Fluxing is the deliberate addition of specific chemical compounds, known as fluxes, to a metallurgical furnace charge or smelting process to facilitate the removal of unwanted impurities (gangue minerals) from the metal or metal oxide of interest. Fluxes work by reacting with impurities to form a low-melting-point, easily separable slag phase that floats above or is tapped separately from the molten metal or matte phase. Fluxing is a foundational practice in pyrometallurgical processing across all major mining sectors.
In iron ore processing and steelmaking, limestone (CaCO₃) and dolomite (CaMgCO₃) are the most commonly used fluxes. They decompose at high temperatures to release CaO and MgO, which combine with siliceous gangue minerals, alumina, and other acidic oxides to form a fluid, pourable blast furnace slag. The basicity of the slag — defined as the ratio of CaO to SiO₂ — is carefully controlled to optimize iron desulfurization and phosphorus removal.
In gold smelting, borax (Na₂B₄O₇), silica (SiO₂), soda ash (Na₂CO₃), and litharge (PbO) are used as fluxes to dissolve base metal oxides, silicates, and other gangue from the gold-silver alloy (doré). In bauxite smelting and aluminum casting, cryolite and aluminum fluoride serve as electrolytic flux media. In diamond recovery operations, fluxing may be used in high-temperature DMS ferrosilicon reclaim processes.
The selection of appropriate fluxes, their ratios, and the resulting slag chemistry are critical metallurgical parameters that directly affect metal recovery, product quality, furnace refractory wear, energy consumption, and overall smelting economics. Fluxing agents must be carefully proportioned and blended into the furnace charge to achieve target slag viscosities, melting points, and liquidus temperatures across varied ore feed compositions.