RHI Magnesita N.V. is a supplier of refractory products, systems and services. It is listed on the London Stock Exchange and is a constituent of the FTSE 250 Index.
MgO (periclase, magnesia) is a key component of refractory materials used for the lining of high-temperature aggregates in the metallic and non-metallic sectors alike. The primary source in production is magnesite (MgCO₃), which is thermally decomposed to magnesium oxide. However, the excellent refractory properties of magnesia are only achieved at specific crystallite sizes, which requires thermal post-treatment of MgO using sintering at temperatures of 1500 to 2200 °C in rotary or a shaft kiln or by fusion well above the melting point in an electric arc furnace, respectively. The present contribution is part of an exploratory study that investigates the possibility of obtaining periclase crystals with dimensions of at least 500 μm using alternative synthesis routes that demand lower temperatures. Therefore, the flux method using two alkali borate-based systems were successfully tested. The investigations included the determination of MgO solubilities in a pure NaBO2 melt and a melt of eutectic composition in the system NaBO2-KBO2. It is worthy to note that synthesis runs employing pure potassium metaborate (KBO₂) as a solvent proved less successful and resulted in the concomitant formation of Mg-containing borates such as Mg3(BO3)2. Temperatures and annealing times were systematically varied to identify the optimal growth conditions through slow evaporation of the solvents. The samples were characterized employing the following methods: optical microscopy, SEM, and PXRD. High-quality octahedra of sufficient size and free from flux inclusions were obtained after 96 hours and 120 hours at 1000 °C using the eutectic mixture. The investigation was conceived as a proof-of-concept study. Some issues related to a potential upscaling of the process are also discussed.
Using titanium dioxide (TiO2) as a sintering additive is common practice in the refractory castable industry, as its lower melting point and ability to increase diffusivity in structures induce densification. In alumina (Al2O3) and magnesia (MgO) castables, titania is capable of forming complex phases, such as spinels and pseudobrookites. The in-situ formation of these phases and the mismatch in their thermal expansion coefficients generate stresses that result in microcracks, increasing the material flexibility. However, there is a crucial trade-off: the higher densification promoted by TiO2 additions increases mechanical strength and, consequently, reduces the structure's ability to accommodate stress, whereas microcracks generated by Ti-containing phases negatively impact mechanical strength, but increase the flexibility of the microstructure. This study aimed to optimize the TiO2 content in the Al2O3-MgO-TiO2 (AMT) system to find the ideal balance point that would maximize thermal shock resistance without compromising mechanical integrity. To do this, compositions were defined using computational thermodynamics (Calphad) and subsequently evaluated for their physical, mechanical, mineralogical, thermomechanical and microstructural features. Microcracking and flexibility of the castables were assessed by in situ elastic modulus and thermal shock resistance tests, respectively. The results showed that increasing TiO2 content intensifies the formation of pseudobrookite, which increases crack formation at the matrix/aggregate interface, besides promoting interaction between pores and cracks. However, the addition of 1 wt% of titania proved to be the ideal content, enhancing the formation of short, controlled microcracks that effectively dissipate stress energy. As a consequence, the composition with 1 wt% of TiO2 (80M-20T) retained 90.2 % of its initial elastic modulus after 10 thermal shock cycles (Delta T = 1000 degrees C), while the flexural strength remained at 31 MPa, compared to 20 MPa for the sample without TiO2.
This study presents an integrated experimental and numerical investigation of freeze-lining (FL) formation during the solidification of molten slag under natural and forced flow conditions. Laboratory experiments involved immersing a gas-cooled probe into molten slag contained in a crucible. Continuous probe cooling promoted FL growth, while controlled crucible rotation imposed forced convection. This setup enabled in situ monitoring of FL evolution under different conditions and provided high-fidelity data for model calibration and validation. Measurements included FL thickness evolution, temperature profiles near the crucible wall and within the solidifying layer, and the thermal response of the internal gas cooling. A CFD model was developed using an enthalpy-based mixture continuum approach, extended to incorporate centrifugal, Coriolis, and Euler forces within the Boussinesq approximation to enable a physically sound simulation of rotating slag systems. The model achieved good agreement with experimental measurements, with minor deviations attributed to thermocouple positioning uncertainties. The simulation results demonstrated that crucible rotation shifts the internal flow from buoyancy-driven recirculation to a toroidal mixing pattern, creating a more uniform temperature field that significantly alters FL evolution. Thermal boundary conditions were determined through systematic inverse modeling, including the convective heat transfer coefficient at the slag–gas interface and the probe heat flux. Notably, the gas-side heat flux measurements was found to overestimate the effective slag-side heat flux by approximately 75 pct, a finding with direct practical implications for future cooled probe experiments. The model validation under both static and rotating conditions advances the predictive capability of FL models and provides a benchmark dataset relevant to refractory protection and energy efficiency in pyrometallurgical processes.
Refractories are used in high-temperature applications, consuming 35 to 40 Mt globally each year and generating 28 Mt of spent refractories (SR), primarily from steel and cement production. This paper addresses knowledge gaps in SR material flows from steel casting ladles (SCL) and cement rotary kilns (CRK) by collecting data from respective industry interviews, reports and trade balance sheets, to estimate Europe's annual refractory demand. Additionally, an approach is presented to easily estimate the amount of SR generated. In 2022, Europe consumed 2.7 Mt of refractories, with 1.36 Mt for steel and 0.14 Mt for cement production, resulting in 326 kt of SR from SCL and 74 kt from CRK. Compared to dated literature, recycling of CRK breakout material increased to 42 %, whereas no data was provided in 2003. Contrary, 52 % of SCL material is being landfilled nowadays compared to no landfilling reported in 2003. Spent refractories recovered by recyclers is utilised in novel refractory production at rates of 64 % for SCL and 81 % for CRK. This study provides the first in-depth analysis of SR handling from cement production, including an average process loss of 45 %. The findings improve the overall knowledge about the final fate of SR.
