Since the 1970s catalytic converters have helped to dramatically improve air quality, particularly in urban areas. As well as turning highly toxic carbon monoxide into inert carbon dioxide they eliminate emissions of unburnt fuel from the exhaust, breaking it down into water and more carbon dioxide. While carbon dioxide emissions have their own issues the immediate problem is much less than spewing out carcinogenic hydrocarbons. Newer models also deal with nitrogen oxides, converting them to oxygen and harmless nitrogen gas. The effect on pollution levels has been dramatic.
Of course any technology also presents some challenges, and catalytic converters are no exception. In this case the main one is the value of the materials they contain. As well as their basic structure, which is usually a metal casing holding an internal structure of heat-resistant ceramic or kanthal, there’s the catalyst itself. This is where most of the potentially reclaimable value of the device lies.
When exhaust gases enter the converter they’re channeled through a matrix of fine channels, lined with the metal catalyst. The catalyst, together with heat supplied by the exhaust system itself, promotes the chemical reactions that turn pollutants into less harmful atmospheric gases. A variety of metals are suitable for use in converters but most of them have some disadvantages, such as longevity or the possibility that under some running conditions new toxins could be created. The three most effective and reliable ones, however, are platinum, palladium and rhodium. These are all precious metals with a value of hundreds, or even thousands, of dollars an ounce. It’s obviously not cost effective to discard them when a vehicle is scrapped. Both to recover value and preserve limited resources, the catalyst needs to be recovered for reuse.
The recycling process begins when either a car is scrapped or an old converter is handed in to a specialist for exchange. There are a number of businesses which specialize in recycling them but a similar process is used throughout the industry. First the casing is cut open and processed as normal scrap. The internal components, which carry the catalyst in foil form, may consist of beads or a lattice. However the foil isn’t pure catalyst; it also incorporates the “wash coat”, which ensures that exhaust gases are evenly spread over the catalyst. Often this is bonded to a ceramic substrate.
To begin the separation process the wash coat and substrate are pulverized, usually with a hammer mill; the result is a fine powder. The wash coat, which is mostly aluminum, is removed by dissolving in caustic soda. Non-metals like ceramic can usually then be floated out, or the metal content removed by dissolving in a strong acid (which the ceramic resists). The precious metal content is then removed from the solution. This can be done by precipitating it out chemically, but a more effective method is electrolysis – a current is passed through the solution between two electrodes, and the catalyst will be deposited as a pure metal around one of them.
A new technology is currently being developed in Japan, based on the plasma arc furnaces that are widely used for processing electronics scrap. This system employs rapid, extreme heating followed by quenching in cold water. The process separates the catalyst from the substrate and wash coat, greatly reducing the use of toxic chemicals.
Catalytic converters are now an established part of any modern vehicle, but they depend on scarce and expensive resources. With the materials in a single unit now worth more than $1,000, recycling them is both cost-effective and environmentally sound.