A fundamental study of the dissolution of gold from refractory ores

Lorenzen, Leon (1992-12)

Thesis (PhD)--Stellenbosch University, 1992.

Thesis

ENGLISH ABSTRACT: The dissolution of gold from refractory ores is a complex kinetic problem involving a number of chemical, mass transport and mineralogical factors. In most Witwatersrand ores in South Africa more than 97 % of the gold is dissolved in cyanide medium after a residence time of about 16 hours in pachuca tanks. This high percentage may be the reason why so little fundamental research has been done into the mechanism and kinetics of the leaching process. With the increasingly lower grades of ore mined, the introduction of backfill mining, and the reduction of profit margins, it has become imperative to increase the efficiency of gold 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. Interferences with the leaching of gold in contact with other minerals or metals could be attributed to the galvanic interaction (electrical conductivity) between the gold and the mineral and to the formation of a surface film on the gold surface. Sulphide minerals and their 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 conductive minerals passivates as a result of the enhanced magnitude of the cathodic cu1Tent. In all experiments the rotating disc of gold passivated so that the rate of dissolution was much slower than that 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, depend largely on the pretreatment of the ore. Pre-elimination of host minerals from the gold bearing ore increases the dissolution rate of gold, and explains 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 composition of these films and precipitates depends on the mineralogy of the sample. These films may be oxides, sulphides, carbonates and cyanide complexes. The complexes can be destroyed, depending on the nature of the film, by interstage dilute acid and/or cyanide washes in an agitated vessel. The destruction of the films exposes the gold surface for cyanidation. A simple distribution function similar to the King liberation model is 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 the free gold in the complete sample. This approach for studying the leaching behaviour of different gold bearing minerals has provided reasons why some ores leach better than others.

AFRIKAANSE OPSOMMING: Die loging van goud vanuit weerbarstige ertse is 'n komplekse kinetiese probleem wat verskeie faktore soos massa-oordrag, chemiese aspekte en mineralogiese ingeweefdheid insluit. Goud ekstraksies so hoog as 97 % in sianied oplossings in Pachuca reaktore na ongeveer 16 uur logingstyd word behaal in die meeste Witwatersrand ertse in Suid-Afrika. Hierdie hoë ekstraksies mag dalk die rede wees vir die min fundamentele navorsing oor die ekstraksie van goud vanuit minerale in 'n spesifieke erts. Die dalende erts grade, die verlaging van winsgrense en die terugplaas van geloogde erts in die myn noodsaak verbeterde goud ekstraksie. Verkeie faktore nl., chemie, diffusie, partikelgrootte en oplosbaarheid van goud in elke monster erts is in hierdie studie ondersoek. Die sentrale tema was om die logingsgedrag van goud vanuit verskeie minerale in 'n erts te bepaal. Galvaniese interaksie (hou verband met elektriese geleidingsvermoeë) en film vorming is die belangrikste faktore wat die loging van goud in kontak met minerale nadelig beïnvloed. Sulfied minerale en hul oksidasie produkte speel die grootste rol in die verlaging van die tempo van goudloging. Galena verhoog die tempo van goudloging as gevolg van die Pb (II)- ione in oplossing. Goud in kontak met geleidende minerale passiveer as gevolg van die verhoogde katodiese stroomdigtheid. In alle eksperimente met die roterende skyf (goudskyf) apparaat, passiveer die goudskyf in so 'n mate dat die logingstempo baie stadiger is as wat voorspel word met die massa-oordrags model. Die onderskeie films wat vorm op die goud- en geassosieerde minerale se oppervlaktes, asook die galvaniese interaksies, is 'n funksie van die voorafbehandeling van die erts. Die selektiewe eliminering van minerale vanuit 'n gouddraende erts verhoog die tempo van goudloging drasties en dit beskryf die kinetika van goudloging op die goudoppervlak in 'n groot mate. Die selektiewe eliminering van minerale deur gebruik te maak van oksiderende suurlogings, vernietig sekere van die minerale wat films op die goudoppervlakte kan veroorsaak deur middel van presipitasie. Die chemiese samestelling van hierdie films hang af van die mineralogie van die monster. Dit bestaan meestal uit oksiedes, sulfiedes, karbonate en sianiedkomplekse en hulle kan vernietig word deur middel van inter-stadia verdunde suur-en/of sianied wasse. 'n Eenvoudige distribusiefunksie, soortgelyk aan die King bevrydingsmodel word voorgestel en eksperimenteel getoets om die logingstap in die multi-stadia logingsmeganisme te beskryf. Vir bevryding deur loging, is die neiging van King se model korrek, maar kalibrasie word benodig vir goeie passing. 'n Potensiele gebruik van die aangepaste model is om vrye of loogbare goud in 'n spesi fieke partikel grootte fraksie van 'n erts te voorspel as die vry goud in die totale fraksie bekend is. Die resultate uit hierdie studie kan gebruik word om die logingsgedrag van gouddraende minerale te voorspel en te beskryf, en dus veduidelik hoekom goud uit sekere ertse beter loog as uit ander.

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