Experience sharing related to fire assay

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Pyrometallurgy – Main text: A method for determining the content of precious metal components in minerals and metal products through melting and roasting. 　　Brief History Fire testing for gold detection existed in ancient times. Around the year 60 AD, the ancient Roman naturalist Pliny the Elder used such a method to identify gold: \"Put one part gold, two parts salt, and three parts yellow vitriol into an earthen pot; then cover the surface with a mixture of two parts salt and one part slate powder. Subsequently, heat it over charcoal to determine whether it is genuine gold.\" ”This method involves converting silver into silver chloride and infiltrating it into slate, after which gold is identified using thermal methods. Wei Boyang, an alchemist from the Eastern Han Dynasty in China, wrote in his book \"Zhou Yi Can Tong Qi\" that \"when gold is placed in intense fire, its color does not lose its luster.\" This method of using fire was also used to distinguish genuine gold from fake gold, which was often an alloy or compound of copper. In his book \"Baowu Yaolan\", Gu Yingtai of the Ming dynasty also recorded that \"when in doubt as to whether an object is genuine or not, and it is necessary to determine its true nature, mix mountain yellow clay with some substance such as vinegar, apply this mixture to the gold object, and then heat it intensely in glowing coals; if the object is fake, it will turn black.\" It also refers to the fire assay. Here it is explained that after being calcined over charcoal fire, pseudo-gold develops a layer of black copper oxide on its surface, whereas the color of true gold remains unchanged. 　　Main steps: ① Sampling – the samples include minerals, refined products, debris, etc.; it is necessary to collect representative samples carefully. Since precious metals are often present in minerals in a disordered dispersed state, a large amount of mineral is usually required to obtain a representative sample, which then has to be carefully crushed. ②Sample melting: An appropriate flux is added to the sample, such as crushed sodium carbonate, borax, silicates, lead monoxide, etc., and then heated to melt the sample and the flux together. Lead monoxide is reduced to metallic lead, which settles to the bottom of the container along with the precious metals. After cooling, it forms metal spheres that contain the precious metals, a large amount of metallic lead, as well as other metal impurities. The slag produced by the reaction is discarded. ③Pyrotesting involves placing metal balls containing precious metals and lead into an oven made from ash (or unglazed porcelain), and then heating this oven in a special furnace that creates a strong oxidizing atmosphere. At this point, the lead and other metal impurities in the metal balls are oxidized; the resulting lead oxide and other metal oxides become slag, or they get trapped in the pores of the baking pot. Only gold and silver do not oxidize and remain in their metallic state, separated from the oxides of lead and other metals. The baked container is removed from the furnace and allowed to cool slowly; the resulting pellets can be used as samples after being washed and dried, or they can be transformed into thin-sheet samples by hammering or rolling. ④Weigh the sample containing gold and silver; by determining its weight, the total weight of gold and silver can be calculated. ⑤To separate gold from silver, treat the sample containing both metals with hot dilute nitric acid, which will dissolve the silver. ⑥Weighing: After removing silver, the sample that contains only gold is washed and dried before being weighed, thereby allowing the weight of gold to be determined. 　　Other precious metals present in the sample, such as platinum, palladium, rhodium, iridium, osmium, and ruthenium from the platinum group elements, undergo the following changes during the fire assay: platinum, palladium, and rhodium dissolve in molten lead. When the sample is heated in a crucible, they do not get oxidized and remain together with gold and silver within the metal spheres. When hot dilute nitric acid is added to dissolve the silver, only palladium dissolves; platinum and rhodium remain intact within the gold, thereby increasing its weight. If iridium is present, it will oxidize in the crucible, forming a black deposit that adheres to the surface of the metal balls. When metal spheres are heated in an oven, osmium and ruthenium form volatile oxides (the boiling point of osmium oxide is 130℃ ； It is lost due to the boiling point of ruthenium oxide being 100°C. For the above reasons, if one wants to analyze the platinum group elements in a sample, the pyrometallurgical method cannot be used; instead, other chemical analysis methods must be employed