A fundamental study of the dissolution of gold from refractory ores

[摘要] ENGLISH ABSTRACT: The dissolution of gold from refractory ores is a complex kinetic problem involving a number of chemical,masstransport andmineralogical factors. InmostWitwatersrand ores inSouth Africa more than97 % of the gold is dissolved in cyanide mediumafter a residence time of about16hours inpachucatanks.Thishigh percentage maybethereasonwhy so littlefundamental research has been done into the mechanismand kinetics of the leaching process. With the increasingly lower grades of ore mined, the introduction of backfill mining, and the reductionofprofitmargins,ithasbecomeimperativetoincreasetheefficiencyofgold dissolution. The effects of the chemistry and particle size on the dissolution of gold in each sample of ore were studied in detail. The emphasis in this study is on the effect of the leaching behaviour of various ore constituents on the rate of gold dissolution.Interferenceswiththeleaching of gold incontactwith othermineralsor metalscouldbe attributed to the galvanic interaction (electrical conductivity) between the gold and the mineral andtotheformationof asurfacefilmonthegold surface. Sulphidemineralsandtheir oxidation products cause the largest decrease in gold dissolution rate. Galena enhances the rate of gold dissolution owing to dissolved Pb(II)-ions.Gold in contact with conductiveminerals passivates as a result of the enhanced magnitude of the cathodic cu1Tent.In all experiments the rotating disc of gold passivated so thattherate of dissolutionwas muchslower thanthat predicted by a mass-transport limiting model.The various films that form on the surface of the gold and associated minerals, as well as the galvanic interaction, dependlargely on the pretreatment of the ore. Pre-eliminationof host mineralsfrom thegold bearing ore increases thedissolution rate of gold, andexplains the kinetics of reaction on the gold surface to a large extent. The selective destruction of the various minerals with oxidative acid leaches destroys and/or decomposes certain minerals which may form films on the gold surface by precipitation. The chemical compositionof these films and precipitates depends on the mineralogy of the sample.Thesefilms may be oxides, sulphides, carbonates and cyanide complexes.The complexes can be destroyed, depending on the nature ofthefilm,byinterstage diluteacidand/or cyanidewashesinanagitated vessel.The destruction of the films exposes the gold surface for cyanidation.A simple distribution function similar to the King liberation modelis proposed and tested to describe the dissolution step in the multi-step leaching mechanism. For the King model, good agreement is shown with experimental results. For the liberation results obtained by leaching in this study, the trend is co1Tect, but calibration is required for a close fit. A potentially important use for the liberation model by leaching is to predict the leachable or free gold in an ore from thefree gold in the completesample. This approach for studying the leaching behaviour of different gold bearing minerals has provided reasons why some ores leach better than others.

[发布机构] Stellenbosch University