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 In2O3 in the scrap, leading to high dissolution of In2O3 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 Na2CO3 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.
Recovery of rare earths from waste cathode ray tube (CRT) phosphor powder by selective sulfation roasting and water leaching
