A Kinetic Study of the Leaching of Iron and Manganese from a Nigerian Tantalite-columbite Ore

MATERIALS AND METHODS

Material: A hard rock tantalite-columbite from Awo, Osun State, Nigeria, was used for all kinetic leaching processes. The chemical analysis of the ore is shown in Table 1. The hydrochloric and sulphuric acids used were reagent grades. They were purchased and used without further purification.

Equipment: A 250 mL three-necked round-bottomed pyrex flask equipped with a thermometer, a mechanical stirrer and a reflux condenser was used as the batch reactor. The reactor was placed on an electrical heating mantle with temperature control.

Table 1:	Chemical analysis of the tantalite-columbite ore

The desired reaction temperature was maintained within ±1°C.

Procedure: For each run, 150 mL of the acid solution of predetermined molarity was charged into the reactor and heated to the required temperature. When the desired temperature was reached, tantalite-columbite (0.5 g) was added to the reactor with continuous stirring. Solution samples of 2 mL were pipetted through a fine frit at after 10 min and then 30 min. Subsequent sampling was done at 30 min intervals. The total time of leaching was 5 h. Iron and manganese were determined by atomic absorption spectrophotometer (Perkin-Elmer model 403).

RESULTS

Effect of stirring speed: The effect of string speed on the rate of leaching of iron and manganese was investigated using 6.0 M HCl solution with the < 63 μm size fraction of the tantalite-columbite at 70°C, at 180, 360 and 540 min^-1. The results in Fig. 1 show that leaching rates depends on the stirring speed as the amount of iron and manganese leached with time increase with increase in stirring speed.

Effect of particle size: The effect of particle size on the leaching of iron and manganese was investigated in 6.0 HCl solution at 70°C, using the three particle size fractions, 0 < diameter (μm) < 63, 63 < diameter (μm) < 171 and 171 < diameter (μm) < 250. The results as shown in Fig. 2 indicated that the rates of leaching of iron and manganese are inversely proportional to the average initial diameter of the particles.

Concentration effect: The effect of HCl and H₂SO₄ concentrations on the leaching of iron and manganese was investigated at 70°C using < 63 μm sized fraction of the tantalite-columbite and a stirring speed of 360 min^-1.

Fig. 1:	Effect of stirring speed on the Leaching of Fe and Mn by 6.0 M HCl solution at 70°C and a particle size of <63 μm

Fig. 2:	Effect of particle size on the leaching of Fe and Mn by 6.0 M HCl solution at 70°C and a stirring speed of 360 rpm

The HCl and H₂SO₄ concentrations used are 4.0, 6.0, 8.0 and 10.0 M. The amount in milligrams of iron and manganese leached versus time plots for various HCl and H₂SO₄ concentrations are shown in Fig. 3 and 4. Concentration of the leachants has a significant effect on the leaching of iron and manganese from tantalite-columbite as the amount leached increases with increase in concentration.

Fig. 3:	Effect of HCl concentration on the leaching of Fe and Mn from the Tantalite-columbite ore at 70°C, a stirring speed of 360 rpm and a particle size of <63 μm

Fig. 4:	Effect of H2SO4 concentration on the leaching of Fe and Mn from the Tantalite-columbite ore at 70°C, a stirring speed of 360 rpm and a particle size of <63 μm

Comparative effect of leachants: The effect of leachants (HCl and H₂SO₄) on the leaching of iron and manganese from the tantalite ore was investigated using 8.0 M HCl and 4.0 M H₂SO₄ at 70°C using the < 63 μm size fraction and stirring speed of 360 min^-1.

Fig. 5:	Effect of hydrogen ion concentration, [H+], of the leachants on the leaching of Fe and Mn at 70°C, a stirring speed of 360 rpm and a particle size of <63 μm

Fig. 6:	Effect of temperature on the leaching of Fe and Mn by 6.0 M HCl solution at a stirring speed of 360 rpm and a particle size of <63 μm

It can be seen from the results in Fig. 5 that both metals were largely leached in HCl within the first 120 min of leaching. Beyond 120 min the leaching of iron was significantly more enhanced in H₂SO₄.

Fig. 7:	Effect of temperature on the leaching of Fe and Mn by 6.0 M H2SO4 solution at a stirring speed of 360 rpm and a particle size of <63 μm

Effect of temperature: The effect of temperature on the leaching of iron and manganese was investigated using 6.0 M HCl and H₂SO₄ solutions at a stirring speed of 360 min^-1 and < 63 μm size fraction at temperature of 30, 50, 70 and 90°C. The results in Fig. 6 and 7 show that leaching rate increases with increase in temperature with both metals being more strongly leached at 90°C.

DISCUSSION

Leaching profiles: The results of the effects of acid concentration on the removal of iron and manganese as presented in Fig. 3 and 4 showed that acid concentrations have significant effect on the leaching. In the two solvent systems, it was observed that the rate of leaching increases with increase in acid concentration. Iron and manganese were leached best by hydrochloric acid within the first 120 mins of leaching. The observed enhanced of leaching of iron by sulphuric acid after 120 min was due to the fact that H₂SO₄ is initially less than 100% dissociated (second proton not as readily released as the first proton).

(1)

(2)

It takes time for H⁺ from reaction (2) to accumulate. Hence as the reaction progresses more ion is leached by H₂SO₄.

Fig. 8:	Graph of 1+2(1-a)-3(1-a)2/3 against time for the leaching of Fe by HCl solution

Fig. 9:	Graph of 1+2(1-a)-3(1-a)2/3 against time for the leaching of Mn by HCl solution

Kinetic models: The leaching rates of iron and manganese were analysed with the shrinking core model for reaction control under the assumption that the ore is a homogeneous spherical solid phase^[12]. Two previously established models were used, expressed by the following equations:

(3)

(4)

Fig. 10:	Graph of 1+2(1-a)-3(1-a)2/3 against time for the leaching of Fe by H2SO4 solution

Fig. 11:	Graph of 1+2(1-a)-3(1-a)2/3 against time for the leaching of Mn by H2SO4 solution

where, ‘a’ is the fraction reacted at time t (min), k₁ (m min^-1) and k₂ (m² min^-1) are the overall rate constants. Equation (3) is applicable to chemically controlled processes and equation (4) to diffusion controlled processes through the porous product layer.

The diffusion controlled kinetic model fits the use of hydrochloric acid and sulphuric acid for both iron and manganese at the temperature range covered except for the leaching of iron by H₂SO₄ solution where deviation was observed at higher temperatures 70 and 90°C, (Fig. 10).

Fig. 12:	Arrhenius plot for the leaching of Fe by HCl and H2SO4 solutions

Fig. 13:	Arrhenius plot for the leaching of Mn by HCl and H2SO4 solutions

At these temperatures (70 and 90°C) thermal motion is greater and diffusion may not significantly be rate determining.

For oxidizing acid like H₂SO₄ the validity of diffusion controlled model may be influenced by formation of metal oxides. It is known that iron (II) oxide, FeO and manganese (II) oxide, MnO, present in the tantalite-columbite matrix in the solid in solution^[13].

Table 2:	Activation energy values for the leaching of iron and manganese

During phase transformation, solid/solution, it is presumed that iron and manganese oxides are separated out of the tantalite-columbite matrix, but are still surrounded by newly formed Fe₂O₃ (hematite) and Mn₂O₃ layer ( products of oxidation of FeO and MnO). Therefore, during leaching the rate of removal of both metals, Fe and Mn is controlled by diffusion through the Fe₂O₃ and Mn₂O₃ layers.

Estimation of activation energies: The reaction between the overall rate constant and temperature from equation (4) may be expressed by the Arrhenius equation, k=Ae^-Ea/RT where, k is the overall rate constant (m² min^-1), Ea is the activation energy (J mol^-1), R is the universal gas constant (8.314 KJ^-1), T is the reaction temperature (k) and A is the preexponential factor (m² min^-1). Rate constants for different temperatures were calculated from the plots in Fig. 8-11 and the Arrheniun plots of ln k vs T^-1 are as shown in Fig. 12 and 13 for iron and manganese respectively. The estimated activation energies from these plots are as stated in Table 2.

The relatively low values of activation energies (< 20 KJ mol^-1) for the leaching of iron and manganese by hydrochloric acid are clearly indicative of a diffusion and not chemically-controlled mechanism^[14]. However, the relatively high values of activation energies (34.99 and 60.78 KJ mol^-1) for the leaching by sulphuric acid are not an indication that the leaching is by chemical control since the best way to predict the leaching mechanism of a reaction is through the kinetic model plots.

CONCLUSION

The following conclusions can be drawn from this study;