Elution of gold from carbon is controlled mainly by the temperature of the solution, very little elution occurs until the temperature is above 70C, high levels of elution require temperatures above 90C, commercial elution is carried out at temperatures around 98 to 105C.
High levels of cyanide on their own will not strip gold from carbon without the higher temperatures.
2% cyanide is around the level used for high intensity cyaniding of coarse gold concentrates, the main difficulty with these levels is maintaining the dissolved oxygen (DO) level required to take advantage of the high cyanide level.
Options are to use a commercial leach promoter which will add oxygen chemically, add gaseous oxygen to the leach, add hydrogen peroxide to the leach or to continuously add air to the solution to maintain the DO level.
The last method is the cheapest but requires a recirculation line where the air is injected into a bleed stream.
2% cyanide will give protective alkalinity levels around pH11 + depending on the water quality and ore type, this removes the need for alkali addition.
The usual cyanide level is around 0.1% sodium cyanide which gives an actual cyanide level of about 0.05% or 500 ppm. Think 1 gram per litre or 1 kg per cubic metre of sodium cyanide.
The protective alkalinity from this level is not high enough for safety so lime or caustic is added to get the pH around 11.
All leaching is a trade off of time versus reagent strength, it depends on your own personal requirements as to how much you need to speed up the leach rate.
Always keep in mind that when you separate the leached ore from the leach liquor there will be a loss of around 200 litres of water per ton of ore, this is the amount of water needed to just wet the ore.
Assuming that you are removing the gold values from the leach liquor by carbon before carrying out the separation you will still lose the cyanide content of this 200 litres per ton.
It is better to minimise these losses and the associated costs by keeping the cyanide level as low as practical without going so low that you need an extended leach time.
Deano
