In the method according to the invention, after leaving the reduction zone proper, the metallic material obtained by direct reduction is subjected to the action of a carburising gas before being discharged from the reduction reactor and used to load a melting system to be converted into liquid steel.

Another characteristic of the invention is that the carburised material, obtained by reduction and subsequent carburising, is discharged from the reactor and can be cooled and subjected to passivation to be converted into granulated iron carbide, by means of depositing in a controlled manner a thin layer of oxide on the reduced pellet.

A further characteristic of the invention is that it is possible to generate hot metallic material to be supplied to the melting system, thus optimising the energy consumption and the costs of the plant.

BACKGROUND OF THE INVENTION It is well known that iron carbide is a very useful material in the production of steel, both as a metallic material to be loaded instead of or in addition to scrap, and also as an auxiliary source of energy to be used in the melting process.

At present however, the use made of this material in processes to produce steel is extremely limited, since the existing processes used in the production of iron carbide by

means of reduction are not able to limit the percentage of Fe3C to intermediate values in the overall composition of the material obtained.

Moreover, iron carbide is normally produced in the form of fine particles which are difficult to use in the electric furnace. Furthermore, there is the problem of energy consumption since it is necessary to deliver energy to the reduction plant or reactor used to obtain the iron carbide.

Document US-A-5,437,708 describes a method to produce iron carbide in a reduction reactor, wherein natural gas (methane) is mixed with the cooling gas and sent inside the outlet cone in the lower part of the reactor to obtain iron carbide in the reduction zone of the reactor.

This solution therefore provides that the material is cooled in the lower tapered zone of the reactor, whereas carburation takes place essentially in the central reduction zone.

This document also provides that the percentage of methane in the gas reintroduced into the reactor is less than 50%.

Moreover, the iron carbide is produced, in the reduction zone, at a temperature of between 650 and 750°C, while the material remains inside the reactor for a period of between 9 and 15 hours, advantageously 12.

The Applicant has verified that, with such working parameters, the productivity yield of the iron carbide is very low and unsatisfactory.

Moreover, with such low reduction temperatures the pellets have high porosity and therefore they cannot be subjected to passivation with a good efficiency.

Moreover, if the material remains in the reactor for such a long time, as is necessary in this case to complete the cooling of the iron carbide, it entails high energy consumption and a very low productivity.

Naturally, it would be very useful to be able to have a process with high efficiency and low energy consumption which would allow to obtain, and then to use directly, iron carbide with a controlled composition, possibly passivated and in large quantities, as a loading material for the melting system without the disadvantages mentioned above.

After thorough studies and experiments, the present Applicant has devised and achieved this invention to achieve this purpose.

SUMMARY OF THE INVENTION The invention is set forth and characterised in the respective main claims, while the dependent claims describe other characteristics of the main embodiment.

The main purpose of the invention is to achieve a high productivity process for the production of iron carbide starting from iron oxides, natural gas and electric energy, in such a manner that the iron carbide can be used directly in the production of steel by means of continuous and successive stages of reduction-melting in a single operation and/or can be subjected to cooling and passivation to be used directly for loading into the furnace or cooled and stored for subsequent use.

A further purpose of the invention is to achieve an integrated plant, comprising at least a reactor of direct reduction and subsequent carburising combined with a melting and/or cooling system, which will be suitable to achieve the method mentioned above.

This invention refers to a method for the direct production of iron carbide starting from iron oxides which provides first for the direct reduction of the iron oxides in the reduction zone of an appropriate reduction reactor, and then the subsequent conversion into iron carbide of the said reduced material in the lower part of the same reactor.

The direct reduction of the iron oxide is achieved in two steps which occur in continuous succession inside the same reduction reactor: -a first step to pre-heat and pre-reduce the load material, which provides to direct onto the iron oxide unloaded into the reactor a partly reducing gassy current which derives from the reduction process itself and which is made to re- circulate in the upper part of the reactor; -a second reduction step which provides to direct onto the iron oxide, partly reduced in the previous step, a mixture of reformed gas, recircled gas and natural gas, injected into the intermediate zone of the reduction reactor.

At the end, the metallised iron oxide which descends due to gravity and passes in the tapered lower zone of the reduction reactor is lapped by a mixture containing natural gas and/or by the gases deriving from the process for a long enough time to allow the controlled deposit of the carbon onto the surface (outer and inner) of the hot metallic material, this passage occurring in a period of time such as to allow the carbon deposited to react, forming the iron carbide.

The iron carbide thus obtained is then discharged from the reactor and transferred to the melting furnace to be converted into liquid steel.

According to one characteristic of the invention, instead of being sent to the furnace, the carburised material is fed to a cooling system outside the reduction reactor.

In this case, the material is subjected to the action of a slightly oxidising gas which allows it to passivate by means of a controlled deposit of a thin layer of oxide on the reduced pellet.

This gas is injected at a temperature preferably between 30°C and 50°C and contains hydrogen, water vapour and

methane in the following volume percentages: hydrogen: 1-7%; water vapour: 2-7%; methane: 50-70%.

The flow rate of the cooling gas in the system outside the reactor is preferably between 400 and 600 Nm3/t.

According to the invention, the reduction inside the reactor is carried out at high temperature, by means of the controlled introduction of oxygen into the central and/or upper part of the reactor. This gives the advantage of a reduction in the porosity of the pellet and therefore better conditions for re-oxidising the pellet.

The plant suitable to achieve the method as described above comprises at least a reactor consisting of a first upper reaction zone, a second intermediate reaction zone and a third lower reaction zone.

In the second reaction zone of the furnace, wherein the reduction of the iron oxides is completed, a gas is generated with a high H2 and CO content, and with an oxidation level of between 0.15 and 0.25, due to the reduction reactions of the iron oxides with H2, CO and CH4.

Once the gas has left the second reaction zone, it enters the first reaction zone, arranged higher, and mixes with the hot gas injected into this first zone to pre-heat and pre- reduce the iron oxides.

The gas emerging from the reduction reactor is partly re- circled and partly used as fuel.

The recircled gas has a composition in volume in the following percentage ranges: -from 20% to 41% hydrogen; -from 15% to 28% carbon monoxide (CO); -from 15% to 25% carbon dioxide (C02); -from 3% to 10% methane;

-from 0% to 8% azote and from 2% to 7% water vapour.

According to the invention, the gas which is fed to the reduction reactor in its various reaction zones consists of a mixture of natural gas, recircled gas produced inside the reactor itself and possibly reformed gas.

The mixture of natural gas and recircled gas to be sent to the reduction zone is pre-heated to a temperature of between 650°C and 950°C, and immediately afterwards it is mixed with reformed gas and oxygen-enriched air.

This mixing produces a partial combustion of the gases until they reach a temperature of between 800°C and 1150°C, preferably between 1000°C and 1150°C, at which temperature the gassy current is introduced into the reduction reactor.

The oxidation level of the feed gas fed to the reaction zone is between 0.06 and 0.45.

The carburising of the metallised material is achieved in continuous succession after the reduction reaction of the metallic oxides, by exploiting the heat which the material is able to yield and controlling the carbon content by controlling the flow of natural gas and/or process gas injected into the carburising zone.

To be more exact, the flow of carburising gas is controlled in such a manner as to define a ratio, with respect to the metallised material, of between 15 and 25 Nm3/t if the carburising agent is natural gas, and between 40 and 60 Nm3/t if the carburising agent is a process gas, for each 1% of carbon which is deposited on the surface of the material.

In the mixture sent to the lower part of the reactor in order to activate the carburation reaction, there is a percentage of natural gas preferably equal to or more than 60%.

So that the carbon deposited on the surface of the metallised material spreads into the iron and interacts therewith to form high concentration iron carbide (Fe3C), it is necessary for the material to transit in the carburation zone for a time of between 30 and 90 minutes, at a temperature of between 550°C and 750°C, preferentially between 600°C and 700°C.

BRIEF DESCRIPTION OF THE DRAWING The attached Figure is a schematic illustration of the different parts of the direct reduction plant according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The method to produce iron carbide from mineral iron according to the invention uses the plant 13 shown schematically in the attached Figure.

The essential component of the plant 13 is a reduction reactor 10 which consists of an upper zone 12, for pre- heating and pre-reduction, an intermediate zone 14, where the reduction proper is achieved, and a lower zone 16 where the previously reduced metallised material is carburised.

The upper zone 12 is associated with a feed inlet 11 through which the ferrous material, normally in the form of oxides, is introduced into the reactor 10; the lower zone 16, on the other hand, is associated with an outlet 15 to discharge the iron carbide obtained by the reduction reactor 10.

The metallic oxides introduced into the reactor 10, which for example can be in the form of little balls, normally have an iron content of between 63% and 68% in weight.

The hot metal iron discharged from the outlet 15 of the reactor 10 contains between 85% and 95% of iron in weight.

The gas discharged from the reactor 10 through the conduit 18 has the following volume composition: from 20% to 41%

hydrogen; from 15% to 28% carbon monoxide (CO); from 12% to 25% carbon dioxide (C02); from 2% to 10% methane; from 0% to 8% azote and from 10% to 30% water vapour.

The temperature of the gas is between 300°C and 700°C and its oxidation level is between 0.30 and 0.5, preferably between 0.4 and 0.5, with a ratio of reducing/oxidising gases of between 1.0 and 2.8.

The oxidation level and the ratio between reducing/ oxidising gases, as will be shown hereafter, are calculated in the following manner: TIO = (C02 + H20)/ (C02 + H20 + CO + H2) (1) and r = (CO + H2)/ (C02 + H20) (2) The gassy current discharged from the reactor 10 is taken to a unit 20, arranged transverse to the conduit 18, with the purpose of recovering the heat which can be yielded.

In the embodiment shown here, along the conduit 18 there is a cyclone device 118 to filter and partly separate the powders.

Then, through a conduit 22, the gassy current is sent immediately to a cooling and cleaning unit 24 so as to be cooled to a temperature of between 40°C and 65°C; in order to remove the water present therein.

The water is discharged through an outlet 25 located below the unit 24.

The quantity of residual water in the gassy current in outlet from the cooling unit 24 is between 2% and 7% in volume.

At outlet from the cooling unit 24 the gas is divided into three currents: -a first current, through a conduit 30 which then divides into two branches, 30a and 30b, is sent partly to a pre- heater 36 and partly to the burners of a reformer 44 to be

used as fuel; -a second current, through a compressor 27 which supplies energy for movement, and a conduit 46, is sent to the reformer 44 after being mixed in a proportion of 4 to 1 with natural gas from a conduit 34 associated with a source of supply; -a third current, which is used as recircled gas, is sent inside the unit 20 to be subjected to pre-heating thanks to the heat exchange with the hot gases emerging from the reactor 10 (the unit 20 thus acts as a cooling unit for the gases emerging from the reactor and as a pre-heating unit for the recircled gases); then, the pre-heated flow of gas is sent through the conduit 32 to the pre-heater 36; finally, the current of gas emerging from the pre-heater 36 is mixed with the reformed gas arriving from the reformer 44 through the conduit 50.

The mixture emerging from the pre-heater 36 is further divided into two parts: -the first part, through the conduit indicated with the reference number 15a, is mixed with oxygen or oxygen- enriched air, fed through the conduit 17, and natural gas (CH4) fed through the conduit 19, and is then sent to the intermediate reaction zone 14; -the second part, through the conduit indicated with the reference number 15b, is mixed with oxygen or oxygen- enriched air supplied by the conduit 17, and natural gas (CH4) fed through the conduit 19, and is then sent to the upper reaction zone 12.

In the pre-heater 36 the mixture of gas is heated to a temperature of between 650°C and 950°C; the flow has a ratio of between 600 Nm3 and 1500 Nm3 for each tonne of hot metal iron.

The consumption of oxygen, which is necessary to raise the

temperature of the reducing gas from 650°C-950°C to 800°C- 1150°C, intended as pure oxygen plus the oxygen contained in the air, in the event that air is also injected, is between 8 Nm3/ton DRI and 60 Nm3/ton DRI, preferably between 20 and 60 Nm3/ton DRI.

The current of gas delivered to the intermediate reaction zone 14 at a speed of flow of between 1000 and 1500 Nm3 for each tonne of hot metal iron has to react with the iron oxide, previously pre-heated and pre-reduced, which is descending from the pre-reduction zone 12.

In this situation, in the intermediate zone 14 the following, extremely endothermic reduction reaction takes place: FeO + CH4 = Fe + 2H2 + CO (1) At the same time, again in the intermediate reaction zone 14, the reduction reactions with hydrogen and carbon monoxide take place: FeO + H2 = Fe + H20 (2) FeO + CO = FeO + C02 (3) One consequence of the endothermic reaction is that the temperature of the gas in the intermediate reaction zone 14 rapidly diminishes to values of between 700°C and 900°C; at the same time, the gas leaving the zone 14 has an oxidation level of between 0.15 and 0.35, and a reduction capacity of between 1.1 and 2.8.

The current of gas delivered to the upper reaction zone 12, at a speed of flow of between 500 and 800 Nm3 for every tonne of hot metal iron, reacts with the iron oxide, pre- heating it and pre-reducing it.

This gas supplies the heat and the quantity of hydrogen and carbon monoxide required to produce the pre-reduction reactions which occur in the upper reaction zone 12: Fe203 + H2 = 2FeO + H20 (4)

Fe203 + CO = 2FeO + C02 (5) The hot metal iron arriving from the intermediate reduction zone 14 of the reduction reactor 10, with a carbon content of between 1 and 1.5%, enters the lower carburising zone 16 at a temperature of between 700°C and 900°C, advantageously in the region of 850°C.

In this zone, which corresponds to the tapered outlet zone of the reactor 10, it is put into contact with the carburising agent, introduced through the conduit 119, in a quantity sufficient to control the carbon content to the levels required for the process of transforming the iron into steel (between 2 and 6%).

Together with the carburising agent, in this case consisting mainly of CH4, it is also possible to inject into the lower zone 16 process gas taken from the conduit 46 and sent to the zone 16 by means of the conduit 146, shown with a line of dashes.

Natural gas and process gas can be mixed before they are introduced into the zone 16.

According to a variant which is not shown here, the process gas, possibly mixed with reformed gas taken from the conduit 50 by means of a conduit 31, and the natural gas are introduced into the zone 16 at two different levels, for example the process gas at a higher level and the natural gas at a lower level.

The ratio of the carburising agent, consisting of natural gas and/or process gas, with respect to the metallised material, is between 15 and 25 Nm3 if it is natural gas, and between 40 and 60 Nm3 if it is process gas, for each tonne of hot metal iron for each 1% of carbon deposited.

The carbon is deposited on the inner and outer surfaces of the metal material according to the following reactions: CH4 = C + 2H2

CO + H2 = C + H20 2CO = C + C02 To obtain a high conversion of the carbon deposited on the metal surface into iron carbide, obtained through the following reactions: C + Fe = FeC FeC + Fe = Fe2C Fe2C + Fe = Fe3C it is necessary for the material to remain in the lower zone 16 within a range of temperatures of between 550 and 750°C, preferably between 600 and 700°C, for a time of between 30 and 90 minutes or more.

To obtain a greater efficiency in conversion, the cooling speed of the material in this zone must be between 50 and 100°C per hour.

In the embodiment shown here, there is a line 29 which serves to extract the excess gas from the lower zone 16 of the reactor 10 and to send it to the inlet of the unit 24.

As we have said, part of the reformed gas can be removed through the conduit 31 and sent, together with the process gas, to the lower zone 16 of the reactor 10 to achieve the carburising of the reduced material.

The carburised material which leaves the lower reaction zone 16 of the reduction reactor 10 can be fed directly to a melting furnace 21 to be converted into liquid steel, or it can advantageously be fed to an outside cooling system 23.

In the outer cooling system 23 the material discharged from the reactor 10 is lapped by a slightly oxidising gas which allows it to passivate in a period of time necessary to cool it to 60°C, thus obtaining cold granulated iron carbide, which is easy to transport.

This passivation process is actuated by means of the controlled deposit of a layer of oxide on the reduced

pellet, and is encouraged by the fact that the reduction process in the reactor 10 occurs at high temperature thanks to the introduction of oxygen, which raises the temperature of the reducing gas in the upper 12 and intermediate 14 zones.

This higher temperature causes a reduction in the porosity of the pellet and therefore a smaller surface which lends itself to re-oxidation.

The gas used for this cooling is a gas obtained from an independent circuit and has a water vapour content of between 2% and 7%, a hydrogen content of between 1% and 7%, and a methane content of between 50% and 70%. The cooling gas is sent at a temperature preferably between 30°C and 50°C, while the flow rate is preferably between 400 and 600 Nm3/t.

As an example of how the invention is applied, we present the results obtained in a trial program, carried out according to the conditions described above with regard to the flow of carburising agent, reaction times and speed of cooling. % carbon carburizing Carburizing Flow reaction time mins. % agent NM3/t in reduced iron 15 30 45 gp metal temp. °C (t=ton % Fe3C % Fe3C % Fe3C % Fe3C 92,6 650-700 of product) 15 2,15 24,14 27,38 28,65 29,94 92,6 650-700 Natural 35 3,27 36, 84 41, 75 43, 71 45, 66 92, 6 650-700 gas 55 4,40 49,53 56,13 58,77 61,42 92,6 650-700 75 5,53 62,22 70,52 73,84 77,15 92,6 650-700 95 6,66 74,91 84,90 88,90 92,89 92,6 650-700 Process 100 100 3, 58 40,01 45,34 47,48 49,61 92,6 650-700 It is obvious that modifications and variants can be made to the invention yet these shall remain within the field and scope thereof, as set forth in the following claims.