Note: Descriptions are shown in the official language in which they were submitted.
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IRON AND MOLYBDENUM CONTAINING COMPACTS
TECHNICAL FIELD
The present invention relates to a process for producing iron and molybdenum
containing
compacts. It also relates to and compacts produced by the process.
BACKGROUND
Ferromolybdenum is an iron molybdenum alloy normally having a molybdenum
content of 60-
80 % by weight.
In most commercial applications ferromolybdenum is produced from molybdenum
trioxide
(Mo03) by a carbothermic reduction, an aluminothermic reduction, or a
silicothermic reduction.
The carbothermic process produces a high carbon ferromolybdenum, while the
latter two
produces a low carbon ferromolybdenum. Low carbon ferromolybdenum is more
common than
the high carbon alloy. Lumps of ferromolybdenum produced by these methods
normally have
densities around 9 gicm3. Dissolving the lumps in the steel melt can be
difficult due to the high
melting point of the lumps, for instance the commercial grade FeMo70 has a
melting point of
1950 C, and since the temperature of the steel melt is considerably lower,
dissolution of the
ferromolybdenum is mainly affected by diffusion processes, which prolong the
dissolution time
of the ferromolybdenum. Another factor is the high cost of raw materials in
the aluminothermic
reduction and silicothermic reductions. Furthermore, around 2 % of the Mo can
be lost in the
slag in these processes.
OBJECTS OF THE INVENTION
It is an object of the invention to provide a novel iron and molybdenum
containing material
suitable for molybdenum addition in melting industries e.g. steel, foundry and
superalloy
industry, and a process for producing such material in a comparably cost
efficient manner.
A further object is to provide a novel iron and molybdenum containing material
that has a
comparably quick dissolving time in a steel melt, and a process for producing
such material in a
comparably cost efficient manner.
A further object is to provide a novel iron and molybdenum containing material
low in carbon
and high in Mo, and a process for producing such material in a comparably cost
efficient
manner.
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A further object is to provide a material that can be easily handled when
added to the melt, and a
process for producing such material in a comparably cost efficient manner.
SUMMARY OF THE INVENTION
At least one of the above mentioned objects is at least to some extent
achieved by a process for
producing an iron and molybdenum containing compacts including the steps of:
a) mixing:
an iron containing powder,
a molybdenum oxide powder,
a carbonaceous powder,
a liquid, preferably water,
optionally a binder, and/or a lubricant and/or a slag former;
b) compacting to provide a plurality of green compacts.
Preferably, the green compacts have a geometric density in the range of 1.0-
4.0 g/cm3. The non-
reduced green compacts may be used as a substitute for traditionally
manufactured
ferromolybdenum alloys or even as a substitute for molybdenum oxide, when
alloying the melt
in industrial production. The iron-and/or molybdenum containing green compacts
can be
produced at lower costs than standard grades of ferromolybdenum. Their porous
structure
facilitates quick dissolving time in a steel melt.
In the present application the term "green" is used for raw or non-reduced
compacts. In the
present applications, the term compacts includes briquettes, filter cakes,
compacted sheets, and
other shapes of compacted agglomerates.
Dry matter composition refers to the composition for a dried specimen, i.e.
excluding any
moisture present in the green compacts. The moisture content is defined as
water present in the
green compacts apart from water of crystallization. The moisture content can
be determined by
a LOD (loss on drying) analysis in accordance to ASTM D2216 ¨ 10.
In some embodiments drying the green agglomerates to reduce the moisture
content to less than
10 % by weight. The moisture content is defined as water present in the green
pellets apart from
water of crystallization. The moisture content can be determined by a LOD
(loss on drying)
analysis in accordance to ASTM D2216 ¨ 10. By drying the green compacts to a
moisture
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content less than 10% by weight, the risk of cracking due to quick
vaporisation of the liquid,
when heated at high temperatures, is minimised. Preferably the green
agglomerates are dried to
have a moisture content less than 5 % by weight, more preferably less than 3 %
by weight.
Green compacts as defined by the pending claims may be produced by the
suggested method.
Reduced compacts as defined by the pending claims may be produced by the
suggested method.
The compacts can substitute for traditionally manufactured ferromolybdenum
alloys, when
alloying with molybdenum in melting practices. The iron- and/or molybdenum
containing
compacts can be produced at lower costs than standard grades of
ferromolybdenum. The iron
and molybdenum containing compacts dissolve quicker than standard grades of
ferromolybdenum. Depending on the reduction time, the relative amount of
carbon in relation to
the amount of reducible oxides, and the reduction temperature ¨ the oxygen
content in the
compacts can be partially or fully reduced. The compacts can be easily
transported on a
conveying belt without the risk of rolling off.
BRIEF DESCRIPTIONS OF THE DRAWINGS
Fig. 1 is a schematic overview of the process of producing iron and molybdenum
containing
briquettes according to the invention.
DESCRIPTION OF THE INVENTION
The invention will now be described in more detail and with reference to the
figures. The
invention is described in relation to the production of briquettes. However,
other kinds of
compacts can be produced by the process by substituting the briquetting
machine to a machine
that can compact powders in other forms such as filter cakes or sheets.
Fig. 1 is a schematic overview of the process of producing iron and molybdenum
containing
briquettes according to the invention. In the mixing station 30, a powder
mixture is prepared by
mixing an iron containing powder, a carbonaceous powder, a molybdenum oxide
powder, and
water. The mixing in the mixing station 30 can be executed batchwise or
continuously.
Before being added to the mixing station 30, the molybdenum oxide powder may
be milled in
the rod mill 10. Of course other mills, grinders, or crushers may be used to
disintegrate the
molybdenum oxide into smaller particles. Furthermore, the iron containing
powder and/or the
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carbonaceous powder may also be disintegrated into smaller particles by
grinding and/or milling
and/or crushing.
The ground and/or milled and/or crushed molybdenum oxide particles may be
sieved in a sieve
20 to provide a desired particle distribution. Naturally, sieving can also be
applied to the iron
containing powder and/or the carbonaceous powder.
In one embodiment the molybdenum oxide powder and the carbonaceous powder are
mixed and
ground together and thereafter the iron containing powder is added and mixed
with the
molybdenum oxide powder and the carbonaceous powder. However, any combination
of mixing
order may be executed.
The molybdenum oxide powder, iron containing powder, and the carbonaceous
powder are each
described under separate headline below. The amount of added powders are
described under the
headline Iron and molybdenum containing green compacts.
Optionally, lubricants and/or binders and/or slag formers can be added when
mixing. The
optional binders may be organic or inorganic binders. The binders may e.g. be
a carbon
containing binders partially replacing the carbonaceous powder. Other binders
may e.g. be
bentonite and/or dextrin and/or sodium silicate and/or lime. Gelatin may also
be used. The
optional slag former may be limestone, dolomite, and/or olivine. The total
amount of optional
lubricants and/or binders and/or optional slag formers can be 0.1-10 % by
weight of the dry
matter content of the mixture, more preferably less than 5 wt%. It may be in
the range of 1-10 %
by weight. The binders are optional since the green briquettes by the water
and iron addition
becomes sufficiently strong to be reduced in the reduction furnace without
severely cracking. If
added the lubricant is preferably added in amounts of 0.1-2 % of the the dry
matter content of
the mixture, e.g. about 0.5-1 % by weight. The lubricant can e.g. be zinc
stearate. However,
other lubricants that are used in powder metallurgy may be added. Preferably
neither binder, nor
lubricant nor slag former are used. The iron containing powder when mixed in
wet condition
strengthens the briquettes, making the use of a binder unnecessary. Thereby
the amount of
impurities can be reduced.
Liquid, preferably water, is preferably added in amounts of 1-10 % by weight
of the dry matter
content of the mixture, preferably 2-7 % by weight. In some embodiments 2-5 %
by weight.
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From the mixing station 30 the prepared powder mixture is transferred to a
briquetting machine
40. In briquetting machine 40 the powder mixture is briquetted to provide a
plurality of green
briquettes.
Preferably the briquetting machine 40 is a roller press. However, other kinds
of briquetting
machines 40 can be used including but not limited to: mechanical piston
presses, hydraulic
presses, screw presses, briquette extruders. Furthermore the briquetting
machine 4 may be
substituted for other machines capable of compacting the mixture. For instance
but not limited
to; filter cakes may be produced in a filter press, flakes or sheets may be
produced between two
counter rotating rollers.
In one embodiment the powder mixture is compacted at a comparably low
pressure. The lower
limit of the compacting pressure may be as low as 20 kg/cm2, but is typically
at least 50 kg/cm2.
Preferably the compacting pressure is in the range of 80-1000 kg/cm2, more
preferably 100-500
kg/cm2. The low compacting pressure has been found out to improve the quality
of the produced
green compacts.
In one embodiment a briquetting machines operates at higher pressures, e.g.
1000-10000
kg/cm2. Higher pressure can be used to increase the geometric density of green
briquettes.
The green briquettes produced from the powder mixture are preferably reduced
in a reduction
furnace 60. Alternatively the non-reduced green briquettes can be used as
alloying additive in
iron and steel making.
Optionally the green briquettes are dried in a dryer 50 before being
transferred to the reduction
furnace 60. Many different kinds of industrial dryers can be used. The
briquettes may also be
dried without active heating, e.g. in ambient air temperature. In a dryer
vapour may be removed
by a gas steam or by vacuum. The green briquettes can be dried until desired
moisture content
has been reached. The green briquettes may be dried to moisture content less
than 10 % by
weight, more preferably less than 5 % by weight, most preferably less than 3 %
by weight. The
green briquettes may be dried at a temperature in the range of 50-250 C, more
preferably 80-
200 C, most preferably 100-150 C. For improved process economy, drying time
is preferably
in the range of 10- 120 minutes, more preferably 20-60 minutes. But longer
drying times are of
course viable. The moisture content is defined as water present in the green
briquettes apart
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from water of crystallization. The moisture content can be determined by a LOD
(loss on
drying) analysis in accordance to ASTM D2216 ¨ 10.
The green briquettes are preferably reduced in a reduction furnace 60. The
reduction furnace is
preferably a continuous furnace but may also be a batch furnace. The
continuous furnace 6
having an inlet 7 and outlet 8, and the briquettes are conveyed during
reduction from the inlet 7
to the outlet 8. In a preferred embodiment a belt furnace is used. Of course
other furnace types
may be used, for instance a walking beam furnace.
The green briquettes are reduced at a temperature in the range of 800-1500 C,
preferably 800-
1350 C. In some embodiments 1000-1200 C. The reduction time at least 10
minutes,
preferably reducing during at least 20 minutes. In some embodiments during at
least 30 minutes.
By monitoring the formation of CO/CO2 it can be determined when the reduction
process is
finished. Preferably the reduction time is at most 10 hours, preferably at
most 2 hours, more
preferably at most 1 hour. Depending on the reduction time, the reduction
temperature, and the
relation between carbon and reducible oxides in the briquettes; the reducible
oxides of the
briquettes can be partially or fully reduced.
Optionally the green briquettes are heat treated at a lower temperature before
reduction. The
green briquettes may be heat treated at a temperature in the range of 200-800
C, more
preferably 400-700 C. Preferably, the optional heat treating at lower
temperature is performed
from 10 minutes to less than 2 hours, preferably less than 1 hour. By heat-
treating at lower
temperatures the optional lubricant can be burned off in a controlled manner.
In addition
molybdenum trioxide may be reduced to molybdenum dioxide. This may be employed
as a pre-
reduction step prior to the reduction described in the previous paragraph or
when producing
partially reduced briquettes. The optional heat treating at 200-800 C, can be
performed in the
same furnace as the reduction. The optional heat treating and optional drying
may also be
combined.
Unexpectedly it has been found that briquettes can be reduced at high
temperatures without
noticeable sublimation losses of Mo03. Accordingly the claimed process results
in a simplified
process resulting in improved yield and higher Mo content in the end product.
I.e. there is no
need to perform a pre-reduction in regards to sublimation losses of Mo03.
During the reduction CO and CO2 can form from reactions with the carbon source
and the
reducible oxides in the briquettes. Additionally remaining moisture may
vaporise. The reduction
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time can be optimised by measuring the formation of CO and CO2; in particular
CO since CO2
is mainly formed during the first minutes of reduction where after CO
formation is dominating
until the carbon source is consumed or all reducible oxides have been reduced.
The reduction reactions are endothermic and require heat. Preferably heat is
generated by
heating means not affecting the atmosphere within the furnace, more preferably
the heat is
generated by electrical heating.
The atmosphere within the furnace 60 is preferably controlled by supplying an
inert or a
reducing gas, preferably a weakly reducing gas, at one end of the furnace and
evacuating gases
(e.g. reaction gases (e.g. CO, CO2, and H20) and the supplied gas) at the
opposite end, more
preferably, supplying the inert or reducing gas counter current at an outlet
side 80 of the furnace
60, and evacuating gases at an inlet side 70 of the furnace 60. I.e. the inert
or reducing gas is
preferably supplied counter flow. The gas supplied may include argon, N2, H2,
CO, CO2 or any
mixture of them. For instance H2/N2 having relations such as 5:95, 20:80,
40:60, 80:20, and
95:5 by vol. . In one embodiment the atmosphere comprises 20-60 vol % of H2
and balance N2.
Such atmosphere may reduce N2 uptake, compared to e.g. H2/N2 (5:95), and it
may increase the
density of the reduced pellets. The atmosphere may also be supplied with CO,
e.g. from burning
natural gas. Of course, other gas mixes being inert or reducing may be
supplied to the furnace.
Preferably the furnace operates at pressure in the range of 0.1-5 atm,
preferably 0.8-2 atm, more
preferably at a pressure in the range of 1.0-1.5 atm, most preferably 1.05-1.2
atm.
At the outlet 80 of the reduction furnace the briquettes are transferred to a
cooling section 90,
for cooling the briquettes in a non-oxidising atmosphere (e.g. reducing or
inert) to a
temperature below 200 C to avoid re-oxidation of the briquettes, more
preferably below 150 C
in an inert atmosphere. The atmosphere may e.g. be argon, N2 H2, or any
mixture of H2/N2 (e.g.
5:95 by vol.). Other atmospheres may also be employed. If it is desirable to
have very low
levels of nitrogen in the briquettes, the briquettes may be cooled in a
nitrogen free atmosphere
such as for example an argon gas atmosphere.
Fig. 2 shows a method how to produce briquettes. In a mixing station 300 a
powder mixture is
prepared by mixing an iron containing powder, a carbonaceous powder, a
molybdenum oxide
powder, and water in a blender 300. A convening belt 110 conveys a tray 120 to
the mixing
station 300. In the mixing station 300 the tray 120 is filled with mixture
from the blender 310.
The tray 120 is thereafter conveyed to a briquetting station 40 and at the
same time another tray
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120 is conveyed to the mixing station 300 to be filled with mixture from the
blender 310. In the
briquetting station 400 the mixture on the tray is stamped by a meshed stamp
410 forming a set
of green briquettes. The pattern seen is indicated by reference number 420.
The tray 120 holding
the green briquettes thereafter continues to a reduction furnace 600, here
schematically shown
as a belt furnace. Of course other furnace types may be used, for instance
walking beam
furnaces. Optionally a drying station may be positioned between the
briquetting station and the
reduction furnace 600.
Molybdenum oxide powder
The molybdenum oxide powder is preferably a molybdenum trioxide powder. The
powder may
also be a molybdenum dioxide powder or a mix of molybdenum trioxide powder and
molybdenum dioxide powders.
The molybdenum powder should include 50-80 % of Mo, the remaining elements
being oxygen
and impurities. The purer the grade of molybdenum oxide is, the purer the iron
and molybdenum
containing compacts can be made. However, purer grades of Mo03 are on the
other hand more
expensive.
In a preferred embodiment technical grade Mo03is used. Such powders are less
costly than
purer grades of Mo03 and may contain oxides that are difficult to reduce in
solid state reduction
with carbon. Examples of such oxides are e.g. A1203, 5i02, and MgO.
Fortunately these oxides
can easily be removed to the slag phase when alloying in steel melts and they
can therefore be
allowed in the product.
Preferably at least 90% by weight of the particles of the molybdenum oxide
powder pass
through a test sieve having nominal aperture sizes of 300 [tm and at least 50
% by weight of the
particles of the molybdenum oxide powder pass through a test sieve having
nominal aperture
sizes of 125 [tm. More preferably at least 90% by weight of the particles of
the molybdenum
oxide powder pass through a test sieve having nominal aperture sizes of 125
[tm and at least 50
% by weight of the particles of the molybdenum oxide powder pass through a
test sieve having
nominal aperture sizes of 45 [tm. Nominal aperture sizes in the present
application are in
accordance with ISO 565:1990 and which hereby is incorporated by reference.
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In one embodiment at least 90 % by weight, more preferably at least 99 % by
weight, of the
particles of the molybdenum oxide powder pass through a test having nominal
aperture sizes of
250 [tm, more preferably 125 [tm, most preferably 45 [tm.
Iron containing powder
The iron containing powder is preferably an iron powder containing at least 80
wt% Fe,
preferably at least 90 wt% Fe, more preferably at least 95 wt% Fe, most
preferably at least 99
wt% Fe. The iron powder can be an iron sponge powder and/or a water atomised
iron powder
and/or a gas atomised iron powder and/or an iron filter dust and/or an iron
sludge powder. For
instance filter dust X-RFS40 from Hoganas AB, Sweden is a suitable powder.
The iron powder may partly or fully be replaced by an iron oxide powder, for
instance but not
limited to: powder consisting of one or more from the group of FeO, Fe203,
Fe304, Fe0(OH),
(Fe203*H20). The iron oxide powder may e.g. be mill scale.
In one embodiment the iron containing powder contains at least 50 % be weight
of metallic iron,
more preferably at least 80 wt% metallic Fe, most preferably at least 90 wt%
metallic Fe.
Preferably at least 90% by weight of the particles of the iron containing
powder pass through a
test sieve having nominal aperture sizes of 125 [tm and at least 50 % by
weight of the particles
of the iron containing powder pass through a test sieve having nominal
aperture sizes of 45 [tm.
In one embodiment at least 90 % by weight, more preferably at least 99 % by
weight, of the
particles of the iron containing powder pass through a test sieve having
nominal aperture sizes
of 125 [tm, more preferably 45 [tm. In one example at least 90 % by weight,
more preferably at
least 99 % by weight, of the particles of the iron containing powder pass
through a test sieve
having nominal aperture sizes of 20 [tm.
Carbonaceous powder
The carbonaceous powder is preferably chosen from the group of: sub-bituminous
coals,
bituminous coals, lignite, anthracite, graphite, coke, petroleum coke, and bio-
carbons such as
charcoal, or carbon containing powders processed from these resources. The
carbonaceous
powder may e.g. be soot, carbon black, activated carbon. The carbonaceous
powder can also be
a mixture of different carbonaceous powders.
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Regarding the choice of carbonaceous powder, the reactivity of the carbon may
be taken into
consideration, since the productivity as well as the yield of Mo depends on
this factor. A high
reactivity is desired. In particular, it is desirable to have a carbonaceous
powder that is reactive
at lower temperatures (preferably < 700 C). For instance German brown coal
(lignite) is
normally reactive at lower temperatures than petroleum coke, and is hence
suitable since it has
comparably high reactivity at low temperatures. Also charcoal, bituminous and
sub-bituminous
coals can exhibit comparably high reactivity. Particularly suitable examples
are soot, carbon
black, and activated carbon. Graphite may also be suitably due to its high
density.
The amount of carbonaceous powder is preferably determined by analysing the
amount of
oxides in the molybdenum oxide powder and optionally the iron containing
powder. Preferably
the amount of reducible oxides is determined. The oxygen content can e.g. be
analysed by a
LECOO TC400. Furthermore the maximum allowed carbon content in the compacts is
preferably also taken into consideration. Preferably the amount is chosen to
stoichiometric
match or slightly exceed the amount of reducible metal oxides in the
molybdenum oxide
powder and the iron containing powder. However, the amount of carbon may also
be sub-
stoichiometric.
The amount of carbonaceous powder can be optimised by measuring the carbon and
the oxygen
levels in the reduced compacts (e.g. by reducing green compacts in a lab
furnace and measuring
carbon and oxygen levels). Based on the measurements the amount of
carbonaceous powder can
be optimised to achieve desired levels of carbon and oxygen in the produced
compacts. Some
oxides, which may be present in the molybdenum oxide powder are difficult to
reduce with
carbon. All oxides with higher affinity to oxygen at the reduction max
temperature will remain
as oxides in the finished product and therefore do not consume carbon in the
reduction process.
Such oxides can for instance be oxides of Si, Ca, Al, and Mg and may e.g. be
present if cruder
grades of molybdenum trioxide are used, e.g. technical molybdenum trioxide.
However, in
many applications of steel metallurgy these oxides can be handled e.g. by
removing them in the
slag of steel melt and they can therefore be allowed in the compacts. If lower
amounts of these
oxides and elements are desired, purer grades of molybdenum trioxide can be
employed, e.g.
grades that contain less or no amounts of these oxides.
By controlling the amount of carbonaceous powder and matching it with the
amount of
reducible oxides in the green compacts; the iron and molybdenum containing
compacts can be
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made that has carbon content (after reduction) less than 10 % by weight,
preferably less than 5
wt%, more preferably less than 1 wt%, most preferably less than 0.5 wt%.
However it is also possible to provide compacts having deliberately high
carbon content after
reduction. Eg. 1-5 wt% C. Such compacts may be used when alloying high carbon
steel.
Preferably, at least 90 % by weight, more preferably at least 99 % by weight,
of the particles of
the carbonaceous powder pass through a test sieve having nominal aperture
sizes of 125 [tm,
and at least 50 % by weight of the particles of the carbonaceous powder pass
through a test
sieve having nominal aperture sizes of 45 [tm.
In one embodiment at least 90 % by weight, more preferably at least 99 % by
weight, of the
particles of the carbonaceous powder pass through a test sieve having nominal
aperture sizes of
45 [tm, and at least 50 % by weight of the particles of the carbonaceous
powder pass through a
test sieve having nominal aperture sizes of 20 [tm. In one example at least 90
% by weight,
more preferably at least 99 % by weight of the particles of the carbonaceous
powder pass
through a test sieve having nominal aperture sizes of 20 [tm.
Iron and molybdenum containing green compacts
The compacts may be briquettes, filter cakes, flakes or other compacted
agglomerates.
The iron and molybdenum containing green compacts may have a dry matter
composition in
weight-% of:
1-25 iron containing powder;
5-30 carbonaceous powder;
Optionally
0.1-10 lubricant and/or binder and/or slag former; and
balance 50-90 molybdenum oxide powder.
According to one embodiment the iron and molybdenum containing green compacts
having a
dry matter composition in weight-% of:
1-15, preferably 1-10 iron containing powder,
5-25, preferably 10-20 carbonaceous powder,
Optionally
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0.1-10 lubricant and/or binder and/or slag former; and
balance at least 50-90 molybdenum oxide powder.
In one embodiment the dry matter composition of the green compacts consists of
in weight-%:
1-15, preferably 1-10 iron containing powder,
5-25, preferably 10-20 carbonaceous powder,
balance 50-90 molybdenum oxide powder.
In regards to elements the iron and molybdenum containing green compacts
preferably have a
dry matter composition in weight % of: 1-25 Fe, 15-40 0, 5-25 C, less than 15
of other elements
besides 0, C, Mo and Fe, and balance being at least 30 Mo. Preferably the dry
matter
composition in weight % is: 1-15 Fe, 15-40 0, 5-25 C, less than 15 of other
elements besides
0, C, Mo and Fe, and balance being at least 30 Mo.
The elements may further be limited to:
- Iron is preferably within the range of 1.5-10 % by weight.
- Carbon is preferably 7-20 % by weight.
- Oxygen is preferably 15-30 % by weight.
- Molybdenum is preferably 40-65 % by weight.
- Other elements are preferably at least 1 % by weight and less than 10 %
by weight, more
preferably at least 2 % by weight and less than 7 % by weight. Other elements
are preferably
only present as impurities.
In subsequent reduction steps, the relative amount of iron and molybdenum will
increase in the
compacts as the reduction progresses. The same may of course true for the
other elements that
remain.
The green compacts can be cost efficient substitutes to Mo03powder or standard
FeMo when
alloying in melting practices, considering price and/or yield of the Mo
addition into melt.
Typically, such addition could be made e.g. into electrical arc furnace (EAF)
and e.g. be a Mo
addition into stainless steel, tool steel or high speed steel.
The green compacts may have a geometric density up to 5 g/cm3 or even up to 6
g/cm3.
Preferably the geometric density is in the range of 1.0-4.0 g/cm3. In other
embodiments the
geometric density may be in the range of 1.2-3.5 g/cm3, or 1.2- 3.0 g/cm3. The
geometric
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density may be less than 4 gicm3. Density can be increased by increasing
compacting pressure.
A lower geometric density results in higher porosity, which is believed to
yield a shorter
dissolution time of the compacts. The geometric (envelope) density can be
measured in
accordance to ASTM 962-08.
Reduced iron and molybdenum containing compacts
The compacts may be briquettes, filter cakes, flakes or other compacted
agglomerates.
The iron and molybdenum containing compacts may have a composition in weight %
of: 2-30
Fe, less than 30 0, less than 20 C, less than 15 of other elements besides 0,
C, Mo and Fe, and
balance being at least 40 Mo, preferably a least 50 Mo.
Suitably, the reduced iron and molybdenum containing compacts have a
composition in weight
% of: 1-20 Fe, less than 10 0, less than 10 C, less than 15 of other elements
besides 0, C, Mo
and Fe, and balance being at least 40 Mo, preferably a least 50 Mo.
Preferably the content of 0 is less than 10 % by weight, more preferably less
than 8 % by
weight, even more preferred less than 6 % by weight, most preferably less than
4 % by weight,
and preferably that only a minority of the oxygen content comes from
molybdenum oxide that
has not been reduced, i.e. a compact that contains Mo0x, where x < 0.5.
Preferably essentially
all of the molybdenum oxide is reduced to Mo, i.e. where x is around 0. Here,
remaining oxygen
content mainly comes from oxides in molybdenum oxide powder and the iron
containing
powder that are difficult to reduce, e.g. oxides of Si, Ca, Al, and Mg. Using
purer grades of the
molybdenum oxide powder, the iron containing powder, and the carbonaceous
powder, the
oxygen content of the compacts can, if desired, be made lower than 2% by
weight. However,
since many of these oxides that are difficult to reduce can be handled in the
steel melt
metallurgy (e.g. removing them in the slag phase), they may be allowed in the
iron and
molybdenum containing compact. The lower limit for oxygen may be about 0% by
weight, but
typically the oxygen is at least 1 % by weight, more typically at least 2 % by
weight.
The molybdenum content in the compacts can be controlled by varying the
relative proportions
of the molybdenum oxide powder in relation to the iron containing powder. For
essentially fully
reduced compacts (i.e. compacts containing MoOx where x < 0.5) the content of
molybdenum is
preferably controlled to be in the range of 60-95 % by weight. More preferably
the content of
Mo is in the range of 65-95 wt%, most preferably the content of Mo is in the
range of 70-95
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wt%. Surprisingly a very high dissolution rate has been found for reduced
compacts having a
molybdenum content of 80-95 % by weight. This result is due to the much higher
specific
surface and is in spite of the very high melting point of these alloys, 2100-
2500 C.
By balancing the carbon addition it is possible to control the carbon content
of the reduced
compacts to be less than 5 wt %, less than 2 wt. %, less than 1 wt. %, less
than 0.5 wt. %, or less
than 0.1 wt. %. Compacts low in carbon can e.g. be used when alloying low
carbon steels.
However, in some applications, for example in the production of high carbon
steels or cast iron,
it may desirable to have a carbon content in the range of 1-5 % by weight.
The iron content of the compacts is preferably within the range of 1-20 % by
weight, more
preferably 2-10 % by weight, most preferably 2-5 % by weight. The iron content
in the
compacts can be controlled by varying the relative proportions of the iron
containing powder in
relation to the molybdenum oxide powder.
The reduced compacts can be cost efficient substitutes to Mo03 powder or
standard FeMo,
when alloying in melting practices, considering price and/or yield of the Mo
addition into melt.
Typically such addition could be made e.g. into an electrical arc furnace
(EAF) and e.g. be a Mo
addition into stainless steel, tool steel or high speed steel.
Depending on the purity of the powder mixture, the compacts may contain
further elements
including oxides that are difficult to reduce. Other elements apart from Mo,
Fe, C and 0 may be
allowed up to less than 15 % by weight. Preferably the total amount of other
elements besides
0, C, Mo and Fe is less than 10 % by weight, more preferably less than 7 % by
weight. The
amount of other elements is mainly controlled by the purity of the molybdenum
trioxide, but
may also come from impurities in the iron containing powder, the carbonaceous
powder, and
from reactions with elements in the surrounding atmosphere during heating,
reduction, or
cooling. Using high purity grades of molybdenum trioxide, iron containing
powder and the
carbonaceous powder; the total amount of other elements besides 0, C, Mo and
Fe can, if
desired, be kept lower than 1 % by weight. If present in the compacts,
elements from the group
of Si, Ca, Al, and Mg are mainly bound as oxides. For instance, in a steel
melt, silicon bound as
silicon oxides may be easier to handle than silicon that is dissolved in the
lattice of the alloy.
The other elements may in some embodiments be limited to at least 1 % by
weight or to at least
2 % by weight. Other elements include impurities.
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Preferably, in some embodiments, the other elements in weight % are limited
to:
max 2 N, more preferably max 1 N;
max 1 S, more preferably max 0.5 S;
max 2 Al, more preferably max 1.5 Al;
max 2 Mg, more preferably max 1 Mg;
max 2 Na, more preferably max 1 Na;
max 4 Ca, more preferably max 2 Ca;
max 6 Si, more preferably max 3 Si;
max 1 K, more preferably max 0.5 K;
max 1 Cu, more preferably max 0.5 Cu;
max 1 Pb, more preferably max 0.1 Pb;
max 1 W, more preferably max 0.1 W;
max 1 V, more preferably max 0.1 V;
and remaining elements is preferably max 0.5 each, more preferably max 0.1
each, most
preferably max 0.05 each.
In some embodiment, the content in weight % of Si is in the range of 0.5-3,
the content of Ca is
in the range of 0.3-2, the content of Al is in the range 0.1 -1, and/or the
content of Mg is in the
range of 0.1- 1.
Preferably, if present, the elements of the group of Si, Ca, Al and Mg are to
at least to 50% by
weight bound as oxides in the compacts, preferably at least to 90 % by weight.
The nitrogen content mainly depends on the nitrogen level in the atmosphere
during reduction
and cooling of the compacts. By controlling the atmosphere in these steps the
nitrogen content
can be made lower than 0.5 wt%, preferably lower than 0.1 wt% and most
preferably lower than
0.05 wt%.
Reduced compacts may be produced with a geometric density up to 6 &in',
preferably less
than 4.5 &in'. It may be less than 4.0 &in'. For quick dissolving in steel
melt it is preferred
that the reduced compacts have a geometric density in the range of 1.0-4.0
&in'. Other possible
ranges includes 1.2-3.5 &in', 1.2-3.0 &in', 1.5-3.9 g/cm'and 2.0-4.0 &in'. The
given upper
and lower limits of the ranges may be combined with one another to form new
ranges. Density
can be controlled by varying the briquetting pressure for the green compacts.
A higher reduction
temperature may also increase density. By controlling process parameters it is
possible to
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produce reduced compacts having geometric density below 2.0 g/cm3 as well as
reduced
compacts having geometric density between 2.0-4.0 g/cm3, or even higher up to
6 g/cm3.
A lower density results in higher porosity, which is believed to yield a
shorter dissolution time
of the compacts. On the other hand a higher density increases the amount of Mo
for a given
volume. The geometric density measured in accordance with ASTM 962-08.
EXAMPLE
A mixture was prepared by mixing 180 g of a fine grained iron powder (< 40 pm,
>99 wt% Fe,
X-RSF40 from Hoganas AB) with 1000 g molybdenum oxide (Mo03 92.5 wt%, Si02 7.5
wt%,
<40 um) and 176 g graphite powder (< 40 um). 7 dl of Water was added to the
mixture. The
mixture was compacted in a briquetting machine using a compaction pressure of
75 kg/cm2.
The green briquettes were thereafter dried at room temperature to a moisture
of 0.5 wt%.
The green briquettes was visually examined and handled. No identification of
cracking was
observed. The green briquettes were reduced in a batch furnace at a
temperature of 1300 C for
a time period of 20 minutes, in a 95 vol-% N2 and 5 vol-% H2 atmosphere.
The reduced briquettes were thereafter allowed to cool to a temperature around
100 C before
evacuating the atmosphere and removal from the furnace. The reduced was
visually examined.
No identification of cracking was observed.
