Decomposition of Niobium Ore by Sodium Hydroxide Fusion Method
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INTRODUCTION
THE niobium resources on the planet are scarce and their grades are low, so that niobium is classified as a rare metal.[1] Niobium has good corrosion resistance, high melting point [2741 K (2468 °C)], good thermal conductivity (0.53 J cm–1 s–1 K–1, under room temperature), good compatibility with the human body, etc., so they are widely used in metallurgy, nuclear, aerospace, electronics, and medical equipment industries.[1–5] The decomposition of the ore is the key step in extracting niobium from niobium ores, and it has been investigated for many years. Several processing methods have been reported, including hydrofluoric acid leaching, chlorination, alkali fusion method, etc. Currently, most niobium ores are processed by the hydrofluoric acid method.[6,7] However, due to its high volatility, about 6 to 7 pct HF is lost during the decomposition process, which is hazardous.[8,9] Furthermore, a large amount of wastewater containing fluoride is generated that needs to be treated. The chlorination method has been studied for many years. Niobium compounds with high purity can be obtained by this method. But in this method, the chlorinating agent is very corrosive, and it may cause serious equipment corrosion and environment pollution. The alkali fusion method was the earliest one used in industry. In alkali fusion, the niobium ore was mixed with sodium hydroxide or potassium hydroxide and roasted. In addition, a certain amount of corresponding carbonate XIU-LI YANG, Ph.D. Student, and CHANG WEI, Professor, are with the Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Yunnan 650093, P.R. China. XIAO-HUI WANG, Research Assistant, and SHI-LI ZHENG, Researcher, are with the National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R. China. Contact e-mail: [email protected] QING SUN, Ph.D. Student, is with the School of Chemical & Environmental Engineering, China University of Mining & Technology, Beijing, Beijing 100083, P.R. China. Manuscript submitted May 22, 2012. Article published online November 16, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
was also added for lowering the melting point and viscosity of smelt. The main disadvantage of this method is the high alkali consumption, which was 6 to 8 times the reaction requirements (the alkali-to-ore mass ratio is about 3:1).[10] The traditional alkali fusion method was based on the following reaction[10]: ðFe; MnÞNb2 O6 þ 10NaOH ! 2Na5 NbO5 þ ðFe; MnÞO þ 5H2 O ½1 The niobium in the ore is mainly transformed into Na3(Ta, Nb)O4. Through the alkali fusion process, excessive sodium hydroxide was added for the complete conversion from (Fe, Mn) Nb2O6 to Na5NbO5. The alkali-to-ore mass ratio is as high as 5:1, which far exceeds the stoichiometric value and causes a high alkali consumption. However, the fusibility diagram of Na2O-Nb2O5[11] and Na2O-Ta2O5 show
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