Production processes

• Ore Attack
• Soda Attack
• Acid Attacks
• Separation and Purification
• Liquid-Liquid Extraction

• Ore Attack

Principle - Monazite, bastnaesite, xenotime and loparite, the primary ores, generally enriched by means of various physical processes (gravimetry, flotation, magnetic separation), are pulverized more or less finely into 50 mm to 1 mm grains, then chemically attacked by acid or basic reagents.

Table of the rare earth content of various ores:

Typical Distribution of Rare Earths in the Primary Marketable Concentrates (1)

There are currently three types of attacks used in the industry:

• soda attack of monazite
• hydrochloric attack of bastnaesite before or after calcination
• sulfuric attack of bastnaesite, as well as loparite.

We would also like to point out the possibility of performing high temperature chlorination to prepare unseparated anhydrous rare earth chloride, used in the manufacture of mischmetal (a mixture of rare earth metals).

Marketable concentrates of unseparated rare earth oxides are simply attacked using acid.

• Soda Attack

It is possible to effectively attack monazite and bastnaesite using soda concentrated under heat. The rare earth hydroxide obtained is separated, then subsequently made soluble using an acid.

• Acid Attacks

A first process consists of precalcinating the bastnaesite in order to oxidize the cerium which is in an insoluble acid form. A carefully managed acid attack then makes it possible to selectively dissolve the trivalent Rare Earths. - Bastnaesite can be attacked using sulfuric acid. This process consists of attacking ground bastnaesite with concentrated sulfuric acid in a furnace at 300-400 °C. The rare earth sulfates obtained are then dissolved in water.

• Separation and Purification

Whatever the analytical or preparation ends, the methods of separation used for Rare Earths are numerous and varied. The most effective are those that successively enable a large number of equilibria to be established and phase transfers to occur, thereby making the most of the minute differences in the chemical properties of these elements.

There are two phases involved in all separation methods used for Rare Earths:

• liquid-solid precipitation or fractional crystallization or ion exchange;
• liquid-liquid during solvent extraction.

Whichever method is used, the separation factors between two Rare Earths (ratio of distribution coefficients of the species considered between the two phases) are low, viz: 1 to 10. Fine separation, therefore, requires the basic splitting operations to be broken down, which may be done continuously or discontinuously.

Fractional crystallization, which makes possible to obtain high purity levels (as in La₂O₃ at 99.99%), was a technique that required numerous stages and recycling that proved difficult on a continuous basis. This was abandoned in the early 1970's.
Ion exchanges applied to rare earth production imposes the development by deplacement technique credited to the Spedding team at the University of Iowa.
As standard resins are not very selective, selective rare earth complexing agents, generally aminopolycarboxylic acids from the EDTA (ethylenediamic tetra-acetic acid) family, must be used.

However the presence of these chelating agents in aqueous phase have some limitations: low solubility; high price; delicate and expensive recovery. When coupled with the disadvantages of using resins, namely: process discontinuity and high theoretical plate equivalent because of low kinetics, this results in high cost for large scale production of Rare Earths.

• Liquid-Liquid Extraction

Continuity of operations was the predominant factor in the rapid expansion of liquid-liquid extraction, qwhich has supplanted both earlier techniques. This extraction lends itself well to total continuity in the separation processes, especially its ease in implementing countercurrent techniques in the decanting mixers with a very high degree of automation.

• Contacts

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