­­Hydro-chemical conversion of galena in FeCl₃-KCl solution

LONG Huai-zhong(龙怀中)¹, CHAI Li-yuan(柴立元)¹, LIU Hui(刘 辉)², QIN Wen-qing(覃文庆)²

1. School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China;

2. School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China

Received 20 May 2009; accepted 10 August 2009

Abstract: The behaviours of complexation and dissolution of PbCl₂ on the surface of galena were investigated to explore the process of hydro-chemical conversion of galena (PbS) in chloride media. By means of solution chemistry calculation, the production and dissolution of the products PbCl₂ were studied. And the passivation of the galena was studied by Tafel curve. The results show that PbCl₄^2- is the main form of PbCl₂ presented in the saturated potassium chloride (KCl) solution. The PbCl₂ crystal is easy to precipitate when the total concentration of chloride ion ([Cl^-]_T) is equal to 0.92 mol/L, and it is inclined to dissolve when [Cl^-]_T is more than 0.92 mol/L. The chloride complexing reaction rate strongly depends on the Fe³⁺ ion concentration when it is less than 6×10^-4 mol/L, while passivation occurs on the surface of the electrode when Fe³⁺ concentration is larger than 6×10^-4 mol/L. The reaction rate increases obviously when KCl is added, since the activity of Cl^- increases; thus accelerates the dissolution of PbCl₂.

Key words: FeCl₃-KCl solution; galena; complexation; solution chemistry

1 Introduction

At present, pyro-metallurgy is still a main method in plumbery. However, traditional sintering technology as melting in the blast furnace, has many disadvantages, such as high energy consumption and serious environment pollution[1]. Recently, the basic study on a direct extract of Pb and preparation of material from original resource has been paid more attention to, with continuously increasing need of Pb and environment protection problem[2]. Chemistry extraction of Pb has attracted much attention because of its low energy consumption and friendly environment. Many methods have been reported in hydrometalurgy ways using FeCl₃ [3], Fe₂(SO₄)₃[4-5], silicofluoric acid[6], HNO₃[7] and EDTA[8]. The study on extraction technique using FeCl₃, especially the thermodynamics and dynamics in the transformation process[9-18] was mostly performed because of its high transformation speed, efficiency and easy manipulation. However, there was less report about the complexation and dissolution behavior of reaction production on the surface of galena (PbS). In this study, the complexation and dissolution of chloride on the surface of galena (PbS) in FeCl₃-KCl media was investigated by means of solution chemistry calculation and corrosion electrochemistry.

2 Experimental

The galena used in this study consists of 82.20% Fe, 16.20% S and 1.6% SiO₂, and was taken from Fankou Mine of Guangdong Province in China. Section cut out from the highly mineralized galena was fashioned into the form of electrodes for electrochemical measurement. The cut section of the mineral was mounted on the tip of a perspex tubule of d 7 mm using epoxy resin and the exposed outer surface was well polished. The exposed surface area of the electrode was about 1 cm². FeCl₃·6H₂O and KCl were analytical reagents. A conventional three-electrode system was used, including a Ag/AgCl reference electrode, a platinum plate as the counter electrode and galena (PbS) as working electrode. An EG&GPARC Model 273 bi-potentiostat and 686 rotating disk electrode were used for electrochemistry experiments. The Tafel experiment was performed at room temperature (298 K) and normal atmosphere with a rotating speed of 500 r/min. The potentials throughout this study were referenced to standard hydrogen electrode(SCE).

3 Results and discussion

3.1 Solution chemistry on surface of galena in chloride medium

In FeCl₃-KCl system, the surface of galena contacts with Fe³⁺ by the following reaction:

PbS(s)+2Fe³⁺(aq)+6Cl^-(aq)→

PbCl₂(s)+2Fe²⁺(aq)+4Cl^-(aq)+S(s)         (1)

The strong oxidation capability of PbCl⁺ and Fe³⁺ facilitates the fracture of Pb—S bond on the surface of galena. S^2- is oxidized to S solid, and Pb(Ⅱ) combines with Cl^- to form PbCl₂ crystal adhering on the surface of galena. The following complexing equilibriums of PbCl₂ crystal exist on the surface of galena in chloride medium:

PbCl₂Pb²⁺+2Cl^-,

                  (2)

Pb²⁺+Cl^- PbCl⁺,

                    (3)

Pb²⁺+2Cl^- PbCl₂(aq),

                   (4)

Pb²⁺+3Cl^-

                   (5)

Pb²⁺+4Cl^-

                    (6)

Associating with Eqs.(2)-(6), the concentration of each component can be calculated by the following equations:

                   (7)

                   (8)

                            (9)

                  (10)

                  (11)

According to Eqs.(7)-(11), the logarithmic solubility curve of PbCl₂ is drawn as shown in Fig.1. It can be seen that the decrease of Cl^- concentration facilitates the dissolution of PbCl₂. As a result, the concentration of Pb²⁺ increases, but the probability of Pb²⁺ complexation with Cl^- to form high-coordination anion decreases because of the low concentration of Cl^-. Until the concentrations of Pb²⁺ and  are equal, the system reaches the balance. On the contrary, if the concentration of Cl^- increases, so that of Pb²⁺ decreases, and the complexation reaction takes place easily, resulting in high concentration of . As a result,  is in a dominant position when the concentration of Cl^- is high in the solution. So, in the FeCl₃-KCl system with saturated KCl medium, PbCl₂ exists as a form of complex .

Fig.1 Logarithmic solubility diagram of lead chloride at 25 ℃

In order to study the formation and dissolution of PbCl₂ in KCl medium, the total concentrations of Pb²⁺ ([Pb²⁺]_T) and Cl^- ([Cl^-]_T) were calculated by the following equations that depended on -lg[Cl^-].

,

                (12)

where

        (13)

If n=4, the total concentration of Pb²⁺ can be calculated from the following equation:

          (14)

The best point for producing PbCl₂ is estimated by

differentiating Eq.(14). Assuming  we

have

Approximately, it can be written as

As a result,

The solubility of PbCl₂ at different values of lg[Cl^-] could be calculated by Eqs.(12) and (13), and the results are listed in Table 1.

Table 1 Calculation results of solubility of PbCl₂

As a result, the relationship of lg[Pb²⁺] and lg[Cl^-] with lg[Cl^-] is shown in Fig.2.

Fig.2 Relationship of lg[Pb²⁺]_T and lg[Cl^-]_T with lg[Cl^-] at 25 ℃

As can be seen in Fig.2, the total concentration of Cl^- increases with increasing lg[Cl^-], while that of Pb²⁺ decreases. When the value of lg([Cl^-]/(mol?L^-1)) is -0.04 and the total concentration of Cl^- is 0.92 mol/L, the concentration of Pb²⁺ reaches the minimal value of  0.002 8 mol/L, then the equilibrium point of production and dissolution occurs. When the value of lg([Cl^-]/(mol?L^-1)) is lower than -0.04, the dissolution rate of PbCl₂ is less than that of production, so the production reaction is dominant. When the value of lg([Cl^-]/(mol?L^-1)) is larger than -0.04, the dissolution rate of PbCl₂ is more than that of production, indicating that at a high concentration of Cl^-, the complexation reaction of Pb²⁺ with Cl^- is dominant, and the dissolution rate of PbCl₂ becomes rapid. As a result, the surface of galena can contact with Fe³⁺ and dissolve continuously.

3.2 Corrosion electrochemistry on surface of galena in chloride medium

Galena has the character of semiconductor. Actually in chloride medium, galena (PbS) reacts with Fe³⁺ as a corrosion primary cell. In this study, the corrosion electrochemistry technique, Tafel curve was used to investigate the corrosion and dissolution behavior of galena in chloride medium.

On the surface of electrode in FeCl₃-KCl solution system, the following reactions occur:

Anode

PbS(s)+2Cl^-→PbCl₂(s)+S⁰(s)+2e              (15)

Cathode

2Fe³⁺(aq)→2Fe²⁺(aq)-2e                     (16)

The Tafel curve test was carried out with the M352 software using 686 rotating electrode. The rotating speed of the electrode was 400 r/min, the scanning rate was 1.5 mV/s, and the step was 0.20 s. The pH value was adjusted to 3.0 at each time and pure N₂ was pumped into the solution to remove the oxygen before the measurement. The electrokinetic potential polarization curves of anode dissolution are shown in Fig.3 and Fig.4, from which the corrosion potential(φ_corr) and corrosion current density(J_corr) were calculated and the results are listed in Table 2.

Fig.3 Tafel curves of lead sulfide electrode under different ferric chloride concentrations without KCl: (a) [FeCl₃]=0;  (b) [FeCl₃]=2×10^-4 mol/L; (c) [FeCl₃]=5×10^-4 mol/L;   (d) [FeCl₃]=8×10^-4 mol/L; (e) [FeCl₃]=1.0×10^-3 mol/L

Fig.4 Tafel curves of lead sulfide electrode in saturated KCl solution under different ferric chloride concentrations:   (a) [FeCl₃]=0; (b) [FeCl₃]=2×10^-4 mol/L; (c) [FeCl₃]=5×10^-4 mol/L; (d) [FeCl₃]=8×10^-4 mol/L; (e) [FeCl₃]=1×10^-3 mol/L

Table 2 Tafel parameters of lead sulfide electrode in saturated KCl solution and without KCl under different ferric chloride oncentrations

It can be seen from Fig.3 and Table 2 that, when the concentration of Fe³⁺ ([Fe³⁺]) is 0-5×10^-4 mol/L, φ_corr and J_corr increase with the increase of [Fe³⁺]. While when [Fe³⁺] is more than 5×10^-4 mol/L, the corrosion current density decreases with the increase of [Fe³⁺]. It is illuminated that there is passivation reaction on the surface of galena. The electrochemistry reaction production PbCl₂ crystal agglomerates on the surface of electrode, resulting in the increase of resistance and decrease of current. At the open circuit voltage ([Fe³⁺]=0), φ_corr and J_corr do not change any more with and without KCl addition, indicating that KCl has little effect on the rest potential. When [Fe³⁺] is 5×10^-4 mol/L, J_corr reaches the maximum value.

From Fig.4 and Table 2, φ_corr and J_corr in saturated KCl system have the same trend of change as the in the system without KCl. That is, with the increase of [Fe³⁺], φ_corr and J_corr increase firstly. When [Fe³⁺] is more than  5×10^-4 mol/L, φ_corr and J_corr decrease. The difference is when [Fe³⁺] is equal to 5×10^-4 mol/L, the current density increase abruptly. The reason is that with the addition of KCl, the concentration of Cl^- increases. The complexation reaction of Pb²⁺ with Cl^- accelerates, which facilitates the dissolution of PbCl₂ crystal on the surface of electrode. As a result, the electrode surface resistance decreases, and the current density increases.

In FeCl₃-KCl system, when [Fe³⁺] is less than 5×10^-4 mol/L, the corrosion rate depends on the existence of Fe³⁺, and increases rapidly with increasing Fe³⁺ concentration. When [Fe³⁺] is more than 5×10^-4 mol/L, passivation reaction takes place on the surface of galena. With the addition of KCl, the complexation reaction of Pb²⁺ and Cl^- facilitates the dissolution of PbCl₂, so the electrode surface resistance decreases, and the corrosion current density increases.

4 Conclusions

1) In FeCl₃-KCl system, the strong oxidation of Fe³⁺ facilitates the transformation of galena to PbCl₂ crystal, which is dissolved by complexation with Cl^-. This equilibrium state is verified by liquid chemistry calculation. PbCl₂ exists as complex compound PbCl₄^2- in saturated KCl solution.

2) When [Cl^-]_T is equal to 0.92 mol/L, PbCl₂ precipitates in solid state. When [Cl^-]_T is more than 0.92 mol/L, PbCl₂ dissolves because of complex reaction.

3) With the addition of KCl, the activity of Cl^- increases, and the complex reaction becomes stronger. Therefore, the electrode surface resistance decreases, and corrosion current density increases.

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Foundation item: Project(50774094) supported by the National Natural Science Foundation of China

Corresponding author: LONG Huai-zhong; Tel/Fax: +86-731-88876765; E-mail: ysxblong@mail.csu.edu.cn

DOI: 10.1016/S1003-6326(08)60445-8

(Edited by YUAN Sai-qian)