By Louis Rumao
Pollutants from internal combustion engines have undesirable effect on air quality, environment, and consequently, on human health. Technologies like fuel-efficient engine design, superior tuning of the combustion process, fuel pre-treatment, or alternative fuels etc. are useful in reducing exhaust emissions. But a catalytic converter is found to be the best option to control HC, CO and NOx emissions from vehicles. A catalytic converter is placed inside the tailpipe and in a 3-way action oxidizes CO & HC into CO2 & H2O, and reduces NOx into N2 and O2.
Catalytic converters came into widespread use after their rollout in the US market in 1975. Since then, catalytic converters have become the standard in exhaust pollution control. Significant developments in automotive catalytic converters have occurred, with three-way catalysts (TWC) representing probably the most important of these developments. Their efficiency at transforming carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons (HC) resulted in a dramatic reduction in air pollution even as more cars were added to the road.
However, major challenges remain, particularly because of regulatory demands for increased performance. Design goals for the next-generation catalytic converters include: (1) high activity and selectivity; (2) very fast activity; (3) high thermal stability; and (4) high oxygen storage capacity.
Diesel engine emission control systems also require particulate filters for removing particulate matter (PM). In the current catalytic converter technology for lean burn diesel engines, the emission control system includes two separate components: A catalytic converter for oxidizing CO and hydrocarbons, and a separate device for capturing and reducing carbon particulate matter (PM) and NOx. As regulations continue to restrict the number of small particles that can be emitted by diesel vehicles, the importance of finding a highly efficient filter that does not compromise performance and fuel mileage becomes increasingly important.
Exposure To Catalyst
The content of the platinum group metals (PGMs) in a converter depends on the engine displacement and the type of fuel used and varies dramatically. The formulation can consist of only Pt, or various ratios of Pt-Pd-Rh, Pt-Rh, and Pd-Rh. 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. Rhodium and palladium have increasingly substituted Pt, and iridium-based catalysts have recently been introduced.
Some research data indicate that platinum concentrations in ambient air from exhaust range from approximately 4 – 100 picogram per cubic meter, depending upon volume of traffic and severity of driving. The nano-crystalline platinum particles are attached to micrometer-sized aluminum oxide particles.
Although the database on these platinum group metals is small, there is no evidence that they pose a health risk to the people.
Recycling Of Converters
Most of the recycled catalytic converters come from cars manufactured 10 to 15 years ago. Catalytic converter recycling is a big business and plays a significant part in the effort to feed the constant demand for, and scarce supply of, platinum group metals (PGMs). There are dozens of companies that purchase spent catalytic converters to recover the PGMs.
According to the 2015 Platinum Group Metals Mineral Commodity Study by the US Geological Survey, an estimated 155,000 kg of platinum, palladium and rhodium was recovered globally from new and old scrap in 2014, including about 50,000 kgrecovered from automobile catalytic converters in the US.
The collected scrap converters’ steel casing is opened to separate the honeycomb-structured ceramic material. The ceramic is then sorted, crushed, milled. In contrast, converters with a metallic substrate are first shredded or milled, and then the metallic parts are separated using magnets. In both cases, the materials are pulverized to a maximum 250µm and then processed to recover the PGM catalyst.
In many recovery techniques, large furnaces are used to melt the catalysts, along with fluxes and strong acids or chemicals. An Australian company, Wintermute Metals, has patented a process which does not use strong chemicals, as reported in the Mining Weekly magazine in August 2012.
The patented Wintermute Process entails a dilute acidic, oxidizing leach of the crushed catalyst, says business development and investor relations director Barry Epstein. “The catalyst is removed from the steel containers, after which it is crushed and leached for a short period in dilute acid, which contains several chlorine containing compounds at a relatively low temperature (90 ˚C) and at atmospheric pressure. This process converts the PGMs to water-soluble metal complexes, which are filtered from the matrix material and precipitated, by using recycled aluminum cans. The acid and reagents are then collected and reused for the next round of processing,” he explains.
Catalytic Converter Theft
Catalytic converters are appealing to robbers as an easy source of quick reward! Unlike, many other automotive components, these generally cannot be traced back to a particular vehicle. Further, a catalytic converter can be stolen in less than two minutes using a simple wrench or a battery-powered reciprocating saw.
Catalytic converter thefts typically happen to vehicles that are parked for prolonged periods in large lots, such as shopping centers, mass transit commuter lots or company parking lots. Vehicles that sit higher from the ground, such as trucks, pick-ups and SUVs, are particularly vulnerable to catalytic converter theft because thieves can slide underneath without having to jack up the vehicle to gain access to the converter.
According to various police departments, who investigate these robberies, thieves can sell catalytic converters to repair shops for about $100-150, while metal recyclers may pay $20-$200 for one depending on whether it contains platinum, palladium, rhodium or gold as the catalyst in it. The recyclers then extract the metal and resell it for as much as $200 a gram, as in the case of rhodium. It costs around $1000 to replace a stolen catalytic converter.
To combat catalytic converter thefts, a number of localities have passed laws tightening the restrictions on metal scrap dealers. In many cases, dealers are required to verify the seller’s identity with a photo ID and maintain complete records of sellers for 5 years.