Advancing Sustainable Practices in Refractories for the Cement Industry

At the end of their lifespan, worn out refractory bricks were typically disposed of or landfilled, potentially incurring additional costs for operators. However, these materials are a valuable source of raw materials, which can be repurposed to create new refractory products. Removing used bricks from the kiln involves standard procedures and machinery, without any additional steps. Before debricking, any coatings or build-ups on the lining should be removed. Depending on the type of magnesia (e.g., low or high iron content) and flexibiliser (e.g., spinel or hercynite), the bricks are separated onsite into predefined categories to control the chemical characteristics of the subsequently generated circular raw materials. A well-defined sourcing concept, as well as an adequate reconditioning process, is key to guarantee excellent and stable product properties [5].

Once the material has arrived at a recycling hub, the conversion from spent refractories to circular raw material begins. This process consists of several sorting and classification steps and starts with chemical analysis to verify the nature and type of collected material. After manual sorting (Figure 1), where any foreign objects (e.g., steel anchors and wood) are also removed, the material is subjected to a patented cleaning process in which the largest share of unavoidable alkali salt infiltrations from the cement process are removed. This treatment requires a certain effort in terms of energy and time; however, it is essential to achieve the desired product properties. After crushing the material, the new circular raw material is used to produce ANKRAL LC-Series bricks.

One of RHI Magnesita’s priorities is developing fully automated identification and separation of used materials, including sensors specifically designed to meet the challenges of refractory recycling. Currently, pilot equipment is being implemented in Austria that will completely change the status quo of spent refractory sorting [4]. 

With the ANKRAL LC-Series, RHI Magnesita has successfully implemented the principles of a circular economy in the production of magnesia spinel bricks for the cement industry. The LC-Series (low carbon) incorporates up to 50% recycled materials. Designed with a significantly reduced carbon footprint, these products aim to assist cement producers in lowering emissions within their supply chain while maintaining technical specifications. To meet the growing demand for sustainable refractories, RHI Magnesita offers a range of LC products, including ANKRAL Q1-LC, ANKRAL R8-LC, ANKRAL Q2-LC, ANKRAL RC-LC, and ANKRAL ZE AF-LC. The different products of the LC-Series have been widely used globally in central burning and transition zones under various operational conditions since their market launch in 2020. During this time, the series has gained recognition as a well-known brand, particularly in Europe.

The primary goal in creating the ANKRAL LC-series was to incorporate circular raw materials, thereby reducing the carbon footprint, while ensuring that product properties remained uncompromised. Although the technical data sheet values for conventional ANKRAL bricks and their corresponding LC-Series counterparts may not be identical, both can be effectively used in the same applications and achieve the same level of performance. Rigorous laboratory tests, combined with meticulous sourcing and raw material processing, guarantee consistent performance even under demanding conditions, which has been proven in numerous applications.

The latest development in the field of sustainable products is ANKRAL RS-LC, where the circular raw material content has been increased up to a remarkable 50%. Despite this high recycling rate, a product with outstanding properties was developed. The brick has an exceptional refractoriness under load (RuL T0,5 > 1700 °C), which is comparable to products based on primary raw materials (Figure 2). Additionally, due to sophisticated processing methods, the brick exhibits a remarkably low content of secondary phases, providing excellent resistance against chemothermal wear. The positive results of this development have already been confirmed in practical applications.

Plant A operates a precalciner kiln (Ø 3.8 m x 59 m long) with a capacity of approximately 1900 tonnes per day (tpd) in Central Europe and an alternative fuel (AF) rate of beyond 90% (including plastics and sewage sludge). ANKRAL ZE AF-LC was installed in January 2022 between 15 m and 17 m in the upper part of the central burning zone (CBZ), where high thermal load in combination with occasional clinker melt infiltration and coating loss are the typical wear mechanisms. After completing a full campaign of one year, the lining was inspected again in January 2023. This revealed a very uniform wear and a residual thickness of 170–180 mm (Figure 3). At the next opportunity to enter the kiln in January 2024, the lining showed a similar appearance, with a residual thickness of 140–160 mm. By mid-2024, at the time of writing this article, the bricks were already in their third campaign, with high expectations for completing a third year. The results of this successful trial confirm ANKRAL ZE AF LC’s full equivalency to any other iron-rich CBZ brick on the market.

The kiln at Plant B (precalciner, Ø 4.0 m x 60 m long with a capacity of 2200 tpd) is another example of a European kiln running with substitution rates close to 100% (e.g., refuse derived fuels, solvents, and biomass). ANKRAL Q1-LC was installed in the lower transition zone (LTZ) between 6.4 m and 8.4 m, followed by ANKRAL Q1 between 8.4 m and 10 m. In previous campaigns, this area was lined completely with the conventional ANKRAL Q1, achieving lifetimes of one year despite high chemothermal load and proximity to the tyre. After one full year in operation, the Lining Evaluation Scan (LES) showed a residual thickness of 140–170 mm and rather uniform wear in the installation area of the LC-series bricks (Figures 4 and 5). The corresponding standard brick performance was largely the same, confirming that the product development goal of not compromising the product properties by adding circular raw material was fully met. Based on these results, the operator decided to enlarge the installation area of ANKRAL Q1-LC in future campaigns.

The plant for case study C is operating a modern precalciner kiln (Ø 5.2 m x 77 m with a capacity of approximately 6000 tpd) in Southern Europe. It is using a fuel mix consisting of coal/pet coke and residue derived fuel (i.e., sewage sludge and tyre chips), reaching AF rates of more than 60%. The refractory lining of this kiln is exposed to high chemothermal stress and increased mechanical load from the centre tyre and in the outlet area. In addition, the lining is affected by frequent kiln stops. In March 2023, ANKRAL RS-LC was installed in the remote upper transition zone of this kiln between 40.5 m and 45.5 m (Figure 6). A LES in September of the same year (after approximately 6 months of service) revealed that the brick was in perfect condition, showing no signs of wear and still having the original thickness (Figure 7).

Another inspection in January 2024, which again included LES, basically showed the same results (Figure 8). Both lining evaluation scans performed by RHI Magnesita in September 2023 and January 2024 showed that the lining area of ANKRAL RS-LC was in perfect condition, without any spalling and with uniform residual thicknesses beyond 200 mm from the original 220 mm (Figure 9).

Another opportunity to inspect the lining arose in April 2024, during a further kiln stoppage. At this time, the ANKRAL RS-LC had been in service for more than 12 months and still did not show signs of massive wear (Figure 10). After the first year of service, the customer confirmed that the bricks were still in satisfactory condition and is very confident that they will remain in operation for at least another year.