UPGRADING OF A NIGERIAN IXIOLITE ORE TO COLUMBITE BY SULPHURIC ACID TREATMENT

Leaching of a Nigerian Ixiolite ore by sulphuric acid solution was carried out and the effect of some parameters such as acid concentration, temperature and particle size was evaluated as a function of time. The experimental results showed that the Ixiolite dissolution increases with increasing acid concentration, temperature, leaching time and, decreases with particle size. With 3 mol/L H2SO4 solution, 71.14% of the ore reacted within 120 minutes at 80°C.The kinetics of dissolution of the mineral was also examined and the observed effects of the operating variables on the leaching rate were consistent with the diffusion controlled kinetic model. The calculated activation energy for the leaching process was 24.40 kJ/mol with the reaction order of 0.5, supporting the proposed model. Detailed characterization of the ore before and after leaching were carried out. At optimal leaching conditions, thermodynamic feasibility of the conversion of raw Ixiolite to Columbite of industrial value as confirmed by X-ray diffraction (XRD) analysis is possible.


Introduction
Ixiolite (Ta,Nb,Sn,Mn,Fe)4O8 is an important ore of niobium-tantalum containing mineral (coltan), commonly associated with igneous rocks including granites, pegmatites, syenites and carbonatites [1].Over the last 20 years, Ixiolite has been of interest to the modern scientists due to its prominence in the production of modern industrial materials and "high-tech" end user products.These ranges from super alloys in the production of jet engines, nuclear reactors and space vehicle frames to hearing aids, laptop computers, mobile phones, video consoles, GPS and digital cameras, among others [2].However, high percentage of Ta2O5 in Ixiolite has appreciably increased its market value than other coltans such as euxenite (Y,Ca,Ce,U,Th)(Nb,Ti,Ta)2O6, pyrochlore (Na,Ca)2Nb2O6(O,OH,F), stuverite (Ti,Ta,Fe)O2, lueshite (NaNbO3) and columbite (Fe,Mn)(Nb,Ta)2)6 [3,4].For example, the market potential for pure tantalum metal has been steadily growing with increasing application in the electrical and electronic fields and in materials for the chemical industry.Tantalum in sheet and tube form is used for lining chemical reaction vessels and in condensers and heat exchangers.The high reliability, excellent rectification properties, high capacitance and the wide temperature range of application of tantalum capacitors are contributing to a phenomenal growth in demand for them all over the world [5].
Apart from Australia known to have the largest coltan ore deposits, other countries with large reserves are Brazil, Canada, Mozambique, Portugal, Thailand, Zaire and Nigeria [6].In Nigeria, large deposits of coltan have been reported in Nasarawa, Osun, Ekiti, Oyo, Cross-river and Federal Capital Territory, Abuja.The deposits in each of these locations are both alluvial and primary in numerous pegmatite bodies.Until recently, coltan were also discovered from a broad 400km long North east -Southwest trending belt stretching from the Wamba -Keffi -Nasarawa area (near the Jos Plateau) through Isanlu -Egbe in Central Nigeria to the Ijero -Aramoko -Ilesha areas of Southwest Nigeria and a few occurrences in the Obudu and Oban massifs of Southeast Nigeria [7][8].
Due to increasing demands of pure coltan products to meeting various diverse industrial needs, efforts of researchers has been geared towards achieving the objectives of the availability of pure coltan products.Within the last decade, ore processors have made considerable progress in the processing of coltan minerals using different methods such as pyrometallurgy, chlorination or hydrometallurgy.It has been observed that during chlorination of Low Grade Concentrate (LGC) and High Grade Concentrate (HGC), coltan products from Low Grade Concentrates are often found contaminated with chloride compounds of manganese and iron, while pure coltan compounds can be obtained from High Grade Concentrate [9].However, hydrometallurgy method which involves the selective leaching using acids such as hydrochloric and nitric acid has been the most economically viable route for the processing of coltan ores [10,11].The appropriate pregnant solution containing the desired metal of interest emanating from the leaching process can further be purified using selective extractants by solvent extraction process [12].
In this work, enrichment of a Nigerian Ixiolite ore for possible conversion to high grade columbite of industrial value by leaching with sulphuric acid solution was examined.The effects of acid concentration, reaction temperature and particle size on the extent of the ore dissolution were examined.

Materials and Method
The Ixiolite ore used for this investigation was sourced from Aba-Paanu, Oluyole Local Government Area of Oyo State, Nigeria.The sample was crushed, grinded and pulverized using ASTM standard species into particle sizes: <0.09 mm, 0.25 mm and 0.30 mm.The <0.09 mm fraction was used in performing all experiments due to its high surface area, unless otherwise stated.
Leaching experiments were conducted in a well-sealed, acid-resistant borosilicate 250 ml glass reactor equipped with a mechanical stirrer for stirring, a thermometer for temperature control and a condenser to prevent heat loss by evaporation.The required temperature of the reactor contents within ±0.5 °C was adjusted by a thermostatically controlled electric heating mantle.For each experiment, the reactor was filled with 100 ml sulphuric acid with pre-determined concentrations (0.1-4.0 mol/L).In each run, 10 g/L of the ore was charged into the reactor and heated to the desired temperature (55 °C) using H2SO4 solution, at various reaction times up to 120 minutes.Preliminary experiments were carried out to determine the optimal leaching conditions.The concentration that gives the maximum dissolution was used for the optimization of other leaching parameters including temperature and particle size.At the completion of the leaching tests, the leached product was filtered, water-washed, and oven-dried to constant weight before being analyzed.
The fraction of the Ixiolite reacted was calculated from the loss in weight of the initial amount of sample dissolved or undissolved during leaching.[10,13].The raw Ixiolite ore and some selected leached products were analyzed using XRF (MINIPAL 4) and XRD (EMPYREAN Diffractometer) and SEM (FEI NOVA NANOSEM 230) with a field emission gun coupled by an EDS detector for preparation of SEM images and analysis.
The X-ray diffraction analysis indicated that the mineralogical constituent is made up of Ixiolite (Ta1.76Fe0.72Sn0.52Mn0.48O8)with considerable amount of Quartz (SiO2) as shown in Figure 1.The EDX analysis of the ore corroborates the result of the EDXRF to give Ta (19.96%),Nb (28.9%),Fe (6.34%), Mn (10.16%), and O (34.64%).The Scanning electron microscopy studies revealed a dominantly flakes of Ixiolite and quartz with dominantly porous ridged surface (A), (B): agglomeration of irregular-like crystals showing high product crystallinity.

A B
Ta Ta

Effect of sulphuric acid concentration
The effect of sulphuric acid solution (0.1-4 mol/L) on the Ixiolite dissolution was studied using a moderate stirring speed and <0.09mm particle size at a temperature of 55 o C. The result obtained is plotted in Figure 5.The result shows that the rate of dissolution increases with time.The rate of dissolution reaches about 50.2% and 51.3% at 3 mol/L and 4 mol/L H2SO4 solution, respectively.However, since the extent of the ore reacted at the above concentrations are almost equal and for economic considerations, 3 mol/L H2SO4 solution was considered as optimum acid concentration and was used for further leaching investigations [14].

Effect of leaching temperature
The effect of temperature on the rate of Ixiolite dissolution was studied over the temperature range of 25 °C -80 °C, using moderate stirring speed (Figure 5).The experimental result obtained showed that the fraction of ore dissolved increases with increasing temperature.It was observed that dissolution rate is sensitive to reaction temperature change.Thus, rate of dissolution increases as a function of temperature, and the dissolution reached 71.4% within 120 minutes at 80°C.The residual product was analyzed and found to contain silica (ɑ-SiO2: 41-1043).However, temperature above 80 °C was not considered in this study, as loss of acid through evaporation was envisaged.

versus leaching time(min) at different H2SO4 concentrations
From figure 8, the slope of each of the lines were calculated and recorded as apparent rate constants kd from which a plot of ln K versus ln [H2SO4] was obtained (Figure 9).

Fig. 9. Plot of ln K versus ln [H2SO4]
From figure 9, the order of the reaction with respect to [H + ] ion concentration from 0.1 -3.0 mol/L H2SO4 was estimated to be 0.5.

Conclusion
In this study, the leaching of an Aba-Paanu (Nigeria) Ixiolite ore was carried out in suphuric acid medium.The results obtained were exemplified and tested with appropriate models.These results have revealed that the rate of Ixiolite dissolution increases with hydrogen ion [H + ] concentration, reaction temperature, leaching time and decreases with particle size.With 3 mol/L H2SO4 solution, 71.4% of the Ixiolite ore was reacted within 120 minutes at optimal leaching conditions.The unreacted component (~29%) was analyzed and found to contain silica.The dissolution kinetics was also analyzed and found to be governed by the shrinking core model for the diffusion controlled process.The reaction order with respect to [H + ] ion and the activation energy for the dissolution process was found to be 0.5 and 24.40 kJ/mol, respectively.Based on the aforementioned data, the results of the sulphuric acid leaching obtained in the study affirmed the possibility of Ixiolite conversion to Columbite at defined optimal conditions.