Simulation-based life cycle assessment of ferrochrome smelting technologies to determine environmental impacts

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Highlights

  • Simulation based life cycle assessment of ferrochrome production demonstrated.

  • Estimation of greenhouse gas emissions for two ferrochrome smelting technologies done.

  • Technology using rotary kiln has higher emissions at 3.72 tCO2/ton ferrochrome.

  • Prereduction reduces smelting energy consumption but not net greenhouse gas emissions.

  • Carbon neutral electricity and biocarbon use recommended for low greenhouse emissions.

Abstract

Decarbonization of metal production is currently a unique challenge for the industry. To gain insights, environmental impacts of ferrochrome smelting technologies were estimated using simulation-based life cycle assessment. Two leading technologies: (1) Steel Belt Sintering-Submerged Electric Arc Furnace (SBS-SAF), and (2) Rotary Kiln-Submerged Electric Arc Furnace (RK-SAF) were investigated. Four environmental impact categories: climate change, acidification, particulate matter, and resource use (minerals and metals), were considered. Results showed that GHG emissions for producing high carbon ferrochrome vary to a greater extent depending on location of processing plant because of differences in electricity emission factors. For example, South African energy grid generates more GHG emission than Finnish energy grid. Furthermore, though prereduction reduced SAF energy consumption, it did not necessarily result in reduced net GHG emissions due to high coal consumption of RK. Acidification and particulate matter were higher when using RK-SAF technology. Ferrochrome production generally had low impact on resource use.

Introduction

Ferrochrome is the most important alloy in the production of stainless steel. It is estimated that 90% of global ferrochrome produced is consumed by the stainless-steel industry (Basson and Daavittila, 2013). For this reason, ferrochrome production is closely linked with stainless steel production. In the past 5 years, high carbon ferrochrome production has seen a steady increase from 10 732 ktonnes in 2015–13713 ktonnes in 2019 (CRU, 2018). This coincide with the stainless-steel demand for the same period.

Chromium alloying (10.5–12%) renders steel stainless by the formation of a thin and tenacious chromium oxide layer, the passive layer, that protects the underlying material from corrosion. (Cunat, 2004). Ferrochrome alloys are classified based on their C and Cr contents, as low carbon (0.015–1% C, 56–70% Cr), intermediate (1–4%C, 56–70% Cr), high carbon (4–6% C, >60% Cr) and charge (6–8% C, 50–60% Cr) ferrochromium (Basson and Daavittila, 2013; Haque and Norgate, 2013). Today, alloys with composition of either high carbon or charge are both considered as high carbon ferrochrome alloys (Basson and Daavittila, 2013). Primary source of ferrochrome is chromite ore. More than 70% of known world reserves of chromite ores are geographically concentrated in South Africa (Basson and Daavittila, 2013). Consequently, a large proportion of processing facilities are in South Africa.

Production of ferrochrome from chromite ores is typically by concentrating, pelletizing of chromite fine ore, and sintering (Basson and Daavittila, 2013; Haque and Norgate, 2013) or prereduction (Basson and Daavittila, 2013; Eric, 2014) before a carbothermic reductive smelting process (Mc Dougall, 2013; Eric, 2013). The smelting process to produce the alloy is conventionally done in a submerged arc furnace (SAF). Smelting in a SAF is an energy intensive process (Basson and Daavittila, 2013). The energy is supplied via an arc at the tip of three electrodes. The reductant is typically carbon and is supplied through metallurgical coke. Most used feed is sintered pellets, prepared using Steel Belt Sintering technology (Hekkala et al., 2004), Rotary kiln (Agarwal et al., 2016; Naiker, 2007) or Shaft Kiln (Harman and Rao, 2007). Smelting is therefore done by the following three technology combinations: (i) SBS-SAF, (ii) RK-SAF, and (iii) SK-SAF (Ramakrishna, G. and Srikakulapu, 2017). Of the three combinations, SBS-SAF and RK-SAF are by far the most widely used around the world, and thus form the basis of this research. The SK-SAF is currently a popular option only in Asia, especially China and India. Pre-smelting steps consume large amounts of carbon which can be in form of coal or metallurgical coke. Because of high energy and carbon consumption, ferrochromium production like other ferroalloy smelting processes produce large amounts of GHG emissions.

Determination of the environmental impact on the natural ecosystem requires accounting for all process input and output materials including emissions and effluents coming out of the process. For a complex process such as smelting, this is a demanding task. However, using established simulation tools such as HSC chemistry software (Roine, 2020) and coupling them to Life cycle assessment (LCA) tools, a reliable and reproducible estimation of impacts can be achieved (Llamas et al., 2019; Reuter and Van Schaik, 2015). Life cycle assessment (LCA) is an internationally standardised and applied method to evaluate environmental impacts. Several commercial tools supporting the quantification are available.

One approach to LCA is collecting data from literature and using it directly to compute environmental impacts. Using this approach, Haque and Norgate have estimated GHG emissions of different ferroalloy production processes with a focus on Australia as location of production plants (Haque and Norgate, 2013). In their study, LCA was done using SimaPro (software and database). When assessing environmental impacts of an industrial process such a direct approach is impracticable because of large number of interacting variables. This research uses a simulation-based approach to life cycle assessment. Steady state simulation of the industrial ferrochrome processes was done using HSC Sim software (version 10.0), an established process modelling tool. Simulation life cycle inventory (LCI) data were directly linked to OpenLCA, for life cycle assessment. Advantages of such an approach is that mass and energy balances of individual units as well as the entire process are simultaneously accounted for and computed in a methodical way. Simulation with HSC takes advantage of thermodynamic data such as enthalpies, heat capacities, entropy, etc. (Reuter and Van Schaik, 2015).

Section snippets

Technologies for ferrochrome production

Smelting of chromite concentrate is done in a submerged arc furnace and in most cases using pelletized feed. However, there are several variations on technologies mostly in the preparation of pelletized feed for smelting. The Steel belt sintering and Rotary kiln for pelletized feed preparation are discussed in detail in this section. The term process technology is used to refer to SBS-SAF or RK-SAF methods:

Process simulation with HSC

Two smelting process technologies were compared based on process simulation of each flowsheet and using thermodynamics capabilities of the HSC chemistry software version 10.0 (Roine, 2020). Each process flowsheet has been simulated with the Simulation Module (SIM) and the pyro distribution units. HSC Chemistry database is equipped with essential thermodynamic data to perform calculations of the model. Simulation with HSC is graphical flowsheet based, meaning the user can create process units

Life cycle assessment

Cradle to gate boundaries of life cycle assessment for ferrochrome production process are depicted in Fig. 3. All process steps with relevant contributions to the environmental impacts were included. The dotted lines show ferrochrome process steps which were included in the HSC simulation flowsheet. Environmental impacts of input streams that are outside doted box were accounted for from the ecoinvent database sources (version 3.5). For sources of materials in ecoinvent database, market sources

Results and discussion

High voltage electricity (HV) in Table 1, Table 2, Table 3 represents the amount of electricity consumed by the electric furnace for carbothermic reduction of chromite pretreated pellets or a combination of pellets and lumpy ore (Table 2). Medium voltage electricity (MV) is utilized for other purposes such as milling of feed before pelletizing. It should be noted that SBS-SAF technology in both cases only use metallurgical coke while RK-SAF additionally use coal (anthracite). The coal is used

Conclusions

Using an HSC Simulation-based life cycle assessment, the environmental impacts of ferrochrome production process have been investigated with focus on four environmental impact categories: climate change, acidification, particulate matter, and resource use. Two locations for technologies have been preselected based on their important roles in the industry. Based on the results, The main sources of GHG emissions for production of high carbon ferrochrome are carbon sources and electricity (ca.

CRediT authorship contribution statement

Joseph Hamuyuni: Data curation, Conceptualization, Methodology, Investigation, Writing – original draft. Hannu Johto: Visualization, Investigation. Ali Bunjaku: Visualization, Investigation. Saija Vatanen: Validation, Writing – review & editing. Tiina Pajula: Validation, Writing – review & editing. Pasi Mäkelä: Supervision, Writing – review & editing. Mari Lindgren: Supervision, Writing – review & editing.

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.

Acknowledgements

This research was financially supported by the Carbon Handprint Project of Business Finland.

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