1758
Ind. Eng. Chem. Res. 1997, 36, 1758-1766
Fundamental Rotating Disk Study of Platinum Recovery from Aqueous Solution by a Reduction/Collection Technique T. N. Angelidis* and K. A. Kydros Department of Chemistry, Box 114, Aristotle University, 54006 Thessaloniki, Greece
S. A. Sklavounos Department of Geology, Aristotle University, 54006 Thessaloniki, Greece
A reduction/collection procedure for the recovery of platinum from aqueous solutions applying metallic iron (in the form of a rotating disk) as reductant and lead or copper ions as reduced platinum collectors was studied. Fundamental kinetic aspects of the process, such as the influence of the pH and of the collector’s concentration on platinum recovery yield, were examined experimentally. The two collectors were compared with respect to the particle size of the precipitant falling from the disk surface. Lead ions seem to give a coarser precipitant compared to copper and increase the pH region at which the reaction takes place. The final product was a mixture of platinum and platinum/lead bimetallic compounds (mainly PbPt). Introduction The occurrence (Fogg and Cornelisson, 1993; Warshawky, 1987) of platinum group metals (PGM) in the Earth’s crust is about 0.01 of a part of million (ppm). Even in platinum-rich deposits (e.g., the Merensky Reef in Transvaal, South Africa) where the PGM concentration is a thousand times higher, it reaches only 10 ppm (Fogg and Cornelisson, 1993; Warshawky, 1987). Yet, despite the low abundance and the very difficult mining conditions and laborious refining process, the world production of PGM is continually rising. Apart from traditional uses in the chemical industry, petroleum refining, the electrical industry, and the glass industry, a new need for platinum-rhodium exhaust gas catalytic converters in automobiles was added in the 1970s. Platinum demand for gas catalytic converters reached 39% of the world platinum demand in 1995 (Cowley, 1996). As a result, the segment of the market with the greatest potential for increased recycling is the automotive market. The efforts to recycle automotive catalysts have been confronted with both technical and economic problems that are only gradually being overcome. As a result, the part of the world platinum demand covered by the recycling of automotive catalysts was only 7% in 1995 (Cowley, 1996). Three general processing routes can be used to recover PGM from scrap autocatalysts (Mehrotra, 1974; D’Aniello, 1982; Hoffman, 1988, 1989; Lakshmanan and Ryder, 1989): hydrometallurgical (selective leaching of platinum or catalyst subsrtate dissolution); hydro-pyrometallurgical (chlorination); pyrometallurgical (plasma fusion, copper collection, copper/nickel smelters). In all cases a PGM concentrate is produced and sent to PGM refineries for final purification. During the preconcentration and refining procedures, main or side acid water streams, which contain small quantities of PGM in soluble form, are produced. Many methods are available for the recovery of platinum from such a solution: ammonia precipitation (Warshawky, 1987), solvent extraction (Warshawky, 1987; Hoffman, 1989), ion exchange (Warshawky, 1987; Hoffman, 1989), sulfide precipitation (D’Aniello, 1982; Hoffman, 1989), and reduction/collection (Hoffman, 1988, 1989). With re* To whom correspondence should be addressed. Telephone/ FAX: +3031997696. E-mail:
[email protected]. S0888-5885(96)00685-9 CCC: $14.00
spect to reduction/collection, reductants commonly employed include sulfur dioxide, stannous chloride, hypophosphorous acid, hydrazine salts, and aluminum scrap (Hoffman, 1988, 1989). The product is a fine metallic platinum powder, which is rather difficult to separate from the water streams. Thus, tellurium oxide (TeO2) is used as a platinum collector. TeO2 is reduced simultaneously and produces platinum telluride (a bimetallic compound of the PtTe type). The simultaneous reduction produces an easily recoverable PtTe sponge. TeO2 is an expensive reagent and a rather complicated recovery procedure (sponge dissolution and TeO2 separation by solvent extraction with tributyl phosphate), follows (Hoffman, 1988, 1989). The main effort of the present research work was to examine fundamental aspects of the use of iron as reductant and lead or copper ions as collectors instead of tellurium oxide. The application of such substances may cause a significant reduction of the separation cost by the use of common and cheap materials. Scrap iron is easily available. Lead and copper are used as platinum collectors in pyrometallurgical practice (Warshawky, 1987; Hoffman, 1989). They are also present on the active surface of automotive catalysts mainly as contaminants (Shelef et al., 1978). Lead especially was accumulated in large quantities on the early automotive catalysts and although unleaded gasoline is now used, the accumulation of lead remains significant (lead concentration up to 1% w/w has been detected in used automotive catalysts) (Hammerle and Graves, 1983; McIntyre and Faix, 1986; Angelidis and Sklavounos, 1995). Lead will pass to the acid water streams and in practice no special addition is required. In order to analyze the kinetics of any reaction in which a solid phase reacts with species dissolved in a liquid, one must be able to describe the hydrodynamics of the reaction system accurately. The most convenient geometry for any fundamental study of such reaction is the rotating disk (Strickland and Lawson, 1973; Miller, 1979). Therefore, a rotating disk of iron was used for the study of the kinetics of the system. Theoretical Aspects In Table 1 the simplified spontaneous reactions involved in the systems under study are presented with the respective standard potentials at 25 °C. The reac© 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1759 Table 1. Simplified Reactions Involved in the Systems under Study and the Respective Standard Potentials at 25 °C (Positive Values Show Thermodynamically Spontaneous Reactions) reaction
E° (V)
Pt-Fe System 1. PtCl62- + 2Fe f Pt + 2Fe2+ + 6Cl2. 3PtCl62- + 4Fe f Pt + 4Fe3+ + 18Cl3. PtCl62- + 2H2 f Pt + 4H+ + 6Cl-
1.145 0.741 0.705
Pt-Pb-Fe System 4. PtCl62- + 2Pb f Pt + 2Pb2+ + 6Cl5. Pb2+ + Fe f Pb + Fe2+ 6. Pb2+ + 2Fe2+ f Pb + 2Fe3+ 7. Pb + 2H+ f Pb2+ + H2 8. Fe + Pb + 2H+ f Fe2+ + Pb + H2 9. 2Fe + Pb + PtCl62- f 2Fe2+ + Pb + Pt + 6Cl-
0.831 0.314 0.896 0.126
Pt-Cu-Fe System 10. PtCl62- + 2Cu f Pt + 2Cu2+ + 6Cl11. Cu2+ + Fe f Cu + Fe2+ 12. 3Cu2+ + 2Fe f 3Cu + 2Fe3+ 13. Cu2+ + H2 f Cu + 2H+
0.368 0.777 0.373 0.337
Side Reactions of Iron 14. Fe + 2H+ f Fe2+ + H2 + 15. 2Fe + 6H f 2Fe3+ + 3H2 16. 4Fe2+ + O2 + 2H+ f 4Fe3+ + 2OH17. 2Fe3+ + Fe f 3Fe2+
0.440 0.036 0.044 1.210
Table 2. Melting Points (Perry and Chilton, 1973) and Hydrogen Overpotentials (1 M H2SO4, 25 °C) (Pourbaix, 1974) for Platinum, Lead, and Copper metal
melting point (°C)
hydrogen overpotential (V)
Pt Pb Cu
1755.0 327.5 1083.0
0.000 0.402 0.190
tion standard potentials were calculated applying thermodynamic data derived from Pourbaix (1974) and Bard et al. (1985). The reduction/collection system is rather complicated due to the simultaneous occurrence of a large number of spontaneous reactions. The following observations are useful for the explanation of the experimental results: a. Lead is reduced in its metallic form by iron (reaction 5) and by ferrous iron ions (reaction 6). b. Copper is reduced in its metallic form by iron (reactions 11 and 12) and by hydrogen (reaction 13). c. Platinum is reduced by iron (reaction 1) as well as by secondary products, such as hydrogen (reaction 3), metallic lead (reaction 4), and metallic copper (reaction 10). d. Iron and lead are dissolved in acidic solution by hydrogen evolution (reactions 14 and 7, respectively) while platinum and copper are not affected. e. Metallic lead in conduct with the iron disk surface is not dissolved, protected by the less noble metallic iron (reactions 8 and 9). f. The ferrous state (bivalent) of iron prevails in solution (reaction 17). In Table 2 two properties of great importance for the systems under study are presented: melting points and hydrogen overpotentials of the respective metals. Melting point is an indication of the energy of crystallization: the higher the melting point the higher is the difficulty of the respective metal to crystallize and form large particles during reduction. Hydrogen overpotential represents the difficulty of hydrogen evolution on the respective metal, which is directly connected to the particle size of the reduced metals. Experimental Section Analytical grade solid hexachloroplatinic acid, lead acetate, or copper sulfate was used for the preparation
of the test solutions. The pH of the solution was adjusted by the addition of hydrochloric acid. Additional quantities of chlorides, in the form of aluminum chloride, were added to ensure a total chloride concentration of 1.5 g-equiv L-1 of solution. This addition was essential in order to keep a constant ionic strength and a substantial excess of chlorides. Chlorides are normally present in all platinum hydrometallurgical streams in the form of aluminum chloride, since the latter is the main product of the autocatalyst washcoat dissolution by hydrochloric acid. The iron disk (diameter 2 cm, area 3.14 cm2) was machined from a pure iron rod. Before each measurement, the active surface of the disk was polished using alumina powder (elementary particle size 0.075 µm), immersed in a 10% w/w hydrochloric acid bath for 30 s to remove any iron oxide formed on the surface and rinsed with distilled water. The upper disk surface was covered by a Teflon cover, so that only the lower surface was free. The disk was mounted on the shaft of a Fischer Stedi-Speed stirrer equipped with a speed controller. An airtight glass vessel, immersed in a temperature-controlled water bath, was used as the reactor. During the experiments, aliquots were withdrawn from the reactor at regular time intervals for chemical analysis. The platinum analysis was carried out by the tin(II) chloride spectrophotometric method (Marzenco, 1976) using a Hitachi U-2000 spectrophotometer, while lead and iron were analyzed by atomic absorption spectroscopy (Perkin-Elmer Model 2380). Unless otherwise stated, the standard experimental conditions were as follows: initial volume of solution 400 mL; disk diameter 2 cm; rotational speed 500 rpm; initial concentration of platinum 50 mg L-1; initial concentration of lead 60 mg L-1; pH 1.5; temperature 25 °C; reaction time 90 min; aliquot volume 8 mL; sampling interval 10 min; number of aliquots 9; aluminum chloride 0.5 M. Additional experiments were carried out at selected conditions in order to examine the following: the morphology of the precipitate on the disk surface by scanning electron microscopy (SEM, JEOL 840-LINK AN10S); the size of the precipitant particles falling from the disk checked by 0.2 and 0.45 µm membrane filters; the qualitative composition of the precipitant by energy dispersion system (EDS, JEOL 840-LINK AN10S, ZAF4); the composition of the precipitant collected in the reactor (after dissolution in aqua regia and chemical analysis); the chemical compounds present in the precipitant by X-ray diffraction (PHILIPS XRD PW1710/ PW1820). The procedures followed for the above examination are described in detail in the respective discussion text. Kinetic Aspects and Presentation of the Results The cementation (metal reduction) reactions are generally assumed to follow first-order kinetics (Strickland and Lawson, 1973; Miller, 1979; Angelidis et al., 1989), while pure metal dissolution follows zero-order kinetics (Angelidis et al., 1993), so the results for platinum, copper, and lead are presented as first-order kinetics plots, log(C/C0) versus t* (where C is the concentration at any time t, C0 is the initial concentration of the respective substance, and t* is the corrected time), and the results for dissolved iron are presented as a zero-order kinetics plot (dissolved iron concentration at any time t versus corrected time). The use of corrected time was essential since the aliquot withdrawn from the reactor causes a significant
1760 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997
volume change during every experimental run (more than 10%). “Corrected time” is that time which would have elapsed if the total volume had remained constant at its initial value. Corrected time is calculated applying the following procedure (Strickland, 1971):
t* ) ∆tSn Sn )
∑{1/[1 - (n - 1)(v/V0)]}
where ∆t ) sampling time interval; n ) number of samples; v ) sample volume, and V0 ) initial solution volume. The cementation system studied is rather complicated (due to simulataneous lead or copper and platinum reduction and the variety of reduction and side reactions shown in Table 1) and strong deviations from first-order kinetics were observed. As shown in Figures 1-4, 8, 10, and 12, six kinetic types were observed (for the experimental time period) in the case of the reduction reactions: type A, one-stage first-order kinetics; type B, twostage first-order kinetics (enchancement); type C, onestage first-order kinetics followed by reaction stop or equilibrium; type D, two-stage first-order kinetics (retardation); type E, two-stage first-order kinetics (reduction followed by dissolution); type F, three-stage firstorder kinetics (enchancement followed by retardation). In the case of iron dissolution types A-D were observed with zero-order kinetics. Types A-D and F are common in all cementation systems where enchancement is attributed to the changes of the anodic and cathodic areas as a substantial quantity of precipitate is accumulated on the disk surface. Retardation or reaction stop occurs when the disk surface is covered by inert reaction products such as hydrogen gas (Miller, 1979; Miller et al., 1990). Type E is a specific case for metalic lead which dissolves in acidic environment when it falls from the disk surface (Table 1, reactions 4 and 7) . The Pt/Pb ratio (Pt and Pb represent the mass of the respective metal recovered from the solution) was used as an indication of the precipitant composition. The Pt/ Fe ratio (Fe represents the mass of iron consumed) was used as an indication of the relative iron consumption. The kinetic type, the calculated rate constants, the time at which the precipitant falls from the disk, the percent yield of platinum recovery in solid form, and the Pt/Pb and Pt/Fe ratios at the end of each experiment (t* ) 98 min) are presented in separate tables. Results and Discussion Effect of Rotational Speed. A series of experiments were carried out in order to study the influence of the rotational speed on the Pt-Pb-Fe system. The results are summarized in Figure 1 and Table 3. At low rotational speeds (up to 100 rpm), no reaction was observed. The entire surface of the disk is covered by hydrogen bubbles. At higher rotational speeds the initially formed hydrogen bubbles are mechanically removed from the surface, due to the increased forces applied to them under these conditions, and reactions take place. The rate of the platinum reduction increases up to 750 rpm and then remains constant. At rotational speeds higher than 500 rpm the precipitant detaches from the surface of the disk is dispersed to a fine powder, due to the vigorous agitation of the solution in the reaction vessel. This powder is not easily separated, since the major part of it passes through the 0.45 µm membrane filter. Therefore, the rest of the experiments were carried out at 500 rpm.
Figure 1. Platinum reduction as a function of time for various rotational speeds of the disk at standard conditions (Pt-Pb-Fe system). Table 3. Influence of Rotational Speed on Platinum Recovery for the Pt-Pb-Fe Systema speed (rpm)
type
KPt1 (min-1)
KPt2 (min-1)
Pt/Fe ratio
Pt/Pb ratio
R%
T (min)
250 500 750 1000
B B B B
0.0008 0.0023 0.0032 0.0032
0.0073 0.0093 0.0133 0.0112
1.066 0.882 0.546 0.710
0.521 1.205 0.746 0.687
49.4 76.7 88.6 85.5
45 43 43
a In the table are shown the reaction kinetic type as defined in the text (type), the first stage rate constant (KPt1), the second stage rate constant (KPt2), the Pt/Fe and the Pt/Pb ratios (at the end of the experiment), the platinum recovery yield (R%, at the end of the experiment), and the time at which part of the precipitant falls from the disk surface (T).
According to Levich (1962) the rate constant of the reaction must be proportional to the square root of the rotational speed, when the reaction is mass transfer (diffusion) controlled. Levich’s equation is applicable when the surface of the disk is completely smooth. In the case under examination, the presence of the precipitant causes significant deviations from Levich’s equation as the roughness of the surface is continually increased (Cornet et al., 1969; Lee et al., 1978; Annamalai and Murr, 1979). The situation becomes more complicated due to the great number of possible reactions (Table 1), the presence of hydrogen bubbles, and the continuation of the reaction in the reaction vesssel, where the mass transfer conditions are completely different. The addition of these steps increases the difficulty of separating the rate-controlling step, although up to 750 rpm the reaction rate is influenced by rotational speed. A rough conclusion is that at low rotational speeds the reaction rate is controlled mainly by the mechanical detachment of hydrogen bubbles, at rotational speeds between 100 and 750 rpm it is controlled mainly by diffusion and the rate of the removal of the hydrogen bubbles (mixed control), and at higher rotational speeds it is under chemical control. Effect of pH: Comparison of Pt-Fe, Pt-Cu-Fe, and Pt-Pb-Fe Systems. The pH is a significant factor in hydrometallurgical practice, since platinum solutions are characterized by relative high acidity. A series of experiments were carried out at pH values
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1761
Figure 2. Platinum reduction as a function of time for various pH values at standard conditions (Pt-Fe and Pt-Cu-Fe systems, first-order kinetics plot).
Figure 4. Dissolved iron production as a function of time for various pH values at standard conditions (Pt-Cu-Fe system, zeroorder kinetics plot). Table 4. Influence of pH on Platinum Recovery for the Pt-Pb-Fe System without and with Lead Predeposition (P) and the Pt-Cu-Fe System (Cu1 Initial Copper Concentration 15 mg L-1 and Cu2 Initial Copper Concentration 60 mg L-1)a pH 0.0 0.5 1.0 1.5 2.0 1.5 (P) 0.0 (P) 1.5 (Cu1) 1.5 (Cu2) 2.0 (Cu2)
KPt1 KPt2 Pt/Fe Pt/Pb type (min-1) (min-1) ratio ratio
R%
C B B B B A C F F B
7.5 51.4 73.7 76.7 74.3 81.0 37.9 88.2 97.3 52.6
0.0011 0.0012 0.0029 0.0023 0.0008 0.0074 0.0057 0.0054 0.0064 0.0009
0.0036 0.0090 0.0093 0.0081 0.0153 0.0389 0.0051
0.084 0.377 0.608 0.882 1.025 0.434 0.073 0.073 0.105 0.064
1.845 1.284 1.450 1.205 1.339 1.204 1.317 3.260 0.866 1.049
T (min)
40 45 50 10 p p
a In the table are shown the reaction kinetic type as defined in the text (type), the first stage rate constant (KPt1), the second stage rate constant (KPt2), the Pt/Fe and the Pt/Pb ratios (at the end of the experiment), the platinum recovery yield (R%), and the time at which part of the precipitant falls from the disk surface (T). p refers to the formation of fine powder from the beginning of the experiment.
Figure 3. Lead reduction as a function of time for various pH values at standard conditions (Pt-Cu-Fe system, first-order kinetics plot).
between 0 and 2 for the Pt-Fe, Pt-Cu-Fe, and PtPb-Fe systems. Pt-Fe System. The results of these experiments for Pt recovery as first-order kinetics plots are shown in Figure 2. For pH values between 0 and 1 the disk is readily covered by hydrogen gas bubbles and platinum concentration changes are not measurable. At pH 1.5-2 recovery reaction is observed during the few first minutes of experiment and then is stopped by hydrogen gas bubble formation. An extremely fine solid Pt powder is produced. This powder disperses in the reaction vessel and passes through the 0.2 µm membrane filter. The above behavior is connected with the low hydrogen overpotential of iron (Pourbaix, 1974) and the fact that Pt is not easily crystallized (Table 2). The reaction that prevails is that of reduction by hydrogen,
which occurs far from the disk surface (Table 1, reaction 3). The results are small recovery yields and a not easily recoverable product. Pt-Cu-Fe System. The results of these experiments for Pt recovery, Cu removal, and Fe dissolution are presented in Figures 2-4 and Tables 4 and 5. The behavior of the Pt recovery the is similar to that of the Pt-Fe system for the 0-1 pH region (absence of reaction in measurable extent and coverage of the active surface by hydrogen gas bubbles). At pH 1.5 reaction takes place and gives considerably higher recovery yields than the respective Pt-Fe system for pH 1.5. The reaction presents enchancement, which is followed by retardation after about 60 min. The latter is followed by hydrogen gas bubble accumulation on the active disk surface. A fine powder is again produced, which disperses in the reaction vessel (it passes through the 0.45 µm membrane filter and only a small part of the total quantity is recovered on the 0.2 µm membrane filter). The presence of metallic copper decreases the hydrogen
1762 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Table 5. Influence of pH on Lead Reduction and Iron Production for the Pt-Pb-Fe System without and with Lead Predeposition (P) and the Pt-Cu-Fe System (Cu1 Initial Copper Concentration 15 mg L-1 and Cu2 Initial Copper Concentration 60 mg L-1)
pH
type Pb
KPb1 (min-1)
KPb2 (min-1)
type Fe
KFe1 (mg L-1 min-1)
0.0 0.5 1.0 1.5 2.0 1.5 (P) 2.0 (P) 1.5 (Cu1) 1.5 (Cu2) 2.0 (Cu2)
E A C F F A F F F B
0.0010 0.0017 0.0037 0.0019 0.0016 0.0029 0.0019 0.0007 0.0032 0.0007
0.0007
C B B B B A B D D A
2.175 0.523 0.328 0.285 0.079 0.903 1.820 8.673 6.194 1.419
0.0056b 0.0041b 0.0056b 0.0166b 0.0296b 0.0036
KFe2 (mg L-1 min-1) 0.766 0.876 0.539 0.583 4.103 3.369 1.817
a In the table are shown the reaction kinetic type as defined in the text (type Pb for lead and type Fe for iron), the first stage rate constant (KPb1 or KFe1), and the second stage rate constant (KPb2 or KFe2). b A third retardation stage was observed.
Figure 6. Comparison of iron dissolution for the Pt-Cu-Fe, CuFe, and Fe systems at standard conditions.
Figure 5. Comparison of copper reduction for the Pt-Cu-Fe and Cu-Fe systems at standard conditions.
overpotential of iron (Table 2) and hydrogen evolution is more severe. The reaction probably continuous through the reduction by hydrogen for both copper and platinum (Table 1, reactions 3 and 13). In Figures 5 and 6 and SEM photos A and B (Figure 7) the Cu-Fe system is compared with the Pt-Cu-Fe at the same condition (pH 1.5, initial copper concentration 15 mg L-1). Copper recovery and iron consumption increase in the presence of Pt, while a less dense precipitant is formed. The above observations confirm the increase of hydrogen gas evolution, due to the decrease of hydrogen overpotential connected with the presence of platinum. At pH 2 the behavior of the system inverts. Both platinum and copper seem to follow a two-stage (enhancement) kinetic type, while only a few bubbles of hydrogen are observed. A part of the precipitant falls from the disk surface after about 40 min and forms a fine powder completely recoverable on the 0.2 µm membrane filter but passing through the 0.45 µm filter. Pt-Pb-Fe System. The results of these experiments are shown in Figures 8-11 and Tables 4 and 5. With respect to the Pt recovery, reaction is observed in the entire pH range studied (0-2). Only at pH 0 the
Figure 7. (a) SEM photo of the disk precipitant for the Cu-Fe system at standard conditions (CCuo ) 15 mg L-1). (b) SEM photo of the disk precipitant for the Pt-Cu-Fe system at standard conditions (CCuo ) 15 mg L-1).
reaction stops due to hydrogen gas evolution; at the higher pH values the reaction continues following the two-stage enhancement kinetic type. The behavior of lead is complicated. At pH 0 a thin metallic powder (lead and platinum mixture) is formed and disperses in the reaction vessel. When the reaction stops, lead is redissolved due to reaction 7 (Table 1). At higher pH
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1763
Figure 8. Platinum reduction as a function of time for various pH values at standard conditions (Pt-Pb-Fe system, first-order kinetics plot).
Figure 9. Platinum reduction rate constants (Pt-Pb-Fe system, K1 for the first reaction stage and K2 for the second reaction stage) as a function of pH.
values the behavior is different. Reaction retardation is observed when part of the precipitant falls from the active disk surface. An explanation of this behavior is that lead is redissolved through reactions 4 and 7 (Table 1) . The latter is connected with the sharp platinum recovery rate enhancement after the detachment of the precipitant. The detached precipitant forms a rather coarse metallic powder (sponge), which is accumulated at the bottom of the reaction vessel and remains there even during the experiment, while the solution is under agitation. This powder is completely recoverable on the 0.45 µm membrane filter. In Figures 12 and 13 the PtPb-Fe, Pb-Fe, and Fe systems are compared with respect to lead reduction. The presence of platinum causes an increase on the reduction rate of lead followed by a third retardation stage due to the higher mass
Figure 10. Lead reduction as a function of time for various pH values at standard conditions (Pt-Pb-Fe system, first-order kinetics plot).
Figure 11. Dissolved iron production as a function of time for various pH values at standard conditions (Pt-Pb-Fe system, zeroorder kinetics plot).
accumulation (lead and platinum) on the disk surface and the fall of the precipitant (followed by dissolution of lead) at earlier stage (Figure 12). With respect to iron consumption, the presence of lead causes a decrease due to the coverage of the surface by metallic lead with high hydrogen overpotential. The simultaneous presence of platinum causes a lesss severe increase than in the Pt-Cu-Fe system (Figure 13). A comparison of the precipitant morphology in the presence and in the absence of platinum is made in Figure 14. The characteristic Pb crystals are formed in the absence of Pt (part A) while in the presence of platinum coarse and flat crystals are formed (part B). Due to the high hydrogen overpotential of lead and the low crystallization energy, reactions 1 and 2 prevail and the crystals
1764 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997
Figure 12. Comparison of lead reduction for the Pt-Pb-Fe and Pb-Fe systems at standard conditions.
Figure 14. (a) SEM photo of the disk precipitant for the Pb-Fe system at standard conditions. (b) SEM photo of the disk precipitant for the Pt-Pb-Fe system at standard conditions.
Figure 13. Comparison of iron dissolution for the Pt-Pb-Fe, PbFe, and Fe systems at standard conditions.
attached to the active surface are increased and detached for mechanical reasons when they have considerably grown. Comparison of Pt-Cu-Fe and Pt-Pb-Fe Systems. Although the Pt-Cu-Fe system seems to give higher reaction rates than the Pt-Pb-Fe system, the latter has other advantages more significant in hydrometallurgical practice. These advantages are the following: 1. The final product, coarse powder, is more easily separated from the reaction solution. 2. The reaction proceeds even at the low pH values observed in the real leaching solutions. 3. Iron consumption is considerably reduced due to the high hydrogen overpotential of lead. 4. Platinum can be easily separated from the produced concentrate by the classic cupelation method
Figure 15. Platinum reduction rate constants (K1 for the first reaction stage and K2 for the second reaction stage) as a function of initial lead concentration.
(Furman, 1963) or by the HOBOKEN process (Warshawky, 1987). 5. Lead is naturally present at substantial quantities in the leaching solution produced from spent automotive catalysts and no additional quantities are required. Therefore, the rest of the present research was focused only on the Pt-Pb-Fe system.
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1765 Table 6. Influence of Initial Lead Concentration on Platinum Recovery for the Pt-Pb-Fe Systema CPbo (mg/L)
type
KPt1 (min-1)
0 10 20 30 40 50 60 70 100
C C C A B B B B B
0.0104 0.0102 0.0035 0.0012 0.0014 0.0024 0.0023 0.0033 0.0041
KPt2 (min-1)
Pt/Fe ratio
Pt/Pb ratio
R%
T (min)
0.0034 0.0064 0.0093 0.0141 0.0186
0.647 0.795 0.395 0.512 0.910 0.900 0.882 1.039 0.968
8.700 2.728 1.141 1.079 1.292 1.205 1.302 0.968
51.9 68.2 43.1 24.2 38.3 64.3 76.7 86.3 91.3
p p p 60 45 48 42
a In the table are shown the reaction kinetic type as defined in the text (type), the first stage rate constant (KPt1), the second stage rate constant (KPt2), the Pt/Fe and the Pt/Pb ratios (at the end of the experiment), the platinum recovery yield (R%, at the end of the experiment), and the time at which part of the precipitant falls from the disk surface (T). p refers to the formation of fine powder from the beginning of the experiment.
Table 7. Influence of Initial Lead Concentration on Lead Recovery and Iron Production for the Pt-Pb-Fe Systema CPbo (mg/L)
type Pb
KPb1 (min-1)
0 10 20 30 40 50 60 70 100
C C D B A F B A
0.0052 0.0049 0.0035 0.0016 0.0029 0.0019 0.0012 0.0029
KPb2 (min-1)
0.0013 0.0041 0.0056b 0.0038
type Fe
KFe1 (mg L-1 min-1)
KFe2 (mg L-1 min-1)
D D D A A B B B B
1.406 1.811 0.730 0.262 0.214 0.216 0.285 0.223 0.234
0.251 0.200 0.464 0.422 0.539 0.501 0.543
a In the table are shown the reaction kinetic type as defined in the text (type) Pb for lead and Fe for iron), the first stage rate constant (KPb1 or KFe1), and the second stage rate constant (KPb2 or KFe2). b A third retardation stage was observed.
Lead Concentration Effect A series of experiments were carried out in order to examine the influence of lead concentration on platinum recovery yield. The results are summarized in Figure 15 and Tables 6 and 7. At low lead concentrations the rate of platinum recovery is high, but the reaction stops after a time period due to hydrogen gas evolution. A fine powder is produced, which passes through the 2 µm membrane filter. The reaction rate decreases as the initial lead concentration increases and reaches a minimum at 30-40 mg of Pb L-1. Then the behavior of the system changes. The recovery rate increases again and a coarse powder is formed when the precipitant falls from the disk surface. An enchancement of the reaction rate was observed at this concentration region. The absence of enchancement on lead reduction confirms the conclusion that lead redissolves after the fall of the precipitant by platinum reduction and hydrogen gas evolution. The Pt/Fe ratio remains almost constant. It is obvious that when the precipitant remains on the disk platinum recovery takes place through hydrogen evolution and iron consumption, since lead is protected by the less noble iron. When the precipitant falls, platinum reduction on lead takes place and gives the sharp enchancement of platinum recovery. The rate constants of platinum recovery increase almost linearly with initial lead concentration for the higher lead concentrations (Figure 15). The platinum recovery yield increases from 38.3% (CPbo ) 40 mg L-1) to 91.3% (CPbo ) 100 mg L-1) at the end of the experimental time in this concentration region (Table 6).
Figure 16. Influence of lead predeposition on platinum reduction (first-order kinetics plot).
Lead Predeposition Since the presence of lead minimizes hydrogen gas evolution due to its high hydrogen gas overpotential, it is obvious that the predeposition of lead on the disk surface may increase the region of pH at which platinum recovery yield is high. To confirm this assumption two experiments were carried out at pH 0 and pH 1.5 after predeposition of lead on the active disk surface (the predeposition conditions were the standard conditions in the absence of platinum). The results are shown in Figures 16. In both cases the platinum recovery yield increases considerably (Table 4): at pH 0 due to the less severe hydrogen evolution and at pH 1.5 increases due to the fall of the precipitant at the initial stages of the reaction and the respective continuation of the reaction by the two parallel reaction paths. Precipitant Composition The precipitant consists of two separate phases: precipitant that remains attached to the disk surface and precipitant that falls in the reaction vessel. Additional experiments at various initial lead concentrations were carried out in order to collect enough precipitant for chemical analysis. The collected precipitant was analyzed after dissolution in aqua regia. The results and the respective conditions are summarized in Table 8. The Pt/Pb ratio is almost constant and independent of the initial lead concentration. The mean value is near the stoichiometric ratio of a bimetallic compound of the type PbPt. To confirm the latter observation, analysis of a precipitant sample by XRD (1.2 °C/min, Cu KR, 35 kV, 25 mA) was carried out. The results are shown in Figure 17. The falling precipitant consists mainly of PbPt and to a lesser extent Pb4Pt and Pt. The precipitant that remains on the disk surface has a different composition, which is close to the composition of the solution. This was confirmed by the mass balance calculations as well as by direct EDS measurements carried out in selected samples.
1766 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Table 8. Composition of the Falling Precipitant (Precipitant), the Solution, and the Precipitant Remaining on the Disk (Disk) precipitant
solution
disk
Pt Pb Pt Pb Pt Pb no.a (mg) (mg) Pt/Pb (mg) (mg) (mg) (mg) Pt/Pbb Pt/Pbc 1 2 3 4
20.0 29.2 32.8 21.3
23.4 33.9 35.2 22.8
0.854 5.0 0.861 7.0 0.931 4.0 0.934 11.3
10.2 15.0 17.8 20.0
25.0 16.4 13.8 21.1 13.2 47.0 17.4 7.2
1.524 0.654 0.280 2.417
1.354 0.554 0.322 2.376
a 1: C -1 Pbo ) 50 mg L , reaction time 2.5 h, volume 1 L, standard conditions. 2: CPbo ) 70 mg L-1, reaction time 2.5 h, volume 1 L, standard conditions. 3: CPbo ) 100 mg L-1, reaction time 2.5 h, volume 1 L, standard conditions. 4: CPbo ) 50 mg L-1, reaction time 1 h, volume 1 L, standard conditions. b Values obtained by mass balance for the Pt-Pb-Fe system. c Values obtained by EDS analysis for the Pt-Pb-Fe system.
Figure 17. XRD pattern of the falling precipitant at standard conditions and the respective possible chemical composition.
Conclusion The fundamental reaction study shows the following: Iron may be effectively used as a reductant for separation of platinum from aqueous solution. Both lead and copper may be used as reduced platinum collectors. Lead seems to have advantages compared to copper, since it extends the pH region of the reduction/collection reaction, it forms a product that is easily separated from the solution, and it minimizes excess iron consumption. The rate of the reaction is a linear function of the initial lead concentration. Predeposition of metallic lead on the iron surface may cause a further increase of the pH region at which the reaction takes place. The product is a metallic platinum concentrate, which mainly consists of a bimetallic compound of the PtPb type. Further research is in progress with respect to the application of the method on real automotive catalyst leaching solutions and of an effective technique for the separation of platinum from the product of the procedure.
Angelidis, T.; Fytianos, K.; Vasilikiotis, G. Lead recovery from aqueous solution and wastewater by cementation utilizing an iron rotating disc. Resour. Conserv. Recycl. 1989, 2, 131. Angelidis, T. N.; Kydros, K. A.; Matis, K. A. A fundamental rotating disc study of gold dissolution in iodine-iodide solutions. Hydrometallurgy 1993, 34, 49. Annamalai, V.; Murr, L. E. Influence of deposit morphology on the kinetics of copper cementation on pure iron. Hydrometallurgy 1979, 4, 57. Bard, J. B.; Parsons, R.; Jordan, J. Standard Potential in Aqueous Solution; Marcel Dekker: New York, 1985. Cornet, I.; Lewis, W. N.; Lightfoot, E. N. The effect of surface roughness on mass transfer to a rotating disc. Trans. Inst.Chem. Eng. 1969, 47, T222. Cowley, A. PLATINUM 1996; Johnson Matthey; May 1996. D’Aniello M. J. Noble Metal Recovery from Spent Automotive Catalysts. SAE 1982, No. 820187, 796. Fogg, C. T.; Cornelisson, J. L. Availability of Platinum and Platinum-Group Metals; U.S. Department of Interior, Bureau of Mines, IC9338, 1993. Furman, N. H. Standard Methods of Chemical Analysis; D. Van Nostrand Co Inc.: New York, 1963. Hammerle, R. H; Graves, Y. B. Lead Accumulation on Automotive Catalysts. SAE 1983, No. 830270, 1. Hoffman, J. E. Recovering Platinum-Group Metals from Auto Catalysts. J. Met. 1988, 6, 40. Hoffman, J. E. Recovery of platinum group metals from automotive catalysts. In Precious and rare earths technologies; Torma, A. E., Gundiler, I. C., Eds.; Elsevier: 1989; p 345. Lakshmanan, V. I.; Ryder, J. Recovery of precious metals from spent automotive catalysts. In Precious and rare earths technologies; Torma, A. E., Gundiler, I. C., Eds.; Elsevier: 1989; p 381. Lee, E. C.; Lawson, F.; Han, K. N. Effect of precipitant roughness on cementation kinetics. Hydrometallurgy 1978, 3, 7. Levich, V. G. Physico-chemical Hydrodynamics; Prentice Hall: Englewood Cliffs, NJ, 1962. Marzenco, Z. Spectrophotometric Determination of Elements; J. Wiley & Sons: New York, 1976. McIntyre, B. R.; Faix, L. J. Lead Detection in Catalytic Emission Systems and Effects on Emissions. SAE 1986, No. 860488, 137. Mehrotra, R. P. Recovery of platinum metals from spent catalysts. Chem. Ind. Dev., July 1974, 26. Miller, J. D. Cementation. In Rate Processes of Extractive Metallurgy; Sohn, H. Y., Wadsworth, M. E., Eds.; Plenum Press: New York, 1979; p 197. Miller, J. D.; Wan, R. Y. ; Parga, J. R. Characterization and Electrochemical Analysis of Gold Cementation from Alkaline Solution by Suspended Zinc Particles. Hydrometallurgy 1990, 24, 373. Perry, R. H.; Chilton, C. H. Chemical Engineers’ Handbook, 5th ed.; McGraw Hill: New York, 1973. Pourbaix M. Atlas of Electrochemical Equilibria in Aqueous Solution; NACE: 1974. Shelef, M.; Otto, K.; Otto, N. C. Poisoning of Automotive Catalysts. In Advances in Catalysis; Academic Press, Inc.: New York, 1978; p 311. Strickland, P. H. The cementation of metals from dilute aqueous solution. Ph.D. Thesis, Monash University, Australia 1971. Strickland, P. H.; Lawson, F. The measurement and interpretation of cementation rate data. International Symposium on Hydrometallurgy; In Evans, D. J. L., Shoemaker, R. S., Eds.; AIME: 1973; p 293. Warshawky, A. Extraction of platinum group metal by ion exchange resins. In Ion Exchange and Sorption Processes; M., Sreat, D., Naden, Eds.; Critical Reports on Applied Chemistry; John Wiley & Sons: New York, 1987; Vol. 19, p 127.
Received for review October 25, 1996 Revised manuscript received January 27, 1997 Accepted January 27, 1997X IE9606855
Literature Cited Angelidis, T. N.; Sklavounos, S. A. A SEM-EDS study of new and used automotive catalysts. Appl. Catal. A: Gen. 1995, 133, 121.
X Abstract published in Advance ACS Abstracts, March 15, 1997.
