Study of dissolution of technogenic waste of heat-resistant nickel alloys in acid solutions

Heat-resistant nickel alloys take the lead among other materials used in machinery, aircraft and other industries. The recycling of disused products consisting of heat-resistant alloys as well as grinding waste from their production is a relevant task, due to the high cost of the components contained in these alloys and the need to return them to production. In addition to nickel, the alloy contains such valuable elements as rhenium, tantalum, cobalt. The hydrometallurgical method of converting alloy components into a solution (usually with acid) followed by their separation allows to solve this problem but it is the acid digestion that constitutes the main problem of the processing. The paper studies the process of acid digestion of grinding waste heat resistant nickel alloy depending on the composition of the acid solution (H2SO4, HCl, HNO3 and their mixtures), temperature (in the range from room to boiling), the acid concentration (from 150 to 300 g/dm3) and the ratio of solid to liquid phases. By mass spectrometry and X-ray fluorescence, sieve method, as well as by scanning electron microscopy, elemental, dispersive and morphological analysis of the waste powder was carried out, respectively. The composition of the solution and conditions of acid digestion of grinding waste with the highest degree of extraction of metals in the solution were determined: the treatment should be carried out in sulfuric acid solution with a concentration of 300 g/dm3 at a temperature of 60–70 °C for 6 hours with solid-to-liquid ratio at 1 : 10. Preliminary heat treatment of the waste powder allows to reduce the amount of organic substances in the material by more than 3 times, thereby reducing the contamination of the solution. It was found that 95–99% of nickel is transferred into solution during the dissolution of the waste powder, and valuable rare-earth elements remain in the undissolved sediment, which is important for the technology of their further separation.