Current density distribution and gas volume fraction in the gap of lantern blade electrodes

https://doi.org/10.1016/j.cherd.2013.10.003Get rights and content

Highlights

  • Improved design of gas-evolving anodes formed by parallel flat blades.

  • Model for prediction of current density distributions.

  • Use of segmented electrodes for determination of local current density.

  • Determination of gas holdup by use of high-speed camera.

  • Comparison of experimental and theoretical current distributions.

Abstract

Electrochemical processes involving gas evolution at appreciable rates have been optimized in their design for the sake of reduced energy consumption. The present investigation was conducted in view to reducing the energy demand of a pilot process for electrolytic reduction of hematite particles to iron metal; attention was paid at the design of the lantern blade anodes where oxygen evolution occurs. An experimental cell consisting on two facing anodes and two remote cathodes has been designed and used for investigation of the gas behavior and current density distribution at the anode blades. The model for prediction of secondary distributions was validated by measurement of the currents at the segmented anodes and the effects of the average current density and the anode gap could be observed. The model was finally applied to the pilot cell for iron production; as expected, larger gaps allow more uniform current distributions at the anode, however without reducing the cell voltage. In contrast, blade lengths in the order of 10–15 × 10−3 m only would allow visible reduction in the cell voltage.

Introduction

Electrochemical processes involving the production of gas have been optimized for decades in their design and operation for minimum energy consumption, by both improvement of the electrode materials and minimization of the ohmic drop, which is to be enhanced by the presence of non-conducting gas bubbles in the cell gap. The design of gas evolving electrodes has been the topics of numerous investigations; a possible design allowing prompt disengagement of gas bubbles from the solution has often consisted in lantern blade electrodes (Kuhn, 1971, Schmitt et al., 1995) formed by a series of inclined parallel blades. Bubbles evolving from the surface of the two faces of the blades leave the cell upward by Archimede's forces, with little influence of the possible convection of the fluid between the anode and the cathode.

In addition to usual electrochemical processes for gas production, e.g. water electrolysis or chlorine-alkali process, gas evolution can occur at the counter electrode of cells designed for other applications such as electrosynthesis or metal deposition. In particular, an electrochemical process has been suggested years ago for production of steel and iron from iron ores such as hematite through their cathode reduction in a 50% NaOH solution at temperature near 110 °C (Allanore et al., 2007, Allanore et al., 2010, Lavelaine and Allanore, 2008); this process far from commercialization had been proposed as an alternative from the conventional carbon-based reduction of iron oxide into iron with tremendous CO2 emissions. As a matter of fact, in the current technology around 1.9 tons of CO2 are produced per ton of iron (Worldsteel Association, 2012); 25% of CO2 emissions from industrial activities originate from iron industry (International Energy Agency, 2007). The following reactions occur in the iron production cell:At the cathode: Fe2O3 + 3H2O + 6e  2Fe + 6OHAt the anode: 6OH  (3/2)O2 + 3H2O + 6eGlobal reaction: Fe2O3  2Fe + (3/2)O2From the differences in free energy and enthalpy of reaction (3), the reversible cell voltage and the thermoneutral voltage are calculated at 1.245 V and 1.422 V at 100 °C respectively. The process still under development is a three-phase process, with the presence of hematite and iron as solids, sodium hydroxide solution, and oxygen bubbles as gas phase. In addition to strongly reduced CO2 emissions – in particular when electricity is not produced by power plants – the electrochemical technique should allow appreciable reduction in the energy consumption; this can be attained by improved current efficiency at the cathode and reduced cell voltage, now in the order of 1.8 V at 1000 A m−2 (Lavelaine and Allanore, 2008). The deviation from the reversible voltage is due to overpotentials of oxygen evolution at the anode and deposition of iron metal at the cathode, and to the ohmic drop in the cell, in the order of 140 mV at 1000 A m−2 in the present conditions. Reducing the irreversibility sources in the cell is to reduce the cell voltage and the energy consumption as consequence. In addition to drastic reduction in CO2 emissions, the electrolytic route is expected to allow iron production with an energy consumption of 3.7 MWh ton−1, in comparison to coal-based processes which consume at least 4 MWh ton−1.

A pilot cell for electrolytic reduction of hematite – a common iron ore – had been imagined and designed by ArcelorMittal for iron ore electrolysis (Allanore et al., 2007, Allanore et al., 2010, Lavelaine and Allanore, 2008); the design has been inspired from current electrochemical processes for gas production (Kuhn, 1971, Schmitt et al., 1995). The industrial pilot cell consists in a flat plate inclined cathode and the anode formed by successive blades oriented perpendicular to facilitate the disengagement of oxygen bubbles formed at their surface in the gap, being a few millimeters broad. Both electrodes have been machined out of nickel. The cell is operated with continuous flow of hematite suspension from the top of the cell (Fig. 1). The inclination of the cell was chosen at 45° for regular transport of the hematite suspension in the cell with possible contact of the particles (with a diameter in the order of 10 μm) at the cathode, and for easier evacuation of the electrogenerated gas. The fed suspension is composed of little soluble hematite particles, sodium hydroxide, and water, each of components at the same weight fraction as made in previous works (Allanore et al., 2007, Allanore et al., 2010). The non-reacted part of hematite exits the cell by the bottom outlet with the liquid phase. Bubbles exit the cell by the top outlet of the cell as shown in Fig. 1. The dimensions of the various cell components had not yet been the topic of a full optimization. It can be imagined that long blades could favor gas-lift circulation of both the fluid and the gas; however, too long anode blades are to be avoided since the current density in electrode regions too far from the counter electrode is to be very low in comparison with that on the blade area facing the cathode.

The present investigation deals with the effect of gas bubbles on the ohmic drop in the cell, more particularly in the gap between two anode blades; the anode design is known to be of great importance in the cell voltage. Therefore, this work has been carried out in view to determining the optimal anode design for the sake of minimal value of the ohmic drop, for fixed conditions of temperature and electrolyte composition. Although carried out for the present case of iron electrolytic production, the methods employed could be used for any electrochemical process allowing gas production. For the present work, a laboratory test cell has been designed and constructed to investigate the effect of the anode gap and the blade length on the gas behavior and its impact on the current distribution. First the volume fraction of the gas – linked to hydrodynamics of gas bubbles between two neighboring blades – has been measured depending on the anode gap. In a second part, current density (cd) distributions in the laboratory cell have been determined by both measurements on segmented anodes embedded in the blades, and calculations taking into account the presence of the gas in the anode gap. The simulation tool was then applied to the pilot cell for calculation of the cd distributions along the anode blade, which led to possible recommendations for the improved design in view to reducing the ohmic drop and the energy consumption.

Section snippets

Methodology

Since the main purpose of this work concerns optimization of the anode geometry, only the anodic reaction, i.e. oxygen evolution in alkaline solution, and phenomena related to bubbles generation and flow will be examined. For the sake of simplicity in the experimental investigation, the test cell to be used for measurements of the current distribution at the anode blades, has been designed as a water electrolysis cell, with oxidation of OH species at the anode, and reduction of water to

Gas volume fraction

Use of the high-speed camera could yield the gas volume fraction at a given height x in different geometrical configurations (e = 2, 4 and 6 × 10−3 m), and for different current densities at the cathode. Coordinate x was defined from the lower edge of the first (lowest) nickel segment. Image processing was carried out with a Matlab program to convert the real image to a binary image. The surface fraction of the gas was calculated from 500 images for each experimental point, with a standard deviation

Conclusion

This work was aimed at investigating the effect on gas bubbles in cells provided with lantern blade electrodes, in view to optimizing the cell design for reduced ohmic drop in an electrochemical process for iron production. Investigations conducted with a laboratory test cell consisting of two facing anodes for oxygen evolution and two remote cathodes, allowed the system to be characterized depending on the current density and the anode-to-anode gap. The current density distributions could be

Acknowledgments

This investigation has been funded by ANR with the project “ASCoPE” (Acier sans CO2 par electrolyse) (ANR-EESI-2009). Thanks are due to the mechanical workshop and to the microelectronic service of the laboratory for construction of the cell and development of multichannel systems for current measurements.

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