GB2176207A - Metal recovery - Google Patents

Abstract

In the recovery of metal, such as a non-ferrous metal, from an aqueous solution thereof by extracting the metal into an organic solvent phase, back extracting the metal into an aqueous phase therefor, and plating out the metal electrolytically, the back extraction and plating out are both carried out in the cathode compartment of an electrochemical cell, e.g. in a rotating cathode contactor. This procedure reduces the number of process operations, improves process effectiveness and gives the metal in a form (e.g. powder) suitable for subsequent removal and processing.

Description

SPECIFICATION Metal recover This invention relates to a method for recovering a metal such as a non-ferrous metal.

Solvent extraction is a well-known method for recovering a non-ferrous metal from an aqueous solution containing the non-ferrous metal (the first aqueous solution) such as may be derived from an ore, mine tailing or waste. For example, the first aqueous solution may be contacted with an organic solventto extract the metal specifically and the resulting organic solvent phase contacted with an appropriate aqueous solution (the second aqueous solution) to backextractthe metal. Such back extraction is commonly referred to in the art as 'stripping'. The second aqueous solution may then be separated completely from the organic solvent phase and electrolyticallytreated in a plating bath for removal ofthe metal in elemental form.Several stages of backextraction may be necessary to maintain an adequate concentration difference between the organic solvent phase entering the above-described back extraction process and organic solvent phase leaving it.

In the present invention, the steps of contacting the organic solvent phase with the second aqueous solution (i.e. strip solution) and of electrodeposition from that solution are combined, resulting in the use of less process plantthan the above-described known process.

The present invention comprises a method for recovering a metal from aqueous solution comprising the sequential steps of (i) extracting the metal from the aqueous solution into an organic solvent phase; (ii) contacting the organic solvent phase with an aqueous phase for back extraction ofthe metal from the organic solvent phase in the cathode compartment of an electrochemical cell of controlled cathode potential whereby the back extracted metal is elec trodeposited onto the cathode ofthe cell; and (iii) physically removing the metal from the organic solvent phase and the aqueous phase.

In step (ii), the organic solvent and aqueous phases in the cathode compartment are preferably stirred to improve contacttherebetween. The cathode may be stationary, though preferably it is a moving cathode such as a known rotating cathode. Where the organic solvent is electrochemically oxidisable,the cathode compartment is preferably separated from the anode compartment to prevent the organic solvent from contacting the anode. The cathode compartment in step (ii) is referred to herein as a "cathode contactor".

In step (ii), the metal may be electrodeposited in powder form which may be protected from oxidation by the organic solvent present. Such powder may be removed in step (iii) by known methods such asfiltra tion orcentrifugation. The abilityto recoverthe metal in powder form may be advantageous,forexample, where it is desired to utilise the powder in powder metallurgy, after comminution if necessary.

Afurther advantage of the invention is that, in step (ii), the concentration of metal in the aqueous phase for back extraction is held continuously low. Metal can hence be removed more completely from the organis solvent phase than in known processes and in a single rather than several stages. Thus, use can be made of a 'stronger' organic solventfrom which metal removal would be impractical in known processes. More complete removal of metal from aqueous solutions is therefore possible.

The invention is applicable to the recovery of any plateable metal capable offorming a complex with an organis solventthat is effectively insoluble in water.

Examples of such metals are non-ferrous metals such as copper, nickel, cobalt, zinc and lead. Also, mixtures of such metals may be recovered by means ofthe present invention. Much of current production of non-ferrous metals by solvent extraction involvesaci- dicextraction, and the method of the invention is particularly suitable forthis as acid is generated in situ by 2 evolution at the anode.

The method ofthe invention may conveniently be carried out as a batch process or continuously. In the latter case, the organic solvent and aqueous phases resulting from step (ii) may be recirculated for re-use, and the various flow rates are balanced in accordance with the rate of production of the metal and the desired metal concentration in the raffinate.

One way of carrying outthe invention will be de- scribed as follows by way of example only. Reference will be made to the accompanying drawing,the sole figure of which is a schematic apparatus and flow scheme for carrying out the method of the invention.

Referring to the figure, an electrochemical cell 1 has a cathode 2 rotatable about axis 3 and a cation exchange membrane4 defining acathodecompart- ment 5 (or cathode contactor). The cathodecompart- ment 5 has an inietflow line 6 and an outlet flow line7 connected to a filter 8. Thefilter 8 has an exit 9 for a solid and an outlet flow line 10 communicating with a settler 11. The settler 11 has an outlet flow line 12for an organic phase and an outlet flow line 13foran aqueous phase communicating with the flow line 6.

In the figure, flow of liquids during operation ofthe apparatus is depicted by arrows wherein unfilled arrows show flow of an aqueous phase and filled arrows show flow of an organic solvent phase.

An organic solvent phase containing metal for recoverythat has been extracted from an aqueous solution by methods known in the art is fed to the cathode compartment 5ofthe cell 1 via flow line 6 as shown by arrows a. Atthe sametime, a strong aqueous acid phase for back extraction of metal is recycled to the cathode compartment 5 along flow line 13 and then along flow line 6 as shown by arrows a'. A potential is applied across the cell 1 and the cathode 2 is rotated about its axis 3.

In the cathode compartment 5, the metal is back extracted from the organic solvent phase into the strong aqueous acid phase and electrodeposited at the cathode 2 to give a powder. The powder and the strong aqueous acid and organic solvent phases are removed from the cathode compartment 5 via flow line 7 and enter the filter 8. The flow of the liquid phases is shown by arrows and b'.

The powder is separated from the liquid phases in thefilter and leaves via exit 9 for collection. The liquid phases leave the filter 8 via flow line 10 as shown by arrows c and c' and enterthe settler 11 where they are separated from one another. The organic solvent phase (stripped of metal) leaves via flow line 12 as shown by arrow dfor reuse in extracting metal from aqueous solution; the strong aqueous acid phase is recycled via flow line 13 as already described.

The apparatus and flow scheme shown in thy figure were used to recover copper from a waste aqueous solution containing 25 g j-1 of Cu2+ and 150 g | -1 of H2S04.

The copper was extracted from the waste solution into an organic solvent phase comprising a 15% (by volume) solution of"Acorga P50" (a mixture of 2hydroxy-5-nonyl-oxime isomers) in "Escaid 100" (a paraffin). The extraction was carried out in a conventional mixer-settlerwherethewaste solution was fed in art a rate of 2 Ih-1 and the organic phase at a rate of 10 Ih-1.

The resulting organic phase together with sulphuric acid solution (300 gl-l; 50 lh-1) wherein the organic phase constituted 10 to 20% (by volume) were fed to the cathode compartment of an "Ecocell" (a rotating cathode metal recovery cell) having a stainless steel cathode of area 70 cm2 and rotation speed up to 20 HZ. Powerwas supplied by a 100 A potentiostat and the currentdensitywas 0.5 A cm -2.

Copperwas produced in the cathode compartment intheform of powder loosely adherentto the cathode. The powder, which was wetted bytheorga- nicsolvent phase, had a particle diameter in the range 1 to 1 0cm. Itwas removed from the cell with the organic phase and the sulphuric acid and separated therefrom byfiltration ata rate of 35 gh-1. The liquid phases were separated in a settler: the organic solvent phase, which contained 2 gI1 Cu2+,was removed at a rate of 10 Ih-1 for use in extracting copper from furtherwaste solution and the sulphuric acid was recycled to the "Ecocel I".

Claims (9)

1. A method for recovering a metal from aqueous solution comprising the sequential steps of (i) extracting the metal from the aqueous solution into an organic solvent phase; (ii) contacting the organic solvent phase with an aqueous phase for back extraction ofthe metal from the organic solvent phase in the cathode compartmentor an electrochemical cell of controlled cathode potential whereby the back extracted metal is elec trodeposited ontothecathode ofthe cell; and (iii) physically removing the metal from the organic solvent phase and the aqueous phase.

2. A method as claimed in ciaim 1 wherein, in step (ii),the organic solvent phase and the aqueous phase in the cathode compartment are stirred to improve contacttherebetween.

3. Amethodasclaimedinclaimi orclaim2 wherein the cathode is a rotating cathode.

4. A method as claimed in any of the preceding claims wherein the organic solvent is electrochemic allyoxidisableand the cathode compartment is separated from the anode compartment.

5. A method as claimed in anyofthepreceding claims wherein the metal is a non-ferrous metal.

6. A method as claimed in claim Swhereinthe non-ferrous metal is copper.

7. A method as claimed in any of the preceding claims when carried out continuously and wherein the organic solvent and aqueous phases resulting from step (ii) are recirculated.

8. A method for recovering a metal from aqueous solution substantially as described herein with refer ence to the example.

9. A metal recovered from aqueous solution by a method as claimed in any ofthe preceding claims.