Leaching of platinum group metals (PGMs) from ... - Wiley Online Library

Research Article Received: 1 November 2012

Revised: 10 January 2013

Accepted article published: 13 February 2013

Published online in Wiley Online Library: 1 April 2013

(wileyonlinelibrary.com) DOI 10.1002/jctb.4057

Leaching of platinum group metals (PGMs) from spent automotive catalyst using electro-generated chlorine in HCl solution Arun K. Upadhyay,a Jae-chun Lee,a∗ Eun-young Kim,b Min-seuk Kim,a Byung-Su Kima and Vinay Kumarc Abstract BACKGROUND: In order to conserve resources without affecting the environment, an energy efficient leaching process has been studied to recover platinum group metals (PGMs) from spent automotive catalyst using electro-generated chlorine as a strong oxidant in a separate reactor. RESULTS: The different process parameters studied showed an increase in the dissolution of PGMs (Pt, Pd, Rh) with HCl concentration, current density, time, and temperature but not pulp density. Leaching efficiencies of 71% Pt, 68% Pd and 60% Rh were obtained under the optimized conditions of 6.0 mol L−1 HCl, 714 A m−2 current density, 363 K, 20 g L−1 pulp density, 700 rpm agitation speed. The PGMs dissolution kinetics followed ash diffusion control with activation energies for Pt, Pd and Rh of 29.6, 26.4, 20.6 in kJ mol−1 , respectively over the temperature range 298–363 K. The leaching studies carried out after treatment with 20% formic acid at room temperature showed a remarkable increase in dissolution efficiencies to 97% Pt, 94% Pd and 90% Rh. CONCLUSIONS: A process for the leaching of PGMs using electro-generated chlorine showed almost total dissolution of PGMs in hydrochloric acid solution after pretreatment of the spent automotive catalyst with formic acid. The proposed leaching system is eco-friendly and has high leaching ability for PGMs from spent materials. c 2013 Society of Chemical Industry Keywords: spent automotive catalyst; PGMs; electro-generated chlorine; leaching; reduction

INTRODUCTION

J Chem Technol Biotechnol 2013; 88: 1991–1999

costly. To conserve the resources and meet the future market demand for metals for advanced technological development, it is necessary to develop economical and environmentally-friendly processes to recover and recycle PGMs from secondary resources, particularly spent automotive catalysts. Extensive efforts have been made to develop energy-efficient and eco-friendly pyroand hydro-metallurgical processes to recover PGMs from spent autocatalysts.7 – 10 The main problems of the pyrometallurgical methods are that they have high energy consumption and must be carried out on a large scale, while the hydrometallurgical methods can efficiently recover the PGMs from such complex materials under ambient experimental conditions.

∗

Correspondence to: Jae-chun Lee, Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 305–350, Republic of Korea. E-mail: [email protected]

a Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon, 305-350, Republic of Korea b Research Institute of Resources and Technology, LS-Nikko Copper Inc., Gyeonggi-do, 463-400, Republic of Korea c Metal Extraction & Forming Division, CSIR- National Metallurgical Laboratory (NML), Jamshedpur, India

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c 2013 Society of Chemical Industry

1991

Platinum group metals (PGMs) are widely used in many chemical processes, electrical and electronic industries, catalysis etc. Due to their chemical resistance, high temperature stability, selectivity and activity for reactants, platinum (Pt), palladium (Pd) and rhodium (Rh) are extensively employed as automobile converter catalysts for converting exhaust emission containing carbon monoxide (CO), hydrocarbons and nitrogen oxide into nonpolluting gases.1 – 3 Pt is mainly responsible for transforming hydrocarbons and CO to water and carbon dioxide (CO2 ), while Pd and Rh are most efficient in reducing nitrogen oxide (NOx ) to nitrogen. The catalytic convertor is composed of a honeycombtype cordierite skeleton (2MgO · 2Al2 O3· 5SiO2 ) and a mixture of base metal additives, mainly oxides of cerium, zirconium, nickel, lanthanum, iron and alkaline earth metals to improve the stability of the catalytic convertor. The PGMs are fixed in the highly porous alumina wash-coat surface by impregnation or coating from a solution of chloro-complexes of Pt, Pd and Rh.4,5 The average PGMs weight content of a catalytic convertor is 1.56 g Pt, 0.62 g Pd and 0.156 g Rh. Between 2009 and 2010, the global demand for Pt, Pd and Rh increased by 16%, 23% and 22%, respectively.6 Their natural resources are limited due to exhaustion of high grade minerals, and their recovery from lean resources is also becoming

www.soci.org In hydrometallurgical processing, leaching is one of the techniques used for the dissolution of PGMs from spent catalysts using different lixiviants, such as sulfuric acid, hydrochloric acid, nitric acid, and aqua regia in the presence of other additives such as hydrogen peroxide, chloride, iodide, etc.11 – 14 Sometimes, the autocatalyst is also thermally treated under oxidizing or reducing atmosphere to remove certain undesired constituents such as hydrocarbons or carbon to enhance leaching efficiency.15,16 Aqua regia has been employed by several researchers17,18 for dissolution of PGMs but the nitric oxide (NO) gas evolved during metal digestion is a major threat to the health and safety of the workplace. Aqua regia has also been employed to leach Pt from the residue of spent catalysts obtained after dissolving other constituents in mineral acids or alkali.13 This process has high acid and alkali consumption for the leaching of metals from spent catalysts. An attempt to leach Pt-Rh / Pt-Pd-Rh from spent catalyst was also studied using a mixed solution of sulfuric acid and sodium chloride to replace the aqua regia or autoclave leaching.19 The metals from the leaching solution were then recovered by precipitation, solvent extraction or ion exchange processing. The possibility of replacing aqua regia with HCl and H2 O2 lixiviant for the dissolution of Pt from spent autocatalyst was suggested by Kizilaslan et al.20 Recovery of 95% Pt was achieved under the optimized condition of lixiviant concentration with oxidizing agent of 10 HCl:1 H2 O2 solution. Aberasturi et al.21 recently proposed a process for the dissolution of PGMs including Pt from different types of automotive exhaust catalytic converters. The PGMs from the catalyst were leached in HCl and H2 SO4 with the addition of hydrogen peroxide. A recovery of 95% was obtained by leaching at 363 K for 6 h after thermal pre-treatment at 523 K for 22 h. Cyanide leaching of PGMs from spent autocatalysts under elevated temperatures and pressures has also been reported.22 The leaching order Pd > Pt > Rh was found to be due to the following order of metal bonding strengths of their complex in Rh(CN)6 3− > Pt(CN)4 2− > Pd(CN)4 2− . Recently, iodine–iodide solution has also been used for the leaching of Pt from spent reforming catalyst as it forms a stable Pt iodide complex.14 In this work, studies have been conducted to leach PGMs from spent autocatalyst using electro-generated chlorine in acidic chloride solution to form chloro-complexes. The proposed leaching is eco-friendly because the electro-generated chlorine can operate in a closed system and chloride concentration can be reduced during leaching.23,24 To suggest an environmentally friendly method for the recovery of PGMs, different parameters such as lixiviant concentration, current density, leaching time, temperature, and agitation speed were studied to optimize the conditions for dissolution of Pt, Pd and Rh from spent catalyst. Furthermore, to improve the dissolution of PGMs, the leaching of the spent catalyst was also carried out after pretreatment with formic acid at room temperature. As the dissolution of PGMs is also affected by the redox potential of the solution, the leaching of the spent autocatalyst was also carried out by treating the sample with formic acid at room temperature to reduce the oxide particles, (PtO, PdO, and Rh2 O3 ) to the metallic state.

experiments a constant electric current was supplied by a programmable DC power supply (IPS-30B10, INTERACT Co., Ltd). Electro-generated chlorine was obtained using electrolytic cells (18 cm × 9 cm × 12.5 cm) made of acrylic and divided into two compartments by an anion exchange membrane (Neosepta AMX, Tokuyama Co.). Two cathodes and two anodes of high purity graphite rods (20 cm (l) × 0.8 cm (d)) were used with an effective electrode area in solution of 14.0 cm2 (Fig. 1). HCl solution (4.0 mol L−1 , 200 mL) was added to the anode compartment and HCl solution (6.0 mol L−1 , 200 mL) to the cathode compartment. The anode compartment was then saturated with chlorine before starting leaching by supplying a constant current across the electrodes. The dissolved chlorine concentration in the leaching solution was maintained at saturation and its concentration was not varied during the process. After the anode compartment of the cell had been saturated by chlorine HCl solution (4.0 − 8.0 mol L−1 , 500 mL) was added to the separate leaching reactor. A catalyst sample (10 g) was added after the required temperature was attained. The electrogenerated chlorine gas was passed from the electrolytic cell to the leaching reactor through a tube. The solution in the reactor was also agitated by a PTFE-coated steel impeller at a controlled speed of 500–900 rpm. The samples were removed from the reactors at predetermined time periods and analyzed by atomic absorption spectrometry (AAnalyst 400, PerkinElmer Inc.).

RESULTS AND DISCUSSION Anodic chlorine generation from hydrochloric acid solution can be represented by the following reaction:25 Anode : 2Cl – → Cl2 (electrode surface) + 2e –

E0 = 1.35 VSHE (1) The generated chlorine gas dissolves in aqueous hydrochloric acid solution as aqueous species such as Cl2(aq) , Cl3 − , and HClO, as shown in Equations (2) − (4).26 – 28 The distribution of these species depends on the HCl concentration and temperature. Cl2 (g) ↔ Cl2 (aq)

(2)

Cl2 (aq) + H2 O ↔ HCl + HClO

(3)

Cl2 (aq) + Cl – ↔ Cl3–

(4)

The chloride ion concentration is an important parameter that influences the dissolution of PGMs. In general, the dissolution of PGMs is based on providing a high oxidation potential and effective complexing ions in solution. Because the pH falls below 2, the dominant species of chlorine are Cl2 (aq), Cl3 − , 26,28 and the leaching reactions of the PGMs with the chlorine in HCl solution occur as shown in Equations (5) − (11). These cell potentials were calculated from the standard electrode potentials for chlorine species (Cl2(aq) , Cl3 − ) and PGMs, i.e. Pt, Pd, and Rh:19 Pt + Cl2 (aq) + 2Cl – = PtCl24 –

EXPERIMENTAL

1992

The spent autocatalyst sample obtained from a South Korean company was ground to < 3.36 mm in a ball mill. A ground sample of −0.212 mm with a chemical composition of 0.18 wt% Pt, 0.12 wt% Pd and 0.016 wt% Rh analyzed by fire assay was used for leaching studies in chloride solution. For the

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A. K. Upadhyay et al.

Pt + Cl3– + Cl – = PtCl24 –

E5 = 0.63 V E6 = 0.69 V

PtCl24 – + Cl2 (aq) = PtCl26 – Pd + Cl2 (aq) + 2Cl – = PdCl24 –


E7 = 0.62 V E8 = 0.74 V

(5) (6) (7) (8)


Leaching of platinum group metals from spent automotive catalyst

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Figure 1. Schematic diagram of the separate leaching reactor.

80

E9 = 0.80 V

Rh + 3/2Cl2 (aq) + 3Cl – = RhCl36 – Rh + 3/2Cl3– + 3/2Cl – = RhCl36 –

(9)

E10 = 0.92 V

(10)

E11 = 0.98 V

(11)

The Cl3 – (Equation (13)) has a slightly higher oxidizing power29 than aqueous chlorine, Cl2 (aq) (Equation (12)) towards PGMs, which is evident from its higher cell voltage, E (Equations (6), (9), and (11)), than aqueous chlorine, Cl2 (aq) (Equations (5), (8), and (10)). −

−

Cl2(aq) + 2e = 2Cl

− − Cl− 3(aq) + 2e = 3Cl

E = 1.36 VSHE

(12)

Eo = 1.42 VSHE

(13)

o

Without electrogenerated Cl2 70

With electrogenerated Cl2

60 Pt leached (%)

Pd + Cl3– + Cl – = PdCl24 –

50 40 30 20 10

Leaching of PGMs To optimize the process parameters for the dissolution of PGMs from the spent autocatalyst, leaching studies were carried out under varying HCl concentration, current density, leaching temperature and time, agitation speed, and pulp density. The results of the studies are described below. Leaching of Pt in the presence and absence of electro-generated chlorine Initially, the leaching of the sample was made in HCl solution (6.0 mol L−1 ) at 20 g L−1 pulp density and 700 rpm for 4 h in the presence and absence of electro-generated chlorine at different temperatures ranging from 298–363 K. The results (Fig. 2) show an increase in Pt leaching with temperature, and a low recovery of Pt (22%) was obtained even at 363 K in the absence of chlorine gas. Subsequent studies carried out in the presence of electrogenerated chlorine obtained using a current of 2 A, i.e. current density (714 A m−2 ) showed a remarkable increase in Pt dissolution (71%) compared with conventional HCl leaching at 363 K due to the presence of the highly oxidizing chlorine in solution.


300

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360

370

Temperature (K) Figure 2. Significance of electro-generated chlorine in platinum leaching.

(6.0 mol L−1 ) at a current density of 714 A m−2 at 363 K for 1–5 h. As shown in Fig. 3, Pt leaching increased with increasing agitation speed from 500 to 700 rpm; thereafter, at speeds higher than 700 rpm for 5 h it remained constant. Therefore, 700 rpm is sufficient to minimize the external diffusion of the ions (metals and leachant) during leaching. Effect of leaching time The leaching of PGMs was studied over time at 363 K under the process conditions: HCl solution (6.0 mol L−1 ); current density (714 A m−2 ); agitation speed (700 rpm), and pulp density (20 g L−1 ). The results (Fig. 4) show that leaching of PGMs increased with time; 55–71% for Pt, 42–68% for Pd and 32–60% for Rh with maximum leaching obtained after 5 h. After this time the leaching rates of Pd and Rh were almost constant, but Pt showed a slightly higher leaching rate with increased time. It has been reported15,30 that part of the PGM might be oxidized to oxides by the exhaust



1993

Effect of agitation speed Leaching experiments were conducted to investigate the effect of agitation speed over the range 500–900 rpm in HCl solution

0 290

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A. K. Upadhyay et al. 80

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60 Pt leached (%)

Pt leached (%)

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40 500 rpm 600 rpm 700 rpm 800 rpm 900 rpm

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50 4.0 mol/L 5.0 mol/L 6.0 mol/L 7.0 mol/L 8.0 mol/L

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30

0 0

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150 200 Time (min)

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20 0

Figure 3. Effect of agitation speed on the leaching of platinum over time (HCl = 6.0 mol L−1 ; current density = 714 A m−2 ; temperature = 363 K; pulp density = 20 g L−1 ).

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Time (min) Figure 5. Effect of HCl concentration on the leaching of platinum with time (current density = 714 A m−2 ; temperature = 363 K; pulp density = 20 g L−1 , agitation speed = 700 rpm).

80 Pt Pd Rh

PGM leached (%)

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60

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20 0

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Time (min) Figure 4. Effect of time on the leaching of PGMs (HCl = 6.0 mol L−1 ; current density = 714 A m−2 ; temperature = 363 K; pulp density = 20 g L−1 , agitation speed = 700 rpm).

emission gas at high temperatures (>873 K) and the leaching of the oxides, PtO, PdO and Rh2 O3 is more difficult than the leaching of the metals. The more reactive metals, Pd or Rh are oxidized more easily than Pt, resulting in their lower leaching rate. This phenomenon has also been observed in other research.19

1994

Effect of HCl concentration The PGMs are dissolved as anionic chloro-complexes in chloride media, i.e. PtCl6 2– , PdCl4 2– and RhCl6 3– (Equations (5) − (11)), and their stability also increases with increase in chloride ion concentration. High chloride ion concentration also improves the dissolution and efficiency of oxidization of the metals. In additional,


low concentration of chloride ion may promote the transformation of Pt(IV) complexes from PtCl6 2– to PtClx (H2 O)6–x (x–2)– , PdCl4 2– to PdClx (H2 O)4–x (x–2)– and RhCl6 3– to RhClx (H2 O)6–x (x–3)– . The aqua complexes of these metals reduce the formation of negatively charged chloro-complexes and decrease the oxidization behavior of metals.31 Therefore, a high concentration of hydrochloric acid (>3 mol L−1 ) is necessary to obtain the maximum recovery of PGMs. Because acid concentration plays a measurable role in the recovery of PGMs, the leaching of Pt as a function of lixiviant concentration in the range 4.0 − 8.0 mol L –1 HCl for different times was studied at 363 K, pulp density 20 g L−1 , and electrogenerated chlorine at 2.0 A current (current density 714 A m−2 ). The leaching of Pt (Fig. 5) increased with time and acid concentration. When the acid concentration was increased from 4.0–6.0 mol L−1 , Pt leaching increased from 59 to 71% over 300 min indicating that better platinum dissolution occurs when more concentrated hydrochloric acid is used with a high concentration of chlorine in solution.27 However, this leaching rate of Pt stayed almost constant at 71 and 72% for 6.0 and 8.0 mol L−1 HCl, respectively. Therefore, to minimize cost, the concentration of lixiviant was selected as 6.0 mol L−1 for further experiments. Effect of current density The leaching of PGMs was studied under varying current density in the range 357 A m−2 (1 A current) to 714 A m−2 (2 A current) while maintaining other process parameters at HCl 6.0 mol L−1 ; 363 K; pulp density 20 g L−1 , and agitation speed 700 rpm. The results (Fig. 6(a), (b) and (c)) show an increase in leaching of Pt, Pd and Rh, respectively, with current density as the higher current densities generate larger quantities of chlorine. For example, Pt leaching increased from 54–71%. The dissolution of metals was also found to increase with time and reached almost zero leaching rates after 5 h at different current densities. The maximum leaching of Pt, Pd and Rh was 71, 68 and 60%, respectively, after 5 h at the high current density (714 A m−2 ).



Leaching of platinum group metals from spent automotive catalyst (a) 75

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357 A/m2 (I = 1.0A) 534 A/m2 (I = 1.5A) 714 A/m2 (I = 2.0A)

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65

65

60 Pd leached (%)

Pt leached (%)

357 A/m2 (I = 1.0A) 534 A/m2 (I = 1.5A) 714 A/m2 (I = 2.0A)

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(c) 70 357 A/m2 (I = 1.0A) 534 A/m2 (I = 1.5A) 714 A/m2 (I = 2.0A)

Rh leached (%)

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Figure 6. Leaching of PGMs at different current densities (a) Pt (b) Pd (c) Rh (HCl = 6.0 mol L−1 ; temperature = 363 K; pulp density = 20 g L−1 , agitation speed = 700 rpm).


Effect of pulp density Pulp density is an important factor in the leaching process. Four solid/liquid (S/L) ratios were studied, in the range 10–30 g L−1 (wt/vol) for leaching in HCl (6.0 mol L−1 ) at 363 K with an applied current density of 714 A m−2 . It was found that leaching increased from 62–73% when the pulp density was decreased from 30–10 g L−1 indicating a faster leaching rate (Fig. 8). The leaching of Pt increased from 51–71% as time was increased from 1–5 h at a pulp density of 20 g L−1 . Above this pulp density the percentage leaching of Pt decreased sharply after 5 h. Therefore, the pulp density 20 g L−1 was considered to be the optimum value for Pt leaching. Similar dissolution behavior of Pd and Rh was obtained under the above conditions. Kinetic models for leaching of PGMs An attempt was made to study the dissolution kinetics of Pt, Pd and Rh during leaching using the shrinking core model, which



1995

Effect of leaching temperature Figure 7 shows the leaching from spent autocatalyst in 6.0 mol L−1 HCl solution as a function of temperature at an applied current density 714 A m−2 , agitation speed 700 rpm and time 5 h. It can be seen that leaching increased with increased temperature from 298–363 K. The percentage leached at 298 K was Pt = 23%, Pd = 20% and Rh = 18%, and increased to 71%, 68% and 60%, respectively, at 363 K after 5 h. The effect of temperature on the leaching was substantial, therefore, 363 K was chosen as the optimum temperature for further leaching experiments under atmospheric conditions. Although the solubility of aqueous chlorine is reported to decrease with increasing temperature,27 the increase in PGMs leaching at higher temperature may be attributed to enhancement in the diffusivity of chlorine/other chloride ions and substantial increase in the leaching rate of PGMs chloro-complexes in HCl media due to the fast kinetics and the high mobility of chlorine.

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80 Table 1. Rate constants of PGM leaching by mass transfer model: 1 − 3(1−X)2/3 + 2(1−X) = kt

Rh Pd Pt

70

PGM leached (%)

60

50

40

Pt

Pd

Rh

(min−1 )

(min−1 )

(min−1 ) ×

Temp k (K) × 10−4

R2

k

298 323 348 363

0.99 0.98 0.98 0.97

1.12 2.25 7.07 6.65

× 10−4 1.65 3.08 7.24 8.24

k

10−4

R2 0.96 0.95 0.96 0.97

R2

1.02 3.28 4.94 6.63

0.96 0.96 0.99 0.99

30 100

20

10 290

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370 80

Figure 7. Effect of temperature on the leaching of PGMs over time (HCl = 6.0 mol L−1 ; current density = 714 A m−2 ; pulp density = 20 g L−1 , agitation speed = 700 rpm).

80

Pt leached (%)

Temperature (K)

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60 Pt leached (%)

Reduction time 30 min 60 min 120 min 180 min 240 min 300 min 0

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150 200 Time (min)

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300

350

Figure 9. Effect of reduction time with formic acid (FA) on the leaching of platinum (HCl = 6.0 mol L−1 ; current density = 714 A m−2 ; temperature = 363 K; pulp density = 20 g L−1 , agitation speed = 700 rpm).

40 10g/L 15g/L 20g/L 30g/L

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350

Time (min) Figure 8. Effect of pulp density on the leaching of platinum over time (HCl = 6.0 mol L−1 ; current density = 714 A m−2 ; temperature = 363 K; agitation speed = 700 rpm).

Pt leached (%)

0

80

70

60

is the most widely used model describing fluid–solid reaction kinetics of dense particles. After examining the experimental results in comparison with various kinetic models, it was found that the kinetics of Pt, Pd and Rh followed the ash diffusion control model (Equation (14)) with maximum linearity and correlation coefficients. 1 − 3 (1 − X)2/3 + 2 (1 − X) = kt

(14)

1996

where X is the fraction of PGMs reacted in time t and k is the apparent rate constant.


10% FA 15% FA 20% FA 25% FA

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40 0

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150 200 Time (min)

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Figure 10. Effect of amount of formic acid (FA) on the leaching of platinum (HCl = 6.0 mol L−1 ; current density = 714 A m−2 ; temperature = 363 K; pulp density = 20 g L−1 , agitation speed = 700 rpm).



Leaching of platinum group metals from spent automotive catalyst

www.soci.org (b) 100

(a)

298 K without FA 298 K with FA 363 K without FA 363 K with FA

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80

Pd leached (%)

Pt leached (%)

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Rh leached (%)

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Figure 11. Impact of formic acid (FA) on the leaching of PGMs (a) Pt (b) Pd (c) Rh (HCl = 6.0 mol L−1 ; current density = 714 A m−2 ; temperature = 363 K; pulp density = 20 g L−1 , agitation speed = 700 rpm).

The reaction rate constant (k) and the correlation coefficient (R2 ) were calculated as a function of temperature (Table 1). Subsequently, the activation energies calculated from the Arrhenius plot in the temperature range 298–363 K were found to be Pt = 29.6, Pd = 26.4 and Rh = 20.6 kJ mol−1 . These results indicate the reaction is controlled by diffusion of ions through the highly porous alumina wash-coat surface of the autocatalyst. A small amount of alumina leached from the washcoat remain in solution, and can be recovered during subsequent hydrometallurgical processing.


PtO + HCOOH → Pt + H2 O + CO2

(15)

PdO + HCOOH → Pd + H2 O + CO2

(16)

Rh2 O3 + 3HCOOH → 2Rh + 3H2 O + 3CO2

(17)

Different process conditions such as reduction time, formic acid concentration, and temperature were studied to examine the



1997

Leaching of PGMs after treatment with formic acid Generally, PGMs are impregnated in the wash-coat in the form of hexachloroplatinic(IV) acid (H2 PtCl6 .6H2 O), palladium chloride (PdCl2 ) and rhodium chloride (RhCl3 ). Then, they are reduced to their metallic form by heat treatment. Leaching of PGM metal

oxides is more difficult than leaching of the metals. Therefore, pretreatment of any PGM oxides present in the used catalyst is necessary to reduce them to the metal. Formic acid (FA) was used as a reducing agent for the pretreatment of PGMs to obtain high recovery of precious metals. Formic acid reduces and stabilizes the PGMs in their metallic form, as shown in Equations (15) − (17): however, it should be used carefully because formic acid can react in the presence of PGMs to form hydrogen which may creates a potential explosion risk.32,33

www.soci.org reduction of the PGM oxides in the spent catalysts, and then leaching studies were carried out under the above mentioned conditions on a sample after formic acid treatment. The results of the study are described below. Effect of reduction time The effect of time from 30–300 min on PGM oxide reduction with 20% formic acid was studied under the optimized leaching conditions of 363 K, current density 714 A m −2 , HCl 6.0 mol L−1 and 5 h. It can be seen that Pt leaching increased from 71–97% with increasing reduction time from 30–240 min (Fig. 9) but the percentage remained approximately constant when the reduction time was increased from 240–300 min. Therefore, 240 min was selected as the optimized reduction time with formic acid for subsequent leaching experiments. Similar leaching behavior was also observed for Pd and Rh. Effect of formic acid concentration Different concentrations of formic acid (from 10–25%) were used to reduce the PGMs. The leaching efficiencies of Pt increased continuously with increasing formic acid concentration, from 10–20% for 240 min under the previously optimized leaching conditions (HCl 6.0 mol L−1 , current density 714 A m−2 , 363 K, agitation speed 700 rpm and time 1–5 h). A Pt recovery of 97% was observed with 20% formic acid concentration (Fig. 10), but above this acid concentration, no significant enhancement occurred with similar behavior observed for Pd and Rh. Effect of temperature The spent catalyst PGMs were reduced with 20% formic acid for 240 min at room temperature, and then the leaching behavior of these samples was observed at different temperatures under optimized experimental conditions. Figure 11(a)–(c) shows the role of formic acid in the leaching process, where Pt leaching increased from 21–38% at 298 K in the reduced sample compared with the unreduced PGM sample. At 363 K, the leaching yield of platinum improved from 71–97% (Fig. 11(a)). A similar leaching pattern was observed for the other two metals with an increase in leaching from 26–42% for Pd and from 23–37% for Rh at 298 K under the optimized leaching conditions (Fig. 11(b) and (c)). After pretreatment with formic acid the percentage leaching was found to improve from 60–94% for Pd, and from 58–90% for Rh at 363 K. This result is especially impressive for Rh, because the percentage of Rh leached from the scrapped autocatalyst by conventional means did not exceed 85%. These results show the impact of formic acid and temperature on the PGM leaching process.

CONCLUSIONS

1998

The leaching behavior of PGMs (Pt, Pd and Rh) from spent autocatalyst was studied using electro-generated chlorine. The effects of the process parameters HCl concentration, agitation speed, leaching time, temperature, and pulp density on the dissolution behaviour of PGMs were studied. Leach percentages of Pt = 71%, Pd = 68% and Rh = 60% were obtained under the optimized conditions of HCl (6.0 mol L−1 ), current density (714 A m−2 ), temperature (363 K), pulp density (20 g L−1 ) and agitation speed (700 rpm) with the dissolution kinetics following ash diffusion control. The activation energies of Pt, Pd and Rh calculated by an Arrhenius plot were 29.6, 26.4 and 20.6 kJ mol−1 ,



respectively, in the temperature range 298–363 K. Furthermore, to improve the dissolution of PGMs by reduction of any oxide particles, (PtO, PdO, and Rh2 O3 ) to the metal, the leaching of the spent catalyst was carried out after pretreatment with 20% formic acid for 240 min at room temperature. The studies showed an increase in leaching efficiencies to Pt = 97%, Pd = 94% and Rh = 90% after 5 h leaching under optimum leaching conditions. Any alumina leached from the wash-coat can be recovered during subsequent hydrometallurgical processing of the leach solution. The proposed leaching system is eco-friendly with high leaching ability of PGMs from the spent autocatalyst.

ACKNOWLEDGEMENTS This study was supported by the R&D Center for Valuable Recycling (Global-Top Environmental Technology Development Program) funded by the Ministry of Environment (Project No.: GT-11-C-01200-0). The authors are very grateful to Professor Michael Cox, Editor of the Journal of Chemical Technology and Biotechnology, for his constructive comments for the improvement of manuscript.

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