Elution and carbon regeneration

Loaded carbon coming off the adsorption circuit now has to give the value back and be made fit to go round again. Two steps do it: elution strips the gold off the carbon into a small, hot, concentrated solution, and regeneration restores the carbon’s activity so it can return to the circuit. Together they close the adsorption loop, and they are where the value the carbon gathered is finally handed to recovery.

Elution: stripping the value back off

Elution reverses adsorption by changing the conditions until the carbon’s hold on the value collapses. A hot caustic-and-cyanide solution — the basis of the common Zadra and AARL routes — drives the gold cyanide complex back off the carbon and into the eluate. The eluate is the prize: small in volume and high in tenor, the concentration step the whole adsorption route exists to reach, and it goes forward to recovery by electrowinning or precipitation. Where the adsorption circuit took the value out of a vast dilute stream, elution puts it into a small rich one.

Regeneration: restoring the carbon

Stripped carbon does not adsorb as well as fresh, because organic foulants and carbonate scale have built up on its surface and in its pores. Thermal regeneration in a rotary kiln, at high temperature in a controlled atmosphere, volatilises and burns off the organic foulants and reopens the pore structure, returning the carbon to near-fresh activity to recirculate. An acid wash ahead of the kiln removes the carbonate scale first. The regenerated carbon rejoins the adsorption circuit; the small inevitable losses are made up with fresh carbon.

The regen kiln is residence-time sizing

A carbon-regeneration kiln is a small rotary kiln, and it is sized by the same logic as any rotary kiln: the mean solids residence time from the kiln length, internal diameter, rotation speed and slope, on the Sullivan, Maier & Ralston (1927) relation. The residence time is what matters because the carbon has to spend long enough at reactivation temperature for the foulants to leave. This is the fourth justification on this path for the kiln residence-time calculator — after roasting, calcination and acid baking — and it is the same arithmetic each time: a kiln is a kiln, and the residence time falls out of its geometry and motion. The calculator below runs the Sullivan form; the worked thread quotes its committed example.