Kinetics of Sulfuric-Acid Nickel Leaching from the Grinding Wastes of Rhenium-Containing Superalloys

The sulfuric-acid leaching kinetics of nickel as the main component of grinding wastes originating from machining a rhenium-containing heat-resistant ZhS-32VI superalloy and containing the impurities of abrasive materials, oils, ceramics and other pollutants has been studied in the agitation mode concentrating refractory metals in the solid residue. The nickel content amounts to 60% and, alongside nickel, there are other metals in the grinding wastes, including rhenium, chromium, cobalt, tungsten, tantalum, molybdenum, hafnium, titanium, and aluminum. The leaching of nickel from wastes with the use of a sulfuric acid solution has been carried out using a thermostated cell at an elevated temperature (55–85°С), the ratio between the wastes and the solution of sulfuric acid (3 M) amounting to 1 : 10 g/mL and the stirring rate being of 200 min –1 . To study the kinetics, a grinding waste fraction (–0.071 mm) obtained with the greatest yield (49.2 wt %) has been used. Convex kinetic curves of nickel leaching from the wastes have been obtained. It has been revealed that, when the temperature changes from 55 to 85°С, the time until the leaching stops decreases from 220 to 140 min, and the level of nickel extraction from the solution increases from 45 to 99%. The kinetic curves are linearized according to shrinking sphere, Gistling–Braunstein and Kazeev–Erofeev equations. The latter has turned to be most suitable for describing the process. Taking into account the assessment of the correlation coefficients of the mentioned anamorphoses, it has been found that the leaching of nickel from the grinding wastes is rate-determined by a chemical reaction, and the process occurs in a kinetic mode. The apparent activation energy E act , calculated using the Arrhenius equation, and the rate constants, obtained via processing linearized kinetic curves according to the shrinking sphere model, amounts to 47.5 ± 0.5 kJ/mol. This value confirms the fact that the leaching process occurs in a kinetic mode and can be intensified via increasing the reaction temperature.