Conversion of lead chloride into lead carbonate in ammonium bicarbonate solution

Accepted Manuscript Conversion of lead chloride into lead carbonate in ammonium bicarbonate solution

Yongming Chen, Longgang Ye, Haotian Xue, Shenghai Yang PII: DOI: Reference:

S0304-386X(16)30933-1 doi: 10.1016/j.hydromet.2017.04.014 HYDROM 4563

To appear in:

Hydrometallurgy

Received date: Revised date: Accepted date:

30 November 2016 29 March 2017 23 April 2017

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Conversion of lead chloride into lead carbonate in ammonium bicarbonate solution Yongming Chena*, LonggangYeb, HaotianXuec, ShenghaiYanga a

School of Metallurgy and Environment, Central South University, Changsha 410083, China; School of Metallurgy and Materials Engineering, Hunan University of Technology, ZhuZhou 412007, China;

c

Qinghai Provincial Research and Design Academy of Environmental Science, Xining 810000, China.

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Abstract: This study aimed to characterize the kinetics and mechanism of the phase transformation from lead chloride to lead carbonate in ammonium bicarbonate solution. The

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dechlorination rate of PbCl2 was consistently high: it increased with increasing concentrations of NH4HCO3 and decreased only moderately with the increasing total concentration of Cl-in the

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solution. The rate also increased systematically with increasing temperatures, and the apparent activation energy was 14.363 kJ/mol. The reaction obeyed the shrinking-core model, which

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involved diffusion through the thickening product layer formed on the PbCl2 particles. The pH of the leaching solution strongly influenced the conversion results. A reduction in the pH led to a decrease in the dechlorination rate owing to the conversion of the carbonate ions into slowly

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reacting bicarbonate species. In contrast, an increase in the pH from 11.0 to 12.0 by the use of

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ammonium hydroxide led to a significant increase in the conversion rate of PbCl2; the main conversion products were Pb3(CO3)2(OH)2 and PbCO3, with a high conversion rate of around 99%.

transformation.

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Keywords: lead chloride; lead carbonate; ammonium bicarbonate; dechlorination kinetics; phase

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1. Introduction

Lead found in the environment has become a global issue of concern owing to its toxicity to organisms and its large anthropogenic input to the environment. Additionally, large amounts of lead-bearing residues are produced annually in the engineering industry(Jha et al., 2012), especially in metallurgical plants(Turanet al., 2004; Şahin and Erdem, 2015). However, a pyrometallurgical process offers the advantages of high efficiency and solidification of dangerous elements, which are important for lead residue disposal. Studies have been conducted on the extraction of lead using a sinter-blast furnace or dross or a short-rotary furnace from lead-bearing residues via oxidation and reduction pyrometallurgical processes(Zheng et al., 2015; Chen et al.,

ACCEPTED MANUSCRIPT 2015). In industry, chlorine is commonly used as a reactant with most metals, oxides, and sulfides, and it exhibits high activity in certain chemical reactions. However, chlorides have a lower melting point and higher volatility than sulfides and oxides and can be dissolved in aqueous solutions. These properties are advantageous for achieving effective metal separation, extraction, and refinement(Ojeda et al., 2002, 2009).Yang et al. (2010) reported that during the process of chlorine

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metallurgy, lead chloride was formed from lead concentrate, residue, and dust and easily emitted because of volatilization. However, direct smelting of lead chloride by pyrometallurgical processes is difficult. Additionally, use of such processes can result in corrosion of the smelting

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furnace(Barbin et al., 2002). Hence, in practice, conversion of lead chloride should be performed

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before smelting.

Some previous works investigated the reaction between PbSO4 and aqueous Na2CO3; they

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reported that the reaction could be completed within 30 min at 20°C or within 15 min at 50°C(Morachevskiiet al., 2001; Arai andToguri, 1984; Ning et al., 2016; Zhang et al., 2016). Gong et al.(1992a, 1992b) and Lyakov et al.(2007) thoroughly established details of the reaction kinetics

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and reaction product formation in their works. The latter group noted that the reaction obeyed the shrinking-core model, which involved diffusion through the product layer formed on the particles. The reaction rate increased with increasing concentrations of Na2CO3 and decreased with

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increasing concentrations of the reaction product Na2SO4. The rapid nature of the reaction

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between PbSO4 and aqueous Na2CO3 is well known. Additionally, Lu and Chen (1986) examined the reaction between PbS and (NH4)2CO3; they found that Pb could precipitate out as PbCO3, yielding high conversion ratios of >90%. Additionally, more than 80% of sulfur was oxidized to

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elemental sulfur.

However, the disposal process of waste residues containing PbCl2 is not yet well established,

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because it involves complex treatment processes and high costs. Chen et al.(2011) investigated the removal of chloride from electric arc furnace dust. Though sulfation roasting is more efficient in reducing chloride content than are other roasting processes, dechlorination of the remaining water-insoluble substance is difficult. Lin et al.(2011) described a longer process for the conversion of PbCl2 in solution containing NaCl and HCl. In this process, PbOHCl was first precipitated out by adding NaOH, and lead carbonate was obtained by increasing the concentration of NH4HCO3. In another study, Tsugita(2003) reported that lead chloride could be converted directly in the presence of CO32-. However, in our exploratory experiments, we noted that the conversion rate was low and the conversion products were impure when Na2CO3was used as a conversion reagent, because of the uncontrollable and vigorous reaction between CO32-and

ACCEPTED MANUSCRIPT Pb2+. As an alternative, NH4HCO3—which is typically used as buffering agent—can yield CO32gently in alkaline solution and can thus overcome the above drawbacks. Thus, the aim of the present work is to develop a new and effective method to directly convert PbCl2 into PbCO3 in NH4HCO3 solution for the disposal of PbCl2-bearing slag in a smeltery and to promote chlorine metallurgy methods. The kinetics and mechanism of the phase transformation of lead chloride in

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ammonium bicarbonate solution were examined. The reaction products were systematically determined at different pH values, and correlations between the various products obtained and the

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reaction conditions employed were established.

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2. Materials and methods

All materials used in this study were of analytical grade. The lead chloride sample was screened

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to obtain a 120–140-mesh fraction (106–125 μm). Distilled water was used for preparation of all solutions.

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Conversion experiments were performed in a 1500mL flat-bottomed flask, with five necked tops for sample extraction and process control. A mechanical stirrer and a mercury thermometer were placed in the solution. The temperature in the flask was adjusted using a thermostatically

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controlled water bath to within ±0.5°C. All tests were conducted at temperatures between 30°C

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and 80°C. Agitation was carried out by the mechanical stirrer, and the stirring speed was fixed at 150rpm. For each test, 1000mL solution containing desired concentrations of NH4HCO3 was added to the conversion reactor. When the temperature reached a desired value, 50g PbCl2 samples were added to solution, and 5mL of slurry was extracted using a sample collector at appropriate

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time intervals for Cl- analysis after its separation into liquor and residue. Ammonium hydroxide

solution.

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and ammonium chloride were employed to regulate the pH and [Cl-]T, respectively, of the reaction

Direct calculation of the percentage of converted lead is difficult because PbCl2 and PbCO3 are insoluble in NH4HCO3solution. Therefore, the dechlorination rate of lead chloride was used to characterize the conversion rate of PbCO3. This calculated dechlorination rate could also be used to calculate the reaction rate. The Cl-concentration in the filter liquor was analyzed by argentometry (Fu, 2004). The phase and morphology of the filter residue were detected by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD studies were performed using the Rigaku D/MAX 24000 diffractometer with Cu/Kα radiation. The morphologies of antimony were determined by SEM (JEOL JSM-6360LV) coupled with energy dispersive X-ray microanalysis (EDX).

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Fig.1. Schematic of reaction setup.

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The dechlorination rate of PbCl2 was calculated using the following equation: (1)

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where xCl- is the concentration of Cl- in filter liquor, g/mL.

3. Results and discussions

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3.1 Effects of operation parameters on conversion of lead chloride

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Parameters that may influence the conversion rate of PbCl2, such as the NH4HCO3 concentration, reaction temperature, and [Cl-]T (i.e., the total content of Cl-in solution), were examined.

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3.1.1 Effect of NH4HCO3 concentration

Fig. 2(a) shows the dechlorination results of PbCl2 particles obtained at different

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NH4HCO3concentrations at the reaction temperature of 50°C,solution pH of 10.0, and mechanical stirring speed of 150 rpm. As observed, the reaction rate in the early stages of the reaction (<30 min) was high, even at a low NH4HCO3 concentration(i.e., 0.2mol/L), and reaction equilibrium was achieved within 40 min. The dechlorination rate increased significantly when the concentration of NH4HCO3 increased from 0.2 to 1.0 mol/L. The dechlorination rate reached 99.96% at the NH4HCO3 concentration of 0.6mol/L within 90 min. 3.1.2Effect of temperature To determine the effect of temperature on the transformation of PbCl2, conversion experiments were conducted at varying temperatures ranging from 30°C to 80°C at 0.6 mol/L NH4HCO3; the results are shown in Fig. 2(b). Similar to the results for the effects of the NH4HCO3 concentration,

ACCEPTED MANUSCRIPT high reaction rates were obtained even at a low reaction temperature of 30°C, and dechlorination rates of 52.81% and 90.29% were achieved within 10 and 40 min, respectively. However, it was noted that the conversion rate increased with an increase in the reaction temperature. 3.1.3 Effect of [Cl-]T In the present work, the influence of the reaction product, NH4Cl, as a measure of [Cl-]T was evaluated in the concentration range of 0.36–0.45 mol/L. The experiments were conducted in 0.6 mol/L NH4HCO3solution at 50°C, and the results are presented in Fig. 2(c). As shown in the figure,

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the dechlorination rate decreased slightly with increasing [Cl-]T concentration. For instance, the dechlorination rate decreased from 99.96 to 94.88% as [Cl-]T increased from 0.36 to 0.45 mol/L.

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Hence, [Cl-]T is expected to have a minor influence only on the diffusion of the carbonate ions to

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PbCl2.

Fig.2.Effects of operation parameters on dechlorination rate of PbCl2:(a) NH4HCO3concentration; (b) temperature; (c) [Cl-]T.

3.2 Kinetics analysis of conversion of lead chloride Fig. 3 shows the phase composition of the products generated at different NH4HCO3 concentrations at a reaction time of 40 min. Despite the short reaction time and relatively mild reaction conditions, diffraction peaks related to PbCO3 could be observed even at a low

ACCEPTED MANUSCRIPT NH4HCO3solution concentration of 0.2 mol/L. However, diffraction peaks related to PbOHCl and PbCl2 could also be observed under these conditions. Their formation, which is indicative of incomplete conversion of PbCl2, was attributed to insufficient dosage of CO32-. PbCO3 was obtained as the only product when the NH4HCO3 solution concentration was greater than

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0.6mol/L.

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Fig.3.XRD patterns of products generated at different NH4HCO3concentrations:(a) 0.2mol/L; (b) 0.4mol/L;

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(c) 0.6mol/L; (d) 0.8mol/L; (e) 1.0mol/L.

Fig. 4 shows the cross-section of two PbCl2 particles after conversion in 0.6 mol/L NH4HCO3 solution (pH 10.0) at 50°C at different reaction times. Despite the short reaction time considered, a

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reaction interface between the PbCl2 core and the reaction product layer could be observed at 1 min. Hence, the reaction could be classified as being topochemical in nature and it involved an in

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situ conversion mechanism. Furthermore, as observed in Fig. 4(b), the thickness of the product layer increased with increasing reaction time, and the area of the product layer was >10% of the total cross-sectional area. These phenomena were consistent with the dechlorination rate achieved (i.e., 20%) within a reaction time of 2 min.

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(b)

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Fig.4.Backscattered electron micrographs of partly reacted PbCl2 particles (polished section) at (a) 1min; (b) 2min.

The topochemical nature of the reaction suggests that the conversion data can be described by a

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shrinking-core model. The conversion data were fitted to both the equations of rate control by the chemical reaction between the PbCl2 core and the NH4HCO3 solution and the equation of rate

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control by diffusion through the constantly thickening product layer formed on the particles: 1-(1-α)1/3=krt+a 1–(2/3)α-(1-α)2/3=kdt+a

(2) (3)

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Here, α is the dechlorination rate, t is the reaction time, and kr and kd are rate constants. A comparison of the correlation coefficients (R2) of these two equations and a glance at the data

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in table1reveals that Eq.(3) is well fitted for the correlation coefficients up to a maximum of 99.33%. Therefore, the diffusion-controlled model fits better. Accordingly, the rate equations for

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every NH4HCO3 concentration fitted to Eq.(3) are listed in table 1, and the derived rate constants (kp) are used as an index of the reaction rate.

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Table 1 Data fittings to shrinking-core model involving different types of rate control for various concentrations of NH4HCO3. 1-(1-α)1/3=kt+a

R2

1-(2/3)α-(1-α)2/3=kt+a

R2

1-(1-α)1/3=0.00347t+0.1706

0.9697

1-(2/3)α-(1-α)2/3=0.00168t+0.0080

0.9753

1-(1-α)1/3=0.00774t+0.2104

0.9673

1-(2/3)α-(1-α)2/3=0.00347t+0.0054

0.9830

1-(1-α)1/3=0.00998t+0.3585

0.9553

1- (2/3)α-(1-α)2/3=0.00461t+0.0156

0.9649

0.8

1-(1-α)1/3=0.00957t+0.3822

0.9599

1-(2/3(α-(1-α)2/3=0.00565t+0.0174

0.9889

1.0

1-(1-α)1/3=0.01029t+0.3769

0.9621

1- (2/3)α-(1-α)2/3=0.00705t+0.0131

0.9933

0.2 0.4 0.6

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NH4HCO3/mol/L

A series of well-fitted plots of 1-(2/3) α-(1-α)2/3 versus time at different NH4HCO3 concentrations, reaction temperatures, and [Cl-]T are also shown in Fig.5.

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Fig.5. Plots of t versus 1-(2/3)α-(1-α)2/3 at different operation parameters: (a) NH4HCO3 concentration;

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(b) reaction temperature.

In order to quantitatively determine the effects of NH4HCO3 concentration, reaction temperature, and [Cl-]T on the reaction kinetics, the following semi-empirical model was

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established:

1-(2/3) α-(1-α)2/3=k0CNH4HCO3a[Cl-]Tbexp[-E/(RT)]t

(4)

where T is the temperature, K; CNH4HCO3 is the concentration of ammonium bicarbonate, mol/L;

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k0 is the apparent reaction rate coefficient, S-1; and E is the activation energy, kJ/mol. At different

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ammonium bicarbonate concentrations, when other parameters remain constant, Eq.(4) can be rewritten as

1-2/3α-(1-α)2/3=K1×CNH4HCO3at

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ln{d[1-(2/3) α-(1-α)2/3]/dt}=lnK1+alnCNH4HCO3

(5) (6)

Here, d[1-(2/3) α-(1-α)2/3]/dt is the slope of the straight lines corresponding to different

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ammonium bicarbonate concentrations in Fig.6(a). The plot of the values of ln{d[1-(2/3) α-(1-α)2/3]/dt} versus lnCNH4HCO3 is a straight line, and from the slope, it is calculated as a=0.87. In a similar way, the empirical reaction order obtained for [Cl-]T is -1.60.The Arrhenius plot is obtained using the apparent rate constant (kd) acquired from Eq.(3), as shown in Fig.6(c). The activation energy is calculated to be 14.3632kJ/mol. This value clearly confirms that this process was controlled by diffusion. Through substitution of the values of a, b, and E into Eq.(4), which is the equation used for fitting the straight lines in Fig.6, the value of k0 is calculated to be about 9.7624; the plotted results are shown in Fig.7. Thus, the rate equation for the conversion of lead chloride into lead carbonate can be expressed by the following equation:

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1-(2/3)α-(1-α)2/3=9.7624[NH4HCO3]0.87[Cl-]-1.6exp(-14363/RT)

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Fig.6. Influence of [Cl-]T concentration on dechlorinationrate of PbCl2.

Fig.7. Graph of 1- (2/3)α-(1-α)2/3 versus [NH4HCO3]0.87[Cl-]T-1.6exp(-14363/RT) for reaction of PbCl2.

ACCEPTED MANUSCRIPT 3.3 Reaction mechanism and reaction products Gong et al.(1992a, 1992b), Tsugita(2003), Woosley and Millero(2013), and Chen et al.(2009) observed that the solution pH is the most complex, yet decisive, factor influencing the conversion process and conversion products obtained, whereas it is the decisive factor for the conversion products. The effect of the solution pH, whose value is within the range of 8.0–12.0, on the conversion of

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PbCl2 in a 0.6 mol/L NH4HCO3 medium at 50°C was investigated; the conversion curves are shown in Fig.8. Within a pH range of 8.0–9.0, the dechlorination rate increased slowly under a

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prolonged reaction. However, the final conversion rate after a reaction time of 100 min was low, ~25%. In contrast, in a higher pH range of 10.0–12.0, the dechlorination rate of lead chloride

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increased considerably, and the final conversion rate approached 99%.

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Fig.8. Influence of solution pH on dechlorination rate (NH4HCO3 0.6mol/L; [Cl-]T 0.36mol/L; 50°C).

To explain the difference in dechlorination rates obtained at different pH values, the phase

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composition of the products generated at different solution pH values was investigated by XRD, as shown in Fig.9. The phase of the products changed significantly.

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♣-PbOHCl

♦-Pb2Cl2CO3

Δ-Pb3(CO3)2(OH)2

●-PbCO3

○-PbCl2

Fig.9. XRD patterns of products at different initial pH values: (a) pH 8.0; (b) pH 9.0; (c) pH 10.0; (d) pH 11.0; (e) pH 12.0.

Complex mixtures comprising PbCl2(CO3) and PbOHCl were obtained at low pH values in the range of 8.0–9.0. As revealed by Gong et al.(1992a) and Woosley and Millero(2013), the formation of such mixtures is controlled by the carbonate ion concentration, which determines the

ACCEPTED MANUSCRIPT carbonate/bicarbonate equilibrium (Eq.(8)) and thus controls the PbCl2 conversion rate in solution at pH values of 8.0–9.0. Figs.9(a) and 9(b) revealed that the main products were PbCl2(CO3), PbOHCl, and some unreacted PbCl2 at pH values of8.0 and 9.0; hence, the dechlorination rates were low at these pH values. The above mentioned obtained products suggest the occurrence of the following conversion reactions: HCO3-(aq)+OH-(aq) →CO32-(aq)+H2O(aq)

(8)

2PbCl2(s)+CO32-(aq)→Pb2Cl2(CO3)(s)+2Cl-(aq)

(9)

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PbCl2(s)+OH-(aq)→PbOHCl(s)+Cl-(aq)

(10)

Clearly, a high pH value of solution could improve Eq.(8)owing to the increase in the

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concentration of the carbonate ions. A high solution pH would instigate a vigorous reaction

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between PbCl2 and NH4HCO3 and subsequently result in a rapid reaction rate. Hence, purePbCO3was obtained as the only reaction product, as noted in Fig. 9(c): (11)

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PbCl2(s)+CO32-(aq)→PbCO3(s)+ 2Cl-(aq)

However, excessively high pH values (0.6 mol/L NH4HCO3solution) led to diverse products, probably because of the excess OH-present in solution, as observed in Figs.9(d) and 9(e). As the

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pH was increased further, progressively larger amounts of Pb3(CO3)2(OH)2 were formed from PbCO3 according to the following reaction:

3PbCO3(s)+2OH-(aq)→Pb3(CO3)2(OH)2(s)+CO32-(aq)

(12)

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This secondary reaction does not affect the dechlorination process, since chlorine is absent from

as shown in Fig.7.

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4. Conclusions

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the precipitate. This is consistent with the high dechlorination rate obtained at pH values of 10–12

The kinetics and mechanism of the phase transformation of lead chloride in ammonium

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bicarbonate solution were evaluated by experimental investigations of the dechlorination rate under various conditions and SEM and XRD analyses of the reaction products. The following conclusions were drawn. (1) The conversion of PbCl2 in NH4HCO3 solution was topochemical in nature and it obeyed the shrinking-core model, which involved diffusion through the thickening product layer formed on the particles. The dechlorination rates of PbCl2 were consistently high under the considered reaction conditions. High concentrations of NH4HCO3 led to increased conversion rates, whereas high [Cl-]T had the opposite effect. The dechlorination rate increased systematically with increasing temperatures, and the apparent activation energy was 14.363 kJ/mol. The reaction kinetics

could

be

described

by

the

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equation:

1-

ACCEPTED MANUSCRIPT (2/3)α-(1-α)2/3=9.7624[NH4HCO3]0.87[Cl-]-1.6exp(-14363/RT). (2) The pH value of the leaching solution strongly influenced the conversion results. Lower pH values (8.0–9.0) led to low dechlorination rates. Additionally, a mixture of products, which included PbOHCl and PbCl2(CO3), was obtained at lower pH values. In contrast, a higher pH value (10.0) resulted in the formation of pure PbCO3, which then transformed to Pb3(CO3)2(OH)2 when the pH was higher than 11.0. Hence, to obtain pure PbCO3 from PbCl2 in NH4HCO3 solution,

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the pH value of the reaction system must be maintained at 10.0.

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Acknowledgements

Development Program of China (No.2011AA061001).

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This work was financially supported by the National High Technology Research and

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Figure 1: Schematic of reaction setup. Figure 2: Effects of operation parameters on dechlorinationrate of PbCl2: (a) NH4HCO3concentration; (b) temperature; (c) [Cl-]T. Figure 3:XRD patterns of products generated at different NH4HCO3concentrations:(a) 0.2mol/L; (b) 0.4mol/L; (c) 0.6mol/L; (d) 0.8mol/L; (e) 1.0mol/L. Figure 4: Backscattered electron micrographs of partly reacted PbCl2 particles (polished section) at (a) 1min; (b) 2min. Figure 5: Plots of t versus 1- (2/3)α-(1-α)2/3 at different operation parameters: (a) NH4HCO3 concentration; (b) reaction temperature. Figure 6: Influence of [Cl-]T concentration on dechlorinationrate of PbCl2. Figure 7: Graph of 1- (2/3)α-(1-α)2/3 versus [NH4HCO3]0.87[Cl-]T-1.6exp(-14363/RT) for reaction of PbCl2. Figure 8: Influence of solution pH value on dechlorinationrate (NH4HCO3 0.6mol/L; [Cl-]T 0.36mol/L; 50°C). Figure 9: XRD patterns of products at different initial pH values: (a) pH 8.0; (b) pH 9.0; (c) pH 10.0; (d) pH 11.0; (e) pH 12.0.

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Graphical abstract

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 The schematic diagram of reaction process and reaction mechanism

ACCEPTED MANUSCRIPT Highlights 

A new cleaning process to transform lead chloride in ammonium bicarbonate solution was presented.

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The dechlorination rate of lead chloride reached 99% at optimum conditions.

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The conversion mechanism of lead chloride to lead carbonate in ammonium bicarbonate with

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different pH value was investigated.