Low temperature refining and formation of refractory metals

#This invention pertains to electrochemical reduction and purification of refractory metals, metal compounds and semi-metals at low temperatures in non-aqueous ionic solvents. Metals and semi-metals form oxides and they also have a significant oxygen solubility. Using the methods described herein below it is possible to produce metals such as titanium from bulk titanium dioxide at significant cost savings. Further, it is possible to reduce or remove the oxides on highly oxidized titanium metal surfaces.

#The Kroll process and Hunter process are methods currently in use for the production of titanium metal from titanium dioxide. In these methods, TiO2 is reacted with chlorine gas to produce titanium tetrachloride, a volatile corrosive liquid. This is reduced to titanium metal by reacting with metallic magnesium in the Kroll process or with sodium in the Hunter process. Both processes are carried out at high temperatures in sealed reactors. Following this, a two-step refining process is carried out which includes two high temperature vacuum distillations to remove the alkali metal and its chloride from titanium metal.

#The refining of titanium by electrochemical means has long been a sought after process. It has been shown in the literature that oxygen could be removed from titanium and titanium alloys using an electrochemical high temperature molten salt method. This has led to the development of a possible new method of extracting and refining titanium directly from the oxide ore and was published by G. Z. Chen, D. J. Fray and T. W. Farthing in Nature 407, 361 (2002), and PCT international application publication number WO 99/64638, 16 Dec. 1999. Both documents are incorporated herein by reference in their entirety. This process involves electrochemistry in a high temperature molten salt, molten CaCl2 at ˜800 C. In these publications two different mechanisms are proposed for the reduction of titanium oxides. In the first mechanism it is proposed that the Ca+2 ions are reduced to metallic Ca at the cathode. Then the Ca metal chemically reacts with the TiOx forming an oxygenated Ca species, CaO, which is soluble in the melt forming Ca+2 and O−2. The second mechanism proposed was the direct electrochemical reduction of the TiOx to Ti metal and an oxygen species such as O−2. This is followed by the migration of the O−2 to the carbon anode where it forms a volatile species such as CO or CO2.

#FIG. 1. shows the voltage window for the production of Ti from TiO2 in a non-aqueous ionized solvent.

#FIG. 2. shows the apparatus used to demonstrate the invention and produce the results shown in FIG. 1.

#FIG. 3. shows XPS data for Ti, and TiO2 recorded on the reduced bulk TiO2 discussed below using the apparatus shown in FIG. 2.

#In this invention TiO2 has been reduced to Ti at room temperature using an electrochemical electrolysis system and a non-aqueous ionic solvent. To accomplish the reduction, or the removal of oxygen from TiO2, current was passed through the system at a voltage predetermined to reduce the metal oxide. In this invention, a compound MX is reacted in an electrochemical system to remove X from MX. X may be an element chemically combined with M as for instance TiO2, or dissolved in M. For instance O may react with M to form oxides, or it may also be dissolved as an impurity in M.

#In this invention M is a metal or a semi-metal, while MX is a metal compound, or a semi-metal compound or a metal or semi-metal with X being dissolved in M.

#The non-aqueous ionic liquid solvent electrolytes used in this invention are mono- and dialkylimidazolium salts mixed with aluminum chloride. This is a class of compounds is known as organochloroaluminates.