Assaying

Assaying, in metallurgy, the process of determining the amount of a specific metal in a mineral, ore, or alloy; it differs from analysis (see Chemical Analysis), in which the entire composition is determined. Assaying is usually reserved for ores of gold, silver, and the platinum group metals.

The first operation in making an assay is to take ore samples that must be typical in all respects of the ore being mined. The samples may be selected either by hand or by machine. Each sample is assayed separately. If the ore content of the samples is about the same, the sampling is taken as correct, and the average of the assays constitutes the final assay.

The fire-assay method utilizes heat and suitable fluxes to separate the metal from gangue (worthless material) in the ore. A sample weighing 1 assay ounce, that is, 29.166 g for gold ore, is blended with a flux mixture containing varying amounts of litharge (lead monoxide), silica, soda ash, borax, and flour. The blended material is fused at about 1000° C (about 1832° F) for approximately 1 hr after which the entire mass is poured into an iron mold and cooled. The mass settles into two distinct layers: a top layer of slag, which when cooled can be chipped away, and a bottom layer consisting of a lead alloy “button” weighing about 20 to 30 g. This button contains all the gold, silver, and other precious metals originally present in the ore sample. If the initial sample contains volatile impurities, such as antimony or arsenic, the fire-assay method employs a refinement called scorification.

The process of removing the lead from the alloy button, leaving the gold and silver in a pure state, is called cupellation. This step is based on the fact that when an alloy of gold, silver, and lead is heated in the presence of air, lead monoxide or litharge is formed. The alloy button is placed in an open, porous container called a cupel, which absorbs the litharge that forms during the heating. Air is forced past the heated cupel until all the lead is absorbed, and the mass of metal remaining contains only gold and silver. This alloy is commonly known as a doré bead. Just before the last traces of lead are removed, the bead changes color, darkening and then finally brightening, thereby indicating the completion of the operation. The cupel is removed from the furnace and cooled, and the bead is cleaned of any foreign particles.

Next the gold is separated from the silver in the bead by a process called parting, which consists of dissolving the silver in dilute nitric acid. Usually, the gold-silver bead must contain at least three times as much silver as gold in order to ensure satisfactory dissolution of the silver. If the natural ratio is less, more silver must be added. The bead is then flattened and boiled in nitric acid to remove the last trace of silver. The result is a spongy mass of pure gold. This gold is weighed and the proportion of gold in the original ore determined. The difference between the weight of the gold after parting and the weight of the gold-silver bead after cupelling gives the proportion of silver that is present in the gold ore.