Method and facility for producing reduced iron

Details Method and facility for producing reduced iron Method and facility for producing reduced iron

1999-11-17

2001-09-04

Kastler, Scott (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Producing or treating free metal

C075S503000

Reexamination Certificate

active

06284017

ABSTRACT:

TECHNICAL FIELD
The present invention broadly relates to production of reduced iron, and production of hot metal therefrom. More particularly, the present invention is concerned with a method of and a facility for producing reduced iron by compacting fine iron oxides, for example, fine iron ore, iron-bearing dust, sludge, scale, and the like which are generated at steel mills, in admixture with powdery solid reductants, for example, coal, charcoal, petroleum cokes, cokes, and the like, into sheet-like shape without agglomerating aforesaid raw materials, and by charging sheet-like compacts into a heated furnace for reduction at a high temperature, and further, with an method of producing hot metal by charging the reduced iron kept in a hot condition into a shaft furnace or a in-bath smelting furnace.
BACKGROUND TECHNOLOGY
With recent growth in production of steel products by means of electric furnaces, much attention has been drawn to a technology of obtaining ferrous material as a feed therefor by solid reduction of iron ores. There has since been disclosed a process, representative of the technology, wherein solid metallized iron is produced by forming agglomerates, so-called “pellets”, from fine iron ore in admixture with powdery solid reductants, and then reducing iron oxides contained in the fine iron ore through heating of the agglomerates at a high temperature (reference: for example, specification of U.S. Pat. No. 3,443,931, and Japanese Patent Laid-open No. 7-238307).
The process of reducing fine iron ore as disclosed in U.S. Pat. No. 3,443,931 described above comprises generally the following steps of:
1) forming green pellets by mixing fine iron ore with powdery solid reductants such as coal, coke, and the like,
2) removing water adhered to the green pellets by heating same in such a temperature range that combustible volatile constituents issued therefrom are not ignited,
3) reducing dried pellets by heating same at a high temperature to raise a metallization ratio, and
4) cooling metallized pellets before discharging same out of a furnace.
The conventional process of producing reduced iron as disclosed in U.S. Pat. No. 3,443,931 described above (for the sake of convenience, referred to as “pelletizing process” hereinafter) has fundamental problems as follows:
1) As an agglomerate (pellet) as merely agglomerated does not have sufficient physical strength to withstand handling during the process, it requires drying before charged into a reduction furnace. This entails installation of a drying unit in addition to a pelletizing furnace of complex construction, involving fairly high costs of operation and maintenance thereof. Furthermore, owing to a longer time required in the process from a step of drying the pellet to completion of a reduction step, production efficiency of the process is low, and it is difficult to hold down a cost of producing reduced iron.
2) It is impossible to avoid generation of particles outside a predetermined size range during the pelletizing process. As it is necessary to recycle undersize particles to a mixing step, and to crush oversize particles before recycling to the mixing step, the production efficiency is poor.
3) Iron oxides generated at steel mills such as iron-bearing dust, sludge, scale, and the like are among precious ferrous materials, however, these are often found in lumpy form when recovered, composed of fine particles bonded together, or in a form too large as pellet feed as in the case of scales. Accordingly, for pelletizing these iron oxides on their own in place of iron ore fine, or in admixture with iron ore fine, it is necessary to pulverize them to a predetermined size beforehand, necessitating installation of a pulverizing apparatus.
It is known that in reduction reaction of pellets, the higher a temperature at which the reaction takes place, the more rapidly the reaction proceeds. Hence, it is essential to heat up the pellets to a predetermined temperature rapidly by increasing a warming rate in order to improve productivity by increasing a reduction reaction rate. The process disclosed in the Japanese Patent Laid-open No. 7-238307 described above is characterized in that for a while after pellets are charged into a furnace, an oxygen containing gas is supplied onto the surface of the charged pellets, causing combustible matter issued from the pellets to be actively combusted so that a temperature on the surface of pellets is elevated to an optimum temperature for reduction by heat of combustion.
The process disclosed in the Japanese Patent Laid-open No. 7-238307 described above, however, belongs to a category of the pelletizing process consisting of steps of mixing, agglomeration, and drying, hardly solving the problems of the pelletizing process described above.
A furnace provided with a horizontally rotatable hearth (referred to as rotary hearth hereinafter) for heating is drawing attention in producing the reduced iron, and a same type furnace (referred to as rotary hearth furnace hereinafter) is used in the process as disclosed in the U.S. Pat. No. 3,443,931.
The rotary hearth furnace is characterized by its low capital cost as opposed to the case of a rotary-kiln furnace which has been in practical use over many years, however, due consideration should be given to charging of raw materials and discharging of a product since the hearth is horizontally rotated in the former case.
FIG. 1
is a schematic representation showing an example of conventional processes of producing the reduced iron by use of the rotary hearth furnace for heating of raw materials. As shown in the figure, fine iron ore
3
crushed to a predetermined size by a crusher
1
, and pulverized coal
4
prepared by a dryer
2
and crusher
1
with bentonite
5
as binder added thereto are kneaded and mixed by a mixer
6
while water
7
and tar
8
are further added thereto. Mixed raw materials thus obtained are agglomerated by a pelletizer
9
or double-roll compactor
10
, transferred to a feeder
12
of the rotary hearth furnace
11
, and charged into the furnace, producing solid metallized iron by reducing iron oxides in the iron ore at a high temperature every time the rotary hearth
13
makes one turn. The metallized iron obtained is discharged from a product outlet
14
. Reference numeral
15
denotes an exhaust outlet.
When the fine iron oxide and powdery solid reductants are kneaded an d mixed after drying and crushing as necessary, a binder such as water, tar, theriac, organic resin, cement, slag, bentonite, quick lime, slightly burnt dolomite, or slaked lime is added thereto if need be.
The mixed raw materials are agglomerated into pellets in the shape of a ball by a desk pelletizer, or briquettes by the double-roll compactor. As the mixed raw materials of a particle size, 0.1 mm or less in diameter, are suitable for pelletizing, and same of a particle size, 1 mm or less, are for briquetting, the materials require prior pulverization to a predetermined size. In some cases, drying or curing treatment is applied to the agglomerates that is, pellets and briquettes) to enhance physical strength thereof.
The agglomerates are sent to an upper part of the rotary hearth furnace via a belt conveyer, and charged via a feeding chute into the furnace so as to be spread in a wide area on the surface of the rotary hearth and smoothed out by a leveler. Subsequently, the agglomerates are heated and reduced while in rotation within the furnace, and turned into metallized iron.
The conventional process of producing reduced iron described above, however, has the following problem. That is, the agglomerates, due to powdering occurring thereto before charged into the rotary hearth furnace, will turn into agglomerates composed of particles of various diameters while generating fines, and charged onto the rotary hearth in such a condition. Then, generated fines are blown off by a combusting gas, and adhered in a molten condition to the wall of the furnace, causing troubles to facilities. In addition, the generated fines adhere in a molten condition to the r

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