Electrochemical process of titanium extraction

Available online at www.sciencedirect.com●CIENCE* @noner.Transactions of骂PNonferrous MetalsSociety of ChinaSciencePressTrans. Nonferrous Met. Soc. China 17(2007) 429-433www.csu.cdu.cn/ysxb/Electrochemical process of titanium extractionCH. RVS. NAGESH, C. s. RAMACHANDRANDefence Metallurgical Research Laboratory, Hyderabad, A P. India 500058Received 20 November 2005; accepted 9 January 2006Abstract: A wide varity of processes are being pursued by resarchers for cost efctive extraction of titanium metal.Electrochemical processes are promising dusimplicity and being less capital intensive. Some of the promising electrochemicalprocesses of itanium exraction were reviewed and the results of labortory scale experiments on electrochemical reduction of TiO2granules were brought out. Some of the kinetic parameters of the reduction process were discussed while presenting the qualityimprovements achieved in the experimentation.Key words: electrochemical reduction; titanium dioxide; titanium; kinetic parametersFused salt electrolysis of TiO2 in floride/phosphate baths1 Introduction0 prepare fitanium metal was not successful due toproblems such as complexities in the cell design, poorTitanium metal has enormous potential for a widesolubility of oxide in the melts, recombination of metal,variety of applications but its usage and consumptionproblems associated with purity of the metal.have. been limited to aerospace and few other areasmainly because of the high cost of metal. Currently2 Electrochemical reduction processestitanium is extracted from titanium oxide ore by a seriesof process steps involving: 1) chlorination of oxideThe current practice of Kroll process for tianiumconcentrate to prepare titanium tetrachloride (TiCL4), 2)spongeproduction has undergone tremendousreduction of TiCl4 by magnesium metal (Kroll process),improvements and advancements, resulting in significant3) vacuum ditillation/leaching of reduced mass.improvement in energy savings as well as quality of theElectrolysis of the by-product MgCl2 is a parallel processproduct[1]. Apparently the Kroll technology has assumedcarried out for recovering magnesium metal and chlorine.stagnation and there is hardly any scope existing forSodium reduction of TiCl4 (Hunter process) had alsobringing down the cost of titanium metal further by thisbeen operated till early 1990s but its commercialmethod. There have been a wide variety of efforts toapplication at present is limited. Fused salt electrolysis ofdevelop an alternate process of titanium extraction toTiCl4 in alkali chloride mixtures has been extensivelyproduce the metal at a cheaper price. Extensive study onpursued in Dow-Howmet, USA and Ginatta, Italy. Butelectrochemical de-oxidation of titanium in moltenthe process could not be commercialized due to severalcalcium chloride electrolyte led to the development ofproblems such as multi-valency of the metal, selection ofelectrochemical reduction processes. The following is amaterials of construction of the cell and otherbriefdescription of important electrochemical/techno-economic factors.electrometallurgical reduction processes that are beingExtraction of titanium directly from titaniumdeveloped.dioxide has been proven to be futile due to high affinityof the metal towards oxygen and high process2.1 FCC process，temperatures. Conventional carbothermic reduction anc中国煤化工AY, FARTHING andmetallothermic reduction methods did not yield titaniumCHECNMH GUK[2- -3], involvesof required purity owing to limiting thermodynamicremoiIUII oULIU titanium dioxide byconstraint and high chemical reactivity of the metal.subjecting it to electrolysis in molten calcium chlorideCorresponding author: CH. RVS. NAGESH; E-mail: nagesh chrv@rifiail.co430CH. RVS. NAGESH, et al/Trans. Noferrous Met. Soc. China 17(2007)bath using graphite as anode. The process involvesTable 1 Other electrochemical processes of titanium extractionpreparation of TiO2 compacts/pellets from the oxideunder studypowder that are sintered and taken as cathode. In thisName/organizationProcessProductprocess it is understood that, oxygen diffuses out of theAnode reduction ofTiO2,oxide through crystal defects under the conditions ofMER Corporation, clectrolysis in molten balide Powder/applied DC voltage that is also aided by the formation ofUSAmixture to deposit titaniumflakeelectrically conductive 'magnelli’ phase at highmetal on cathodetemperature. Oxygen ions pass through molten CaCl2BHP Biliton processElectrochemical reduction ofSpongeTiO2 in calcium chloride bathelectrolyte to form CO/CO2 gas at the graphite anode.ElectronicallyElectrolytic reduction ofTiO2The process is vigorously being pursued by TIMET,Mediated Reactionwithout physical contact ofUSA at California University, Berkeley for scale up and(EMR)/Molten Salt oxide and calcium usingPowdercommercial adaptation[4].Electolysis (MSE)Ca-Ni alloy and CaCh2electrolyte2.2 OS processThe calciothermic reduction of TiO2 and in-situelectrolysis of CaO in molten calcium chloride wasTiOpowder-Binderextensively studied by ONO and SUZUKI[5] forpreparing titanium sponge granules. The OS process,Mixing |thus combines the calciothermic reduction at about 900C and fused salt electrolysis of CaO in CaCl2 at DCGranulationvoltage in the range 2.7-3.2 V in a single set upGranulescomprising a reduction zone and an electrolytic zone. Ina conceptual cell design proposed by this process,Cathodetitanium dioxide powder. is continuously. fed into a.titanium crucible and from the bottom of the crucible、Elcrolysisin molten CaCl2molten CaCl2 with dissolved CaO is taken out and fedinto the electrolytic compartment. Calcium metal is fedMetallized granulesinto the crucible from the electrolytic chamber. Theprocess is under investigation for improvements andWashingscale up.Titanium sponge2.3 OTT liquid titanium processFig.1 Steps in electrochemical reduction process for titaniumQuebec Iron and Titanium(QIT)， Canada hassponge productionrecently patented a new titanium extraction process[6], inwhich titania slag as obtained by beneficiation ofInitially bench scale experimental work wasconducted in which different pre-forms of titaniumilmenite can be treated in an electric arc furmace toremove impurity oxides and then subsequently subjecteddioxide were subjected to electrochemical reduction into electrolysis using moiten CaF2 as electrolyte tccalcium chloride bath at a temperature range of 800 900electrowin titanium that can be tapped out in liquidC to study the important parameters of the process, andcondition. A few other processes for titanium extractionthe details of which were presented elsewhere[7]. Thisbased on electrometallurgy that are being tried onhas led to the current activity of cell operation on a batchsize of 100-200 g of sponge.laboratory scale are given in Table 1.The schematic of the cell is shown in Fig.2 thatconsists of a stainless steel retort beated by an electrical3 Experimentalresistance furmace. The retort is closed by a lid thathas provision of nozzles for electrode leads, argon gasAt the Defence Metallurgical Research Laboratory,supply and vent gas outlet. TiO2 granules prepared fromHyderabad(DMRL) experimental work on laboratorythe oxide powder are sintered and taken into a stainlessscale has been carried out to study the electrochemicalsteel中国煤化工rted into a graphitereduction of TiO2 in molten calcium chloride bath (Fig.1).crucil: help of a steel rod.Studies are also under way at DMRL to explore theThe:IYCH.。C NMH Gare conee tothekinetics of the process. The following is a description oftwo terminals of a DC power source. Typicalexperimental work and discussion of the results.experimental procedure involves: 1) loading of weighedCH. RVS. NAGESH, et a/Trans. Nonferrous Met. Soc. China 17(2007)431quantity of calcium chloride into the graphite crucibletitanium dioxide to the oxide-salt interace; 2) transportand melting; 2) loading of weighed quantity of oxideof oxygen ions through molten clectrolyte towardsgranules into the steel basket and assembly of the systemgraphite anode; and 3) liberation of CO/CO2 gas at theso that the basket with granules lies well above the meltanode. Among these steps oxygen diffusion in solidlevel; 3) melting of salt and lowering of the basket to atitanium dioxide is the slowest and becomes the ratepre-determined level; and 4) application of DC voltagecontrolling step. Diffusion of oxygen in the oxideand continuation of electrolysis for selected time.depends on parameters such as defects concentration,During the electrolysis argon gas is purged into thetemperature, and electrical conductivity. The ionicsystem and vent gases are allowed to pass through amobility may be computed employing a simple model[8]bubbler flled with baryta solution. At the end of thesuch as = Dq82 /kT， where 0 is vibration frequency,electrolysis, the basket is lifted up above the melt levelq is charge of ion and δ is mean free path. Electricaland the system is cooled to room temperature underconductivity of titanium dioxide was measured in theargon gas cover. At room temperature the basket withtemperature range of 25- -1000 C. The conductivitymtallized granules is thoroughly washed with water,measurement method followed the standards of setup asacetic acid and dilute HC1 to remove the adhered saltwell as procedure listed in the ASTM standard D257-99.completely. The metallized granules are then dried andTypical measurement involves placing of the oxidetaken for characterization.specimen in a tubular furnace and heating to hightemperature and application of DC voltage of the order- 000000of milli volts and measuring current response (in milli-16 mm stcel rodamps) employing Ketheley instrument continuouslyVent gasesevery few seconds. It is found that there is a sharp+ -000000Retort top closerincrease in conductivity (Fig.3) at a temperature of海Retort supportabout 930 C, which is explained by the formation offlangehighly conductive 'magnelli' phase[3]. The rate ofoxygen ion removal from the oxide-salt interface mightalso influence the bulk diffusion of oxygen within theoxide.， Inconelretort12 r- Fumnace108+Molten尼6S.s._levef号6basket_TiO24granulesT 1501.D.”Graphite82crucible”100 300 500 700 900Fig.2 Schematic of experimental set-upTemperature/CA number of experiments were conducted to studyFig.3 Variation of electrical conductivity of TiO2 withvarious parameters such as bathtemperature,temperatureinter-lectrodedistance,voltage. Some4 Discussionimprovements in the cell configuration could beincorporated for achieving better results.In an electrochemical reduction process, the overallDuring the reduction process the cell current isreaction can be described as: O(Metal)+e=03- (Moltenfound中国煤化工le the cell voltage issalt). At graphite anode, 0 (Molten salt)+C=CO/CO2.keptMYHFained by the fall inThe reduction process involving complete removal ofresistCNMHGomliaion.Theoxygen from the oxide is viewed to take place by theell current comprises current due to ionic conductivityfollowing steps: 1) diffusion of oxygen from bulknd current due to electronic conductivity of the oxide.432CH. RVS. NAGESH, et al/'Trans. Nonferrous Met. Soc. China 17(2007)The ionic currents are expected to reduce as themetallization progresses. The electronic current isexpected to increase as the electrical resistance of thesystem falls at high temperature. However, detailed studyof these aspects only can bring out the current patterns.As the electrolysis is initiated, formation of whiteprecipitate is noticed in the baryta bubbler indicating theoccurrence of desired de-oxidation reactions. On thebasis of conversion of oxygen removed from the oxide toCO/CO2 that is absorbed in baryta and change in rate offormation of carbonate further insight into the kinetics of0the process may be determined.SEM and EDAX characterization of the washed andEnergy/keVdried granules show a network of metallic species, ie.titanium particulate with very low levels of oxygen asFig.5 Typical EDAX profile of sponge granuleseen from Figs.4 and 5. Photographs of initial oxidegranules and metallized species generated after theexperiment and a button (measuring approximately 50mm in diameter and 5 mm in thickness) melted out of thesponge by non-consumable arc melting are shown inFig.6. Characterization of the button by hardness testing,chemical analysis and metallography reveals goodmetallization in the samples. The hardness of the buttonis found to be as high as BHN 270, which must be due tohigher oxygen and carbon contents that were analyzed toGRANULESbe about 1% and 0.6% respectively. However, higher2●oxygen content in the button could be due to the fact thatit is very diffcult to separate partially metallized600 gramsgranules from the fully metallized granules which are6)30 mm中国煤化工YHCNMHG2cmFig.4 SEM photographs of sponge formed by electrochemicalFig.6 Photographs of sintered oxide granules(a), titaniumreduction ofTiO2sponge granules(b) and button melted out of sponge granules(c)CH. RVS. NAGESH, et al/Trans. Nonferrous Met. Soc. China 17(2007)33melted into button. The degree of metallization is foundTable 2 Chemical analysis of titanium button pieces (massto be improved with increased electrolyte temperature.fraction, %)However, at higher temperatures it is found that theElementInitial meltRecent meltcarbon content in the metal increases. It should be notedF0.60.028that pick up of carbon is noticed even in the earlyN0.70.010experimental work reported on de-oxidation of titaniumS.2by electrochemical means[9]. Some improvements in cellC0.10.035parameters could lead to reduced carbon impurityA0.4_0.031content of the metal as seen from Fig.7, which showscarbon X-ray images taken by EPMA of initial melts andscope to improve the process and quality of the product.recent melts with improvements. There are someVarious fundamental aspects of the process includingimproverments with respect to other impurity elerments askinetics are being explored for further understanding ofseen from Table 2 that shows the chemical analysis of thethe process.button samples of the initial and recent melts.AcknowledgementsThe authors are thankful to Dr. A. M. SriramMurthy, Director, DMRL for permitting to publish thiswork. They are grateful to Defence Research &Development Organization(DRDO) for initiating thisactivity and extending financial support. They also wishto acknowledge the help received from Dr. V. V. BhanuPrasad and Sir V. V. Rama Rao of CCG & EPMA groupsof DMRL. Technical support to carry out the resistancemeasurement of TiO2 received from Dr. A. K. Suri, .BARC is gratefully acknowledged.References1] HYADO T, ICHIHASH H. Establishment of the manufacture of 5Nsuper purity titanium billets by Kroll process [A]. LUTJERING G,ALBRECHT J. Titanium'2003 Science and Technology [C].Wiley-Vch Verald GmbH, 2004: 141-148.2] FRAY, FARTHING, CHEN. Removal of oxygen fom metal oxidesand solid solutions by clectrolysis in a fused salt [P]. IntemnationalPatent, WO 99/64638, 1999.CHEN, FRAY, FARTHING. Direct clctrochemical reduction oftitanium dioxide to titanium in molten calcium chloride [0. Nature,2000, 407: 361-364.4] 'Switched- on taium', MBM July 2003, 20.[5] SUZUKI R O, TERANUMA K O H, ONO K. Calciothermicreduction of titanium oxide and in-situ clctrolysis in moltcn CaCh[J]. Met Mat Trans B, 2003, 34B: 287- -295.6] KRAFT E H. Sunmary of emerging titanium cost reductiontechnologies [R]. Report by EHK technologies. Vancouver, WAFig.7 Carbon X-ray images of titanium samples: (a) Initial2004.melts; (b) Recent mclts7] RAMMOHAN RAO A v, NARASIMHA RAO R v L, NAGESH CR, BHANU PRASAD V V, RAMACHANDRAN C s. Titaniumsponge production by avel electrochemical reduction of TiO2 [R].5 ConclusionsDMRL technical report, DMR TR 378, 2005-07.8] sErz F. The Modern Theory of Solids [M]. McGraw Hill Book Co,The results obtained on electrochemical reduction9] OKABE T H, NAKAMURA M, OISHI T, ONO K. Electrochemicalprocess have been very encouraging. There is a lot ofde-oxidation of tianium [J]. Met Trans B, 1993, 24B: 449 -454.(Edited by YUAN Sai-qian)中国煤化工MYHCNMHG