Catalytic converter recycling with XRF technology
A catalytic converter is a device used to convert toxic vehicle emissions to less harmful substances by way of catalysed, or accelerated, chemical reactions. Most present-day vehicles that run on gasoline, including automobiles, trucks, buses, trains, motorcycles, and planes, have exhaust systems employing a catalytic converter.
The catalyst component of a catalytic converter is usually platinum (Pt), along with palladium (Pd), and rhodium (Rh). All three of these platinum group metals, or PGMs, are extremely rare but have a broad range of applications in addition to catalytic converters. Platinum, for example, is used in laboratory and dental equipment, electrical contacts and electrodes, and jewellery, while palladium plays a key role in fuel cell technology. With numerous applications and limited supply, these valuable metals are an attractive target for recovery and reuse from spent catalytic converters. In 2010, the total value of Pt, Pd, and Rh reclaimed from the recycling of spent catalytic converters was $3 billion.
Determining the recovery value of the PGMs in a catalytic converter begins with knowing the composition and ratios of the metals used. Automotive catalyst material is made either of a ceramic substrate, mostly cordierite coated with a precious metal containing a wash coat, or of a metallic substrate with an aluminium oxide wash coat also containing precious metals. The average concentration and the ratio of Pt and Rh were constant 20 years ago, so a simple weighting was sufficient to arrive at a good estimation of the precious metal content. However, the price of these three elements has fluctuated strongly over the last 20 years, depending on the supply, demand, and speculation. These variations, as well as the tightening of emission legislation, have had a direct correlation on the composition of the catalysts, which themselves have had a strong influence on demand.
Currently, the composition, which depends on the engine displacement and the type of fuel used, varies dramatically. The formulation can consist of only Pt, or various ratios of Pt-Pd-Rh, Pt-Rh, and Pd-Rh. Most of today’s recycled catalytic converters come from cars manufactured, on average, 10 to 15 years ago. The recoverable amounts of Pt, Pd, and Rh in each can range from 1-2 grams for a small car to 12-15 grams for a big truck in the US.
The corresponding value in recoverable PGMs ranges from $25 to a few hundred dollars per vehicle. Moreover, the trade of ground-up material sold as catalysts can be very dangerous because of possible alterations, which can mean inclusion of lead or spent nickel-cadmium batteries.
To avoid considerable financial losses, companies need to quickly and accurately determine the contents of Pt, Pd, and Rh in spent catalytic converters at the collector’s site or in the refineries. But there is no technique for analysing non-homogeneous materials like catalytic converters directly without sample preparation. Therefore, the collected catalysts with ceramic substrate undergo a “de-canning” operation, which is the extraction of the honeycomb-structured ceramic material from the steel case. All the ceramic is then sorted crushed, milled, and mostly blended with other catalysts. In contrast, converters with a metallic substrate are first shredded or milled, and then the metallic parts are separated using magnets and winnowing from the wash coat powder containing precious metals. Because of this enrichment, the PGM content of these wash coat samples is usually much higher than that for milled ceramics. In both cases, the materials are pulverised to a maximum 250µm and loaded into sample cups or sample bags for analysis, and then analysed using x-ray fluorescence (XRF) technology.