Catalysts may become spent over time and be unable to catalyse. This may be caused by contaminants building up on the substrate. When this happens the catalyst must be recovered or regenerated. Typically, this is done via incineration, leading to aloss of precious metals and/or pollution.
The Application
Spent carbon-based catalysts which contain precious metals have traditionally been treated by incineration or pyrolysis as a first step in the recovery of the precious metal content. The incineration step is employed to destroy the organic component of the catalyst and leaves ash. This incineration process is sometimes incomplete on the first pass and subsequent burns are required to reduce the carbon content within the metal ash residue to acceptable levels. After incineration, but before chemical recovery is started, the ash is sampled to determine the precious metal content. In general, there is no certainty that metals have not been lost in the incineration process. In the case of rhenium-based catalysts, the incineration process is particularly difficult because rhenium rhenium oxide will volatilise at 350 °C leading to the possibility of significant rhenium loss.
Feed Preparation
SCFI carried out oxidation trials at the Validation Test Centre in Cork. Material was delivered to the pilot plant in steel drums filled with hard, dry pellets. These pellets were approximately 1.4mm x 3.4 mm and were difficult to break down.
It was decided that carboxyl methyl cellulose (CMC) would be required to thicken the water to keep the catalyst in suspension. The feed tank was connected to the grinder pump and water was added to the mixture. This prepared feed was processed in the SCWO reactor.
The Success
The trial work proved the ability of supercritical water oxidation to recover precious metals from spent catalyst. The temperature of the process is above 500 °C and all metals present formed metal oxides. This stream was subsequently cooled to ambient temperature causing all condensable material to separate into the liquid phase. The result was excellent recovery of rhenium as perrhenic acid.
We achieved full recovery of the catalyst, however the first generation of the Aquacritox SCWO was not a practical solution due to reactor fouling. Since these trials, SCFI have developed the Aquacritox SCWO Gen 3 to overcome the issue of blockage.
