Metallurgical apparatus – With means treating or handling gases exhausted by treating... – By means recycling exhaust gas
Patent
1995-09-11
1997-05-27
Kastler, Scott
Metallurgical apparatus
With means treating or handling gases exhausted by treating...
By means recycling exhaust gas
266197, 266 44, C21B 1102
Patent
active
056329539
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a coke-fired cupola having a recirculating gas circuit and to a process for melting iron metallurgical materials, especially low-carbon oxidation-susceptible iron metallurgical charge materials, such as steel scrap, for example, for the production of cast iron.
DESCRIPTION OF THE PRIOR ART
A known distinction is generally made among classic cupolas on the basis of process-related and technological factors, identifying them as those operated with air (DD 299917 A7 and DE 3437911 C2), with oxygen enrichment, and with direct oxygen injection (U.S. Pat. No. 4,324,583). The direct injection of a secondary gas containing oxygen (50% to 100% oxygen) at supersonic speed, the gas being directly injected by nozzles separately from the primary air blast containing oxygen, which is disclosed, for example, in U.S. Pat. No. 4,324,583, leads to improved coke combustion and Si pick-up as well as to higher C contents and lower heat losses via the cupola mantle. In keeping with the Jungbluth diagram, only the supply of an optimal quantity of air is advantageous in coke-fired cupolas or blast melting furnaces, from the point of view of heat engineering, in terms of technical process. This means that enriching the air blast with oxygen is useful only for those furnace areas that do not achieve optimal heat conditions. Enrichment of the air blast with oxygen leads, as is known, to an increase in melting rate. In the case of an oxygen supply of up to 100%, a five fold increase in melting rate must be expected. At the same time, the combustion ratio is lowered, i.e., the CO share of the reaction gas in the furnace increases, the heat conditions deteriorate and can lead to the point of melting interruption. Enriching the air blast with oxygen while maintaining melting rate limits of practical value therefore necessitates a reduction in the air blast quantities; i.e., in the case of a cupola operated with a hot blast, a reduction in the hot blast quantity, and thus a reduction in the quantity of furnace gas, is required. The reduction in furnace gas quantity leads to a reduction in gas speeds in the furnace, as a result of which the share of the heat transfer as a function of the gas quantity in the fill drops and melting-off is hampered. Furthermore, high oxygen concentrations are created by the injection of oxygen in the fill area, which, due to the coarseness of the pieces of coke, cannot be completely converted with carbon when the concentration limit ranges are exceeded. The unconverted oxygen reacts with iron to form FeO and leads to limitations in the melting process.
That hot-blast cupola, the hot blast of which is generated in the recuperator and acted upon by flue gas created during top gas consumption, is currently the most widespread hot-blast cupola design. This furnace is also suitable for the manufacture of cast iron using high shares of steel scrap. It is also known that in hot-blast cupolas, via the temperature-dependent Boudouard equilibrium CO.sub.2 +C.fwdarw.CO furnace areas arise thermodynamically which act on the charge materials in both an oxidating and a reducing manner. The overheating of the molten iron thereby depends primarily on the level of the coke bed, i.e., on the drops-through time of the melted iron and on the temperature profile realized in the coke bed. In order to achieve the greatest possible overheating of the cast iron melt, the maximum gas temperatures are striven for in the oxidation zone during the coke conversion. This is realized in practice by preheating the combustion air up to 600.degree. C. or by introducing additional oxygen.
In this way, gas temperatures from 2000.degree. to 2200.degree. C. are achieved. Higher temperatures are not attainable, due to the increasing tendency of the combustion products to disassociate and the high flow speeds in the overheating zone. During melting of low-carbon metal scrap, the hot-blast cupola has the disadvantage, in terms of technical process, that a strongly oxidizing furnace atmosphere is cr
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"Entwicklungen beim Schmelzen von Eisengusswerkstoffen . . ." Giesserei 79, Dec. 1992, Nr. 4, pp. 134-143.
"Developments of Cupola Melting" Dr. Rachner, The Foundryman 84, Mar. 1991, pp. 114-118.
Feustel Hans U.
Kohler Karl-Heinz
Mallon Joachim
Neumann Gerhard
Ruschitzka Ludwig
Feustel Hans U.
Kastler Scott
Mallon Joachim
Schaaf Michael
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