Eco-threat Minimization in HCl Leaching of PGMs from Spent

Jun 24, 2017 - Keywords: Automobiles catalytic converter; HCl leaching; HCOOH prereduction; Platinum group metals; Recycling; Sustainability; X-ray ph...
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Research Article pubs.acs.org/journal/ascecg

Eco-threat Minimization in HCl Leaching of PGMs from Spent Automobile Catalysts by Formic Acid Prereduction Ha Bich Trinh,†,‡ Jae-chun Lee,†,‡ Rajiv R. Srivastava,†,§ Sookyung Kim,*,†,‡ and Sadia Ilyas∥ †

Resources Recycling, Korea University of Science and Technology (UST), 217, Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea ‡ Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), 124, Gwahak-ro, Yuseong-gu, Daejeon 34132, Republic of Korea § Research and Product Development, Tae-Hyung Recycling, Nongnam-ro, Gimcheon-si, Gyeongsangbuk-do 740-872, Republic of Korea ∥ Mineral and Material Chemistry Laboratory, Department of Chemistry, University of Agriculture Faisalabad (UAF), Main Road, Faisalabad, 38040, Pakistan ABSTRACT: Reclamation of spent automobile catalysts via aqueous processing for the efficient recovery of Pt, Pd, and Rh (PGMs) has remained a challenge. In this research, the effect of prereduction by HCOOH on PGMs leaching using mild acid was investigated and compared to the typical leaching procedure that uses concentrated HCl. Prereduced samples with particle sizes of 4.0 M HCl), the prereduction effect on PGMs extraction was negligible compared to leaching without HCOOH prereduction. A detailed study of the influential parameters revealed the optimal prereduction conditions to be HCOOH concentration, 15 vol %; pulp density, 10%; temperature, 60 °C; and time, 1 h. Postreduction leaching in 2.0 M HCl at 90 °C for 2 h yielded > 80% Pt, > 85% Pd, and >62% Rh in leach liquor. A subsequent study on the addition of oxidant during the leaching step further enhanced the extraction efficacy up to ∼95% PGMs by introducing ≥1.5 M NaClO3. The results revealed that a HCOOH prereduction step can significantly minimize the environmental impact and cost of reagents with the maximum yield of PGMs in a less acidic solution. KEYWORDS: Sustainability, Platinum group metals, Recycling, Automobiles catalytic converter, HCOOH prereduction, HCl leaching, X-ray photoelectron spectroscopy

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milling,17 which are major advantages of recycling spent autocatalysts. Currently, reclamation of spent autocatalysts is achieved via high temperature smelting using Cu/Ni/Fe to collect the PGMs alloy.16,18 The concentrated PGMs in the collector-alloy then undergo chemical dissolution and separation to produce pure metals. Generation of a large quantity of slag and the consumption of a significant amount of energy in the smelting process are the major drawbacks.16 To overcome these limitations, hydrometallurgical routes with low calorie inputs have been explored by several researchers. However, the chemical inertness of PGMs and the refractory cordierite substrate limit their dissolution in an ordinary acid environment, and hence, aggressive HCl leaching with a high dosage of oxidizing agents (HNO3/halogens/NaOCl/NaClO3/H2O2) is commonly applied.19−25 The use of concentrated HCl and

INTRODUCTION

Mandated by environmental legislation on the gaseous emission of automobiles, the use of a three-way catalytic converter is essential for transforming (i) CO to CO2, (ii) NOx to N2, and (iii) unburnt HC to CO2 and H2O.1−3 High exposure to gaseous emissions causes serious health problems; thus, emissions need to be regulated within the limits stated in Table 1.4−11 Gaseous emissions are controlled by the catalytic reactivity of platinum group metals (PGMs) in which Pt and Pd mainly catalyze transformation steps (i) and (iii) and Rh performs step (ii).12 The newly imposed “Euro 6” and “Tier 3 vehicle emission and fuel standard program” may further increase PGMs consumption from its current figure of ∼10 million ounces.13−15 In view of the limited natural resources, reclamation of PGMs from spent automobile catalysts is therefore of vital importance.16 Notably, the recovery of hundred-folds more concentrated PGMs from spent automobile catalysts instead of mining primary ores can save ∼41% of the energy costs from ore mining and reduce water consumption by 391.5 m3 for each kilogram of PGMs in ore © 2017 American Chemical Society

Received: May 16, 2017 Revised: June 21, 2017 Published: June 24, 2017 7302

DOI: 10.1021/acssuschemeng.7b01538 ACS Sustainable Chem. Eng. 2017, 5, 7302−7309

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ACS Sustainable Chemistry & Engineering

Table 1. Level of Toxicity and Impact on Human Health of Gaseous Emissions from Motor Vehicles with Their Standard Regulatory Limits toxicity to human health gaseous emission carbon monoxide (CO)

regulation for emissions standard (effective from)

level (0−4)

impact on human health

3

>35 ppm causes headache, dizziness, nausea, convulsions, and loss of senses >800 ppm

a

Tier 3 (2017) Euro 6 (2014) c Japan (2009)

2.61 1.0 1.15

>20 ppm causes irritation, bronchitis, and pneumonia; >100 ppm leads to death due to methemoglobin formed in blood

a

0.04 0.06 0.05

HCs attack the liver, kidneys, lungs, and heart; >5 ppm benzene causes chromosomal and neural defects

a

b

nitrogen oxides (NOx)

hydrocarbons (HCs; C6H6, toluene, etc.)

3

2

limit (g/km)

Tier 3 (2017) Euro 6 (2014) c Japan (2009) b

Tier 3 (2017) Euro 6 (2014) c Japan (2009) b

0.056 0.068 0.05

a U.S. emission standards for light duty gasoline vehicles. bEU emission standards for gasoline passenger cars. cJapan emission standards for gasoline and LPG vehicles.

(2Al2O3·2SiO2·5MgO).22 Though the PGMs are resistant to oxidation in air, they have a remarkable ability to adsorb gaseous O2 on their surfaces, and under critical conditions, they can be oxidized.32 The standard Gibbs free energy change vs temperature diagram (Figure 1) reveals that the spontaneous oxidation of Pt, Pd, and Rh occurs at different temperatures.

H2SO4 in the presence of H2O2 recovers >95% of PGMs,23 whereas extraction of 87% Pt, 97% Pd, and 68% Rh can be achieved with a mixture of NaClO3, H2O2, and HCl.20 The standard potentials of PGMs dissolution in chloride media can be reduced to negative values via the formation of chlorocomplexes, which requires an oxidant with a reduction potential >0.74 V.12 The use of aqua regia (8 M HCl + 3.5 M HNO3) yields a lower leaching efficiency of Rh (83%)19 than chlorine leaching (gaseous Cl2 through 6 M HCl), which has an efficacy of ∼90% Pt and Rh.22 Moreover, such a process creates acute environmental problems due to the formation of toxic and hazardous gaseous products (NOCl, Cl2, and NOx), and a large amount of unconsumed acid and oxidants are generated in the effluent after the downstream processing of the leach liquor.26 In addition to acidic media, cyanidation pressure leaching has also been explored.27,28 From a high pulp density (PD) sample charge of 50% in an autoclave above 160 °C and ∼200 psi pressure, a 1 vol % sodium cyanide solution yielded 97% Pd, 94% Pt, and 98% Rh.28 The environmental problems associated with the toxicity of free cyanide are again a drawback of this process; note that an exposure >500 μg/L cyanide is unsafe.29 The World Health Organization (WHO) has established limits of 20−100 mg/day of chloride, 10 mg/L of nitrate, and 3 mg/L of maximum nitrite uptake through drinking water.30,31 Higher levels may cause severe health problems (gastrointestinal irritation, methemoglobinemia, and stomach cancer).26 This provides the motivation of the present study to develop a sustainable process by reducing the acid consumption while applying a less aggressive/biodegradable reagent, HCOOH. We report herein a simple formic acid prereduction followed by HCl leaching to extract PGMs from three-way spent automobile catalysts. The effect of prereduction on the efficacy of PGMs extraction has been determined by comparison to the leaching of samples in HCl without any prereduction. Accordingly, the influence of various parameters on prereduction and leaching has been investigated, including the concentration of reagents, particle size, temperature, time, pulp density (PD), and the oxidant dosage for yielding the maximum amount of PGMs in the leach liquor.

Figure 1. Change in the standard Gibbs free energy (ΔG°) for the formation of PGM-oxides as a function of temperature (°C).

As stated elsewhere, the formation of PdO and Rh2O3 takes place at 800−840 and 600 °C, respectively.33 Achieving such a high temperature during operation (that can reach 1150 °C)12 is possible inside the catalytic converter, which leads to transformation of a portion of the PGMs to their oxides depending on their service life. Unlike PtO2, which readily converts to their metallic form at 380−400 °C,33 Rh2O3 forms a protective layer on the surface over time, which restricts the dissolution of PGMs in acid leaching.21 Under such conditions, a pretreatment step (prior to leaching) that reduces the PGMoxides to the elemental forms becomes essential for achieving a high extraction yield from spent catalysts. Though the hydrogen reduction is commonly examined,23 the associated risk in handling and the high temperature requirement (sometimes with pressure) are well-known problems of this system. In contrast, simple handling with biodegradable characteristics presents formic acid as a suitable reducing

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THERMODYNAMICS OF REACTIONS In a three-way catalytic converter, PGMs are contained in a highly porous γ-alumina wash-coat on the surface of cordierite 7303

DOI: 10.1021/acssuschemeng.7b01538 ACS Sustainable Chem. Eng. 2017, 5, 7302−7309

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Methods. All of the leaching experiments were carried out in a 200 mL Pyrex glass reactor fitted with a water bath (Lab. Companion CW10G). A fixed agitation of 250 rpm was provided by magnetic paddle stirring for proper mixing of the solid and liquid phases. A known amount of ground sample (to maintain the desired pulp density of 10%) was charged at a fixed temperature under stirring. After completion of the prereduction, a solid−liquid separation was completed using a Buckner funnel under vacuum. Thus, collected mass was subjected to HCl leaching under conditions mentioned separately in the manuscript using the same experimental setup as described for the prereduction step. To determine the effect of formic acid treatment (with different concentration) on PGMs, X-ray photoelectron spectroscopy was performed on the untreated and pretreated samples using a KRATOS AXIS NOVA instrument (performed under the conditions: power 150 W, X-ray source monochromatic Al Kα, vacuum 10−9 to 10−8 Torr, Al anode voltage 15 keV). Furthermore, to understand the direct influence of prereduction on the leaching yield of PGMs, experiments were also performed without formic acid prereduction in which a sample of a particular size fraction was directly charged into an HCl solution of a desired concentration. After solid−liquid separation, the leach liquor was analyzed for the PGMs content. Inductively coupled plasmaatomic emission spectroscopy (ICP-AES; Model: JY-38 plus, JobinYvon Ltd.) was used for the analysis purpose. The percentage extraction of PGMs in each step was calculated as

agent. Possible reactions involved in the prereduction of PGMs are given below.25 PtO2 + 2HCOOH → Pt + 2H 2O + 2CO2 ° = −110.7kcal/mol ΔG298K

(1)

PdO + HCOOH → Pd + H 2O + CO2 ° = −45.1kcal/mol ΔG298K

(2)

Rh 2O3 + 3HCOOH → 2Rh + 3H 2O + 3CO2 ° = −129.0kcal/mol ΔG298K

(3)

PGMs, being noble metals, are difficult to leach, and their efficient recovery mainly depends on lowering the redox potential of ions in the presence of Cl− ions to form stable aqua chloro-complexes of PGMs.12,20,21 Being a strong acid, HCl completely dissociates into34 HCl ↔ H+ + Cl−

° = − 29.6kcal/mol ΔG298K

(4)

PGMs form multiple chloro-complexes depending on the chloride concentration in aqueous solution:24,35 Pt + 4Cl− ↔ PtCl24 − + 2e−

E 0 = −0.75V

PtCl24 − + 2Cl− ↔ PtCl 62 − + 2e−

Pd + 4Cl− ↔ PdCl24 − + 2e−

Rh + 6Cl− ↔ RhCl 36 − + 3e−

E 0 = −0.77V

(6)

E 0 = −0.59V

PdCl24 − + 2Cl− ↔ PdCl 62 − + 2e−

⎛ M − Mf ⎞ %Leaching = ⎜ i ⎟ × 100 ⎝ Mi ⎠

(5)

where Mi and Mf are the metal mass in the initial feed sample and in the leach liquor (after leaching at a given time), respectively.

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(7)

E 0 = −1.26V

RESULTS AND DISCUSSION Influence of HCl Concentration and Particle Size on PGMs Extraction. An increase in the chloride concentration under an acidic environment increases the stability of chlorocomplexes of PGMs with the consequent enhancement in their dissolution. Hence, the influence of HCl concentration on leaching behavior of PGMs was primarily investigated with all the particle size of samples (without HCOOH prereduction) under the invariable leaching condition of 10% PD, 90 °C temperature, and 2 h duration. The experimental results presented in Figure 2a−c clearly demonstrate the significance of HCl concentration on PGMs extraction. As shown in Figure 2a, a 23.5% leaching efficiency of Pt could be enhanced to the maximum 88.2% by increasing the HCl concentration from 1.0 to 8.0 M in the lixiviant by using the sample of particle size Rh. A greater degree of Pd leaching can be ascribed to the differences between the standard potentials (eqs 5−9) for chloro-complex formation of the PGMs despite PdO having the least negative value of free energy for reduction with formic acid (eqs 1−3). Although the standard potential of Rh is more favorable for RhCl63− formation, the lower leaching efficiency 7307

DOI: 10.1021/acssuschemeng.7b01538 ACS Sustainable Chem. Eng. 2017, 5, 7302−7309

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ACS Sustainable Chemistry & Engineering Pt + 2Cl 2 + 2Cl− → PtCl 62 −

can be attributed to the more refractory nature of Rh which hinders the leaching in mild acid (2.0 M HCl) solution. Effect of Adding Oxidant in Leaching Step. The effect of prereduction (at HCOOH concentration 15 vol %, PD 10%, temperature 60 °C, and time 1 h) on the leaching of PGMs in HCl (at 2.0 M HCl, PD 10%, temperature 90 °C, and time 2 h) was successfully optimized to reduce the consumption of acid and additional reagents. Nevertheless, the yield of PGMs was lower than the level that is considered to be satisfactory (>90%). Since the commercial values of PGMs are much higher, to further maximize the efficiency of PGMs extraction the effect of adding a suitable chloro-oxidant, NaClO3, was investigated. For this study, the dosage of oxidant was varied between 0.5−2.0 M NaClO3, while the optimized parameters for leaching were kept unchanged. The leaching efficiencies obtained as a function of NaClO3 concentration are presented in Table 3. An increasing trend in the extraction of PGMs was

(14)

Pt + 2Cl−3 → PtCl 62 −

entry

oxidant added

Pt, %

Pd, %

Rh, %

a b c d e f g

without reduction with HCOOH reduction with HCOOH reduction with HCOOH reduction with HCOOH reduction with HCOOH reduction without reduction

no addition no addition 0.5 M NaClO3 1.0 M NaClO3 1.5 M NaClO3 2.0 M NaClO3 2.0 M NaClO3

42.5 81.3 87.2 90.8 94.9 95.1 70.6

62.2 86.6 90.7 92.3 94.6 94.9 65.2

38.8 62.1 85.7 93.4 95.1 95.2 45.4

Pd + Cl−3 + Cl− → PdCl24 −

° = −38.1kcal/mol ΔG298K ° = −133.7kcal/mol ΔG298K (17)

Rh + 3/2Cl 2 + 3Cl− → RhCl 36 − ° = −62.2kcal/mol ΔG298K

(18)

Rh + 3/2Cl−3 + 3/2Cl− → RhCl 36 − ° = −205.5kcal/mol ΔG298K

(19)

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CONCLUSIONS The hydrometallurgical process described herein demonstrated the role of HCOOH prereduction in the enhancement of Pt, Pd, and Rh leaching in a lower concentration HCl solution. The prereduction of PGMs was found to increase the leaching efficacy up to 60 °C, after which a slight decrease in PGMs extraction attributed to thermal decomposition of formic acid (>60 °C) was observed. The least improvement in the PGMs leaching at lower HCl concentration was observed for larger size particles (0.59−1.0 mm). The pretreatment conditions for particles >0.212 mm were determined to be HCOOH, 15 vol %; PD, 10%; temperature, 60 °C; and time, 1 h followed by leaching with 2.0 M HCl at 90 °C for 2 h; the optimized yield was 81% Pt, 87% Pd, and 62% Rh in the leach liquor. An improved extraction efficiency of approximately 95% PGMs was successfully achieved by introducing ≥1.5 M NaClO3 as a suitable oxidant during the leaching step. The enhanced leaching in the presence of an oxidant was mainly observed for Rh with an increase of up to 33%, which may be attributed to the significant contribution of in situ formation of soluble species of chlorine in the HCl−NaClO3 system. Thus, the formic acid prereduction followed by leaching in the presence of a lower oxidant concentration could substantially reduce HCl consumption presents an ideal and sustainable reclamation process for the extraction of PGMs from spent automobile catalysts.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +82 42 868 3418. ORCID

Sookyung Kim: 0000-0003-4048-5690 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the R&D Center for Valuable Recycling (Global-Top R&BD Program), Ministry of Environment, Republic of Korea (Project No. 2016002250004). The authors thank Mr. Bae Muki (UST-KIGAM) for his support with the XPS analysis of the samples.

° = −28.4kcal/mol ΔG298K (12)

NaClO + 2HCl → NaCl + Cl 2 + H 2O ° = −69.6kcal/mol ΔG298K

(15)

(16)

observed upon raising the concentration of NaClO3 to 1.5 M, after which the effect was negligible. As seen, the contribution of 0.5 M NaClO3 to the extraction of Pt and Pd was only 5.9% and 4.6%, respectively, albeit the Rh extraction was remarkably enhanced (23.6%). A further addition of NaClO3 (≥1.5 M) led to a total PGMs extraction of ∼95%, with approximate contributions of 8% Pd, 14% Pt, and 33% Rh during leaching in a mild acid (2.0 M HCl) environment. Moreover, for getting a clear effect of the prereduction step in leaching with lower acid concentration, the sample without formic acid reduction was also leached in the presence of 2.0 M NaClO3. The obtained results for PGMs extraction (Table 3g; 70.6% Pt, 65.2% Pd, and 45.4% Rh) was lower than the leaching yields obtained with the prereduced samples either in presence or absence of the oxidant (Table 3). The extraction of PGMs in the presence of HCl and oxidizing agents can be correlated to the in situ evolution of gaseous chlorine, Cl2(g), which easily dissolves in HCl solution as Cl2(aq), and Cl−3(aq).43 As reported elsewhere, the distribution of Cl−3(aq) increases with ≥2.0 M HCl,44 which provides a highly oxidizing atmosphere that accelerates the formation of aqua chloro-complexes of PGMs, albeit in a comparatively less acidic solution. The plausible reactions include18,20,25 NaClO3 → NaClO + O2

° = −247.1kcal/mol ΔG298K

Pd + Cl 2 + 2Cl− → PdCl24 −

Table 3. Comparative Yield of PGMs from Ground Sample (particle size −0.212 mm) as a Function of Oxidant Dosage Added during (2.0 M) HCl Leaching types of sample used in HCl leaching

° = − 55.9kcal/mol ΔG298K

(13) 7308

DOI: 10.1021/acssuschemeng.7b01538 ACS Sustainable Chem. Eng. 2017, 5, 7302−7309

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DOI: 10.1021/acssuschemeng.7b01538 ACS Sustainable Chem. Eng. 2017, 5, 7302−7309