Abstract:
The invention disclosed provides a process for recovering niobium values from aqueous hydrofluoric acid solutions by evaporating said solution to dryness and baking the residue at temperatures of 150° to 250° C. The solid residue is niobium oxyfluroide (NbO 2  F) which may then be exothermically reduced with aluminum in the presence of an alloying element such as iron, nickel, or chromium to produce a niobium alloy. The evaporated hydrofluoric acid may be recovered by condensing or absorbing the evolved vapors.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for the aluminothermic reduction of niobium oxyfluoride in the presence of an alloying metal powder. The niobium oxyfluoride is prepared by evaporating a hydrofluoric acid solution of niobium to dryness and baking at 150° C. or greater. 
     BACKGROUND OF THE INVENTION 
     This invention relates to the treatment of aqueous hydrofluoric acid solutions of niobium emanating from the liquid-liquid extraction and purification of niobium. 
     The usual purification and recovery of niobium from its ores involves taking the ore into hydrofluoric acid solution and separating the niobium from tantalum and gangue materials by its differential solubility in organic extractants such as methyl-isobutyl-ketone, di-alkyl-phosphate, tributyl-phosphate, trioctyl-amine and the like. The niobium fluoride loaded organic phase is then contacted with water whereby the niobium values are preferentially soluble in the aqueous phase. This aqueous niobium fluoride solution is then treated with ammonia to precipitate the niobium as the hydroxide along with chemisorbed ammonium fluoride. This slurry is then filtered thus separating the ammonium fluoride solution from the precipitated niobium hydroxide. The niobium hydroxide filter cake is then dried and calcined at 875° to 1000° C. to remove moisture and absorbed ammonium fluoride. The ammonium fluoride solutions create a waste disposal problem and must be treated with lime to react with the ammonium fluoride according to the reaction: 
     
         2NH.sub.4 F+ CaO→CaF.sub. 2 + 2NH.sub.3 + H.sub.2 O (I) 
    
     generally, this reaction takes place in an ammonia stripping column whereby the released ammonia is recovered in the overhead fraction. 
     The calcined Nb 2  O 5  can then be alumino-thermically reduced in the presence of an alloying element such as iron, nickel, or chromium to produce a vacuum-grade niobium-containing alloy. 
     It has now been found that by recovering niobium values from aqueous hydrofluoric acid strip solutions which are evaporated to dryness following which the residue is baked at a temperature from about 150° C. to 250° C., niobium oxyfluoride (NbO 2  F) is recovered which can be alumino-thermically reduced to a vacuum-grade alloy. The process is performed in a simple efficient manner and avoids numerous disadvantages of the prior art. In addition, about 1.5 pounds hydrogen fluoride per pound of niobium is recovered in the process. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Practice of the present invention provides an effective means by which niobium alloys may be produced by alumino-thermic reduction of niobium oxyfluoride, NbO 2  F, which may be prepared by evaporating a hydrofluoric acid solution containing niobium values to dryness following which the dried residue is baked for conversion to niobium oxyfluoride. Alternatively, niobium oxyfluoride may be converted to niobium pentoxide, Nb 2  O 5 , by calcining in the presence of water vapor. 
     In an embodiment of this invention, there is disclosed the treatment of aqueous hydrofluoric acid solutions of niobium emanating from the liquid-liquid extraction and purification of niobium. 
     It has now been found that by the present invention there is provided a method of recovering the niobium values from an aqueous niobium fluoride solution and at the same time recovering the hydrofluoric acid values from the solution. The process involves evaporating an aqueous solution of niobium fluoride from its initial concentration of about 70 to 140 grams of niobium per liter to about 350 to 700 grams of niobium per liter in a closed evaporator attached to a condenser to recover the vapors emanating from the evaporation. The condensed vapors normally contain about 8 to 12% by weight hydrofluoric acid. The concentrated niobium fluoride solution is then placed in a suitable tray, for example, nickel, and evaporated to dryness at a temperature greater than 150° C. The vapors from this evaporation are also recovered as an approximate 50% by weight hydrofluoric acid solution. The residue cake that remains in the tray is niobium oxyfluoride (NbO 2  F). The recovered niobium oxyfluoride can be calcined in the presence of water vapor at about 500° to 1000°  C. to form niobium pentoxide (Nb 2  O 5 ) by the following reaction: 
     
         2 NbO.sub.2 F+ H.sub.2 O→ Nb.sub.2 O.sub.5 + 2 HF    (II) 
    
     an advantage of this process is that silicon and antimony, common impurities in niobium pentoxide produced in the customary way, are removed. 
     Vacuum grade ferro-, nickel-, or chromium-niobium are customarily produced from high purity niobium pentoxide (Nb 2  O 5 ) which has been made by ammonia precipitation of a high purity niobium fluoride solution. The precipitate is filtered, washed, dried and calcined at about 1000° C. The calcined niobium pentoxide is then mixed with aluminum powder and either iron, chromium or nickel powder, lime or fluorspar for fluxing and sodium chlorate as a heat booster. This mixture is ignited in a suitable container to produce the desired alloy by the alumino-thermic reaction: 
     
         3 Cb.sub.2 O.sub.5 + 10 Al→ 6 Cb+ 5 Al.sub.2 O.sub.3 (III) 
    
     now, however, it has been found that niobium oxyfluoride (NbO 2  F) can be used in place of niobium pentoxide for the alumino-thermic production of niobium alloys. The niobium oxyfluoride can be produced without the high temperature calcination step which is a source of contamination of the high purity niobium pentoxide. The fluorine content of the niobium oxyfluoride passes into the slag where it acts as an efficient flux and assists in metal collection during the reduction step. In addition, the aluminum reduction of niobium oxyfluoride generates more exothermicity than the reduction of niobium pentoxide resulting in the elimination of a chemical heat booster such as sodium chlorate. The addition of lime (CaO) or the like in sufficient quantity to react approximately stoichiometrically with the fluorine in the niobium oxyfluoride (NbO 2  F) is desirable and is found necessary. Without the lime addition, aluminum fluorides would be volatilized thus agitating the molten system and, at the same time, removing heat and leaving a high melting point slag, Al 2  O 3 . 
     The overall reaction for the reduction of niobium oxyfluoride with aluminum in the presence of lime follows: 
     
         6 NbO.sub.2 F+ 10 Al+ 3 CaO→ 6 Nb+ 5 Al.sub.2 O.sub.3 + 3 CaF.sub.2 (IV) 
    
     the reaction probably takes place in two steps: 
     
         6 NbO.sub.2 F+ 10 Al→6 Nb+ 4 Al.sub.2 O.sub.3 + 2 AlF.sub.3 (V) 
    
     
         Δ f.sub.2000 = -468 kcal. 
    
     and 
     
         2 AlF.sub.3 + 3 CaO→Al.sub.2 O.sub.3 + 3 CaF.sub.2   (VI) 
    
     
         Δ f.sub.2000 = -114 kcal. 
    
     
         Δ F.sub.2000 (Overall)= -582 Kcal. 
    
     which can be contrasted to the Nb 2  O 5  reaction: 
     
         3 Nb.sub.2 O.sub.5 + 10 Al→6 Nb+ 5 Al.sub.2 O.sub.3  (VII) 
    
     
         Δ f.sub.2000 = -450 kcal. 
    
     Thus, it can be seen that at thermite reaction temperatures the free energy of reaction is about 29% greater for the NbO 2  F system than the Nb 2  O 5  system. 
     Of course, other oxides capable of reacting with aluminum fluoride at thermite temperatures can also be used in place of calcium oxide. The list includes materials such as strontium oxide, barium oxide, magnesium oxide, and lithium oxide as follows: 
     
         2 AlF.sub.3 + 3SrO→ Al.sub.2 O.sub.3 + 3 SrF.sub.2   (VIII) 
    
     
         Δ f.sub.2000 =-150 kcal. 
    
     
         2 AlF.sub.3 + 3 BaO→ Al.sub.2 O.sub.3 + 3 BaF.sub.2  (IX) 
    
     
         Δ f.sub.2000 =-156 kcal. 
    
     
         2 AlF.sub.3 + 3 MgO→Al.sub.2 O.sub.3 3 MgF.sub.2     (X) 
    
     
         Δ f.sub.2000 =-69 kcal. 
    
     
         2 AlF.sub.3 + 3 Li.sub.2 O→Al.sub.2 O.sub.3 + 6 LiF  (XI) 
    
     
         Δ f.sub.2000 =-177 kcal. 
    
     Typically, the reactions proceed using stoichiometrical amounts of reactants, and preferably a slight excess of the oxides. 
     Thus, by the present process alloys such as ferro-niobium, nickel-niobium, or chromium-niobium can be effectively prepared by the alumino-thermic reduction of niobium oxyfluoride, without the use of a chemical or thermal heat booster. 
     The following examples illustrate practice of the present invention in greater detail. In the examples, as well as in this specification generally, all parts and percentages are given by weight unless indicated otherwise. 
     Example 1 
     A solution of niobium fluoride emanating from the liquid-liquid extraction of niobium was found to contain: 
     
         ______________________________________Nb            =      134.8    grams/literHF            =      238      grams/literF.sup.- /Nb Mole Ratio         =      8.2Antimony      =      550      ppm (Sb/Nb Basis)Silicon       =      1200     ppm (Si/Nb Basis)______________________________________ 
    
     About 3,100 gallons of this solution was evaporated to 893 gallons. The boiling point at the end of evaporation was 122° C. The concentrated niobium solution was found to contain: 
     
         ______________________________________Nb                =       468     grams/literHF                =       625     grams/literF.sup.- /Cb Mole Ratio             =       6.2______________________________________ 
    
     The condensed vapors from the evaporation were collected. The condensate consisted of 70 gallons of methylisobutylketone plus 2,400 gallons of hydrofluoric acid solution containing 75 grams/liter HF. This amounts to a recovery of 0.43 lbs. 100% HF per pound of niobium in the original solution. 
     The concentrated niobium-bearing solution was found to be stable with no crystallization or precipitation of solids on prolonged storage at ambient conditions. A number of niobium-bearing solutions were evaporated to test the recovery of hydrofluoric acid and methylisobutylketone under varying concentrations of niobium. These are reported in Table I. 
     
                       TABLE I______________________________________CONCENTRATION OF NIOBIUM-BEARING SOLUTIONSRUN NO.      1       2       3     4     5______________________________________Nb-SolutionNb (gm/l.)   103.5   123     134   147   163HF (gm/l.)   223.5   227     237   252   263F/Cb Mole Ratio        10      8.55    8.2   7.95  7.52CondensateEvaporated(% by Wt.)   77.5    81.7    78.5  79.2  75.5HF (gm/l.)   92.4    109.5   118   116   99.5HF/Nb (lb/lb)        0.68    0.71    0.67  0.61  0.45MIBK/Nb (lb/lb)        0.17    0.10    0.12  0.09  0.08Conc. Cb-SolutionNb (gm/l.)   460     668     625   756   665HF (gm/l.)   680     757     682   735   772F/Cb MoleRatio        6.85    5.27    5.06  4.5   5.4______________________________________ 
    
     The concentrated niobium-bearing solution was then evaporated to dryness in a nickel tray contained in a carbon-lined externally heated oven maintained at 232° C. The resulting dry product was identified by X-ray diffraction to be NbO 2  F. Analysis of the product was: 
     
         ______________________________________Ingredient    Percent by Weight                    Impurity ppm______________________________________Nb =     61.95           Sb       = 183(Sb/CbF.sup.-  =    13.8            Si       = &gt;100______________________________________ 
    
     As can be seen, about 64% by weight of the antimony and greater than 90% of the silicon were removed by the evaporation step.  The evaporated vapors from the oven varied during the course of the evaporation from about 15% HF to greater than 90% HF at the end. Because of this variation in HF concentration, the use of a contact condenser to recover the HF is impractical. A Venturi-type scrubber which can quench and condense the evolved vapors is preferred although an absorption tower could also be used. The recovered HF solution averages about 50% HF. 
     An example of the above data is shown in a typical process flow sheet, shown in the drawing. Thus, 2,057 gallons of niobium solution having 140 gram/liter of Nb values and 240 grams/liter of HF are evaporated, the overhead condensate being composed of 41 gallons MIBK and about 1400 gallons of HF solution. The concentrate Nb solution is 588 gallons which is baked in an oven to yield 3717 pounds of NbO 2  F. Vapors recovered from a Venturi scrubber yield 2577 pounds HF and 2696 pounds of water, i.e., approximately 49% by weight HF solution. ##STR1## 
     EXAMPLE 2 
     A number of evaporations to dryness were made to determine the temperature required for optimum conversion to NbO 2  F. These results are shown in Table II. The data show a lower fluorine content in the NbO 2  F with increasing temperature. For this set of experiments a contact condenser was used to recover the HF solution and an indeterminant amount of HF was lost to the atmosphere. If the NbO 2  F product is to be calcined to Nb 2  O 5 , an even lower temperature can be used. For example, in one experiment at 125° C. the product contained 15.8% F. but also still contained water which, of course, would be removed by calcination. The absorbed water would, however, be counter-productive if the NbO 2  F were to be used directly in aluminothermic reactions. 
     
                       TABLE II______________________________________EVAPORATION OF CONCENTRATEDCOLUMBIUM-BEARING SOLUTIONS          RUN NO.           1    2       3       4______________________________________ Evaporation Temp. ° C.            204     218     232   232ChargeGallons          4.5     4.63    4.5   6.75Pounds           66.4    66.0    63.5  93.3Pounds Niobium   19.4    18.2    17.1  24.6Pounds HF        25.1    23.4    21.6  28.7CondensateGallons          2.98    3.42    3.29  4.67Pounds           29.3    33.0    31.5  45.2Pounds HF        14.7    14.7    12.2  18.7Pounds HF/Pound Nb            0.76    0.81    0.71  0.76HF Concentration, % by Wt.            50.2    44.3    38.8  41.3Solid ResiduePounds           30.6    30.3    29.0  41.6F.sup.- (% by Wt.)            15.9    15.04   10.23 11.41Niobium (% by Wt.)            61.6    64.3    62.3  62.1X-Ray Identification            CbO.sub.2 F                    CbO.sub.2 F                            CbO.sub.2 F                                  CbO.sub.2 F______________________________________ 
    
     EXAMPLE 3 
     A number of corrosion tests were made to determine acceptable tray materials. In general, nickel and nickel-containing alloys such as Monel and Hastelloy A, B, and C were found to be satisfactory. 
     A portion of the NbO 2  F were calcined at 1000° C. in the presence of water vapor and was converted to Nb 2  O 5  according to the reaction: 
     
         2 NbO.sub.2 F+ H.sub.2 O→Nb.sub.2 O.sub.5 + 2 HF 
    
     example 4 
     a quantity of NbO 2  F that had been produced at 232° C. and which contained 62.1% Nb was crushed to 8 mesh by down and blended with aluminum powder, iron powder, lime and sodium chlorate. Three small scale reactions were made varying the amount of aluminum from 90% to 110% by weight of the amount stoichiometrically required to react with the CbO 2  F and the sodium chlorate. These results are given in Table III. 
     
                       TABLE III______________________________________SMALL-SCALE ALUMINO-THERMIC REDUCTIONSRun No.           1        2        3______________________________________Wt. Reagents, gramsNbO.sub.2 F       980      980      980Iron Powder       330      330      330Aluminum Powder   291      321      351Sodium Chlorate   50       50       50Lime, CaO         170      170      170Total             1821     1851     1881% Aluminum Stoichiometry             90       100      110Wt. Alloy, grams  695      686      705Alloy Analysis% Nb              58.2     59.9     58.1% Al              0.64     1.02     4.04ppm O.sub.2       1470     1265     750ppm N.sub.2       450      540      470______________________________________ 
    
     These results indicate that an excess amount of aluminum is not required. The ignition and firing proceeded smoothly with very little splashing. Due to the small scale of the reactions, the molten charge cooled rapidly precluding complete metal collection. This phenomenon is typical of all small scale thermite reactions. Excellent slag-metal separation was observed. 
     EXAMPLE 5 
     A second set of experiments was designed to determine if lime was necessary for the reaction. The same NbO 2  F was used as the previous set of experiments of Example 4 and aluminum stoichiometry was maintained at 100%. These results are shown in Table IV. 
     
                       TABLE IV______________________________________CaO-FREE THERMITE REDUCTIONS OF NbO.sub.2 FCharge, Grams       1          2          3______________________________________NbO.sub.2 F 980        980        980Fe Powder   330        330        330Al Powder   296        321        331NaClO.sub.3  0          50         70Total       1606       1681       1711Slag Condition       Thick      Thick      ThickFiring Time,        75         40         30 SecondsFiring Conditions       Very Smoky Very Smoky Very SmokyMetal Collection       None       None       None______________________________________ 
    
     Conclusions drawn from this set of experiments were that during the reaction aluminum fluoride is volatilized and does not furnish a slagging effect. The use of lime in approximately stoichiometric quantity to the fluorine contained in the system is recommended. 
     EXAMPLE 6 
     In another set of experiments, the reaction of NbO 2  F was compared to Nb 2  O 5  on a 63 lb. niobium scale. These results are shown in Table V. 
     
                       TABLE V______________________________________COMPARISON OF NbO.sub.2 F AND Nb.sub.2 O.sub.5       Mix 1       Mix 2______________________________________NbO.sub.2 F, lbs.         100 (63 lb. Cb)                       --Nb.sub.2 O.sub.5, lbs.         --            90 (63 lb. Cb)Al Powder     32            38.4CaO           17            17NaClO.sub.3   --            16Fe Powder     26.9          26.9Total, lbs.   175.9         188.3Wt. Alloy, lbs.         81.5          80.0% Nb          66.7          67.5% Nb Recovery 86.3          85.7______________________________________ 
    
     The results of Table V show that, indeed, NbO 2  F can be alumino-thermically reduced to ferro-columbium without a heat booster. Niobium recoveries were comparable with those from Nb 2  O 5 . 
     EXAMPLE 7 
     The process of Example 6 was further scaled up to a 350 lb. niobium contained set of reactions. These results are shown in Table VI. 
     
                       TABLE VI______________________________________COMPARISON OF NbO.sub.2 F AND Nb.sub.2 O.sub.5ON A 350 LB. NIOBIUM SCALE      NbO.sub.2 F Nb.sub.2 O.sub.5______________________________________NbO.sub.2 F, lbs.        565 (350 lb. Cb)                      --Nb.sub.2 O.sub.5        --            500 (350 lb. Nb)Fe Powder    169           170Al Powder    172           193CaO          115           --CaF.sub.2    --            100NaClO.sub.3  --            42Total, lbs.  1021          1005Wt. Alloy, lbs.        493           490% Nb         64.4          65.0% Nb Recovery        90.7          91.0______________________________________ 
    
     These results of Table VI show comparable recoveries. It will be noted that the Nb 2  O 5  required 21 lbs. more aluminum to react with the 42 lbs. NaClO 3  added as a heat booster. The NbO 2  F reaction proceeded mildly with reduced fume emission. The slag, weighing 531 lbs., contained 13.6% fluorine for a 92.6% fluorine retention in the slag. The results show that NbO 2  F can be used for the preparation of ferro-, nickel-, or chromium-columbium provided lime is added to the mix to react with essentially all of the fluorine introduced in the NbO 2  F. 
     From the foregoing, it will be readily apparent to those skilled in the art that various modifications and changes may be effected without departing from practice of the presently disclosed invention. An example of such a modification would be to introduce iron oxide, chromium oxide or nickel oxide instead of the metallic elements into the mix. Under these conditions aluminum would preferentially reduce the metallic oxides and the resulting metals would be available to alloy with the niobium as the reaction proceeded.