GB2125825A - Metal recovery - Google Patents

Abstract

A cell for the recovery of gold and other metals from solution comprises generally coaxial electrodes 5, 6 with a relatively narrow gap between them, means for passing a metal-containing solution through the said gap in a generally spiral or helical path, and means for inducing turbulence in the solution in the said path. In the cell shown, gold containing solution enters through tangential inlet 7 and leaves through tangential outlet 8, flowing in a helical path through the cell. The inner electrode 5 has a rough surface, being of expanded metal mesh, and this induces turbulence in the solution. <IMAGE>

Description

SPECIFICATION Metal recovery This invention reiates to the recovery of metals from solutions, in particular precious metals such as gold.

According to the present invention, a cell for the recovery of gold and other metals from solution comprises generally coaxial electrodes with a relatively narrow gap between them, means for passing a metal-containing solution through the said gap in a generally spirai or helical path, and means for inducing turbulence in the solution in the said path.

We have found that such a cell has a high metal recovery efficiency coupled with low power consumption.

In another aspect, the present invention provides a metal-recovery cell comprising a container, an upright electrode or electrode array mounted in the container, a top cover for the container, and a second electrode or electrode array attached to the cover and arranged so as to be spaced from the first electrode or electrode array when the cover is in position. Such a cell can provide accurate relative placing of the electrodes and excellent sealing with a very simple and inexpensive construction.

By way of example only, the present invention will be further described with reference to the accompanying drawings, in which: Figure 7 is a plan view of a gold-recovery cell with its cover omitted, and Figure 2 is a schematic sectional side view of the cell.

The illustrated cell for the recovery of gold, for example from electro-plating solutions, has a cylindrical container 1 which is open at the top and is provided with a cover 2. The container and cover are made of any suitable material resistant to the solutions to be processed, for example plastics materials.

The cell forms part of a system which also includes a holding tank, a circulation pump and a filter, connected in series so that the solution to be processed is circulated from the holding tank through the cell. The system is so arranged that if the pump stops, the solution in the cell will drain from the cell, to prevent metal already deposited in the cell from being dissolved back into the solution.

The cover is fastened to the top of the casing by any suitable means for example clamping screws, and is sealed by a simple O-ring 3.

A static cylindrical anode assembly is mounted centrally on the base of the cell container. It consists of a drum 4 of any suitable resistant material, and a tubular sleeve 5 of platinised titanium mesh, for example expanded metal mesh.

Coaxial with the anode is a static cathode 6, for example of brass foil or thin plate. This is attached to and hangs from the cover 2, close to the internal surface of the cell container wall. The anode and cathode are respectively connected to the poles of an electric power supply and control system suitable for the metal recovery to be performed.

The diameter of the inner electrode is not much less than that of the outer electrode, so that the radial gap between them is relatively narrow. By way of example, a cell has been constructed with a cathode of 20 cm diameter and 15.2 cm length, and an anode of 14.5 cm diameter and 14.5 cm length, so that the gap between the electrodes was about 2.7 cm.

The gold-containing solution to be treated enters the cell through a tangential inlet 7 between the electrodes at the bottom of the cell, and leaves the cell through the tangential outlet 8 near the top of the cell, the brass outer electrode being cut away to provide access to the outlet. The solution therefore flows at a relatively high rate in a generally helical path through the cell. Because the gap between the electrodes is narrow, and because the inner electrode has a rough surface, being of expanded metal mesh, substantial turbulence is induced in the solution. Because of this and the absence of any moving parts or brush contacts, as are required in cells with rotating electrodes, excellent conditions for efficient mass transport are achieved. The use of a large-diameter anode close to the cathode also provides an extremely even current density, further improving efficiency.Such a cell can easily operate down to a gold concentration of the order of 500 ppm, and with extended treatment it is possible to reduce the gold concentration to below 0.5 ppm.

The close electrode spacing also minimises electrolyte power consumption.

The described cell construction also facilitates manufacture and maintenance. There are no moving parts, and no need for special measures to mount and locate the outer electrode, as this is located automatically when the cover is fitted to the cell.

Opening and closing the cell and electrode changing therefore need nothing more than operation of a few simple screws.

A cell as shown in the drawings, with electrodes of the sizes mentioned above, was used to treat a solution initially containing 360 ppm of gold. The cell voltage was substantially constant at 4.0 to 4.26 volts and the cell current was substantially constant at 4.45 to 5.05 amps. Eight hours treatment reduced the gold concentration to 28 ppm. Afurther eight hours led to a gold concentration of 2.4 ppm, and a third treatment of eight hours reduced the gold concentration to 0.55 ppm. The pH increased progressively from 6.45 to 6.67. The increase tends to discourage formation of hydrogen cyanide.

For recovery of gold, the cathode may be a stainless steel cathode, or a disposable brass cathode. The cathode deposit can easily be measured by weighing. Furthermore it can be made easily visible, by the use of transparent plastics as the cell cover.

The cell may be provided with means for dividing the electrolyte space into catholyte and anolyte regions divided by a porous membrane of the type used as battery separators. This membrane may be removable. Typical materials include polyolefines or ion-separating membrane materials e.g. those of DuPont.

The inclusion of such a membrane enables the cell ta be used for electrolysis of materials which give rise to an unwanted redox reaction, for example the electrolysis of photographic bleach fix, containing ferricyanide, or the electrolysis of silver in a solution of ferric nitrate. In both cases, an unwanted redox reaction arises, short-circuiting the cell current and swamping the intended metal recovery reaction at the cathode. The incorporation of a porous ionseparating membrane between the anode and cathode, with a liquid-tight seal, can prevent the unwanted passage of the positive and negative ions between the anode and cathode.

The modified cell will now be described with reference to the accompanying drawings, in which Figures 1a and 2a correspond to Figures 1 and 2 but contain added features.

In the illustrated cell a cylindrical porous membrane (DuPont) 10 with ion-separating properties is placed between the anode and cathode and a liquid-tight seal is provided by a plastics disc 9 across the top of the anode and the membrane.

The anolyte chamber within the membrane communicates with inlet and outlet pipes 11, 12 to allow external pumping for purging the chamber of evolved gases.

Claims (7)

1. A cell for the recovery of gold and other metals from solution, comprising generally coaxial electrodes with a relatively narrow gap between them, means for passing a metal-containing solution through the said gap in a generally spiral or helical path, and means for inducing turbulence in the solution in the said path.

2. A cell according to claim 1, comprising a container, an upright electrode or electrode array mounted in the container, a top cover for the container, and a second electrode or electrode array attached to the cover and arranged so as to be spaced from the first electrode or electrode array when the cover is in position.

3. A cell according to claim 1 or claim 2, in which the inner electrode or electrode array is the anode in operation.

4. A cell according to claim 3, in which the anode has a rough surface so as to induce turbulence in the electrolyte.

5. A cell according to any of claims 1 to 4, in which the gap between the electrodes is less than 20% of the diameter of the inner electrode.

6. A cell according to any of claims 1 to 5, comprising means dividing the electrolyte space between the electrodes into anolyte and catholyte regions separated by a porous membrane.

7. A cell according to any of claims 1 to 5, substantially as described with reference to any of the accompanying drawings.