Development of Isostatically Pressed Flow Control Products Containing MgO-Al₂O₃-C Recycling Material to Reduce the Carbon Footprint

The continuous casting process plays a crucial role in producing the required steel grades for various customer specifications. Key refractory products used in this process include ladle shrouds, monoblock stoppers, submerged entry nozzles (SENs), and submerged entry shrouds [4]. Additional functional products—such as nozzle changers, monotubes, starter tubes, and starter rings—are continually being developed to further enhance process efficiency and steel quality.

As the production of isostatically pressed products is technically complex and their application in continuous casting is both critical and highly demanding, the introduction of circular raw materials into this product group has been limited. The stringent performance requirements and severe operating conditions necessitate a stepwise strategy and carefully controlled approach to ensure that the use of recycled materials does not compromise product reliability or steel quality. Consequently, high levels of circular MgO-Al₂O₃-C raw material were initially used in the manufacture of starter tubes (Figure 1) and later extended to the production of SEN bodies for short casting sequences.

Starter tubes are installed in the tundish and used with metering nozzles, nozzle changers, or tundish slide gates (Figure 2). Their main purpose is to delay the flow of liquid steel into the casting channel when the tundish is still empty, cool, or cold. This controlled delay allows the refractory lining to preheat and the steel bath to reach the desired level before casting begins. As a result, the ferrostatic height and elevated temperature in the casting channel enable operators to start the continuous casting process at the desired casting speed, with minimal risk of steel freezing and the need for oxygen lancing. Early flow stabilisation in the sequence also helps to prevent vortex formation and improves mould level control. Another key function of starter tubes is to promote inclusion flotation, which is essential for clean steelmaking.

As the bath level rises, slag and loose particles from the newly lined tundish float on the steel surface. The use of a floating starter ring, in combination with the starter tube, helps prevent these impurities from entering the casting channel and is common practice in most operations (Figure 3). Once the bath reaches its nominal height, the starter tube has fulfilled its function and typically breaks, floats up, or gradually dissolves into the slag.

In multi-strand tundishes, the height of the starter tubes typically varies between strands to stagger the start-up sequence. The central strands usually start first with shorter starter tubes, while the outer strands use progressively longer ones (Figure 4a). This time-shifted start helps the steel bath temperature to homogenise before full casting operation begins. In cases with higher superheat, the arrangement is reversed—the centre strands are equipped with the longest starter tubes (Figure 4b). Another advantage of using starter tubes is eliminating the need for filler sand.

Reclaimed refractory materials are routinely processed at RHI Magnesita’s Bhiwadi plant (India), where spent refractories are cleaned, crushed, and sieved for reuse in a range of flow control products. This long-established practice of circular raw material utilisation provides both significant sustainability benefits and cost advantages, aligning with the company’s recycling targets and forming a key element of the plant’s production strategy. To maximise the reuse of all available circular material fractions, new product formulations are continuously developed and optimised. One such development is the DELTEK M832 grade, containing >65 wt.% fine circular MgO-Al₂O₃-C raw material and designed for the production of isostatically pressed starter tubes as well as SEN bodies used in short casting sequences.

Stringent quality-control measures are applied throughout the recycling and production processes, for example regarding the chemical composition, carbon content, and particle size distribution of the circular MgO-Al₂O₃-C fractions. An example of recycled MgO-Al₂O₃-C raw material produced at the plant is summarised in Table I, illustrating the conformity of a 1.5-tonne batch with some of the established quality parameters.

To enable incorporation of >65 wt.% circular MgO-Al₂O₃-C raw material into DELTEK M832, the grain size distribution required careful optimisation. For example, finer circular raw material was found to provide improved mixing and uniform consistency, enabling the necessary specifications to be achieved. Table II provides representative chemical and physical properties of DELTEK M832 alongside the specifications.

As this grade was also developed for SEN applications, achieving an appropriate pore-size distribution (e.g., D10 ≈ 0.03 μm and D50 ≈ 0.65 μm) was essential and measured as a standard parameter. Dilatometry confirmed that the thermal expansion behaviour of the new grade was compatible with the other materials used in the product. Additionally, X-ray diffraction analysis identified the mineral phases present, which included periclase, corundum, and graphite.

Optical microscopy revealed a homogeneous distribution of the periclase and corundum grains within DELTEK M832, together with a uniform dispersion of graphite in which antioxidants were finely distributed (Figure 5). The matrix structure and phase distribution were further examined using scanning electron microscopy and elemental mapping, which confirmed the overall microstructural uniformity of the material (Figure 6).