Containing Solutions"

Field of the Invention

The present invention relates to a method for the pretreatment and separation of metals from cyanide containing solutions. The method of the present invention is particularly applicable to the separation of metal cyanide complexes in the treatment of gold leaching solutions, to minimise cyanide waste.

Background Art

Cyanide is a common lixiviant for gold and other metals in the metallurgical industry. It is utilised in a number of gold leaching processes, including carbon in leach (CIL) and carbon in pulp (CIP), as well as heap leach operations. These processes in particular involve the addition of activated carbon to leach solutions and gold is thus adsorbed on the carbon, which enables it to be separated from other species in solution and recovered. Unfortunately, other cyanide consuming metals, such as copper, are also commonly present in these solutions and these can interfere with the gold recovery process as they may compete with the gold by absorbing onto the activated carbon. It is necessary to bleed some of the barren solution to prevent build-up of the impurity metals like copper. Thus some gold is lost to waste because the gold cannot be separated from the impurity metals. Cyanide is also lost to waste because of the bleed stream.

As copper and other impurity metals form cyanide complexes, a significant quantity of the cyanide is lost as a result of reaction with these metal species. Thus, gold leaching processes often operate using as little cyanide as possible because any excess cyanide would simply be lost in this way rather than recovered. An additional cost for gold leaching operations are the environmental regulations associated with discharging cyanide and heavy metals to the environment and the decreasing discharge limits associated with any water that is released to the environment from the process.

It is desirable to minimise cyanide losses whilst still maintaining levels in the process sufficient to leach the gold efficiently. It is also desirable that any water stream discharged to the environment be substantially free of contaminants for example, heavy metals and/or cyanide. The method of the present invention has one object thereof to substantially overcome one or more of the abovementioned problems associated with the prior art, or to at least provide a useful alternative thereto. It is one object of the present invention to overcome substantially the abovementioned problems of the prior art, or to at least provide a useful alternative thereto.

The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

Throughout the specification and claims, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Disclosure of the Invention

In accordance with the present invention there is provided a method for the pretreatment and separation of metals from cyanide containing solutions, the method characterised by the step of: directing a feed stream containing cyanide, particulate matter and metal values to a pre-treatment step to produce an aqueous metal cyanide stream and a substantially particulate stream, wherein the pre-treatment step is capable of retaining fine carbon particulates in the substantially particulate stream.

Preferably, the substantially particular stream is passed to a metal recovery step.

Still preferably, the aqueous metal cyanide stream is passed to a separation step to separate multi-valent metal cyanide complexes from mono-valent metal cyanide complexes, wherein at least a portion of a mono-valent metal cyanide stream formed thereby is recirculated back to the feed stream for further processing.

A multi-valent metal cyanide stream formed by the separation step is preferably directed to a recovery step.

Further, at least a portion of the mono-valent metal cyanide stream is preferably directed to at least one concentration step to produce a filtrate stream and a concentrated mono-valent metal cyanide stream, the filtrate stream being substantially free of cyanide. Still further preferably, at least a portion of the concentrated mono-valent metal cyanide stream is directed back to the feed stream for further processing and the multi-valent metal cyanide stream is directed to a recovery step.

According to one aspect of the present invention the method is further characterised by the steps of: passing the aqueous metal cyanide stream to a separation step to separate multi-valent metal cyanide complexes from mono-valent metal cyanide complexes to form an impurity metal stream and a mono-valent metal cyanide stream; directing at least a portion of the impurity metal cyanide stream to a recovery step; directing at least a portion of the mono-valent metal cyanide stream to at least one concentration step to produce a filtrate stream and a concentrated mono-valent metal cyanide stream; directing at least a portion of the concentrated mono-valent metal cyanide stream back to the feed stream for further processing; and passing the filtrate stream through a cyanide removal step to remove substantially all residual cyanide and produce a treated permeate stream, wherein the treated permeate stream is substantially free of cyanide.

Preferably, the treated permeate stream is passed to a metal removal step to produce a polished water stream, which is substantially free of heavy metals.

The polished water stream is preferably directed to a pH adjustment step to produce a final water stream, which meets environmental discharge requirements.

Preferably, the metal removal step is carried out using ion exchange techniques.

Preferably, at least a portion of the concentrated mono-valent metal cyanide stream is directed to a recovery step.

Preferably, at least a portion of the concentrated mono-valent metal cyanide stream is also directed back to a leaching step prior to the pre-treatment step.

Preferably, the final water stream has a pH of between about 6.5 and 9.5 which is deemed acceptable for discharge to the environment. More preferably, the final water stream has a pH within the range of about 6.5 and 9.0.

Preferably, the feed stream includes but is not limited to, leach solutions resulting from a gold leach circuit, including Carbon in Pulp (CIP) and Carbon in Leach (CIL) circuits, and heap leach circuits.

The feed stream preferably comprises dissolved or complexed impurity metals including but not limited to nickel, copper, iron and cobalt. Preferably, the particulate material is provided in the form of activated carbon from one or more of a CIP or CIL circuit, or a heap leach circuit.

The pre-treatment step preferably includes backwashable screens and/or filters.

More preferably, the pre-treatment step includes an ultra-filtration (UF) membrane system.

Still preferably, the UF membrane system comprises one or more of hollow fibre, spiral, tubular or flat sheet UF membranes.

The UF system is preferably capable of retaining particulates having a particle size within the range of about 0.01 pm to 50 pm. More preferably, the UF system is capable of retaining particulates having a particle size of between about 0.01 pm and 10 pm.

Preferably, the driving pressure of the feed stream through the UF membrane system is controlled.

More preferably, the driving pressure of the feed stream through the UF membrane system is maintained within the range of about 1 .5 psi(g) and 145 psi(g).

The particulate stream is preferably directed to a metal recovery step to recover adsorbed gold.

Alternatively, where there is little or no gold to be recovered (such as where the particulates are not carbon), the particulate stream may be directed back to a gold leach process or sent to the tails dam.

Preferably, the separation step involves the use of a membrane that is capable of fractionating multi-valent metal cyanide complexes from mono-valent metal cyanide complexes. More preferably, the membrane is provided in the form of a reverse osmosis (RO) membrane of type known in the art. The driving pressure of the feed stream through the RO membrane is preferably within the range of about 50 psi(g) and 1500 psi(g).

The separation step is preferably capable of retaining between about 50% to 99% of multi-valent metal cyanide complexes in the impurity metal stream, whilst allowing between about 50% to 99% of the mono-valent metal complexes and between about 50% to 99% of the free cyanide to pass through the membrane.

More preferably, the impurity metal stream contains between about 70% and 99% of impurity metals originally present in the aqueous metal cyanide stream.

The mono-valent metal cyanide stream preferably contains between about 70% and 99% of mono-valent metal cyanide, and between about 70% and 99% of the free cyanide, originally present in the aqueous metal cyanide stream.

The impurity metal stream is preferably directed to a recovery step to recover metal values and cyanide.

Preferably, the concentration step is provided in the form of a reverse osmosis (RO) membrane.

More preferably, the concentration step is provided in the form of a seawater tight RO membrane.

The driving pressure of the feed stream through the seawater tight RO membrane is preferably within the range of about 50 psi(g) and 1500 psi(g)

Alternatively, the membrane of the concentration step is a polymer membrane in the form of a polyamide thin-film composite.

After the concentration step the concentration of the mono-valent metal cyanide complex and free cyanide in the concentrated mono-valent metal cyanide stream is at least about 0.1 ppm and 50,000ppm, and 10 ppm and 100,000 ppm, respectively. More preferably, the concentration of the mono-valent metal cyanide complex is within the range of about 0.1 ppm and 10,000 ppm, and free cyanide within the range of about 100 ppm and 50,000 ppm, respectively.

After the concentration step the concentration of free cyanide in the filtrate stream is preferably less than about 1 ppm.

Preferably, at least a portion of the concentrated mono-valent metal cyanide stream is directed to a recovery circuit.

The cyanide removal step is preferably provided in the form of Ultra Violet (UV) light or ozone or a combination thereof, to oxidise cyanide in the filtrate stream.

Alternatively the the cyanide removal step is preferably in the form of ion exchange or carbon absorption.

Preferably, the pH is maintained within the range of about 10-12 during the pre- treatment step through to the cyanide removal step. More preferably, the pH is maintained above at least about 10.5 during the pre-treatment step through to the cyanide removal step.

Brief Description of the Drawings

The present invention will now be described, by way of example only, with reference to two embodiments thereof and the accompanying Figures, in which:- Figure 1 is a diagrammatic representation of a flow sheet depicting a method for the pretreatment and separation of metals from cyanide containing solutions in accordance with a first embodiment of the present invention; and

Figure 2 is a diagrammatic representation of a flow sheet depicting a method for the pretreatment and separation of metals from cyanide containing solutions in accordance with a second embodiment of the present invention. Best Mode(s) for Carrying Out the Invention

In Figure 1 there is shown a flowsheet for a method 10 for the pretreatment and separation of metals from cyanide containing solutions in accordance with a first embodiment of the present invention. A feed stream 12 comprising a leach liquor exiting a gold leaching process using carbon (not shown), is directed to a pre-treatment step 14. The pretreament step 14 includes back-washable screens and filters 16 capable of separating particulates within the size range of about 75 pm to 200 μιτι, for example a 75 pm screen followed by an ultra filtration (UF) membrane 18. The UF membrane may, for example, be provided in the form of a hollow fibre, spiral, tubular or flat sheet UF membrane. The ability to be back-washed reduces the instance of blockage and scaling from, for example, suspended solids such as colloids or insoluble calcium carbonate and calcium sulphate. The UF membrane 18 is in the form of a polymeric or ceramic membranes known in the art, for example, any one of polyvanyladinefluoride (PVDF), Teflon™, polysulfone, polyacrylonitrile (PAN), cellulose, cellulose acetate hydrolysed cellulose acetate, polyethersulfone, or silicon carbide. Some polymers are more suitable than others because of surface charge, surface smoothness and pH tolerance. For gold leaching processes, PVDF, cellulose, polysulfone or ceramic membranes are understood to be preferred.

Alternatively, the UF membrane 18 may be provided in the form of a non- polymeric membrane such as a ceramic membrane, for example silicon carbide or titanium dioxide.

The UF membrane 18 is capable of retaining particles present in the feed stream 12 having a size within the range of about 0.01 pm to 50pm, for example between about 0.01 pm and 10pm. This is fine particulate matter, generally in the form of fine carbon particulates, which would otherwise not be recovered from a typical leach circuit, as particles of this size would invariably be directed ultimately to tails and lost. The driving pressure of the feed stream 12 through the UF membrane 18 is maintained at a relatively low level, for example between 1.5 psi(g) and 145 psi(g).

The permeate exiting the pre-treatment step 14 is in the form of a primary aqueous metal cyanide stream 20 containing metal values. The retentate is in the form of a particulate stream 17. Where the particulate stream is in the form of a heap leach solution, or where the particulates are in the form of carbon or activated carbon comprising adsorbed metal values (for example, from a CIP or a CIL process or a heap leach process after carbon extraction), the particulate stream 17 is directed to a metal recovery step 23 such as an elution column known in the art. For example, gold is eluted from the activated carbon loaded with adsorbed metal values using concentrated sodium hydroxide at elevated temperature and pressure to recover metal values and regenerate activated carbon. The regenerated activated carbon can then be returned to the leach process for re-use, although generally the carbon is too fine and is preferentially sent to waste. If the particulates are not carbon (for example leach solutions resulting from a heap leach and/or an agitated leach) and/or there are no metal values to be recovered, the particulate stream 17 may be directed back to the leach process or sent to tails. The primary aqueous metal cyanide stream 20 contains various impurity metal cyanide complexes such as copper, nickel, iron and cobalt, together with complexed gold and free cyanide. The primary aqueous metal cyanide stream 20 is directed to a separation step 24. The separation step 24 comprises a membrane that is capable of fractionating multi-valent metal cyanide complexes (particularly di- and tri- valent complexes), including but not limited to Cu, Ni, Fe and Co, from mono-valent metal cyanide complexes, for example, Au. The separation step 24 may comprise a reverse osmosis (RO) membrane or a nanofiltration membrane of type known in the art.

Although the separation achieved will be dependent upon the various metal solubilities and the operating pressures employed, the separation step 24 retains at least about 50%, for example between about 70% to 99%, of the multi-valent metal cyanide complexes initially present in the primary aqueous metal cyanide stream 20, to produce an impurity metal stream 26. The impurity metal stream 26 contains less than about 1% to 50%, for example less than 10% of the complexed gold initial present in the aqueous metal cyanide stream 20.

The driving pressure of the feed stream through the RO membrane of the separation step 24 is maintained within the range of about 50 psi(g) and 1500 psi(g) in order to minimise the volume of concentrate produced. Thus, the size of downstream processes, such as an impurity metal recovery step 27, is minimised. Capital and operating costs are therefore also minimised.

The permeate from separation step 24 is in the form of a mono-valent metal cyanide stream 28, for example silver and/or gold cyanide, which also contains free cyanide. The mono-valent metal cyanide stream 28 contains at least about 50% of the complexed gold originally present in the primary aqueous metal cyanide stream 20, for example between 70% and 99%. Furthermore, the monovalent metal cyanide stream 28 also contains at least about 50% of the free cyanide originally present in the primary aqueous metal cyanide stream 20, for example, between about 70% and 99%.

The impurity metal stream 26, containing the impurity metal complexes, can be directed to a metal recovery process 27, including but not limited to a SART (sulphidation, acidification, recycling and thickening) process. Essentially, copper or other impurity metals are precipitated using sulphide and cyanide is recovered by reducing the solution pH and recovering volatilised cyanide as sodium cyanide in a caustic bath. Other processes such as AVR (acidification-volatilization- regeneration) and MNR (Metallgesellschaft Natural Resources), known in the art can be utilised. Further, electrowinning can be used to recover Cu and CN, and there are a number of other recovery processes known in the art utilising exchange resins and activated carbon, which could be utilised. The mono-valent metal cyanide stream is passed through a concentration step 30 in order to concentrate the complexed gold and cyanide into a concentrated metal cyanide stream 32, while producing a filtrate stream, which is substantially barren of any metals and cyanide. The concentration step 30 is in the form of at least one reverse osmosis (RO) membrane, for example two membranes. The membrane itself is in the form of, for example, a seawater-tight RO membrane or a polymer membrane such as a polyamide thin-film composite membrane.

Where two membranes are used in the concentration step 30, as depicted in Figure 1 , the concentrate stream 32 from a first membrane 31 is passed to a metal recovery step 33, such as a Merrill-Crowe process to recover the gold and silver values, or carbon process in order to recover the gold values. Alternately the concentrate stream 32 may be returned to the gold leaching process.

A primary filtrate stream 34 is produced and directed to a second membrane 35 of the concentration step 30, producing a secondary concentrated metal cyanide stream 36. The secondary concentrated metal cyanide stream 36 is recirculated to be combined with the mono-valent metal cyanide stream 28 and passed again through the concentration step 30.

The concentrate stream 36 contains the mono-valent metal cyanide species, such that its concentration is within the range of about 0.1 ppm and 50,000 ppm, for example 0.1 ppm to 10,000 ppm. The concentrate stream also contains free cyanide within the range of about 10 ppm and 100,000 ppm, for example 100 ppm to 50,0000 ppm.

The driving pressure of the feed stream through the RO membrane/s 31 and 35 of the concentration step 30 is maintained within the range of about 50 psi(g) and 1500 psi(g) in order to minimise the volume of the concentrate stream 32 and/or the secondary concentrate stream 36. Reducing the volume of the concentrate streams 32 and 36 invariably results in higher concentrations of mono-valent metals in these streams, such as gold. As a result, these factors minimise the size of the gold recovery step 33, as well as reducing the capital and operating costs.

A secondary filtrate stream 38 containing less than about 0.1 ppm free cyanide, can then be redirected to the mine for use as process water, or it can be sent to a tailings dam or discharged to the environment. Alternatively, as shown in Figure , the secondary filtrate stream 38 is passed through a cyanide removal step 40, to remove any trace quantities of cyanide still present. The cyanide removal step 40 is an oxidation treatment step such as Ultra-Violet (UV) light or ozone or a combination of ozone and UV, in which the secondary filtrate stream 38 is exposed to UV light sourced from, for example, a UV generator, or ozone sourced from, for example, from an ozone generator, or a combination of the two. Using such generated oxidation treatment such as UV light or ozone is more efficient and does not require large holding ponds to maximise exposure to sunlight (and therefore maximise UV exposure), or use of chemicals such as chlorine or hydrogen peroxide which must be transported to the plant site. This produces a treated permeate 42, which is substantially a water stream and contains cyanide at levels of less than about 0.01 ppm, such that it would meet strict water quality standards.

Alternatively, the cyanide removal step 40 may be a CN removal step such as a carbon absorption column or an anion ion exchange column which removes cyanide from the solution. This option also produces a treated permeate 42, which is substantially a water stream and contains cyanide at levels of less than about 0.01 ppm such that it would meet strict water quality standards.

The pH of any cyanide-containing solutions are maintained at a pH between about 10 and 12, for example at least 10.5. This ensures that cyanide remains in solution as NaCN and losses of cyanide through volatilisation are minimised.

Prior to discharge of the stream 42 to the environment it is passed through a metal removal step 45 in the form of, for example a cation ion exchange column, which is used to remove any trace heavy metals. This produces a polished water stream 46, which is substantially a water stream and contains heavy metals at levels of less than about 0.01 ppm such that it would meet strict water quality standards.

The polished water stream 46 then undergoes a pH adjustment step 48 to produce a final water stream 49. The the pH is adjusted to between about 6.0 to 9.5, for example pH between about 6.5 to 9.0. The pH is adjusted by injection of an acidic material if the pH is above 9.0, for example, carbon dioxide or a mineral acid such as sulphuric acid or hydrochloric acid. If the pH is below 6.0, an alkaline material is injected, for example, sodium hydroxide or lime, to meet the required environmental pH discharge limit.

In Figure 2 there is depicted a method 1 1 for the separation of metals from cyanide containing solutions in accordance with a second embodiment of the present invention. The method 1 and the method 10 described above are substantially similar in many respects and like numerals denote like parts/steps. In the method 1 1 the particulate stream 17 undergoes further treatment in a second ultra filtration (UF) membrane 21 for example, a hollow fibre, spiral, tubular or flat sheet UF membrane.

The UF membrane 21 is capable of retaining particles present in the feed stream 17 having a size within the range of about 0.01 pm to 50μιτι, for example between about 0.01 pm and 10pm. This is fine particulate matter, generally in the form of fine carbon particulates, which would otherwise not be recovered from a typical leach circuit, as particles of this size would invariably be directed ultimately to tails and lost.

The driving pressure of the particulate stream 17 through the UF membrane 21 is maintained relatively low, for example between about 1.5 psi(g) and 145 psi(g).

The permeate exiting the second UF step 21 is in the form of a secondary aqueous metal cyanide stream 22 containing metal values. The retentate is in the form of a secondary particulate stream 19. Where the secondary particulate stream 19 is in the form of a heap leach solution, or where it contains carbon or activated carbon having adsorbed metal values (for example, from a Carbon in Pulp (CIP) or Carbon in Leach (CIL) process, or a heap leach process after carbon extraction), the secondary particulate stream 19 is directed to a metal recovery step 23 such as an elution column known in the art. It is understood that the benefit provided by the second UF membrane step 21 is to further concentrate the particulate matter present in the primary particulate stream 17, into a smaller volume which is then treated in the metal recovery step 23. This, in turn, reduces the size of the metal recovery step 23.

It is also understood that this also allows for additional recovery of the various impurity metal cyanide complexes such as copper, nickel, iron and cobalt, together with complexed gold and free cyanide, which are found in the primary particulate stream 17 and which report to the permeate stream 22. The permeate stream 22 is in turn combined with stream 20 and then directed to the separation step 24. It should be noted that the membrane separation process described herein has the particular advantage that it enables the use of membranes that are stable and operate well at high pH. Membranes of the prior art such as nanofiltration membranes degrade quickly at higher pH levels, for example at or above approximately pH 10, and thus require frequent (and costly) replacement. It is envisaged that the mono-valent metal cyanide stream 28 exiting the separation step 24 could be directed back to the leaching process without undergoing a concentration step 30. This would have the effect of recycling free cyanide present in the mono-valent metal cyanide stream 24 and also raising the gold content in the feed stream 12 to improve recovery. It is further envisaged that the primary and secondary filtrate streams 34 and 38 could be directed to tails waste or to the environment in light of their low cyanide content, without passing through the cyanide destruction step 40. Without the pretreatment step 14 the utilisation of membrane separation to separate metal values from an aqueous metal cyanide stream 16 would not be economical as the remaining carbon particulates, together with silica and other colloidal particles (such as fine clays) would cause fouling of the downstream membranes, for example in the separation step 24, which would then require frequent replacement. The fine carbon particulates retained by the pre-treatment step 14 generally contain adsorbed gold, which would traditionally be lost to tails. The Applicant has discovered that the additional gold, which can be recovered through this process, is an economical quantity. It is understood that this process produces concentrated copper, gold, silver and cyanide streams, together with high quality water (in the form of the primary and secondary filtrate streams 34 and 38) that would meet world discharge standards. Alternatively, given that streams 38 and/or 42, 46, 49 have low TDS (total dissolved solids), they form an excellent water source for use in flotation circuits, as low TDS enhances flotation. It is believed a new greenfields operation adopting this process would meet worldwide water discharge standards thereby allowing the mining operation to start almost immediately. This is also understood to reduce the water input requirements of the operation as quality water can be recirculated to various areas of the operation. It is envisaged that this process is particularly suitable for use in new and existing gold processing operations that treat ores containing high levels of cyanide consuming metals such as copper. In the past, such ore bodies could not be processed because consumption of cyanide was so high that processing was rendered uneconomical. The method of the present invention, which recovers and recirculates cyanide, is understood to allow such ore bodies to be processed to economically recover gold, whilst also producing a saleable copper product.

It is understood that the method of the present invention may also be utilised in gold processing operations prior to an activated carbon gold recovery step such as is employed in the Sceresini process and the Oretek CPC process [Developments in Mineral Processing, Chapter 32, Volume 15, 2005, Pages 789 to 824], in which copper or other interfering metal ions may be removed from the leach solution. This provides the benefit of improving gold leaching and subsequent adsorption onto the activated carbon. The membrane process could similarly treat a leach solution stream to remove copper and other interfering metal ions ahead of the activated carbon adsorption step

Further, unlike earlier methods that utilise membranes, this method can place the membrane separation step after the solution has been contacted with carbon.

The use of multiple membrane systems for filtration, separation and concentration also provides the advantage that the various feed solutions are split into retentate and permeate streams. This means that the volumes that need to be treated in the various areas of the operation at one time (e.g. separation step 24, the impurity metal recovery process 27, the concentration step 30, the gold recovery step 33 and/or the cyanide destruction/removal step 40) are significantly reduced. This in turn provides a significant reduction of capital expenditure. The method of the present invention may be further understood with reference to the following non-limiting examples:

EXAMPLE 1

In Example 1 is demonstrated the method 10 for the pretreatment and separation of metals from cyanide containing solutions in accordance with the first embodiment of the present invention and as shown in Figure 1 , as applied to a barren gold leach solution.

A barren gold leach solution is pumped through a pre-treatment step firstly comprising a back-washable screen to remove particulates above 75 pm, followed by an ultra filtration membrane operating at approximately 1 bar feed pressure. The membrane is provided in the form of a hollow fibre polysulfone membrane with 0.01 pm pore size. The retentate stream contains predominantly fine ore and colloidal clay particles up to 75 μητι and has no metal values to be recovered. It may further be directed back to the leach process or sent to tails.

The permeate stream is pumped through a membrane separation step operating at a pressure of about 65 bar. The membrane is a thin film composite reverse osmosis (RO) membrane. The retentate stream retains approximately 97% of the copper ions, whilst approximately 98% of the gold and approximately 97% of the free cyanide from the feed solution is found in the permeate stream. The retentate stream can then be sent to a copper recovery process, i.e. a SART process or electrowinning in which copper can be recovered.

The permeate stream from the separation step is then pumped through a two stage membrane concentration step. The membranes used in the concentration step are thin film composite seawater tight RO membranes. The first membrane concentration step operates at about 65 bar, while the second membrane concentration step operates at 55 bar. The gold ions and free cyanide are recovered to the retentate stream from the concentration step which is suitable to be sent to a metal recovery process, for example a carbon absorption process to recover the gold, while the recovered free cyanide can be recycled back to the leach process. The permeate from the concentration step is clean water and contains very low levels of metals or free cyanide. The permeate is then sent to a UV treatment unit to further reduce free cyanide in the solution to below 0.01 ppm. The final solution after UV treatment is suitable for discharge to the environment or can be recycled back to the process. Results are summarised in Table 1 and presents the amount of copper, gold, and free cyanide recovered from a barren leach solution from a gold mining operation. Table 1

EXAMPLE 2

In Example 2 is demonstrated the method 11 for the pretreatment and separation 5 of metals from cyanide containing solutions in accordance with the second embodiment of the present invention and as shown in Figure 2, as applied to a barren gold heap leach solution after carbon extraction.

The barren gold heap leach solution contains approximately 200 ppm of fine carbon particles which were not retained in the carbon extraction process. The 0 carbon particles contain approximately 500 ppm of absorbed gold. If this solution is returned to the heap leach there potential for the fine carbon to accumulate within the heap leach bed and block the flow of solution through the heap as well as reducing the gold that is recovered from the heap.

Prior to returning the barren solution to the heap it is pumped through a pre- 5 treatment step firstly comprising a back-washable screen to remove particulates above 100 μιη, followed by a first ultra filtration membrane operating at approximately 1 bar feed pressure. The membrane is a hollow fibre polysulfone membrane with 0.02 μιτι pore size. The permeate stream contains less than 10 ppm fine carbon, while the retentate stream contains the majority of the fine carbon particles. The retentate is then pumped through a second ultra filtration membrane operating at approximately 1 bar feed pressure. The membrane is a hollow fibre polysulfone membrane with 0.02 μηη pore size. The permeate stream contains less than 10 ppm fine carbon and is mixed with the permeate from the first UF membrane step, while the retentate stream contains the majority of the fine carbon particles. The fine carbon concentration in the second retentate stream has increased to 77,000 ppm and there is approximately 99% recovery of the fine carbon and gold to the final retentate. The final retentate is then sent to a gold recovery process.

Results are summarised in Table 2 below and show the amount of gold recovered from 1000 L of the barren leach solution.

Table 2

The two permeate streams from Example 2 are mixed and then pumped through a membrane separation step operating at a pressure of about 40 bar. The membrane is a thin film composite RO membrane. The retentate stream retains approximately between 95-99% of the multi-valent metal ions, copper, iron, cobalt, nickel while the monovalent metal ions predominately pass through the membrane into the permeate stream; approximately 99% of the gold and 90% of the silver from the feed solution are found in the permeate stream, as well as approximately 94% of the free cyanide. The retentate stream can then be sent to a copper recovery process, for example a SART process or electrowinning in which copper can be recovered.

Details of the separation step are shown in Table 3 below and are based on treatment of 1000 L of solution collected after the pre-treatment step. The volume of permeate and retentate collected were approximately 930 L and 70 L respectively.

Table 3

Stream No 20+22 26 28

Feed to Membrane Retentate from Permeate from

Stream ID

Separation Step Separation Step Separation Step

Volume (L) 1 ,000 70 930

Composition

Cu

242 1833 2.21

(ppm)

Fe

852 6135 7.04

(ppm)

Co

49.9 358 0.97

(ppm)

Ni

68.3 174. 4.32

(ppm)

Au

1.15 0.05 1.33

(ppm)

Ag

0.87 0.32 0.84

(ppm)

CN

206 179 208

(ppm) The permeate stream from the separation step is then pumped through a two stage membrane concentration step where the monovalent metals and free cyanide are recovered to the retentate stream. The permeate from the concentration step is clean water and contains very low levels of metals or free cyanide. The membranes used in the concentration step are a seawater tight RO membrane. The first membrane concentration step operates at about 45 bar, while the second membrane concentration step operates at 35 bar.

The retentate stream can be sent to a metal recovery process, for example a Merrill Crowe process to recover both gold and silver, while the recovered free cyanide can be recycled back to the leach process. The permeate is then sent to a UV treatment unit to further reduce free cyanide in the solution to below 0.01 ppm. The final solution after UV treatment is suitable for discharge to the environment or can be recycled back to the process.

Results are summarised in Table 4 and presents the amount of gold, silver and free cyanide recovered from 000L of the barren leach solution collected after the separation step described above.

Table 4

Stream No 28 32 36 38 42

Feed to

Retentate from Recycle from Permeate from

Membrane Discharge

Stream ID Concentration Concentration Concentration

Concentration Solution

Step Step Step

Step

Volume (L) 1 ,000 80 80 920 920

Composition

Cu (ppm) 2.21 79.30 0.07 <0.05 <0.05

Fe (ppm) 7.04 58.70 0.62 <0.05 <0.05

Co (ppm) 0.97 4.72 0.09 <0.05 <0.05

Ni (ppm) 4.32 57.50 0.64 0.10 0.10

Au (ppm) 1.33 8.15 3.14 0.05 0.05

Ag (ppm) 0.84 6.86 0.62 O.02 <0.02

CN (ppm) 208 2512.50 8.75 0.06 <0.01 Modifications and variations such as would be apparent to the skilled addressee are understood to fall within the scope of this invention.