Gold Assay

Metallurgical processes need to know the metallic content of the different products obtained in recovery and refining processes. If we have to know about gold assay, it is important to mention the Fire Assay Process that has been employed in the mining industry for a long time. Basically the ore is mixed with a mixture of fluxes and during the smelting process precious metals are liberated and most of heavy metals are trapped by the fluxes. However, lead is the key element due to collect precious metals. The latter product is reprocessed by a cupellation stage so that we can have a gold-silver alloy that will be leached selectively to calculate the gold content.


Furnace	Crucible	Cupel

One aspect very important is to know the minerals present in the sample to be assayed due to they have influence on the flux to be added. Obviously, this knowledge is fruit of experience and study. Basically, minerals can be identified by specific gravity, color, streak, luster, hardness, magnetic susceptibility, and reaction with any special reagent such as hydrochloric acid. The number of minerals is large and assayers have to be focused in the main minerals such as sulphides or sulphides. Carbonates react easily with nitric or hydrochloric acid and this is a common characteristic of alkaline samples.

Identifying Minerals by Vanning

A good procedure to estimate the minerals is to perform a gravimetric separation by using a watch glass or a curved plate. This procedure is called vanning. In this case, is necessary to take 10 to 30 g of sample and mix it with water in the watch glass. The idea is to separate minerals according to their specific gravity like a panning gold operation. Heavy minerals stay in the center and the lighter minerals at the border. When the vanning is well done, it is possible to identify minerals of different color and densities due to they are distributed in different zones. The size of these zones will tell us the main minerals and the possible nature of the sample.

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