As an important
 secondary resource with abundant platinum group metals (PGMs), spent catalysts demand recycling for both economic and environmental benefits. This article reviews the main pyrometallurgical processes for PGM recovery from spent catalysts. Existing processes, including smelting, vaporization, and sintering processes, are discussed based in part on a review of the physiochemical characteristics of PGMs in spent catalysts. The smelting technology, which produces a PGM-containing alloy, is significantly influenced by the addition of various collectors, such as lead, copper, iron, matte, or printed circuit board (PCB), considering their chemical affinities for PGMs. The vaporization process can recover PGMs in vapor form at low temperatures (250–700°C), but it suffers high corrosion and potential environmental and health risks as a result of involvement of the 
hazardous gases, mainly Cl2 and CO. The sintering process serves as a reforming means for recycling of the spent catalysts by in situ reduction of their oxidized PGMs components. Among these processes, the smelting process seems more promising although its overall performance can be further improved by seeking a suitable target-oriented collector and flux, together with proper pretreatment and process intensification using an external field.

Pyrometallurgical Recovery of Platinum Group Metals from Spent Catalysts

Performance of trickle-bed and up-flow reactors was studied experimentally and theoretically for an exothermic multistep hydrogenation of 1,5,9-cyclododecatriene (CDT) in n-decane as solvent over 0.5% Pd/alumina catalyst. Intrinsic kinetics was studied in a batch stirred slurry reactor using the powdered catalyst, and a Langmuir-Hinshelwood-type rate model is proposed. Using this rate equation, a trickle-bed-reactor model was developed that incorporates contributions of partial wetting and stagnant liquid holdup, in addition to the external and intraparticle mass transfer for the gas-phase reactant (hydrogen). It was also modified to describe the behavior of the upflow reactor. Experimental data were obtained in both upflow (trickle-bed) and down-flow modes at different liquid velocities, pressures, and inlet feed concentrations at 373-413 K. Reactor performance of the two modes was compared in terms of global hydrogenation rate, CDT conversion, selectivity to cyclododecene and the maximum temperature rise observed in the catalyst bed. The conversions and global hydrogenation rate were significantly higher in a trickle-bed reactor than in the upflow reactor. Similarly, a significant temperature rise in the catalyst bed was observed for the downflow operation compared to the upflow mode, which is explained from wetting characteristics of the catalyst bed. Model predictions for both reactors agreed well with experimental data.

Hydrogenation of 1,5,9-cyclododecatriene in fixed-bed reactors: Down- vs. upflow modes

The kinetics of hydrogenation of 2,4-dinitrotoluene (2,4-DNT) using a 10% Ni supported on zeolite Y (10% Ni/HY) powdered catalyst (dp = 1 × 10-5 m) was studied experimentally in a semibatch slurry reactor over a temperature range of 333−363 K. The effects of reaction temperature, H2 pressure, concentration of 2,4-DNT and catalyst loading on the concentration−time and H2 consumption−time profiles were studied under isothermal conditions. To explain the rate behavior of this complex, consecutive, and parallel reaction, several rate expressions were derived based on Langmuir−Hinshelwood type rate mechanisms. The rate equations that were derived assuming the reaction between competitively adsorbed organic species and dissociatively adsorbed hydrogen as the rate-limiting step were found to represent the kinetics best. Quantitative analysis of the experiments performed under the chosen conditions (temperature, 333−363 K; H2 pressure, 1.3−5.4 MPa; 2,4-DNT concentration, 0.14−0.55 kmol/m3) indicated that the rate...

Hydrogenation of 2,4-Dinitrotoluene Using a Supported Ni Catalyst: Reaction Kinetics and Semibatch Slurry Reactor Modeling

https://hal.archives-ouvertes.fr/hal-00735867/document

Selective hydrogenation of 1,5,9-cyclododecatriene in up- and down-flow fixed-bed reactors: experimental observations and modeling

High initial conversions for the cyclododecatriene (CDT) hydrogenation in a slurry reactor at 433 K and 1.2 MPa were obtained from a supercritical (sc) CO2-cleaned 0.5% Pd/Al2O3 catalyst. A recovered intermediate selectivity was achieved by the sc CO2-cleaned catalyst at the higher CDT conversion values, suggesting an effective cleaning of Pd sites and a possible reconstruction of the residual coke layers onto the surface caused by the solvent nature of sc CO2. SEM pictures show that the cleanup of the used catalyst with sc CO2 removes most of the coke and oily materials from the surface. Pd SEM X-ray microanalysis evidenced a decrease of the Pd dispersion on the surface of the used-activated catalyst. Coke was also characterized using elemental analysis and TGA in air, indicating that about 80 wt % of the deactivating materials were efficiently removed from the surface by sc CO2. The formation of the oligomerized and stabilized residual coke (remaining 20%) should be presumably avoided if the catalytic hydrogenation had taken place at sc (in situ) conditions where oligomerization times are minimized. The SCF conditions are then highly challenging at the catalytic hydrogenation of polyunsaturated hydrocarbons.

Coking and ex situ catalyst reactivation using supercritical CO2 : A preliminary study

The kinetic modeling of the hydrogenation of 1,5,9-cyclododecatriene has been studied using a Pd/Al2O3 catalyst in a slurry reactor on the basis of a complete isomers analysis. A reaction scheme involving multistep hydrogenation and reversible isomerization reactions has been considered and a kinetic model of Langmuir−Hinshelwood type proposed. The kinetic parameters were estimated from experimental batch slurry reactor data. The model predictions of cyclododecene selectivity at different pressures agree well with the experimental data.

Galo Carillo Le Roux

Papers

Kinetic Modeling of the Hydrogenation of 1,5,9-Cyclododecatriene on Pd/Al2O3 Catalyst Including Isomerization

Nonlinear programming reformulation of dynamic flux balance analysis models

Integrated ethanol and gasoline supply chain planning under environmental constraints: A case study of Brazil