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Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.4, pp 261-270, 2009
jmmce.org Printed in the USA. All rights reserved
Model for Predictive Analysis of the Concentration of Phosphorus Removed
during Leaching of Iron Oxide Ore in Sulphuric Acid Solution
C. I. Nwoye*
and S. Ndlu
1. Department of Materials and Metallurgical Engineering Federal University of
Technology, Owerri, Imo State.
2. School of Process & Materials Engineering, University of the Witwatersrand,
Johannesburg, South Africa.
*Corresponding Author: email@example.com
Model for predictive analysis of the concentration of phosphorus removed (relative to the initial
and final pH of the leaching solution) during leaching of iron oxide ore in sulphuric acid
solution has been derived. It was observed that the validity of the model is rooted in the
mathematical expression; (P/N)
) where both sides of the relationship are almost equal.
The model; P = 4.25(e
shows that the concentration of phosphorus removed is dependent on
the values of the initial and final pH of the leaching solution. In all, the positive or negative
deviation of the model-predicted phosphorus concentration from its corresponding value
obtained from the experiment was found to be less than 29%, which is quite within the
acceptable deviation limit of experimental results hence establishing the validity and precision of
the model in its application for predicting quantitatively the concentration of phosphorus
removed during the leaching process.
Key Words: Model, Prediction, Phosphorus Removed, Sulphuric Acid, Iron Oxide Ore, Leaching.
Vol.8, No.4 Model for Predictive Analysis of the Concentration of Phosphorus 262
Lee et al.  reported that the leaching of 3g/L pure haematite (98.2% purity, 105-140µm size
range) using 0.048-0.48M oxalic acid at 80-100
C passed through a maximum peak at pH 2.5.
Dissolution of haematite was found  to be slower than magnetite (FeO.Fe
) and other
hydrated iron oxide such as goethite (α-FeOOH), lapidochrosite (γ-FeOOH) and iron hydroxide
The dissolution of iron oxide is believed to take place via a photo-electro chemical reduction
process, involving a complicated mechanism of charge transfer between the predominant oxalate
species, namely ferric oxalate Fe(C
, ferrous oxalate Fe(C
acting also as an auto
catalyst, and the oxalate ligand on the iron oxide surface .
The dissolution of iron oxides in oxalic acid was found to be very slow at temperatures within
the range 25-60
C, but its rate increases rapidly above 90
C . The dissolution rate also
increases with increasing oxalate concentration at the constant pH values set within the optimum
range of pH 2.5-3.0. At this optimum pH, the dissolution of fine pure haematite (Fe
140µm) follows a diffusion-controlled shrinking core model .
Taxiarchour et al  reported that it took close to 40h to dissolve 80% of pure haematite slurry
(97% purity, 0.022% w/v or 0.21% g/L Fe
) at pH 1. He stated that even at 90
C, it required
close to 10h to achieve 95% dissolution of iron of the slurry at pH 1. They also dissolved iron
using 0.1-0.5M oxalic acid (pH1-5) to dissolve iron from a 20% w/v slurry (83% of particle size
in the range 0.18-0.35mm, containing 0.029% Fe
). The iron oxide concentration in the leach
is equivalent to 0.058g/L Fe
The speciation of Fe(III) oxalate and Fe(II) oxalate has been found  to be governed by pH and
total oxalate concentration. For a having pH › 2.5 and an oxalate concentration higher than 0.1M,
the most predominant Fe(III) complex ion existing is Fe(C
. At these conditions, (pH › 2.5
and an oxalate concentration higher than 0.1M) the predominant Fe(II) complex species is
263 C. I. Nwoye and S. Ndlu Vol.8, No.4
Nwoye  derived a model for quantitative analysis of dissolved iron in oxalic acid solution in
relation to the final pH of the solution during leaching of iron oxide ore;
γ = 0.5 K
= Dissolution constants of Fe and Fe
γ = pH of leaching solution after time t (mins.).
The values of the dissolution constants compared with those of % Fe and % Fe
experiment  indicate clearly that the constants K
are numerical equivalence of the
chemical resistance to the dissolution of Fe and Fe
(respectively) in oxalic acid solution. It
was found that K
indicating twice chemical resistance to the dissolution Fe compare to
that of Fe
. This expression agreed with the higher percentage of Fe
dissolved compared to
that of the corresponding Fe.The model also predicted the final pH of the leaching solution when
the concentrations of Fe and Fe
dissolved (at a temperature of 30
C and average ore grain
size; 150µm) are known.
Nwoye et al  derived a model for predicting the concentration of dissolved iron during
leaching of iron oxide ore in sulphuric acid solution. The model is stated as;
%Fe = 0.35(α/T)
T = Solution temperature at the time t, when the concentration of dissolved iron is
0.35= (pH coefficient for iron dissolution in sulphuric acid solution during the leaching
process) determined in the experiment .
α = Final pH of the leaching solution at the time t, when the concentration of
dissolved iron is evaluated.
The model (formulated at conditions; leaching temperature of 25
C, initial solution pH 5.0 and
average grain size; 150µm) is dependent of the final pH and temperature of the leaching solution.
The model shows that the concentration of iron dissolved during the leaching process is directly
proportional to the third power of the ratio of final leaching and temperature.
Vol.8, No.4 Model for Predictive Analysis of the Concentration of Phosphorus 264
It has been found [8, 9] that the final pH of the leaching solution depend on the leaching time,
initial pH for the leaching solution and the leaching temperature. Past reports [10-15] have been
expounded researches carried out to remove phosphorus from steel during steel making. Findings
from all these works carried out, pointed out high oxygen activity and low treatment temperature
as the only essential and unavoidable process conditions which can enhance the rate of
Removal of phosphorus from iron has been reportedly achieved only by oxidation during steel
making, under the influence of a basic slag which is principally made up of CaO . Model has
been derived  for predicting the concentration of phosphorus removed during leaching of
iron oxide ore in oxalic acid solution. The model is expressed as;
P = 150.5 (3)
P =Concentration of phosphorus removed during the leaching process (mg/Kg)
(µ) = Weight input of iron oxide ore (g)
(α) = Final pH of the leaching solution at the time t when P is evaluated
150.5 = (pH coefficient for phosphorus dissolution in oxalic acid solution during the
process) determined in the experiment .
The model  predicted the concentration of phosphorus removed, with high degree of
precision being dependent on the final pH of the leaching solution and weight input of the iron
oxide ore. It also shows that the concentration of phosphorus removed (at a temperature of 25
average ore grain size; 150µm and initial leaching solution pH 5.5) is inversely proportional to
the product of the final pH of the leaching solution and the weight input of the iron oxide ore.
It has been reported  that phosphorus could be removed from Itakpe iron oxide ore through a
leaching process using sulphuric acid solution. Chemical analysis carried out on the iron ore
indicates that the percentage of phosphorus in the ore is about 1.18%, which from all indication
is quite high and could cause embrittlement and other adverse effects on the service life of
engineering materials made from such iron ore. This was the basis for the dephosphorization
265 C. I. Nwoye and S. Ndlu Vol.8, No.4
process carried out . Phosphorus was removed (at a temperature of 25
C, average ore grain
size; 150µm and initial leaching solution pH range 0.47-0.53) and then dissolved in the acid
solution in the form of phosphorus oxide.
The aim of this work is to derive a model for predictive analysis of the concentration of
phosphorus removed relative to the initial and final pH of the solution during leaching of Itakpe
(Nigeria) iron oxide ore using sulphuric acid solution. This derivation is embarked on in
furtherance of the previous work .
The ore is assumed to be stationary in the reaction vessel during the leaching process and
contains the un-leached iron as part of reaction remnants. The ore is attacked by hydrogen ions
from sulphuric acid within the liquid phase, and in the presence of oxygen.
2.1 Model Formulation
Results from experimental work  carried out at SynchroWell Research Laboratory, Enugu
were used for the model derivation. These results are as presented in Table 1.
Computational analysis of these experimental results  shown in Table 1, resulted to Table 2
which indicate that;
) (approximately) (4)
P/N = (
P = N (
Introducing the value of N into equation (6)
P = 4.25(
P = Concentration of phosphorus removed during the leaching process (mg/Kg)
Vol.8, No.4 Model for Predictive Analysis of the Concentration of Phosphorus 266
N= 4.25; (pH coefficient for phosphorus dissolution in sulphuric acid solution.)
determined in the experiment.
α = Final pH of the leaching solution at the time t when the concentration of removed
phosphorus is evaluated.
γ = Initial pH of the leaching solution just before the leaching process started.
Equation (7) is the derived model.
Table1: Variation of initial and final pH of leaching solution with concentration of
(α) P (mg/Kg)
Table2: Variation of (P/N)
267 C. I. Nwoye and S. Ndlu Vol.8, No.4
3. BOUNDARY AND INITIAL CONDITION
Iron oxide ore was placed in cylindrical flask 30cm high containing leaching solution of
sulphuric acid. The leaching solution is non flowing (stationary). Before the start of the leaching
process, the flask was assumed to be initially free of attached bacteria and other micro organism.
Initially, the effect of oxygen on the process was assumed to be atmospheric. In all cases, weight
of iron oxide ore used; 2g. The initial pH range of leaching solutions used; 0.47-0.53 and
leaching time of 3 hrs were used for all samples. A constant leaching temperature of 25
used. Sulphuric acid concentration at 0.1mol/litre and average ore grain size; 150µm were also
used. Details of the experimental technique are as presented in the report .
The leaching process boundary conditions include: atmospheric levels of oxygen (considering
that the cylinder was open at the top) at both the top and bottom of the ore particles in the gas
and liquid phases respectively. A zero gradient was assumed for the liquid scalar at the bottom of
the particles and for the gas phase at the top of the particles. The sides of the particles were
assumed to be symmetries.
4. MODEL VALIDATION
The formulated model was validated by calculating the deviation of the model-predicted
concentration of phosphorus removed from the corresponding experimental values. The
deviation recorded is believed to be due to the fact that the surface properties of the ore and the
physiochemical interactions between the ore and leaching solution which were found to play
vital roles during the leaching process  were not considered during the model formulation. It
is expected that introduction of correction factor to the predicted concentrations of P, gives
exactly the experimental values of P.
Deviation (Dv) (%) of model- predicted P values from experimental P values is given by
Dv = P
x 100 (8)
Where Pp = Predicted P values
= Experimental P values
Vol.8, No.4 Model for Predictive Analysis of the Concentration of Phosphorus 268
Since correction factor (Cr) is the negative of the deviation,
Cr = - Dv (9)
Substituting equation (8) into equation (9) for Dv,
Cr = -100 P
It was observed that addition of the corresponding values of Cr from equation (10) to the model-
predicted concentrations of P gave exactly the corresponding experimental P values. 
5. RESULTS AND DISCUSSION
The derived model is equation (7). Insignificant positive and negative deviations were found to
have resulted on comparing values of P from the experimental data and those from the model,
hence depicting the reliability and validity of the model. This can be deduced from Table 3. The
positive and negative deviations of the model-predicted P values from those of the experiment
were found to be less than 29% which is quite within the acceptable deviation limit of
experimental results. The validity of the model is believed to be rooted on equation (4) where
both sides of the equation are almost equal. Table 2 also agrees with equation (4) following the
values of (P/N)
) evaluated after statistical and computational analysis carried out on
experimental results in Table1.
The model predicts the concentration of phosphorus removed (relative to the initial and final
solution pH) during leaching of Itakpe iron oxide ore. The validity of the model is believed to be
rooted on equation (4) where both sides of the equation are almost equal. In all cases, the
deviation of the model-predicted P value from that of the corresponding experimental P value is
less than 29% which is quite within the acceptable deviation limit of experimental results.
269 C. I. Nwoye and S. Ndlu Vol.8, No.4
Table 3: Comparison between concentrations of phosphorus removed as predicted by model
and as obtained from experiment .
(mg/Kg) Dv (%) Cr (%)
= P values from experiment 
= P values predicted by model.
The authors thank Dr. Ekeme Udoh and Pearl Bassey, modelling experts at Linkwell Modelling
Centre Calabar for his technical inputs. The management of SynchroWell Nig. Ltd. Enugu is also
appreciated for permitting and providing the experimental data used in this work.
 Lee, S. O., Oh, J. K., Shin, B. S., 1999, Dissolution of Iron Rust Materials using Oxalic Acid.
J. Min. Metall. Inst. Jpn., Vol. 115, pp. 815-819.
 Taxiarchour, M., Panias, D., Doumi, I., Paspaliaris, I., Kontopoulos, A., (1997a) Removal of
Iron from Silica Sand by Leaching with Oxalic Acid, Hydrometallurgy, 46, 215-227.
 Lee, S.O, Tran, T., Park Y.Y., Kim S.J., and Kim, M. J. (2006) Study on the Kinetics of Iron
Leaching by Oxalic Acid.Int. J .Miner Process, 80, 144-152.
 Taxiarchou, M., Parnias, D., Doumi, I., Paspaliaris, I., and Kontopoulous, A. (1997b)
Dissolution of Haematite in Acidic Oxalate Solutions. Hydrometallurgy, 44, 287-299.
Vol.8, No.4 Model for Predictive Analysis of the Concentration of Phosphorus 270
 Panias, D., Taxiarchou, M., Paspaliaris, I., Kontopoulos, A. (1996) Mechanism of
Dissolution of Iron Oxides in Aqueous Oxalic Acid. Hydrometallurgy, 42, 257-265.
 Nwoye, C. I. (2008) Model for Quantitative Analysis of Dissolved Iron in Oxalic Acid
Solution during Leaching of Iron Oxide Ore, Inter. Res. J. Eng. Sc. Tech., 5(1): 37-41.
 Nwoye, C. I., Amara, G. N., and Onyemaobi, O. O. (2008) Model for Evaluating Dissolved
Iron during Leaching of Iron Oxide Ore in Sulphuric Acid Solution, Inter. J. Nat. Appl. Sc., 4(2):
 Pinches, A. (1975) Bacterial Leaching of an Arsenic Bearing Sulphide Concentrate. The
Institute of Mining and Metallurgy, England, 34.
 Nwoye, C. I. (2008) Ph.D Thesis, Metallurgical and Materials Engineering Department,
Federal University of Technology,Owerri,178.
 Turkdogan, E.T., and Pearson, J., (1953) J. Iron and Steel Inst., 221, pp.393-401.
 Decker, A., Sevrin, R., and Scimar, R., (1962) Open Hearth Proceedings, 45, pp. 421- 456.
 Duke, D.A., Ramstad, H. F. and Meyer, H. W (1962) Open Hearth Proceedings, 45, pp.81-
 Kootz,T., and Neuhaus, H., (1961) Stahl u. Eisen, 81, pp. 1810-1815.
 Kootz, K., Behrens, K., Maas, H., and Baumgarten, P. (1965) Stahl u. Eisen , 85, pp 857-
 Edneral, F. P., (1979) Electrometallurgy of Steel and Ferro- alloys, MIR Publisher, 5th
edition Moscow. pp 30-239.
 Zea,Y.K. (1945) J. Iron and Steel Inst., 151, pp. 459-504.
 Nwoye, C. I., Agu, P.C., Mark, U., Ikele, U.S., Mbuka, I. E., and Anyakwo, C. N. (2008)
Model for Predicting Phosphorus Removal in Relation to Weight of Iron Oxide Ore and pH
during Leaching with Oxalic Acid Inter. J. Nat. Appl. Sc., 4(3): 106-112.
 Nwoye, C. I. (2008) SynchroWell Research Work Report, DFM Unit, No 2000116, 36-50.