Abstract: The steel industry is transitioning toward decarbonization and increased circularity, driving a growing dependence on scrap. However, limited availability of high-quality scrap forces producers to use more mixed or lower grade scrap streams, increasing variability in tramp element content—especially chromium. As a result, maintaining controlled chromium levels in liquid steel has become challenging. Chromium behavior during steelmaking is governed by a complex interplay of thermodynamics and mass transfer processes. Yet much of the mechanistic understanding of chromium distribution derives from high alloy and stainless steelmaking systems, where slag chemistries and oxygen potentials differ markedly from low carbon steel production, making direct transfer of knowledge challenging and often nontrivial. In parallel with controlling chromium in steel, the oxidation of chromium into the slag introduces additional sustainability concerns, since Cr-bearing slags may limit future slag applications unless chromium is effectively immobilized, increasing resistance to form environmentally harmful Cr⁶⁺ species. This critical literature review synthesizes current knowledge on chromium distribution in the blast-furnace–basic-oxygen-furnace (BF–BOF) route, examining the thermodynamics of chromium oxidation, the influence of Cr²⁺/Cr³⁺ on slag-network formation and oxide activities, the stability of Cr-containing phases, and the kinetics of chromium transfer at the metal–slag interface and within the slag phase. By integrating these thermodynamic, structural, and kinetic perspectives, the review identifies the mechanisms controlling chromium behavior and highlights key research needs, defining a chromium retention capacity concept for slags to support improved impurity management and more sustainable process operation. ... Read more

Steel is an important product in many engineering sectors; however, steelmaking remains one of the largest CO2 emitters. Therefore, new governmental policies drive the steelmaking industry toward a cleaner and more sustainable operation such as the gas-based direct reduction–electric arc furnace process. To become carbon neutral, utilizing more scrap is one of the feasible solutions to achieve this goal. Addressing knowledge gaps regarding scrap heterogeneity (size, shape, and composition) is essential to evaluate the effects of increased scrap ratios in basic oxygen furnace (BOF) operations. This review systematically examines heat and mass transfer correlations relevant to scrap melting in BOF steelmaking, with a focus on low Prandtl number fluids (thick thermal boundary layer) and dense particulate systems. Notably, a majority of these correlations are designed for fluids with high Prandtl numbers. Even for the ones tailored for low Prandtl, they lack the introduction of the porosity effect which alters the melting behavior in such high temperature systems. The review is divided into two parts. First, it surveys heat transfer correlations for single elements (rods, spheres, and prisms) under natural and forced convection, emphasizing their role in predicting melting rates and estimating maximum shell size. Second, it introduces three numerical modeling approaches, highlighting that the computational fluid dynamics–discrete element method (CFD–DEM) offers flexibility in modeling diverse scrap geometries and contact interactions while being computationally less demanding than particle-resolved direct numerical simulation (PR-DNS). Nevertheless, the review identifies a critical gap: no current CFD–DEM framework simultaneously captures shell formation (particle growth) and non-isotropic scrap melting (particle shrinkage), underscoring the need for improved multiphase models to enhance BOF operation. ... Read more

As a result of the global transition towards green steel production, integrated steel plants are optimizing the existing processes and/or adopting new steelmaking methods to minimize their carbon footprint. These adjustments affect desulfurization conditions and requirements. The present study examines how the desulfurization is impacted for both hot metal and liquid steel across three routes: the traditional blast furnace (BF) - basic oxygen furnace (BF-BOF) with increased scrap ratio, the direct reduced iron (DRI) - electric arc furnace (EAF) with varying scrap to DRI ratios, and the DRI - electric smelting furnace (ESF) - BOF. This work discusses challenges and possible solutions for each route. ... Read more

In this study, different existing methods to remove alloyed and coated copper from steel are summarized, compared, and discussed. None of these methods have been scaled up industrially so far. Characterization of industrial steel scrap will indicate in which forms and quantities copper is present. Based on environment, economics and process efficiency, the most promising techniques will be selected for further investigation. The study will then focus on ways to bring the process efficiency to the level required for industrial application, by better understanding of thermodynamic limits and the reaction kinetics. Possible removal of other undesired elements from the scrap will be taken into account as well. ... Read more

In this PhD thesis, the desulphurisation in 21st century iron- and steelmaking is investigated. The current state of the art in sulphur removal in ironmaking and oxygen steelmaking is discussed (Part I of this thesis) and optimisation of the hot metal desulphurisation (HMD) slag, which is an important aspect of present day desulphurisation, is investigated (Part II of this thesis). Furthermore, since the steelmaking industry will change significantly as a result of the global climate change mitigation, desulphurisation of hot metal from HIsarna, a new low-CO2 ironmaking process, is studied (Part III of this thesis). Finally, an overview of the main conclusions of this work, as well as an outlook about desulphurisation in iron- and steelmaking for the coming decades, based on the research presented in this thesis, is given in Part IV of this thesis. Sulphur is an unwanted impurity in steel that lowers the formability and weldability of steel and it makes steel more brittle. Therefore, steelmakers try to limit the concentration of sulphur in the steel. In 2021, globally roughly two third of the steel is produced via the BF-BOF steelmaking process, where iron ore is reduced by carbon (coal and coke) in the blast furnace (BF), and the hot metal from the BF is refined in the basic oxygen furnace (BOF, or converter). Sulphur can be removed at different process steps in the steelmaking process chain, like at the HMD, at the converter, or at the secondary metallurgy processes. Because of the low oxygen activity in hot metal, sulphur is most efficiently removed at the HMD process. In Chapter 2, the different sulphur removal steps in the steelmaking process chain are discussed. Here also the different HMD processes that are globally being used are discussed. The two most important HMD processes are the co-injection process (where desulphurising reagents, typically Mg and CaO or CaC2, are injected into the hot metal) and the Kanbara reactor (KR; where calcium-based reagents are mixed through the hot metal with an impellor). Currently, the co-injection process is globally the most commonly used process of the two and it is dominant in Europe and North America. Typically, magnesium and lime are used as reagents in the co-injection HMD process. Magnesium dissolves in the hot metal and reacts with the dissolved sulphur to form solid MgS. Although some lime directly reacts with the sulphur, its main task is to react with the MgS to form the more stable CaS, which moves to the slag phase. When the reagent injection is finished, the slag is removed with a skimmer. During the removal of the slag, some iron is lost with it. The amount of iron lost per heat is typically 0.5-2.5 wt% of the total hot metal weight, which is a major cost for the HMD process. In Chapter 3 it is explained that iron loss is governed by two mechanisms: colloidal loss (iron present in the slag in a colloidal form, which is removed together with the slag) and entrainment loss (iron being entrained with the slag during the slag removal). Entrainment loss can be minimised by optimising skimming conditions like an experienced operator, a clean skimmer paddle and a well-controlled skimmer. Colloidal loss can be minimised by decreasing the apparent viscosity of the slag, which under typical HMD conditions means that the solid fraction of the slag should be minimised. This can be achieved by either increasing the slag temperature (in practice minimising the temperature loss) or by decreasing the slag’s basicity, which lowers the melting temperature of the slag. Furthermore, in Chapter 3 it is shown that the HMD slag also needs a B2 basicity (ratio of the concentrations of CaO and SiO2) of at least 1.1 and enough lime to convert all present sulphur to CaS, in order to have a sufficient sulphur removal capacity. The optimal HMD slag has a B2 basicity high enough to allow all the removed sulphur to stay in the slag (sufficient sulphur removal capacity), but its basicity is low enough to keep the slag’s melting temperature below the actual temperature of the slag (typically 1300-1450 °C), ensuring a mostly liquid slag, resulting in a low colloidal loss. In Chapter 4, these findings are evaluated and supported with a Monte Carlo simulation based on thermodynamic data from FactSage, melting point and viscosity measurements with artificial HMD slags and plant data analysis. The temperature of the slag has the strongest influence on the colloidal loss and total iron loss, where a lower temperature leads to a slag with a higher solid fraction and, thus, a higher iron loss. From the typical HMD slag components, MgO has the largest influence on the slag’s melting temperature, where a higher concentration of MgO leads to a higher melting temperature (thus to a higher iron loss). In an industrial setting, it is difficult to increase the temperature of the slag (which is typically 1300-1450 °C). Also, it is difficult to influence the HMD slag composition, because 60-80 wt% of the HMD slag is carryover slag from the BF (changing that would require changing the BF process) and the rest is determined by the reagents injected to remove a certain amount of sulphur (resulting in a certain amount of CaO, MgO and CaS being added to the slag). A more practical method to change the HMD slag composition for a lower viscosity is to add a slag modifier. In Chapter 5, fly ash and nepheline syenite are investigated as suitable slag modifiers for the HMD process. Fly ash contains SiO2 and Al2O3 and decreases the basicity of the slag and, thus, its melting temperature. Nephelene syenite contains SiO2 and Al2O3 as well, but it also contains Na2O, which is a basic network modifier that decreases the slag’s viscosity. Melting point and viscosity experiments with synthetic HMD slags show that both fly ash and nepheline syenite are viable slag modifiers and are a good alternative to the fluoride-based slag modifiers, which are common in industry. Fluoride-based slag modifiers lower the slag’s melting temperature and the viscosity of the liquid fraction. However, fluoride leads to health and environment issues and it decreases the desulphurisation efficiency of magnesium as well. As a result of the global climate change mitigation, the steel industry has to lower its CO2 emission. One new process that can contribute to a lower CO2 footprint of the steelmaking industry is the HIsarna process, which is being developed at Tata Steel in IJmuiden, the Netherlands. Like a BF, HIsarna produces hot metal, but with a 20 % lower CO2 emission. Even an 80 % lower CO2 emission can be achieved when using carbon capture and storage or usage, due to the concentrated CO2 off gas. Compared to a BF, HIsarna produces hot metal with a lower temperature and with lower carbon, manganese and phosphorus concentrations. HIsarna hot metal contains almost no silicon and titanium. However, compared to a BF, HIsarna produces hot metal with roughly 3-4 times more sulphur (typical sulphur concentration in hot metal is around 0.1 wt%). This means that for HIsarna hot metal more sulphur needs to be removed compared to typical BF hot metal. The consequences for desulphurisation of HIsarna hot metal are discussed in Part III of this thesis. Typically, due to cooling, hot metal from a BF is supersaturated in carbon by the time it arrives at the HMD. This carbon supersaturation leads to graphite formation, also known as kish. The formed graphite flakes can form a layer between the slag and the hot metal. Earlier research suggested that this graphite layer could hamper the HMD process, as it would block MgS formed in the hot metal, thus it cannot reach the slag phase and form the more stable CaS. This would result in a lower desulphurisation efficiency. Since HIsarna hot metal contains less carbon, this effect could be smaller or even non-existent for desulphurisation of HIsarna hot metal. However, as is explained in Chapter 6, this hampering effect of precipitated graphite on the efficiency of the HMD process is very small, for both HIsarna and BF hot metal. Analysis of plant data shows that there is only a small correlation between expected graphite formation and HMD efficiency. Only for heats with a low initial sulphur concentration (below 225 ppm sulphur) showed a significant correlation. This means that there is no significant benefit for desulphurisation of low-carbon HIsarna hot metal, compared to carbon-saturated BF hot metal. The lower temperature and the higher initial sulphur concentration of HIsarna hot metal, compared to BF hot metal, do influence the HMD process, as is discussed in Chapter 7. A literature study, a thermodynamic analysis with FactSage and plant data analysis show that the lower temperature and higher initial sulphur concentration lead to a lower specific magnesium consumption. The lower temperature, typically 50 °C colder than BF hot metal, thermodynamically favours the desulphurisation reaction with magnesium. The higher initial sulphur concentration leads to a higher sulphur activity, which enhances the desulphurisation reactions. However, it should be noted that the higher efficiency caused by the initial sulphur concentration is only valid for the surplus of sulphur, compared to BF hot metal. The total amount of sulphur that has to be removed is still 3-4 times higher for HIsarna hot metal than for BF hot metal. Therefore, the total magnesium consumption for desulphurisation of HIsarna hot metal is higher as well. This also means that desulphurisation of HIsarna hot metal will take longer than desulphurisation of BF hot metal, which could lead to the HMD becoming the bottleneck in a steel plant. It is estimated that the oxygen concentration in HIsarna hot metal (~6 ppm) is roughly 5-10 times higher than in BF hot metal (0.5-1 ppm). A high oxygen concentration leads to a lower desulphurisation efficiency. However, since the oxygen concentration in HIsarna hot metal is still low, the expected extra magnesium consumption as a result of the higher oxygen concentration is limited to about 2-3 kg for a 300 t heat. The absence of silicon and titanium in the hot metal will not influence the efficiency of a magnesium-based HMD process. However, a lime-based HMD process, like KR, will have a lower efficiency, since silicon reacts with the oxygen in lime as the calcium reacts with sulphur to form CaS. Furthermore, since HIsarna produces hot metal without slag, an alternative for the carryover slag from the BF needs to be found. A slag based only on the injected magnesium and lime and the formed CaS would be solid at HMD temperatures (see Chapter 3). Therefore, the use of acidic slag additions (like SiO2 and Al2O3) is required, to keep the slag liquid and minimise the iron loss. Still, the higher slag volumes as a result of more sulphur that has to be removed will lead to a higher iron loss, compared to desulphurisation of typical BF hot metal. Because the HIsarna produces, and taps, hot metal continuously with a constant composition and temperature, it is ideal for continuous hot metal desulphurisation. Therefore, at Tata Steel in IJmuiden, the Netherlands, a new continuous hot metal desulphurisation (CHMD) process is being developed. In Chapter 8, this novel CHMD process is introduced. The CHMD process is based on the magnesium-lime co-injection HMD process. It uses several reactors in series (process simulations suggest three reactors in series are required to desulphurise typical HIsarna hot metal to typical post-HMD sulphur concentrations), to limit the total reactor volume. The desulphurisation efficiency of the process is increased by optimising the reactor dimensions to a height to diameter ratio of 5:1, whereas a typical hot metal ladle, used for the batch HMD, has a height to diameter ratio of 1.5:1. It is expected that this leads to a reduction in reagent consumption of ~20 %. Furthermore, the continuous nature of the process allows for a foxhole-type slag skimming (separating slag and hot metal by their density difference), which will lead to an estimated 60 % lower total iron loss, compared to the skimming method of the batch HMD process (a remote-controlled skimmer arm, raking off the slag). Based on the cost estimation for iron loss and reagent costs, the cost for desulphurising one tonne hot metal with the CHMD process will be approximately € 2 lower than with the state-of-the-art batch HMD process. However, according to the current calculations, the residence time of the hot metal in the CHMD is 3-4 times longer than in a batch HMD process, leading to a higher temperature loss and, possibly, a higher CO2 footprint, as a lower temperature allows for less scrap being charged at the converter. Given the already lower temperature of HIsarna hot metal, compared to BF hot metal, this is an issue that needs to be solved before the CHMD process can be used in industry. Currently the development of the CHMD process is still in the conceptual design phase. The changes in the global steel industry as a result of the climate change mitigation will not stop after 2030. Finally, in 2050, the steel industry should be CO2-neutral. In Chapter 10, an outlook is given for the expected changes in the steel industry between now and 2050 and its impact on sulphur removal in iron- and steelmaking. The amount of carbon used to reduce iron ore will gradually decrease and so will the demand for HMD, as the carbon sources coal and coke are the largest source of sulphur in hot metal. However, it is unlikely that carbon can be fully replaced by hydrogen or electricity. It is expected that in 2050 still a significant amount of steel will be produced via carbon-utilising smelting processes, like HIsarna, in combination with carbon capture and usage. Besides, scrap contains sulphur that needs to be removed as well. It is expected that the share of scrap as a source of iron in the steelmaking industry will increase in the coming decades. Therefore, steel desulphurisation will remain necessary in every steel plant and hot metal desulphurisation will be required as well for the carbon-utilising plants. ... Read more

In hot metal desulphurisation (HMD) the slag will hold the removed sulphur. However, the iron that is lost when the slag is skimmed off, accounts for the highest costs of the HMD process. These iron losses are lower when the slag has a lower viscosity, which can be achieved by changing the slag composition. A lower slag basicity decreases the viscosity of the slag, but also lowers its sulphur removal capacity, therefore optimisation is necessary. In this study, the optimal HMD slag composition is investigated, considering both the sulphur removal capacity and the iron losses. In part I the theory is discussed and in part II the optimal slag is validated with plant data, laboratory experiments and a thermodynamic analysis. ... Read more

The HIsarna process is one of the emerging low-CO₂ ironmaking processes that could help the steel industry in achieving their carbon footprint goals. HIsarna hot metal contains 3–4 times more sulfur than hot metal from blast furnaces (BFs). Therefore, a literature study, a thermodynamic analysis, and plant data analysis from Tata Steel, IJmuiden, are used herein to investigate the consequences of HIsarna hot metal for the current hot metal desulfurization process. Although the high sulfur concentration and low temperature of HIsarna hot metal lead to a higher total reagent consumption, compared with desulfurization of BF hot metal, the specific magnesium consumption decreases. The higher oxygen concentration in HIsarna hot metal only leads to a small increase in reagent consumption. ... Read more

The optimal hot metal desulphurisation (HMD) slag is defined as a slag with a sufficient sulphur removal capacity and a low apparent viscosity (η_slag) which leads to low iron losses. In part I of this study, the fundamentals behind the optimal slag were discussed. In this part these fundamentals are explored by a Monte Carlo simulation, based on FactSage calculations, plant data analysis and melting point and viscosity measurements of the optimal slag. Furthermore, the applicability of knowing the optimal slag composition for an industrial HMD is discussed. ... Read more

To lower the iron losses of the hot metal desulphurisation (HMD) process, slag modifiers can be added to the slag. Slag modifiers decrease the apparent viscosity of the HMD slag. Most common slag modifiers in industry contain fluoride as a fluidiser. However, fluoride leads to a higher magnesium consumption and has health, safety and environment issues. Fluoride-free alternatives like nepheline syenite (NS) and fly ash (or pulverised fuel ash, PFA) can decrease the slag’s apparent viscosity. Experiments with HMD slags containing CaF₂, NS and PFA and without slag modifier were performed for slags with a high and an average basicity. The melting points of the slags and their viscosities 1250–1600°C were measured. The experimental results are compared with FactSage calculations. PFA and NS are viable alternatives in the industrial HMD process, as reasonable amounts are sufficient to reach the same lower apparent viscosities and melting points as with CaF₂. ... Read more

During the magnesium-lime co-injection process for hot metal desulphurisation, graphite can precipitate as a result of carbon oversaturation. The formed graphite is known to form a layer between the slag and the hot metal. This potentially blocks the sulphides, that are formed during the desulphurisation process, to reach the slag phase thus hampering the desulphurisation efficiency. In this research it was aimed to obtain experimental evidence for the postulated hampering effect of graphite on the hot metal desulphurisation efficiency at an industrial process. In 2018 at Tata Steel in IJmuiden, the Netherlands, the carbon concentration in the hot metal was measured after the reagent injection, instead of being calculated assuming carbon saturation of the hot metal. This provides the opportunity to predict graphite formation during the process. Although a correlation is found between graphite formation and specific magnesium consumption, which is a measure for desulphurisation efficiency, the effect could not be directly proven, as too many other parameters could have influenced the desulphurisation process. Data analysis does show that the observed correlation could not be attributed to temperature and initial sulphur concentration of the hot metal, which are the two most important factors for the desulphurisation efficiency. The observed correlation between graphite formation and desulphurisation efficiency is significant at low initial sulphur concentrations (<225 ppm) and insignificant at high initial sulphur concentrations (>225 ppm). Key Words Hot metal desulphurisation, magnesium consumption, graphite precipitation, kish ... Read more

Carbon may precipitate during the hot metal desulfurization (HMD) process as a result of carbon oversaturation because of temperature decrease. The precipitated carbon flakes form a layer between hot metal and slag. It is postulated that this carbon layer hampers desulfurization with magnesium by preventing MgS particles from reaching the slag phase. At Tata Steel in IJmuiden, the Netherlands, carbon in hot metal is measured in 657 heats after reagent injection. With this data, it can be determined whether the hampering effect of precipitated carbon on MgS flotation has a significant effect on the performance of the industrial HMD process. Plant data show a correlation between the precipitated carbon and the specific magnesium consumption for hot metal with a low initial sulfur concentration (below 225 ppm). This correlation cannot be found for hot metal with a higher initial sulfur concentration (above 275 ppm). Furthermore, a sulfur mass balance is made over the converter process, that shows no effect of carbon precipitation during HMD on resulfurization in the converter. The limited experimental accuracy of the plant data prevents a quantitative description of the hampering effect. The measurements do suggest that the effect is small. ... Read more

The slag in the hot metal desulphurisation (HMD) process should have a high sulphide capacity to capture the sulphur and a low viscosity to minimise the iron loss; in particular the emulsion loss. Although the slag composition changes during the HMD process as a result of reagent injection, the initial slag from the blast furnace (BF) is still the main constituent of the slag after HMD. Therefore, an average composition of BF slag is used as a starting point in this study. Using FactSage 7.1 thermochemical software, the influence of slag composition and temperature on the sulphide capacity is calculated. The slag basicity (CaO/(Al2O3+SiO2)) has the strongest influence on the sulphide capacity. Furthermore, the influence of CaO and temperature on the liquid viscosity and solid fraction of the slag is calculated and compared with plant data from Tata Steel, The Netherlands. Although the addition of CaO decreases the viscosity of the liquid part of the slag, neglecting the solid particles, it strongly increases the solid fraction of the slag. Based on the Einstein-Roscoe theory, more CaO leads to a higher slag viscosity and consequently a higher iron loss. ... Read more

At the hot metal desulphurisation (HMD) stations of Tata Steel’s BOS plant in Port Talbot (UK), KAlF4 is injected together with magnesium in order to increase the fluidity of the slag and thus to decrease the iron loss. Although KAlF4 is successfully contributing to a lower iron loss during HMD, it also has a negative effect on the desulphurisation efficiency of magnesium. Analysis of slag samples from the plant and FactSage calculations showed that the fluorine in KAlF4 reacts with the magnesium, thus preventing the magnesium from reacting with the sulphur. ... Read more

HIsarna is a smelting reduction ironmaking process that is currently in the pilot plant development phase. HIsarna produces hot metal with higher sulphur, lower phosphorus and manganese, almost no silicon and titanium and a lower temperature compared to the blast furnace. Because of that, desulphurisation of the HIsarna hot metal is one of the challenges to ensure its use for steelmaking. Plant data from different Tata Steel plants in Europe and India was used to study the effect of carbon, silicon, phosphorus, manganese, titanium, chromium
and temperature on hot metal desulphurisation by magnesium lime co-injection. The analysis of the plant data implies that the composition of HIsarna hot metal will be in favour of sulphur removal. Furthermore significant correlations were found between carbon, silicon and desulphurisation efficiency, that needs further research. ... Read more

Sulphur removal in the ironmaking and oxygen steelmaking process is reviewed. A sulphur balance is made for the steelmaking process of Tata Steel IJmuiden, the Netherlands. There are four stages where sulphur can be removed: in the blast furnace (BF), during hot metal (HM) pretreatment, in the converter and during the secondary metallurgy (SM) treatment. For sulphur removal a low oxygen activity and a basic slag are required. In the BF typically 90% of the sulphur is removed; still, the HM contains about 0.03% of sulphur. Different HM desulphurisation processes are used worldwide. With co-injection or the Kanbara reactor, sulphur concentrations below 0.001% are reached. Basic slag helps desulphurisation in the converter. However, sulphur increase is not uncommon in the converter due to high oxygen activity and sulphur input via scrap and additions. For low sulphur concentrations SM desulphurisation, with a decreased oxygen activity and a basic slag, is always required. ... Read more