Role of EDTA on metal removal from refinery waste catalysts

Waste spent hydro-processing catalysts contains significant quantities of heavy metals such as Mo, Ni, V, and Co. The disposal of waste refinery catalysts is a serious environmental concern because of the presence of heavy metals. Hence, an attempt has been made to recover the metals from a refinery waste ARDS catalyst using organic acids such acetic acid, oxalic acid, citric acid, and ethylenediaminetetraacetic acid (EDTA) as leaching agents. EDTA has been found to be a most reactive agent for extraction along with the recycling of the leachant. EDTA acts as an auxiliary complexing agent and binds selectively to the metals particularly Ni, Co, V, and Mo. Hence EDTA is the most active complexing agent for the selective binding of metals in a variety of matrixes. The effect of different process conditions such as pH, temperature (35-60°C), concentration (2-10 wt.%), reaction time (1-6 h), and solid to liquid ratio (1:15-1:40) on metal recovery was investigated. It was found that the EDTA can remove 97% of Mo, 95% of Ni, and 94% of V under optimum process conditions. Subsequently, the effluents of the process are also separated as metal salts, solvent, and EDTA.