Scientists from the Luleå University of Technology in Sweden have developed a new method for extracting molybdenum from spent iron-molybdate catalysts, a common type of industrial waste in the chemical industry. Their paper shows how this valuable and strategically important metal could be recycled without using energy-intensive and complex methods. While molybdenum is widely used in the production of high-strength steels, superalloys and chemical products, its production is concentrated in a few countries, which is why supply remains vulnerable. Meanwhile, the catalysts contain more than 50% molybdenum by weight, effectively acting as urban deposits that are often richer than natural ore.
The biggest technical challenge faced by the researchers was how to effectively dissolve the molybdenum while preserving the iron, which is also present in the catalyst, as a solid residue. The conventional approach (leaching with pure ammonia) required a very high reagent concentration (5 mol/liter), providing an efficiency of less than 90%. The Swedish team took a different approach: they used a buffer mixture of ammonium hydroxide and ammonium sulfate. This novel approach allowed them to more than halve ammonia consumption (to 2 mol/liter), simultaneously increasing the molybdenum yield into solution to 92%, with iron leaching into the liquid at a rate of less than 5%. This effect can be explained by the fact that the buffer system maintains an optimal pH (about 10) and a negative oxidation-reduction potential, at which molybdenum forms highly soluble ammonium molybdates and iron precipitates as hydroxide.
The most interesting reactions began after the molybdenum entered the solution. As a rule, it is precipitated by evaporation or heating, which requires a large amount of energy. Instead, the authors of the study used an antisolvent crystallization method by adding ethyl alcohol to an aqueous ammonia solution. Ethanol, which has a much lower dielectric capacity than water, disrupts the solvation shell around the molybdate ions, sharply reducing their solubility. Crystals start growing at room temperature, without any heating. The scientists selected the optimal process conditions: they used roughly one and a half parts alcohol for one part solution, stirring the mixture at 400 rpm for about 45 minutes. As a result, they managed to extract up to 95% of the molybdenum as pure ammonium heptamolybdate crystals.
Crucially, the purity of the resulting product exceeded 99.9%, with the crystals turning out small and uniform at approximately 3.7 micrometers. This material is sought after in industry: it can be used to produce catalysts, coatings, fertilizers and other products. It should be noted that the entire technology cycle works at room temperature, without heating. In addition, the scientists managed to reduce ammonia consumption by about 2.6 times: while the standard process requires 1.82 grams per gram of molybdenum, this process requires some 0.7 grams.
