Solvent Extraction of Rare Earth Elements from a Nitric Acid Leach Solution of Apatite by Mixtures of Tributyl Phosphate
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NTRODUCTION
APATITE is the main source of phosphate fertilizers and phosphoric acid, but it also contains up to 1.0 pct rare earth elements (REEs).[1] REEs are substituted in the apatite lattice for calcium ions or are present as mineral inclusions in the ore.[2] In spite of its low rare earth content, apatite could become an important source of rare earths because it is processed in large quantities and also can be found all over the world.[3] For extracting REEs from apatite, sulfuric, nitric, and hydrochloric acids have been used as leaching reagents.[4–6] When leaching is conducted by nitric acid, most of the rare earths in the apatite ore will go into the solution, and the rate of REE recovery is essentially
ALI FERDOWSI and HOSSEIN YOOZBASHIZADEH are with the Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave., Tehran, PO Box 11155-9466, Iran. Contact e-mail: [email protected] Manuscript submitted November 16, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS B
higher than when using sulfuric acid.[4,7] Besides, nitrate solutions are preferable for the extraction and separation of the REEs by solvent extraction.[8] The main constituent elements of apatite are Ca and P. It also contains impurities such as Fe, Mg, and Na, and compared with other REE minerals, the REE concentration in apatite is low. As a result, the leaching solution of apatite contains relatively low concentrations of REEs and high concentrations of base metals and phosphate. Hence, the selective extraction and purification of rare earth elements from a leaching solution are difficult. Different extractants have been used for recovery and separation of REEs from leach liquors.[9,10] Among them, tributyl phosphate (TBP) and di-(2-ethylhexyl) phosphoric acid (D2EHPA) are common and widely used in the industrial process for the recovery of rare earths.[11] The separation factor of REEs by D2EHPA is more suitable than by TBP, but the loading capacity of REE by TBP is greater than that of D2EHPA[12] Extraction of rare earth by TBP was studied by Majdan,[13] Preston et al.,[11] and Jorjani et al.[14] and by D2EHPA was studied by Geist et al.,[15] Sato,[16]
Morais and Ciminelli,[17] Yin et al.,[18] Khaironie et al.,[19] Rabie,[20] Preston et al.,[21] Rodrigues et al.,[22] and Takip et al.[23] Mg and Na can be separated from REEs by TBP and D2EHPA, but Fe, Ca, and P will be co-extracted by REE into the organic phase. However, Ca and P could be separated by other methods such as precipitation,[21] but Fe remains in the organic phase, and stripping it from the organic phase is too difficult.[24] As a result, by recycling the extractant in the solvent extraction circuit, the concentration of Fe in the organic phase increases and the loading capacity of the extractant decreases. Although it is reported that effective stripping of Fe is possible when mixtures of TBP and D2EHPA are used as extractants,[25,26] there are fewer data on the extraction of lanthanides by the D2EHPA-TBP system, and even these few data are cont
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