Copper and other cyanide complexes are removed from the resin with a high strength solution of sodium chloride. The resin is converted to chloride form, and the copper eluate is discharged to the metal precipitation stage. Precipitation is effected by the addition of sulphuric acid and NaHS. The precipitate is thickened and filtered to produce a washed cake for transport and sale. The product is a Cu 2 S / CuS concentrate containing up to 65% Cu, depending on the eluate composition. The cyanide that was associated with the copper and other eluted complexes is recycled to the leach plant. RECYN III Gold is adsorbed in the ReCYN metal circuit along with WAD cyanides. If the ReCYN plant follows a CIL or CIP circuit, then the resin acts in a gold scavenging role. However, in some circumstances, the carbon circuit can be eliminated, and ReCYN becomes the primary gold recovery method, known as a ReCYN III circuit. A typical loaded resin from a ReCYN III adsorption circuit treating a copper/gold ore will assay 50kg/t Cu, 100kg/t NaCN and 5kg/t Au. Depending on the adsorption solution feed chemistry, there can be many other cyanide complexes that are adsorbed onto the resin, including Zn, Ag, Ni, Fe, SCN etc. (cyanate is not adsorbed). These complexes are desorbed to different degrees in the various elution circuits. Elution rates for each elution circuit are determined by consideration of the total quantity and type of complexes on the loaded resin and the elution efficiency of each species. The resin concentration in adsorption is controlled by varying the elution rate for each elution step. For example, considering a 3 Mtpa ore treatment rate, cyanide elution may require an elution rate of 2 t/hr resin, which gives a resin-in-slurry concentration of 5 g/l. For metal cyanide elution, the elution rate may be 1 t/hr, resulting in a resin-in-slurry concentration of 2.5g/l. These are typical average concentrations and will vary according to solution chemistry and other factors as determined by testwork. Gold behaves differently to WAD metals and is categorised as a SAD complex, thus, a different elution method is employed using Na 2 ZnCN 4 . The other major difference is the gold concentration in solution, which is usually very much lower than the WAD metals, sometimes by a factor of 100:1 or more. The relatively low concentration of gold in solution exiting the leach train also impacts the equilibrium loading of gold compared to copper. To avoid the necessity for a counter-current adsorption circuit, as with carbon, residual gold is scavenged in the cyanide adsorption circuit to ensure low residual gold in final tails solution. ReCYN III has two adsorption and three elution circuits, producing cyanide, base metals and gold. All three recovery systems have been demonstrated on a commercial scale, but the Mt. Morgan Tailings project is the first plant designed as a ReCYN III, incorporating the simultaneous recovery of cyanide, copper and gold in one plant. ReCYN I & II parts of the ReCYN III circuit operate as described in previous sections. For gold recovery, a bleed of metal loaded resin is sent to the gold elution circuit on a batch basis, the quantity determined by the gold loading on the resin. Gold elution is carried out batch-wise in a combined elution/electro-winning circuit, similar to a Zadra elution process. The gold elution time is much longer than the free and complexed cyanide elution steps (ie; 30 hours for gold compared to 2 hours for free and complexed cyanide elutions). Gold is eluted with a ZnCN 4 2- solution at 60 o C, which is also very effective at eluting the copper cyanide complex. The copper is substantially eluted before the gold, allowing the first bed volume of eluate containing most of the copper to be diverted to the copper recovery circuit with minimal loss of gold. Subsequent bed volumes of eluate are circulated through an electro-winning circuit for gold recovery. High efficiency cells are necessary to minimise the gold returning in spent electrolyte to the elution column.
ALTA 2020 Gold-PM Proceedings
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