A METHOD OF RECOVERING BASE METALS FROM LOW GRADE ORES AND RESIDUES

[0001] This Invention relates to a method of recovering metals from low grade ores and residues. More particularly, the invention is concerned with increasing the economic viability of recovering base or other metals by means of cyanide leaching of low grade ores and residues by providing a method for the effective recovery of cyanide, contained in a lixiviant to the leach process, which allows for the recovered cyanide to be recycled.[0002] Without being restrictive “low grade ores”, as used herein, includes oxide and sulphide ores, especially ores containing high acid-consuming gangue materials such as caScite, dolomite, calcium, magnesium, aluminium, manganese and iron.[0003] The recovery of base metals such as copper, nickel, cobait and zinc from low grade ores by means of sulphuric add leaching is often considered uneconomical, at least for the following reasons:1. the amount of acid, which is a costly reagent, consumed during the leaching of gangue materials such as the aforementioned is reiativeiy high-2. base metal ores or residues containing sulphides are not leached efficiently in sulphuric add and, for most sulphide materials, an oxidant such as ferric must be employed. The cost of the oxidant Is normally unacceptably high; and3. during sulphuric add leaching precious metals such as gold, sliver and platinum are not leached and an add residue must be further processed by cyanidation to recover these metals.[0004] Although the cyanidation of low grade ores can he very effective in recovering base metals and precious metals, the high levels of free cyanide that are required to ensure effective leaching have a detrimental effect on the economic viability of the process.[0005] Several methods for recovering cyanide, following a cyanidation process, have been developed so that the recovered cyanide can be recycled in order to make the cyanidation process more economical. One method, referred to as the SART process, used for recovering gold from copper-gold ores, has shown promise.A basic flow sheet of the SART process Is shown in Figure 1. The process includes the following steps: leaching the ore with cyanide to form a solution which is subjected to a first soiiti/liquid separation step to produce a pregnant leach solution (PLS) and solids;acidification of the PLS using H₂S04_; followed by suiphidisation by means of the addition of H₂S, to produce a solution containing a metal sulphide complex, which is subjected to a second solid/liquid separation step, such as activated carbon filtration or ion exchange, to form a filtrate and a filtered liquor. Copper, gold and silver are recovered from the filtrate and the pH of filtered liquid is adjusted before recycling to the leaching step. The solids from the first solid/liquid separation step are washed and subjected to a cyanide destruction step to form a detoxified product which is disposed of in a slimes dam.[0007] The SART process is primarily used for gold recovery. To the applicant’s knowledge the SART process has not been considered for the treatment of low grade base metal ores or residues, for the quantity of base metal which would be recovered would not normally justify the high cost of the reagents.[0008] An object of the present invention is to provide a method which, at least partially, addresses the problem associated with these high costs.

[0009] The Invention provides a method for the recovery of at least one metal from a low grade ore or residue, wherein the method includes the following steps:a) cyanide-leaching the low grade ore or residue, at an alkaline pH, to produce a pregnant leach solution which contains at least one metal cyanide;b) upgrading the pregnant leach solution by removing leached or barren solids from the pregnant leach solution containing the metal cyanide;c) subjecting the upgraded pregnant leach solution to ultrafiitration to produce a clarified liquor containing the metal cyanide;d) subjecting the clarified liquor to nano-filtration to produce a metal cyanide concentrate and a permeate;℮) subjecting the metal cyanide concentrate to acidification and then to sulphidisaiion to produce a solution which contains a metal-sulphide and hydrogen cyanide;f) subjecting the solution formed in step e) to a solid liquid separation step thereby to recover at least some of the metal suiohide and at least some of the hydrogen cyanide; andh) recycling the recovered hydrogen cyanide to the cyanide leaching (step a)).[0010] The cyanide leaching step a) is conducted at a pH which preferably is between 10 and 11.[0011] The removal of the leached or barren solids from the pregnant leach solution in step b) may be effected by means of counter-current decantation, filtration or the like.[0012] in order to optimise the recovery of the metal sulphide and the hydrogen cyanide, the leached or barren solids may be subjected to a washing step. A resulting solution is then separated into a wash water which is recycled to the upgraded pregnant leach solution, and a washed leach residue which may then be subjected to a cyanide destruction step.[0013] The nano-filtration in step d) may be carried out using any suitable membrane having an appropriate pore size to prevent the metal cyanide concentrate from permeating the membrane.[0014] The permeate produced in step d) may be recycled to the cyanide leaching step a).[0015] In the acidification of the metal cyanide concentrate (in step ℮)) the pH of the concentrate may be lowered to a value of between 2 and 6. This may be done by the addition of sulphuric acid. The lowering of the pH causes the meiai cyanide concentrate to dissociate and form a solution which contains metal-sulphate and hydrogen cyanide.[0018] in the subsequent suiphidisation process, which is preferably effected by adding sulphide in the form of hydrogen suiphid℮ or sodium sulphide, the metal sulphate In the solution is sulphidised to form the stream which contains metal sulphide.[0017] The permeate which is produced in step d) is, in a variation of the invention, subjected to a reverse osmosis process to form a high quality water stream and a NaCN concentrate which is recycled to the cyanide leaching step a).[0018] The metal may be extracted from the rnetai sulphide by any suitable means known in the art. The metal recovered may be base metal, such as copper, nickel, cobalt or zinc, or precious metal, such as gold or sliver.

'he invention is further described by way of examples with reference to the accompanying drawings in which:Figure 1 is a flow sheet of the SART process and has already been described In the preamble to this specification;Figure 2 Is a flow sheet of a modified SART process according to the Invention; and Figure 3 illustrates a variation to the process In Figure 2.

[0020] Figure 2 of the accompanying drawings illustrates steps in a method Τ3 according to the invention for the recovery of metals from low grade ores and residues 12. Typically the ores and residues are found in the tailings of sulphide and oxide ores and contain high levels of acid-consuming gangue materials such as calcite, dolomite, calcium, magnesium, aluminium, manganese and iron. The metals in these ores and residues may include precious metals such as gold and silver and base metals such as copper, nickel, cobalt and zinc,[0021] In a first step 14 the ores and residues 12 are subjected to cyanide leaching at an alkaline pH by the addition of cyanide 16. Preferably the pH Is between 10 and 11 pH values which are regarded as optimal. The outcome of the cyanide leaching step Is a pregnant leach solution 18 which contains at least one metal cyanide i.e. a cyanide of a target metal which may be any one of the aforementioned metals.[0022] The pregnant leach solution 18 is subjected to a soSid/Siquid separation process 20 which may be effected by means of counter-current decantation, filtration or the like. The process 20 removes barren or leached solids 22 from the solution and produces an upgraded pregnant leach solution 24 which contains the metal cyanide.[0023] To optimise the recovery of the base metal and the free cyanide the solids 22 are washed in a step 28 and a resulting solution is filtered to produce a wash water 28 and washed solids 30.[0024] The washed solids are subjected to a cyanide destruction step 32 to produce s detoxified product 34 which is sent to a slimes dam 38.[0025] The wash water 28 which may contain some of the metal cyanide, and the upgraded pregnant leach solution 24 are subjected to an ultrafiltration or clarification step 38 which produces a clarified liquor 40 which contains the metal cyanide. This is followed by a nano-filtration step 44 during -which the metal cyanide is removed from the clarified liquor 40. The nano-filtration step 44 is carried out using an appropriate membrane which has a chosen pore size which allows a permeate 46 of water and sodium cyanide (NaCN) to pass through the membrane. The permeate is then recycled to the step 12.[0026] l he pore size of the membrane prevents the metal cyanide from permeating the membrane. The metal cyanide which is thereby concentrated constitutes the retentate 48.[0027] In an acidification step 50 sulphuric acid 52 is added to the retentate 48 to lower the pH of the retentate to a value of between 2 and 6. Once the pH is sufficiently low, the metal cyanide complex dissociates and forms a solution 54 which contains a metal sulphate complex and dissolved hydrogen cyanide.[0028] In a subsequent suiphidisation step 56 hydrogen sulphide 58 (or sodium sulphide in a salt form) is added to the soiution 54 to produce a solution 60 which contains a metal sulphide complex 62. This complex 62, which for example may be a copper, gold or silver sulphide product, Is recovered from the soiution 60 by means of a solid/liquid separation step 64. The metal content may be recovered from the complex using any suitable technique.[0023] The pH of a liquid 66 produced by the step 64 is adjusted in a step 68, for example by the addition of lime 76 (CaO), to a value of between 10 and 11, le. to the alkaline operating range of the cyanide leaching step 14. Hydrogen cyanide 74 emerging from the step 68 is recycled to the cyanide leaching step 14.[0030] i he Invention provides a number of significant benefits which include the following:1. the nano-filtration step 42 effectively removes the high add consuming free cyanide, thereby reducing the amount of sulphuric acid (56) which is subsequently needed to acidify the retentate;2. as the permeate 46 is recycled to the step 14 before the alkalizing step 48, the permeate remains at the correct alkaline pH. This reduces the amount of calcium oxide needed to alkalize the liquid 66 and thus to recover the hydrogen cyanide 74;3, due to the decrease in chemical processing a substantial amount of sodium cyanide Is recovered in the permeate, and is recycled to the step 14 to provide a significant reduction in the reagent.4. the discharge of harmful waste waters in to the environment Is reduced.[0031] The aforementioned benefits produce a substantial saving in operating and capital costs. Table 2 shows the savings in operating expenditure (ΟΡΕΧ) produced by the process according to Figure 2 when compared to a conventional SART process of the type shown in Figure 1.Table 2