Recovery by hydrometallurgical extraction of the platinum-group metals from car catalytic converters

Minerals Engineering 24 (2011) 505–513

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Recovery by hydrometallurgical extraction of the platinum-group metals from car catalytic converters D. Jimenez de Aberasturi, R. Pinedo, I. Ruiz de Larramendi, J.I. Ruiz de Larramendi, T. Rojo ⇑ Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco UPV/EHU, Apdo.644, 48080 Bilbao, Spain

a r t i c l e

i n f o

Article history: Available online 8 January 2011 Keywords: Precious metals Hydrometallurgy Recovery Car catalytic converters

a b s t r a c t The car industry is one of the technological applications which more platinum-group metals (PGM) employs. Therefore, the recovery of the PGMs from the car catalytic converters could be an important source to obtain these precious metals, with economic and environmental consequences. In this work, the car catalytic converters were characterized through different techniques as X-ray diffraction, scanning electron microscopy and inductively coupled plasma. In order to suggest an environmentally friendly method for the recovery of PGMs, some conditions such as the reagents concentration and composition, energy optimization, pre-treatment in hydrogen atmosphere and a thermal pre-treatment were studied. In addition, a new alternative to recover at least the 95% of the PGMs present in the car catalytic converters by the application of lees aggressive, corrosive or expensive reagents and conditions is proposed. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction In order to comply with the European Standard and Directive 94/12/EEC, on ambient air quality, all the motor vehicles produced since 1993 must be fitted with catalytic converters, which reduce the carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxide (NOx) emissions below the legislated levels. These catalytic converters contain precious metals such as platinum, palladium and rhodium, which are the main active components in the oxidation of carbon monoxide (CO) and hydrocarbons (HC), and the reduction of nitrogen oxides (NOx) into nitrogen (N2). The most common car catalytic converters are the honeycomb type cordierite skeleton (2MgO2Al2O35SiO2), with a density of 60–120 cells/cm2 and walls between cells of about 150 lm thickness (Cuif et al.,1997; Lucena et al., 1999). So as to increase the contact area between the catalytic converter and the car exhaust fumes, the inner surface of the cells is rough and it is coated with a highly porous washcoat of 50–200 lm formed of about 90% c-Al2O3 and a mixture of base metals additives, mainly oxides of Ce, Zr, La, Ni, Fe and alkaline-earths. These oxides improve the stabilization of the catalytic converter (Palacios et al., 2000). The platinum-group metals (PGMs) are located in the surface where they are exposed to the gases. Generally these PGMs are fixed in the washcoat surface by impregnation or coating from a solution of hexachloroplatinic (IV) acid (H2PtCl66H2O), palladium chloride

⇑ Corresponding author. Tel.: +34 94 6012458; fax: +34 94 6013500. E-mail address: [email protected] (T. Rojo). 0892-6875/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2010.12.009

(PdCl2) and rhodium chloride (RhCl3). Then they are reduced to their metallic form (Moldovan et al., 1999). Although there are a wide variety of possible combinations of these metals in the catalytic converters, in all the cases the total content of PGMs is always lower than 0.1%. As a result of this measure and to reduce the pollution caused by the automobiles, nowadays the car industry has become in one of the technological applications which more PGMs employs. However, the use of PGMs has also some disadvantages such as the high price, the non-renewability of these metals and the high environmental impact caused in their extraction. Due to scarcity and high value of these metals, there is an increasing interest towards their recovery from wastes, such as spent catalyst, with economic and environmental consequences (Angelidis and Skouraki, 1996; Nowottny et al., 1997; Baghalha et al. 2009). Recycling automotive catalysts to recover their PGMs has received much attention in recent years and several works have been dedicated to the recovery of metals using different methods (Kim et al., 2000; Faisal et al., 2008; Fontas et al., 2002). In the typical PGMs recovery process, PGMs are extracted from scraps by smelting them with collector metals such as copper, or dissolving the scraps in strong acids (Kayanuma et al., 2004). The extraction methods can be classified in three groups: hydrometallurgical, volatilization and pirometallurgical methods (Mishra, 1993). The method we have proposed belongs to the hydrometallurgical methods group (Gupta and Mukherjee, 1990). The applied hydrometallurgical processes are based on the selective dissolution of the washcoat (Angelidis, 2001) where the

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PGMs are dispersed. The problem emerged with this method is that precious metals are hardly dissolved in ordinary acids because of their chemical inertness. Therefore, a large amount of strong acid with strong oxidant is required for the dissolution and an equally large amount of waste water containing heavy metals and strong acid is generated. Moreover, several hours are required for the complete dissolution of PGMs. These facts indicate that it is important to develop an efficient dissolution process for the recovery of PGMs from automotive catalyst scrap. The effort of this study is to recover the PGMs present in used car catalytic converters by the application of less aggressive and corrosive reagents, optimizing the required energy in the process. First, the chemical compositions of the catalysts were obtained by X-ray powder diffraction and field emission scanning electron microscope (FESEM). After verified the presence and distribution of PGMs in the samples, a hydrometallurgical extraction method was studied to recover those PGMs.

Table 2 Crystallographic parameters of the different phases present in the catalyst used in this study. Phase

Space group

Spatial group

Al2O3 CeO2 Cordierite MgO ZrO2 La(AlO3) Ce(AlO3) La2O3 CeO2–ZrO2

Orthorhombic Cubic Orthorhombic Cubic Orthorhombic Triclinic Tetragonal Hexagonal Cubic

P n a 21 F m 3 m C ccm F m 3 m Pnam R 3 c I 4/m c m P 63/m m c F m 3 m

2.2.2. Morphological characterization The microstructure of the catalyst was examined using a field emission scanning electron microscope (FESEM) JEOL JMS-7000-F equipped with an EDX detector (Oxford Instruments INCA Energy 350). Samples for SEM observation were catalysts scraps of 5  5  10 mm encapsulated in an epoxy resin.

2. Experimental 2.1. Materials

2.3. Procedure

Different types of catalysts were used in this study (see Table 1). All of them are honeycomb monolith supports which are formed by a cylindrical body traversed by a multitude of straight channels and coated with a washcoat onto which the PGMs are adsorbed.

The effort of this study is to obtain a new and environmentally friendly hydrometallurgical method, to recover at least the 95% of the PGMs present in the car catalytic converters (Bonucci and Parker, 1984) by the application of less aggressive and corrosive reagents and optimizing the required energy in the process. PGMs in general are very resistant to acid dissolutions. For solubilisation of platinum, palladium and rhodium metals from catalysts in aqueous chloride media forming corresponding chloro2 2 3 complexes (PtCl6 , PdCl4 , RhCl6 ), the standard electrode potentials for the half reactions are (CRC Handbook, 1995):

2.2. Characterization To confirm the presence of these metals, their distribution and their evolution, the characterization of the different car catalytic converters was carried out. The employed characterization techniques are X-ray powder diffraction (XRD) and scanning electron microscopy (FESEM) combined with an electron backscatter diffraction (EBSD) with energy dispersive X-ray spectroscopy (EDX) for quantitative and qualitative analyses. With the aim of obtaining the exact concentration of the metals, an analysis by inductively coupled plasma with a mass spectrometer (ICP-MS) (Agilent 7500ce) was performed.

2



PtCl6 þ 4e $ Pt þ 6Cl ½E ¼ 0:74 V

ð1Þ

2



ð2Þ

3



ð3Þ

PdCl4 þ 2e $ Pd þ 4Cl ½E ¼ 0:62 V RhCl6 þ 3e $ Rh þ 6Cl ½E ¼ 0:44 V

This fact implies that for a mixture of Pt, Pd and Rh in aqueous chloride media, the standard potentials for the formation of the platinum, rhodium and palladium chloro- complexes require an oxidizing agent with a reduction potential >0.74 V (Mishra, 1993). Therefore good candidates to act as oxidizing agents could be HNO3 and H2O2 (Burkin, 2001):

2.2.1. Structural characterization The crystalline structure of the catalytic converter samples was studied by means of X-ray powder diffraction using a Philips PW1710 difractometer. The scans were performed in 2h range of 5–70° with a 0.020° step. Structure refinements were carried out using FullProf suite (Rodriguez-Carvajal, 1993). The car catalytic converters are formed by an alumina-coated ceramic structure which supports the PGMs that carry out the catalytic action. The ceramic structure is insulated by a Teflon cover and enclosed in a stainless steel casing, and it is coated with c-alumina where the PGMs are incorporated. This alumina layer, which contains the PGMs, is called washcoat and usually embodies oxides such as ZrO2, CeO2 and some others (Fornasiero et al., 1996; Colon et al., 1998). The samples studied by X-ray powder diffraction were taken from the inner surface of the cells where the washcoat is located. The different phases present in the samples, together with their crystalline systems and spatial groups, are summarized in Table 2.

NO3 þ 4Hþ þ 3e $ NO þ 2H2 O ½E ¼ 0; 96V

ð4Þ

H2 O2 þ 2Hþ þ 2e $ 2H2 O ½E ¼ 1:77 V

ð5Þ

Taking into account these reduction potentials, HNO3 and H2O2 should be able to solubilise Pt, Pd, and Rh metals from car catalytic converters providing favourable reaction kinetics (Barakat and Mahomoud, 2006). In case of PGM dissolution with aqua regia, the formed nascent chlorine (Cl2) and nitrosyl chloride (NOCl) provide high oxidation potential and the high chloride-ion concentration acts as the complexing agent. It is believed (Massuci et al., 1999) that HNO3 and HCl in aqua regia undergo through the Reactions (6) and (7).

Table 1 The different types of car catalytic converters used in this study. Car converter code

CC1

CC2

CC3

CC4

CC5

CC6

Type Fuel

1 Way Diesel

1 Way Diesel

3 Ways Gasoline

3 Ways Gasoline

3 Ways Gasoline

3 Ways Gasoline

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HNO3 þ 3HCl $ NOCl þ Cl2 þ 2H2 O

ð6Þ

NOCl þ H2 O $ HNO2 þ HCl

ð7Þ

The platinum and palladium are completely dissolved in aqua regia medium. The dissolution of them from the catalyst is a redox reaction that undergoes according to Reactions (8) and (9). By the way, the addition of H2SO4 as acidity source to the leaching increases the extraction of rhodium (Angelidis, 2001; Mahomoud, 2003):

3PtðsÞ þ 18HClðacÞ þ 4HNO3 ðacÞ $ 3½PtCl6 2 ðacÞ þ 6Hþ ðacÞ þ 4NOðgÞ þ 8H2 O

ð8Þ

3PdðsÞ þ 12HClðacÞ þ 2HNO3 ðacÞ $ 3½PdCl4 2 ðacÞ þ 6Hþ ðacÞ þ 2NOðgÞ þ 4H2 O

ð9Þ

2RhðsÞ þ 12HClðacÞ þ 2HNO3 ðacÞ $ 2½RhCl6 3 ðacÞ þ 6Hþ ðacÞ þ 2NOðgÞ þ 4H2 O

ð10Þ

With the oxidizing agent H2O2 platinum and palladium are completely dissolved owing to its high oxidizing capacity. In this case, the addition of H2SO4 as acidity source to the leaching also increases the extraction of rhodium. The dissolution of them undergo through the next reactions:

507

PtðsÞ þ 6HClðacÞ þ 2H2 O2 ðacÞ $ ½PtCl6 2 ðacÞ þ 2Hþ ðacÞ þ 4H2 O ð11Þ PdðsÞ þ 4HClðacÞ þ H2 O2 ðacÞ $ ½PdCl4 2 ðacÞ þ 2Hþ ðacÞ þ 2H2 O ð12Þ 2RhðsÞ þ 12HClðacÞ þ 3H2 O2 ðacÞ $ 2½RhCl6 3 ðacÞ þ 6Hþ ðacÞ þ 6H2 O

ð13Þ

Thermodynamics applied to hydrometallurgy is used to calculate conditions of chemical equilibrium between species. Predominance area diagrams show the conditions in which the selected species are dominant in a system. To confirm the major species in the corresponding solutions, HSC chemistry software (Outokumpu Research) was implemented. Fig. 1 summarizes the Eh–pH diagrams for the studied systems at 90 °C. In both systems the Cl stabilizes the oxide metals as soluble halogen derivates. When HCl is used in the systems as stabilizer of chloro-complexes in the presence of the different oxidant agents, the oxidation reactions of the PGMs are alike. This fact explains that the diagrams which represent the formation of the chloro-complexes in both cases are similar. The studied concentrations were around 103 M of platinum and palladium, 104 M of rhodium, 10 M of hydrochloric acid, 1.5 M of nitric acid and 0.5 M of hydrogen peroxide. For the N–Cl–Pt, N–Cl–Pd and N–Cl–Rh systems the distribution of nitrogen species in Fig. 1a shows that at low pHs and in the presence of NO 3 in solution, the redox potential of solution must be

Fig. 1. Eh–pH diagrams of (a) nitrogen species; (b) O2–H2O system; (c) chlorine species; (d) platinum species; (e) palladium species; and (f) rhodium species in the studied systems using HSC software.

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close to 1.0 V (vs. the Standard Hydrogen Electrode). In hydrochloric acid media the corresponding chloro-complexes of the metals are formed. For the Eh values of 1.0 V and pHs close to zero, 2 Fig. 1d shows that the major platinum specie is PtCl6 . Fig. 1e shows that the major palladium specie is PdCl42- and Fig. 1f shows 3 that the rhodium specie formed in this area is RhCl6 . No NOCl was identified by the software as the major nitrogen/chlorine species. For the oxidation of PGMs in the presence of hydrogen peroxide as oxidant, Fig. 1b shows that at low pHs, the redox potential of solution must be close to 1.8 V. Fig. 1d–f shows that in hydrochlo2 2 3 ric acid media the species formed are PtCl6 , PdCl4 , RhCl6 . The Eh–pH diagrams have confirmed that the thermodynamic requirements of the process have been satisfied and they confirm the possible dissolution of platinum, palladium and rhodium as chloro-complexes in HNO3/HCl and H2O2/HCl media. In this work, the optimum concentration of these reactants was examined. Besides, some other variables with potential influence on the PGMs extraction such as hydrogen pre-treatment, thermal pre-treatment and the optimization of the energy consumption (temperature and time) were analyzed. The effort was to optimize the process by measuring the influence of the parameters on the recovery yield. The applied experimental procedure consists of several steps is shown in Fig. 2. The first step involved the optimization of the oxidation leaching and the influence of a hydrogen and thermal pre-treatment on the extraction of PGMs from catalysts. The leaching experiments were carried out in 250 ml flasks equipped with magnetic stirrers under refrigeration and heated at different temperatures. In order

to maintain constant the reaction temperature, the vessel was immersed in an oil bath. To calculate the extracted PGMs, the obtained liquid phases were analyzed by ICP-MS. However, in order to prove the effectiveness of the method, which has to dissolve at least the 95% of the PGMs with the oxidation leaching, it was necessary a second step to dissolve the undigested part of car catalytic converter. Amounts of PGMs remaining in these residues were determined by complete digestion of the sample using a hydrothermal method with two stages. First of all, the a-Al2O3 of the sample was attacked with H3PO4 and H2SO4 at 150 °C for 100 h. Then the solid residue was dissolved with a mixture of HF and HNO3 at 150 °C for 100 h. The final phase residue was analyzed by ICP-MS.

2.3.1. Sample preparation As car catalytic converters are used for a long time, several ageing deactivation mechanisms that reduce catalytic activity could take place in them and a portion of the PGMs could be released from them. Some studies show that the central part of the catalyst is less damaged (Moldovan et al., 2003). Because of that, the samples employed for the experiment have been taken out from this central part of the spent catalysts, where more PGMs are supposed to be. In addition, it is really important to determinate the presence and content of PGMs in the sample before recovery them. The scraps of the different ceramic supports were crushed and milled using a ball miller, until a maximum particle size of 0.1 mm was achieved (Wisecarver et al., 1992; Barakat and Maho-

Fig. 2. Simplified flowsheet of the proposed procedure.

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moud, 2004). The amount of catalyst used for each experiment was 25 g.

509

content of PGMs in the samples was always lower than 0.1%. By X-ray powder diffraction it was not possible to identify phases with so low concentration.

3. Results and discussion 3.1. Characterization of automotive catalysts 3.1.1. Structural characterization X-ray powder diffraction (XRD) was used to investigate the bulk phases present in the sample and to determine the ageing-induced solid–solid phase transformations. Several difficulties are associated with the local analysis of cordierite catalysts using XRD, because of the complex elemental composition of the washcoat, with many phases apart from the PGM. The obtained diffractograms were fitted using Fullprof suite without structural model (Pattern Matching). The diffractograms of two of the car catalytic converters studied are shown in Fig. 3. The main phases were identified as cordierite (2MgO 2Al2O35SiO2), alumina (Al2O3), and some secondary phases as CeO2, La2O3 and ZrO2 corresponding to the 20%. Only the 5% correspond to some other elements that were added to the catalytic converters as additives and whose percentages depend on the car manufacturers. The concentrations of platinum-group metals in catalysts vary widely depending on the manufacturer. In new car catalytic converters platinum, which is generally present in larger amounts than palladium and rhodium, ranges in concentration from 300 to 1000 lg g1; in the case of palladium, the concentrations vary from 200 to 800 lg g1; and for rhodium, they vary from 50 to 100 lg g1 (Faisal et al., 2008). Therefore in all the cases the total

Fig. 3. Diffractograms of (a) CC1 and (b) CC4 car catalytic converters: experimental (squares), fitted (line) and difference between them (lower line) profiles.

3.1.2. Morphological characterization The scanning electron microscopy was employed to check the morphology of the car catalytic converters and to verify the presence of PGMs in the car catalytic converters by EBSD and EDX analysis. As it has been indicated before, the ceramic material had a honeycomb structure, being the cordierite the main component. The skeleton was coated with a highly porous washcoat of 50– 150 lm. This fact can be observed in Fig. 3, as well as the wall thickness of the cells which can be different depending on the model. A longitudinal cross section of the honeycomb structure analyzed by FESEM revealed the width of the channels to be 1 mm. The different coats of the honeycomb cells are represented in Fig. 4. The microscopy analysis showed that the washcoat was divided in several porous and roughness coats. The roughness provides an increase of the contact area between the catalytic converter and the car exhaust fumes allowing a better performance of the reactions (Fig. 4). The chemical composition of the coat was quite similar (mostly Al2O3, CeO2, ZrO2), but the grain size decreases as the coat was more external, increasing therefore the surface area.

Fig. 4. FESEM images of the honeycomb structure. (a) Cross section image of CC3 catalyst. (b) Image of the washcoat of CC2 catalyst. (c) Surface of the catalytic converter.

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NH4F attacks the union surface between the PGM and the catalytic converter, the addition of 0, 0.26 and 1.52 g of NH4F was studied. In all the cases 25 g of milled catalytic converter were employed. The operating temperature was 90 °C and the reaction time was 6 h. The car catalytic converter used for this study was a Pt/Pd/ Rh three way gasoline-engine catalyst (CC4) where the main precious metal was the Pt. The results obtained are shown in Table 3. For PGMs extraction, HCl 12 M, 8 ml of HNO3, 5 ml H2SO4 volumes produced the best results in terms of maximum recovery. The addition of NH4F did not improve significantly the extraction rate of Rh and the extraction of platinum got worse.

Another important fact was that although the PGMs were present in all the coats, the majority of them were located in the external one, where they were more exposed to the exhaust fumes. 3.2. PGMs extraction method After verified that the PGMs were present in the catalysts by these different characterization techniques, a study of different variables was done to check the influence of them in the recovery of the precious metals. Two different compositions of reagents, a pre-treatment of the sample with hydrogen and the optimization of the energy consumption were the analyzed variables.

3.2.1.2. Composition B. With reference to Composition B, the addition of different concentrations of HCl, H2O2, H2SO4 and NH4F were studied. In this reagent composition the nitric acid was replaced by hydrogen peroxide (H2O2). The first proposed composition of reagents includes the addition of 10 ml of hydrogen peroxide (H2O2), 80 ml of hydrochloridric acid (HCl) and 10 ml of sulphuric acid (H2SO4). The reactions that take place are Eqs. (11)–(13). In this case the hydrogen peroxide acts as oxidizing agent, oxidizing the metals which are present in the sample. The HCl stabilizes the oxidized metals as soluble halogen derivates. The F ion provided by NH4F also favours the attack of the interface between the PGMs and the catalytic converter. The studied values were 6, 8 and 12 M for the HCl, 0, 8 and 17 ml for the HNO3 and 0, 5 and 15 ml for H2SO4. The addition of 0, 0.26 and 1.52 g for NH4F was also studied. The operating temperature was 90 °C and the reaction time was 6 h. The car catalytic converter used for this study was a Pd/Rh three way gasoline–engine catalysts (CC3). This car catalytic converter had Pd and Rh as majority precious metals. According to the analysis by ICP-MS the quantity of Pt was below 0.01 mg in 25 g sample with a 0.06 ppm quantification limit. The obtained results for the Composition B are shown in Table 4. For PGMs extraction HCl 12 M, 8 ml of H2O2 and 5 ml H2SO4 volumes produced the best results. The addition of NH4F did not improve significantly the extraction rate.

3.2.1. Leaching composition The influence of two different oxidizing agents for the recovery of PGMs from car catalytic converters was studied. Thus, two different reagent compositions were proposed Composition A with HNO3 and Composition B with H2O2. 3.2.1.1. Composition A. In this case, the addition of different concentrations of HCl, HNO3, H2SO4 and NH4F were studied. The first proposed composition of reagents included the addition of 25 ml of nitric acid (HNO3), 65 ml of hydrochloridric acid (HCl), and 10 ml of sulphuric acid (H2SO4). The reactions that take place are Eqs. (8)–(10). The nitric acid oxidates the metals present in the samples. The reaction medium was a solution of HCl. This acid has two effects, on one hand the HCl allows the formation of aqua regia, which increases the oxidizing capacity of the media, and on the other hand the oxidized metals are stabilized as soluble halogen derivates by the HCl (Weast et al., 1988; Angelidis, 2001). The F ion provided by NH4F favours the attack of the interface between the PGMs and the catalytic converter. In order to determinate the optimum values of the reagent composition, the volumes and concentrations of the initial composition reagents were modified. After each modification, the dissolutions obtained in each attack were analyzed by ICP-MS to determinate the most adequate volume or concentration of reagents. The first modified variable was the HCl concentration, and the studied values were 6, 8 and 12 M. To continue with the study different volumes of HNO3 (15 M) were also analyzed. The studied volumes of HNO3 were 0, 8 and 17 ml. The same study was also carried out for H2SO4 (18 M) with volumes of 0, 5 and 15 ml. As the

3.2.2. Hydrogen pre-treatment The influence of hydrogen pre-treatment in the PGMs extraction rate was studied. Hydrogen stabilizes the PGMs in their metallic form. When PGMs are in their metallic form it is easier to

Table 3 Obtained quantities of the recovered PGMs for the A composition. PGMsa

Pt (mg) Pd (mg) Rh (mg) a

HCl [65 ml] (M)

HNO3 [15 M] (ml)

H2SO4 [18 M] (ml)

NH4F (g)

6

8

12

0

8

17

0

5

15

0

0.26

0.54

43.13 0.49 5.75

43.54 0.55 5.15

44.04 0.61 6.43

41.02 0.57 6.31

44.38 0.64 7.13

43.31 0.62 6.41

36.42 0.59 6.11

45.57 0.66 7.11

42.16 0.60 6.13

45.55 0.66 6.92

40.17 0.69 7.34

40.86 0.71 6.10

Feed material: 25 g sample.

Table 4 Obtained quantities of the recovered PGMs for the B composition. PGMa

Pt (mg) Pd (mg) Rh (mg) a *

HCl [80 ml] (M)

H2O2 [110 vol] (ml)

H2SO4 [18 M] (ml)

NH4F (g)

6

8

12

0

8

17

0

5

15

0

0.26

0.54

–* 55.19 5.23

–* 56.56 5.26

–* 57.85 6.09

–* 57.02 7.13

–* 58.29 7.31

–* 57.27 5.14

–* 54.79 6.58

–* 58.98 7.46

–* 57.24 6.94

–* 58.97 7.52

–* 58.35 7.63

–* 58.82 8.76

Feed material: 25 g sample. Trace metal: obtained values below 0.01 mg (quantification limit 0.06 ppm).

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dissolve them with the proposed reagents. Only a portion of rhodium could be found as its oxide so this pre-treatment was thought to be especially effective for this metal (Bolinski and Distin, 1992; Williams et al., 1998). The sample was treated for 22 h with a constant hydrogen gas flow (7% H2) at different temperatures. The studied temperatures were room temperature, 80 and 250 °C. The study was done with the two reagent compositions mentioned before. The partial results corresponding to the hydrogen pre-treatment are shown in Table 5. The Table 5 shows that the pre-treatment with hydrogen in Compositions A and B at 250 °C induced the best results, but these results could also be attributed to the thermal treatment.

Table 7 Recovered PGMs using different temperatures and reaction times. PGMa

Temperature

Time

65 °C

90 °C

2h

4h

6h

8h

10 h

Comp. Ab Pt (mg) Pd (mg) Rh (mg)

44.21 0.01 6.19

47.01 0.74 7.40

22.13 0.06 4.37

43.82 0.58 6.89

47.00 0.74 7.39

47.03 0.74 7.40

47.04 0.76 7.41

Comp. Bc Pt (mg) Pd (mg) Rh (mg)

–* 49.37 4.91

–* 59.73 9.53

–* 28.14 2.06

–* 55.63 4.27

–* 59.74 9.54

–* 59.75 9.54

–* 59.77 9.56

a

Feed material: 25 g sample, 22 h at 250 °C pre-treatment. Leaching conditions: 12 M HCl 65 ml, 15 M HNO3 8 ml, 18 M H2SO4 5 ml. c Leaching conditions: 12 M HCl 80 ml, 110 vol. H2O2 5 ml, 18 M H2SO4 5 ml. Trace metal: obtained values below 0.01 mg (quantification limit 0.06 ppm).

b

3.2.3. Thermal pre-treatment Due to these results, it became necessary to investigate the effect in the sample of a thermal pre-treatment on the extraction of platinum, palladium and rhodium. A sample thermal pre-treatment at 250 °C was carried out. The results were compared with the previous results obtained when the sample was treated with a hydrogen gas flow at 250 °C. The results are plotted in Table 6 and indicate that metal extraction increased with a thermal pre-treatment of the sample at 250 °C. This improvement was attributed to the elimination of the hydrocarbons and charcoal present on the catalysts surface.

Table 5 Obtained results applying a pre-treatment in hydrogen atmosphere. H2c 25 °C

80 °C

250 °C

Comp. Ab Pt (mg)a Pd (mg)a Rh (mg)a

45.37 0.64 6.91

45.53 0.69 7.26

47.01 0.77 7.44

Comp. Bd Pt (mg)a Pd (mg)a Rh (mg)a

–* 58.92 6.62

–* 58.89 7.59

–* 59.73 9.88

a

Feed material: 25 g sample. Leaching conditions: 12 M HCl 65 ml, 15 M HNO3 8 ml, 18 M H2SO4 5 ml, 90 °C,

b

6 h. c

Gas flow: H2 7%, 22 h. Leaching conditions: 12 M HCl 80 ml, 110 vol. H2O2 5 ml, 18 M H2SO4 5 ml, 90 °C, 6 h. * Trace metal: obtained values below 0.01 mg (quantification limit 0.06 ppm). d

Table 6 Comparison of the recovered PGMs obtained by the application of a hydrogen pretreatment at 250 °C and by a thermal pre-treatment in air at 250 °C. 250 °C

a

H2c

Air

Comp. Ab Pt (mg)a Pd (mg)a Rh (mg)a

46.91 0.78 7.44

47.02 0.72 7.39

Comp. Be Pt (mg)d Pd (mg)d Rh (mg)d

_* 59.71 9.88

_* 59.68 9.53

Feed material: 25 g sample, 22 h at 250 °C pre-treatment. Leaching conditions: 12 M HCl 65 ml, 15 M HNO3 8 ml, 18 M H2SO4 5 ml. c Gas flow: H2 7%, 22 h. d Feed material: 25 g sample. e Leaching conditions: 12 M HCl 80 ml, 110 vol. H2O2 5 ml, 18 M H2SO4 5 ml, 90 °C, 6 h. * Trace metal: obtained values below 0.01 mg (quantification limit 0.06 ppm). b

*

As the quantity of rhodium oxide in the sample is small hydrogen pre-treatment did not improve the results significantly and the addition of it supposed an extra cost. 3.2.4. Energy consumption optimization In order to reduce the energy consumption during the process, the optimum values of temperature and reaction time were studied. The studied values of temperature were 65 and 90 °C respectively. The reaction time values were 2, 4, 6, 8 and 10 h, and the studies have been done with the two reagent composition (A and B). The partial results corresponding to the optimization of the energy consumption are summarized in Table 7. At 90 °C the extraction efficiencies of platinum, palladium and rhodium increased. These results indicated that metal extraction improved by increasing reaction time but plateaus above 6 h. 3.3. Selection of the optimal composition The analysis of the different variables gave rise to two different reagents compositions as potentially adequate for the PGMs extraction: Composition A HCl HNO3 H2SO4 Thermal pre-treatment Reaction temperature Time reaction

65 ml 8 ml 5 ml 250 °C 90 °C 6h

12 M 15 M 18 M 22 h (in Air)

Composition B HCl H2O2 H2SO4 Thermal pre-treatment Reaction temperature Time reaction

80 ml 5 ml 5 ml 250 °C 90 °C 6h

12 M 110 vol 18 M 22 h (in Air)

The development of a method capable of extracting the PGMs of any type of car catalytic converter is a crucial step. Thus, different car catalytic converters of several car-makes were studied employing the two reagents compositions mentioned above. The results of the PGMs extraction rates with the two compositions probed in different car-makes are summarized in Table 8. The results in terms of extraction were quite similar comparing the two compositions. Bearing in mind that the Composition B is more environmentally friendly and that the extraction results were as good as with the other composition, the more adequate and therefore chosen composition was the Composition B.

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Table 8 Recovered quantities of PGMs in different car catalytic converters with A and B compositions. Catalytic convertera

Pt (mg) Feed Material

CC2 CC3 CC4 CC5 CC6

51.32 _* 47.44 38.49 3.33

Pd (mg) b

A

c

50.37 _* 47.01 37.66 3.49

B

d

49.67 _* 47.09 37.54 3.23

Rh (mg) b

c

Feed Material

A

1.77 60.65 0.94 0.27 65.18

1.44 56.77 0.74 0.22 63.49

B

d

1.46 59.74 0.76 0.23 65.01

Feed Materialb

Ac

Bd

0.56 10.50 7.68 5.69 7.44

0.28 8.42 7.40 5.36 7.18

0.33 9.54 7.66 5.50 7.27

a

Feed material: 25 g sample, 22 h at 250 °C pre-treatment. Total mg in the sample. A leaching conditions: 12 M HCl 65 ml, 15 M HNO3 8 ml, 18 M H2SO4 5 ml. d B leaching conditions: 12 M HCl 80 ml, 110 vol. H2O2 5 ml, 18 M H2SO4 5 ml. Trace metal: obtained values below 0.01 mg (quantification limit 0.06 ppm). b

c

*

Fig. 5. Recovery percentages of the PGMs with composition B. [Feed material: 25 g sample, 22 h at 250 °C pre-treatment; Leaching conditions: 12 M HCl 80 ml, 110 vol. H2O2 5 ml, 18 M H2SO4 5 ml. Trace metal: obtained values below 0.01 mg (quantification limit 0.06 ppm.)]

3.3.1. Determination of the chosen composition extraction capacity Samples of different catalytic converters were treated with the chosen composition and its extraction capacity was determined. In order to know the amount of PGMs that was not recovered with the attack, the PGMs remaining in the residue were determined by a complete digestion of the residue using the hydrothermal method mentioned before. After calculating the total amount of PGM in the sample, it was possible to measure the recovery rate with the proposed method. The Fig. 5 shows the recovery percentage obtained using different models of car catalytic converters. As it is observed in Fig. 5 with Composition B the extraction percentage of PGMs present in the different car catalytic converters was up to 95% in all cases. Therefore a new method was developed by the application of less aggressive, corrosive or expensive reagents and conditions, extracting more than 95% of PGMs present in the catalysts. 4. Conclusions In present work the main phases of the car catalytic converters were characterized by X-ray powder diffraction and ICP-MS techniques, allowing the knowledge that how they are divided up. Besides, the scanning electron microscopy was used in order to observe the distribution of the PGMs. This study revealed that cordierite and alumina are the main components of the catalysts, and that PGMs are mostly located on the inner surface of the cells which compose the car catalytic converters. In addition two different compositions of reagents for the PGMs extraction from car catalytic converters were studied. It was demonstrated that both of them were able to recover more than 95% of PGMs present in the catalysts. The recovery of the 95% of the metals from the car catalytic converters supposes an important advance to reduce the extraction

rate of platinum, palladium and rhodium and allows the performance of one of the premise of the sustainable development. This fact indicates the importance of the reutilization of the non renewable natural resources. In this case, with the determination of the optimum values of the composition, the use of reagent is minimized and others with less potential damage for the environment are presented. The new alternative method that it is proposed includes the addition of H2O2 as oxidant agent which implies the reduction of the pollutant gases and also the environmental impact of the chemical process. The final reagent composition which is proposed contains 80 ml of HCl (12 M), 5 ml of H2O2 (110 vol), and 5 ml of H2SO4 (18 M), at 90 °C for 6 h, with a previous thermal pre-treatment at 250 °C during 22 h of the sample. With this treatment more than 95% of the PGMs present in different car catalytic converters can be recovered. Acknowledgements This work has been partially financed by the Spanish CiCyT under Project MAT2007-66737-C02-01 and by the Government of the Basque Country under Project IT-312-07. D. Jimenez de Aberasturi thanks the Eusko Jaurlaritza/Gobierno Vasco for her predoctoral fellowship and R. Pinedo thanks the University of the Basque Country Universidad for his predoctoral fellowship. I. Ruiz de Larramendi thanks the Government of the Basque Country for funding her research activities as postdoc within the Project GIC07/126-IT312-07. The authors thank the technicians of SGIker, for the ICPMS measurements. References Angelidis, T.N., 2001. Development of a laboratory scale hydrometallurgical procedure for the recovery of Pt and Rh from spent automotive catalysts. Topics in Catalysis 16/17, 1–4. Angelidis, T.N., Skouraki, E., 1996. Preliminary studies of platinum dissolution from a spent industrial catalyst. Applied Catalysis A: General 142, 387. Barakat, M.A., Mahomoud, M.H.H., 2004. Recovery of platinum from spent catalyst. Hydometallurgy 72, 179–184. Barakat, M.A., Mahomoud, M.H.H., 2006. Recovery and separation of palladium from spent catalyst. Applied Catalysis A: General 301, 182–186. Baghalha, M., Khosravian Gh, H., Mortaheb, H.R., 2009. Kinetics of platinum extraction from spent reforming catalysts in aqua-regia solutions. Hydrometallurgy 95, 247–253. Bolinski, L., Distin, P.A., 1992. Platinum group metals recovery from recycled autocatalyst by aqueous processing. Publications of the Australasian Institute of Mining and Metallurgy, 9/92. Bonucci, J.A., Parker, P.D., 1984. Recovery of PGM from automobile catalytic converters. In: Precious Metals: Mining, Extraction and Processing, Proceedings of International Symposium, pp. 463–481. Burkin, A.R., 2001. Chemical Hydrometallurgy: Theory and Principles. ISBN: 186094-184-2. Colon, J.L., Tejada, G.A., Maldonado, I., Hofmann, C., 1998. Sol–gel synthesis of zirconium phosphates layered materials. In: Book of Abstracts, 216th ACS National Meeting, Boston, INOR-478.

D. Jimenez de Aberasturi et al. / Minerals Engineering 24 (2011) 505–513 Lide, D.R., CRC Handbook of Chemistry and Physics. CRC Press, London, 1994–1995. Cuif, J.P., Blanchard, G., Touret, O., Seigneurin, A., Marczi M., Quéméré, E., 1997. Society of Automotive Engineers, pp. 35–47 (Paper 970463). Faisal, M., Atsuta, Y., Daimon, H., Fujie, K., 2008. Recovery of precious metals from spent automobile catalytic converters using supercritical carbon dioxide. Asia-Pacific Journal of Chemical Engineering 3, 364–367. Fontàs, C., Salvadó, V., Hidalgo, M., 2002. Separation and Concentration of Pd, Pt, and Rh from automotive catalytic converters by combining two hollow-fiber liquid membrane systems. Industrial and Engineering Chemistry Research 41, 1616– 1620. Fornasiero, P., Balducci, G., Kaspar, J., Meriani, S., di Monte, R., Graziani, M., 1996. Metal-loaded CeO2–ZrO2 solid solutions as innovative catalysts for automotive catalytic converters. Catalysis Today 29 (1–4), 47–52. Gupta, C.K., Mukherjee, T.K., 1990. Hydrometallurgy in Extraction Processes, vol. I. CRC Press, ISBN 0-8493-6804-9. Kayanuma, Y., Okabe, T.H., Mitsuda, Y., Miyake, M., Maeda M., 2004. Metal vapour treatment for platinum group metals from spent automotive catalyst. In: Waste Processing and Recycling in Mineral and Metallurgical Industries –Fifth International Symposium, Japan, pp. 169–180. Kim, C.H., Woo, S.I., Jeon, S.H., 2000. Recovery of platinum-group metals from recycled automotive catalytic converters by carbochlorination. Industrial and Engineering Chemistry Research 39, 1185–1192. Lucena, P., Vadillo, J.M., Laserna, J.J., 1999. Mapping of platinum group metals in automotive exhaust three-way catalysts using laser-induced breakdown spectrometry. Analytical Chemistry 71, 4385–4391. Mahomoud, M.H.H., 2003. Leaching platinum-group metals in sulfuric acid/chloride solution. Journal of the Minerals, Metals and Materials Society 55 (4), 37–40.

513

Massucci, M., Clegg, S.L., Brimblecombe, P., 1999. Equilibrium partial pressures, thermodynamic properties of aqueous and solid phases, and Cl2 production from aqueous HCl and HNO3 and their mixtures. Journal of Physical Chemistry A 103, 4209–4226. Mishra, R.K., 1993. A review of platinum group metals recovery from automobile catalytic converters. Precious Metals, 449–475. Moldovan, M., Gómez, M.M., Palacios, M.A., 1999. Determination of platinum, rhodium and palladium in car exhaust fumes. Journal of Analytical Atomic Spectrometry 14, 1163–1169. Moldovan, M., Rauch, S., Morrison, G.M., Gómez, M., Palacios, M.A., 2003. Impact of ageing on the distribution of platinum group elements and catalyst poisoning elements in automobile catalysts. Surface and Interface Analysis 35, 354–359. Nowottny, C., Halwachs, W., Schürgerl, K., 1997. Recovery of platinum, palladium and rhodium from industrial process leaching solutions by reactive extraction. Separation and Purification Technology 12, 135–144. Palacios, M.A., Moldovan, M., Gomez, M., 2000. The automobile catalyst as an important source of PGE in the environment. Anthropogenic Platinum-Group Element Emissions, 3–14. Rodriguez-Carvajal, J., 1993. Recent advances in magnetic structure determination by neutron powder diffraction. Physica B: Condensed Matter 192, 55–69. Weast, R.C., Astle, M.J., Beyer, W.H. (Eds.), 1988. CRC Handbook of Chemistry and Physics. CRC Press, Boca Raton, FL. Williams, C.T., Chen, E.K.-Y., Takoudis, C.G., Weaver, M.J., 1998. Reduction kinetics of surface rhodium oxide by hydrogen and carbon monoxide at ambient gas pressures as probed by transient surface-enhanced Raman spectroscopy. Journal of Physical Chemistry B 102, 4785–4794. Wisecarver, K.D., Ningsheng, Y., Kuo Ying Amanda, W., 1992. Dissolution of platinum and palladium from automotive catalysts. Precious Metals 16, 29–37.