GB2172586A - Metallurgical composites - Google Patents

Abstract

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

SPECIFICATION Metallurgical composites and processes This invention relates to metallurgical composites and processes utilising these composites.

We have described in Australian patent application 24294/84 (published 23rd August 1984) and 52590/86 (PG 9283) processes of upgrading brown coal involving shearing the coal and compacting it.

In the invention metallurgical composites are made by (a) subjecting brown coal to shearing forces to produce a plastic mass, (b) admixing finely divided metal compound with the coal either during or after step (a), compacting the mixture produced in step (b) to produce a compacted mass and (d) drying the compacted mass to produce the metallurgical composite.

The metal compound can be combined with the brown coal after the coal has been converted to the plastic mass or it can be admixed during the conversion of the coal to the plastic mass. The metal compound may be introduced in the desired finely divided state or it may be introduced in a coarser state, for instance before subjecting the brown coal to shearing forces, and may be comminuted to the desired finely divided state during the admixture with the coal.

The metal compound is preferably a compound that is reducable upon heating in a reducing environment and generally comprises a metal oxide.

The metal oxide is generally in the form of a metallic oxide ore or it may be a metallic oxide concentrate, for instance as obtained by concentrating an ore by conventional concentration procedures.

The invention includes processes in which a composite made by the described process wherein the metal compound is a reducable metal compound is heated to a temperature at which the metal compound is reduced. Thus the invention includes smelting processes.

The preferred metal compound is iron ore but others that may be used in the invention include chromite ore and zinc ore or concentrates thereof.

Step (a) of the process of the invention can be conducted in the same manner as the brown coal upgrading/densification process described in the above-mentioned Australian patent applications.

The process converts soft friable raw brown 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 final densified product.

Shearing performs 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 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 80% in 24 hours in still air at 20"C) 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-inbarrel machine which produces either 3 or 10 mm diameter cylindrical specimens which may be cut to any desired length. Application of pressure during extrusion is believed to be significant in forcing the freshly cleaved 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 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 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 of the attritioning machine, the intensity of the shearing action achieved by the machine and of the efficiency of the machine in forcing the coal constantiy into the shearing zone.

In respect of very short shearing times the water content of the coal can be critical; if too low, machine 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 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 successfuliy converted to extrudable plastic states in periods of 30 seconds shearing attritioning.

However 30 seconds should not be regarded as the minimum time 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 shearing-attritioning times giving limited size reduction of the coal particles may be compensated to some extent by the subsequent use of high extrusion pressures. In fact a relatively dry plastic mass leads to the development of high pressures in the nozzle region of the extruder.

A further preferred embodiment of the present 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 lowspeed (20-40 revolutions per minute) sigma4ype shearing-attritioning machine. The configuration 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 extrusion head designed to give the required extrusion 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 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 cleaved/sheared coal surfaces are available. This leads to a further improvement providing a continuous process in which the 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 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 400"C volatiles in the form of water vapour and low molecular weight organic substances (principally phenols) are evolved. Above about 500"C permanent gases only (principally hydrogen, carbon monoxide, carbon dioxide and methane) are produced. Our investigation of the densified brown coal has indicated its potential usefuiness in certain metallurgical 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 comminuted metal ore or concentrates, (b) The fine 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 powerfui reluctant, (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 500or 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 addition to the reactive carbon in the densified brown coal, hydrogen, and particularly the nascent form of hydrogen present, enormously enhance reduction reactions.

We have established by extensive experimental investigation that finely divided ores and concentrates, particularly dxidic iron ores, mix readily with the wet plastic coal and, when added during attrition of the latter, a 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 product shows rather reduced airdried strength but this is frequently recovered on pyrolysis. In other cases 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 of 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 generated during preliminary heating of the composites. Polyhdyroxy phenols are likely to be major contributors of pyrolytic hydrogen but other reactive species may also be involved. Evolution of atomic hydrogen in close proximity to the phase to be reduced has the potential for extremely fast and efficient reduction of the solid ore particles.

In summary, composites according to this invention offer significant advantages in that they have the capacity to provide: (a) Effective bonding - in the cold - of fine particulate ores or concentrates, (b) Sufficient strength in the green composite pellets or briquettes to enable satisfactory handling for drying and subsequent feeding to pre-heating or 'pyrolysis' furnaces, (c) Fast and efficient reduction of oxide ores, par ticuiarly 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.

Example 1 In this preliminary experiment densified brown coal - iron ore composite pellets were prepared as described below and then heated to determine the type and amounts of gases evolved.

Dried densified brown coal - iron ore composite 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 400"C 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 9000C. Samples of the gases evolved at three different temperatures were removed for analysis on a mass spectrometer. The principal gases were found to be hydrogen, carbon monoxide, carbon dioxide, methane and a small amount of water vapour. The approximate relative partial pressures of the first four products at the three temperatures are shown in Figure 1.

At 600"C hydrogen was the most abundant constituent followed by carbon monoxide and carbon dioxide (approximately equal) with methane the least abundant. As the temperature was raised to 900"C hydrogen evolution became even more dominant while that of carbon monoxide also increased. Carbon dioxide diminished markedly and methane to a lesser extent.

It is evident from this experiment that the densified pellets produce a strongly reducing atmosphere on heating to high temperatures. This 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 (60% water) was kneaded for 4 hours in a sigma-type kneader as described in our pending application Aust. 24294/ 84. 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 pellets initially of 10 mm diameter (about 8 mm after drying) and varying from 10 - 20 mm in length. The pellets were permitted to dry and harden in still laboratory air at 20 C for 7 days.The dried pellets were next subjected to pyrolysis in a stream of nitrogen gas, initially being maintained for one hour in the temperature range 300 - 400"C to eliminate residual water and low molecular weight organic volatiles, followed by further heating for one hour with an increase of temperature to 700"C. This latter period of heating was designed to determine whether detectable reduction had commenced in the temperature range concerned. In one instance (see below) pellets were heated to 1070"C, on this occasion in the reducing atmosphere generated by the coal pyrolysis.

Pellets were 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 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 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 occurried.

The compressive strength a, was calculated from the Force F (determined from the maximum load the pellet withstands) according to the following formula: a, = (4F/1TD2)(H/D)05 All of the composites were strongly magnetic (particularly the 75:25 ore: densified coal blend) after pyrolysis to 700"C, indicating the production of reduced iron.

in one experiment pellets containing 75% Fe203 were placed in a silica tube attached to a vacuum system. The tube was pumped free of all gases whilst being heated to 5000C. The tube was then isolated from the pumps and the pressure change observed as the temperature was further increased at an approximately steady rate to 10700C. The results of these measurements are shown in Figure 2. At about 900"C a very rapid pressure rise commenced and it became necessary to pump gas away to maintain the total pressure below one atmosphere. Substantial gas evolution contained until the experiment was terminated. The phenomena described in this experiment are characteristic of pellets containing iron oxide, and are indicative of chemical reactions between the oxide and species derived from the coal.

At 800"C the principal reaction appears to be reduction of the iron oxide by evolved hydrogen with production of water. This reaction appears to be supplemented at about 900"C 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 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 preliminary pyrolysis to 700"C as described above, were further tested by immersion in a bath of liquid iron maintained at 1500 C. Gas evolution commenced immediately on immersion and continued throughout the dipping period of 30 seconds. The pellets did not disintegrate but continued to evolve gas whilst dissolving rapidly in the liquid iron. Rate of dissolution was greatest on that side of the pellets which has sustained the highest temperature by contact with the furnace wall during preliminary pyrolysis; presumably more reduced iron was present in that zone of the pellet thus 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.

Claims (10)

1. Process for production of metallurigcal composites which comprises the following steps: (a) subjecting brown coal to shearing forces to produce a plastic mass; (b) admixing finely divided metal compound with the coal either during or after step (a); (c) compacting the mixture produced in step (b) to produce a compacted mass; (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 or claim 2 in which the drying step (d) is effected at or near ambient temperature.

4. Process according to any preceding claim in which the metal compound is iron ore or chromite ore.

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

6. A process in which the metal compound is a reducible metal compound and a composite made by a process according to any of claims 1 to 5 is heated to a temperature at which the metal compound is reduced.

7. 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.

8. An iron smelting process characterised by heating composites produced by the process of claim 7 to a temperature 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 7 in a bath of liquid iron.

10. An iron smelting process characterised by subjecting composites produced by the process of claim 7 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.