High-temperature industrial sectors—such as those producing steel, ceramics, cement, and glass—consume vast quantities of refractory materials, resulting in a substantial volume of refractory waste. Furthermore, the manufacturing process for refractory products itself generates significant amounts of scrap and waste material.
While a tiny fraction of this massive volume of refractory waste can be returned to the production line for reuse, the vast majority is either sent to landfills or relegated to lower-value, “downcycled” applications.
Moreover, discarded refractory materials pose environmental and health risks, primarily through: (1) dust generation; (2) the risk of silicosis caused by crystalline silica; (3) radioactivity associated with zirconia-based raw materials; (4) the carcinogenicity of hexavalent chromium (Cr⁶⁺); (5) the carcinogenic potential of refractory fibers and asbestos; (6) pollution from volatile organic compounds (VOCs) released by pitch and resins; and (7) the land footprint occupied by the waste.
If the millions of tons of used refractory materials generated annually in my country could be fully utilized as secondary resources, it would not only conserve a significant amount of resources but also yield social benefits exceeding 10 billion yuan per year.
This article uses reclaimed ladle nozzle blocks and porous plug blocks—both types of steel-mill waste—as examples. Through technical research and development, these materials (amounting to tens of thousands of tons) can be transformed into high-quality corundum-based castables. Such secondary utilization is not only environmentally friendly but also creates tangible value. Specific applications are as follows:
1. Test
1.1 Raw Materials and Experimental Procedure
Corundum-based castables were prepared using 95-grade brown fused alumina, recycled steel-ladle nozzles, and porous purging plugs as aggregates; white fused alumina powder, alumina micropowder, and 92-grade silica fume as the matrix; and Secar-71 cement as the binder. Table 1 presents the chemical composition of the raw materials. Based on the experimental proportions provided in Table 2, the influence of varying amounts of recycled materials on the overall performance of the corundum-based castables was investigated.
1.2 Specimen Preparation and Performance Testing
Specimens were prepared in accordance with YB/T 5205.1—2003. Table 3 presents the water content required for the castables to achieve similar flow values. For each mixture, nine test specimens (160 mm × 40 mm × 40 mm) were cast. After curing and demolding, the specimens were dried at 110°C for 24 hours, followed by the measurement of their apparent porosity, bulk density, flexural strength, and compressive strength. The dried specimens were then subjected to heat treatment—held at 1100°C for 3 hours and 1500°C for 3 hours, respectively—after which their apparent porosity, bulk density, flexural strength (at room temperature), compressive strength, and linear change after firing were determined. Additionally, a slag resistance test was conducted on crucible-shaped specimens that had been fired at 1500°C for 3 hours. A 50 g charge of slag was used, with the following chemical composition (by mass fraction, w): Al₂O₃ 11.81%, Fe₂O₃ 1.05%, SiO₂ 36.82%, CaO 22.65%, MgO 21.79%, and MnO 3.79%.
2. Results and Analysis
2.1 Effect of the Amount of Recycled Well Block Material on the Ambient-Temperature Properties of Castables
Figure 1 illustrates the effect of the amount of recycled well block material on the ambient-temperature properties of castable specimens. It can be observed that as the content of recycled ladle nozzle well blocks and porous plug well blocks increases, the water requirement for the corundum-based castables rises, the apparent porosity increases, and the bulk density decreases. This is primarily because the crushed recycled well blocks consist largely of “pseudo-particles” (aggregates formed by smaller particles), which exhibit higher water absorption than virgin raw materials; consequently, more water is required to achieve proper wetting and meet the requirements for shaping. As the proportion of recycled well block material increases, the ambient-temperature strength of the specimens shows a downward trend. This is because, after drying at 110°C for 24 hours, the strength of the corundum-based castables is primarily derived from the hydration products of the cement—specifically, the formation of a cohesive-crystalline network consisting of CAH10, C2AH8, and C3AH6 crystals, along with AH3 (alumina gel). At this temperature, the increased water requirement leads to higher porosity, which in turn results in lower strength. Following heat treatment at 1100°C for 3 hours, the hydrated mineral phases lose some or all of their crystalline water and undergo partial phase transformations. These transformations release a significant amount of free water, which is subsequently lost, causing the specimen structure to become loose and porous, thereby reducing the strength of the castable. Conversely, after heat treatment at 1500°C for 3 hours, an extensive, interlocking network of CA2 and CA6 crystals forms within the matrix; this strengthens the bonding between the matrix and the aggregate particles, leading to an increase in strength.
2.2 Effect of the Addition of Recycled Well-Block/Purging-Plug Material on Slag Resistance
After being held at 1500°C for 3 hours, it is observed that the slag resistance of the corundum-based castable gradually decreases as the proportion of recycled material—derived from ladle well blocks and purging plugs—is increased. The higher addition of this recycled material leads to a reduction in the bulk density of the corundum-based castable and an increase in its porosity; this facilitates the penetration of slag (from the tundish) into the specimens, thereby compromising their slag resistance. When the content of the recycled well-block/purging-plug material reaches or exceeds 45% (by weight), the slag resistance of the specimens drops significantly, failing to meet operational requirements. To address this issue, experiments involving the addition of chromium oxide were conducted.
2.3 Effect of Chromium Oxide Powder on Corundum-Based Self-Flowing Castables
Using formulation 3#—which incorporates 45 wt% of recycled aggregate derived from steel ladle nozzles and porous purging plugs—chromium oxide powder was added at concentrations of 0.5 wt%, 1 wt%, and 2 wt%, respectively. The resulting samples are designated as 5#, 6#, and 7#. Following the specimen preparation procedure described in 1.2, slag resistance tests were conducted; the results are presented in Figure 3.
The slag resistance of corundum-based castables—incorporating recycled ladle nozzles and seating blocks—is significantly improved by the addition of chromium oxide (Cr₂O₃) powder. While increasing the Cr₂O₃ content beyond 1 wt% yields further improvements in slag resistance, the marginal gain is limited. This phenomenon occurs because, at temperatures reaching 1400°C, chromium oxide begins to form a solid solution with aluminum oxide (Al₂O₃). This solid solution promotes the sintering of the castable’s matrix, effectively bonding the corundum grains to the matrix and enhancing the material’s high-temperature strength. Furthermore, as the amount of Cr₂O₃ in the solid solution increases, the liquidus temperature of the system also rises. Upon contact with molten slag, the Al₂O₃ within the solid solution gradually dissolves into the slag, whereas the Cr₂O₃ precipitates from the refractory material as a chromium-rich (Cr, Al)₂O₄ solid solution. This forms a protective, chromium-rich (Cr, Al)₂O₄ layer at the interface between the refractory and the slag, which inhibits further slag penetration. Consequently, the strategic addition of chromium oxide powder can substantially enhance the slag resistance and service life of the castables. Taking into account both practical on-site application and economic considerations, the optimal addition of chromium oxide powder is determined to be 1 wt%.
3.Conclusion
For corundum-based castables prepared using recycled ladle nozzles and porous purging plugs, the optimal formulation—balancing practical applicability and cost-effectiveness—consists of 45 wt% recycled nozzle/plug aggregates and 1 wt% chromium oxide powder. The resulting castables were tested in 120-ton steel ladles at the new plant of Kunming Iron & Steel (Kungang) and were found to meet all operational requirements.