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.

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.

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.

Physicochemical behaviour of the pellets, sinters and its mixture (60% pellets: 40% sinter) is investigated by a series of smelting and quenching experiments. For all ferrous raw-material beds, three distinct stages of bed shrinkage occur due to indirect reduction, softening and melting. However, the characteristic nature (displacement, temperature and permeability) differ with the ferrous raw-material type. In mixed ferrous bed, the first and third stages are found to be controlled by the pellets (individual particle shrinkage) and sinter (slow melting rate), respectively. Second stage behaviour is initially observed to be close to the pellet and later to that of sinter. In mixed bed (upto 1505°C), the interaction between the pellet and sinter is limited to the interface only. The sinter slag is observed to control the melting and dripping properties of the mixed bed.These results gives an understanding of individual and mixed burden behaviour under blast furnace conditions.

Effect of nut coke addition with ferrous burden (pellet and sinter mixture) is experimentally investigated under simulated blast furnace conditions. Nut coke mixing degree was varied (0, 20 and 40 wt-%) as a replacement of the regular coke. During smelting, the ferrous bed evolves through three distinct stages of shrinkage due to indirect reduction, softening and melting, respectively. Nut coke increases the reduction kinetics, limits softening and enhances iron carburization in the ferrous bed to affect all three stages. Additionally, nut coke physically hinders the sintering among the ferrous burden to keep the interstitial voids open, which exponentially increases the gas permeability. A significant impact of nut coke mixing occurs in the cohesive zone temperature range, which is decreased by 77°C upon addition of 40 wt-% nut coke. Various experimental results give supports for the extensive utilization of nut coke as a replacement of regular coke in the blast furnace.

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.

One of the primary causes that limit the blast furnace productivity is the resistance exerted to the gas flow in the cohesive zone by the ferrous burden. Use of nut coke (10–40 mm) together with ferrous burden proves beneficial for decreasing this resistance. In present study, effect of nut coke addition on the olivine fluxed iron ore pellet bed is investigated under simulated blast furnace conditions. Nut coke mixing degree (replacement ratio of regular coke) was varied from 0 to 40 wt% to investigate the physicochemical characteristics of the pellet bed. Three distinct stages of bed contraction are observed and the principal phenomena governing these stages are indirect reduction, softening and melting. It is observed that nut coke mixing enhances the reduction kinetics, lowers softening, limits sintering and promotes iron carburisation to affects all three stages. In the second stage, the temperature and displacement range is reduced by 60°C and 24%, respectively upon 40 wt% nut coke mixing. Addition of nut coke exponentially increases the gas permeability (represented by pressure drop and S-value). A higher degree of carburisation achieved on the pellet shell (iron) is suggested to be the principal reason for decrease in the pellet melting temperature. The pellets softening temperature increases by approximately 4°C, melting and dripping temperature drops by 11°C and 12°C, respectively, for every 10 wt% nut coke addition. Consequently, the nut coke addition shortens the softening, melting and dripping temperature ranges, which shows improved properties of the cohesive zone.

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.

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.

Reduction of CO₂ emissions in blast furnaces is an important problem for the steel industry. Operating a blast furnace at lower CO₂ levels requires a reduction in the amount of coke that is used to maintain gas permeability in the cohesive zone. Therefore, gas permeability in the iron-ore layer of the cohesive zone should be improved. In this study, gas permeability through a packed bed with liquid was measured using an experimental sponge ball packed bed as a model. The pressure drop of the sponge ball packed bed without liquid was proportional to the square of gas flow velocity. Furthermore, it was affected by the shrinkage ratio of particles. The pressure drop of the deformed packed bed with liquid was mostly affected by liquid that overflowed from the sponge balls into vacancies in the packed bed during the deformation process. This setup can simulate the phenomenon of rising pressure drop within sinter ore at the cohesive zone. The effect of sponge ball arrangement was tested using sponge balls filled with much liquid and sponge balls with smaller amount of liquid. These sponge balls simulate gas permeability of the ore layer containing acid pellets and sintered ore in the cohesive zone. The results indicate that both the mixed arrangement and longitudinal arrangements are effective in maintaining higher gas permeability.

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.