Electrochemical Reduction of Metal Oxides

Abstract

A metal oxide feed material for an electrochemical reduction process is disclosed. The feed material is in the form of powders and/or pellets that have sufficient porosity, typically 35-60%, to enable penetration of molten electrolyte into the powders and/or pellets during the course of an electrochemical reduction process in an electrolytic cell and subsequent washing of electrolyte from the powders and/or pellets after the powders and/or pellets are discharged from the cell. A process for preparing the metal oxide feed material is also disclosed.

1 . A metal oxide feed material for electrochemically reducing metal oxides in an apparatus that includes an electrolytic cell that includes a bath of molten electrolyte and an anode and a cathode comprising one of a powder or pellet that includes a sufficient porosity to enable penetration of molten electrolyte into the one of a powder or pellet during the course of an electrochemical reduction process and subsequent washing of electrolyte from the one of a powder or pellet after the one of a powder or pellet is discharged from the cell.  
     
     
         2 . The feed material defined in  claim 1  wherein the one of a powder or pellet includes any one or more than one of: 
 a. a density that is higher than the density of the molten electrolyte so that the one of a powder or pellet of metal oxide feed material does not float in the bath;    b. a size that is sufficiently large so that the one of a powder or pellet can sink at a reasonable rate within the bath if supplied to the bath from above the surface of the bath;    c. minimum fines that could restrict access of molten electrolyte to the one of a powder or pellet of metal oxide feed material and minimise reduction rates during the course of the process; and    d. sufficient strength so that there is minimal breakdown of the one of a powder or pellet in the process, including handling the one of a powder or pellet pre- and post-processing in the cell.    
     
     
         3 . The feed material defined in  claim 1  wherein when the one of a powder or pellet is a titanium oxide material and the electrolyte is a CaCl 2 -based electrolyte that contains CaO when in a molten state, the one of a powder or pellet has an open connected pore structure with a porosity in the range of about 35 to about 60% by volume.  
     
     
         4 . The feed material defined in  claim 3  wherein the porosity is at least about 40 vol. %.  
     
     
         5 . The feed material defined in  claim 3  wherein the porosity is at least about 55 vol. %.  
     
     
         6 . The feed material defined in  claim 3  wherein the porosity is in a range of about 40 to about 50 vol. %.  
     
     
         7 . The feed material defined in  claim 3  wherein at least about 25 vol. % of the pores have a size of about 0.005 microns to about 10 microns as measured by Mercury Intrusion porosimetry.  
     
     
         8 . The feed material defined in  claim 7  wherein the remainder of the pores have a size of less than about 0.005 microns measured by gas absorption.  
     
     
         9 . The feed material defined in  claim 3  wherein when the one of a powder or pellet is a titanium oxide material and the electrolyte is a CaCl 2 -based electrolyte that contains CaO when in a molten state, the one of a powder or pellet includes any one or more than one of: 
 (i) a density of about 2.5 to about 3.5 g/cc in air,    (ii) a size of about 1 mm to 3 mm,    (iii) a minimum mechanical strength to minimise decrepitation of the one of a powder or pellet prior to, during, and after processing the one of a powder or pellet in the cell; and    (iv) a minimum strength to withstand thermal shock associated with introducing the one of a powder or pellet into the cell.    
     
     
         10 . The feed material defined in  claim 9  wherein the one of a powder or pellet has a size in the range of about 1 mm to about 2.5 mm.  
     
     
         11 . The feed material defined in  claim 9  wherein the one of a powder or pellet has size in the range of about 1.5 mm to about 2 mm.  
     
     
         12 . A process for preparing a metal oxide feed material for electrochemically reducing metal oxides in an apparatus that includes an electrolytic cell that includes a bath of molten electrolyte and an anode and a cathode, which process includes the steps of: 
 a. forming one of a powder or pellet of metal oxide feed material having a required porosity;    b. sintering the one of a powder or pellet from step (a) to a required strength; and    c. washing the sintered the one of a powder or pellet to remove fines attached to the one of a powder or pellet.    
     
     
         13 . The process defined in  claim 12  includes a further step of separating the one of a powder or pellet formed in step (a) into at least two fractions on the basis of size.  
     
     
         14 . The process defined in  claim 13  includes supplying a larger size fraction to step (b).  
     
     
         15 . The process defined in  claim 13  includes returning a smaller size fraction to step (a).  
     
     
         16 . The process defined in  claim 12  wherein step (a) includes forming the one of a powder or pellet of metal oxide feed material to have an open connected pore structure and a porosity of about 35 to about 60 vol. %.  
     
     
         17 . The process defined in  claim 12  wherein step (a) includes forming one of a powder or pellet of metal oxide feed material to have one or more of a required size or a required density.  
     
     
         18 . The process defined in  claim 12  wherein step (a) includes forming one of a powder or pellet of metal oxide feed material by agglomerating particles of metal oxide material.  
     
     
         19 . The process defined in  claim 18  wherein the particles of metal oxide material for agglomeration are fully dense particles.  
     
     
         20 . The process defined in  claim 18  wherein the minimum dimensions of the particles of metal oxide material for agglomeration are less than about 50 microns.  
     
     
         21 . The process defined in  claim 18  wherein the minimum dimensions of the particles of metal oxide material for agglomeration are less than about 30 microns.  
     
     
         22 . The process defined in  claim 18  wherein the minimum dimensions of the particles of metal oxide material for agglomeration range from nanometer size up to about 15 microns.  
     
     
         23 . The process defined in  claim 18  wherein step (a) includes forming the one of a powder or pellet of metal oxide feed material by agglomerating particles of metal oxide material with a binder.  
     
     
         24 . The process defined in  claim 18  wherein step (a) includes forming the one of a powder or pellet of metal oxide feed material by agglomerating one or more of 
 (i) particles of metal oxide material,    (ii) fines washed from the sintered one of a powder or pellet in step (c), and    (iii) the smaller size fraction from the separation step, wherein the agglomeration is optionally conducted with a binder.    
     
     
         25 . The process defined in  claim 18  wherein step (a) includes agglomerating particles of metal oxide material in a mixer that is capable of high impact and high velocity mixing of metal oxide material.  
     
     
         26 . The process defined in  claim 18  wherein step (a) includes agglomerating particles of metal oxide material by supplying particles of metal oxide feed material and water to the mixer and operating the mixer and forming agglomerates of a required porosity.  
     
     
         27 . The process defined in  claim 12  wherein when the metal oxide feed material is a titanium oxide, the required porosity of the one of a powder or pellet formed in step (a) is an open pore structure with about 35 to about 60% porosity.  
     
     
         28 . The process defined in  claim 27  wherein the one of a powder or pellet formed in step (a) has a size of about 1 mm to 4 mm and a density of about 2.5 to about 3.5 g/cc.  
     
     
         29 . The process defined in  claim 12  wherein, in situations in which the metal oxide feed material is titania, step (b) includes sintering the one of a powder or pellet formed in step (a) at a temperature in the range of about 850 to about 1400° C.  
     
     
         30 . The process defined in  claim 29  wherein step (b) includes sintering the one of a powder or pellet formed in step (a) in air.  
     
     
         31 . A process for electrochemically reducing metal oxides in an apparatus that includes an electrolytic cell that includes a bath of molten electrolyte and an anode and a cathode using the metal oxide feed material defined in  claim 1 .  
     
     
         32 . A process for electrochemically reducing metal oxides in an apparatus that includes an electrolytic cell that includes a bath of molten electrolyte and an anode and a cathode, which process includes the feed preparation process defined in  claim 12.