Abstract

A study to leach and recycle rhenium from CMSX-4, a second generation ultrahigh-strength single crystal nickel-based superalloy, containing 3% rhenium is presented. Experimental factors involved in the leaching process of rhenium from CMSX-4 using aqua-regia solution have been investigated and reported. Experimental factors such as concentration of aqua-regia, the use of sonication, leaching period, solid-to-liquid ratio and stirring speed have been optimised and the leaching kinetics of rhenium and other component metals of superalloy CMSX-4 have been studied. Increasing aqua-regia concentration to 100% (v/v) resulted in more than 30% gain in recovery which can be attributed to the increased availability of leaching reagents. The leaching kinetics of rhenium from superalloy CMSX-4 in 100% aqua-regia solution fit into a chemical reaction controlled kinetics model for the first period of leaching. For the leaching period after 480 minutes, the leaching kinetics of rhenium fits a diffusion through the product layer model. Application of ultrasonic waves to the leaching process has proven useful to slightly increase yield. Given the small increase, however, sonication may not be economically feasible for recovery of rhenium on an industrial scale. Precipitation of other metals away from the rhenium rich solution has also been explored to prepare the solution for a downstream rhenium recovery process. It was found that that two unique precipitates can be recovered from the solution. The first precipitate, recovered at pH 5.05, has been determined to be mainly composed of the oxides of aluminium, chromium, molybdenum, and titanium. A green precipitate of mixed hydroxides of cobalt and nickel (MHP) which has commercial value can then be recovered at pH 7.0 leaving a rhenium enriched solution for further processing.