Abstract:
A process for recovering the vanadium present in the chromium ore chromite. as ammonium metavanadate, as a by-product of the fusion of the chromium ore with alkali and its work-up to produce sodium chromate solution and sodium dichromate.

Description:
[0001]    The invention relates to a process in which the vanadium present in the chromium ore chromite is recovered as ammonium metavanadate during the course of the fusion of the chromium ore with alkali and its work-up to produce sodium chromate solution and the important chromium chemical sodium dichromate.  
         BACKGROUND OF THE INVENTION  
         [0002]    All processes used for producing sodium dichromate Na 2 Cr 2 O 7 -2H 2 O via sodium chromate solution employ a procedure having the same principle:  
           [0003]    Chromium spinel or chromite is mixed with residue sodium carbonate and/or sodium hydroxide and iron oxide (recycled ore) residue and heated at 1000-1100° C. in the presence of oxygen. The sodium chromate produced is leached from the resulting reaction mixture by means of water at a controlled pH. During this procedure, the vanadium present in the chromite also goes into solution as sodium vanadate. Control of the pH is necessary to suppress the dissolution of iron, aluminium, silicon and magnesium. In general, addition of an acid, e.g. a dichromate solution, is necessary to adjust the pH. After leaching with water is complete the sodium chromate solution produced is converted into a sodium dichromate solution by addition of sulphuric acid or preferably of carbon dioxide under pressure. Solid sodium dichromate is recovered from the solution by evaporation and crystallization. This process is described in BUCHNER, SCHLIEBS, WINTER, BÜCHEL “Industrielle Anorganische Chemie”, Weinheim 1984, and in Ullmann&#39;s Encyclopedia of Industrial Chemistry, Fifth ed., Vol A 7, Weinheim 1986, p. 67-97.  
           [0004]    It has now been found that the vanadium content of the sodium dichromate (about 0.2% of V 2 O 5  in the Na 2 Cr 2 O 7 .2H 2 O) interferes in various applications of the sodium dichromate and its downstream products, so that purification of the sodium chromate fusion solution to remove the vanadium before conversion into sodium dichromate is desirable.  
           [0005]    The removal of vanadium is preferably carried out by addition of calcium oxide to the sodium chromate solution produced by leaching and filtration to remove insoluble material. The solution is in this way brought to a pH of 12-13 (EP-A-0 047 799, EP 0 453 913 B1), resulting in precipitation of a filterable calcium hydroxyvanadate Ca 5 (OH)(VO 4 ) 3 . The removal of the calcium which has been introduced in excess is carried out by subsequent precipitation of calcium carbonate from the sodium chromate solution, as described in EP 0 453 913 B1.  
           [0006]    A consequence of the precipitation of calcium hydroxyvanadate from a solution having a high concentration of chromate ions is the high contamination of the calcium hydroxyvanadate by coprecipitated calcium chromate and sodium chromate and by entrained calcium oxide. The V 2 O 5  content of the dried “calcium vanadate” precipitate is about 10-20% and is thus significantly below the V 2 O 5  content of pure calcium hydroxyvanadate Ca 5 (VO 4 ) 3 OH of 48.7%, but also significantly higher than the V 2 O 5  content of naturally occurring vanadium-containing ore of not more than 2.4% of V 2 O 5  (cf. Ullmann&#39;s Encyclopedia of Industrial Chemistry, 5 th  ed., Vol. A27, p. 370). The “calcium vanadate” precipitate, having a V 2 O 5  content of 10-20%, is thus an attractive starting material for producing vanadium oxide, due to its high vanadium content as well as its high reactivity which results from its finely divided nature and low crystallinity. In addition, the disposal of such a material as waste in a landfill is not acceptable, because chromate and vanadate would be released to the environment over time, and the material therefore would have to be made inert by treatment with reducing agents such as iron(II)sulphate or sulphur dioxide or the like before it could be disposed of in a landfill.  
           [0007]    In the processing of this calcium vanadate precipitate, its valuable constituents, namely vanadium as V in the oxidation state +5 and chromium as Cr in the oxidation state +6, have to be obtained as separate or easily separable species, the vanadium in particular in a readily usable and commercial form and the chromium in a utilizable form, e.g. one that is able to be reintroduced into the chromate production process, i.e. recyclable, species or solution. Readily soluble sodium compounds, which are undesirable in the wastewater, are to be recovered as a utilizable, recyclable species and the calcium as insoluble species which can be disposed of in a landfill or as precursor of a reusable calcium oxide or reusable calcium chromate or calcium dichromate.  
           [0008]    Solutions containing sodium ions and chromium in the oxidation state +6 to be reintroduced into the chromate production process can be so utilized only if they do not contain appreciable amounts (e.g. &gt;0.1%) of interfering extraneous material. However, any elements and oxidation states which are not already present to an appreciable extent in the product stream into which this solution is to be introduced will interfere. For this reason, only water H 2 O, hydroxide OH- or hydroxonium H 3 O +  ions, carbonate, bicarbonate, carbon dioxide, chromate, dichromate, polychromate, chromic acid, sodium and also calcium and vanadate in very minor concentrations are acceptable as constituents of the solutions which are to be reintroduced into the chromite production process. Depending on the pH, these solutions can be introduced into the acidification steps of the sodium chromate process (e.g. after or during leaching of the furnace clinker) or the alkalization steps (e.g. before or during vanadate precipitation).  
           [0009]    The digestion of the calcium vanadate precipitate by means of sulphuric acid, separation of the precipitated calcium sulphate from the solution in accordance with a treatment of insoluble calcium salts which is frequently practiced in industry and subsequent precipitation of the vanadium as V 2 O 5  from the filtrate by-means of sulphuric acid is prior art.  
           [0010]    However, a consequence is that the chromium is obtained as a polychromate or chromic acid solution having a high sulphate content. As the many proposals for removing sulphate from sodium chromate and dichromate demonstrate, the amounts of sulphate always introduced as sulphuric acid in earlier processes for producing sodium chromate and sodium dichromate are nowadays totally undesirable for further processing of these products (EP 0 453 913 B1).  
           [0011]    Another possible way of treating sparingly soluble salts is digestion of the calcium vanadate precipitate by means of sodium carbonate in aqueous solution and subsequent precipitation of the vanadium as ammonium metavanadate (NH 4 ) 4 V 4 O 12 , also referred to as NH 4 VO 3  for short, by addition of an excess of ammonium salts. Ammonium metavanadate is a versatile intermediate, in particular in the route to the most important vanadium chemical V 2 O 5  (cf. Ullmann&#39;s Encyclopedia of Industrial Chemistry, 5 th  edition, Vol. A27, Weinheim 1996). However, in this case too, the resulting solution containing the chromium as chromate is contaminated with the ions of the precipitation reagent, i.e. with ammonium ions and the associated anions, and can therefore not be reused or recycled without problems. Furthermore, the proportion of the vanadium in the calcium hydroxyvanadate which is dissolved by means of sodium carbonate is less than 50% and therefore completely unsatisfactory unless economically nonviable sodium carbonate excesses are employed.  
           [0012]    The routes known from the prior art are therefore not suitable for dissolving vanadium from the calcium vanadate precipitate obtained from the sodium chromate production process in such a way that an introduced, readily utilizable and commercial grade vanadium chemical can be obtained and the valuable chromium present obtained as a usable or recyclable product which would meet the above-described requirement.  
           [0013]    The present invention now provides a route by means of which the vanadium present in the leaching solution from the fusion of chromium ore with alkali can be obtained as valuable ammonium metavanadate (NH 4 ) 4 V 4 O 12  and the valuable constituents chromium in the oxidation state +6 and sodium ions are made available as utilizable and recyclable chemicals.  
         SUMMARY OF THE INVENTION  
         [0014]    The present invention relates to a process for recovering the vanadium present in the chromium ore chromite as ammonium metavanadate (NH 4 ) 4 V 4 O 12  in the course of the preparation of sodium chromate and sodium dichromate, characterized by  
           [0015]    1. precipitation of a calcium vanadate precipitate from the sodium chromate solution (after pH-controlled removal of the residue from the chromium ore) by addition of calcium oxide, calcium hydroxide, calcium dichromate or calcium chromate at pH 12-13 set by addition of sodium hydroxide to the sodium chromate solution,  
           [0016]    2. separation of the calcium vanadate precipitate from the solution by customary solid/liquid separation operations, e.g. by filtration or centrifugation, and, if appropriate, washing of the precipitate with water or with water which has been made alkaline by means of dilute sodium hydroxide solution,  
           [0017]    3. treatment of the calcium vanadate precipitate with 1.5 to 10-fold quantity of water (ratio based on the dry weight of the residue), preferably from 2 to 4-fold quantity of water, and (a) at least that quantity of sodium carbonate which is stoichiometrically equivalent to the calcium content of the precipitate or an excess of sodium carbonate and the quantity of carbon dioxide or sodium bicarbonate necessary to set the pH to 8.5-12.3, preferably 9-11, or (b) with a molar quantity of sodium bicarbonate which is at least 1.0 times the molar quantity of calcium in the precipitate (stoichiometry based on the CO 2  content) or an up to 3-fold molar excess of sodium bicarbonate, preferably from 1.3- to 2-fold amount of sodium hydrogen carbonate, or (c) with carbon dioxide in an at least stoichiometric amount based on the calcium content and sodium hydroxide in the amount necessary to set a pH of at least 8, with this treatment preferably being carried out at an elevated temperature in the treatment solution, e.g. from 50 to 110° C., for a time of from about 0.1 to 5 hours, preferably from 0.5 to 1.5 hours,  
           [0018]    4. separation of the calcium carbonate formed from the solution by means of customary solid/liquid separation operations and washing of the calcium carbonate with water in such a way that the vanadium concentration in the aqueous filtrate is from 5 to 100 g of V/litre, preferably 20-50 g of V/litre, particularly preferably 35-45 g of V/litre, and, if desired, further washing of the calcium carbonate,  
           [0019]    5. addition of ammonium salts selected from the group consisting of ammonium carbonate, ammonium hydrogen carbonate, ammonium carbamate, ammonium hydrogen carbonate-ammonium carbamate double salt, ammonium chromate (ammonium monochromate), ammonium dichromate, ammonium polychromate or mixed sodium-ammonium salts of the above-mentioned, associated polyvalent anions as pure substances or mixtures among one another or as aqueous solutions of the above-mentioned substances, and of ammonia, in such a way that a pH of from 7.5 to 11, preferably from 9.5 to 10.3, is established, to the aqueous phase obtained after step 4, with the molar amount of ammonium salt added being selected so that it is from 2.5 to 20 times, preferably from 3 to 6 times, the molar amount of vanadium, and maintenance of the resulting mixture at from 0° C. to 50° C., preferably from 15 to 40° C., for at least one hour, preferably for from 3 to 5 hours,  
           [0020]    6. separation of the precipitated ammonium metavanadate (NH 4 ) 4 V 4 O 12 /(NH 4 VO 3 ) formed from the solution by means of customary solid/liquid separation operations and, if desired, washing with water or aqueous solutions,  
           [0021]    7. alkalization of the remaining aqueous phase to a pH of at least 10.5, heating of the solution to the boiling point while maintaining a pH of 10.5 or above until all the ammonium liberated has been driven off, if desired absorption of this ammonia in water or aqueous solutions and recovery of the solution which has been freed of ammonium ions and ammonia for the sodium chromate production process and recirculation and  
           [0022]    8. if desired, drying of the ammonium metavanadate produced in step 6.  
           [0023]    If desired, part of the carbonate or bicarbonate which has been used in excess is recovered as sodium bicarbonate after step 4 and before step 5 by passing carbon dioxide into the solution at a pH of from about 7 to 9 under a carbon dioxide pressure of from 0.9 to 10 bar and cooling the solution to from about −10 to 20° C., preferably from 0 to 5° C., and separation of the precipitated sodium bicarbonate from the solution.  
         DETAILED DESCRIPTION  
         [0024]    In the case of the ammonium salts to be added in step 5, it is of no consequence whether they are added as such, in solution or in undissolved form, or as their precursors which immediately form the desired ammonium salts in the reaction solution. Thus, instead of adding ammonium carbonate or bicarbonate or carbamate, it is possible to introduce a corresponding amount of carbon dioxide which reacts immediately with the ammonia introduced to form ammonium carbonate and/or bicarbonate. Similarly, the ammonium chromates indicated above can be produced by addition of chromic acid, sodium polychromate or sodium dichromate to the reaction solution, without introducing fresh impurities.  
           [0025]    The washing in step 6 of the ammonium metavanadate produced in step 5 can be carried out using either water or aqueous solutions having a pH of 7.5-11, preferably 9.5-10.3, when the pH adjustment is carried out by means of ammonia or sodium hydroxide and only the salts which are also suitable for step 5 are introduced. The alkalization in step 7 can be carried out using sodium hydroxide or sodium carbonate.  
           [0026]    The ammonia which has been liberated and driven off in step 7 can be absorbed in water or in aqueous solutions containing the anions to be used in step 5 as acid component if internal reuse in the ammonium metavanadate process is sought.  
           [0027]    The calcium carbonate obtained in step 4 can be passed to calcium oxide production for use in step 1, or can be converted into calcium chromate or calcium dichromate or calcium polychromate by reaction with sodium dichromate or sodium dichromate/sodium polychromate solution or sodium polychromate/chromic acid solution. However, it can also, if desired, be deposited in a landfill as a nonhazardous, insoluble waste material or be used as an auxiliary in steel production. All other streams obtained can be fed back into the sodium chromate production process directly and without further treatment. The washing water which may be obtained in step 2 together with the filtrate generated previously forms the sodium chromate stream for further conversion into sodium dichromate. The sodium bicarbonate produced in the step which may optionally be inserted between steps 4 and 5 is added to the sodium bicarbonate which is formed in large quantities in the acidification step in which sodium chromate is converted into sodium dichromate by means of carbon dioxide under pressure and the combined sodium bicarbonate is passed to sodium carbonate production for the chromite fusion. The solution obtained in step 7 which is free of ammonium ions and ammonia contains only chromate and carbonate and hydroxide anions and residual vanadate ions and sodium cations in appreciable amounts. It is added to the sodium chromate solution obtained by leaching of the furnace clinker in the sodium chromate production process as alkalizing agent prior to the calcium vanadate precipitation with complete recovery of the chromium and sodium present.  
           [0028]    Likewise, the alkaline washing water obtained in step 4, which contains sodium chromate and sodium vanadate and sodium(hydrogen)carbonate, is added to the sodium chromate solution prior to the calcium vanadate precipitation.  
           [0029]    The entire process and also the individual steps can be carried out either batchwise or continuously.  
           [0030]    The invention is illustrated by the following examples:  
       
    
    
     EXAMPLE 1  
     Preparation of Calcium Vanadate  
       [0031]    About 50 kg of the calcium vanadate precipitate (more accurately: calcium hydroxy-vanadate) precipitated from sodium chromate solution at pH 12.5 (setting of pH by means of sodium hydroxide, addition of Ca as CaO) are dried at 110° C. for 5 hours and is then found to have the following analytical composition:  
                                                           10.2% V   =   18.2% V 2 O 5              7.3% Cr   =   14.0% CrO 3             29.1% Ca   =   40.7% CaO            5.2% Na   =    7.0% Na 2 O            1.3% CO 3   2−                        
 
         [0032]    Main constituents are therefore about 38% of calcium vanadate Ca 5 (VO 4 ) 3 OH, about 10% of calcium chromate CaCrO 4 , about 18% of CaO and about 13% of sodium chromate Na 2 CrO 4 .  
       EXAMPLE 2  
     Digestion of Calcium Vanadate  
       [0033]    200 g of the dry calcium vanadate precipitate prepared in Example 1 is in each case mixed with 400 g of water and 155 g of sodium carbonate. The mixture was heated to 60° C. while stirring. To establish the desired pH values, carbon dioxide from a pressure bottle was introduced. The reaction mixture was stirred at 60° C. for 2 hours while continuing to monitor and adjust the pH. The mixture was then filtered and the filtrate was analyzed for chromium(VI) and vanadium(V). The filter residue was stirred twice in succession with 250 ml in each case of water at 60° C. for 10 minutes and then filtered off again. The washing water obtained as filtrate was likewise analyzed for chromium(VI) and vanadium(V). The filter residue which remained was dried at 110° C.  
         [0034]    The results of the experiments carried out at pH 9, pH 10 and pH 111 are shown in the following table.  
       EXAMPLES 2a, 2b, 2c  
       [0035]    [0035]                                                                                                                                   V yield [%]   Cr yield [%]                Example   pH   Filtrate   Washing water   Total   Filtrate   Washing water   Total   Amount of residue [g]                    2 a    9   45.2   44.7   89.9   49.2   41.6   90.8   237       2 b   10   46.0   42   88.0   52.4   43.0   95.4   223       2 c   11   60.0   32.3   92.3   65.4   27.0   92.4   193                            
         [0036]    In a comparative experiment (Example 2 d) without introduction of carbon dioxide and thus without control of the pH, a vanadium(V) yield of less than 50% was obtained.  
       EXAMPLE 3  
     Digestion of Calcium Vanadate  
       [0037]    The procedure described in Example 2 was repeated, except that 90° C. instead of 60° C. was selected as temperature for the digestion mixture (reaction mixture) and for the washing water.  
         [0038]    The results with respect to the chromium(V) and vanadium(V) recovery in the experiments at pH 9, pH 10 and pH 11 are shown in the following table.  
       EXAMPLES 3a, 3b, 3c  
       [0039]    [0039]                                                                                                                                   V yield [%]   Cr yield [%]                Example   pH   Filtrate   Washing water   Total   Filtrate   Washing water   Total   Amount of residue ([g]                    3 a    9   63.8   27.9   91.7   65.7   27.8   93.5   196       3 b   10   79   16.8   95.8   74.0   18.1   92.4   169       3 c   11   77.9   19.3   97.2   77.4   18.5   95.9   164                            
         [0040]    Without introduction of carbon dioxide and thus without pH control, vanadium(V) yields of less than 50% are obtained.  
       EXAMPLE 4  
     Digestion of Calcium Vanadate  
       [0041]    The procedure described in Example 2 was repeated, except that the reaction mixture was prepared from 200 g of dry calcium vanadate precipitate, 800 g of water and 155 g of sodium carbonate. The results at a reaction temperature and washing water temperature of 60° C. and at the pH values of 9, 10 and 11 are shown in the table of Example 5.  
       EXAMPLE 5  
     Digestion of Calcium Vanadate  
       [0042]    The procedure described in Example 4 was repeated, except that the reaction temperature and the temperature of the washing water was increased to 90° C. The results are shown in the following table.  
                                                                                                                                           V yield [%]   Cr yield [%]                Example   Temperature [° C.]   pH   Filtrate   Washing water   Total   Filtrate   Washing water   Total   Amount of residue [g]                    4 a   60    9   75.2   21.2   96.8   73.7   20.3   94.0   171       4 b   60   10   84.7   11.4   96.1   85.0   12.1   97.1   165       4 c   60   11   84.7   12   96.7   83.9   12.0   96.9   164       5 a   90    9   74.9   21.0   95.9   70.3   22.6   92.9   170       5 b   90   10   80.6   11.9   92.5   85.6   13.2   98.8   166       5 c   90   11   83.0   12.7   95.7   84.8   13.2   97.0   166                  
 
       EXAMPLE 6  
       [0043]    The procedure described in Example 2 was repeated, except that the reaction mixture was prepared from 200 g of dry calcium vanadate precipitate, 1200 g of water and 155 g of sodium carbonate. The results at a reaction temperature of 60° C. are shown in the table of Example 7.  
       EXAMPLE 7  
       [0044]    The procedure described in Example 6 was repeated, except, that the reaction temperature and the temperature of the washing water was increased to 90° C. The results are shown in the following table.  
                                                                                                                                           V yield [%]   Cr yield [%]                Example   Temperature [° C]   pH   Filtrate   Washing water   Total   Filtrate   Washing water   Total   Amount of residue [g]                    6 a   60    9   87.1   11.3   98.4   85.9   12.3   98.3   168       6 b   60   10   89.5   8.7   98.2   87.1   9.3   96.4   168       6 c   60   11   90.1   7.8   97.9   90.4   7.6   98   170       7 a   90    9   73.9   22.9   96.8   74.0   22.2   96.6   166       7 b   90   10   90.5   7.1   97.6   88.9   7.7   96.6   169       7 c   90   11   89.5   8.0   97.5   88.0   9.7   97.7   173                  
 
       EXAMPLE 8  
       [0045]    500 g of the dry calcium vanadate precipitate prepared in Example 1 is in each case mixed with 1100 ml of water and increasing amounts of sodium bicarbonate and heated to 90° C. In the case of complete conversion of the bicarbonate into carbonate ions by means of alkali, 305 g was the amount of sodium bicarbonate stoichiometrically equivalent to the calcium content. Over the course of one hour, the pH of the mixture rises as a result of consumption of the bicarbonate (neutralization by bound hydroxide or oxide, evolution of the carbon dioxide liberated). After one hour at 90° C., the mixture is filtered and the filtrate is analyzed. The filter residue is stirred twice in succession with 625 ml each time of water at 90° C. for 10 minutes and then filtered off again. The chromium(VI) and vanadium(V) contents determined in the washing water obtained in this way were added to the values for the filtrates.  
         [0046]    The results are recorded in the following table.  
                                                                                 V yield [%]   Cr yield [%]                Amount of NaHCO 3     Final   in the       in the           Example   used [g]   pH   filtrate   Total   filtrate   Total               8 a   610    9.2   78   98   77   94       8 b   375   12.1   77   98   75   95       8 c   300   12.5   58   74   66   86                  
 
       EXAMPLE 9  
       [0047]    500 g of the dry calcium vanadate precipitate prepared in Example 1 are mixed with 1100 ml of water and 400 g of sodium bicarbonate and heated to 90° C. while stirring vigorously. Over the course of one hour at this temperature, the pH rises to 11.8.  
         [0048]    After this time, the resulting calcium carbonate precipitate is filtered off. The filtrate is isolated and treated with carbon dioxide gas while cooling. The final temperature of 5° C. in the stirred mixture is maintained for 30 minutes. While continuing the treatment with carbon dioxide gas, the sodium bicarbonate precipitate formed is filtered off and subsequently converted into sodium carbonate by drying at 110° C. The final weight of sodium carbonate is 100 g (corresponds to 177 g of dry sodium carbonate), and the contents of vanadium(V) and of chromium(VI) are 3.0% of V and 2.5% of Cr.  
         [0049]    The calcium carbonate precipitate which has been filtered off is stirred twice in succession with 625 ml in each case of water at 90° C. for 10 minutes and then filtered off again. The washing water obtained in this way is collected separately and stored.  
       EXAMPLE 10  
       [0050]    The procedure of Example 9 is repeated, except that 600 g of sodium bicarbonate are used in the digestion mixture.  
         [0051]    After precipitation of calcium carbonate and filtration, the filtrate is treated in the same way as in Example 9.  
         [0052]    The sodium carbonate obtained by filtration and drying of the precipitated sodium bicarbonate weighs 255 g.  
       EXAMPLE 11  
       [0053]    5 kg of the dry calcium vanadate precipitate prepared in Example 1 is mixed with 11 l of water, 3700 g of sodium bicarbonate and 1000 g of sodium carbonate and heated to 90° C. while stirring vigorously. After one hour at this temperature, the calcium carbonate precipitate formed is filtered off and the filtrate is isolated. The precipitate is stirred twice in succession with 6.5 l each time of water at 90° C. for minutes and then filtered off again. The washing water obtained is isolated.  
         [0054]    The filtrate from the calcium carbonate precipitation is transferred to a closable stainless steel vessel and in this vessel is cooled and at the same time treated with carbon dioxide gas. After displacement of the air by the carbon dioxide gas, the stainless steel vessel is closed and further carbon dioxide gas is introduced in the amount necessary to maintain a pressure of 3 bar. After a temperature of −5° C. has been reached, the mixture is stirred for another 30 minutes, the stirrer is then turned off and the contents of the stainless steel vessel are then pushed out through a filter by means of the carbon dioxide atmosphere.  
         [0055]    The sodium bicarbonate which has been filtered off is dried at 110° C. and then weighs 1990 g as sodium carbonate.  
         [0056]    The filtrate from the sodium bicarbonate precipitation is combined with the washing water from the calcium carbonate precipitation, and the liquid mixture obtained serves as starting material in some subsequent experiments. The vanadium(V) content was determined as 19.3 g of V/litre, and the chromium(V) content was determined as 13.3 g of Cr/litre.  
       EXAMPLE 12  
       [0057]    The filtrate from experiment 8 a, containing 39 g of V and 28 g of Cr, is diluted with water to 6.5 l, resulting in a solution containing 6.0 g of V/litre and 4.3 g of Cr/litre, and then mixed with 690 g of ammonium carbonate.  
         [0058]    The pH of 9 is set by addition of small amounts of sodium hydroxide. The reaction mixture is stirred at 35° C. for 4 hours, then stirred further at 20° C. for 4 hours and the ammonium metavanadate precipitate formed is filtered off. The precipitate is washed with 40 ml of water and dried. The vanadium content of 44.0% and the ammonium content of 15.0% determined are close to the theoretical values and indicate the purity of the ammonium metavanadate produced. As impurities, only 0.23% of chromium, 0.23% of sodium, 0.001% of calcium and 0.44% of carbonate were found.  
       EXAMPLE 13  
       [0059]    The filtrate (1219 g) from experiment 8b, containing 38.4 g of V and 27.4 g of Cr, is mixed with 160 g of ammonium carbonate at 35° C. The pH is set to 9 by addition of carbon dioxide. After 1 hour at 35° C., the mixture is stirred at 20° C. for 4 hours. The precipitate is filtered off, washed with 20 ml of water at +1° C. and then dried at 110° C. The contents of 18% of vanadium, 20% of sodium, 30% of carbonate and 4% of chromium and only 3% of ammonium determined by analysis indicate that predominantly sodium metavanadate and sodium bicarbonate are precipitated under the condition of the high sodium content.  
       EXAMPLE 14  
       [0060]    The filtrate from the sodium bicarbonate precipitation in Example 9 is mixed at 35° C. with an approximately three-fold stoichiometric amount relative to the vanadium content of ammonium ions in the form of ammonium dichromate in aqueous solution, i.e. 285 g of (NH 4 ) 2 Cr 2 O 7 , brought to a pH of 9.3 by means of ammonia gas and then stirred at 20-30° C. for 4 hours.  
         [0061]    The ammonium metavanadate which has been filtered off weighs 95 g after drying, the vanadium content is 35.8%, i.e. 34 g of vanadium corresponding to 85% of the vanadium which was present, the chromium content is 5.7% and the sodium content is 0.84%.  
         [0062]    A mother liquor from a chromic acid crystallization is diluted to approximately half its concentration. 1300 ml of this solution then contain 247.5 g (0.837 mol of Na 2 Cr 2 O 7 .2H 2 O) of sodium dichromate and 170.5 g of chromic acid (1.705 mol of CrO 3 ), which corresponds to an acid removal of 50.5%. This cooled solution is employed for collecting and binding the ammonia liberated from the filtrate from the ammonium metavanadate precipitation. This liberation of ammonia is brought about by adding 50% strength sodium hydroxide solution to this filtrate until a pH of 13.5 has been reached and then heating this mixture to boiling. After 8-10 ml of water have distilled over with the ammonia, heating is stopped. Ammonia and ammonium ions are no longer detectable in the heated mixture. Aqueous concentrated ammonia solution is then added to the solution which has taken up the ammonia driven off until a pH of 10 has been reached. The solution contains ammonium chromate (2.54 mol) and sodium chromate (0.84 mol) and excess ammonia to adjust the pH.  
       EXAMPLE 15  
       [0063]    The filtrate from the sodium bicarbonate precipitation of a repetition of Example 9 is mixed at 35° C. with 140 g of ammonium carbonate (NH 4 ) 2 CO 3 . A pH of 8 is established. After 75 minutes, precipitation of ammonium metavanadate commences. After stirring at 20° C. for 4 hours, the mixture is filtered. The amount of filter residue after drying is 40.3 g, and its vanadium content is 40.0%, i.e. 16.1 g of V, corresponding to a vanadium yield of 40.3%. The chromium content is 1.2% and the sodium content is 1.3%.  
         [0064]    The filtrate from the ammonium metavanadate precipitation is admixed with sodium hydroxide until the pH is 13.7 and is then heated. The ammonia which is given off is introduced into 30 ml of water in a glass flask provided with a pH electrode and a dropping funnel containing 40% strength chromic acid solution. This chromic acid is added dropwise at such a rate that a pH of 8.5 is maintained in the ammonium chromate solution which forms. This treatment is continued until the pH remains stable for 2 minutes while water continues to be distilled over.  
         [0065]    The alkaline sodium chromate solution remaining after heating is free of ammonia and ammonium ions, and the residual vanadium content is 19.1 g/l.  
       EXAMPLE 16  
       [0066]    The filtrate from the sodium bicarbonate precipitation of a repetition of Example 9 is admixed at 35° C. with 140 g of ammonium carbonate and the resulting solution is then brought to a pH of 10 by addition of ammonia solution. The precipitation of the ammonium metavanadate commences after 10 minutes. After stirring at 20° for 4 hours, the mixture is filtered. After drying, 65 g of solid residue containing 42.9% of vanadium, i.e. 27.9 g of V corresponding to a yield of 69.7% of the vanadium which was present, are obtained. The chromium content is 0.19% and the sodium content is 0.13%.  
       EXAMPLE 17  
       [0067]    The filtrate from the sodium bicarbonate precipitation of a repetition of Example 9 is admixed at 38° C. with 280 g of ammonium carbonate and the resulting solution is brought to a pH of 10 by means of ammonia gas. The precipitation of the ammonium metavanadate commences immediately. After stirring for 4 hours at 30° C. at first and then at 20° C., the precipitate is filtered off. The solid weighs 83 g after drying, and its vanadium content is 42.2%, i.e. 35.0 g of V corresponding to a yield of 87.6% of the vanadium which was present. The chromium and sodium contents are each 0.77%.  
         [0068]    After alkalization of the filtrate from the ammonium metavanadate precipitation and driving off of the ammonia in a manner analogous to that described in Example 15, a sodium chromate solution having a residual vanadium content of 3.5 g/litre remains.  
       EXAMPLE 18  
       [0069]    The filtrate from the sodium bicarbonate precipitation of a repetition of Example 9 is admixed at 35° C. with 140 g of ammonium carbonate and the resulting solution is then brought to a pH of 10 by means of ammonia solution. The ammonium metavanadate precipitation commences after 15 minutes. While continually cooling the solution to room temperature, it is stirred for a total of 1 hour, and the precipitate is then filtered off and dried. The weight is 35.9 g and the vanadium content is 42.3%, i.e. 15.2 g of V corresponding to 38% of the vanadium present. The chromium content is 0.28% and the sodium content is 0.16%.  
       EXAMPLE 19  
       [0070]    650 ml of the solution obtained in Example 11 from the filtrate from the sodium bicarbonate precipitation and washing water from the calcium carbonate precipitation are diluted with water to 1000 ml, so that the vanadium concentration is now 12.54 g/l (total of 0.246 mol of V). 1180 g of ammonium carbonate are introduced and the solution is treated with ammonia gas at 33° C. until a pH of 10 has been reached. The ammonium metavanadate precipitation commences immediately during this treatment. After stirring at 20° C. for 4 hours, the mixture is filtered and the filter residue is dried. It weighs 190 g, the vanadium content is 43.5% and sodium and chromium are present in amounts of less than 0.1%. The vanadium yield is therefore 65.9%.