Scabies ( English dross ), also called "dross", or according to as ash after melting ( stannic oxide ) denotes arises during melting of metals from their ores, as well as the melting of metals or their alloys under atmospheric or technology-related influence. In contrast to slag , which occurs in molten form in these processes, the impurities known as dross have a higher melting point than the metal in question and are therefore in solid form. Ingress of oxygen causes oxide formation - occasionally also burn-offcalled - to varying degrees, depending on the oxygen affinity of the melt components. In addition to the oxidic dross lying on the melt, impurities primarily adhering to the melted material or process-related reaction products with generally lower density than the melt and therefore themselves rising to the bath surface, otherwise driven out by a "flushing" melt treatment.
Since the oxides of the melt constituents are of lower density, but have a higher melting point than they themselves, the dross lying on the melt is usually highly viscous, which favors the inclusion of melt constituents.
Melts must be freed of process slag before they can be used further, "purged" or "scraped off" (see cast and melt treatment ). In this respect, the formation, like the presence of dross, always means a loss of metallic melt. This applies to a steelworks as well as to recycling smelters and foundries and gives slag processing, such as dross recycling, a field of activity within the framework of the required sustainable handling of metallic raw materials.
The formation of dross due to oxidation can be avoided by vacuum melting . The cost of this technology usually only allows it to be used for high-quality melt material, such as precious or rare metals.
- slag . In: Römpp Chemie Lexikon USB . Thieme, Stuttgart 2008.