In the mining explorations the samples obtained in the field are analyzed in order to determine the amount of gold that they contain, this the samples are sent to a laboratory, the essay term commonly used to this analysis.
There are many types of sample tests available, however the method used by each exploration company is determined by the amount of gold that the sample is expected to contain, and each method has a certain range of precision according to the values it may contain.
The expected gold content in a sample is calculated according to the exploration stage where it is taken, for example when analyzing sediment samples from a stream, or from a soil, they will be measured in parts per billion (ppb), and the samples obtained through a drilling program are obtained in parts per million (ppm).
The samples or tests that are sent to the laboratory are usually between 2 and 3 kilograms, although the amount that is actually analyzed is 30 to 50 grams only, it is very important that this sample is representative, so it must be ensured that the analysis is not obtained from a particularly rich or sterile part.
The most used method today is the fire assay, where high temperatures are used, this procedure involves taking 30 to 50 pulp in a small clay cup, the ‘crucible’, together with chemicals to help melt the pulp and liberate gold and, in a furnace, it is brought to a temperature of 1150 Celsius degrees to release the gold, here lead is added to collect the molten gold.
The next step of this process is where the molten contents are cooled leaving a “button” of lead at the base of the silica slag, this button is then placed in a cup or “cupel” of magnesium oxide where it will heat up again and the lead will be absorbed in the cup, from here you get a small prill of gold and usually silver that were contained in the sample.
After this procedure an actual measurement of the gold contained in the sample is made, this procedure is called finishing.
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