Pregnant gold solutions are passed through activated carbon, where the gold is adsorbed on carbon.
We supply several grades of activated carbon for improved gold recovery. We pride ourselves in ensuring the following carbon properties are enjoyed by our clients:
- Our carbon is coconut shell-based giving it low susceptibility to attrition because of its hardness.
- It has excellent activity making gold adsorption easy
- We also offer activated carbon in several size fractions which gives an array of properties that can suit any particular process.
- With good carbon management practices, our activated carbon has proven to have longer life in the circuit. This is attributed to its ease of regeneration, hardness even under severe CIP conditions as well as maintenance of high levels of activity.
Carbon loading varies from 300g/ton to as much as 20 000g of gold/ton of carbon
There are several grades of activated carbon, but for gold processing E612 and E816 are the most commonly used.
The main sizes of activated carbon for gold processing are given in the table below:
Form
|
Mean Particle Size (mm)
|
Size Fraction (US sieve no.)
|
Raw Material
|
Granular
|
2.5
|
6*12
|
Coconut Shell
|
Granular
|
2.1
|
6*16
|
Coconut Shell
|
Granular
|
1.7
|
8*16
|
Coconut Shell
|
There are 3 main methods of adsorbing gold on activated carbon in gold leaching:
- Carbon-in-leach (CIL) – in CIL, leaching and CIP are integrated into a single process. Gold is thus adsorbed on the carbon almost as soon as it is dissolved in cyanide. CIL is the preferred method where there are preg-robbing elements in the ore which would otherwise compete for gold with carbon. The activated carbon has to have a high activity to adsorb the gold before any other competing carbon forms do so.
- Carbon-in-Pulp (CIP) – in CIP, there are several agitated tanks where NaCN, lime and oxygen are added to dissolve the gold. The carbon is placed in inter-stage screens and moves counter current to the flow of pulp through the tanks.
- Carbon-in-Column (CIC) – in these solution flows through a series of fluidized beds which can process solution with as low as 2% solids. CIC is mainly used to recover gold from heap leach circuits.
The 3 size fractions in the table above all have individual advantages and disadvantages.
Generally because of increased surface area, the smaller the size fraction, the higher the gold loading capacity. Inter-stage screen management in CIP especially because of the high attrition forces the carbon is exposed to is very important. Large size fractions are recommended to ensure one never losses gold through the screens. Of the three sizes above 6*12 would thus be recommended in CIP and to an extent CIL.
8*16 can be safely used in plants employing CIC.
Factors Affecting Adsorption rate
- Fouling – this is the contamination of activated carbon during use. Major foulants are some flotation reagents, greases used to oil agitators or mills
- Time of contact – the longer this is the more efficient the adsorption
- Temperature – rate increases slightly with temperature, with slightly slower rates likely to be experienced in cold seasons
- Pulp density – rate of adsorption decreases with increasing slurry density. However, suspension of carbon is difficult if pulp density is too low
- Gold concentration – rate increases with increase in gold concentration
Elution
Loaded carbon from CIC, CIL or CIP is removed and washed before undergoing elution at high temperature and pH. Typical solution concentrations for pressure stripping consist of % NaCN and % NaOH. Elution brings back the gold into solution before it is captured by wirewool in electrowinning cells.