EP0053139B1

EP0053139B1 - Agglomerates, a process for producing thereof and use thereof

Info

Publication number: EP0053139B1
Application number: EP81901494A
Authority: EP
Inventors: Rolf Linder
Original assignee: SSAB Division Gruvor
Current assignee: SSAB Division Gruvor
Priority date: 1980-06-05
Filing date: 1981-06-05
Publication date: 1987-12-16
Also published as: WO1981003499A1; DE3176577D1; EP0053139A1

Abstract

A process for improving the reactivity, permeability and/or similar characteristics of an ore charge being subjected to down-draught sintering, characterized by including into said charge an active quantity of a micropelletized product produced by balling a fine-grained ore material with the addition of a minor quantity of hydraulic binder and water to a maximum agglomerate size of up to 10 mm (preferably up to 6 mm) which prior to charging onto said sintering device is cured to in average at least50% of the maximum achievable cured strength or of the cured strength obtainable by curing for 28-30 days at room temperature.

Description

This invention is related to a process for improving the reactivity, permeability and/or similar characteristics of an ore charge being subjected to down-draught sintering, characterized by including into said charge an active quantity of a micropelletized product produced by balling a fine-grained ore material with the addition of at least 0.5% and at most 4% by weight of a fine-grained hydraulic binder with a specific surface area of at least 2000 cm²/gram consisting of blast furnace slag or a similar metallurgical slag and optionally up to 50% of cement, up to 50% of said slag and/or cement optionally being replaced with slaked lime, and water, to an agglomerate size of at least 75% below 5 mm, which prior to charging onto said sintering device is cured to in average at least 50% of the cured strength obtainable by curing for 28-30 days at room temperature. The invention is also related to the micropelletized product defined above. The ore is e.g. iron ore, especially hematitic or magnetitic iron ore, e.g. with at least 60%, at least 75% or at least 85% of its iron present as magnetite. For curing (bonding) the micropellets can be spread in a layer of comparatively low height or thickness, e.g. at most 15 metres, preferably at most 10 metres or at most 5 metres or even at most 3 metres in order to prevent that the agglomerates in the lower part of said layer are crushed, for the period of time required for hardening or bonding to a strength which permits subsequent transport by loading into a railway wagon or a similar treatment and charging into a device for performing a metallurgical process, e.g. a draft sintering device of the travelling grate type or a similar device, without forming an unacceptably large amount of fine-grained material, as is defined in the following with reference to testing methods, such as drop test in free fall or dropping through a tube against a hard bottom surface or against a layer of the tested material with a drop height of at least 15 metres, and by treatment in a rotating drum of defined dimensions for a certain period of time. Curing in said store, also called low store, is performed preferably for 1 to 14 days, especially from 2 to 7 days. Thereafter further storing can be performed under conditions which permit continued curing preferably for a total storing and curing time after the production of at least 15 days, preferably at least 30 days and optionally at least 60 or at least 100 days. Depending upon the conditions the storing can in some cases be restricted to at most 45 days, particularly at most 30 days or at most 15 days or even at most 7 days prior to transportation from the production location to the location of use or prior to charging into the metallurgical device for the intended use if the product is used at the production location. Usually the transport from the production location to the location of use is performed in lorries, railroad wagons, transport belts, as a suspension or dispersion in a carrying gas or liquid in tubes, in ships or in a similar way under conditions which would cause the micropellets produced without the hydraulic binder or without the curing in the low store or the final curing after low store treatment to be weared and denuded at said transport to such an extent that the product does not fulfil the required test standards, especially those stated above.
Micropelletizing and micropellets are well known concepts which are explained in the literature. Thus the EP-Al-4 637 comprises a process for converting fine iron or manganese ores to a material suited for sintering by pelletizing to a pellet size below 6 mm and discloses in the example the use of an agglomerating agent consisting of a combination of 3% by weight of sugar cane molass and 5% by weight of lime.
The micropellet product according to the instant invention comprises agglomerated particles of varying size with a largest particle size of 5 mm, under certain conditions a largest particle size of 4 mm or 3 mm. This means that at least 75%, especially at least 85% or 90%, optionally at least 95% or 98% or even 100% of the quantity by weight of agglomerated particles or agglomerated particles and unagglomerated feed material, which has been subjected to the agglomeration treatment without being agglomerated, has a particle size within the stated upper limits. The lower limit of the agglomerate size is especially the grain size of the feed material but it is also possible to settle a lower limit of the agglomerate size, e.g. by sieving or by controlled agglomeration, such as not below 0.1 mm, not below 0.5 mm, not below 1 mm, not below 2 mm, not below 3 mm or not below 5 mm. Said limit means that at least 75%, preferably at least 85% or at least 90%, optionally at least 95% or 98% or even 100% of the quantity by weight of agglomerated particles or agglomerated particles and unagglomerated feed material has a size which exceeds the stated lower limit.
The agglomeration is preferably performed on an agglomerating disc with sloping axis but also any other suitable agglomerating device, e.g. of agglomerating roll type of similar devices can be used.
The agglomerates preferably are of essentially spherical shape which can be achieved e.g. by rolling. Preferably the particles exhibit a ratio largest diameter: smallest diameter (through the geometrical centre or middle point of the balls) of at most 2, preferably at most 1.5 and especially at most 1.3 or 1.2, said value being fulfilled by at least 50% and preferably at least 75% of the particles, based on the weight, especially within the intermediate 50% range of the agglomerate size interval.
The agglomeration can be performed in one or more steps, e.g. for building up micropellets of different composition within different parts, e.g. with a larger or smaller quantity of binder and/or combustible material, especially carbonaceous material, mixed into the outer part of the agglomerates, in relation to the inner part. Said division into layers can preferably be performed with agglomerates within the upper 50% range of the agglomerate size range. The outer part of the agglomerates may especially comprise up to 50% of the weight of the agglomerate and optionally comprise at least 10% or at least 25% of said weight, and e.g. at least 50% of the particles showing said layers of different composition may fulfil said request.
The material used as feed material for agglomeration consists of a finely divided ore material, especially metal ore material. The agglomerated material consists preferably of a metal ore, e.g. essentially oxidic or sulphidic metal ore, preferably comprising one or more of the metals iron, chromium, copper, lead, zinc, tin, cobalt, tungsten, manganese, titanium.
Preferably the material consists of oxidic or hydroxidic iron ore, especially hematite and/or magnetite. Especially preferred is iron ore which to at least 50%, especially at least 75% or at least 85% or even at least 90-95% or 100% consists of magnetite as an iron carrier. Especially suitable is a benefication concentrate of the iron ores stated above as well as other ores which have been disintegrated by grinding. Preferably the grain size of the material prior to agglomeration may be at most 0.5 mm, especially at most 0.2 mm or at most 0.1 mm and optionally at most 0.05 mm. This is intended to mean that at least 75%, preferably at least 90% and especially at least 95% or 100% of the metal ore material or similar material has a grain size below said upper limit. The lower limit of the particle size is normallly the particle size obtained by grinding, but it is also possible to separate a fine-grained part falling below a certain limit value prior to agglomeration by sieving or by similar methods. Thus, the lower limit may be selected to 0.05 mm or to 0.01 or 0.04 mm so that e.g. at least 75%, optionally at least 80% and possibly at least 90 or 95% and under certain conditions at least 100% of the quantity by weight has a grain size exceeding a stated lower limit value.
For the ores mentioned above, especially for iron ores such as hematite and/or magnetite, a grain size of 8Fr-100% below 0.1 mm and a specific surface area of at least 500, preferably at least 1000, at least 1200 or at least 1400, under certain conditions at least 1500 or even at least 2000 cm²/g is suitable. The upper limit of the specific surface area can usually be selected arbitrarily and may e.g. amount to 6000, up to 5000 or up to 4000 or even up to 3000 and in some cases lower, such as up to 2800 or up to 2500 cm²/ g. Usual ranges of the ores stated above is e.g. 1500-2800 or 550-2200 cm²/g, said values of the specific surface area being calculated according to the "Svensson-method" disclosed in "Jernkon- torets Annaler", vol. 133, issue 2, 1949, pages 33-86.
The bonding of the micropellet material is performed with hydraulic binder in finely divided shape, said binder being mixed into the agglomerates, preferably by mixing the binder material with the inorganic feed material, especially iron ore, prior to supplying the ore to the pelletizing equipment, e.g. in a mixing drum or mill being arranged prior to the pelletizing equipment. The binder may also be grinded together with the inorganic starting material with simultaneous mixing with said material. Suitable binders which can be used alone or together with portland cement and similar are metallurgical slags, such as blast-furnace slag, e.g. acid or basic blast-furnace slag, slag from the LD-process, the Kaldo-process, Martin-furnace slag, Thomas slag, slag obtained when performing refining in electric steel furnaces for steel production, as well as slag derived or obtained from melting other metals, such as processes for producing lead, copper, zinc, tin, etc., starting e.g. from oxidic and sulphidic ores. The metallurgical slag may also be used in slag cements. Cement materials of various types can be used as hydraulic binders together with the slag, such as portland cement which is produced by intimately mixing lime- and clay-containing products or other feed materials comprising Si0₂, A1₂0₃ and CaO in suitable quantities, said materials being fired and sintered. One may use a so-called cement clinker or a further treated grinded clinker which optionally is mixed with further binders, such as up to 2-4% of gypsum. An embodiment of portland cement is quicksetting cement or special cement which quickly reaches high physical strength values, usually by comprising a higher content of tri- calciumsilicate or a lower content of dicalcium- silicate than a corresponding normal portland cement and optionally being more finely disintegrated or grinded. Other binders are pozzolanic cement, slag cement and aluminate cement. Slag cement is preferably based on blast-furnace slag, preferably in combination with lime or lime-supplying materials, such as lime slag cement produced from blast-furnace slag and lime, eisenportland cement (iron portland cement) made from about 70% of portland cement and about 30% of blast-furnace slag, and blast-furnace cement (Hochofenzement) made from about 30-70% of blast-furnace slag and 70-30% of portland cement. One may also use aluminate cement prepared by firing to melting of a mixture of lime and bauxite and finely grinding of the molten product. One may also use non- portland cement, e.g. lime and hydraulic limes obtained by burning limestone, preferably at about 1000°C or limestones comprising substantial or effective quantities of A1₂0₃, Si0₂ and Fe₂0₃ as impurities which after burning can be cured with water without interaction of carbon dioxide from the air. Said products are also called hydraulic limes.
The slag used, especially blast-furnace slag, may especially be vitreous, water-granulated blast-furnace slag or blast-furnace slag which in other ways has achieved corresponding characteristics, especially reactivity and hydraulic curing and bonding power. The slag should preferably be basic, especially with a basicity CaO/ Si0₂ = 1.0-2.0 or above, e.g. 1.0-1.5. Examples of compositions or analysis values is 28-40%, e.g. 35--40% Si0₂, 5-17%, e.g. 8-12% AI₂0₃, 29―48%, e.g. 35―45% CaO and 2-13%, e.g. 4-8% MgO. The slag as well as other hydraulic binder constituents should preferably have a low content of Na₂0 + K₂0, especially when producing agglomerates intended for charging in blast-furnaces and steel- furnaces, such as a content of said compounds below 2%, preferably below 1% or 0.5%, especially below 0.1 or 0.05%. Examples of suitable binders are calcium aluminate cement, CA- cement and calcium ferrit cement (C,AF), having the arbitrary composition 4CaO.AI₂0₃-Fe₂O₃.
The quantity of hydraulic binder should be restricted to the lowest quantity which gives the desired strength. The upper limit is at most 4%, in some cases at most 3% or even at most 2% based on the weight of the agglomerated solid materials.
Combinations of the two or more hydraulic binders used are e.g. portland cement, slag cement, aluminate cement, etc. plus blast-furnace slag or a similar metallurgical slag, lime plus blast-furnace slag, and cement plus blast-furnace slag plus lime. A suitable combination is about 10-50% of cement and about 90-50% av slag, such as blast-furnace slag, preferably about 1/3 cement plus about 2/3 slag, especially blast-furnace slag. Said slag and/or cement may also up to 50% be substituted with lime (slaked lime), e.g. up to 50% of the cement may be substituted with slaked lime and/or up to 50% of the slag, especially blast-burnace slag, may be substituted with slaked lime. A particular type of blast-furnace slag is blast-furnace slag which has been purified from sulphur.
A suitable mixture is 10-20% cement, 10-20% lime (burned or slaked), 60-80% slag, such as blast-furnace slag. The hydraulic binder is grinded to a fine particle size, corresponding to a specific surface area, according to the definition above, of at least 2000, preferably at least 3000 and especially at least 5000 cm²/g. Usually reactivity is improved with increasing specific surface area and therefore also an even larger specific surface area, such as at least 6000 and even at least 7000 cm²/g or even at least 8000 or 10000 cm²/g may be preferable. As regards suitable hydraulic binders which can be used according to the invention and the characteristics of said binders reference is also made to the Swedish Published Application No. 324 166 and the Swedish Patent No. 226 608.
Fuel can also be included into the agglomerates, preferably coke powder, anthracite powder or similar carbonaceous materials, preferably with a relatively low content of volatile constituents. When producing micropellets of iron ore for down draft sintering, e.g. on a travelling sinter grate or a similar device, the entire quantity of fuel required in the process can be included into the agglomerate, preferably in finely disintegrated state of essentially the same grain size range as the inorganic agglomerated material and/or the binder. Also a minor part of the total required quantity of fuel, e.g. 25-75% of said quantity, can be included in the agglomerates and the remainder may conventionally be included as particles, especially as somewhat coarser particles, e.g. particles with a size of essentially above 1 mm, e.g. 1-5 mm or 1-3 mm, which are mixed with the agglomerates and optional other charged constituents.
The agglomeration of the fine-grained inorganic material with the binder and optionally other constituents is performed in a wet state, usually with a water content of 5-15%, e.g. 7-10%, whereof a minor quantity is usually sprayed onto the surface of the agglomerated charge when rolling the micropellets on an agglomerating disc or similar device.
In the micropelletizing treatment it is also possible to introduce cores (nuclei), preferably particles, e.g. recirculated agglomerates, having a particle size above about 1-2 mm which grow in size through repeated passage through the pelletizing device. Optionally balling (rolling, pelletizing) may be performed in several steps, e.g. two or more steps, with increased binder addition in the last step, e.g. to the contents stated above. Optionally at least 2/3 or the entire quantity of the binder may be added in the last step.
The formed agglomerates, optionally after sieving to remove undersized particles and/or oversized particles or for separating cores or nuclei intended to be recirculated, are transferred to a store for curing, preferably a store in which the pelletized material is laid down in a low layer thickness in order to prevent crushing of the agglomerates in the lower part of said layer, e.g. a layer thickness of at most 15 m, at most 10 m, at most 5 m or even at most 3 m, preferably at least 2 m. The uncured (unhardened) micropelletized material can be spread in said layer from a conveyor belt, e.g. as an elongated or annular heap, e.g. by scraping off from an elongated conveyor belt and spreading out in a direction transverse to said conveyor belt so that an elongated heap is gradually formed. The elevation from which the material is dropped is preferably restricted to at most 15 m, preferably at most 10 or at most 5 m. The uncured or green agglomerates may prior to storing be mixed with starting material which is free from binder or with cured or partly cured agglomerates, optionally after separation of a finer or coarser fraction of said materials, in order to prevent a tendency of the uncured or green material to form lumps in the curing treatment. Preferably up to 40% or up to 30% of such materials are added, e.g. at least 5% or at least 10%, e.g. 10-20%, based on the weight of the uncured or green material. The storing time in the low store should be at least sufficient for giving a curing strength which permits further transportation and handling of the agglomerates, e.g. at least 1-2 days up to 5 or 10 days, preferably so that the agglomerated material when dropped in free fall from an elevation of 15 m through a tube against a layer of said material or against a concrete floor shows an increase of the quantity of material with a particle size below 0.42 mm and/or below 0.15 mm of at most 20%, preferably at most 15% or at most 10%, said values being obtained after dropping four times with a height of fall of 15 m. After the agglomerates have achieved said strength they can be transported from the store and/or stored for a further period of time prior to the final use in an intended process, especially draft sintering.
The curing temperature is suitably ambient temperature or room temperature, or about 10-40°C.
The strength of the agglomerates can also be stated or measured as the compression strength of separate micropellets when crushing the agglomerates between flat surfaces. Suitable values for fraction of a diameter size of 4-6 mm is e.g.: minimum strength after storing in a low store: at least 0.2, preferably at least 0.5, at least 1, at least 2 or at least 5 kilograms. Compressive strength after final storing prior to use: at least 0.3, preferably at least 0.5, at least 1, often at least 2 or at least 5 kilograms, preferably at least 0.2 or at least 0.5 or 1 kilograms higher strength when after the storing in the low store.
After curing the agglomerates to the stated strength values and/or for the stated minimum period of time the agglomerates are used in the metallurgical process, especially downdraught sintering of iron ore on a travelling sinter grate or similar device. The cured micropellet material may according to the invention comprise up to 100% of the quantity of charged iron carrier, especially together with recirculated material from downdraught sintering in e.g. commonly used quantities or may comprise up to 80% and often up to 60% or up to 40% or 20% of the iron carrier in the charge. The lower limit for achieving the desired effect may e.g. amount to 5% or 10% or even 20% or more.
Cured micropellet material can according to the invention be arranged homogeneously distributed in the charge bed of a downdraught sintering device or arranged in one or more layer in said bed, e.g. with at least 60% or at least 75% or even at least 90% of the quantity of the cured micropellet material distributed in the lower half or one third of said bed or alternatively in the upper half or one third of the bed or in the central half or one third of the bed height in order to control or improve the carrying capacity, resistance against disintegration in the heating step and the reactivity so that optimum values are obtained within different parts of the bed thickness. Improved permeability and reactivity makes possible an increase of the bed thickness, e.g. to above 30 cm, preferably above 35 cm and optionally to above 40 or 50 cm and/or a reduction of the sintering time to a corresponding degree, said comparison especially being made with the same starting material when being micropelletized without addition of the hydraulic binder.
The following is an example of the invention: In a process comprising sintering on a sintering band the charge in run A comprised 63.8% fine-grained magnetite concentrate, 27.2% iron ore having a grain size which was suitable for sintering, 1.1% iron sponge ash, 1.5% LD-slag, 2.7% gabbro, 3.7% burnt lime, together 100%, and furthermore 5.0% coke breeze, 4.0% limestone and 30.0% recycled material from the sintering process.
In a run B the magnetite concentrate was substituted with a mixture of 20% by weight of said concentrate and 80% by weight of cured micropellets of said concentrate bonded with 1% of portland cement clinker and 2% of blast-furnace slag which were grinded to a specific surface area of about 6000 cm²/g.
In a run C said magnetite concentrate was substituted entirely with the cured micropellet material.
The production amounted in run A to 31.8, in run B to 35.5 and in run C to 35.9 tons/square meter.24 hours. The product quality was in all said runs satisfactory.
Corresponding experiments were performed with the addition of 1% cement and 1% blast-furnace slag with similar results.
Further experiments were performed with the same binder additives but with micropellets of hematite ore concentrates and mixtures of magnetite and hematite ore concentrates with similar improvements of production results.
Corresponding experiments with hematitic ores, especially such ores with more than 80% hematite or tropical hematitic ores give similar good results. Examples of such tropical or subtropical hematite ores are South American-ores, e.g. hematites from Brasilia, e.g. from Minas Gerais, hematites from Venezuela, e.g. hematites of the orinoco-type. Other examples are West African-hematites, e.g. from Liberia and the Mauretania, e.g. from the Nimba-mine, and Australian hematites, e.g. from the North-West Territory. Such hematites may in addition to hematite also comprise e.g. martite. Thus, the invention can with good result be used also for hematitic ores, e.g. such ores comprising at least 70%, at least 80%, at least 90% or even at least 95% of the Fe as Fe₂0₃.
For measuring the physical strength of the micropelletized material according to the invention one may preferably use a device of the type denoted "ISO-Tumbler", i.e. a drum with the diameter 1000 mm and length 500 mm comprising two internal lifters with a height (breadth) of 50 mm. Said drum is charged with 15 kg of dry material and is rotated 200 revolutions at a speed of 25 revolutions/minute. The micropelletized material should preferably be cured to a physical strength at which after the treatment in the ISO-Tumbler stated above the fraction of the material with a grain size below 0.15 mm shows an increase with less than 30%, preferably less than 20% and optionally also less than 15% or 10%, based on the weight of the material being tested in the drum. Said increase values are also suitable limit values at the test disclosed above which comprises dropping two or four times with a fall height of 15 m mentioned above.
It is often preferable to expedite the bonding (curing) or the hydraulic binder, especially during the first part of the bonding (curing) reaction, and/ or to achieve a supplementary bonding (curing) especially during the first part of bonding after forming the micropellet material. This can be achieved in several ways, e.g. by

a) adding a binding accelerator (curing accelerator) to the hydraulic binder, e.g. chlorides, such as calcium chloride or sodium chloride, sodium carbonate and water glass. The quantity of said additives may e.g. amount to up to 5% of the weight of the binder, e.g. 0.5―4% and specially 1-3%, said ranges being valid especially for chlorides, such as calcium chloride, CaCI₂.2H₂0, and a preferred amount of said and other chlorides is about 2%;
b) by decreasing the quantity of calcium sulphate in the cement, e.g. to at most 50% or at most 20% of the quantity of calcium sulphate normally present in portland cement;
c) by adding light burnt MgO, preferably in the quantities stated for the accelerator according to a) above, e.g. about 2%;
d) by adding so called "silica dust" from e.g. electro steel furnaces or ferro silicon furnaces, i.e. mainly from the gas phase separated silicon oxide, especially silica. The quantity of said additive can likewise be selected within the limits stated for the accelerator according to a) above;
e) by subjecting the binder to fine-grinding to a large specific surface area, e.g. at least 6000 cm²/g or at least 8000 or 10,000 cm²/g or above, e.g. at least 12,000. preferably at least 15,000 or even at least 20,000 c_m ² _/g;
f) by adding residual liquor from cellulose digesting processes, e.g. residual liquor from the sulphite process or inorganic constituents obtained from treating said liquor;
g) by increasing the temperature in the binding step, e.g. by preheating one or more of the constituents used in the agglomerating step. Grinding of the binder to a fine particle size may contribute and give a preheating of the binder, e.g. to about 200°C. Furthermore, the ore concentrate or the micropellets may be heated prior to, during or after the agglomerating treatment, e.g. with hot combustion flue gases. A suitable temperature increase is at least 10° and preferably at least 20° or at least 30°C above the ambient temperature or the temperature of the starting materials;
h) by carbonate hardening through a reaction with C0₂, especially a reaction with C0₂ in combustion flue gases used for heating the material.

The measures for expediting curing or hardening may of course also be combined so that two or more such measures are used simultaneously.
It is also possible to add to the agglomerates other constituents which increase the thermal stability, such as coal in various forms, e.g. coke, dolomite, A1₂0₃ in various forms, bauxite, limestone etc. The agglomerates may also be neutral, acid or basic, calculated e.g. from the ratio CaO + MgO/Si0₂ which especially for iron ore agglomerates may be within the ranges below 1, 0.5 to 1.5, 1 to 2 or 1.5 to 2.5 or above 2.
Calcium hydroxide or calcium oxide in various forms, e.g. burnt lime, slaked lime, is not used alone as a binder but may form a constituent of the hydraulic binder together with e.g. a slag which is reactive with the lime.

Claims

1. A process for improving the reactivity, permeability and/or similar characteristics of an ore charge being subjected to down-draught sintering, characterized by including into said charge an active quantity of a micropelletized product produced by balling a fine-grained ore material with the addition of at least 0.5 percent and at most 4 percent by weight of a fine-grained hydraulic binder with a specific surface area of at least 2000 cm²/g, said hydraulic binder consisting of blast furnace slag or a similar metallurgical slag, and optionally up to 50 percent of cement, up to 50 percent of said slag and/or cement optionally being replaced with slaked lime, and water, to an agglomerate size of at least 75 percent by weight below 5 mm, which prior to charging onto said sintering device is cured to in average at least 50 percent of the cured strength obtainable by curing for 28-30 days at room temperature.

2. A process according to claim 1, characterized in that the quantity of hydraulic binder in the cured micropelletized material is at least 1 percent and at most 3 percent, based on the weight of the agglomerated material.

3. A process according to claim 1 or 2, characterized in that the hydraulic binder comprises about 80-50 percent slag and 90-50 percent cement, optionally partly replaced by lime.

4. A process according to any of claims 1-3, characterized in that the hydraulic binder comprises about 10-20 percent cement, 10-20 percent lime and at least 60 percent slag.

5. A process according to any of the preceding claims, characterized in that the hydraulic binder comprises slag cement.

6. A process according to any of the preceding claims, characterized in that the ore consists of iron ore.

7. A process according to any of the preceding claims, characterized by subjecting the micropelletized material immediately after pelletizing to storing in a layer with a layer height of at most 10 m for at least 1 day until the increase of the quantity of material with an agglomerate size below 0.42 mm when subjected to free fall four times from a fall height of 15 m is below 20 percent, said storing being performed prior to transportation of the micropelletized material to and charging on a down-draught sintering device.

8. A process according to any of the preceding claims, characterized in that the micropellets in one or more steps are formed with a larger or smaller quantity of the hydraulic binder mixed into the outer part of the agglomerates in relation to the inner part, said outer part comprising up to 50 percent of the weight of the agglomerate.

9. A process according to any of the preceding claims, characterized by including into the micropellets coke powder anthracite powder or similar carbonaceous materials.

10. A process according to any of the preceding claims, characterized in that the bonding of the pellets is improved by carbonate hardening through a reaction with C0₂.

11. A process according to any of the preceding claims, characterized in that the hydraulic binder has a specific surface area of at least 3000 cm²/g.

12. A process according to any of the preceding claims, characterized by curing the pellets at a temperature of 10-40°C or at ambient temperature.

13. A process according to any of the preceding claims, characterized by charging the micropelletized material in said down-draught sintering process with a bed height of above 35 cm and in a quantity of at least 10% of the charged iron carrier.

14. A micropelletized product suited for carrying out the process according to any of the preceding claims, characterized by comprising a fine-grained ore material balled to a micropellet size of at least 75 percent below 5 mm, said micropelletized product being bonded with at least 0.5 percent and at most 4 percent by weight of a fine-grained hydraulic binder with a specific surface area of at least 2000 cm²/g consisting of blast furnace slag or a similar metallurgical slag, and optionally up to 50 percent by weight of cement, up to 50 percent of said slag and/or cement optionally being replaced with slaked lime, and cured to in average at least 50 percent of the cured strength obtainable by curing for 28-30 days at room temperature.

15. A micropelletized product according to claim 14, characterized in that the ore consists of iron ore.

16. A micropelletized product according to claim 14 or 15, characterized by comprising at most 3 percent by weight of the hydraulic binder.