Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Special medium during sintering
Reexamination Certificate
2000-01-24
2002-09-17
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Special medium during sintering
Reexamination Certificate
active
06451250
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method for operating a sintering plant so as to reduce the emission of dioxins.
A sintering plant is a large-scale industrial plant used in the metallurgical industry and serves to convert fine grain or fine dust mixtures of metal, metal oxide or metal sulfide solids into lumps, so that they can be used for smelting. Only solids mixtures which have been made lumpy can be used in a blast furnace.
In the sintering plant, the material to be sintered, or material for sintering, is consolidated by sintering, i.e. by being heated to close to the melting point, with surface softening and partial formation of melt and slag. For this purpose, the material for sintering, which in the case of smelting of ore may, for example, be a fine ore, calcined pyrites, flue dust or a fine dust from a metal-processing industry, is placed on a so-called traveling grate, if appropriate together with recycled material, slag-forming additives or a solid fuel mixture. The traveling grate is generally designed as an endless chain of individual grate carriages, the chain running over corresponding diversion rollers in the operating direction of the sintering plant.
At an inlet position, the traveling grate carriages are filled with the material for sintering. In this process, in order to protect against the material for sintering falling through, the base of the grate carriage, which is designed as a grating, is first covered with previously roasted recycle from the sintering plant, and then the material for sintering, which can be mixed with fuel, is added. Then, the filled grate carriages pass by an ignition furnace, the material for sintering or the fuel contained therein being ignited at the surface by means of an ignition flame. Then, the combustion and sintering operation in the individual grate carriages continues automatically into the inside of the grate carriage as a result of self-combustion by means of combustion air which is fed through by an extractor or forced draught fan, while the grate carriages are transported to an outlet position.
Abraded material in the form of fine grains or fine dust from filing, grinding or drilling processes used in the metal-processing industry represents a large part of the material for sintering which is processed in the sintering plant. However, it is a disadvantage that this abraded material is often contaminated with oily drilling aids, lubricants or coolants, which include not inconsiderable quantities of halogenated hydrocarbons and aromatics. Therefore, the thermal processes during the sintering operation also produce dioxins in the sintering plant, which can pass into the environment via the off-gas. In this context, the term “dioxins” is used herein as a collective term for the group of cyclic halogenated aromatic ethers and polyethers. These include in particular the cyclic ethers (furans) and the cyclic diethers (the actual dioxins).
Polychlorinated dibenzodioxins (PCDD) and polychlorinated dibenzofurans (PCDF) can be mentioned here as particularly toxic representatives of these two groups.
GB 2,155,457 A has disclosed a method for operating a sintering plant in which an oxidation catalyst is admixed with the material for sintering prior to sintering in order to reduce the amount of hydrocarbons present.
Catalyst components mentioned are inorganic or organic compounds of the transition metals, such as Ni, Cu, Fe, V or Cr.
EP 0,645,172 A1 proposes a noble-metal-based dioxin catalyst for treating exhaust gases.
In comparison with other large-scale industrial plants used in the metal processing and extractive metallurgy industry, sintering plants therefore represent the major source of the emission of dioxins. Thus, in the off-gas from a sintering plant, dioxin levels of up to 60 ng TE/m
3
are found (TE=Toxic Equivalent). In order to keep the amount of dioxins released into the environment from a sintering plant below the legally prescribed limit level (currently 0.1 ng TE/m
3
), it is known from W. Weiss: “Minderung der PCDD/PCDF-Emissionen an einer Eisenerz-Sinteranlage [Reduction of the PCDD/PCDF emissions from an iron ore sintering plant]”, VDI Reports No. 1298 (1996), pp. 249 ff, to add a mixture of calcium hydroxide Ca(OH)2 and coal (in the form of hearth-furnace coke or activated charcoal) as an additive to the off-gas from a sintering plant for the purpose of dioxin adsorption and to remove the additive, which is to some extent laden with dioxins, from the off-gas again by means of a fabric filter and then to return it once again to the off-gas. Furthermore, it is known from G. Mayer-Schwinning et al.: “Minderungstechniken zur Abgasreinigung fur PCDD/F [Reduction techniques for off-gas cleaning for PCDD/F]”, VDI Reports No. 1298 (1996), pp. 191 ff, to use zeolites as dioxin adsorbers in the off-gas from a sintering plant.
However, it is a disadvantage that the laden dioxin adsorbers must ultimately be land-filled, which brings about considerable costs and presents a threat to the environment.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method for operating a sintering plant that overcomes the above-mentioned disadvantages of the prior art devices and methods of this genera type, in which the dioxin emission when operating a sintering plant is reduced, without presenting additional hazards to the environment.
With the foregoing and other objects in view there is provided, in accordance with the invention a method for reducing the dioxin content of the off-gas in operating a sintering plant, in which prior to sintering a material catalytically active in decomposing dioxins and in the form of fine grains or dust is admixed with the material for sintering, and is sintered together with the material for sintering.
It has also been found, according to the invention, that foreign material admixed with the material for sintering, such as impurities, admixed slagging aids (silicates) or combustion aids (e.g. coke) is incorporated in the agglomerate of sintered material formed during the sintering operation in the sintering plant. Then, the incorporated foreign material passes into the blast furnace operation together with the sintered material, where it has no adverse effect on the formation of slag. On the contrary, the foreign material is even insolubly incorporated in the slag formed in the blast furnace.
Although as a rule the low off-gas temperatures of less than 200° C. mean that a catalytic reduction of the dioxin content in the off-gas from a sintering plant cannot be achieved, the incorporation of foreign material in the slag allows dioxins to be catalytically eliminated by adding catalytically active material to the material for sintering. On the one hand, material introduced in this way does not have any adverse effect on the sintering operation and on the subsequent smelting process of the sintered material. On the other hand, the introduction of catalytically active material as early as at the location where the dioxins are formed, i.e. during the combustion process in the material for sintering, causes the dioxins produced to be effectively reduced, and at the same time further formation of dioxins is prevented.
Since, in a sintering plant, the combustion air is sucked or blown through the material being sintered, the combustion gas or off-gas produced during combustion must inevitably flow past the catalytically active particles which have been admixed with the material for sintering.
In this way, a catalytically active material which is in the form of fine dust or fine grains and is admixed with the material for sintering makes it possible to achieve an effective reduction of the dioxin content in the off-gas flowing through. Sodium, potassium, magnesium, calcium, barium, zinc, nickel, lead, titanium, copper, iron, aluminum, platinum, vanadium, tungsten, molybdenum, rhenium or cerium, individually or in combination and in the form of fine dust, can be admixed with the material for sintering as th
Hums Erich
Spielmann Horst
Greenberg Laurence A.
Locher Ralph E.
Mai Ngoclan
Siemens Aktiengesellschaft
Stemer Werner H.
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