Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Patent
1997-10-14
2000-02-08
Andrews, Melvyn
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
75500, 75669, 75670, 205364, 420590, C22C 2102, C25C 306
Patent
active
060223960
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a process for treatment of filter ashes, which arise during waste disposal, in particular through a two-stage reduction of the metal compounds obtained, reusable metal and silicon-containing alloys being recovered.
For reduction of the pollution burden in the vicinity of waste incineration plants, these plants are equipped with so-called flue gas cleaning filters. The dust and products which arise at these plants from the further flue gas cleaning are heavily polluted by heavy metals and are therefore toxic (see Table 1). At the present time the so-called filter ashes are treated using conventional, wet-chemical recycling processes so that the aforementioned toxic substances can be extracted for reuse.
For the described residues, ashes and dust there are three treatment principles known today: non-leachable substances, whereby the heavy metals are still present in the toxic, ionized form. Belonging here are consolidation methods with preferably cement or clays. Also belonging to this category are thermal (smelting) processes in which the metals are embedded in a stable, leach-proof glass matrix. suitable elutriation or respectively distillation methods and separate treatment of the eluates or respectively distillates. The inert matrix, depleted of heavy metals, can be sent for reprocessing if the toxic organic compounds have been previously destroyed. The separated metal compounds are still toxic and are considered special waste. Therefore they cannot be disposed of without further conditioning. which can be reused, are extracted from toxic substances and compounds. A known process of this kind treats residues from combustion in an iron melt first by oxidation, then by reduction at temperatures over 1300.degree. C. or respectively over 1450.degree. C.
Owing to the declining market for heavy metals and the energy-intensive, costly processes, treatments of this kind are at present not very attractive for economic reasons. Furthermore the components remain partly in a toxic form even after the treatment. Therefore there is a need for other solutions to the problem of disposal of filter ashes, solutions without the aforementioned drawbacks, which allow an economical and environmentally sound reprocessing of toxic and environmentally relevant components.
It has been discovered that the said residues can be treated economically if they are subjected to a fused salt electrolysis in an aluminum furnace. It has been further discovered--so that less electrical energy is required--that the heavy metals can be reduced and extracted economically with a prior reduction using carbon.
The object of the present invention is therefore a process for the treatment of filter ashes, which arise during waste disposal and are polluted with heavy metals and/or heavy metal compounds, by means of a two-stage reduction of the obtained metal compounds with formation of reusable metals and metal and/or silicon-containing alloys.
The process according to the invention with the reduction of residues, such as filter ashes, in two stages, leads to the following advantages:
It can be used on many kinds of starting substances, such as ashes, filter ashes, dusts, foundry sands, sludges, contaminated earth, etc.
Not only are the toxic heavy metals bound more or less firmly in a matrix, as in the case of immobilization processes, but they are also put into a non-toxic. reduced form.
The lowering of the melting temperature of residues from combustion through suitable substances saves energy.
The greatest consumer of current during the electrical reduction is silicon (compare Table 1), amounting to practically 80 percent of the reduction current. If silicon is reduced with inexpensive coal and/or calcium carbide, the process becomes less expensive and considerably more interesting economically since expensive electric current can be saved.
The materials used in exhaust gas cleaning, coke, coal dust or activated charcoal serve, after charging, as reduction means.
The slags which arise can be mixed with substances which re
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Andrews Melvyn
Refista Recycling AG
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