Wet recycling
Decomposition with sulfuric acid or sodium hydroxide under pressure in an alkaline medium dissolves the carrier. Precious metals remain in the residue after dissolving, and are then leached with chlorine and hydrochloric acid to bring the platinum group metals into solution. In the alkaline process, the contained SiO2 remains undissolved, preventing further processing of the precious metal. It is not advisable to regenerate the bulk support by this type of method because the γ-Al2O3 has been converted to insoluble α-Al2O3 during the active use of the catalyst.
On the other hand, various methods for dissolving precious metals and the recovery rate of precious metals have a large variation, which are well known, such as dissolving methods with hydrochloric acid and chlorine, hydrochloric acid and nitric acid, or hydrochloric acid and hydrogen peroxide. One of the main problems with all these methods is the difficulty in separating platinum group metals from non-ferrous metals in dilute solution. The recoveries of these methods, especially of rhodium, are unsatisfactory.