电化学直接还原

In situ synchrotron diffraction of the electrochemical reduction

pathway of TiO 2q

R.Bhagat a,*,D.Dye b ,S.L.Raghunathan b ,R.J.Talling b ,D.Inman b ,B.K.Jackson b ,

K.K.Rao c ,R.J.Dashwood a

a

University of Warwick,Coventry CV47AL,UK b

Imperial College,London SW72AZ,UK c

Metalysis,Rotherham S635DB,UK

Received 2November 2009;received in revised form 17May 2010;accepted 20May 2010

Available online 16June 2010

Abstract

Despite over ten years of work into the low-cost electrowinning of Ti direct from the oxide,the reduction sequence of TiO 2pellets in molten CaCl 2has been the subject of debate,particularly as the reduction pathway has been inferred from ex situ studies.Here,for the first time white beam synchrotron X-ray diffraction is used to characterize the phases that form in situ during reduction and with $100l m resolution.It is found that TiO 2becomes sub-stoichiometric very early in reduction,facilitating the ionic conduction of O ions,that CaTiO 3persists to nearly the end of the process and that,finally,CaO forms just before completion of the process.The method is quite generally applicable to the in situ study of industrial chemical processes.Implications for the industrial scale-up of this method for the low-cost production of Ti are drawn.

Ó2010Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.

Keywords:Electrochemistry;Titanium;Synchrotron radiation;XRD;Phase transformation kinetics

1.Introduction

In the FFC Cambridge process,metal oxides are electro-chemically reduced to metal using a molten chloride flux [1].Currently the Kroll process is used to produce Ti from rutile [2];potentially,the FFC process may lead to a step change in the cost of extraction of this and other alloy systems.

The process involves the progressive reduction and deox-idation of a porous TiO 2pellet cathode in a molten halide salt [1].At the cathode TiO 2is reduced to Ti.The oxide ions dissolve into CaCl 2and then migrate to a C anode,forming

CO 2and CO.This reduction pathway has been studied using pellet reductions [3],metal-cavity electrode [4–6]and thin-film electrode experimentation [7,8].Using cyclic voltamme-try and X-ray diffraction (XRD)analysis of thin films after

each event,the electrochemical events C4,C3,C20

,C2and C1,corresponding to reactions (1)–(6),respectively,have been identified during reduction.Despite being observed in XRD,the formation of CaTiO 3was not observed on the vol-tammograms and was therefore believed to form via a chem-ical reaction [9].

2TiO 2þ2e À!Ti 3O 5þO 2Àð1Þ2Ti 3O 5þ2e À!3Ti 2O 3þO 2À

ð2ÞCaTiO 3þ2e À!Ca 2þþTiO þ2O 2Àð3ÞTi 2O 3þ2e À!2TiO þO 2Àð4ÞTiO þ2e À!2TiO þO 2Àð5ÞTiO 2þCa 2þþO 2À!CaTiO 3

ð6Þ

1359-6454/$36.00Ó2010Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.doi:10.1016/j.actamat.2010.05.041

q

R.J.D.and D.D.conceived and designed the study.B.K.J.,R.J.D.and D.D.designed the apparatus,which B.K.J.built and commissioned.R.B.,R.J.D.,D.D.,S.L.R.,R.J.T.and K.K.R.conducted the experiments.R.B.and S.L.R.analysed the data with support from R.J.T.,R.J.D.and D.D.The paper was written by R.B.with contributions and editing from R.J.D.,D.D.,D.I.,R.J.T.and S.L.R.*

Corresponding author.

E-mail address:r.bhagat@ (R.Bhagat).

/locate/actamat

Acta Materialia 58(2010)

5057–5062