Novel waste processing by means of mechanochemical treatment

As the extension of mineral processing, recycling metals from wastes is very important for a sustainable society. We have been working on mechanochemistry and its engineering applications. One of the applications is to recover and separate useful components from different kinds of wastes emitted in our society. When a waste sample is subjected to grinding in air so called mechanical treatment, it changes its structure to disordered system, resulting in chemical reactions with other substances when it takes over the certain level of energy. Depending on the existing states of target elements in the wastes, mechanical activation and mechanochemical (MC) reaction can be applied for the recycling of useful compositions and a process based on MC treatment has been developed. We will report several examples from our research experiences at the conference. The first example is to recover rare earths from fluorescent powders in waste lamps. The waste is firstly subjected to dry grinding to cause amorphization of their structures. This amorphization makes it possible to dissolve the rare earths from the waste at high yield by leaching with mild acid solution at room temperature. Similar phenomenon can be seen in the case of ITO (indium tin oxide) scrap when it is ground, followed by leaching with acid solution. In this case, dry grinding the scrap induces disordering the In ₂O₃ in the scrap, leading to high dissolution of In ₂O₃ by leaching with weak acid solution at room temperature. The presence of alumina (α-Al₂O₃) in the scrap plays a significant role to the amorphization. Another advanced waste processing is to recover molybdenum (Mo), vanadium (V) and nickel (Ni) sulphide in catalysts in oil refineries. The processing is based on MC reactions between the sulphides and additives. That is, the sulphides are subjected to dry grinding with additives such as CaO, MnO₂ and Na₂CO ₃ to transform them into molybdate and vanadate. Subsequent water leaching allows us to recover Mo and V from the ground product. Other successful example is dry grinding metals or their oxides with polyvinyl chloride (PVC) to transform into chlorides, which dissolve easily in water at ambient condition. The waste processing described above is now in the investigation on industrial applications, and this is a great expectation in the field of industries which emit such waste materials.