Gold assaying: Fire assays
As mentioned, the fire assay method is the preferred way to determine a deposit’s gold grade because it is generally the most accurate. A drawback to fire assays is that they are very disruptive; however, if performed on exploration-stage properties that is typically not a problem.
The first step in the fire assay process is to fuse or dissolve the rock sample being examined in a crucible using a lead glass flux. The flux is a mixture of materials selected by a chemist, and usually includes: sodium bicarbonate, potassium carbonate, borax, litharge (lead oxide) and flour.
Once combined, the flux and rock sample are heated to encourage a reaction. The reaction is allowed to go to completion, and then the crucible is removed from the heat source. At this point, the flux mixture and rock sample have reacted to form two parts: molten glass, which sits at the top of the crucible, and metal, which sits at the bottom. The molten glass is poured off, as it does not contain any valuable metals, and the metal is poured into a mold and allowed to cool until it solidifies.
Once cooled, the solid metal is removed from the mold. This metal generally contains lead, gold and silver. The mass of metal is then placed on a cupel made of bone ash — a material that readily absorbs lead oxide, but does not absorb metal. The metal is then heated, and the lead metal is oxidized to lead oxide. The oxide is either absorbed into the cupel, or volatized off into the atmosphere.
What is left is a tiny bead containing all of the gold and silver that was in the original sample. The bead is weighed and the total weight of the gold and silver together is then known.
The final step is to separate the gold and silver (in the rare case of a sample without silver, the separating process may be skipped). The gold and silver are parted by flattening the bead and putting it in a solution of diluted nitric acid, as the acid will dissolve the silver but not the gold. The remaining purified gold is then weighed. The weight of the silver in the assay is calculated by subtracting the weight of the gold from the weight of the bead that contained both the gold and the silver.
Using the known original weight of sample that was used in the assay, as well as the weight of the gold and the weight of the silver, the contents of the ore sample in ounces per tonne can be calculated.
Post time: Jul-28-2017