Plant for the direct reduction of particulate...

Details Plant for the direct reduction of particulate... Plant for the direct reduction of particulate...

2002-01-15

2003-05-27

Andrews, Melvyn (Department: 1742)

Metallurgical apparatus

With means treating or handling gases exhausted by treating...

With heat exchanger, e.g., waste heat boiler

C266S156000, C266S172000

Reexamination Certificate

active

06569377

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a process for the direct reduction of particulate iron-oxide-containing material by fluidization, wherein synthesis gas such as reformed natural gas is introduced as a reducing gas into several fluidized bed zones consecutively arranged in series for the reducing gas and is conducted from one fluidized bed zone to another fluidized bed zone in counterflow to the particulate iron-oxide containing material, and wherein heating of the iron-oxide-containing material is effected in the fluidized bed zone arranged first in the flow direction of the iron-oxide-containing material and direct reduction is carried out in the further fluidized bed zone(s), as well as a plant for carrying out the process.
A process of this kind is known from U.S. Pat. No. 5,082,251, WO-A-92/02458 and EP-A-571 358. According to U.S. Pat. No. 5,082,251, iron-rich fine ore is reduced in a system of fluidized bed reactors arranged in series by aid of a reducing gas under elevated pressure. The thus produced iron powder is then subjected to hot or cold briquetting.
The reducing gas is produced by catalytic reformation of desulfurized and preheated natural gas with superheated water vapor in a conventional reformer furnace. After this, the reformed gas is cooled in a heat exchanger and, subsequently, the H
2
portion in the reducing gas is increased by CO conversion by aid of an iron oxide catalyst. After this, the CO
2
forming as well as the CO
2
coming from the reformer are eliminated in a CO
2
scrubber.
This gas is mixed with the reducing gas (top gas) consumed only partially, heated and the fine ore is reduced in three steps (three fluidized bed reactors) in counterflow.
The ore flow starts with drying and subsequent screening. Then the ore is transferred into a preheating reactor in which natural gas is burnt. In three consecutive reactors, the fine ore is reduced under elevated pressure.
From EP-A 0 571 358 it is known to realize the reduction of fine ore not exclusively via the strongly endothermic reaction with H
2
according to
Fe
2
O
3
+3H
2
=2Fe+3H
2
−&Dgr;H,
additionally via the reaction with CO according to
Fe
2
O
3
+3CO=2Fe+3CO
2
−&Dgr;H,
which is exothermic. It is thereby feasible to considerably lower operating costs and, in particular, energy costs.
According to the prior art, direct reduction, because of the kinetics of the known processes, involves magnetite formation during direct reduction in a layer constantly growing from outside towards inside and forming on each particle or grain of the iron-oxide-containing material. It has been shown in practice that the formation of magnetite has an inhibiting effect on direct reduction with a reducing gas. Thus, it is feasible only at elevated expenditures, i.e., by increasing the consumption of reducing gas, to obtain a more or less complete reduction of the iron-oxide-containing material charged. In particular, it is necessary to make available a reducing gas having a high reduction potential even in the fluidized bed zones arranged first.
SUMMARY OF THE INVENTION
The invention aims at avoiding these disadvantages and difficulties and has as its object to further develop a process of the initially defined kind with a view to lowering the energy demand by fully utilizing the chemical potential of the reducing gas. In particular, operating costs are to be considerably lowered by utilizing the reducing gas to an optimum degree both in terms of reduction potential and in terms of sensible heat.
In accordance with the invention, this object is achieved
in that a temperature of the iron-oxide-containing material of either below 400° C. and, preferably, below 350° C.
or above 580° C. and, preferably about 650° C.,
or a temperature ranging from 400 to 580° C. is adjusted in the first fluidized bed zone,
wherein, at a temperature adjustment to below 400° C., the temperature range between 400° C. and 580° C. in the fluidized bed zone following the first fluidized bed zone in the flow direction of the iron-oxide-containing material is passed through within a period of 10 minutes and, preferably, within 5 minutes, and
wherein, at a temperature adjustment to above 580° C., the temperature range between 400° C. and 580° C. is passed through within a period of maximally 10 minutes and, preferably, 5 minutes, and
wherein, furthermore, at a temperature adjustment in the range of from 400° C. to 580° C., the iron-oxide-containing material remains within that temperature range for a maximum of 10 minutes and, preferably, 5 minutes and is passed on into the fluidized bed zone following next immediately after having reached the desired temperature.
By these measures, it is feasible to effectively avoid or reduce to an acceptable extent, the formation of magnetite layers. The formation of a magnetite layer occurs very rapidly, i.e., the more rapidly the closer the temperature of the iron-oxide-containing material approaches to the limit temperature of about 580° C. A magnetite formed on the surface of a particle of iron-oxide-containing material or an ore grain is denser than the ore itself, thus increasing the diffusion resistance of the interface between reducing gas and iron ore. As a result, the reaction speed is reduced. According to the Baur-Glaessner diagram, such a formation of a dense magnetite layer on the surface of an iron ore grain primarily occurs up to a temperature of the iron ore of 580° C. At a temperature of the iron ore below 400° C., the formation of magnetite is again slowed down and, as a result, dense magnetite layers are formed less rapidly.
The reaction kinetics of magnetite formation is influenced by the composition of the gas and of the solid. The molecules of the reducing gas must travel from the outer gas flow through the adhering gas border layer and through the macropores and micropores to the site of reaction. There, the dissociation of oxygen takes place. The oxidized gas returns via the same manner. The ore grains is, thus, reduced from the outside towards the inside. Thereby, its porosity increases, since the dissociated oxygen leaves behind hollow spaces and the original volume of the ore grain does not shrink significantly. The reaction front migrates from the outside towards the inside into the ore grain. With dense layers, the concentration of the reducing gas decreases from the outside towards the inside. The gas at first diffuses from the outside through the already reduced shell as far as the to the reaction front, where it is reacted and then diffuses back as a reaction product. With porous surfaces, the phase border reaction occurs on the walls of the pores within the reaction front, while the gas at the same time also may diffuse inside. With dense magnetite layers on the surface of the ore grain, the reaction kinetics is inhibited because the reducing gas is impeded from diffusing by the magnetite layer and the mass transfer of the reducing gas thus cannot occur in the same manner as with porous grains.
The basic idea of the invention is to be seen in accomplishing the transition of the temperature of the iron-oxide-containing material during heating from 400 to 580° C. within as short a period of time as possible and avoiding maintenance within that critical temperature range. When rapidly passing that temperature range, the formation of a magnetite layer is extremely modest. If at all, wuestite is formed, which is not disadvantageous to reduction. Hence result substantially enhanced reduction conditions for the fluidized bed zone arranged first in the flow direction of the iron-oxide-containing material.
Advantageously, the iron-oxide-containing material in any event is transferred to the consecutively arranged fluidized bed zone immediately after having reached the desired temperature.
According to a preferred embodiment, the temperature range between 400° C. and 580° C. is passed through while avoiding a residence time, the average temperature gradient within the range of between 400° C. and 580° C. amounting to at

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