The size and mode of occurrence of free gold and gold-bearing minerals is usually determined microscopically and may have an important influence on the recovery process and the economics of the operation. Basically, in most cases secondary minerals from gold deposits are soft and friable and occur as coatings and fillings on grains and in fissures and cracks of the host rock. In this way, the mineralogy of some ore zones indicate that gold-bearing minerals are readily exposed to the cyanidation solution by grinding only as fine as is required to separate the grains. Gold occurring in fine-grained minerals and penetrating small cracks and pore spaces, even in relatively coarse grains, may need finer grinding for exposure. Fine grinding may also be needed when to treat slimy ores due to solutions will not decompose calcareous materials which may be coating or encapsulating part of the gold minerals. Primary gold mineralization in veins is often more massive than in secondary deposits, but primary occurrences as disseminations in fine crack network of the host rock usually needs very fine grinding. It is important to get the optimum point between power draw and gold liberation due to consume unnecessary energy increase the operating cost of the cyanidation plant.
Basically, the suitability of a gold ore to mechanical methods of beneficiation and upgrading may be determined during the preliminary ore examination. With exception of flotation, such methods depend on the occurrence of the gold minerals at a relatively coarse size and with sufficient physical strength to resist handling without degradation. These characteristics are found in some gold deposits. In this way, associations of gold with copper and iron minerals with special characteristics may indicate a means of concentration and must be keep in mind. Some gangue minerals present in the gold ore may have influence over process selection and recovery plant performance. Carbonaceous minerals are a poison for cyanidation and need a previous treatment such chlorination. Oxide copper minerals consume cyanide.
Clay minerals are troublesome in some gold ores causing high consumption of cyanide, high viscosity slurries that are difficult to settle. These problems occur when some organic compounds are present and it is necessary a pre-treatment or identify its presence to avoid problems. In this case the operator must be aware of the potential and how to solve the possible problems; otherwise, the efficiency of the process will be affected and the costs associated to the operation will increase. A required ore grind size is determined by the results of the metallurgical tests (e.g. cyanidation, flotation) and standard procedures may be employed to determine the right equipment in the crushing, grinding, flotation and cyanidation circuits.
Free gold particle