Process and device for melting iron metallurgical materials in a coke-fired cupola

A coke-fired cupola, including: a furnace shaft having a well at one end, a charging device connected to the shaft, a lower-mouth exhaust connected to the shaft below the charging device, and a furnace gas exhaust ring arranged at the shaft below the charging device and the lower-mouth exhaust. Additionally, at least one nozzle is connected to the shaft above the well. The at least one nozzle has a centrally run oxygen lance. A gas recirculating circuit is provided for connecting the furnace gas exhaust ring to the at least one nozzle. The gas recirculating circuit includes an exhaust device connected to the furnace gas exhaust ring, a recirculating gas ring connected to the at least one nozzle and a recirculating gas duct connected between the exhaust device and the recirculating gas ring.

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 created in the nozzle 
area, which causes a silicon metal loss of up to 30%. 
The cause of this is the heterogeneous combustion reaction of the coke. It 
is also disadvantageous that large specific top gas quantities arise 
during melting, which require high plant expenditure for the gas economy. 
A special design of the coke-fired cupola is known from (U.S. Pat. No. 
2,788,964). This cupola is a hot-blast furnace for producing pig iron 
through the reduction of iron ores; however, it is also supposed to be 
usable as a remelting furnace for pig iron and iron/steel scrap. It is 
characteristic that the hot blast is blasted at up to 1000.degree. C. 
through a ring-shaped line and is brought from there via lines into the 
reduction zone of the shaft in the downward direction through nozzles. Due 
to the combustion, a mixture of ore and anthracite drizzles through the 
reduction zone, which is filled with coarse coke. In the reduction zone, 
the ore is reduced, and temperatures of up to 1900.degree. C. are reached. 
The gas outlet in the hearth area directly above the melting bath permits 
the out-take of a main stream of exhaust gases, which are fed to a special 
construction of a hot-blast generator and thus heat the fresh blast air. 
The temperature of the exhaust gases is to be above 1600.degree. C. A 
reduction of the CO content of the reduction gas is to be achieved in the 
reduction zone of the furnace by virtue of the fact that the hot blast, 
which is an oxygen carrier, burns carbon monoxide to form carbon dioxide. 
The blast furnace is furthermore provided with a line and a ventilator, 
through which, as an auxiliary flow, an upwardly directed gas stream rich 
in hydrocarbons (i.e., carbon monoxide poor) is exhausted from an exhaust 
duct in the upper shaft region and fed back into the combustion and 
reduction zone via a ring line through nozzles below the nozzles for 
supplying the hot blast. The forced flow of the major part of the furnace 
gases, i.e., the so-called downwardly directed combustion, which is also 
known as reverse firing, should permit temperatures of 1800.degree. C. to 
1900.degree. C. to be produced in the reduction zone, thus making possible 
the reduction of iron ore. The disadvantage of this hot-blast furnace is 
that the exothermic processes which are decisive for coke-fired and/or 
coal-fired blast melting furnaces are arranged downward from the hot-blast 
nozzles, and only there do the hot reaction gases necessary for the heat 
transfer to the fixed charge arise. Thus when the alternating-current 
principle, effective and proved for heat transfer, is changed to the 
ineffective direct-current principle for the heat transfer, only a 
shortened shaft portion is available, corresponding to a reduction to 1/4 
of the normal heat transfer zone, which is not compensated for through 
other measures in (U.S. Pat. No. 2,788,964.) Because the attained furnace 
room temperatures from 1800.degree. C. to 1900.degree. C. correspond to 
the usual level of cold-blast or hot-blast furnaces, melting an iron 
metallurgical charge is not reliably possible under these conditions. 
The upwardly directed auxiliary flow created above the hot-blast nozzles 
consists of hydrocarbons, mainly methane formed via the water gas reaction 
and the methane reaction. The conditions for forming methane according to 
this mechanism, however, call for temperatures greater than 700.degree. C. 
at normal pressure and, under technical conditions, for temperatures 
preferably of 1000.degree. C. and an atmosphere saturated with water 
vapor. However, the furnace according to U.S. Pat. No. 2,788,964 rules out 
precisely these conditions, so that an upwardly directed flow of gaseous 
hydrocarbon cannot be created and the circulatory principle thus cannot be 
implemented. 
In terms of heat engineering, feeding hydrocarbons to a level below the 
hot-blast nozzles is meaningless, because this level is too far removed 
from the blast nozzles. As a result, the air-oxygen of the hot blast has 
already converted with the carbon of the coke bed and/or coal bed and is 
no longer available for the methane combustion. Methane is thus taken out 
at the exhaust opening located directly below without being used. If 
portions of the C--H compounds nonetheless combust to CO.sub.2 and H.sub.2 
O, these gas components, together with the CO.sub.2 arising from the 
combustion of the coke and/or the coal, are endothermically reduced, below 
the nozzles to CO and H.sub.2 by the carbon of the coke and/or the coal, 
in keeping with the effect of the Boudouard equilibrium, as a result of 
which a lowering of the temperature in the reduction zone and a 
consumption of the coarse coke occur as effects. 
Furthermore, it is also disadvantageous in the case of the furnace 
according to U.S. Pat. No. 2,788,964 that, due to the unheated shaft above 
the hot-blast nozzles, the exothermic indirect reduction that is 
particularly important to the blast-furnace process and dominates with 55% 
to 60% of the total reduction, which rests on the formation of expanded 
temperature zones between 800.degree. C. and 1000.degree. C., is 
suppressed. Above 1000.degree. C., direct endothermic reduction begins. 
The burden placed in the furnace is first heated suddenly to temperatures 
greater than 1000.degree. C. on the nozzle level and then immediately 
subjected to the conditions for direct reduction. The substantially higher 
demand for heat connected with this can no longer be met by the standard 
described burden, as a result of which the heat budget of the furnace 
collapses and the process may come to a standstill. 
Also disadvantageous is the geometric design of the furnace zone below the 
throat, which in the short, compacted design shown cannot make available 
the necessary time reserves for the heating and diffusion processes. It is 
also disadvantageous that the coarse-granular coke bed is consumed before 
the air nozzles and is replaced only by small-piece burden and coke and/or 
coal. In this way, the gas distribution, heating up and diffusion 
necessary to the process are additionally hampered in a granulometric 
fashion. The blast-furnace process according to the process in (U.S. Pat. 
No. 2,788,964) is, like the use of the depicted recirculating gas process 
and the melting of the metal charge, not possible for reasons of heat 
engineering, geometry and flow engineering. 
All currently known new developments also have disadvantages during the 
melting of low-carbon, oxidation-susceptible iron metallurgical materials. 
In the furnace developed by the TUPI Company in Brazil, the iron 
metallurgical charge materials are charged in a central shaft. The coke 
and lime are fed to a well via six filling shafts arranged symmetrically 
around the perimeter. The combustion air, which is blasted in at high 
speed through special nozzles, is not preheated. The combustion air is 
enriched, however, with up to 1.5% oxygen. The furnace is controlled in 
such a way that the combustion gases always flow through the central shaft 
and in this way heat the iron metallurgical charge materials thoroughly. 
Through a secondary row of nozzles, the gases containing CO are 
post-combusted prior to entry into the central shaft. The disadvantages of 
the furnace are that a large number of channels form through the bed, 
through which gas flows and which cannot be supplied with hot combustion 
air in an even manner. As a result, no complete mixing of combustion air 
and combustion gas occurs and thus no complete conversion of the CO into 
CO.sub.2 takes place. The exhaust gas still has a CO content of &gt;0.1% by 
volume. In order to avoid environmental pollution, relatively high 
expenditures for the gas economy are accordingly also required here. In 
hot-blast cupolas having plasma burners, the combustion air is 
traditionally additionally preheated, due to the high CO shares and the 
required post-combustion. The plasma burner is subsequently used for 
heating a partial flow of the combustion air to temperatures of 
3000.degree. to 5000.degree. C., whereby any desired mixing temperature 
can be set. The initial temperature of the combustion air may be up to 
1400.degree. C. These high temperatures require a reducing furnace 
practice. The disadvantages of the hot-blast cupolas having plasma burners 
are found in the relatively high electricity costs, because here melting 
is carried out indirectly in electrical fashion. The CO.sub.2 output also 
is large. 
In order to avoid the large gas output, conceptual models of this type of 
furnace have been developed as an idea for the future (Sonderdruck 
GieBerei 79 (1992) 4, pp. 134-143). A closed gas circuit is to be formed, 
in that a highly heated gas is produced as a heat carrier, e.g., via a 
plasma burner, in partial flux, and is then brought to the desired 
temperature by being mixed with the remaining gas. The top gas is 
exhausted completely and, after being purified, is fed again to the plasma 
burner for heating. It is disadvantageous that in order to realize this 
conceptual model, high equipment expenditures, in conjunction with large 
space requirements, are needed; furthermore, the melting process, 
conditioned on the production of an artificial furnace gas composition, 
can be reliably carried out only through additional measurement and 
control technology. 
The object of the invention is to develop a simply functioning coke-fired 
cupola having a recirculating gas circuit and a process characterized by 
low melting costs and the avoidance of silicon metal loss for reliably 
melting iron metallurgical materials, especially low-carbon, 
oxidation-susceptible iron metallurgical charge materials, such as steel 
scrap, for the production of cast iron, which allows a low and CO-free top 
gas quantity to develop and thus sharply reduces environmental pollution. 
According to the inventive process, the above mentioned object is attained 
in that furnace gas having a temperature &gt;400.degree. C. is partially 
drawn off in the preheating zone and is fed back into the melting and 
overheating zone together with &gt;23%, preferably 33% to 48% oxygen, 
relative to the supplied gas quantity. A further feature of the inventive 
process is that the partially withdrawn furnace gas quantity equals up to 
70% of the furnace gases arising during the melting process. 
Advantageously, the exhaust gas volume is reduced, because the ballast gas 
nitrogen of the blast present in blast-operated cupolas is replaced by the 
energetically useful furnace gas (as circulating gas) and is usefully 
utilized in the melting process. The furnace gas here acts as a gaseous 
fuel and combusts with the supplied oxygen thus, intensifying the reducing 
conditions in the furnace. The device-related expense for the gas economy 
is low. 
The coke-fired cupola according to the invention, is used exclusively with 
oxygen as the combustion medium and conveyor medium for the CO-rich 
furnace gas that is drawn off below the throat and is not subsequently 
combusted. In this way, the subsequent combustion to CO-free exhaust gas, 
which is the prerequisite for producing a hot blast from fresh air (c. 21% 
O.sub.2, 79% N.sub.2) via heat exchange from the hot furnace gas and in 
classic cupolas is carried out outside of the bedding, is moved in the 
cupola, according to the invention, into the bedding directly before the 
oxidation zone. The described and practically-confirmed effects of the 
furnace are thus based on the fact that the furnace is operated in 
principle without the nitrogen carrier air, i.e., exclusively with oxygen 
as the combustion means. Advantageously, the use of the energy contents of 
the furnace gases containing CO occurs directly, without going through the 
hot-blast production in the melting and overheating zone. Only in this way 
can the process-typical displacement of the Boudouard equilibrium toward 
CO be achieved. 
The design according to the invention of the nozzle area in the furnace 
mantle permits the furnace gas to be mixed with oxygen and a conversion of 
CO+O.sub.2 to CO.sub.2 to begin already at the nozzle mouth. This leads to 
the desired furnace atmosphere and metallurgical effects. The invention is 
described in greater detail below in reference to the example of melting 
two grey cast iron alloys.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a coke-fired cupola having an exhaust smoke stack 4, in 
section. It shows that below the charging device 6 and the lower mouth 
exhaust 7 in the area of the preheating zone 3, there is a furnace gas 
exhaust ring 9, and above a well 1 in a area of the melting and 
overheating zone 2 there are four nozzles 13, which have centrally-run 
oxygen lances 12, whereby the furnace gas exchange ring 9 is connected to 
the nozzles 13 by a radial ventilator 10, a recirculating gas duct 14 and 
a recirculating gas ring 11. 
FIG. 2 shows a further embodiment of the coke-fired cupola, in section. It 
is characteristic here that below the charging device 6 and the lower 
mouth exhaust 7, there are four furnace gas exhaust openings 15 arranged 
around the perimeter of the furnace shaft 5. The opening 15 are connected 
by ducts 16 to the nozzles 13 above the well 1, nozzles 13 which also have 
the centrally-run oxygen lances 12. 
FIG. 3 shows the design according to the invention of the nozzle area in 
the furnace mantle, in section. The furnace mantle 17 has, in the area of 
the nozzle acting in direction 18, a calotte-type widening 19, and the 
oxygen lances 12 which run centrally in the nozzle 13 have a distance a of 
18 mm to the furnace mantle 17. The distance a is variable but must be at 
least 15 mm. An arrangement of this type of nozzle and oxygen lance acts 
as a suctioning injector, like, for example, a gas jet compressor. For the 
melting according to the invention of grey cast iron alloys, the process 
is as described below. 
EXAMPLE 1 
Via the charging device 6, the coke-fired cupola is charged with iron 
metallurgical charging material consisting of 50% scrap castings and 50% 
returns with charge coke and slag-forming constituent. Charge coke is that 
portion of fuel which is fed to the cupola as a replacement for the share 
of the fuel bed coke during the melting and overheating process. Bed coke 
is that portion of the fuel which, prior to the beginning of melting and 
overheating, is brought into the cupola for building up a coke bed. This 
coke bed forms the melting and overheating zone as well as the area of 
carbon pick-up for the melted iron metallurgical charge material. 
Slag-forming constituents are mineral components that are needed to slag 
the coke ashes which are created when the coke is burned. The quantity in 
which slag-forming constituent is added is specified in percent to the 
added charge coke quantity in kg/charge. The charge coke quantity is 
specified in percent to the weight of the iron metallurgical charge 
material. The bed coke is specified in percent to the weight of the iron 
metallurgical charge during one melting series, i.e., taking into 
consideration the number of the melted iron metallurgical charge material 
quantity over the entire melting time. The charge make up analysis results 
in 3.42% C, 1.85% Si, 0.62% Mn, 0.50% P, 0.11% S and &gt;1% trace elements as 
well as up to 100% Fe. The iron oxide share of the charge material is 2%. 
Per charge, 10.43% charge coke, lump size 60/90 is added. The proportional 
share of bed coke, lump size 80/100, is 1.81%, and 20% limestone of the 
charge coke is used as a slag-forming constituent. Through the radial 
ventilator 10, 60% of the furnace gas, which is composed of approximately 
63% CO and 37% CO.sub.2 as well as of residual gas (H, and H.sub.2 O), is 
fed back to the furnace from the furnace gas exhaust ring 9 of the nozzles 
13. This furnace gas has a temperature of 550.degree. C. and burns through 
the simultaneous supply of 33% oxygen, relative to the supplied gas 
quantity. The resulting metallurgical processes with a reducing effect in 
the melting and overheating shaft lead to a molten iron analysis of 3.64% 
C, 2.03% Si, 0.58% Mn, 0.5% P and &gt;0.11% S. The carbon pick-up rate of 
this make-up which is free of steel scrap, thus leads to 7% relatively, 
the silicon pick-up to 10% relatively, and the manganese metal loss to 
approximately 5% relatively. For carbon pick-up, 0.27% of the charge coke 
is needed, and for reducing the two-percent iron oxide share, 0.23% of the 
charge coke is need. The effective charge (melting) coke share is 11.75%. 
As a result of the alkaline furnace operation, a successive sulphurization 
of the cast iron is avoided. The invention causes a minimum of dust 
output, at the level of roughly 40% of the usual dust emissions from 
coke-fired cupolas. The remaining furnace gases are extracted by the 
lower-throat exhaust 7, whereby the ignition device 8 ensures a constant 
ignition of the top gas and thus complete subsequent combustion. 
Accordingly, the dust-removing devices installed downstream are smaller. 
EXAMPLE 2 
Via the charging device 6, the coke-fired cupola is charged with 25% cast 
chips, 40% steel scrap, including 30% shredder scrap, 32% returns, 0.22% 
FeSi formed bodies and 0.22% FeMn formed bodies. The make-up analysis 
reveals 2.09% C, 1.18% Si, 0.55% Mn, 0.34% P and 0.08% s and &lt;1% trace 
elements as well as up to 100% Fe. The iron oxide supplement of the charge 
material is 2%. Per charge, 10.43% charge coke, lump size 60/90 is added. 
The proportional share of bed coke, lump size 80/100, is 1.81%. 20% 
limestone of the charge coke share is used as a slag-forming constituent. 
Through the furnace gas exhaust device 9, 60% of the furnace gas at a 
temperature of 480.degree. C. is fed back through the nozzles 13 to the 
melting and overheating zone 2, whereby this burns through the 
simultaneous supply of 33% oxygen, relative to the supplied gas quantity. 
This yields a molten iron analysis of 3.55% C, 1.4% Si, 0.6% Mn, 0.40% P 
and &gt;0.10% S. The carbon pick-up rate of this make-up, which consists only 
of hard-melting recycled materials leads to 71% relatively. The relative 
Si pick-up is approximately 10% and the relative manganese metal loss is 
approximately 5%. The sulphur pick-up results at 33% from the sulphur of 
the coke using an acidic furnace operation. For carbon pick-up, 1.67% of 
the charge coke is needed, and for reducing the iron oxide share, 0.23% of 
the charge coke is consumed. The effective charge (melting) coke share is 
10.34%. The cupola tapping temperature is 1500.degree. C. By drawing off 
of 60% furnace gas below the mouth from the packed bed and a combustion 
with oxygen in the well area the furnace, the dust output is minimized to 
approximately 40% of the dust emission usually existing during the cupula 
melting process. The remaining furnace gases which are not drawn off are 
subsequently completely combusted.