WO2004022796A1

WO2004022796A1 - Process and apparatus for recovery of cyanide and metals

Info

Publication number: WO2004022796A1
Application number: PCT/AU2003/001171
Authority: WO
Inventors: Peter Douglas Macintosh
Original assignee: Occtech Limited
Priority date: 2002-09-09
Filing date: 2003-09-08
Publication date: 2004-03-18
Also published as: AU2002951283A0

Abstract

Disclosed is a process for recovering components from a liquor containing a complex or compound electrochemically dissociable to form a cation (C) and an oxidisable species for recovery including: (a) delivering the liquor containing said complex or compound to an electrochemical cell having an anode compartment containing an anode and a cathode compartment containing a cathode, the compartments being separated by a boundary layer or porous diaphragm; (b) electrochemically dissociating the compound or complex; (c) directing a controlled portion of the liquor through the anode compartment and a controlled portion of the liquor through the cathode compartment; wherein said oxidisable species is recovered from a catholyte. The process allows for recovery, for example, of cyanide from weakly acid dissociable (WAD) metal cyanide complexes such as copper WAD complexes.

Description

PROCESS AND APPARATUS FOR RECOVERY OF CYANIDE AND METALS FIELD OF THE INVENTION

The invention relates to processes and apparatus for the recovery of oxidisable species, such as cyanide, and/or cations such as metals from solutions containing these in compound or complex form. The processes and apparatus may involve electrochemical and membrane processes which prevent or minimize oxidation of chemical species in the feed liquor. BACKGROUND OF THE INVENTION

Cyanide is used extensively in gold processing plants for the leaching of gold from a milled ore containing it. Certain ores, concentrates and oxidation residues also contain copper, zinc and/or other base or precious metals that, like gold and silver, form complexes with cyanide, chloride or the like. As a result, mixtures of dissolved complexes, including Cu(CN)₂ ^", Cu(CN)₃ ^2" and Cu(CN)₄ ^3" may also be formed in a gold cyanide leach solution. Such complexes may be stable, weak acid dissociable (or WAD) complexes.

When this occurs, large excesses of cyanide may be required to ensure that sufficient "free" cyanide is present to leach the gold. This results in high cyanide consumption, and loss of process efficiency which is costly. Moreover, the complexed species are not useful for the further leaching of gold, and cannot be readily recycled to the leach. In certain situations, such complexes cannot be discharged to a tailings dam or alternative disposal system due to prohibitions under environmental regulations. Useful metals may also be lost in the form of copper containing streams that are disposed of, perhaps following gold recovery from a pregnant liquor solution from the copper/gold ore leach. As cyanide is a potent toxic contaminant in waste streams, particularly those associated with gold processing operations, technology has been developed for managing cyanide. Destruction and recovery strategies for cyanide and other chemical species may be employed. An electrolytic reactor for destruction of chemical species, such as cyanide, has been described in US Patent No. 4585539.

Oxidation may be used to destroy cyanide species known as weak acid dissociable (WAD) cyanide from waste waters. Such oxidation techniques commonly use of an oxidant to destroy cyanide. Hydrogen peroxide has been widely used for this purpose with WAD cyanide being oxidised to cyanate. Photooxidation and electrochemical destruction are other techniques that may be employed.

Recovery techniques which involve chemical processing of cyanide streams to recover costly reagent for re-use are also known. Options may involve ion exchange or acidification-volatilisation-regeneration (AVR) processes. An AVR treatment circuit involves acidification of cyanide liquors or slurries to lower the pH from an alkaline range to pH 3-5 resulting in conversion of free cyanide and weak complexes such as zinc, cadmium and nickel complexes to HCN which is then volatilised by passing air bubbles through the liquor or pulp. An air/HCN stream may then be scrubbed in a lime slurry or NaOH (or like alkali metal hydroxide) solution to neutralise the HCN and recover sodium cyanide, a source of free cyanide that can be recycled to a leach. AVR processes for free cyanide recovery are described, for example, in US Patent Nos. 5961833 and 6335175. While the AVR process is an option for treating moderate or strong cyanide liquors (over 500ppm weak acid dissociable cyanide), copper is precipitated as CuCN during the acidification stage. Such product is unsaleable and retains up to a third of the available cyanide complexed with the copper, lowering the possible cyanide recovery. The Cyanisorb process is similar to the AVR process above but differs in that clear solutions or slurries are processed at near neutral pH.

The MNR process was developed by Metallgesellschaft Natural Resources and involves sulphidisation using NaSH and acidification, to pH 5 or less, of copper/cyanide rich liquors to precipitate synthetic chalcocite (Cu₂S). The liquor may then be re-causticised to produce sodium cyanide or acidified further to form HCN which may be recovered by absorption or the methods described above with regard to AVR routes. The copper by-product is saleable. Nevertheless, material handling of the chalocite and potential co- precipitation of CuCN and CuSCN may be issues in plant design. Solvent extraction and ion exchange techniques are also possible alternatives for treatment of liquors containing free cyanide.

In International Publication WO 01/62993, the contents of which are hereby incorporated herein by reference, the present Applicant provides a process for recovery of cyanide and metals involving electrochemical and membrane processing steps. In the electrochemical step, a metal cyanide complex, such as a copper WAD cyanide complex, was electrochemically dissociated in an electrochemical cell to allow metal recovery. The spent electrolyte was then subjected to a membrane processing step for recovery of free cyanide. Such recovered free cyanide could advantageously be recycled to a leach process.

In WO 01/62993, there is also described an electrowinning unit of filter press type in which a number of electrochemical cells are held together in a press. The flow circulating means of the unit circulated a high velocity turbulent flow of electrolyte through each cell. The Applicant found that cyanide recovery using such an electrochemical cell had certain disadvantages.

In US 5,411 ,575, there is disclosed an electrochemical cell for recovery of copper metal in which anode(s) are kept isolated from the catholyte by a cation exchange membrane, preferably a Nation® membrane, to prevent anodic oxidation of cyanide ions.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide a process which allows recovery of cations, such as metal ions, and cyanide, or other oxidisable species which may - for example - be used in hydrometallurgical processes. Such a process addresses, at least to some extent, the disadvantages of the prior art.

With this object in view, the present invention provides a process for recovering chemical components from a feed liquor containing a complex or compound electrochemically dissociable to form a cation (C) and an oxidisable species for recovery comprising:

(a) delivering the feed liquor containing said complex or compound to an electrochemical cell having an anode compartment containing an anode and a cathode compartment containing a cathode, the compartments being separated by a boundary layer or porous diaphragm;

(b) electrochemically dissociating the compound or complex; (c) directing a controlled portion of the feed liquor through the anode compartment and a controlled portion of the feed liquor through the cathode compartment; wherein said oxidisable species is recovered from a catholyte. The catholyte may be passed through a membrane treatment step for said recovering said oxidisable species.

The oxidisable species may be hydrometallurgically active and useful for hydrometallurgical extraction or other hydrometallurgical unit operations. It may be complexed or bonded to a metal in an advantageous embodiment. The oxidisable species of most interest is cyanide, such being a common leaching agent used in leaching of precious metal containing ores. In that case, the liquor is a solution that may contain copper cyanide (with cyanide being cation (C)) or other base metal cyanide complexes. Such solutions include process streams throughout hydrometallurgical plants as well as tailings liquors, including tailings streams prior to tailings dam or in recovered water from a tailings dam These solutions may contain weakly acid dissociable complexes such as copper WAD complexes, electrochemical dissociation of which deposits copper liberating a portion of the WAD cyanide and free cyanide. Generally, the process is suitable for treatment of any aqueous solution for recovery of cyanide and metals from complexes of these contained in the solution. High salt content solutions or liquors typical in Western Australian gold circuits, may be treated by the process. Cyanide is recovered from a metal depleted catholyte following recovery of metal at the cathode.

The oxidisable species could be any lixiant, solvent or other component that associates with a metal or cation (C). Cation (C) may be non-metallic, for example, ammonium ion. The oxidisable species could be molecule(s) of a resin which could be recovered for re-use.

All feed solution containing the compound or complex of the cation, for example, metal and oxidisable species may, and advantageously is, fed into the cathode compartment of the electrochemical cell. The feed flow is advantageously not a turbulent flow. Turbulent flow is desirably avoided. The boundary layer or porous diaphragm may be a polymer, ceramic, fabric or textile such as a filter cloth. The diaphragm may be of any desired thickness. It may be permeable or semi-permeable to the oxidisable species.

Partitioning of feed liquor into controlled portions of liquor directed to the anode and cathode compartments is achieved by suitable flow control means controlling flow through the anode and cathode compartments, for example by pressure difference between them fix volume pump or flow control devices. A pressure difference may be induced across the porous diaphragm to prevent mixing between the anode and cathode compartments on filling of the electrochemical cell. However, such a pressure difference allows only a small flow (0 to 20%, for example 1%) of the feed solution to travel to the anode where some anodic oxidation and loss of the oxidisable species, such as cyanide, may occur. Oxidation reaction products favoured at the anode are chlorine, hypochlorite, chlorates and the like. The reactions at the anode may produce some low pH residual oxidants which must be disposed of or used elsewhere. This flow may be regarded as a sacrificial or bleed flow which forms the minor portion of the feed flow. The bleed or sacrificial flow is controlled to typically represent between 0 and 20%, preferably less than 10% and more preferably less than 5%, and still more preferably less than 1%, of the volume of feed liquor delivered to the electrochemical cell in step (a) flow. The sacrificial or bleed flow is removed from the anode compartment through one or more anolyte outlets.

This bleed or sacrificial flow may also enable prevention of build up of impurity components in the solution, such impurity components including components such as calcium and iron which might be undesirably concentrated in subsequent membrane treatment steps.

A solution may also be circulated through the anode compartment to reduce or supplement the bleed or sacrificial flow to the anode compartment. This may be described herein as an anolyte solution or stream. If there is no or negligible bleed or sacrificial flow across the porous diaphragm such a solution is advantageously circulated through the anode compartment. Such a solution may, for example, be a sodium chloride solution or any alkali or brine or other species independent of the feed liquor that can be oxidised at the anode. The supplemental solution or stream may consist of salt brine, which may be sodium chloride or alkali metal chloride, and may contain sodium hydroxide, or other alkali or alkaline metal hydroxides, so as to provide an alternative oxidisable anolyte and alkalinity for pH control.

The porous diaphragm may have structure to prevent back diffusion of the oxidising hypochlorite or other oxidising species present in the anode compartment to the cathode compartment. Oxidation of the sensitive oxidisable species in the cathode compartment may thus be minimised.

The flow control means, or anolyte solution flow, if an anolyte solution is employed, may require to be adjusted as pressure difference across the porous diaphragm may vary as operation of the electrochemical cell proceeds with time.

The anode and cathode compartments are provided with inlets, outlets and each compartment accommodates a volume of solution. Multiple inlets and outlets may be employed. The compartments advantageously are void, not including any packing material. The anode and cathode may be of any desired form. The anode is desirably a dimensionally stable anode and the cathode is preferably of large surface area and multiple cathodes may be used to achieve this. Both anode and cathode are connected to a source of current. The current may be varied as a function of the metakcyanide ratio, particularly copper:cyanide ratio (which may also affect membrane processing) and general conductivity of the solution and electrode materials in the electrochemical cell. This ratio may also be time variant.

The feed stream to the electrowinning cell(s) may be concentrated by membrane treatment or other steps prior to delivery to the electrochemical cell(s). Such pre-concentration will reduce the hydraulic loading to electrowinning cell(s). The degree of concentration achievable is limited by the degree of saturation with respect to sparingly soluble salts such as calcium sulphate (gypsum) and whether or not scale inhibiting additives or pre-softening of the liquor is employed prior to entry to the cell(s). Solids may be removed first from the liquor by decantation, such as counter-current decantation, or filtration. Microfiltration (MF) or multi- media sand (MMS) filters may be used to reduce total suspended solids depending on solution quality. Barren liquor from a CIP/CIL process may be washed to remove free and WAD cyanide from solids, for example, by a counter- current decanter (CCD) circuit. Feed may be delivered to a number of electrochemical cells arranged in parallel or in series. Two cells may be arranged in a bank with two anodes in the middle of the bank with cathodes on opposed sides of the anodes. As many cells, as desired, may be employed in the electrochemical step. Metal depleted catholyte is removed from the cathode compartment by a catholyte outlet, one or more of which may be provided in the cathode compartment. The catholyte contains the oxidisable species such as free cyanide (these terms being used interchangeably in the following discussion) in controlled amount as the sacrificial or bleed flow to the anode compartment is controlled such that only a minor amount of cyanide in the feed stream is anodically oxidised under desired operating conditions. The catholyte containing the major amount of free cyanide may have an increased level of free cyanide relative to the feed liquor. A membrane treatment step external to the electrochemical cell is advantageous and suitable for recovering and/or concentrating cyanide from the catholyte. Such membrane treatment step may be conducted in a recirculation flow to the electrochemical cell but free cyanide recovery is possible on a "once through" basis without recirculation. Spent electrolyte not for direct recirculation to the electrochemical cell may be subjected to the membrane treatment step for the recovery of the oxidisable species. Prior membrane concentration or other concentration process of a free cyanide and bound cyanide containing stream to remove excess water may be conducted as a preliminary or pre-treatment stage.

The membrane process may be pressure or diffusion driven, being characterised as a process of ultrafiltration, nanofiltration or reverse osmosis, differences between which are described in Lein, L, "Nanofiltration: Trend of the Future!", Water Conditioning and Purification, September 1992, pp 24 to 27 and Cheryan,M et al, "Consider Nanofiltration for Membrane Separations", Chemical Engineering Progress, March 1994, pp 68 to 74, both of which are incorporated herein by reference. These references further describe specific characteristics of membranes for use in these processes though many membranes are proprietary in nature. Nanofiltration may be the key membrane treatment step employed. However, the other membrane processes may be adopted in accordance with processes of the invention. Such processes, as nanofiltration, ultrafiltration or reverse osmosis may further be applied to the step of concentrating the recovered oxidisable species stream to enable re-use if desired, as described in the Applicant's PCT Application No. WO 01/62993. In the case of cyanide, for example, recovered cyanide stream or permeate may contain a low concentration of cyanide in comparison to that required in a hydrometallurgical process, such as a leach process. Concentration or reconstitution or make up of recovered permeate with fresh material may therefore be a pre-requisite to recycle.

Concentration of free cyanide in the free cyanide containing stream or permeate may be upgraded by further membrane modules in one or more steps in which the cyanide is selectively retained by membranes of decreasing pore size or increasing "tightness". A number of stages may be required to sufficiently concentrate free cyanide for re-use in a leaching process.

Pre-treatment of liquors, such as tailings liquors, prior to processing to recover free cyanide, may be required involving steps such as pre-softening or organic removal. The pre-treatment to be employed may be developed following chemical and physical evaluation of the liquor. Many tailings liquors, for example, require such pre-treatment.

In another aspect, the present invention provides apparatus for conducting the above described process. An electrochemical cell for use in the process of the invention forms another aspect of the invention.

In one such embodiment, the invention provides an electrochemical cell for electrochemically dissociating a complex or compound to a cation (C) and an oxidisable species to be recovered comprising an anode compartment containing an anode and a cathode compartment containing a cathode, the compartments being separated by a boundary layer or porous diaphragm permeable or semi- permeable to said oxidisable species and wherein flow control means are included for directing, in use, a controlled portion of a feed liquor to the cell to the anode compartment of the cell and a controlled portion of the feed liquor to the cell to the cathode compartment of the cell for limiting oxidation at the anode of the oxidisable species and enabling a major portion of said oxidisable species to be recovered from a catholyte stream. The portion of feed directed to the anode compartment may be a minor portion of the feed liquor. If provision is made for a supplemental stream or solution to be directed to the anode compartment, this portion may be zero. The major portion or all of the feed liquor is then directed to the cathode compartment. Flow of the minor and major portions of the feed liquor within the electrochemical cell is advantageously and desirably non-turbulent, particularly adjacent the porous diaphragm.

The process and apparatus may be conveniently and advantageously used for recovery of hydrometallurgically active oxidisable species such as cyanide or chloride, from solutions containing complexes of the oxidisable species and metals. Precious or base metal leaching processes may produce such solutions. Additionally, the process and apparatus provides a means for removing impurities from solutions which might otherwise build up possibly deleteriously affecting a metallurgical leach process BRIEF DESCRIPTION OF THE DRAWINGS

The process and apparatus of the present invention may be more fully understood from the following description of preferred embodiments thereof made with reference to the accompanying drawings in which:

Figure 1 is a flowsheet showing the cyanide and metal recovery process and apparatus of one embodiment of the present invention in schematic form;

Figure 2 is a detailed schematic of the electrochemical cell employed in the process and apparatus of the embodiment of the invention shown in Figure 1 ; and

Figure 3 is a schematic flowsheet for a process for concentrating cyanide in accordance with the concentration step of Figure 1. DETAILED DESCRIPTION OF THE DRAWINGS Referring now to Figure 1 , there is shown a flowsheet for conducting a hydrometallurgical process in which cyanide and copper metal are recovered from an alkaline aqueous feed liquor stream 110 containing copper cyanide complexes including Cu(CN)₂ ^", Cu(CN)₃ ^2" and Cu(CN)₄ ^3" complexes or weakly acid dissociable copper complexes (copper WAD complexes). Cyanide may complex with copper up to around 4:1 free cyanide to copper mole ratio. Total cyanide, in excess of that equating with a CN:Cu mole ratio of 3.5:1 may be considered free. Feed stream 110 may contain more or less than 1 ,000 mg/L [CN1 total. Stream 110 may be a liquor generated during the leaching of a copper gold ore. More specifically, stream 110 may be a gold leach tail solution following adsorption of gold onto activated carbon or an ion exchange resin from a pregnant liquor from the leach plant and recovered from a CCD circuit. Stream 110 may also be a stream following cold stripping of copper from activated carbon or ion exchange resin. Stream 110 may be pretreated, such as by softening or organic removal as necessary. Broadly, a stream containing complexes of base metal and cyanide may be treated in the process and apparatus in accordance with the present invention. Stream 110 is delivered to electrochemical cell 120 following pre-filtration for removal of suspended solids, if desired. Use of MF or MMS filters may reduce total suspended solids from 100 mg/L, for example for return water from a tailings dam, and significantly lower, less than 50% mg/L. A reverse osmosis or membrane treatment step may be conducted for removal of excess water thereby concentrating the solution and reducing the hydraulic loading to cell 120. Degree of concentration achievable may be limited by the degree of saturation with respect to sparingly soluble salts such as calcium sulphate (gypsum) and whether or not scale inhibiting additives or pre-softening are used in a pretreatment step. Referring to Figure 2, cell 120 includes a cathode compartment 121 containing a cathode 123 and an anode compartment 126 containing an anode 129. Cathode compartment 121 is separated from anode compartment 126 by a porous diaphragm 128. A number of cells 120 could be employed arranged in bank(s) and one or two cells could be arranged to have two anodes in the middle and cathodes on opposed sides of the anodes. The cathode 123 may be any desired form of cathode, such as flat plate, mesh and so on. It is desirable that the cathode 123 have surface area maximised to recover metal from a potentially dilute solution of the copper cyanide complex. The cathode may be of porous or foam kind. It may be of reticulated metal foam. Two or more cathodes could be used within a single electrochemical cell. In the embodiment shown, there is provided a three dimensional cathode comprising a bundle of steel wool for maximising surface area for recovery of copper metal. The anode 129 may also be any desired form of anode. Preferred is a dimensionally stable anode, for example, one having a titanium base with an oxide coating. The oxide coating may be of ruthenium oxide or iridium oxide. Anode 129 is stable under the solution conditions in anode compartment 126. Cathode 123 and anode 129 are electrically connected together and to a current source (not shown). The current source may be a DC current source. Capacity for reverse polarity of the electrodes is not required.

Porous diaphragm 128 is desirably of a fabric, textile or cloth which may be suitable as a filter cloth. Solution passes through the porous diaphragm 128 and used to limit the migration of cyanide to the anode compartment 126 where anodic oxidation would otherwise occur.

Cathode compartment 121 has a solution inlet 122 and a catholyte outlet 124. Solution inlet 122 allows feed stream 110 to enter the electrochemical cell 120. Solution inlet 122 may comprise a number of inlet ports configured to minimise turbulence in the solution adjacent porous diaphragm 128. Catholyte outlet(s) 124 allow catholyte, depleted of copper due to copper deposition on cathode 123 during electrowinning, to pass to the next stage of the process for recovery of free cyanide or on to a subsequent cell similar to cell 120. This catholyte may be called a copper depleted catholyte. In a pilot run, the concentration of copper of the catholyte decreased by 30%. Free cyanide concentration in the same catholyte increased by 120%.

Anode compartment 126 is provided with outlet(s) 127 from which components oxidised in the anode compartment 126 during operation of the electrochemical cell 120 may be removed as stream 170. This stream may be used as part of the detox of CCD bottom solids forwarded out to tailings. Anode compartment 126 may also be provided with one or more inlets to enable an independent anolyte solution, such as a sodium chloride solution, to be circulated through it, if desired, as shown by dashed flow line 126a. This is done if a bleedflow through porous membrane 128 is at or near zero. It may also be done to supplement bleed flow across porous diaphragm 128.

The inlets and outlets to the anode and cathode compartments 121 and 126 of electrochemical cell 120 may be fitted with flow control means, such as valves, orifices, weirs or the like, to control flow of feed solution, anolyte and catholyte through the cell 120. These flow control means may be set to create a pressure difference across porous diaphragm 128 to enable a minor controlled or regulated portion of the feed solution to enter the anode compartment 126 from the cathode compartment 121 as a bleed or sacrificial flow during steady state flow of solution to electrochemical cell 120. The major portion or bulk of the feed solution flow passes through the cathode compartment 121.

On start up, all of feed stream 110 enters the cathode compartment 121 of the electrochemical cell 120.

As diaphragm 128 is porous, solution also flows through diaphragm 128 to fill the anode compartment 126 subject to flow resistance across porous diaphragm 128. Under steady state conditions, with a pressure difference across porous diaphragm 128 set to allow a controlled bleed or sacrificial flow into the anode compartment 126, as above described, the major portion of the solution flow passes only through the cathode compartment 121. A minor portion only of the solution flow passes through anode compartment 126.

The bleed or sacrificial flow represents a minor portion, between 0 and 20%, most preferably less than 1%, of the feed flow to the electrochemical cell 120. The bleed or sacrificial flow contains, in consequence, a minor portion of the cyanide (or other oxidisable species) contained in the feed solution. As oxidising reactions occur at the anode, a minor portion of the cyanide is anodically oxidised to cyanate. If a supplemental sodium chloride stream is employed as an anolyte solution, where the bleed or sacrificial flow across porous diaphragm 128 is at or near zero or otherwise, a hypochlorite solution is produced in the anode compartment 126 and this may be discharged following any required treatment. The pH of the anolyte leaving the anode compartment 126 may be adjusted to prevent discharge of excessively acidic or excessively low pH level streams from electrochemical cell 120. Cyanide reaction by-products production is to be avoided by regulating the solution pH and composition. This may be done using supplemental anode stream 126a. The major portion of the cyanide contained in the feed stream passes through cathode compartment 121 such that copper deposits on cathode 123 and or stream or catholyte is formed containing an increased level of free cyanide, substantially protected from oxidants in the anode compartment 126. The free cyanide reports in a copper depleted catholyte leaving cathode compartment 121 through catholyte outlet(s) 124. The porous diaphragm 128 prevents the major portion of cyanide circulating about the anode 127 and being oxidised other than as described above with reference to the sacrificial or bleed flow. As the sacrificial or bleed flow is controlled, the portion of free cyanide in the copper depleted catholyte is also controlled.

The copper depleted catholyte is directed to a membrane treatment step 140 involving reverse osmosis, ultrafiltration or nanofiltration steps in which free cyanide is caused to permeate through a suitable membrane under imposition of a pressure gradient across the membrane.

The membrane modules for membrane separation of free cyanide may be of spiral, fibre, flat plate, tubular or other known membrane unit type operated at a suitable pressure, for example, in the range of 50 kPa to 8,000 kPa (0.05 to 8 MPa). A number of membrane modules, which may be arranged in stages may be used for this process.

The nanofiltration process is advantageously employed as the membrane treatment step. The nanofiltration membrane allows the free cyanide and most monovalent ions to pass through, while rejecting virtually all of the Cu(WAD) and other divalent metal ions in solution. As a result, permeate streams 150 would contain free cyanide at a similar pH, say about 11 , as feed stream 110. This permeate 150 can be recycled back to the leach plant suitable for use as a leach liquor.

A recirculating flow 155 of part or all of the electrolyte may be maintained through electrochemical cell 120. The reject stream 153 after treatment of electrolyte in membrane treatment step 140 for cyanide recovery may also be recirculated to the electrochemical cell 120 or further treated. Membrane step 140 need not be located in a recirculating stream, recovery is possible on a "once through" basis or using a series of electrochemical cells and membrane processes. Cyanide containing permeate stream 150 may be directed to a further concentration step 160 for upgrading concentration of cyanide for re-use in the gold leaching process. Stream 157 may be recycled to the leaching process. Lean stream 154, lean in copper and cyanide, may be directed to further treatment, reuse or disposal. Figure 3 shows a schematic of a cyanide concentration step 160 which itself involves membrane processes which selectively separate cyanide as retentate (concentrate) or otherwise. Stream 150, containing for example 1 g/L cyanide, is directed first to membrane concentration unit 162 with concentrate stream 152, having concentration 5 g/L cyanide, being directed to further membrane concentration unit 164 to generate concentrate stream 154 of concentration sufficient for recycle to a gold or other leaching process, say 15 g/L concentration free cyanide.

The lean cyanide stream 153 is directed to membrane concentration unit 166 with concentrate stream 175 being recycled to membrane concentration unit 162. The lean cyanide stream 157 containing a low concentration, say 0.01 g/l cyanide, may be directed to disposal or other treatment steps.

Cyanide concentration may therefore involve two concentration steps but other concentration process flowsheets could be developed to perform the same duty. More concentration or separation steps could be employed as necessary to improve metal recovery and the amount of cyanide recoverable, for example, from tailings.

In the embodiment described above, description was made of free cyanide and copper metal recovery from liquors containing complexes of these. However, the process of the invention is also suitable for recovery of cyanide, other hydrometallurgically active oxidisable species, and other base or precious metals from complexes thereof. The process would also be suitable for recovery of cyanide and silver from complexes thereof. The process is further suitable for recovery of free cyanide from other waste water streams. However, the process may be applied to any liquor stream, for example arising in a metallurgical plant. The process may be implemented in the tailings circuit prior to discharge to the tailings pond or in a returned water circuit recovering water from a tailings dam. In the latter application, while less equipment may be required for solids/liquids separation, residence time in the tailings pond may allow some loss of cyanide and reduce the overall recovery efficiencies possible.

Modifications and variations to the process and apparatus of the present invention will be apparent to the skilled reader of this disclosure. Such modifications and variations are within the scope of the invention.

Claims

CLAIMS:

1. A process for recovering chemical components from a feed liquor containing a complex or compound electrochemically dissociable to form a cation (C) and an oxidisable species for recovery comprising: a) delivering the feed liquor containing said complex or compound to an electrochemical cell having an anode compartment containing an anode and a cathode compartment containing a cathode, the compartments being separated by a boundary layer or porous diaphragm; b) electrochemically dissociating the compound or complex; c) directing a controlled portion of the feed liquor through the anode compartment and a controlled portion of the feed liquor through the cathode compartment; wherein said oxidisable species is recovered from said catholyte.

2. The process of claim 1 wherein said feed liquor contains, as said complex or compound, a complex of a base metal and cyanide and a controlled portion of said liquors is directed from said cathode compartment to said anode compartment for anodic oxidisation of a minor amount of said cyanide while a major amount of said cyanide is recovered from said catholyte while base metal deposits at said cathode.

3. The process of claim 1 or 2 wherein all of said feed liquor is delivered to said cathode compartment.

4. The process of any one of claims 1 to 3 wherein a pressure difference is induced across said boundary layer or porous diaphragm.

5. The process of claim 2 wherein said controlled portion of said feed liquor directed to said anode compartment represents less than 5% of the volume of liquor delivered to said electrochemical cell in step (a).

6. The process of claim 5 wherein said controlled portion of said feed liquor directed to said anode compartment represents less than 1% of the volume of feed liquor delivered to said electrochemical cell in step (a).

7. The process of any one of claims 1 to 6 wherein an anolyte solution, different from said feed liquor, is circulated through said anode compartment to control pH and reduce or supplement said controlled portion of feed liquor directed to said anode compartment.

8. The process of claim 7 wherein said anolyte solution contains an alkali or brine.

9. The process of claim 8 wherein said alkali or brine is selected from the group consisting of sodium chloride, alkali metal chloride, sodium hydroxide, alkali metal hydroxide and alkaline metal hydroxide.

10. The process of any one of the preceding claims wherein back diffusion of oxidising species from said anode compartment to said cathode compartment is prevented by said boundary layer or porous diaphragm.

11. The process of any one of the preceding claims wherein said oxidisable species is recovered by a membrane treatment step.

12. The process of claim 1 wherein said cation (C) is non-metallic.

13. The process of claim 1 wherein said controlled portion of feed liquor directed to said anode compartment is varied as operation of the electrochemical cell proceeds with time.

14. The process of an one of claims 7 to 13 wherein flow of anolyte solution through said anode compartment is varied as operation of the electrochemical cell proceeds with time.

15. The process of claim 2 wherein said anode and cathode are connected to a source of current and said current is varied as a function of the base metal to cyanide ratio.

16. The process of claim 15 wherein said current is varied as a function of time.

17. An electrochemical cell when used in the process as claimed in any one of the preceding claims.

18. An electrochemical cell for electrochemically dissociating a complex or compound to a cation (C) and an oxidisable species to be recovered comprising an anode compartment containing an anode and a cathode compartment containing a cathode, the compartments being separated by a boundary layer or porous diaphragm permeable or semi-permeable to said oxidisable species and wherein flow control means are included for directing, in use, a controlled portion of a feed liquor to the cell to the anode compartment of the cell and a controlled portion of the feed liquor to the cell to the cathode compartment of the cell for limiting oxidation, at the anode of the oxidisable species and enabling a major portion of said oxidisable species to be recovered from a catholyte stream.

19. The electrochemical cell of claim 18 wherein said boundary layer or porous diaphragm is permeable or semi-permeable to said oxidisable species.

20. The electrochemical cell of claim 18 wherein said flow control means creates a pressure difference across the boundary layer or porous diaphragm.

21. The electrochemical cell of claim 18 including an inlet through which, in use, an anolyte solution different from said feed liquor is directed to said anode compartment for reducing or supplementing said controlled portion of feed liquor directed to said anode compartment.

22. The electrochemical cell of claim 18 wherein said anode compartment has at least one outlet through which, in use, a stream containing anodically oxidised oxidisable species flows.