Electrochemical oxidation of pyrrhotute in aqueous solution

QIN Wen-qing(覃文庆), LI Quan(黎 全),QIU Guan-zhou(邱冠周), XU Ben-jun(徐本军)

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

Abstract: The anodic surface oxidation of natural pyrrhotite in 0.3mol/L KCl and HCl solution (pH 4.0) and0.1mol/L Na₂B₄O₇ solution (pH 9.18) respectively was investigated by using cyclic voltammetry, Tafel plot, and chronoamperometry. In 0.3mol/L KCl and HCl solution (pH 4.0), at potential less than 0.5V(vs SHE), the production of anodic oxidation on pyrrhotite surface can not maintain a stable phase to form a passive film. In 0.1mol/L Na₂B₄O₇ solution (pH 9.18), when the electrode potential increases to more than 0.5V (vs SHE), part of S is oxidized to sulfate, making the passive film somewhat porous, but elemental S and metal oxidates Fe(OH)₃ still remain on the electrode surface, and the passive film can not be broken down totally. According to PARCalc Tafel analysis, the corresponding corrosion current density (J₀) is 5.34μA/cm², which is also the exchange current density of the oxidation reaction on pyrrhotite electrode surface in 0.1mol/L Na₂B₄O₇ solution (pH 9.18). The electrochemical dynamics equation of the oxidation was determined.

Key words: pyrrhotite; electrochemical oxidation; aqueous solution; electrochemistry CLC number:

Document code: A

1 INTRODUCTION

In Guangxi Province of China, complex sulfide ores exist, which is the major source of zinc, lead, antimony and tin. The valuable minerals include marmatite(ZnFeS), jamesonite and cassiterite(SnO₂), which are associated with a greater abundance of pyrrhotite^[1].

Pyrrhotite (Fe_1-xS) is a kind of non-stoichiometric sulfur rich compound compared with stoichiometric FeS, whose chemical components are not fixed^[2]. The iron atom of in pyrrhotite is imperfect, the structural formula could be illustrated as Fe_1-xS(x=0-0.223), while x is equal to zero the iron atom is perfect in the crystal, the only example is troilite. The reason for the imperfect crystal of pyrrhotite is commonly considered to be that part of Fe²⁺ is oxidized to form Fe²⁺_x/3Fe³⁺_(x/3S. Pyrrhotite forms a defect structure with a deficiency of iron with respect to sulfur, terminating in a monoclinic structure Fe₇S₈. Some iron in pyrrhotite presents as Fe³⁺ that stabilizes the lattice^[3]. And it was considered that this stabilization changed with the decrease of rest potential of pyrrhotite as iron content increased, indicating that “iron-rich” pyrrhotite is more reactive than Fe₇S₈. When the iron content increases from 46% to 50%(mole fraction) the measured rest potential of natural and synthetic pyrrhotite at pH 3.0 decreased from -0.1 to -0.6V(vs SHE).

Pyrrhotite oxidation has been extensively studied, and it is generally believed that the oxidation of both the mineral and the collector play important roles in flotation process. Majima and Peters^[4] defined that pyrrhotite as the most reactive sulfide for a wide range of minerals in acid solution according to the oxidation rate of pyrrhotite with pressurized O₂. Burkin^[5] considered that in alkaline solution the pyrrhotite could be coated with the iron oxide that was pseudomorphic with the original sulfide lattice. The iron oxide formed a coherent layer in which slow, solid-state diffusional processes controled its oxidation process. But very few references were available on the electrochemistry kinetics of oxidation happening on pyrrhotite surface in aqueous solution, and few researches are carried out on how the oxide layer modulates the oxidation kinetics of the pyrrhotite. Buckley and Woods^[6] thinks it a solid state diffusinal process in which S^2- ion moves through the oxide layer for further oxidation on the external surface. This may be the reason why the surface of pyrrhotite can be rapidly oxidized to form a thin ferric hydroxy-oxide lay which overlays an iron-deficient sulfide lattice.

Therefore the study of reactions occurring on sulfide mineral surface in aqueous solution is important. The effect of pulp potential(φ_p) on the flotation of sulfide minerals and precious metals is commonly considered important^{[7, 8]}. It is well established that the development of hydrophobicity or hydrophilicity on sulfide minerals arises from an anodic process which is coupled with a cathodic process such as reduction of oxygen^{[9, 10]}. It is essential to determine the kinetics parameters of electrode process of oxidation of sulfide mineral on the surface, and to deeply understand the mechanism involved in the oxidation on mineral surface, then to establish the kinetics equilibrium of this electrode process and calculate the kinetic parameters^{[11, 12]}.

In this paper, the authors aim to study the electrochemical oxidation of pyrrhotite to get better insight into the mechanism of electrode process on pyrrhotite surface.

2 EXPERIMENTAL

Table 1 lists the results of magnetic separation of a pyrrhotite collected from Dachang Mine, Guangxi Province, China. The crystal form of the pyrrhotite is monoclinic. Sections cut out from the highly mineralized pyrrhotite were fashioned into form of electrodes for electrochemical measurements. The tested electrode was mounted on epoxy resin, grounded with SiC paper, then polished with 0.3μm and 0.05μm alumina. The exposed surface area of the electrode was about 1cm². The mounted electrode was then washed with deoxygenated deionized water to remove the adhering alumina particles.

Table 1  Chemical composition of pyrrhotite after magnetic separation(mass fraction, %)

   Two kinds of electrolyte were prepared by using analytical grade KCl and Na₂B₄O₇ with deionized water of resistivity larger than 6.0×10⁵Ω·m. The electrochemical experiments were carried out in a three-compartment cell, including a graphite counter electrode, pyrite electrode and saturated calomel reference electrode (SCE). All potentials in this study are quoted in volts, with respect to a standard hydrogen electrode (SHE).

An Electronics Potentiostat Model 273A and Model 636 electrode rotator were used in all electrochemical measurements. The EG&G corrosion measurement system (Princeton Applied Research Model 352) and Analysis Software (Model 270) were used for the analysis of electrochemical experimental results.

3 RESULTS AND DISCUSSION

3.1 Cyclic voltammetry

The cyclic voltammogram curves shown in Fig.1 is obtained in 0.3mol/L KCl and HCl solution (pH 4.0) for a freshly grounded surface, stationary pyrrhotite electrode starting from -0.20 to 0.9V (vs SHE). This indicates that, before the strong oxidation at high electrode potential, there is a pre-wave region (-0.20 - 0V vs SHE), in which the oxidation process occurs at lower rate. There is a weak current density peak at an electrode potential around -0.08V (vs SHE). In this electrode potential region, the primary reactions are as

Fe₇S₈→5Fe²⁺+2Fe³⁺+8S⁰+16e(1)

Fe²⁺→Fe³⁺+e(2)

Fig.1  Voltammogram curve of pyrrhotite electrode in 0.3mol/L KCl at 298K

   It shows the production of anodic oxidation can not represent a stable phase at the pyrrhotite surface forming a passive film. When the electrode potential is increased to larger than 0.5V (vs SHE), the oxidation current density increases abruptly, indicating that the following reaction can take place:

Fe₇S₈+12H₂O→   7Fe³⁺+S₂O^2-₃+24H⁺+37e(3)

The cyclic voltammogram curves shown in Fig.2 is obtained in 0.1mol/L Na₂B₄O₇ solution (pH 9.18) in the electrode potential range of -0.20 to 0.9V (vs SHE). It indicates that, before the main oxidation current density peak, there is a pre-wave region (-0.20 - 0.05V vs SHE), which is assigned to the reactions as:

Fe₇S₈+16H₂O→5Fe(OH)₂+2Fe(OH)₃+8S⁰+16H⁺+16e(4)

Fe₇S₈+21H₂O→   7Fe(OH)₃+8S⁰+21H⁺+21e(5)

Fig.2  Voltammogram curve of pyrrhotite electrode in 0.1mol/L Na₂B₄O₇ at 298K

   When the electrode potential is increased to higher than 0.3V (vs SHE), the following reaction occurs:

Fe₇S₈+33H₂O→  7Fe(OH)₃+4S₂O^2-₃+45H⁺+37e(6)

When the electrode potential is increased to higher than 0.5V (vs SHE), the following reaction occurs:

Fe₇S₈+53H₂O→  7Fe(OH)₃+8SO^2-₄+85H⁺+69e(7)

It is obvious that, in the high electrode potential region, part of S being oxidized to sulfate can make the passive film somewhat porous, but the elemental S and metal oxidates Fe(OH)₃ still remain on the electrode surface, and the passive film can not breakdown totally. So the oxidation current density of the pyrrhotite in alkaline solution is smaller than the in acid solution.

3.2 Tafel plots of pyrrhotite electrode

The Tafel plot of pyrrhotite electrode in 0.1mol/L Na₂B₄O₇ solution (pH 9.18) is shown in Fig.3. The entire potential scan is programmed to start from -225mV to 225mV of rest potential with a scan rate 0.2mV/s. Using the PARCalc Tafel analysis, the oxidation rate of pyrrhotite can be calculated. The corresponding corrosion current density(J₀) is 5.34μA/cm², and it is also the exchange current density of the oxidation reaction occurring on pyrrhotite electrode surface.

Fig.3  Tafel plots of pyrrhotite electrode in 0.1mol/L Na₂B₄O₇ solution (pH 9.18)

Fig.4  Curves of current density vs time stepped from rest potential to 0.3, 0.35, 0.40, 0.45V (vs SHE) in 0.1mol/L Na₂B₄O₇ at 298K(pH 9.18)

3.3 Chronoamperometry

In chronoamperometry, the potentiostat applies a constant potential for a specified duration and monitors the resulting current density, which can be used for many different types of analyses or to investigate electrode kinetics.

Fig.4 shows the current density vs time transients in response to a step in electrode potentials from rest value to 0.3, 0.35, 0.40, 0.45V(vs SHE), respectively. When the electrode potential increases to higher than 0.3V(vs SHE), the half cell reaction for pyrrhotite in 0.1mol/L Na₂B₄O₇ aqueous (pH 9.18) is assigned to reaction (6).

The relationship between over potential of reaction (6) and the reaction time can be described as

η=[-2.303RT/(βnF)]lgJ₀+[2.303RT/(βnF)]lgJ_t→0(8)

where η is the over potential of oxidation occurring at pyrrhotite electrode surface; T is the temperature(298K); J₀ is the exchange current density; J_t→0 is the current density when the concentration polarization of reaction can not exist; t is the time of reaction; β is the transmission coefficient; F is the Farady constant.

The data in Fig.4 increase approximately linearly with the square root of time, as shown in Fig.5, from which we can get the value of J_t→0, as about 1.6, 0.375, 0.09, 0.02mA/cm², respec-tively. Fig.6 shows the resulting curve of over-

Fig.5  Relation between current density and square root of time in 0.1mol/L Na₂B₄O₇ at 298K

Fig.6  Relation between over potential and lgJ_t→0

potential η versus J_t→0; it gives the plot of over potential (η) versus lgJ_t→0.

So the relationship between η and lgJ_t→0 can be expressed by

η= 0.18-0.08 lgJ_t→0(9)

Eqn.(9) is the electrochemical dynamics equation of reaction(6). Compared with Eqn.(8), the electrochemical kinetics parameters can be calculated from Eqn.(9) as J₀ =5.63μA/cm², which is similar to the value got from the Tafel plot.

4 CONCLUSIONS

From the cyclic voltammetry, anodic oxidation of pyrrhotite in 0.3mol/L KCl and HCl aqueous (pH 4.0) and 0.1mol/L Na₂B₄O₇ aqueous (pH9.18) can not result in passivating a stable film when the electrode potential is lower than 0.5V. When the electrode potential is increased to more than 0.5V vs SHE, part of the S being oxidized to sulfate can make the passive film somewhat porous, but the elemental S and metal oxidants still remain on the electrode surface, and the passive film can not break down totally. Using the Tafel plots , the exchange current density of the oxidation reaction occurring on pyrrhotite electrode surface in 0.1mol/L Na₂B₄O₇ aqueous (pH 9.18) (J₀) was determined as about 5.34μA/cm², which was similar to the value 5.63μA/cm² calculated by chronoamperometry.

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(Edited by LONG Huai-zhong)

Foundation item: Project(50204013) supported by the National Natural Science Foundation of China

Received date: 2004-09-16; Accepted date:2005-02-24

Correspondence: QIN Wen-qing, Professor, PhD; Tel: +86-731-8830346; E-mail: QWQ@mail.csu.edu.cn