Abstract:
A method for reducing titanium and other metal oxides to metal is disclosed. The method comprises providing a titanium or other metal oxide exposing the metal oxide to a non-thermal plasma, and reducing at least a portion of the titanium oxide to provide titanium metal. The non-thermal plasma may be formed using a first inert gas species, and a second reactive gas species.

Description:
STATEMENT REGARDING FEDERAL RIGHTS 
       [0001]    This invention was made with government support under Contract No. DE-AC52-06NA25396 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     BACKGROUND OF INVENTION 
       [0002]    The present application relates generally to processes for reducing metal salts. In particular, the present application relates to the reduction of titanium oxides. 
         [0003]    Before titanium can be made into useful metal products, the pure material must be extracted from its ore state (typically rutile or ilmenite) by a difficult and costly procedure, commonly called the Kroll process. To accomplish this, the basic ore is converted to a spongy material by chlorinating the ore with chlorine gas, thereby yielding titanium tetrachloride. Rutile and ilmenite are oxides of titanium (e.g., Ti02, etc.). The oxygen is removed as CO2 or CO, resulting in a colorless-liquid form of TiCI4 that must be purified by continuous fractional distillation. The purified product is then reacted with either magnesium or sodium under an inert atmosphere to yield a metallic titanium sponge and magnesium or sodium chloride. The chloride is then reprocessed and recycled. Next, the titanium sponge is crushed and pressed before being melted in a consumable-electrode vacuum arc furnace at very high temperature and heat consumption. Melted ingots, each weighing as much as a few tons, are then allowed to solidify under vacuum conditions in the furnace. 
         [0004]    This standard Kroll process is largely the reason for the high cost of titanium metal. Another reason for the high cost of titanium is the fact that titanium is difficult to form into basic shapes. 
         [0005]    The invention described here specifically overcomes the first source of high cost, namely the Kroll process, by using a direct plasma process and also partially addresses the second source of high cost, the forming of titanium into workable shapes. 
         [0006]    There have been attempts to create more efficient processes for reducing Ti metal. Past attempts, by others, typically employ thermal plasmas, which are energy-wasteful and non-selective in converting oxides of Ti into free metal. Such processes, while they reduce the need for some expensive and/or toxic materials, are highly energy inefficient. Thermal plasmas heat the entire plasma volume which wastes energy. 
         [0007]    Accordingly, it would be desirable to provide a method for the reduction of Ti oxides to Ti metal without the used of certain expensive and/or toxic materials. Further, it would be desirable to provide a method for the reduction of Ti oxides to Ti metal that is more energy efficient. It would also be desirable to provide a method for the reduction of Ti oxides to Ti metal that yields a useful form of Ti. 
       SUMMARY OF INVENTION 
       [0008]    One embodiment of the present application relates to a method for reducing titanium oxides to titanium metal. The method comprises providing a titanium oxide, exposing the titanium oxide to a non-thermal plasma, and reducing at least a portion of the titanium oxide to provide titanium metal. The non-thermal plasma may be formed using a first inert gas species, and a second reactive gas species. 
         [0009]    Another embodiment relates to a method for reducing metal oxides to metal. The method comprises providing a metal oxide, exposing the metal oxide to a non-thermal plasma, and reducing at least a portion of the metal oxide to provide a metal. The non-thermal plasma is formed using a first inert gas species, and a second reactive gas species. The second reactive gas species comprises a gas selected from the group consisting of hydrogen gas, carbon monoxide, one or more hydrocarbon gasses, and mixtures thereof. 
         [0010]    Yet another embodiment relates to a method for reducing titanium oxides to titanium metal. The method comprises providing a titanium oxide, exposing the titanium oxide to a non-thermal plasma selected from the group consisting of an electrical-discharge plasma, a microwave-driven non-thermal plasma, a dielectric barrier plasma, a pulsed corona discharge plasma, a pulsed homogeneous/quasi-homogeneous plasma, a glow discharge plasma, and an atmospheric pressure plasma jet. The plasma may be formed using a first inert gas species, and a second reactive gas species. At least a portion of the titanium oxide is reduced to provide titanium metal powder. The second reactive gas species comprises a gas selected from the group consisting of hydrogen gas, carbon monoxide, one or more hydrocarbon gasses, and mixtures thereof. 
     
    
     DETAILED DESCRIPTION  
       [0011]    This application pertains to the chemical reduction of oxides of titanium to its base metal by means of a non-thermal plasma process. For example, an electrical-discharge or microwave-driven non-thermal plasma in a selected process-gas medium is employed for the process. In non-thermal plasmas, the electrons are ‘hot’, while the ions and neutral species are ‘cold’-—which results in little waste enthalpy being deposited in a process gas stream. This is in contrast to thermal plasmas, where the electron, ion, and neutral-species energies are in thermal equilibrium (or ‘hot’) and considerable waste heat is deposited in the process gas. In general, non-thermal plasmas may be considered medium to low temperature plasmas. 
         [0012]    The non-thermal plasma process is a single step process thus greatly reducing the cost of the process. Also, unlike the Kroll process, expensive magnesium, poisonous chlorine gas, sodium chloride, or vacuum furnaces are not needed to reduce the TiO 2  to free metal. The process is functional on a benchtop scale. 
         [0013]    In larger scales, the resulting form of the metal in our plasma process is expected to be a powder which has advantages for forming into useful shapes by a range of process such as hot, iso-static pressing (HIPing), sintering and others. 
         [0014]    Non-thermal plasmas processes could be used as a continuous (non-batch), economical processes to produce free titanium metal. Non-thermal plasma conditions are quite selective chemically and can be low in energy consumption. Also, the non-thermal plasma route could yield titanium powder (not sponge) which has important part-forming implications (i.e., direct sintering to near-net-shape). 
         [0015]    In the primary example, it is believed hydrogen gas is cracked into atomic hydrogen which then is used to scavenge oxygen from TiO 2 , ultimately resulting in the formation of free titanium and water. While a microwave plasma, which is a mid-range temperature plasma, has been used in promising laboratory partial reductions to date, other plasmas are proposed including “cold” plasmas such as dielectric barrier plasmas, pulsed corona discharge plasmas, pulsed homogeneous/quasi-homogeneous plasmas, glow discharge plasmas, atmospheric pressure plasma jets, and others. 
         [0016]    Enhancement of the reduction reaction rate in the plasma can be achieved by use of secondary applied electric fields (bias fields) to aid in cleavage of the titanium-oxygen bond. This includes various strategies to place the TiO2 as a target on a neutral or charged surface to attract electron and/or ion bombardment. Other reducing gases can be added to the mixture such as carbon bearing gases like methane, hydrocarbons and others. 
         [0017]    Inert gases, such as argon can be added to help maintain plasma stability and to adjust the average electron density and concentration to that more optimal for bond cleavage. 
       Examples 
       [0018]    In exploratory experiments, it was demonstrated that a microwave-driven hydrogen/argon plasma could reduce titanium oxides to sub-oxides and titanium metal in a very simple apparatus at relatively low temperatures and in relatively short periods of time. 
         [0019]    TiO2 reduction experiments were carried out using a conventional quartz tube. TiO 2  powder was put in the flit for microwave treatment. Argon plasma was used at a 1.5 lpm flow. The experiment was mainly divided in two cases. One of them used pure argon plasma and the other used 2% H 2  in the argon plasma. In the first case no apparent change was observed in the TiO 2 . In the second case, as plasma treatment time passed, the more black the reaction product appeared. A black product which was a mixture of metallic titanium and other reduction products was obtained. While higher amounts of H 2  gas may result in greater heat loss, reaction rates may increase. In some embodiments, the amount of H 2  gas may range from about 0.5% to 10%. In other embodiments, the amount of H 2  gas may range from about 1% to 5%. 
         [0020]    Currently, thought the exact mechanism for the titanium reduction is not clear, it is believed to follow one of the mechanisms discussed below. As some surface processes, such as adsorption-desorption of 0 2 , may be assisted by plasma species to obtain titanium metal may be involved, and because plasma chemistry is an unusual chemistry it will be difficult to be certain about the specific mechanism(s) at this stage. 
         [0021]    However, it is possible to go back to basic chemistry to give a first insight to understand and maybe improve the process. 
         [0022]    In the overall process we have Argon and Hydrogen gases entering into the plasma region. In the pure argon plasma case, at the end of the process Argon has to return to neutrality, so it is not being involved in electron transfer, that is, it is a non-influent species in chemical terms. This is in accordance with our experimental results: pure Argon plasma had no effect on TiO 2  over the same time that Ar/H 2  plasma actuated on the material. 
         [0023]    Again, considering the overall process, a possible but very general explanation for a theoretical complete reduction of the TiO 2  that could ideally be obtained with the Ar/H 2  plasma is provided. At first, because of the evident reduction of Titanium, there must be an oxidation implicated in the process. We can have two possibilities for this: 
         [0024]    1. Pure Argon Plasma
       4e-+Ti 4+ →Ti°   20 2 −→0 2 +4e-   Overall: TiO 2 →Ti+02
 
However, the Ti oxides are likely to be thermodynamically preferred at the plasma conditions and the reduction reaction would be highly reversible. This may account for the lack of reduction observed in the Ar plasma experiment.
       
 
         [0028]    2. Ar/H 2  Plasma
       4e-+Ti 4+ →Ti°   2H 2   − →4H + +4 e-      Overall: TiO 2 +2H 2 →Ti+2H 2 0       
 
         [0032]    In both cases a reduction of TiO 2  could be explained, but practically we can see that for some energetic reason the TiO 2  dismutation is not favored in the pure Argon plasma case. 
         [0033]    In the second case, we can see that, since TiO 2  needs electrons to achieve the reduction, H2 is there to release them and take oxygen (which basically does not suffer any change) from TiO2 to make water. 
         [0034]    Based on the Ar/H 2  plasma experiments, any species that can be oxidized, that is, release electrons, could theoretically be useful for the TiO 2  reduction. For example, an Ar/CO plasma may also be a possible candidate for the process. In that case, the mechanism may be: 
         [0035]    Ar/CO Plasma 
         [0036]    4e-+Ti 4+ →Ti° 
         [0037]    2C 2+ +20 2− →2C 4+ +4e- 
         [0038]    Overall: TiO2+2C0Ti+2CO2 
         [0039]    Other gasses could possibly be used. For example, hydrocarbons, e.g., methane etc., may theoretically be used. However, such gasses may adversely affect the economics of the process. 
         [0040]    Thus, there has been shown and described several embodiments of a method for reducing titanium oxides to titanium metal which methods fulfill all of the objects and advantages sought therefore. Many changes, modifications, variations and other uses and applications of the present invention will, however, become apparent to those skilled in the art after considering this specification. All such modifications, variations and other uses and applications which do not depart from the spirit and scope of the present invention are deemed to be covered by the present invention which is limited by the claims which follow.