Research On Preparation Titanium By Thermoelectric Reduction

Titanium has excellent physical and chemical properties and has broad application prospects in areas of aviation, aerospace, metallurgical industry and daily life. Nowadays, the industrial production method of titanium is Kroll process, this method long production period and high cost, which has seriously restricted the titanium extensive application. Direct electrochemical reduction of solid titanium dioxide to titanium in molten salt is a revolutionary titanium production method with advantages of short flow and lower energy consumption, having a great development room. However, nowadays research focus on using TiO2block as cathode in CaCl2-CaO molten salt, because of the limitation of the diffusion of the oxygen ion from the oxide to electrolyte resulting low current efficiency and high residual oxygen content in the electrolysis products and precipitated calcium metallic leading to a high background current. In this study, metallic titanium was prepared by the combination of strontium reduction of TiO2powder and electrolysis of reduction product (SrO) to recovery of reducing agent (Sr) in SrCl2-KCl melt, as metallic strontium has both suitable density and high reducibility, which aimed to achieve stable and continuous electrolysis process in chloride molten salt. As chloride molten salt with disadvantages of easy hygroscopic, high metal solubility, product easily polluted by carbon contamination and chlorine evolution, titanium preparation by direct electrochemical reduction in SrF2-NaF eutectic molten salt system was studied.TiO2and SrO solubility, liquidus temperature and density of SrCl2-KCl molten salt were measured. The results show that TiO2is basic insoluble in molten salts and SrO solubility increases with increasing temperature and SrCl2content. The liquidus temperature of80%SrCl2-20%KCl (mass fraction, same below) molten salt is638.1℃, the solubility of TiO2being7.67%and the density being2.515g/cm3at800℃, and the density decreases with rising temperature and addition of SrO.Theoretical decomposition potential drops in the turns of SrCl2、KCl、SrO、TiO2in1100～1300K when graphite used as anode. Electrochemical tests show oxygen ionization reduction of TiO2powder conducts difficultly and strontium thermal reduction plays a dominant role in the system; TiO2transforms to SrTiO3very early, then reduced to TixO and finally becomes metallic titanium. Stable SrTiO3phase and removal of dissolved oxygen in TixO are control steps in reduction process.Metallic titanium containing0.58%oxygen was produced after5h electrolysis with0.5%SrO added to the melt, a graphite crucible with corundum tube inside, molybdenum sheets bottom used as electrolyser at1123K and3.0V cell voltage in the80%SrCl2-20%KCl bath. Current efficiency is31.13%. Adding too much amount of TiO2will hinder the reduction process because the powder covers on the molybdenum sheets cathode. The "fluidized bed" concept was introduced to electrolysis process by inletting argon to the melt, making the TiO2particles suspended in the melt and accelerating the reduction process.Titanium preparation by direct electrochemical reduction in SrF2-NaF eutectic was studied with TiO2pellet and TiO2powder as raw material. Porosity increases about5%with additive content increasing10%within the scope of0%-100%. Porosity is the key role that determines the rate of reduction process. Metallic titanium was obtained in SrF2-NaF eutectic after6h electrolysis with60%NH4HCO3added to the pellet of56.8%porosity at3.0V and950℃. High porosity will reduce the strength of the pellets and make it difficult to ensure the smooth progress of the electrolysis process. TiO2pellet is reduced in sequence of SrTiO2.6, Ti2O3, Ti2O, and finally convert to titanium. TiO2powder-stainless steel mesh cathode with high porosity can facilitate the ionic conduction of O ions, accelerating the process of electrolysis and the current efficiency increases up to31.08%compared with16.67%when TiO2pellet used as cathode.