A method to produce direct reduced iron briquettes

Abstract

The invention is related to a method to produce an agglomerated direct reduced iron product (14) comprising the reduction of iron ore (10) in a direct reduction furnace (1) using a reducing gas (11) to produce a hot direct reduced iron (12) having a temperature from 500°C to 1000°C, agglomeration of the hot direct reduced iron (12) together with added solid carbon (13) to form an agglomerated direct reduced iron product (14).

Description

A method to produce direct reduced iron briquettes

[0001] The invention is related to a method to produce direct reduced iron briquettes and to the associated equipment.

[0002] Direct reduced iron (DRI) products or hot briquetted iron (HBI) products result from the direct reduction of iron ores. Those products are traditionally melted in electric arc furnace (EAF) to produce liquid steel or in electric smelting furnaces (ESF) to produce hot metal in steel production.

[0003] The carbon content of these products is an important characteristic as it plays several favorable roles in the steelmaking process. For example, in EAF, carbon addition in the DRI will improve slag foaming which will protect the refractory linings of the furnace and reduce electrodes consumption and heat losses of the process by covering the arcs. Proper amount of carbon in DRI will not only efficiently reduce the residual Wustite in DRI or HBI to Fe which will increase the DRI-metal yield rate, but also accelerate the DRI or HBI melting inside EAF which will minimize the re-oxidation of reduced iron. Additionally, carbon addition through DRI or HBI charging in the bath of liquid steel or molten metal will also improve its stirring by formation of CO bubbles and thus improve removal of impurities, especially N2. Meanwhile when the CO rises up through the molten slag layer, it will reduce FeO present in the slag and thus further increase the metal yield.

[0004] In the classical direct reduction processes, i.e., Midrex® and HyL®, the carbon content of DRI or HBI produced varies in a wide range and, quite often, the carbon contents are lower than that required by EAF steelmaking operations and external carbon additions into the EAF are needed either through charging lump carbon such as coke or injecting carbon powder through dedicated lances.

[0005] This is illustrated in Figure 1 which represents the carbon content of DRI products manufactured in a commercial direct reduction plant. Two periods may be identified, a period of low carbon content (line-1 range) wherein the average carbon content is of 2.05 in weight%, with the standard deviation of 0.18% and the range (Max-min) of 1.64% and a period of high carbon content (line-2 range) wherein the average carbon content was of 2.35 in weight %, with the standard deviation of 0.19% and the range (Max-min) of 2.10%.

[0006] The inconsistent carbon content of DRI or HBI would cause difficulties for EAF operation. The operators would need more time to adjust the external carbon addition via charging carbon and / or injecting carbon; and adjust the oxygen injection for refining thecarbon content of the liquid steel. All these would prolong the tap-to-tap time and decrease the EAF productivity. Meanwhile, it is exceedingly difficult to produce DRI or HBI with a consistent carbon content that meets the requirement from EAF steelmaking operation all the time, especially, for cases without charging carbon and / or injecting carbon.

[0007] Moreover, during steelmaking process, an EAF vessel is fully filled with oxidizing gases, including carbon dioxide (CO2), oxygen (02) and water vapor (H2O). Charging, or injecting carbon into the EAF is thus not very efficient. In the case of charging lump carbon, the lump carbon tends to remain on the surface of the molten slag and will be oxidized by those oxidizing gases. In the case of carbon powder injection, efficiency is highly dependent on the location and number of the injection means and thus certain portions of DRI, and its associated Wustite content, which are located farther to the injection areas will never be reached by the carbon, and no reduction by the added carbon will occur. Both charged lump carbon and injected carbon powder will not reach the Wustite inside the DRI product until it is melted. Reduction efficiency of the carbon and thus the metal yield are not optimal.

[0008] Additionally, in order to reduce CO2 footprint of the steelmaking process, natural gas, currently used in direct reduction process, will be progressively replaced by hydrogen as reducing gas. The increased use of hydrogen implies that less carbon will be deposited on the reduced iron ore and thus the DRI product at the exit of the direct reduction furnace will have a very low carbon content.

[0009] There is so a need for a method allowing to control the carbon content of the direct reduction products. There is furthermore a need for a method allowing to improve the efficiency of carbon in the electric arc furnace and / or the smelting furnaces, notably for metal yield increase and slag foaming.

[0010] This problem is solved by a method to produce an agglomerated direct reduced iron product comprising the steps of reducing iron ore in a direct reduction furnace using a reducing gas to produce a hot direct reduced iron having a temperature from 500°C to 1000°C, agglomerating the hot direct reduced iron together with added solid carbon to form an agglomerated direct reduced iron product, wherein the amount of added solid carbon fulfils equation (1):„ _ '-BHIC '-DRI -add n fl BHlC100 where CBHIC is a targeted carbon content to be reached in the agglomerated direct reduced iron product, CDRI is an amount of carbon in the hot direct reduced iron and r|c isthe fraction of the added solid carbon retained in the agglomerated direct reduced iron product.

[0011] The method of the invention may also comprise the following optional characteristics considered separately or according to all possible technical combinations: the added solid carbon is chosen at least among one of a biomass-based carbon, a biochar, a recycled-carbon, a by-products of graphite materials, coke breeze, petroleum coke, the added solid carbon is a biomass-based carbon, the added solid carbon has a size from 1 mm to 8 mm, the added solid carbon has a size lower than 1 mm, the solid carbon is in a natural form or in an agglomerated form, before being agglomerated with said hot direct reduced iron, the agglomeration step is a briquetting step, the reducing gas comprises more than 40% by volume of hydrogen.

[0012] The invention is also related to a steelmaking method wherein liquid steel is produced using an agglomerated direct reduced iron product manufactured with a method according to anyone of the previous claims.

[0013] The invention is also related to an agglomerated direct reduced iron product comprising from 2.3 to 10 by weight % of carbon, at least 50% in weight of said carbon being free carbon.

[0014] Other characteristics and advantages of the invention will emerge clearly from the description of it that is given below by way of an indication, and which is in no way restrictive, with reference to the appended figures in which:Figure 1 is a diagram illustrating the percentage of carbon in a direct reduction product with a method according to prior artFigure 2 illustrates an embodiment of an equipment allowing to perform a method according to the inventionFIG. 3 is the curves showing the Gibbs free energies of the oxidations of free carbon and the carbon in cementite by oxygen into carbon dioxide (CO2) under different reaction temperatures.FIG. 4 is the curves showing the Gibbs free energies of the oxidations of free carbon and the carbon in cementite by oxygen into carbon monoxide (CO) under different reaction temperatures.FIG. 5 is the curves showing the Gibbs free energies of the oxidations of free carbon and the carbon in cementite by carbon dioxide (CO2) into carbon monoxide (CO) under different reaction temperatures.FIG. 6 is the curves showing the Gibbs free energies of the oxidations of free carbon and the carbon in cementite by water vapor (H2O) into carbon monoxide (CO)

[0015] Elements in the figures are illustration and may not have been drawn to scale.

[0016] Figure 2 illustrates an embodiment of an equipment allowing to perform a method according to the invention. In the method according to the invention, iron ore 10 is charged into a direct reduction furnace, or shaft furnace. The iron ore is reduced in the furnace 1 by a reducing gas 11 introduced into the furnace and flowing counter-currently to the iron ore, thereby forming a hot direct reduced product or hot DRI 12 with a temperature higher than 500°C, preferably from 500 to1000°C. The iron ore 10 may be charged in the form of pellets, briquettes and / or lump ore.

[0017] This hot DRI 12 is then charged into an agglomeration device 2 together with solid carbon 13. The hot DRI 12 is then agglomerated together with the solid carbon 13 to form an agglomerated DRI product 14.

[0018] The solid carbon 13 is added to the agglomeration device in a given amount Cadd which fulfils the following equation (1):Equation (1)CBHIC is a targeted carbon content to be reached in the agglomerated product, it is preferably expressed in % weight and defined according to the subsequent steelmaking requirements. It is preferably comprised from 2.3 to 10 by weight %.CDRI is the amount of carbon in the hot DR1 12. It can be measured by sampling, calculated via mass-balance, or estimated by process mathematical models or simulations. It is expressed in the same unit as CBHIC. r|c is the fraction of the added solid carbon actually retained in the agglomerated product 14. It depends on the temperature of the hot DRI 12, the type and size distribution of the solid carbon, as well as on operational conditions of the agglomeration step. It is preferentially from 0.80 to 1.00. It may be calculated by simulation or determined by experimentation.

[0019] The agglomeration device 2 may be a compactor or a briquetting device. It is preferably located directly at the exit of the furnace 1 to minimize the loss and reoxidationof the hot DRI 12 by avoiding any additional transportation. In another embodiment the hot DRI 12 may be re-heated before being subjected to the agglomeration step.

[0020] The added carbon may be biomass-based Carbon, e.g., biochar, recycled-carbon including graphite refractory, by-products of graphite materials (breeze), coke breeze, petroleum coke. It is preferentially biochar. By Biochar it is meant a charcoal that is produced by pyrolysis of biomass in the absence of oxygen. Biomass is renewable organic material that comes from plants and animals. Biomass sources for energy include wood and wood processing wastes — firewood, wood pellets, and wood chips, lumber and furniture mill sawdust and waste, and black liquor from pulp and paper mills, agricultural crops and waste materials — corn, soybeans, sugar cane, switchgrass, woody plants, and algae, and crop and food processing residues, biogenic materials in municipal solid wastepaper, cotton, and wool products, and food, yard, and wood wastes and animal manure and human sewage.

[0021] The particle size of solid carbon to be added to the hot DRI 12 is preferentially lower than 8 mm, in the forms of natural particles or agglomeration of carbon powders.

[0022] When the agglomeration step is a briquetting step, the particles size of solid carbon has a strong impact on the ratio r|c, for example when particles size is from 1 mm to 8mm the ratio r|c is from 0.95 to 1.00 while it is from 0.80 to 0.90 when the particles size is smaller than 1 mm. This means that with smaller particles more carbon needs to be added to reach the same carbon content in the agglomerated DRI product 14. Other influencing parameters are the pressure and the temperature of the briquetting step.

[0023] The solid carbon 13 may be supplied with any device adapted to the size of the solid carbon to be added. It may be for example a two-ways piston feeder or a screw feeder.

[0024] With the method according to the invention it is possible to control the carbon content of the agglomerated DRI product 14 to be used in the subsequent steelmaking steps and thus to reduce any detrimental impact on the productivity of such subsequent steps.

[0025] Moreover, the inventors have discovered that the direct reduced products manufactured according to the invention has a higher efficiency in terms of metal yield, slag foaming and impurities removal than the existing DR products.

[0026] In the DR products according to prior art, carbon is present in the form of cementite Fe3C while in the DR products according to the invention it is present in the form of free carbon.

[0027] In FIG. 3 to FIG. 6, which illustrate the Gibbs free energies of oxidations of cementite (curve A) and free carbon (curve B) by the different oxidants, the common point is that the two curves cross each other at a temperature of about 800 °C.

[0028] In all these four figures, when the reaction temperature is lower than the temperature of the cross-point, curves A are all below curves B. This indicates that inside the DRI product, the free carbon is more stable than the carbon in cementite when the temperature is below 800 °C. During charging DRI product into an EAF or ESF, the DRI or HBI product will be exposed to the oxidizing gases until it descends through the molten slag layer. The oxidation of the carbon in cementite would be easier than that of free carbon when the temperature is below 800 °C, which is the start temperature for the reduction of Wustite by carbon. Therefore, thermodynamically, the free carbon in the agglomerated DRI product according to the invention will have higher efficiency and higher utilization for reducing the residual wustite and foaming slag than those of the carbon in the cementite. On the other hand, kinetically, the solid carbon inside the agglomerated DRI product according to the invention will have much larger particle size and much less reaction surface than those of the carbon in cementite. The agglomerated DRI product manufactured with the method according to the invention allows thus to increase the metal yield rate, slag foaming, and impurities, notably nitrogen, removal. The agglomerated DRI product according to the invention comprises from 2.3 to 10 by weight % of carbon, preferably more than 5% of this carbon being free carbon, more preferably more than 50%.

[0029] An additional advantage of the invention is that the agglomerated DRI product has a lower content in fines (particles having a size lower than 4mm) than the products according to prior art. When charging a DRI product with a high content of fines in an EAF / ESF, some of the fines will be carried out via the off gas and some of them would be oxidized on the top or upper portion of the molten slag layer as they would be more prone to float, which will decrease the metal yield. With the method according to the invention it is thus possible to increase the metal yield of the steelmaking process.Trial

[0030] In a first trial, a hot DRI product, reduced with a reducing gas and containing 1.45% in weight of carbon and 93.33% in weight of total iron with a metallization degree of 95.84%, remainder being impurities brought by the iron ore, is subjected to a briquetting step at a temperature of 700°C and a pressure of 165 kN / cmL. The target content of carbon CBHIC in the agglomerated product is of 2.5 wt.% and the estimated fraction of the solid carbon thatwould actually be retained in the agglomerated product in the current conditions would be of 0.975. Thus, the amount of carbon added is 1.10 kg carbon per 100 kg DRI.

[0031] The resulting agglomerated direct reduced iron product contains 2.5% in weight total carbon, 43% of which being free carbon, and 92.31% in weight total iron with the same metallization degree as the initial product.

[0032] In another trial the DRI product has been produced with a reducing gas containing more than 80% in volume of H2 and it contains 0.23% by weight of carbon, 92.35% in weight of total iron with a metallization degree of 94.72%, remainder being impurities brought by the iron ore. This hot DRI product is subjecting to an agglomerating step according to the invention, said agglomerating step being a briquetting step at a temperature of 700°C and a pressure of 165 kN / cmL.

[0033] The target content of carbon CBHIC in the agglomerated product is of 2.70 wt.% and the estimated fraction of the solid carbon that would actually be retained in the agglomerated product in the current conditions would be of 0.965. Thus, the amount of carbon added is 2.63 kg carbon per 100 kg of DRI.

[0034] The resulting agglomerated direct reduced iron product contains 2.7% in weight of total carbon, 97% of which being free carbon, and 90.06% in weight of total iron with the same metallization degree as the initial product.

Claims

CLAIMS1. A method to produce an agglomerated direct reduced iron product (14) comprising:- Reducing iron ore (10) in a direct reduction furnace (1 ) using a reducing gas (11 ) to produce a hot direct reduced iron (12) having a temperature from 500°C to 1000°C,- Agglomerating the hot direct reduced iron (12) together with added solid carbon (13) to form an agglomerated direct reduced iron product (14), wherein the amount of added solid carbon, in kg per 100 kg of hot direct reduced iron, fulfils following equation:whereCBHIC is a targeted carbon content to be reached in the agglomerated direct reduced iron product (14)CDRI is an amount of carbon in the hot direct reduced iron (12) r|c is the fraction of the added solid carbon retained in the agglomerated direct reduced iron product (14).

2. A method according to claim 1 wherein the added solid carbon (13) is chosen at least among one of a biomass-based carbon, a biochar, a recycled-carbon, a byproducts of graphite materials, coke breeze, petroleum coke.

3. A method according to claim 2 wherein the added solid carbon (13) is a biomass-based Carbon.

4. A method according to anyone of claims 1 to 3 wherein the added solid carbon (13) has a size from 1 mm to 8 mm.

5. A method according to anyone of claims 1 to 3 wherein the added solid carbon (13) has a size lower than 1 mm.

6. A method according to anyone of claims 1 to 5 wherein the solid carbon (13) is in a natural form or in an agglomerated form, before being agglomerated with said hot direct reduced iron (12).

7. A method according to anyone of the preceding claims wherein the agglomeration step is a briquetting step.

8. A method according to anyone of the previous claims wherein the reducing gas (11 ) comprises more than 40% by volume of hydrogen.

9. A steelmaking method wherein liquid steel is produced using an agglomerated direct reduced iron product (14) manufactured with a method according to anyone of the previous claims.

10. An agglomerated direct reduced iron product (14) comprising from 2.3 to 10 by weight % of carbon, at least 50% in weight of said carbon being free carbon.

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