Browsing by Author "Piedras, Francisco Javier vega"
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- Hydrometallurgical approach for recovery of pgms from spent automobile catalytic convertersPublication . Piedras, Francisco Javier vega; Paiva, Ana Paula; Costa, Maria ClaraThe recovery of platinum-group metals (PGMs) from secondary sources, such as spent devices like automobile catalytic converters (ACC), has been gaining interest in the last decades due to their scarcity in the earth crust and their high demand. Their extended applications among different industries, together with their difficulty to be replaced by other materials, has created the urge to develop new technologies for PGMs recovery that can help with the supply requirements, while being environmentally friendlier than the existing processes. Hydrometallurgical techniques represent a possible strategy to recover PGMs and avoid the high energy consumption of smelting processes in pyrometallurgical plants. In this study, a hydrometallurgical approach was adopted, starting with a leaching step for the solubilization of the metals into an aqueous phase, employing the use of an oxidizing agent for the formation of metal chlorocomplexes, thus facilitating their dissolution; after this, the recovery of PGMs was intended by using a solvent extraction (SX) step with appropriate extractants, to transfer the selected metals into a more purified aqueous phase, from which the PGMs recovery and transformation into forms with commercial interest would be easier. Two spent ACCs were collected (H98 and I95), and their initial metal compositions were evaluated by an X-ray fluorescence (XRF) analysis. Two different elemental compositions were found, leading to a separate treatment of each sample. In the first stage of the work, the leaching process, the most relevant variables such as temperature (°C), acid concentration (M), L/S ratio (L/kg), time (h), stirring (min-1) and particle size (mm) were evaluated experimentally and through a factorial design methodology (FDM), to establish the individual and joint contributions of the relevant parameters to the leaching efficiency. As a result, the optimised conditions for the leaching of both catalyst samples were T= 60 ºC, [HCl]= 11.6M, L/S= 2L/kg, t= 3h, stirring= 250 min-1, and particle sizes Dp=0.397mm and Dp=0.409mm for H98 and I95, respectively. The FDM treatment showed that time represented the most determinant parameter in terms of leaching efficiency for the H98 sample. For the I95, time, temperature, acid concentration, and the pair time-temperature were the main interactions to be considered, according to the FDM, in order to obtain a higher efficiency of the leaching process. For the second step, the SX, four commercial compounds and/or mixtures (tributyl phosphate (TBP), Cyanex® 471X, trioctylphosphine oxide (TOPO) and Adogen® 464), and one ionic liquid (Cyphos® 101), were employed as extractants, using toluene as diluent. The concentrations of each extractant were calculated according to the concentration of the targeted metals in the solutions. The first part of the SX step was the development of a scheme for each sample using model solutions, prepared with the metals expected to be in higher concentrations in the real leaching solutions obtained. The results achieved with the model solutions showed that the appropriate SX scheme, when using traditional extractants, would involve an approximate 6M HCl concentration for the aqueous phases and the use of TBP to remove Fe(III) in the first extraction, followed by an extraction with Cyanex® 471X to remove Pd(II). In the third extraction, TOPO could be used to remove Pt(IV), and the final extraction could be carried out with Adogen® 464 to remove Rh(III). The above loaded organic phases were subjected to a stripping procedure, to transfer the metals to new aqueous phases and to evaluate the reusability of the organic solutions. The stripping agents chosen were: a 0.1M HCl solution to strip Fe(III) from TBP, a stabilized 1M Na2S2O3 solution to strip Pd(II) from Cyanex® 471X, and a 0.4M malonic acid solution to strip Pt(IV) from TOPO. Due to the low content of Rh(III) in the solutions, the decision of not stripping this metal from Adogen® 464 was taken. Regarding the application of the ionic liquid Cyphos® 101 for the SX of the model solutions, it was found that Pd(II), Pt(IV) and Fe(III) were quantitatively extracted from both 3M and 6M HCl solutions. Two stripping agents were tested, e.g., a 0.1M KSCN to remove Pt(IV), and a 0.1M CH4N2S in 5% v/v HCl to strip Pd(II). Finally, the SX schemes previously developed were tested in the real leaching solutions for each catalyst, and a completely different behavior was found. The extraction and stripping efficiencies were very low compared to the results from the model solutions. This unexpected behavior is attributed to the complexity of the real leaching solutions, to the large number of metals involved that were not considered for the composition of the modelsolutions, and to possible interactions of the extractants with those unanalysed metals