Analysis of the Causes of Burn-Through in Blast Furnace Hearths and Bottoms, and Proposed Improvement Measures

1. Inherent Design Flaws
1.1 Insufficient Hearth Cooling Intensity: A Mismatch with the Thermal Conductivity of Carbon Bricks and the Level of Smelting Intensity
A 3,200 m³ blast furnace at a certain ironworks employs a "ceramic cup" structure. The second section of the hearth utilizes cast iron cooling staves; the cast iron has a thermal conductivity of 34 W/(m·K), and the cooling water flow rate ranges from 960 to 1,248 m³/h. The refractory material forming the walls of the ceramic cup possesses the lowest thermal conductivity, ranging from 4 to 6 W/(m·K). The two types of adjacent, small-block, molded carbon bricks, however, exhibit very high thermal conductivity: NMD carbon bricks range from 40 to 80 W/(m·K), while NMA carbon bricks are rated at 20 W/(m·K). The carbon ramming layer has a thickness of 60 mm and a thermal conductivity of 6 to 10 W/(m·K). In a hearth structure of this configuration, should the ceramic cup erode away or cracks develop in its walls, the molten iron comes into direct contact with the carbon bricks; consequently, the carbon ramming layer—with its relatively low thermal conductivity—and the cooling staves—with their insufficient cooling capacity—effectively act as "thermal resistance layers." This phenomenon occurs because the thermal conductivity of the NMD carbon bricks is twice that of the cast iron staves; furthermore, given the relatively low water flow rate through the cooling staves, the radial heat transfer within the hearth becomes impeded. As the temperature of the hot face of the carbon bricks equilibrates with that of the molten iron, it becomes difficult to form a protective layer of slag and iron; moreover, since neither the NMD nor the NMA carbon bricks are of the microporous variety, they are highly susceptible to erosion by the molten iron. This vulnerability is particularly pronounced in the NMD carbon bricks, whose primary constituent is electrode graphite—a material that is readily dissolved into carbon-unsaturated iron-carbon melts. Graphite-rich carbon bricks struggle to retain a protective layer of slag and iron, rendering them ill-equipped to withstand the penetrative erosion caused by molten iron; this significantly increases the likelihood of a "burn-through" failure occurring at specific locations within the hearth.

Based on the above analysis, the carbon bricks selected for the hearth must be chosen with careful consideration given to both their thermal conductivity and their microporous structure. Furthermore, the heat dissipation capacity of the cooling staves—specifically their thermal conductivity and the volume of cooling water flow—must be enhanced to ensure proper thermal matching with the carbon bricks. The thermal conductivity of the carbon ramming layer should be comparable to that of the carbon bricks to prevent it from acting as a "thermal resistance layer." For newly constructed blast furnaces, the hearth structure should incorporate carbon bricks featuring a microporous texture and excellent resistance to erosion by molten iron; moreover, the design must ensure a gradual increase in heat transfer efficiency from the hot face of the carbon bricks (the surface in contact with the molten iron) to the water-cooling pipes in the hearth bottom, thereby preventing the formation of thermal resistance layers and facilitating the efficient dissipation of heat. Post-campaign inspections of blast furnaces possessing these characteristics have revealed that, following a full furnace campaign, the refractory materials in the hearth bottom remain virtually un-eroded.

In recent years, the smelting intensity (or utilization coefficient) of blast furnaces in China has increased significantly compared to the 1980s; consequently, the thermal load per unit area and per unit time on the blast furnace walls has also risen sharply. Therefore, it is no longer appropriate to adhere to outdated operational philosophies characterized by low cooling water flow rates or reliance on water-spray cooling for the furnace shell. Cooling staves featuring low water flow rates, small pipe diameters, and limited cooling surface areas are no longer suitable for modern blast furnaces operating under high-intensity smelting conditions. Based on analyses regarding heat transfer, system pressure loss, and the prevention of vapor lock, the water velocity within the cooling staves should be maintained at or above 2.0 m/s. Furthermore, the ratio of the cross-sectional area of ​​the cooling water pipes to the total surface area of ​​the cooling stave should exceed 1.0. For the hearth section, the temperature rise of the cooling water across each individual cooling stave segment must be strictly controlled to within 0.5°C. Operational experience from various blast furnaces in my country indicates that for hearths constructed with large-block carbon bricks (possessing a thermal conductivity of 20–25 W/(m·K)) and utilizing cooling staves, the heat flux intensity should be maintained below 10,000 W/m². A "red alert" warning should be triggered once the heat flux intensity reaches 12,000 W/m²; should the intensity further escalate to between 18,000 and 21,000 W/m², a hearth burn-through becomes virtually inevitable. A case in point is Blast Furnace No. 14 at the Gary Works in the United States; its hearth, which utilized a jacket-type cooling system, suffered a burn-through incident due to insufficient cooling capacity. The heat flux intensity detected immediately prior to the burn-through was approximately 12,880 W/m².

1.2 Inadequate Monitoring Capabilities

The limited number of temperature measurement points within the hearth brick lining—coupled with a lack of monitoring capabilities for parameters such as cooling-wall water temperature differentials, water flow rates, and heat flux intensity—often prevents the timely detection of anomalies in the hearth and furnace bottom, thereby hindering the implementation of appropriate remedial measures. Consequently, this can sometimes lead to sudden accidents involving blast furnace burn-throughs, or even result in the escalation of such incidents. In the cases presented in this chapter, two blast furnaces—which possessed comparatively superior monitoring capabilities—did not suffer burn-throughs; instead, they experienced only minor iron seepage. Because warning signs were detected prior to the incidents, remedial actions were taken promptly, thereby preventing the accidents from escalating further.

1.3 Improper Selection of Carbon Bricks
At the Yangchun Ironmaking Plant, merely 15 days after the startup of its 1250 m³ blast furnace, the temperature of the carbon bricks in the hearth's annular wall rose above 600°C. After eight months of operation, iron penetration reached over 70 tons; fortunately, effective management measures were implemented, preventing a catastrophic burn-through.
Upon cutting open the cooling staves for inspection, gaps of 30 to 70 mm were observed between adjacent carbon bricks, indicating that the bricks had deformed and contracted under thermal stress. The excessive size of these gaps was likely attributable to insufficient firing temperatures during the manufacture of the carbon bricks used, or possibly to poor quality during the masonry construction.
The aforementioned example underscores the critical importance of selecting appropriate carbon bricks for the hearth and furnace bottom. The following points should be carefully considered when selecting carbon bricks:
(1) For areas in direct contact with molten iron—or areas expected to come into contact with molten iron toward the end of a furnace campaign—graphitic or semi-graphitic carbon bricks should be avoided. Carbon bricks with a high graphite content are prone to carburization reactions, which leads to rapid erosive wear of the bricks.
(2) Graphite bricks exhibit poor affinity for slag and molten iron, making it difficult for a protective slag layer to adhere to their surfaces; however, it is highly desirable to maintain a protective slag lining in the hearth zone to shield the furnace refractory. In blast furnaces abroad, graphitic carbon bricks used in the lower stack are often interspersed with silicon carbide bricks during masonry construction, as the latter facilitates the adhesion of a protective slag layer. As another illustrative example, crucibles used in all slag viscosity tests are typically fabricated from graphite material, specifically to exploit graphite's inherent resistance to slag adhesion.
(3) When evaluating carbon bricks, one must prioritize not only their thermal conductivity but, more importantly, their microporosity characteristics and resistance to erosion by molten iron. Some carbon brick manufacturers, in an effort to achieve high thermal conductivity, incorporate excessive amounts of graphite during production; this practice, however, compromises the bricks' resistance to molten iron erosion, thereby jeopardizing the operational safety of the furnace hearth.

1.4 Improper Taphole Arrangement
In some blast furnaces, the two iron tapholes are positioned at a 90-degree angle to each other. Aside from the tendency for uneven flow patterns to develop during furnace operation, this arrangement also exacerbates the erosive circulation of molten iron within the hearth, thereby compromising the safety of the hearth structure. In other blast furnaces, burn-throughs have occurred in the taphole zone; this is often attributed to significant disparities in slag runner lengths, where—during critical phases such as furnace start-up, shut-down, wind-off periods, or accident recovery operations—molten iron is frequently tapped through the taphole corresponding to the shorter slag runner, thereby accelerating hearth erosion in that specific taphole zone.
2 Deficiencies in Cooling Wall Manufacturing and Installation
The quality of cooling wall manufacturing and installation is paramount to the longevity of the blast furnace hearth; should a leak occur in the cooling wall, it could precipitate a major industrial accident. The manufacturing and installation of cooling walls require careful attention to the following points:
(1) Prior to the masonry installation of the hearth refractory lining, the cooling system must undergo water circulation and leak testing; bricklaying should commence only after the pressure testing has been successfully completed.
(2) Currently, drilled-copper cooling walls—fabricated via a drilling process—are widely utilized. However, this manufacturing method necessitates the welding and plugging of numerous inlet/outlet pipes and process holes. Should these welds fail and leak during installation or subsequent furnace operation, the resulting water ingress can cause the oxidation and deterioration of the carbon bricks, thereby creating a high risk of accidents. In recent years, however, certain blast furnaces in Japan and within China have adopted cast-copper cooling walls for their hearths, effectively eliminating the aforementioned drawbacks.
(3) With the advancements in the machining precision of carbon bricks—whether utilizing large-block or small-block configurations—the masonry joint gaps should be strictly controlled to within 0.5 mm; it is recommended that the standards for masonry installation be appropriately elevated.
(4) The design of the cooling and gas-channeling structures within the taphole zone should be improved, as should the selection of packing materials situated between the hearth cooling walls and the furnace shell. These measures aim to prevent molten iron splashing during tapping operations and to ensure that the tapholes maintain sufficient depth.
(5) The carbon ramming mix used to fill the interface between the carbon bricks and the cooling walls should possess a thermal conductivity comparable to that of the carbon bricks themselves, ideally reaching a range of 15 to 20 W/m².

3. Deficiencies in Operation and Maintenance Following Commissioning
3.1 Strictly Control the Influx of Harmful Elements into the Furnace
Extensive research has been conducted on the detrimental effects of harmful elements—such as potassium, sodium, lead, and zinc—on the furnace structure; these findings have also been corroborated by investigations into damage sustained by various blast furnaces. According to my country's blast furnace process design standards, the content of these elements in the furnace charge must be restricted to K + Na < 3.0 kg/t and Zn < 0.15 kg/t. These harmful elements circulate and accumulate within the furnace; not only do they disrupt the stable and smooth operation of the blast furnace and reduce coke strength, but they also react with refractory materials to form compounds that undergo significant volume expansion—in some cases by as much as 50%—leading to the rapid deterioration of the hearth lining bricks. Therefore, the influx of the aforementioned harmful elements must be strictly controlled, and measures for their periodic removal should be implemented. Particular attention should be paid to this issue by enterprises that lack adequate inspection capabilities for incoming raw materials and fuels.
3.2 Ensure Smooth Furnace Operation and Prevent Leaks in Cooling Equipment
Water leaks from tuyere coolers or upper-level cooling staves can seep down along the furnace shell into the hearth, causing the carbon bricks to oxidize and disintegrate into powder. This constitutes a major cause of damage to the carbon lining in the hearth. Upon detecting a leak in any cooling component, immediate remedial action must be taken to ensure the leakage ceases. Some blast furnace operators, in an effort to prioritize production output, fail to replace damaged tuyeres promptly; instead, they allow multiple damaged units to accumulate before replacing them all at once. Such a practice is ultimately counterproductive and results in a net loss.
3.3 Maintain Economically Optimal Smelting Intensity and Utilization Coefficients
For any given blast furnace operating under specific smelting conditions, there exists an optimal smelting intensity and an optimal fuel ratio. If one blindly attempts to increase smelting intensity without regard for these specific conditions, the fuel ratio will inevitably rise. Operating a blast furnace in this manner not only prevents the maximization of economic benefits but also inflicts significant structural damage upon the furnace body. Analyses of blast furnaces that have achieved service lifetimes of 15 to 25 years reveal that their average utilization coefficient over a single furnace campaign typically does not exceed 2.3 V (m³/d). Such operations are characterized by stable production and low energy consumption—thereby meeting the requirements for low-carbon smelting—and result in lower overall operating costs.
3.4 Utilizing Vanadium-Titanium Ore for Furnace Protection
When the temperature of the carbon lining bricks rises to critical levels, the primary remedial measure typically employed is the addition of vanadium-titanium ore to protect the furnace structure; this method has proven to be highly effective in achieving the desired protective results. However, two points are worth noting: First, the addition rate of vanadium-titanium ore should typically be controlled to maintain a titanium content of at least 0.10% in the hot metal, while the silicon content should be kept slightly higher—preferably above 0.5%. Second, it is recommended to take preventive measures in advance; specifically, furnace protection using vanadium-titanium ore—spanning a period of approximately one week—should commence about six months after the furnace is blown in, and subsequently be carried out once annually.
3.5 Enhancing the Technical Proficiency of Hearth Grouting

In recent years, when high temperatures have been detected in the carbon bricks of blast furnaces, some operators have adopted a practice of drilling holes in the furnace shell (specifically in the gaps between adjacent cooling staves) to inject anhydrous carbonaceous slurry into the space between the carbon bricks and the cooling staves. This procedure serves to seal gaps and eliminate localized thermal resistance layers, yielding a certain degree of effectiveness. This method is particularly well-suited for blast furnaces that have been in operation for a relatively short period, exhibit substandard construction quality, possess a non-compacted ramming layer, or utilize ramming materials characterized by high volatile content and significant shrinkage upon heating. However, this method also presents drawbacks: if the injection technique is improper, the injection pressure is excessive, or the slurry material is of poor quality, the procedure may inadvertently crush the already-thinned brick lining. Furthermore, if high-volatile slurry is forced into the furnace interior and comes into contact with the molten iron, the resulting volumetric expansion could trigger a "furnace blast"—a sudden, violent eruption within the furnace. In 2010 alone, three domestic blast furnaces experienced such blasts during slurry injection operations; these incidents resulted in employee injuries and subsequently triggered furnace hearth burn-throughs and molten iron breakouts. Such occurrences warrant the utmost attention and concern. Consequently, the application of slurry injection treatments should be approached with extreme caution, accompanied by concerted efforts to enhance the technical proficiency of the injection process. The optimal strategy remains the rigorous control of furnace construction quality, with the ultimate objective of completing an entire furnace campaign without the need for any slurry injection interventions.
3.6 Maintaining Adequate Taphole Depth Over the Long Term
A meticulous analysis of historical instances of furnace hearth burn-throughs reveals that the majority of such failures originate at or in the immediate vicinity of the taphole. This phenomenon is attributable to the exceptionally harsh operating environment within the taphole zone, which subjects the lining to severe erosion. Therefore, a critical objective in blast furnace operations is to consistently maintain an adequate taphole depth over the long term. Insufficient taphole depth allows molten iron to readily infiltrate the joints between the bricks via the taphole channel, thereby accelerating the erosive degradation of the carbon brick lining.