J. Cent. South Univ. Technol. (2007)01-0028-04

DOI: 10.1007/s11771-007-0006-3               

Influencing factors of pyrite leaching in germ-free system

OU Le-ming(欧乐明), HE Rong-quan(何荣权), FENG Qi-ming(冯其明)

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

Abstract: The effect of mineral particle size, pulp potential and category of oxidant on pyrite leaching was studied. The results show that a smaller mineral particle size leads to a higher leaching rate of pyrite, and the optimum result with pyrite leaching rate of 2.92% is obtained when mineral particle size is less than 0.037 mm. The pulp potential reflects the leaching process. The increase of pulp potential can improve pyrite leaching. The leaching rate and velocity of pyrite can be enhanced rapidly by adding strong oxidant. The kind and the method of adding oxidant have important effect on the pyrite leaching. Appropriate concentration of Fe³⁺ can enhance pyrite leaching but the precipitation generated by high concentration of ferric ion covers the surface of pyrites and prevents the leaching process. The leaching rate increases with the constant addition of H₂O₂.

Key words: pyrite; leaching rate; mineral particle size; pulp potential; oxidant

1 Introduction

Pyrite (FeS₂) is of reducibility, which can be oxidized when exposed in air or in aqueous solution. The oxidation of pyrite has both advantage and disadvantage for some practical applications. For example, the pyrite oxidation should be inhibited to reduce acid waste water in waste water treatment process, but when it comes to desulphurization from coal, the pyrite is expected to be fully oxidated so that sulphur can be removed completely^[1]. During bioleaching      and acid leaching processes, pyrite is expected to be oxidized as fast as possible to increase the leaching velocity^[2]. In flotation process, the oxidation of pyrite should be encouraged in some cases, but sometimes it should be inhibited^[3]. In a word, whether the pyrite should be oxidized or not is determined by practical applications. At present, researchers have paid more attention to the influencing factors of oxidation of pyrite, such as temperature, leaching time, pulp concentration, mineral particle size, pH value and so on^[4-6]. Reports on influencing factors in bioleaching of pyrite are available^[7-8], but there are few reports on germ-free acid leaching. Therefore, in this study, the influencing factors of pyrite leaching in the germ-free system were investigated.

2 Experimental

2.1 Preparation of samples

All pyrites used in the experiments were bought

from Changsha mineral market. The crystals of pyrite were intact and contained few impurities. The ores were

crashed and ground by porcelain mill and then sieved into four different sizes: ＜0.037 mm, 0.037-0.050 mm, 0.050-0.074 mm and 0.074-0.154 mm. These samples were vacuum dried and finally saved in sealed glass bottles for preparation. The chemical analysis indicates that the pyrite contains 49.02% S and 40.11% Fe.

2.2 Experimental method

The experiments were carried out in shake flasks, which were put in an oscillator. The ratio of liquid to solid was 10?1. The leaching solution was sulfuric acid prepared before the experiments. Analytically pure sulfuric acid was used to adjust the original pH value of the leaching solution. The rotational speed of the oscillator was set to be 160 r/min, and the temperature was set at 30 ℃. All the water used in experiments was distilled water.

During the experiments, the contents of Fe²⁺ and total iron ions(TFe) in the leaching solution and the pulp potential were tested. The concentration of Fe²⁺ was tested by titration with potassium dichromate. The content of TFe was tested by atomic absorption spectrometry. The pulp potential was monitored with platinum reference electrode. The value of pulp potential, E_m, was converted to standard hydrogen electrode potential, E, by the equation of E=E_m+244.5 mV.

3 Results and discussion

3.1 Effect of mineral size on leaching of pyrite

Mineral particle size is one of the important influencing factors during the leaching of pyrite, which should be paid much attention especially during bioleaching^[9]. Therefore, mineral particle size was investigated at first in the experiments. The results are shown in Fig.1.

Fig.1 Relationship between leaching rate of pyrite and leaching time at original pH value of 2.0

Mineral particle size/mm: 1—＜0.037; 2—0.037-0.050; 3—0.050-0.074; 4—0.074-0.154

From Fig.1 it can be seen that the leaching rates of pyrite with various sizes increase with lengthening leaching time. Especially when the particle size is less than 0.037 mm, the leaching rate increases in a bigger margin than others, and a leaching rate of 2.92% is obtained after leaching for 7 d. It is also shown that the smaller the particle size, the higher the leaching rate. After leaching for 7 d, the leaching rate of pyrite with size less than 0.037 mm is 2.22% which is higher than that of the mineral with size of 0.074-0.154 mm. The smaller mineral particle size means larger specific surface and more surface activity site, which make the leaching easier. This result is identical with the effect of mineral particle size on bioleaching investigated by YUAN et al^[9].

3.2 Effect of pulp potential on leaching of pyrite

It has been proved that the pyrite leaching is an electrochemical process by thermodynamics analysis and previous experiments. Therefore, pulp potential is a key influencing factor for leaching of pyrite. LAN et al^[10] suggested that the releasing rate of SO₄^2- was very slow when the pulp potential was lower than 0.35 V, and so was the oxidation processing of pyrite. When pulp potential enhanced, the oxidation would be accelerated, and the pH value would drop rapidly. Based on some correlative reports, the following conclusion can be drawn that the leaching rate of pyrite is related to the pulp potential, when the pulp potential is higher than 0.60 V, the oxidation of pyrite occurs fiercely, however, when it is lower than 0.60 V, the reaction is very weak.

It is discovered that the pulp potential ties with the leaching system through the measure of pulp potential. Fig.2 shows that the average pulp potential decreases with increasing the mineral particle size. Based on the analysis of Fig.1, the conclusions that mineral particle size influences the pulp potential, and thus influences the leaching efficiency are obtained. The finer the particle, the higher the potential and the leaching rate.

Fig.2 Relationship between pulp potential and leaching time of pyrite with different sizes

Particle size/mm: 1—＜0.037; 2—0.037-0.050; 3—0.050-0.074; 4—0.074-0.154

Fig.3 shows the effect of oxidant on relationship between leaching time and pulp potential, indicating that when there is no oxidant in the leaching solution, the pulp potential increases firstly with the time prolonging and then keeps stable; when Fe³⁺ is added, the pulp potential decreases with increasing the time and then keeps stable; when hydrogen peroxide is added, the potential decreases in the first three hours, and then increases gradually to a stable value. The different changing trend indicates different mechanism of reactions. Therefore, by controlling the pulp potential the leaching of pyrite can be controlled.

3.3 Effect of oxidant on leaching of pyrite

Oxidation plays an important role in the leaching of pyrite. The effects contain two aspects. The first is the kind of oxidant such as Fe³⁺, dissolved oxygen and H₂O₂. MOSES^[11] suggested that the effect of Fe³⁺ on pyrite oxidation was greater than that of dissolved oxygen. During the leaching, the quantity of Fe²⁺ created by the effect of Fe³⁺ is 15 times larger than that of dissolution oxygen. The second is the effect of oxidant concentration on pyrite leaching, generally, the higher the oxidant concentration, the higher the leaching rate.

Fig.3 Effect of oxidant on relationship between leaching time and pulp potential

1—0.16 mol/L Fe³⁺; 2—0.08 mol/L Fe³⁺; 3—0.04 mol/LFe³⁺;  4—0.01 mol/L Fe³⁺; 5—No oxidant; 6—1 mL/d H₂O₂; 7—1 mL H₂O₂

3.3.1 Effect of Fe³⁺ on leaching of pyrite

Based on Ref.[10] and the analysis of thermo- dynamics, Fe³⁺ can improve the leaching of pyrite. Therefore, the experiments using ferric sulfate as oxidant were carried out. The results are shown in Figs.4 and 5.

Fig.4 shows that the concentration of Fe²⁺ increases with prolonging leaching time, but in the meantime the increasing extent slows down, and with increasing concentration of Fe³⁺, the concentration of Fe²⁺ change remarkably. Fig.5 shows the content of total iron, TFe, almost keep unchangeable. When the concentration of Fe³⁺ is low, the content of total iron presents an increasing trend, and when the concentration of Fe³⁺ is high, the content of total iron presents a decreasing trend. With increasing concentration of Fe³⁺, the hydrolysis of Fe³⁺ results in precipitation. From Figs.4 and 5, the addition of Fe³⁺ can increase the content of Fe²⁺ rapidly. Fe²⁺ is generated by the reduction of Fe³⁺ and the oxidation of pyrite.

Fig.4 Relationship between mass concentration of 

Fe²⁺(ρ(Fe²⁺)) and leaching time at different concentrations of Fe³⁺(c(Fe³⁺))

c(Fe³⁺)/(mol?L^-1): 1—0.16; 2—0.08; 3—0.04; 4—0.01; 5—0

Fig.5 Relationship between ρ(TFe) and leaching time at different c(Fe³⁺)

c(Fe³⁺)/(mol?L^-1): 1—0.16; 2— 0.08; 3—0.04; 4—0.01; 5—0

Figs.3-5 show that the pulp potential and the concentration of Fe²⁺ correlate each other when c(Fe³⁺)=0.16 mol/L. With prolonging leaching time, the pulp potential decreases and the concentration of Fe²⁺ increases. When the pulp potential keeps stable, the concentration of Fe²⁺ presents a stable trend. The relationship between pulp potential and mass concentration of ρ(Fe²⁺) indicates that pulp potential changes with the process of leaching. Therefore, the conclusion that an oxidation process occurs during the leaching of pyrite can be made. At the same time, it can be seen that the content of Fe²⁺ keeps stable after leaching for 5 d, but the content of total iron presents a decreasing trend. The reason may be the precipitation of Fe(OH)₃. The precipitation covers the surface of pyrite and prevents the leaching.

3.3.2 Effect of H₂O₂ on leaching of pyrite

H₂O₂ is also an effective oxidant which can improve the leaching of pyrite besides Fe³⁺. The experiments using H₂O₂ as oxidant were investigated. The amounts of H₂O₂ were 0, 1 mL and 1 mL/d, respectively. The results are shown in Fig.6, indicating that the addition of H₂O₂ enhances the leaching velocity obviously. However, the leaching rate with H₂O₂ is almost close to that without H₂O₂ after leaching for 8 d, and the difference between the two is only 0.03%. From Fig.6 (curve 2), the leaching rate comes to a stable value after leaching 4 d with the addition of H₂O₂, because S⁰ or some dense sulfur layers are generated on the surface of pyrite, which hinder the leaching. Comparing curve 2 with curve 3, the gap between the two is very small, and the leaching rate keeps increasing when H₂O₂ is added constantly. The leaching rate keeps stable if the H₂O₂ is added at one time, this is because the constant addition of H₂O₂ provides dissolved oxygen for the reaction, which results in the disappearance of S⁰ on the surface of pyrite, and the effect of inhibition is eliminated.

Fig.6 Effect of H₂O₂ on leaching rate of pyrite

Amount of H₂O₂: 1—0; 2—1 mL; 3—1 mL/d

4 Conclusions

1) Particle size is an important factor influencing the leaching of pyrite. Finer particle size will improve the leaching. When particle size is less than 0.037 mm, the leaching rate of pyrite is 2.92%.

2) Pulp potential is also a key influencing factor for the leaching of pyrite. The pulp potential is related to particle size, concentration of oxidant and categories of oxidant. There is a certain functional relation between pulp potential and leaching rate, and also the leaching velocity of pyrite.

3) The oxidant such as Fe³⁺ and H₂O₂ can enhance the pulp potential effectively, thus enhance the leaching rate. H₂O₂ is better than Fe³⁺ for the leaching. After leaching for a certain time, the leaching process will be hindered by the precipitation of excessive Fe³⁺. The leaching rate comes to a stable value after leaching for 4 d with the addition of H₂O₂, but constant addition of H₂O₂ can provide dissolved oxygen for the reaction and can eliminate the inhibition effect of S⁰ on the surface of pyrite.

References

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(Edited by CHEN Wei-ping)

Foundation item: Project(2004CB619204) supported by the National Key Fundamental Research and Development Program of China; Projects(50321402, 50574101) supported by the National Natural Science Foundation of China

Received date: 2006-04-28; Accepted date: 2006-06-02

Corresponding author: OU Le-ming, PhD; Tel: +86-731-8830913; E-mail: ou-leming@163.com