Recovery of platinum group metals from automobile catalytic converters

Abstract

Platinum has played important roles in human life due to their specific physical and chemical properties. The demand of platinum was reported to be continuously increasing, especially the use in automobile catalytic converters, while natural resources of platinum becomes limited. Therefore, the study of platinum recovery from spent automobile catalytic converter, which is an important secondary resource of platinum, becomes significant. In this study, platinum recovery process consists of two stages; platinum extraction by leaching with aqua regia and then platinum separation from the solution by co-precipitation. In the platinum extraction process, influences of time and temperature were investigated to identify optimum condition for the leaching process. The result showed that the maximum platinum extraction was obtained when leaching at 90 °C for 6 hours. However, at this condition, aluminum which is the major component in catalytic converter was also highly extracted. The coexistence of aluminum in the leachate will retard the co-precipitation of platinum (IV) with iron hydroxides, which is the second stage of platinum recovery. Hence, the optimum leaching condition here was considered to be leaching at 90°C for 1 hour. In the second stage of platinum recovery, platinum co-precipitation with various metal hydroxides was selected for this study. The metal hydroxides used in this study were hydroxides of Al, Mg, Fe (II) and Fe (III), which are major components in the catalytic converter and also dissolved in the leachate. Influences of pH and precipitation time were investigated. The result showed that almost all platinum (IV) was effectively co-precipitated with iron hydroxides at pH 6 in 1 hours. The chemical state of platinum investigated by XPS showed that co-precipitated platinum was partly reduced to platinum (II) with iron (III) hydroxide and some was further reduced to metallic platinum with iron (II) hydroxide. These facts suggest that iron hydroxide is considered as an effective carrier for co-precipitation of platinum.