INTRODUCTION

This invention concerns an electrochemical system and process and more particularly a system and process which will enable many electrochemical processes to dissolve and recover metals from their natural or artificial compounds at a commercial scale and on a continuous basis.

DISCUSSION OF PRIOR ART

Pyrometallurgical processes such as the KIVCET process and the ISASMELT process and hydrometallurgical processes such as the solvent extraction process; the Arbiter process using ammonia; the EIMCO Electro Slurry Process and the DEXTEC Copper Process among others were introduced in the last three or so decades for the extraction of metals from their ores. Most of these processes require considerable inputs of energy and produce considerable waste products or are limited in their commercial application.

The most appealing in terms pf environmental effect, low cost and good recoveries and quality of product are electrolytic processes. These were the claims for the DEXTEC copper process which was developed in Australia. In this process the electrochemical dissolution and precipitation of the copper is carried out in a single cell where the dissolution of the fine sulfide copper ore using air and electricity in the anode section is separated by a diaphragm from the cathode section where copper is deposited by electrolysis. On a small scale the DEXTEC process produces copper metal in simple steps at low energy and reagent cost with minimum environmental effect. It appears, however, that the DEXTEC process is not suitable for commercial scale production due to the following difficulties.

1. The rate of the reaction appears to be controlled by the rate of travel of the copper ions through the diaphragm bag and is probably too slow for commercial application.

2. Problems of maintaining the small openings in the diaphragm bag

from being blocked up by small ore particles or by scale build-up.

3. Difficulty in maintaining the shape of and supporting the diaphragm bag.

4. Impurities dissolved at the anode section could not be removed and are precipitated with the copper at the cathode.

The electrical chemical system proposed in this invention overcomes the shortcomings of the conventional diaphragm cell described above and allow electrochemical reaction similar to the DEXTEC process to be carried out on a continuous commercial scale.

SUMMARY OF THE INVENTION

In one form therefore the invention is said to reside in an electrochemical metal recovery cell comprising an anode tank for containing a slurry of a metal ore and having an anode immersed therein, a cathode tank for containing a catholyte and having a cathode immersed therein, electrical connection between the cathode tank and the anode tank, means to supply an electrical current between the cathode and the anode, means to withdraw reacted slurry from the anode tank and to transfer it into a liquid solid separation chamber and means to transfer the liquid portion of the reacted slurry in the liquid solid separation chamber to the cathode tank as the catholyte.

It will be seen that by this device there is provided an arrangement where between the anode tank and the cathode tank the reacted slurry is withdrawn, has a liquid solid separation operation carried out on it and the liquid portion from the separation stage is then returned to the cathode tank for deposition of the metal. By this separation stage much of the problems of transfer of impurities into the cathode tank are removed and there will be no problems with blocking of a diaphragm between the anode tank and the cathode tank.

There may be further provided one or several purification stages to purify the separated liquid before it is transferred to the cathode tank. The purification may comprise solvent extraction, hydrogen sulphide precipitation, carbonation, cementation or other known purification processes. The liquid portion may also have solution conditions such as temperature and pH

adjusted and have additives added to improve the subsequent electrolysis.

The metal recovery cell may further include a reaction container for withdrawn reacted slurry before the liquid solid separation stage so as to allow for complete reaction to the slurry before separation. The reaction section is optional only as some anode reactions may be fast or the liquid solid separation stage may provide sufficient time for the anode reactions to be completed.

The electrical connection between the cathode tank and the anode tank may be provided by a porous diaphragm comprising at least part of a common wall between the two tanks or it may comprise electrical conductors immersed into each tank and electrically connected outside the tanks. Such electrical conductors may be graphite or carbon rods or be a common wall made of carbon or graphite.

The anode may be provided by a plurality of carbon rods to provide sufficient surface area for the anode reaction to occur.

In a preferred form the cathode may be comprised of pure metal electrodes of the metal to be recovered.

There may be provided means to provide air sparging into the slurry to provide agitation of the slurry within the anode tank and to provide oxidation conditions. The air may be heated to provide and maintain a suitable reaction temperature in the anode tank.

In an alternate form the invention may be said to reside in a process for electrochemical recovery of a metal from its ore comprising the steps of reacting a slurry of the ore in an anode tank including an anode of an electrochemical cell, withdrawing leached slurry from the anode tank and separating the liquid portion from the solid residue of the leached slurry and passing the liquid portion to a cathode tank including a cathode of the electrochemical cell and providing electrical current between the anode and the cathode to effect a depositing of the metal at the cathode.

In a preferred embodiment there may be further included a step of introducing air in the form of bubbles at a bottom of the anode tank to provide agitation of

the slurry and to provide oxidation conditions in the anode tank. The air may be heated to provide heating for the reaction to a desired temperature.

There may be included the further step of allowing reaction to take place in a reaction container before the liquid solid separation after removal of the leached slurry from the anode tank.

There may be further Included the step of purification of the liquid portion before transferring it to the cathode tank.

The slurry in the anode tank may include a halite - acid or other suitable electrolyte solution. The spent liquor from the cathode tank may be used as make-up solution for making up further slurry before supplying the slurry to the anode tank.

The anode may be comprised of a plurality of carbon or graphite rods and the cathode may be comprised of a pure metal electrode of the same metal as that to be recovered in the electrochemical cell.

It will be seen that the main feature of the electrochemical cell of this invention is a system where the metals are dissolved in an anode section fitted with inert electrodes such as graphite electrodes and containing a slurry of the fine metal compound. The electrolyte or anolyte is agitated by the addition of air such as hot air from the bottom of the anode tank.

Products from the anode section are continually treated in a liquid solid separation stage and preferably the separated liquid is purified before it is returned to the cathode section of the electrochemical cell of this invention where the principle metal is electrolytically deposited.

Metals which may be recovered by the electrochemical process of the present invention include copper, nickel, cobalt, lead, zinc, iron, chromium, aluminium, titanium, gold, silver, manganese and other metals with similar electrical propertied from their compounds or ores.

An important feature of the present invention is that both anode and cathode reactions are happening at the same time but are separated by diaphragm or electrical conductor wall. Unlike the prior art diaphragm cell, the diaphragm in

the cell of the present invention is used only to prevent the anolyte from mixing with the catholyte and does not require that metal ions migrate across the diaphragm for the cathode reaction to occur. Problems of blockage of the diaphragm by solids can be eliminated by maintaining a slight hydraulic head in the cathode tank over the anode tank.

BRIEF DESCRIPTION OF THE DRAWINGS

This then generally describes the invention but to assist with understanding reference will be made to the accompanying drawings which show preferred embodiments of apparatus according to the invention and an example.

In the drawings:

FIG. 1. shows a first embodiment of electrochemical cell according to this invention,

FIG.2 shows an alternative embodiment electrochemical cell according to this invention, and

FIG. 3 shows a commercial scale electrochemical process according to this invention including multi-stage processing.

DETAILED DESCRIPTION OF THE DRAWINGS

Now looking more closely at the drawings to be seen in particular in relation to FIG 1 that the electrochemical cell according to this system includes an anode tank 1 having anodes 2 therein and a cathode tank 3 having cathodes 4 therein. The anode tank 1 includes the supply of air 5 at its bottom end and appropriate porous material or sparging nozzles 6 to allow bubbles of air to pass through the slurry in the anode tank. Reacted slurry is drawn out through line 7 to an optional reaction container 8. After sufficient reaction time the slurry is passed through line 9 to a liquid solid separation stage 10. In this stage a solid leach residue 11 is produced and a liquid portion 12 is also produced. The liquid portion is transferred through line 12 to a solution purification stage 13. Purified solution is passed through line 14 to the cathode tank 3 and metal is deposited at the cathode 4 and the lean solution is withdrawn through line 15. A DC power source 16 is used to provide power

to the anode and the cathode. The wall 17 between the cathode tank and anode tank is either porous to provide solution contact between the cathode tank and the anode tank or electrically conductive to allow solution contact between the anode tank and the cathode tank.

It may be noted that the metal product produced in the cathode tank may be in a plate form deposited in the cathodes if a low current density is used or in a powder form if a high current density is used in the cathode.

Looking at FIG 2 it will be seen that an alternative embodiment of electrochemical cells provided for which there are two anode sections either side of a cathode section. Although in this embodiment flow of anolyte is shown to be countercurrent to the flow of catholyte the flow may be either co- current or countercurrent.

Slurry is prepared in slurry preparation stage 25 and passed into the two anode sections 20 and 21 including anodes 30. Reacted slurry is passed into an optional reaction section 26 and after sufficient reaction time passed to a liquid solid separation stage 27. Liquid from the liquid solid separation is passed to a solution purification stage 28 which may include adjustment of other properties such as pH and the pure rich liquid is passed into the cathode section 22. After depositing of the metal in the cathode section either onto cathode 29 or as a powder to the bottom of the cathode cell, lean liquid is withdrawn through line 32 to the slurry preparation stage for reuse. An electrically conductive wall 23 is provided either side of the cathode section 22 to provide solution contact between the anode tank 20 and the cathode tank 22. The electrical power to the anode and cathode is provided by power supply 31.

FIG 3 shows an alternative embodiment of electrochemical system according to this invention in which a three stage process is used.

The slurry is prepared is prepared in slurry preparation stage 40 by adding finely ground metal ore 59, acid and reagents 60 and liquid from the solution storage tank 56 and is then passed into a first anode section 41. After reacting the leached slurry is passed to first thickener 42 from which liquid is passed to a solution purification stage 43 before being returned to the cathode section of the first stage 44. Lean liquid from the cathode 44 and thickened slurry from

the thickener 42 is passed to mixing stage 45 which may include addition of acid and reagents before being passed to the anode section 46 of a second stage.

Once again leached slurry is passed to a thickener 47. Liquid from the thickener 47 is passed through a solution purification stage 48 and into the second stage cathode section 49. Lean solution from the cathode section 49 is mixed with thickened slurry from the thickener 47 in mixer 50 with any required acid or reagents and this mixed slurry is passed into the anode section 51 of the third stage.

Leached slurry of the third stage is passed to thickener 52 and liquid from this stage is passed through solution purification 53 before being passed to the cathode section of the third stage 54. The lean solution from the cathode section 54 is passed by means of line 55 to solution storage 56 and subsequently use for new slurry preparation in slurry preparation stage 40. Slurry underflow from the thickener 52 is washed in wash stage 58 and the residue discarded. The wash liquid may require some evaporation on evaporation stage 62 to remove some water to maintain process water balance before it is transferred solution storage for further use.

A certain amount of spent liquor may be discarded to prevent build up of undesirable salts.

It will be noted that metal product is produced from the cathode section of the first, second and third stages and this multistage system may be used to obtain a better recovery of a single metal or to separate the extraction of several metals.

EXAMPLE

A mixture of fine copper ore and anolyte which contains near saturated halite, about 12 grams per litre of copper, and sulfuric acid to keep the pH at about 2 to 2.5 is introduced into the anode section of an electrochemical cell according to this invention where graphite electrodes are immersed. Hot air is introduced through a disperser at the bottom of the anode section to provide agitation for the slurry, oxygen for the oxidising reactions at the anode, and heat to maintain the slurry temperature at 85 to 95 degrees Centigrade.

A low voltage at a low current density is applied to the graphite anodes where copper and other metals dissolve through the removal of electrons. Dissolved iron is converted to iron oxide precipitate and the sulfur remains as elemental sulfur.

The electro-leached slurry containing the dissolved metals is transferred to a reaction section to allow the oxidising reactions from the anode section to be completed to avoid interference in the cathode reactions. This reaction section is optional as some anode reactions may be fast enough and also, the liquid solid separation step may allow the anode reactions to be completed.

The leach residues are separated from the anolyte solution in the liquid solid separation step which may consist of thickening ahead of filtering and washing, or counter-current decantation with washing. The washings may require multi-stage evaporation before returning to the circuit to maintain the process water balance. The solids may go to waste or to further valuable metal or sulphur recovery.

The leach liquor may then go to solution purification for the removal of impurities such as silver, zinc, iron etc., if these interfere with the required quality of the copper metal deposit. It is also possible that solvent extraction is applied to remove the impurities, or to collect the copper from the impurities and the stripped solution containing the copper is then transferred to the cathode section for the electrolytic recovery of the copper.

In the normal case the purified copper solution is fed to the cathode section of electrochemical cell according to this invention where the copper is deposited on copper electrodes through the addition of electrons. Some reagents may be added to improve the purity of the copper deposited or to prevent problems such as growth of dendrites. The copper may be collected as a powder from the bottom of the cathode section if high current densities are used, and as sheets of copper on copper starter sheets if low current densities are used. Steam may be injected into the cathode section to provide heating and agitation. The hydraulic gradient in the cathode section is kept just above that in the anode section to give a minimal flow through the diaphragm from the cathode section to the anode section to prevent blinding of the diaphragm.

The diaphragm between the anode section and the cathode section is used to

maintain solution contact between the anode section and the cathode section but to prevent the mixing of the anolyte and the catholyte. In one embodiment graphite rods may be used to provide the solution contact between the anode section and the cathode section but under certain additions scale deposits on the graphite rods may break the contact. A diaphragm or a conductive material such as graphite may provide the solution contact between the anode section and the cathode section.

The lean solution discharged from the cathode section may be transferred to slurry feed preparation for the anode section or to a solution storage tank. Part of this lean solution may need to be discarded or treated to prevent the build-up of unwanted salts.