Process for the separation of rare earth elements and usable gypsum from radium-bearing phosphogypsum

Abstract

Systems and methods are provided for separation of gypsum and/or rare earth elements and yttrium from phosphogypsum. Gypsum can be extracted from the phosphogypsum by dissolution with a basic aqueous medium that can include salinity, followed by reprecipitation of the calcium sulfate dihydrate (gypsum) and optional recycling of the solvent. A further extraction, of rare earth elements and yttrium, can be performed on the intermediate residue from the gypsum extraction step. The further extraction can be done by using one or more acidic, aqueous media on the intermediate residue, leaving a final residue that contains most or all of the radium that was present in the initial phosphogypsum. Optionally, the radium can be extracted from the residue for medical isotopes.

What is claimed is: 
     
         1 . A method for separation of gypsum from phosphogypsum, the method comprising:
 i) dissolving the phosphogypsum in a first aqueous medium to give a first solution;   ii) precipitating gypsum from the first solution to give extracted gypsum and an intermediate residue;   iii) treating the intermediate residue with a second aqueous medium that is acidic to give a second solution and a second residue,   wherein the second solution comprises rare earth elements and yttrium, and   wherein the extracted gypsum has a concentration of radium that is lower than that of the phosphogypsum.   
     
     
         2 . The method according to  claim 1 , further comprising:
 iv) treating the second residue with a third aqueous medium that is acidic to give a third solution and a final residue.   
     
     
         3 . The method according to  claim 2 , wherein the final residue comprises radium. 
     
     
         4 . The method according to  claim 2 , wherein the third aqueous medium is nitric acid. 
     
     
         5 . The method according to  claim 1 , wherein the first aqueous medium has a basic pH. 
     
     
         6 . The method according to  claim 1 , wherein the concentration of radium in the extracted gypsum is at least 75% less than that of the phosphogypsum. 
     
     
         7 . The method according to  claim 1 , wherein the concentration of radium in the extracted gypsum is at least 95% less than that of the phosphogypsum. 
     
     
         8 . The method according to  claim 1 , wherein the precipitating of the gypsum from the first solution comprises using evaporation of the first solution. 
     
     
         9 . The method according to  claim 1 , wherein steps i) and ii) are performed in a first tank, and wherein the first tank is optionally stirred. 
     
     
         10 . The method according to  claim 9 , wherein the first tank is a first sedimentation tank. 
     
     
         11 . The method according to  claim 9 , wherein step iii) is performed in a second tank, and wherein the second tank is optionally stirred. 
     
     
         12 . The method according to  claim 11 , wherein the second tank is a second sedimentation tank. 
     
     
         13 . The method according to  claim 11 , wherein the first tank is the same as the second tank. 
     
     
         14 . The method according to  claim 11 , wherein the first tank is different from the second tank. 
     
     
         15 . The method according to  claim 1 , wherein the second aqueous medium comprises at least one mineral acid, and
 wherein a concentration of the at least one mineral acid is in a range of from 0.01 molar (M) to 3 molar.   
     
     
         16 . The method according to  claim 15 , wherein the at least one mineral acid comprises at least one of nitric acid, hydrochloric acid, and sulfuric acid. 
     
     
         17 . The method according to  claim 1 , wherein step iii) is performed at ambient temperature. 
     
     
         18 . The method according to  claim 1 , wherein the second solution comprises at least 80% by mass of the rare earth elements that were present in the phosphogypsum. 
     
     
         19 . The method according to  claim 1 , wherein the extracted gypsum has a radium activity of 5 picocuries per gram (pCi/g) or less. 
     
     
         20 . The method according to  claim 1 , wherein the extracted gypsum has a radium activity of 1 pCi/g or less.