WO1982002061A1

WO1982002061A1 - The manufacture of a product to be sintered from fine-grain ironoxide material

Info

Publication number: WO1982002061A1
Application number: PCT/SE1981/000353
Authority: WO
Inventors: Olle Bostroem; Karl G Goerling
Original assignee: Olle Bostroem; Karl G Goerling
Priority date: 1980-12-08
Filing date: 1981-12-04
Publication date: 1982-06-24
Also published as: BR8109010A; SE439327B; AU545430B2; SE8008612L

Abstract

A product suitable as a constituent in suction-sinter charges is produced from fine-grain iron-oxide material, such as iron-ore concentrates, by agglomerating the material in the presence of moisture and an hydraulic binder, with subsequent hardening. The mixture of moist fine-grain material and binder is formed, under densification, into cakes or shaped bodies whose size substantially exceeds the desired maximum particle size of the product to be sintered. The cakes or shaped bodies are disintegrated after hardening, to form a relatively coarse particulate product having a particle size below about 8 mm, suitably below about 6 mm.

Description

THE MANUFACTURE OF A PRODUCT TO BE SINTERED FROM FINE-GRAIN IRON-OXIDE MATERIAL

The present invention relates to a method for manufacturing a product which is suitable as a constituent of suction-sinter charges from fine-grain iron-oxide materials, such as iron-ore concentrates, including agglomerating the material in the presence of moisture and a hydraulic binder, with subsequent hardening.

Practically all the crude iron in the world is produced in blast furnaces. The starting material used is predominantly an agglomerated iron-oxide material produced by the suction sintering method, in which the starting material, in mixture with fuel and water, is fired on a grate while drawing air therethrough.

In achieving an economic capacity per unit of grate area per unit of time, the most important factor is the gas-permeability of the mixture charged. Consequently, it is not possible from an economic aspect to include fine-grain material in the charge in any large quantities, because such material will tend to clog the charge, i.e. render it less pervious. In many parts of the world, iron-oxide is highly enriched in order firstly to increase the iron content and secondly to remove contaminents, such as phosphorous for example. In such working-up processes, prior to being enriched the ore must be ground down to such a small particle size as to render the resultant fine concentrate unsuitable for suction sintering. Examples of such concentrates include the extremely fine MidSwedish and Northern-Swedish concentrates which have been purified of phosphorous. In order to find a market for such materials, they are normally sintered to pellet form. Sintered pellets, however, are not ideal agglomerates for treatment in blast furnaces, neither with respect to shape nor chemical composition, and in many instances the sinter obtained from suction sintering processes is preferred, not least because it can be made self-fluxing, i.e. the sinter can be made to include constituents necessary for the blast furnace.

Attempts have been made to produce from fine-grain concentrates micro-pellets which can be mixed with the sinter charge in sintering plants, and micro-pellets have also be en, produced right at the enrichment plants, in an attempt to produce a product which lends itself to transportation and handling and which can be sold to the sintering plants. This requires the inclusion of a binding agent. The manufacture by micro-pelleting processes of a product having a grain-size distribution suitable for products to be sintered, however, presents certain problems. In order to form micro-pellets, it is necessary to add moisture, which results in bogging, i.e. the adhesion of one pellet to another, during storage, before hardening of the binding agent is complete. This necessitates the application of special measures. In addition, because of the high moisture content of the finegrain material, the binder is slow to harden and more binder must be used than would otherwise be the case.

An object of the present invention is to provide a novel and advantageous method for producing a product which is suitable as a constituent of suction-sinter charges from a fine-grain iron-oxide material, such as fine-grain iron-ore concentrates, cinders, dust originating from oxygen-blowing processes, and other metallurgical intermediate products, which method at least substantially eliminates the aforementioned disadvantages.

To this end it is proposed in accordance with the invention that the mixture of moist fine-grain material and binder is formed, under densification, into cakes or shaped bodies having a size which substantially exceeds the desired maximum particle size of the product to be sintered and that the cakes or shaped bodies, after hardening, are disintegrated to form a relatively coarse particulate product having a particle size below approximately 8 mm, preferably below approximately 6 mm.

In order to further increase the strength of the product to be sintered, said product can be subjected to a heat-treatment process in a fluidized bed. A marked effect is already obtained when heating the product to 200 - 300°C.

The method most suited to the manufacture and hardening of the cakes or shaped bodies is dependent upon the properties of the starting material and upon the amount of binder available and its hydraulic properties. If sufficient binder is added, it will fill the interstices or pores exhibited by the starting material and when hardening form bridges between respective particles, without requiring the material to be compacted to the greatest density. The mixture can thus be placed in pulp form in moulds or on belts and hardened. One way of forming a lump product is to subject the mixture to a suction process on a drum filter and to break the resultant cake into pieces, said pieces then being hardened. Cakes may also be formed by vibration.

One advantageous manner of producing bodies of the desired shape from the fine-grain starting material is to compact said material between smooth or profiled, pressure-loaded rolls. This enables the amount of binder required to be kept at a low level, because of low porosity of the compacted material. It should be ensured when compacting the material that the amount of moisture (water) used in the starting material corresponds as far as possible to the amount of binding agent used and its ability to absorb water. It should also be ensured, however, that the amount of moisture present is always less than the pore volume of the cakes or shaped bodies formed by compaction, so that no free moisture is present on the surfaces of said cakes or bodies, since this would render compaction difficult. The cakes are given a relatively small thickness, for example 4-12 mm, so that they harden relatively quickly. The cakes or shaped bodies can be hardened on permeable belts or in stacks or in bunkers through which hot air or hot flue gases can be passed to accelerate the hardening process. The hardened cakes or shaped bodies can be broken up by means, for example, of jaw crushers, rolling crushers or impact mills, suitably to a largest particle size of 6 mm. The finest fraction of the material can be screened off and returned to the compacting stage, although this is not normally necessary from a sintering aspect, but may be desirable in order to avoid generating dust when transporting and handling the material. If the material to be compacted is too moist, for example in the case of a concentrate arriving straight from a filtering station, the moisture content can be reduced by mixing with said material a material which has already been compacted and chemically dried by hardening. When the compaction is carried out at the sintering plant, return products from said plant can normally be added to the material to be compacted.

The binder used in forming the cakes or shaped bodies may be one which contains substances desirable as slag formers in the manufacture of crude iron, preferably from the group comprising burnt or slaked line, burnt or slaked dolomite, cement, blast-furnace slag, slag obtained from crude-iron and steel manufacturing processes, dust obtained from basic steel refining processes and fly ash from coal combustion processes.

The method according to the invention will now be illustrated with reference to the following example: An extremely fine concentrate adapted for use as a starting material for sintered pellets and having a specific surface area of 1600 cm²/g and containing about 6 % by weight moisture and 2.5 % by weight burnt lime was compacted between a pair of smooth rolls having a diameter of 350 mm with a pressure between the rolls of 750 kg/cm per length of roll utilized.

The rolled cakes (cake thickness about 8 mm) were allowed to fall from a downwardly sloping supporting surface arranged in connection with the rolls onto a bottom surface, about 1 m, whereat the cakes disintegrated into a product were 59.6 % by weight had a particle size > 5 mm. After being hardened in air for some days, particles having a size > 5 mm were screened off and crush- ed in a jaw crusher having a discharge gap of 5 mm. The crushed material was returned to the remaining screened material, there being obtained a product to be sintered having the following particle size distribution:

> 5 mm 0.2 by weight 5-3 mm 27.4 3-1 mm 35.3

< 1 mm 37.1

The particles were very strong and were found to be totally water resistent. The product was very suitable for mixing into sinter charges and was of such strength as to be well suited for transportation by boat or motor vehicles, for example between an enrichment plant and a sintering plant.

When practicing the method according to the invention it is not necessary to roll or press the cakes or shaped bodies to obtain a high degree of strength, since sufficient strength is obtained by the hardening process. Thus, it is not possible to disintegrate the cakes etc. to the particle size distribution required prior to hardening. The roll pressure required when practicing the method according to the invention is only about 1/10th of the roll strength required when rolling is to immediately provide the requisite strength, and hence when practicing the method according to the invention the heavy wear on the rolls, which is prohibitive in many other cases, does not occur. As before mentioned, the amount of binder required is also low, for example about 1-5 % by weight calculated as ingoing CaO or MgO respectively.

Claims

C l a i m s

1. A method for manufacturing a product which is suitable as a constituent of suction-sinter charges from fine-grain iron-oxide material, such as iron-ore concentrates, by agglomerating the material in the presence of moisture and a hydraulic binder, with subsequent hardening, characterized in that the mixture of moist finegrain material and hydraulic binder is formed, under densification, for example by subjecting the mixture to a suction process oh a filter, by vibration or by compaction, into cakes or shaped bodies having a size which substantially exceeds the desired maximum particle size of the product to be sintered, said cakes or shaped bodies being disintegrated, after hardening, to form a relatively coarse particulate product having a particle size below about 8. mm, suitably below about 6 mm.

2. A method according to Claim 1, characterized in that said product is further hardened by subjecting the same to a heat treatment process.

3. A method according to Claim 1 or 2, characterized in that the binder used is one which contains substances which are desirable as slag formers in a sinter, preferably taken from the group comprising burnt or slaked lime, burnt or slaked dolomite, cement, blast furnace slagbasic slag originating from crude-iron and steel manufacturing processes, dust from basic steel refining processes and fly ash from coal combustion processes .

4. A method according to any one of Claims 1-3, characterized in that the cakes or shaped bodies are formed by compacting the starting material between smooth or profiled pressure-loaded rolls.

5. A. method according to Claim 4, characterized in that the amount of moisture used is less than the pore volume of the cakes or shaped bodies formed by compaction.

6. A method according to any one of Claims 1-5, characterized by returning material which has been chemically dried by hardening, preferably undersize material, and mixing said returned material with the fine-grain starting material to be agglomerated.

7. A method according to any one of Claims 1-6, characterized by mixing with the fine-grain starting material to be agglomerated undersize return material from a sintering operation.

8. A method according to anyone of Claims 1-7, characterized in that hardening of the cakes or shaped bodies is effected with hot flue gases.