The extraction of precious metals from an alkaline cyanided medium by granular activated carbon

[摘要] ENGLISH ABSTRACT: A 2 stage heap leach process to extract base and precious metals from the Platreef ore is currently being investigated industrially. A first stage bioleach is used to extract the base metals. In the 2nd stage, cyanide is used as the lixiviant at high pH to extract the platinum group metals and gold. By analogy with current gold recovery practices, the present study investigates the preferential and quantitative adsorption of precious metals (Pt, Pd, Rh and Au) over base metals (Cu, Ni and Fe) from an alkaline cyanide medium, by means of granular activated carbon.Experiments were designed statistically to optimise the process parameters using synthetic alkaline cyanide solutions close in composition to those expected from plant leach solutions. The statistical approach allowed the development of a reliable quantitative approach to express adsorption as a response variable on the basis of a number of experiments. A 2IV(7-2) fractional factorial design approach was carried out in a batch adsorption study to identify significant experimental variables along with their combined effects for the simultaneous adsorption of Pt(II), Pd(II), Rh(III) and Au(I). The adsorbent was characterized using SEM-EDX, and XRF. Precious metals adsorption efficiency was studied in terms of process recovery as a function of different adsorption parameters such as solution pH, copper, nickel, free cyanide ion, thiocyanate, initial precious metal (Pt, Pd, Rh and Au) ion and activated carbon concentrations.It was shown that adsorption rates within the first 60 minutes were very high (giving more than 90% extraction of precious metals) and thereafter the adsorption proceeds at a slower rate until pseudo-equilibrium was reached. Among the different adsorption parameters, at 95% confidence interval, nickel concentration had the most influential effect on the adsorption process followed by the adsorbent concentration. Adsorption of Ni was found to proceed at approximately the same rate and with the same recovery as the precious metals, showing a recovery of approximately 90% in two hours. The kinetics of Cu adsorption were slower, with less than 30% being recovered at the 120 minute period. This suggests that the co-adsorption of Cu can be minimised by shortening the residence time.Adsorption of Fe was found to be less than 5%, while the recovery of Rh was negligibly small. The effect of thiocyanate ion concentration was not as important as the effect of free cyanide ion concentration but still had some influence. The correlation among different adsorption parameters was studied using multivariate analysis.The optimum experimental conditions resulted in a solution with pH of 9.5, [Cu(I)] of 10 ppm,[Ni(II)] of 10 ppm, [CN ] of 132.44 ppm, [SCN ] of 98.95 ppm, [PMs] of 2.03 ppm and [AC]of 10 g/L. Under these conditions, predicted adsorption percentages of Pt, Pd and Au wereapproximately 98, 92 and 100%, at the level of 95% probability within two hours as aneffective loading time. The negative values of ΔG° for all ions under optimum conditionsindicate the feasibility and spontaneous nature of the adsorption process. Chemisorption wasfound to be the predominant mechanism in the adsorption process of Pt(II), Pd(II) and Au(I).Based on their distribution coefficients, the affinity of activated carbon for metal ions followsthe selectivity sequence expressed below.Au(CN) > Pt(CN) > Pd(CN) > Ni(CN) > Cu(CN)Finally, it is important that additional research and development activities in the future shouldprove the economic viability of the process. Future work is also needed to investigate theadsorption of precious metals (PMs) by comparing the efficiencies and kinetics of adsorptionwhen using sodium hydroxide (in this study) or lime, respectively, in order to control the pH.

[发布机构] Stellenbosch University