Establishing the process mineralogy of gold ores
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This paper discusses the classification of gold ores and gold minerals, mineralogical factors affecting gold extractive metallurgy, and study procedures and techniques commonly employed in gold process mineralogy.A case study of a refractory gold ore is provided to show the need and importance of process mineralogy in gold extractive metallurgy.
From the perspective of metallurgical processing, gold ores can be classified into free-milling and refractory ores. Typically, free-milling ores are defined as those where over 80% of gold can be recovered by conventional cyanide leaching, such as placers, quartz vein gold ores, oxidized ores, silver-rich ores, copper sulphide ores, and some iron sulphide and arsenic sulphide ores. Refractory ores are defined as those that give low gold recoveries or give acceptable gold recoveries only with the use of either significantly more reagents or more complex pre-treatment processes. They typically include iron sulphide ores, arsenic sulphide ores, antimony-, bismuth-, and telluride-bearing gold ores, and carbonaceous sulphide ores.
The deportment of gold in each ore is different and has to be determined individually in order to select the optimum technique or a combination of techniques for gold ore processing. Mineralogically, the extractive metallurgy of gold is influenced by gold particle size, association with other minerals, coatings and rimmings, the presence of cyanicides, oxygen consumers and preg-robbers, and the presence of refractory gold minerals and invisible gold in sulphide and sulpharsenide minerals.
The gold deportment and the various mineral processing requirements of the several ore types can be summarized as follows: (1) gold in placers, quartz vein gold ores, and oxidized ores are easily liberated, and can be recovered by gravity, flotation, and/or direct cyanide leaching; (2) gold in copper sulphide ores is often coarse-grained and associated with copper minerals, and can be recovered into a copper concentrate by flotation; (3) gold in silver-rich ores is often present as electrum or associated with silver minerals, which can be recovered by gravity, flotation, and/or direct cyanide leaching (the problem in recovering gold from silver-rich ores is that the greater reactivity of silver can influence the behaviour of gold in flotation, leaching, and/or recovery processes by the formation of silver sulphide or silver sulphate coatings); (4) gold in non-refractory sulphide ores is medium-to coarsegrained and can be recovered by whole-ore cyanidation, or some combination of gravity, flotation, and cyanidation; (5) gold in antimony, bismuth sulphide, and telluride gold ores often occurs as aurostibite, maldonite, or various gold tellurides, which are somewhat refractory due to their slow dissolution kinetics or association with sulphides. To recover gold from these ores, pre-oxidation and/or ultra-fine grinding are often required; (6) gold in carbonaceous sulphide ores is fine grained and predominantly associated with the sulphides (pyrite or arsenopyrite) that are present in carbonaceous ores. Gold in these types of ores is usually recovered by pre-oxidation to dissolve the host sulphides, followed by cyanidation with or without prior concentration by flotation. Gold recovery from the carbonaceous sulphide ore is more difficult because the gold is “robbed” from the cyanide solution by the native carbonaceous matter.
Gold process mineralogy studies deportment of gold in various ores and metallurgical products and helps address issues and problems related to gold ore processing. It is widely used as a predictive and trouble-shooting tool in gold ore processing, and provides useful information on process selection, flowsheet development, recovery improvement, and reagent consumption optimization.