Abstract:
The invention relates to a metal boride precursor mixture comprising a metal oxide and a boric oxide combined in such a manner so as to produce intimately linked clusters wherein the boric oxide is found within the metal oxide. Furthermore, the invention discloses a carbon composite material made with the metal boride precursor mixture and a carbonaceous component. Finally, the invention also teaches the process for preparing the metal boride precursor mixture comprising steps of providing a metal oxide and a boron oxide, mechanically mixing the metal oxide and the boron oxide at a temperature that liquefies the boron oxide and may impregnate the metal oxide to produce an intimately linked cluster of metal oxide and boric oxide.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to the production of a metal boride precursor mixture comprising components which when combined with a carbonaceous component, produce a carbon composite material suitable for use in a cathode or cell lining of an aluminium electrolysis cell. 
     2. Description of the Prior Art 
     Metal borides, such as TiB 2  are used in an admixture with carbon components to form ramming pastes, cell linings and cathodes for electrolysis cells. Metal borides are known to improve surface wettability of the electrolytic cell components into which they are added. Although titanium diboride is preferred from the point of view of superior performance, it has the considerable disadvantage that it is very expensive. 
     Titanium diboride is generally formed by combining equal mole quantities of titanium oxide and boron oxide with a reducing agent. Carbon and aluminum, for example, are both known reducing agents. This reduction of the oxides to titanium diboride is also known to be possible at the start-up of an aluminium producing electrolysis cell, therefore producing TiB 2  in situ. However, it is known that in situ formation of the metal boride from the precursors proceeds suitably only if the reactants are thoroughly mixed one with the other, and in particular, contact between the metal oxide and the boron compound is required for the production of the metal boride. 
     Khazai et al. in U.S. Pat. No. 5,160,698 teach a process for producing metal borides using finely comminuted mixtures of reactants. The mixture of particulate reactants comprises: a metal compound, carbon, and a boron source having a particle size of about 200 microns which is heated to convert the mixture to a metal boride having an average particle size of about 0.05 to about 0.5 micron. The metal compound is one which can be converted to a corresponding metal boride by a reaction with carbon and the boron source under controlled conditions and at temperatures from 1200° C. to 2500° C. 
     Mirtchi et al. in European Patent EP 1 141 446 B1 teach a wettable and erosion/oxidation-resistant carbon composite material for use in the production of aluminium. Mirtchi et al. provide carbon composite materials which may be fashioned into cathode blocks, jointing pastes or the like that are erosion/oxidation resistant and wettable by molten aluminium when used in electrolysis cells while being relatively inexpensive and easy to produce. Mirtchi et al. utilize a precursor mixture that is preferably finely divided having average particle sizes sufficiently small to allow effective reaction of the precursor during the cell start-up to produce corresponding metal borides. 
     In preparing carbon composite mixtures containing the metal oxide and the boron compound, segregation of the metal oxide and boron compound may occur even when the particles are finely divided. Furthermore, boron oxide seems to have a detrimental effect on the carbon binder used in block production. Thus, mixing and baking is difficult and leads to lower quality products. It is the object of this invention to overcome at least some of the deficiencies in the prior art. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, there is provided a metal boride precursor mixture comprising: a metal oxide, and boric oxide (B 2 O 3 ), wherein the metal oxide and the boric oxide are physically linked in clusters and the boric oxide is intimately supported by the metal oxide. 
     In accordance with another aspect of the invention, there is provided a carbon composite material for use in an aluminium electrolysis cell as at least one of a cathode and a cell wall material, the composite material comprising: a carbonaceous component, and a metal boride precursor mixture comprising a metal oxide, and boric oxide, wherein the metal oxide and the boric oxide are physically linked in clusters and the boric oxide is intimately supported by the metal oxide. 
     In accordance with yet another aspect of the invention, there is provided a process for producing a metal boride precursor mixture comprising the steps of: providing a metal oxide wherein the metal oxide has a particle surface providing a boron component; mechanically mixing the metal oxide and the boron component to produce a particulate mix, and heating the particulate mix to a temperature where the boron component becomes a liquefied boric oxide and impregnates the particle surface to produce a physically linked cluster of metal oxide and boric oxide, and the boric oxide is intimately supported by the metal oxide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
         FIG. 1  is a process diagram for producing a metal boride precursor mixture according to a preferred embodiment of the invention; 
         FIG. 2   a  is a micrograph of a green carbon composite block (Block A) made using a metal boride precursor mixture according to a preferred embodiment of the invention, showing small TiO 2  particles and a B 2 O 3  binder-like phase; 
         FIG. 2   b  is a micrograph of a green carbon composite block (Block B) made using finely mixed but separate TiO 2 /B 2 O 3  materials similar to those of EP 1 141 446 B1; 
         FIG. 3   a  is a photomicrograph of a carbon composite material microstructure of block A including a TiO 2 /B 2 O 3  precursor mixture according to the invention; 
         FIG. 3   b )i) is a photomicrograph of a carbon composite material microstructure at low magnification of block B with dispersed boron oxide in the binder phase and no TiO 2  in proximity; 
         FIG. 3   b )ii) is a photomicrograph of a carbon composite material microstructure at low magnification of block B with dispersed TiO 2  in the material and no boron oxide in proximity; and 
         FIG. 4  is a photomicrograph of a transformed precursor mixture particle after several days of electrolysis in an aluminium reduction cell. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a block flow diagram according to one embodiment of a process for the production of a metal boride precursor mixture of the invention. The precursor mixture includes a metal oxide and boric oxide. 
     The metal oxide of the invention may be selected from, but not necessarily limited to, titanium oxide (TiO 2 ), zirconium oxide (ZrO 2 ), hafnium oxide (HfO 2 ), vanadium oxide (V 2 O 5  and V 2 O 3 ), niobium oxide, tantalum oxide, chromium oxide and molybdenum oxide, and combinations thereof. In a preferred embodiment of the invention, the metal oxide is TiO 2  or ZrO 2 , and in a particularly preferred embodiment is TiO 2 . 
     The boric oxide of the precursor mixture in a preferred embodiment is produced from a boron component selected from the group consisting of ortho-boric acid (H 3 BO 3 ) and meta-boric acid (HBO 2 ). Clearly, boric oxide (B 2 O 3 ) may also be used as the boron component or starting material. 
     In a preferred embodiment, the metal boride precursor of the invention will have a metal oxide/boric oxide in a theoretical weight percentage needed to combine stoichiometrically under cell operations to form metal borides. Therefore, if the metal oxide is TiO 2 , the amount of B 2 O 3  needed to react to produce TiB 2  via Reaction 1 is 53 wt %/47 wt % TiO 2 /B 2 O 3  respectively. Although these weight % are preferred, small variations from these weight ratios are permissible. In particular, small excesses of the boron compound may compensate for volatilization which may still occur to a limited extent.
 
3TiO 2 +3B 2 O 3 +10Al→3TiB 2 +5Al 2 O 3   Reaction 1
 
     Where the starting boron component is not boric oxide, the weight ratio of the starting components will be different. For example, in the case of TiO 2 /H 3 BO 3 , the ratio is 40/60 wt %/wt % respectively, that produces the required 53 wt %/47 wt % TiO 2 /B 2 O 3  in the metal boride precursor. 
     In a preferred embodiment, the metal oxide and the boron component are provided as particulate materials having an average particle size of less than 200 micron. In the first step of the process, there is a particulate mixing step  10 , where the two starting materials are mechanically mixed to produce a particulate mix. In a preferred embodiment, the metal oxide includes a high level of porosity into which the boron component may enter. 
     The second step of the process is a heating/baking step  20 , wherein the particulate mix is heated progressively up to 600° C. 
     In one embodiment of the process to produce the metal boride precursor mixture of the invention, the temperature may be progressively increased in the following step-wise manner:
         a) from 25° C. to 120° C.; where the temperature of 120° C. is maintained for 15 minutes—at this temperature any surface moisture is evaporated;   b) from 120° C. to 400° C., where the temperature of 400° C. is maintained for 15 minutes. At this temperature the reaction to produce the metal boride precursor may begin with the decomposition of the boron component and the evolution of water vapor; and   c) from 400° C. to 600° C., where the temperature is maintained for more than 6 hours, or in a preferred embodiment for more than 20 hours. These longer time periods help to ensure: that the reaction goes to completion, that the B 2 O 3  produced is in liquid form, and that the water vapor evolved in any decomposition reaction will have time to diffuse out of the metal boride precursor mixture. The skilled practitioner would understand that the precursor mixture can be heated in many ways to obtain a liquefied boron oxide, and up to at least about 450° C. where boron oxide becomes liquid.       

     After the heating/baking step  20 , the precursor mixture is solidified in a mold, by reducing the temperature to 25° C. In this solidifying step  30 , the molten B 2 O 3  that has been liquefied in the heating/baking step  20 , will solidify and become intimately supported on the metal oxide particles e.g. TiO 2  crystals. In the case where TiO 2  particles include pores, the B 2 O 3  may enter into these pores. Therefore, the metal oxide and the boric oxide produce a physical connection, and in a preferred embodiment, the boric oxide is found within pores of the particle surface of the metal oxide. 
     The solidified precursor mixture is removed from a mold, i.e. demolded. This is followed by a crushing step  40  and a pulverizing step  50 . In a preferred embodiment the crushing step  40  is performed in a jaw crusher. The pulverization step  50 , is preferably performed in a rod mill or any apparatus with low heat production so as not to exceed the boron oxide melting point. The desired product size from pulverization  50 , may be screened; and in a preferred embodiment, the metal boride precursor mixture has an average particle size less than 150 micron (−100 mesh). 
     The reactions which occur in heating/baking step  20  that produce the metal boride precursor mixture will now be discussed in greater detail by reference to the preferred embodiment in which the metal oxide is titanium oxide. If the starting boron compound is ortho-boric acid (H 3 BO 3 ) a percentage of approximately 40% TiO 2  to 60% H 3 BO 3  (by weight) or more exactly 39.3 wt % to 60.7 wt %, produces the appropriate weight % in the metal boride precursor (i.e. 53 wt %/47 wt % TiO 2 /B 2 O 3  respectively). However, to compensate for volatilization of the B 2 O 3  in the final composite, as noted above, a percentage of 35% to 40% TiO 2  to 60% to 65% H 3 BO 3  can be used. The heat treatment after mixing leads to the decomposition of H 3 BO 3  following the approximate reaction below: 
     
       
                 
         
             
             
         
      
     
     Thus water vapor is evolved at two temperatures, approximately 170° C. and 300° C. Upon baking the metal oxide and the boron component, the global reaction from room temperature to 600° C. is given by the expression: 
     
       
                 
         
             
             
         
      
     
     Therefore, if TiO 2 /H 3 BO 3  proportions of 40/60 wt %/wt % are used as starting materials, after completion of the Reaction 3 (above), the resulting proportions of TiO 2 /B 2 O 3  in the final mixture will be 53/47 wt %/wt % respectively. This final weight percentage of TiO 2 /B 2 O 3  equates to a theoretical loss of water of 26.5% from the starting TiO 2 /H 3 BO 3  mixture. This final weight percentage of TiO 2 /H 3 BO 3  corresponds to a molar ratio of 1:1, that is required to form one mole of TiB 2  from one mole of the precursor in Reaction 1 (previously defined). 
     At temperatures above 450° C., B 2 O 3  is a liquid (B 2 O 3  has a melting point of approximately 450° C.) and wets the TiO 2  particles. It should be mentioned that metal oxides have higher melting points, in the case of TiO 2 , over 1800° C., and would remain in a solid crystal form. 
     Under process conditions, the molten B 2 O 3  coats and impregnates the TiO 2  particles. The B 2 O 3  is thus intimately mixed and physically bound to the metal oxide, and produces a semi-solid. In an embodiment where the TiO 2  particles includes pores, these pores will permit better impregnation by B 2 O 3  into the TiO 2 , and so as to maximize the TiO 2 /B 2 O 3  contacts. It is believed that this intimate contact between TiO 2 /B 2 O 3  limits the B 2 O 3 /carbonaceous (binder or pitch) interaction. 
     It has been further found that a portion of the boron component can be Na 2 B 4 O 7 . This has a melting point in excess of 700° C. so cannot be used as the sole source, but as a portion of the boron component, it becomes mixed with the liquid boron oxide and coats and impregnates the TiO 2 . As this compound is less volatile at higher temperatures than B 2 O 3 , the loss of boron can be controlled more effectively. Although not a preferred embodiment, the boron component could be solely Na 2 B 4 O 7  if higher processing temperatures are used. Similarly Borax (the hydrated form of Na 2 B 4 O 7 ) could be used in place of Na 2 B 4 O 7 . 
     In order to minimize boron losses during baking, a small part of the boron oxide may be replaced by boron carbide (B 4 C), which does not volatilize at normal baking temperatures. In small amounts, boron carbide will react with the titanium oxide and aluminium to form TiB 2 . 
     The pulverized metal boride precursor mixture thus produced can now be made into carbon composite materials suitable for aluminium electrolysis cells. 
     The precursor is combined with a carbon material that may be graphite or calcined anthracite and a pitch binder. Preferably 10 to 40% by weight of the precursor mixture is used. More preferably 20 to 35% by weight of precursor mixture is used. The proportion of pitch to carbon material is adjusted to facilitate manufacture by normal means well known to the skilled worker. 
     In certain embodiments, titanium diboride may be added as well to the carbon composite mixture. This may be advantageous when a particular particle size or morphology combination is desired in the composite after it has been exposed to the cell environment, where this cannot be achieved by reaction of the precursor mixture alone. For example, combinations using from 10 to 40% by weight of precursor mixture with from 40 to 0% by weight of titanium diboride may be used. 
     In a typical composite material, 30% of the metal boride precursor mixture by weight is added to 51% graphite by weight, and combined with roughly 19% by weight of binder-pitch. 
     50 wt % of the metal boride precursor mixture has a particle size of −100+200 mesh, while the other 50 wt % is −200 mesh. 
     The carbon composite mixtures produced may be further processed by conventional forming and baking operations to form carbon blocks typically used as the cathode of an electrolytic cell. Such formed components are generally baked at temperatures of about 1100 to 1200° C. prior to use. Such temperatures are not sufficient to cause the carbon present to reduce the metal boride precursor mixture to the metal boride. The carbon composite mixture may also be used directly in the cell (as a “ramming mixture” to fix gaps around and between the carbon blocks, in which case the baking occurs during cell startup. 
     EXAMPLE 1 
     Comparison of Carbon Blocks 
     Table 1 presents a comparison between composite carbon blocks made using two different metal boride precursor materials. Block A was prepared using the precursor mixture of the present invention while Block B used finely mixed but separate TiO 2 /B 2 O 3  materials (i.e. no thermal treatment and crushing prior to mixing with the carbonaceous components) similar to that of EP 1 141 446 B1. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Block A 
                   
               
               
                   
                 C Composite made from 
                 Block B 
               
               
                   
                 material of the precursor 
                 C composite made of finely 
               
               
                   
                 mixture of the present 
                 mixed separated oxide 
               
               
                 Components/Parameters 
                 invention 
                 materials (Prior Art) 
               
               
                   
               
             
             
               
                 Oxide mixture 
                   
                   
               
               
                 TiO 2   
                 15.9% - 100 mesh* 
                 15.9% - 100 mesh* 
               
               
                 H 3 BO 3   
                 14.1% - 100 mesh 
                 — 
               
               
                 B 2 O 3   
                 — 
                 7.05% - 200 mesh 
               
               
                   
                   
                 7.05% - 100 to 200 mesh 
               
               
                 Oxide treatment 
                 See metal boride precursor 
                 Mechanically mixed 5 
               
               
                   
                 mixture preparation 
                 minutes 
               
               
                 Carbon composite 
               
               
                 Graphite 
                 51% wt 
                 51% wt 
               
               
                 Binder-pitch 
                 19% wt 
                 19% wt 
               
               
                 Oxides 
                 15% - 100 + 200 mesh 
                 30% 
               
               
                   
                 15% - 200 mesh 
               
               
                 Mixing temperature 
                 150 to 160° C. 
                 150 to 160° C. 
               
               
                 Mixing time 
                 30 minutes 
                 30 minutes 
               
               
                 Forming method 
                 Vibro-compacted 
                 Vibro-compacted 
               
               
                 Forming time 
                 2 minutes 
                 2 minutes 
               
               
                 Heat treatment temperature 
                 1150° C. 
                 1150° C. 
               
               
                 Heat Treatment ramp 
                 approx. 25° C. per hour 
                 approx. 25° C. per hour 
               
               
                 Heat treatment time 
                 7 days 
                 7 days 
               
               
                 (including heating and cooling) 
               
               
                 Time at 1150° C. 
                 40 hours 
                 40 hours 
               
               
                   
               
               
                 *The TiO 2  particles used in this experiment are fine - 1 micron spheres loosely agglomerated in small clusters. 
               
             
          
         
       
     
     During mixing in both Block A and Block B pitch hardening was observed in both cases but to a much higher degree in Block B. In Block B, the binder had lost most of its fluidity at the end of the mixing step and formed small very hard spheres of material that were very difficult to shape into a block form by standard procedures. The appearance of both Block A and Block B in their green (pre-baked) state is presented in  FIGS. 2   a  and  2   b  respectively. It can be seen that beside the upper face Block A in  FIG. 2   a , there are no signs of major imperfections while there appears to be porosity observed in Block B ( FIG. 2   b ). 
     Not wishing to be bound by any theory, it is believed that the hardening of the binder pitch during mixing is presumably due to pitch-B 2 O 3  contacts at elevated temperatures. In fact, B 2 O 3  is an acid compound known for its ability to catalyze organic reactions (Kirk-Othmer  Encyclopedia of Chemical Technology , vol. 4, 5 th  Edition, John Wiley and Sons, pg. 249). This has a consequence of a rapid polymerization of the medium length carbonaceous chains into longer less mobile ones which may in some way explain the loss of fluidity at the mixing step. After baking, both carbon composite materials were examined. It was observed that Block B prepared from the finely mixed separated oxide materials included important macro-porosity. 
     Table 2 presents some properties of the Blocks A and B measured after baking (in Table 2, S.D. is the standard deviation of the average values). 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Properties of Blocks A and B after baking (three samples) 
               
             
          
           
               
                   
                   
                 Electrical 
                 Compressive 
               
               
                   
                 Bulk density 
                 resistivity 
                 strength 
               
               
                   
                 (g/cm 3 ) 
                 (μΩ m) 
                 (MPa) 
               
             
          
           
               
                 Material 
                 Average 
                 S.D. 
                 Average 
                 S.D. 
                 Average 
                 S.D. 
               
               
                   
               
               
                 A 
                 1.7 
                 0.004 
                 58.0 
                 3.28 
                 23.4 
                 2.81 
               
               
                 B 
                 1.7 
                 0.002 
                 74.5 
                 2.19 
                 28.5 
                 1.12 
               
               
                   
               
             
          
         
       
     
     Both blocks A and B have similar bulk densities and identical nominal compositions, this means that both blocks also have very similar levels of porosity after baking. The room temperature compressive strength of Block B was somewhat superior to that of Block A. This is believed to be due to the different room temperature microstructures arising from the presence of separate “glassy” B 2 O 3  throughout that may improve room temperature strength. Note however that this may also result in lower mechanical strength at higher temperature since B 2 O 3  will then be in its liquid state. 
     We see that Block B exhibits a much higher electrical resistivity than Block A. Electrical resistivity is a property that depends mostly on the nature of the raw materials and the material microstructure. Both materials have the same nominal composition and exhibit similar porosity, thus the difference is mostly due to the microstructure. In Block B the presence of a large fraction of B 2 O 3  within the conductive binder phase is believed to act as an insulator and thus to considerably lower the ability of the latter to conduct electricity. Because electrical conductivity is a major property of the cathode block, this is one of the most important drawbacks of using a dry-mixed oxide. 
       FIGS. 3   a ,  3   b )i) and  3   b )ii) represent an overview of material microstructure of Blocks A ( FIG. 3   a ) and B ( FIGS. 3   b (i) and (ii)). In  FIG. 3   b (i) the B 2 O 3  is found dispersed in the pitch phase with no TiO 2  particles in the proximity. In  FIG. 3   b (ii) the TiO 2  is dispersed in the carbon mixture and we see that there are no boron oxides particles in the vicinity of the TiO 2 . 
     On the other hand, B 2 O 3  is almost always found surrounding the TiO 2  particles in Block A. In Block A as seen in  FIG. 3   a , instead of being spread inside the matrix, the oxide remains mostly located within clusters, and this is presumed to enable the formation of a continuous carbon network. This carbon network eventually provides the composite material of the present invention with a higher electrical conductivity. 
     The use of the metal boride precursor mixture of the invention in the carbon composite material has proven to be efficient in producing in situ TiB 2 .  FIG. 4  is a photomicrograph of a transformed precursor mixture particle after several days of electrolysis in an aluminium reduction cell. The micrograph shows TiB 2  particles in a matrix of alumina, which is a side product of the transformation reaction. Carbon, which is a component of the material, is also found in the vicinity of the TiB 2  particles. Observations both from laboratory specimens and samples from industrial sized cell cathodes show that TiB 2  may be formed quickly at the material surface by a reaction with metal aluminium. However, TiB 2  was also observed deep beneath the surface of the block after 200 days in a reduction cell. This may be by a reaction with the aluminium dissolved in the bath and transported through the cathode. 
     The metal boride precursor mixture, the carbon composite material produced from the metal boride precursor mixture and the process for producing the metal boride precursor mixture clearly offer the potential of greatly lower fabrication costs of the materials while avoiding problems encountered when simple mechanical mixtures of the components are used. 
     The embodiment(s) of the invention described above is (are) intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.