Precious metal refining involves purifying metals through pyrometallurgy or hydrometallurgy. Preparation includes sampling and creating an assay. Pyrometallurgy involves heating and melting to separate impurities, while hydrometallurgy dissolves metals in a solvent. Zone refining is a highly effective but expensive purification process. Hydrometallurgy involves leaching, purification, and recovery of precious metals through various methods.
Precious metal refining is a technique used to purify precious metals. There are several methods of refining precious metals, but the two main methods are pyrometallurgy, which involves heating, and hydrometallurgy, which involves dissolving metals in a solvent. After purification, the metals are cleaned to complete the refining process.
Preparation for refining precious metals often involves taking a sample of the metal to be refined and creating an assay. During sampling, the metal is weighed, measured and melted into a liquid, then samples are taken. Typically one sample is held by the refiner and another is delivered to the customer. After sampling, a test is performed to determine the exact quantities and concentrations of the precious metals and impurities to be removed.
Once the sample and analysis have been performed, the process of refining the precious metals can begin, usually through pyrometallurgy or hydrometallurgy. During pyrometallurgy, metal is heated and melted to separate out various impurities. In some metalworking processes, a form of gas will be used to bubble through the molten metal and the targeted precious metals or impurities will oxidize, changing consistency to allow for removal. In some cases, impurities and precious metals melt at significantly different temperatures, which allows for easy separation by physical means.
A special type of pyrometallurgy, known as zone refining, is an expensive but highly effective purification process. This precious metal refining technique involves stretching the metal to be refined into a long bar. The bar is then heated sequentially, starting at one end, and the impurities remain in liquid form and migrate along the bar as the next section is heated. At the end of the bar the impurities are collected in the last segment, allowing for easy removal. The process can be repeated several times, creating a loss of a significant portion of the metal, but leaving the rest of the metal purified.
Hydrometallurgy is the other main type of precious metal refining and involves the use of a solvent to dissolve the metal to remove impurities. The first stage of this process is known as leaching, during which the metal is dissolved using a solvent known as a leachate which is either acidic or basic. Subsequently, the solution is purified through processes of precipitation, extraction or ion exchange. The last step is to recover the precious metals from the purified solution and the techniques for this step include precipitation, electrorefining and gas reduction. Hydrometallurgy is a much more variable process and the wide range of different methods allows it to be tailored to the individual sample that needs to be refined.
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