Method for selectively recovering the rare earths from an aqueous acid sulfate solution rich in aluminum and phosphates

Abstract

The present invention relates to a process for the selective recovery of the rare earth metals from an acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, characterized in that it comprises the following successive stages: a) neutralization at a pH of between 3 and 4 of an acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, the solution having a molar ratio Al/P>1 and a concentration of sulfates>100 g/l, by addition of a base, so as to precipitate the phosphate and the aluminum and the possible titanium, b) liquid/solid separation between the precipitate formed by the phosphate and the aluminum and the possible titanium and the aqueous sulfate solution, c) recovery of the aqueous sulfate solution, d) addition of phosphates to the aqueous sulfate solution obtained in stage c) such that the molar ratio of the solution obtained PO 4 /REs>4, so as to precipitate the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates, e) liquid/solid separation between the precipitate formed by the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates and the aqueous sulfate solution, f) recovery of the precipitate formed by the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates.

1 . A process for the selective recovery of the rare earth metals from an acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, wherein it comprises the following successive stages:
 a) neutralization at a pH of between 3 and 4 of an acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, the solution having a molar ratio Al/P>1 and a concentration of sulfates>100 g/l, by addition of a base, so as to precipitate the phosphate and the aluminum and the possible titanium,   b) liquid/solid separation between the precipitate formed by the phosphate and the aluminum and the possible titanium and the aqueous sulfate solution,   c) recovery of the aqueous sulfate solution,   d) addition of phosphates to the aqueous sulfate solution obtained in stage c) such that the molar ratio of the solution obtained PO 4 /REs>4, so as to precipitate the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates,   e) liquid/solid separation between the precipitate formed by the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates and the aqueous sulfate solution,   f) recovery of the precipitate formed by the heavy rare earth metal phosphates and the possible medium rare earth metal phosphates.   
     
     
         2 . The process as claimed in  claim 1 , wherein, in stage a), the base is chosen from MgCO 3  and a basic calcium compound. 
     
     
         3 . The process as claimed in  claim 2 , wherein, in stage a), the base is a basic calcium compound. 
     
     
         4 . The process as claimed in  claims 1 , wherein, in stage d), the phosphate is chosen from Na 3 PO 4 , K 3 PO 4 , (NH 4 ) 3 PO 4  and their mixtures. 
     
     
         5 . The process as claimed in  claim 1 , wherein the acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, iron(II) and titanium, is the leachate obtained by acid attack on a pyrochlore ore in a sulfate medium. 
     
     
         6 . The process as claimed in  claim 1 , wherein the recovery yield of the heavy rare earth metals is greater than 50%. 
     
     
         7 . The process as claimed in  claims 1 , wherein the acidic aqueous sulfate solution comprising phosphates, aluminum, heavy rare earth metals and medium rare earth metals, and possibly titanium and iron(II), additionally comprises light rare earth metals and in that the process comprises, before stage a), a prior stage A) of double salt precipitation of the light rare earth metals, so as to recover an acidic aqueous sulfate solution depleted in light rare earth metals and comprising phosphates, aluminum, heavy rare earth metals and medium rare earth metals, and possibly iron(II) and titanium. 
     
     
         8 . The process as claimed in  claims 1 , wherein the acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, light rare earth metals, iron(II) and titanium, additionally comprises iron(III) and in that the process comprises, before stage a) and after the optional stage A), a stage B) of reduction of the iron(III) to give iron(II). 
     
     
         9 . The process as claimed in  claims 1 , wherein, before stage a), the molar ratio Al/P of the acidic aqueous sulfate solution comprising phosphates, aluminum and heavy rare earth metals, and possibly medium rare earth metals, light rare earth metals, iron(II), titanium and iron(III), is <1 and in that the process comprises, before stage a) and after the optional stages A) and B), a stage C) of doping of the solution with aluminum, so as to obtain a molar ratio Al/P>1. 
     
     
         10 . The process as claimed in  claim 3 , wherein the base is a basic calcium compound chosen from CaCO 3 , CaO, Ca(OH) 2  and their mixtures. 
     
     
         11 . The process as claimed in  claim 10 , wherein the base is CaCO 3 . 
     
     
         12 . The process as claimed in  claim 4 , wherein in stage d), the phosphate is Na 3 PO 4 . 
     
     
         13 . The process as claimed in  claim 6 , wherein the recovery yield of the heavy rare earth metals is greater than or equal to 60%. 
     
     
         14 . The process as claimed in  claim 8 , wherein the stage B) of reduction of the iron(III) to give iron(II) is carried out by addition of Fe(0) or SO 2 .