In order to establish a gold particle size classification, gold particles can be classified as visible, microscopic, submicroscopic and surface bound. The first class is referred to free gold particles detected on placer deposits or some hydrothermal deposits. The human eye can identify these gold particles without difficulty. The liberation of gold particles is close to 100% and the presence of any carrier is not necessary is most the cases. These kind of gold particles are affected by erosion, attrition and deformation. The latter one is probably a very important factor to be considered on gold recovery due to gold is malleable and particles can occur as flattened, square or any irregular form. These shapes are responsible for the metallurgical performance of gravity processes.
The second class includes the gold particles detected by using a regular microscope. It means that is not necessary a special microscope type. In this way we can detect free gold particles, Calaverite, sylvanite, krennerite, petzite, nagyagite and others. It believed that some gold deposits have a problem related to the decomposition of tellurides to oxidized compounds and native gold. This problem is major when the deposit content pyrite and/or arsenopyrite. The microscope helps to detect the presence of sulphides and gold particles associated or not. In this way we can study several ore types and the shape of gold particles. Even, some inclusions can be detected without having an important impact the host rock. The key factor is to detect the mineral with gold.
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| Figures show two different gold grain sizes with different degree of liberation | |
The third class is related to gold particles detected by special techniques. Perhaps one of the last techniques is the QemScan. Basically, the detection is possible by using microscopic scanning procedures. Gold particles are so insignificant is size that the observation under the microscope must be done several time in order to sure about the results. These gold particles are called invisible due to they are difficult to detect. Some carbonaceous deposits (Carlin) present these particles, but most the time base metal deposits present these gold particles. Perhaps, copper and copper-lead-zinc deposits are typical examples. The problem is noted when the head assay reports gold values and at the moment of detected the gold under a microscope, results are negative, even trying to pre-concentrate the sample. Gold particles are very small, 0.2 µm or smaller.
Probably, there are two sulphides mineral with the main probability of carrying gold particles, pyrite and arsenopyrite. These ores are treated most the time by flotation and/or cyanidation. If we consider flotation, gold values are reported on tailings (80-90%) and the relationship with pyrite and/or arsenopyrite is notable. The particles sent to the concentration or leaching process have an important effect. For example, coarse particles need fine liberation in order to make successful the process performance; porous particles will be useful to cyanidation because sodium cyanide solution can penetrate the particle and dissolves the gold; disseminated particles require fine liberation for flotation and cyanidation. Obviously, the regrind process has restrictions and some ores can’t be reground excessively fine as they need. In this case, the ore is considered refractory is the recovery is very low.
Surface bound particles are present in form of very small stains onto the minerals surfaces. They were formed during the mineralization process. In this way, the geochemical reactions have an impact on the formation of this type of gold particles. The main minerals that act as carrier of gold are iron oxides, clayed minerals and organic compounds.