Calciothermic reduction of titanium oxide and <Emphasis Type="Italic">in-situ</Emphasis> electrolysis in molten CaCl<Subscript>2</Subscript>

A concept for calciothermic direct reduction of titanium dioxide in molten CaCl₂ is proposed and experimentally tested. This production process consists of a single cell, where both the thermochemical reaction of the calciothermic reduction and the electrochemical reaction for recovery of the reducing agent, Ca, coexist in the same molten CaCl₂ bath. A few molar percentages of Ca dissolve in the melt, which gives the media a strong reducing power. Using a carbon anode and a Ti basket-type cathode in which anatase-type TiO₂ powder was filled, a metallic titanium sponge containing 2000 ppm oxygen was produced after 10.8 ks at 1173 K in the CaCl₂ bath. The optimum concentration of CaO in the molten CaCl₂ was 0.5 to 1 mol pct, to shorten the operating time and to achieve a lower oxygen content in Ti.     

Preparation of titanium metal by electrochemical reduction of titanium dioxide

Preparation of titanium metal by direct electrolytic reduction of TiO₂ in molten CaCl₂ bath was studied using sintered TiO₂ pellets as cathode. The process was carried out at 950°C under normal atmosphere without using any inert gas. Titanium was obtained in the form of powder/compact product depending on the method of preparation of the pellets and the reaction conditions. The product was characterized by XRD, SEM and EPMA. The purity of the product was >99% as evidenced from EPMA analysis.     

Green Electrochemical Process Solid-Oxide Oxygen-Ion-Conducting Membrane (SOM): Direct Extraction of Ti-Fe Alloys from Natural Ilmenite

The direct electrochemical extraction of Ti-Fe alloys from natural ilmenite (FeTiO₃) in molten CaCl₂ is reported in this article. The sintered porous pellet of natural ilmenite acted as the cathode of the electrochemical system, and the carbon-saturated liquid tin contained in a solid-oxide oxygen-ion-conducting membrane (SOM) tube served as the anode of the electrolytic cell. The electrochemical process was carried out at 3.8?V, under 1223?K and 1273?K (950?°C and 1000?°C). Oxygen was ionized continuously from the cathode and discharged at the anode; solid porous Ti-Fe alloys were obtained at the cathode. The electro-deoxidation procedure of the ilmenite was characterized by analyzing partially electro-deoxidized samples taken periodically throughout the electro-deoxidation process. The findings of this study are as follows: (1) The electro-deoxidation process followed these steps: Fe₂TiO₅????FeTiO₃????Fe₂TiO₄????Fe, Ti (and/or Ti-Fe alloys); and TiO₂????CaTiO₃????Ti; and (2) two types of particle growth pattern are observed in the experiments. The first pattern is characterized with particle fusion and second pattern is interconnection of particles to form porous structure. A microhole oxygen-ion-migration model is suggested based on the experimental evidence.     

Calciothermic reduction of titanium oxide in molten CaCl<Subscript>2</Subscript>

Titanium oxides were reduced to metallic titanium using the liquid calcium floating on the molten CaCl₂. A part of Ca dissolved into CaCl₂ and reacted with TiO₂ settled below CaCl₂. The by-product CaO also dissolved by about 20 mol pct into CaCl₂, which was effective in reducing the oxygen concentration in the obtained Ti particles. The compositional region in the Ca-CaCl₂-CaO system was examined for the less oxygen contamination in Ti and the better handling in leaching. A large amount of the residual calcium oxidized the titanium powder in leaching. The metallic Ti powder less than 1000 mass ppm oxygen could be obtained only for 3.6 ks using 5 to 7 mol pct Ca-CaCl₂ at 1173 K. The powder was slightly sintered like sponge, and contained approximately 1500 ppm Ca. The anatase phae, the intermediate product in the refining process of TiO₂, could be also supplied as raw material as well as rutile.     

Electrochemical deoxidation of titanium

Removal of oxygen in titanium using an electrochemical technique was examined at temperatures around 1223 K with the purpose of obtaining nearly oxygen-free titanium. Titanium and carbon electrodes, immersed in molten CaCl₂, served as cathode and anode, respectively, with an external DC source. CaCl₂ was employed to produce the deoxidant calcium and to facilitate the reaction by decreasing the activity of the by-product CaO. By applying about 3 V between the electrodes, the calcium potential in CaCl₂ was increased at the titanium cathode surface and titanium samples of the cathode could be deoxidized by the electrolytically produced deoxidant calcium or by calcium of high activity in the CaCl₂ flux. Resulting O²⁻ species, mainly present as the deoxidation product CaO in the flux, reacted at the carbon anode to form CO (or CO₂) gas which was removed from the system. Titanium wires containing 1400 mass ppm oxygen were deoxidized to less than 100 mass ppm, whereas the carbon concentration increased by about 50 mass ppm. In some cases, the oxygen concentration in titanium samples was lowered to a level less than 10 mass ppm that could be determined by conventional inert gas fusion analysis. The behavior of contaminants, such as carbon and nitrogen, is also discussed.     

Upgrading Titanium Ore Through Selective Chlorination Using Calcium Chloride

To develop a simple and effective process for upgrading low-grade titanium ore (ilmenite, mainly FeTiO₃), a new selective chlorination process based on the use of calcium chloride (CaCl₂) as the chlorine source was investigated in this study. Titanium ore and a titanium ore/CaCl₂ mixture were placed in two separate crucibles inside a gas-tight quartz tube that was then positioned in a horizontal furnace. In the experiments, the titanium ore in the two crucibles reacted with either HCl produced from CaCl₂ or CaCl₂ itself at 1100 K (827 °C), leading to the selective removal of the iron present in the titanium ore as iron chlorides [FeCl _x (l,g) (x = 2, 3)]. Various kinds of titanium ores produced in different countries were used as feedstock, and the influence of the particle size and atmosphere on the selective chlorination was investigated. Under certain conditions, titanium dioxide (TiO₂) with purity of about 97 pct was directly obtained in a single step from titanium ore containing 51 pct TiO₂. Thus, selective chlorination is a feasible method for producing high purity titanium dioxide from low-grade titanium ore.     

Calciothermic co-reduction of TiO2-Cr2O3 oxides in molten CaCl2: Effect of particle size of starting materials

In this research, the effect of particle size of starting materials (TiO₂ and Cr₂O₃) was investigated on calciothermic co-reduction of oxides and in-situ dissolution of the CaO by-product in molten CaCl₂. The particle size of starting oxides, molar ratio of molten salt and holding time of reaction at elevated temperature affected the progress of reduction reactions and the remaining oxygen. In the best optimized condition, the remaining oxygen in the obtained TiCr₂ was 1020 and 802 ppm using 388 and 120 nm particle sizes of starting oxides, respectively. The formation of TiCr₂ in calciothermic co-reduction process was considered to be mutual diffusion phenomena. Also, the kinetics of hydrogen adsorption for the co-reduced sample was better than that of the arc-melted sample. Particle sizes of the obtained products decreased with-the decrease of the particle sizes of the starting oxides.     

Cathodic deoxygenation of the alpha case on titanium and alloys in molten calcium chloride

The oxygen-enriched alpha case on titanium and alloys was successfully deoxygenated to satisfactory levels by electrolysis in molten CaCl₂, in which the cathode was made from the metal to be refined. The oxygen distribution in the metal before and after electrolysis was characterized by microhardness tests, scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX). The electrolysis has been carried out at voltages sufficiently below that for the decomposition of CaCl₂, and the results obtained suggest that the alpha case deoxygenation follows a simple oxygen ionization mechanism in which the oxygen in the metal is simply ionized at the cathode/electrolyte interface, dissolves in the molten salt, and then discharges at the anode. It is shown that by applying the electrochemical method, the alpha cases on both commercially pure titanium (CP Ti) and the Ti-6Al-4V alloy can be effectively deoxygenated. In particular, due to the removal of oxygen, the original alpha case (single phase) on the Ti-6Al-4V alloy has been converted back to the two-phase microstructure.     

Metal Anode Performance in Low-Temperature Electrolytes for Aluminum Production

An investigation has been undertaken into the performance of metal alloy anodes used to produce aluminum via an electrochemical method. Alumina was electrolyzed in NaF/AlF₃ and KF/AlF₃ electrolytes and mixtures thereof with copper-nickel-iron (Cu:Ni:Fe) alloy anodes and titanium diboride (TiB₂) cathodes. The operating temperatures of the electrochemical cells ranged from 973 K to 1123 K (700 °C to 850 °C), with an anode current density of 5000 A/m². Cells ranged in current capacity from 10 to 300 amperes, with oxygen gas formed at the anode and molten aluminum collected from the cathode. Posttest anodes were sectioned, and elemental maps were performed to characterize the distribution of the chemical phases, including the metal electrodes, bath phases, and aluminum metal production, which were used to determine the reaction mechanisms of the cell. The metal alloy slowly corroded and formed an adherent, electronically conducting nickel ferrite plus copper scale during the operation of the cell. The proposed mechanisms of the anode performance are described herein.     

Titanium powder prepared by magnesiothermic reduction of Ti2+ in molten salt

A process to produce titanium powder from Ti²⁺ in the molten salt was confirmed experimentally. It consists of two steps. In the first step, titanium (IV) chloride gas dissolves in the molten salt as the titanium (II) chloride by reacting chemically with the metallic titanium or magnesium. In the second step, this molten salt is exposed to the metallic reductant Mg for the final reduction to the metallic Ti powder. Experimentally, the feed of TiCl₄ gas and the subsequent reaction with Ti prepared the molten salt containing 5 to 10 mass pct Ti²⁺. By Mg reduction of this salt, the well-isolated fine Ti particles were recovered. The powder morphology and particle size depended on the Ti²⁺ concentration, reduction temperature, time, and concentration of the by-product MgCl₂. The stirring by argon gas bubbling effectively grew the fine and round particles to a few tens of microns in size.     

Understanding the Magnesiothermic Reduction Mechanism of TiO<Subscript>2</Subscript> to Produce Ti

Titanium dioxide (TiO₂) powders in the mineral form of rutile were reduced to metallic and an intermediate phase via a magnesiothermic reaction in molten Mg at temperatures between 973 K and 1173 K (700 °C and 900 °C) under high-purity Ar atmosphere. The reaction behavior and pathway indicated intermediate phase formation during the magnesiothermic reduction of TiO₂ using XRD (X-ray diffraction), SEM (scanning electron microscope), and TEM (transmission electron microscope). Mg/TiO₂ = 2 resulted in various intermediate phases of oxygen containing titanium, including Ti₆O, Ti₃O, and Ti₂O, with metallic Ti present. MgTi₂O₄ ternary intermediate phases could also be observed, but they were dependent on the excess Mg present in the sample. Nevertheless, even with excessive amounts of Mg at Mg/TiO₂ = 10, complete reduction to metallic Ti could not be obtained and some Ti₆O intermediate phases were present. Although thermodynamics do not predict the formation of the MgTi₂O₄ spinel phase, detailed phase identification through XRD, SEM, and TEM showed significant amounts of this intermediate ternary phase even at excess Mg additions. Considering the stepwise reduction of TiO₂ by Mg and the pronounced amounts of MgTi₂O₄ phase observed, the rate-limiting reaction is likely the reduction of MgTi₂O₄ to the Ti_tO phase. Thus, an additional reduction step beyond thermodynamic predictions was developed.     

Production of Titanium Powder by Sodiothermic Reduction in CaCl2 Molten Salts

A new sodiothermic reduction process of TiO₂ in CaCl₂ melt was proposed aimed at fine Ti powder preparation. The chemical analysis and direct potentiometric methods were used to investigate the reaction pathway of sodiothermic reduction in CaCl₂ melt. The as-prepared samples were characterized by X-ray diffraction and scanning electron microscopy. It was found that when reductant of Na was added into the CaCl₂ melt, Ca²⁺ was reduced to Ca by Na and Ca dissolved in the CaCl₂ melt. The whole melt would have the reducing power with dissolved Ca. Using this melt as a reaction medium, fine and uniform Ti powder with a purity of around 99 mass pct was successfully produced at 1173 K (900 °C). In addition, as the CaCl₂ melt could dissolve about 20 mol pct CaO, it was found that the molar ratio of TiO₂ and CaCl₂ should be 1:20 to eliminate the by-product CaO from the reaction interface within the experimental period to continue the reduction.     

Hydrothermal formation and conversion of calcium titanate species in the system Na2O-Al2O3-CaO-TiO2-H2O

The hydrothermal formation and conversion of titanate species in the system Na₂O-Al₂O₃-CaO-TiO₂-H₂O was investigated based on the thermodynamic calculation and systematic experiments. Experimental results show that the reaction product of anatase with aluminate solution is sodium tri-titanate (Na₂O·3TiO₂), and that kassite (CaO·2TiO₂·H₂O) is formed by the reaction of sodium tri-titanate with calcium hydroxide (Ca(OH)₂) and sodium aluminate solution with high free caustic concentration at elevated temperatures. The results also show that there are two reaction paths for the formation of perovskite (CaTiO₃), one is the direct reaction of sodium tri-titanate with calcium hydroxide, and the other is the conversion of kassite with calcium hydroxide and sodium aluminate solution at elevated temperatures. In addition, the aluminum species in the sodium aluminate solution play a very important role in the process of kassite formation and its conversion to perovskite. The findings provide a potential method for the efficient preparation of titanate species and enhance the understanding of the scaling of heat exchanger surfaces in the Bayer process alumina refinery which is a result of titanate formation.     

Activities of titanium in molten copper at dilute concentrations measured by solid- state electrochemical cells at 1373 K

In order to obtain the activities of titanium in molten copper at dilute concentrations,i.e., between 5 x 10^-6 and 3.4 x 10^-3 titanium mole fractions, liquid copper was brought into equilibrium with molten {CaCl₂ + Ti₂O₃} slag saturated with Ti₂O₃ (s) at 1373 K and the equilibrium oxygen partial pressures were measured by means of a solid-oxide galvanic cell of the type Mo/Mo + MoO₂/ZrO₂(MgO)/(Cu + Ti)_alloy + Ti₂O₃ + CaCl₂ + Ti₂O_{3 slag}/Mo The free energy change for the dissolution of solid titanium in molten copper at infinite dilution referred to 1 wt pet was determined as Ti (s) = Ti(1 wt pet in Cu) ΔG°/J = -86,100 ± 8900 at 1373 K     

Effect of TiO2‐content on Reduction of Iron Ore Agglomerates

The effect of titanium oxide on iron ore agglomerates is studied by the use of test sinter, test pellets and synthetic briquettes under laboratory conditions. Titanium favours secondary hematite rather than magnetite, which is the main phase in the sinter of Rautaruukki's Raahe plant. Additionally, the effects of sinter RDI and pellet LTD on the blast furnace process are evaluated using the test results of basket trials in LKAB's Experimental Blast Furnace. The effect of titanium in synthetic hematite is studied as hematite is reduced to magnetite in the RDI test. This occurrence causes deterioration in burden permeability. Synthetic titanium‐bearing iron oxides under controlled conditions are investigated at the University of Oulu. The effect of TiO₂, in solid solution in magnetite, on the magnetite to hematite oxidation is studied separately in order to simulate the final stage of the sintering process. In other experiments, hematite samples doped with various contents of TiO₂ are studied using thermogravimetry under a controlled gas atmosphere (CO/CO₂/H₂/N₂). The TiO₂ content of hematite has a clear effect on reduction degradation. Also increasing content of TiO₂ in solid solution in magnetite radically accelerates the oxidation rate. In the pilot tests, TiO₂ content has a similar negative effect on the reduction strength of both sinter and pellets     

Effect of substituting SiO2 with TiO2 on viscosity and crystallisation of mould flux for casting titanium stabilised stainless steel

Abstract

During casting of titanium stabilised stainless steel, a gradual increase in TiO₂ content in the molten mould slag from absorption of TiO₂ inclusions causes changes in properties of the mould flux, such as viscosity, solidification temperature and crystallisation behaviour. To simulate this effect mould fluxes were prepared with SiO₂ being substituted with 5 and 10%TiO₂ and viscosity and crystallisation behaviour studied. Experimental results indicate that the substitution of SiO₂ with TiO₂ leads to suppression of crystallisation of cuspidine and formation of perovskite in glass quenched from mould fluxes. The crystallisation tendency is reduced by the introduction of TiO₂. The high temperature viscosity of mould fluxes shows a decrease with increasing TiO₂. At lower temperature, the viscosity of mould fluxes with 10%TiO₂ is greater than that of TiO₂ free slag, which could be attributed to precipitation of solid particles.     

Reaction Behavior of Stratified SiO2 Granules during Electrochemical Reduction in Molten CaCl2

The electrochemical reduction behavior of stratified SiO₂ granules in molten CaCl₂ at 1123 K (850 °C) was investigated to develop a new process for producing solar-grade silicon. The cross sections of the electrolyzed electrode prepared from a graphite crucible filled with SiO₂ granules were observed and analyzed. The visual and SEM observations indicate that the overall reduction proceeds via two different routes. In one route, the reduction proceeds along the granule surfaces from the SiO₂ near the conductor to the distant SiO₂. In the other route, the reduction proceeds from the granule surface to the core of each granule. The reduction along the granule surfaces is faster than that from the surface to the core. Fine SiO₂ granules are expected to be favorable for a high reduction rate.     

Electrochemical Reduction of Tungsten Compounds to Produce Tungsten Powder

The production of tungsten by direct current reduction has been investigated. Experimental studies involved the electrochemical reduction of the solid tungsten compounds tungsten trioxide (WO₃) and calcium tungstate (CaWO₄) in the form of an assembled cathode of porous pellets attached to a current collector. Molten calcium chloride and a molten solution of calcium chloride and sodium chloride at eutectic composition, 48 pct mol NaCl, were used as the electrolytes. Reduced samples were characterized by means of X-ray diffraction analyses and scanning electron microscopy. The results of X-ray analyses, supported with thermodynamic computations, showed that WO₃ cannot be used without loss in processes that involve the use of CaCl₂ at high temperatures because it reacts with CaCl₂ by releasing volatile tungsten oxychloride. In the electrochemical reduction of CaWO₄, X-ray diffraction results indicated the presence of tungsten with significant concentrations of calcium compounds. Metallic tungsten was obtained after treating the reduced samples with dilute hydrochloric acid solutions.     

Influence of Cr2O3 on the Viscous Flow Behavior and Structure of Cr-Containing Molten Titanium Slag

Herein, the influence of Cr₂O₃ on the viscous flow behavior and structure of Cr-containing molten titanium slag is investigated in this work. The melting and viscosity properties are studied by hemisphere method and rotating cylinder method, respectively. In addition, the structure of Cr-containing molten titanium slag is investigated by Raman spectroscopy. It is shown in the results that the melting temperature and viscosity both increase with the increase of Cr₂O₃ content. The activation energy for viscous flow increases with increasing Cr₂O₃ content, which is in the same trend as viscosity. When the Cr₂O₃ content increases from 0% to 3%, the simpler 6-coordinated Ti⁴⁺ and TiO₄⁴⁻ monomer unit transformed into more complex O–Ti–O deformation and Ti₂O₆⁴⁻ chain unit. Furthermore, the fraction of relatively complex silicate structural units shows an increasing trend. Cr₂O₃ behaves as a network former and enhances the degree of polymerization of network structure, and the variation of Cr-containing molten titanium slag structure is consistent with the increasing trend of viscosity.     

Selection of the Optimum Electrolysis Bath Composition for the Synthesis of Holmium–Iron Triad Metal Intermetallics

The results of high-temperature electrochemical synthesis of holmium–iron triad metal intermetallics in chloride melts are presented. The influence of the current density, the composition of an electrolysis bath, and the synthesis time on the electrolysis processes and the composition of the end product is studied. The electrolysis of the molten KCl–NaCl mixture containing 0.5–2.5 mol % holmium trichloride and 0.1–2.5 mol % nickel (cobalt) dichloride at a current density of 0.5–2.0 A/cm², a temperature of 973–1073 K, and an electrolysis time of 30–90 min is shown to cause the formation of a cathode deposit in the form of a “metal–salt pear” on a tungsten electrode. This pear consists of a mixture of metallic nickel (cobalt) and HoNi, HoNi₅, and HoNi₃ (HoCo₂, HoCo₃, HoCo₅, Ho₂Co₁₇) intermetallics. The intermetallic compound content in the cathode deposit is found to increase at a constant current density (1.2 A/cm²) and when the holmium chloride content in a melt or the ratio of the holmium chloride concentration to the nickel (cobalt) chloride concentration increases. Only a mixture of holmium–nickel (cobalt) intermetallics can exist in the cathode deposit if the electrolysis bath composition and the electrolysis parameters are controlled. The electrochemical synthesis of holmium–iron intermetallics was performed under galvanostatic conditions in molten KCl–NaCl–HoCl₃. Iron ions are introduced in a melt via the anodic dissolution of metallic iron. The results of X-ray diffraction analysis of the electrolysis products demonstrate the fundamental possibility of synthesizing holmium–iron intermetallics. The optimum conditions of electrosynthesis of holmium–iron triad metal intermetallics are determined.