Niobium and tantalum beneficiation from tantalite ore

English: Tantalum and niobium occur in nature in the form of minerals such as tantalite and columbite. Recent advances in technology have increased the demand for these elements and their chemical compounds. Tantalum is extensively used in the electronics industry, especially in cell phones and laptops while niobium is used mainly in the production of super metal alloys for the use in the construction industry. The presence of U and Th in these Nb/Ta containing minerals (NORMs) however complicates the beneficiation process of the minerals immensely. Regulatory constraints on the quantity of material that can be handled at one time, processing equipment, transportation and waste disposal of these materials put more strain on the processing of these minerals. This strain adds a financial burden on the total beneficiation value chain when U and Th are present. The overall aim of this study was to investigate the selective removal of U and Th from these minerals prior to their complete dissolution in order to increase the safety of processing and/or transportation procedures. The methods included acid leaching (time and temperature variation), anion precipitation and ion exchange chromatography. The results obtained from these studies indicated varying degrees of success. The acid leaching study involved the use of sulphuric acid, phosphoric acid, hydrochloric acid, nitric acid, perchloric acid and aqua regia, of which the first three of these acids proved to be the most effective in the removal of the majority of U and Th from the mineral. Analytical results obtained for while using concentrated sulphuric acid as reagent indicated that an increased amount of radioactive material was removed from the mineral. The Th content removed from the mineral ore increased from 68.82% at 50°C to approximately 100% at 100°C after 3 hours of leaching in both cases. Additionally, the U content in the filtrate increased from 64.37% at 100°C to 81.27% at 150°C and finally to 90.08% at 200°C. Phosphoric acid was less effective in the removal of uranium and thorium at 50°C while an increase in temperature to 150 °C resulted in 85.10 and 98.77 % of U and Th respectively being removed after 3 hours. On the other hand, hydrochloric acid exhibited the same trend as sulphuric and phosphoric acid with an increasing amount of U and Th present in the filtrate as time and temperature were increased. At 90°C and 3 hours of leaching, hydrochloric acid succeeded in removing 51.53 and 91.09% of U and Th respectively. The main disadvantage of the use of these concentrated acids as leaching reagents at elevated temperatures and extended times, is the simultaneous removal of Ta and Nb from the mineral. Analytical results indicated that 9.70 and 29.11% of Ta and Nb respectively were dissolved at 150°C after 3 hours using sulphuric acid. The use of sulphuric acid as reagent, a temperature of 50°C and 2 hours leaching time were selected as the most suitable conditions for the subsequent beneficiation process as a compromise between the maximum U and Th removal and the minimum amount of Ta and Nb removed from the NORMs. The selective removal of the radioactive elements using anion precipitation and ion exchange was also investigated. The selective precipitation using several different anions indicated that sulphate (SO42-) was the most successful anion in the selective removal of U and Th from the mineral solution compared with all of the acids investigated. The sulphate resulted in the precipitation of 5.11 % of the total U precipitated along with 82.58 % of the Th, 80.13 % of the Nb and 99.21 % of the Ta. The ion exchange chromatography method, however, was unsuccessful in the separation of uranium and thorium from the tantalum and niobium as there was no selectivity in retention and/or elution of these elements. The final step in this study was to investigate the possible beneficiation of the tantalite ore by combining the acid leaching with the magnetic removal and microwave dissolution of the mineral. The mineral sample which was investigated possessed minute magnetic properties and small amounts of Ta and Nb (1.79 and 1.69 % respectively) together were removed together with Fe by magnetic separation. The acid leaching resulted in consistent but lower U and Th removal. Interestingly, the microwave digestion preferentially removed U, Th and Nb, as indicated by their presence in the filtrate, leaving the precipitate with significantly more Ta. The ratio of niobium to tantalum changed from 1:1 in the initial mineral sample to 3:1 and 1:2 in the filtrate and the precipitate respectively. The final insoluble mineral contained 0.13 % Th (initially 0.50 %), 1.33 % U (initially 3.07 %), 12.18 % Nb (initially 28.1 %) and 21.41 % Ta (initially 27.71 %). The filtrate, on the other hand, contained 0.14 % and 0.98 % of thorium and uranium as well as 13.00 % niobium and 3.99 % tantalum.