Main steps
①Sampling, samples include minerals, refined products, debris, etc., requiring careful collection of representative samples. Because precious metals often exist in minerals in a random dispersion state, it is often necessary to use a large amount of minerals to take out a representative sample, and finally the sample must be carefully crushed.
② To melt the sample, add appropriate flux to the sample, such as crushed sodium carbonate, borax, silicate, lead monoxide, etc., and then heat to melt the sample and flux. Lead monoxide is reduced to metallic lead and sinks into the bottom of the container together with the precious metal. After cooling, it becomes a small metal ball, which contains precious metal and a large amount of metallic lead, as well as other metal impurities. The slag produced by the reaction is discarded.
③Baking pot test gold, placing precious metal and lead metal balls in a baking pot made of bone ashes (or biscuit), and then placing the baking pot in a special furnace that can provide a strong oxidizing atmosphere heating. At this time, the lead and other metal impurities in the metal ball are oxidized, and the resulting lead oxide and other metal oxides become slag, or are immersed in the pores of the roasting pot. Only the gold and silver are not oxidized and still maintain the metallic state. Lead is separated from other metal oxides. Take out the roasting bowl from the stove and let it cool down slowly. The resulting pellets can be used as a sample after being washed and dried. The pellets can also be made into a thin sample by hammering or rolling.
④Weigh, weigh out the weight of the sample containing gold and silver, then the total weight of gold and silver can be measured.
⑤Separate gold and silver. Treat the sample containing gold and silver with hot dilute nitric acid to dissolve the silver.
⑥ Weigh, wash and dry the sample containing only gold after removing the silver, and then weigh the gold.
Other precious metals in the sample, such as platinum, palladium, rhodium, iridium, osmium, and ruthenium in the platinum group elements, their changes in the fire assay are: platinum, palladium, and rhodium will dissolve in the molten lead metal When the samples are heated in a roasting bowl, they will not be oxidized, but they exist in the metal balls together with gold and silver. When adding hot dilute nitric acid to dissolve the silver, only palladium will dissolve, platinum and rhodium It does not dissolve but stays in the gold, increasing the weight of the gold.
If iridium is present, it will be oxidized in the roasting bowl to form a black deposit that adheres to the surface of the metal ball. When the metal balls are heated in a roasting bowl, osmium and ruthenium form volatile oxides (boiling point of osmium oxide is 130°C; the boiling point of ruthenium oxide is 100°C) and is lost. For the above reasons, if you want to analyze the platinum elements in the sample, you cannot use the fire assay method, but use other chemical analysis methods.
Post time: Nov-09-2020