Sustainable Tundish Lining—Reducing Energy Consumption and CO₂ Emissions While Achieving Cost Savings

With respect to sustainability and cost efficiency, CO₂ emission reduction, energy consumption as well as CO₂ emission allowance and energy costs are becoming increasingly more important, with both the steel and refractory industries challenged to achieve change. This article focuses on the tundish, as the most relined unit in the steel plant, to demonstrate how RHI Magnesita products decrease energy consumption and CO₂ emissions and how this translates into cost benefits for the customer.

In continuous casting, the tundish serves as a reservoir for distributing molten steel to the moulds while also fulfilling essential metallurgical functions. The vessel comprises a steel shell and anchors, with an insulation layer, permanent lining, and wear lining (Figure 1). In this article refractories for the permanent lining are described that highlight the multiple advantages of using sol-bonded castables for this application, including energy savings. Furthermore, the various wear lining approaches are reviewed with a focus on cold-setting (i.e., self-hardening) mixes, which also provide benefits including reduced CO₂ emissions through a decreased gas requirement and increased tundish availability. 

To quantify the savings that can be achieved by adopting sol-bonded and cold-setting mixes, trials were conducted in various steel plants to determine the gas and CO₂ emission allowance costs per tonne of installed refractory mix. Prompted by the substantial natural gas price increases that started in 2021, the case studies presented in this article using 2024 gas and CO₂ emission allowance costs clearly demonstrate the advantages of using sol-bonded and cold-setting mixes.

Typically known as the permanent or safety lining, this refractory layer is the last defence to prevent a breakout of liquid steel through the tundish steel shell; therefore, the aim is to minimise any wear and keep it in operation for as long as possible. Ideally, and with appropriate maintenance practices, the lifetime can be extended to years. Currently, the most commonly used refractories for the permanent lining are hydraulically bonded low cement castables (LCC), which are mainly composed of mullite-rich raw materials or bauxite.

For a standard LCC, 7% water is added during mixing and the castable is applied using a template equipped with vibrators. During processing the water reacts with calcium aluminate phases to form hydrates, which result in the necessary bonding and the template can be removed after dimensional stability has been reached (~24 hours). For safe operation and to prevent a vapour burst the added water must then be removed. Since the undried cement-bonded castable is dense and has low gas permeability, the first heat up must be performed very carefully and slowly so initially the physically bonded water can evaporate, followed by release of the chemically bonded water with increasing temperature [1]. For a 150 mm thick permanent lining, this time- and energy-consuming drying can take ~55 hours, with a stepwise temperature increase to 600–700 °C (Figure 2a).

Renowned for their easy and fast heat-up procedure, sol-bonded mixes are a sustainable alternative for the permanent lining. Processed in a similar manner to LCCs, the binder is added during the mixing procedure in the form of a liquid sol (i.e., DIVASIL) that contains nano-sized silica particles [1]. As with LCCs, sol-bonded castables can be applied using a template equipped with vibrators, although free-flowing and gunnable versions are also available for additional flexibility. After setting, water in the sol-bonded mix is not chemically bonded and as a result the main water component in the gel structure is easily removed at very low temperatures. A comparison of the vaporisation rates for a sol-bonded mix and a LCC is shown in Figure 3. This thermogravimetric analysis clearly shows that the sol-bonded mix reaches zero vaporisation at 100–110 °C while the LCC does not release all the water until 450 °C. In addition, the pore size distribution of sol-bonded castables results in a high permeability even at low temperatures, which further facilitates dehydration. As a result, the drying time of a 150 mm thick sol-bonded lining can be reduced to ~20 hours (see Figure 2b), increasing tundish availability and decreasing gas demand. There is also an option to avoid any gas consumption by using alternative heat sources such as electric drying.

In summary, the main advantages of sol-bonded mixes include:

• Fast and easy heat-up procedure.
• Decreased gas demand for drying and the associated CO₂ emission reductions.
• Option for drying with green electricity.
• Increased tundish availability.
• Various application methods (i.e., vibrating, self-flowing, and gunning mixes).
• Longer shelf life than LCCs as the dry mix does not contain binder.
• Reduced sensitivity to incorrect dosing of the mixing liquid.
• Higher refractoriness compared to equivalent LCCs.
• Longer operational lifetime than LCCs.

There is a wide range of sol-bonded mixes available from RHI Magnesita and the positive experiences regarding performance, installation, and handling over the years have resulted in their increasing application in diverse areas by multiple industries (e.g., steel, cement, and nonferrous). Table I details the chemical composition and main raw materials of three sol-bonded castables recommended for the tundish permanent lining.

To withstand casting sequences that can be over 100 hours, the alumina silica based permanent lining is protected by a sacrificial basic refractory wear lining. Unlike the permanent lining, the wear lining is directly exposed to various chemical and thermomechanical wear mechanisms during steel casting (e.g., from slags and cover powders). Consequently, it must be removed after use (via deskulling or tilting) and replaced. Over the last decades, three main technologies have become the industrial standard for wear lining applications, namely:

• Slurry gunning mixes.
• Dry-setting mixes.
• Cold-setting mixes.

As the most conventional type of wear lining, slurry gunning mixes require ~20–35% water content to reach a pumpable consistency. After continuous water addition, the wet mix is conveyed to a collecting hopper and then pumped through a hose to the nozzle. Compressed air, injected directly at the front end of the spraying nozzle, propels the mix towards the target area (e.g., tundish wall). Characterised by their broad flexibility in terms of the desired lining thickness, slurry gunning mixes are still the primary tundish wear lining mix type used in most steel plants. Due to the relatively high water addition, a drying process is crucial prior to steel casting, which can take place at a controlled drying station in ~3–5 hours. To decrease or even eliminate this drying step, RHI Magnesita developed the FAST TO CAST gunning mixes [2].

Dry-setting and cold-setting mixes are widely used alternatives to slurry gunning mixes. For dry-setting mixes, the dry mix is filled into a gap between the template and the permanent lining. During curing at 200–300 °C, the organic or inorganic binders are thermally activated and result in a stable wear lining after ~1–2 hours of total preparation. In contrast, cold-setting mixes are applied via a continuous mixing machine, for example the ANKERTUN CS machine shown in Figure 4, whereby the addition of 1–3% water initiates a chemical cold-setting reaction. Simultaneously, the mix is filled into a gap between a vibratable template and the permanent lining. After approximately 30–60 minutes the mix hardens without further actions, such as temperature treatment. After template removal, a cold-setting lining can be used directly for steel casting as the added water is already at a reasonably low level. Since no drying or preheating cycles must be applied to cold-setting tundish linings, the total gas consumption and associated CO₂ emissions can be reduced to a bare minimum. 

Cold-setting mixes can be defined as mixes that set or harden at room temperature without any external heat, as a result of starting a chemical reaction during the mixing process. Typically, this is achieved by using either a two-component binder system based on the well-known reaction of liquid sodium silicate with a hardener or the patent protected self-hardening (SH) system where only minimal water is required to activate binding. For the wear lining case study examples in this article, the focus is on ANKERTUN SH mixes as they represent an environmentally friendly, state-of-the-art tundish lining solution where the liquid additive is free of chemicals.

The SH binder consists of a solid acid component, in combination with a solid inorganic basic material. This SH binder is mixed with the desired refractory raw materials (e.g., sintered magnesia and olivine) in a dry state before delivery to the customer. During application at the customer and due to the slight water addition, the SH binder forms a metal-organic complex leading to the formation of binder bridges with the refractory material. At this stage the water is in a nonchemically bonded form and can easily evaporate from the lining at ambient temperature [3]. The chemical composition and main raw materials in commonly used ANKERTUN SH wear lining grades are detailed in Table II.

Gas costs and CO₂ emission allowance prices

In 2021, natural gas prices increased dramatically due to various reasons such as the Ukraine war [4]. As this energy source is primarily used to dry tundish linings and preheat the vessel, the case studies described in this section were initiated to accurately determine the financial benefits that can be achieved using sol-bonded and cold-setting ANKERTUN SH mixes. Despite the recent decline in gas prices, for example in Europe (Figure 5), the studies clearly show the cost savings that can be achieved per tonne of installed refractory with these lining technologies as well as the reduced susceptibility to future fuel price volatilities. Additionally, as approximately 2 kg of CO₂ are emitted for every cubic metre of gas consumed [5], the reduced gas consumption also has a positive environmental impact as well as an economic advantage in regions with carbon pricing schemes. As the European Union Emission Trading System (EU ETS) is the world’s first and largest carbon market [6], the EU CO₂ emission allowance price was used to calculate the monetary implications. Although this allowance price has fluctuated during the last years (Figure 6), since the EU ETS is based on a “cap and trade” principle, CO₂ allowance costs are predicted to increase over time and become even more relevant regarding these types of cost calculations. For the studies, April 2024 natural gas and CO₂ allowance prices of €26.2/MWh and €61.1/tonne CO₂, respectively, were used.

Permanent lining comparison

For the permanent lining case study, three customers were selected to compare the costs associated with drying either LCC or sol-bonded linings. The maximum drying temperature and time, tundish capacity, as well as the amount of mix per lining at each customer are provided in Table III. In the case of customer C, a direct comparison could be made as the lining technology was recently changed from LCC to sol-bonded mixes. Regarding gas consumption, it is important to state that the equipment for tundish lining is often on the same gas loop as the ladle preheating stands or other gas consuming equipment. Therefore, the gas consumption for the lining drying was measured indirectly and it was necessary to make certain assumptions for any missing customer data, including whether the equipment was new or old as well as the tundish dimensions and lining thickness. Using this approach, figures for the specific gas consumption (m³/tonne_refractory) to dry the permanent lining were determined that show sol-bonded mixes required significantly lower amounts of gas (Figure 7). For example, a gas saving of 70% was achieved at customer C by changing from LCCs to sol-bonded mixes and there is the possibility to further decrease this by optimising the drying curve, as illustrated by customer B. In this case, after improving the drying schedule at customer B, an 86% lower specific gas consumption was achieved compared to customer A using LCCs, although the amount of mix installed per lining was similar.

To determine the financial implications, the emitted CO₂, as well as the gas and CO₂ emission allowance costs per tonne of installed refractory were calculated from the specific gas consumption (Table IV). A value of 0.0105 MWh/m³ was used to convert the gas consumption (m³) to MWh. These results indicate that ~€45/tonne_refractory can be saved by changing the permanent lining from a LCC to a sol-bonded mix (Figure 8), as well as a time saving of 60 hours in the case of customer C. 

Wear lining comparison

In the wear lining case study, the focus was on comparing traditional slurry gunning with the cold-setting ANKERTUN SH mixes. Two of the customers selected were also part of the permanent lining evaluation, while the third customer D provided a direct comparison of the lining technologies as both mix types are used in this steel plant. Details of the wear lining type, maximum drying temperature, maximum preheat temperature, tundish capacity, and amount of installed refractory for the three customers are given in Table V. The gas consumption for drying the slurry gunning wear linings and preheating the tundishes was directly measured as it was possible to ensure that no other equipment on the same gas line was concurrently in operation. Figure 9 shows that a >40% specific gas consumption saving was achieved with the ANKERTUN SH mixes, as no lining drying step was required and the preheating time was slightly shorter than for slurry gunning mixes. Following calculation of the emitted CO₂, and gas and CO₂ emission allowance costs per tonne of refractory (Table VI), potential savings ranging from €40/tonne_refractory up to €90/tonne_refractory were determined for ANKERTUN SH mixes, depending on the specific preheat practice (Figure 10). Furthermore, since it is possible to use the ANKERTUN SH mixes with a cold start practice at the caster, the specific gas consumption and financial savings can be considerably more.

In conclusion, the case studies show that sol-bonded and ANKERTUN SH mixes can significantly reduce the specific gas consumption required in the tundish area for drying and preheating, thereby providing both financial and environmental benefits. Furthermore, since certain steps during the tundish turnaround cycle are shorter or can be eliminated, tundish availability is also increased by using these refractory technologies. For example, comparison of typical time schedules for slurry gunning and ANKERTUN SH mixes, from wear lining installation to transfer to the casting platform, show that 210 minutes can be saved using ANKERTUN SH mixes (Table VII), with additional time savings possible from implementing a cold start practice.

Greenhouse gas (GHG) emissions, such as CO₂, are subdivided into three categories [10]:

• Scope 1: Direct emissions of an organisation (e.g., fuel combustion).
• Scope 2: Indirect emissions of an organisation directly associated with the provision of energy (e.g., emissions from a coal electricity power plant).
• Scope 3: Indirect emissions upstream or downstream of an organisation (e.g., purchased raw materials and third party transport).

In the case of sol-bonded and ANKERTUN SH mixes, the decreased natural gas consumption directly impacts scope 1 emissions at the steel plant. However, there are additional decarbonisation initiatives that enable scope 3 emissions to be tracked and decreased.

Product carbon footprint and recycling

In line with RHI Magnesita’s corporate climate strategy, it was decided to create 100% transparency regarding the GHGs emitted during refractory production. Therefore, the product carbon footprint (PCF) of our products is calculated in accordance with the principles of ISO 14067 and disclosed on the technical data sheets (Figure 11). The PCF covers all GHG emissions from upstream sources and the direct manufacturing of a product (i.e., cradle-to-gate) [10], enabling customers to make purchasing decisions based on this sustainability criterion. Furthermore, RHI Magnesita is committed to reducing the GHG intensity of its products by becoming more circular and using recycled raw materials. This has resulted in the tundish mix portfolio including grades that contain appropriate amounts of circular raw materials, which have a decreased PCF while maintaining the required performance.

Local-for-local

With a global network of 47 main production sites, RHI Magnesita follows a local-for-local strategy by using production facilities located as close as possible to the customer. This translates into scope 3 benefits, as third party transport to the customer can be minimised. In addition, by operating our own raw material production sites across several continents, the uncertainties and volatility of raw material supply and price fluctuations can be mitigated. However, when considering raw materials for tundish wear mixes, local raw material sources can differ from standard raw materials available on the world market (Table VIII). To establish the most cost efficient and sustainable usage at the customer, the local raw material compositions must be considered and the grades optimised for individual customer applications. This might lead to rethinking well-known standard concepts based on Chinese raw material sources, opening up the possibility to use high-quality local raw materials with slightly different chemical compositions and colours.