Extraction of highly pure nickel hydroxide from spent NiO/Al2O3 catalyst: Statistical study on leaching by sulfuric acid lixiviant and selective precipitation

In this work, extraction of nickel from spent NiO/Al2O3 catalyst was investigated through a leaching process with the aid of sulfuric acid. The impact of the influential parameters including leaching time (30-240 min), temperature (50-90.), catalyst mesh size (7-200) and leachant concentration (1-4 M) were studied by a design of experiments (DOE) technique. A hybrid framework constructed from irregular factorial and D-optimal designs was employed to build up a set of experimental runs to ascertain the influence of these factors affecting nickel extraction from active phase and undesired aluminium co-extraction from support. The significance of the main effects, binary and ternary interactions along with quadratic form of parameters on the yields of metals extraction was evaluated by analysis of variance (ANOVA). As a result, two comprehensive models were developed to predict nickel and aluminium recoveries with R-2 > 0.99. The optimization study revealed that maximum nickel recovery (100 %) with restricted aluminium dissolution was attained for the catalyst with mesh size of 7-12 (1.68-2.83 mm) employing 4 M concentrated acid solution at 90. after 240 min of digestion. Separation of nickel from accompanying aluminium in mother liquor (purification) was examined by two precipitants including ammonia and caustic soda solutions. Nickel hydroxide with purity > 98 % was successfully deposited using NaOH solution in excess amount (>= 100 % of the required solution to obtain pH = 14).