Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Metallurgical composites and processes
This invention relates to metallurgical composites and
p~-ocesses utilizing those composites.
According to one aspect of the invention there is
provided a process for production of metallurgical composites
which comprises the following steps: (a) subjecting brown
coal to shearing forces to produce a wet, compactible plastic
mass; ~b) admixing finely divided ore and/or concentrate with
the coal either during or after step (a); (c) compacting the
mixture produced in step (b) to produce a compacted mass; and
~d) drying the compacted mass to produce the metalluryical
composite.
Another aspect to the invention relates to the
metallurgical composites produced by the above process.
Processes for treatment of the composites to reduce the
metallic oxides therein, including smelting processes, are
also contemplated by the invention.
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The upgraded brown coal utilized in the present
invention is preferably a product of the invention described
in our copending Canadian patent application Serial No. 447,360
filed on February 14, 1984 and/or Canadian patent application
Serial No. 500,721 filed on January 30, 1986.
The brown coal upgrading/densiication process
described in the abovementioned copending patent applications
is a procedure which converts soft friable raw bro~n coal with
an as-mined water content of about 60% to a hard,
attrition-resistant, black solid of a water content of about
10~. In the procedure the brown coal, with as-mined water
content, is subjected to shearing/attritioning in a selected
kneading device for periods which may vary from five minutes
or less to an hour or more depending on the hardness required
in the fin~l densified product.
Shearing peFforms several functions which are of
importance in the present context. The coal is converted to
fine particulate form, part at least of the water content,
originally finely dispersed in the porous structure of the
20 ` coal, is converted to a bulk liquid phase which causes the
coal to become wet and plastic and finally very large numbers
and areas of freshly cleaved coal surfaces are produced.
These freshly cleaved surfaces participate in inter-particle
bridge bonding processes which ultimately cause the coal mass
to harden and become much denser with simultaneous exclusion
and loss of most of the original water. Density increases
from about 0.8 to 1.4 are not uncommon. Water loss occurs
rapidly (for example ~0% in 24 hours in still air at 20C) and
maximum hardness is attained within three to four days. After
attritioning the now plastic coal is subjected to compaction
under appreciable pressure through suitable extrusion or high
pressure briquetting devices, e.g., a ringroll press. In a
particular example the compacting device is in the form of a
screw operated piston-in-barrel machine which produces either
3 or 10 mm diameter cylindrical specimens which may be cut to
any desired length. Application of pressure durinq extrusion
is believed to be significant in forcing the freshly cleaved
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surfaces of coal particles into close proximity thus
facilitating bridge bonding and so greatly enhancing the rate
at which bonding occurs. The use of higher pressures during
extrusion permits coal attritioning times to be greatly
5 reduced. Times as short as five minutes or less become
practicable particularly if an efficient attritioning machine
is used.
The least time required for shearing-attritioning of
raw brown coal in the densification process is that which is
lO sufficient to generate perceptible moistness and plastic
character in the mass of the coal. In practice the required
condition is verified by- visual observation based upon
experience. The period of time is a function of the rate of
operation ~f the attritioning machine, the intensity of the
15 shearing action achi~,ved by the machine and of the efficiency
of the machine in forcing the coal constantly in.to the
shearing zone.
In respect of very short shearing times the water
content of the coal can be critical; if too low, machine
20 efficiency decreases severely. Experience indicates that
brown coals with water contents of about 60% by weight display
optimum shearing-attritioning characteristics whereas water
contents in the vicinity of 54% (or less) are unsatisfactory.
Using a sigma kneading machine operating with
25 kneading shaft speeds of 40 and 20 r.p.m. and a rotor-wall
clearance of 0.3 mm, various brown coals of Victorian and
German origin have been successfully converted to extrudable
plastic states in periods of 30 seconds shearing attritioning.
~owever 30 seconds should not be regarded as the minimum time
30 covered by the present claim since the time will be governed
to a significant degree by the effectiveness of the available
machine. Any period sufficient to convert the raw brown coal
to an extrudable plastic state will be appropriate.
It should be noted that in practice short
35 shearing-attritioning times giving limited size reduction of
the coal particles may be compensated to some extent by the
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subsequent use of high extrusion pressures. In fact a
relatively dry plastic mass lea~s to the development ~f high
pressures in the nozzle region of the extruder.
A further preferred embodiment of the present
5 invention provides a continuous shearing-extrusion process.
The very short attritioning times permit continuous operation
in which brown coal in small lumps (5mm or less) is fed
continuously to a low-speed (20-40 revolutions per minute)
sigma-type shearing-attritioning machine. The configuration
10 of this machine is designed to give a residence time for the
coal in the shearing zone of the required order (as defined
above) before being extracted by a suitably located discharge
screw. The discharge screw feeds the moist attritioned coal
to an extr~sion head designed to give the required extrusion
15 pressure and provide pellets sufficiently firm to withstand
reasonable loads immediately after formation.
A machine which performs the functions described
above and has a discharge screw and extruder fitted integrally
is Sigma Knetmachine HKS 50 manufactured by Janke & Kunkel
20 GmbH & Co. KH IKA-Werk Beingen.
Although we do not wish to be limited by any
postulated or hypothetical mechanism for the observed
beneficial effects, we believe that densification will begin
to proceed at an appreciable rate as soon as sufficient
25 cleaved/sheared coal surfaces are available. This leads to a
further improvement providing a continuous process in which
t~e coal has a residence time in the attritioning (shearing)
zone just sufficiently to produce material capable of being
effectively extruded in a high pressure extrusion or pressing
30 device.
Investigation of the properties of dried densified
brown coal pellets produced in this manner has shown that they
retain their form and often become much harder on progressive
heating to higher temperatures. At between 300 and ~00C
35 volatiles in the form of water vapour and low molecular weight
organic substances (principally phenols) are evolved. Above
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about 500C perrnanent gases only (principally hydrogen,
carbon monoxide, carbon dioxide and methane~ are produced.
Our investigation of the densified brown coal has indicated
its potential usefulness in certain metallurgical
S applications, e.g. composite pellets.
Although we do not wish to be limited by any
postulated or theoretical mechanism for the observed
beneficial effects of the present invention, we believe that
the following considerations are significant:-
~a) Attritioning the raw coal to produce the
aforesaid damp or wet plastic mass provides a
suitable vehicle for effective incorporation of
finely divided particulate materials, such as
comm n,uted metal ore or concentrates,
(b) The Eine state of subdivision of the
attritioned coal is conducive to a very close
physical association of particles of metal ore
with particles of plasticised coal, the latter
acting as a powerful reductant,
(c) Spontaneous evaporative water loss occurs from
the pellets during the densification reaction
so producing hardened, dry pellets which are
particularly suitable for relatively rapid
heating for metallurgical purposes,
(d) On heating above about 500C the densified
brown coal evolves substantial quantities of a
gas mixture which is of a strongly reducing
character,
(e) After pyrolysis or low temperature
carbonisation the pellets provide a residual
carbon which is in a highly reactive form which
- is very closely associated with the phases to
be reduced. In this context it should be noted
that brown coal chars are known to be effective
and rapid metallurgical reductants. In
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addition to the reactive carbon in the
densified brown coal, hydrogen, and
particularly the nascent form o~ hydrogen
present, enormously enhance reduction
reactions.
We have established by extensive experimental
investigation that finely divided ores and concentrates,
particularly oxidic iron ores, mix readily with the wet
plastic coal and, when added during attrition of the latter, a
10 smooth homogeneous mix results. Such mix is readily extruded
or briquetted and the pellets or briquettes so produced dry
and harden to a surprising extent. In some instances the
hardened p~oduct shows rather reduced air-dried strength but
this is frequently reçovered on pyrolysis. In other cases
15 there is an apparent reaction between the inorganic phase and
coal constituents resulting in significant increases in the
strength of the dried product.
The metallurgical behaviour of various composites
will be described in the examples given later.
In the course of our work it has become apparent
that very rapid rates oE reduction can be effected in the
brown coal composite pellets. As stated it seems likely that
a substantial contribution to the reducing power of the system
is provided by freshly evolving atomic or nascent hydrogen
25 generated during preliminary heating of the composites.
Polyhydroxy phenols are likely to be major contributors of
pyrolytic hydrogen but other reacti~ve species may also be
involved. Evolution of atomic hydrogen in close proximity to
the phase to be reduced has the potential for extremely fast
30 and efficient reduction of the solid ore particles.
In summary, composites according to this invention
oEfer significant advantages in that they have the capacity to
provide:-
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(a) Effective bonding - in the cold - of fine
particulate ores or concentrates,
(b) Sufficient strength in the green composite
pellets or ~riquettes to enable satisfactory
handling for drying and subsequent feeding
to pre-heating or 'pyrolysis' furnaces,
(c) Fast and efficient reduction of oxide ores,
particularly iron oxide ores, but including
others, such as, e.g. chromite ores,
(d) An ideal means for conveying simultaneously
both partially or substantially metallized
pellets or briquettes together with carbon
to smelting furnaces, particularly to those
using recent new bath smelting technologies,
(e) Reduced/metallized pellets or briquettes
which can be easily handled transported and
stored without the risk of re-oxidation or
of displaying pyrophoric behaviour as is
experienced with various types of pre-
reduced iron ore composites now available.
Useful composites containing certain base metal
ore and concentrates, e.g. zinc concentrates may also be
produced.
In the Examples that follow reference is made to
~5 the drawings in which:
Figure 1 is a graph showing the approximate
relative partial pressures of the first four products
evolved in the process of Example 1 at the three
temperatures; and
Figure 2 is a graph showing the results of the
experiment conducted in Example 2.
Example 1
In this preliminary experiment densified brown
coal - iron ore composite pellets were prepared as
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described below and then heated to determine the type and
amounts of gases evolved.
Dried densified brown coal - iron ore cornposite
pellets containing 75~ iron oxide were prepared using the
procedure described in Example 2. Loy Yang coal from the
Latrobe Valley, in Victoria, Australia deposits was used.
After preliminary pyrolysis in a nitrogen atmosphere at 400C
to remove water and low molecular weight organic volatiles,
the pellets were placed in a silica tube attached to a
vacuum system~
When preliminary pumping had removed all of the
air, the pellets were raised progressively in temperature
to 900C. Samples of the gases evolved at three different
temperatures were removed for analysis on a mass spectro-
meter. The principal gases were found to be hydrogen,
carbon monoxide, carbon dioxide, methane and a small amount
of water vapour. ~he approximate relative partial pressures
of tlle ~irst four products at the three temperatures are
shown in Figure 1.
At 600C hydrogen was the most abundant
constituent ~ollowed by carbon monoxide and carbon dioxide
(approximately equal) with methane the least abundant. As
the temperature was raised to 900~ hydrogen evolution
became even more dominant while that of carbon monoxide
also increased. Carbon dioxide diminished markedly and
methane to a lesser extent.
~t is evident from this experiment that the
densified pellets produce a strongly reducing atmosphere
on heating to high temperatures. ~his atmosphere exerts a
strong reducing effect additional to any direct reduction
by the solid reactive carbon or the nascent hydrogen
within the composite pellets or briquettes.
Example 2
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Composite pellets were made with various
proportions of fine iron oxide and coal from Morwell,
Victoria, Australia (N3372 bore hole).
In each case 200 g of raw coal t60% water) was
kneaded for 4 hours in a sigma-type kneader as described
in our pending application Serial No. 447,360 mentioned
earlier. Fifteen minutes before terminating kneading,
selected weights of fine iron oxide (laboratory reagent
material) were added to the plastic mass and kneading
then continued for long enough to give a thoroughly
mixed smooth plastic mass. This was then extruded with
a hand operated screw extruder to provide cylindrical
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pellets initially of 10 mm diameter (about 8 mm after drying)
and varying from 10 - 20 mm in length. The pel]ets were
permitted to dry and harden in still laboratory air at 20C
for 7 days. The dried pe]lets were next subjected to
5 pyrolysis in a stream of nitrogen gas, initially being
maintained for one hour in the temperature range 300 - 400C
to elimina~e residual water and low molecular weight organic
volatiles, followed by further heating for one hour with an
increase of temperature to 700C. This latter period of
10 heating was designed to determine whether detectable reduction
had commenced in the temperature range concerned. In one
instance (see below) pellets were heated to 1070C, on this
occasion in the reducing atmosphere generated by the coal
pyrolysis~'
Pellets w~ e made with 10, 30, 50 and 75% by weight
(based on dry coal weight) of iron oxide. The 10~ composites
gave an average crush strength of 17 MPa compared with 30 MPa
for comparable pellets containing no iron oxide; on pyrolysis
the 10~ ferric oxide pellets displayed an increase to an
20 average crush strength of 20 MPa indicating the development of
further bonding during pyrolysis.
The compressive/crush strengths of the dried
densified coal pellets were determined following measurement
of the height (H) and diameter (D) of pellets to be tested
25 with a micrometer.
The pellets were then placed on the anvil of a
universal testing machine (Tirius Olsen Testing Machine Co.,
Willor Grove, Pa.), and an axial load was applied across the
plane ends until failure occurred.
The compressive strength ~c was calculated from the
Force F (determined from the maximum load the pellet
withstands) according to the following formula:-
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a = (4F/~D )(H/D) -
All of the composites were strongly magnetic
(particularly the 75:25 ore: densified coal blend) after
pyrolysis to 700C, indicating the production of reduced iron.
In one experiment pellets containing 75% Fe2O3 were
placed in a silica tube attached to a vacuum system. The tube
was pumped free of all gases whilst being heated to 500C.
The tube was then isolated from the pumps and the pressure
change observed as the temperature was further increased at an
10 approximately steady rate to 1070C. The results of these
measurements are shown in Figure 2. At about 900C a very
rapid pressure rise commenced and it became necessary to pump
gas away t~ maintain the total pressure below one atmosphere.
Substantial gas evol~tion continued until the experiment was
15 terminated. The phenomena described in this experiment are
characteristic of pellets containing iron o~ide, and are
indicative of chemical reactions between the oxide and species
derived from the coal.
At 800C the principal reaction appears to be
20 reduction of the iron oxide by evolved hydrogen with
production of water. This reaction appears to be supplemented
at about 900C by reduction reactions involving carbon
monoxide and carbon with a net substantial increase in total
gas pressure. At the end of this experiment the pellets while
25 being strongly ferro-magnetic, did not display visible
metallic iron. When the temperature was raised still further
using pellets as electrodes in a DC arc in an inert atmosphere
globules of malleable iron were produced very rapidly.
Composite pellets containing 75~ Fe2O3 after
30 preliminary pyrolysis to 700C as described above, were
further tested by immersion in a bath of liquid iron
maintained`at 1500C. Gas evolution commenced immediately on
immersion and continued throughout the dipping period of 30
seconds. The pellets did not disintegrate but continued to
35 evolve gas whilst dissolving rapidly in the liquid iron. Rate
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of dissolution was greatest on that side of the pellets which
has sustained the highest temperature by contact with the
furnace wall during preliminary pyrolysisi presumably more
reduced iron was present in that zone of the pellet thus
5 enhancing the rate of attack by the liquid iron. This
experiment demonstrates that the composite iron pellets in a
pre-reduced state may be used as feed material to supply both
iron and carbon to steelmaking furnaces by a new bath smelting
technology.
It will be clearly understood that the invention in
its general aspects is not limited to the specific details
referred to hereinabove.
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