U.S. patent application number 10/508546 was filed with the patent office on 2005-09-29 for ferronickel and process for producing raw material for ferronickel smelting.
Invention is credited to Harada, Takao, Kobayashi, Isao, Miyahara, Itsuo, Sugitatsu, Hiroshi, Tanaka, Hidetoshi.
Application Number | 20050211020 10/508546 |
Document ID | / |
Family ID | 32105125 |
Filed Date | 2005-09-29 |
United States Patent
Application |
20050211020 |
Kind Code |
A1 |
Sugitatsu, Hiroshi ; et
al. |
September 29, 2005 |
Ferronickel and process for producing raw material for ferronickel
smelting
Abstract
The present invention provides a process that is useful in
producing ferronickel having a high Ni content at low cost with
high efficiency and reproducibility even if a low-grade feedstock
containing nickel oxide is used. In particular, a feedstock
containing nickel oxide and iron oxide is mixed with a carbonaceous
reductant, the mixture is formed into agglomerates with an
agglomerator, and the agglomerates are heated and reduced in a
moving hearth furnace, whereby reduced agglomerates in which the Ni
metallization degree is 40% or more and the Fe metallization degree
is at least 15% less than the Ni metallization degree are prepared
by adjusting the retention time of the agglomerates placed in the
moving hearth furnace. The reduced agglomerates, in which the Ni
component has been primarily reduced as compared with the Fe
component, are smelted in a smelting furnace, whereby ferronickel
having a high Ni content is obtained.
Inventors: |
Sugitatsu, Hiroshi; (Hyogo,
JP) ; Tanaka, Hidetoshi; (Hyogo, JP) ; Harada,
Takao; (Hyogo, JP) ; Miyahara, Itsuo; (Hyogo,
JP) ; Kobayashi, Isao; (Hyogo, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
32105125 |
Appl. No.: |
10/508546 |
Filed: |
September 28, 2004 |
PCT Filed: |
September 19, 2003 |
PCT NO: |
PCT/JP03/11960 |
Current U.S.
Class: |
75/484 |
Current CPC
Class: |
C21C 5/5264 20130101;
C21B 13/105 20130101; Y02P 10/20 20151101; C21B 13/0046 20130101;
Y02P 10/216 20151101; C21B 13/006 20130101; C21B 13/143
20130101 |
Class at
Publication: |
075/484 |
International
Class: |
C21B 013/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 18, 2002 |
JP |
20002-304575 |
Claims
1. A process for producing a ferronickel, comprising: mixing a
feedstock comprising nickel oxide and iron oxide with a
carbonaceous reductant to prepare a mixture; heating and reducing
the mixture in a moving hearth furnace to prepare a reduced
mixture; and smelting the reduced mixture in a smelting furnace to
prepare the ferronickel.
2. The process for producing a ferronickel according to claim 1,
wherein the retention time of the mixture placed in the moving
hearth furnace is adjusted such that the Ni metallization degree of
the reduced mixture is 40% or more and the Fe metallization degree
of the reduced mixture is at least 15% less than the Ni
metallization degree thereof.
3. The process for producing a ferronickel according to claim 2,
wherein the Ni metallization degree is 85% or more.
4. The process for producing a ferronickel according to claim 1,
further comprising cooling the reduced mixture to a temperature
ranging from 450.degree. C. to 1100.degree. C. in the moving hearth
furnace to maintain the reduced mixture at that temperature for 17
seconds or more or discharging the reduced mixture from the moving
hearth furnace to place the reduced mixture in another vessel to
cool the reduced mixture to a temperature ranging from 450.degree.
C. to 1100.degree. C. in the vessel to maintain the reduced mixture
at that temperature for 17 seconds or more.
5. A process for producing a ferronickel, comprising: mixing a
feedstock comprising nickel oxide and iron oxide with a
carbonaceous reductant to prepare a mixture; heating and reducing
the mixture in a moving hearth furnace to prepare a reduced mixture
wherein the Ni metallization degree is 40% or more and the Fe
metallization degree is at least 15% less than the Ni metallization
degree to heat and melt the reduced mixture to prepare a reduced
melt; cooling the reduced melt in the moving hearth furnace to
prepare a reduced solid or discharging the reduced melt from the
moving hearth furnace to cool the reduced melt to prepare a reduced
solid; and separating the reduced melt into a metal and a slag to
prepare the ferronickel.
6. The process for producing a ferronickel according to claim 5,
wherein the Ni metallization degree is 85% or more.
7. A process for producing a feedstock for ferronickel production,
comprising: mixing a composition comprising nickel oxide and iron
oxide with a carbonaceous reductant to prepare a mixture; and
heating and reducing the mixture in a moving hearth furnace to
prepare the feedstock for ferronickel production wherein the Ni
metallization degree is 40% or more and the Fe metallization degree
is at least 15% less than the Ni metallization degree.
8. The process for producing a feedstock for ferronickel production
according to claim 7, wherein the Ni metallization degree is 85% or
more.
Description
TECHNICAL FIELD
[0001] The present invention relates to processes for producing
ferronickel and particularly relates to a process for producing
ferronickel or a feedstock for ferronickel production with high
efficiency using low-grade nickel oxide ore.
BACKGROUND ART
[0002] Examples of a process, currently used in our country, for
producing ferronickel from nickel oxide ore include electric
furnace processes and the Krupp-Ren process, and the electric
furnace processes are categorized into a complete reduction process
and a selective reduction process (see The Iron and Steel Institute
of Japan, "TEKKO BINRAN fourth edition", Volume 2, Chapter 7,
Section 3, Subsection 4, published by The Iron and Steel Institute
of Japan, published on Jul. 30, 2002).
[0003] In the complete reduction process, which is one of the
electric furnace processes, nickel oxide ore is mixed with coal,
the mixture is calcined in a rotary kiln, and the resulting mixture
is melted and reduced in an electric furnace, whereby ferronickel
is produced. In the selective reduction process, nickel oxide ore
is mixed with coal, the mixture is preliminarily reduced in a
rotary kiln, and the resulting mixture is melted and further
reduction is completed in an electric furnace, whereby ferronickel
is produced.
[0004] In the Krupp-Ren process, nickel oxide ore is mixed with
anthracite, the mixture is pressed into briquettes, and the
briquettes are heated and reduced in a rotary kiln, whereby a
semi-solid containing luppe (metal) and slag are prepared. The
semi-solid is water-granulated, and the luppe is then isolated and
recovered from the semi-solid by a magnetic separation process or a
flotation process, whereby ferronickel is produced.
[0005] In the above processes, rotary kilns are used and there are
many problems, described below, characteristic of such rotary
kilns. That is, since the rotary kilns are based on the principle
that raw materials are moved by rolling motion, a large amount of
dust is generated; hence, there is a problem in that a dam ring is
apt to be formed. In order to prevent the dam ring from being
formed, a technique of limiting the content of slag in a feedstock
has been proposed (see, for example, Japanese Examined Patent
Application Publication No. 48-43766, second page); however, there
is a problem in that the degree of freedom in feedstock selection
is low. Furthermore, in order to operate a kiln without trouble,
the outlet temperature must be relatively low; hence, the kiln must
be have a large size so as to provide a long retention time (see
Japanese Unexamined Patent Application Publication No. 9-291319,
second page). Therefore, there is a problem in that the fuel
consumption is high because the kiln has a large surface area and
the heat release is therefore high.
[0006] Furthermore, in the above processes, since nickel oxide and
iron oxide in nickel oxide ore are reduced into metals at the same
time but only the nickel oxide cannot be reduced into a metal,
there is a problem in that ferronickel having a high Ni content
cannot be produced from ore having a low Ni content. In order to
solve that problem, techniques described below have been
proposed.
[0007] One of the techniques is similar to the Krupp-Ren process.
In this technique, nickel oxide ore is pretreated, the resulting
ore is reduced in a firing furnace while the ore is in a
semi-molten state, and metallic Fe and metallic Ni are recovered
from the ore, whereby ferronickel is produced. In the step of
reducing the ore in the firing furnace while the ore is in a
semi-molten state, an Fe component and an Ni component are reduced
in a strongly reducing atmosphere and only the Fe component is then
oxidized in a strongly oxidizing atmosphere, whereby the content of
Ni in the luppe is relatively increased and ferronickel having a
high Ni content is produced (see Japanese Unexamined Patent
Application Publication No. 5-186838).
[0008] The other one is also similar to the Krupp-Ren process. In
this technique, nickel oxide ore is pretreated, the resulting ore
is reduced in a firing furnace while the ore is in a semi-molten
state, and metallic Fe and metallic Ni are recovered from the ore,
whereby ferronickel is produced. In the step of reducing the ore in
the firing furnace while the ore is in a semi-molten state, a
predetermined amount of additional coal necessary to reduce
(metallize) an Ni component and an Fe component to a desired level
is divided into several portions, which are fed into the furnace
intermittently or continuously, whereby the content of Ni in the
luppe is relatively increased and ferronickel having a high Ni
content is produced (see Japanese Unexamined Patent Application
Publication No. 5-247581).
[0009] If an attempt to use the rotary kiln as a firing furnace is
made to commercialize the above techniques (Japanese Unexamined
Patent Application Publication Nos. 5-186838 and 5-247581) for
producing ferronickel from the low-Ni content ore, the attempt is
not successful due to the structure thereof. This is because it is
necessary for the techniques to feed a gas or solid material into
the kiln though a side wall of the kiln that is usually rotating
and the structure of the kiln is therefore complicated; hence,
trouble frequently occurs during the operation and the plant cost
is high.
[0010] Even if the structural problem of the kiln is solved, the
following problems that are characteristic of the rotary kiln still
remain: a large amount of dust is generated, the dam ring is formed
in the kiln, the plant area is large, and a large amount of heat is
released from walls, which causes an increase in fuel
consumption.
DISCLOSURE OF INVENTION
[0011] Under the circumstances described above, it is an object of
the present invention to provide a process for producing
ferronickel having a high Ni content from low-grade nickel oxide
ore (a feedstock containing nickel oxide) at low cost with high
efficiency without trouble.
[0012] According to a first aspect of the present invention, a
process for producing ferronickel includes a mixing step of mixing
a feedstock containing nickel oxide and iron oxide with a
carbonaceous reductant to prepare a mixture, a reducing step of
heating and reducing the mixture in a moving hearth furnace to
prepare a reduced mixture, and a smelting step of smelting the
reduced mixture in a smelting furnace to prepare ferronickel.
[0013] In the process, since the moving hearth furnace is used to
heat and reduce the mixture, the amount of dust generated is
greatly decreased and the dam ring caused by the dust deposited on
furnace walls is prevented from being formed. Thus, in order to
prevent the dam ring from being formed, the slag content of the
feedstock need not be adjusted; hence, the degree of freedom in
feedstock selection is high. Since the retention time of the
mixture placed in the moving hearth furnace is uniform, a large
size apparatus such as a rotary kiln is not necessary and the plant
is compact; hence, the plant area is small and the heat release is
low.
[0014] In the process, the retention time of the mixture placed in
the moving hearth furnace is preferably adjusted such that the Ni
metallization degree of the reduced mixture is 40% or more
(preferably 85% or more) and the Fe metallization degree of the
reduced mixture is at least 15% less than the Ni metallization
degree thereof.
[0015] When the Fe metallization degree of the reduced mixture is
adjusted to a value that is at least 15% less than the Ni
metallization degree by controlling the retention time of the
mixture placed in the moving hearth furnace, nickel oxide in ore
having a low Ni content is primarily metallized but iron oxide
therein is slowly metallized; hence, ferronickel having a high Ni
content can be readily produced with high efficiency by smelting
the reduced mixture in the smelting furnace. When the Ni
metallization degree of the reduced mixture is 40% or more, the
amount of heat necessary to reduce nickel oxide remaining in the
reduced mixture in the smelting furnace is small; hence, there is
an advantage in that the energy consumption of the smelting furnace
can be reduced. The Ni metallization degree and the Fe
metallization degree are defined as follows:
Ni metallization degree (%)=[(metallic Ni content (% by
mass))/(total Ni content (% by mass))].times.100
Fe metallization degree (%)=[(metallic Fe content (% by
mass))/(total Fe content (% by mass))].times.100
[0016] The process preferably further includes a reduced
mixture-retaining step of cooling the reduced mixture to a
temperature ranging from 450.degree. C. to 1100.degree. C. in the
moving hearth furnace to maintain the reduced mixture at that
temperature for 17 seconds or more or discharging the reduced
mixture from the moving hearth furnace to place the reduced mixture
in another vessel to cool the reduced mixture to a temperature
ranging from 450.degree. C. to 1100.degree. C. in the vessel to
maintain the reduced mixture at that temperature for 17 seconds or
more.
[0017] When the reduced mixture is maintained at a temperature
ranging from 450.degree. C. to 1100.degree. C. for a predetermined
time, a reaction in which nickel oxide contained in the reduced
mixture is reduced with metallic iron and metallic nickel and iron
oxide are thereby formed is promoted, the reaction being expressed
as NiO+Fe.fwdarw.Ni+FeO; hence, the Ni metallization degree can be
increased but the Fe metallization degree can be decreased. That
is, Ni can be more primarily reduced.
[0018] According to a second aspect of the present invention, a
process for producing ferronickel includes a mixing step of mixing
a feedstock containing nickel oxide and iron oxide with a
carbonaceous reductant to prepare a mixture, a reducing and melting
step of heating and reducing the mixture in a moving hearth furnace
to prepare a reduced mixture in which the Ni metallization degree
is 40% or more (preferably 85% or more) and the Fe metallization
degree is at least 15% less than the Ni metallization degree to
heat and melt the reduced mixture to prepare a reduced melt, a
solidifying step of cooling the reduced melt in the moving hearth
furnace to prepare a reduced solid or discharging the reduced melt
from the moving hearth furnace to cool the reduced melt to prepare
a reduced solid, and a separating step of separating the reduced
melt into metal and slag to prepare ferronickel.
[0019] According to this process, since ferronickel having a high
Ni content can be produced by reducing and melting ore having a low
Ni content only in the moving hearth furnace, no melting furnace is
used; hence, plant cost and the energy consumption can be greatly
decreased.
[0020] Furthermore, a process for producing a feedstock for
ferronickel production according to the present invention includes
a mixing step of mixing a feedstock containing nickel oxide and
iron oxide with a carbonaceous reductant to prepare a mixture and a
reducing and melting step of heating and reducing the mixture in a
moving hearth furnace to prepare a ferronickel feedstock in which
the Ni metallization degree is 40% or more (preferably 85% or more)
and the Fe metallization degree is at least 15% less than the Ni
metallization degree.
[0021] In the feedstock-producing step, as well as the process of
the first aspect, since the moving hearth furnace, which is of a
stationary type, is used to heat and reduce the mixture, the amount
of dust generated is greatly decreased and the dam ring caused by
the dust deposited on furnace walls is prevented from being formed.
Thus, in order to prevent the dam ring from being formed, the slag
content of the feedstock need not be adjusted; hence, the degree of
freedom in feedstock selection is high. Since the retention time of
the mixture placed in the moving hearth furnace is uniform, a large
size apparatus such as a rotary kiln is not necessary and the plant
is compact; hence, the plant area is small and the heat release is
low. When the Fe metallization degree is adjusted to a value that
is at least 15% less than the Ni metallization degree by
controlling the retention time of the mixture placed in the moving
hearth furnace, nickel oxide in ore having a low Ni content is
primarily metallized but iron oxide therein is slowly metallized;
hence, a feedstock for producing ferronickel having a high Ni
content can be readily produced with high efficiency. Furthermore,
when the Ni metallization degree of the reduced mixture is 40% or
more (preferably 85% or more), the amount of heat necessary to
reduce nickel oxide in the smelting furnace in a subsequent step is
small; hence, the energy consumption can be decreased.
[0022] Embodiments of the present invention will now be described
in detail with reference to the accompanying drawings.
FIRST EMBODIMENT
[0023] FIG. 1 shows steps for producing ferronickel according to an
embodiment of the present invention. In the figure, reference
numeral 1 represents a feedstock containing nickel oxide and iron
oxide (hereinafter simply referred to as "a feedstock"), reference
numeral 2 represents a carbonaceous reductant, reference numeral 3
represents an agglomerator, reference numeral 4 represents
agglomerates (mixture), reference numeral 5 represents a moving
hearth furnace, reference numeral 6 represents reduced agglomerates
(reduced mixture), reference numeral 7 represents a smelting
furnace, reference numeral 8 represents metal (ferronickel), and
reference numeral 9 represents slag.
[0024] Examples of the feedstock 1 containing nickel oxide and iron
oxide include a nickel oxide ore and a residue, generated in
ferronickel- or nickel-manufacturing steps, such as kiln dust
generated from ferronickel-manufacturing plants. Examples of the
nickel oxide ore include garnierite usually used and low-grade
nickel ores such as nickel-containing laterite and limonite. Those
ores and residue may be used in combination. Since a rotary kiln is
not used but the moving hearth furnace 5 that is of a stationary
type is used, a dam ring is prevented from being formed and there
are no limitations on slag components; hence, the degree of freedom
in feedstock selection is high. When the feedstock 1 contains a
large amount of water, the feedstock 1 is preferably dried in
advance. The dryness of the feedstock 1 may be determined depending
on the type of a mixing means (the agglomerator 3 is used in this
embodiment) used in a subsequent mixing step. The carbonaceous
reductant 2 contains fixed carbon, and any reductant such as coal,
coke, charcoal, waste toner, or carbonized biomass may be used.
Those materials may be used in combination.
[0025] The content of the carbonaceous reductant 2 in the
agglomerates (mixture) 4 may be determined based on the amount of
carbon necessary to reduce nickel oxide and iron oxide contained in
the feedstock 1 in the moving hearth furnace 5, the amount of
carbon consumed in reducing nickel oxide remaining in reduced
agglomerates (reduced mixture) 6 in the smelting furnace 6, and the
amount of carbon remaining in ferronickel.
[0026] [Mixing Step]: A mixer, which is not shown, is preferably
used to mix the feedstock 1 and the carbonaceous reductant 2. The
mixture may be directly fed into the moving hearth furnace 5;
however, the mixture is preferably agglomerated with the
agglomerator 3. This is because the agglomeration reduces the
amount of dust generated from the moving hearth furnace 5 and the
smelting furnace 7 and enhances the heat-transfer efficiency of the
agglomerates (mixture) 4 placed in the moving hearth furnace 5 to
increase the reduction rate. The agglomerates (mixture) 4 may
contain an auxiliary feedstock such as flux. Examples of the
agglomerator 3 include a compression molding machine such as a
briquette press, a rotary pelletizer such as a disk pelletizer, and
an extruder. When the content of water in the obtained agglomerates
(mixture) 4 is high, the agglomerates (mixture) 4 may be dried
before they are fed into the moving hearth furnace 5.
[0027] [Reducing Step]: The agglomerates (mixture) 4 are fed into
the moving hearth furnace 5 and then heated and reduced at an
atmospheric temperature ranging from 1000.degree. C. to
1400.degree. C.
[0028] Examples of the moving hearth furnace 5 include known moving
hearth furnaces having at least one bed on which the agglomerates
(mixture) 4 are place and which horizontally moves in the furnace
and also have a heating means for reduction or the like and there
are no limitations on such moving hearth furnaces. Examples of the
moving hearth furnaces include a rotary hearth furnace, a straight
furnace, and a multi-hearth furnace. In those moving hearth
furnaces, the amount of dust and the like generated is small
because heating objects (agglomerates are herein used) are kept
stationary. Furthermore, plant cost is relatively low, operation
troubles hardly occurs, and the heat release is small because the
surface are is less than that of rotary kilns; hence, the reduction
efficiency is high.
[0029] The retention time of which the agglomerates (mixture) 4
heated to a temperature ranging from 1000.degree. C. to
1400.degree. C. is preferably adjusted within a range of 3 to 20
minutes such that the relationship between the Ni metallization
degree and the Fe metallization degree is satisfied as described
below. That is, in the moving hearth furnace 5, the agglomerates
(mixture) 4 are reduced such that the Ni metallization degree of
the agglomerates (mixture) 4 is 40% or more (more preferably 50% or
more and further more preferably 85% or more) and the Fe
metallization degree of the agglomerates (mixture) 4 is at least
15% (more preferably at least 20%) less than the Ni metallization
degree thereof. The Fe metallization degree is determined based on
the Ni content and Fe content of the feedstock 1 and the target
content of Ni in the ferronickel 8, which is a product. In order to
produce the ferronickel 8 having an Ni content of, for example,
16%, the Ni metallization degree and Fe metallization degree shown
in FIG. 2 must be achieved depending on the kinds of nickel oxide
ore (see Table 1) used for the feedstock 1. For the standard ore,
except for the high-grade ore, the Fe metallization degree must be
15% to 20% less than the Ni metallization degree. For the low-grade
ores, the Fe metallization degree must be less than the above.
1 TABLE 1 Kinds of Nickel Content (% by mass) Oxide Ore Ni Fe
Standard Ore 2.4 14.7 High-grade Ore 2.5 9.8 Low-grade Ore 1 1.7
15.7 Low-grade Ore 2 1.3 34.5
[0030] FIG. 3 shows the target Fe metallization degree determined
based on the Ni content and Fe content of the feedstock 1. The
target Fe metallization degree is in proportion to the Ni content
of the feedstock 1 but is in inverse proportion to the Fe content
of the feedstock 1. In FIG. 3, the content of Ni in the ferronickel
8 is 16% and the Ni metallization degree of the reduced
agglomerates (reduced mixture) 6 is 90%. In order to increase the
Ni content of the ferronickel 8 from 16% to, for example, 20%, the
Fe metallization degree must be further decreased.
[0031] The Ni metallization degree and the Fe metallization degree
may be adjusted by varying the heating temperature and the
retention time based on a difference in reduction rate between both
components. In general, the Ni component is more readily reduced
(metallized) than the Fe component because the affinity of Ni to
oxygen is less than that of Fe. Although the difference between the
Ni metallization degree and the Fe metallization degree is small
when the retention time is short, an increase in the retention time
allows the Ni metallization degree and the Fe metallization degree
to approach 100% if there is a sufficient amount of a reductant.
Furthermore, an increase in heating temperature promotes the
reduction of FeO, whereby the difference between the Ni
metallization degree and the Fe metallization degree is decreased
(see FIGS. 5 and 6). Therefore, the heating temperature and the
retention time are adjusted depending on the reduction efficiency
such that the Ni metallization degree slightly exceeds the Fe
metallization degree.
[0032] On the other hand, as described in examples described below,
a decrease in the surplus of carbon (%) causes a decrease in the Fe
metallization degree (see FIG. 8) and an increase in the strength
(for example, the crushing strength) of reduced agglomerates,
whereby the agglomerates can be readily handled and the yield of a
smelting operation is increased. Thus, the surplus of carbon is
preferably 0% or less, more preferably -2% or less, and first more
preferably -4% or less.
[0033] The Ni metallization degree and Fe metallization degree of
the reduced agglomerates (reduced mixture) 6 can be controlled by
varying the content of the carbonaceous reductant 2 in the
agglomerates (mixture) 4 and can also be controlled by varying the
retention time. When the metallization degrees are controlled by
varying the content and the retention time, the degree of freedom
in feedstock selection and the Ni content of the ferronickel 8 can
be increased.
[0034] The reduced agglomerates (reduced mixture) 6 treated in the
moving hearth furnace 5 are usually cooled to about 1000.degree. C.
with a radiant cooler or coolant sprayer placed in the moving
hearth furnace 5 and then discharged with a discharging unit.
[0035] [Step of Retaining Reduced Mixture]: Unreduced nickel oxide
is reduced by metallic Fe while the reduced agglomerates (reduced
mixture) 6 are cooled. In particular, a reaction expressed as
NiO+Fe.fwdarw.Ni+FeO occurs to cause an increase in the Ni
metallization degree but a decrease in Fe metallization degree,
whereby the Ni component in the reduced agglomerates (reduced
mixture) 6 is primarily reduced. In order to actively use the
reaction, the reduced agglomerates (reduced mixture) 6 are
preferably cooled to a temperature ranging from 450.degree. C. to
1100.degree. C. and maintained at that temperature for 17 seconds
or more while the reduced agglomerates (reduced mixture) 6 are
placed in the moving hearth furnace 5 or placed in another vessel,
which is not shown, after the reduced agglomerates (reduced
mixture) 6 are discharged from the moving hearth furnace 5. The
reason for setting the lower limit of the temperature to
450.degree. C. is that the reaction rate is too low and the
advantage is therefore low when the temperature is less than
450.degree. C. The lower limit is more preferably 650.degree. C. In
contrast, the reason for setting the upper limit of the temperature
to 1100.degree. C. is that iron oxide and the carbonaceous
reductant remaining in the reduced agglomerates (reduced mixture) 6
react each other and the chain reactions expressed as
FeO+CO.fwdarw.Fe+CO.sub.2 and CO.sub.2+C.fwdarw.2CO are promoted
when the temperature is more than 1100.degree. C., whereby the Fe
metallization degree is increased. The upper limit is more
preferably 1000.degree. C.
[0036] The reason for setting the lower limit of the retaining time
to 17 seconds during which the reduced agglomerates (reduced
mixture) 6 are maintained at a temperature within the above range
is described below. The following equation has been obtained by
formulating the correlation between the reduction time and Ni
metallization degree (see FIG. 6) obtained from a reduction
experiment (an atmospheric temperature of 1300.degree. C.)
performed in an example described below:
MetNi=83.9.times.[1-exp(-t/46)]+15.3
[0037] wherein MetNi represents the Ni metallization degree (%) and
t represents the reduction time (s).
[0038] According to the above equation, it takes 17 seconds to
reduce 30% of unreduced nickel oxide (NiO) contained in the reduced
agglomerates (reduced mixture) 6 into metallic Ni; hence, the lower
limit of the retaining time is specified as 17 seconds. Considering
that the upper limit (1100.degree. C.) of the cooling temperature
is lower than the atmospheric temperature (1300.degree. C.) of the
reduction experiment, the lower limit of the retaining time is
preferably set to 20 seconds, which is slightly longer than 17
seconds. Furthermore, according to the equation, it takes 32
seconds to reduce 50% of unreduced nickel oxide (NiO) contained in
the reduced agglomerates (reduced mixture) 6 into metallic Ni;
hence, the lower limit of the retaining time is more preferably 32
seconds and further more preferably 40 seconds. As long as the
reduced agglomerates (reduced mixture) 6 are maintained at a
temperature within the above range, the retaining time may include
the time elapsed during the transfer of the reduced agglomerates
(reduced mixture) 6 that is discharged from the moving hearth
furnace 5 or the vessel and then fed into the smelting furnace
7.
[0039] [Melting Step]: The reduced agglomerates (reduced mixture) 6
discharged from the moving hearth furnace 5 or the vessel are
preferably fed into the smelting furnace 7 directly in such a
manner that the reduced agglomerates (reduced mixture) 6 are not
further cooled but maintained at a high temperature. The smelting
furnace 7 may be directly connected to an outlet of the moving
hearth furnace 5 or the vessel with a chute placed therebetween.
The reduced agglomerates (reduced mixture) 6 may be fed into the
smelting furnace 7 using a conveying unit such as a conveyer or
using a container for temporarily storing the reduced agglomerates
(reduced mixture) 6. When the smelting furnace 7 is not placed
close to the moving hearth furnace 5 or is not operated, the
reduced agglomerates (reduced mixture) 6 may be cooled to
atmospheric temperature and then treated as a semi-product
(feedstock for ferronickel production) during the storage or the
transportation. Alternatively, the reduced agglomerates (reduced
mixture) 6 may be hot-briquetted to reduce the surface area in such
a manner that the reduced agglomerates (reduced mixture) 6 are not
cooled but maintained at a high temperature, whereby the surface
area thereof is deceased. The hot-briquetted reduced agglomerates
(reduced mixture) 6 are cooled and then treated as a semi-product
(feedstock for ferronickel production) having high re-oxidation
resistance during the storage or the transportation.
[0040] Examples of the smelting furnace 7 include an electric
furnace. When the electric furnace is used, the content of carbon
in molten metal, the voltage of the electric furnace, the positions
of electrodes arranged in the electric furnace, the amount of
oxygen, and the amount of agitation gas are preferably adjusted
such that the nickel yield is increased and the reduction of iron
is suppressed. A smelting furnace using fossil energy such as coal,
fuel oil, natural gas may be used. The flux or the like may be fed
into the smelting furnace 7 according to needs and the reduced
agglomerates (reduced mixture) 6 are melted at a high temperature
ranging from 1400.degree. C. to 1700.degree. C., whereby the
reduced agglomerates (reduced mixture) 6 are separated into the
metal 8 and the slag 9. The metal 8, which corresponds to the
ferronickel 8, is withdrawn and then additionally refined according
to needs, whereby commercial ferronickel is produced. The slag 9
can be used for concrete aggregates.
SECOND EMBODIMENT
[0041] FIG. 4 shows steps of producing ferronickel according to
another embodiment of the present invention. In the figure,
reference numeral 11 represents a feedstock containing nickel oxide
and iron oxide (hereinafter simply referred to as "a feedstock"),
reference numeral 12 represents a carbonaceous reductant, reference
numeral 13 represents an agglomerator, reference numeral 14
represents agglomerates (mixture), reference numeral 15 represents
a moving hearth furnace, reference numeral 16 represents a reduced
solid, reference numeral 17 represents a screen, reference numeral
18 represents metal (ferronickel), and reference numeral 19
represents slag.
[0042] In the second embodiment, the feedstock 11, the carbonaceous
reductant 12, the agglomerator 13, and the agglomerates (mixture)
14 are the same as the feedstock 1, carbonaceous reductant 2,
agglomerator 3, and agglomerates (mixture) 4 of the first
embodiment, respectively, and a mixing step is also the same as
that of the first embodiment; hence, the description is
omitted.
[0043] [Reducing/Melting Step]: The agglomerates (mixture) 14 are
fed into the moving hearth furnace 15 and then heated and reduced
at an atmospheric temperature ranging from 1000.degree. C. to
1400.degree. C. According to the same concept as that described in
the first embodiment, the retention time of the agglomerates
(mixture) 14 heated to a temperature ranging from 1000.degree. C.
to 1400.degree. C. may be adjusted within a range of 3 to 20
minutes such that the relationship between the Ni metallization
degree and the Fe metallization degree satisfies conditions
described below. That is, in the above temperature range, the
agglomerates (mixture) 14 are reduced in reduced agglomerates
(reduced mixture) such that the Ni metallization degree of the
reduced agglomerates (reduced mixture) is 40% or more (more
preferably 50% or more, further more preferably 85% or more, most
preferably 90% or more) and the Fe metallization degree thereof is
at least 15% (more preferably at least 20%) less than the Ni
metallization degree. Subsequently, the reduced agglomerates
(reduced mixture) are heated and melted at an atmospheric
temperature ranging from 1100.degree. C. to 1500.degree. C., which
is higher than that described above, in the moving hearth furnace
15, whereby reduced melt is prepared. The retention time of the
reduced agglomerates (reduced mixture) heated to an atmospheric
temperature ranging from 1100.degree. C. to 1500.degree. C. may be
adjusted within a range of 0.5 to 10 minutes such that the reduced
agglomerates (reduced mixture) are completely melted and separated
into metal and slag. The atmospheric temperature of the moving
hearth furnace 15 is varied in two steps in the above procedure but
may not be varied, and the agglomerates (mixture) 14 may be heated
to an atmospheric temperature ranging from 1100.degree. C. to
1500.degree. C. in one step, whereby the agglomerates (mixture) 14
are melted and reduced at the same time. The latter procedure
provides the reduced melt in a shorter time. Both the metal and
slag may be melted and only either one may be melted. For example,
only the metal may be melted, whereby the metal is isolated from
the slag.
[0044] [Solidifying Step]: The reduced melt is cooled to about
1000.degree. C. in or outside the moving hearth furnace 15, whereby
the reduced melt is solidified into the reduced solid 16. Examples
of a cooling/solidifying unit placed in the moving hearth furnace
15 include the radiant cooler and coolant sprayer described in the
first embodiment. In order to cool and solidify the reduced melt
discharged from the moving hearth furnace 15, a method such as
water granulation may be used.
[0045] [Separating Step]: The reduced solid is separated into the
metal (ferronickel) 18 and the slag 19 with the screen Metal
components may be recovered from the resulting slag 19 by a method
such as a magnetic separation process or a floatation process
according to needs. The resulting metal 18 is then additionally
refined according to needs, whereby commercial ferronickel is
produced. Alternatively, the metal 18 may be used as a semi-product
(feedstock for ferronickel production), which is fed into a
smelting furnace. In comparison between the semi-products of the
functions as embodiments, the reduced agglomerates (reduced
mixture) of the first embodiment contain slag components but the
metal 18 of the second embodiment does not contain such slag
components. Therefore, in the second embodiment, energy consumed in
melting the slag components in the smelting furnace is not
necessary, whereby energy consumption of the smelting furnace is
greatly reduced. Furthermore, the semi-product has reduced weight
depending on the weight of the removed slag components and the
storage cost and transportation cost thereof can therefore be
reduced; hence, it is preferable to use a process of the present
invention in areas where nickel oxide ore is produced. For
convenience of storage and transportation, agglomeration or the
like may be performed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0046] FIG. 1 is a flow chart showing steps of producing
ferronickel according to a first embodiment of the present
invention.
[0047] FIG. 2 is a graph showing the relationship between the Ni
metallization degree and Fe metallization degree of a reduced
mixture when the content of Ni in ferronickel is 16% by mass.
[0048] FIG. 3 is a graph showing the relationship between the Ni
content of a feedstock and the Fe metallization degree and the
relationship between the Fe content thereof and the Fe
metallization degree when the content of Ni in ferronickel is 16%
by mass and the Ni metallization degree of a reduced mixture is
90%.
[0049] FIG. 4 is a flow chart showing steps of producing
ferronickel according to another embodiment of the present
invention.
[0050] FIG. 5 is a graph showing the relationship between the
retention time and the Ni metallization degree and the relationship
between the retention time and the Fe metallization degree when the
atmospheric temperature is 1200.degree. C.
[0051] FIG. 6 is a graph showing the relationship between the
retention time and the Ni metallization degree and the relationship
between the retention time and the Fe metallization degree when the
atmospheric temperature is 1300.degree. C.
[0052] FIG. 7 is a graph showing the relationship between the
retention time and the Ni metallization degree and the relationship
between the retention time and the Fe metallization degree when the
atmospheric temperature is 1200.degree. C.
[0053] FIG. 8 is a graph showing the relationship between the
surplus of carbon (%) and the Ni metallization degree and the
relationship between the surplus of carbon (%) and the Fe
metallization degree when the atmospheric temperature is
1200.degree. C.
[0054] FIG. 9 is a graph showing the relationship between the
heating temperature and the Ni metallization degree and the
relationship between the heating temperature and the Fe
metallization degree when the retention time is 15 minutes.
REFERENCE NUMERALS
[0055] 1 and 11: feedstock containing nickel oxide and iron
oxide
[0056] 2 and 12: carbonaceous reductant
[0057] 3 and 13: agglomerator
[0058] 4 and 14: agglomerates (mixture)
[0059] 5 and 15: moving hearth furnace
[0060] 6: reduced agglomerates (reduced mixture)
[0061] 7: smelting furnace
[0062] 8 and 18: metal (ferronickel)
[0063] 9 and 19: slag
[0064] 16: reduced solid
[0065] 17: screen
BEST MODE FOR CARRYING OUT THE INVENTION
EXAMPLE 1
[0066] In order to assess the reduction state of a feedstock
mixture smelted in a moving hearth furnace according to the present
invention, a reduction experiment described below was performed
using a small-sized furnace for laboratory use.
[0067] A feedstock, containing nickel oxide and iron oxide, having
the composition shown in Table 2 was mixed with a carbonaceous
reductant containing coke powder (a content of fixed carbon: 77.7
mass percent), the ratio of the feedstock to the carbonaceous
reductant being 85.7 to 14.3 on a mass basis. A certain amount of
water was added to the mixture and the resulting mixture was formed
into pellets having a diameter of 13 mm with a small-sized disk
pelletizer. Those pellets were dried, fed into the small-sized
furnace in a batch mode, and then heated and reduced in such a
manner that the atmospheric temperature is maintained constant and
the retention time is varied. The reduced pellets were chemically
analyzed for composition, whereby the Ni metallization degree and
the Fe metallization degree were determined. A nitrogen atmosphere
was used and the atmospheric temperature was varied in two levels:
1200.degree. C. and 1300.degree. C.
2TABLE 2 (Unit: mass percent) T.Fe FeO M.Fe T.Ni 27.5 3.0 5.5
2.5
[0068] FIGS. 5 and 6 each show a correlation between the retaining
time (retention time) and the Ni metallization degree and a
correlation between the retaining time and the Fe metallization
degree, those correlations being obtained from the reduction
experiment. The correlations shown in FIG. 5 were obtained at an
atmospheric temperature of 1200.degree. C. and the correlations
shown in FIG. 6 were obtained at an atmospheric temperature of
1300.degree. C. Both figures illustrate that a Ni component is
primarily reduced as compared with a Fe component. Furthermore, the
reduction rate determined at 1300.degree. C. is greater than that
determined at 1200.degree. C. For example, FIG. 5 illustrates that
the Ni metallization degree reaches about 90% and the Fe
metallization degree remains at about 60% when the atmospheric
temperature is 1200.degree. C. and the retaining time (retention
time) is 2 minutes. Thus, a semi-product in which the Ni
metallization degree has been maximized and the Fe metallization
degree has been minimized can be obtained by appropriately
adjusting the retention time depending on heating conditions such
as the atmospheric temperature. Since the reduction state of the
feedstock mixture in an actual moving hearth furnace is affected by
the difference in heating rate due to the shape or size of the
furnace and the gas composition of the atmosphere, the optimum
retention time can be determined by actually measuring the Ni
metallization degree and the Fe metallization degree, with the
retention time varied in the moving hearth furnace.
EXAMPLE 2
[0069] A mixture consisting of 94 parts by mass (dry basis) of
nickel oxide ore and 6 parts by mass (dry basis) of coal was formed
into briquettes having a volume of 5.5 cm.sup.3 with a briquette
press. The nickel oxide ore had a total Ni content of 2.4%, a total
Fe content of 14.7%, a SiO.sub.2 content of 35.5%, and an MgO
content of 25.8% and the coal had a fixed carbon content of 74.0%,
a volatile matter content of 15.5%, and an ash content of 10.5% on
a mass basis. The briquettes were fed into a rotary hearth furnace
and then reduced into a semi-product (reduced briquettes) at an
atmospheric temperature ranging from 1100.degree. C. to
1300.degree. C. with a retention time of 5 minutes, the Fe
metallization degree of the semi-product being about 60%. In the
above operation, the yield of the semi-product (reduced briquettes)
obtained from the rotary hearth furnace was 88 parts by mass, the
Ni metallization degree of the semi-product being about 98%.
[0070] The semi-product (reduced briquettes) maintained at
1000.degree. C. was fed into an electric furnace and then smelted,
whereby 11 parts by mass of crude ferronickel having an Ni content
of 20% to 21% by mass and 80 parts by mass of slag having an FeO
content of about 10% by mass were obtained. The electric
consumption of the electronic furnace was 13000 kWh per ton of Ni.
This value is less than that of a known electric furnace process
(selective reduction process) using a rotary kiln as a pre-reducing
furnace. In the process, the electric consumption is about 20000
kWh per ton of Ni.
EXAMPLE 3
[0071] The same feedstock and carbonaceous reductant as those used
in the Example 2 were used. A mixture consisting of 93 parts by
mass (dry basis) of the nickel oxide ore and 7 parts by mass (dry
basis) of coal was mixed with a certain amount of water, and the
resulting mixture was formed into pellets having a diameter of 18
mm with a disk pelletizer. The pellets were dried with a dryer, fed
into a rotary hearth furnace, and then heated and reduced at an
atmospheric temperature ranging from 1350.degree. C. to
1350.degree. C., whereby a Ni component was substantially
completely metallized. After the Fe metallization degree reached
about 60%, the resulting pellets were heated at an atmospheric
temperature ranging from 1350.degree. C. to 1450.degree. C.,
whereby the pellets were melted.
[0072] The melt was then cooled and solidified with a chill plate
(radiant cooler) placed in the rotary hearth furnace, and the solid
was discharged from the rotary hearth furnace and then separated
into metal (crude ferronickel) and slag with a screen. As a result,
the following crude ferronickel and slag were obtained: 11 parts by
mass of the crude ferronickel having a Ni content of 20%, a Fe
content of 74%, and a C content of 2% and 77 parts by mass of the
slag having a FeO content of 10% on a mass basis.
[0073] In the Examples 2 and 3, no binder was used for
agglomeration; however, an appropriate binder may be used when the
agglomerates have an insufficient strength.
EXAMPLE 4
[0074] A mixture consisting of 96.5 parts by mass (dry basis) of
nickel oxide ore that is difficult in reducing and 3.5 parts by
mass (dry basis) of coal was formed into tablets having a diameter
of 25 mm with a tablet press. The nickel oxide ore had a total Ni
content of 2.1%, a total Fe content of 18.8%, a SiO.sub.2 content
of 35.0%, and an MgO content of 19.5% and the coal had a fixed
carbon content of 72%, a volatile matter content of 18%, and an ash
content of 10% on a mass basis. The tablets were fed into a rotary
hearth furnace and then reduced at an atmospheric temperature of
1200.degree. C. FIG. 7 shows a correlation between the retaining
time (retention time) and the Ni metallization degree and a
correlation between the retaining time and the Fe metallization
degree, those correlations being obtained from the reduction
experiment.
[0075] The experiment shows that the Ni component is more rapidly
reduced as compared with the Fe component. Since the nickel oxide
ore used in the experiment is difficult in reducing, the Ni
metallization degree is saturated when it is reached to about 56%.
However, if the ore is heated and reduced at 1200.degree. C. for 6
minutes or more, the Ni metallization degree is 40% or more and the
Fe metallization degree is at least 15% less than the Ni
metallization degree.
EXAMPLE 5
[0076] A mixture was prepared in such a manner that the same
feedstock and coal as those used in Example 4 were used but the
blend ratio of the nickel oxide ore to the coal was varied, and the
mixture was formed into tablets having a diameter of 25 mm. The
tablets were fed into a rotary hearth furnace and then reduced at
an atmospheric temperature of 1200.degree. C. for 12 minutes
(retention time), whereby the difference between the Ni
metallization degree and the Fe metallization degree was
determined. The results are shown in FIG. 8.
[0077] The results indicate that a decrease in "the surplus of
carbon (%)" specified below causes a decrease in the Fe
metallization degree. When the surplus of carbon is small, the
reduced tablets have high strength (for example, crushing strength)
and are therefore easy in handling and the melting yield is
high.
the surplus of carbon (%)=(the mass percentage of carbon in an
unreduced mixture)-(the mass percentage of oxygen bonded to Fe or
Ni in the unreduced mixture).times.12/16
EXAMPLE 6
[0078] The same feedstock and coal as those used in Examples 4 and
5 were used. A mixture consisting of 90.5 parts by mass of the
nickel oxide ore and 9.5 parts by mass of the coal was formed into
tablets having a diameter of 25 mm. The tablets were fed into a
rotary hearth furnace and then reduced at an atmospheric
temperature of 1200.degree. C., 1250.degree. C., or 1300.degree. C.
for 15 minutes, whereby the difference between the Ni metallization
degree and the Fe metallization degree were determined. The results
are shown in FIG. 9.
[0079] The results indicate that an increase in temperature causes
a decrease in difference between the Ni metallization degree and
the Fe metallization degree. However, considering these results and
the results of the above examples, even if the heating and
reduction temperature is 1300.degree. C., the Fe metallization
degree can be adjusted to a value that is at least 15% less than
the Ni metallization degree by decreasing the retention time and/or
controlling the content of the reductant (controlling the surplus
of carbon).
INDUSTRIAL APPLICABILITY
[0080] As described above, the present invention provides a process
that is useful in producing ferronickel having a high Ni content at
low cost with high efficiency and reproducibility even if a
low-grade feedstock containing nickel oxide is used.
* * * * *