GB2027452A - Removing metals from solutions electrolytically - Google Patents

Abstract

A method of recovering an electrodepositable metal from a solution containing ions of the metal which includes the steps of locating an anode and an electrically conducting member in the solution, connecting the member as a cathode relative to the anode and passing an electrical current therebetween to electroplate the metal onto the member from the solution, removing the member having the electroplated layer of metal thereon from the solution, inserting the member into an electrorefining cell, connecting the member as an anode relative to a cathode, and passing an electrical current therebetween to remove the metal from the member and to deposit the metal onto the cathode. The member may be formed of expanded titanium. <IMAGE>

Description

SPECIFICATION Removing metals from solutions This invention related to methods of recovering electrodepositabie metals from solutions thereof, particularly aqueous solutions. The invention has particular but not exclusive reference to the recovery of metals from dilute solutions, by which is meant solutions containing less than 20g/l of the metal.

In numerous circumstances, it is desired to recover metals from dilute solutions. In the plating industry or in pickling operations, the plating or pickling tanks are followed by rinse baths to remove remnants of the liquors from the articles being treated. There is a gradual accumulation of metal ions in the rinse bath.

Because of pollution problems, it is not possible to throw the bath liquors away but the metal ions must be removed in some way.

Another arrangement in which it is desired to remove metal ions is when liquors from electrorefining baths are treated. In an electrorefining bath, for example for electrodefining copper, there is a gradual build-up in the liquor of impurities which come from the impure anode starting material. To prevent contamination of the electrodeposit on the cathode and maintain high current efficiencies, it is necessary to remove a portion of the liquor and to top up the bath with fresh liquor, thereby lowering the impurity levels.

The liquors from the electrorefining tanks then have to be depleted of their metal values.

The initial removal of copper from the liquor is done in a liberator cell but this will eventually give rise to a dilute solution of copper together with other elements which may be regarded as impurities. Several methods can be used to recover metals from these dilute solutions such as ion exchange, chemical precipitation, electrolytic deposition, solvent extraction and reverse osm sis. Apart from the electrolytic method, these methods are only capable of producing compounds of the metal such as an oxide, hydroxide, sulphide, or a solution containing the metal ions.

Even with an electrolytic recovery system, however, the problem has been to deal with the material electrodeposited onto the cathode. One system has been to use a rotating cathode and to recover the metal in the form of a powder. This powder is then melted down to form a conventional anode for electrorefining purposes. Another proposal by C L Lopez-Cacicedo (see I. Chem. E Symposium Series No 42, pages 29.1 to 29.7) had been to deposit the metal onto a mesh electrode of either titanium or copper (the metal being deposited was also copper). The copper eiectrodes had the advantage of being capable of being melted down together with their deposit to form a suitable anode which could subsequently be refined.It is believed that the deposition onto titanium was tried to establish whether or not the invention would work and the use of copper as the cathode was subsequently considered to be the most desirable material because this enabled the copper plating to be recovered. The cell described in the paper by Lopez-Cacicedo is also described in British Patent Specification No 1423369 (= British Patent Application No 44650/73).

By the present invention, there is provided a method of recovering an electrodepositable metal from a solution containing ions of the metal which includes the steps of locating an anode and an electrically conducting member in the solution, connecting the member as a cathode relative to the anode and passing an electrical current there between to electroplate the metal onto the member from the solution, removing the member having the electroplated layer of metal thereon from the solution, inserting the member into an electrorefining cell, connecting the member as an anode relative to a cathode, and passing an electrical current therebetween to remove the metal from the member and to deposit the metal onto the cathode.

The member is preferably a metallic member and is further preferably a member capable of resisting anodic dissolution when formed as an anode. The member may be formed of a film-forming metal or alloy thereof or may be formed from stainless steel.

By "film-forming metal" is meant a metal chosen from the group titanium, zirconium, niobium, hafnium and tantalum.

The member may be in the form of a foraminate sheet and is preferably formed of expanded metal.

The solution is preferably an aqueous solution.

When the member is connected as a cathode, it is preferably utilised in a cell of the type described in British Patent Specification No 1423369. Such a cell basically comprises a tank having a porous false bottom. The solution may be forced through the porous bottom and bled off at the top. A plurality of small balls or balletini (hollow spheres) of insulating material are located in the tank and are maintained in a fluidised condition by the circulating solution. The member and the anode are located in the fluidised bed and an electrical current passed therebetween.

The expanded metal titanium member may be secured to a copper hanger bar and may be of a size suitable for subsequent immersion in a conventional electrorefining tank. A suitable arrangement would be a hanger bar somewhat in excess of 1 m in length, having a 1 m square (or any other suitable dimension) expanded metal working electrode dependent therefrom. The hanger bar may be formed from a copper-cored titanium rod.

The electrical current is suitably a direct current. During removal step when the elec trodeposited metal is removed from the member, a current reversal may be given so that the member is principally connected as an anode but is on occasion connected as a cathode. Such a current reversal acts to prevent passivation of the anode.

By way of example, embodiments of the present invention will now be described with reference to the accompanying drawings, of which: Figure 1 is a schematic view of an electrodepletion cell; Figure 2 is a perspective schematic view of an electrorefining cell; and Figure 3 is a schematic cross-section of an alternative form of electrorefining cell.

The Lopez-Cacicedo cell as described in British Patent Specification No 1 423369 is essentially a cell which operates in a batchwise manner. Referring to Fig. 1, the cell comprises a tank 1 which has a porous false bottom 2. An overflow 3 at the top of the tank returns electrolyte to a reservoir 4 from which the electrolyte solution is passed by line 5 to pump 6 for injection through the porous false bottom 2 into the body of the tank.

Within the tank 1 is a mass of small spheres, typically glass beads of 500 microns diameter. The glass beads may be hollow. Located within the tank 1 are titanium mesh cathodes 7 and lead anodes 8. The cathodes are in the form of titanium mesh connected to a suitable hanger bar 9 formed of copper or coppercored titanium.

In one example, a prototype Lopez-Cacicedo cell was used, an experimental cell of laboratory size six litre-and an electrolyte which contained 9g/l of copper plus many impurities was pumped through the cell. An electrical current was passed between the anode and cathode until the copper level dropped to 300ppm. A chemical analysis of the material deposited on the cathode is as follows: Zinc L 1 Oppm Tin L 2ppm Lead 200ppm Iron L Sppm Nickel L 1 ppm Manganese L 1 ppm Bismuth 80ppm Antimony L Sppm Arsenic L 1 ppm Tellerium L Sppm Remainder copper Three points particularly are worth noting.

Firstly, the high level of lead which would be unacceptable for normal commercial sales of copper. Secondly, the low level of nickel, although the liquor supplied to the cell initially contained approximately 25g/l of nickel.

Thirdly, the high level of bismuth which again would be unacceptably high for commercial use.

The electrodes were then subsequently removed from the depleted electrolyte and the electrode on which the copper had been deposited was then connected as an anode in an electrorefining cell. The temperature of the electrolyte was maintained at approximately 60"C and in this case copper starter cathodes were used with an anode to cathode spacing of 2.5cm. With a cell voltage of 100 to 120 millivolts, two tests were run: one with a current density at the cathode of 200 amps/m2 and the other with a current density at the cathode of 1 86 amps/m2. In the first case, the current efficiency was 94.9% and in the second case, the current efficiency was 98.7%.On analysis, the cathode deposit contained the following impurities: Zinc L LlOppm Tin L 2ppm Lead L 2ppm Iron L Sppm Nickel L 1 ppm Manganese L 1 ppm Bismuth L 2ppm Antimony L Sppm Arsenic L 2ppm Tellerium L 5ppm In both tests, the current was maintained through the cell until the cell voltage rose from 100 millivolts to 1 Ovolts. It can be seen that there is a dramatic lowering in the lead and bismuth impurities so as to make the copper eminently saleable as first-class cathode copper.

There was considerable concern before the experiments were carried out that during the dissolution of the copper from the titanium anode onto a conventional cathode, passivation might take place, This was not found although the comparative lack of "slime-forming elements" in the anode could have been a problem. When there is a relatively small amount of slime-forming elements present, it is possible that an adherent slime forms but an insufficient amount forms to fall off and hence passivation occurs. This has been found to happen in practice at certain plants but has not proved to be a problem with the system of the present invention.

Fig. 2 shows the conventional electrorefining cell in which a titanium mesh anode 10 coated with copper alternates with conventional cathodes 11. The conventional cathode would be normally a copper starter blank. In Fig. 3, there is shown a pair of titanium expanded metal anodes 1 2 and 1 3 which were originally covered with copper which have been "cleaned" so that the copper has been deposited onto conventional starter sheets 14.

Because the method of the invention does not require impure metal recovered from the electro-depletion cell to be melted to form an anode to be refined, a complete melting step is eliminated. This means that there is a saving in the recovery of the metal.

It will be appreciated that the material for the anode/cathode could be stainless steel rather than titanium. Any other suitable material such as graphite could also be used.

Materials other than copper might be electrodeposited in the same manner such as nickel, zinc etc. It will also be appreciated that the invention could be used for the removal of contaminants from effluent prior to the effluent being passed to an effluent treatment plant.

Claims (9)

1. A method of recovering an electrodepositable metal from a solution containing ions of the metal which includes the steps of locating an anode and an electrically conducting member in the solution, connecting the member as a cathode relative to the anode and passing an electrical current therebetween to electroplate the metal onto the member from the solution, removing the member having the electroplated layer of metal thereon from the solution, inserting the member into an electrorefining cell, connecting the member as an anode relative to a cathode, and passing an electrical current therebetween to remove the metal from the member and to deposit the metal onto the cathode.

2. A method as claimed in Claim 1 in which the member is a metallic member.

3. A method as claimed in Claim 2 in which the metallic member is capable of resisting anodic dissolution when formed as an anode.

4. A method as claimed in Claim 3 in which the metal is a film-forming metal, preferably titanium.

5. A method as claimed in any one of Claims 1 to 4 in which the member is a foraminate sheet.

6. A method as claimed in Claim 5 in which the member is formed of expanded metal.

7. A method as claimed in any one of Claims 1 to 6 in which there is applied a current reversal step when the member is connected as an anode so that for a time not exceeding 15%, the anode is connected as a cathode to prevent passivation of the anode.

8. Metal when recovered by the method claimed in any one of Claims 1 to 7.

9. A method of recovering an electrodepositable metal substantially as herein described with reference to and as illustrated by the accompanying drawings.