AU2012100138A4 - Process for treating a cyanide containing solution - Google Patents

Description

1 Process for Treating a Cyanide Containing Solution This invention relates to a process for treating a cyanide containing solution which assists recovery of cyanide and metal values contained in the 5 solution. Cyanidation is a commonly practised process for the recovery of precious metals from a wide range of precious metal containing materials. The economic extraction of precious metals from such materials is dependent on the grade (quantity) of the precious metals contained therein and the presence or absence 10 of one or more of a number of interfering elements. The economics of cyanidation processes become more challenging for lower grade materials if cyanide consumption increases as a consequence of the presence of cyanide consuming metals other than precious metals to the point where such materials may be termed "refractory" or resistant to cyanidation. 15 That is, the value of recoverable metal fails to deliver satisfactory profit once production costs are allowed for. Cyanide reagent costs are a significant cost to be covered. Such materials may include both oxidised (including carbonate) and sulphidic ores. Significant amounts of precious and base metal cyanide complexes plus some free cyanide may be present in tailings from earlier 20 precious metal extraction processes posing a long term threat to the environment. However, particularly where the base metal involved is copper resulting in formation of copper containing cyanide copmplexes, effective recovery of cyanide, precious metal and copper values would present both environmental and economic opportunities. 25 One process, the Clarke Process, for treating gold-copper ores and tailings involves cyanidation to generate a pregnant liquor containing gold and copper cyanide complexes as well as other precious and base metal complexes. The pregnant liquor, a cyanide containing solution, is treated with a precipitant to co-precipitate gold and copper as insoluble cyanides. The precipitate comprises 30 a major proportion of copper and a minor proportion of gold. The bulky copper precipitate acts as a collector for the gold and any other precious metals that may be present. The precipitate may then be separated in a solid/liquid separation 2 step, such as filtration, and roasted to produce a gold rich copper oxide which can readily be further processed for extraction of gold and copper in copper refineries. The Clarke Process employs a copper salt, particularly copper sulphate, as a precipitant. Presence of a reducing agent, such as sodium metabisulphite or 5 sulphur dioxide, increases precipitation efficiency. The process removes copper and destroys cyanide and therefore may be considered benign and suitable for operation in environmentally sensitive areas. The barren liquor from the Clarke Process is pH modified with an alkali before addition of cyanide and return to the leaching process. This recycling of 10 the stream to the leaching process makes the process an efficient user of water, an important consideration in many areas. The Clarke Process requires a minimum copper tenor in solution of about 100 ppm copper to produce sufficient bulk of precipitate to act as an efficient collector for the precious metals. As the cyanide soluble copper content of the 15 material being treated increases, the cyanide consumption also increases as it is necessary to maintain a ratio of 3.5 parts NaCN to 1 part Cu so as to maintain a satisfactory dissolution rate of the precious metals. The increasing cyanide consumption affects the economics so that ores and tailings containing lower grades of precious metals become un-economic to treat, and higher levels of 20 copper cyanide complexes increase the environmental liability of storing the residues. It is an object of the present invention to provide a process for treating a cyanide containing solution such as a solution generated by processing of ores, tailings and other metal bearing materials (including electronic scrap, metal scrap 25 and so on) to enable more economic metal recovery or to enable environmental remediation or detoxification. With this object in view, the present invention provides a process for treating a cyanide containing solution wherein said cyanide containing solution is treated, in a precipitation stage with cuprous chloride in an amount equal to or 30 greater than stoichiometric for reaction with copper containing cyanide complexes present in the solution to co-precipitate cyanide and metal values as a material treatable for recovery of metal values and free cyanide.

3 Stoichiometry is the branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers. Cyanidation is performed under alkaline 5 conditions with the usual alkaline reagent of choice being lime providing calcium hydroxide (Ca(OH) 2 ) in solution on a cost effective basis. In the case of the reaction between the calcium copper cyanide complex and cuprous chloride, four molecules of calcium copper cyanide complex (CaCu(CN) 3 ) plus four molecules of cuprous chloride (Cu2Cl2), the minimum to be reacted to achieve stoichiometry 10 (i.e react at least with calcium copper cyanide complex to form copper cyanide and calcium chloride) yield twelve molecules of copper cyanide (CuCN) and four molecules of calcium chloride (CaCl 2 ): 4 CaCu(CN) 3 + 4 Cu 2

CI

2 P, 12CuCN + 4CaC 2 (1) Similarly, in the case of the reaction between a sodium copper cyanide 15 complex and cuprous chloride, where caustic or NaOH is used, two molecules of sodium copper cyanide complex (Na 2 Cu(CN) 3 ) plus two molecules of cuprous chloride (Cu2CI2), the minimum to be reacted to achieve stoichiometry (i.e react at least with sodium copper cyanide complex to form copper cyanide and sodium chloride) yields 6 molecules of copper cyanide (CuCN) and four molecules of 20 sodium chloride (NaCI): 2 Na 2 Cu(CN) 3 + 2 Cu 2

CI

2 -> 6 CuCN + 4 NaCI (11) Mixtures of caustic and lime may be used as well in which case both reaction schemes are relevant. An at least stoichiometric quantity of cuprous chloride is added for reaction 25 with the copper containing cyanide complexes including the above identified sodium copper cyanide complex and calcium copper cyanide complexes during the process. However, identifying the exact chemical nature of all copper containing cyanide complexes in the solution is not necessary. The amount of cuprous chloride required is readily determined by analytical techniques as 30 known in the art (and including wet chemical and spectrometric techniques), and, because the copper product of all reactions is insoluble, is confirmed by a minimum copper value remaining in solution following precipitation. It follows from this that the cuprous chloride requirement can be determined with reference to a 4 target maximum copper tenor following precipitation (at end point). This endpoint copper tenor should be less than 100 ppm, preferably less than 50 ppm and most preferably less than 25 ppm. Sufficient cuprous chloride is added to react in reaction schemes, particularly the above schemes I and II, to precipitate copper 5 and meet the copper tenor target. Control, particularly feedback control, over cuprous chloride addition may be achieved by linking the cuprous chloride addition rate with target copper tenor following precipitation. Other metal endpoint targets could also be set as desired. The precipitation reaction, conducted as above described, should produce 10 sufficient bulk co-precipitate material to act as a collector for precious metals. Advantageously, the process is used to treat solutions from a tailings resource such as a tailings storage facility ("TSF") which may include tailings dams and other stocks of tailings. Typically, the tailings would be a waste product from processing of ores containing precious and base metals by 15 cyanidation. As precious metals are often found in association with copper, such solutions may be contaminated with significant amounts of copper cyanide as well as free cyanide. Complex copper cyanides are particularly persistent and pose a long term threat to the environment should the integrity of the TSF be compromised in any way. The process of the invention can be applied to 20 reducing this environmental hazard through detoxification. However, the process could also be used as part of a process for extracting precious and base metals from materials, such as ores, overburden from mining operations, tailings or scrap (such as from electronic or other applications) containing these metals, by cyanidation or cyanide leaching, sodium 25 cyanide typically being the reagent of choice. Such materials may be low grade and/or classified as "refractory" to conventional cyanidation because of an elevated copper content. The ability to recover copper values and alkaline cyanide for re-use in the cyanide leach process as well as the ability to regenerate the cyanide compound precipitant, cuprous chloride, may benefit 30 economics of such extraction process to the extent that materials previously considered too refractory or un-economic to process by cyanide leaching may be economically treated for metal recovery. At the same time, an environmental 5 benefit of reducing output of cyanide contaminated materials to the environment, albeit contained in a tailings storage facility may be achieved. The cuprous chloride is in stable soluble form (Cu 2

CI

2 ) which can be achieved by making up a solution containing an excess of chloride ions. Various 5 alternatives are available for achieving this chloride excess. One scheme involves introducing water insoluble CuCl to a hydrochloric acid solution, advantageously a dilute hydrochloric acid solution. This solution could also contain NaCI or other soluble chloride salts. It may be possible to introduce the cuprous chloride in highly saline water which is available in some mining areas. 10 In addition, the solution may advantageously contain a reducing agent, such as sodium metabisulphite, sulphur dioxide or other reducing agent to maintain copper in cuprous state. The co-precipitate material containing metal cyanides, including cuprous cyanide, is conveniently separated from the treated solution by a solid liquid 15 separation process which may involve thickening, clarification and/or filtration. In a leach situation, clarified overflow or barren liquor from a thickening step may be recovered and returned to the cyanide leach stage after adjusting pH to alkaline range as required for safe and effective cyanidation leach practice. If necessary, make up cyanide - conveniently in the form of sodium cyanide - can be added in, 20 or prior, to the leach stage. If the process is used to treat cyanide containing solution from a tailings storage facility, or other source of cyanide containing solution, not involving an active leach operation, the above mentioned clarified overflow may be pH modified and returned to the TSF with or without additional water to assist 25 recovery of further contaminants. Options for the regenerated cyanide solution then include selling the cyanide solution if there is a buyer or rendering the cyanide solution harmless by a suitable known process, such as through oxidation reaction with hydrogen peroxide. In this case, metal recovery from co precipitate material - as described below - may help to defray the costs of 30 rendering the regenerated cyanide containing solution safe. Where there is an active cyanide leach operation, cyanide containing solution can be returned typically in combination with cyanide regenerated as described below - to cyanide storage for the leach operation and re-use.

6 Separated co-precipitate material is then beneficially treated to regenerate cyanide and form a metal value containing product directed to metal value extraction steps. Such metal value extraction steps may involve pyrometallurgical and/or hydrometallurgical processes including electrowinning. 5 One process scheme would involve digestion of the co-precipitate with reagents, including chlorides, to form hydrogen cyanide gas, convertible - for example by alkaline scrubbing with a hydroxide, into an alkaline cyanide stream directed to leaching, and metal value containing streams which can be subjected to further unit operations, including smelting for precious metals or electrowinning 10 for copper, to extract metal values. As the process is well suited to treatment of refractory gold-copper ores or tailings from gold-copper ore processing, the metal values would include copper and gold. However, other precious metals, and base metals including cobalt may be extracted as well. Electrowinning and other hydrometallurgical schemes may be used for recovery of base metals and 15 especially copper. Precious metals, particularly gold, can be recovered as bullion through a pyrometallurgical process to produce a Dore bullion. Hydrogen cyanide gas, for subsequent conversion to alkaline cyanide as above described, can be produced by acid treatment of the co-precipitate material, for example with hydrochloric acid. This process has a significant 20 advantage of also regenerating stable cuprous chloride solution which can be returned to the precipitation stage. One scheme would involve chloride leaching or digestion under reducing conditions, desirably using acid chloride solution with the leaching reagent including hydrochloric acid, NaCI and a reducing agent such as sodium metabisulphite or sulphur dioxide to create reducing conditions under 25 which cuprous ion is stable. A key reaction in this scheme, and that critical to regeneration of cyanide and cuprous chloride, is: 6 CuCN + 6 HCI + NaCI --* 3 Cu 2

CI

2 + 6 HCN + NaCI (III) The NaCI does not participate in the reaction as such but is present to achieve an excess of chloride ions. 30 The process for treating a cyanide containing solution of the invention may be more fully understood from the following description of two preferred embodiments thereof made with reference to the figures in which: 7 Fig. 1 is a process flow diagram for a first embodiment of the process used to treat cyanide containing solutions from a tailings storage facility. Fig. 2 is a process flow diagram for a second embodiment of the process used within a cyanide leaching process used in extracting copper, gold and other 5 precious metals from a copper-gold ore. Referring to Fig.1, the illustrated plant 10 treats solution from the tailings storage facility (TSF) 105 of a disused copper-gold ore processing operation, these tailings being heavily contaminated with cyanide and cyanide complexes arising from cyanidation with sodium cyanide (NaCN) under alkaline conditions 10 using lime(Ca(OH) 2 ) so sodium copper cyanide and calcium copper cyanide complexes are present. The most commonly encountered sodium copper cyanide complex, though not the exclusive one, is the complex with the formula Na 2 Cu(CN) 3 . The most commonly encountered calcium copper cyanide complex is the complex with the formula CaCu(CN) 3 . 15 In particular, plant 10 treats cyanide containing solutions from the TSF 105, in a remediation process, for copper and gold recovery. Gold is recovered through processes involving precipitation, digestion and smelting stages. Copper is recovered through processes involving precipitation, digestion and electrowinning stages. 20 In order to commence the process, a starter solution of cuprous chloride (Cu 2

CI

2 , the stable soluble form of cuprous chloride) must be prepared, for example by dissolving the following reagents in 1000 litres of water: 125 kg technical grade CuSO 4 .5H 2 0 200 kg technical grade NaCI 25 75 I commercial grade HCI 68 kg technical grade Na 2 S20 5 Once in operation, no further copper sulphate will be required and the proportions of the remaining chemicals are adjusted to suit the pregnant liquor being treated, consistent with maintaining sufficient acidity to neutralise the 30 incoming liquor, sufficient excess of chloride ions to ensure a stable cuprous chloride (Cu 2

CI

2 ) solution and sufficient sodium metabisulphite, or some other reducing agent, to maintain a reducing action.

8 The TSF 105 is equipped with one or more decant towers 107 to assist with recovery of water for re-use in the process. However, other methods may be used. The solution to be treated is recovered from the TSF 105 by a decant 5 tower 107, stored until required for processing in storage tank 109 and mixed with the starter cuprous chloride solution or recycle cuprous chloride stream 22 in an agitated mixing vessel of precipitation stage 20 of plant 10 for sufficient residence time to enable practically complete precipitation. The retention time and degree of agitation of the mixing vessel are adjusted to ensure the ensuing precipitate is 10 free settling with a clear supernatant. Cloudiness, if experienced, would represent a loss of values and corrective action would be required, for example by means of a small addition of a flocculating agent such as potassium ferrocyanide or an organic flocculating agent. No neutralisation of cyanide solution is required prior to the precipitation stage 20. 15 The cyanide, in the form of free cyanide or sodium, calcium and other copper containing cyanide complexes is co-precipitated in precipitation stage 20 as cuprous cyanide (CuCN) and gold cyanide complexes among other complex metal cyanides corresponding with metals present in the co-precipitate material. Key reaction schemes, which involve the predominant copper containing cyanide 20 complexes are: 2 Na 2 Cu(CN) 3 + 2 Cu 2

CI

2 -> 6 CuCN + 4 NaCI ; and 4 CaCu(CN) 3 + 4 Cu2CI2 p, 12CuCN + 4CaCl 2 Other copper cyanide containing complexes are likely to be present in the solution but their exact nature need not be identified to properly perform the 25 treatment process as described below. At least stoichiometric quantity of cuprous chloride is reacted with the above sodium, calcium and other copper containing cyanide complexes during the process and this quantity may be determined based on the copper tenor of the solution to be treated. The amounts of the predominant cyanide complexes 30 present in the cyanide containing solution are determined by analytical techniques as known in the art and stoichiometric quantity of cuprous chloride added to precipitate copper cyanide and a chloride salt in accordance with at least the above reaction schemes. The end point of the precipitation process is 9 reached when a minimum amount of copper remains in solution. In practice, this endpoint copper tenor should be < 25ppm and preferably <15ppm Cu. The endpoint copper tenor may be analysed by wet chemical techniques or spectrometric techniques, such as atomic absorption spectroscopy (AAS). This 5 could be done using an on-stream AAS analyser allowing feedback control over cuprous chloride addition as a function of endpoint copper tenor. Sufficient cuprous chloride is added, for example, to maintain an endpoint copper tenor of 15 ±1.5 ppm The process does not require neutralisation with sulphuric acid or addition 10 of a further excess of 15-25% of the amount of cuprous chloride added once the precipitation end point has been reached. Settling alone of the co-precipitate, for example in a thickening process, may produce sufficiently dense or bulky product to allow the co-precipitate to be sent from a thickener 30 as underflow 32 directly to a regeneration stage 15 including a digester 40 in which hydrogen cyanide, convertible to alkaline cyanide for re-use in cyanidation, and cuprous chloride are regenerated and gold precipitated in metallic form. In this case, a further solid-liquid separation step such as filtration following the precipitation stage 20 may be eliminated and the process may be run in continuous mode. 20 The overflow 34 from the thickener 30, also called barren liquor, has its pH adjusted to alkaline by addition of lime in pH adjustment stage 35 and is returned, together with additional water, to the TSF 105 to flush further cyanide compounds to the decant tower/s 107. This process would continue, probably intermittently, until the cyanide level in the TSF was at a satisfactory level. Options for the 25 cyanide solution 52 recovered from the scrubber unit 50 include on-site re-use if there is an on-going operation, sale if there is a buyer nearby or rendering the cyanide solution harmless by a suitable known process. Sodium cyanide is readily de-toxified by reacting with hydrogen peroxide to form sodium cyanate (NaCNO) 30 Underflow 32, including co-precipitate material in sufficient bulk to act as an efficient collector of gold and copper for recovery, is heated with reagents hydrochloric acid, sodium chloride and sodium metabisulphite, under acidic conditions, in a digester 40 forming part of a cyanide regeneration stage. An 10 acidic recycle stream 62 from copper electrowinning stage 60 may also be directed to digester 40. This process results in formation of hydrogen cyanide and soluble cuprous chloride (Cu 2

CI

2 ) and so enables regeneration of the cuprous chloride precipitant for re-use in precipitation stage 20. Heating of underflow 32 5 in digester 40 is to temperature 70 0 C or above, preferably above 90 0 C. In continuous operation mode, heating to boiling point temperature of 113 0 C is preferred. Sodium metabisulphite is added as a reducing agent in sufficient concentration to keep copper in cuprous state. The cuprous chloride is always in excess, in proportion to the copper 10 contained in the feed solution 12 originating from the decant tower/s 107, the excess copper being removed preferentially by electrowinning in copper electrowinning stage 60. Recycling the stripped solution to the digester 40 conserves approximately 33% of the digester 40 inputs, the stoichiometry of the process dictating that 2/3 of the Cu 2

CI

2 containing solution 42 is returned to 15 precipitation stage 20, as recycle stream 22, and 1/3 to electrowinning stage 60. Two mols of a dibasic copper salt (CuCO 3 ), as would be found in an oxidic (carbonate) copper ore, produces after cyanidation 2 mols of copper cyanide complex (Na 2 Cu(CN) 3 ) this is contained in stream 12 from decant tower 107. 2 mols of copper cyanide complex react with 2 mols of cuprous chloride stream 22 20 in the precipitator 20 to produce 6 mols of copper cyanide CuCN and 4 moles of sodium chloride. In the digester 40 the six moles of copper cyanide react with 6 moles of hydrochloric acid and 4 moles of sodium chloride (formed during precipitation) to form 3 mols of Cu 2

CI

2 2 mols of which are returned to the precipitation stage 20, the remaining mol of Cu 2

CI

2 which corresponds to the 25 copper contained in the 2 mols of copper cyanide complex. The digestion reaction scheme is: 6 CuCN + 6 HCI + NaCI -- * 3 Cu 2

CI

2 + 6 HCN + NaCI Copper may also be recovered by any other suitable method involving precipitation, including, but not limited to, dilution followed by filtration when it will 30 be recovered as insoluble cuprous chloride (CuCI). Hydrogen cyanide stream 44 is passed through a cyanide recovery unit 50 and scrubbed with an alkaline hydroxide, advantageously NaOH because NaCN is a favoured cyanidation reagent, according to a known scheme for regenerating 11 alkaline cyanide from hydrogen cyanide. Hydrogen cyanide gas is highly toxic and digester 40 and cyanide recovery unit 50 are fully enclosed to avoid environmental release and safety hazard. Regenerated NaCN stream 52 may be sold or rendered harmless as above described. In this case, metal recovery from 5 co-precipitate material - as described below - may help to defray the costs of rendering the cyanide rich solutions safe. Hydrogen cyanide gas may advantageously be extracted as stream 44 from digester 40 which is operated under slight vacuum. This slight vacuum gives rise to a vigorous boiling action at about 1 10 C which can obviate the need 10 to provide digester 40 with an agitator. Digester 40 should be equipped with a solids trap or solids filter (not shown) because any coincidental precious metals in the feed solution will be precipitated at this stage and thus gold may be recovered, by solid/liquid separation as slimes stream 45, and fed to Dore furnace 70 from which a 15 precious metal bullion may be obtained and further refined at a gold refinery. The process, substantially as above described, can also be used as part of a process for extracting precious and base metals from materials, such as ores or tailings containing these metals, by cyanide heap leaching. Such a process is schematically shown by the process flow diagram of Fig. 2. 20 A heap 110 of copper-gold ore, which would normally be considered "refractory" to cyanide leaching, that is too expensive to leach using an alkaline cyanide leaching process is prepared on a leach pad. This ore could be sulphidic or oxidic. Alkaline cyanide, NaCN, is percolated through the heap 110, according to 25 usual practice, causing gold and copper values to solubilise forming complexes of the two metal values with cyanide. The cyanide leaching process, and solubilisation, follow the chemistry typical of alkaline cyanide leaching of copper gold ores as understood by those skilled in the art. Pregnant liquor 115 containing copper and gold solubilised by the cyanide leaching process is 30 directed to precipitation tank 120. A cuprous chloride solution is introduced to precipitation tank 120 as recycle solution stream 122 following the description provided above.

12 Copper and gold cyanides are caused to co-precipitate as above described and the remaining steps of the process mirror those described, with reference to Fig. 1, for treatment of the solution from a TSF. It will be noted, in this regard, that stage reference numerals used in Fig. 2 are the same as for the TSF 5 treatment scheme of Fig. 1 other than being prefixed with numeral "1". Figure 2 illustrates how the process might be applied to a conventional heap leach operation but where the gold leaching kinetics dictate that the material being treated must be subjected to a grinding process prior to cyanidation, it is necessary to perform a solid/liquid separation on the leached pulp. The solution 10 so obtained equates to stream 115, the solids, now cyanide free, can be safely deposited in a TSF 105. A scheme for doing this would involve recovery of pulp from cyanidation leaching using, for example, a continuous belt filter to separate solids and liquids. The pulp would be subjected to multi-stage washing to remove adhering cyanide solution. A semi-dried product could then be discharged to a 15 TSF 105. Alternatively, a counter-current decantation process could be used to produce a substantially copper and cyanide free tailing in a slurry form. There are some relatively minor differences between the process, when applied for treatment of TSF (flowsheet of Fig. 1) and when applied to an existing leaching operation (flowsheet of Fig. 2). The key difference is the ability to 20 recycle regenerated cyanide solution, as stream 152, to heap leach stage 110 though this does not preclude sale or treatment of excess cyanide in accordance with the options described for the Fig. 1 flowsheet. The metal recovery stages 160 and 170 are also likely to require sizing to higher capacity for an existing leach operation as compared with a TSF treatment operation. 25 The ability to recover gold and base metal values and alkaline cyanide for re-use in the cyanidation leach process as well as the ability to regenerate the cyanide compound precipitant, cuprous chloride plus the elimination of the carbon inventory and losses associated therewith may benefit economics of such extraction process to the extent that materials previously considered too 30 refractory or un-economic to process by cyanidation may be economically treated for metal recovery. At the same time, an environmental benefit of reducing output of cyanide contaminated materials to the environment, albeit contained in a tailings storage facility may be achieved.

13 Modifications and variations to the process for treating cyanide solutions of the present invention may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present invention. 5

Claims (5)

1. A process for treating a cyanide containing solution wherein said cyanide containing solution is treated, in a precipitation stage with cuprous chloride 5 in an amount equal to or greater than stoichiometric for reaction with copper containing cyanide complexes present in the solution to co precipitate cyanide and metal values as a material treatable for recovery of metal values and free cyanide. 10

2. A process of claim 1 wherein said cyanide containing solution is a solution from a tailings storage facility.

3. A process of claim 1 wherein said solution is obtained from an extraction process for extracting precious and base metals from materials by 15 cyanidation.

4. A process of any one of claims 1 to 3 wherein said metal values are copper and precious metals, preferably gold. 20

5. A process of claim 3 or 4 wherein acid treatment of said co-precipitated material produces hydrogen cyanide gas which is subsequently converted to alkaline cyanide. WATERMARK PATENT & TRADE MARK ATTORNEYS