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The basic principle of the fire assay (Principle of Method)

2021/12/24

There are many types and different operating procedures, including lead test, bismuth test, tin test, antimony test, nickel sulfide test, copper sulfide test, copper-iron-nickel test, copper test, iron test, etc. However, the smelting principles and reactions in the various new assay methods still have many similarities with the lead assay method. Among all the fire assays, the most commonly used and most important is the lead assay. Its advantage is that the resulting lead can be blown. The combination of lead assay and soot blowing technology can enrich the precious metals in dozens of grams of samples in aggregates weighing several milligrams. In the lead assay, the collection rate of Au is> 99%, and there is still a high recovery rate for Au as low as 0.2 ~ 0.3g/t. The analysis accuracy of the lead assay for macro and trace precious metals is very high. The following is a brief description of the principle of fire assay with lead assay as an example.

 

The lead fire assay is mainly divided into 3 stages:

 

(1) Melting. It uses solid reagents to mix with rocks, ore or smelted products, heat and melt in a crucible, and use lead to capture gold, silver and precious metals in a molten state to form lead alloys (generally called lead buttons, also called precious lead). Due to the high specific gravity of the lead alloy, it sinks to the bottom of the crucible. At the same time, the base metal oxides and gangue in the sample react with fluxes such as silica, borax, and sodium carbonate to form slag such as silicate or borate, which floats on top due to their small specific gravity. , To separate the gold and silver from the sample. Therefore, during the fire assay process, both functions of decomposing samples and enriching precious metals are played.


(2) Ash blowing. Put the obtained lead alloy in a ashtray at an appropriate temperature for ash blowing to remove lead. During ash blowing, the lead is oxidized to lead oxide and penetrates into the porous ashtray, thereby removing the lead in the lead button and a small amount of base metal, gold Silver and precious metals are not oxidized and remain in the ash dish to form gold and silver particles.

 

(3) Gold separating residue . Use nitric acid to dissolve the gold-silver composite particles to dissolve the silver while the gold remains solid. After quenching, weigh the obtained gold particles to calculate the gold content. The silver content can be calculated based on the difference between the quality of the gold-silver composite particles and the gold quality .


After the fire assay method completes the separation and enrichment of gold, silver and precious metals, in addition to the above-mentioned gravi metric method for determining gold and silver, after dissolving the gold and silver particles with aqua regia, a variety of chemical analysis methods can be used to determine gold, silver and other precious metals.


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