Hydrometallurgical Process to Extract Niobium from Tin Slag Through Alkaline Treatment and Sulfuric Acid Leaching

Niobium and tantalum are critical metals that are important for technological development. Their main applications are in the production of alloys for the civil construction, electronics, nuclear, and aerospace industries, and in catalysis. Tin reduction slag is a possible secondary source of niobium and tantalum, containing 3.7% and 0.5% of Nb and Ta, respectively. The slag matrix is mainly composed of calcium silicate, a low-reactivity material that prevents contact between the leaching solution and the metals to be extracted; therefore, it is necessary to previously react the material with molten NaOH. This reaction converts calcium silicates into sodium silicates, which are more reactive and water-soluble, and converts the metals into oxyanions, niobates, and tantalates, which are more reactive species. After treatment with molten hydroxide, the material is then solubilized in water; this reaction removes part of the soluble materials and also fragments the silicate matrix. Nb and Ta remain in the solid phase during the water washing step and then undergo acid leaching, where, after the parameters are evaluated, Nb extraction of 96% and Ta leaching of less than 3% are achieved, using a concentration of 10 mol/L H2SO4, a time of 2 h, a temperature of 90 °C, and a liquid–solid ratio of 50.