J. Cent. South Univ. Technol. (2006)06−0673−05 DOI: 10.1007/s11771−006−0003−y

Removal of phosphorus from iron ores by chemical leaching

JIN Yong-shi(金勇士), JIANG Tao(姜 涛), YANG Yong-bin(杨永斌), LI Qian(李 骞), LI Guang-hui(李光辉), GUO Yu-feng(郭宇峰)

（School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China）

Abstract: Alkali-leaching and acid-leaching were proposed for the dephosphorization of Changde iron ore, which contains an average of 1.12% for phosphorus content. Sodium hydroxide, sulfuriced, hydrochloric and nitric acids were used for the preparation of leach solutions. The results show that phosphorus occurring as apatite phase could be removed by alkali-leaching, but those occurring in the iron phase could not. Sulfuric acid is the most effective among the three kinds of acid. 91.61% phosphorus removal was attained with 1% sulfuric acid after leaching for 20 min at room temperature. Iron loss during acid-leaching can be negligible, which was less than 0.25%.The pH value of solution after leaching with 1% sulfuric acid was about 0.86, which means acid would not be exhausted during the process and it could be recycled, and the recycle of sulfuric acid solution would make the dephosphorization process more economical. Key words: iron ore; dephosphorization; acid-leaching; alkali-leaching CLC number: TD925.6 Document code: A

1 INTRODUCTION

With the rise of the price of iron ores in the global market, the iron and steel companies in China have to increase the proportion of domestic iron ores in iron-making production. However, iron ores produced in China commonly contain many impurities, such as phosphorus, sulphur, silica, etc[1]. Phosphorus is one of the main harmful elements to ferrous metallurgy, and it will affect the quality of iron and steel products[2]. At present, China has explored some large mines, which bear tremendous iron ore with high phosphorus content. For examples, Meishan iron ore contains an average of 0.38% for phosphorus content, which of Ningxiang iron ore is 0.5%[3−6].

The method for removal of phosphorus from iron ores involves smelting process, physical separation and chemical leaching. Smelting process is effective for dephosphorization but with very high cost, and it is still under fundamental research. For physical separation, crushed ores should be fine ground until the phosphorus minerals were dissociated from iron minerals, and then dephosphorized with flotation or magnetic separation[7]. Low phosphorus extraction, high grinding cost and iron

loss are the major disadvantages of the method. Chemical approach, in which the ore is leached with a suitable solution, is a relatively simple process as it can directly treat the sinter fines without strict requirements for the particle size[8]. Forssberg, et al[9−12] investigated dephosphorization with acid leaching. In their studies, the acid concentrations were very high and low phosphorus extractions were obtained. The cost of chemical leaching process mostly depends on the consumption of the leaching agents[9−12]. In this study, the feasibility of dephosphorization for Changde phosphorous iron ores by alkali-leaching and acid-leaching was investigated.

2 EXPERIMENTAL 2.1 Materials

The iron ore samples were provided by a mining company of Changde, China. All the samples were crushed and milled, and then mixed. The size distributions of the mixed samples are given in Table 1. Chemical analysis (Table 2) indicates that the phosphorus content of the ore is up to 1.12%. X-ray diffraction pattern of the ore is presented in Fig.1, which indicates that the ore is mainly composed of hematite,

Received date: 2006−01−06; Accepted date: 2006−02−05 Foundation item: Project (50321402) supported by the National Natural Science Foundation of China; project(2004CB619204) supported by Major State

Basic Research Development Program of China Corresponding author: JIANG Tao, Professor, PhD; Tel: +86-731-8879260; E-mail: jiangtao@mail.csu.edu.cn

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SiO2 and kaolinite. 22.3% phosphorus occurs as apatite, 67.9% phosphorus is in the iron phase, and only 9.8% phosphorus is in the silicate phase.

Table 1 Size distribution of iron ore sample (mass fraction, %)

＜0.075 mm 0.106−0.075 mm 0.15−0.106

mm ＞0.15 mm

45.60 11.90 32.45 10.05

Table 2 Chemical compositions of iron ore sample (mass fraction, %)

FeO Fe Al2O3 SiO2 P 3.08 47.79 6.33 17.32 1.12 CaO MgO Pb As S 4.78 0.29 0.022 0.002 0.028

▼—Hematite; ●—SiO; ◆—Kaolinite

Fig.1 X-ray diffraction pattern of iron ore

In the study, sodium hydroxide， sulfuric acid, hydrochloric acid and nitric acid were used for the preparation of leach solutions. All the reagents are all analytically pure agent.

2.2 Methods and equipment The X-ray analysis results indicate that the iron ore is

mainly composed of hematite. According to the previous research, it is unnecessary to take heat pretreatment to remove the phosphorus from hematite[13]. Most of the experiments were carried out in beakers at room temperature and agitators whose rotational speed can be controlled were used to stir the slurry. At the end of the experiment, the slurry was vacuum filtered and the leaching residues were processed for chemical analysis.

3 RESULTS AND DISCUSSION 3.1 Alkali leaching

3.1.1 Effect of sodium hydroxide concentration Fig.2 shows the phosphorus extraction in different

sodium hydroxide concentrations after leaching for 2 h at

room temperature. The slurry concentration was 10% (ratio of ore mass to sodium hydroxide solution volume), and the speed of agitator was 620 r/min. Fig.2 indicates that the extraction of phosphorus is only enhanced from 20.18% to 27.96% when hydroxide concentration increased from 0.1 mol/L to 1.0 mol/L. It can be concluded that the enhancement of sodium hydroxide concentration is not very effective to the extraction of phosphorus.

Fig.2 Effect of concentration of sodium hydroxide on

dephosphorization

3.1.2 Effect of leaching time

Base on Fig.2, 0.2 mol/L was chosen as the concentration of sodium hydroxide in this test. Fig.3 shows that increasing leaching time was not effective to improve the extraction of phosphorus. After leaching for 6 h, the phosphorus extraction was only 25.3%. On the contrary, the phosphorus extraction dropped when it was leached over 6 h, which is due to the reprecipitation of phosphorus with some metal ions released during the process.

The above results indicate that alkali leaching is not suitable for dephosphorization of the iron ores, and the best phosphorus extraction obtained is only 27.96%. According to phosphorus phase in the iron ore, there was

Fig.3 Effect of leaching time on dephosphorization

JIN Yong-shi, et al: Removal of phosphorus from iron ores by chemical leaching

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22.3% phosphorus occurring as apatite, 9.8% in the silicate phase, and others in the iron phase. The former two together is 32.1%, which reaches 27.96% to some extent. Therefore, it is concluded that it is difficult to extract the phosphorus in the iron phase with sodium hydroxide. Apatite reacts with sodium hydroxide following the reaction[4]:

Ca5(PO4)3F(s)+10NaOH+5SiO2= 3Na3PO4+NaF+5CaSiO3(s)+5H2O (1)

3.2 Acid leaching

Acid leaching in normal atmosphere is one of the typical approaches for mineral separation[14]. In this research, sulfuric acid, hydrochloric acid and nitric acid were used. 3.2.1 Effect of leaching time

The effects of acid leaching time on dephosphcriz- ation of ore are shown in Fig.4. Fig.4 indicates whatever the acid is, the phosphorus extraction increases with time in the first 60 min. After leaching for 60 min, the phosphorus extraction almost keeps unchanged. It can be seen that sulfuric acid is the most efficient to remove the phosphorus from the ore. 91.61% phosphorus was extracted after leaching for 20 min with sulfuric acid. And the best result obtained was 93.31% at the time of 60 min.

Fig.4 Effect of acid leaching time on dephosphorization

(Acid concentration 1%, slurry concentration 10%, speed of agitator 620 r/min, room temperature)

1—H2SO4; 2—HNO3; 3—HCl 3.2.2 Effect of acid concentration

According to Fig.4, phosphorus extraction increased only by 2.3% when leaching time increased from 20 min to 60 min. Therefore, 20 min is chosen as leaching time when sulfuric acid is used in this experiment, 60 min is chosen when hydrochloric acid and nitric acid are used.

The effect of acid concentration on dephosphoriz-

ation is shown in Fig.5. Fig.5 indicates that phosphorus extractions increase remarkably when the concentrations of the three kinds of acid increase from 0.2% to 1.0%. But when the concentration exceeds 1%, it presents a stable trend. And the phosphorus extraction at 1.0% acid concentration for sulfuric, hydrochloric, nitric acids is 91.61%, 84.96%, 89.37%, respectively. No matter how the concentrations of hydrochloric acid and nitric acid increase, the phosphorus extraction is lower than that with sulfuric acid of 1% concentration. As is known, hydrochloric acid can react with lots of metallic compound and generate corresponding solvable metallic chloride, which will result in consumption of acid, its reacting competence is higher than sulfuric acid[14], and the volatility of hydrochloric would also reduce its acidity, which make it less effective than other acids. Moreover, the following reaction would occur under various operating conditions when nitric acid is used[15]:

)g(NOFeOHNOH)s(FeO 23

23 ++⎯→⎯++ +−+ (2)

++−+ ++⎯→⎯++ 43

23 NHFeOHNOH)s(FeO (3)

Fig. 5 Effect of acid concentration on dephosphorization 1—H2SO4; 2—HNO3; 3—HCl

These reactions represent consumption of the nitric acid as they can not be simply reversed, which makes nitric less effective than sulfuric acid.

The mechanism of acid leaching can be summarized as follows[16]:

( ) ⎯→⎯+ +H20)s(XPOCa 6410

XHCa10POH6 22

43 ++ + (4) When sulfuric acid is used as the leaching acid, the

precipitation reaction occurs, i.e.,

)s(OHCaSOOHSOCa 24224

2 nn ⋅=++ −+ (5)

where n=0, 0.5 or 2, depending upon the acidity of the processing solution.

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The formation of OHCaSO 24 n⋅ would promote the metathetical reaction given as Eqn.(3), which also makes sulfuric acid more effective to dephosphorization than other acids. 3.2.3 Effect of speed of agitator Sulfuric acid was used as leaching agent, and the result is shown in Fig.6. From Fig.6, it can be seen the suitable speed of agitator is 620 r/min. However, when the speed exceeds 620 r/min, the phosphorus extraction decreases. That is because the iron ore samples are fine ground, the ore particles could be easily driven by the whirlpool of the leaching solution at high speed of agitator, and in this way, the relative refresh velocity of the leaching solution on the surface of the ore particles decreases.

Fig.6 Effect of speed of agitator

3.2.4 Iron loss during leaching

Iron loss after leaching is listed in Table 3. It can be seen that iron content increases to some extent after leaching with three kinds of acid, and iron loss is less than 0.25%, which indicates iron loss during leaching can be negligible.

Table 3 Iron loss during leaching (mass fraction, %)

w (Fe)/% Acids Before

leaching After leaching Iron loss/%

H2SO4 50.12 0.17 HNO3 50.61 0.24 HCl

47.79 47.79 47.79 50.03 0.15

Note: The leaching conditions are as follows: acid concentration 1.0%, slurry concentration 10%, speed of agitator 620 r/min, room temperature; leaching for 20 min with sulfuric acid, leaching for 60 min with other acids.

3.2.5 Acid consumption cost

The feasibility of removal of phosphorus from iron ores in acid-leaching approach depends very much on the acid consumption. Sulfuric acid is the most effective leaching agent, and it is the most economical one.

According to this work, approximately 10 t of sulfuric acid (technical grades: 95%−98%) are needed to treat 100 t of iron ores. The price of 10 t of sulfuric acid is about 1000 RMB. Therefore, the cost of acid consumption for dephosphorization is 10 RMB/t. During the experiments, it was found that the pH value of solution was about 0.86 after leaching with 1% sulfuric acid, which means acid would not be exhausted during the process and it could be recycled. The recycle of sulfuric acid solution would undoubtedly decrease the cost of acid consumption. In some researches, the recovery of the removed phosphorus as a by-product was proposed, indicating that an incorporated process to produce high purity phosphoric acid would make the whole process more economical[16]. 4 CONCLUSIONS

1) Alkali-leaching is ineffective to remove phospho- rus from iron ores. With 0.2 mol/L and 1.0 mol/L sodium hydroxide, only 24.41% and 27.96% phosphorus was removed respectively for a iron ore from Changde, China. A prolonged alkali-leaching would go against dephosphorization because of the reprecipitation of phosphorus with other metal ions released during the leaching.

2) The distribution of phosphorus phase in iron ore has great effect on the efficiency of alkali-leaching. Phosphorus in the iron phase can hardly be removed by alkali-leaching process.

3) Acid-leaching is an effective method to remove phosphorus from iron ores. Sulfuric acid is more effective to dephosphorization from iron ores than hydrochloric acid and nitric acid. 91.61% phosphorus is extracted with 1.0% sulfuric acid after leaching for 20 min at room temperature. The optimum stirring speed is 620 r/min. The suitable leaching time for sulfuric-acid-leaching is 20 min. Iron loss during the acid-leaching can be negligible, which is less than 0.25%.

4) The consumption of acid is important to the application of acid leaching process in production. The recycle of sulfuric acid solution and an incorporated process to produce high purity phosphoric acid as a by-product would make the whole process for dephosphorization more economical.

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(Edited by YANG Hua)