Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2000-03-21
2002-07-23
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
Reexamination Certificate
active
06423116
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and a device for the direct reduction of iron ore with coal.
Such methods and devices are employed in the production of iron or steel from iron ore, ie in metallurgy for example.
The methods applied for by far the greatest part of iron production from ore utilise the carbon in coal or coke for two processes which proceed simultaneously.
Carbon acts first of all as a chemical reducing agent and takes over the oxygen from the iron oxide.
Secondly, by combustion with oxygen (usually from the air) carbon supplies the necessary heat for the process.
For the best utilisation of the coal each of these two process steps requires thermodynamic conditions which are mutually exclusive (“thermodynamic dilemma”).
For the reduction, the chemical potential of the oxygen in the atmosphere must be as low as possible, ie high CO content, low CO
2
content and low O
2
content in equilibrium with CO.
For the most complete possible combustion of the carbon to CO
2
, an O
2
content in the combustion atmosphere meeting at least the stoichiometric requirements is needed.
In the coal reduction methods currently used in industry compromises are entered into. Accordingly, more energy is expended than is needed for the actual iron ore production process. At the same time the methods result either in reduction gas which does not have the full capability for reducing the oxide because before it is used for reduction the gas has already taken up oxygen by partial combustion or they result in the heat needed not being produced with optimum efficiency since the gas cannot be burned completely to CO
2
.
Depending on the process, the unused portion of energy is dissipated in the form of carbon monoxide and hydrogen or as perceptible heat or both. The more or less advantageous use of the coupled product, energy, from these non-autothermic processes has a substantial effect on the economic efficiency of the overall process. In the blast furnace method the excess energy is given off in chemical form as furnace gas usually to usage loads located in the works. In the case of the rotary-tube furnace method the utilisation of the excess energy is difficult because frequently no usage loads are available. Occasionally the excess energy is converted to electrical energy.
SUMMARY OF THE INVENTION
In order to improve the energy yield in the context addressed here two routes have been taken. The compromise referred to has been improved or processes have been proposed which proceed without excess energy and are thus autothermic.
The characteristics of known methods affecting energy demand are here examined in more detail and compared.
For this purpose the RIST diagram in
FIG. 1
is used. This shows on the abscissa the oxidation of the carbon as the molar ratio from O/C=0 to O/C=2, ie from carbon C to carbon dioxide CO
2
, and on the ordinate the origin of the required oxygen with reference to one mol of Fe and thus expressed by the molar ratio O/Fe.
This oxygen taken up by the carbon can come from the ore (central region of the ordinate) or it can be free oxygen for precombustion (lower part) or afterburning (upper part). The terms precombustion and afterburning refer to the combustion of C or CO before and after, respectively, these have been used for reduction.
The removal of oxygen from the oxide illustrating the reduction becomes more clearly discernible if the number of mols of oxygen present in the oxide in question during the reduction process is always related tob
1
mol of iron instead of the conventional 1, 2 or 3 mols of iron. The following notations then emerge:
haematite, Fe
2
O
3
, becomes FeO
1.5
;
magnetite, Fe
3
O
4
, becomes FeO
1.33
; and
wüstite, FeO, becomes FeO
1.05
.
The quantity of oxygen required for the combustion of a portion of the coal or CO (precombustion/afterburning) is also related to 1 mol of iron. Precombustion is given a negative sign and is plotted to −1.5. If, for example, 1.5 mols of oxygen/mol of Fe are needed for precombustion the diagram shows that just as much oxygen for precombustion as for reduction, ie 1.5 mol O per mol Fe in each case and thus 3 O/Fe in total, is removed from the reactor with the flue gas.
A to D are process lines for different reduction processes. The gradient of a process line, that is the quotient O/Fe/O/C, abbreviates to C/Fe the specific carbon consumption. The latter is particularly important for the deliberations pursued here and is specified in the legend of the figure for the methods under consideration both in terms of mol C/mol Fe and kg C/t Fe.
Process line B represents the exchange of oxygen between iron oxide and carbon as well as the further take up of oxygen by the carbon in afterburning for an ideal autothermic process for the production of directly reduced iron (DRI). The carbon demand is 1.4 mol C per mol Fe or 299 kg C/t Fe and is thus almost the lowest possible for a method based only on carbon for reduction and heat generation. The diagram shows that the reduction of the iron oxide oxidises the carbon completely to CO and beyond that converts 10% of it to CO
2
. The remaining gas containing 90% of CO is afterburnt and the heat liberated covers the remaining enthalpy requirement of the reaction.
Process line A represents the rotary-tube method. The energy input required is greater than that for an autothermic process as can be seen from comparison of the gradients of the process lines. The actual energy demand for reduction in the rotary tube can naturally not be greater than that of an autothermic process but as a consequence of the process substantially more carbon must be fed in, that is to say 2.1 mol C/mol Fe, ie 451 kg C/t Fe, so that 152 kg C/t Fe are converted into heat lost in the process. The reason for this is that the CO formed in the reduction reaction must be burnt over the bed of coal and ore in order to generate the heat required for the Boudouard reaction. This combustion reaction, however, cannot be carried out completely to CO
2
because the CO
2
would impair the reduction process. It follows from this that any supply of heat without material separation of the gases in the reduction chamber and combustion chamber results for theoretical reasons in a consumption of carbon and energy which must be higher than that of a hypothetical autothermic process.
Process line C represents the oxygen exchange in a blast furnace operating only with coke. Due to the production of liquid end products as opposed to solid end products in processes A and B the consumption values have only limited comparability. In the blast furnace both precombustion (in the tuyere level) and afterburning (in the air heater) are employed. In the calculation of the carbon consumption the 0.2 C/Fe which leaves the furnace in the pig iron containing 4.3% C have already be subtracted.
Due to the partial combustion of the carbon in the tuyere level (precombustion) to produce the enthalpy needed for melting and heating the charge and for the Boudouard reaction the starting point of the process line is lowered by comparison with processes A and B from the origin of the coordinate system to −1.30 O/Fe. Due to this preliminary oxidation of the carbon the gas in the upper section of the blast furnace contains more CO
2
than originates from the reduction reaction. The gas is thus “diluted” with oxygen and is no longer as well suited for reducing as it would be without this take-up of oxygen. Accordingly, in contrast with A and B the process line C passes close to the “forbidden region” of the diagram marked by W and M. Since the compositions of the reducing gas lying in this region no longer result in the formation of metallic iron a process line cannot run through this zone.
Since the carbon consumption of the blast furnace is higher than that of an autothermic process possibilities for lowering the consumption could be sought. This would mean that the slope of the process line is reduced. However, it can immediately be seen from the diagram that this would only be p
Andrews Melvyn
Young & Basile P.C.
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