The selective dissolution of alumina, cobalt and platinum from a calcined spent catalyst using different lixiviants

Abstract

Metal catalysts are widely used in the petrochemical processing industry such as in the Fischer–Tropsch (FT) process to produce hydrocarbons from synthesis gas (syngas). The deactivated cobalt-containing catalyst from the FT-reaction is subsequently discarded as a spent catalyst. Due to the high cost of the precursor metal salts coupled with the toxicity of cobalt, the disposal of spent FT catalyst is uneconomic and environmentally unacceptable. This paper describes the selective recovery of alumina, high value cobalt and platinum from the spent catalyst as an alternative to disposal. For the purpose of the selective dissolution of high value metals such as Co and Pt, the leaching of the calcined spent catalyst was investigated using different lixiviants such as sodium hydroxide, nitric acid, hydrochloric acid, sulphuric acid and aqua-regia. The effects of concentration of sodium hydroxide, temperature, pressure and time on the extraction efficiencies of metal species from spent FT catalyst during the leaching of the calcined spent catalyst were investigated. It was found that the selective dissolution of aluminium species with sodium hydroxide solution under leaching conditions from the spent cobalt-containing catalyst was effective. Approximately 89% of the aluminium species present in the calcined spent catalyst was selectively dissolved in sodium hydroxide solution (6.25 M) when using elevated pressure (about 9 bar) and temperature (170 °C) during two consecutive leaching steps. As expected, platinum species still remain insoluble in the sodium hydroxide solution. During the leaching step, both aluminium and cobalt species dissolved simultaneously in the mineral acids such as HCl, H 2SO 4 and HNO 3 to form metal salts in the leach liquor. In conclusion, almost all of Al, Co and Pt species present in the original spent catalyst were respectively dissolved in the aqueous phase during the sodium hydroxide–nitric acid–aqua regia leaching process.