Leaching of a Nigerian Columbite in Hydrochloric Acid: Dissolution Kinetics

This paper presents the kinetics of dissolution of a Nigerian columbite mineral ore sample in hydrochloric acid media. The effects of acid concentration, process temperature, stirring rate and particle size on the dissolution rate were examined. Experimental results showed that the columbite dissolution increases with increasing acid concentration, temperature, stirring speed and decreases with particle size. W ith 4.0 M HCl solution, about 15.37% of columbite was dissolved within 120 minutes using < 0.15 mm particle size at a temperature of 80°C and a stirring speed of 360 rpm. The result of the study indicated that the leaching data fitted a diffusion model. Values of 0.5, 22.40 kJmol and 1.51 X 10 min were calculated as reaction order, activation energy and Arrhenius constant, respectively for the dissolution process.


Introduction
Colu mb ite is a b lack mineral group that is an ore of niobium and tantalum [1,2]. It contains a higher percentage of niobium as compared to tantalum and as a result it is taken exclusively as niobiu m mineral. The mineral Niobiu m is therefore used mainly as an alloy of steels, heat sensitive detective devices called barometer for jet engines and other aircraft co mponents. It is used in electronic devices like cell phones and DVD players. High purity niobiu m is utilized to fabricate radio frequency cavities fo r the acceleration of electrons in quantum physics. It is also employed in the field of app lied superconduct iv ity , main ly includ ing energy-rel ated app licat ions, transportat ion, co mput ersand instrumen tation. Niobiu m alloys such as NbTi alloys and Nb3Sn intermetallics, have been the superconducting materials commercially available [3,4].
The processing route for tantalum/niobiu m ores hasbeen th e d iss o lu tion o f the min eral in h o t, co ncentrated hydrofluoric acid , the metals are then separated either by fract ion al crystallisat ion as K2M F7 salts or by solv ent extraction [5]. Authors have therefore investigated the effect of mechanical activation on the leaching of columbite and have found th at the mechan ical act ivat io n of n iob iu m /tantalum concentrates marked ly affects the rate and extent of the d issolution inhydrofluo ric acid med ia [6] and in a s od iu m f lu o rid e an d h y d ro flu o ric acid s o lu tio n [5].
Rodriguez et al. [7] also investigated the leaching of columb ite in hydrofluoric and carbo xylic acids media. It is therefore interesting to know the leaching mechanism of Nb and Ta bearing minerals i.eco lu mbite in acid solutions that are not as strong as HF. Ayanda et al. [1] therefore reported the dissolution kinetics of a Nigerian colu mbite mineral ore sample in nitric acid and values of 15.100 kJ mo l -1 and 9.42 × 10 -5 min -1 were obtained for the activation energy and Arrhenius constant of the columbite dissolution in nitric acid, respectively and the order of reaction was 0.35.
The present investigation was therefore aimed at establish ing the conditions for the leaching of the same columb ite mineral o re in hydrochloric acid media.

Material
Colu mb ite sample used for this study was collected from a mining site inDaba-Lema, Edu Local Govern ment of Kwara State. All reagents used were of analar grade and were all products of BDH. De-ion ized water was used for all the analytical preparat ions.

Leaching Procedure
0.5g of colu mb ite samp le was leached in100 ml of hydrochloric acid. The fract ion of co lu mbite samp le dissolv ed for each process was calculated by the expression [8,9]:

Kinetic Anal ysis
For this study, three shrinking core models [1,10,11] were investigated on the result obtained on the effect of temperature. 1 (4) where X is the fraction of colu mbite dissolved; t is contact time (s); k r , k d and k m are the reaction rate constants. Equation 3 is based on the chemically controlled reaction and its slope corresponds to the apparent rate constant k r . Equation 4 on the other hand is based on diffusion controlled and its slope gives the apparent rate constant k d . Lastly, Equation 3 is based on a mixed controlled reaction and apparent rate constant k m .
The slope that gave the best correlation coefficient was chosen as the model that fit perfectly with the colu mb ite dissolution

3.1.1.Effect of Concentration
The effect of concent ration on co lu mb itesamp le dissolut ion was studied over the concentration range of 0.1M-8.0M. The results obtained are as shown in Figures 1. It was observed fro m Figure 1 that an increase in the concentration of hydrochloric acid was accompanied with increase in the rate of colu mbite dissolution. i.e the rate of colu mb ite dissolution is affected directly by hydrogen ion[H + ] concentration. This agrees with several reported work on acid dissolution of solid mineral ores [12][13][14][15]. The result also showed that 11.04% colu mbite was dissolved by contacting columb ite samp le with 8.0M HCl at a temperature of 55℃ and a contact time of 120 min.

3.1.2.Effect of Temperature
The effect o f temperatu re was stud ied o ver the temper ature ranges of 28 -80 ℃ and investigated in different concentrations of leachants ranging from 0.5M -8.0M. The results obtained are presented in Figure 2. The figure showed that increase in temperature accelerates the reaction rate and this leads to increase in the amount of columb ite dissolved. 13.46% of colu mbite sample was dissolved in 8.0M HCl at a temperature of 80 ℃ and a contact time of 120min.

3.1.3.Effect of Particle Size
The result of the effect of particle size on the dissolution of columb ite sample as investigated in 4.0M HCl is represented in Figure 3. A graph of the fraction of co lu mbite d issolved versus contact time for the different part icle size fraction as represented in Figure 3, showed that the fraction with the smallest particle size (<0.15mm) gave the h ighest percentage of colu mbite dissolution (12.86%). Th is is due to the highest surface area of the smallest particle size fraction.

3.1.4.Effect of Stirring Speed
The percentage of columbite d issolved (%) versus stirring rate (rp m) as investigated between 0 -540 rp m is as shown in Figure 4.
The result showed that the amount of columb ite dissolved increases with stirring speed between 0 -360rp m. The percentage dissolved appears to be practically constant afterwards. About 15.37% of colu mb ite was dissolved in 4.0 M HCl, at a temperature of 80℃, and a stirring speed of 360 rpm.

Kinetics of Dissolution
The rate constant values k r , k d and k m calculated fro m Equat ion (2), (3) and (4) respect ively, and their correspon dng correlation coefficients are given in Tab le 1.
The values of the correlat ion coefficients indicated that the dissolution rate of colu mb ite in hydrochloric acid is d iffusion controlled. The application of the diffusion kinetic model is as shown in Figure 5.

Activation energy
This model was chosen for fu rther studies. Table 2 shows the values for the apparent rate constants and ln of the apparent rate constants for HCl at various temperatures and the Arrhenius plot is illustrated in Figure 6.
Fro m the slope of Figure 6, using Equation 5, the activation energy (E a ) for the dissolution of colu mb ite in HCl is 22.40 kJmo l -1 .   The extrapolation of Figure 6 to the y-axis[ Figure 7] gave a value of -11.1 min -1 wh ich is equivalent to lnA in Equation 5. The Arrhenius constant (A) for the columbite dissolution in HCl is 1.51 X 10 -5 min -1 .

Order of reaction
The order of reaction fro m the slope for colu mb ite dissolution in hydrochloric acid with respect to[H + ] was 0.5. Table 3 shows the values for the apparent rate constants for the hydrogen ion concentration while Figure 8 shows the plot of lnk d2 versus ln[H + ] for hydrochloric acid concentrations.

Conclusions
In the present study, the dissolution of colu mbite in hydrochloric acid was studied. It was found that the rate of columb ite dissolution increases with acid concentration, temperature, stirring speed and decreases with particle size. The dissolution of colu mbitein hydrochloric acid was found to be controlled by the shrin king core model for a diffusion-controlled process. Values of 22.40kJmo l -1 and 1.51 X 10 -5 min -1 were obtained for the activation energy and Arrhenius constant of columbite dissolution in hydrochloric acid respectively; and the order of reaction was 0.5.