[0001] This invention relates to a noble and base metal recovery process where cyanide is used.

[0002] The invention is concerned with such a process in which the comminuted ore is added to a cyanide leach circuit and the solution from the leach circuit is treated with activated carbon which becomes loaded with metal such as gold. The loaded carbon is delivered to an elution plant or desorption circuit where the metal is recovered from the carbon. The carbon is then regenerated for re-use.

[0003] This process is widely and successfully used and sophisticated developments have been made to the process. However, in most if not all such processes the cyanide used is the most expensive reagent used in the process and substantial amounts of cyanide are used making the process expensive. [0004] According to the invention there is provided a process as aforesaid wherein the solids and solution from the cyanide leach circuit are treated and copper is added to the slurry or solution in a mixing vessel, preferably an upflow mixing vessel to enable the copper to react with the cyanide and to adsorb on to activated carbon. Acid can also be added, preferably sulphuric acid, to lower its pH to between pH 8 to pH 9 and to maintain it at this pH for maximum adsorption efficiency onto the carbon. The loaded copper cyanide carbon is separated from the leach circuit and is passed to a batch or semi- continuous cold elution plant. [0005] From the elution plant the eluted carbon is transferred back to the cyanide adsorption circuit for re-use and the eluate is processed further.

[0006] The copper in the eluate is then precipitated with a reagent, preferably sulphuric acid at pH 2, to produce CuCN precipitate (or at pH 4.5 with NaSH to produce Cu ₂ S). The precipitated CuCN can be filtered and will then be available for re-use by returning to the Adsorption vessels in the cyanide removal process, or it can be treated further.

[0007] The cyanide that was complexed with the copper returns after neutralisation and is re-used as free cyanide in the leach circuit. In this way a substantial amount of the cyanide is saved for re-use. The eluate preferably has acid added thereto to reduce its pH to pH 2 and CuCN crystals are then precipitated which can subsequently be converted to Cu ₂ 0 crystals by a multistage counter current precipitation and filtration process by adding caustic solution and raw water. If desired the final Cu ₂ 0 crystals can be re dissolved by adding sulphuric acid followed by copper metal electrowinning. The metallic copper is available for sale without additional processing.

[0008] An embodiment of the invention will now be described by way of example with reference to the accompanying drawing which is a circuit diagram of the process of the invention.

[0009] Referring now to Figure 1 there is shown the mined feed resource 10 leading the ore to comminution 12. The comminuted ore solids are transferred to a cyanide leach circuit 14 including activated carbon all contained at about pH 10 to pH 11. The solids and solution from the leach circuit 14 are transferred to cyanide recovery adsorption circuit 16 which uses 1 to 6 mixing vessels or upflow vessels which improves the mixing and adsorption efficiency and increases precious and base metal recovery.

[0010] Copper, which may be Cu, CuCN, Cu ₂ 0 or CuO, is added from a feed 18 in the circuit 16 to react with the cyanide to form predominately cuprocyanide complexes i.e., Cu (CN) ₂ or Cu (CN) ₃ ^2_ . Sulphuric acid can be added to reduce the pH to between pH 8 and 9.5 to improve the adsorption of the cuprocyanide and precious metals onto the activated carbon. The carbon loaded with copper cyanide and precious metals is then passed to a batch or semi-continuous copper elution plant 22. [0011] The tailings after cyanide removal are directed to final tails disposal 24.

[0012] After the batch or semi-continuous copper elution in step 22 is completed, the eluted carbon is recycled along line 26 back to the cyanide adsorption circuit. CuCN precipitation is achieved by adding H ₂ S0 ₄ to lower the pH to 2 in step 28. The copper can also be precipitated with H ₂ S0 ₄ and NaSH at pH 4.5 to precipitate Cu ₂ S.

[0013] The eluted carbon is recycled along line 24 back to the cyanide adsorption circuit 16. After sulphuric acid is added to the copper-cyanide solution which would lower the pH to pH 2, the solution is now passed via means for providing solution de-gasification (HCN removal) in a de-aeration tower to extract maximum HCN. The filtrate is now neutralised to a pH above 10 and passed back via line 30 to the cyanide leach circuit 14 for further re use as free cyanide.

[0014] The batch or semi-continuous elution of carbon to elute the copper can be accomplished using ambient pressure and 25° to 50° centigrade temperature. The solution used for elution is sodium cyanide and sodium hydroxide (caustic). The use of semi-continuous elution at 21 will maximise the use of cyanide in the elution process and maximise the grade of copper in the eluate. The objective is converting the copper to the trivalent ad tetra complex (Cu (CN) ₃ ^2_ / Cu (CN ₄ ³ )), or to achieve a greater than 6:1 cyanide to copper ratio in the eluate.

[0015] After elution the copper and cyanide eluate at 5 are precipitated as CuCN crystals as a result of the lowering of the pH to less than pH 2.5 with the addition of sulphuric acid in a pipe reactor 22, using a de-aeration scrubber for HCN gas removal to improve the precipitation of CuCN.

[0016] The CuCN crystals from step 28 are separated from the remaining solution via pressure or vacuum filtration. The CuCN precipitate can then be mixed with water and re-used in the cyanide removal process to react with the free cyanide. Alternatively, the CuCN precipitate is then transferred to step 32 for conversion from CuCN to Cu ₂ 0. Filtrate from pressure filtration in step 38 is used in step 32 for the reaction of approximately 75% conversion of the CuCN to Cu ₂ 0. The NaCN filtrate from step 32 is passed via line 34 to the elution plant 22. Caustic (NaOH) is fed from a supply shown at 36 to the step 38 to convert approximately 15% of the CuCN to Cu ₂ 0 precipitate tank 38.

[0017] The precipitate from step 40 where approximately 10% of the CuCN is converted to Cu ₂ 0 precipitate can be further processed in step 44. In step 44 sulphuric acid is added to dissolve the Cu ₂ 0 crystals and recovered as copper metal by means of electrowinning.

[0018] The copper metal can now be sold without further processing.

[0019] The filtrate from step 40 is transferred counter current to step 38 and the filtrate from step 38 is transferred counter current to step 32.

[0020] Gold and other metals is/are recovered from the carbon in conventional manner which is not described herein.

[0021] It will be seen that the cyanide is recycled for re-use after passing through the circuit and not discarded as hitherto done. It has been found that depending on the ore, that approximately 50% and more of the free cyanide together with any Cu-CN complexes present left in the tails stream can be recovered which reduces the cost of the process significantly compared to conventional detox. It will also be seen that additional precious metals (gold and silver) are recovered from the solids and solution being treated in the process, improving the financials.

[0022] The invention is not limited to the precise constructional details or steps as set forth above, for example the separated CuCN crystals can be converted to Cu ₂ 0 crystals in more than 3 stages counter current precipitation or indeed in less than the three stages. [0023] The acid addition after copper addition before adsorption can also be excluded or included in the process.

[0024] The use of the upflow vessels has a very big benefit and improves the efficiency of the additional precious metal recovery.

[0025] The process of the invention can be used for recovery of free and weak acid dissociable (WAD) cyanide where cyanide is used in the process. Cyanide is typically used for gold, silver and copper processing.