Trends and Advances in Blast Furnace Casthouse Technologies with a Focus on Refractory Lining Performance, Operational Safety and Decarbonisation

In steelmaking, the blast furnace (BF) is the most widespread globally used route to produce hot metal as it offers several advantages, such as high productivity, continuous operation for long periods of time, and fuel consumption optimisation [1]. Nevertheless, with the intense growth of steel production, the level of associated CO₂ emissions has increased dramatically, imposing a series of challenges for this sector, including the transformation of its process conditions and the use of hydrogen as a fuel. Figure 1 presents the relationship between the increase in steel production and the emissions produced, with the blast furnace-basic oxygen furnace route leading the ranking. Consequently, a 25% reduction of CO₂ emissions is required based on current global projections relative to a 2-degree warming scenario (i.e., 2DS target), which corresponds to limiting temperature growth to avoid significant and potentially catastrophic changes due to global warming [2].

The refractory industry plays an important role in this decarbonisation process by reducing scope 1, 2, and 3 emissions, through incorporating recycled materials into products (i.e., circular economy), and by developing and implementing safe, integrated, and innovative solutions that enable customers to reduce their CO₂ footprint without compromising operational performance [3].

This article addresses the latest refractory technologies, maintenance practices, and project improvements recently implemented in South America’s BF casthouse area that contribute to the current demands regarding energy efficiency, CO₂ emission reductions, and safety.

The main physical and mechanical property results for CARSIT and CARSITAL K31C-19-BR samples are presented in Table I. Firstly, it is important to note that even though both refractories have the same formulation, there is a difference in water content due to the type of installation and processing requirements. Water is a key ingredient in such processes and plays a major role by wetting the ceramic particle surfaces, generating strong capillarity forces between large and fine particles (i.e., wall effect), and acting as a crucial component of binding systems [4]. Nevertheless, the increase in porosity associated with the higher water content also promotes a reduction in the mechanical properties besides influencing corrosion resistance. In this context, better mechanical strength levels for the precast block grade were observed as a result of its lower porosity, higher density, and optimised water content.
Although a significant amount of water reacts with the cement to form hydraulic bonding phases, a residual unreacted part facilitates proper installation of the refractory castable, generating porosity after firing [5] and a lower hot modulus of rupture.

During operation, the refractory applied in BF runners is subjected to extreme temperature conditions (1490–1530 °C) inherent to the steelmaking process. For example, when in contact with hot metal and slag at high temperatures, the lining is exposed to combined wear mechanisms such as corrosion and thermal shock [6]. In order to assess the materials’ corrosion resistance, tests were performed using both an induction and rotary furnace. The results (Figure 2) revealed similar refractory wear for both materials in the induction furnace—a test used to evaluate the wear at the hot metal/slag interface. However, in the rotary test (widely used to evaluate the wear due to slag chemical attack and its erosive effect on the lining), a higher wear was obtained for the castable product. This result is in accordance with the higher apparent porosity observed (i.e., 16.33% at 1400 °C), which would increase the slag penetration rate, and the lower hot strength (0.70 MPa) that could be associated with decreased erosion resistance.

The thermal shock resistance of both refractories was also evaluated by measuring the elastic modulus (E) after successive cycles of thermal shock (i.e., 1400 °C for 30 minutes). The results showed a lower decrease of E for the monolithic block grade in comparison to the refractory castable (Figure 3), which confirms a superior thermal shock resistance of the CARSITAL K31C-19-BR.

In addition to the beneficial material properties of CARSITAL K31C-19-BR, the refractory secondary runner maintenance schedule is significantly shorter when using precast blocks. This is illustrated by the following real case scenario for a South American customer where a typical runner wear lining installation with castables requires a 36-hour mould assembly stage before the application starts. This step is time consuming because the secondary runner borders are irregular, with no anchoring points to install the moulds. Another important stage when installing castables is the curing (10 hours), which needs to be completed before removing the moulds as the hydration process that takes place during this period is responsible for the castable’s green mechanical strength development. In contrast, no mould assembly is required for precast blocks and only ~2 hours are needed to remove assembly parts after installation of the blocks. Moreover, as the refractory block has already been cured and dried (350 °C/12 hours) during its production, these processing steps are not necessary. Nevertheless, a heating up curve is required for both technologies as the refractory needs to reach a minimal temperature to minimise the thermal shock while ensuring suitable flowability of the slag and hot metal. Table II summarises the overall time requirement for each stage, comparing the installation of 8 blocks (~20 tonnes) with applying a castable in the secondary runners.

Regarding performance, independent of the refractory technology used, a typical secondary runner campaign can last up to 12 months with only minor (hot) repairs required during this time. However, when installing precast blocks in secondary runners, the amount of maintenance material used for repairs is ~20% lower when compared with that required for repairing castables. Therefore, due to its superior properties, the precast block technology can optimise the overall refractory maintenance time and increase the secondary runners’ availability.

Although BFs have been the most effective route to produce iron for over 200 years, several issues can affect their efficiency and stability, such as the runners’ lining condition. Among the main digital solutions implemented, 3D laser scanning has proved to be one of the most effective methods to measure the lining thickness throughout the campaigns, providing a large amount of discrete data points in a very short time. To accomplish this, the scanning equipment is positioned next to the runner and a reference measurement of the original refractory lining is taken. Once the runner reaches at least half of the planned production, a new inspection is performed to assess the remaining refractory lining thickness and establish a new production target. Additionally, in steel plants where the slag line determines the campaign, there is no need for full runner drainage.

The result is a mobile-friendly report automatically generated in about 20 minutes, enabling the operator to have all the data on only one platform called Steel Lining Evaluation Scan (LES), which leads to fast decision making [7]. Figure 5 shows an example of measurements taken for the residual refractory lining in the slag line between tapping.

In a real case scenario, the scanner was successfully used to safely extend a main runner’s campaign following hot inspection of the slag line. Through the implementation of this new technology in a BF with two tapholes and a daily hot metal production of 4200 tonnes, it was possible to increase the campaign by 5 days without draining the runner although the refractory thickness had decreased from 150 mm to 80 mm, taking into consideration the reliability of the data acquired. Furthermore, in a situation where the equipment had an average hot metal production of 90000 tonnes, the 3D laser solution enabled refractory maintenance to be postponed until the hot metal production had reached 108732 tonnes (an increase of 20% in comparison to the original planned campaign). Moreover, the refractory specific consumption (kg of refractory/tonne of hot metal) decreased by 10%, leading to increased annual productivity due to better maintenance practices [7].

It is also important to highlight that by decreasing the number of inspection drainages, a reduction in the CO₂ emissions during hot metal production is expected. As a reference, for each tonne of hot metal produced, 2 tonnes of CO₂ are released [8]. This means that considering a coke BF with four main runners (operating in pairs) and a campaign of 28 days, eliminating drainage for inspection would represent up to a 3120-tonne CO₂ reduction of the annual emissions (26 drainages of a main runner with a hot metal capacity of 60 tonnes were considered for the calculation).

Another digital solution implemented in South American BFs is improved thermal control through integrating continuous hot metal temperature measurements. By replacing the conventional thermocouple procedure (i.e., manual measurements are taken by immersing the thermocouple in the molten bath) with the novel optical pyrometer methodology, it was possible to continuously measure the temperature, improve data reliability, eliminate manual measurements that are a high safety hazard, and to have better control of the BF operation by anticipating necessary actions for process stability [7].
Figure 6 shows a pyrometer being used in the BF casthouse during high-temperature hot metal tapping.

By implementing these digital technologies, it is possible to increase productivity and achieve higher operational stability of casthouse processes. Furthermore, the online hot metal monitoring process associated with the precise control of runner refractory wear are part of the key initiatives that will make the construction of reliable prediction models possible and ultimately the creation of a fully integrated systemic analysis of BF runner campaigns.