Molten Salts Chemistry and Technology

Marcelle Gaune-Escard, Geir Martin Haarberg

• Publisher: Wiley
• Year: 2014
• Print ISBN: 9781118448731
• eBook ISBN: 9781118448823
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Written to record and report on recent research progresses in the field of molten salts, Molten Salts Chemistry and Technology focuses on molten salts and ionic liquids for sustainable supply and application of materials. Including coverage of molten salt reactors, electrodeposition, aluminium electrolysis, electrochemistry, and electrowinning, the text provides researchers and postgraduate students with applications include energy conversion (solar cells and fuel cells), heat storage, green solvents, metallurgy, nuclear industry, pharmaceutics and biotechnology.

Part 1

Aluminium Electrolysis

Chapter 1.1

Formation of CO2 and CO on Carbon Anodes in Molten Salts

J. Thonstad¹ and E. Sandnes²

¹Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), Norway

²Primary Metal Technology, Hydro, Norway

1.1.1 Introduction

There is a great body of information on the anode product on carbon anodes in cryolite-alumina melts, which is the electrolyte used in aluminium electrolysis. A typical electrolyte composition can be cryolite, that is Na₃AlF₆, with 12 wt% AlF₃ + 5 wt% CaF₂ + 2–6 wt% Al₂O₃, operating at around 960 °C. The consumable carbon anodes are baked into solid bodies from petroleum coke together with pitch, serving as binder.

For the use of carbon anodes in chloride melts containing oxides the information is much more scant. Sandnes et al. [1] presented polarisation curves on graphite anodes in NaCl-Na₂O and NaCl-CaCl₂-CaO melts. The content of CO and CO₂ in the anode gas was determined. As shown in the following, the CO₂/CO ratio varied widely. The work by Sandnes et al. [1] will be used to throw some light on the anode products as a function of composition and applied potential.

1.1.2 Results

1.1.2.1 Fluoride melts

The molten cryolite-based electrolyte (see above) is dissociated into Na⁺ cations and AlF₆³⁻, AlF₄⁻, F⁻ and various oxyfluoride anions, like Al₂O₄F₄⁶⁻. For aluminium electrolysis there are two possible cell reactions:

1.1.1

1.1.2

The reversible E⁰ of these reactions at 1000 °C are −1.187 and −1.065 V, respectively [2]. The difference in favour of Equation 1.1.2 reflects the fact that the so-called Boudouard reaction:

1.1.3

is strongly shifted to the right at this temperature.

The question whether CO₂ or CO is the primary anode product has been studied extensively [2 3] and it has been shown that, except at very low current densities, the primary product is CO₂ [2]. This conclusion has been based on carbon consumption studies (Equation 1.1.2) requires twice as much carbon per Faraday as Equation 1.1.1) and careful gas analysis, by avoiding disturbing side reactions, for example by using a diaphragm to separate the anode and cathode compartments [4]. Side reactions are reactions between CO₂ and carbon, either within pores in the interior of the anode, with carbon particles dispersed in the electrolyte, or with metal dissolved in the melt. In all cases the reaction product of these side reactions is normally CO. Dissolved aluminium can even reduce CO₂ all the way to carbon [5]. By bubbling CO₂ underneath a graphite anode it was shown [5] that, while Equation 1.1.3 did occur at zero current, the reaction ceased when the electrode was anodically polarised, even at quite low current densities (0.05–0.1 A cm⁻²).

Most of the studies cited above were conducted in the time period between 1950 and 1980. However, as early as in 1936 Drossbach [6] demonstrated in a very elegant way how thermodynamic arguments could be applied to clarify the problem of identification of the primary anode product. If we consider the equilibrium:

1.1.4

it is seen that the CO₂/CO ratio defines a partial pressure of oxygen. In principle we can consider the primary cell reaction being:

1.1.5

The standard reversible potential of this reaction is E⁰ = −2.213 V at 970 °C [2]. On an inert anode the anode product would be pure oxygen, so the anode potential (provided zero overvoltage) would be 2.213 V positive to an aluminium electrode. On a carbon anode the oxygen will react with carbon, and the partial pressure of oxygen will be much reduced and so would the anode potential. For the case of unit activity of alumina, the Nernst equation simply relates anode potential and oxygen partial pressure:

1.1.6

The so-called depolarisation, ΔE, that is the reduction in potential with respect to E⁰ of Equation 1.1.5, can then be writte...

Table of contents

• Cover
• Title Page
• Copyright
• List of Contributors
• Foreword
• Preface
• Part 1: Aluminium Electrolysis
• Part 2: New Processes for Electrowinning
• Part 3: Modeling and Thermodynamics
• Part 4: High-Temperature Experimental Techniques
• Part 5: Electrochemistry in Ionic Liquids
• Part 6: Nuclear Energy
• Part 7: Energy Technology
• Index
• End User License Agreement