Study On Preparation Of Titanium And Titanium-aluminum Alloys By Electrolysis In Molten Salt

With the boom of aviation and aerospace fields,increasing demands for metallic titanium and titanium alloys greatly promote the development of titanium industry in China.Many researchers have devoted themselves to molten salt electrolysis methods,hoping to correct current shortcomings during the production process of metallic titanium and its alloys,i.g.,high cost and discontinuous production.This article is to systematically study the electrochemical theories and laboratory electrolysis processes related to "molten salt electrolysis methods to prepare metallic titanium powders".Using the "molten salt electrolysis process",Ti-Al alloys are prepared in two ways:codeposition of titanium and aluminum ions in molten salts,and direct alloying of titanium ions with metallic aluminum as liquid cathode.Further discussion is made on the feasibility of the corresponding preparation process of Ti-Al alloy and on the control of Ti-Al alloy phases and their compositions.In this article,electrochemical behaviors of Ti(?),Ti(?),and Ti(?)ions in the molten NaCl-KCl system were investigated using cyclic voltammetry(CV),chronopotentiometry(CP),square wave voltammetry(SWV)and open-circuit chronopotentiometry(OCCP)on the molybdenum electrode at 750?.After addition of NaF,we investigated the reduction process of Ti(IV)in the molten NaCl-KCl-NaF system.We concluded that,in the molten NaCl-KCl system with K2TiF6as titanium source,Ti(?)was reduced in three steps,in which the redox process of Ti3+?Ti2+was irreversible and incompletely controlled by diffusion;in the NaF added molten NaCl-KCl-K2TiF6 system,Ti(?)was reduced in two steps suggesting addition of F-went against the stable existence of Ti(?),in the molten NaCl-KCl-TiCl3 system,Ti(?)was reduced in two steps,in which metallic titanium was precipitated on the molybdenum electrode.We investigated the electrocrystallization process and principle of titanium ions in the molten salt,and the effects of electrolytic parameters on the topography and purity of titanium powders.We concluded that titanium ions were electrodeposited on the molybdenum electrode in three stages:? the charging process of the electric double layer on the electrode,? the nucleation process of electrocrystallization,?the crystal nucleus growth process.Results of chronoamperometry showed an overall increment in current density with an increase of titanium ion concentration,and an increment in electrocrystallization nucleation in the molten salt system with a higher titanium ion concentration.XRD results showed,on the molybdenum electrode,the metal phase of titanium was obtained by electrolysis in the molten NaCl-KCl-TiCl2 system at a constant voltage of-1.8 V.SEM results showed effects of electrolysis temperature,electrolysis time,initial current density and titanium ion concentration in molten salts on topography of electrocrystallized titanium powder.Conclusions were made as follows.? When Ti(II)was at a low level(<0.5 wt%)in electrolytes,no product was obtained basically on the molybdenum electrode;products were gradually precipitated and changed from powders to granules followed by dense granules as Ti(II)level increased in electrolytes.? The deposited granule size of metallic titanium increased with the current density,but after the current density increased to a certain value,it provided energy not only for crystal nucleus growth but also for production of dendritic crystal of metallic titanium.? Electric quantity increasing over electrolysis time led to a decrement in titanium ions aggregated on the electrode surface,resulting in the inability of the mass-transfer rate of the titanium ion to match with its reduction rate.Titanium ions followed the principle of nearby nucleation and growth within the distribution range of titanium ion level.This article investigated the feasibility of chlorination of titanium oxides to TiCl3 in liquid phase using AlCl3 as chlorinating agent in the molten NaCl-KCl system.Compared with chlorination results of TiO,Ti2O3 and TiO2 and based on the thermodynamic analysis of reactions of AlCl2 with TiO,Ti2O3 and TiO2,Ti2O3 was selected to chlorinate to TiCl3.Different electrochemical testing means were used to investigate the alloying process of Al(?)and Ti(?)on the molybdenum electrode and the feasibility of preparation of TiAl2 and TiAl3 alloys by codeposition.Alloys were analyzed using XRD,SEM and ICP.We concluded that:in the NaCl-KCl system chlorinating Ti2O3 as TiCl3,codeposited alloys were only present in TiAl2 and TiAl3 phases;in the NaCl-KCl system where TiCl4 reacted with titanium sponge to TiCl3,the codeposited products included three phases:TiAl2,TiAl3 and TiAl.Results of ICP analysis suggested that changing concentrations of TiCls and AICl3 could control the content of Ti and A1 in alloys.Thus,it is feasible to prepare composition-controllable Ti-Al alloys by molten salt electrolysis directly.In the molten NaCl-KCl system,electrochemical behaviors of Ti(?)ions on the liquid aluminum electrode were investigated by CV.We concluded that the reduction process of Ti(?)ions to metallic Ti on the liquid aluminum electrode was done in two successive steps,with reduction potentials of-0.8 and-1.2 V,respectively.The feasibility of preparation of Ti-Al alloys by alloying via Ti(?)ions on the active liquid aluminum cathode was investigated by different electrochemical testing means;the resulting products containing Al2Ti and Al3Ti were obtained by electrolysis at a constant voltage of-1.3 V;alloys were analyzed using XRD,SEM,ICP and MLA.We also investigated the formation processes of Ti-Al alloys in molten salts.MLA results showed that:metallic aluminum was encapsulated in metallic titanium powders;alloying began when the titanium amount was sufficient;a small quantity of alloys were produced as long as the time was long enough and the electric quantity supplied was sufficient,which alloyed the whole liquid aluminum cathode to Ti-Al alloy.