Electrolytic Iron Production from Alkaline Bauxite Residue Slurries at Low Temperatures
Electrolytic Iron Production from Alkaline Bauxite Residue Slurries at Low Temperatures
Carbon-free electrochemical process for the production of metallic iron
Primary iron metallurgy is characterised by significant direct carbon dioxide emissions, due to the carbothermic reduction of the iron ore. This paper deals with the electrification of primary iron production by developing a new and innovative process for the carbon-free production of metallic iron from bauxite residue which is a byproduct of the alumina industry. It is based on the electroreduction of iron oxides from bauxite residue suspensions in concentrated sodium hydroxide solutions, at low temperature and normal pressure. The iron oxide source used in the present study is bauxite residue provided by MYTILINEOS SA, Metallurgy Business Unit-Aluminium of Greece. The research study is a preliminary screening of bauxite residue as a potential raw material for iron production by performing experiments in a small-scale electrolysis cell. The first results presented here show that iron can be produced by the reduction of iron oxides in bauxite residue with high Faradaic efficiency (>70%). Although significant optimisation is needed, the novel process shows great promise.
At present, primary iron metal is commonly produced through the CO2 intensive carbothermic reduction of iron oxides in blast furnaces at a temperature of around 1600°C. Since carbon is used as both reducing agent and fuel for the process, blast furnace pig-iron cannot eliminate its CO2 emissions. Therefore, in recent years, carbon-free electrochemical processes have been widely investigated as potential green alternative routes for the production of iron and iron-base alloys (1–4) (assuming renewable electricity).
A large project that has been focused on alternative ways of producing iron was Ultra Low CO2 in Steelmaking (ULCOS) in which new smelting reduction concepts were studied. The ULCOWIN electrolytic production of iron from suspensions of iron oxide particles in a highly concentrated sodium hydroxide solution at 110°C was demonstrated at laboratory scale. It has been shown that the iron particles are reduced in the solid state, which differs from the conventional electrowinning processes where the metal is deposited through the reduction of dissolved metal cations. Previous works with this process achieved high Faradaic yield (80–95%) (1–4).
Based on the above-mentioned studies, the technology for alkaline pulp iron electrowinning is being studied for the first time from a secondary mineral source, namely bauxite residue from the alumina refining industry. This technique is referred to in the SIDERWIN project (5), which aims among others to produce iron from alternative low-grade iron sources, currently incompatible with the conventional steel making processes.
Bauxite ore is treated within the Bayer process to produce metallurgical grade alumina which is the raw material for aluminium production. Bauxite ore depending on its origin contains 40–60% alumina and the rest is a mixture of iron (20–30%), silicon and titanium oxides. When bauxite ore is treated with caustic soda, the aluminium hydroxides or oxides contained within are solubilised, with approximately 50% of the bauxite mass being transferred to the liquid phase, while the remaining solid fraction constitutes the bauxite residue, often termed as ‘red mud’ due to its colour. Depending on the grade of the bauxite ore used, bauxite residue, on a dry basis, is produced from 0.9 to 1.5 mass ratio to the alumina product (6). The high volume and alkalinity of this byproduct make its valorisation a major challenge worldwide (7).
Bauxite residue is an untapped secondary raw material source considering the presence of valuable substances such as iron (30–45 wt%), aluminium (15–25 wt%), silicon, calcium, titanium and sodium oxides as well as smaller concentrations of critical or industrially important elements such as rare earth elements (REEs) (mainly cerium, lanthanum, scandium, yttrium and neodymium), vanadium, chromium and others (6). The recovery of the major metals from bauxite residue has never yet been implemented while a lot of processes have been proposed. Despite the laboratory-scale success of much of the work so far, currently the industrial utilisation of bauxite residue is estimated at just 2–4 million tonnes, accounting for less than 3% of the annual bauxite residue production (8).
The main constituent of bauxite residue is iron oxide and it can make up to 45% of the mass of the bauxite residue. In fact, the red colour of bauxite residue is caused by iron(III) oxides (mostly haematite, Fe2O3) (9). In general, due to its high alkaline (Na2O ≈ 10%) and its titanium (TiO2 ≈ 4–15%) content, bauxite residue is not suitable for use as an iron ore substitute in blast furnaces.
Bauxite residue reductive smelting processes can be applied by several technologies (Corex, Finex®, HIsmelt, Romelt, AusIron and electric arc furnace (EAF)) for the production of pig iron (10, 11). So far, two methods have been applied at pilot scale for bauxite residue reductive smelting: the Romelt method (12) and the EAF (13–16). The Moscow Institute of Steel and Alloys (MISA) (Russia), with National Aluminium Company Ltd (NALCO) and the joint venture company Romelt-Steel Authority of India Ltd (RSIL) (India), studied the pyrometallurgical process of bauxite residues using the Romelt method (12). The advantage of this method is that materials can be used with moisture levels of up to 10 wt%. The main disadvantage is the high energy consumption and the poor quality of pig iron with a high concentration of sulfur and phosphorus (10). In EAF reductive smelting, a mixture of bauxite residue, carbon and fluxes is treated at 1500–1700°C to form pig iron with higher than 95% iron recovery (6, 13, 17). Recovery of residual iron can be further improved by later magnetic separation in slag dust (18). Post-melting slag can be used to produce rockwool or building materials (6, 19) and for the recovery of non-ferrous metals and REE (9, 18, 20, 21). However, such methods have not been industrialised as they are not competitive to established iron and steel making processes.
The present paper describes a highly promising electrochemical method for sustainably extracting the iron from bauxite residue, in an alkaline environment, which is in-tune with the alkaline environment of the Bayer process.
2. Materials and Methods
The process is conducted in an electrolysis cell consisting of a borosilicate glass beaker (250 ml) closed with a specially configured cylindrical silicon bung (45 cm diameter). The experimental apparatus is shown in Figure 1.
A three-electrode configuration was used; the cathode was a rectangular shaped stainless steel V2A plate (110 mm height, 10 mm length, 1 mm width), the anodes were two rectangular shaped nickel plates (100 mm height, 10 mm length, 2 mm width). All electrodes were centred according to the silicon bung’s cylindrical axis in specially configured holes so that the electrodes were in certain positions and at fixed distance to each other. The working electrode’s surface area that was immersed in the solution was defined to be 8 cm2. The reference electrode that was used was a commercial Hg | HgO | NaOH (1 M) electrode (RE-61AP, ALS Co Ltd, Japan) which was immersed in a distinct glass.
Bauxite residue was supplied by MYTILINEOS-Aluminium of Greece. Samples were solubilised via fusion method according to which a quantity of bauxite residue remained at 1000°C for 1 h with a mixture of Li2B4O7/KNO3 followed by direct dissolution in 6.5% nitric acid solution. Chemical analysis was performed by atomic absorption spectroscopy (AAS) with the use of a PerkinElmer 2100 atomic absorption spectrometer. The chemical analysis of the samples used in this study is shown in Table I. The chemical analysis showed that Fe2O3 is the main content of bauxite residue.
|Fe2O3||Al2O3||SiO2||TiO2||CaO||Na2O||Loss on ignition (LOI)|
Mineralogical analysis of bauxite residue (Figure 2) showed that the main iron oxide phase is haematite while a small amount of goethite also exists. The haematite to goethite mass ratio in bauxite residue is 4.2 (22).
Prior to each experiment, the stainless-steel cathode and the nickel anodes were polished with sandpaper and were rinsed with demineralised water. Furthermore, the cathode was weighed. The electrolyte was a 50 wt% NaOH aqueous solution corresponding to molarity of 25 mol kg–1, to which was added 10 wt% bauxite residue solid particles. Typically, a mixture of 152.4 g solid NaOH with purity higher than 99% (CHEM-LAB NV, Belgium) and 152.4 g demineralised water were mixed under stirring for about 30 min. After the electrolyte’s homogenisation, 33.9 g of bauxite residue were slowly added to the solution within 5 min.
The slurry temperature was measured via a probe that was wrapped in a polytetrafluoroethylene (PTFE) shrink tubing to avoid current leakage. The slurry was stirred using a 1 cm ringed cylindrical magnetic bar and magnetic hot plate (IKA® RCT basic, IKA Works Inc, USA) at a rotational speed of 500 rpm to keep bauxite residue particles suspended. The small size of the magnetic bar was chosen to minimise the attraction of magnetite particles to the stirrer. The cathode, the anodes and the reference electrode were connected via their respective terminal to a potentiostat (SourceMeter® 2461 Series, Keithley, Tektronix Inc, USA).
Cyclic voltammetry and electrolysis tests, under chronopotentiometry mode, were performed in a three electrode cell connected to a potentiostat (2461 Series, Keithley) and the obtained experimental data were analysed. Regarding cyclic voltammetry, it should be noted that both working and counter electrode were platinum wires while the reference electrode was the above mentioned Hg/HgO commercial electrode.
Electrolysis tests were performed under galvanostatic mode. The duration of the experiment was 2 h. After the end of each experiment, the cathode was thoroughly rinsed with distilled water in order to remove the remaining electrolyte, and was dried at 100°C for 24 h, before being weighed. The difference between the mass of the cathode prior to and after the end of electrolysis was considered as the mass of the deposit. This value was used to deduce the current efficiency according to Faradaic law.
2.1 Determination of Metallic Iron
According to previous studies, the haematite reduction mechanism to form metallic iron on the cathode has magnetite as an intermediate product (2). For that reason, a quantitative determination of the deposit took place to identify the percentages of both metallic iron and magnetite phases.
The principle of the method is based on the selective dissolution of metallic iron from a 2% bromine solution in ethanol in a mixture with its oxides. According to the procedure, 100 ml of bromine (Acros OrganicsTM, Thermo Fisher Scientific, USA) solution 2% in ethanol (EMSURE®, Merck, Germany) was prepared and 0.2 g of sample powder was added in a 250 ml conical flask. The solution was let to stir at ambient temperature for 90 min and then filtered using a 47 mm diameter glass fibre filter (Whatman®, Cytiva, USA) (23). The resulting solution was titrated into a 500 ml flask with deionised water. Finally, the solution was diluted and measured in an atomic absorption spectrometer (PerkinElmer 2100).
3.1 Cyclic Voltammetry
A voltammogram of bauxite residue (10 wt%) with 50 wt% NaOH solution at 110°C and scan rate 100 mV s–1 is shown in Figure 3. Iron oxide from bauxite residue (mainly haematite) is reduced to metallic iron in the region of cathodic potentials from –1.2 V to –1.4 V with a peak at –1.36 V. Hydrogen evolves at more negative potentials lower than –1.4 V. The plateau observed at cathodic potentials between –1.0 V and –1.2 V is attributed to reduction of haematite to magnetite which is always taking place in the system under study as is seen in Raman spectra of a typical deposit in Figure 4. The peak observed at anodic scanning at about –0.7 V is attributed to the reversible oxidation of iron.
The electrochemical reactivity of the iron oxides of bauxite residue was evaluated by galvanostatic electrolytic experiments. The parameters that were tested were current density and slurry temperature.
3.2 Effect of Current density
Galvanostatic experiments were performed in different applied currents (62.5 A m–2, 156.3 A m–2, 312.5 A m–2, 625 A m–2, 937.5 A m–2, 1250 A m–2) while all other factors remained constant (50 wt% NaOH, 10 wt% bauxite residue, temperature 110°C and stirring rate 500 rpm). The open circuit potential between the working electrode and the reference electrode was –0.968 V.
The cathodic potential vs. Hg/HgO is shown in Figure 5. As expected, the increase of the applied current results in more negative values of cathodic potential indicating more intense reductive conditions at the cathode. The applied currents between 62.5 A m–2 and 312.5 A m–2 gave constant cathodic potential values for the whole duration of electrolysis tests in the range of –1.2 V to –1.4 V which coincide fully with the region of haematite reduction to metallic iron as shown in the voltammogram of Figure 3. Applied current densities higher than 312.5 A m–2 push the cathodic potential towards the hydrogen evolution region (<1.4 V) as seen in Figure 3. In all experiments the cathodic potential was lower than the open circuit potential and therefore the cathodic material was stable and not corroded.
The calculated Faradaic efficiency for each experiment, taking into account the purity of metallic iron in the deposit that was determined to be 89–91% in all experiments, is shown in Table II. Particles of magnetite which are formed by the reduction of haematite appeared as impurities on the cathode surface. In conclusion, the metallic iron produced is extremely pure and the only processing needed is a melting process to produce pure iron ingots.
|Current density, A m–2||Current efficiency, %|
As seen, the lower the applied current, the higher the Faradaic efficiency. The lowest applied currents in the region of 62.5 A m–2 to 312.5 A m–2 gave the highest current efficiencies which were very close to 50%. Even this current efficiency is very low indicating the strong presence of parallel unwanted cathodic reactions such as the hydrogen evolution reaction as well as unavoidable cathodic reactions such as the reduction of haematite to magnetite that is always taking place in this system. The intense hydrogen evolution even at the lowest applied currents where the measured cathodic potentials was higher than that of hydrogen reduction indicates a cathode with non-uniform potential. This is reasonable because the cathode is covered by electroactive haematite particles as well as non-electroactive particles coming from the bauxite residue. The electroactive species are the iron oxides haematite and goethite and the non-electroactive particles are all the other phases in bauxite residue such as diaspore, cancrinite, calcite, hydrogarnet, perovskite, gibbsite, boehmite and anatase. Therefore, the current distribution on cathode is non-uniform giving rise to areas with charge accumulation (located where the non-electroactive species are concentrated) and thus more negative potential in relation to the other areas where the electroactive haematite particles are located.
3.3 Effect of Temperature
The temperature effect was studied in the range 70–135°C while the other electrolysis parameters were kept constant (50 wt% NaOH, 10 wt% bauxite residue and stirring rate 500 rpm). The applied current density was selected to be 156.3 A m–2 as this value resulted in the highest Faradaic yield (48.84%) in the previous experimental series. The cathodic potentials vs. Hg/HgO are shown in Figure 6. As seen, the cathodic potentials within the whole duration of all electrolysis experiments remained in the region from –1.2 V to –1.4 V which coincides with the region where the haematite is reduced to metallic iron (Figure 3). In addition, there is a tendency for less negative cathodic potentials (milder reductive conditions) as the process temperature increases. The calculated Faradaic efficiencies are shown in Table III. The increase of temperature from 70°C to 130°C resulted in a steady almost linear increase of Faradaic efficiency from 11.23% to 71.58%. Step-up temperature to 135°C caused a slight decrease in current efficiency to 59.76% which is still higher than that at 120°C.
|Temperature, °C||Current efficiency, %|
As mentioned above, the cathode surface is covered with electroactive and non-electroactive particles coming from the bauxite residue. The electroactive particles are strongly attached to the cathode surface due to their partial reduction to metallic iron while the non-electroactive particles are loosely attached. The temperature increase causes an increase in the rate of heterogeneous nucleation of water bubbles on the particles’ surface due to vapour pressure increase. Therefore, the probability of removing the loosely attached non-electroactive particles from the cathode surface increases and thus the probability of replacing the non-electroactive with electroactive ones increases. At higher temperatures the percent coverage of the cathode with electroactive species increases and thus the Faradaic efficiency increases. At temperatures close to the boiling point of 50 wt% NaOH solution (that is 143°C), the water bubbling is so intense that the electroactive particles also start to detach from the cathode surface and therefore decrease the Faradaic efficiency. A compromise is achieved at an intermediate temperature which in this case is 130°C.
In any case, the Faradaic efficiencies achieved during the bauxite residue pulp electrolysis are substantially lower than those achieved in iron ore pulp electrolysis (3) where the cathode is uniformly covered by only electroactive particles.
The present work has demonstrated the possibility to electrochemically reduce iron from bauxite residue in alkaline pulps. The process temperature proved to be the most crucial parameter that substantially affects the Faradaic process efficiency. At the current level of process development, a Faradaic efficiency of 71.58% was achieved at pulp density of 10 wt% bauxite residue in a 50 wt% NaOH solution at 130°C. The applied current has to create a cathodic potential higher than –1.4 V vs. Hg/HgO (in 1 M NaOH) in order to avoid the hydrogen evolution which takes place at cathodic potentials lower than –1.4 V.
The cathode in the case of bauxite residue pulps electrolysis is not uniformly covered by electroactive iron oxide particles (mainly haematite) and therefore there is not a uniform cathodic potential on the whole cathode surface. This affects the current efficiency of the process which is always substantially lower than that observed in pure haematite ore electrolysis.
Bauxite residue is produced as a byproduct of the Bayer process, an alkaline leaching process taking place at temperatures 120–250°C. Therefore, the present process has great potential for integration in the established Bayer process as a symbiotic step to valorise the (currently wasted) iron portion of the bauxite ore.
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Sevasti Koutsoupa studied Mining and Metallurgical engineering at National Technical University of Athens, Greece, and currently she is a PhD candidate in the School of Mining and Metallurgical Engineering on the topic of “Iron oxide Electrowinning from Bauxite Residue”. Currently she is involved in the research of SIDERWIN and SCALE projects.
Stavroula Koutalidi is a Chemical Engineer from the National Technical University of Athens. She obtained a Master of Science in Industrial Pharmacy from the National Kapodistrian University of Athens. She is currently a senior researcher in the School of Mining and Metallurgical Engineering in the field of Electrometallurgy and she is involved with SIDERWIN and SCALE projects.
Evangelos Bourbos is a graduate of the Chemical Engineering Department of the National Technical University of Athens. He obtained a Master of Science, for which he received an award from the Limmat foundation, in Protection of Monuments, Sites and Complexes with emphasis in Conservation Interventions: Techniques and Materials directed by the School of Architecture Engineering of the National Technical University of Athens. He is a PhD candidate in the School of Mining and Metallurgical Engineering in the field of electrometallurgy under the topic of “Electrorecovery of Rare Earth Metals from Low Temperature Electrolytes as an Alternative to Molten Salts Electrolysis”. He has worked as a researcher in one national and three European projects (EURARE, SCALE, SIDERWIN) over the past six years. He has four scientific publications in peer reviewed scientific journals and books and has also actively participated in various international and European conferences and training courses. He is currently working for Titan Cement SA.
Efthymios Balomenos studied Mining and Metallurgical engineering at National Technical University of Athens and received his PhD degree in thermodynamics in the same school in 2006. Since 2008 he has been working in the Laboratory of Metallurgy as a postdoctoral researcher focusing on sustainable process development, CO2 mitigation strategies, exergy analysis and resource utilisation efficiency. He was involved in the research management of the ENEXAL and EURARE projects and is involved in the ongoing SCALE, ENSUREAL, REMOVAL, SIDERWIN, BIORECOVER and AlSiCaL research projects. He has more than 40 research publications in journals and conference proceedings with more than 170 citation and an h-index of 8. Since 2015 he is also working for MYTILINEOS SA Metallurgy Business Unit, focusing on promoting sustainable solutions for the valorisation of bauxite residue.
Dimitrios Panias is a metallurgical engineer. He graduated from the School of Mining and Metallurgical Engineering of the National Technical University of Athens in 1984. He completed his doctoral thesis on gold pyrometallurgy at National Technical University of Athens in 1989. Currently, he is Professor in Extractive Metallurgy at National Technical University of Athens teaching Chemistry, Non-Ferrous Metals Extractive Metallurgy and Transport Phenomena. His research interests include but are not limited to extractive metallurgy, electrometallurgy, waste valorisation, wastewater treatment, geopolymerisation as well as chemical processing of ores and metallurgical wastes with ionic liquids.