Recycling red mud to develop a competitive desulfurization flux for Kanbara Reactor (KR) desulfurization process

Highlights

• A new desulfurization flux was designed by blending lime and red mud.

• Desulfurization of liquid iron was performed by the flux under mechanical stirring.

• The red mud containing flux showed highly competitive performance.

• A desulfurization index was proposed to assess the desulfurization performance.

Abstract

One of the industrial wastes, red mud, was assessed as an additive to lime, as a new desulfurization flux for the hot metal in the steelmaking process. Since typical red mud contains some amount of iron oxide, a preliminarily reduced red mud was mainly investigated. Pre-reduction of the red mud and execution of mechanical stirring to the hot metal were effective to enhance desulfurization efficiency (extent and rate of the reaction). However, adding more than 20 pct of red mud in the lime/red mud (pre-reduced) flux did not increase the desulfurization rate. In order to assess the desulfurization efficiency in a more quantitative manner, a “DeS Index” was defined as: $\text{DeS Index}=\log \left[{\left(\frac{{[\text{pct S}]}_{\mathrm{f}}}{{[\text{pct S}]}_0}\right)}^{-1}\left(k\prime \right){\left(\frac{W_{\mathrm{CaO}}}{W_{\mathrm{S}}}\right)}^{-1}\right]$. This takes into account the extent, rate, and cost of the desulfurization simultaneously. With this index, it was found that the presently developed red mud (pre-reduced)/lime mixtures showed almost similar desulfurization efficiency compared with those of commercially used desulfurization fluxes. It was even better than a typical lime/fluorspar mixed flux, which is now prohibited from being used in many companies due to its environmentally harmful character. It is emphasized that the desulfurization efficiency should be assessed under the mechanical stirring in order to be used in the KR type desulfurization process.

Introduction

Red mud is one of the industrial waste residues discharged during the production of alumina from the bauxite (Wang et al., 2021, Pontikes and Angelopoulos, 2013, Pérez-Villarejo et al., 2012, Zhang et al., 2016, Tuazon and Corder, 2008, Liu et al., 2014). More than 95 pct of the alumina is produced from the bauxite extraction by the Bayer process, sintering process, or a combination of these two processes (Mukiza et al., 2019, Dentoni et al., 2014, Zhang et al., 2020). In the Bayer process, the bauxite is dissolved by NaOH, and the slurry waste residue called red mud is separated after the dissolution (Zhang et al., 2020). Therefore, the red mud is a high alkaline material, and is classified as hazardous waste residue (Zhang et al., 2021). About 1–1.5 tons of red mud are produced for every 1 ton of alumina produced, which is equivalent to nearly 160 million tons of red mud produced in 2015 (Lima et al., 2017). China produced 88 million tons in 2016. Australia, Brazil, and India are also major red mud producers in the world (Mukiza et al., 2019). In South Korea, only one company (KC corp.) produces the red mud, and its annual production capacity is 0.3 million tons.

Due to its hazardous character, it is now one of the important issues to dispose of it safely or to recycle it as a valuable resource in any field. Due to its high alkalinity, its disposal without caution can cause serious environmental troubles including groundwater pollution (Balomenos et al., 2011). Also it sometimes causes air pollution (Deelwal et al., 2014). However, its safe disposal requires considerable cost as high as 2–5 pct of the total alumina price (Tsakiridis et al., 2004, Kumar et al., 2006). It costs approximately 10 USD per ton only for the “safe disposal”. Therefore, the utilization of the red mud in an environment-friendly and economical way in any industry is the urgent need for resources recycling.

It is mainly composed of Al₂O₃, Fe₂O₃, SiO₂, TiO₂, Na₂O and CaO (Tsakiridis et al., 2004, Liu et al., 2014, Liu et al., 2014, Pérez-Villarejo et al., 2012). In some cases, V₂O₅ and MnO are reported (Lima et al., 2017). Its color is dark red due to its high content of Fe₂O₃ (Tsakiridis et al., 2004, Rai et al., 2012) in a wide range of 5–50 pct by mass (Liu et al., 2014, Liu et al., 2014, Mukiza et al., 2019). Since the bauxite itself is not strictly specified for its chemical composition, the composition of red mud discharged from the bauxite also varies depending on its origin, extraction processes, storage methods, etc. (Lima et al., 2017). Several researchers have investigated to use of the red mud in the cement industry (Liu and Zhang, 2011, Tsakiridis et al., 2004, Singh et al., 1996), building materials using specific waste components (Yang and Xiao, 2008, Tsakiridis et al., 2004, Liu et al., 2007), and catalysts for waste gas and liquid cleaning (Yang et al., 2009, Liu et al., 2007, Liu et al., 2007, Tsakiridis et al., 2004). However, there have been quite a few studies on its utilization in metallurgical processes.

Recently, there have been a few attempts to use the red mud as desulfurization flux of hot metal or liquid steel. Fundamental desulfurization reaction of the hot metal or the liquid steel is written as (Fincham and Richardson, 1954):

$$\underline{\mathrm{S}}+({\mathrm{O}}^{2-})=\underline{\mathrm{O}}+({\mathrm{S}}^{2-})$$

where (O²⁻) is the free oxygen in the desulfurization flux and S is the dissolved S in the hot metal. A desulfurization process within a steelmaking process is then utilizing the above reaction as efficiently as possible. After the blast furnace ironmaking process, the desulfurization reaction occurs in three different stages: during hot metal pretreatment, in the converter, and during the secondary metallurgy treatment (Schrama et al., 2015, Schrama et al., 2017). The hot metal pretreatment is the most efficient stage to desulfurize (Kitamura, 2014). Various process technologies have been developed: Torpedo desulfurization, co-injection, Kanbara Reactor (KR), magnesium mono-injection, etc. (Emi, 2015, Schrama et al., 2017). These processes employ various kinds of fluxes: calcium carbide, soda ash, magnesium, lime with fluorspar, etc. Among these processes, the KR process was developed in 1965 in the Hirohata works of Nippon Steel (Emi, 2015). It is a successfully industrialized process that employs an impeller immersed and rotating in a ladle to disperse the flux in hot metal. The impeller rotates at 60–120 rpm (Schrama et al., 2017). This process was rapidly adopted in Asian steel industries (Schrama et al., 2017). An overview of prior modeling efforts relevant to the KR process is available in a recent review paper (Visuri et al., 2020). The desulfurization flux of the KR process is usually composed of lime and some additives to facilitate melting the lime. Therefore, the additives can dissolve (O²⁻) from the lime in the flux. Traditionally, fluorspar was used as a very efficient additive for the melting lime. However, due to its toxic character, the present steel industry has tried to replace it with other materials. Due to the high Na₂O content in the red mud, it is an attractive additive to the lime. Indeed, there were several reports that Na₂O is beneficial for enhancing desulfurization of hot metal or liquid steel (Choi et al., 2001, Cho et al., 2010, Chan and Fruehan, 1986). Since the desulfurization reaction is basically electrochemical in nature (Ramachandran et al., 1956, Ohtani and Gokcen, 1961), applying electricity also enhances the extent of desulfurization reaction (Ward and Salmon, 1963, Kim et al., 2018a,Kim et al., 2018b, Lee and Min, 2020b,Lee and Min, 2020a, Kim and Kang, 2021). In addition to this, enlarging interfacial reaction area by forming liquid iron droplets was found to be also another effective way in the enhancement of the desulfurization reaction rate (Kang et al., 2003, Alba et al., 2015). The KR process also takes an advantage of enlarging the interfacial reaction area by forming flux droplets in the hot metal due to the dispersion of the flux by the impeller rotation. All these approaches requires the development of competitive desulfurization fluxes, which enhance the desulfurization efficiency.

Here, the desulfurization efficiency should be defined in a more clear manner. In an industrial sense, it is also often mentioned as the desulfurization rate, the extent of the desulfurization reaction (percentage of a final S content in hot metal compared to its initial content). In the present study, however, the desulfurization rate will be referred to as the speed of the desulfurization reaction (kinetics). The desulfurization efficiency is referred to as a combinatorial aspect, indicating both the extent of the reaction and the speed of the reaction. Moreover, in the other industrial point of view, the cost of the desulfurization process is also an interesting issue. It is partly determined by the price of the desulfurization flux. Therefore, in the present study, the desulfurization efficiency means the combinatorial aspect of the desulfurization process: the extent of the reaction, rate (speed) of the reaction, and cost of the flux. It will be discussed more in Section 5, in the course of the assessment of various desulfurization fluxes.

Li et al. tried to use the red mud in the desulfurization flux of hot metal by both laboratory-scale and pilot-scale tests (Li et al., 2017). They pointed out that the use of sintered red mud-based flux was beneficial by lowering the melting point of the flux. Jeong and Park used the red mud in order to replace fluorspar in the desulfurization flux of molten steel (Jeong and Park, 2020). They found that too much addition of red mud was not recommended, because the flux composition changed during the desulfurization reaction. They attributed to two undesirable phenomena for the desulfurization: decreasing sulfide capacity of the flux (C_S) and increasing the viscosity of the flux. They suggested that a small amount (less than 10 mass pct) of the red mud can be added to the desulfurization flux, in order to partially replace the fluorspar. Zhang et al. also tried to recycle the red mud on hot metal desulfurization (Zhang et al., 2020). Contrary to the study of Jeong and Park (Jeong and Park, 2020), they used pre-reduced red mud. The composition of the flux was adjusted with lime and alumina in order to recover iron in the red mud and to improve the extent of the desulfurization. They found that the extent of the desulfurization using the red mud was similar to that of the conventional lime-fluorspar-based flux. However, it should be stressed that all these investigations were carried out under still condition – without a mechanical stirring to hot metal or liquid steel. On the other hand, the KR process is carried out under mechanical stirring.

The present authors recently reported that assessment of the desulfurization efficiency should be careful when the mechanical stirring is executed such as KR process (Jeong et al., 2021). The acting mechanisms of the desulfurization with and without the mechanical stirring are different. Without the stirring, the extent and the rate of the desulfurization could be enhanced by increasing the C_S and by decreasing the flux viscosity (Jeong and Park, 2020). However, with the stirring, the rate of the desulfurization increased significantly by enlarging reaction interfacial area (Nakai et al., 2013). This was more severe when the stirring was executed by an impeller, which broke down the flux in smaller size aggregates (Jeong et al., 2021, Nakai et al., 2013). Since many modern steel plants carry out the desulfurization of hot metal in the KR process, in particular in Asia, it is now important to understand how the red mud addition affects the desulfurization efficiency under the mechanical stirring. Investigating this topic will be interesting in view of the possibility of using the red mud in commercial hot metal desulfurization processes. Therefore, in the present study, hot metal desulfurization tests were carried out in the present authors’ laboratory along with the mechanical stirring device. It will be shown that the red mud is indeed very effective and promising industrial waste to be recycled at no or very low cost for the desulfurization of hot metal.