Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/26—Electrolytic production, recovery or refining of metals by electrolysis of melts of titanium, zirconium, hafnium, tantalum or vanadium
  • C25C3/28—Electrolytic production, recovery or refining of metals by electrolysis of melts of titanium, zirconium, hafnium, tantalum or vanadium of titanium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B34/00—Obtaining refractory metals
  • C22B34/10—Obtaining titanium, zirconium or hafnium
  • C22B34/12—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08
  • C22B34/129—Obtaining titanium or titanium compounds from ores or scrap by metallurgical processing; preparation of titanium compounds from other titanium compounds see C01G23/00 - C01G23/08 obtaining metallic titanium from titanium compounds by dissociation, e.g. thermic dissociation of titanium tetraiodide, or by electrolysis or with the use of an electric arc
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C14/00—Alloys based on titanium
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts

Definitions

• This invention describes a method, apparatus and means for production of a metal, metal alloy or metal composite in an electrolysis cell.
• Titanium and its alloys exhibit excellent mechanical properties, unrivalled corrosion resistance and outstanding biocompatibility; however, annual global titanium production is dwarfed by commodity metals such as steel and aluminium.
• Kroll process One needs only to examine the complex and discontinuous production method, the Kroll process, to correlate the high price of titanium with its low consumption.
• alternate processing routes have been sought, in vain; titanium oxides are extremely stable compounds that are bound with increasing tenacity to oxygen as the latter concentration decreases.
• a high solubility for oxygen in metallic titanium necessitates carbo-chlorination of titanium dioxide to produce an oxygen-free, chloride feedstock (TiCI 4 ), which is subsequently metallothermically reduced with liquid magnesium.
• Claim 1 of US Patent 6,712,952 states a method that is operated by "...applying a voltage between the electrode and the anode such that the potential at the electrode is lower than a deposition potential for the cation at a surface of the electrode and such that the substance dissolves in the electrolyte.”
• the electrolyte in the process described by these groups acts merely as a solvent for the
• the Ca metal deposited on the cathode is soluble in the electrolyte and can dissolve in the electrolyte and thereby migrate to the vicinity of the cathode titanium oxide."
• line 41 the Ca metal that deposited on electrically conductive sections of the cathode was deposited predominantly as a separate phase .
• a separate phase is equivalent to a condensed phase and calcium metal at unit activity.
• Patent application US 2004/0237711 A1 describes, in paragraph [0049], "Moreover, the electrolysis of calcium oxide in calcium chloride between the anode made of a consumable carbonaceous anode material and the cathode made of non-consumable cathode material forms calcium-saturated calcium chloride either saturated with dissolved calcium or coexisting with pure calcium in the vicinity of the cathode."
• the calcium formed by electrolysis is maintained at the saturation concentration as may be understood by the formation of "pure calcium” or Ca metal, which may be interpreted as either a discrete solid or liquid, depending on the operating temperature.
• Claim 1 of the same patent application states "...the molten salt in the reaction region is electrolysed thereby converting the molten salt into a strongly reducing molten salt ".
• someone skilled in the art would appreciate that the higher the extent of calcium saturation, the greater the reducing strength of the electrolyte. Consequently, the use of a "calcium-saturated" calcium chloride describes an extremely reducing electrolyte that has reached the solubility limit for dissolved calcium metal.
• Figure 1 depicts the current response to the potential waveform and there are clearly electrochemical processes, labeled A, B, C and D, occurring at potentials significantly positive of the reductive limit (E) of the electrolyte, where calcium metal would form.
• electrochemical processes were attributed to the formation of Ti 3 Os, Ti 2 O 3 , TiO and Ti-O, respectively.
• metallic titanium was produced, significant quantities of calcium were also generated during the reduction. Thus, it has been established that a calcium-saturated melt or metallic calcium is not necessary to effect reduction of TiO 2 to the lower oxides of titanium or metallic titanium containing oxygen.
• the applicant has extensively investigated the electrochemistry of the CaCI 2 -CaO-Ti-O system in an attempt to rationalise the disagreement in mechanistic explanations.
• the experimental work conducted by the applicant has proven that it is impossible to operate under conditions where the electrolyte is not decomposed and the cation of the electrolyte is not deposited, owing to the thermodynamic and physical properties of the electrolyte system.
• the applicant has established that it is preferable to conduct the electrolysis under conditions where the cation deposition process results in a dilute and controlled concentration of dissolved calcium, which may effectively reduce the metal oxide or deoxidize the metal.
• the present invention describes a process in which, a metal, M 1 , is produced in an electrolytic cell consisting of a molten electrolyte, M Z Y - M Z O, at least one anode and at least one cathode, characterised in that the passage of current between said anode(s) and cathode(s) through said electrolyte, produces a metal, M 1 , from a raw material, M 1 X, containing a non-metallic species, X, under conditions such that the potential at the cathode causes the reduction of the M z cation and the formation of M z at activities less than one.
• the M z produced in this manner reduces the raw material, M-
• the raw material feed may also contain both species M 1 and X, in a ternary or higher order oxide, of the form M 2 M 1 X, by way of example.
• molten electrolyte (M Z Y - M Z O) for conducting such electrolysis is CaCI 2 -CaO electrolyte.
• This molten salt used by both the applicant and other organisations is comprised of Ca 2+ cations with Cl " and O 2' anions.
• the CaO dissolved in the electrolyte is present as Ca 2+ and O 2" ions and there is no distinction between Ca 2+ cations that originate from CaO and those from CaCI 2 .
• the consequence of this is that the cathode potential for reduction of Ca 2+ is unchanged by addition of CaO. Decomposition of the electrolyte to form elemental calcium will occur once the cathode potential exceeds a certain threshold value, which may be determined from known thermodynamic data for a given temperature.
• the standard state potential for the reduction of the cation Ca 2+ to calcium is 3.211 V negative of the standard chlorine electrode. It has been established that many alkali and alkaline earth metals exhibit high solubility in their chloride salts. Unsurprisingly, calcium exhibits a high solubility in the CaCI 2 -CaO electrolyte system (Fischbach). This has profound consequences for the cathodic process, as the form in which the cations of the electrolyte are present following reduction is no longer restricted to a discrete solid or liquid (depending on the electrolysis temperature).
• E is measured in volts versus the standard state potential for the reduction of Ca 2+ to Ca 0
• R is the universal gas constant (8,3144 J-mol "1 -K "1)
• T is the temperature in Kelvin
• Table 1 lists the standard state reduction potential (100% saturation of calcium in the electrolyte) of Ca 2+ at 900 2 C and the potentials calculated from Equation 1 , corresponding to selected dissolved calcium activities less than one (less than 100% saturation of calcium in the electrolyte).
• the electrochemical spectrum that represents the continuum of calcium activities from infinitely low values, (ostensibly, an activity of zero) to saturation (unit activity) may be further defined by a second variable of interest, such as melt calcium oxide activity.
• a second variable of interest such as melt calcium oxide activity.
• Predominance diagrams for the Ti-O-Ca-Cl system were constructed by Dring et al (J. Electrochem. Soc. v152, #10 (2005):D184), in the manner described by Littlewood (J. Electrochem. Sod 965). These maps delineate the regions of stability for the experimentally observed phases over a range of electrode potentials and melt CaO contents. This representation of the Ca-Cl-O system is shown graphically in Figure 2 (J. Electrochem. Soc.
• v152, #10 (2005):D184 may be considered the molten salt equivalent of Pourbaix diagrams.
• the x-axis is chosen as the negative, base-ten logarithm of the CaO activity, (-log 10 [a Ca o]) in the melt, which is denoted by p ⁇ 2' .
• possible electrode reactions for oxide anions in the presence of a graphite anode are shown.
• the low cell voltage, relative to the standard state decomposition potential of CaCI 2 , reported by the inventors of competing processes, that may be applied in order to observe electrolysis current is due to the fact that oxide anions are oxidised at less positive potentials than chloride anions, and the potential for CO and CO 2 evolution on graphite anodes is significantly less positive than for O 2 evolution on inert anodes at the same CaO content.
• Figure 3 shows the regions of stability of various titanium-oxygen-calcium compounds as a function of both melt calcium oxide activity and electrode potential, which is, as shown earlier, synonymous with calcium activity.
• Figure 3 illustrates that the reduction of TiO 2 to all of its lower oxides and even low-oxygen metal occurs without calcium metal at unit activity (either solid or liquid) present, and that this may occur at calcium activities below 10 ⁇ 3 in melts having low CaO contents.
• the melt contains neither calcium metal as a discrete phase nor the high concentrations of Ca 0 necessary to constitute a "strongly reducing molten salt".
• Operation of the electrolysis cell such that the cathode potential is less negative than that corresponding to saturated calcium formation, which results in the reduction of the electrolyte cation to produce calcium in the solvated state, is equally effective for reduction of the metal oxide.
• the exothermic reaction with the oxide precursor may result in the sintering of the feed material, with possible entrapment of reaction products and/or slowing of subsequent reduction rates.
• Any calcium reductant that does not reduce the titanium oxide at the cathode is able to diffuse away from the raw material, where it cannot do useful electrochemical work. Consequently, the calcium may chemically react with the anode material or anode off-gases resulting in excessive anode consumption/erosion and, if graphite anodes are used, the generation of free carbon or calcium carbides.
• increasing contents of calcium in the electrolyte impart a high degree of electronic conductivity and create a short circuit path for electrons within the electrolyte, which should ideally function as an ionic conductor only. This final drawback is of the greatest concern, since current efficiency is significantly worsened as a consequence.
• the increased energy consumption leads not only to an increase in financial costs, but added environmental burdens arising from the prerequisite power generation.
• the applicant has determined that since high amounts of calcium are detrimental to the efficiency of the electrolysis, it is preferable to ensure that throughout the reduction process the concentration and activity of calcium and calcium oxide are controlled.
• the preferred manner of control is via the use of a reference electrode whose potential is not affected by changes in the electrolyte composition, specifically the CaO concentration. By controlling the potential at the cathode with respect to this reference electrode potential, a fixed calcium activity/concentration may be obtained. This serves to maximize the reduction rate since the rate at which calcium is supplied to the raw material can be controlled to match the rate at which the reduction of metal oxide occurs.
• the melt calcium oxide content must also be controlled, and this may be understood in the context of an equilibrium between oxide ions in the titanium-containing raw material, and in the electrolyte (Reaction 5).
• this exterior surface may be reduced all the way to metal, which may then act as an additional surface area for the cathodic reduction of the Ca 2+ cations in the melt. This would exacerbate the situation further, since even more of the cathode would be producing calcium at high activities.
• the applicant has resolved that the optimum operating conditions are such that the consumption of calcium reductant by the raw material is matched by the rate of calcium generation at the cathode.
• the cathode potential may be maintained at potentials corresponding to the activities of calcium needed to effect each of the reduction process in order to proceed from TiO 2 to metallic titanium. This is accomplished using a stable reference electrode, which does not vary as the melt CaO composition changes.
• the benefits of this are two-fold: a lower cell voltage may be used, thus consuming less power; and significantly less calcium is formed The latter effect avoids the numerous disadvantages described above.
• the applicant believes that operation of the electrolysis cell in the manner described above is the only way to achieve a high quality metal, alloy or composite product at an acceptable price. Table of figures:
• Figure 1 is a current versus potential plot of TiO 2 and Mo in CaCI 2 at 900 3 C.
• Figure 2 is a predominance diagram showing the conditions of electrode potential and melt oxide content corresponding to a given electrochemical reaction for a system with a CaCI 2 -CaO electrolyte and a graphite electrode(s).
• Figure 3 is a predominance diagram showing the conditions of electrode potential and melt oxide content corresponding to a given electrochemical reaction for a system with a CaCI 2 -CaO electrolyte, a graphite anode(s), and a cathode consisting of titanium oxide.
• FIG. 4 is a schematic diagram of the electrochemical cell used in conjunction with the present invention.
• Figure 5 is the x-ray diffraction pattern of cathode material produced after 24 hours with a cell voltage of 50OmV.
• Figure 6 is the x-ray diffraction pattern of cathode material produced after 24 hours with a cell voltage of 75OmV.
• Figure 7 is the x-ray diffraction pattern of cathode material produced after 24 hours with a cell voltage of 100OmV.
• Figure 8 is an optical image of partially reduced cathode material exhibiting a metallic shell and a core consisting of oxides.
• Figure 9 is a potential versus time plot for TiO 2 reduced under constant current
• FIG. 10 Scanning electron micrograph of Ti-10V-2Fe-3AI alloy produced via the present invention.
• Table 1 lists the standard state reduction potential for Ca 2+ to Ca 0 , and the potentials calculated from Equation 1 corresponding to selected dissolved activities of Ca 0 at 900 g C.
• a molten salt reactor depicted in Figure 4, was assembled using vertical tube furnace with temperatures recorded using a thermocouple (1 ) within the cell and a PC-based data acquisition unit.
• a sealed lnconel reaction (2) vessel housed alumina crucibles (3), which contained the CaCI 2 -CaO electrolyte (4). This electrolyte was obtained by mixing thermally dried CaCI 2 -2H 2 O and 1 wt% CaO, and was subsequently heated in the retort under flowing argon (5, 6) to 1173 K.
• Example 2 Reduction of TiO? under conditions of constantly high Ca activity
• the experimental apparatus used in Example 1 was reproduced identically, except for the electrolysis voltage, which was fixed at 3V, and the duration of electrolysis, which was 12 hours.
• the sample was removed from the electrolyte, allowed to cool, and washed in water.
• a cross section of the sample ( Figure 8) revealed a metallic ⁇ -titanium case that enclosed a darker powder, which was identified by x-ray diffraction as a titanium sub-oxide.
• the thickness of the metallic layer was approximately 100-200 microns, which effectively acted as a diffusion barrier preventing full reduction of the titanium dioxide pellet.
• Example 2 An identical reactor to that used in Example 1 was employed to reduce 10-micron thick TiO 2 layers thermally formed on a titanium substrate.
• a potentiostat was used in conjunction with a graphite counter electrode, nickel/nickel chloride reference electrode and TiO 2 working electrodes.
• a constant reduction current was applied to the working electrode and the potential, with respect to the reference electrode, was recorded over time ( Figure 9). The reduction current was terminated when the working electrode potential reached a steady state value that did not continue to decrease over a long period of time. Since the TiO 2 layer was of finite thickness, the reduction current at the conclusion of the experiment must have been comprised primarily of calcium formation.
• Example 4 Production of conventional titanium alloy (Ti-10V-2Fe-3AI) Reagent grade oxide powders from Alfa Aesar (TiO 2 99.5 %, FeTiO 3 99.8 %, AI 2 O 3 99.9 % and V 2 O 5 99 %, 1-2 ⁇ m particle size) were mixed, as-received, with a small amount of distilled water, which acted as a binding agent, to achieve a final composition of 10 wt% V, 2 wt% Fe, 3 wt% Al with the balance of titanium. The powder was then ground with a mortar and pestle for 5 minutes to break down large agglomerates prior to uniaxial compaction on a 15 mm diameter die at 100 MPa to obtain the desired preform shape.
• Alfa Aesar TiO 2 99.5 %, FeTiO 3 99.8 %, AI 2 O 3 99.9 % and V 2 O 5 99 %, 1-2 ⁇ m particle size

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• 2006
  • 2006-06-14 NO NO20062776A patent/NO20062776L/no not_active Application Discontinuation
• 2007
  • 2007-05-30 US US12/308,367 patent/US20100006448A1/en not_active Abandoned
  • 2007-05-30 WO PCT/NO2007/000183 patent/WO2007145526A1/en active Application Filing
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