Overview

Many metallurgical residues containing valuable metals occur as oxides. The recovery of metals from them is usually done by direct reduction of the oxides at elevated temperatures exceeding 1000 C. This is done using carbon (or any carbonaceous material like coal, coke, etc.) as reducing agent. In addition, to facilitate the melting process, a flux is usually added with the high-melting components. Its function is to combine with and neutralize the gangue and products of decomposition, apart from gases, making a product that is fusible and easy to handle and separate from the metal at the operating temperature. The flux forms the slag with lower melting point. For example, lime is added as flux to combine with silica (which has melting point of 1728 C) present in the recycle material. It forms calcium silicate whose melting point is around 1500 C.

The oxides and other insolubles separating from a molten metal are referred to as dross. Refining operations such as cooling to allow separation of insolubles and blowing with air or steam (sometimes the metal is poled with green timber) of lead and tin are called drossing. It may also include addition of other elements (like aluminum to separate antimony, arsenic and copper from tin), to produce insolubles, or to increase insolubility. The basic feature of drossing is that the solubility of the constituent being removed decreases with decreasing temperature.

Both slag and dross are inevitable products generated in metal extraction processes and were considered...