Cyanide Geochemistry. Outline. Introduction to Cyanide Cyanide in the beneficiation of gold Heap Leach Process Cyanide tank leach and CIP circuits Optimum Conditions for CN leaching Extraction of gold from the CN solution (a) Merrill Crowe Process (b) CIP Process Cyanide Analysis
NH2CH2COOH = HCN + CO2 + 2H2
O2 + 2 H2O + 2 e- => H2O2 + 2 OH-
Au => Au+ + e-
Au+ + 2CN- => Au(CN)2-
4Au + 8NaCN + O2 + 2H2O = 4NaAu(CN)2 + 4NaOH
Or Adamson’s 2nd reaction
2Au + 4NaCN +2H2O = 2NaAu(CN)2 + H2O2 + 2NaOH
NaAu(CN)2+ Zn = NaZn(CN)2 + Au
CN is difficult to analyze because of the difference in solubility of the various complexes.
1. Weak acid dissociable (WAD) cyanide.
Reaction between cyanide and water
produces HCN gas
CN- + H2O = HCN + OH-
72 complexes with varying solubilities are possible from 28 elements. These rapid reactions immediately remove CN- from solution.
NaCN, KCN and Ca(CN)2, Hg(CN)2 dissolve in water to give cyanide anions
NaCN = Na+ + CN-
Ca(CN)2 = Ca2+ + 2CN-
Insoluble Neutral Cyanide Compounds
Zn(CN)2, Cd(CN)2, CuCN, Ni(CN)2, AgCN
Cyanide complexes form in order of increasing number of CN ligands with successively higher CN concentration
Fe(CN)64- + H+ = Fe(CN)53- + HCN
the compound is a deep “Prussian” blue.
Sx2- + CN- = Sx-12- + SCN-
S2O32- + CN- = SO32- + SCN-
HCN + 2H2O = NH4COOH (ammonium formate)
HCN + 2H2O = NH3 + HCOOH (formic acid)
HCN + O2 = 2 HCNO
HCNO + 0.5 O2 + H2O = NH3 + CO2
CN + H2S = HCNS + H+
HCN + HS = HCNS + H+
CN content e.g. Landusky heap leach remediation