Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR IMPROVING THE REDUCTION DEGREE IN THE SMELTING
OF FERROALLOY
This invention relates to a method for improving the reduction degree of metal
components in a material to be treated when smelting ferroalloy, as
ferrochrome suitable for manufacturing of stainless steel. According to the
method nickel-bearing material is fed into ferroalloy.
It is known from the WO patent publication 2010/092234 a method wherein
nickel ore and/or nickel concentrate or an intermediate product precipitated
from solutions of nickel ore and/or nickel concentrate is agglomerated in the
manufacturing process of ferrochrome so that it is first produced from nickel-
containing material together with iron-containing chromite concentrate and
binder agent pellets, and the drying and calcination of nickel-containing
material
is carried out advantageously within one-stage heat treatment of pellets,
sintering. With the heat treatment of pellets the object are strengthened so
that
the heat treated objects are conveyable, when desired, essentially complete
between separate process stages. If needed, the pellets can be preheated
before sintering. Heat treated objects can be conveyed, when desired,
essentially complete between separate process stages. Heat treated objects
can be downsized, when desired, when conveying object between separate
process stages or process units. Sintered and thus strengthened pellets are
used as material in a smelting process carried out in reducing conditions, in
which case it is received as a smelting product nickel-containing ferroalloy,
ferrochromenickel.
The above mentioned WO patent publication 2010/092234 thus relates mainly
to the production of nickel-containing pellets by sintering. Instead, smelting
conditions of the sintered pellets are not exactly described. When describing
the energy efficiency it is, however, mentioned that nickel containing in
pellets
catalyses chromium reduction in pellets and thus decreases the specific
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consumption, advantageous carbon, of the reducing agent in the ferroalloy
production.
It is now surprisingly observed that nickel containing in pellets not only
catalyses the reduction of chromium in chromite pellets, but nickel containing
in
the feed of a furnace used for smelting of chromite improves in the smelting
process the reduction of all essential metal components, iron, chromium and
nickel, containing in the feed of the smelting furnace. The object of the
present
invention is to utilize this surprizing finding and to achieve a more
effective
method than before for increasing the reduction degree in the smelting process
of chromite material in which method the reduction of metal components in
chromite during the smelting is improved by alloying into the material to be
gone into smelting nickel-containing material and simultaneously to achieve a
prealloy, ferrochromenickel, suitable to the production of stainless steel.
The
essential features are enlisted in the appended claims.
According to the invention, it is alloyed into the raw material, as chromite,
to be
smelted in the ferroalloy production before the smelting nickel-containing
material, in which case nickel-containing improves the reduction of metal
components containing in the feed material simultaneously when nickel-
containing material itself is managed to be reduced as a metallic component in
the ferroalloy. According to the invention, by means of the nickel amount to
be
added into the ferroalloy it can advantageously be adjusted the reduction
degree of metal components in the ferroalloy and simultaneously be achieved a
ferroalloy containing the desired nickel content, as ferrochromenickel alloys
having different nickel contents. Ferrochromenickel alloys containing desired
nickel contents can be used for instance for the production of different
stainless
steels, as austenitic or duplex stainless steels.
In the method according to the invention it can be used as a nickel-containing
raw material at least partly nickel oxide, at least partly nickel ore and/or
nickel
concentrate or at least partly a nickel-containing intermediate product
achieved
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by the leaching and/or by precipitating of nickel ores and/or nickel
concentrates.
The nickel-containing raw material is fed into a smelting process together
with
ferrochrome raw material. Before feeding into a smelting furnace the nickel-
containing raw material is pretreated either so that sintered pellets are
formed
from the nickel-containing material together with the ferrochrome raw material
or so that the nickel-containing raw material is pretreated separately to
chromite
pellets. It is possible to carry out the pretreatment of the nickel-containing
raw
material also so that one part of the nickel-containing raw material to be fed
into
the smelting furnace is pretreated together with chromite pellets and one part
of
the nickel-containing raw material is pretreated separately to chromite
pellets.
Thanks to different pretreatments the nickel-containing raw material to be fed
into the smelting furnace and promoting the reduction of different metal
components can be for instance partly nickel-containing hydroxide intermediate
product, partly sulphidic or lateritic nickel concentrate.
The nickel-containing raw material to be utilized in the method according to
the
invention is advantageously a nickel-containing hydroxide intermediate product
from mines or other hydrometallurgical processes, which intermediate product
is precipitated from solutions of lateritic and/or sulphidic nickel ores
and/or
nickel-containing concentrates of sulphidic ores. This kind of nickel-
containing
hydroxide intermediate product is for instance a nickel-containing
intermediate
product from pressure leaching, atmosphere leaching or heap leaching of
lateritic or sulphidic nickel ores or nickel concentrates as well as a nickel-
containing precipitated product of solvent extraction solutions, stripping
solutions or refining solutions received from solvent extraction processes or
ion
exchange processes of nickel-containing materials. In the method of the
invention it can as a raw material be used also carbonate or sulphate nickel
materials. Further, a sulphidic nickel concentrate itself and a
hydrometallurgically precipitated nickel sulphide intermediate product are
suited
for the nickel-containing raw material of the method.
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According to the invention, the amount of the nickel-containing material to be
fed into a smelting furnace is adjusted in the range of 5 ¨ 25 weight /0,
preferably 10 ¨ 20 weight %, from the total mass of the pretreated material to
be fed into the smelting furnace. When adjusting the amount of the nickel-
containing to be fed into the smelting furnace it is considered the
achievement
of the energy-economically favourable reduction conditions and/or the
production of a prealloy, ferrochromenickel, suitable the production of
favourable stainless steel in each case. Using a small addition of nickel-
containing raw material, the reduction degree remains low, in which case it is
created a ferroalloy with low nickel content, ferrochromenickel. This kind of
ferroalloy with a low nickel content is a favourable prealloy especially to
the
production of duplex stainless steel grades. Using a greater addition of
nickel-
containing raw material the reduction degree increases and also the nickel
content in the smelting product is greater. This kind of ferrochromenickel
with a
greater nickel content is favourable to use to the production of austenitic
stainless steel grades having a high nickel content.
In the pretreatment of nickel-containing raw material to be fed into a
smelting
furnace in accordance with the method of the invention it is advantageously
considered the composition and the microstructure of the nickel raw material.
If
the nickel-containing raw material is for instance a nickel-containing
intermediate product of mines or other hydrometallurgical processes
precipitated from solutions of nickel-containing solutions, which intermediate
product requires to carry out as a pretreatment among others calcination at a
higher temperature, the pretreatment of the nickel-containing raw material is
carried out together with the production of chromite pellets and sintering of
pellets. Instead, if the nickel-containing raw material of the method
according to
the invention is material, as for instance nickel oxide, nickel ore and/or
nickel
concentrate, which does not require in addition to a possible drying any other
essential pretreatment at a higher temperature, then the nickel-containing raw
material is possible to feed into a smelting furnace with the feeding of
chromite
pellets. The microstructure and composition of the nickel-containing raw
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material can also be such that it is advantageous to pretreat the raw material
separately from chromite pelletizing and to feed the nickel-containing raw
material into sintering of chromite pellets before feeding into a smelting
furnace.
5 In the method according to the invention it is used advantageously a
smelting
furnace which is provided with a preheating equipment so that the feed going
into the smelting furnace is conducted through the preheating equipment into
the smelting furnace. According to the invention the pretreated nickel-
containing
raw material is conducted also into the preheating equipment wherein the
nickel-containing will come at the latest into contact with other material to
be fed
into the smelting furnace. In the smelting furnace the nickel-containing
together
with chromite pellets are smelted to ferrochromenickel having a desired
composition, which ferrochromenickel can be utilized in accordance with its
composition advantageously for instance in the production of austenitic or
duplex stainless steels.
When according to the invention smelting of the nickel-containing raw material
is carried out advantageously in a closed submerged arc furnace, carbon
monoxide gases generated in the reduction and smelting can be utilized in one
hand for instance in the sintering of chromite pellets and in possible other
pretreatment and preheating, in another hand for instance in different steps
of
the production path of stainless steel produced from the smelting product,
ferrochromenickel.
The method according to the invention is described in more details by means of
the appended example.
EXAMPLE
From a chromite concentrate containing iron and chromium and an
intermediate product containing nickel it was formed a mixture, into which
mixture it was added as a binder 1,2 weight % bentonite and 3 weight % slag
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forming material, flux, either limestone or wollastonite. In the table 1 it is
presented the contents of chromium, iron, nickel, carbon and sulphur as weight
% in mixtures, into which was added 10 weight % (Test 1) and 20 weight %
(Test 2) nickel hydroxide. Further, in the table 1 it has as a reference
material
(REF) a mixture, into which mixture nickel hydroxide was not added.
Cr weight % Fe weight % Ni weight % C weight % S weight %
REF 28,3 18,3 0,3 0,12 0,06
Test 1 26,5 16,8 5,3 0,10 0,03
Test 2 24,4 15,1 10,1 0,10 0,03
Table 1
The mixtures containing a binder and representing each material of the table 1
were pelletized and sintered. A part of sintered pellets was fed
representatively
into a smelting furnace with a slag former and a reducing agent.
The materials according to the table 1 were smelted, and in the table 2 it is
presented the contents of chromium, iron, nickel, carbon and silicon in
smelting
products in question and further the recovery of the metal components,
chromium, iron and nickel, into the smelting product. The carbon content is
composed in accordance with the composition and the equilibrium of the metal
alloy. The feed batch has carbon so much that carbon is some enough also for
the reduction of silicon into the smelting product. The feed alloy has silicon
oxide in raw material and in production bulk supplies.
Contents (weight %) Recoveries
C r /0 F e % N i /0 C% S i % C r /0 F e
% N i /0
REF 53,5 33,4 0,36 8,1 2,4 88,9 90,3 -
Test 1 49,8 30,1 7,1 6,7 2,8 86,6 88,7 86,0
Test 2 46,2 26,9 13,3 6,1 4,2 91,5 90,1 88,6
Table 2
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For one part of sintered pellets it was made in the laboratory scale
thermogravimetric measurements in order to monitor the reduction degree of
the metal components, chromium, iron and nickel, of pellets in the conditions
representing the smelting process at different temperature zones with the
maximum temperature of 1550 C. In the table 3 it is presented the results of
the thermogravimetric measurements for the reduction degree of chromium
(Crmet/Crtot), iron (Femet/Fetot) and nickel (Nimet/Nitot) at the temperatures
of 1400
C and 1550 C.
(Crmet/Crtot) /0 (Femet/Fetot) ok (Nimet/Nitot)
O/0
REF (1400 C) 1,1 16,8 -
REF (1550 C) 6,1 47,2 -
Test 1 (1400 C) 2,6 37,4 67,3
Test 1 (1550 C) 15,4 70,6 78,9
Test 2 (1400 C) 5,2 56,7 79,1
Test 2 (1550 C) 57,4 94,3 99,1
Table 3
The addition of the nickel-containing raw material into pellets increases the
reduction degree of chromium and iron at the temperature of 1550 C
substantially, chromium more than 15 % and iron more than 70 %
simultaneously when the reduction degree of nickel increases near to 100 %
with the Test 2 nickel content. The increase of the reduction degree for all
metal
components, chromium, iron and nickel in sintered pellets by means of the
addition of a nickel-containing raw material simultaneously decreases the need
of coke used as reducing agent in the achievement of the reduction conditions
of the smelting process.
