In-cell manganese ore reduction
FIELD OF THE INVENTION 
     This invention relates to an improved process for producing manganese metal by employing the heat content of a fused salt medium to reduce higher manganese oxides contained in manganese ore to lower manganese oxides using a pelletized blend of crushed manganese ore and a solid carbonaceous fuel which is heated by a fused salt electrolytic bath. 
     BACKGROUND OF THE INVENTION 
     Manganese metal may be produced by electrolysis in a fused salt electrolytic bath. A fused salt electrolytic bath for the production of manganese is described in U.S. Pat. No. 3,018,233 by J. Y. Welsh, M. W. Milberg and H. R. Peterson, the disclosure of which is incorporated herein by reference. Such a fused salt electrolytic bath contains a mixture of oxides and also contains a fluoride. The fluoride, preferably calcium fluoride, is necessary in order to provide sufficient electrical conductivity and fluidity to the fused electrolyte. It is preferred that the bath contain at least 50% by weight of a fluoride; but it is possible to operate a fused salt electrolytic bath with an electrolyte containing less than 50% by weight of fluoride. The mixture of oxides in the fused salt electrolytic bath may include B 2  O 3 , Al 2  O 3 , TiO 2 , SiO 2 , CaO, K 2  O, Na 2  O, BaO, MnO, or a combination of these or other similar oxides. The composition of the electrodes and the electrical operation of the electrolytic cell are fully described in the U.S. Pat. No. 3,018,233. 
     The fused salt electrolytic process described in U.S. Pat. No. 3,018,233 reduces a lower oxide of manganese such as manganese (II) oxide to manganese metal. The present invention provides an improved process for reducing higher manganese oxides, such as manganese (IV) dioxide, contained in a manganese ore to manganese metal via manganese (II) oxide, using a pelletized blend of crushed manganese ore and a solid carbonaceous fuel heated by a fused salt electrolytic bath. 
     SUMMARY OF THE INVENTION 
     An improved process has been discovered for producing manganese metal from manganese ore by means of a fused salt electrolytic process. In the present invention, a pelletized blend of crushed manganese ore and solid carbonaceous fuel is heated in a fused salt electrolytic bath for the production of manganese metal. The heat contained in the fused salt electrolytic bath effects reduction of the manganese ore with carbon contained in the solid carbonaceous fuel to produce manganese (II) oxide. 
     The MnO produced by the reduction of the manganese ore is useful for the production of manganese metal by the electrolytic process described in U.S. Pat. No. 3,018,233, the disclosure of which is incorporated hereby by reference. In this electrolytic process, the MnO produced by heating the pelletized blend of crushed manganese ore and solid carbonaceous fuel is dissolved in the molten electrolytic bath which is in contact with the surface of a pool of molten manganese. An electric current is passed from a carbon anode partially immersed in the molten electrolytic bath, through the electrolytic bath, to the surface of the pool of molten manganese which acts as a cathode. The electric current effects reduction of the manganese (II) oxide to manganese metal. 
     The present invention encompasses the production of the pelletized blend of crushed manganese ore and a solid carbonaceous fuel, the process for reducing manganese (IV) dioxide to manganese (II) oxide by heating this pelletized blend of crushed manganese ore and a solid carbonaceous fuel, and the process of preparing manganese metal in a fused salt electrolytic bath using manganese (II) oxide formed from the pelletized blend of crushed manganese ore and a solid carbonaceous fuel. 
     It is an advantage of the present invention that it provides an efficient method of using the heat content in a fused salt electrolytic bath to effect reduction of manganese (IV) dioxide to manganese (II) oxide. 
     It is a further advantage of the present invention that it provides a process for reducing manganese (IV) dioxide to manganese (II) oxide in which the ratio of manganese ore to the solid carbonaceous reducing agent is predetermined and constant, thereby obviating the need to maintain control of the ratio during the reduction process. 
     It is another advantage of the present invention that the process for reducing manganese (IV) dioxide to manganese (II) oxide is particularly efficient because of the high surface area and intimate contact between the crushed manganese ore and solid carbonaceous fuel which acts as a reducing agent in the pelletized blend. 
     Still other advantages of the invention will be readily apparent to those of ordinary skill in the art from the FIGURE and detailed description of the invention which follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE is a flow diagram illustrating an in-cell manganese ore reduction process according to the present invention. 
    
    
     While the invention will be described in connection with preferred procedures, it will be understood that it is not intended to limit the invention to those procedures. On the contrary, it is intended that the present invention encompass all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the present invention, a pelletized blend of crushed manganese ore and solid carbonaceous fuel is heated in a fused salt electrolytic bath for the production of manganese metal. 
     The pelletized blend of crushed manganese ore and solid carbonaceous fuel is formed by the process illustrated in the FIGURE. The manganese ore used is primarily in the form of MnO 2 . Manganese ore may contain, for example, about 50 to about 55% by weight of manganese. The solid carbonaceous fuel may be selected from materials such as coke, coal, charcoal, carbon, and wood chips. Of these, coke and coal are preferred carbonaceous fuels. 
     In the first step of the pellet making process, manganese ore is mixed or blended with sufficient carbonaceous fuel to reduce the MnO 2  contained in the ore to MnO. The ratio of manganese ore to carbonaceous fuel can be varied from about 8 to 1 up to about 18 to 1. At ratios below about 8 to 1, the pellets subsequently produced are too soft and susceptible to disintegration in subsequent handling. Disintegration of the pellets is undesirable inter alia because of dusting and excessive losses when the pellets are used to feed an electrolytic cell for the production of manganese metal. At ratios above about 18 to 1, there will be insufficient solid fuel to effect the complete reduction of MnO 2  in the ore. It is preferred that the ratio of manganese ore to carbonaceous fuel be between about 10  to 1 and about 13 to 1. It is especially preferred that the ratio be about 11 to 1. 
     In the second step of the pellet making process, the blended manganese ore and carbonaceous fuel are dried to remove free moisture. The temperature of the drying step is kept sufficiently low to prevent ignition of any dust from the solid fuel. Typically, the blended ore and fuel are dried at a temperature of about 100° C. 
     In the third step of the pellet making process, the blended and dried manganese ore and carbonaceous fuel are ground, for example in a mill, to a particle size which will produce a pellet having good physical properties. The size of the grind may vary from about -30 mesh×D to about -100 mesh×D. Grinding the blended and dried ore and fuel to a particle size of about -60 mesh×D is generally preferred. If the grind is too coarse, the agglomeration step (which follows) will produce a pellet having poor physical properties due to its low density. Conversely, if the grind is too fine, the agglomeration step will produce a pellet having serious dusting problems. For these reasons, sizing the grind from about -30 mesh×D to about -100 mesh×D is preferred. The symbol &#34;mesh×D&#34; refers to mesh sizes by Down. 
     In the fourth step of the pellet making process, the ground, dried blend of ore and fuel is mixed with a clay-type binder, for example bentonite, to strengthen the pellets produced in the following agglomeration step. The amount of clay-type binder mixed with the blend may vary from about 0.25% up to about 4% by weight. Generally, mixing about 1% to about 3% by weight of a clay-type binder with the ground, dried blend is preferred. It is desirable to strengthen the pellets produced in the following agglomeration step so that they may be subjected to subsequent processing and handling without physical disintegration. 
     In the fifth step of the pellet making process, the ground, dried blend of ore and fuel, mixed with a clay-type binder, is compacted or agglomerated to form pellets, for example in a balling dish. Other means for compacting the blend mixed with clay-type binder may also be used, for example pelletizing or briqueting presses. Typically, the pellets thus produced are between about 0.25 and about 0.5 inches in diameter. A liquid binder may be added during the compacting to facilitate agglomeration. Suitable liquid binders include sodium silicate solution and aqueous molasses solution, as well as other liquid binders which will be readily apparent to those in the field of agglomeration. It is generally preferred to add about 1% by weight of a liquid binder to the ground, dried blend mixed with a clay-type binder, although from about 0.5% to about 5% by weight of a liquid binder may be usefully added to facilitate agglomeration. 
     In the sixth and last step of the pellet making process, pellets formed by the compacted blend of ore and fuel mixed with binders are hardened or indurated by heat treatment at a temperature between about 200° F. to about 350° F. for a period of about 10 to about 60 minutes. In general, heating at a temperature of at least about 325° F. for a period of at least about 40 minutes is sufficient to harden the pellets so that the pellets can withstand subsequent handling without physical disintegration. 
     Hardened pellets made by the above-described process may be stored in storage bins until needed for use in the production of manganese metal by electrolysis in a fused salt electrolytic bath. A suitable fused electrolytic bath for the production of manganese is described in U.S. Pat. No. 3,018,233 by J. Y. Welsh, M. W. Milberg and H. R. Peterson, the disclosure of which is incorporated herein by reference. As described therein, it is a feature of the operation of the electrolytic cell that the heat balance of the cell must be such that the molten electrolytic bath is in contact with the surface of a molten pool of manganese metal, which is the product of cell&#39;s operation. 
     The present invention uses heat contained in the molten electrolyte bath to effect the reduction of manganese ore by carbon as represented by the following equations: 
     
         2MnO.sub.2 →Mn.sub.2 O.sub.3 +1/20.sub.2            (1) 
    
     
         C+1/20.sub.2 →CO                                    (2) 
    
     
         Mn.sub.2 O.sub.3 +CO→2MnO+CO.sub.2                  (3) 
    
     
         Mn.sub.2 O.sub.3 +H.sub.2 →2MnO+H.sub.2 O           (4) 
    
     The first equation represents the partial thermal reduction of MnO 2  contained in the pelletized crushed manganese ore to Mn 2  O 3 . This reaction requires a substantial temperature, at least about 800° K. or 527° C. Furthermore, since this reaction is endothermic, the molten electrolytic bath must supply sufficient additional heat to maintain the reaction. The oxygen released from the above reaction then reacts with carbon in the pelletized blend of crushed manganese ore and solid carbonaceous fuel to produce carbon monoxide gas. The carbon monoxide gas produced in the interior of the pellet reacts with Mn 2  O 3  as indicated in the third equation to produce MnO and CO 2 . The fourth equation is applicable only if the solid carbonaceous fuel used in the pelletized blend is coal, which contains residual volatile hydrocarbons. When exposed to the intense heat of the molten electrolyte bath, these hydrocarbons generate hydrogen as well as carbon monoxide. If hydrogen is present in the interior of the pellet, it reacts with Mn 2  O 3 , as indicated in the fourth equation, to produce MnO and H 2  O. These reactions are discussed in more detail in U.S. Pat. No. 4,010,236 by J. Y. Welsh, the disclosure of which is incorporated herein by reference. 
     The MnO produced by the reduction of manganese ore as set forth in the above equations is useful as a starting material for the production of manganese metal by the electrolytic process described in U.S. Pat. No. 3,018,233. In this process, a lower oxide of manganese, such as MnO, is dissolved in a molten electrolytic bath (as previously discussed) which is supported by a pool of molten manganese. An electric current is passed from a carbon anode partially immersed in the molten electrolytic bath, through the electrolyte bath containing dissolved MnO to the surface of the pool of molten manganese (which acts as a cathode) to reduce the manganese oxide according to the equation: 
     
         MnO+2e.sup.- →Mn+O.sup.2- 
    
     Periodically the molten manganese produced is tapped from the pool. The product manganese metal, as tapped from the electrolytic cell, generally assays over 91.0% Mn. 
     The pelletized blend of crushed manganese ore and solid carbonaceous fuel ore made by the process of the FIGURE are fed to the electrolytic cell to form layers on the surface of the molten electrolytic bath. As the heat from the molten bath warms the nearest pellets, they undergo the reduction reactions discussed above. When the pellets reach the temperature of the molten bath, they melt and dissolve in the bath. Successively, the next layer of pellets containing MnO 2  moves down toward the surface of the molten electrolytic bath, is warmed and reduced to form MnO, dissolves in the bath, and is electrolyzed as discussed above to form manganese metal. 
     The invention will now be illustrated with the following specific examples, which should not be considered as limiting in any way: 
     EXAMPLE NO. 1 
     Ore: Australian Ore; 52% Mn, typical 
     Solid Fuel: Pennsylvania Rice Coal 
     Ore:Fuel Ratio: 10:1 
     Grind: -60 mesh 
     Binder System: 2% (Bentonite/20% Sodium Silicate) solution 
     Table of Results: 84,581 pounds total 
     
         ______________________________________                                    
Avg. Daily                                                                
        Avg. Cell No. of  % Mn  Metal Analysis                            
Feed, Lbs.                                                                
        Volt.     Days    Eff.  % Mn  % Si  % C                           
______________________________________                                    
7,050   8.2V      12      98.9  91.6  0.43  0.18                          
______________________________________                                    
 
    
     EXAMPLE NO. 2 
     Ore: Australian Ore; 52% Mn, typical 
     Solid Fuel: Pennsylvania Pea Coke 
     Ore:Fuel Ratio: 10:1 
     Grind: -60 mesh 
     Binder System: 1% (Bentonite/50% Molasses) solution 
     Table of Results: 37,756 pounds total 
     
         ______________________________________                                    
Avg. Daily                                                                
        Avg. Cell No. of  % Mn  Metal Analysis                            
Feed, Lbs.                                                                
        Volt.     Days    Eff.  % Mn  % Si  % C                           
______________________________________                                    
6,300   8.0V      6       94.7  91.8  0.24  0.20                          
______________________________________                                    
 
    
     EXAMPLE NO. 3 
     Ore: Australian Ore; 52% Mn, typical 
     Solid Fuel: Pennsylvania Rice Coal 
     Ore:Fuel Ratio: 12:1 
     Grind: -60 mesh 
     Binder System: 2% (Bentonite/50% Molasses) solution 
     Table of Results: 73,039 pounds total 
     
         ______________________________________                                    
Avg. Daily                                                                
        Avg. Cell No. of  % Mn  Metal Analysis                            
Feed, Lbs.                                                                
        Volt.     Days    Eff.  % Mn  % Si  % C                           
______________________________________                                    
6,640   8.1V      11      90.1  91.3  0.27  0.21                          
______________________________________                                    
 
    
     Examples 1 and 2 compare the process of the present invention using coal, which contains residual volatile hydrocarbons, to coke. It will be seen that the overall manganese efficiency of the process in Example 1 using coal is higher than the efficiency of the process in Example 2 using coke. It is believed that coal is a more effective reducing agent since, when exposed to the intense heat of the molten electrolytic bath, the residual volatile hydrocarbons in coal generate hydrogen as well as carbon monoxide. The hydrogen present in the interior of the pelletized blend according to the present invention reacts with Mn 2  O 3  to produce MnO and H 2  O. When coke is used, as in Example 2, more carbon monoxide is required and the reducing effect of hydrogen is lost. Example 3 illustrates a loss in overall manganese efficiency when the ore:fuel ratio is increased from 10:1 to 12:1. 
     It is understood that various other modifications will be apparent to and can readily be made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty which reside in the present invention, including all features which would be treated as equivalents thereof by those skilled in the art to which the invention pertains.