Merrill-Crowe Precipitation (2D)

Reagent Dosing

Zinc dust dosage 900 g/t

Deaeration vacuum 92%

Lead nitrate activator 8 ppm

Feed gold grade (PLS) 6.0 ppm

Live Readout

Residual O₂

0.0 ppm

Precipitation eff.

0.0%

Tail solution Au

0.00 ppm

Zn : Au consumption

0.0 : 1

Au precipitated

0 nuggets

Zn excess ratio

0.0x

How it works

The Merrill-Crowe process recovers gold (and silver) from a clarified cyanide pregnant leach solution (PLS) by cementation: metallic zinc dust reduces the soluble gold cyanide complex to solid gold.

2 Au(CN)2- + Zn -> 2 Au(s) + Zn(CN)4(2-)

Two things control how completely that reaction runs:

Dissolved oxygen — a vacuum deaeration tower strips O₂ out of the PLS before the zinc box, because O₂ re-oxidises precipitated gold and wastes zinc on a parasitic side reaction (2 Zn + O₂ + 2 H₂O → 2 Zn(OH)₂). Low vacuum leaves O₂ near its ~8 ppm saturation value and starves the real reaction.
Zinc excess & lead nitrate — plants dose zinc well above the ~0.166 g Zn per g Au stoichiometric ratio, and add a few ppm of lead nitrate, which deposits a lead film on the zinc surface (a galvanic Pb–Zn couple) that keeps the reaction fast instead of passivating.

This panel drives a simplified rate model: effective zinc = dosage − zinc lost to residual O₂; the precipitation efficiency rises with the zinc excess ratio and the lead-nitrate activation factor, and falls as residual O₂ climbs. The pipeline animation moves gold-complex ions from the clarifier through the deaeration tower into the zinc cone, where each ion is captured as a solid gold nugget with a probability equal to the current efficiency — the rest continue on as barren tailings.

Au(CN)2- ion Zinc dust Precipitated gold O₂ bubble