AU598906B2 - Technique and device for the separation of metal ions from aqueous solutions - Google Patents

Description

COMMONWEALTH OF AUSTRALIA COMMONWEALTH OF AUSTRALIA Patents Act 1952 COMPLETE SPEC I F I CATION

(ORIGINAL)

Application Number Lodged Complete Specification Lodged Accepted Published e^ DI-^C. This document contains t e amendments rnade undr' Section 49 and is correct fo, printing.

Priority :11 February 1987 Related Art Name of Applicant Address of Applicant S Actual Inventor/s Address for Service :KERNFORSCHUNGSZENTRUM KARLSRUHE

GMBH

:Postfach 3640, D-7500 Karlsruhe 1 Federal Republic of Germany :Herr Prof. Dr. Klaus Heckmann Frau Christine Rieger Herr Dr. Reinhard Kroebel F.B. RICE CO.

Patent Attorneys 28A Montague Street, Balmain N.S.W. 2041 Complete Specification for the invention entitled: TECHNIQUE AND DEVICE FOR THE SEPARATION OF METAL IONS FROM AQUEOUS SOLUTIONS The following statement is a full description of this invention including the best method of performing it known to us/re:- 2 The present invention relates to an improved method and a device for metal ion separation from aqueous solutions in at least one electrolytic cell.

It is known in the art to separate metal ions from an aqueous solution using the following steps:immersion of at least two electrodes into an aqueous solution and by setting an electrode potential in said aqueous solution, so that metal ions are selectively brought to a level of oxidation; the metal ions form anionic complexes with anions present in the solution, or with anions previously added to the solution; to the solution is added one, or several, cation tensides prior to, during, or after, setting of the electrode potential; the complexes formed and precipitated in the reactions of the anionic metal ions complexes are 00 floated together with the cation tensides by means of o a gas and separated in this way from the solution.

S° 20 A technique falling under the foregoing generic description has been known from EP-OS 0 004 953 where the selective separation of uranium from its accompanying metals by means of precipitate flotation of suitable cation tenside precipitates of the anionic chloro or 25 sulfato complexes of uranium was described.

However, this technique fails in nitric and nitrate solutions ore dressing brines) in cases where uranium occurs as U0 2 2+ because complex formation of this cation with NO 3 ions is such that the soluble products of the cation tenside precipitates of the anionic "0 onitrato complexes are tco excessive in number to be precipitated. However, unlike U02 U 4 forms very effectively complexes with NO 3 and the anionic nitrato uranates (IV) with suitable cation tensides to form precipitates which are difficult to dissolve and can ;R

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r; r. Ir.i- _r -L-rriri 3 be well floated. Thus, if uranium is to be removed from nitric or nitrate brines by means of precipitate flotation, uranium has to be reduced to U 4 beforehand and it must be taken into account that in the presence of atmospheric oxygen reoxidation according to the formula U0 2 2+ U 4 very easily occurs which might result in considerable losses in precipitate yield when the process in not conducted intelligently. For this reason, it is advisable to precipitate and eliminate U 4 immediately after it has been generated and complexed. As cation tenside precipitates of U 4 are much more resistant to oxidation by atmospheric oxygen.

Precipitate flotation of anionic metal complexes is in many cases the cheapest and simplest method of obtaining valuable materials. The status of this technology has not undergone fundamental changes since EP-OS 0 004 953 was laid open although several new publications have dealt with this topic. The method has been associated with the drawback that in some cases the o 00 0 0 20 valency of the metal cation allowed complexing with o anionic constituents of the solvent either not at all, or, but inadequately. In such situations it became necessary to modify the valency of the respective metal ion by 2 chemical interventions in such a manner than complexing °oo 25 and precipitation became possible.

S° The present invention seeks to provide an improved method and a device for separating metal ions from aqueous solutions by means of precipitate flotation of anionic metal complexes which are amenable to coprecipitation with tensides while avoiding interfering chemical reactions, such as reoxidation of metal ions etc., at less expenditure than is typical of the technique corresponding to the status of the art, and achieving the desired changes in valency of metal ions so completely that complete complex formations and precipitations can be 4 guaranteed and possible application of chemical correction reactions supplementing the technique falling under the generic description can be avoided.

According to the present invention, there is provided an improved method for separating metal ions from an aqueous solution comprising new and novel method steps:- .Soale 9; belsg jCP.ced forming an electrolytic flotation gason at least one of the electrodes; and setting the level of oxidation of at least one metal ion, required for precipitation, on at least one of the electrodes.

In an embodiment of the present invention the electrode potential can be set with direct voltage. If several species of metal ions are present, then in another embodiment the electrode potentials, although they can be set with direct voltage, can be set as well with polar o Oo changes made at irregular or regular time intervals. In a yet another embodiment an alternating voltage can be 0 applied as electrolytic voltage. In an embodiment of the method according to the present invention the aqueous solution (the electrolyte) is passed through several electrolytic cells arranged in cascade.

Although electroflotation as a means of eliminating finely distributed solids from industrial liquid effluents °B 25 is an already known technique 0 (Umweltschutz-Stadtereinigung, Volume 7 (1970), No. 3, pp. 56 to 59), use of electrolytic flotation gas has not yet been studied in a technique similar to that in this generic description. The generation by electrolysis of a flotation gas, according to the present invention, compared with mechanical gas supply, offers the advantage that the bubble size and bubble flow can be set to be largely independent of each other and that, if a suitable electrode geometry is used, extrenely small bubbles are generated which rise but slowly and can hence be loaded particularly well with hydrophobic particles.

Electrolytic setting of the valency combined with precipitate flotation using the electrolytic flotation gas according to the present invention gives an extremely 1 simple and effective novel separation technique and opens up broad applications to flotation engineering which go beyond the familiar fields. By electrolytic setting and valency the addition of oxidants and reductants and hence the ballast produced by the addition of reagents are avoided. Use of the electrolytic gas for flotation adds the advantages of electro-flotation to the overall process; the continuous elimination by flotation of the tenside precipitates continuously removes the desired metal ion from any redox equilibria established.

By proper selection of the conditions of electrolysis it is possible, to achieve in a surprisingly convenient way reduction at the cathode of U0 2 2+ to

U

4 and the immediately following precipitation of the anionic nitrato uranates If the cell is suitably designed the uranium containing precipitates can be floated out of the electrolytic solution (the initial solution) immediately and withdrawn from the head of the equipment as a relatively dry foam. If the process is properly conducted no more uranium can be detected in the electrolytic solution with the help of currently employed methods. Very unexpectedly, the long-chain jI alkylpyridinium cations which, on account of their capability of easy crystallization, are favored as precipitates for anionic nitrato uranates (IV) are not attacked during electrolysis, neither on the cathode nor on the anode at least not to a measurable extent.

By appropriate modification of the conditions underlying this combined Electro-Precipitation Flotation process (EPF) this method becomes applicable to other metal '6 ions, other solvents and, finally, anodic oxidation processes Mex+ Me( x l as well. Accordingly, the new and simple EPF method can be applied to all ions which can be made floating via setting of a defined stage of valency. For instance, the metal ions can practically be completely removed from a nitric solution of Ce 4 and Ce 3 if one converts them into Ce 4 at the anode and precipitates and floats them still in the anode space. As cerium simulates plutonium well, it can be expected that in the course of nuclear fuel reprocessing not only uranium but also plutonium can be floated in nitric and nitrate containing solutions, respectively, while cation tenside is added provided that it occurs as trivalent cation in the waste stream and is oxidized to Pu 4 at the anode.

,From the precipitates separated with the EPF technique the metal ions after processing may be converted, into metal oxides while the tenside used as a reagent for precipitation can be practically fully i 20 recovered and recycled.

The present invention also seeks to provide a device utilizing the improved method as hereinbefore described and which device provides at least one electrolytic cell in which one of the electrode spaces is made as a rising pipe. The rising pipe is the simplest variant of a flotation pipe. In an embodiment of the device according to the present invention, the electrodes consist of concentrically arranged cylinder shaped bodies. The electrodes are, in an embodiment of the present invention, made as platinum nets with a relatively high density of meshes. The electrode space which is not designed as a flotation pipe can be closed towards the outside, with the exception of a gas outlet. A diaphragm may, in an embodiment of the present invention, be provided between the two electrode spaces. In another embodiment of the

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r -7- 4 4I 4I 4 4 04 0 04 C'QO t 0 4 0440 410 4 00440 0 0 44 4 4I 4 44 404 device of the present invention, the electrolytic cell is equipped with a cooling or heating jacket. Furthermore, in yet another embodiment of the device, the transition of the electrolytic cell to the rising pipe is made as a mixing chamber with at least one feed pipe. The mixing chamber in another embodiment of the present invention is equipped with a cooling or heating jacket. Moreover, the rising pipe in an embodiment of the present invention is surrounded by a cooling or heating jacket. In an embodiment of the present invention the electrolytic cell is provided with a stirrer which extends upwards from the electrolytic cell into the mixing chamber. The electrolytic cell, the mixing chamber and the rising pipe can in an embodiment of the present invention be made as individual elements detachable from each other which, are composed to a unit by means of flanges. In an embodiment of the present invention there is provided, a foam withdrawal device located above the rising pipe and containing agent which destroy the foam, said foam 20 comprising, gas, solid particles and liquid, together with a liquid discharge line.

Embodiments of the present invention will now be further described below with reference to the accompanying drawings, wherein:- 25 Fig. 1 is a schematic representation of an embodiment of apparatus employed in the present invention; Fig. 2 is a schematic representation of a column head employed in an embodiment of the present invention; and Fig. 3 is a schematic representation of a stirrer employed in an embodiment of the present invention.

In Fig. i, the platinum nets 3 and 4 are used as acid or corrosion resistant electrode materials for the electrolytic reduction and oxidation, respectively, of, actinides and fission product ions, respectively, in nitrate and sulfate acid solutiofis, respectively. Via the I- I il diameter of the wires and the density of meshes, respectively, of the platinum nets the size of the gas bubbles can be adjusted. Very small bubbles are preferred for flotation, nets having a high density of meshes are used.

A diaphragm 7, preferably made of borosilicate glass, is able to separate from each other the anole space 2 and the cathode space 5, and the flotation gases generated there by electroylsis, mainly oxygen at the anode and hydrogen at the cathode. The electrolytic cell 1 consists of three concentrically arranged glass cylinders: thermosheath 8, cylinder 5' surrounding the external electrode space 5, and cylinder 2' comprising the inner electrode space 2. In case of anodic flotation the innermost glass pipe with also the diaphragm 7 fused into a it makes up the anode space. By means of the flotation 0 o gases generated at the anode the metal/tenside precipitate S developing after precipitation with the tenside can be .u removed. In case of flotation at the cathode the inner 20 space 2 is converted into the cathode space by polar reversal, and flotation is then achieved by the gas bubbles generated by electrolysis at the cathode. The inner electrode 4 is supplied with electricity via the connection 18. In the inner electrode space 2 the net o'a 25 shaped platinum electrode contacts the diaphragm 7 in a cylindrical configuration. The inner electrolyte may be recirculated, if applicable, via connection 20. Besides, <there is a supply 21 for the addition of reagents hydrazine) into the inner electrode space 2. In the external electrode space 5 again in a coaxial configuration with respect to the diaphragm 7 the at ,f external electrode 3 is provided; it is also made as a platinum net and again fitting closely the diaphragm 7.

Via connection 19 the external electrode 3 is supplied electricity. The gases generated in the external 9 electrode space 5 are discharged via the rising pipe 6 into which via connection 21 fresh solution can be supplied.

The metal ions formed in the inner electrode space 2 are precipitated using tenside in the mixing chamber The tenside solution is injected via the inlet 11. The mixing chamber 10 can likewise be controlled by thermostat and is also equipped with a stirrer 9 which can be driven from the bottom of the electrolytic cell 1 via a shaft.

The same shaft can be used, if needed, to effect stirring in the inner electrode space 2.

The bubble flow generated in the electrolytic cell 1 rises as a laminar flow in the flotation tube 13. If required by the conditions for precipitation, the stay time of the tenside in the solution can be extended or reduced by making the flotation tube longer or shorter (modular design).

In the column heads 15, 23, as shown in Fig. 2, the foam generated (metal/tenside precipitate) is sucked off 20 and any metal not yet reacted electrolytically and precipitated, respectively, is fe' into the same or a succeeding electroflotation device via connection In the inner space 24 of the column head 23 provided with holes, a cylindrical closely fitting glass fiber sleeve 25 with open bottom is inserted. While the liquid column rises the solution is pressed through the glass fiber sleeve 25. Freed from the metal/tenside precipitate the solution then runs through the openings 26 of the inner space in the discharge line 27 and from there into the electrolytic cell of the same or a second apparatus.

Rising of the liquid column, which causes circular flow if the means of reflow is provided, occurs spontaneously due to the flow of the rising electrolytic gas (gas lift).

The effect can be intensified by installation of a pump 35 (not represented in the figures).

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4 4 6 B 4 499 f 9190 4 a Kl FOCY ii 1 il_ .I~-I Tranquilization segments (not represented) may be installed between the electrolytic cell 1 and the mixing chamber 10, between the mixing chamber 10 and the flotation tube 13 and in the flotation tube. At higher flow densities they improve the laminar pattern of the bubble flows.

The stirrer 9, as shown in Fig. 3, with the plate shaped stirrer foot 28, stands on the internal bottom of the electrolytic cell 1 and is provided with a plurality of bores 29 to accommodate magnetic stirrer rods (not represented). Via a detachable plug-in connection 30 the stirrer foot is connected with an adaptor 31 which can be screwed onto the stirrer shaft 32. In the zone of the inner electrode space 2 in the electrolytic cell 1 the shaft 32 is provided with longitudinal sheet metals 33 which keep moving the electrolyte. At the upper end of the stirrer shaft 32 in the zone of the mixing chamber a stirrer blade holder 34 with several stirrer blades is provided. The stirrer blades 35 can be fixed in an 20 inclined position at the stirrer blade holder 34 in order o to ensure laminar upward movement of the foam.

Dimensions and Properties 4001o0 Electrodes: PtlOIr nets °o wire thickness: 0.12 mm number of meshes: 225 meshes/cm 2 4 4 044 0 0 0 0 11 External Internal Pt electrode Diameter (cm) Height (cm) Geometric surface (cm2) Leads: 4.0 6.6 93.7 Pt1OIr wires 2.9 6.6 68.0 wire thickness 1 mm Viton 0-rings: Diaphragm: for flanges for tranquilization segments VIOR 57-3 (manufactured by Freudenberg, Nuremberg) THOMAPOR universal filter candle made of borosilicate glass maximum pore width: 10 20 um length 95 mm (manufactured by Reichelt, Heidelberg) No. 603 Gh inner diameter: 36 mm outer diameter: 40.5 mm height: 60 mm (manufactured by Schleicher Schill, Dassel) Glass fiber sleeve: 0 4 -f--w-ventin wi-l-l--b-c-r-i be d be 10 practical example. However, the invention is no stricted to this example and the species of met ons used in it, S3 respectively.

Example: ria''odic _el P trroflott Q f R vit r- a n ^-Biu -E I QV 0 'dtiors Li. tr I la The present invention will now be described in more detail by means of a practical example. However, the present invention is not restricted to this example and the species of metal ions used in it, respectively.

Example: Cathodic electroflotation of nitric uranium (IV) solutions.

a a I a 1 t 0 a o a oa aac Ia 444414 a a a t-

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12 With a view to a flotation which is to take place either 2+ 4 during reduction of UO2 U or thereafter, the following facts have to be considered: Electrolyses performed at constant low voltage givesgood current yields and result in complete reduction of uranium(VI) with little current loss for side reactions.

If flotation is to be carried out during reduction, electrolyses at constant potential are not possible because during the whole electrolysis the gas evolution observed is poor.

ooo If electrolysis is performed at constant current the evolution of nitrogen starts at an intensified rate after uranium(VI) reduction.

The concentration of nitric acid in the cathode space must be controlled during reduction. It should not fall below 5.5 molar if it is to be ensured that only hexa- Snitrato uranates(IV) are formed and not uranium(IV) tetranitrate which is of no use for flotation. If necessary, nitric acid must be supplied in addition to the 0 cathode space during electrolysis.

After electrolytic reduction of uranium(VI) to uranium(IV) in 6 M nitric acid the anionic hexanitrato uranate(IV) complex is precipitated using cationic tensides, preferably N-cetylpyridinium nitrate (CPNO 3 suspended with the electrolytically generated hydrogen as the carrier gas and entrained as foam. However, as only U 4 and not UO 2 2 can be precipitated from nitric acid or nitrate containing solutions using CPNO 3 and with nitrate concentrations 10 M, selective separation 0- of U 4 from U22+ is possible under the conditions stated above. The precipitations occur at a temperature immediately below the Krafft point. For this purpose, the temperature in the equipment is set to 30 oC. The temperature of the tenside solution (6 M HNO 3 tenside) is 45 "C so that a temperature gradient develops in the mixing chamber which ensures that precipitation can take place just below the Krafft point.

Test 1 Test 2 Uranium(VI) /mole/l/ 1.2 x 10 2 3.2 x 10 /mmole/l/ 16 41 o Reduction time t /min/ 90 110 ae oe.

Amount of tenside /mmole/ 46 114 Duration of flotation t /min/ 40 113 Residual uranium(VI) /mole/l/ 4 x 10- 4 no U(VI) concentration detectable o a Q o I 6 A T 35

°C

CHNO 6.35 mole/1 0r4 20 CHNO 110 mmole/l

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2 5 3 V

Claims (13)

1. 2. A method as claimed in claim 1 wherein, the electrode potential is set with direct voltage.
2. 3. A method as claimed in claim 1 wherein, the electrode Spotential is set with direct voltage and with polar changes performed at irregular or regular time intervals.
3. 4. A method as claimed in claim 2 wherein, the electrode potential applied is an alternating voltage. A method as claimed in claim 1 wherein, the aqueous solution (the electrolyte) is passed through several electrolytic cells connected in cascade.
4. 6. A device for applying the method as claimed in any one of claims 1 to 5 wherein, in the at least one >-Ic- -a 1~ AC electrolytic cell, comprising at least two electrodes, there is one electrode space configured as a rising pipe.
5. 7. A device as claimed in claim 6 wherein, the at least two electrodes are made of concentrically arranged cylinder-shaped bodies.
6. 8. A device as claimed in claim 6 wherein, the at least one electrolytic cell has another electrode space which is made closed towards the outside with the exception of a gas outlet.
7. 9. A device as claimed in claim 8 wherein, a diaphragm is provided between the two electrode spaces. A device as claimed in any one of claims 6-9 wherein, the at least one electrolytic cell is equipped with a cooling or heating jacket.
8. 11. A device as claimed in claim 6 wherein, a transition space from the at least one electrolytic cell to the rising pipe is made as a mixing chamber with at least one feed pipe. o12. A device as claimed in claim 11 wherein, the mixing o o o chamber is equipped with a cooling or heating jacket. G. 13. A device as claimed in 6 wherein, the rising pipe is equipped with a cooling or heating jacket.
9. 14. A device as claimed in claim 11 wherein, the at least 0 one electrolytic cell is provided with a stirrer which extends upwards from the internal base of the at least one electrolytic cell into the mixing chamber.
10. 15. A device as claimed in any one of claims 6 to 14 wherein, the at least one electrolytic cell, the mixing a4* o chamber and the rising pipe are made as single elements detachable from each other.
11. 16. A device as claimed in any one of claims 6 to wherein, above the rising pipe there is provided, a foam withdrawal device which contains agents for destroying the °a foam, said foam comprising gas and solid particles and 4 i liquid, together with a liquid discharge pipe. 4 4C I 16
12. 17. A method for the separation of metal ions from an aqueous solution substantially as hereinbefore described with reference to the accompanying example.
13. 18. A device for the separation of metal ions from an aqueous solution substantially as hereinbefore described with reference to any one of the accompanying figures. DATED this 15th day of January 1990 KERNFORSCHUNGSZENTRUM KARLSRUHE GMBH Patent Attorneys for the Applicant: F.B. RICE CO. 0 1o of 0 6 01 0 1a-- -I 0 0 0F\C