Method for producing direct reduced iron briquettes

Abstract

The invention relates to a method of producing a pelletized direct reduced iron product (14), the method comprising reducing iron ore (10) in a direct reduction furnace (1) using a reducing gas (11) to produce hot direct reduced iron (12) at a temperature of 500°C to 1000°C; pelletizing the hot direct reduced iron (12) together with added solid carbon (13) to form a pelletized direct reduced iron product (14).

Description

Technical Field

[0001] This invention relates to a method and related equipment for producing direct reduced iron briquettes. Background Technology

[0002] Direct reduced iron (DRI) products, or hot-pressed iron (HBI) products, are produced by the direct reduction of iron ore. These products are traditionally melted in an electric arc furnace (EAF) to produce molten steel, or in an electric melting furnace (ESF) to produce molten iron in steel production.

[0003] The carbon content of these products is an important characteristic because it plays several beneficial roles in the steelmaking process. For example, in EAF (Extended Equivalent Furnace), the addition of carbon to DRI (Distilled Reduction Iron) improves foamy slag, which protects the furnace refractory lining and reduces electrode consumption and heat loss during the process by covering the arc. Appropriate amounts of carbon in DRI not only effectively reduce residual ferrite in DRI or HBI to Fe, thus increasing DRI metal yield, but also accelerate the melting of DRI or HBI in EAF, minimizing the re-oxidation of reduced iron. Furthermore, adding carbon by charging DRI or HBI into the molten steel or molten metal pool also improves agitation by forming CO bubbles, thereby improving the removal of impurities, especially N2. Simultaneously, as CO rises through the slag layer, it reduces FeO present in the slag, further increasing metal yield.

[0004] In classic direct reduction processes, namely Midrex® and HyL®, the carbon content of the produced DRI or HBI varies widely and is usually lower than the requirements for EAF steelmaking operations. Therefore, external carbon needs to be added to the EAF by charging block carbon such as coke or by spraying carbon powder through a special spray gun.

[0005] This shows Figure 1 middle, Figure 1 This indicates the carbon content of DRI products manufactured in commercial direct reduction facilities. Two periods can be identified: a low-carbon-content period (line-1 range) with an average carbon content of 2.05 wt%, a standard deviation of 0.18%, and a range (Max-min) of 1.64%; and a high-carbon-content period (line-2 range) with an average carbon content of 2.35 wt%, a standard deviation of 0.19%, and a range (Max-min) of 2.10%.

[0006] Inconsistent carbon content in the DRI or HBI can cause difficulties in EAF operation. Operators will need more time to adjust external carbon addition by charging and / or injecting carbon; and to adjust oxygen injection to refine the carbon content of the molten steel. All of this will prolong tap-to-tapping time and reduce EAF productivity. Meanwhile, producing a DRI or HBI with a consistent carbon content that always meets the requirements of EAF steelmaking operation is extremely difficult, especially when carbon is not charged and / or injected.

[0007] Furthermore, during the steelmaking process, the EAF vessel is completely filled with oxidizing gases, including carbon dioxide (CO2), oxygen (O2), and water vapor (H2O). Therefore, charging or injecting carbon into the EAF is not very efficient. With lumpy carbon, it tends to remain on the surface of the slag and be oxidized by these oxidizing gases. With injected carbon powder, efficiency is highly dependent on the location and number of injection devices; therefore, certain portions of the DRI located far from the injection area, along with their associated aragonite content, will never be contacted by carbon, and no reduction caused by the added carbon will occur. Neither lumpy carbon nor injected carbon powder will contact the aragonite within the DRI product before it melts. The reduction efficiency of carbon, and consequently, the metal yield, is not optimal.

[0008] Furthermore, to reduce the CO2 footprint of the steelmaking process, the natural gas currently used in direct reduction will be gradually replaced by hydrogen as the reducing gas. Increased hydrogen use means less carbon will be deposited on the reduced iron ore, resulting in very low carbon content in the DRI product at the direct reduction furnace outlet.

[0009] Therefore, a method is needed that allows control over the carbon content of direct reduction products. Furthermore, a method is needed that allows for improvement of carbon efficiency in electric arc furnaces and / or electric melting furnaces, particularly for increasing metal yield and improving foamy slag. Summary of the Invention

[0010] This problem is solved by a method for producing agglomerated direct reduced iron products, which includes the following steps: reducing iron ore with reducing gas in a direct reduction furnace to produce hot direct reduced iron at a temperature of 500°C to 1000°C; agglomerating the hot direct reduced iron with added solid carbon to form agglomerated direct reduced iron products, wherein the amount of added solid carbon satisfies equation (1):

[0011]

[0012] Where C BHIC The target carbon content, C, to be achieved in agglomerated direct reduced iron products. DRI It is the amount of carbon in thermally reduced iron, η CIt is the proportion of added solid carbon retained in the agglomerated direct reduced iron product.

[0013] The method of the present invention may also include the following optional features, either individually or according to all possible combinations of techniques:

[0014] - The added solid carbon is selected from at least one of the following: biomass-based carbon, biochar, recycled carbon, graphite material byproducts, coke powder, and petroleum coke.

[0015] - The added solid carbon is biomass-based carbon.

[0016] - The added solid carbon has a size of 1 mm to 8 mm.

[0017] - The added solid carbon has a size of less than 1 mm.

[0018] - Before agglomeration with the thermally reduced iron, solid carbon exists in its native or agglomerated form.

[0019] - The reunion step is the pressing step.

[0020] - The reducing gas contains more than 40% hydrogen by volume.

[0021] The present invention also relates to a steelmaking method in which molten steel is produced using an agglomerated direct reduced iron product manufactured using the method according to any one of the preceding claims.

[0022] The present invention also relates to an agglomerated direct reduced iron product containing 2.3% to 10% by weight of carbon, wherein at least 50% by weight of the carbon is free carbon. Attached Figure Description

[0023] Other features and advantages of the invention will become apparent from the following illustrative and in no way limiting description given with reference to the accompanying drawings, in which:

[0024] - Figure 1 This is a chart showing the percentage of carbon in the direct reduction product according to existing techniques.

[0025] - Figure 2 Embodiments of devices that allow the execution of the method according to the invention are shown.

[0026] - Figure 3 It is a curve showing the Gibbs free energy of oxygen oxidizing free carbon and carbon in cementite into carbon dioxide (CO2) at different reaction temperatures.

[0027] - Figure 4 The curves show the Gibbs free energy of oxygen oxidizing free carbon and carbon in cementite to carbon monoxide (CO) at different reaction temperatures.

[0028] - Figure 5 It is a curve showing the Gibbs free energy of carbon dioxide (CO2) oxidizing free carbon and carbon in cementite to carbon monoxide (CO) at different reaction temperatures.

[0029] - Figure 6 This curve shows the Gibbs free energy of the oxidation of free carbon and carbon in cementite to carbon monoxide (CO) by water vapor (H2O).

[0030] The elements in the accompanying drawings are schematic and may not be drawn to scale. Detailed Implementation

[0031] Figure 2 An embodiment of an apparatus that allows the execution of the method according to the invention is shown. In the method according to the invention, iron ore 10 is charged into a direct reduction furnace or shaft furnace. The iron ore is introduced into the furnace 1 and reduced by reducing gas 11 flowing countercurrently to the iron ore, thereby forming a hot direct reduction product or hot DRI 12 at a temperature above 500°C, preferably 500°C to 1000°C. The iron ore 10 may be charged in the form of pellets, briquettes, and / or lump ore.

[0032] The hot DRI 12 is then loaded together with solid carbon 13 into the agglomeration device 2. The hot DRI 12 and solid carbon 13 are then agglomerated together to form agglomerated DRI product 14.

[0033] Solid carbon-13 is used in a given amount of C add The addition of this amount C to the aggregation device add The following equation (1) must be satisfied:

[0034] Equation (1)

[0035]

[0036] C BHIC This refers to the target carbon content to be achieved in the agglomerated product, preferably expressed as a percentage by weight (%), and defined according to subsequent steelmaking requirements. It is preferably between 2.3% by weight and 10% by weight.

[0037] C DRI This refers to the carbon content in thermal DRI 12. It can be estimated through sampling measurement, mass balance calculations, or process mathematical models or simulations. It is expressed in relation to C. BHIC The same units are used.

[0038] η CThis refers to the proportion of added solid carbon actually retained in the agglomerated product 14. It depends on the temperature of the thermal DRI 12, the type and size distribution of the solid carbon, and the operating conditions of the agglomeration step. It is preferably between 0.80 and 1.00. It can be determined through simulation calculations or experiments.

[0039] The agglomeration device 2 can be a compactor or a briquetting device. It is preferably located directly at the outlet of furnace 1 to minimize the loss and re-oxidation of the hot DRI 12 by avoiding any additional transport. In another embodiment, the hot DRI 12 can be reheated before undergoing the agglomeration step.

[0040] The added carbon can be biomass-based carbon, such as biochar, recycled carbon including graphite refractories, graphite byproducts (powder), coke powder, and petroleum coke. Biochar is preferred. Biochar refers to charcoal produced through the pyrolysis of biomass under anaerobic conditions. Biomass is a renewable organic material derived from plants and animals. Biomass sources for energy include wood and wood processing waste—firewood, wood pellets and sawdust, sawdust and waste from wood and furniture factories, and black liquor from pulp and paper mills; agricultural crops and waste—corn, soybeans, sugarcane, switchgrass, woody plants and algae; agricultural and food processing residues; biomass materials in municipal solid waste paper, cotton and wool products; and food, yard and wood waste, animal manure, and human sewage.

[0041] The particle size of the solid carbon to be added to thermal DRI 12 is preferably less than 8 mm, and it is in the form of natural particles or carbon powder agglomerates.

[0042] When the agglomeration step is a briquetting step, the particle size of solid carbon is proportional to η. C It has a significant impact, for example, when the particle size is between 1 mm and 8 mm, the proportion η C The ratio η ranges from 0.95 to 1.00, while when the particle size is less than 1 mm, the ratio η is... C The carbon content is between 0.80 and 0.90. This means that with smaller particle sizes, more carbon needs to be added to achieve the same carbon content in the agglomerated DRI product 14. Other influencing parameters are the pressure and temperature of the briquetting process.

[0043] Solid carbon 13 can be supplied using any device suitable for the size of solid carbon to be added. For example, it can be a bidirectional piston feeder or a screw feeder.

[0044] Using the method according to the invention, the carbon content of the agglomerated DRI product 14 used in subsequent steelmaking steps can be controlled, thereby reducing any adverse effects on the productivity of such subsequent steps.

[0045] Furthermore, the inventors have discovered that the direct reduction product manufactured according to the present invention is more efficient than existing DR products in terms of metal recovery, foam slag and impurity removal.

[0046] In DR products according to the prior art, carbon exists in the form of cementite Fe3C, while in DR products according to the present invention, it exists in the form of free carbon.

[0047] The Gibbs free energy of cementite oxidized by different oxidants (curve A) and free carbon (curve B) is shown. Figures 3 to 6 In the two cases, the common point is that the two curves intersect each other at a temperature of about 800°C.

[0048] In all four graphs, curve A is lower than curve B when the reaction temperature is below the crossover point. This indicates that free carbon is more stable than carbon in cementite within the DRI product at temperatures below 800°C. During the loading of the DRI product into the EAF or ESF, the DRI or HBI product is exposed to oxidizing gases until it descends through the slag layer. When the temperature is below 800°C (the starting temperature for carbon reduction of argentite), the oxidation of carbon in cementite is easier than that of free carbon. Therefore, thermodynamically, the free carbon in the agglomerated DRI product according to the invention will have higher efficiency and utilization in reducing residual argentite and reducing frothing slag than carbon in cementite. On the other hand, kinetically, the solid carbon within the agglomerated DRI product according to the invention will have a much larger particle size and a much smaller reaction surface than carbon in cementite. The agglomerated DRI product manufactured using the method according to the invention thus allows for improved metal yield, frothing slag, and the removal of impurities, particularly nitrogen. The aggregated DRI product according to the present invention contains 2.3% to 10% carbon, preferably more than 5% and more preferably more than 50% of the carbon as free carbon.

[0049] Another advantage of this invention is that the content of fine powder (particles smaller than 4 mm) in the agglomerated DRI product is lower than that in products according to the prior art. When a DRI product with a high content of fine powder is charged into the EAF / ESF, some of the fine powder is carried away by the exhaust gas, while some of the fine powder is oxidized on top of or above the slag layer because it is more likely to float, which reduces the metal yield. Therefore, the metal yield in the steelmaking process can be improved using the method according to the invention.

[0050] test

[0051] In the first test, a hot DRI product, reduced by the reducing gas and containing 1.45% carbon by weight and 93.33% total iron by weight, with a metallization rate of 95.84% and the remainder being impurities from the iron ore, underwent a briquetting step at a temperature of 700°C and a pressure of 165 kN / cmL. The target carbon content (C) in the agglomerated product was... BHIC The carbon content is 2.5% by weight, and the estimated proportion of solid carbon actually retained in the aggregated product under current conditions is 0.975%. Therefore, the amount of carbon added is 1.10 kg carbon per 100 kg DRI.

[0052] The resulting agglomerated direct reduced iron product contains 2.5% total carbon by weight, of which 43% is free carbon, and 92.31% total iron by weight, with the same metallization rate as the initial product.

[0053] In another experiment, a DRI product was produced using a reducing gas containing more than 80% H2 by volume, and it contained 0.23% carbon by weight, 92.35% total iron by weight, and a metallization rate of 94.72%, with the remainder being impurities from the iron ore. This hot DRI product was then subjected to an agglomeration step according to the invention, which was a briquetting step performed at a temperature of 700°C and a pressure of 165 kN / cmL.

[0054] Target carbon content C in agglomerated products BHIC The carbon content is 2.70% by weight, and the estimated proportion of solid carbon actually retained in the aggregated product under current conditions is 0.965%. Therefore, the amount of carbon added is 2.63 kg carbon per 100 kg DRI.

[0055] The resulting agglomerated direct reduced iron product contains 2.7% total carbon by weight, of which 97% is free carbon, and 90.06% total iron by weight, with the same metallization rate as the initial product.

Claims

1. A method for producing agglomerated direct reduced iron product (14), comprising: - Iron ore (10) is reduced in a direct reduction furnace (1) using reducing gas (11) to produce hot direct reduced iron (12) at a temperature of 500°C to 1000°C. - The thermal direct reduced iron (12) is agglomerated together with added solid carbon (13) to form agglomerated direct reduced iron product (14). The amount of solid carbon added, expressed in kg per 100 kg of thermally reduced iron, satisfies the following equation: in C BHIC The target carbon content to be achieved in the agglomerated direct reduced iron product (14) is... C DRI It is the amount of carbon in the thermally reduced iron (12). η C It is the proportion of the added solid carbon retained in the agglomerated direct reduced iron product (14).

2. The method according to claim 1, wherein the added solid carbon (13) is at least selected from one of biomass-based carbon, biochar, recycled carbon, graphite material by-products, coke powder, and petroleum coke.

3. The method according to claim 2, wherein the added solid carbon (13) is biomass-based carbon.

4. The method according to any one of claims 1 to 3, wherein the added solid carbon (13) has a size of 1 mm to 8 mm.

5. The method according to any one of claims 1 to 3, wherein the added solid carbon (13) has a size of less than 1 mm.

6. The method according to any one of claims 1 to 5, wherein the solid carbon (13) is in natural or aggregated form prior to agglomeration with the thermally reduced iron (12).

7. The method according to any one of the preceding claims, wherein the agglomeration step is a briquetting step.

8. The method according to any one of the preceding claims, wherein the reducing gas (11) comprises more than 40% hydrogen by volume.

9. A steelmaking method wherein molten steel is produced using an agglomerated direct reduced iron product (14) manufactured using the method according to any one of the preceding claims.

10. An agglomerated direct reduced iron product (14) comprising 2.3% to 10% by weight of carbon, wherein at least 50% by weight of the carbon is free carbon.

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