Recovery of Nickel from Spent Industrial Catalysts Using Chelating Agents

The extraction of nickel from a spent primary steam reformer catalyst from an ammonia plant was carried out by chelation using ethylenediaminetetraacetic acid (EDTA) as the chelating agent. Ni recovery was optimized by varying the particle size distribution of catalyst (pretreatment of spent catalyst), stirring speed, temperature (particularly in an autoclave, where temperatures ranging from 100 to 200 °C were used), EDTA concentration, and solid-to-liquid ratio. Approximately 95% Ni recovery was achieved in the Ni extraction carried out under hydrothermal conditions in an autoclave, at temperatures of 150 °C and higher, over a 4-h period. The resulting Ni−EDTA complex was then "dechelated" using a mineral acid (H2SO4 and HNO3), resulting in the formation of a nickel nitrate or sulfate solution and the precipitation of EDTA (about 97% of the initial weight of EDTA was recovered). However, the chelation performance of Ni was shown to decrease with every successive recovery of EDTA (in the case of dechelation using H2SO4). EDX analysis of fresh and recovered EDTA established that fresh EDTA is a disodium salt whereas recovered EDTA is protonated. EDX analysis also indicated sulfur in the recovered EDTA when sulfuric acid was used for dechelation. TGA data showed a much larger weight loss in recovered EDTA in comparison to the fresh sample, probably because of a combination of two factors: the presence of sulfur species and the protonation of EDTA after recovery. It is likely that differences in recovered EDTA as evidenced by EDX analysis and TGA are responsible for the lowering the Ni chelation efficiency. This possibility is being investigated further as part of ongoing research.