How to Efficiently Recover Residual Gold from Loaded Activated Carbon

The recovery of gold from spent activated carbon is a critical and valuable final step in the carbon-in-pulp (CIP) or carbon-in-leach (CIL) processes. While the primary elution strip removes the majority of gold, a significant residual value can often remain trapped within the carbon's complex pore structure. Extracting this remaining gold efficiently requires a methodical approach that combines optimized chemical treatment, improved desorption conditions, and targeted recovery techniques. Success hinges on understanding how to condition the carbon, enhance the elution process itself, and then capture the gold from the resulting solution.

Moving beyond basic procedures allows operations to maximize yield and improve overall economics. The following guide outlines a practical, step-by-step framework for advanced recovery of residual gold from activated carbon.

Preparing the Carbon for Elution

Effective recovery begins with preparing the carbon to ensure the eluant can access the trapped gold. Pre-treatment cleans the pores and can regenerate the carbon's activity.

A standard first step is an acid wash pre-treatment. Soaking the carbon in a 5-10% solution of nitric or hydrochloric acid helps dissolve inorganic scales—like calcium carbonate or metal oxides—that may be clogging the pores. This is followed by a thorough rinse with deionized water. For a more advanced regeneration, microwave-assisted treatment shows promise. Combining microwave heating with an oxidant like hydrogen peroxide can rapidly break down organic foulants and help restore the carbon's porosity, leading to better elution performance in the next cycle.

Optimizing the Elution Process

The choice of eluant and the conditions under which it is applied are the most significant factors in stripping residual gold. The goal is to transfer the gold from the carbon surface into a solution as completely as possible.

The traditional and highly effective eluant is a sodium cyanide solution (0.1-1% NaCN), which must be maintained at a high pH of 10-11 with caustic soda for safety and efficacy. For operations seeking a non-cyanide alternative, thiourea in a sulfuric acid solution (e.g., 5% thiourea in 1M H₂SO₄) or thiosulfate at near-neutral pH are viable, more environmentally friendly options. Regardless of the eluant, improving the conditions increases yield. Elevating the temperature to 50-70°C significantly accelerates the desorption kinetics. Reducing the flow rate or using a pulsed flow pattern increases contact time between the eluant and the carbon. Furthermore, adding a small amount of an oxidant like hydrogen peroxide (e.g., 0.1%) can help destabilize the gold complex and promote its release from the carbon.

Enhancing Gold Recovery from the Eluate

Once gold is in the pregnant solution, robust methods are needed to recover it in a solid, marketable form. This stage focuses on efficient and complete precipitation or plating.

A direct and elegant method is electrowinning, where electrodes (typically stainless steel wool or titanium mesh) are placed directly into the eluate. Applying a low current density causes the gold to plate directly onto the cathode. The classic alternative is chemical displacement. Adding zinc powder (at about twice the theoretical requirement) to the acidified eluate causes gold to cement out. For faster kinetics, more active reagents like aluminum powder or sodium sulfite can be used. If the stripped solution still shows traces of gold, a final polishing step using a fresh batch of activated carbon or an ion-exchange resin can capture these remnants.

Processing the Carbon Residue and Final Residues

After the primary elution and recovery steps, the carbon itself may still contain traces of gold that are uneconomical to strip chemically. Recovering this requires destroying the carbon matrix.

The most straightforward method is high-temperature ashing. Heating the spent carbon to 800°C in a controlled furnace burns away the carbon, leaving behind a mineral ash that concentrates any residual gold. This ash can then be digested with aqua regia. For a wet chemistry approach, aqua regia digestion is definitive: refluxing the carbon residue in a 3:1 mixture of hydrochloric and nitric acid will dissolve even trace gold, which can be detected and recovered from the resulting solution. A milder alternative is the thiourea-ferric salt method, which uses an acidic thiourea solution with an oxidant (Fe³⁺) to leach micro-disseminated gold without using strong acids.

Implementing a Complete Recovery Strategy

Maximizing gold recovery from loaded carbon is not a single-step operation but a synergistic process. Best practice involves a logical sequence: begin with proper carbon pre-treatment to open pores, proceed with an optimized elution stage using the most suitable eluant and conditions, ensure complete recovery from the solution via electrowinning or cementation, and finally, account for trace values through carbon digestion or ashing. By paying attention to each of these stages, operators can significantly improve their overall recovery efficiency, turning what was once considered "spent" material into an important source of additional revenue.