| Abstract: | Nickel extraction from an uncrushed spent nickel-tungsten catalyst was studied using sulphuric acid leaching in a column. The effects of flowrate, temperature and acid concentration on the cumulative nickel recovery was investigated using a three-variable, two-level factorial design. Statistical analysis indicated that among these variables only acid concentration was significant (at the 95% confidence level) in the ranges investigated. The catalyst can be leached either by a two-step process or in a single step following catalyst preoxidation. In either case extractions exceeding 95% can be obtained. With oxidized samples, single-step acid leaching at 82°C yielded significantly better nickel extraction than at 95°C. This was attributed mainly to tungstic acid precipitation in the catalyst pellets, an effect which occurs rapidly at 95°C and retards the leaching of nickel. Nickel extraction was also suppressed following preoxidation at over 300°C. This was thought to be due to formation of slow-leaching nickel oxide-tungstate. Following preoxidation at the optimum of 300°C, 97% of the nickel could be leached in 4 h at 82°C using (by volume) 3% sulphuric acid and 0.6% nitric acid flowing at 1.5 litres/h. About 70% of the nickel could be extracted from preoxidized catalyst using only water for 40 h at room temperature. These findings are incorporated in a simple proposed flowsheet. |
