag film often exists between the iron tapper material and the metal at the affected sites. The composition of this slag film varies in the vertical direction, creating an interfacial tension gradient. This interfacial tension gradient induces movement of the slag film, which effectively promotes mass transport within the expansion layer and also leads to increased wear of the refractory materials.
For example, Kazuo Mukai and colleagues studied local melting at the (powder-metal) interface of Al₂O₃-C continuous casting submerged nozzles using CaO-Al₂O₃-SiO₂ and CaO-Al₂O₃-CaF₂ melts. Using an X-ray transmission system, they directly observed the conditions near the three-phase boundary (tundish material–slag–metal) during localized melting loss. The results revealed that at the slag–metal interface in the localized melting loss area, the interface repeatedly underwent up-and-down motion as shown in Figure 1, while simultaneously causing localized melting loss. When the slag-metal interface, as shown in Figure 1, is in its descending phase, slag infiltrates the space between the nozzle material and the metal, forming a slag film; oxides generated by the nozzle material dissolve into this film.
If the surface of the nozzle material becomes graphite-enriched, the slag film—which does not adhere well to graphite—will split open and retreat. Subsequently, the metal, which adheres well, causes the nozzle material surface to stick, leading to an upward movement of the slag-metal interface (Figure 1). During this upward movement of the slag-metal interface, the graphite in direct contact with the metal rapidly dissolves into the metal. Conversely, if the surface of the nozzle material becomes an oxide-enriched zone, the slag—which adheres well to oxides—will penetrate from the upper slag phase and reform a slag film. As this process repeats, localized erosion continues. Clearly, the shorter the time it takes for the slag-metal interface to complete one cycle of upward and downward movement, the greater the rate of localized erosion.
In actual continuous casting processes, due to the low carbon concentration in the metal, graphite dissolves rapidly into the metal. The rising phase at the slag-metal interface is very short compared to the falling phase; consequently, this rising phase becomes the primary driving phase for localized melting loss. This finding provides important guidance for material design regarding how to control localized melting loss at the slag-metal interface in continuous casting nozzles and extend their service life.
It is evident that elucidating the mechanism of localized melting lays the foundation for establishing measures to prevent it. Furthermore, from a broader perspective, expanding and deepening research on high-temperature interface phenomena is also crucial for developing strategies to suppress localized melting of refractory materials.
It now appears that different strategies can be adopted to suppress the rate of localized melting in refractory materials depending on the specific application.
1. Material Improvements
In actual continuous casting, based on the observation that the rising phase of the slag-metal interface during localized melt loss in Al₂O₃-C submerged nozzles is significantly shorter than the falling phase, it is believed that the best way to suppress localized melt loss is to eliminate the rising phase of the slag-metal interface while prolonging the falling phase. This principle has guided improvements to nozzle materials. The most compelling example in this regard is the work by Kusunoki Hiroshi et al., who added BN—which has better slag adhesion than graphite—to the Al₂O₃-C submerged nozzle material. As a result, the rising phase was virtually eliminated, and the surface of the nozzle material at the site of localized erosion was frequently covered by slag. This prevented direct contact between the graphite in the nozzle material and the metal, thereby suppressing localized erosion of the Al₂O₃-C submerged nozzle.
2. Increasing the Proportion of Low-Solubility Components in Refractories
It has long been recognized that MgO–Cr₂O₃ refractories exhibit good resistance to erosion by low-alkalinity slag; consequently, they are widely used in molten steel refining furnaces such as VOD and RH furnaces, as well as serving as important candidate materials for refractories in molten reduction furnaces. In these applications, given that an increase in the Cr₂O₃ content of MgO–Cr₂O₃ refractories reduces the depth of localized melting, increasing the Cr₂O₃ content in the mix can reduce the extent of localized melting and extend service life.
3. Development of New Materials
Taking submerged nozzles for continuous casting as an example, given that ZrO₂ dissolves slowly into the slag and can form fine particles suspended in the slag film—thereby increasing the apparent viscosity of the slag film and suppressing its movement—the rate of oxide dissolution in the nozzle material during the descent phase is reduced, and overall localized melting is effectively suppressed. Consequently, a ZrO₂ -C submerged nozzles to replace Al₂O₃-C submerged nozzles, thereby effectively controlling localized erosion.
4. Slag Control
Takaki Yoshitomi et al. suggest that for significant localized erosion occurring at the slag-molten iron interface—such as in the lining of blast furnace iron-tapping channels—the following measures can be taken to control it by suppressing slag film movement:
(1) Suspend carbon in the slag film. When the carbon concentration in the metal approaches that of the saturation region, local melting loss decreases significantly as the carbon concentration increases; therefore, it is believed that local melting loss can be suppressed through carbon-enrichment operations.
(2) Add oxides with low solubility and high melting points to the slag, or introduce phases with low solubility, such as ZrO₂. In other words, controlling the slag can effectively suppress localized melting damage to refractory materials.
5. Modifying Lining Design
To address significant melting loss occurring at the interfaces between different types of refractories, a buffer zone is primarily established. For example, when MgO-Cr₂O₃ bricks (magnesium-chromium bricks) are used between MgO-C lining bricks and high-alumina lining bricks in a ladle, local melting loss does not occur; the problem of significant local melting loss at the interfaces between different types of refractories can also be controlled by altering the position of the joints between dissimilar refractories.
Types of heat treatment kilns commonly used for magnesium-carbon bricks:
The binder in magnesium-carbon bricks is primarily liquid thermosetting phenolic resin. The drying process for magnesium-carbon bricks is essentially the curing process of the thermosetting phenolic resin; therefore, magnesium-carbon bricks must undergo drying treatment after molding.
As the temperature rises, the thermosetting phenolic resin undergoes a highly complex curing reaction. This reaction depends not only on the reaction temperature, the structure of the raw materials, and the reactivity of the ortho- and para-hydroxyl groups on the phenol, but also on the catalyst selected during resin synthesis. The heat curing of thermosetting phenolic resin generally occurs in two stages.
Stage 1:
When the temperature is below 170°C, the primary reactions involve molecular chain elongation, in which hydroxymethyl groups react with active hydrogen atoms on other molecules to release one molecule of water and form a methylene bond. Additionally, hydroxymethyl groups react with hydroxymethyl groups on other molecules to release one molecule of water and form dibenzyl ether. These two processes constitute the main reactions of the first stage of curing, though other reactions also occur to some extent.
Stage Two:
When the temperature exceeds 170°C, the reactions in Stage Two become vigorous and continue up to approximately 200°C. Under high-temperature conditions, the reactions of phenolic resins are highly complex. They primarily involve the further reaction of dibenzyl ether; the dibenzyl ether bond is unstable and readily decomposes into a methylene bond, releasing formaldehyde. Additionally, the phenols and alcohols that did not fully react in Stage One continue to react. A characteristic of the second-stage reaction is that the cured product turns reddish-brown; in some cases, the color may be darker, approaching dark brown. At this stage, the primary products are methylene benzoquinone and its polymers. The amount of solid material generated by the reaction increases steadily, and viscosity rises accordingly. The primary bonding form is the methylene bond, and the curing temperature ranges from 170 to 250°C.
Based on the curing principle of thermosetting phenolic resins, the heat treatment temperature for magnesium-carbon bricks is generally set at 200°C. The holding time is typically 10 hours or more. The reason the heat treatment temperature is set at 200°C is that this temperature ensures complete resin curing without consuming excessive thermal energy; moreover, it can be easily achieved using ordinary fuels and kilns, making it a relatively economical and practical temperature.
To ensure a heat treatment temperature of 200°C, various companies have designed many different types of kilns based on their specific characteristics. Depending on the heating method, kilns can be classified into coal-fired kilns, resistance-heated kilns, and microwave heat treatment kilns. Each of these three types has its own characteristics, which will be introduced one by one below.
Types of Heat Treatment Furnaces Commonly Used with Magnesium-Carbon Bricks
1. Coal-Fired Kiln
The coal-fired kiln was the first type of heat treatment kiln adopted by early manufacturers of magnesia-carbon bricks. In the design of a coal-fired kiln, flue channels are distributed throughout the kiln, as shown in Figure 1. The section that has just been repaired is the damaged flue channel. When coal burns, high-temperature flue gas passes through the flue channels, thereby raising the temperature inside the kiln. Coal-fired kilns were designed with virtually no electrical auxiliary equipment; they featured only one thermometer at each end of the kiln—at the head and tail—to monitor the internal temperature.
For coal-fired kilns to have been adopted as a heat treatment method, they must have possessed certain distinctive characteristics. When magnesium-carbon bricks first appeared, people did not place much emphasis on their appearance; rather, it was their performance that captured attention. Consequently, this very primitive type of kiln came into use—as long as it achieved the desired heat treatment results for magnesium-carbon bricks, many of its shortcomings were overlooked.
2. Resistance Wire Furnace
With the development of magnesium-carbon bricks, demands on heat treatment furnaces grew increasingly stringent. At this point, many shortcomings of coal-fired furnaces began to surface, such as unstable furnace temperatures, unattractive product appearance, and severe soot and smoke marks. Consequently, resistance wire kilns were designed to replace coal-fired kilns. Figure 2 shows the internal arrangement of the resistance wires used for heating in a resistance wire kiln. Compared to coal-fired kilns, resistance wire kilns offer many advantages:
(1) Rapid heating, uniform temperature distribution, and precise temperature control;
(2) A clean production environment with no dust or soot marks;
(3) Fully automated control throughout the process, requiring no human intervention and ensuring high safety;
(4) Since resistance wire kilns heat the entire kiln uniformly, maintaining consistent and stable temperatures, the kiln structure has a long service life and generally requires no maintenance throughout its operation; the only component that needs replacement is the resistance wire.
From the above points, it is clear that the adoption of resistance wire kilns represents the trend in modern production, and their complete replacement of coal-fired kilns is just around the corner—a fact that has already been proven. Of course, coal-fired kilns are not entirely without merit compared to resistance wire kilns; they have one significant advantage that cannot be overlooked: lower heat treatment costs. Based on actual production comparisons, the cost of heat treating one metric ton of magnesia-carbon bricks in a coal-fired kiln is 30–50 yuan lower than that in a resistance wire kiln. At present, aside from a few relatively backward manufacturers and those pursuing low costs who still retain some coal-fired kilns, virtually all other manufacturers have switched to resistance-wire kilns.
3. Microwave Heat Treatment Kilns
Microwave heat treatment kilns are a new type of heat treatment equipment. Although their theoretical basis was established long ago and they have long been used in microwave ovens, they have only recently been put into industrial production. Consequently, the technical knowledge and experience associated with them are not as extensive as those for resistance wire kilns, particularly regarding the heat treatment process for magnesium-carbon bricks, which involves not only the evaporation of moisture but also the curing of phenolic resin.
When it comes to simply drying out moisture, microwave heat treatment kilns have the following characteristics:
(1) Fast drying speed, saving time. Conventional methods—such as steam drying, electric heating drying, and hot-air drying—require at least twenty-odd hours to dehydrate green bodies with a moisture content of around 30% to below 1%, whereas the new microwave heat treatment equipment can complete this in just over twenty minutes; Drying green bodies with a moisture content of about 10% to below 1% takes more than ten hours using conventional methods, whereas the new microwave heat treatment equipment requires only a little over ten minutes; similarly, drying green bodies with a moisture content of about 5% to below 1% takes six to seven hours using conventional methods, but can be completed in just a few minutes with microwave heat treatment equipment.
(2) High energy efficiency and ease of operation. Since microwaves act directly on the material, there is no additional heat loss. In contrast, conventional heat-generating drying equipment suffers from significant heat loss due to environmental factors, particularly during winter. Furthermore, microwave heat treatment equipment heats up quickly and can be turned on as needed, eliminating the need for the heating and cooling cycles required by conventional heat-generating drying equipment, thereby conserving heat. At the same time, the power is adjustable, making operation convenient.
(3) Excellent results; preserves the material’s original color and prevents cracking. Microwave heating does not require a medium for heat transfer; the microwaves act directly on the material, causing it to generate heat on its own. This results in rapid heating and uniform temperature distribution, preventing material cracking.
(4) Assembly-line style operation and a better working environment. Compared to conventional heat treatment methods, microwave heat treatment equipment is simple and straightforward, requires minimal floor space, and has few auxiliary components; production can begin with only the most basic utilities—water and electricity. Compared to conventional methods, this approach typically saves 30% to 50% in electricity consumption, optimizes working conditions, and reduces the required floor space. Furthermore, the equipment is simple to operate—requiring only about three people to run the entire system—and operates with low noise levels, significantly improving the on-site working environment.
Characteristics of Magnesium-Carbon Brick Manufacturing:
There are two methods for manufacturing magnesium-carbon bricks: fired oil-impregnated magnesium-carbon bricks and unfired magnesium-carbon bricks. The former involves a relatively complex manufacturing process and is rarely used; therefore, this section will only briefly describe the characteristics of the unfired magnesium-carbon brick manufacturing process.
Preparation of the slurry. The selection of the critical particle size during formulation is crucial. Finer aggregate particles can reduce open porosity and enhance oxidation resistance. However, smaller aggregate particles increase closed-porosity and reduce bulk density. Additionally, fine-grained MgO aggregate readily reacts with graphite; a particle size of 1 mm is generally considered optimal. When high-pressure molding equipment is available, magnesia particles tend to be finer. Since molding equipment in China typically operates at lower pressures, many manufacturers use particles with diameters of 5 mm or larger to increase the density of the refractory bricks.
The quality and quantity of graphite added to the mixture are critical. Generally speaking, increasing the graphite content in refractory bricks improves their slag resistance and thermal shock stability, but reduces both strength and oxidation resistance. If the carbon content in magnesia-carbon bricks is too low (<10%), a network framework cannot form within the bricks, and the benefits of carbon cannot be effectively realized. Therefore, a carbon content within the range of 10–20% is considered appropriate.
During the mixing process, to ensure that graphite is evenly distributed around the magnesia particles, the order of addition should be: magnesia particles → binder → graphite → fine magnesia powder and additive powder. Since graphite has a high content and low density, and the amount of additives is very small, achieving uniform mixing requires a relatively long time. However, excessive mixing time can cause the graphite and fine powder surrounding the magnesia particles to fall off; therefore, the mixing time must be appropriately controlled.
There are two methods for manufacturing magnesium-carbon bricks: fired oil-impregnated magnesium-carbon bricks and unfired magnesium-carbon bricks. The former involves a relatively complex manufacturing process and is rarely used; therefore, this section will only briefly describe the characteristics of the unfired magnesium-carbon brick manufacturing process.
Preparation of the slurry. The selection of the critical particle size during formulation is crucial. Finer aggregate particles can reduce the open porosity and enhance oxidation resistance. However, smaller aggregate particles increase closed-porosity and reduce bulk density. Additionally, fine-grained MgO aggregate readily reacts with graphite; a particle size of 1 mm is generally considered optimal. When high-pressure molding equipment is available, magnesia particles tend to be finer. Since molding equipment in China typically operates at lower pressures, many manufacturers use particles with diameters of 5 mm or larger to increase the density of the refractory bricks.