To diminish the discharge of heavy metals and lahthanides by the phosphoric acid industry, these impurities have to be removed from the mother liquor before their incorporation in the gypsum crystals. This can best be achieved by means of solvent extraction or ion exchange during the recrystallization of hemihydrate to dihydrate gypsum. Various commercial carriers and two ion-exchange resins were screened for their efficiency and selectivity. Light and heavy lanthanide ions are extracted from the recrystallization acid by didodecylnaphthalenesulfonic acid (Nacure 1052) and di-(2-ethylhexyl)phosphoric acid (D2EHPA), and the heavy-metal ions by bi:s(2,4,4-trimethylpentyl)dithiophosphinic acid (Cyanex 301) and by bis(2,4,4-4-trimethylpentyl)monothiophosphinic acid (Cyanex 302). Mercury is also extracted by the an- ion carriers tri(C8-C10)amine (Alamine 33-65 and tri(C8-C10) monomethyl ammonium-chloride (Aliquat 336). Both Dowex C-500 and Amberlite IR-120 extract lanthanide and heavy-metal ions. Unfortunately, D2EHPA, Nacure 1052, and the two ion-exchange resins also show affinity for ions present in much higher concentrations, like calcium or iron ions.
ASJC Scopus subject areas
- Chemical Engineering(all)
- Filtration and Separation
- Process Chemistry and Technology