R&D And Application Of Rapid Repair Refractories For Blast Furnace Iron Troughs

The blast furnace cast house is a crucial part of the ironmaking process. With the upsizing of blast furnaces and the intensification of smelting, the amount of slag-iron has increased, and the temperature of the slag-iron has risen. As a result, the service environment of the refractories in the working layer of the cast house has become increasingly harsh, and these refractories have gradually become a limiting factor in ironmaking production.

This research focuses on the development of supporting rapid repair materials for the working layer of blast furnace iron trough castables. The results from experiments and industrial applications show that the newly developed iron trough gunning mix and iron trough ramming mix can significantly extend the service life of the cast house working layer, increase iron throughput, and reduce refractory consumption per ton of iron.

In addition, waste Al₂O₃-SiC-C bricks from mixer furnaces and hot metal ladles, as well as waste silicon carbide saggers, are reused as the main raw materials for the iron trough ramming mix. This practice not only reduces production costs for iron and steel enterprises and refractory manufacturers but also minimizes resource waste, yielding both economic and social benefits.

Development of Gunning Mix for Iron Troughs

The development of this gunning mix for iron troughs primarily aims to solve the problem of local rapid hot-state repairs during the operation of large or extra-large blast furnace iron troughs. The temperature range for hot-state repair is 1000–1450°C. This imposes the following performance requirements on the iron trough gunning mix:

(1)It should have good adhesion during gunning construction, without flowing or bursting.

(2Within the temperature range of 1000–1450°C, it should cure and sinter rapidly, develop high strength, and bond quickly and effectively with the trough lining.

(3)It should possess excellent resistance to molten iron scouring, molten slag erosion, and high-temperature oxidation, as well as maintain good high-temperature strength.

Among these requirements, the first two are primarily related to construction performance. To address them, qualitative tests were conducted in the laboratory, and raw materials were compared and screened. The effects of several bonding methods-including cement bonding, silica sol bonding, aluminum sol bonding, and aluminum dihydrogen phosphate bonding-on both the construction performance and service performance of the gunning mix were studied.

The test results show that: the gunning mix with the cement bonding system has poor explosion resistance; the one with the silica sol bonding system has poor adhesion; and the sample with the aluminum sol bonding system is prone to swelling. After selection, aluminum dihydrogen phosphate solution was finally determined to be the most effective binder. Taking advantage of its characteristics-strong adhesion, good explosion resistance, and excellent bonding with the hot trough lining-it can significantly improve construction quality and shorten construction time. The specific gravity of the aluminum dihydrogen phosphate solution is generally 1.3–1.4 g/cm³. To meet construction performance requirements, the specific gravity of the binder should be minimized, or the amount of binder used should be reduced as much as possible.

Selection of the Optimal Curing Agent

Through experimental research, it was found that medium-grade magnesia powder or fused magnesia powder reacts more appropriately with aluminum dihydrogen phosphate and also exhibits better purity and curing effectiveness. Therefore, 200-mesh medium-grade magnesia powder or fused magnesia powder was selected as the curing agent for this material. Further on-site construction experiments showed that when 1%–2% magnesia powder is added to the powder material, it mixes effectively with the aluminum dihydrogen phosphate solution sprayed from the gun nozzle. When sprayed onto the repair surface of the iron trough, the mixture does not flow, cures and sinters quickly, and effectively meets the needs of emergency repairs for the damaged working surface of the iron trough.

To meet the third performance requirement, while ensuring the construction performance, fused corundum, silicon carbide, spherical pitch, anti-explosion fibers, as well as metallic silicon powder, metallic aluminum powder, boron carbide, and other materials were primarily used in the raw material composition. Based on the previous research results of this project, the addition ratios of various raw materials were determined. The main raw materials and their ratios for the iron trough gunning mix in this research are shown in Table 1, and the physical and chemical properties are listed in Table 2.

Role of Spherical Pitch and Antioxidant System in Gunning Mix

The primary role of spherical pitch in the gunning mix is to enhance slag erosion resistance. However, spherical pitch oxidizes when the ambient temperature exceeds 660°C, while the service temperature of the gunning mix ranges from 1000°C to 1450°C. To address this, silicon carbide powder, metallic aluminum powder, metallic silicon powder, and boron carbide powder are added as antioxidants for different temperature ranges, with a total addition amount of 2%–6%:

Boron carbide powder primarily acts as an antioxidant between 700°C and 1000°C.

Metallic silicon powder is effective between 1000°C and 1400°C.

Silicon carbide powder functions above 1300°C.

Additionally:

Metallic aluminum powder serves multiple roles: introducing gas to prevent explosions, aiding low-temperature sintering, forming ceramic bonds with carbon, and enhancing strength and toughness.

Boron carbide powder assists in medium-temperature sintering.

Metallic silicon powder promotes high-temperature sintering, forms ceramic bonds with carbon, and improves mechanical properties.

Development and Application of Ramming Mix for Iron Troughs

This research focuses on the secondary recycling and utilization of waste refractories in the development of a ramming mix for iron troughs. The goal is to prepare a new type of iron trough ramming mix using a large amount of waste refractories as the main raw material, without significantly reducing the product's performance and service life, thereby reducing costs and conserving resources.

Currently, most ramming mixes for iron troughs on the market widely use premium high-alumina bauxite clinker as the main raw material. In this study, however, the primary raw materials are waste Al₂O₃-SiC-C bricks, waste SiC saggers, and SiC dust collection powder. The waste Al₂O₃-SiC-C bricks come from hot metal ladles, mixer furnaces, and discarded materials from the demolition of the permanent layer of iron troughs. After sorting, these bricks are crushed into particles of various sizes (8–5 mm, 5–3 mm, 3–1 mm, and 1–0 mm) using a jaw crusher.

A comparative experiment was conducted between the iron trough ramming mix prepared with waste materials as the main raw material (Sample D2) and the one prepared with premium high-alumina bauxite clinker as the main raw material (Sample D1). The chemical compositions of the main refractory raw materials used in the experiment are shown in Table 3, and the raw material ratios of the samples are shown in Table 4.

The ramming mix samples, prepared according to the mix ratios in Table 4, were tested for chemical composition, bulk density (GB/T2997-2000), linear change rate (YB/T5203-1993), flexural strength (GB/T5072-1985), compressive strength (GB/T3001-2000), and other indicators. The test results are shown in Table 5.

As shown in the data in Table 5, compared with the iron trough ramming mix prepared with 85-grade premium high-alumina bauxite clinker as the main raw material (Sample D1), the ramming mix prepared with waste Al₂O₃-SiC-C brick particles (Sample D2) shows no significant difference (or reduction) in various indicators, and some indicators are even slightly higher. This is primarily because the used waste Al₂O₃-SiC-C brick particles and waste SiC sagger materials have undergone repeated and long-term high-temperature action (sintering) during their previous service. The initial sintering process between particles, as well as between the particles and the matrix in the Al₂O₃-SiC-C bricks, has essentially been completed, forming a strong bond. Therefore, the large-sized particles of waste Al₂O₃-SiC-C bricks, after crushing, exhibit high strength, and this performance is retained in the iron trough ramming mix (Sample D2).

The post-sintering linear change rate of Sample D2, shown in Table 5, is even smaller than that of Sample D1. This is because the expansion of the ramming mix occurs due to mullitization during the sintering process. Since the Al₂O₃-SiC-C bricks have already undergone effective sintering during their previous service, the expansion caused by sintering has been fully exhibited. Therefore, when waste Al₂O₃-SiC-C brick particles are incorporated into the ramming mix, the post-sintering linear expansion is no longer significant.

The addition of waste Al₂O₃-SiC-C brick particles and waste SiC sagger materials to the iron trough ramming mix (Sample D2) can be seen as an increase in SiC content (which helps improve erosion resistance) in both the aggregate and matrix of the ramming mix. These waste materials-Al₂O₃-SiC-C brick particles and SiC sagger materials-have uniform chemical composition, stable performance, wide availability, and are easily accessible. When applied to the ramming mix for routine iron trough maintenance, they fully meet the repair requirements.

In addition, ultra-fine SiC dust collection powder (collected from the crushing process of SiC production) with a particle size of less than 5 μm and a SiC content of 97.5% is added to the ramming mix (Sample D2). The ultra-fine particle size of the dust collection powder gives it higher reactivity, which helps fill pores, achieves dense packing of the ramming mix, improves construction performance, increases ramming density, and thus enhances its high-temperature slag erosion resistance.

In the iron trough ramming mix developed in this research, the amount of waste Al₂O₃-SiC-C brick particles reaches 67%, while the addition of waste SiC sagger materials and recycled 95-grade SiC dust collection powder reaches 8%, with the total waste material content at 75%. This iron trough ramming mix has shown good application results in the 1350 m³ and 630 m³ blast furnaces at Chengde Jianlong Iron and Steel Co., Ltd. The secondary utilization effect of the waste materials is significant, achieving the goal of "turning waste into treasure."

Conclusions

(1)A new type of rapid hot-state repair material for iron troughs has been successfully developed by using aluminum dihydrogen phosphate as the binder, selecting appropriate anti-explosion agents, antioxidants, slag resistance modifiers, and setting accelerators, and adjusting the particle gradation and the amount of aggregates and matrix. This material exhibits strong adhesion, excellent bonding with the hot trough lining, good quick-drying and anti-explosion performance, superior oxidation resistance, high scouring resistance, and slag erosion resistance, along with a long service life. It can be put into use without baking after the completion of hot-state construction.

(2)The iron trough ramming mix developed with waste Al₂O₃-SiC-C brick particles, fine powder, waste silicon carbide (SiC) sagger materials, and ultra-fine SiC dust collection powder as the main raw materials shows stable performance and good application effects. It not only significantly reduces product costs and saves energy but also reduces the land occupied by waste stacking and mitigates environmental pollution, thus achieving substantial economic and social benefits.

(3)The two rapid repair materials developed in this research-the iron trough ramming mix and the iron trough gunning mix-when combined with innovative maintenance and construction technologies, can effectively extend the service life cycle of refractories in the cast house, reduce the number of iron trough maintenance operations, shorten maintenance time, and lower refractory consumption per ton of iron. Additionally, they reduce the downtime of blast furnace shutdowns and soaking, providing a guarantee for the normal production of blast furnaces.