Comparative Kinetics of Iron Ore Dissolution in Aqueous HCl-HNO 3 System

The dissolution kinetics of the dissolution of iron ore in aqueous HCl/HNO3 solution was studied. The elemental composition of the ore was carried out using the inductively coupled plasma-optical emission spectrophotometer (ICP-OES). The result showed that the iron ore contain; Fe (62.1%), O (21.7%), Cu (11.1%), Mg (2.39%), Na (1.51%), Mn (1.47%), K (0.78%), Ca (0.58%) and Zn (0.01%). It was determined that the dissolution rate increased with increased solution concentration, temperature, time and decreased particle size of the ore. The optimum conditions for effective dissolution of 88% of the iron ore were found to be 8 M of the solution, 353 K, 100 min and ore particle size of less than 75 μm. The kinetic evaluation of the dissolution process was studied using three different shrinking core models (SCM); Film diffusion: kft = XB; interfacial chemical reaction krt =   3 1 1   B X and ash/product layer diffusion: kdt =   3 2 1 1 3    B B X X for spherical materials was performed. The results obtained showed that the rate determining step for the dissolution process was the product layer diffusion and therefore, the reaction followed this mechanism. The apparent activation energy (Ea) and the order of reaction were found to be 20.48 kJ/mol and 0.7 respectively.


Introduction
Dissolution processes have been extensively utilized to separate valuable minerals from their ores.Several researches [1][2][3][4][5] have been carried out on either the dissolution or both the dissolution and kinetic study of iron ore in different reagents such as HCl, H 2 SO 4 , oxalic acid et cetera.It was observed that the dissolution of iron ore or even the extraction of iron from an ore containing it increases as the concentration, temperature, time and stirring speed increases but decreases as the particle size increases due to its decreasing surface area [3][4][5].Baba et al. [1,2] observed that iron ore dissolves easily and more efficiently in HCl than in H 2 SO 4 or HNO 3 and it was believed to be as a result of ferric-chloride complexes formation.It therefore required less energy for the reaction to occur comparing the activation energies of 13.63 kJ/mol in HCl and 38.29 kJ/mol in H 2 SO 4 respectively.The kinetic study of these dissolutions reaction processes were carried out using the shrinking core models to interpret the processes involved in either the leaching or dissolution [1,3,4,6].During the leaching process, three major steps occur; diffusion or mass transfer through the liquid film surrounding a solid particle, chemical reaction on the surface of the un-reacted core, and diffusion through the ash/inert solid layer.The slowest between these steps is considered the rate determining step.The model first developed by Yagi and Kunii [6][7][8] helped to derive the rate determining step from the experimental data using the equations below:   where K f , K d , and K r are the rate constants for the liquid film diffusion, ash/inert solid layer diffusion, and surface chemical reaction respectively.X B is the fraction reacted, M S is the molecular weight of the solid, C A is the concentration of the dissolved lixiviant A in the bulk of the solution, "a" is the stoichiometric coefficient of reagent in the leaching reaction, r 0 is the initial radius of the solid particle, ρ s is the density of the solid, D is the diffusion coefficient in the porous product layer, K g is the mass transfer coefficient between fluid and solid particle, "K" is the first-order rate constant for the surface reaction and t is the contact time.This research work is intended to study the dissolution of iron ore in a 3:1 HCl-HNO 3 system (similar to aqua regia) which is a well known reagent for dissolving un-reactive metal minerals such as gold, silver, et cetera and also the kinetic evaluation using three different shrinking core models (SCM).So far, there is no data to this effect viz-a-viz the use of Nigerian iron ore is concerned.

Materials
Iron ore from the Toto Muro iron ore deposit in Nasarawa State of Nigeria was used for this study.Chemical analysis of the sample ore was done with Leeman Model of Inductively coupled plasma-optical emission spectrophotometer.Different particle sizes (<75 µm, 75 µm, 125 µm and 150 µm) were obtained with the use of 8 inch diameter size mechanical sieve shaker.Distilled water and analytical grade chemicals (BDH) were used as provided to prepare all the solutions.

Methods
Particle sizes of less than 75 µm sample of the iron ore was used for the experiment at a constant concentration HCl-HNO 3 solution and a fixed temperature of 331 K but at different contact time.1.0 g of the given particle size sieve sample was weighed and then transferred into a 250 ml beaker containing 15 ml of 8 M HCl and 5 ml of 8 M HNO 3 and made to 100 ml with distilled water.The mixture was agitated manually with a glass stirrer.The mixture was stirred and heated to 331 K for various contact times of 20, 30, 60 and 100 min.At the end of each period, the solution was cooled and filtered into a 100 ml standard flask and analyzed.
The same procedure was also repeated for different acid (HCl-HNO 3 ) concentrations of 4 M, 2 M and 1 M at 331 K for 20, 30, 60 and 100 min.This same procedure described above was repeated for 343 K and 353 K for 20, 30, 60 and 100 min with 8 M acid concentration.The following particle size fractions were examined 75 µm, 125 µm and 150 µm at a fixed temperature of 353 K for 30 min with 8 M acid concentration.The kinetics of the ore dissolution was also investigated for the effects of temperature, acid concentration, particle size and contact time.

Chemical Analysis
The iron ore was analyzed chemically using the inductively coupled plasma-optical emission spectrophotometer and the results are shown in Table 1.The primary elements found in the sample included 62.10% Fe, 21.7% O, 11% Cu.Other elements such as Mn, Mg, K, Ca et cetera make up the 5.2% of the sample.From these analytical results, it is concluded that the ore was an iron ore.

Effect of Acid Concentration
The effect of 3:1 HCl-HNO 3 solution concentration on the dissolution of the iron ore was studied using the following concentrations; 1 M, 2 M, 4 M, and 8 M. Figure 1 is the graphical presentation of the result.From the figure, it can be seen that the iron dissolution rate increases as the concentration of the HCl-HNO 3 system increases from 1 M to 8 M at the same contact time.The optimum dissolution of 81% iron was achieved at the system concentration of 8 M.

Effect of Temperature and Time
The effects of temperature and contact time on iron dissolution were studied at varying temperatures (331 K, 343 K and 353 K) and at varying time (20 min, 30 min, 60 min, and 100 min).The results are shown in Figure 2. From Figure 2 it is observed that as the temperature increases, dissolution of the iron increased with tempera-   ture and period of contact.When the temperature was increased from 331 K to 343 K at a particular period (20 min) only 2.5% increase in the quantity of iron leached was achieved.At temperature 331 K, there was 8.5% increase in the quantity of iron leached.Optimum temperature and period was found to be 353 K and 100 min respectively in which 88% of the iron was leached.constants k f , k r and k d and their respective regression correlation coefficients (R 2 ).From the values of the apparent rate constants and their respective R 2 values, it is observed that the experimental data best fit the product/ash layer diffusion control mechanism and therefore is the rate determining step.

Effect of Particle Size
Table 2 shows the experimental data when different particle sizes were investigated at 8 M HCl-HNO 3 solution, temperature of 353 K and contact time of 60 min.Four particle sizes were studied-<75 µm, 75 µm, 125 µm and 150 µm and from the data it was observed that the smallest particle size (<75 µm) gave the highest percentage of iron leached (87%) which is expected due to higher surface area for smaller particle size.

Activation Energy
The relationship between the rate constant k d and the temperature is given by the Arrhenius equation given by the expression;

Kinetic Analysis
where A is the frequency factor and Ea is the apparent activation energy.The dissolution of the iron ore under study was analyzed using the shrinking core model (SCM) based on the assumption that the material is spherical [3,6,9] and the linearization of a plot of iron dissolved at various temperature and at various contact time was investigated using the three common shrinking core model for spherical material [6].The Equations ( 1), (2), and (3) for the mod-The rate constants were calculated from Figure 5 which is the slopes from the graph and are given in Ta- ble 5. To determine the activation energy for the dissolution of the iron ore in HCl-HNO 3 system, an arrehnius graph of lnk d vs 1/T (K −1 ) was plotted which an Arrhenius plot is obtained and the activation energy also obtained from the slope of the graph (Figure 6) using Equation ( 4) to obtain Equation (5).
for the experimental data in which the slowest step is the rate determining step.Where Equations ( 1)-( 3) are film diffusion control mechanism, interfacial chemical reaction control mechanism and ash/product layer diffusion control mechanism respectively with k f , k r and k d as their respective rate constants, t is contact time and X B is the fraction of iron leached.The activation energy and the order of reaction were also calculated.This is equivalent to y = mx + c and therefore the slope from the graph of lnk d vs 1/T (K −1 ) is equivalent to E a /R, which means E a = slope × R.
From Figure 6 the apparent activation energy was calculated to be 20.46 kJ/mol which is within the range of activation energy for product layer diffusion controlled mechanism.

Determination of the Reaction Mechanism 3.8. Order of Reaction
The fraction of iron dissolved (X B ) obtained from the Figure 2 with respect to the various contact time for various temperature was substituted into Equations ( 1)-( 3) and the results are recorded in     order of reaction was calculated to be the slope of the plot and equal to 0.7.Therefore the order of reaction for the dissolution of the iron ore in HCl-HNO 3 solution is 0.7 indicating a pseudo first order reaction.

Conclusion
From the kinetic evaluation for the dissolution of iron ore obtained from Toto Muro iron ore deposit in Nasarawa ( Table 3. Data of three control mechanisms at varying times with respect to temperature.State) in HCl-HNO 3 solution, it was observed that the rate of dissolution increases with increasing system concentration, temperature, and time but with decreasing particle size.The optimum conditions for the effective dissolution of 88% of the iron ore in HCl-HNO 3 solution were obtained at concentration of 8 M, temperature of 353 K, and contact time of 100 min and particle size of less than 75 µm.Also the reaction was found to be controlled by the product layer diffusion controlled process and this conformed to the shrinking core model equation: . The apparent Activation energy (E a ) and the order of reaction for the dissolution of 1 g of the iron ore in HCl-HNO 3 system were found to be 20.48 kJ/mol and 1 respectively.

Figure 7
to obtain the k d values.The k d values were used to plot a graph of lnk d vs ln[HCl-HNO 3 ] as shown in Figure 8 from which the

Figure 3 .
Figure 3. Plot of X B vs contact time with respect to temperature.

Figure 4
Figure 4. Plot of

Figure 5
Figure 5. Plot of

Table 3 .
The respective plots of the film diffusion, interfacial chemical reaction and product/ash layer diffusion control mechanisms vs contact time with respect to temperature is given in Fig- ures 3-5.Table 4 give a comparative values of the rate

Table 6 .
They were used in making a plot against time in