Application of the process of Adsorption of complex aurocyanide

Adsorption of aurocyanide complex can be done using strong base resin and weak base resins. Strong base resins have high loading capacity and good kinetic but its selectivity is bad. The selectively of adsorption of the different metal cyanides is pH dependant. At low pH, metal cyanide salts are precipitated which reduces competition for resin adsorption sites. Selective precipitation of base metal prior to precious metal extraction by strong base resins is an option. The main disadvantage of strong base resin is the elution, but has been shown that thiocyanate is a good alternative.

Several elution tests with thiocyanate were carried out by C. Fleming.[10] Employing a concentration 0.5 to 5.0 M found different elution rates, obtaining the best result with a solution 5.0 M. the concentration of gold on the resin being reduced from 4500 to less than 20 g/t in 4 hours under ambient conditions. However a solution 2.0 M can be adequate. See Figure 8.

The rates of pH during elution when were studied from 2 to 12 showed small variation. The ph value apparently has little effect on the rate of elution and small variations can be occurred by secondary aspects such as contamination of recycled solution due to corrosion. Thus, a good ph value for the elution should be major than 10. As can be appreciated application of strong base resins is feasible. Once eluted the resin, the obtained solution with high gold concentration can be sent to electrowinning.

Weak base resin are selective and can be eluted easily but have low capacity, approximately 25-50% of a strong base resin. Loading and selectivity are depended of pH. Selectivity improves with increasing pH; however, both the capacity and adsorption rate are reduced markedly as the pH is increased above 8. These resins can be eluted by simple hydrolysis with sodium hydroxide, the rate increases with increasing the NaOH concentration to an optimum concentration of 0.5 M. At higher concentrations the rate is reduced, maybe to the precipitation of some hydroxides. Then weak base resins can be employed in the gold recovery from cyanidation.

Some pilot tests were carried out at Mintek some time ago. Some results are presented in Table 3. The performance of the RIP and CIP plants are very similar, with stage efficiencies of between 70 and 75% under the chosen operating conditions. This is interesting, since one of the claimed disadvantages of the resin for gold is its non-selectivity.

Table 3. Extraction results from RIP and CIP plant. [10]

Extractant	Results	Feed	Stage 1	Stage 2	Stage 3	Stage 4	Stage 5
Strong base resin	Gold in solution, ppm Extraction, % Gold on resin, ppm	2.63	0.38 85 5290	0.11 71 1240	0.03 73 550	0.008 73 230	----
Weak base resin	Gold in solution, ppm Extraction, % Gold on resin, ppm	2.06	0.41 80 4125	0.068 83 1210	0.018 74 380	0.006 67 84	----
Activated carbon	Gold in solution, ppm Extraction, % Gold on resin, ppm	2.81	0.83 70 6080	0.23 72 1580	0.066 71 700	0.018 73 320	0.005 72 190

The rate of elution is an aspect very important in the efficiency of the RIP process. The best results with strong base resins can be obtained in the range 40-60 ^oC.

A schematic flowsheet for the RIP process is showed in Figure 9. The final design is strongly dependent of the elution method and the type of resin employed in the adsorption stage.