Introduction
Solid hazardous waste materials derived from a wide variety of sources (e.g. incinerator
residues, discharged batteries, automobile shredder residues, contaminated soil, and
by-products from the metallurgical industry) contain toxic compounds, principally heavy
metals, which cannot be discharged to the environment. These materials are dumped in
hazardous waste storage sites for an indeterminate period of time, where they have to be
monitored, or they are processed in energy intensive thermal and other processes, or
vitrified in non-leaching slags. The current fossil-fuel-based recycling techniques using
high-temperature furnaces or electricity for the extraction of the toxic compounds are
characterized by their high energy consumption and their conconmitant environmental
pollution. Using concentrated solar radiation for supplying the process heat can
significantly reduce these emissions.
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Feedstock
Two important types of hazardous waste materials are examined: Electric Arc Furnace Dust
(EAFD) and Automobile Shredded Residue (ASR). EAFD is a byproduct of the metalurgical
industry. Production of 1 ton of steel produces 10-20 kg of EAFD. About 1.6 millions of
tons EAFD are estimated to be produced annually in North America, Western Europe and
Japan. EAFD is a toxic material due to its high amount of heavy metals. It is dumped into
landfill sites, or recycled in conventional high-temperature or hydrometalurgical
processes. Fossil fuel based recycling of EAFD produces about 2.3 tons of CO2 per ton
EAFD processed, while solar energy based processes would produce only about 0.3 tons of
CO2 per ton EAFD.
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ASR is a fluffy residue derived from car shredding at the end of the cars life. It is
traditionally placed in a landfill. It contains about 50% organics (textiles, rubber,
plastic, wood etc), wires, and small pieces of metal, stones, glass and other materials.
ASR is considered as hazardous waste material in some countries due to its high content
of heavy metals (especially copper, zinc, lead) and other toxic compounds.
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Solar recycling processes
The scope of the project is to develop a solar recycling process, where the fossil fuels,
used in current recycling processes are replaced with concentrated solar energy. A 5 kW
recycling reactor is being tested at the PSI High Flux Solar Furnace, where the only
source of process heat is concentrated solar radiation. Two processes are studied: the
carbothermal reduction of metal oxides abundant in EAFD and the evaporation of zinc, lead
and cadmium, and the pyrolysis of ASR. The carbothermal reduction can be described
by:
MxOy + C --> x M + y CO(g)
where MxOy denote the metaloxide and M the corresponding metal.
These reactions are highly endothermic and require temperatures above 1000K for the
metaloxides in EAFD.
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The solar reactor is operated at temperatures above 1000K. The reactants (1 kg EAFD,
reducing agent carbon, and glass) are fed into the recycling reactor. The carbothermal
reduction of the metaloxides abundant in EAFD takes place after heating the reactor with
concentrated solar radiation to the reaction temperatures. The reaction products zinc,
lead, CO(g) and CO2(g) exit the reactor and undergo quenching. The products
can be used either as fuels or as material commodities. The oxidised fuels and the waste
materials are sent again to the recycling reactor.
Experiments with the recycling of EAFD have been conducted at temperatures up to 1300°C.
Heavy metals (zinc, lead, cadmium) have been evaporated and collected. The reaction
products are a mixture of Zinc and Lead, and a residue depleted of heavy metals.
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Experimental setup in the PSI High Flux Solar Furnace
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Aperture (opening for the incomming concentrated solar radiation) of the reactor after an
experiment.
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One way to process ASR is the pyrolysis of the organic material to get pyrocoke,
synthesis gas and a metal fraction. Synthesis gas and pyrocoke can both be used as
feedstock for a wide range of processes. First solar experiments of the pyrolysis of ASR
at moderate temperatures yield a mixture of CO, CO2, CH4,
H2O and other gases, pyrocoke, tar and oil and a metal fraction.
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related publications
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Recycling of Hazardous Solid Waste Material Using High-Temperature Solar
Process Heat. 2. Reactor Design and Experimentation
Schaffner, B., Meier, A., Wuillemin, D., Hoffelner, W. and Steinfeld, A.
Environmental Science & Technology, Vol. 37, 165-170, 2003.
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Recycling of Hazardous Solid Waste Material Using High-Temperature Solar
Process Heat - I. Thermodynamic Analysis
Schaffner B, Hoffelner W. and Steinfeld A.
Environmental Science & Technology, Vol. 34, No. 19, pp. 4177-4184,
2000.
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Solarthermisches Recycling von festen schwermetallhaltigen
Sonderabfällen
Schaffner, B.
Ph. D. Thesis, No. 14743, ETH Zürich, 2002. link to ETH e-collection
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Recycling of Hazardous Solid Waste Material Using High-Temperature Solar
Process Heat
Schaffner, B., Meier, A., Wuillemin, D., Hoffelner, W. and Steinfeld, A.
PSI Annual Report 2002 - Annex V, 30-32, 2002. pdf file 432 kB
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Solar Recycling of Hazardous Solid Waste Materials
B. Schaffner, W. Hoffelner, A. Steinfeld
PSI Annual Report 2000 - Annex V, 29-30, 2000. pdf file 227 kB
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Hochtemperaturrecyling mittels konzentrierter Solarstrahlung
Schaffner B., Steinfeld A., Hoffelner W.
PSI Spectrum 4/2000, p. 19.
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