Atom was contacted by a client whose tailings resource in the Gold Fields area of Western Australia had a good head grade of 1.8g/t, but contained 0.55% (5,500 ppm) copper of which approximately 50% was cyanide soluble.
High copper concentrations pose significant problems with gold cyanidation as copper is leached and adsorbed onto activated carbon (the carrier medium) preferentially. There are a number of possible solutions to this problem, including splitting the process into separate copper and gold streams. However, most existing solutions require complex and expensive processes which, when amortised over the tonnage of the available resource, were not economically viable.
Previous research has shown that copper concentrations of up to 300ppm do not affect gold loading significantly, but with increasing Cu concentrations, gold adsorption is correspondingly reduced. At a copper concentration in solution of 2,750ppm (compared to 1.8 ppm gold), the adsorption onto carbon would consist almost entirely of copper.
To ensure viability for the limited resource, Atom set the following project goals:
The challenge of developing a viable process with strict constraints required breaking away from conventional approaches to gold processing.
The key task was to find a method of adsorbing soluble copper before the gold adsorption process without diminishing the gold recovery. With this objective in mind, Atom researched the following methods of copper removal:
i. “Cold stripping” of copper: The key feature is the use of an ambient temperature intensive cyanide wash which involves targeting copper, which had already been adsorbed on to activated carbon (along with gold), being selectively removed from the carbon by repeatedly soaking it with an intense cyanide solution. The gold adsorbed on the carbon is not affected by this “cold wash” and therefore remains. Whilst this method had been well proven, it required high volumes of cyanide which increased costs and environmental liabilities.
ii. Post CIL/CIP recovery of cyanide: This method requires that cyanide is recovered after adsorption using a copper powder to complex with free cyanide and be converted to a dissolvable form of copper cyanide that can be precipitated as Cu(OH)2 by an increase in pH due to oxygen reduction during copper dissolution. This method would impose additional burden on the carbon adsorption, elution circuit and electrowinning process.
iii. Ion Exchange Resin methods: These rely on the ability of specially constructed IX Resins to be able to adsorb metal complexes including copper, zinc, iron, nickel, gold and silver and then selectively elute the non-precious metals with sodium cyanide followed by the precious metals with acidic thiourea. There are many variants of this method using a variety of derivatives of the technology. In practice, some have been reasonably successful with certain mineralogies and others have not, but they are generally complicated to control and optimise without specialised skill sets being available.
iv. Solvent Extraction methods: This is a method of separating metal complexes based on their relative solubilities in two different immiscible liquids such as water and an organic solvent such as kerosene. The feed solution and the solute (in this case copper and gold) would be mixed the solvent (i.e. kerosene or similar) in a controlled manner which allows the target metal (i.e. gold) to be dissolved into the solvent whilst the gangue (i.e. copper) is left in the aqueous solution as a raffinate. For large plants separating metals such as copper and cobalt, and uranium and plutonium this is a standard process – however, for a limited resource with low grade copper (i.e. of little value) it could not be scaled down efficiently.
It was clear from this research that applying mainstream methods such as the above would be unlikely to have commercially unsuccessful outcomes.
Atom engaged with Bruno Sceresini, inventor of the Sceresini Process, which is based on the principle of removing the copper cyanide complexes before the gold leaching process by restricting the supply of cyanide so that the dissolved copper is starved of free cyanide.
This structured approach to solving a complex problem enabled a more appropriate solution for the client to be found and entered into the process design.
