CA1281907C - Metallurgical composites and processes - Google Patents

Abstract

A B S T R A C T

A process for production of metallurgical composites comprises the steps: (a) subjecting brown coal to shearing forces to produce a 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 metallurgical composite. Step (c) is preferably effected by extrusion. The invention also provides a smelting process in which the composite is heated to a temperature at which the ore or concentrate is reduced to metal.

Description

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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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' , - 3 - ~ '7 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 .::,. 0~
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~81907 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 5 ~ L9~:)7 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 OE

6~ 7 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 OE

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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:-~ OE

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'7 (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 :'; ' . , ' ~ ' L9~7 - 8a -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 . .
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 - 1 o ~ )7 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:-OE

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 OE

- 12 ~ 7 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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Claims (10)

1. 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 metallurgical composite.

2. Process according to claim 1 in which the compacting step (c) is effected by extruding the said mixture.

3. Process according to claim 1 in which the drying step (d) is effected at or near ambient temperature.

4. Process according to claim 1 in which the ore is iron ore or chromite ore.

5. Process according to claim 1 in which the ore or concentrate is a zinc ore or concentrate.

6. Process for production of metallurgical composites containing iron ore and upgraded brown coal which comprises the following steps:
(a) subjecting brown coal to shearing forces to produce a plastic mass;
(b) admixing finely divided iron ore with the coal either during or after step (a);
(c) extruding the mixture produced in step (b) to produce a compacted extrudate in the form of pellets; and (d) drying the pellets at ambient temperature.

7. Metallurgical composites comprising upgraded brown coal and comminuted metal ore and/or concentrate, produced by the process of any one of claims 1 to 3.

8. An iron smelting process characterised by heating composites produced by the process of claim 6 to a tempera-ture at which the iron ore is reduced to metallic iron.

9. An iron smelting process characterised by heating composites produced by the process of claim 6 in a bath of liquid iron.

10. An iron smelting process characterised by subjecting composites produced by the process of claim 6 to a preliminary pyrolysis up to a temperature of about 700°C, followed by immersion in a bath of liquid iron at a temperature of about 1500°C.