A Case Study on the Service-Life Extension of Refractory Materials in Various Sections of a Steelworks' Blast Furnace

01 Improvements to the Tuyere Section
The hearth and bosh of the 3200 m³ blast furnace utilize a total of five segments of smooth-surface, low-chromium cast iron cooling staves. Specifically, the fifth segment—situated at the uppermost level of the hearth—features openings at its mid-section for the installation of tuyeres. The first components of the blast furnace lining to show signs of erosion were the brick lining and cooling staves within the tuyere section. In October 2011, following a furnace shutdown necessitated by market conditions, an inspection conducted after lowering the burden level revealed severe erosion of the furnace lining in the fifth segment (the tuyere section). This erosion extended from the upper rim of the main tuyere casing down to the lower edge of the copper cooling staves; aside from a residual layer of composite tuyere bricks, all other corundum bricks and sprayed refractory materials had completely spalled off. The cast iron body of the cooling staves in this specific area suffered severe thermal damage; the cast iron on the hot face had been largely eroded away, exposing at least one or two of the three water pipes embedded in each stave. Furthermore, the copper cooling staves in the overlying belly section of the furnace were left unsupported and suspended, resulting in the formation of a deep, concave cavity.
Based on on-site inspections and subsequent analysis, it was concluded that the primary causes of this erosion were poor coke quality, high pressure differentials, and the insufficient cooling capacity of the cast iron cooling staves in this specific section. For an extended period, the blast furnace charge included a blend of purchased rammed coke and coke produced in-house by 4-meter small-scale coke ovens, resulting in overall poor coke quality. To intensify the smelting process, the pressure differential was maintained at a relatively high level; the daily average pressure differential consistently hovered around 185 kPa, with hourly averages occasionally exceeding 200 kPa. This excessively high pressure differential caused deformation of the raceway zone, which expanded outward and upward. This expansion shifted the high-temperature zone closer to the tuyeres and the furnace walls; given that this section was equipped with cast iron cooling staves—which possess limited cooling capacity—the result was premature thermal burnout of the staves.

At the time, due to the limited timeframe for the shutdown, the cooling staves in Section 5 were not replaced; instead, only the tuyere assembly bricks were relaid, and a protective lining was applied via gunning. Following the furnace restart, and based on the findings of the prior analysis, the quality of the coke was significantly improved: the use of lower-quality stamped coke was discontinued, and a new Dry Quenching System was constructed, raising the proportion of dry-quenched coke from 0% to 80%. Concurrently, the control range for the differential pressure was lowered, reducing the average differential pressure to between 160 kPa and 170 kPa.
In February 2017, the burden level was lowered once again to allow for an internal inspection. After five years of operation, the cooling staves in this specific section were found to be completely covered by a protective layer of slag; notably, no further erosion had occurred, and the relaid assembly bricks remained in excellent condition. These observations demonstrate that improving coke quality and reducing differential pressure effectively mitigated erosion in this critical area. Of course, for blast furnaces where improving coke quality remains a challenge, it is recommended to modify the structural design of the cooling staves in the tuyere zone. Specifically, the tuyere-zone cooling staves could be converted to a two-stage configuration: the lower section would retain the existing cast-iron cooling staves, while the upper section—spanning from above the tuyere centerline up to the copper cooling staves in the belly zone—would utilize a transitional arrangement of two to three layers of cooling plates or copper cooling staves. This approach can effectively and permanently resolve issues related to premature burnout in this area caused by insufficient cooling.

02 Refurbishment of the Belly-to-Lower Stack Section
In the 3200 m³ blast furnace, four tiers of copper cooling staves were installed across the sections subjected to the highest thermal loads—specifically, the 6th-tier belly, the 7th-tier bosh, and the 8th through 9th tiers of the lower stack. Each tier consisted of 48 staves, comprising a total of 192 water-cooling channels. Following the furnace's commissioning, these copper cooling staves performed reliably; however, after the first water channel leak occurred in September 2015, the rate of deterioration accelerated rapidly. By February 2017—when the furnace was shut down for minor repairs—the number of damaged water channels had reached 101. This figure represented 53% of the total circumferential channels, with the damage concentrated exclusively within the 7th-tier bosh section. The failure mode observed in these copper cooling staves differed significantly from the gradual, progressive deterioration typical of conventional cast-iron cooling staves. For approximately six years following commissioning, not a single channel leaked; yet, within a span of just over one year after the initial failure, widespread and concentrated damage emerged—particularly during the three months immediately preceding the shutdown, when leaks occurred at an average rate of one channel every 2.5 days. Furthermore, these damaged staves proved extremely difficult to maintain; once the water supply was cut off, they rapidly burned away and detached, leaving behind only the outer furnace shell plate—a situation that posed a severe risk to both production operations and safety.

Extensive investigations were conducted regarding the damage to the copper cooling staves. Observations of cylindrical copper core samples—extracted via drilling during a furnace shutdown using a columnar micro-cooling device—revealed that the inner surfaces of all damaged cooling staves in the furnace bosh zone were smooth and flat, with their original dovetail grooves completely worn away. The thickness of these cooling staves had diminished from an original 110 mm to a range of 50 mm to 85 mm. Based on these findings, it was hypothesized that the failure of these cooling staves was caused by the continuous abrasion and thinning of their hot-face surfaces; specifically, when the thickness dropped below 59 mm, the internal cooling channels would become exposed, resulting in water leakage. Furthermore, even those cooling staves that had not yet begun to leak had already thinned significantly and were susceptible to failure at any moment. This explains the observed phenomenon wherein the staves would remain intact for extended periods following the furnace's startup, yet once damage began, it would rapidly escalate into widespread failure across a large area within a very short timeframe. Observations made in February 2017—following a blast furnace shutdown and the subsequent lowering of the stockline—corroborated these earlier hypotheses. Specifically, the entire circumferential band spanning from the lower two-thirds of the Level 7 copper cooling staves in the furnace bosh to the upper one-third of the Level 6 staves in the furnace belly exhibited severe wear, confirming that all instances of water leakage were a direct consequence of this material thinning.

Based on the analysis of the causes of damage, and prior to the furnace shutdown for the replacement of cooling staves, the blast furnace primarily implemented measures aimed at reducing wear. First, efforts focused on stabilizing the peripheral gas flow to minimize abrasion. The 3200 m³ blast furnace suffered from insufficient central gas flow; furthermore, the upper burden distribution regime had long utilized a "forced-edge-packing" strategy, resulting in an excessively heavy peripheral burden. This led to instability in the slag crust within the belly and bosh zones; consequently, the unprotected copper cooling staves frequently came into direct contact with the burden and furnace gas, resulting in wear. To address this, the furnace burden regime was adjusted: the previous "platform-plus-funnel" edge-packing method was replaced with an "O98764332C110293837261561 double-coke-layer" regime. This adjustment simultaneously fostered central gas flow while stabilizing the periphery. Following this adjustment, gas flow became unimpeded, the periphery stabilized, slag crust stability improved, and instances of "red spots" (localized overheating) on ​​the furnace shell were significantly reduced. Second, cylindrical "micro-coolers" were installed by drilling holes at the locations of damaged cooling staves; each cooling stave was fitted with 12 to 16 micro-coolers. Subsequently, approximately 200 kg of hard-setting grout was injected through the grouting ports on these micro-coolers. These cylindrical coolers, with a diameter of approximately 100 mm, could be inserted directly into the furnace shell via drilled holes, facilitating easy installation. An insertion depth slightly exceeding the designed thickness of the furnace wall proved sufficient to effectively stabilize the hard-setting material and promote slag adhesion. Upon inspecting the surfaces of the old copper cooling staves removed during the furnace shutdown, it was confirmed that the micro-coolers had indeed performed excellently in promoting slag adhesion and protecting the staves: the surfaces of the staves fitted with micro-coolers were covered by a stable layer of slag crust, whereas the surfaces of the staves without micro-coolers remained smooth and slag-free, exhibiting severe wear. Similarly, analogous conditions were observed on sections of the furnace shell that had detached from the copper cooling staves.
In February 2017, the 3200 m³ blast furnace underwent a scheduled shutdown for maintenance and the replacement of cooling staves; during this period, modifications were implemented on the new copper cooling staves. All cooling staves within segments 6 through 8—which had previously featured dovetail grooves—were converted into "brick-embedded" copper cooling staves. Furthermore, the surface profile of the embedded bricks was modified from a smooth finish to a dovetail-groove configuration, thereby further enhancing the capacity for slag adhesion. These modifications yielded highly positive results. It has been 15 months since the blast furnace was blown in. The temperature of the copper cooling staves has remained highly stable, with an average water temperature differential of only 1.5°C. Compared to the previous generation of blast furnaces utilizing older-style copper cooling staves—which exhibited a long-term water temperature differential of approximately 3.2°C following blow-in—this represents a significant reduction in the temperature differential. This indicates that the slag layer is highly stable, effectively protecting the copper cooling staves and extending their service life.

03 Hearth and Bottom Refurbishment
The 3200 m³ blast furnace employs a water-cooled bottom structure, overlaid with five layers of carbon bricks and two layers of ceramic cups, with each layer having a thickness of 400 mm. The hearth is constructed using circumferentially laid, hot-pressed small-block carbon bricks from UCAR (USA), with small-block ceramic cups installed on the inner side. Three layers of domestically produced large-block microporous carbon bricks are circumferentially laid in the upper section of the hearth. Since commissioning, the hearth bottom has operated stably; only portions of the ceramic cups and the uppermost layer of ultra-microporous carbon bricks have detached, while the remaining four layers of carbon bricks remain intact. In terms of erosion rate, the hearth bottom is eroding relatively slowly and is unlikely to become a limiting factor affecting the blast furnace's service life. The critical limiting factor for the blast furnace's bottom and hearth structure lies in the hearth side walls.

Currently, the erosion profile of the 3200 m³ blast furnace's bottom and hearth resembles an "elephant's foot"; however, the zone of most severe erosion has shifted upward relative to the typical "elephant's foot" pattern, now centering on the middle section of the dead-man iron layer. The dead-man iron layer in the 3200 m³ blast furnace has a depth of 2.75 m, and the area of ​​most severe erosion is located precisely 1.3 m below the centerline of the No. 2 taphole. Based on calculations, the carbon bricks—which originally had a thickness of 1258 mm—now measure approximately 630 mm. Regarding the progression of erosion, very little erosion occurred during the initial four years of operation, a phenomenon likely attributable, at least in part, to the protective effect of the ceramic cups. Starting in August 2013, periodic erosion events began to occur; a total of six such events have been recorded, specifically during the periods of July–October 2013, January–April 2014, September–November 2015, February–March 2016, August 2017, and April 2018. Each erosion episode was relatively brief in duration—ranging from as little as a dozen days to approximately three months—with the most recent instances lasting around one month each. For the majority of the remaining time, the carbon bricks in the blast furnace's bottom and hearth remained covered by a thick layer of solidified iron, resulting in virtually no erosion. Furthermore, the eroded area during each event was relatively small and localized in scope—typically affecting only two or three thermocouple points beneath a specific taphole, or manifesting as a circumferential band of erosion situated 1.3 m below a taphole. During erosion, the thermocouple temperature rises rapidly; subsequently, the temperature at one or two specific points breaches historical maximums, signaling the complete detachment of the solidified iron layer, after which the carbon bricks begin to erode. In response to such erosion, blast furnaces typically implement measures such as raising the furnace temperature, increasing slag basicity, or even curtailing production; however, the efficacy of these measures is generally quite limited, and the erosion process ultimately tends to cease spontaneously. This erosion phenomenon is characterized by its rapid progression, localized nature, and inherent difficulty in control.

To address the characteristics of hearth erosion, daily operations focus primarily on maintaining hearth uniformity and activity. This involves the proactive use of blast air—while minimizing oxygen enrichment—to increase blast velocity and kinetic energy, as well as establishing control standards for harmful elements to prevent them from exceeding permissible limits. Concurrently, hearth monitoring and management have been strengthened; through measures such as temperature surveillance and calculations of the remaining hearth lining thickness, the extent of lining erosion is tracked in real time. During scheduled blast interruptions, additional monitoring thermocouples were installed in areas beneath the tapholes that exhibited severe erosion. A total of 16 new thermocouples were added, increasing the thermocouple density in this region from one per 3 m² to one per 1.5 m². In February 2017, during a minor maintenance outage, the cooling systems for 18 cooling staves—specifically the nine staves located to the sides and beneath each of the severely eroded Tapholes #2 and #3—were upgraded from soft-water cooling to high-pressure water cooling. This upgrade increased the cooling water flow velocity from 2.1 m/s to over 7.0 m/s, thereby significantly enhancing cooling intensity. Should a rapid rise in the temperature of the hearth sidewalls occur, countermeasures are implemented—based on the estimated thickness of the solidified iron layer within the furnace—including raising the furnace temperature, prioritizing the production of prime-grade iron, controlling metallurgical intensity, and appropriately increasing the volume of taphole clay injected. In the event of further hearth erosion in the future, additional protective measures—such as the use of titanium-bearing ores to safeguard the furnace lining—may be considered to ensure the continued safe operation of the blast furnace.

04 Conclusion
(1) With the widespread adoption of copper cooling walls, their inherent drawback—namely, their susceptibility to wear following the detachment of the protective slag layer—has gradually become apparent. By implementing novel brick-inlay designs and optimizing in-furnace operational practices, the stability of the slag layer can be enhanced, thereby effectively extending the service life of the copper cooling walls.

(2) The design of the refractory lining and cooling walls spanning the zone from the centerline of the blast furnace tuyeres down to the lower edge of the belly has long presented a significant challenge in blast furnace engineering. Through continuous refinement, the overall thickness of this section has been reduced while its cooling capacity has been enhanced; nevertheless, during prolonged periods when the quality of raw materials—such as coke—is substandard, this zone remains susceptible to premature erosion. Consequently, it is advisable to consider utilizing copper cooling walls or copper cooling plates to serve as a transitional lining in this specific region.

(3) As the dead-iron layer within the hearth deepens over time, the carbon bricks lining the side walls in the central section of the hearth are frequently the first to suffer erosion during the mid-stage of a furnace campaign. This erosion compromises the safety of blast furnace operations and even poses a risk of burn-through, potentially necessitating a premature major overhaul of the furnace. Therefore, during the design phase, it is imperative to reinforce the erosion resistance of the side walls in this critical zone and to enhance their cooling intensity.