Industrial To Pamela, Heather and Steven; Gill, Linda and Ian Industrial Electrochemistry

SECOND EDITION

Derek Pletcher Department of Chemistry, University of Southampton and

Frank C. Walsh Department of Chemistry, Portsmouth Polytechnic First edition 1982 Paperback edition 1984 Second edition 1990 Paperback edition 1993

© 1982, 1984, 1990, 1993 Derek Pletcher and Frank C. Walsh Originally pub1ished by Blackie Academic & Professiona1 in 1993 Typeset in 1 0/12pt Times by Macmillan India Ltd, Bangalore 560 025

ISBN 978-0-7514-0148-6 ISBN 978-94-011-2154-5 (eBook) DOI 10.1007/978-94-011-2154-5

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers at the Glasgow address printed on this page. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data available Contents

Preface Vlll

Symbols Xl

1 Fundamental concepts 1 1.1 Electron transfer 8 1.2 Mass transport 18 1.3 The interplay of electron transfer and mass transport control 30 1.4 Adsorption 32 1.5 Electrocatalysis 38 1.6 Phase formation in electrode reactions 48 1.7 Chemical reactions 50 1.8 The properties of electrolyte solutions 51 1.9 The assessment of cell voltage 54 1.10 Electrochemistry at surfaces on open circuit 55 Further reading 58

2 Electrochemical engineering 60 2.1 General considerations 60 2.2 Costing an electrolytic process 64 2.3 Performance and figures of merit 70 2.4 r.arameters 91 2.5 Principles of cell design 95 2.6 The additional technology of electrolytic processes 109 2.7 TypicaL-cell designs 141 2.8 Laboratory data and scale-up 166 Further reading 171

3 The chlor-alkali industry 173 3.1 General concepts of brine electrolysis 175 3.2 Modern technological developments 177 3.3 cell technologies 184 VI Contents

3.4 The production of hydroxide 208 Further reading 209

4 The extraction, refining and production of metal 210 4.1 211 4.2 Cementation 228 4.3 Electrorefining 231 4.4 Electrodeposition of metal powders 245 Further reading 247

5 Other inorganic electrolytic processes 249 5.1 249 5.2 electrolysis 256 5.3 chlorate and sodium bromate 269 5.4 Peracids and their salts 274 5.5 Potassium permanganate 275 5.6 Potassium dichromate and chromic acid 278 5.7 peroxide 279 5.8 Ozone 282 5.9 Manganese dioxide 288 5.10 Cuprous oxide 290 5.11 Synthesis of metal salts via anodic dissolution 291 Further reading 292

6 Organic electrosynthesis 294 6.1 The hydrodimerization of acrylonitrile 298 6.2 Other commercial electro synthetic processes 311 6.3 Indirect electrosynthesis 326 6.4 The future of electro synthesis 329 Further reading 330

7 Water purification, effluent treatment and recycling of industrial process streams 331 7.1 Metal ion removal and metal recovery 333 7.2 Hypochlorite and low-tonnage chlorine electrolysers 353 7.3 Electrodialysis 358 7.4 The treatment of liquors containing dissolved chromium 364 7.5 Electrolytic methods of phase separation 374 7.6 Flue-gas desulphurization 379 7.7 Other electrochemical processes 382 Further reading 384

8 Metal finishing 385 8.1 Electroplating 386 8.2 Electroless plating 424 Contents VB

8.3 Conversion coatings 434 8.4 Electrophoretic painting 441 8.5 Other related surface-finishing techniques 447 Further reading 448

9 Metals and materials processing 451 9.1 Electroforming 451 9.2 Electrochemical machining 457 9.3 Electrochemical etching 468 Further reading 479

10 Corrosion and its control 481 10.1 Fundamentals of corrosion 483 10.2 The thermodynamics of corrosion 489 10.3 The kinetics of corrosion reactions 498 10.4 Corrosion problems in practice 509 10.5 Corrosion prevention and control 518 10.6 Corrosion problems in electrolytic processing 536 10.7 Corrosion measurement and monitoring 538 Further reading 541

11 Batteries and fuel cells 543 11.1 Battery characteristics 546 11.2 Battery specifications 551 11.3 Evaluation of battery performance 554 11.4 Battery components 555 11.5 Present battery systems 559 11.6 Batteries under development 584 11.7 Fuel cells 590 Further reading 595

12 Electrochemical sensors and monitoring techniques 596 12.1 Electrochemical procedures 596 12.2 Polarography to anodic stripping voltammetry 596 12.3 Ion-selective electrodes 603 12.4 Portable and on-line devices 609 12.5 Electrochemical biosensors 618 12.6 Electrochemical detector cells for high-performance liquid chromatography (HPLC) 624 12.7 Miscellaneous 634 Further reading 636

Index 639 Preface

The objective of this second edition remains the discussion of the many diverse roles of electrochemical technology in industry. Throughout the book, the intention is to emphasize that the applications, though extremely diverse, all are based on the same principles of electrochemistry and electrochemical engineer• ing. Those familiar with the first edition will note a significant increase in the number of pages. The most obvious addition is the separate chapter on electrochemical sensors but, in fact, all chapters have been reviewed thoroughly and many have been altered substantially. These changes to the book partly reflect the different view of a second author as well as comments from students and friends. Also, they arise inevitably from the vitality and strength of electrochemical technology; in addition to important improvements in tech• nology, new electrolytic processes and electrochemical devices continue to be reported. In the preface to the first edition it was stated: ... the future for electrochemical technology is bright and there is a general expectation that new applications of electrochemistry will become economic as the world responds to the challenge of more expensive energy, of the need to develop new materials and to exploit different chemical feedstocks and of the necessity to protect the environment. The preparation of this second edition, seven years after these words were written, provided an occasion to review the progress of industrial electro• chemistry. To our great pleasure, the conclusion is that despite the fact that energy has not become more expensive, the progress in terms of both improved technology and completely new processes and devices is very substantial. Improved membrane cell technology for the chlor-alkali industry, new processes for the manufacture of low-tonnage organic and inorganic chemicals, the appearance on the market of new batteries and a variety of sensors, the coming of age of cathodic electropainting, many electrolytic processes for effluent treatment, the commercial availability of several families of electro• chemical tells, etc. are all symptoms of a healthy technology. Preface IX

Less satisfactory are the status of electrochemistry and electrochemical engineering as academic disciplines. They remain insufficiently taught at both undergraduate and postgraduate levels. Moreover, even when they appear within the syllabus, all too frequently one aspect of the subjects is covered to the exclusion of all others. It is a prime hope of both authors that this book will encourage many more teachers to take up the challenge of teaching an integrated applied electrochemistry course. Following the two introductory chapters, we have tried to use a similar approach for the discussion of the various groups of applications. We have sought to relate the technology to the underlying principles and to discuss current thinking and practice within the industries as well as to comment on likely future trends. We would wish to emphasize, however, that it is never our purpose to compare the technologies, cells or devices available from competing companies; the examples selected are based on our personal experiences and are in the text for illustration. We have also sought to describe only technology which has already reached industrial usage. Hence, we have always tried to avoid the temptation to outline the many other processes which have only been demonstrated in the laboratory or on a small pilot scale (otherwise the book would be in many volumes). We have attempted thereby, to produce a readable account of real industrial electrochemistry, useful to both students and those already engaged in the investigation of some aspect of the subject. In writing this book, many compromises had to be made. There have been many lively (but friendly) debates between the two authors and topics discussed have included the depth of treatment, the balance between fundamental and applied aspects and the choice of illustrative material. By far the most vexed topics were, however, those relating to signs, symbols and other conventions; throughout we were aware that the established practices of electrochemists, electroplaters, corrosion engineers, materials technologists and chemical en• gineers were quite different and they also depended on the country of origin. In these very unfortunate circumstances, authors are bound to offend most readers. In some desperation we decided to follow a system which will be most readily acceptable to electrochemists since we expect them to be our largest group of readers. Most of all, we have endeavoured to achieve uniformity. Finally, it is a pleasure to thank the many who have helped us in the preparation of this book. The most obvious are those who have kindly persuaded their organizations to release the photographs which illustrate the text. There are, however, many other industrialists and academics who have provided source material. We also feel that we owe much gratitude to the many who have stimulated our interest in applied electrochemistry. In the case of one of us (D.P.) special mention should be made of Professor Martin Fleischmann (University of Southampton) and Dr Gordon Lewis (associate and guide during a survey of industrial electrochemistry carried out in 1979). The other (F.C.W.) would wish to single out Dr Des Barker (Portsmouth Polytechnic), x Preface

Dr David Gabe (University of Loughborough) and Professor Martin Fleisch• mann, as particularly strong influences in his training. We are both also aware of our debt to several companies who have given us practical training in the most acceptable way - they have paid us as consultants! Thanks are due to Susan Neale and David Jackson of the Frewen Library, Portsmouth Polytechnic who checked the lists of further reading at the end of each chapter. Our typists are also remembered with many thanks. Lastly, we wish to acknowledge the debt to our families, especially our wives who have suffered extra duties to allow the completion of this book, but also our parents and children for their contributions to our contented writing.

Derek Pletcher Frank Walsh Symbols

Symbol Definition Typical units a+ Activity of cation Dimensionless a_ Activity of anion Dimensionless a± Mean ionic activity Dimensionless A Electrode area m2 As Electroactive area per unit reactor volume m- l l AE Electroactive area per unit electrode volume m- b Cost of a unit of electrical power £W-lS- l B Width of flow channel m 3 Cj Concentration of species i molm- 3 C~1 Concentration of species i in the bulk solution molm- c'! 3 1 Concentration of species i at the electrode molm- surface c(O) Initial concentration of reactant molm- 3 3 C(I) Concentration of reactant at time t molm- C(lN) Concentration of reactant at inlet to reactor molm- 3 3 c(OUT) Concentration of reactant at outlet of reactor molm- C Capital invested £ C Capacitance Fm- 2 C Capacity of a battery electrode As CE Electrolysis power cost £ CF Fixed capital £ C1 Variable reactor investment cost £ CL Land capital £ Cs Cost of electrolyte agitation £ Cw Working capital £ CSCRAP Total scrap value of plant £ Cp Heat capacity at constant pressure Jkg- l K- l de Equivalent diameter of a flow channel m D Depreciation £ year- l Xll Symbols

Symbol Definition Typical units 2 S-1 Dj Diffusion coefficient of species i m E Measured or applied electrode potential V Ee Equilibrium electrode potential V E~ e Standard electrode potential V C Ee Equilibrium cathode potential V EA e Equilibrium anode potential V EC Cathode potential V EA Anode potential V

ECELL Equilibrium cell potential (E~ - E~) V C ECELL Cell potential (E - EA) V EM Membrane potential V ECORR Corrosion potential V ETRANS Potential at which transpassivity first occurs V Epozoco Point of zero charge potential V Etn Thermoneutral cell potential V F Faraday constant. A smol- 1 g Acceleration due to gravity ms- 2 1 Gj Total Gibbs free energy for species i J mol- G~ 1 1 Gibbs free energy for species i in the absence J mol- of a potential field LlG Gibbs free energy change J mol- 1 LlG ADS Standard Gibbs free energy of adsorption J mol- 1 LlG±- Gibbs free energy of activation for the J mol- 1 forward process h Heat transfer coefficient Wm- 2 K- 1 H Enthalpy J mol- 1 Current A i~ Exchange current for the cathodic process A oM 10 Exchange current for the metal dissolution A reaction iCRIT Critical current for the onset of passivation A ipAss Current in the passive range A iL Mass transport controlled limiting current A I Current density; 1= i/ A Am- 2 10 Exchange current density Am- 2 I~ Exchange current density for the hydrogen Am- 2 evolution reaction I~ Exchange current density for the metal Am- 2 dissolution reaction 1 Reduction (cathodic) partial current density Am- 2 1 Oxidation (anodic) partial current density Am- 2 h Limiting current density (under mass transport Am- 2 control due to diffusion or convective diffusion) Symbols Xlll

Symbol Definition Typical units

IOPT Optimum current density Am- 2 Ix Current density at a distance x along the Am- 2 electrode k Rate constant for first order chemical process S-l k Rate constant for the forward (cathodic) process ms- 1 k Rate constant for the reverse (anodic) process ms- 1 ko Rate constant for an electron transfer process at ms- 1 E = 0 V vs. the reference electrode k" Standard rate constant for an electrode process ms- 1 1 kL Mass transport coefficient ms- 2 S-l kh Averaged, overall heat transfer coefficient m K' Kohlrausch constant ohm - 1 m 7/2 mol- 3/2

K j Selectivity constant for species i Dimensionless I Characteristic length m L Length of a plate electrode m m+ Molality of cation molkg- 1 m_ Molality of anion molkg- 1 m Number of moles of electro active species mol mo Initial molar amount of reactant mol mt Molar amount of reactant at time t mol m(IN) Molar amount of reactant at the inlet to a mol reactor m(OUT) Molar amount of reactant at the outlet of a mol reactor mp Molar amount of product mol M Molar mass kgmol- 1 n Number of electrons Dimensionless np Number of moles of product Dimensionless N Projected lifetime of plant years Nc Chromatographic plate number Dimensionless N m Mass flow kgs- 1 q Electrical charge As Q Volumetric flow rate m 3 S-l Q Charge density A sm- 2 Qn Heat flow W r Radius of disc or cylinder electrode m R Gas constant JK -1 mol- 1 R Electrical resistance ohm

RCELL Cell resistance ohm RCIRCUIT Resistance of electrical circuit, including bus bars ohm Rp Linear polarization resistance ohm s Space-velocity S-l S-l Sn Normalized space-velocity XIV Symbols

Symbol Definition Typical units S Entropy JK- 1 S Separation of electrodes m Sp Overall selectivity Dimensionless t Time s t' Time to discharge battery s t' Critical time in batch processing s tR Retention time s t+ Transport number of cation Dimensionless L Transport number of anion Dimensionless T Temperature K 2 1 1 Ui Mobility of species i m s- V- v+ Number of positive ions Dimensionless v_ Number of negative ions Dimensionless v Velocity of electrolyte flow ms- 1 vx Solution velocity in the x direction ms- 1 jj Mean flow velocity ms- 1 V Volume m3 VR Reactor volume m3 VT Reservoir volume m3 VM Molar volume m3 mol- 1 VE Electrode volume m3 w Mass of material kg W Power required for electrode/electrolyte W movement WCELL Electrolytic power requirement W x Distance, thickness or penetration m X A Fractional conversion Dimensionless Zi Electrical charge on species i Dimensionless Z Frequency factor for an activation controlled ms- 1 process

Dimensionless Groups gLlpL3 Gr Grashof Number: Gr=--z- Dimensionless v p vi Re Reynolds Number: Re=- Dimensionless v Sc Schmidt Number: Sc =~ Dimensionless ' kLi Sh Sherwood Number: Sh=V Dimensionless Symbols xv

Symbol Definition Typical units Greek

!J. Transfer coefficient Dimensionless

!J. A Anodic transfer coefficient Dimensionless !J. c Cathodic transfer coefficient Dimensionless f3 Inverse of the slope of a log Ii I vs. E (or log Ii I V vs. '1 plot) YG Energy yield Dimensionless 'YH Thermal energy yield Dimensionless Y Effectiveness factor for mass transport control Dimensionless Y+ Activity coefficient of cation Dimensionless Y- Activity coefficient of anion Dimensionless Y± Mean ionic activity coefficient Dimensionless 2 4 <5SB Stefan-Boltzmann constant = 5.67 x 10- 8 Wm- K-

<5 N Nernstian, concentration boundary layer m thickness e Emissivity Dimensionless '1 Overpotential (E - Ee) V '1' Polarization (E - ECORR ) V '1c Overpotential at cathode V '1A Overpotential at anode V {} Surface coverage Dimensionless {}p Material yield Dimensionless K Electrolytic conductivity ohm- 1 m- 1 A+ ohm -1 m2 mol- 1 0 Ionic conductivity of cation at infinite dilution ohm -1 m2 mol- 1 A-0 Ionic conductivity of anion at infinite dilution A Thermal conductivity Wm- 1 K- 1 A Molar conductivity of electrolyte ohm- 1 m2 mol- 1 Ao Molar conductivity of electrolyte at infinite ohm- 1 m2 mol- 1 dilution /1 Dynamic viscosity kgm -1 S-l v Kinematic viscosity; (/1/ p) m2 S-l P Density kgm- 3 L1p Difference in density between solution at kgm- 3 electrode surface and bulk PST Space-time yield kgm- 3 S-l PN Normalized space-time yield kgm -3 S-l (12 Peak variance Dimensionless , Detector time constant Dimensionless , Residence time s

CST Space-time s ¢ Peak fidelity Dimensionless XVI Symbols

Definition Typical units Current efficiency Dimensionless Absolute potential at the electrode surface V Overall conversion related yield Dimensionless Absolute potential of the bulk solution phase V Absolute potential at the plane of closest V approach of cations Potential field strength w Rotation rate