a new technique for extraction of platinum group metals by pressure cyanidation

(2006) 164–171www.elsevier.com/locate/hydromet

Hydrometallurgy 82

A new technique for extraction of platinum group metalsby pressure cyanidation

Jing Chen ⁎, Kun Huang

College of Chemistry and Material Engineering, Yunnan University, Kunming 650092, PR China

Available online 27 April 2006

Abstract

At room temperature and pressures, the reaction between sodium cyanide and platinum group metals (PGMs) does not occurbecause of poor kinetics. However, at elevated temperatures between 120 and 180 °C, PGMs can be leached by sodium cyanidelike the reaction of gold. In this work, a new technique to treat Pt–Pd sulfide flotation concentrates and spent auto-catalysts bypressure cyanidation is put forward, and the results of 5 kg-scale batch experiments in a 50 L autoclave are reported for processdevelopment. The cyanide leaching reaction mechanism is also discussed. For flotation concentrates containing about 80 g/t Pt andPd, after pre-treating by pressure acid leaching, followed by two steps of pressure cyanide leaching, up to 90–94% Pt and 99% Pdextraction could be achieved. Final concentrates obtained from cyanide leaching solution using zinc cementation contained 70–90% of precious metals. For spent auto-catalysts containing ≈1000–2000 g/t Pt+Pd+Rh, after a pre-treatment process to removethe wrapping of the catalyst carrier and to rid surface accumulated carbon and gasoline contaminants, followed by two steps ofpressure cyanide leaching, the recoveries of Pt, Pd and Rh were 95–96%, 97–98% and 90–92%, respectively.© 2006 Elsevier B.V. All rights reserved.

Keywords: Platinum group metals; Flotation concentrates; Spent auto-catalysts; Pressure cyanidation; Leaching mechanism

1. Introduction

The content of platinum group metals (PGMs) inproterozoic platinum ores is about 1–10 g/t, while inCu–Ni sulfide ores containing PGMs, the content isonly 0.1–1 g/t. By flotation, the content of PGMs isconcentrated to more than 100 g/t. For flotationconcentrates, the traditional treatment method is mattesmelting where the PGMs are concentrated in themattes. The matte is then hydrometallurgically treated to

⁎ Corresponding author. Tel./fax: +86 871 5032180.E-mail address: [email protected] (J. Chen).

0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.hydromet.2006.03.041

leach and separate Fe, Co, Ni and Cu leaving a slimecontaining 10–50% of precious metals.

Direct hydrometallurgical processing of flotationconcentrates has been considered impossible (Liu,2001), because of the complicated solution componentsand low recoveries. At room temperature and atmosphericpressure, sodium cyanide cannot also react with PGMslike the reaction of gold, because of poor kinetics (Feather,1978; Dawson, 1984). It has been reported (Bruckard etal., 1992; Bruckard, 1998; McInnes et al., 1993;Duyvesteyn et al., 1994) that when concentrates arereacted with sodium cyanide at 100–125 °C and pH 9.0–10.0 for 6 h, the Pd and Pt leached could reach 90% and80%, respectively. However, the ore used in the best casewas from Coronation Hill after amalgamation to extract

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http://dx.doi.org/10.1016/j.hydromet.2006.03.041

Table 2Typical content of PGMs in spent auto-catalysts fromMultimetco, Inc.

Elements Pt (g/t) Pd (g/t) Rh (g/t)

Content 818.3 516.7 213.8

165J. Chen, K. Huang / Hydrometallurgy 82 (2006) 164–171

gold. This ore is highly oxidized with a sulfur assay ofonly 0.1%. Clearly, the mineral composition of mostoxidized ores is largely different from that of sulfideflotation concentrates. For sulfide flotation concentrates,it is almost impossible to extract PGMs economicallyusing sodium cyanide. Therefore, there have been noreports so far on the treatment of such concentrates bycyanidation.

Smelting has serious environmental impacts and thelengthy overall flowsheet results in unavoidable lossesof PGMs. When the mattes are acid leached to separatebase metals, the content of PGMs in residues is neverabove 50%, leading to difficulties in the PGM refinery.To avoid these limitations a new all hydrometallurgicaltwo-stage process for the extraction of PGMs fromsulfide flotation concentrates has been suggested (Chenet al., 2005). The sulfides are pre-oxidized by pressureacid leaching to remove the base metals and exposePGM minerals; the residue is then subjected to pressurecyanidation to recover the PGMs. Up to 90–94% Pt and99% Pd extraction could be achieved, and cementationof the final solution with zinc powder resulted in a PGMconcentrate containing about 70–90% of preciousmetals.

Automobile catalysts have been one of the mainconsumers of PGMs so far, and much attention has beenpaid on PGM recovery from such wastes (Mishra, 1993;Hoffmann, 1988; Huang and Chen, 2004a,b). Oxidizedacid leaching of spent auto-catalysts was consideredunsatisfactory (Guo et al., 1999; Wu et al., 1993), partlybecause of high reagent consumption, severe pollution,instability of PGM extraction, and low recoveries ofrhodium etc. The US Bureau of Mines (U.S.B.M.) hadconducted some successful exploration trials on recov-ering of PGMs by high-temperature cyanide leaching(Atkinson, 1992; Desmond, 1991). However, therecoveries reported were only 80–85% Pt and Pd, and70–75% Rh, respectively. The consumption of cyanidein the leach process was also high because the cyanidewas affected by the composition of the catalyst substrateincluding metal, carbon and gasoline contaminants onthe surface. Thus appropriate pre-treatment methods arerequired. A new pre-treatment procedure (Chen andHuang, 2001) was suggested, which was different tothose reported elsewhere. The spent auto-catalysts are

Table 1Chemical analysis results of typical Jinbaoshan flotation concentrates

Elements Pt Pd Cu Ni Co Fe S MgO SiO2

(g/t) (%)

Content 34 52.4 3.45 3.86 0.24 14.8 13.4 19.3 26.9

pre-treated by pressure alkaline leaching, followed bytwo steps of pressure cyanide leaching, which achievesrecoveries of Pt, Pd and Rh of 95–96%, 97–98% and90–92%, respectively.

In present paper, the results of pre-treating andleaching a copper–nickel sulfide flotation concentrateand an automobile catalyst are compared and thereaction mechanism of pressure cyanide leaching ofPGMs is discussed.

2. Experimental

2.1. Samples tested

2.1.1. Flotation consTable 1 shows the typical chemical composition of a

copper-nickel sulfide flotation concentrate samplekindly provided by Jinbaoshan Platinum Mine (YunnanProvince, PRC).

2.1.2. Auto-catalystsTable 2 shows the typical content of PGMs in spent

auto-catalysts samples, provided by Multimetco, Inc.(Anniston, Alabama 36207, USA).

2.2. Testing apparatus

A 50 L autoclave ( TFYXD-50 type, 316 L stainlesssteel, electrical heated) was used for the tests. The maxdesign pressure, temperature and agitation speed for theautoclave are 7.0 MPa, 350 °C and 500 rpm, respectively.

The mineralogical investigations of leaching residueswere carried out byX-ray Diffraction (XRD; DMAX-RC,Japan), Scanning Electron Microscopy (SEM; JEOL,JSM-5610) and X-ray Energy Dispersive Analysis(EDAX; ThermoNoran, Vantage DSE), respectively.

2.3. Testing procedures and typical autoclaveleaching conditions

2.3.1. Flotation consThe new all hydrometallurgical technique for treating

sulfide flotation concentrates containing PGMs includesthe following procedures: flotation concentrates→pre-tre-atment by pressure oxidized acid leaching→ two stages ofpressure cyanide leaching→Zn cementation→precious

Table 3Results of pressure oxidized acid leaching of Cu, Ni and Co (see Experimental 2.3 for conditions)

Batch Samples Weight(kg)

Content (%) Metal amount (g) Recovery (%)

Cu Ni Co Cu Ni Co Cu Ni Co

Flotation con 5.00 4.28 3.86 0.3 214 193 15No. 1 Acid slag 2.04 0.07 0.02 b0.005 1.43 0.41 b0.10 99.3 99.8 N99.3No. 2 Acid slag 2.19 0.08 0.03 b0.005 1.75 0.66 b0.10 99.2 99.7 N99.3No. 3 Acid slag 2.18 0.07 0.02 b0.005 1.52 0.44 b0.10 99.3 99.7 N99.3

166 J. Chen, K. Huang / Hydrometallurgy 82 (2006) 164–171

metal concentrates. The 5.0 kg batch leaching tests werecarried out under the following conditions:

Pressure acid leach: temp. 200 °C, 12.5 g/L H2SO4, 6 h,O2 pressure 1.8 MPa, L:S ratio 4:1;

Two-stage cyanidation: temp. 160 °C, 6.25 g/L NaCN,1 h, O2 pressure 1.5 MPa, L:Sratio 4:1;

Zinc cementation: temp. 60–80 °C, pH 9.5–10, time2 h, 1 atmos.

2.3.2. Auto-catalystsThe new technique for extraction of PGMs from

spent auto-catalysts includes the following procedures:spent auto-catalysts→pre-treatment by pressure alka-line leaching→ two stages of pressure cyanide lea-ching→Zn cementation.

The 5.0 kg batch tests were carried out at thefollowing conditions of:

Alkaline pressure: temp. 160 °C, 25 g/L NaOH, 2 h, O2

pressure 2.0 MPa, L:S ratio 4:1;Two-stage cyanidation: temp. 160 °C, 6.25 g/L NaCN,

1 h, pressure 1.5 MPa, L:Sratio 4:1;

Zinc cementation: temp. 60–80 °C, pH 9.5–10, time 2 h,1 atmos.

2.4. Analysis methods and calculation of metal recovery

Precious metals in dried solid residue samples wereanalyzed by both fire assay and solution analysis

Table 4Results on two stages of pressure cyanide leaching of Pt and Pd (see Exper


Content (g/t)

Pt

Flotation con 5.00 36.73No. 1 Cyanide residue 1.01 7.3No. 2 Cyanide residue 1.00 6.6

Flotation con 5.00 32.14No. 3 Cyanide residue 1.00 6.7

methods (ICP). To prepare precious metals solutionsamples from the residues, the residues were alkali-fused, then dissolved in 50% HCl+50% H2O2 solution.The ICP results were compared with the results from fireassay and were quite close. The extraction recovery ofprecious metals was calculated by either fire assayresults or back-calculated from solution results. Othermetals (Cu, Ni, Co) from the solution samples wereanalyzed by AAS and/or ICP.

3. Results

3.1. Extraction of PGMs from a sulfide flotationconcentrate

The 5 kg-scale batch experimental results of thepercentage of Cu, Ni and Co oxidized and leached byacid and the percentage of Pt and Pd leached by cyanidefor a process development unit are presented in Tables 3and 4.

As shown in Table 3, under pressure acid leachingconditions, the Cu, Ni and Co in the flotationconcentrates were almost completely leached intosolution with N99% extracted. Whilst Table 4indicates that, after two stages of pressure cyanideleaching, the weight of cyanide leach residue was only20–25% of the flotation concentrates, and the Pt andPd content in the residues were less than 10 g/t. Thepercentage of Pt and Pd leached from the two sampleswere 95.8–96.4% and 99.3–99.4%, respectively, andthis was subsequently precipitated by the addition ofzinc dust.

imental 2.3 for conditions)

Metal amount (mg) Recovery (%)

Pd Pt Pd Pt Pd

51.42 183.65 257.101.76 7.37 1.78 96.0 99.31.52 6.6 1.52 96.4 99.450.31 160.70 251.551.75 6.7 1.75 95.8 99.3

Table 5Analysis of Zn cementation precipitates re-dissolved in HCl+H2O2

solutions (mg/L)

Solutions Pt Pd Rh Ir Au Ag Cu Ni Co Fe Zn

No. 1 29.6 43.0 1.4 0.8 4.5 27.6 3 1 b0.5 14 0.6No. 2 32.0 42.6 1.4 0.8 4.4 28.8 3 1 b0.5 11 0.8No. 3 26.8 41.8 1.5 0.9 4.6 30.2 3 1 b0.5 12 0.8

Note: each solution has the volume of 5.00 L.Zn cementation precipitates were pre-leached by 1% HCl to removeunreacted Zn.

Table 7Recovery of PGMs from auto-catalysts

Pt Pd Rh

Present work(pre-treated)

Specimen, g/t 718.3 594.7 173.1Content inresidues1, g/t

50.0 27.5 22.8

% metalextraction

95.75 97.18 91.97

U.S. Bureau of Mines'(non-pre-treated)


135.63 75.57 11.55

% metalextraction

82 72 77


83.4 36.0 9.9

% metal extraction 90 78 88

Contrast between this work and U.S. Bureau of Mines' work(Desmond, 1991).1 — two steps of pressure cyanide leaching.2 — four steps of pressure cyanide leaching.Present cyanidation: temp. 160 °C, 6.25 g/L NaCN, 1 h, pressure1.5 Mpa, L:S ratio 4:1;U.S.B.M cyanidation: temp. 160 °C, 5% NaCN, 1 h, pressure 1.5 Mpa,L:S ratio 5:1.


Because the content of Pt and Pd in 5 kg flotationconcentrate was only about 400 mg, and the amount ofzinc cementation precipitate was smaller, the preciousmetal precipitate was dissolved by 50% HCl+50%H2O2 solution at 80–90 °C. By analyzing the concen-tration of PGMs in solution, the content of preciousmetals in zinc cementation precipitates could becalculated.

As shown in Table 5, the general content of Pt andPd in the zinc cementation precipitates were 56–59%,and that of other precious metals were 27–30%.Compared with their content in the flotation concen-trates, Pt and Pd were enriched about 6000 times whichwas more favourable for the later refinery processes.Furthermore, Rh, Ir, Au and Ag could also berecovered satisfactorily.

3.2. Extraction of PGMs from spent auto-catalysts bypressure cyanidation

The 5 kg-scale batch experimental results from theprocess development unit are given in Table 6.

As shown in Table 6, the percentage of Pt, Pd andRh leached by cyanide could reach 96.0%, 97.8% and92.0%, respectively. The new pre-treatment procedurenot only eliminates the carbon and oil-pollution on thesurface of the catalysts, but it also uncovers the silicahoneycomb carriers of PGMs. The cyanide leaching of

Table 6Leaching of Pt, Pd and Rh by two-stage pressure cyanidation from spent au


Content (g/t)

Pt Pd Rh

No. 1 Auto-cat. CHJ-1 5.00 727.2 593.1 182.9Cyanide residue 3.02 48.5 25.7 29.3



See Experimental 2.3 for conditions.

PGMs was apparently higher than that reported byDesmond (1991), as shown in Table 7.

4. Discussion of reaction mechanism

4.1. Treatment of sulfide flotation concentratescontaining PGMs

In low-grade Pt–Pd flotation concentrates, themineralogical investigations on Pt and Pd mineralsindicated that they existed in ultra-fine micro-grains andwere encapsulated in FeNi2S4, CuFeS2 and FeS2. AnEDAX analysis of a typical Pt/Pd ore grain showed thepresence of Cu, Ni, Zn, Fe and S. In pressure acidleaching processes, the following chemical reactionsoccur with these sulfide minerals that produces acid and

to-catalysts after alkaline pressure pre-treatment

Metal amount (mg) Recovery (%)

Pt Pd Rh Pt Pd Rh

3636.0 2965.5 914.5146.5 77.6 88.5 96.0 97.4 90.3

4956.5 2306.5 1158.0250.5 50.1 100.5 95.0 97.8 91.3

3591.5 2973.5 865.5152.5 83.9 69.5 95.8 97.2 92.0


iron oxides when the acid is neutralised by lime ormagnesia.

CuFeS2 þ 2H2SO4 þ O2

¼ CuSO4 þ FeSO4 þ 2S0 þ 2H2O ð1Þ

FeNi2S4 þ 2H2SO4 þ 3O2

¼ 2NiSO4 þ FeSO4 þ 3S0 þ 2H2O ð2Þ

2FeS2 þ 2H2O þ 7O2 ¼ 2FeSO4 þ 2H2SO4 ð3Þ

4FeSO4 þ 2H2SO4 þ O2 ¼ 2Fe2ðSO4Þ3 þ 2H2O ð4Þ

Fe2ðSO4Þ3 þ 3H2O ¼ Fe2O3 þ 3H2SO4 ð5Þ

Fe2ðSO4Þ3 þ 4H2O ¼ 2α� FeOOH þ 3H2SO4 ð6Þ

MgO þ H2SO4 ¼ MgSO4 þ H2O ð7Þ

CaO þ H2SO4 ¼ CaSO4 þ H2O ð8ÞWhen reacted at high-temperatures (N180 °C) and

high oxygen pressures for a long time, the base metalsulfides were oxidized to sulfate completely, and S0 inabove reactions was further oxidized as SO4

2−. Afterpressure H2SO4 leaching, Cu and Ni sulfides in theleach residues disappeared and the XRD of the residueshowed only patterns corresponding to Fe2O3, FeO(OH)and SiO2. This was verified by SEM imaging andEDAX analysis, which gave no evidence of residualsulfides. Similarly, the main minerals identified in thepressure cyanide leach residues were still Fe2O3 andSiO2.

4.2. Treatment of spent auto-catalysts

The experimental results indicated that the encapsu-lated PGMs micro-grains in the host catalyst carrierminerals could be exposed out after pre-treatmentprocedure. An EDAX analysis of exposed PGMs grainsshowed a complex series of peaks with Pt and Pd barelydetectable from a background matrix of Cu, Ni, Mg, Al,Ce, Na, Si and other minor elements. After the pre-

treated leach residues were pressure cyanide leached,the SEM image and EDAX analysis of the residueshowed only oxides and chlorides of Si, Mg, Al and Cecorresponding to the rare earth oxide and alumino-silicate support matrix. There was no evidence of PGMsor base metals.

4.3. Behaviour of PGMs in pressure cyanide leachingprocess

For pressure cyanide leaching of PGMs, thefollowing chemical reactions occurred.

2Pt þ 8NaCN þ O2 þ 2H2O¼ 2Na2½PtðCNÞ4� þ 4NaOH ð9Þ

2Pd þ 8NaCN þ O2 þ 2H2O¼ 2Na2½PdðCNÞ4� þ 4NaOH ð10Þ

4Rh þ 24NaCN þ 3O2 þ 6H2O¼ 4Na3½RhðCNÞ6� þ 12NaOH ð11Þ

The reactions (9)–(11) are the same as that for goldcyanide leaching. Wadsworth et al. (2000) have reportedthat the gold cyanide reaction mechanism was mainlycontrolled by a surface chemical reaction of gold, andthat the rate was the function of cyanide and oxygenconcentration. Similarly, in the cyanide dissolution ofplatinum group metals, the reaction rate is proposed tobe controlled by a surface chemical reaction. What isdifferent is that the metallic bonding strength of PGMsis higher than that of gold, and a surface oxidepassivating layer is present. Therefore, the cyanideleaching of PGMs occurs only at higher temperatures.

The melting point of the precious metals has anaffinity with their metallic bonding energy. The meltingpoint of Pd, Pt and Rh is 1552, 1772, and 1966 °C,respectively (Mai and Zhou, 2001). Their reactivity withacid also decreases in this order. For example, the finePd powders can be dissolved by concentrated HNO3 orconcentrated H2SO4, whilst Pt can only be dissolved byaqua regia, and Rh cannot be dissolved by aqua regia.Therefore, the order of cyanide dissolution of Pt, Pd andRh should be PdNPtNRh. In the foregoing experimentson cyanide leaching of PGMs from spent auto-catalysts(Huang, 2004), it was found that no matter how thereaction temperature, oxygen pressure and initial NaCNconcentration was changed, the order of cyanideleaching order was still PdNPtNRh.

0.0 0.5 1.0 1.5 2.0 2.5 3.080

82

84

86

88

90

92

94

96

98

100

Pd

Pt

Rh

Per

cent

Ext

ract

ion

, %

PO2 MPa

Fig. 2. Effect of PO2on percent PGM leaching (NaCN 6.25 g/L,

160 °C, 1 h).

0 5 10 15 20 2575

80

85

90

95

100

Pd

Pt

Rh

Per

cent

Ext

ract

ion

, %

Initial NaCN Concentration, g/L

Fig. 3. Effect of NaCN concentration on PGM leaching (160 °C, 1 h.,O2 pressure 1.5 MPa).

100 120 140 160 180

86

88

90

92

94

96

98

100

PdPt

Rh

Per

cent

Ext

ract

ion

, %

Reaction Temperature, Co

Fig. 1. Effect of reaction temperature onPGM leaching. (NaCN6.25 g/L,1 h., O2 pressure 1.5 MPa).


As shown in Fig. 1, when the reaction temperaturewas higher than 160 °C, the percentage of Pd leacheddecreased rapidly because Pd(CN)4

2− is not stable at hightemperature and is easily decomposed to Pd metal. Incontrast, the Pt and Rh cyanide complexes remainedrelatively stable in solution at 180 °C but show a slightdecrease in concentration. At these high temperatures,free cyanide is readily hydrolysed and oxidized and isdecomposed more quickly than the complexed cyanide.The thermal decomposition reactions of Pt, Pd and Rhare as follows.

PdðCNÞ2−4 þ 3=2O2 þ 7H2O¼ Pd þ 4NH3 þ 4CO2 þ 2OH− ð12Þ

PtðCNÞ2−4 þ 3=2O2 þ 7H2O¼ Pt þ 4NH3 þ 4CO2 þ 2OH− ð13Þ

RhðCNÞ3−6 þ 9=4O2 þ 21=2H2O¼ Rh þ 6NH3 þ 6CO2 þ 3OH− ð14Þ

Under higher oxygen pressure, Fig. 2 shows that thepercentage of Pt, Pd and Rh leached all decreased —presumably due to greater surface oxide passivation.However, more rapid oxidation of free cyanide mayhave an effect on the leaching rate. Optimum recoverywas obtained with an oxygen pressure between 1.0 and1.5 MPa. Similarly, there was a small decrease inrecovery at higher concentrations of NaCN as shown inFig. 3.

The reason for the decreased leaching with theincrease of NaCN concentration as shown in Fig. 3, isbelieved to be due to the surface chemical reactionprocess where both CN− and O2 need to be absorbed onthe surface of metal. If the concentration of CN− is toohigh, many of active sites on the metal surface will beoccupied by CN−, which will not favour the absorptionof O2. The existence of an optimum ratio between theconcentration of cyanide and oxygen was also observedin the cyanide leaching of gold (Gu, 1994).

The higher temperature stability of Pt(CN)42− than that

of Pd(CN)42−, can be explained from the reported views


of Chen (1995). He notes that the heavy platinum groupmetal complexes have higher thermodynamic and kineticstability than the light platinum group complexes withthe same valence state, same complexing agent and samecomplex ion geometrical structure. On the other hand,the higher stability of Rh(CN)6

3− relative to Pd(CN)42−

can be explained from the chemical reactivity of thecyanide complex having different geometrical structure.Pd(CN)4

2− has a planar square structure, allowing O2 toattack the central ions along the Z axis. However, Rh(CN)6

3− has an octahedral structure, and the central ion issurrounded by the cyanide ion. Therefore, breakdown ofbond between CN− and central atom is demanded firstbefore reaction with O2 can occur (Huang and Chen,2004a,b).

The above discussion only gives some simple explana-tions for the reaction phenomena involved in the cyanideleaching of PGMs. The actual reaction may be morecomplicated, involving the atomic structure of platinumgroupmetals, the metal bonding strength (Chen, 2001), thecharacter of atoms on the metal surface, the complexationenergy of ions, activation energy of absorbed transition-state and the electron transfer in electrochemical reactions.

5. Conclusions

A new technique for the extraction of PGMs from Pt–Pd sulfide flotation concentrates and spent auto-catalystsby pressure cyanidation has been put forward and the5 kg-scale batch experimental results reported. Forflotation concentrates containing about 80 g/t Pt and Pd,after a pressure acid leaching pre-treatment process, twostages of high-temperature pressure cyanide leaching at160 °C achieved 90–94% and 99% extraction of Pt andPd, respectively. A final precipitate containing 70–90%of the precious metals were obtained from the cyanideleach solution by using zinc cementation. For spent auto-catalysts containing ≈1000–2000 g/t Pt+Pd+Rh, afterpressure alkaline leaching pre-treatment and two stagesof pressure cyanide leaching, the recovery of Pt, Pd andRh were 95–96%, 97–98% and 90–92%, respectively.

A mineralogical investigation on the leaching process-es indicated that the pre-treatment procedure before thepressure cyanide leaching was very important to obtain ahigher overall PGM extraction. In the pressure oxygenH2SO4 leaching process, all of sulfides are oxidized andconverted to sulfates and oxides, therefore the PGMsencapsulated in the sulfides were exposed. After analkaline pressure pre-treatment process of auto-catalysts,the surface accumulated carbon and gasoline contami-nants could be also eliminated, and the same time exposethe honeycomb substrate carrier to PGMs.

For a further understanding of the behaviour of PGMsin the pressure cyanide leaching process, some newexplanations are proposed in the present paper. Thecyanide leaching order of PtNPdNRh appears to berelated to the metal bonding strength. The highertemperature stability of Pt(CN)4

2− relative to that ofPd(CN)4

2− can be explained from the higher thermody-namic and kinetic stability of the heavier platinum groupcomplex. Whilst the higher stability of Rh(CN)6

3− relativeto Pd(CN)4

2− can be explained from the chemical reac-tivity of cyanide complex ions having different geomet-rical structure. This investigation has deepened ourunderstanding of the reaction mechanism for the ex-traction of platinumgroupmetals by pressure cyanidation.

Acknowledgements

This work was supported by National Natural ScienceFoundation of China (50404004) and (50374060 ).

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a new technique for extraction of platinum group metals by pressure cyanidation

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