Selective extraction of cerium from other metals

Abstract

Methods for the extraction of cerium and/or thorium from metal compounds and solutions. A single step or two-step extraction method may be applied to selectively precipitate thorium and/or cerium as hydroxides under controlled pH conditions such that a substantially thorium-free and/or cerium-free rare earth element (REE) solution may be formed, such as for the subsequent separation of individual rare earth elements.

1 . A method for the extraction of cerium from a cerium-containing acidic solution, comprising the steps of:
 contacting the cerium-containing acidic solution with ammonium hydroxide to precipitate at least a portion of the cerium as cerium hydroxide and form a cerium-depleted solution and a cerium hydroxide product; and   separating at least a portion of the cerium hydroxide product from the cerium depleted solution.   
     
     
         2 . The method recited in  claim 1 , wherein the cerium-containing acidic solution is formed by digesting a cerium-containing solid product in an acid. 
     
     
         3 . The method recited in  claim 1 , wherein the cerium-containing acidic solution comprises nitric acid. 
     
     
         4 . (canceled) 
     
     
         5 . The method recited in  claim 1 , wherein, before the step of contacting the cerium-containing acidic solution with ammonium hydroxide, the cerium-containing acidic solution has a free acid content that is sufficient to dissolve at least about 95% of the cerium and maintain at least about 95% of the cerium as Ce 4+ . 
     
     
         6 . The method recited in  claim 1 , wherein, before the step of contacting the cerium-containing acidic solution with ammonium hydroxide, the cerium-containing acidic solution has a free acid content of at least about 18%. 
     
     
         7 . (canceled) 
     
     
         8 . (canceled) 
     
     
         9 . The method recited in  claim 1 , wherein the step of contacting the cerium-containing acidic solution with ammonium hydroxide comprises contacting the cerium-containing acidic solution with a sufficient amount of ammonium hydroxide to stabilize at least about 95% of the cerium in the liquor as a cerium (IV) ammonium nitrate complex. 
     
     
         10 . The method recited in  claim 8 , wherein the step of contacting the cerium-containing acidic solution with ammonium hydroxide comprises contacting the cerium-containing acidic solution with a sufficient amount of ammonium hydroxide to raise the pH of the cerium-containing acidic solution to at least about pH 4.5. 
     
     
         11 . The method recited in  claim 8 , wherein the step of contacting the cerium-containing acidic solution with ammonium hydroxide comprises contacting the cerium-containing acidic solution with a sufficient amount of ammonium hydroxide to raise the pH of the cerium-containing acidic solution to not greater than about pH 6.0. 
     
     
         12 . The method recited in  claim 1 , wherein at least about 60% of the cerium in the cerium-containing acidic solution is precipitated as cerium hydroxide. 
     
     
         13 . (canceled) 
     
     
         14 . The method recited in  claim 1 , wherein the cerium depleted solution comprises not greater than about 10 wt. % cerium. 
     
     
         15 . (canceled) 
     
     
         16 . The method recited in  claim 2 , wherein the cerium-containing solid product comprises a compound selected from cerium oxalate and cerium oxide. 
     
     
         17 . (canceled) 
     
     
         18 . The method recited in  claim 16 , wherein
 the cerium-containing solid product comprises cerium oxide and wherein the cerium oxide is formed by the step of:   calcining a cerium-containing oxide precursor at a temperature that is sufficient to convert substantially all of the REEs to RE-oxides and to convert substantially all of the cerium to CeO 2 .   
     
     
         19 . The method recited in  claim 18 , wherein the calcining is carried out at a temperature of at least about 710° C. 
     
     
         20 . The method recited in  claim 2 , wherein the cerium-containing product further comprises rare earth elements in addition to cerium. 
     
     
         21 . (canceled) 
     
     
         22 . (canceled) 
     
     
         23 . The method recited in  claim 20 , wherein the rare earth elements comprise at least three additional rare earth elements selected from the group consisting of dysprosium, europium, terbium, lanthanum, neodymium, praseodymium and yttrium. 
     
     
         24 . The method recited in  claim 20 , wherein the additional rare earth elements comprise at least one rare earth element selected from the group consisting of neodymium, europium, praseodymium and terbium. 
     
     
         25 . The method recited in  claim 20 , wherein the cerium hydroxide product comprises not greater than about 10 at. % non-cerium rare earth element compounds. 
     
     
         26 . (canceled) 
     
     
         27 . The method recited in  claim 20 , wherein at least about 90 wt. % of the non-cerium rare earth elements in the cerium-containing acidic solution remain solubilized in the cerium depleted solution after the step of contacting the cerium-containing acidic solution with ammonium hydroxide and precipitating cerium. 
     
     
         28 . (canceled) 
     
     
         29 . The method recited in  claim 1 , wherein the step of contacting the cerium-containing acidic solution with ammonium hydroxide comprises:
 first contacting the cerium-containing acidic solution with a ammonium hydroxide to raise the pH to a first pH to precipitate cerium as cerium hydroxide and form an intermediate cerium depleted solution and a first cerium hydroxide product; and   second contacting the intermediate cerium depleted solution with the ammonium hydroxide to raise the pH to a second pH, wherein the second pH is greater than the first pH, to precipitate additional solubilized cerium as cerium hydroxide and form the cerium depleted solution and a second cerium hydroxide product.   
     
     
         30 . (canceled) 
     
     
         31 . The method recited in  claim 29 , wherein the first pH is at least about pH 4.0. 
     
     
         32 . The method recited in  claim 29 , wherein the first pH is not greater than about pH 4.5. 
     
     
         33 . (canceled) 
     
     
         34 . The method recited in  claim 29 , wherein the second pH is at least about pH 5.5. 
     
     
         35 . The method recited in  claim 29 , wherein the second pH is not greater than about pH 6.0. 
     
     
         36 . The method recited in  claim 29 , further comprising the step of:
 separating from the intermediate cerium depleted solution, before the second contacting step, at least a portion of the precipitated cerium hydroxide formed in the first contacting step.   
     
     
         37 . The method recited in  claim 36 , wherein the cerium-containing acidic solution further comprises thorium. 
     
     
         38 . The method recited in  claim 37 , wherein the concentration of solubilized thorium in the cerium-containing acidic solution is at least about 50 mg/l and is not greater than about 2000 mg/l. 
     
     
         39 . (canceled) 
     
     
         40 . The method recited in  claim 37 , wherein at least about 95% of the thorium in the cerium-containing acidic solution is co-precipitated with the cerium hydroxide as thorium hydroxide in the cerium hydroxide product. 
     
     
         41 . (canceled) 
     
     
         42 . (canceled) 
     
     
         43 . The method recited in  claim 37 , wherein the cerium depleted solution comprises not greater than about 0.01 at. % thorium. 
     
     
         44 . The method recited in  claim 37 , wherein the cerium depleted solution comprises not greater than about 0.002 at. % thorium. 
     
     
         45 . (canceled) 
     
     
         46 . The method recited in  claim 1 , wherein the cerium-containing acidic solution further comprises base metals. 
     
     
         47 . The method recited in  claim 46 , wherein substantially all of the base metals remain in the cerium depleted solution.