Separating gold from other metals through melting is a process used for small-scale recovery or preliminary purification. This method leverages distinct physical properties of gold and other metals to achieve separation.
Understanding the Principles of Separation by Melting
Separating gold from other metals by melting relies on fundamental differences in their physical properties. Gold has a relatively high melting point, approximately 1,064°C (1,947°F), compared to many common base metals. This allows for heating a mixture to a temperature where impurities become liquid or react, while gold remains solid or separates distinctly when molten.
A significant density difference also aids separation. Gold has a density of about 19.32 grams per cubic centimeter (g/cm³), making it considerably denser than metals like silver or copper. When a mixture of molten metals cools, denser gold settles at the bottom, while lighter impurities or slag float to the top. This natural stratification facilitates physical separation.
Fluxes, which are specific chemical compounds, are added to the melt to interact with impurities. They form a molten, glassy layer known as slag, which is less dense than gold and floats on the surface. This isolates impurities for removal. Fluxes also help in lowering the melting points of impurities, improving their fluidity, and preventing oxidation of the desired metal.
Preparing for the Melting Process
Careful preparation is necessary before melting gold for separation. Essential equipment includes a crucible, which is a container designed to withstand extreme temperatures, typically made of graphite or clay. A high-temperature heat source is also required, such as an electric induction furnace for precise control or a high-powered propane torch for smaller operations. Other tools like sturdy tongs for handling hot materials, ingot molds for shaping the recovered gold, and a stirring rod are also important.
The materials to be processed can vary, including scrap jewelry, electronic components, or gold concentrates. Personal protective equipment (PPE) is paramount due to the high temperatures and potential for hazardous fumes. This includes heat-resistant gloves, eye protection (goggles or a face shield), and a heat-resistant apron. Adequate ventilation is also necessary to avoid inhaling fumes, especially when melting materials that might contain zinc or other metals that vaporize.
Specific fluxes are chosen based on the impurities present in the material. Borax is a common flux that helps lower the melting point of impurities and binds with metal oxides to form a removable slag. Silica can be added to control slag viscosity and prevent gold from sticking to the crucible. Sodium carbonate or soda ash assists in removing sulfides and other impurities by reacting to form a fluid slag. These fluxes facilitate the separation of non-gold materials into a manageable, floating layer.
The Melting and Initial Separation Process
The actual melting process begins with preparing the material. Large pieces of metal scrap might need to be cleaned or crushed to fit into the crucible and promote more uniform melting. Once prepared, the material is carefully placed into the crucible, and the appropriate flux mixture is added on top.
The crucible is then placed within the chosen heating apparatus, such as an electric furnace, and heated gradually. The temperature must be raised sufficiently to melt the gold and the impurities, allowing the fluxes to react and form a separate slag layer. As the material heats, impurities begin to oxidize and combine with the flux, causing the slag to form and float on the surface of the molten gold. Observing the melt can indicate when the process is complete, as the molten gold will appear distinct from the slag.
Once the metals are fully molten and the slag has separated, the molten material is carefully poured into a preheated mold. Preheating the mold helps prevent thermal shock and ensures a smooth pour. As the molten mixture cools and solidifies in the mold, the denser gold settles at the bottom, while the lighter slag forms a distinct layer on top. After cooling completely, the solidified gold button can be separated from the slag layer, often by gently tapping or breaking the brittle slag away.
Challenges and Limitations of Melting for Purity
While melting can effectively separate gold from many common metals, it has limitations regarding the ultimate purity achieved. This method primarily relies on differences in melting points and densities, meaning it may not remove all impurities, especially those that readily alloy with gold or have similar physical properties. For example, silver and copper often remain mixed with gold after a simple melt, as they form alloys with gold and have melting points relatively close to or higher than gold’s.
Platinum group metals (PGMs) can also remain in the gold after melting due to their high melting points and chemical properties. Impurities like zinc, while some may vaporize during melting, can still affect the overall composition. The resulting gold, often referred to as “doré” metal, typically has a purity of around 90% after this initial process.
For higher purity levels, such as 99.9% or 99.99%, additional refinement steps are necessary beyond simple melting. These advanced processes, such as the Miller chlorination process or electrolytic Wohlwill process, involve chemical or electrochemical techniques designed to remove remaining trace impurities. Therefore, while melting is an important initial step in gold recovery and purification, it is generally insufficient for producing investment-grade gold.