U.S. patent application number 10/480256 was filed with the patent office on 2004-08-12 for method for producing granular metal.
Invention is credited to Ito, Shuzo, Tsuge, Osamu.
Application Number | 20040154436 10/480256 |
Document ID | / |
Family ID | 26617135 |
Filed Date | 2004-08-12 |
United States Patent
Application |
20040154436 |
Kind Code |
A1 |
Ito, Shuzo ; et al. |
August 12, 2004 |
Method for producing granular metal
Abstract
A method for making metal nuggets comprises heating a material
containing a metal-oxide-containing substance and a carbonaceous
reductant to reduce metal oxide in the material and then further
heating the produced metal so as to melt the metal while allowing
the metal to separate from a by-product slag component and to form
granular iron. A cohesion accelerator for the by-product slag is
mixed into the material to produce metal nuggets having a high
metal purity, a large diameter, and superior transportation and
handling qualities at a high yield and high productivity.
Inventors: |
Ito, Shuzo; (Hyogo, JP)
; Tsuge, Osamu; (Hyogo, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
26617135 |
Appl. No.: |
10/480256 |
Filed: |
December 17, 2003 |
PCT Filed: |
June 13, 2002 |
PCT NO: |
PCT/JP02/05872 |
Current U.S.
Class: |
75/476 ;
75/484 |
Current CPC
Class: |
C21B 13/008 20130101;
C21B 13/105 20130101; C22B 1/245 20130101; C21B 13/0046 20130101;
C22B 23/00 20130101; Y02P 10/20 20151101; C22B 5/10 20130101; C22B
34/32 20130101; C22B 7/02 20130101 |
Class at
Publication: |
075/476 ;
075/484 |
International
Class: |
C21B 011/06; C21B
013/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 18, 2001 |
JP |
2001-183932 |
Jan 16, 2002 |
JP |
2002-7854 |
Claims
1. A method for making metal nuggets, comprising heating a material
comprising a metal-oxide-containing substance and a carbonaceous
reductant to reduce metal oxide contained in the material and
further heating the resultant so as to melt metal while allowing
the metal to separate from a by-product slag component and allowing
the by-product slag component to undergo cohesion, wherein a
cohesion accelerator is blended into the material to accelerate
cohesion of the by-product slag.
2. A method according to claim 1, wherein the content of the
cohesion accelerator in the material is in the range of 0.2 to 2.5
percent by mass.
3. A method according to one of claims 1 and 2, wherein the
material is a mixture of powders of the metal-oxide-containing
substance, the carbonaceous reductant, and the cohesion
accelerator.
4. A method according to claim 3, wherein the mixture is formed
into pellets or briquettes, or press-formed into a compact, and
used as the material.
5. A method according to one of claims 1 to 4, wherein the cohesion
accelerator comprises at least one of calcium fluoride, boron
oxide, sodium carbonate, and sodium oxide.
6. A method according to claim 5, wherein fluorite is used as the
cohesion accelerator.
7. A method according to one of claims 1 to 6, wherein a moving bed
furnace is used to heat and reduce the material.
8. A method according to one of claims 1 to 6, wherein a rotary
hearth furnace is used to heat and reduce the material.
9. A method according to one of claims 7 and 8, wherein a layer of
a carbonaceous material is bed on a hearth in advance, and then the
material is fed to be heated and reduced on the hearth.
10. A method according to one of claims 1 to 9, wherein iron oxide
is used as the metal oxide.
11. A method according to one of claims 1 to 10, wherein the
metal-oxide-containing substance comprises at least one of iron
ore, steel-making dust, steel-making waste materials, and metal
scrap.
12. A method according to claim 11, wherein dust generated during
producing stainless-steel ingots is used as the steel-making dust
so as to recover valuable metals contained in the dust.
Description
TECHNICAL FIELD
[0001] The present invention relates to methods for manufacturing
metal nuggets. In particular, the present invention relates to an
improved method for manufacturing high-purity metal nuggets such as
iron nuggets that have a large diameter and superior transportation
and handling qualities, the method including heating a mixture of a
metal-oxide-containing substance and a carbonaceous reductant to
reduce the metal oxide, in which an auxiliary material for
increasing the fluidity of and promoting the cohesion of molten
by-product slag is added to the material. The improved method
achieves a high yield and high productivity.
[0002] This specification mainly describes a method for
manufacturing iron nuggets to which the present invention is most
effectively applied. However, the present invention is not limited
to the manufacturing of iron nuggets. The present invention can
also be effectively applied to the manufacture of ferrochromium or
ferronickel by heating and reducing chromium-containing ore or
nickel-containing ore, for example. In the present invention, the
term "nuggets" does not necessarily mean those spherical in shape
and includes those having an elliptical shape, an oval shape, and
slightly deformed shapes thereof.
BACKGROUND ART
[0003] An example of a known method for manufacturing metallic iron
by heating and reducing an iron oxide source is a direct
iron-making process for obtaining reduced iron by directly reducing
iron ore or iron oxide pellets using a carbonaceous material or a
reducing gas. In particular, a shaft furnace process such as the
Midrex process is known in the art. In the direct iron-making
process, a reducing gas made from natural gas or the like is blown
from a tuyere at the lower portion of the shaft furnace so as to
obtain reduced iron by reducing iron oxide using the reducing power
of the reducing gas.
[0004] Recently, a process for making reduced iron using a
carbonaceous material such as coal as the reductant instead of
natural gas has received much attention. For example, a so-called
SL/RN process of heating and reducing sintered pellets of iron ore
or the like with coal powder in a rotary kiln has already been put
into practice.
[0005] The publication of U.S. Pat. No. 3,443,931 discloses yet
another process for making reduced iron, the process including
making blocks from a mixture of a carbonaceous material and iron
oxide powder and heating and reducing the blocks on a rotary
hearth. In this process, the reduction is performed by heating the
blocks of a mixture of the carbonaceous material and powdered iron
ore in a high-temperature atmosphere.
[0006] The reduced iron made by the processes described above may
be directly used as an iron source for an electric furnace or may
be subjected to briquetting or the like before it is used as the
iron source. With the recent trends toward the recycling of scrap
iron, the reduced iron made by the above processes is drawing much
attention as an agent to decrease the ratio of metal impurities in
the scrap, since the resulting reduced iron has a low content of
metals other than iron.
[0007] A reducing melt process, such as a direct iron ore smelting
(DIOS) reduction process, for making reduced iron by directly
heating and reducing iron ore is also known in the art. In this
process, the iron ore is pre-reduced to approximately 50 percent by
mass (hereinafter denoted as "%") or less of the metallic iron, and
the pre-reduced iron is then allowed to directly react with carbon
in an iron bath to make reduced metallic iron. However, since this
process requires two stages, i.e., a pre-reduction and a final
reduction in an iron bath, the operation is complicated. Moreover,
since molten iron oxide (FeO) existing in the iron bath directly
comes into contact with refractories, the refractories undergo
significant wear, which is a problem. This is because iron oxide in
a molten state significantly corrodes the refractory such as
alumina or magnesia.
[0008] The publications of U.S. Pat. No. 6,036,744, Japanese
Unexamined Patent Application Publications Nos. 09-256017,
2000-144224, and 11-131119, and the like disclose processes for
making nuggets or blocks of high-purity metallic iron, the
processes comprising heating and reducing a mixture of powders of a
carbonaceous reductant and an iron oxide on a rotary hearth to
prepare reduced iron and further heating the resulting reduced iron
to separate the reduced iron from the by-product slag utilizing a
difference in the specific gravities. The mixture may be pre-formed
into blocks before it is heated on the rotary hearth, if
necessary.
[0009] According to this process, the reduced iron made mainly by a
solid-phase reduction is heated, and the melting point of the
reduced iron was decreased by carburization of the reduced iron so
as to promote melting. In this manner, the productivity of the
metallic iron can be improved while avoiding the problem of
significant refractory wear caused by the molten iron oxide
(FeO).
[0010] However, when high-purity iron nuggets are made according to
this process from a substance such as iron ore having a high gangue
content, the amount of by-product slag increases, thereby degrading
the cohesion property and decreasing the purity of the resulting
metallic iron and yield. Moreover, when a substance containing CaO
(for example, CaCO.sub.3) is added to the material to adjust the
basicity of a material having a high SiO.sub.2 content or to
desulfurize a material containing a carbonaceous reductant having a
high sulfur content, the amount of by-product slag further
increases, thereby further degrading the cohesion property of the
by-product slag.
[0011] When the cohesion property of the by-product slag is
degraded, separation of the metallic iron, produced by the
reduction, from the by-product slag becomes difficult, formation of
nuggets or blocks of the metallic iron is inhibited, and metallic
iron enclosing slag is produced. Furthermore, a large amount of
small metallic iron nuggets are produced and obstruct the
separation from the by-product slag. Thus, the yield of the
products having a size within a suitable range is decreased.
[0012] Moreover, the degradation of the cohesion property of the
by-product slag frequently occurs even when the gangue content of
the iron oxide source is low, depending on the type of iron oxide
source, the type of slag components, and the slag composition.
DISCLOSURE OF INVENTION
[0013] The inventors have conducted research to overcome these
problems and found that iron nuggets having a relatively large size
can be made when a carbonaceous reductant having a high fixed
carbon content is used in the process that includes the step of
reducing the metal oxide in a solid state included in a material
containing a carbonaceous reductant and an iron-oxide-containing
substance by means of reducing melt in a furnace and the step of
further heating the resulting metallic iron so as to melt the
metallic iron while allowing the metallic iron to separate from
by-product slag and form nuggets by cohesion. Based on this
finding, a patent application has been filed.
[0014] Here, it has been confirmed that relatively large iron
nuggets can be made at a high yield by controlling the fixed carbon
content in the carbonaceous reductant to 73% or more, the volatile
matter content in the material to 3.9% or less, and the amount of
the carbonaceous reductant in the material to 45% or less of that
of the iron oxide in the material, and by adjusting the temperature
for melting the metallic iron prepared by the solid-reduction in
the furnace to 1,400.degree. C. or more.
[0015] The above-described process is valuable since it examines
the effect of the carbonaceous reductant on the cohesion property
of metallic iron as a factor for making relatively large metallic
iron nuggets at a high yield. However, a large degree of limitation
is imposed as to the types of the applicable carbonaceous
reductant.
[0016] Furthermore, CaCO.sub.3 may be added to the material for the
purpose of adjusting the basicity of slag component when the gangue
content in the material, and particularly in the iron oxide source,
is high, when the SiO.sub.2 content in the material is high, or
when desulfurization is required due to the carbonaceous reductant
having a high sulfur content. In these cases, the amount of
by-product slag increases, and the degradation of the cohesion
property of the slag is inevitable, which is a problem.
[0017] Moreover, the cohesion property of the by-product slag may
also decrease even when the gangue content in the material is low,
depending on the types of the iron oxide source, the slag
components, and slag composition. An improvement as to this point
is also desired.
[0018] The present invention is made based on the above knowledge.
An object of the present invention is to establish a process for
making high-purity metal nuggets, the process comprising heating
and reducing a material containing metal oxide such as iron oxide
and a carbonaceous reductant such as coke, and further heating so
as to melt the metal and to allow the by-product slag to undergo
cohesion while separating the slag from the metal, the process that
can improve the cohesion property of the by-product slag produced
during cohesion and separation of the molten slag, increases the
purity of the resulting granular metal produced by heating and
reducing, makes large metal nuggets having a uniform size, and
reliably enhances the yield of the end product of the metal nuggets
regardless of the type and amount of the gangue component, which
varies according to the type of the metal oxide source.
[0019] A method for making metal nuggets according to the present
invention overcomes these problems. The method includes heating a
material containing a metal-oxide-containing substance and a
carbonaceous reductant to reduce metal oxide in the material, and
subsequently further heating the produced metal to melt the metal
while allowing the metal to separate from by-product slag and to
form granular iron, and is characterized in mixing a cohesion
accelerator for the by-product slag in the above material.
[0020] In implementing the method of the present invention, the
content of the cohesion accelerator in the material is preferably
in the range of 0.2 to 2.5 mass %, and more preferably in the range
of 0.4 to 2.0 mass %.
[0021] The above-described material is preferably a mixture of
powders of the metal-oxide-containing substance, the carbonaceous
reductant, and the cohesion accelerator. The mixture to be used is
preferably uniform. More preferably, a mixture formed into pellets,
briquettes, or the like, or press-formed into a compact is
preferably used.
[0022] Preferably, calcium fluoride (CaF.sub.2), boron oxide
(B.sub.2O.sub.5), sodium carbonate (Na.sub.2CO.sub.3), or sodium
oxide (Na.sub.2O) is used as the cohesion accelerator. These may be
used alone or in combination, if necessary. Among these components,
fluorite which contains CaF.sub.2 as the primary component is
particularly preferable as the cohesion accelerator in overall view
of the cost and the cohesion acceleration effects.
[0023] In implementing the method of the present invention, the
material is preferably heated and reduced continuously using a
moving bed furnace or a rotary hearth furnace. During the process,
a layer of a carbonaceous material may be bed on the hearth in
advance, and then the material may be fed to be heated and reduced
on the hearth. In this manner, the atmosphere adjacent to the
materials on the hearth can be always kept at a high reducing
potential, and thus the reoxidation of the reduced iron due to
oxidative exhaust gas (such as carbonic acid gas and steam) from a
burner, which is used to heat the furnace, can be reliably
prevented.
[0024] In this method, metal nuggets can be manufactured by using
iron oxide as the metal oxide and effectively using at least one of
iron ore, steel-making dust, steel-making waste materials, and
metal scrap as the metal-oxide-containing substance. Here, dust
generated during the production of the stainless-steel ingots,
hereinafter referred to as "stainless-steel dust" is preferably
used as the steel-making dust together with an iron oxide source
such as iron ore and mill scale. In this manner, valuable metals
such as Ni, Cr, and Mo contained in the dust can be captured by the
iron nuggets and thus be recovered effectively.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 includes photographs of products showing the state
immediately after reducing melt according to an example and a
comparative example. FIG. 2 includes photographs that show the
appearance of metallic iron nuggets prepared by the example and the
comparative example. FIG. 3 includes photographs of products
showing the state immediately after reducing melt according to
another example and another comparative example. FIG. 4 includes
photographs that show the appearance of metallic iron nuggets
prepared by yet another example and yet another comparative
example. FIG. 5 is a graph showing the cumulative mass
distributions, by size, of the metal nuggets prepared by a
comparative example and an example.
BEST MODE FOR CARRYING OUT THE INVENTION
[0026] As described above, in the present invention, a component
that promotes cohesion of the by-product molten slag produced
during the step of heating and reducing the metal oxide source is
added to the material, and the resulting mixture is further heated
after reduction to promote the separation of the by-product slag
from the molten metal so as to form relatively large metal nuggets
having a uniform size and a high purity at high productivity and
high recovery rate. The present invention is described below using
a process for making metallic iron nuggets from iron oxide, which
is a representative example of a metal oxide, as an example.
[0027] The present invention uses the same processes for making
high-purity metallic iron nuggets or blocks disclosed in
publications such as U.S. Pat. No. 6,036,744, and Japanese
Unexamined Patent Application Publications Nos. 09-256017,
2000-144224, and 11-131119, the processes comprising preparing a
mixture of powders of a carbonaceous reductant and a
iron-oxide-containing substance, forming the mixture into blocks if
necessary, heating and reducing the mixture on a rotary hearth, and
further heating the mixture so as to separate the molten metallic
iron from the by-product slag by utilizing the difference in the
specific gravities thereof. The present invention is an improved
method based on these processes.
[0028] In particular, in the known processes described above,
CaCO.sub.3 or the like is added, if necessary, to the material
which essentially contains a powder of an iron-oxide-containing
substance and a carbonaceous reductant in order to adjust the
basicity or to perform desulfurization. When the material needs to
form a compact, an adequate amount of binder is further added to
the material. The present invention is characterized in that a
cohesion accelerator preferably containing at least one of calcium
fluoride (CaF.sub.2), boron oxide (B.sub.2O.sub.5), sodium
carbonate (Na.sub.2CO.sub.3) and sodium oxide (Na.sub.2O) is added
to the material in order to promote the cohesion of the molten
slag.
[0029] The above powder material mixture containing the cohesion
accelerator may be directly fed to a furnace such as a rotary
hearth furnace. Preferably, the powder material is formed into
pellets using a pelletizing plant of a pan type, a drum type, or
the like, press-formed into briquettes, or lightly pressed-formed
into a compact before it is fed to the furnace so as to efficiently
promote heat conduction and accelerate reductive reaction inside
the material mixture.
[0030] In order to efficiently perform solid reduction while
maintaining the material mixture containing the cohesion
accelerator in a solid state in the furnace without causing partial
melting of the iron oxide component in the material, a two-stage
heating process is preferably performed. The two-stage heating
process includes: reducing the material chiefly by means of solid
reduction while maintaining the furnace temperature at a
temperature in the range of 1,200 to 1,500.degree. C., and more
preferably, 1,200 to 1,400.degree. C.; and then reducing the
remaining iron oxide at an increased furnace temperature in the
range of 1,400 to 1,500.degree. C. while allowing the generated
metallic iron (reduced iron) to melt and to form granular iron.
Under these conditions, iron nuggets can be reliably produced at a
high yield. The time normally taken for this process is
approximately 8 to 13 minutes. The solid reduction, melting, and
cohesion of iron oxide can be completed in such a short time.
[0031] In the course of the process, the molten metallic iron forms
granular iron and grows larger. Since the granular iron grow while
excluding the by-product molten slag, the resulting molten iron
nuggets contain substantially no slag and thus exhibit a high Fe
purity. When these molten iron nuggets are cooled, solidified, and
separated from the slag using a screen or by magnetic separation,
metallic iron nuggets exhibiting a high Fe purity can be
obtained.
[0032] The cohesion of the by-product slag can be promoted, and the
problem of inhibiting cohesion of the molten metallic iron produced
during the melting step after reduction can be avoided by adding a
cohesion accelerator to the substance containing a metal oxide
source and a carbonaceous reductant, even in the instances
below:
[0033] 1) when an iron oxide inherently having a poor cohesion
property is used as the metal oxide source;
[0034] 2) when a low iron grade material having a high gangue
content is used;
[0035] 3) when large amounts of auxiliary materials are used to
adjust the basicity of a material having a high SiO.sub.2 content;
and
[0036] 4) when CaCO.sub.3 or the like is added to a material
containing a carbonaceous reductant having a high sulfur content
(for example, low-grade coal powder) for the purpose of
desulfurization. Accordingly, the quality of the iron nuggets and
the manufacturing cost therefor are not adversely affected even
when an iron oxide inherently exhibiting a poor cohesion property
is used in the material or when the amount of by-product slag is
increased due to the addition of CaCO.sub.3. The effect of
promoting cohesion of the molten slag also leads to the promotion
of cohesion and separation of the molten metallic iron. Thus,
metallic iron nuggets can be obtained at a high yield, and the
manufacturing cost of therefor can be reduced.
[0037] The effects of using the cohesion accelerator, which is the
feature of the present invention, are as follows.
[0038] When a cohesion accelerator is added to the material, the
melting point of the by-product slag produced during the melting
stage decreases while the fluidity of the slag dramatically
increases. As a result, when molten metallic iron forms iron
nuggets, the molten slag does not inhibit or obstruct the cohesion
of the dispersed molten metallic iron. The molten metallic iron can
efficiently undergo cohesion while excluding the by-product slag
and thus forms relatively large nuggets. This effect negates
factors that inhibit cohesion of the molten metallic iron
regardless of the amount of the by-product slag, because the
addition of the cohesion accelerator decreases the melting point of
the by-product slag and increases the fluidity and the cohesion
property of the by-product slag. Accordingly, the cohesion property
of the molten metallic iron can be increased, and iron nuggets
having a high iron purity, a large diameter, and improved
transportation and handling qualities can be efficiently
manufactured while achieving a high yield even when an iron oxide
source having a low cohesion property is used or when a low-grade
iron-oxide-containing material and a low-grade carbonaceous
reductant are used.
[0039] Another effect of using the cohesion accelerator is as
follows.
[0040] Since the cohesion property of the molten metallic iron is
increased as described above, the rate of large metallic iron
nuggets in the resulting metallic iron nuggets increases. The
generation of micro particles of metallic iron, which are difficult
to separate, and the generation of inseparable metallic iron
nuggets enclosing slag resulting from cohesion of the molten
metallic iron when the molten metallic iron is not adequately
separated from the by-product slag, can be prevented as much as is
feasibly possible. Accordingly, when the metallic iron nuggets and
the by-product slag are discharged from a reduction melting furnace
such as a rotary hearth furnace, micro metallic iron particles,
slag, and the mixtures of metallic iron nuggets and slag that pass
through the clearance of a discharger such as a screw or a scraper
and the hearth surface do not return again to the heating zone of
the furnace. Moreover, the micro metallic iron particles, slag, and
the mixtures of metallic iron nuggets and slag can be prevented
from being pressed into a hearth protecting layer, hearth
refractories, or a bedding layer on the hearth. These effects
prevent damage such as infiltration and erosion of the hearth
refractories by the micro metallic iron particles, slag, and the
mixtures of the metallic iron particles and the slag, and
deterioration and enlargement of the hearth refractories resulting
from severe heat cycles. Accordingly, the life of the hearth
refractories can be prolonged, a problem of blockage of the product
discharge section can be resolved, and long-term continuous
operation can be reliably performed.
[0041] When the fluidity of the by-product slag is improved by
using the cohesion accelerator, the carburization reaction of the
metallic iron during the heating and reducing stage is promoted,
thereby also accelerating the melting of the by-product slag, and
the time taken to reduce and melt the material is shortened. Since
the fluidity of the by-product slag is increased, the rate at which
the molten metallic iron and molten slag grow into iron nuggets
also increases. As a result, the time from the beginning of melting
the metallic iron prepared by solid reduction to the completion of
the formation of molten metallic iron nuggets can be shortened, in
addition to the above-described reduction in the time required for
reduction melting. Thus, productivity can be dramatically improved
while maintaining a high yield of high-purity, large metallic iron
nuggets.
[0042] Any type of cohesion accelerator can be suitably used in the
present invention as long as the above-described effects and
advantages can be achieved. However, a cohesion accelerator that
enters the reduced metal and decreases the purity of, for example,
reduced iron, should be avoided. A cohesion accelerator which
combines with slag components and readily separates from the
metallic iron is preferably selected. Preferable examples of the
cohesion accelerators include calcium fluoride (CaF.sub.2), boron
oxide (B.sub.2O.sub.5), sodium carbonate (Na.sub.2CO.sub.3) and
sodium oxide (Na.sub.2O). These components may be used alone or in
combination, if necessary. Among these components, CaF.sub.2, and
more particularly, fluorite containing CaF.sub.2 as the primary
component, is particularly preferable in view of cost and cohesion
acceleration effects.
[0043] Among these components, sodium carbonate (Na.sub.2CO.sub.3)
and sodium oxide (Na.sub.2O) are preferable when coal powder or
coke powder is used as the carbonaceous reductant, since sodium
carbonate and sodium oxide capture the sulfur component derived
from these carbonaceous reductants. As a result, the sulfur content
in the resulting metal nuggets can be reduced.
[0044] No limit is imposed as to the amount of the cohesion
accelerator. The cohesion accelerator does not exhibit sufficient
effects when the amount of the cohesion accelerator is
insufficient. The effects of promoting cohesion are saturated at an
excessive amount of cohesion accelerator. Thus, an excessive amount
of the cohesion accelerator is not economical and increases the
cost of processing an increased amount of by-product slag. In view
of the above, the content of the cohesion accelerator is preferably
in the range of 0.2 to 2.5%, and more preferably, 0.4 to 2.0%, of
the material. The content of the cohesion accelerator based on the
slag component in the material is preferably in the range of 1 to
11%, and more preferably, 3 to 8%.
[0045] No limit is imposed as to the apparatuses to which the
present invention can be applied. In other words, no limit is
imposed as to the structure of the reducing melt furnace. For
example, the present invention can be applied to various types of
reducing melt equipment disclosed in publications such as U.S. Pat.
No. 6,036,744, and Japanese Unexamined Patent Application
Publications Nos. 09-256017, 2000-144224, and 11-131119. Use of a
moving bed furnace or a rotary hearth furnace is particularly
preferable since the process of heating and reducing the material,
melting the reduced metal, forming granular iron, and separating
the by-product slag from the metallic iron can be continuously and
effectively performed using these furnaces.
[0046] In this invention, no limit is imposed as to the type of
metal oxide used as a metal source. For example, nickel-containing
ore, chromium-containing ore, recycled scrap containing nickel or
chromium, or the like may be used. The most common source of iron
oxide when making iron nuggets is iron ore, but steel-making scrap
and waste materials, such as steel-making dust, produced during the
iron-making process in an iron-making plant, and recycled scrap may
also be used as the iron source. These materials can be used either
alone or in combination, if necessary. For example, stainless-steel
dust produced when making stainless-steel ingots in an electric
furnace or the like contains large amounts of nonferrous valuable
metals such as Ni, Cr, and Mo. By using the stainless-steel dust,
either alone or in combination with an iron source such as iron ore
or mill scale, as a metal oxide source, Ni, Cr, Mo, and the like
can be captured inside the iron nuggets and can be efficiently
recovered.
[0047] When a metal source, such as steel-making slag, that
contains large amounts of slag components is used, an excessive
amount of by-product slag relative to the metal nuggets is produced
and inhibits the cohesion of the metal, thereby decreasing the
recovery rate of large metal nuggets. Accordingly, in order to
prevent an increase in the amount of the by-product slag and to
improve the recovery rate of the large metal nuggets, the slag
component content in the material mixture should be as low as is
feasibly possible. In particular, the slag component content in the
material mixture is preferably controlled so as to limit the amount
of the slag to approximately 500 kg per ton of the metal
nuggets.
[0048] In implementing the present invention, in order to reduce
the metal oxide by means of solid reduction as much as possible and
to prevent generation of molten metal oxide which significantly
erodes hearth refractories, CaO or CaCO.sub.3, which adjust the
basicity of the by-product slag as shown in the examples below, may
be added to the material depending on the type of
metal-oxide-containing substance and the composition of the slag
component contained in the carbonaceous reductant. It is within the
technical scope of the present invention to use CaO or CaCO.sub.3
during the operation.
EXAMPLES
[0049] The configuration, the effects, and the advantages of the
present invention are described below by way of examples. These
examples do not limit the scope of the present invention. Various
modifications are possible within the scope of the spirit of the
invention described herein. The technical scope of the present
invention includes those modifications.
Example 1
[0050] In this example, magnetite iron ore having a high gangue
content, and more specifically, a high SiO.sub.2 content of 5.0% or
more, was used as an iron oxide source. The chemical composition of
the ore is shown in Table 1.
1TABLE 1 Chemical Composition of Material Iron Ore Having a High
SiO.sub.2 Content (mass %) T.Fe Fe.sub.2O.sub.3 Fe.sub.3O.sub.4
SiO.sub.2 Al.sub.2O.sub.3 CaO MgO Other Total 67.21 1.15 91.77 5.4
0.28 0.81 0.45 0.14 100
[0051] The designed composition of high-SiO.sub.2 iron ore, a
carbonaceous reductant, a binder (wheat flour), CaCO.sub.3
(basicity adjustor), and CaF.sub.2 (cohesion accelerator) in the
material pellets was adjusted as shown in Table 2.
2TABLE 2 Designed Composition of the Material Pellets (mass %) Case
A Case B (Comparative (Comparative Case C Case D Composition
Example) Example (Example) (Example) High-SiO.sub.2 78.88 72.33
73.42 71.23 iron ore Coal 19.62 18.17 17.08 17.77 (carbonaceous
reductant) Binder 1.5 1.5 1.5 1.5 CaCO.sub.3 0 8 7 8.5 CaF.sub.2 0
0 1 1 Total 100 100 100 100
[0052] In Case A, the basicity was not adjusted. In Cases B and C,
an adequate amount of CaCO.sub.3 was used to adjust the basicity of
the slag (CaO/SiO.sub.2) to approximately 1.15. In Case D, an
adequate amount of CaCO.sub.3 was used to adjust the basicity of
the slag (CaO/SiO.sub.2) to 1.3 or more. Cases C and D were the
examples implementing the present invention, and contained a small
amount of fluorite (CaF.sub.2) to enhance the fluidity of the
by-product slag. The designed amounts of slag were 127.8 kg/ton
(iron nugget) in Case A, 215.7 kg/ton (iron nugget) in Case B,
212.7 kg/ton (iron nugget) in Case C, and 233.8 kg/ton (iron
nugget) in Case D.
[0053] In Cases A and B, the compositions were adjusted to examine
the effect of the by-product slag on the cohesion property of the
iron nuggets. In Cases B, C, and D, the compositions were adjusted
to examine the effect of adding a small amount of a cohesion
accelerator (CaF.sub.2) on the improvement of the cohesion property
of the iron nuggets. The addition of a small amount of the cohesion
accelerator is the feature of the present invention. The material
pellets were substantially spherical and had a diameter of
approximately 16 to 20 mm.
[0054] In each of these cases, 250 to 265 grams of the
above-described material pellets were placed on a carbonaceous
bedding material (coke powder) provided on the entire surface of a
alumina foam board, were charged into a box furnace, and were
heated for 12 to 14 minutes in a 100% nitrogen atmosphere at an
atmosphere temperature of 1,430 to 1,450.degree. C. The recovery
rate of the metallic iron nuggets of the above cases is shown in
Table 3.
[0055] The recovery rate of the metallic iron nuggets is the ratio
of the mass of the resulting metallic iron nuggets actually
produced to the theoretical mass of the metallic iron nuggets
calculated based on the analytical value of the total Fe content in
the material pellets. In the description below, the recovery rate
of the metallic iron nuggets to the mass of the iron contained in
the material pellets was evaluated from two aspects: (1) the mass
ratio of the metallic iron nuggets that have a mass of
approximately 0.2 gram or more per nugget; and (2) the mass ratio
of the metallic iron nuggets that have a mass of 1.0 gram or more
per nugget.
3TABLE 3 Recovery Rate of Metallic iron nuggets (mass %) Case A
Case B (Comparative (Comparative Case C Case D Composition Example)
Example) (Example) (Example) CaCO.sub.3 content 0.0 8.0 7.0 8.5 (%)
CaF.sub.2 content 0.0 0.0 1.0 1.0 (%) (1) Recovery 96.8 84.6 101.7
99.7 rate of metallic iron nuggets having a mass of approximately
0.2 g or more (%) (2) Recovery 92.86 80.02 100.1 96.2 rate of
metallic iron nuggets having a mass of approximately 1.0 g or more
(%)
[0056] Case A was compared with Case B. Case B, in which the amount
of the by-product slag was increased due to the addition of 8% of
CaCO.sub.3, clearly demonstrated a tendency of an increase in the
production rate of metallic iron nuggets having a small diameter.
The recovery rates of the metallic iron nuggets of (1) 0.2 g or
more and (2) 1.0 gram or more were clearly low. Particularly the
recovery rate of the metallic iron nuggets having a mass of 1.0 g
or more decreased by 12% or more. It should be noted that in Case
B, large amounts of micro particles of metallic iron adhered onto
the surface of the by-product slag, and those metallic iron nuggets
that existed separately from the slag enclosed slag particles.
Thus, the separation of the metallic iron nuggets from the slag was
difficult.
[0057] On the other hand, when Case B was compared with Case C,
although the amount of the by-product slag was increased in both
Case C and Case B, Case C demonstrated that the cohesion of the
molten metallic iron could be dramatically improved by adding
CaF.sub.2. In particular, the yields of both metallic iron nuggets
having a mass of 0.2 g or more per nugget and metallic iron nuggets
having a mass of 1.0 g or more per nugget were dramatically
improved. In Case C, the recovery rate was improved by 20 to 26%
compared to Case B, achieving the recovery rate exceeding 100%. The
recovery rate exceeded 100% because the total iron content of the
input material was used as the denominator and the amount of crude
iron that partially contains recovered carbon, silicon, or the
like, was included in the numerator when calculating the recovery
yield.
[0058] In Case D, the CaCO.sub.3 content was increased to 8.5%, and
an increase in the amount of the slag resulted in the tendency of
decrease in the yield. However, the recovery rate of the metallic
iron nuggets having a mass of 0.2 g or more was approximately 100%,
still achieving a high yield. The improvement effect is obvious
when compared to Case A in which the amount of the by-product slag
is small. Even a difference of 1.0% in the recovery rate of the
metal nuggets has a significantly large impact on the cost in
actual operations, demonstrating the distinct feature of the
present invention.
[0059] Photographs showing the appearance of the metallic iron
nuggets and the slag in Cases B and C are shown in FIG. 1.
Photographs that show the size distribution of the metallic iron
nuggets in Cases B and C are shown in FIG. 2. Note that the
photographs showing the appearance of the metallic iron nuggets and
the slag show the state of the produced metallic iron nuggets and
the slag in a tray after reducing melt.
[0060] As is apparent from FIG. 1, Case B, in which 8% of
CaCO.sub.3 was used, clearly demonstrated a tendency of an increase
in the rate of producing metallic iron nuggets having a small
diameter. In particular, as is apparent from the figure, a large
number of micro particles of metallic iron adhered onto the surface
of the resulting slag, and those metallic iron nuggets that existed
separately from the slag enclosed slag particles and firmly bonded
to these slag particles. Thus, the separation of the metallic iron
nuggets from the slag was difficult.
[0061] Unlike in Case B, in Case C, in which CaF.sub.2 was added as
a cohesion accelerator, the rate of production of small metallic
iron nuggets did not decrease, and nearly all were large metallic
iron nuggets. Substantially no small metallic iron particles, which
adhere onto the surface of the slag nuggets to be firmly bonded
thereto, were found. In Case B, the produced slag was glassy and
blackish green. The dimensions of the produced slag increased as
the basic unit of the slag increased. However, the amount of the
small metallic iron particles adhering onto the surface of the slag
also increased except for Cases C and D in which CaF.sub.2 was
added.
[0062] Tables 4 and 5 below show the results of the chemical
analysis of the produced metallic iron nuggets and slag. Since the
metallic iron nuggets did not substantially contain heavy metals
other than the main gangue components such as SiO.sub.2 in the ore,
only five primary elements are shown in these tables.
4TABLE 4 Results of Chemical Analysis of Metallic iron nuggets
(mass %) C Si Mn P S Case A 3.23 0.32 0.09 0.040 0.127 (Comparative
Example) Case B 3.41 0.20 0.13 0.036 0.078 (Comparative Example)
Case C 3.74 0.21 0.18 0.037 0.059 (Example) Case D 4.01 0.17 0.23
0.034 0.033 (Example)
[0063]
5TABLE 5 Results of Chemical Analysis of Slag (mass %) T.Fe FeO
SiO.sub.2 CaO Al.sub.2O.sub.3 MgO S Basicity Case A 4.76 1.64 64.33
10.21 9.82 6.50 0.072 0.159 (Compar- ative Example) Case B 3.97
0.39 40.54 45.04 6.41 4.04 0.442 1.111 (Compar- ative Example) Case
C 1.58 0.28 40.42 46.73 6.46 4.09 0.408 1.156 (Example) Case D 4.22
0.25 35.09 47.72 5.70 3.71 0.558 1.360 (Example)
[0064] As is apparent from Tables 4 and 5, Cases C and D, in which
7.0 to 8.5% of CaCO.sub.3 and 1.0% of CaF.sub.2 as a cohesion
accelerator were used, maintained a high recovery yield of the
metallic iron nuggets while achieving a low sulfur content in the
metallic iron nuggets. Particularly, in Case D, high-quality
metallic iron nuggets having a distinctively low sulfur content of
0.033% were obtained.
Example 2
[0065] In this example, iron ore having a poor cohesion property,
i.e., hematite ore, was used in the material to confirm the effect
of adding approximately 1.0% of CaF.sub.2 as the cohesion
accelerator. The experiments of reducing melt were conducted as in
Example 1 above.
[0066] Table 6 shows the primary chemical composition of the
hematite ore used in the experiment. FIGS. 3 and 4 include
photographs showing the appearances of the state of the products
immediately after the reducing melt (FIG. 3) on an aluminum board
tray and the recovered metallic iron nuggets (FIG. 4) obtained in
Case F which implements the present invention and uses CaF.sub.2 as
the cohesion accelerator. FIGS. 3 and 4 also include photographs
showing the appearances of the state of the products immediately
after the reducing melt (FIG. 3) on an aluminum board tray and the
recovered metallic iron nuggets (FIG. 4) obtained in Case E which
is a comparative example that does not use CaF.sub.2.
6TABLE 6 Chemical Composition of Hematite Ore (mass %) T.Fe FeO
Fe.sub.2O.sub.3 SiO.sub.2 Al.sub.2O.sub.3 CaO MgO Other Total 68.01
0.1 97.13 1.08 0.47 0.08 0.06 1.13 100
[0067] As in Example 1, no increase in the production amount of the
small metallic iron nuggets was observed when CaF.sub.2 was used.
Nearly all were large metallic iron nuggets. The improvement effect
of adding the cohesion accelerator was apparent.
Example 3
[0068] In this example, Na.sub.2CO.sub.3 was used as the cohesion
accelerator, replacing CaF.sub.2. The conditions of the experiment
were substantially same as in Example 1 above, and the composition
of the material is shown in Table 7 below. Table 7 also describes
the composition of the material of the comparative example.
7TABLE 7 Designed Compound Ratio of Material Pellets (mass %)
Hematite ore Coal Binder CaCO.sub.3 Na.sub.2CO.sub.3 Example 3
74.41 20.09 1.50 4.0 -- Comparative Example 71.88 21.02 1.50 4.0
1.6
[0069] FIG. 5 shows a cumulative mass distribution of the metallic
iron nuggets obtained in Example 3 above and Comparative Example.
In determining the cumulative mass distribution, the recovered
metallic iron nuggets were classified by a screen, and the number
was added from those having the largest size to those having the
smallest size. The figure also shows the cumulative mass percentage
of the metallic iron nuggets that have a mass of 0.2 g or more and
1.0 g or more.
[0070] As is apparent from the figure, when the cumulative mass
ratio of relatively large metallic iron nuggets having a mass of
1.0 g or more is compared, that of Comparative Example was 54.34
percent by mass whereas that of Example 3 was 95.74 percent by
mass, which is significantly higher. As for the cumulative mass
ratio of the metallic iron nuggets of 0.2 g or more, that of
Comparative Example was 58.34 percent by mass. This shows that
although this ratio is slightly higher than the 54.34 percent by
mass of the metallic iron nuggets of 1.0 g or more, the improvement
is substantially none, which plainly points out how large the
production rate of the micro particles of metallic iron is in
Comparative Example. In Example 3, the cumulative mass ratio of the
metallic iron nuggets of 0.2 g or more was 95.9 percent by mass,
which is significantly high. This demonstrates that large metallic
iron nuggets having excellent handling quality can be obtained at a
high yield in Example 3.
Example 4
[0071] Stainless-steel dust and mill scale having the compositions
described below were used as an iron oxide source. A powder mixture
of a carbonaceous reductant (coal powder) and a binder (wheat
flour) and a powder mixture of the carbonaceous reductant, the
binder, and an CaF.sub.2, i.e., a cohesion accelerator, whose
compound ratios are shown in Table 8 was added to this iron-oxide
source so as to make pellets having a diameter of approximately 16
to 20 mm.
[0072] The main components, by mass %, of the stainless-steel dust
were as follows:
[0073] T. Fe: 25.7%, M. Fe: 1.54%, SiO.sub.2: 6.0%,
Al.sub.2O.sub.3: 0.54%, CaO: 3.66%, MgO: 1.3%, M. Ni: 0.27%, NiO:
7.91%, M. Cr: 0.15%, Cr.sub.2O.sub.3: 16.07%, M. Mn: 0.23%, MnO:
6.44%, MoO: 6.44%, ZnO: 5.53%, and C: 0.56%.
[0074] The main components, by mass %, of the mill scale were as
follows:
[0075] T. Fe: 72.2%, M. Fe: 12.6%, SiO.sub.2: 1.95%,
Al.sub.2O.sub.3: 0.42%, CaO: 1.5%, MgO: 0.1%, MnO: 0.9%.
8TABLE 8 Composition of Material Pellets Stainless- Mill steel dust
scale Coal Binder CaF.sub.2 Invention 40.20 40.20 16.63 1.48 1.50
Example Comparative 40.84 40.84 16.82 1.50 0 Example
[0076] The reducing melt experiment using a box furnace was
conducted as in Example 1 above using 350 grams (approximately 40)
of each type of the material pellets. The recovery rate of the
metallic iron nuggets having a mass of 0.6 g or more was then
compared.
[0077] The results showed that in Comparative Example that did not
use CaF.sub.2, a large number of small iron nuggets having a mass
of less than 0.6 g was produced, and the recovery rate of the large
iron nuggets having a mass or 0.6 g or more was approximately 62%.
In contrast, in Invention Example using CaF.sub.2 as the cohesion
accelerator, the amount of the small metallic iron nuggets produced
was remarkably small, and the recovery rate of the large iron
nuggets having a mass of 0.6 g or more was distinctively high,
i.e., 98.3%.
[0078] When the material having the above composition is used, 300
to 400 kg of slag is produced per ton of metallic iron nuggets and
significantly affects the melting and cohesion of the reduced iron.
In Comparative Example above, the fluidity of the slag was poor,
and thus the slag obstructed the cohesion of micro particles of
reduced iron produced by heat reduction. As a result, a large
amount of micro particles of metallic iron was produced. In
contrast, in Invention Example using 1.5% of CaF.sub.2 as a
cohesion accelerator, the fluidity of the by-product slag was
increased, thereby promoting the cohesion of micro particles of
reduced iron, and nearly all of the produced reduced iron combined
into large iron nuggets by cohesion.
Example 5
[0079] Both stainless-steel dust and mill scale produced when
making stainless-steel ingots using an electric furnace were used
as an iron oxide source at compound ratios shown in Table 9 below.
A carbonaceous reductant (coal powder), a binder (wheat powder),
and 1.5 percent by mass of CaF.sub.2 as a cohesion accelerator were
added to this iron oxide source to prepare a mixture powder. This
mixture powder was formed into pellets having a diameter of
approximately 16 to 20 mm. The reducing melt experiment was
performed in a box furnace as in Example 4 above using 350 g
(approximately 40) of each type of material pellets. The amount of
the slag produced as the by-product, and the state of the iron
nuggets and the by-product slag were examined. Note that in this
experiment, 1.5 percent by mass of CaF.sub.2 was used as a cohesion
accelerator in all of the cases.
[0080] The purpose of the experiment was to examine the effects of
the compound ratio of the electric-furnace dust as the iron source
on the amount of the by-product slag. In other words, when reducing
melt is performed using carbon-containing pellets using
electric-furnace dust (stainless-steel dust) as an iron source, the
amount of slag has a large influence on the stable operation of the
process. To be more specific, the amount of the by-product slag
increases as the compound ratio of the electric-furnace dust in the
material increases. As a result, the slag may completely cover the
iron nuggets and obstruct the cohesion of micro iron particles.
Moreover, the dissolution of the hearth refractories may be further
accelerated by the increased amount of the molten slag. In this
experiment, the ratio of the electric-furnace dust was varied to
approximately 25 mass %, 30 mass %, 40 mass %, and 50 mass % in
Cases 1, 2, 3, and 4, respectively, so as to examine the state of
the iron nuggets generated by reducing melt and the by-product
slag. The amount of slag in each of these cases is also shown in
Table 9.
[0081] The main components, by mass %, of the stainless-steel dust
was as follows:
[0082] T. Fe: 23.5%, M. Fe: 15.7%, SiO.sub.2: 8.2%,
Al.sub.2O.sub.3: 2.7%, CaO: 14.2%, MgO: 3.6%, M. Ni: 0.8%, NiO:
3.9%, M. Cr: 0.3%, Cr.sub.2O.sub.3: 10.8%, M. Mn: 0.6%, MnO: 3.2%,
MoO: 1.1%, ZnO: 9.6%, and C: 0.9%.
[0083] The main components, by mass %, of the mill scale were as
follows:
[0084] T. Fe: 75.1%, M. Fe: 0.07%, SiO.sub.2: 0.01%,
Al.sub.2O.sub.3: 0.07%, CaO: 0.02%, MgO: 0.01%, MnO: 0.29%.
9TABLE 9 Composition of Material Pellets (mass %) Amount of by-
Stainless- product slag steel Mill (kg/ton of dust scale Coal
Binder CaF.sub.2 iron nuggets) Case 1 23.59 55.05 18.38 1.48 1.5
217.2 Case 2 31.53 47.3 18.19 1.48 1.5 315.4 Case 3 40.18 40.18
16.66 1.48 1.5 487.8 Case 4 48.14 32.09 16.79 1.48 1.5 613.7
[0085] As is apparent from Table 9, the amount of the by-product
slag clearly increased as the amount of the stainless-steel dust
increased. In Cases 1 and 2, the amount of by-product slag was not
significantly large compared to the amount of iron nuggets, and the
iron nuggets after heat-reduction was sufficiently distinguishable
from the by-product slag on the alumina tray. The separation of the
iron nuggets from the by-product slag was also easy. In Case 4,
however, the amount of the by-product slag was significantly large
compared with the produced iron nuggets, and substantially all of
the iron nuggets were covered with the by-product slag. Moreover,
it was confirmed that the iron nuggets concentrated at the lower
side of the alumina tray. In such a state, there is a danger of the
hearth refractories coming into direct contact with the molten
metallic iron, thereby accelerating the dissolution of the hearth
material.
[0086] In Case 3, the stainless-steel dust and the mill scale were
used at a ratio of 50:50 as an iron oxide source. This ratio is
substantially considered to be the limit of the ratio that allows
effective separation of the iron nuggets from the by-product slag
while preventing the deterioration of the hearth refractories. In
this case, the amount of the by-product slag relative to a ton of
the iron nuggets produced was approximately 500 kg. These results
show that when an iron source, such as stainless-steel dust, having
a high slag component is used as the iron source, the composition
is preferably adjusted so that the amount of the by-product slag
does not exceed approximately 500 kg per ton of the iron nuggets
produced.
[0087] In Cases 1 to 4 of this example, the recovery rates of the
Cr, Ni, and Mo in the stainless-steel dust were examined. The
results are shown in Table 10. These results also demonstrate that
the recovery rate of the valuable metals other than iron can be
increased by limiting the amount of the by-product slag as small as
possible during the step of preparing the material.
10TABLE 10 Recovery Rate of Valuable Metals in Stainless-steel Dust
(mass %) Cr Ni Mo Case 1 88.10 84.3 90.99 Case 2 71.83 78.0 86.50
Case 3 51.31 70.2 89.62 Case 4 50.70 63.4 81.31
INDUSTRIAL APPLICABILITY
[0088] According to the present invention having the
above-described features, the separation of the molten metal,
produced by reducing melt, from the by-product slag can be
accelerated by adding a cohesion accelerator such as fluorite
(CaF.sub.2) to the material as an auxiliary material, the cohesion
accelerator having an effect of improving the fluidity of the
by-product molten metal. As a result, large-size high-quality metal
nuggets having a high purity of metal such as iron and superior
transporting and handling qualities can be manufactured at a high
yield and high productivity.
* * * * *