EP0033284A1 - Process and device for the preparation of an ore mixture in an agglomerating plant - Google Patents

Abstract

Improvements to the processes for agglomerating a nut-shaped mixture of ore and fuel in an agglomeration plant on a continuous line. Without ignition of the fuel, the mixture is almost instantaneously preheated throughout its thickness to a temperature above 300 DEG C by drawing gases heated to a temperature of at least 700 DEG C through the surface of the ramp formed by the nuts spilling over from the charging hopper, the temperature of the drawn gases and the rate of spillage of the nuts being controlled so that the said nuts are dried, hardened and preheated almost instantaneously while they roll out onto the surface of the ramp and without the fuel contained having the time to ignite. The invention applies especially to the agglomeration of iron ore. <IMAGE>

Description

The subject of the invention is improvements to the methods of agglomeration of a mixture of ore and fuel on a grid in the form of an endless chain and an improved installation for the implementation of the method.

It is known that the agglomeration of ore on a grid makes it possible to transform a mixture of fine ore and fuel into an agglomerated product, porous and solid, capable of being handled, loaded and reduced in an apparatus for reducing metal oxides such as an iron ore processing blast furnace.

The fresh mixture composed of metallic ore, of siliceous or calcareous addition, and of various metallic or non-metallic recovery wastes, is intimately mixed with the quantity of fuel necessary for the agglomeration operation and humidified in order to obtain a final mixture as permeable as possible.

The agglomeration installation essentially consists of a grid forming an endless chain which is driven so that its upper strand passes from an upstream end to a downstream end in front of means for the vertical circulation of an oxidizing gas consisting normally from a series of suction or supply air boxes. At the upstream end of the chain is placed a distributor hopper which allows the humidified mixture to be deposited in a regular layer, of varying thickness depending on the case and generally called "cake". The cake thus deposited, which is entrained - downstream by the movement of the chain, first passes under an ignition hood, the temperature of which is adjusted to a sufficient level and the weakly oxidizing fumes make it possible to ignite , at the top of the cake, a very thin film of mixture.

Thanks to the air circulation means such as boxes placed under the chain and connected to one or more fans which make it possible to create a depression under the grid of the order of 1000 to 2000 mm CE, the ambient air is forced to up and down through the thickness of the cake.

The constantly renewed air then maintains the combustion started under the ignition hood.

The thin lit film, called the "flame front" then produces a reaction zone which moves up and down as the solid fuel is consumed.

During this movement, the various constituents of the mixture are dried, dehydrated, decarbonated, desulfurized, prereduced. The solid products of these reactions fuse and form the primary agglomerate when they are crossed by the "flame front".

The primary agglomerate is then reoxidized by the renewal air.

When the "flame front" has completely passed through the cake, the baking operation is complete. The temperature of the cake which depends on that of the flame front is of the order of 500 to 800 ° C at this time.

The gaseous products thus resulting from the reactions indicated above, as well as the rest of the transfer air, pass through the cold lower layer of mixture, and heat it. During this path, the water vapor contained in these gases produced by the "flame front" condenses in the coldest part of the mixture and a series of successive equilibria is established along the chain between liquid water and steam water; this is called the "water front". This "water front" therefore depends on the temperature of the lower part of the mixture and generally extends over 50% to 60% of the length of the cooking zone, zone going from the loading point to the cooking point. where "the flame front" reaches the grid.

"The water front" only disappears when the mixture has reached at least 100 ° C and if the permeability of the cake allows it. Its disappearance leads to a strong variation in the permeability of the temperature cake and the composition of the fumes.

When it exists, the "water front" considerably reduces the permeability of the cake under the depression under consideration. From the point of loading to the point where the "water front" disappears, there is a decrease in the speed of aspiration of the air above the cake and an increase in this speed from the point of disappearance of the "water front" to the point where the cake is completely baked at the end of the cooking zone.

The suction speed of the transfer and combustion air above the cake can be measured using a portable anemometer. There is, from the loading point to about two-thirds of the cooking - zone -, a decrease in this speed and then a gradual increase in it. This reduction in speed corresponds to the existence of the "water front". The decrease in the air suction speed above the cake in the area where the "water front" exists compared to the air suction speed above the cake in the area the "water front" does not exist is of the order of 15 to 30% or more, depending _on the mixtures the average temperature and depression of the caissons, and in particular the height of the mixture layer. Indeed, the greater the layer height, the longer the mixture located under the "flame front" to heat up by the hot gases resulting from the reactions and therefore the greater and longer the "water front" to disappear.

It is therefore understandable that the area where the "water front" exists will have a much lower productivity than that where it does not exist. Pro- d _u ctivity in the area where there is the "water front" will be reduced to an extent equivalent to the reduction of air above the cake speeds, i.e. in a proportion of the order from 15 to 30%.

This phenomenon of the "water front" has been known for a long time and attempts have been made to reduce its importance by reheating the agglomeration mixture with hot materials, for example hot return fines or steam, or with cold materials such as lime which, when hydrated, heats the agglomeration mixture but these processes above all make it possible to limit the energy loss of the installation, productivity being, at best, only slightly improved.

It has also been proposed (French Patent No. 1,215,231), to subject the load, before ignition, to preheating to a temperature sufficient to vaporize the water contained in the mixture, at most 300 ° C., by suction of hot gases introduced into a hood covering the upper part of the cake upstream of the ignition hood, these hot gases can be constituted, at least in part, by the gases sucked under the grid in a cooling zone of the 'agglomerated, after the cooking point. As preheating takes place by suction of the hot gases from top to bottom, the water vapor formed in the upper layers condenses in the lower layer and the water front is therefore not normally removed before the hood d 'ignition unless giving the preheating hood a significant length and therefore increasing the length of the chain. Moreover, the goal sought by this improvement is not the elimination of the water front, but to obtain a more resistant and larger chipboard.

The subject of the invention is a new improvement to the agglomeration methods of ore on the grid allowing the elimination of the water front and an increase in the productivity of the grid without appreciable increase in the length thereof.

In accordance with the invention, the combustion is carried out without inflammation tible contained in the load, almost instantaneous preheating of the latter over its entire thickness, at a temperature above 300 ° C, no suction of gases brought to a temperature of at least 700 ° C through the surface of the embankment formed by the pellets discharging from the loading hopper, the temperature of the gases sucked in and the rate of discharge of the pellets being regulated so that said pellets are dried, hardened and preheated almost instantaneously while they flow onto the surface of the embankment, without the fuel having time to ignite.

The device for implementing the method according to the invention comprises means for direct heating of the surface of the slope formed by the mixture pouring from the loading hopper at the upstream end of the cake, constituted by a hood with an oblique axis inclined downward in a direction substantially perpendicular to the slope and the lower part of which opens widely through an orifice covering at least the greater part of the slope, for the suction through the latter of hot gases produced inside the hood at a temperature above 700 ° C.

It had already been proposed in 1936, in the German patent n ° 655.490, to place an inclined hood for auxiliary ignition in front of the lower part of the slope to carry out an ignition in two stages, the lower layer being ignited by the inclined hood and the layer upper by the main ignition hood located downstream of the loading hopper.

In practice, the charge is thus ignited over its entire thickness from the start of the grid. But when the mass mixture is thus ignited, there is a considerable widening of the flame front which occupies the entire thickness of the layer. However, the flame front of a cake, which normally represents 5 to 10% of its height, causes a pressure drop of the order of a third or half of the vacuum applied under the chain in order to ensure the flow and the passage of hot gases in the cake and maintain the flame front. If the entire thickness is on, a very high vacuum will be required under the chain in order to maintain productivity unless you agree to lose part of this productivity.

Thus, in this patent No. 655.490, we had not seen all the advantage that could be drawn from the use of an oblique hood covering the entire surface of the slope by adjusting the rate of discharge of the pellets and the temperature of the gases produced in the hood so that the pellets are heated almost instantaneously% without ignition of the combustible, at a very high temperature.

The invention will now be described with reference to a particular embodiment, given by way of example and shown in the accompanying drawings.

• Figure 1 is a schematic view of an agglomeration installation provided with the preheating system according to the invention, Figure la being a detail view of an alternative embodiment.
• Figure 2 is a longitudinal sectional view of the preheating hood.
• Figure 3 is an elevational view of the end of the chain provided with the preheating hood.

FIG. 1 schematically represents a conventional agglomeration installation comprising a continuous chain 1, the upper strand of which moves between an upstream drum 11 and a downstream drum 12 above a series of suction boxes 2.

At the upstream end of the chain, there is, conventionally, a hopper 3 for loading the mixture into a layer for the formation of a "cake" 5 on the chain 1. This hopper 3 is preceded by a hopper 31 of loading of a protective layer 51 of the grid and followed by a hood 32 provided with burners for lighting the surface layer of the cake formed by the hopper 3. At the downstream end 12 of the chain, the cake is dumped, after cooking, into an unloading hopper 13.

In the embodiment shown in the figure, the chain comprises a cooking zone A which goes from the ignition hood to a point C known as the cooking point where the "flame front" disappears after having passed through the whole of the thickness of the cake and a cooling zone B which goes from the cooking point to the downstream end 12 of the chain and which makes it possible to pour into the hopper 13 an agglomerate already cooled.

The porosity of the cake 5 being different during baking and during cooling, to each zone of the chain, respectively of baking and cooling, corresponds a series of boxes connected by a collector, respectively 21 and 22, and by means of 'A dedusting installation, respectively 23 and 24, to a suction fan, respectively 25 and 26, which discharges the aspirated gases either to a chimney or, possibly to a recycling circuit.

In any event, if an agglomeration installation comprising chain cooling has been shown in the figure, the v _e ntion also applies to more conventional installations in which the chain carts overturning soon after the firing point to pour the agglomerate on a separate generally circular cooler.

According to the invention, whether the cooling is carried out in a conventional manner or on the chain, the installation comprises a hood 4 for preheating the slope 50 formed by the pellets pouring out from the loading hopper 3.

The hood 4 is shown in more detail in FIGS. 2 and 3. It is limited by an envelope in the shape of a pyramid trunk comprising a bottom 41, an upper wall 42 substantially parallel to the chain, a lower wall 43 inclined and two walls vertical sides. The median plane P perpendicular to the bottom 41 is inclined with respect to the mixing slope 50 by an angle (a) equal to or a little less than 90 °. In addition, the plane of the orifice 44 of the hood, constituting the base of the pyramid is parallel to the slope 50.

The loading hopper 3 which contains the mixture 52 is provided at its base with a distribution roller 31 which discharges the mixture onto the grid 1 in the form of a sheet 53. This is retained by a flap 14 of height adjustment H of the mixing layer. The latter is formed as is known, of pellets which roll from the flap 14 to the grid 1 covered with the protective layer 51 by forming an embankment 50 which, if the discharge rate of the mixture is properly adjusted according to the speed of movement of the chain, remains substantially in a fixed position, the spilled mixture being continuously driven by the movement of the chain. Similarly, the slope of the slope, which depends in particular on the discharge rate, the particle size and the density of the pellets, remains approximately constant and, depending on the mixture, of the order of 38 to 45 ° relative to the 'horizontal.

As indicated, a very hot gas brought to the temperature of at least 700 ° is produced inside the hood, which is therefore entirely covered with an insulating and / or refractory interior coating. vs. This hot gas can simply be formed by heating fumes or hot air having passed through the cake 5 to the desired temperature, and regulated by a circuit 212 connected for example to the last cooking box 210 or to the first cooling box. 220, so as to draw the hottest gases, However, generally, the bottom 41 of the hood will be provided with a burner 6 supplied on the one hand with combus gas tible by a pipe 61 and on the other hand by oxidizing gas such as air by a pipe 62. This burner can be of any conventional type and opens into the hood by a diffusion plate of either circular or oblong shape. to have a width corresponding to that of the hood.

Below the slope 50 is placed a suction box 26 connected either to the suction circuit of the cooking boxes by a pipe 261, or to an independent fan 27 by a pipe 262. Thus the hot gases produced inside from the hood 4 are sucked through the embankment 50 according to the arrows indicated in FIG. 2. When the hood 4 is provided with one or more burners 6 the latter can receive as oxidizing gas cold or hot air or highly oxidizing fumes. The fuel can be of different types. Thus, the temperature of the combustion fumes can be adjusted between 700 and 1800 °. The oxidizer / fuel ratio must be adjusted to obtain the quantity of oxidizing fumes or not, necessary for the vaporization of the water and the heating of the mixture throughout its thickness up to 400 ° C and possibly at a higher temperature but compatible with the absorption capacity of the cake. The regulation of the flue gas temperature is carried out in a conventional manner by maintaining in the hood a pressure of burnt gases very close to atmospheric pressure taking into account the absorption capacity of the cake.

The spraying and preheating operation of the mixture can be done without igniting the fuel it contains because the pellets regularly discharged by the distribution hopper 3 are constantly renewed and the heat exchange takes place over a very large area, the fuel normally not having time to light up. Indeed, the geometric exchange surface exposed to the hot fluid, which depends on the particle size of the pellets and the speed and width of the chain, can be 200 to 500 times greater than that exposed to the hot fluid when the cake goes under the ignition hood. Indeed, the pellets roll very quickly down the slope, and are immediately covered by other pellets.

In addition, this geometric surface is even less than the actual surface of contact of the pellets with the flame or hot fumes, especially if they are prepared from concentrated ores. Thus, the heat exchange between the wet pellets, the flame and the fumes is almost perfect and very fast. As a result, the Wet pellets are dried, hardened and preheated almost instant ly _year when roll front of the hood opening. Due to the regular advancement of the chain they become immobile and do not deteriorate, the largest pellets descending the slope lower than the finest, coming to accumulate above the protective layer.

The heat flux per m2 of geometrical surface is of the order of half that usually observed under the fume hoods. Of course, it is even lower if we consider the real surface of the pellets subjected to the flame. This flow can be increased or decreased depending on the mixture used and the preheating that one wishes to achieve.

The thermal quantity introduced per ton of finished agglomerate can be large, and at least 100 therms per ton of agglomerate, so that it is possible to vaporize all the water and even to preheat the mixture of agglomerate with -from 400 ° almost instantly over its entire thickness.

The products of combustion of the gases or, when a burner is not used, the hot fluids introduced into the hood, as well as the water vapor produced by the heating of the mixture, are sucked by the box 26 which as indicated above, will advantageously be connected by a conduit 262 to a wet or non-humid dust collector 263 and to an independent fan27. In this case the risk of corrosion of the main suction circuit will be practically eliminated.

The production of gaseous fluids is significant per m2 of surface area of the box 26. The extraction of these gaseous fluids produced and the fumes from the feeds which pass through the cake is easy thanks to the very high average permeability of the mixture at this location. In fact, due to the inclination of the slope, the average height of the mixture to be crossed is half the height of the cake and the average permeability is therefore constantly greater than the cold permeability obtained throughout the thickness of the mixture. A plate 16 seals under the chain and prevents the intake of parasitic air.

Thanks to the arrangements which have just been described, the temperature of the fumes drawn into the box 26 can be at least 300 ^° C. so that the water front is eliminated instantly, the water contained in the mixture being vaporized at the moment the layer is formed on the grid. It is even possible to preheat at a higher temperature, without going as far as igniting the mixture, within the limit of the absorption capacity of the slope and possibly of the thermal efficiency of the exchange between the preheating fumes and mixing.

By carrying out the preheating to a temperature of 400 to 450 ^° C. for example, not only is the water front instantly eliminated, but in addition, the speed of vertical movement of the flame front is accelerated and consequently the productivity of the installation.

Indeed, thanks to the suction of the cooking air through the layer, the combustion progressively propagates from top to bottom, each grain of coke forming a flame directed downwards which heats then ignites the grains of coke placed in below. If the charge is already preheated in its mass to a temperature above 400 ° C, the ignition of the coke grains occurs earlier and the flame front propagates faster in the thickness of the charge. It can even be seen that the ignition is almost instantaneous if the temperature of the charge obtained at the slope discharge reaches 700 ^° C. and it is then possible to limit the ignition hood to a single ramp, which compensates for the energy expenditure in the preheating hood.

For the same grid length, it is therefore possible to increase the thickness of the mixture layer, or even, for equal thickness, to increase the speed of the grid. Productivity is therefore increased.

The arrangements which have just been described can be improved in several ways. Thus, an auxiliary cover 33 can be placed above the cake between the adjustment flap 14 and the ignition hood 32 so as to avoid cooling of the mixture before it is ignited by suction into the box 26 of the hot fumes produced by the ignition hood 32 and extended by the cover 33. On the other hand, the mixing cake being limited by this auxiliary cover 33 and by the longitudinal walls 34 (Figure 3) which hold it laterally, the box 26 thus sucks all fumes produced in the preheating hood 4.

These preheating fumes can be isolated from the main cooking air suction circuit by the circuit 262 and, after wet treatment in a device 263, be recycled through the pipe 264 opening into a hood 265 placed downstream of the cooking point C, on the cooling zone B of the chain and in particular at the entrance to this zone, in order to accelerate the cooling of the cake 5 after baking. In fact, these fumes, which contain water vapor and water in fine droplets, will have a more effective cooling action than that of air. On the other hand, by doing so, there is no risk of deteriorating the quality of the chipboard by thermal shock by sucking in cold air immediately after baking.

We thus use in a particularly rational way the water contained in the initial mixture and sucked before lighting the cake. In addition, it results from this prior elimination of the water that the cooking suction circuit no longer contains water vapor and that the electro-filter 23 can be replaced by a less expensive mechanical dedusting system. , for example a cyclone.

A further improvement is shown in Figure 3. In fact, the hood 4 is mounted pivotally on a support system 45 about an axis 46, and its inclination can be changed by means of a screw _has been Oriental _i on 47. It is thus possible to adjust the inclination of the hood first of all in the event of variation of the inclination of the slope, but to orient the axis of the hood preferably towards the upper part of the slope so to get the best performance of the heat exchange, in par- _cu l _i st in the case of very large thicknesses of mixing layers. In addition, a screw (or hydraulic) system 48 allows the longitudinal positioning of the hood according to the thickness of the mixing layer.

On the other hand, in the example shown, the mixture is discharged downstream from the outlet of the hopper 3, the feed direction of the _c h _a ine 1 but by changing the output of the hopper 3 , one could also reverse the direction of rotation of the distributor roller 31 to effect the discharge upstream of the hopper 3. In this case, as shown schematically in Figure la, the preheating hood 40 could advantageously be of dimensions larger so as to cover not only the embankment 50 but also the sheet 53 formed by the pellets discharged by the drum 31. In this way, the contact surface of the pellets with the hot fumes produced in the preheating hood will be further increased since the preheating action is acting not only on the pellets resulting in the slope surface, but _an equal lies on the web of the pellets 53 discharged from the drum 31, i.e. from the outlet of the hopper 3.

Of course, the invention is not limited to details of the mode of réali- tion and _its variants that have just been described, means equivalents may be used to obtain the desired result.

In addition, if the invention is intended especially for the agglomeration of iron ores on a grid, it is quite certain that it could be applied to other minerals.

Claims (14)

1.- Improvements to the agglomeration processes of a mixture of ore and fuel in an agglomeration installation comprising a grid in the form of a continuous chain, the upper strand of which runs from an upstream end to a downstream end in front of means vertical circulation of an oxidizing gas, a loading hopper in a layer of a pelletized wet mixture placed above the upstream end of the chain for the formation on said upper strand of a continuous cake and means conventional ignition of the mixture forming a cooking zone which passes vertically through the cake, the latter undergoing before drying, a preliminary drying, characterized by the fact that almost instantaneous preheating of the mixture is carried out without ignition of the fuel its thickness, at a temperature above 300 ° C by suction of gases brought to a temperature of at least 700 ° C through the surface of the slope formed by the pellets pouring out of the hopper loading, the temperature of the sucked gases and the rate of discharge of the pellets being adjusted so that said pellets are dried, hardened and preheated almost instantaneously while they flow on the surface of the embankment and without the fuel contained having time to light up. 2.- improved agglomeration method according to claim 1, characterized in that the preheating gases sucked through the surface of the slope are produced by a burner at a temperature between 700 ° C and 1800 ° C. 3.- improved agglomeration method according to one of claims 1 and 2, characterized in that one sucks the preheating gases from the slope by a circuit separate from the oxidant gas circuit ensuring the baking of the cake. 4.- improved agglomeration method according to claim 3, characterized in that the gases sucked through the slope are recycled downstream of the cooking point to serve for cooling the agglomerated cake. 5.- improved agglomeration method according to claim 1, characterized in that the mixture to be agglomerated is discharged upstream by the loading hopper in the opposite direction to the direction of movement of the chain and that the preheating gases are sucked up not only through the embankment but also through the mixing sheet pouring out of the hopper. 6.- improved agglomeration installation for implementing the method according to one of the preceding claims, comprising a continuous chain (1), the upper strand of which runs from an upstream end to a downstream end, in front of means (2) for vertical circulation of an oxidizing gas, a hopper (3) for loading a layer of a pelletized mixture placed above the upstream end of the chain (1) for the formation on said upper strand of a continuous cake (5) a hood (32) for ignition of the mixture forming a cooking zone which passes vertically through the cake , the latter being unloaded at the downstream end of the chain, and an inclined auxiliary hood (4), placed in front of the slope formed at the upstream end by the mixture pouring out of the loading hopper, characterized in that the auxiliary hood (4) opens widely at its lower part by an orifice (41) covering at least the largest part of the slope (50) for the suction through the slope of the hot gases produced inside the hood ( 4). 7.- agglomeration installation according to claim 6, characterized in that the part of the cake (5) located immediately downstream of the loading hopper is covered by a cover (33) extending downstream to the ignition hood (32). 8.- Agglomeration installation according to claim 6, characterized in that it comprises insufflation means inside the preheating hood (4) of hot gases from the installation by a circuit of recycling (212). 9.- agglomeration installation according to claim 6, characterized in that the preheating hood (4) is provided with. at least one burner (6) at its upper part for the production in the hood (4) of gas at a temperature between 700 ^° C. and 1800 ° C. 10.- Agglomeration installation according to claim 6, characterized in that the hood (4) for preheating the slope is pivotally mounted about a horizontal axis (46) and provided with means (47) for adjusting its inclination . 11. Agglomeration installation according to claim 6, characterized in that the preheating hood is associated with a separate circuit (262) of suction through the smoke cakes produced in the hood (4), and comprising a box (26) placed under the slope and connected to a suction fan. 12.- agglomeration installation according to claim 11, characterized in that it comprises a pipe (264) for recycling in a hood (265) the fumes charged with humidity sucked in by the box (26) and the circuit ( 262) associated with the preheating hood, said hood recycling (265) being placed downstream of the cooking point C of the cake. 13.- agglomeration installation according to claim 6, characterized in that the loading hopper (3) is associated with a drum (31) for discharging the mixture (32), upstream, in opposite direction movement of the chain (1) the preheating hood (4) being of sufficient dimensions to cover not only the upstream slope (50) of the cake (5) but also the web (53) formed by the mixture poured out by the drum (31).

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