Multi-stage solvent extraction of metal value

Details Multi-stage solvent extraction of metal value Multi-stage solvent extraction of metal value

1998-02-17

2001-05-29

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S644000, C210S649000, C210S660000, C423S008000, C423S020000, C423S021100

Reexamination Certificate

active

06238566

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a multi-stage solvent extraction process of a metal value such as rare earth elements or, more particularly, to an improvement in a multi-stage solvent extraction process of a metal value from an aqueous acidic solution containing the metal value into a water-immiscible organic extractant solution.
The process of solvent extraction or liquid-liquid extraction as a kind of the technologies for material separation is widely employed in the industries of non-metallic and metallic materials. For example, a metal value in a trace concentration can be efficiently recovered or collected by the solvent extraction method and a metallic element can be purified or separated from impurities or accompanying metallic elements, which can hardly be separated by a conventional chemical method, by solvent extraction. The basic procedure of solvent extraction method of a metal value is conducted by bringing an aqueous solution containing the metal value in the form of a water-soluble salt into contact with a water-immiscible organic solvent or an organic solution containing an extractant agent dissolved therein. The extractant compound is specific to the particular metallic element so that the metal value is selectively transferred from the aqueous phase to the organic phase.
The process of solvent extraction can be classified into two types depending on the nature of the extractant agent. In the solvent extraction process of the first type, the extractant agent is a neutral compound, such as tributyl phosphate (TBP) and quaternary ammonium compounds, with which the metallic element forms a complex compound to be extracted into the organic phase. Taking a tervalent metallic element denoted by M, such as a rare earth element, in the form as a nitrate of the metallic element and TBP as the extractant compound, the process of solvent extraction of this type can be expressed by the equation:
[M(NO
3
)
3
]
A
+3[TBP]
O
=[(TBP
3
). M(NO
3
)
3
]
O
,
in which the subscript A or O at the right-hand side of the respective square brackets means that the species inside of the square brackets exists in the aqueous phase or in the organic phase, respectively.
In the solvent extraction process of the second type, which is a so-called ion-exchange solvent extraction process, the extractant agent is exemplified by phosphorus-containing organic acids, carboxylic acids, and &bgr;-diketone compounds with which the metallic ions are combined with release of an acid or hydrogen ions H
+
. Denoting a phosphorus-containing organic acid soluble in an organic solvent by HP, the solvent extraction process of a tervalent metallic ions M
3+
can be expressed by the following equation:
[M
3+
]
A
+3[HP]
O
=[M(P)
3
]
O
+3[H
+
]
A
,
in which the subscripts A and O at the right-hand side of the square brackets each have the same meaning as explained above. The phosphorus-containing acidic extractant compound can be in the form of a dimer or trimer. Although the metal value in the above explanation is assumed to be in the form of a simple cation, the same principle of mechanism is applicable to complex ions formed between the metallic ions and anionic complex-forming ions.
In the solvent extraction process of the latter type, which is more efficient than the solvent extraction process of the former type, as is understood from the above given equation and in contrast to the solvent extraction process of the former type, the hydrogen ion concentration in the aqueous phase is increased as the extraction of the metallic ions proceeds leading to establishment of a state of extraction equilibrium between the aqueous phase and organic phase so that the process of extraction cannot proceed any further. A conventional method for accomplish further proceeding of the solvent extraction process is to decrease the hydrogen ion concentration in the aqueous phase by the addition of an alkaline compound, such as alkali metal hydroxides and ammonia water, to the aqueous phase thus to neutralize a part of the free acid therein. Needless to say, neutralization of a free acid, e.g., hydrochloric and nitric acids, with an alkaline compound produces a salt such as sodium chloride and ammonium nitrate. Accordingly, the solvent extraction process of the latter type, when practiced in a large scale, has disadvantages due to the large consumption of the alkaline compounds as a neutralizing agent and by-production of a large amount of salts to be disposed without causing any environmental pollution resulting in a great increase in the production costs.
SUMMARY OF THE INVENTION
The present invention accordingly has an object to provide a novel and improved method to accomplish a high degree of extraction in a solvent extraction process of a metal value from an aqueous solution containing the metal ions in the form of a water-soluble salt into an organic extractant phase according to the mechanism of the above explained latter type without consumption of a large amount of an alkaline compound as a neutralizing agent for the free acid contained in the aqueous phase.
Thus, the present invention provides an improvement, in a multi-stage solvent extraction process consisting of at least two successive stages of unit extraction treatments, in each of which an aqueous feed solution containing a metal value in the form of metal ions in the presence of an acid is contacted with a water-immiscible organic solution of an extractant agent capable of combining the metal ions with release of an acid to effect extraction of a part of the metal ions from the aqueous solution into the organic solution and the aqueous solution separated from the organic solution is, as an aqueous feed solution to the succeeding stage, further contacted with a water-immiscible organic solution of an extractant agent for the metal ions to effect extraction of a part of the remaining portion of the metal ions from the aqueous solution into the organic solution, which improvement comprises removing at least a part of the acid contained in the aqueous feed solution after the extraction treatment in a first stage by ion exchange dialysis through an ion exchange membrane prior to contacting with the organic solution in a second stage.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
As is understood from the above given description, the present invention is particularly applicable to the solvent extraction process of the latter type in which an acid is released into the aqueous phase from the extractant agent in the organic phase as the extractant molecules are combined with the metal ions. Such an ion-exchange solvent extraction process is widely practiced for: (1) collection, purification or concentration of a metal value by the selective extraction of the metal value from an aqueous phase into an organic phase followed by recovery of the metal value from the organic phase; (2) purification of a metal value by the selective extraction of impurities accompanying the metal value in an aqueous phase into the organic phase; and (3) separation of different metal values from each other by utilizing the difference of the extraction coefficients or, namely the separation factor, between the metallic elements. The solvent extraction process of the third type is widely practiced for the separation of metallic elements which can hardly be separated each from the others by a conventional chemical separation procedure as for the separation of rare earth elements and separation of nickel and cobalt. In such a solvent extraction process, the metal value transferred from the aqueous feed solution into the organic extractant solution is re-transferred by using an acid solution as the so-called scrubbing agent in order to facilitate further processing of the metal value. The present invention can be applied to any of the solvent extraction processes of the above described three types.
The most characteristic feature of the present inven

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