• Principles
• Extraction Solvents
• Selectivity of Extraction Agents
• Commercial extractants available for Rare Earths separation

• Principles

Separation of two Rare Earths or a group of Rare Earths is typically performed on an extraction battery.

The mixture to be separated is introduced at an intermediate stage in the counter-current -system, thus results in two separation zones, extraction- and washing, on both sides of the feed point. At each end of the equipment, there is partial reflux of the solvent properties used. Effectiveness of separation depends on the number of stages and the separation factor.

Using this process, two pure products or perfectly separated fractions are produced continuously from the rest of the Rare Earths.

Calculating the separations requires modeling, which is dependent, of course, on the type of solvent used.

• Extraction Solvents

The organic phase (solvent) is formed of a mixture of various products: extractant, diluent, and possibly modifier.

The extractant or extraction agent is the active chelating substance of the Rare Earths responsible for the transfer of rare earth ions from the aqueous phase to the organic phase.
It is generally a very viscous product or even a solid that is dissolved in a diluent such as kerosene to ensure good contact between the two phases. In some cases, a modifier is added to the organic phase (heavy C₈-C₁₂ alcohol), improving the system's hydrodynamics.

Currently, a large number of different extraction systems are used or may be used for Rare Earths separation. There are, in fact, several types of extraction agents available with varied properties, depending on the type of Rare Earths salt present in the aqueous phase.

There are three distinct classes described below, the selectivity of which is given for the different Rare Earths.

• Acid Extractants or Cationic Exchangers

  These are organic compounds that possess one (or possibly several) acid function(s) of variable force.
  In the exchange reactions shown below, the chemical species overlined are in the organic phase.

  If RH represents the extraction agent in its diluent, the rare earth cations are exchanged according to the reaction:

  

Acidity, therefore, exerts considerable influence on extraction, and it is on this parameter, of course, that intervention takes place to control the separations operation.

Moreover, conditions for use are dependent on the extraction agent. Di- (2-ethylhexyl) phosphoric acid (HDEHP) is viewed as a stronger extractant than the carboxylic acids. Rare earths are separated in an acid medium (pH < 1). It is similarly true for the di- (2-ethylhexyl) phosphoric acids (HEHEHP), useable in a slightly less acid medium. Carboxylic acids must be operated at a pH of approximately 3 to 5.

• Basic Extractants or Anionic Exchangers

  These are the long chain organic compounds containing the primary, secondary and tertiary amino functions or quaternary ammonium salts.
  Primary aminos preferentially extract rare earth sulfates ; tertiary or quaternary aminos extract nitrates and thiocyanates.
  
  For instance, with a quaternary ammonium nitrate R₄N+NO₃- such as Aliquat 336, the simplified equilibrium law may be written thus:

  
  

  Concentration of the nitrate ions in the aqueous phase is an important action parameter on the partition coefficients

• Neutral Extractants or Solvation Agents

  Due to the low basicity of the oxygen atom they most often contain, solvation is used to extract neutral molecules (rare earth salts). Many solvation compounds are used to extract rare earth nitrates, the most frequently used is tri (n-butyl) phosphate (TBP).
  
  Thus, the extraction law:

  
  

  Here again, the concentration of nitrate ions determines the partition coefficients. There is little interest in extraction of other rare earth salts (perchlorates, chlorides, thiocyanates) using these solvents because their partition coefficients are too low or selectivity is insufficient

• Selectivity of Extraction Agents

This is one of the essential parameters used in selecting extraction agents.

HDEHP and HEHEHP are the most selective of all extraction agents, with a variation of 105 to 106 units between the extreme partition coefficients, and an average separation factor of 2.3 to 2.7.
The partition coefficient increases as the ionic radius decreases. Carboxylic acids are selected only for ceric Rare Earths, and the same holds true for TBP. Quaternary ammonium salts (as Aliquat 336) display intermediate selectivity, thus the order of extraction in nitrate medium is opposite to that observed in thiocyanate medium where chelation in aqueous phase occurs extensively.

• Commercial extractants available for Rare Earths separation

Albright and Wilson = A; Daïhachi Chemical = D; Bayer =B; Cytec=Cy; Exxon Chemical = E; Shell Chemical = S; Henkel = He; WITCO= W; and AK = AKZO; CL=Clariant.

• Contacts

If you would like more informations, please contact:
ec-general@eu.rhodia.com