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Why are fluorspar and lime added as fluxes in ferroniobium production?

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niobium processing Four one-millimetre thick samples of niobium metal foil.

In the production of ferroniobium via the aluminothermic process, fluorspar and lime are added as fluxes to promote fluidity and facilitate the removal of objectionable impurities.12

During the reaction, these substances help ensure that gangue impurities—including thorium and uranium oxides—are effectively separated from the metal and incorporated into the molten slag, which can then be tapped off.12