Process development for the recovery of high grade calcium tungstate from alkaline leach liquor of spent [sbnd]HDS catalyst

Abstract

Molybdenum is often associated with nickel-tungsten based spent HDS catalyst and must be removed from tungstate solution to obtain commercially acceptable tungsten products. In order to address this problem, a different approach has been proposed in the present work. A two step process based on selective thionization of Mo using Na2S at alkaline condition followed by W precipitation as calcium tungstate (CT) using lime (Ca(OH)2) was studied. Various operating parameters were tested and optimum conditions for Mo thionization and W precipitation were developed. The CT produced by the conventional process was used as a reference to check the improvement in the proposed process with respect to Mo impurity. Under the optimum conditions, 92.7% Mo was rejected to the CT filtrate and 99.7% W converted to CT. The CT obtained from the proposed process contained 0.53% Mo, which was much less as compared to the CT obtained from the conventional process having 2.5% Mo using the same leach solution. High-grade ammonium paratungstate (APT) containing <100 ppm Mo was obtained by treating the CT, which can be used for the tools industry and other applications.