Up to now, the recovery of precious metals used as catalysts in chemical processes has involved the use of incineration. Now, however, a British-Swedish joint venture has developed a process which uses supercritical water oxidation instead, which provides many economic and environmental advantages. By Steve Minett and Curt Jonsson.

Precious metals are used extensively in catalysts in a wide range of industrial chemical processes. Sometimes they are used in a homogeneous form, but more often fixed to a solid support for ease of handling. In many applications only a precious metal catalyst can provide the necessary speed or selectivity of the reaction, whilst in others these attributes, together with a long catalyst lifetime, make the overall system the most cost-effective choice. Because these metals, like platinum, palladium or rhodium, constitute a huge investment, a key factor in the economics of these processes is the ability to recover the precious metal content from the catalyst once it reaches the end of its life.
Quick and efficient retrieval of the metals is, therefore, very important. Spent process catalysts, however, are typically contaminated with organic materials that were present in the reaction mixture. These organics, which are often hazardous, as well as any carbon present in the support, have to be removed before the exact quantity of metal present can be established and the complex process of metal recovery can begin.
Up till now this recovery process has almost exclusively been handled through incineration - to destroy the organic content of the catalyst material - followed by a chemical treatment of the remaining metal oxides. Now, however, Johnson Matthey (JM), in the UK, recently announced that they have developed a new technology together with the Swedish company Chematur Engineering AB. The new process utilises SCWO (supercritical water oxidation) a technique previously patented by Chematur. This new catalyst recovery process takes place in an entirely enclosed system, where all organic materials are 'burnt' within minutes by extremely hot and pressurised water in a supercritical condition. The only gaseous emissions from this process consist of carbon dioxide and nitrogen at room temperature.
JM has spent three years developing this new process. "We call this system AquaCat?", says Martin Durney, JM's Vice President, Global Sales & Marketing, Chemicals. "From the environmental and economic points of view; it's dramatically more energy-efficient because it needs no external source of energy, it eliminates the need for complex and expensive exhaust gas treatment and it reduces the amount of physical handling of the catalyst materials, which again makes containment easier. But from our customers' point of view, maybe the biggest advantage of AquaCat over incineration is that it allows pre-treatment sampling of the material. This is important because these precious metal catalyst materials are very expensive; rhodium, for example, has a current price of around £22260 per kilogram (E36350/US$32000). It's essential, therefore, to know exactly how much is being processed and to move it through as quickly as possible - speed can save a lot of money.'
The AquaCat process, which is patented by JM and Chematur, is based on a further development of Chematur's AquaCritox process, where water is heated up to approximately 400ûC and pressurised to about 250bar. It then enters a supercritical condition with unique properties, where it can 'burn' organic materials extremely quickly and efficiently (by adding an oxidant), leaving no harmful residues. In the AquaCat application hydrocarbons are converted to carbon dioxide and water, with a guaranteed extent of conversion of at least 99.99percent.
As is often the case, the origin of this innovative process was quite coincidental. Lars Stenmark, Chematur's Director of Development, recalls that he was introduced to some representatives from Johnson Matthey at a conference in Nottingham, England, in 1999. While they were talking during an interval, someone suggested that Chematur's AquaCritox system might be suitable for catalyst recovery.
"Most of the materials that come to us for catalyst recovery are heterogeneous in form," explains Martin Durney. "In the AquaCat recovery process, these heterogeneous materials are made into a 'slurry' in water. After sampling to determine precious metal content the mixture is pumped to supercritical pressure and heated to supercritical temperature. Oxygen is added, which means that the carbon is immediately burnt together with any organic contamination present on the catalyst." Martin Durney says there were several problems which had to be solved in order to apply the new technique. "We needed to optimise the design of the sampling vessel, as well as the stirring and other equipment," he explains.
Martin Durney says that while JM's first commercial-scale AquaCat plant will be based in the UK, they expect to install more of the plants in their facilities worldwide in the near future. A pilot plant has already been extensively tested. "Installation of the commercial plant is well underway and we will be starting customer trials in the near future."
Does AquaCat signal the end of incineration in this field? Martin Durney does not believe so: "Whilst we believe that the AquaCat process will be the benchmark for organic catalyst recovery there is still a role for classical incineration. Certain materials, particularly those contaminated with extraneous material or plant debris, will not be suitable for AquaCat and will continue to be burnt in the traditional way.
Chematur is very optimistic about further developments for their AquaCritox technology: one area that Lars Stenmark mentions is the treatment of sludge from municipal treatment plants. "Farmers are very reluctant to domestic sewage sludge as a fertiliser in their fields, even though it might be an ecologically sound alternative. There have been too many scandals with metals, bacteria, dioxins etc being present in the sludge. Further, the food industries don't particularly want rumours to get around that 'the food you're eating was fertilised with sludge from a sewage plant'." Also in this area, incineration is in bad repute, whereas the AquaCritox method could be a suitable solution.

For more information, visit www.chematur.se