Process for the utilization of vanadium in chromium ore as ammonium metavanadate

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.

We claim:  
     
         1 . Process for recovering the vanadium present in the chromium ore chromite, as ammonium metavanadate (NH 4 ) 4 V 4 O 12 , as a by-product from the vanadium-containing chromate solution produced in the process of preparing sodium chromate and sodium dichromate from said ore, which comprises 
 1. precipitating calcium vanadate from the sodium chromate solution by adding calcium oxide, calcium hydroxide, calcium dichromate or calcium chromate at a pH of 12-13, said pH being established by addition of sodium hydroxide to the sodium chromate solution,    2. separating the calcium vanadate precipitate from the solution to form a low-vanadium content sodium chromate solution,    3. treating the calcium vanadate precipitate with 1.5 to 10-fold quantity of water, based on the dry weight of the precipitate, and (a) at least quantity of sodium carbonate which is stoichiometrically equivalent to the calcium content of the precipitate or an excess of sodium carbonate and quantity of carbon dioxide or sodium bicarbonate sufficient to establish a pH of 8.5-12.3, 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, or (c) with carbon dioxide in an at least stoichiometric quantity based on the calcium content and sodium hydroxide in a quantity sufficient to establish a pH of at least 8, to form a calcium carbonate precipitate,    4. separating the calcium carbonate precipitate from the solution,    5. adding 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, mixed sodium-ammonium salts of the foregoing, and associated polyvalent anions as pure substances or mixtures among one another or as aqueous solutions of the above-mentioned substances, and adding ammonia in an amount sufficient to establish a pH of from 7.5 to 11, to the solution remaining after separation of the calcium carbonate in step 4, with the molar amount of ammonium salt added being from 2.5 to 20 times the molar amount of vanadium, and maintaining the resulting mixture at from 0° C. to 50° C. for at least one hour, to form a sparingly soluble ammonium metavanadate (NH 4 ) 4 V 4 O 12 /(NH 4 VO 3 ) in the solution,    6. separating the precipitated ammonium metavanadate (NH 4 ) 4 V 4 O 12 /(NH 4 VO 3 ) from the solution,    7. alkalizing the remaining solution to a pH of at least 10.5, heating the solution to the boiling point while maintaining a pH of 10.5 or above to liberate such ammonia or ammonium compounds as are present and continuing until all the ammonia and ammonium compounds liberated have been driven off, and recovering the solution which has been freed of ammonium ions and ammonia and recycling it to the sodium chromate production process.    
     
     
         2 . Process according to  claim 1 , wherein an excess of carbonate or bicarbonate is present after step 4, and part of the carbonate or bicarbonate 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. and separating precipitated sodium bicarbonate from the solution.  
     
     
         3 . Process according to  claim 1 , comprising the further step of drying the ammonium metavanadate produced in step 6.  
     
     
         4 . Process according to  claim 1 , wherein the ammonia or ammonium compounds liberated from the solution in step 7 is absorbed by a sodium dichromate solution, a sodium polychromate solution, an ammonium chromate solution, an ammonium polychromate solution, or a chromic acid solution to form a solution which is enriched in ammonium and chromate ions, and said solution which is enriched in ammonium and chromate ions is used as an ammonium source for the precipitation of the ammonium metavanadate in step 5.  
     
     
         5 . Process according to  claim 1 , wherein said calcium vanadate precipitate obtained in step 2 is washed with water or with water which has been made alkaline by sodium hydroxide.  
     
     
         6 . Process according to  claim 1 , wherein the treatment of calcium vanadate precipitate with water in step 3 is washing said calcium vanadate precipitate with from 2 to 4 times its amount of water, based on the dry weight of the precipitate.  
     
     
         7 . Process according to  claim 1 , wherein the treatment with carbon dioxide in part (c) in step 3 is carried out at elevated temperature in the treatment solution for a time of from about 0.1 to 5 hours.  
     
     
         8 . Process according to  claim 6 , wherein the treatment with carbon dioxide in part (c) in step 3 is carried out at elevated temperature in the treatment solution for a time of from about 0.1 to 5 hours.  
     
     
         9 . Process according to  claim 1 , wherein after the calcium carbonate has been separated off in step 4, it is washed with water to produce a wash water having a vanadium concentration of from 5 to 100 g of V/litre.