Precious metals mainly refer to metallic elements such as gold, silver, and platinum group metals (ruthenium, rhodium, palladium, osmium, iridium, and platinum). These metals are mostly bright in color, aesthetically pleasing, resistant to chemical corrosion, and generally do not undergo chemical reactions. Substances containing precious metals can be broadly divided into two categories: the first is mineral resources, i.e., primary resources; the other is renewable resources, mainly found in discarded precious metal products during use, with content ranging from tens of thousands to near-pure metal, belonging to renewable resources. This article summarizes the current development status of precious metal extraction technology and equipment based on the characteristics of precious metals and other resources.
In traditional industrial production processes, the extraction process for precious metals such as gold, platinum, and palladium involves repeated melting and crystallization, and repeated reduction and purification, but the operation is complex and the extraction efficiency is low. The new extraction process consists of three parts: enrichment, separation, and refining. First, gold, platinum, and palladium ores are dissolved in an aqueous solution using a chlorination method. After dissolution, they are reduced with sulfur dioxide or sodium sulfate. By controlling the reduction potential (increasing the potential to control the molar concentration), gold is reduced to produce crude gold powder, which is then used for smelting gold. Platinum, palladium, zinc, bismuth, and other metallic elements in the remaining liquid were separated using the ammonium method. Ammonium chloride was added, causing palladium to enter the liquid phase and platinum to enter the solid phase, completing the rough separation of platinum and palladium. Then, the mixture was dissolved in an aqueous chloride solution for palladium extraction. The loaded organic phase after extraction was washed with hydrochloric acid and then back-extracted with pure water or sodium hydroxide. The raffinate after palladium extraction was used to extract platinum with N235. This extractant showed a high extraction rate for platinum (99.95%), but a low extraction rate for base metals such as copper, iron, nickel, and cobalt. The raffinate after platinum extraction was used to extract base metals with P2O4. The rhodium-iridium solution after gold, palladium, and platinum extraction was precipitated with ammonium chloride, calcined to obtain platinum powder, and then melted into platinum ingots.
Based on the actual operation process, the extraction of metal ions from precious metal solutions is closely related to their valence state and solution properties. Studies on the extraction mechanism and process show that the valence state and condition of chloride complex anions in the solution are mainly affected by the central ion and solution properties, and changes in these factors lead to significant changes in extraction behavior.
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