Ind. Eng. Chem. Res. 2000, 39, 1185-1192
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Recovery of Platinum-Group Metals from Recycled Automotive Catalytic Converters by Carbochlorination Choong-Hyon Kim,† Seong Ihl Woo,*,† and Sung Hwan Jeon‡ Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea, and National Institute of Environmental Research, Seoul 122-706, Korea
The carbochlorination behavior of scrapped honeycomb-type automobile catalysts was investigated using a chlorine and carbon monoxide gas mixture to fully extract platinum and rhodium in the catalysts. The recoveries of platinum, rhodium, and base metals are monitored by ICPAES analyses. Upflow type of fixed-bed carbochlorination experiments were performed between 250 and 700 °C. The effects of flow rate, time, and partial pressures of chlorine and carbon monoxide were also determined. After optimization of these parameters, the recoveries of about 95.9% of platinum and 92.9% of rhodium were obtained at 550 °C. The recovered condensate was contaminated by various base metal chlorides. The chlorides generated from the base metals could be minimized by decreasing the flow rate of the gas mixture without any deterioration of PGM recoveries, but at the expense of the volatility. The carbochlorination of scrapped automobile catalysts could be an efficient way for the profitable recovery of platinum and rhodium chlorinated compounds at comparatively low temperatures. 1. Introduction About 34% of total platinum, 55% of total palladium, and 95% of total rhodium demand is now used for the production of autocatalysts. The automotive industry consumed these metals worth more than about 2.2 billion dollars in 1998.1 The overall demand for these metals is expected to increase in the near future, especially with the introduction, in 1998, of mandatory national low-emission vehicle (NLEV) limits in the United States and Stage III emissions legislation in the European Union. Despite the rapidly growing platinum group metals (PGM) demand in an autocatalyst, less than 10% of PGM from scrapped converters are currently recycled.1 Hence, it is necessary to develop an economic and environmental-friendly recovery process of PGM.2-4 An autocatalyst essentially comprises a refractory oxide support on which two or more precious metals are dispersed in very low concentrations (0.1-0.3 wt % of the monolith). The monolith honeycomb-type body is typically made of cordierite (2MgO‚2Al2O3‚5SiO2) and washcoat (10-30 wt % of cordierite), a mixture of predominantly γ-Al2O3 and various proprietary base metals, to provide a high surface area film on which the catalytic component (PGM) is highly dispersed. When spent autocatalysts are subjected to the recovery process, it is very important to reclaim the full amount of PGM in a spent catalyst in view of reclamation economics.5 In general, it is thought that the overall recovery level of PGM should be more than 95% for a profitable process with the permissible level of secondary pollution. Main process technologies for the recovery of PGM can be categorized as follows: (1) Hydrometallurgical or solution extraction,6,7 (2) pyrometallurgical, and (3) gasphase volatilization or selective chlorination processes.5 * Corresponding author. Telephone: +82-42-869-3918. Fax: +82-42-869-3910. E-mail:
[email protected]. † Korea Advanced Institute of Science and Technology. ‡ National Institute of Environmental Research.
Several review articles were devoted to discuss these recovery processes.5,8-10 Recycled autocatalysts are commonly added to copper or nickel smelting feeds. However, a selective chlorination process has several advantages over the other processes, economically and environmentally.8 Especially, the rhodium recovery by the hydrometallurgical process is known to be far below the economic level. In this study, we propose a novel carbochlorination process which decreases the chlorination temperature in the chlorination process and increases the recovery rate. In the chlorination or carbochlorination process, valuable metals are converted to their corresponding chlorides and then separated on the basis of the difference in volatility between the metal chlorides11 or could be separated by repulp washing12,13 or by using adsorption on the activated carbon bed.14 Carbon monoxide could be used as a reducing agent which makes the overall reaction thermodynamically favorable (vide infra).15 There has been no academic report on the chlorination of spent autocatalysts. Only few articles were reported about the chlorination of the spent hydrotreating catalysts.16,17 On the other hand, the chlorination or carbochlorination process is utilized to some extent for some refractory metals, such as zirconium, titanium, beryllium, and tantalum.18 The main advantage of using metallic chlorides to produce pure metals is based upon the fact that they are easily separable, purified through distillation, sublimation, or crystallization, and finally reduced to metals in a simple way like electrolysis. In this study, almost all of the experiments were conducted using unroasted spent catalysts. The hydrocarbons and carbon present in the unroasted spent catalysts could be used as reducing agents and a source of energy. In addition, the roasting step will increase the capital investment and operating cost. Finally, it was shown that the roasting leads to physicochemical modifications in the sample, leading to the lower extraction rates of the valuable elements during the carbochlorination step.16
10.1021/ie9905355 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/24/2000
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Table 1. Chemical Analysis of the Spent Automotive Catalysts by ICP-AES elements PGM (ppm wt) Pt
Rh Pd
2300 530
base metals (wt %) Ce
Zr
Ba
Mg
Al
6.153 1.467 1.155 5.893 20.27
remarks 105
miles of aging tested
In this work, the optimization of the carbochlorination process of unroasted scrapped autocatalysts are attempted by varying such operating parameters as flow rate, reaction time, reactant gas composition, and reaction temperature. 2. Experimental Work 2.1. Materials. The samples after a 100 000 miles (160 930 km) aging test at the dynamo were used for the recovery of PGM in the carbochlorination tests. The front and back bricks of monolith catalysts in the container were crushed and ball-milled for 24 h to make the powder smaller than 105 µm (150 mesh). The chemical composition of the main elements (more than 1 wt %) of this powder sample is given in Table 1. This analysis was done by the ICP-AES method using microwave-assisted mixed acids (HF + aqua regia) digestion pretreatment.19 Besides, there were a number of elements in the sample, that is, Ti, La, Pb, S, P, Zn, Mn, Ca, Na, Fe, Ni, Cu, and Cr. Pb and S would be from fuel. P, Zn, Mn, Ca, and Na could be originated from the lubricants, whereas Fe, Ni, Cu, and Cr would be metallic parts of the engine and exhaust gas system.5,20 Nitrogen (99.95%), Cl2 (97+%), and CO (99.95%) gases were used without any further purification. 2.2. Apparatus and Procedure. Although most of the investigation was devoted to the optimization of carbochlorination, it was necessary to perform the preliminary aqua regia leaching and carbochlorination experiments to obtain the typical hydrometallurgical results and to select appropriate pretreatment steps. The preliminary aqua regia leaching apparatus consisted of a 250-cm3 flask equipped with a water-cooled condenser, an oil bath emmersion heater, and a stirrer. Second, the preliminary carbochlorination experiment was performed at the vertical tubular reactor equipped with coldfinger or a typical boat-typed horizontal reactor. The optimization study of the carbochlorination reaction was carried out in a fixed-bed reactor system that consisted of a gas inlet system, a quartz reactor, and a product gas-cooling system as shown in Figure 1. The fixed-bed reactor consisted of an inverted “L”shaped vertical quartz tubular reactor 25-cm long with a 6-cm inside diameter. The reactor was externally heated in an electrical heater. The temperature of the bed was measured by a thermocouple immersed in a thermowell which is immersed in the bed. The experimental procedure of carbochlorination is as follows: After the required amount of spent automotive catalyst was introduced into the reactor, it was heated in a N2 atmosphere until the desired temperature was reached. After the removal of moisture in the sample for 1 h, the reactant gases (CO and Cl2) were introduced to the reaction zone for the predetermined reaction time. Then, the reactor system was cooled to ambient temperature in a N2 atmosphere. The volatile PGM chlorides and/or carbonyl chlorides and other volatile elements were separated from the exhaust gas by their condensation on the reactor wall and condenser
Figure 1. Schematic diagram of the experimental apparatus for carbochlorination. Table 2. PGM Recoveries by Aqua Regia Leachinga leaching temperature (°C) 25 Pt Rh a
32.3 12.1
50 Recovery (%) 48.6 26.7
80
95
53.6 37.6
55.8 43.3
Aqua regia extraction condition: 10 g/30 mL, 4 h.
wall. Finally, the exhausted gas containing unreacted Cl2 was neutralized by an aqueous 2.5 M NaOH solution before venting. The resulting chlorination condensate and remaining solid residue containing carbochlorination products were then hydrolyzed separately for compositional analysis. Chlorination condensate was hydrolyzed at room temperature with 1 M HCl, using a solid-to-liquid ratio of 30 gsample/100 cm3 to dissolve out Pt, Pd, and other volatile chlorides. Solid residue was hydrolyzed with hot 1 M HCl for 30 min, using a solidto-liquid ratio of 30 gsample/30 cm3 to dissolve out Rh, Pt, Pd, and other chlorides. This solid residue was then filtered. The remaining residue was dried in air at about 100 °C before elemental analysis. The metal content of dissolved solution was measured with ICP-AES (ICPS1000III, Shimadzu, Japan). 3. Results and Discussion 3.1. Preliminary Aqua-Regia Test. Thirty cubic centimeters of aqua regia (1HNO3/3HCl (v/v) mixture) is added to 10 g of catalyst sample and leached for 4 h at various temperatures. The effect of temperature on leaching is shown in Table 2. PGM recoveries increased with the extraction temperature. However, the maximum obtainable PGM recoveries were 55.8% for Pt and 43.3% for Rh, respectively. Hydrometallurgical treatment of our sample with aqua regia at typical reaction conditions resulted in economically unfeasible PGM recoveries. 3.2. Role of CO in Carbochlorination. The standard Gibbs free energy changes of chlorination and carbochlorination reactions at 500 °C are calculated with the assumption that all the base metals involved are present in the spent catalysts as oxides, as summarized in Table 3. The base metals were selected such that the contents of those metals were found to be more than that of PGM. Because of the lack of data concerning the chlorides of cordierite, only simple constituting oxides of this material are considered. The data of PGM also indicate that the oxides are thermodynamically more stable than the corresponding metals for chlorina-
Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000 1187 Table 3. Main Chlorination and Carbochlorination Reactions of the Spent Automobile Catalysts and Their Standard Gibbs Free Energy Change at 500 °C, in kJ/mol Cl215,21-27 reactions Pt + Cl2 PtO + Cl2 PtO + Cl2 + 1/2C PtO + Cl2 + CO Pd + Cl2 PdO + Cl2 PdO + Cl2 + 1/2C PdO + Cl2 + CO 2/ Rh + Cl 3 2 1/ Rh O + Cl 3 2 3 2 1/ Rh O + Cl + 1/ C 3 2 3 2 2 1/ Rh O + Cl + CO 3 2 3 2 1/ SiO + Cl 2 2 2 1/ SiO + Cl + CO 2 2 2 1/ Al O + Cl 3 2 3 2 1/ Al O + Cl + CO 3 2 3 2 MgO + Cl2 MgO + Cl2 + CO 2/ CeO + Cl 3 2 2 2/ CeO + Cl + 4/ CO 3 2 2 3 1/ ZrO + Cl 2 2 2 1/ ZrO + Cl + CO 2 2 2 BaO + Cl2 BaO + Cl2 + CO FeO + Cl2 FeO + Cl2 + CO NiO + Cl2 NiO + Cl2 +CO PbO + Cl2 PbO + Cl2 + CO Na2O + Cl2 Na2O + Cl2 + CO ZnO + Cl2 ZnO + Cl2 + CO 1/ TiO + Cl 2 2 2 1/ TiO + Cl + CO 2 2 2 CaO + Cl2 CaO + Cl2 + CO 1/ La O + Cl 3 2 3 2 1/ La O + Cl + CO 3 2 3 2
) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) )
∆G° at 500 °C, kJ -34.7 -28.7 -227.2 -244.7 -45.8 -8.6 -207.2 -224.7 -25.9 +98.6 -99.9 -117.4 +113.2 -102.9 +117.7 -98.3 +3.8 -212.2 +195.8 -92.2 +187.2 -28.8 -114.7 -330.7 -32.8 -248.8 -20.2 -236.2 -101.2 -317.2 -180.1 -396.1 -37.4 -253.4 +58.9 -157.1 -128.4 -344.5 -39.6 -255.6
PtCl2 PtCl2 + 1/2O2 PtCl2 + 1/2CO2 PtCl2 + CO2 PdCl2 PdCl2 + 1/2O2 PdCl2 + 1/2CO2 PdCl2 + CO2 2/ RhCl 3 3 2/ RhCl + 1/ O 3 3 2 2 2/ RhCl + 1/ O 3 3 2 2 2/ RhCl + CO 3 3 2 1/ SiCl + 1/ O 2 4 2 2 1/ SiCl + CO 2 4 2 2/ AlCl + 1/ O 3 3 2 2 2/ AlCl + CO 3 3 2 MgCl2 + 1/2O2 MgCl2 + CO2 2/ CeCl + 2/ O 3 3 3 2 2/ CeCl + 4/ CO 3 3 3 2 1/ ZrCl + 1/ O 2 4 2 2 1/ ZrCl + CO 2 4 2 BaCl2 + 1/2O2 BaCl2 + CO2 FeCl2 + 1/2O2 FeCl2 + CO2 NiCl2 + 1/2O2 NiCl2 + CO2 PbCl2 + 1/2O2 PbCl2 + CO2 2NaCl + 1/2O2 2NaCl + CO2 ZnCl2 + 1/2O2 ZnCl2 + CO2 1/ TiCl + 1/ O 2 4 2 2 1/ TiCl + CO 2 4 2 1 CaCl2 + /2O2 CaCl2 + CO2 2/ LaCl + 1/ O 3 3 2 2 2/ LaCl + CO 3 3 2
tion at 500 °C. The data for the thermodynamic calculations are based on the values obtained from refs 15 and 21-27. It can be deduced from their standard free energy change that these reactions will be enhanced in the presence of CO or C. 3.3. Preliminary Chlorination Test. The effects of pretreatment and reaction temperature were examined, as shown in Table 4. The effect of pretreatment on the recovery rate was explored at 250 °C, CO/Cl2 ) 2, and total flow rate ) 300 cm3/min for the 10 g of spent
catalysts. These experiments were performed in a coldfinger-type reactor and the reaction period was comprised of 10 cycles of CO/Cl2 flow for the initial 10 and 50 min in the stagnant atmosphere of CO/Cl2. PGM was recovered as a volatilized condensate and leachate of solid residue. When the sample is carbochlorinated at 250 °C without any pretreatment, the obtained PGM recovery rates were 59% for Pt and 58.3% for Rh. When the sample is calcined at 500 °C for 6 h in air before carbochlorination, the only noticeable change was the decrease in the portion of the volatile condensate and the slight decrease in the total recovery of Pt. From this result, it was decided to perform the following experiments without any pretreatment. The smaller recovery of Pt from the calcined sample might be due to the absence of coke which could be utilized as a reducing agent. Reduction pretreatment of the sample enhanced the recoveries of both Pt and Rh a little, as shown in Table 4. The reaction temperature of 250 °C has been selected because Pt can react with CO and Cl2 to form Pt carbonyl chlorides at this temperature.28,29 However, the profitable recovery level of PGM could not be attained. Therefore, the reaction temperature was increased to 350 and 500 °C. As the reaction temperature increased, the recovery rate of Pt and Rh increased significantly, as shown in Table 4. About 85% of Pt and 98% of Rh were recovered from a spent autocatalyst sample. This high recovery rate stimulated us to investigate further the potential possibility of the commercialization of the carbochlorination process. 3.4. Optimization of the Process Variables of Carbochlorination. 3.4.1. Effect of Gas Flow Rate. The effects of the total gas flow rate on the recoveries of PGM and base metals were examined in the flow rate range between 30 and 480 cm3/min for 30 g of sample at 500 °C. Figure 2 shows PGM recoveries with the gas flow rate. Total Pt recovery was around 75%, regardless of the gas flow rate. On the other hand, an increased gas flow rate caused the increase of volatile condensate portion. In the case of Rh, maximum Rh recovery of about 90% was achieved at the flow rate of 120 cm3/ min. Most of the rhodium was recovered in the chlorinated solid residue which is in agreement with the fact that the volatilization of rhodium chloride requires a temperature above 700 °C.11,30 The existence of the maximum Rh recovery implies that there is an optimum contact time for conversion of rhodium to rhodium chlorides. Base metals recoveries with respect to flow rate were also examined (Table 5). Al, Mg, Ce, Zr, and Ba, which
Table 4. PGM Recoveries by Preliminary Carbochlorination Experiments; Effects of Pretreatment and Temperaturea,b recoveries Pt experimental conditions pretreatment: none 250 °C, 10 h, CO/Cl2 ) 2 pretreatment: 500 °C, 6 h in air 250 °C, 10 h, CO/Cl2 ) 2 pretreatment: 300 °C, 6 h in CO 250 °C, 10 h, CO/Cl2 ) 2 pretreatment: none 350 °C, 10 h, CO/Cl2 ) 2 pretreatment: none 500 °C, 1 h 15 min, CO/Cl2 ) 4/6
Cc
Ld
16.5
42.5
4.7
Rh total
C
L
total
remarks
59
3.5
54.8
58.3
coldfinger type
48.2
52.9
1.5
57.8
59.3
coldfinger type
9.8
58.5
68.3
2.8
73.7
76.5
coldfinger type
44.3
26.2
70.5
2.9
70.7
73.6
coldfinger type
25
59.7
84.7
0.5
97.2
97.7
boat type
a Total flow rate: 300 cm3/min. b Residue leaching conditions: 10 g/1 M HCl 250 mL, 80 °C, 4 h. c C ) recovery originated from condensate portion. d L ) recovery originated from leaching filtrate portion.
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Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000
Figure 2. Effect of total gas flow rate on the PGM recovery (30 g of sample, reaction temperature ) 500 °C, reaction time ) 1 h, CO/Cl2 ) 4/6). Table 5. Effect of Flow Rate of Reactant Gas Mixtures on the Base Metals Extractiona total flow rate (cm3/min) 30 Al (20.3%)d Mg (5.89%) Ce (6.15%) Zr (1.47%) Ba (1.16%)
120
240
480
Base Metals Recovery (%) Cb 1.3 3.1 9.9 Lc 2.4 3.4 3.0 total 3.7 6.5 12.9 C 0.04 0.06 0.3 L 0.4 0.7 1.3 total 0.44 0.76 1.6 C 0.7 0.8 1.2 L 21.8 41.4 53.1 total 22.5 42.2 54.3 C 73.6 66.6 84.8 L 0.2 0.4 0.9 total 73.8 67.0 85.7 C 0.6 0.7 1.2 L 37.8 39.8 54.7 total 38.4 40.5 55.9
60
15.9 1.7 17.6 0.6 1.6 2.2 1.6 33.4 35.0 86.1 0.4 86.5 1.4 44.9 46.3
19.6 3.5 23.1 0.7 1.8 2.5 2.3 38.5 40.8 80.9 6.2 87.1 1.8 43.2 45.0
a Fixed conditions: 30 g of sample, temperature ) 500 °C, time ) 1 h, CO/Cl2 ) 4/6. b C ) recovery originated from condensate portion. c L ) recovery originated from leaching filtrate portion. d For convenience, elemental compositions are listed in parentheses.
were the main constituting elements of more than 1 wt %, were selected and analyzed. In the case of Al, the recovery increased with the total flow rate, as could be seen in Table 5. At the flow rate of 480 cm3/min, 23.1% of Al was extracted almost as volatile condensate (Al content in the sample was 20.3 wt. %), indicating that most of the Al from alumina washcoat was recovered by volatilization. Mg recovery also increased with the gas flow rate. However, only 2.5% of Mg was recovered at 480 cm3/min (the content of Mg was 5.89 wt %). Because Mg is originated exclusively from the cordierite support, the low recovery rate of Mg means that the extent of the carbochlorination of cordierite support is practically negligible. This result also agrees well with the reported results.11 The recovery behaviors of Ce and Ba were similar to those of Rh. Almost all of the Ce and Ba were recovered from the solid residue, and the
Figure 3. Effect of reaction time on PGM recovery (30 g of sample, reaction temperature ) 500 °C, total flow rate ) 120 cm3/min, CO/Cl2 ) 4/6).
maximum recoveries of Ce and Ba were 54.3% and 55.9%, respectively, at the condition 120 cm3/min of flow rate. In the case of Zr, the recovery is practically not affected by the flow rate. It was recovered almost only as a volatile condensate and the maximum recovery approached 90%. In summary, Al was the most prominently responding element to the total flow rate variable. 3.4.2. Effect of Reaction Time. Figure 3 shows the effect of reaction time on PGM recovery at 500 °C; 120 cm3/min of the total flow rate was selected because maximum Rh recovery was obtained. After 2 h of reaction time, practically all of the Rh was recovered. This is a very impressive result because Rh recovery extracted from the scrapped autocatalyst by conventional methods could not reach above 85%. This high recovery rate of Rh is the main advantage of carbochlorination considering the high price of Rh. The recovery of Pt was increased up to 60 min with the increase of reaction time. After that time, the recovery was maintained at a constant value around 75%. Therefore, it can be concluded that the carbochlorination reaction of Pt was completed after 60 min at this condition. The volatile condensate portion was also increased with the reaction time. The maximum Pt and Rh recoveries obtained were 76% and 101.8%, respectively. The recovery exceeding 100% is thought to come from the inhomogeneity between the samples or experimental error. To help in the interpretation of experimental data, their mathematical formulation was tried using the following equations:31,32
kt ) X
(1)
kt ) 1 - (1 - X)1/3
(2)
kt ) 1 - 3(1 - X)2/3 + 2(1 - X)
(3)
where k is a constant, t is the chlorination time (min),
Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000 1189 Table 6. Effect of Reaction Time on the Base Metals Extractiona reaction time (min) 10 Al (20.3%)d Mg (5.89%) Ce (6.15%) Zr (1.47%) Ba (1.16%)
20
40
60
Base Metals Recovery (%) Cb 0.1 0.9 4.8 9.9 Lc 1.0 1.6 1.9 3.0 total 1.1 2.5 6.7 12.9 C 0.02 0.03 0.13 0.3 L 0.4 0.8 1.2 1.3 total 0.42 0.83 1.33 1.6 C 1.2 0.9 1.1 1.2 L 6.3 29.1 37.1 53.1 total 7.5 30.0 38.2 54.3 C 2.3 28.2 87.5 84.8 L 1.4 0.8 3.2 0.9 total 3.7 29.0 90.7 85.7 C 1.3 0.9 1.0 1.2 L 25.6 32.9 41.3 54.7 total 26.9 33.8 42.3 55.9
120
240
19.9 1.4 21.3 0.7 3.2 3.9 2.1 84.5 86.6 99.3 0.03 99.6 1.8 65.8 67.6
27.0 1.3 28.3 0.8 3.4 4.2 1.7 90.0 91.7 94.5 1.3 95.8 1.5 61.2 62.7
a Fixed conditions: 30 g of sample, temperature ) 500 °C, flow rate ) 120 cm3/min, CO/Cl2 ) 4/6. b C ) recovery originated from condensate portion. c L ) recovery originated from leaching filtrate portion. d For convenience, elemental compositions are listed in parentheses.
Figure 4. Plot for the kinetic interpretation of carbochlorination of Pt and Rh (conditions the same as those in Figure 3).
X is the extent of reaction (ratio of weight of the reacted fraction to the initial weight). Equation 1 describes a film diffusion control. Equation 2 can be used for a reaction controlled by the chemical reaction in shrinking nonporous spherical particles (with or without a solid porous product) and porous spherical particles with unchanged overall sizes. It also applies for pore diffusion control in the case of complete gasification of porous solids. Equation 3 applies for pore diffusion control in a reaction of porous or nonporous spherical particles with a porous product layer. At 500 °C, the best correlation of the experimental data for Pt was obtained by eq 3, while that for Rh could be obtained either by eqs 2 or 3, as illustrated in Figure 4. Thus, the controlling mode of carbochlorination of Pt could be described as ash diffusion controls, while that of Rh is intermediate between reaction controls and ash diffusion controls. It seems that the rate of carbochlorination of Pt is less inhibited by diffusion than Rh. For the higher recovery rate of PGM, therefore, an increase of the partial pressure of the reactant gases or decrease of the particle size of the sample might be necessary to facilitate the diffusion through the ash layer. The effect of reaction time on the base metals extraction was also considered. The recovery of Al increased with the reaction time (Table 6). After 4 h of reaction, 28.3% of Al is recovered and this means that practically all of the washcoat alumina is recovered. In the case of Ce, Zr, and Ba, the recoveries were significantly increased with the reaction time. After 4 h, practically all the Ce and Zr were recovered, and Ba was recovered up to about 70%. 3.4.3. Effect of Chlorine and Carbon Monoxide Partial Pressures. Experiments were conducted to investigate the effect of partial pressures of the reactant gases on metal recoveries. The flow rates of the reactant gases were varied to give different partial pressures. Obtained results regarding PGM recoveries are shown in Figure 5. In the case of Pt, the recovery rate was varied significantly with the CO/Cl2 ratio. If CO is absent, Pt recovery drops to about 50%. In the range of
Figure 5. Effect of feed composition on the PGM recovery (30 g of sample, reaction temperature ) 500 °C, reaction time ) 0.5 h, total flow rate ) 240 cm3/min).
2/8 to 7/3 of CO/Cl2, Pt recovery maintains the value around 80%. If the CO/Cl2 ratio exceeds 5/5, then almost all of the Pt was recovered as volatile condensate. This phenomenon may be caused by the formation of a kind of volatile Pt carbonyl chloride compound. Because no significant decrease in Pt recovery was observed, it was thought that the reduction of Pt chloride to Pt metal by CO did not occur under current experimental condition. The behavior of Rh recovery also had peculiar characteristics. If the CO partial pressure is absent, Rh recovery decreases below 30%. If 10 vol % of CO is added to the system, the recovery is changed to near 80%. Therefore, the importance of CO in the chlorination of PGM is clear, as shown previously in Table 3. Rh recovery increases up to about 90%, as the partial pressure of CO in the CO/Cl2 mixture increases up to 50 vol %. However, if CO exceeds 50 vol %, Rh recovery
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Table 7. Effect of Feed Composition on the Base Metals Extractiona CO/(CO + Cl2) (%) 0 Al (20.3%)d Cb Lc total Mg (5.89%) C L total Ce (6.15%) C L total Zr (1.47%) C L total Ba (1.16%) C L total
10
40
50
60
Base Metals Recovery (%) 0 0.9 7.9 7.0 9.6 2.1 1.4 1.1 1.8 1.4 2.1 2.3 9.0 8.8 11.0 0 0.2 0.3 0.4 0.4 0.9 1.4 1.5 2.6 1.5 0.9 1.6 1.8 3.0 1.9 0 3.6 1.3 1.6 1.7 2.4 49.4 61.1 85.5 48.9 2.4 53.0 62.4 87.1 50.6 0 22.2 90.4 91.3 87.0 1.5 0.2 3.1 2.0 2.4 1.5 22.4 93.5 93.3 89.4 0 0.7 1.2 1.4 1.3 28.6 50.8 44.2 71.6 47.3 28.6 51.5 45.4 73.0 48.6
70
80
90
6.5 1.3 7.8 0.2 1.1 1.3 1.0 40.8 41.8 83.2 0.3 83.5 0.9 40.7 41.6
4.8 1.5 6.3 0.2 0.9 1.1 1.3 37.4 38.7 77.0 1.7 78.7 1.1 39.8 40.9
2.2 1.1 3.3 0.3 0.6 0.9 1.5 40.2 41.7 79.8 0.4 80.2 1.9 43.9 45.8
a Fixed conditions: 30 g of sample, temperature ) 500 °C, time ) 0.5 h, flow rate ) 240 cm3/min. b C ) recovery originated from condensate portion. c L ) recovery originated from leaching filtrate portion. d For convenience, elemental compositions are listed in parentheses.
decreases below 50% and maintains the value to the condition of 90 vol % of CO. The low recovery of Rh at the condition exceeding 50 vol % CO could be attributed to the partial reduction of RhCl3 to Rh metal which could not be hydrolyzed in the following procedures. The effect of feed composition on base metals recovery was also considered. In the case of Al, the selected set of conditions seems to be efficient to some extent in suppressing extraction of Al (Table 7). In general, the total recoveries of base metals were maximized when the CO/Cl2 composition ratio approaches unity. The maximum attainable recoveries of Ce and Zr were around 90% and that of Ba was 73%. In the case of Ce and Ba, the recovery behaviors are very similar to those of Rh. They increased fast up to the condition of 50 vol % of CO and then maintained the level below 50% of recovery. 3.4.4. Effect of Temperature. The effect of reaction temperature on metal recoveries was examined, and the results on PGM recoveries are shown in Figure 6. The reaction temperature was varied from 250 to 700 °C. As the reaction temperature increased, both Pt and Rh recoveries reached their maximum at 550 °C and then decreased. The obtained maximum recoveries of Pt and Rh were 95.9% and 92.9%, respectively. Total PGM recovery was 95.4%. This recovery rate is high enough to be commercialized. Because the selected flow rate (30 cm3/min) was rather small, the volatile portion of Pt was also very small. Generally, it is known that platinum dichloride is decomposed to metal itself via monochloride above 581 °C in Cl2.30,33 Pt metal reduced in this way cannot be dissolved in the following process and does not have volatility, Accordingly, the slight decrease in Pt recovery above 580 °C could be partially attributed to the partial decomposition of platinum dichloride to platinum metal. However, in the case of Rh, its chloride is decomposed to metal between 920 and 950 °C.30,33 Therefore, it is thought that the speculation causing a decrease of Pt recovery with temperature discussed above would not be a major contribution. The decrease in PGM recovery above 550 °C could then be explained by the anomaly which has a local
Figure 6. Effect of reaction temperature on PGM recovery (30 g of sample, reaction time ) 1 h, total flow rate ) 30 cm3/min, CO/ Cl2 ) 4/6).
maximum at about 550 °C brought by the photochemical formation of phosgene (COCl2) in the gas feeding line at ambient conditions, known to be a more effective chlorinating agent than the CO/Cl2 gas mixtures,34 accompanied by the dissociation of COCl2 above 550 °C. This hypothesis agrees with the conclusions of other authors who have observed a similar anomaly during the carbochlorination of other oxides such as V2O5,32 R-Al2O3,34 and ZrO2.35 In their data, the abnormal behavior starts at ≈560-600 °C and was persistent to about 850 °C in the majority of the cases. The fact that V2O5, R-Al2O3, and ZrO2 exhibit similar anomalous effects in chlorination reactions with CO/Cl2 mixtures is a strong indication that the observed anomaly is independent of the solid reactant. Especially, Soleiman and Rao34 have extensively investigated whether the phosgene is formed by the ultraviolet irradiation of the CO/Cl2 mixture at room temperature and provided experimental evidence for such an effect. Proper mixing of the gas mixture and effective UV irradiation by a high-pressure mercury arc lamp could result in more than a 10-fold increase in the reaction rate at the local maximum of the the anomaly. They concluded that no special radiation equipment may be necessary in practice because of the much longer exposure time than the mercury vapor lamp and it may suffice to expose the CO/Cl2 gas mixture directly to the sunlight which can effectively initiate the photochemical reaction and thereby ensure the formation of COCl2. They also argued that the local maximum, that is, the temperature above which COCl2 dissociation becomes significant, depended on the reactor geometry and the flow dynamics of the gases entering the reactor. In our case, the gas feeding line was made of a nearly transparent PTFE tube. Therefore, the maximum PGM recoveries obtained at a comparatively low temperature (550 °C) are thought to benefit from the formation of COCl2. Intensive UV irradiation and premixing should be investigated further to lower the temperature of local maximum which has a profitable recovery rate.
Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000 1191 Table 8. Effect of Reaction Temperature on the Base Metals Extractiona reaction temperature (°C) 250 Al (20.3%)d Lc total Mg (5.89%) L total Ce (6.15%) L total Zr (1.47%) L total Ba (1.16%) L total
Cb 2.3 3.7 C 1.1 2.6 C 13.9 14.7 C 2.9 4.4 C 45.6 46.0
400
500
550
580
Base Metals Recovery (%) 1.4 0.7 1.3 1.8 1.2 4.8 2.4 4.5 4.5 3.8 5.5 3.7 6.3 5.7 5.1 1.5 0.05 0.04 0.1 0.1 2.9 0.4 2.1 2.0 2.5 2.95 0.44 2.2 2.1 2.53 0.8 0.9 0.7 2.1 1.8 44.2 72.6 69.9 90.2 99.0 45.1 73.3 72.0 92.0 99.8 1.5 69.5 73.6 41.4 16.7 0.7 0.2 1.1 0.7 0.2 70.2 73.8 42.5 17.4 37.5 0.4 0.9 0.6 2.1 1.6 70.0 37.8 63.8 49.3 64.7 70.9 38.4 65.9 50.9 65.5
600
700
1.3 0.5 1.4 1.9 0.03 0.06 11.3 11.36 0.8 0.5 84.0 84.5 37.3 36.9 1.3 38.2 0.8 0.5 42.5 43.0
a Fixed conditions: 30 g of sample, time ) 1 h, flow rate ) 30 cm3/min, CO/Cl2 ) 4/6. b C ) recovery originated from condensate portion. c L ) recovery originated from leaching filtrate portion. d For convenience, elemental compositions are listed in parentheses.
The effect of reaction temperature on the base metal recoveries was such that the Al recovery is suppressed significantly under the selected condition (Table 8). Mg seems to begin to react substantially if the reaction temperature reaches 700 °C. This means cordierite does the same thing. As the reaction temperature increases, Ce was almost completely recovered at 600 °C. The optimum temperature for the recovery of Zr was 500 °C which gives the recovery of 73.8%. Ba was recovered up to about 70%. 4. Conclusions As an improved gas-phase separation method, the carbochlorination method was developed for the recovery of PGM from the spent automotive catalysts. Various process parameters such as total gas flow rate, reaction time, partial pressures of CO/Cl2 gas mixtures, and temperature were examined so as to get insight into the characteristic behaviors in terms of recovery and volatility of involved metals and to establish the dominating factors in obtaining the economically feasible level of recovery. From this optimization study, we achieved the recoveries of platinum and rhodium as 95.9% and 92.9%, respectively, with a relatively small amount of base metal chlorides at the experimental conditions of 550 °C of reaction temperature, 4:6 of CO: Cl2 composition, 100 cm3/min/100 gsample of flow rate, and 1 h of reaction time for the raw, spent automotive catalysts after a 160 000-km aging test. Acknowledgment The authors wish to express their gratitude to the National Institute of Environmental Research of the Ministry of Environment of Korea for providing the financial support (1995-1997) that made this work possible. Part of the research fund was also kindly provided by Tae Sung Metal Co., Ltd. The ICP analysis was done at the Korea Basic Science Institute. Literature Cited (1) Cowley, A. Platinum 1999; Johnson Matthey: London, 1999.
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Received for review July 21, 1999 Revised manuscript received November 30, 1999 Accepted February 3, 2000 IE9905355
