Dephosphorization Treatment of High Phosphorus Oolitic Iron Ore by Hydrometallurgical Process and Leaching Kinetics

Abstract

Dephosphorization process of high phosphorus oolitic iron ore by acid leaching and leaching kinetics were investigated in the paper. The high phosphorus ore samples (51%T.Fe, 0.52%P) used in this work were analysed by SEM-EDS and XRD, which showed their oolitic structure and mineral phases. Among the three kinds of acids (H₂SO₄, HCl and HNO₃), sulfuric acid was the most appropriate one for dephosphorization, and structure changes after leaching were also investigated through the SEM and EDS analysis. The effects of acidity, particle size, stirring speed and temperature were researched in detail. Through acid leaching, phosphorus could be removed effectively, and iron loss was negligible, which was also studied by thermodynamic calculation. The experimental data could be well described by the unreacted shrinking core model and rate controlling step was found to be chemical reaction between apatite and acid. The apparent activation energy was calculated as 45.02 kJ/mol.

1. Introduction

China is the largest producer and consumer of the iron and steel products in the world. In 2012, China’s crude steel production reached 7.16 billion tons, which made up half the world’s total output. However, high grade iron ores are not rich in China, and the import price of the ores has been rising dramatically, which has threatened the national sustainable development of iron and steel industry. To solve the crisis of resources, utilization of the refractory ores is an urgent need,^1,2) such as oolitic iron ore. The oolitic iron ore is a typical complex resource, which mainly distributes in China, USA, Canada, and Egypt.^3,4,5) The reserves of oolitic iron ore resources in China reach 10 billion tons, which are mainly located in Hubei and Hunan province. The oolitic iron ores in those areas mostly contain high iron content, but these usable reserves are contaminated by the presence of phosphorus, which would lower the quality of the steel products. Therefore, the abundant high phosphorus oolitic iron ore reserves have remained largely unexploited. The dephosphorization of high phosphorus iron ore has been researched in many countries, such as physical separation,^6,7,8,9,10) bio-leaching,^11,12) flotation,^13,14) hydrometallurgical process.^15,16) But the oolitic structure of the ore prevents the effective use of physical concentration and flotation separation, and necessitates the use of chemical leaching. Cheng et al.,¹⁷⁾ Xia et al.¹⁸⁾ and Zhang et al.¹⁹⁾ investigated the dephosphorization process by sulfuric acid, hydrochloric acid and nitric acid leaching respectively. Their focus was mainly on high dephosphorization rate and optimum condition. However, the detailed study on the difference of three acids leaching and ore structure changes after leaching was little, and iron loss was only listed in their paper lacking theoretical analysis. Moreover, the sulfuric acid leaching kinetics of typical oolitic iron ore with high phosphorus was also rarely stated in previous studies,^{15,16,17,18,19)} which was crucial for improving dephosphorization rate effectively.

In this work, the structure of high phosphorus iron ore was analyzed first. Different acids leaching processes were researched in detail and the effect of experiment factors on dephosphorization process was investigated with further study about leaching kinetics. Besides, analysis of iron loss was also involved.

2. Experimental

2.1. Iron Ore Sample Analysis

The high phosphorus oolitic iron ore used in this research was obtained from western Hubei in China. Table 1 shows the chemical composition of the ore examined by chemical analysis, indicating that iron content reached 51%, and phosphorus content was 0.52%. If phosphorus could be removed appropriately, the ores of this kind would be of great value in use. The fundamental characterization of the high phosphorus iron ore samples was examined by XRD and SEM-EDS method. XRD analysis (Fig. 1) discovers that the iron ores are mainly composed of hematite, quartz, apatite and chamosite. The structure of high phosphorus iron ores is displayed in Fig. 2 and the SEM image shows that the ores consist of many oolitic units. Figure 3 indicates the internal structure and elemental composition of an oolitic unit. The observation let us know that phosphorus was concentrated in the center or interlayers of the oolitic unit in the form of apatite. The layer was so thin that phosphorus could not be removed through physical separation effectively. Therefore, dephosphorization by leaching was investigated in this paper.

Table 1. Chemical Composition of the High Phosphorus Iron Ore (mass%).

TFe	SiO2	Al2O3	MnO	TiO2	P	S	Cu
52	12.4	5.3	0.21	0.18	0.52	0.15	0.055

Fig. 1.

XRD pattern of the oolitic iron ore.

Fig. 2.

SEM image of the oolitic ore.

Fig. 3.

SEM and EDS images of the oolitic unit (a-center; b-interlayer).

2.2. SEM-EDS Analysis of Lumpy Sample Before and After Acids Leaching

To investigate the dephosphorization characterization at the same place by sulfuric acid, nitric acid and hydrochloric acid respectively, the iron ore blocks (around 0.5 cm) were mounted in epoxy resin, and the mounted samples were polished by SiC paper from 400# up to 2000#, followed by 0.5 μm diamond paste on a polishing wheel. The surfaces of the samples were observed by SEM and EDS and then operated by acids leaching under the conditions of acid concentration 0.2 mol/L, leaching time 1 h and stirring speed 200 rpm. SEM-EDS was employed to analyze the surface structural characteristic after leaching again, so the structure changes in the original site of the iron ores before and after leaching were studied.

2.3. Leaching Experiment of Powdery Sample

The high phosphorus iron ores were crushed by ball mill and screened to select specific particle sizes for leaching experiment. The effects of different acids, acid concentrations, ore particle sizes and experiment temperatures on leaching process were investigated respectively. The acid–ore reactions were conducted in 250 mL round-bottom flask with constant temperature bath and the slurry was stirred by a digital stirrer. The samples of the solution were taken out by adjustable volume pipettor at a specified interval from the flask and each sampling volume was 0.25 mL (The effect of solution volume reduction could be ignored.). The P content and Fe content of liquid samples were examined by ICP-OES. Dephosphorization rate was defined as the ratio of the P content in acid solution to P content in raw ore and iron loss meant the ratio of the Fe content in acid solution to Fe content in raw ore. After the leaching, the slurry was operated through vacuum filtration and the residues were dried at 393 K for 24 h in the vacuum oven. The P content of the residues was also analyzed by the ICP-OES method.

3. Results and Discussion

3.1. Characterization of Iron Ore Structure Before and After Acids Leaching

The changes of iron ore structure after leaching were rarely reported visually before. The researches^{15,16,17,18,19)} mostly paid attention to the dephosphorization effect and chemical content of iron ore after leaching. However, the study on structure changes is beneficial to leaching kinetic analysis. Figures 4,5 and 6 show SEM-EDS results to investigate the structure changes of iron ore before and after leaching by sulfuric acid, nitric acid, and hydrochloric acid respectively. The mechanism of dephosphorization can be described as follows: Ca₅ (PO₄)₃X(s) + 7H⁺→5Ca²⁺ + $3{\text{H}}_{\text{2}}{\text{PO}}_{\text{4}}^{-}$ + HX (X-F, Cl or OH). From Fig. 4, phosphorus is mainly distributed in position 1 (Fig. 4(a)) before leaching process. However, after sulfuric acid leaching, there is no phosphorus observed in this area, and only a small amount of CaSO₄ appears in particular corrosion region (Fig. 4(b) position 2) but not in all corrosion regions (Fig. 4(b) position 1). It is because when sulfuric acid is used as leaching agent, the precipitation reaction may occur as follows, Ca²⁺ + ${\text{SO}}_4^{2-}$ + nH₂O → CaSO₄ · nH₂O(s) n=0, 0.5, or 2. Figure 5 shows the effects of nitric acid leaching, and phosphorus is also not detected after leaching experiment. Figure 6(a) shows phosphorus exists in position 1, and after leaching, there is still a little residual phosphorus along with Fe_xO_y and gangues phase exposed after leaching in position 1 (Fig. 6(b)) through EDS analysis. Some calcium phosphates even hardly react with acid at all such as position 2 (Fig. 6(b)). Furthermore, after observation of the images above, the corroded areas became porous and hematite phase was seldom leached and surface structure was unbroken.

Fig. 4.

SEM and EDS images of the ore before and after sulfuric acid leaching.

Fig. 5.

SEM and EDS images of the ore before and after nitric acid leaching.

Fig. 6.

SEM and EDS images of the ore before and after hydrochloric acid Leaching.

3.2. Effect of Experimental Factors on Leaching Process

3.2.1. Effect of Different Acids

The experiments were conducted under the conditions of acid concentration 0.2 mol/L, L/S ratio (Liquid-Solid ratio) 100 ml: 8 g, particle size 74–105 μm and stirring speed 200 rpm at 298 K. Sulfuric acid, hydrochloric acid and nitric acid were used as the leaching agents for to study the dephosphorization process respectively. Figure 7(a) shows the dephosphorization rate of the different acid solutions. It indicates that dephosphorization rate rises sharply in the first 15 min. After leaching for 20 min, dephosphorization rates hardly change and are kept around at 60%, 79% and 84%, and P content of the iron ore after leaching are 0.23%, 0.11% and 0.088% for hydrochloric acid, nitric acid and sulfuric acid solutions respectively. Hydrochloric acid solution is the least effective for dephosphorization, which could also be observed through SEM-EDS analysis (Sec.3.1). The previous researches²⁰⁾ find that hydrochloric acid can react with lots of metallic compounds to form corresponding solvable metallic chloride and its chemical reactivity is higher than sulfuric acid, which would consume lots of acid. Moreover, the volatility of hydrochloric acid and the decomposability of nitric acid would also reduce acidity of the solution, which made them less effective than sulfuric acid on dephosphorization reaction. Thus, the residual phosphorus after hydrochloric acid leaching (Fig. 6(b)) could also be explained reasonably. Figure 7(b) shows iron loss obtained from the same experiment. The highest iron loss is only about 0.6%, which could be ignored. Considering the availability and acid cost, sulfuric acid is chosen as the most appropriate leaching agent.

Fig. 7.

Effect of different acids on dephosphorization rate (a) and iron loss (b) of leaching process with L/S ratio 100 ml: 8 g, stirring speed 200 rpm, particle size 74–105 μm at 298 K.

3.2.2. Effect of Acid Concentration

Figure 8(a) shows the effect of sulfuric acid concentrations on the dephosphorization reaction at 298 K with particle size 74–105 μm, stirring speed 200 rpm and L/S ratio 100 mL: 8 g. With the increase of the acid concentrations, dephosphorization rate rises obviously, and meanwhile, the iron loss also increases correspondingly shown in Fig. 8(b). When acid concentration was low, iron loss could hardly be detected, but dephosphorization rate was also not well enough. Considering these two aspects comprehensively, 0.2 mol/L could be set as the appropriate acidity.

Fig. 8.

Effect of sulfuric acid concentrations on dephosphorization rate (a) and iron loss (b) of leaching process with L/S ratio 100 ml: 8 g, stirring speed 200 rpm and particle size 74–105 μm at 298 K.

3.2.3. Effect of Particle Size

The high phosphorus iron ores were grinded and screened to select 149–210 μm, 105–149 μm, 74–105 μm, 53–74 μm and 37–53 μm particles sizes. The iron ore particles chosen were operated by acid leaching respectively under the conditions of sulfuric acid concentration 0.2 mol/L, stirring speed 200 rpm and L/S ratio 100 mL: 8 g at 298 K. Figure 9(a) shows that dephosphorization rate rises obviously at some extent with the particle sizes decreased, and the same as the iron loss (Fig. 9(b)). It is known that the smaller the particle size, the larger the specific area, so the contacts of solid-liquid are more effective, which leads to a higher dephosphorization rate. Considering real effect and cost of grinding, 74–105 μm was the preferable particle size.

Fig. 9.

Effect of particle sizes on dephosphorization rate (a) and iron loss (b) of leaching process with L/S ratio 100 ml: 8 g, stirring speed 200 rpm and sulfuric acid concentration 0.2 mol/L at 298 K.

3.2.4. Effect of Stirring Speed

Experiments were conducted to investigate the effect of stirring speed (100 rpm, 200 rpm, 300 rpm, 400 rpm) on the rate of phosphorus removal from oolitic iron ore. The results are shown in Fig. 10(a). Though high stirring speed can promote mass transfer, dephosphorization rates are improved slightly with increase in speed of rotation, which means stirring speed does not have a significant influence on dephosphorization reaction. At the same time, the differences of iron loss shown in Fig. 10(b) are also not obvious with the stirring speed changed from 100 rpm to 400 rpm.

Fig. 10.

Effect of stirring speeds on dephosphorization rate (a) and iron loss (b) of leaching process with L/S ratio 100 ml: 8 g, sulfuric acid concentration 0.2 mol/L, particle size 74–105 μm at 298 K.

3.2.5. Effect of Temperature

The temperatures chosen for dephosphorization reaction were 298 K, 308 K, 318 K, 328 K respectively. For the convenience of leaching kinetic analysis follwed, experiments factors were set as sulfuric acid concentration 0.1 mol/L, stirring speed 200 rpm, particle size 710–840 μm (Dephosphorization reaction reached balance soon from the results above, and using bigger particle sizes could lower reaction rate so as to take more samples before equilibrium), liquid-solid ratio 150 mL: 1 g (The changes of the acid concentration could be ignored during leaching). The effect of temperature on leaching process is given in Fig. 11. With the temperature increased, the rates of phosphorus removal rise obviously, which indicates that dephosphorization reaction is sensitive to the temperature. Meanwhile, iron loss increased correspondingly.

Fig. 11.

Effect of temperatures on dephosphorization rate (a) and iron loss (b) of leaching process with L/S ratio 150 ml: 1 g, particle size 710–840 μm, stirring speed 200 rpm and sulfuric acid concentration 0.1 mol/L.

3.3. Leaching Kinetics Analysis

Most of the earlier studies on dephosphorization by acid leaching were only carried out to investigated the effect of time, temperature, acidity and so on.^15,18,21) The leaching kinetics of the typical high phosphorus oolitic iron ore was little described. In this part, leaching process and kinetic analysis were researched in detail.

SEM-EDS images (Fig. 3) show that apatite is embedded in the center or interlayer of oolitic unit. The apatite was removed effectively by sulfuric acid leaching and the products (such as CaSO₄) of dephosphorization process take off the reaction surface (Fig. 4). Furthermore, hematite phase was hardly corroded and could be regarded as inert materials. For liquid-solid reaction process, hydrogen ion diffused from the bulk solution through liquid film to the outer surface of iron ore, where it reacted with apatite to give products in turn diffuse in the opposite direction (back into the bulk solution). With the zone of reaction moving in to the solid, the inert iron ore particle became porous and allowed diffusion of fluid reactants.

In order to evaluate the kinetic parameters and rate controlling process, the experimental data were analyzed on the basis of the unreacted shrinking core model.²²⁾ The reaction rate may be controlled by: 1) diffusion through liquid film, 2) diffusion through the ash (converted solids and/or inert material), or 3) the dephosphorization chemical reaction at the surface of the core of unreacted apatite.

Case 1. Liquid film diffusion control   

$$\frac{t}{{\tau }_1}=x$$(1)

where   

$${\tau }_1=\frac{{\rho }_b{R}_1}{3b{k}_L{C}_A}$$(2)

Case 2. Diffusion through porous iron ore particle control   

$$\frac{t}{{\tau }_2}=1–3{\left(1–x\right)}^{2/3}+2\left(1–x\right)$$(3)

where   

$${\tau }_2=\frac{{\rho }_b{R}_1^2}{6b{D}_e{C}_A}$$(4)

Case 3. Chemical reaction control   

$$\frac{t}{{\tau }_3}=1–{\left(1–x\right)}^{1/3}$$(5)

where   

$${\tau }_3=\frac{{\rho }_b{R}_1}{b{k}_s{C}_A}$$(6)

Various plots of dephosphorization rate (x) versus reaction time (t) in Fig. 10(a), show that rate of phosphorus removal is insensitive to stirring speed, which indicates that liquid film diffusion is not the rate-controlling step in the overall reaction. To determine the rate-controlling step, the experimental data were plotted on the basis of Eqs. (3) and (5), and a reasonably good fit was achieved with Eq. (5), which reflected that chemical reaction between acid and apatite was rate-controlling step. Typical results of model fitting are shown in Fig. 12. The apparent rate constant (k) was obtained from the slope of the line. According to the Arrhenius equation $lnk=\frac{–E}{RT}+B$ , the plot of lnk versus 1/T is shown in Fig. 13, and apparent activation energy is found to be 45.02 kJ/mol, which also demonstrates that leaching rate is controlled by chemical reaction (E>41.8 kJ/mol).²³⁾ Therefore, the dephosphorization rate can be improved by increasing the leaching temperature, initial concentration of the leaching solution and reducing the original sizes of the iron ore particles. Moreover, the treated ore may contain still higher level of phosphorus compared to the ore used in the paper, to reach a lower phosphorus concentration after leaching, and the smaller particle sizes, higher acid concentration and longer leaching time are needed during the dephosphorization process.

Fig. 12.

Plot of $\text{1}–{\left(1–x\right)}^{1/3}$ versus time t for different temperatures.

Fig. 13.

lnk-1/T graph with liner fitting.

3.4. Iron Loss Analysis

From the results above, the iron loss could be ignored, even when the acid concentration reached 0.4 mol/L, iron loss was less than 1.6% (Fig. 8(b)). SEM-EDS analysis (Sec.3.1) also showed that the hematite phase was not corroded. Meng et al.²⁴⁾ ignored the test of iron loss during experiment. Though Cheng et al.¹⁷⁾ got low iron loss (less than 0.20%) in their experiments, but reason was not discussed. The iron loss reaction is Fe₂O₃+6H⁺→2Fe³⁺+3H₂O, and the reaction thermodynamic data are shown in Fig. 14.

Fig. 14.

ΔG^θ – T graph of the iron loss reaction.

The reason of the low iron loss could be explained by the following example with thermodynamic calculation. When the leaching experiment was operated under the initial acid concentration 0.2 mol/L (Sec.3.2.2), acid concentration remained stable at around 0.15 mol/L (the value was tested by pH meter) after 20 min. It was assumed that reaction equilibrium concentration of the H⁺ ( $c_{H^{+}}$ ) was 0.15 mol/L at 298 K. Through the isothermal equation $\mathrm{\Delta }{G}^{\theta }=–RT\text{ln}\frac{C_{{\text{Fe}}^{3+}}^2}{C_{H^{+}}^6}$ and thermodynamic datas, it was calculated that $c_{F{e}^{3+}}$ ≈ 8.8E – 3 mol/L, and iron loss was about 1.2 wt%. This assumption was made under ideal situation, and considering the real dynamic conditions, the iron loss should be lower, which agreed with the experimental results on the whole (Actual iron loss was around 0.5 wt%).

4. Conclusions

The structure of high phosphorus iron ore from western Hubei in China was oolitic, and phosphorus distributed in successive layers or the center of the oolitic units in the form of apatite.

Hydrochloric acid leaching process got the lowest dephosphorization rate and iron loss, because of its chemical reactivity and volatility. Sulfuric acid and nitric acid leaching could remove phosphorus more effectively, and some CaSO₄ produced during sulfuric acid leaching process may absorb on a certain corrosion region but not in all corrosion regions. Considering the real effect and cost, sulfuric acid was appropriate for dephosphorization.

After leaching operation, phosphorus was removed effectively, and iron loss could be ignored, which was discussed through thermomechanical analysis. Increasing temperature, initial acidity and reducing the original sizes were effective to enhance the reaction rate.

Leaching kinetics of high phosphorus oolitic iron ore was well described by the unreacted shrinking core model. The process of phosphorus removal was controlled by chemical reaction of interface between apatite and acid. The apparent activation energy for dephosphorization reaction was found to be 45.02 kJ/mol.

Acknowledgements

This work was sponsored by National Natural Science Foundation of China (No.51234001).

Nomenclature

b: stoichiometric coefficient

_CA: concentration of acid solution (kmol/m³)

_De: diffusivity through porous iron ore particle (m²/s)

_kL: mass transfer coefficient between liquid and particle

_ks: first-order rate constant for the surface reaction

_R1: average radium for particle (m)

_t: reaction time (s)

_x: fractional conversion

_ρb: molar concentration of apatite (kmol/m³)

_τ: time required for complete conversion (s)

E: activation energy

R: gas constant (8.314 J/K·mol)

B: constant

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