Full length articleThe production of high value pig iron nuggets from steelmaking by-products – A thermodynamic evaluation
Graphical abstract
Introduction
Iron rich by-product dusts and sludges are an unavoidable consequence of ironmaking and steelmaking activity. The recycling of these fine dusts is usually simple, reincorporating the material into the process through agglomeration in a sinter plant (Umadevi et al., 2013) or via cold bonded briquetting/pelletization (Andersson et al., 2019); however, dusts recovered from the off-gas dedusting systems in the blast furnace (BF) and basic oxygen steelmaking (BOS) plants at integrated works are often contaminated with zinc. The deleterious effect of zinc on the blast furnace is well known (Besta et al., 2013; Narita et al., 1981) and even relatively low concentrations of zinc (100–120 g Zn per ton of hot metal) in feedstock material can have negative effects on process stability and thus the economics of production. The removal of zinc from steelmaking materials via hydrometallurgical means (Cantarino et al., 2012; Jha et al., 2001; Oustadakis et al., 2010; Shawabkeh, 2010; Trung et al., 2011; Zhang et al., 2017) and pyrometallurgical means (Abdel-Latif, 2002; Assis, 1998; Brocchi and Moura, 2008; Mombelli et al., 2016a; Stewart and Barron, 2020) is well documented but these materials are often still stockpiled and underutilized by steel producers. Landfilling of these materials is not a long term economically and environmentally sustainable solution. The steel industry and the governments of countries the industry operates within are actively looking to avoid the negative environmental consequences of huge stockpiles of solid waste on defunct steel manufacturing sites, that must be remediated at huge expense before the land may be reutilized, as has been the case in many high profile plant closures (ENDS Report, 1992). Pyrometallurgical separation of zinc from these dusts in an on-site rotary hearth furnace (RHF) is becoming more commonplace at newer integrated works. These facilities utilize carbon in coal or in the by-products themselves to reduce, volatilize and capture the zinc from the materials in the form of a crude zinc oxide.
The major advantage of a carbothermal reduction route such as the RHF is the iron product is partially reduced and is known as direct reduced iron (DRI). High quality DRI can be >90% metallized (McClelland and Metius, 2003) and charging DRI back to the BF can reduce the coke rate of the furnace, which carries an economic and environmental benefit (Chatterjee, 2012). Unfortunately, DRI produced from recycled materials is often limited in quality. High gangue content due to the nature of the input materials is inevitable and carries penalties such as increased slag volumes, but another key value driver of DRI is sulfur content. As DRI is substantially metallized, it can also be used to displace scrap within a BOS vessel or an electric arc furnace (EAF). Scrap steel is substantially more expensive per unit of iron than virgin ore (Yellishetty et al., 2011) and DRI that is of high enough quality to be recycled to the BOS vessel rather than the BF will have substantially higher value in use (VIU) to the steelmaker.
The BOS process is sensitive to sulfur addition in the scrap charge, because despite the presence of a basic slag, the high oxygen activity in the converter makes the dissolution of sulfur into the hot metal (HM) thermodynamically favorable (Schrama et al., 2017). Many high-performance steels require extremely low levels of residual sulfur within the HM, with the production costs sharpening steeply as ultralow sulfur concentrations (<0.001 wt.%) become necessary (Pehlke and Fuwa, 1985). As such, low sulfur scrap substitutes are expensive and thus low residual sulfur levels within DRI are desirable for the DRI producer.
Next generation RHF processes such as ITmk3 go one step further than the purely solid-state reduction of iron oxides. Through careful manipulation of slag chemistry, the iron in the ITmk3 process is sufficiently carburized to liquify and coalesce into pig iron nuggets (Fig. 1), completely separate from an immiscible, liquid slag phase (Kikuchi et al., 2010; Kobe Steel ltd, 2010). Similar nugget making processes include E-Iron, which uses an adapted linear tunnel furnace to produce iron nuggets from non-zinc bearing mill wastes (Simmons, 2015) and Hi-QIP which uses an un-agglomerated feed on a bed of carbonaceous reductant (Ishiwata et al., 2009). The production of pig iron nuggets from zinc bearing mill wastes has been reported previously (Birol, 2019; Wang et al., 2010; Zhao et al., 2012), implying the feasibility of the production of nuggets based on experimental laboratory conditions in inert gas atmospheres.
Sulfur control in DRI production is mostly achieved by raw material input. DRI produced using coal as a reductant is too high in sulfur (0.44 wt%) to be utilized in a BOS/EAF melt shop (Seetharaman, 2014) and thus is usually utilized via a blast furnace, which is less sensitive to sulfur. However, the addition of a melting step and a molten slag, such as in ITmk3, offers an opportunity to sequester sulfur from the metallic iron within a distinct slag phase. This is not dissimilar to the mechanism by which sulfur is removed within the blast furnace (Schrama et al., 2017). Another potential avenue to add value to iron nugget production is by the simultaneous reduction of manganese oxides often found in BOS dust in small, but not insignificant amounts, into the pig iron nugget product (Longbottom et al., 2016; Mombelli et al., 2016a). Manganese is a common alloying element in the production of steel and is often added on a supplementary basis with the scrap charge to a BOS vessel or EAF in the form of ferromanganese alloys. This is a cost to the steel producer, therefore simultaneously reducing Mn into the hot metal produced during pig iron nugget making would be economically desirable. Srivastava reported the reduction of MnxOy to the liquid iron phase during nugget making reactions using a manganiferous ore with minimal losses of Mn to the slag phase (18% at 1450 °C for 40 min) (Srivastava, 2014). However, this was performed using a high manganese iron ore rather than with steelmaking by-products and used graphite and polyethylene as a reducing agent.
Herein we propose that an exploration of the thermodynamics of the complicated slag-iron system present in nugget production, the ability for the nugget making process to desulfurize the produced pig iron and reduce manganese into the hot metal at a range of temperatures, slag chemistries and reductant levels can be determined. Three conditions need to be met to produce pig iron nuggets in a manner similar to the ITmk3 process or E-Iron;
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a low liquidus temperature slag, to allow for molten iron droplets to coalesce,
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sufficient carbon to reduce iron and zinc oxides to their elemental form, and
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sufficient carbon to further carburize the metallic iron such that the melting point is low enough to form molten pig iron at the reaction temperature.
Using BF dust and BOS dust from Tata Steel Port Talbot in the United Kingdom as a case study, the effect of various blends and temperatures is explored with respect to the thermodynamics of the chemical system. The study is limited in its scope to thermodynamics, without exploring the complicated reaction kinetics of the multiphase system and a comparison is made to experimental studies from literature performed on similar material. By comparing results of a thermodynamic study such as this to the experimental observations made in the literature to determine whether the relatively short reaction times in pig iron nugget making (<60 min) are sufficient to allow the slag-metal system to reach equilibrium or whether kinetic factors play a substantial role in determining Fe yield and Mn/S partition between the hot metal and the slag.
Section snippets
Methods
Calculations were performed using the equilib module of FACTSAGE 7.3. This module uses the Gibbs free energy minimization method to determine the equilibrium composition of complex heterogeneous systems. The input chemistry of the model for BOS dust and BF dust is shown in Table 1, based on material obtained from Tata Steel Port Talbot UK. Reducible oxygen associated to the iron within the material is denoted by OFe. For equilibrium calculations, all pure solid and gaseous species possible from
Low liquidus temperature slag
The Al2O3-SiO2CaO-FeO system is well studied, due to its usefulness in describing the behavior of slags within the blast furnace. In blast furnace ironmaking, siliceous ores are fluxed with limestone to reduce the overall liquidus temperature of the gangue to encourage lower temperature separation from the iron (Geerdes et al., 2015).
The calculated ternary phase diagram of the SiO2CaO-Al2O3 system is shown in Fig. 2 The projection shows the liquidus projection front at temperatures
Iron recovery
Iron recovery is effectively the molar yield of the iron nugget making process. Some iron loss to slag in the emulsion loss (iron droplets unable to coalesce to a size separable from the immiscible slag phase) is an inevitability and controlled by kinetic factors such as reaction time, but in this context solely represents the solubility of Fe in the slag phase at a given BF dust: BOS dust ratio and temperature.
Iron recovery is calculated using Eq. (1), with losses occurring either by the
Introduction of fluxing agents
As explored above, the optimum ratio to produce iron nuggets from BF dust and BOS dust appear to be in a proportion of 37:63. This provides sufficient carbon to reduce and carburize iron oxides present, reduces oxygen activity sufficiently to promote desulfurization and simultaneously recover Mn in the pig iron without incorporating excess carbon into the system which may impede the kinetics of production. However, the temperatures required to fully fluidize the gangue in the system to produce
Conclusions
This study was undertaken to determine the feasibility of an adapted pig iron nugget process as a viable recycling route for BOS dust and BF dust from an integrated steel plant, from a thermodynamic perspective.
The main outcomes of the work are:
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The production of pig iron nuggets from steelmaking by-products is thermodynamically feasible at reasonable operating conditions of 1450 °C.
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The addition of SiO2 and MgO as fluxing agents allows for a reduction in process temperature, and has benefits for
CRediT authorship contribution statement
Daniel J.C. Stewart: Conceptualization, Methodology, Formal analysis, Investigation, Writing – original draft, Investigation, Data curtion. David Thomson: Writing – review & editing, Formal analysis. Andrew R. Barron: Conceptualization, Resources, Writing – review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Financial support was provided by Materials and Manufacturing Academy (M2A) that has been made possible through funding from the European Social Fund via the Welsh Government, the Engineering and Physical Sciences Research Council (EPSRC), and Tata Steel Europe. Additional support is provided by the Reducing Industrial Carbon Emissions (RICE) operations funded by the Welsh European Funding Office (WEFO) through the Welsh Government.
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