Zadra strip is the most common method for eluting (desorbing) gold from loaded activated carbon in an ADR plant. Loaded carbon is contacted with hot (90–95°C) caustic-cyanide-alcohol solution at atmospheric pressure for 24–36 hours. Gold transfers from the carbon back into solution, which is then sent to electrowinning.
How It Works
After carbon loads to maximum gold capacity in the adsorption columns, it is transferred to a strip vessel. Hot strip solution — typically 1% NaOH + 0.1% NaCN + 20% ethanol or methanol — is circulated through the vessel. The hot caustic conditions weaken the gold-cyanide-carbon bond, releasing the gold-cyanide complex back into solution. The pregnant strip solution flows to electrowinning cells.
Atmospheric vs Pressure
Zadra is atmospheric — runs at 90–95°C at 1 atm. AARL strip is pressurized — 110–130°C at 2–4 bar — and is faster (4–8 hours vs. 24–36 hours) but more equipment-intensive. Most KCA modular ADR plants use Zadra for simplicity.
Sizing
KCA delivers Zadra strip vessels at sizes from 1.5 ton (Nixon Fork, Alaska) to 5 ton (Hycroft, Nevada). The size is matched to the carbon-loading rate in the adsorption circuit and the desired strip cycle frequency.
Reagent Recovery
After electrowinning recovers the gold, the lean strip solution is recirculated. Periodic make-up of NaOH, NaCN, and alcohol is required. Carbon is then acid-washed (HCl) to remove calcium and base-metal contaminants before returning to adsorption.
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