AU719637B2 - Reuse of metallurgical fines - Google Patents

Description

WO W96/31630 PCT/GB96/00821 REUSE OF METALLURGICAL FINES The invention relates to a method of use of a wide range of metal containing materials which are otherwise used with difficulty or just dumped as waste.

One suitable material is iron ore fine dust. This is difficult to dispose of. During pelletising fine ores having a grain diameter of under about 0.2 mm are wetted and mixed with binders, e.g. bentonite, to form lumps. The pellets are subjected to flame hardening or sintering because this is the only way in which such fines can only be used in a subsequent metallurgical process. This is similar to the preparation of fines by direct reduction, known as the names of COREX MIDREX or HYL®. For example in the case of the MIDREX® process the fine ore is acted upon by reduction gases in fluidised bed reactors at temperatures between 500 and 800 0 C. The reduction gases contain H 2 and CO and also C0 2

H

2 0, CR 4 and N 2 Preparation takes place in a series of fluidised bed reactors, connected in series, in which the degree of reduction increases from reactor to reactor to reach values of metallisation of between 92 and 94% at the exit of the last fluidised bed reactor. The fines reduced in this manner are then hot briquetted in order to obtain the lump form which is necessary for the subsequent metallurgical process. Both during such sintering and during preparation of the fines, ferrous dusts accumulate, which are very fine and extremely reactive. The WO 96/31630 PCT/GB96/00821 2 dusts also contain proportions of non-converted carbon. Such fines cannot be blown or injected into a metallurgical furnace because they have too high a density.

Other heavy metal containing dusts or slurries are produced as by-products in the manufacture and processing of high quality steels. By high quality steel is meant a grade of steel which is intended generally for heat treatment such as hardening and tempering. High quality steel has a higher purity than quality or basic steel (cf.

Brockhaus, Naturwissenschaften und Technik (Natural Science and Technology), special edition 1989, volume 1, key word). Such steels are produced in electric arc furnaces from steel scrap and alloying constituents. The dust contained in the smelting gases is very finely divided and has a high specific weight more than 4 grams per cubic centimetre. The dust is separated in the electric filters installed beyond the electric furnace. It is also usual to wash out the dust from the smelting gases using water to produce a heavy metal containing slurry. The scrap charge usually contains zinc which evaporates at the high temperatures in the metallurgical vessel and is deposited in the flue gas. These dusts (loaded with the heavy metal zinc) from dry dust extraction or slurries in the case of wet dust extraction can only be deposited at special dumps or must be subjected to expensive separation measures.

For the separation preparation process of this kind of Zn-loaded dusts or slurries it is of great economic advantage if the Zn content is enriched to over 25% e.g. 40% or more by recycling.

-3- High quality steel may be treated in a rolling mill. In this case an extremely heavy rolling mill scale slurry results, which besides the metallic components also contains impurities in lump form from metal parts, organic material or plastics. Larger pieces of metal, such as screws, and cleaning rags, cigarette ends and working gloves, yoghurt cartons and plastic bags can also be dumped by untidy workmen in the rolling mill scale slurry.

All metal dusts and slurries arising during the production of basic, quality and high quality steel are finely divided (fine granulometry in the grain size range of a few microns up to 1 mm) and contain alloying metals. These alloying metals include chiefly chromium, cobalt, nickel, lead, manganese, tungsten, titanium, vanadium, zinc and molybdenum, which are added to the individual grades of steel in differing amounts according to pre-set recipes and together with the iron, are responsible for the heaviness of the dusts and slurries. Although these dusts and slurries contain valuable substances it has not been possible to reclaim and re-use these substances to a satisfactory extent .••oei 15 because of their inherent fineness and heaviness. In many cases the dusts containing 0.o valuable substances are dumped but this is bad for environmental protection and •o o coo* wasteful.

4 It is an object of the present invention to overcome or ameliorate at least one of the .00 disadvantages of the prior art, or to provide a useful alternative.

It is one preferred object of this invention to provide a method of recovering the valuable components of a wide variety of waste materials and introducing them in a particularly convenient way into a metallurgical vessel for re-use.

20071-OO.DOCjIlfnb -3a- According to a first embodiment, the present invention consists in a method of treating a waste material containing heavy metals, the method comprising mixing the waste material and directly reduced iron fines and water so as to cause an exothermic reaction and form relatively dry agglomerates, sieving the agglomerates to isolate those having a maximum particle size of 6 mm and pneumatically injecting the isolated agglomerates into a metallurgical vessel containing molten metal under a reducing atmosphere.

According to a second embodiment, the present invention consists in a pneumatically injectable feed material for use as an additive to the molten metal in an electric arc furnace, the material comprising relatively dry agglomerates formed as the reaction product of directly reduced iron fines and a relatively less dense particulate oxidecontaining material and water, the agglomerates having a maximum particle size of 6 mm oe• o 20071-00.DO/mnb WO 96/31630 PCT/GB96/00821 .4 According to the invention in one aspect there is provided a method of treating a waste material containing heavy metals, the method comprising mixing the waste material and a relatively less dense particulate material containing metal oxide material and water so as to cause an exothermic reaction and thereby form relatively dry agglomerates; sieving the agglomerates to isolate those having a size in the range of from about 0mm to about 6mm; and pneumatically injecting the isolated agglomerates into a metallurgical vessel containing molten metal and under a reducing atmosphere.

The oxide material may be burnt lime or directly reduced iron. Most preferably the agglomerates are injected with carbon (which is required for other purposes in the metallurgical vessel such as a reduction reaction). The carbon may be provided from a variety of sources and may already be in the waste material.

Where fines having little or no carbon are used, a carbon-bearing carrier material may be used. Suitable materials are coke consisting of coal or lignite or brown coal, or petroleum coke because these cokes, owing to their surface structure and porosity, are particularly suitable to carry the fines on to their surface. However, finely divided or dust form fractions of coal or lignite are also suitable as carrier material, just as are finely divided lightweight fractions from the shredding of plastics. The latter have a tendency to form aggregates similar to flakes and are not injectable in this form.

One criterion for the selection of suitable carrier material is the foaming of the slag.

Consequently. for injection into electric arc furnaces or cupola-type furnaces, carboncontaining carrier materials which exhibit a low proportion of volatile constituents are preferred. These include lignite coke, anthracite, petroleum coke and coke breeze. For injection into a blast fuirnace, carbon-containing carrier materials having volatile constituents are best. The amount of volatiles in the total carbon content of the carrier materials should be greater than Lignites as well as other low coalified types of coal, wood chippings, plastic chips, or the like can also be used.

For the promotion of the foaming of the slag, the product should have a carbon content between 20 and 40 percent by weight. The carrier materials may be selected and determined by means of pre-analysis.

For the sake of completeness, it should be established that the mixed, reaction and combined products still exhibit amounts of Fe, FeO, Fe 2 0 3 CaO and CaCO 3 and the residual humidity is between 5 and 15 percent by weight.

S" The invention also makes use of the reactivity of the fines. According to the degree of metallisation, the fines react with water to form iron oxide, releasing heat. When mixing •15 a slurry with the fines iron oxide forms. The heat released during the exothermic reaction causes part of the moisture to evaporate and allows the agglomerates to become crumbly.

The invention also provides a method for preparing fines for use in a subsequent metallurgical process, the fines being from at least partly reduced iron ore in dust form, comprising mixing the fines with a fine-grained carrier material having a low density to produce crumbly product results and sieving the product to provide material between 0 and 6 mm grain size suitable for injection in the metalluhirgical process.

20071-00DOC For injection the agglomerates must have a density of about 1.2 to about 4 grams per cubic centimetre. The delivery cross-sections of standard injection devices in the order of between 1/2 (about 12 mm) and 2 inches (about 50 mm) made it necessary to sieve the 9 *9 9 9 20071-00.DOC WO 96/31630 PCT/GB96/00821 6 products to grain sizes of below about 6 mm in diameter. Standard injection pressures are about 4 to 5 bars for electric arc furnaces and about 5 to 6 bars for blast furnaces.

Mixing of dry carrier material and the fines may be carried out both directly and indirectly and in any sequence of steps. For example, fines and carrier material are first mixed together and then if applicable with water or with a moist carbon-containing slurry; or carbon-containing slurry and carrier material are mixed together first and then mixed with the fines. It is possible to mix fines, carrier material and carboncontaining slurry at the same time.

Evaluations injecting agglomerates consisting of fines, oil-containing rolling mill scale slurry, fine coal, petroleum coke or electric furnace filter dust. and quick lime or caustic lime into an electric arc furnace have revealed that finer grain fractions of the sieved combined product of below 4 mm in grain size produced a better conversion of the injection product in the molten metal than a grain fraction of under 6 mm in diameter; which is attributed to the larger overall surface area of the smaller grain fraction.

The crumbly agglomerate is readily storable. As soon as it has reacted, it becomes inert and is suitable for pneumatic conveying. It is possible to inject product into the blast furnace, electric arc furnace, convener or cupola-type furnace and even into rotary cement kilns, (although in this respect it is no longer a metallurgical process in the narrower sense). In principle, it is possible to use slurry of any kind. that is also metal-containing and mineral-containing slurries but in this respect it must be considered whether, when using metal-containing slurries the alloy arising in the WO 96/31630 PCT/GB96/00821 7 metallurgical process still conforms to the required recipe and whether, when using mineral-containing slurries the amount of slag resulting does not have a negative effect on the profitability of the steelmaking process. In the simplest case it is therefore also possible to treat the fines with coal slurry, of the type from washing coal. However such coal slurry has only a low calorific value (because it exhibits a high proportion of sterile mine waste material which only increases the amount of slag of the metallurgical process, but contributes nothing to the foaming of the slag).

Preferably energy-rich slurries are used, such as result, from oil-containing rolling mill scale slurry, Fuller's earth or slurries which contain lignite dust or coke dust from coal or lignite or waste material slurries from petrochemistry. Such slurries at the same time reduce the energy required in the subsequent metallurgical process, because they provide part of the carbon requirement. Although rolling mill scale slurry in particular is regularly interspersed with foreign bodies, as is well known, such impurities play no role in the present invention, because the foreign bodies separate from the slurries during treatment of the fines and after the thorough mixing of the components are removed by sieving. This applies also to the lumps or aggregates which may occasionally arise during mixing.

It is also suitable to use slurries which result from wet dust-extraction of ore reduction plants. The dusts from dry dust-extraction are suitable for admixing to the fines insofar as they do not already themselves form these fines. Also suitable are slurries which result from the wet dust-extraction of blast furnaces. cupola-type furnaces, electric arc WO 96/31630 PCT/GB96/00821 8 furnaces, converters, sintering plants and surface treatment plants for the grinding/and polishing of metals. The corresponding dust from dry dust-extraction may be admixed to the fines insofar as they do not already themselves form these fines.

In a variation, the additive may include sufficient lime for the lime requirement of the furnace by the addition of substances which exhibit free quicklime, CaO. So according to the invention the amount of slurry necessary to form the reaction product is increased beyond the actual requirement and fly ash is added to the reaction product in addition. The free quicklime of the fly ash reacts with the residual moisture of the slurry to form calcium hydroxide Ca(OH) 2 by which heat is also liberated. The limestone favours the subsequent metallurgical process and the liberated heat makes the combined product crumbly.

Preferred is fly ash from the flue gas in the electric filter behind the power station boiler when burning low sulphur fossil fuels, and this includes primarily lignite from the Rhine. However, that fly ash which accumulates during the treatment of fuels according to the dry additive process is also suitable if limestone has been added to the fuel to bind the sulphur before combustion. These so-called DAP ashes are usable.

The use of dust or sand from blast furnace top gas is also possible: this accumulates when the gas is cleaned from the blast furnace flue. Both still contain the residue of non-converted carbon and limestone as well as amounts of iron and iron oxide.

WO 96/31630 PCT/GB96/00821 9 During the preparation of heavy, metal-containing dusts, (and besides those already mentioned this also includes those which result during the grinding of steels), in the first step of the method, by means of mixing with finely-divided quicklime or dolomite the fluidity of the slag is increased in the subsequent metallurgical process. The preparation of heavy, metal-containing slurries also includes, besides those already mentioned, grinding slurries for use in the first step of the method. By means of the preferred use of finely divided fly ash the usually impenetrable slurries are dried at first and thereby made easy to handle. The amount of quicklime in the fly ash or slag from the ladle furnace assists drying since the free quicklime reacts exothermally with the moisture of the slurry and a considerable part of the moisture inherent in the slurry evaporates thereby; the lime addition which is advantageous for the subsequent metallurgical process remains unaffected.

Another waste material useful in the present invention is made up of different forms of aluminium. The addition of aluminium is known to be useful in deoxidation and in the fluxing of slag. Certain forms of aluminium can attack the lining of a metallurgical vessel, and others are costly. According to this invention waste aluminium oxide or aluminium metal, e.g. oily wet grindings, can be included. Such aluminium may also be present in the flyash. Where aluminium is present and the agglomerate has other suitable ingredients, e.g. a high content of iron. the agglomerates are useful in the cement industry.

WO 96/31630 PCT/GB96/00821 By means of the sieving in the second stage, a uniform granulation band of under 6 mm in diameter is obtained. At the same time foreign bodies are retained, having been exposed in the first step by the so-called "rolling up" more particularly of the rolling mill scale slurry. These foreign bodies and larger lumps may be ground up if applicable and can be inserted into the method cycle again at a suitable point.

In the third step of the method the sieved off fine fraction comprising carboncontaining dusts and lime-stone-containing agglomerates is mixed thoroughly with these. This final step contributes substantially lowering the density until they become blowable or injectable under air or other gas pressure.

In the case of the added carbon-containing dusts the use of lignite coke dust, petroleum coke or anthracite or dusts from slightly or low volatile coal or mixtures thereof is preferred, wherein their respective grain size is below 1 mm in diameter. The addition of carbon is known to promote the foaming of slag in the subsequent metallurgical process. Where the waste material has an organic fraction, e.g. an oil, as in the case of metal-containing slurries from surface machining, such as, e.g. grinding slurries, or in the case of rolling mill scale slurries less carbon dust will be required than for slurries which do not bring with them per se this kind of organic admixtures, e.g. with slurries from the wet cleaning of gas from molten metals.

WO 96/31630 PCT/GB96/00821 11 The addition of carbon-containing dusts alone may not be enough to lower the density to the point where the product can be injected pneumatically. Instead a carrier material in which the mixture consisting of metal-containing dust, calcium and carbon can be added may be present in order to reduce its density further. Limestone-containing agglomerates in sphere form. as they result from the preparation of the boiler feed water from steam power stations, which operate with steam turbines have proved suitable for this purpose. These agglomerates are small, light, stable spheres of an average diameter of 1 to 2 mm. which are extremely resistant to compression and abrasion; the grain size ranges from 0.5 to 6 mm. These spheres result from the removal of calcium and iron from the boiler feed water suitable for the steam-driven turbines. In the centre of the spheres there is a tiny quartz grain on which the lime extracted from the boiler feed water and the iron in crystalline form have accumulated.

The colour of these spheres is beige, they have a smooth surface and exhibit a high compressive strength. Normally they are disposed of on the dump.

In the case of the present invention the spheres are suitable preferably as carrier for the mix consisting of heavy metal particles, lime and carbon. Bonding to the carrier is ensured by electrostatic forces as well as the moisture from the water preparation.

Their content of silicon, lime and iron are helpful for the subsequent metallurgical process.

In other aspects the invention provides: WO 96/31630 PCT/GB96/00821 12 a pneumatically injectable feed material for use as an additive to the molten metal in an electric arc furnace, the material comprising the reaction product of a waste particulate material containing heavy metal elements and oily mill scale, the material being in the form of agglomerates having a particle size from about Omm to about 6mm; a pneumatically injectable feed material for use as an additive to the molten metal in an electric arc furnace, the material comprising the reaction product of a waste particulate material containing heavy metal elements and directly reduced iron fines, the feed material being in the form of agglomerates having a particle size from about Omm to about 6mm; either of these feed materials, and including a source of carbon in particulate form; and either of these feed materials and in which burnt lime was present and participated in the reaction In order that the invention may be well understood it will now be discussed by way of example only and with reference to the accompanying diagrammatic drawings in which: WO 96/31630 PCT/GB96/00821 13 Figure i shows schematically one process of the invention, and Figure 2 is graphs showing exothermic reactions.

As shown in Figure 1, a high quality steel slab 2 is rolled into a sheet 3 at a rolling mill.

Water 5 under pressure is added to the rolling process via a nozzle 4. The rolling mill scale 6 is collected as rolling mill scale slurry 7 in a trough 8 underneath the rolling mill stand 1.

Separately coal 10 is burned in a power station boiler 9. The hot flue gas 11 arising is passed over heat exchanger surfaces 12 in which boiler feed water 13 circulates and which is converted into steam 14 on the heat exchanger surface 12. The flue gas 11 leaves the boiler via an electric filter 33, where the finely divided fly ash 15 which is carried along and separated from the flue gas before it can be discharged through the chimney 16 into the atmosphere.

The rolling mill scale slurry 7 and separated fly ash 15 are fed to a first mixer 17, wherein the rolling mill scale slurry 7 and the fly ash 15 are intimately mixed. During mixing with the fly ash 15 the moist rolling mill scale slurry 7 dries out to form an agglomerate product having a crumbly consistency. The reaction is exothermic and the water evaporates. The first mix 18 leaving the mixer 17 has a temperature above ambient and passed through a sieve 19. where a fine fraction 20 is sieved off. The WO 96/31630 PCT/GB96/00821 14 oversize 21 from the sieve 19 contains the foreign bodies and impurities present previously in the rolling mill scale slurry 7: these are discarded. The fine fraction having a grain size range of under 1 mm flows to a second mixer 22. where it is mixed with lignite coke dust 23 and lime-containing spherical agglomerates whose average grain size is between 0.5 and 2 mm.

The spheres 24 have been derived from a filter 25, where the boiler feed water 13 of the power station boiler 9 is prepared for passage to the turbines. After the water vapour 14 has been stress relieved in the steam turbine 34, it flows through a condenser 26, where it condenses again to form boiler feed water 13. The condensed boiler feed water 13 is forced by feed water pump through a filter 25. Quartz grains 28 are added to the filter 25, where the lime contained in the boiler feed water 13 and the iron accumulate, in order to form the spheres 24. Losses of boiler feed water 13 are compensated for by fresh water introduced via the pipe 29.

The agglomerated product 30, consisting of finely divided metals, lime and carbon, which leaves the second mixer 22 has a dry, powder-like consistency with grain sizes between 1 and 6 mm and average specific weight of 2g/cm 3 and is injected by blower 32 into a blast furnace 33.

In an evaluation to confirm that direct reduced iron could be used. differing amounts of water were added one after the other to a quantity of 1 kg. of ultra-fine sponge iron and then stirred. The reaction of the iron oxide with the water was measured by the WO 96/31630 PCT/GB96/00821 temperature increase observed in the process. The measured values are recorded in the Tables I to III and illustrated in the corresponding graphs of Figure 2.

Initial Material: 1 kg ultra-fine sponge iron in a beaker at I Addition of 5 per cent by weight of water corresponding to 50 grams.

Time (minutes) Temperature 0 2 23 4 23 6 24 8 24 24 The results are also shown in Figure 1 from which it can be seen that the temperature rises by 4 0 C in the course of 6 minutes and remains constant thereafter.

II Another 50 grams of water is added to the mix from I and both are stirred by hand for 4 minutes.

Time (Minutes) Temperature (C) 0 2 26 WO 96/31630 PCT/GB96/00821 27 27.5 28 29 31 32 32 32 32 The results are also shown in the graph. The temperature rises by another 7 0

C

in the course of 16 minutes and remains constant thereafter.

III Another 50 grams of water is added to the mix from n and both stirred by hand for 3 minutes. The temperature was measured.

Time (Minutes) 0 2 4 6 8 12 Temperature 32 32.5 33 33.5 34 34 34.5 WO 96/31630 PCT/GB96/00821 17 14 16 18 35.5 35.5 22 35.5 24 35.5 Results are also shown in the graph. The temperature rises slowly by a further in the course of 18 minutes and remains constant thereafter.

As can be seen therefrom, the most violent reaction starts in mixing phase II and fades gradually in mixing phase III.

Claims (6)

1. A method of treating a waste material containing heavy metals, the method comprising mixing the waste material and directly reduced iron fines and water so as to cause an exothermic reaction and formn relatively dry agglomerates, sieving the agglomerates to isolate those having a maximum particle size of 6 mm, and pneumatically injecting the isolated agglomerates into a metallurgical vessel containing molten metal under a reducing atmosphere.

2. A method according to Claim 1, wherein the formed agglomerates are injected into an electric arc furnace.

3. A method according to Claim 1 or 2, wherein the waste material is a rolling mill scale slurry.

4. A method according to any one of the preceding claims, including the step of

99.. 9 o* 9 9 adding a carbonaceous material to the mixture. A method according to Claim 4, wherein to promote the foaming of the slag 999*99 15 present in the vessel having the reducing atmosphere the content of the carbonaceous material is between 20 and 40% by weight. 6. A-method according to any one of the preceding claims, wherein a source of 9999 aluminium is included in the agglomerates. 9°oO9 7. A method according to any one of the preceding claims, wherein burnt lime is I 20 included in the agglomerates. 8. A pneumatically injectable feed material for use as an additive to the molten metal in an electric arc furnace, the material comprising relatively dry agglomerates formed as directly reduced iron fines and a relatively less dense particulate oxide-

20071-OO.DOC/In b -19- containing material and water, the agglomerates having a maximum particle size of 6 mm. 9. A feed material according to Claim 8, including a carbonaceous material. A feed material according to Claim 8 or 9, including aluminium. 11. A method of treating waste material substantially as herein described with reference to any one of the examples, but excluding the comparative examples. 12. A pneumatically injectable feed material substantially as herein described with reference to any one of the examples, but excluding the comparative examples. DATED this 15th day of March 2000 HECKETT MULTISERV PLC Attorney: RUTH M. CLARKSON Fellow Institute of Patent Attorneys of Australia of BALDWIN SHELSTON WATERS o 9 a o *oo 20071-oo.Doc/mnb