Recovery of valuable metals from molybdenum-removal sludge by reverse sulfurization leaching

Selective precipitation is the main process used to remove molybdenum from tungstate solutions by the tungsten industry in China. The process exploits the fact that molybdenum is a sulfophile whereas tungsten is an oxyphile. First, the sulfur ions replace oxygen and convert the molybdate to thiomolybdate, then a moderate amount of additives (copper compounds) are reacted with the thiomolybdate to form complex precipitates. We call the filtered wet precipitate “molybdenum-removal sludge”. The main components of the sludge are molybdenum, copper and sulfur, infiltrated by some amount of tungsten due tomechanical entrainment. Based on the fact that copper is a typical sulfophile, it is possible that copper ions can react withthe sulfur of the thiomolybdate, thus converting it to soluble molybdate. This is actually the reverse of molybdenum sulfuration process, so we therefore call it the “reverse sulfuration process.” Based on the leaching thermodynamics and structure characteristics of this kind of sludge, a new treatment process was developed in this work. About 90% of tungsten and 15% of molybdenum were washed out with ammonia water when the pH > 9. The recovery rate of molybdenum was close to 70% when a stoichiometric amount of copper ions (with sodium hydroxide 4 mol/L) were utilized for the reverse sulfurization leaching. The leaching rate of molybdenum obtained on leaching with 2 mol/L sodium hydroxide for 120 min at 80 °C and then fixing with copper chloride was 94.4%, and 87.67% of sulfur ions were fixed in the form of an amorphous Cu S compound.