Diesel particulate filters present precious metal recycling companies with new challenges

of 27 March 2018
SiC Dieselpartikelfilter / diesel particulate filter

• Since the end of 2017, the number of diesel particulate filters being recycled has increased; numbers are continuing to rise and will have a lasting impact on recycling companies in the future
• Exhaust gas catalytic converter or diesel particulate filters – what is the difference?
• Smelting operations face new challenges

Increasing numbers have a lasting impact on recycling companies
Since their introduction between 2006 and 2009, diesel particulate filters (DPF) have helped the environment by filtering out the harmful particulate matter, i.e. the fine soot particles, from diesel exhaust fumes. In this process, the carbon soot deposited in the filter is burnt at regular intervals by higher exhaust gas temperatures. Precious metals are used to generate the necessary temperature increases and to convert harmful emissions.

Now, 10 years later, precious-metal-bearing diesel particulate filters are entering the recycling circle in ever increasing numbers. This means that with almost 50 % of all vehicles in Europe being diesel vehicles, soon every second catalytic converter that goes to be recycled will be a diesel particulate filter. The quantity will increase rapidly in the next few years as vehicles that were built from 2006 onwards are gradually scrapped.

Difference: Automotive catalytic converter or diesel particulate filter
The carrier material of a diesel particle filter differs considerably from normal automotive catalytic converters (ACC). Conventional automotive catalytic converters which have been installed since the early nineties consist of approx. 1kg of cordierite. This ceramic consisting of aluminium, magnesium and silicon oxide can be very easily melted in electric high-temperature furnaces. The precious metals are separated off from the ceramic in this process and bound in a so-called collector metal. Smelting takes place under reducing conditions (oxygen release) and ensures high yields of precious metals.

In these diesel particulate filters, the carrier material usually comprises approx. 3kg silicon carbide (SiC). This material behaves very differently to the cordierite in the smelting process. To this end, an oxidising melt (oxygen release) is used to convert the carbon (carbide) to carbon dioxide and the silicon to silicon oxide. Only then is efficient precious metal recovery possible. If both material types ACC and DPF are now mixed, it becomes harder to recover the precious metals and is almost impossible if a specific concentration is reached.

Challenges for analysis and smelting
The capacities for processing this high-carbon material are currently limited and many smelting operations will face difficulties, not just because of limited capacities, but also because of the higher costs associated with this process (energy expenditure, refining, longer processing cycles). The first market participants are already refusing to recycle material mixtures.

Separate analysis and reconditioning is easy as separate commercial processes are available for both types of material. For some time now, the Hensel Recycling Group has also been working with its partners on a special processing process which will provide additional capacities in the future.

Defining features
To ensure sustainable and economic processing of the material, it is important for the materials to be separated at an early stage of the recycling circle. Once the catalytic converters or filters have been dismantled, separation is nigh on impossible. Together with the visual inspection and the weight check, the carbon analysis provides clear answers about the material composition and about the smelting process to be used. Hensel Recycling has the technical capabilities and the expertise to determine the SiC content or total carbon content in accordance with DIN EN ISO 21068-2:2008 and can, therefore, provide a reliable basis. The recycling company has set up a mobile database at https://kabimobile.hensel-recycling.com/frontend/ to provide information on identifying and evaluating diesel particulate filters.

Differences between ACC and SiC at a glance

Weight:

• A regular monolith or a DPF based on aluminium titanate weighs approx. 1 kg
• A DPF made of SiC ceramic usually weighs 2 – 3 kg

Visual inspection:

•SiC – usually grey surface and black ceramic, bonded cuboids with light edges

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