There are numerous reasons why a process plant may miss recovery targets. Gold can be more encapsulated in insoluble material, other elements can consume important reagents -limiting availability for gold-, and sometimes the ore is characterized as “Preg Robbing.” Refractory ore is an issue and has been for a while. Notable mines with refractory ore are Prestea, Tarkwa, and Carlin.
What does this mean?
Sometime ores contain carbon. This carbon absorbs gold particles which would have otherwise been absorbed onto the “reagent” carbon. In “The Chemistry of Gold Extraction,” that author states that as little as 0.1% carbon may produce preg-robbing properties.
Carbonaceous ores can be separated into two categories: mildly and highly carbonaceous.
- Mildly Carbonaceous: contain small quantities of organic carbon, typically less than 1%.
- Highly Carbonaceous: contain carbon >1% and contains carbon that has a strong gold absorbing tendency. This material can reduce gold recoveries to below 80%.
Ways to deal with preg-robbing:
- Use a CIL process rather than CIP: a carbon in leach flowsheet completes the leach and absorption process in the same location simultaneously. In completing this process simultaneously, there is less time for the organic carbon to absorb the dissolved gold.
- Add Kerosene: hydrocarbons can passivate the surface of carbon, decreasing the propensity to absorb gold.
- Chlorine Treatment: Aqueous solutions of chlorine have strong oxidizing capabilities. It’s not well understood why this process works. Between a pH of 3 and 5 hypochlorous species are predominant and are thought to passivate the carbon surface.
- Roasting: Burn it off!
John O.Marsden, I. H. (n.d.). The Chemistry of Gold Extraction. SME.