An Emission-Free Vacuum Chlorinating Process for Simultaneous

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An emission-free vacuum chlorinating process for simultaneous sulfur fixation and lead recovery from spent lead-acid batteries Kang Liu, Jiakuan Yang, Sha Liang, Huijie Hou, Ye Chen, Junxiong Wang, Bingchuan Liu, Keke Xiao, Jingping Hu, and Jin Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05283 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 20, 2018

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Environmental Science & Technology

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An emission-free vacuum chlorinating process for

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simultaneous sulfur fixation and lead recovery from spent

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lead-acid batteries

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Submitted to

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December, 2017

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Kang Liu†, Jiakuan Yang*,†,‡, Sha Liang†, Huijie Hou†, Ye Chen†, Junxiong Wang†,

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Bingchuan Liu†, Keke Xiao†, Jingping Hu†, Jin Wang §

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†School of Environmental Science and Engineering, Huazhong University of Science

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and Technology (HUST), Wuhan, Hubei, 430074, China

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‡State Key Laboratory of Coal Combustion, Huazhong University of Science and

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Technology (HUST), 1037 Luoyu Road, Wuhan, Hubei 430074, China

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§Hubei Jinyang Metallurgical Incorporated, Co., Ltd., Xiangyang, Hubei 441000,

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China

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* Corresponding author. Tel: +86-27-87792207, Fax: +86-27-87792101.

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E-mail: [email protected].

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1

ACS Paragon Plus Environment


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ABSTRACT

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Spent lead-acid battery recycling by using conventional technologies is usually

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accompanied with releases of lead-containing wastewater as well as emissions of

21

sulfur oxides and lead particulates that may potentially cause secondary pollution.

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This study developed a vacuum chlorinating process for simultaneous sulfur fixation

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and high-purity lead chloride (PbCl2) recovery from spent lead paste by using calcium

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chloride (CaCl2) and silicon dioxide (SiO2) as reagents. The process train includes

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pretreatment, simultaneous PbCl2 production and sulfur fixation, and PbCl2

26

volatilization. The pretreatment eliminated chlorine emission from direct chlorinating

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reaction of PbO2 in the initial S-paste (PbSO4/PbO2/PbO/Pb). During the subsequent

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PbCl2 production and sulfur fixation step, lead compounds in the P-paste (PbSO4/PbO)

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was converted to volatile PbCl2, and sulfur was simultaneously fixed to the solid

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residues in the form of CaSO4 to eliminate the emission of sulfur oxides. The final

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step, PbCl2 volatilization under vacuum, is a physical phase-transformation process of

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ionic crystals, following a zeroth-order kinetic model. A cost estimate indicates a

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profit of USD $ 8.50/kg PbCl2. This process offers a novel green lead recovery

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alternative for spent lead-acid batteries with environmental and economic benefits.

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TOC Art

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INTRODUCTION

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Lead-acid batteries (LABs) hold a large share of the battery market, because of low

41

manufacturing cost and high operational safety.1 Lead (Pb) usage in LABs accounts

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for 82% of global lead consumption.2 Spent and discarded LABs pose serious threats

43

to both environment and human health without proper management and disposal.3

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According to World Health Organization (W.H.O.), 1.2 billion people are threatened

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by lead over-exposure.4, 5 Secondary lead production provides a practical strategy to

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ease the pressure on shortage of primary resource. Consequently, lead recovery from

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spent lead paste, which accounts for 30 ~ 40 wt% of spent LABs, is of great interests.6

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Spent lead paste contains a small amount of lead metal (~3 wt%) and a variety of

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lead-containing compounds: PbSO4 (~60 wt%), PbO2 (~28 wt%), PbO (~9 wt%),

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generated from charge and discharge of active materials on both positive and negative

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plates.7 The most commonly-used method for spent LAB recycling in practical

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applications is a crushing and separating process followed by a pyrometallurgical

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route.8 However, decomposition of PbSO4 requires relatively high temperatures (>

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1,000 °C), and using coal or coke as the reducing agent would emit sulfur oxides

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gases and lead particulates in the pyrometallurgical process.9 Alternative routes for a

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green recovery of spent LABs with low energy consumption, less pollution risk, and

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high metal recovery efficiency are of great interests.10

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Vacuum metallurgy technique has become an acceptable metal recovery method for

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e-wastes in recent years.11, 12 Compared to metallurgical processes under atmosphere,

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vacuum metallurgy technique has advantages of low energy consumption and

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significant environmental benefits.13, 14 Vacuum chlorinating metallurgy is emerging

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as an attractive alternative for extraction of metals from ores and e-wastes because of 4


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high volatility of metal chlorides with suitable chlorinating agents.15,

16

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volatilization rates of metal chlorides ensure rapid recovery of final product; and the

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vacuum state reduces generation of by-products and hence guarantees high product

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purity.16, 17 Indium recovery from waste liquid crystal display panels, with NH4Cl as

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the chlorinating agent, achieved 98.02 wt% indium chloride as the final product.18

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Germanium recycling from coal fly ash, by a two-step chlorinating process,

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demonstrated a higher efficiency and environmental compliance when compared to

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conventional recovery processes.19 However, spent lead paste has a more complex

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composition (PbSO4/PbO2/PbO/metallic Pb) with presence of multiple impurities (e.g.,

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Sb, Ba, Cu, and Fe). It would be very challenging to have lead recovered with a final

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product free from impurities.20, 21 In addition, due to the substantial content of PbSO4

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in spent lead paste (~60 wt%), green recovery without sulfur emission is an obstacle

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for a pyrometallurgical recovery process to overcome.22 No researches on lead

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recovery from spent lead paste by vacuum chlorinating process were found in our

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literature search. How to achieve high-purity lead product recovery and simultaneous

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sulfur fixation with no emission of hazardous gases are great challenges with the

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vacuum chlorinating process for spent lead paste recycling.

High

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This study developed a new approach for green conversion of spent lead paste to

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high-purity lead chloride (PbCl2) with simultaneous sulfur fixation using a vacuum

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chlorinating process. The objectives of this study were to (1) evaluate its feasibility, (2)

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determine reaction paths of different lead species with calcium chloride (CaCl2) and

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silicon dioxide (SiO2) as the chlorinating reagents, and (3) elucidate its technical,

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economic and environmental advantages.

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MATERIALS AND METHODS

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Materials and reagents

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Spent lead paste was provided by Jinyang Metallurgical Co. Ltd., Hubei, China. The

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spent lead paste was separated from spent LABs by using a grinding-sorting

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pretreatment system (Engitec Technologies Co. Ltd., Italy). The main ingredients of

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the spent lead paste specimens were 64.5 wt% PbSO4, 29.5 wt% PbO2, 4.5 wt% PbO

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and 1.0 wt% metallic Pb, with 0.5 wt% impurities. The chemical compositions of the

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spent lead paste were measured by disodium edetate (EDTA-2Na) titration method

94

(Figure S1) (Please see more details about EDTA-2Na titration method in Supporting

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Information). Chemical reagents (CaCl2, SiO2, hydrofluoric acid (HF), hydrochloric

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acid (HCl), and nitric acid (HNO3)) used were all of analytical grade and purchased

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from Chemical Reagent Company of Beijing, China. Deionized water was used for

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preparation and dilution of chemical solutions.

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Vacuum chlorinating process S-paste (PbSO4/PbO2/PbO/Pb) Pretreatment (Pb+PbO2=2PbO) (PbO2=PbO+0.5O2 (g)) P-paste (PbSO4/PbO) Vacuum chlorinating process (PbSO4+CaCl2=PbCl2+CaSO4) (PbO+CaCl2+SiO2=PbCl2+CaSiO3)

Reagent (CaCl2+SiO2) Regenerated reagent (CaCl2)

Intermediate product (PbCl2/CaSO4/CaSiO3/SiO2/CaCl2)

Evaporation

PbCl2 vacuum volatilization (Zeroth-order kinetic model)

Filtration

Product (PbCl2)

Solid residues (CaSO4/CaSiO3/SiO2/CaCl2)

Dissolution

Separated solid residues (CaSO4/CaSiO3/SiO2)

100 6


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Figure 1 Process flow diagram of the vacuum chlorinating process for

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recovery/recycling of spent lead paste.

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[Figure 1]

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The flow diagram of the developed vacuum chlorinating process for spent lead paste

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recovery/recycling is shown in Figure 1. The starting spent lead paste (S-paste) was

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pretreated under vacuum (1 Pa) at different temperatures (200, 300, 400, and 500 °C)

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to convert PbO2 and Pb to PbO (Please see more details about pretreatment of S-paste

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in Supporting Information). The pretreated lead paste was named as P-paste hereafter.

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P-paste of 2.0 g was well mixed with different amounts of CaCl2 and SiO2, and each

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prepared paste mixture was then sheeted by a sheeting mill, and placed into a vacuum

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furnace for the chlorinating reaction with 100 mL sodium bicarbonate (NaHCO3) (C0

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= 1.0 M) as the gas absorption solution. In addition, S-paste without pretreatment

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directly used as the raw material for vacuum chlorinating served as the control. The

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schematic diagram of the vacuum chlorinating equipment is shown in Figure S2. The

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chlorinating equipment was vacuumed to the set pressure (1, 101, 102, 103, 104, or 105

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Pa) with chlorinating temperatures of 400, 500, 550, 600, and 650 °C at a heating rate

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of 10 °C/min. The furnace was held for 10, 15, 20, 25, or 30 min before natural

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cooling. Lead chloride evaporated in the high-temperature zone would condense

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rapidly in the low-temperature zone on the quartz tube wall. After the vacuum

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chlorinating, the lead chloride product that coagulated on the wall of quartz tube was

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collected by gently scraping the inter wall of the quartz tube. The solid residues after

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the chlorinating reaction were dissolved in deionized water. Separation of the solid

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residues and the liquid containing CaCl2 was achieved by vacuum filtration. CaCl2

124

could be regenerated after evaporating water at 105 °C.

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Analytical methods

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Recovery percentage of lead in the form of PbCl2 was calculated as follow:23, 24

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R% (PbCl2) =

M0 ିM M0

× 100%

Eq.(1)

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where M0 and M are the molar contents of lead in the initial sample (mixture of the

129

P-paste, CaCl2 and SiO2), and the chlorinated samples, respectively.

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Emission percentage of sulfur or chlorine was calculated as follow:

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E% (Sulfur/Chlorine) =

E E0

× 100%

Eq.(2)

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where E0 is the mass of sulfur/chlorine in the initial sample (mixture of the P-paste,

133

CaCl2 and SiO2) before vacuum chlorinating; and E is the final mass of sulfur/chlorine

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in the NaHCO3 absorption solution.

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Metal contents of samples before and after the chlorinating reaction were

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determined by an inductively coupled plasma-optical emission spectrometer

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(ICP-OES, OPTIMA 8300, PerkinElmer, USA) after the HNO3-HCl-HF digestion

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(Please see more details about the digestion method in Supporting Information).

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Impurities of the PbCl2 products were measured by an ICP-OES after digestion with

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aqua regia (HNO3:HCl = 1:3, v/v) at 180 °C. The contents of sulfur and chlorine in

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the NaHCO3 absorption solution were determined by ion chromatography (IC, Dionex

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ICS2000, USA). All experiments were repeated three times with the average values

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reported.

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The crystal phase of the PbCl2 product was characterized by powder X-ray

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diffraction (XRD, PW 1700, Philips, USA) using Cu Kα radiation (γ = 1.5418 Å) with

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a voltage of 30 kV and current of 30 mA. The morphological property of the PbCl2

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product was examined by a scanning electron microscope with energy dispersive

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X-ray

analysis

(SEM-EDX,

Hitachi

S-3000N,

Japan).

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Thermogravimetric

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analysis-differential scanning calorimetry (TG-DSC) experiments were conducted and

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analyzed using a simultaneous TG-DSC instrument (Mettler-Toledo Inc., Switzerland)

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under vacuum.

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RESULTS AND DISCUSSION

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Pretreatment of spent lead paste (a)

(5) PbSO4=PbO+SO2(g)+0.5O2(g) 200

∆ G (KJ/mol)

(4) PbSO4=PbO+SO3(g) 100

(2) PbO=Pb+0.5O2(g)

0

(3) PbO2=PbO+0.5O2(g)

-100 (1) Pb+PbO2=2PbO 100

Weight (wt%)

(b)

100

200

300

400

500

600

700

Temperature (°C) Stage II Stage III

Stage I

99 98 97 96 400

500

600 700 800 Temperature (°C) ∇ ∗ PbO•PbSO4 ∗ ∇ PbSO4 ♦ PbO2 ∗ ⊕ Pb

Intensity (a.u.)

(c)

∗ P-paste ∇ ∗ ∗ ∇ ∇ ∗ ∗∗ ∇ ∗∇ ∗∗∗ ∗∇ ∇∗∇ ∗ ∇∇ ∗∗∇ ∗ ∇ ∗ ∗∗ ∗ ∗∗∗∗ ∗ ∗∗∗∗∗∗∇∗∗ ∗∗∗ ∗∗∗ ∗∗ ♦∇ ∇ ∇♦ S-paste ∇ ∇ ∇∇ ∇ ♦ ⊕ ∇ ∇ ∇ ∗ ∇♦∗∇ ∇∇ ∇ ∇ ∇ ∗ ∗∗∗ ∗ ∗ ∗ ⊕ ♦∇ ♦∇♦∇∗∗∇∇∇ ∇ ∇∇∇ ∇∇ ♦ ∇ 10

154 155

20

30

40

50

60

70

2θ (degree)

Figure 2 (a) Gibbs free energy values (∆G) of possible thermodynamic reactions 9



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during the thermal pretreatment of S-paste at 1 Pa; (b) TG curve for thermal

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pretreatment of S-paste (heating rate of 10 °C/min and vacuum pressure of 1 Pa); and

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(c) XRD patterns of the S-paste and the P-paste (after pretreating S-paste at 500 °C).

159

[Figure 2]

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Gibbs free energy values (∆G) of possible thermodynamic reactions during

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pretreatment of S-paste at 1 Pa are shown in Figure 2(a) and the reaction equations

162

are as follows:

Pb + PbO2 = 2PbO

(1)

PbO = Pb + 0.5O2 (g)

(2)

PbO2 = PbO + 0.5O2 (g)

(3)

PbSO4 = PbO + SO3 (g)

(4)

PbSO4 = PbO + SO2 (g) + 0.5O2 (g)

(5)

163

ΔG values of reactions (1) and (3) are negative at temperatures between 100 and

164

700 °C, indicating that the conversion of metallic Pb to PbO and the decomposition of

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PbO2 into PbO would proceed spontaneously. The saturated vapor pressure curves of

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metallic lead and its compounds (Figure S3) imply that both metallic Pb and PbO are

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difficult to volatilize below 700 °C at 1 Pa. The TG curve of S-paste during vacuum

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pretreatment demonstrated a 3-stage profile with distinguished behaviors under

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thermal treatment (Figure 2(b)). Stage I corresponded to decomposition of PbO2 into

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PbO, followed by stage II, where a plateau of weight loss appeared between 500 and

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650 °C, indicating a complete decomposition of PbO2 in the S-paste. Further increases

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in temperature beyond 650 °C led to stage III, where PbO began to volatilize (Figure

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S3) and PbSO4 was decomposed into PbO (Figure S4). The phase transformations of

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metallic Pb and lead compounds during the pretreatment were further identified by the

175

XRD analysis (Figure 2(c)). With an increase in temperature, the diffraction peaks of 10


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PbO2 completely disappeared after vacuum pretreatment at 500 °C, showing only

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peaks of crystalline PbO and PbSO4. The XRD patterns of the P-paste after

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pretreating S-paste at 200, 300, and 400 °C are shown in Figure S5. Thus, 500 °C was

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considered as the proper temperature for pretreatment of S-paste, where S-paste in a

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more complex form (PbSO4/PbO2/PbO/Pb) was converted to a simpler form of

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PbSO4/PbO mixture (P-paste).

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The vacuum chlorinating process

Emission percentage of chlorine (wt%) Emission percentage of sulfur (wt%)

Recovery percentage of lead (wt%)

P-paste/(CaCl2+SiO2) 100 (a)

90

80

70

60 100

(b)

80 60 40 20 0

1.0

(c)

0.5

0.0 400

183

P-paste/CaCl2

450

500 550 600 Temperature (°C)

11


650


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Figure 3 Effects of types of chlorinating reagents on (a) lead recovery, (b) sulfur

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emission, and (c) chlorine emission during vacuum chlorinating of the P-paste. (molar

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ratios of Si:Pb = 60:1 and Cl:Pb = 60:1, reaction time = 30 min, and pressure = 1 Pa)

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[Figure 3]

188

Recovery percentage of lead with CaCl2 and SiO2 as the chlorinating reagents reached

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99.7 wt% at 550 °C, compared to 84.1 wt% with CaCl2 as the sole reagent (Figure

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3(a)). Although similar recovery percentage of lead could be achieved at a higher

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temperature at the expense of more energy input, addition of SiO2 could strongly

192

promote the volatilization of lead in P-paste in the lower temperature range (450 to

193

600 °C). Sulfur and chlorine were successfully fixed while no sulfur or chlorine

194

emission was detected in the temperature range of 450 to 650 °C with CaCl2 as the

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sole chlorinating reagent. SiO2 addition induced both sulfur and chlorine emissions at

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over 550 °C (Figures 3(b) and 3(c)). Considering lead recovery, sulfur fixation and

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chlorine emissions all together, 550 °C was selected as the optimal temperature for the

198

vacuum chlorinating process.

199

Thermodynamic analysis (6) (7) (8) (9) (10)

∆G (KJ/mol)

50

0

-50

-100

400 200

450

500 550 600 Temperature (°C) 12


650

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201

Figure 4 Thermodynamic analysis of the plausible transformation paths of Pb species

202

in the vacuum chlorinating stage of the P-paste.

203

[Figure 4]

204

Gibbs free energy calculations were introduced to analyze possible thermodynamic

205

reactions in the vacuum chlorinating stage of the P-paste.25 ΔG values in Figure 4

206

shows that, with CaCl2 as the chlorinating reagent, both PbO and PbSO4 in the P-paste

207

are preferentially converted to more volatile PbCl2 (Reactions (6) and (7)).

208

PbO + CaCl2 = PbCl2 + CaO

(6)

209

PbSO4 + CaCl2 = PbCl2 + CaSO4

(7)

210

With the addition of SiO2, PbSO4 would still preferentially react with CaCl2 as shown

211

in Reaction (7) (ΔGReaction (7) < ΔGReaction (10) < ΔGReaction (9)). However, PbO would be

212

converted to CaSiO3 as shown in Reaction (8) (ΔGReaction

213

addition of SiO2 would strongly promote the conversion of PbO to PbCl2. This is in

214

consistent with the experimental results shown in Figure 3(a). Thus, the product of

215

lead species in the vacuum chlorinating stage of the P-paste would be PbCl2.

216

PbO + CaCl2 + SiO2 = PbCl2 + CaSiO3

(8)

217

PbSO4 + CaCl2 + SiO2 = PbCl2 + CaSiO3 + SO3 (g)

(9)

218

PbSO4 + CaCl2 + SiO2 = PbCl2 + CaSiO3 + SO2 (g) + 0.5 O2 (g)

(10)

219

Moreover, a thermodynamic analysis indicates that ΔG values of Reactions (S1)-(S7)

220

are all positive between 400 and 650 °C (Figures S4). Thus, sulfur could be fixed to

221

the chlorinated solid residues in the form of CaSO4 after the chlorinating reaction. It

222

indicates that no sulfur or chlorine would be spontaneously emitted from the vacuum

223

chlorinating process at 400-650 °C. However, the results of the thermodynamic

224

analysis on sulfur and chlorine are not in consistent with the experimental results

225

shown in Figures 3(b) and 3(c). Thermodynamic analysis in Figures S6 indicates 13


(8)

< ΔGReaction

(6)).

The


226

that at the temperature of >550 oC, the most possible path for sulfur and chlorine

227

emission was Reaction (S7): CaSO4 + CaCl2 + 2SiO2 = 2CaSiO3+ SO2 (g) + Cl2 (g).

228

In this reaction, the emission of gases of SO2 and Cl2 affected the chemical

229

equilibrium, which promoted the chemical equilibrium of the reaction to move

230

towards the right direction, prompting the reaction to occur at a lower temperature.

231

Further reducing the dose of SiO2 can reduce the emission of sulfur and chlorine

232

(Figure S7).

233

Pretreatment of the S-paste is critical for control of chlorine emission during the

234

vacuum chlorinating process. The results of vacuum chlorinating reaction of S-paste

235

are shown in Figures S8 and S9. The thermodynamic analysis (Figure S8) shows that

236

different lead components in S-paste tended to be converted to volatile PbCl2.

237

However, the experimental results in Figure S9(c) show that the chlorinating

238

reactions of S-paste continuously emitted chlorine in the temperature range of 450 to

239

650 °C. Based on the ΔG values of plausible thermodynamic reactions with

240

compounds contained in S-paste (Figure S8), the reaction that might be responsible

241

for chlorine emission would be: PbO2 + 2CaCl2 + 2SiO2 = PbCl2 + 2CaSiO3 + Cl2 (g)

242

(Figure S8(c)). The pretreatment successfully eliminated PbO2 by decomposing it

243

into PbO, and hence inhibited the chlorine emission. Furthermore, the sulfur fixation

244

and PbCl2 recovery were not affected by the pretreatment.

14


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245

Parameter optimization (a) 100 90 80 0

12

24

36

48

60

(b) 100

95

90 12

24

36

48

60

(c) 100 90 80 70 60 0

1

2

3

4

5

(d) 100

95

90 10

15

20

25

30

246 247

Figure 5 Effects of (a) Si:Pb molar ratio, (b) Cl:Pb molar ratio, (c) pressure, and (d)

248

reaction time on PbCl2 recovery percentages in the vacuum chlorinating process of

249

P-paste. Reaction temperature (T) was 550 °C for all experiments. Other parameters

250

were: (a) Cl:Pb molar ratio = 60:1, t = 30 min, and P = 1 Pa; (b) Si:Pb molar ratio =

251

36:1, t = 30 min, and P = 1Pa; (c) Si:Pb molar ratio = 36:1, Cl:Pb molar ratio = 60:1,

252

and t = 30 min; and (d) Si:Pb molar ratio = 36:1, Cl:Pb molar ratio = 60:1, and P =

253

1Pa.

254

[Figure 5] 15



255

Parameters, including the molar ratios of Si/Pb and Cl/Pb, pressure (P) and reaction

256

time (t), influenced the PbCl2 recovery in different ways. Increasing the Si/Pb molar

257

ratio significantly increased the PbCl2 recovery, and the PbCl2 recovery percentage

258

increased from 84.5 to 99.7 wt% when the Si/Pb molar ratios increased from 0 to 36:1,

259

then reached a constant afterwards (Figure 5(a)). The effect of the Cl/Pb molar ratio

260

was much less significant than that of the Si/Pb molar ratio (Figure 5(b)), showing

261

only a 6.0 wt% increase on the PbCl2 recovery for the increase of the Cl/Pb molar

262

ratios from 0 to 60:1. Increasing of system pressure was unfavorable for PbCl2

263

volatilization (Figure 5(c)). ΔG values of the reactions of PbCl2 generation would

264

increase with an increase in pressure, while the volatilization rate of PbCl2 would

265

decrease. The PbCl2 volatilization reaction preceded efficiently and the PbCl2

266

recovery percentage reached 94.9 wt% within 10 min, indicating that the generation

267

and volatilization rates of PbCl2 were extremely fast (Figure 5(d)). In summary,

268

PbCl2 recovery percentage, as high as 99.7 wt%, was successfully obtained under the

269

optimal operation conditions (T = 550 °C, Si:Pb molar ratio = 36:1, Cl:Pb molar ratio

270

= 60:1, P = 1 Pa, and t = 10 min). In addition, no chlorine or sulfur emission was

271

detected, which greatly reduced the potential of secondary pollution when compared

272

to using conventional hydrometallurgy and pyrometallurgy technologies.

273

Lead chloride volatilization

16


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Figure 6(a) TG-DSC curves of PbCl2 (analytical reagent) at a heating rate of

275

10 °C/min (P = 1Pa); (b) TG curves of PbCl2 (analytical reagent) at heating rates of 10,

276

  20, 30, and 40 °C/min (P = 1Pa); (c) ln  β1.92  vs. 10,000/T as a function of  Tα



277

conversion; (d) SEM image of the PbCl2 product (PbCl2-II) after the purification step;

278

(e) EDX analysis of the PbCl2-II product from the selected zone of the above SEM

279

image; and (f) XRD patterns of the PbCl2-II product and the commercial analytical 17



280

Page 18 of 26

reagent (PbCl2-III).

281

[Figure 6]

282

PbCl2 volatilization under vacuum was a physical phase-transformation process of

283

ionic crystals (Figure 6(a)). Two endothermic peaks could be observed in the DSC

284

curve during PbCl2 volatilization. The first strong endothermic peak occurred at

285

495.1 °C, agreeing with the melting point of PbCl2, 501 °C. This endothermic peak

286

was the phase-transformation of PbCl2 from the solid phase to the liquid phase. The

287

second endothermic peak was at 643.1 °C, weaker than the first one, agreed well with

288

the temperature when the maximum volatilization rate of PbCl2 occurred. PbCl2 is an

289

ionic crystal, and the solid phase PbCl2 has a higher free energy than that of its liquid

290

phase. When the temperature rises to a specific temperature under vacuum, it would

291

result in an unstable solid phase transformation to the liquid phase. Further increases

292

in temperature would result in a gradual volatilization of the liquid phase.

293

Kinetic reaction equation of PbCl2 volatilization was obtained in two steps by

294

combining the Kissinger-Akahira-Sunose (KAS) method and the Coats-Redfern (CR)

295

method.26 Firstly, the reliable apparent activation energy (Ea) data was calculated by

296

the KAS method, and the mechanism function was then screened according to the

297

values of the Ea and the correlation coefficient (R2) calculated by the CR method. The

298

calculation equation of the KAS method is shown as Eq. (3) below:

299

 β   E  ln  1.92  = Const − 1.0008  α   Tα   RTα 

300

where β is the heating rate (°C/min); α is the PbCl2 conversion rate; T is the

301

thermodynamic temperature (K); and R = 8.314 J/mol·K. Using Eq. (3), the linear

302

 β  fitting curve of ln  1.92  versus 10,000/T could yield the value of Ea. According to  Tα 

Eq. (3)

18


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303

the heating curves at different heating rates (Figure 6(b)), the kinetic data of PbCl2

304

volatilization were plotted against α from 0.1 to 0.9 (Figures 6(c)). The Ea and R2

305

values were then determined and provided in Table S1. The average Ea was found to

306

be 141.12 kJ/mol. The linear relationship between different mechanism functions

307

ln(G (α ) / T 2 ) and 10,000/T could be obtained by the CR method, and the Ea would

308

be deduced from the slope of the fitting line. Table S2 shows the values of Ea and R2

309

corresponding to the mechanism function involved in the screening as well as the

310

mechanism function at a heating rate of 10 °C/min. As shown, Ea obtained by the

311

No.1 mechanism function was mostly closed to that obtained by the KAS method,

312

with R2 value greater than 0.99. Therefore, the mechanism function of PbCl2 vacuum

313

volatilization was one-dimensional phase boundary reaction: f (α ) = (1−α )0 , which

314

was in consistent with the zeroth-order kinetic model for phase transformation of ion

315

crystals.

316

Product identification

317

PbCl2 product obtained from the chlorinating volatilization reaction (named as

318

PbCl2-I) contained few impurities (i.e., Ga, Al, Fe, Sb, Zn, and As), which might be

319

caused by evaporation of chlorides formed with the metallic impurities in the spent

320

lead paste (Table S3). Some impurities of PbCl2-I product could be separated in the

321

form of metal chlorides through purification at 350 °C under vacuum to obtain the

322

purified PbCl2 product (named as PbCl2-II). According to Figure S10, the saturated

323

vapor pressures of GaCl4, SbCl3, AlCl3, FeCl3, ZnCl2, and AsCl3 are much lower than

324

that of PbCl2. Thus, the purity of the PbCl2-II product increased to 99.8 wt%, which is

325

comparable to that of PbCl2 of analytical reagent (named as PbCl2-III). The PbCl2-II

326

product consisted of irregular spherical of particulates with sizes of 10 to 30 µm

327

(Figure 6(d)). The atomic Pb and Cl contents were 32.21 and 67.79 %, respectively 19



328

(Figure 6(e)). It indicates that the atomic ratio of Pb/Cl in the PbCl2 product is close

329

to the stoichiometric atomic ratio of Pb/Cl. The X-ray diffraction peaks of the PbCl2-II

330

product were well matched with those of PbCl2 of analytical reagent (JCPDF

331

#26-1150), indicating that the synthesized PbCl2 product has high purity (Figure 6(f)).

332

The identification of solid residues and the Toxicity Characteristic Leaching Potential

333

(TCLP) test results are shown in Figure S11 and Table S4, respectively.27

334

Environmental implications for spent lead-acid battery recycling

335

An overview map of the spent lead paste recycling and CaCl2 regeneration process is

336

shown in Figure S12. On the basis of our previous studies, the vacuum chlorinating

337

process was assessed on the aspects of technical, economic and environmental

338

benefits (Table S5). Our vacuum chlorinating process takes place at a low reaction

339

temperature of 550 °C, and has advantage of a high lead recovery and high PbCl2

340

purity without secondary pollution. It has obvious advantages over smelting with

341

operating temperatures exceeding 1,000 °C and emissions of SOx and lead particulates.

342

In addition, CaCl2 and SiO2 are more environmental-friendly, efficient and applicable

343

reaction reagents when compared to inorganic/organic acids and alkali commonly

344

used in hydrometallurgy. These reagents would not pose potential secondary risk to

345

the environment and the practitioners in industrialized production. Therefore,

346

compared with the traditional processes for spent lead paste recovery, the proposed

347

vacuum chlorinating process has significant environmental and technical benefits.

348

We economically evaluated the production of 1 kg of PbCl2 product based on the

349

lab scale (Table S6). The whole process was divided into three parts. It cost USD

350

$5.00 for the energy consumption, $2.30 for the spent lead paste and $4.50 for

351

chemical reagents (CaCl2 and SiO2) respectively for the recovery of 1 kg of PbCl2

352

($20.30). The total revenue of the 1 kg PbCl2 product gained from spent lead paste is 20


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353

USD $8.50. It is noteworthy that the recycling of CaCl2 greatly reduces the cost of

354

chemical reagents. Thus, the proposed vacuum chlorinating process for treating spent

355

lead paste is economically feasible to be applied in industrial applications. A pilot

356

scale experiment would be necessary to predict the economic benefit more precisely

357

for actual large scale industrial applications in the future. In summary, the vacuum

358

chlorinating process could successfully recover lead from the spent lead paste in a

359

green manner, eliminate wastewater discharge, control generation of hazardous solid

360

waste, and prevent emission of harmful gases.

361

ASSOCIATED CONTENTS

362

Flow chart of the determination of different components in spent lead paste (Figure

363

S1); Schematic diagram of vacuum chlorinating equipment (Figure S2); Saturated

364

vapor pressure curves of metallic lead and its compounds (Figure S3); The emission

365

percentage of sulfur during the pretreatment of S-paste at different temperatures

366

(Heating rate = 10 °C/min, retention time = 10 min, and P = 1 Pa) (Figure S4); XRD

367

patterns of the P-paste samples after pretreating the S-paste at 200, 300, and 400 °C

368

(Figure S5); Thermodynamic analysis of the plausible transformation paths of sulfur

369

and chlorine species in the vacuum chlorinating stage of P-paste (T = 400-650 °C, P =

370

1 Pa) (Figure S6); Effect of Si:Pb molar ratio on the emission percentage of sulfur

371

and chlorine (Temperature = 550-650 oC, Cl:Pb molar ratio = 60:1 , t = 30 min, and P

372

= 1 Pa) (Figure S7); ΔG values of plausible reaction paths of S-paste in the presence

373

of CaCl2 and SiO2: (a) Pb, (b) PbO, (c) PbO2, and (d) PbSO4 (T = 400-650 °C, P = 1

374

Pa) (Figure S8); Effects of CaCl2 and SiO2 on (a) lead recovery, (b) sulfur emission,

375

and (c) chlorine emission in the vacuum chlorinating of the S-paste (Figure S9);

376

Relationship between vapor pressure of metal chlorides and temperature (Figure S10); 21



377

XRD patterns of the separated solid residue after CaCl2 regeneration with additions of

378

different chlorinating reagents: (a) CaCl2 and SiO2, and (b) CaCl2. (Figure S11); An

379

overview map of the spent lead paste recycling and CaCl2 regeneration process

380

(Figure S12); The Ea calculated by the KAS method and the linear correlation

381

coefficient (R2) (Table S1); The differential f(α) and the integral form G(α) of the

382

mechanism function involved in the screening (Table S2); Compositions of different

383

PbCl2 products (Table S3); Pb2+ concentrations in the TCLP tests of the solid residues

384

after different vacuum reactions (Table S4); Technical and environmental benefits of

385

different recovery processes for the spent lead paste (Table S5); Economic evaluation

386

of the vacuum chlorinating volatilization process for spent lead paste recycling (Table

387

S6); Supporting Information is available free of charge via Internet at

388

http://pubs.acs.org.

389

AUTHOR INFORMATION

390

Corresponding author

391

*Jiakuan Yang, Phone: +86 27 87792102. Fax: +86 27 87792101. Email:

392

[email protected].

393

Notes

394

The authors declare no competing financial interest.

395

ACKNOWLEDGMENTS

396

The research is supported by the National Science and Technology Support Program

397

(2014BAC03B02), Wuhan Yellow Crane Talents (Science) Program, the National

398

Natural Science Foundation of China (51508214), and the Project of Innovative and

399

Interdisciplinary Team of HUST (2015ZDTD027). We would also like to thank the 22


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400

Analytical and Testing Center of Huazhong University of Science and Technology for

401

providing experimental measurements.

402

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An Emission-Free Vacuum Chlorinating Process for Simultaneous

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