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Catalytic converters are a device used in cars and on other engines to cut down emissions. They are made up of a catalyst sprayed onto a ceramic wash coat on a ceramic or stainless steel substrate. The catalyst contains various amounts of platinum, palladium and rhodium. For the hobby refiner it is possible to leach the metals and extract it but when compared with selling the cat to a buyer the price of acids and the time it takes usually can't compete. In large scale processing the converters are melted either whole and the steel works like a collector for the PGM: or the ceramics are extracted and melted with copper as a collector. Usually an electric arc oven or a plasma oven is used for the melt.
For a home refiner it is impossible to refine catalytic converters economically because of cost of acids, the large waste stream created and low extraction rates. The only sensible thing to do is to either selling the converters as they are to a collector or with medium sized lots extract the ceramics, mill, sample, assaying and send it to a specialized refinery.
But there will always be those that want to do the refining them self, out of ignorance, for fun or to learn the skills. For those there are two main paths for refining and one theoretical.
- Chemical leaching of catalytic converters : The PGM:s are dissolved in a bath of aqua regia, HCl + Cl or some other bath. After the PGM:s have been leached the solution is denoxed and the PGM chloride mixture is treated with zinc to give a black mixed PGM powder or more selective methods for each metal is used. See PGM refining from concentrate. Drawbacks are low extraction rates, especially of rhodium, and large volume of liquids.
- Pyrometallurgical refining of catalytic converters : Basically the crushed catalytic converters are melted with a flux and a metallic collector, for example silver or copper and held liquid long enough for the PGM:s to dissolve in the molten collector. This can be done at relatively moderate of 1500-1700 °C compared to the melting point of the PGM:s. The PGM:s are recovered as a byproduct of refining the collector metal, often in an silver or copper cell.
- Gas phase transport : This is not an easy thing to set up so nothing for the home refiner. Basically gases reacts with the PGM:s which in turn is converted to a chemical that is a gas at the temperature and pressure used. As a gas it is transported out of the reaction vessel and either converted back into metal or made to condense. This transports the PGM:s from the converters into a collection vessel. The PGM:s are then refined in a later step.
Foil based converters
One version of catalytic converters uses high aluminium stainless steel foils as a substrate. Dissolving the steel in HCl leaves a lot of sediments. Aqua regia dissolves some of the content but there will still be quite a lot of undissolved sediments, probably the wash coat and aluminum oxide.
The yield numbers of cats are hard to give. It varies with the age, if pieces have fallen out and how much PGM:s it contained in the beginning. But there are approximately 1-2 g of PGM per converter.
- GRF : Kurtak on large vs small scale leaching of catalytic converters
- GRF : Great thread on leaching cats
- GRF : Leaching honeycomb by kadriver
- GRF : Steve on a couple of ways to leach catalytiv converters
- GRF : A long discussion on cats and platinumill, but in the beginning there are a few yield numbers
- GRF : Scattered posts about gas phase extraction
- Dissolving a metal core catalytic converter
- Umicore : Introduction to different types of catalytic converters
- REMOVAL OF PLATINUM GROUP METALS (PGMs) FROM THE SPENT AUTOMOBILE CATALYST BY THE PYROMETALLURGICAL PROCESS, Saša Z. Ivanovic et. al.
- Archive.org : The Possibilities of Reusing the Ceramic Carriers Coming from Used Auto Catalytic Converters
- Development of Ferritic Stainless Steel Foil as Metal Support for Automotive Catalytic Converter (pdf)