Novel Aqueous Processing of the Reverted Turbine-Blade Superalloy for Rhenium Recovery

A preliminary study on the hydrometallurgical recovery of rhenium from an engine-reverted superalloy, CMSX-4, was performed. The novelty of this work dealt with (i) the applicability of electro-generated Cl 2 in rhenium leaching and (ii) the maneuvering of the organic extractants based on the metal speciation in leach liquor. In a two-step HCl leaching, more than 98% rhenium was selectively leached in the presence of dissolved chlorine (Cl 2aq. and Cl 3aq. – ) in the second step, which was performed after a leaching of thermal-barrier and base-alloy elements in 4 mol L –1 HCl. The leaching kinetics of rhenium indicated a shift from surface diffusion to a chemically controlled reaction with a variance in temperature from 313 to 353 K. The first use of phosphinic acid in molybdenum separation from rhenium in the leach liquor yielded a high separation value (β Mo/Re = 839) using 0.067 mol L –1 extractant; whereas the mixed-phosphine oxides showed a tremendous selectivity for separating rhenium from the base-metal impurities in the Mo-depleted solution. The extraction of rhenium increased from 9% to 98% with an increase in extractant concentration from 0.005 to 0.08 mol L –1 in the organic phase. On a metal-to-metal basis, the recovery value of 93% rhenium as NH 4 ReO 4 (9.5 g L –1 Re in the solution) demonstrated the potential of the present process.