Production of iron and oxygen in molten K2CO3–Na2CO3 by electrochemically splitting Fe2O3 using a cost affordable inert anode

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Abstract

Iron oxide was electrochemically split into iron and oxygen gas in molten Na2CO3–K2CO3 at 750 °C using a solid iron oxide cathode and a Ni10Cu11Fe alloy inert anode. Fe2O3 was electrochemically reduced to Fe on the cathode, releasing oxygen anions into the electrolyte and which were oxidized on the anode to generate O2. The cathodic current efficiency was as high as 95% and the energy consumption for producing 1 kg iron was 2.87 kWh, only half of the current industrial energy consumption of blast-furnace steel production. Due to the cost-affordable inert anode and the high energy efficiency, the method demonstrated in this work shows promise as a practical “green” iron production process.

Graphical abstract

Highlights

► Fe and O2 were produced simultaneously by electro-splitting of Fe2O3 in molten K2CO3–Na2CO3. ► A nickel alloy (Ni10Cu11Fe) was demonstrated a satisfactory inert anode in the melt. ► The current efficiency for iron production was as high as 95%. ► The process could be applied to produce other metals (Ni, Co Mn) and stainless steel.

Introduction

The modern iron and steel industry annually produces more than 1 × 109 tonnes of steel and releases more than 2 × 109 tonnes of CO2 accounting for 3%–4% of the total world greenhouse gas emissions [1]. Production of iron utilizing carbon-free processes would eliminate this major greenhouse gas emission source. Along with the development of renewable electricity, electrolytic production of iron in high temperature molten salts has drawn increasing attention in recent years [2], [3], [4], [5], [6], [7]. Reported methods for electrolytic production of iron in molten salts can be divided into three techniques: (1) electrodeposition of solid iron from molten salt such as molten lithium carbonate [5]; (2) electrowinning of liquid iron from high temperature molten oxide [6]; (3) electro-deoxidation of solid iron oxide in molten calcium chloride or molten alkaline hydroxide [3], [4], [7]. Solid iron produced by electrodeposition in molten salts is dendritic and difficult to separate the product from the molten salt [5]. Production of molten iron and oxygen using an iridium inert anode was recently realized by electrochemical decomposition of molten oxide above liquidus-temperature of iron [6], but a cost-affordable inert anode is still absent. Although the electrochemical reduction of solid iron oxide to iron in molten calcium chloride was demonstrated as a high energy efficiency process [7], an inert anode in the halogen molten salts is also very challenging [8], [9], [10], [11], [12]. Efforts to replace molten CaCl2 with molten hydroxide were performed [3], [4], but it was theoretically limited by the narrow electrochemical potential window and ambiguous mass transfer kinetics of oxygen anions in the melt.

An ideal supporting molten electrolyte for production of iron by electroreduction of solid iron oxide should meet the requirements of: (1) low solubility of iron oxide; (2) wide potential window; (3) facile mass transfer of oxygen ions; and (4) compatibility with an inert anode. Molten Na2CO3–K2CO3 has a wide electrochemical potential window [13], [14], [15], good oxygen anion conductivity and a weak ability to dissolve the transition metal oxides [5] so that it might be an ideal molten electrolyte for iron production. In this paper, we aimed to develop a more practical “green” iron production process in molten Na2CO3–K2CO3 by electrochemically splitting solid iron oxide and test the stability of a low-cost material (nickel alloy) as an inert anode.

Section snippets

Electrolyte and electrodes

About 500 g anhydrous Na2CO3–K2CO3 (Na2CO3:K2CO3 = 59:41 mol%) was put into an alumina crucible which was located in a closed-end sealed steel reactor. After drying at 150 °C for 12 h, the temperature was slowly raised up to 750 °C in argon to melt the carbonate. Porous Fe2O3 pellets (20 mm in diameter) were prepared by die-pressing of Fe2O3 powder and sintering in air at 750 °C for 2 h. All chemicals (AR grade) were purchased from Shanghai Chemical Regent Company. The pellets were attached to an iron

Results and discussion

The electrochemical potential window of molten K2CO3–Na2CO3 at the cathodic end is limited by the deposition of alkaline metals or carbon. The cyclic voltammograms (CVs) recorded from iron electrode and FCE were shown in Fig. 1a. The decomposition of the electrolyte takes place when the applied potential is shifted to − 1.5 V as evidence of the continuous increasing of the reduction current. For FCE, there are two more reduction peaks (c1 and c2) prior to the decomposition of the electrolyte,

Conclusions

Direct electrochemical production of iron and oxygen from solid Fe2O3 in Na2CO3–K2CO3 eutectic at 750 °C was realized using a cost-affordable inert anode. The Fe2O3 can be electrochemically reduced to low carbon iron in two steps. This work offers a green process for production of iron and oxygen from iron oxide in molten carbonate with high current efficiency and low energy consumption. Furthermore, this process can be also applied to production of other metals such as Ni, Co, Mn and stainless

Acknowledgments

The authors thank the NSFC (grant nos. , ), MOE of China (NCET-08-0416) and the Fundamental Research Funds for Central Universities of China for financial support. The authors also extend their thanks to Dr. James Yurko for his kind help on the English polishing.

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