A method to produce a composite briquette and associated composite briquette

Abstract

A method to produce a briquette of composite material comprising the steps of providing biochar (2) and an iron material (1) comprising at least 30% of iron oxides, mixing the biochar (2) and the iron material (1) in appropriate amounts to obtain a predefined molar Fe / C ratio in the range from 3 to 6.5 and briquetting the obtained mixture to form a briquette of composite material (6).

Description

A method to produce a composite briquette and associated composite briquette

[0001] The invention is related to a method to produce a composite briquette comprising oxidized iron and biochar and to the use of such a briquette.

[0002] Steel can be currently produced at an industrial scale through two main manufacturing routes. Nowadays, most used production route consists in producing pig iron in a blast furnace, this process relying on two main raw materials; sintered iron ore to be reduced and coke as reducing agent. Both those raw materials require respectively a sinter plant and a coking plant to be produced, both of those plants being big CO2, and more globally pollutants, emitter.

[0003] An alterative route is the direct reduction of iron ore in the solid state by carbon monoxide and hydrogen derived from natural gas or coal. Such a direct reduction process also requires the use of fossil-based carbon and is a CO2 emitter.

[0004] In order to reduce its environmental footprint steel industry is looking for solutions to produce iron and carbon-bearing materials without the need of the sintering and coking processes and to promote use of renewable carbon materials.

[0005] There is so a need for a method to produce a composite iron-carbon material to be used in the iron or steelmaking which has a reduced environmental footprint.

[0006] This problem is solved by a method according to the invention, comprising the steps of providing biochar and an iron material comprising at least 30% of iron oxides, mixing the biochar and the iron material in appropriate amounts to obtain a predefined molar Fe / C ratio in the range from 3 to 6.5 and briquetting the obtained mixture to form a briquette of composite material.

[0007] The method of the invention may also comprise the following optional characteristics considered separately or according to all possible technical combinations:- the iron material is chosen among at least one of iron ore, iron oxides concentrate, direct reduction fines, oily mill sludge, iron mine tailings, electric arc furnace dust and sintering fines,- the mixing step further comprises the addition of a binder to the biochar and to the iron material,- the method further comprises a preliminary step of pyrolyzing biomass to produce a biochar, a biogas and a pyrolysis oil, the biochar being then provided in step A,- the biomass is lignocellulosic biomass,- the lignocellulosic biomass is chosen among wheat straw, miscanthus, sawdust, eucalyptus, apple bagasse, or out-of-use wood,- the binder comprises pyrolysis oil,- the ratio Fe / C is from 3 to 4.4,- the ratio Fe / C is from 4.5 to 5.9,- the ratio Fe / C is from 6.0 to 6.5,- before step A, the biochar is subjected to a crushing step,- the biochar is provided at a granulometry where 90% of the particles have a size inferior to 3mm.

[0008] The invention is also related to a briquette consisting of biochar and an iron material comprising at least 30% by weight of iron oxides, the amount of biochar and iron material in said briquette being such that the molar ratio Fe / C of the briquette is from 3 to 6.5.

[0009] 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 illustrates a method to produce a briquette according to the inventionFigure 2 illustrates another embodiment of a method according to the inventionFigure 3 is a picture of a sample of solidified slag produced in a steelmaking method without specific additionFigure 4 is a picture of a sample of solidified slag produced in a steelmaking method including the addition of a composite briquette according to the invention

[0010] Elements in the figures are illustration and may not have been drawn to scale.

[0011] In the method as illustrated in Figure 1 iron material 1 comprising at least 30% by weight of iron oxides and biochar 2 are charged together in the hopper 3 of a briquetting machine 4.

[0012] The amount of each material to be charged in the hopper depends on a previously defined molar ratio Fe / C in the range from 3 to 6.5.

[0013] The mix of iron material 1 and biochar 2 is then forced through a gap between dual cylindrical rollers 5 rotating horizontally in opposite directions around parallel axes. The two rollers are arranged in such a way that a small gap exists between them, and the gap depends on factors such as the mix type, the particle size, the moisture content, and the optional addition of binders. The mix is then pressed into a die forming the briquettes 6, which come out on the exit side of the rollers. The type of roller or die used determines the shape of the briquettes 6. In this embodiment a roller press is described but other briquetting technologies can be used.

[0014] The iron material comprising at least 30% by weight of iron oxides maybe iron ore, a concentrate of iron oxides, direct reduction fines, oily mill sludge, iron mine tailings, electric arc furnace dust, sintering fines, or a mixture of any of those materials. The iron oxides may be hematite Fe2O3, magnetite Fe3O4, goethite FeO(OH), limonite FeO(OH)n(H2O), siderite FeCO3 or a mixture of those different oxides. In a preferred embodiment the iron material is a low-quality material comprising less than 67% by weight of iron oxides.

[0015] 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 — com, 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.

[0016] In a preferred embodiment, the biomass is lignocellulosic biomass. The term 'Lignocellulosic’ is understood herein to mean any of several closely related substances consisting essentially of cellulose and hemicellulose in a lignin framework. Such lignocellulosic biomass can be found within forestry products and by-products, agricultural products and by-products (including residues such as straw and chaff wastefrom harvesting crops) and / or energy crops such as sorghum, switchgrass and sugar cane (as sugar cane bagasse).

[0017] This biomass material is preferably chosen among wheat straw, miscanthus, sawdust, eucalyptus, apple bagasse, or out-of-use wood.

[0018] The biochar is preferably provided at a granulometry where 90% of the particles have a size inferior to 3mm (90% < 3mm). This allows having a homogenous distribution of the biochar within the briquette. To reach this granulometry, the biochar may be subjected to a crushing step before being provided to the briquetting machine 4.

[0019] In another embodiment of the invention, as illustrated in figure 2, biomass 10 is subjected to pyrolysis in a pyrolysis reactor 11. This pyrolysis of biomass produces biochar 2, a biogas 12 and a pyrolysis oil 13. This pyrolysis oil 13 is at least partly used as a binder into the briquetting process of the biochar 2 and the iron material 1. All features described in the embodiment of figure 1 are applicable to the embodiment of figure 2. Binders are added to the material mix when the material mix to be agglomerated would not have sufficient compressive and impact resistance, after agglomeration.

[0020] Use of pyrolysis oil 13 as a binder allows recycling a byproduct of the pyrolysis process and thus further reducing the environmental footprint of the overall process. The pyrolysis oil 13 preferably comes from the production of the biochar used in the mix but may come from other biochar production according to availabilities.

[0021] In all previously mentioned embodiments, the amount of each material to be charged in the hopper 3 depends on a previously defined molar ratio Fe / C to be obtained.

[0022] The molar Fe / C ratio of the briquette produced with a method according to the invention is set in a range from 3 to 6.5. Below 3, the Fe / C ratio will be too loo to have a reducing effect. Above 6.5, to the iron content would be too low for the subsequent steel and / or ironmaking steps.

[0023] The main role of the carbon addition to the briquette is to have a reducing effect on iron oxides, which means that the Fe / C ratio is predefined notably depending on to the type of oxides present in the iron material which contains at least 30% of iron oxides.

[0024] Different reduction reactions may occur, their general expression being:(1 ) FexOy + zC -► x Fe + CzOyWhich for hematite Fe2O3 may give following reactions:(2) Fe2O3 + 3C -> 2 Fe + 3CO(3) Fe2O3 + 2C - 2 Fe + CO + CO2(4) 2 Fe2O3 + 3C 4 Fe + 3CO2

[0025] The molar ratio of Fe / C depends also on the subsequent use of the briquette. Indeed, carbon may also be used for other actions than reducing iron oxides. In particular, when used in a direct reduction step at solid state, such carbon can enrich the DRI product obtained. Such direct reduction may be performed in a standard DRI furnace such as a Midrex® or HYL® furnace, preferably operated with hydrogen as reducing gas. The use of the briquette allows in that case to provide carbon for the reduction but also to increase the carbon content of the resulting DRI product. Indeed, one of the issues when operating with an H2 DRI process is that the resulting DRI product will have a low carbon content which may raise issue in the subsequent steps of the steelmaking process.

[0026] This solid reduction may also be performed without the use of any reducing gas but by heating of the briquette to a temperature from 800°C to 1000°C to bum off the carbon and oxygen and thus reduce the oxidized iron to metal iron. In that case, the molar Fe / C ratio can be adapted, so that, the carbon of the briquette may also be used, on top for the reduction of the iron oxides, to provide heat to the process. This solid reduction may be performed in Rotary Hearth Furace (RHF), preferably in an inert atmosphere. If the Fe / C ratio is chosen only to fulfill the needs of the reduction reactions to occur, then the required heating energy for driving this process may be provided by electrical heating and not by combustion of the carbon.

[0027] This solid reduction state process may be performed at lower temperatures, from 550 to 600°C to directly reduce magnetite to iron with the aid of catalysts such as cobalt or nickel materials.

[0028] When added to an electric arc furace (EAF), an electrical smelting furnace (ESF), a submerged arc furace (SAF), or an oxygen bath smelting furace (OSBF), the briquette produced with the method according to the invention will be melted either alone, or in combination with steel scrap and / or DRI products. If the briquette is dense enough to reach the metal melt beneath the slag, use of this briquette will allow notably to provide carbon to the melt, carbon which is necessary to carburize the molten metal and forms CO bubbles by reaction with oxygen. These bubbles can stir the bath and promote reduction of impurities. If the composite has a lower density and stays in the slag, due to its significant content of volatile matters, this briquette may also generatemore gases than standard DRI which will enhance slag foaming. Slag foaming is essential for energy efficiency of the electric arc furnace.

[0029] The density of the briquette can notably be adjusted by changing the molar ratio Fe / C.

[0030] A Fe / C molar ratio from 3 to 4.4 allows reducing more iron oxides and thus reaching a higher metallization yield after reduction. This moreover allows having a reduced iron product with a high carburizing potential for the subsequent steelmaking steps.

[0031] A Fe / C molar ratio from 4.5 to 5.9 implies that the reducing output is lower than with the previous ratio, but a high metallization yield may still be obtained thanks to the participation of the carbon.

[0032] A Fe / C molar ratio from 6.0 to 6.5 allows increasing the density of the briquette and thus of the resulting reduced iron product but there is a higher risk of reoxidation during the reduction step due to the production of CO2.

[0033] The use of the briquette produced with a method according to the invention in an iron or steelmaking process also allows the use of sintered ore and coke to be reduced, thereby reducing the emissions of NOx, SOx and PAHs associated with sintering and coking plants. Furthermore, when used as a coke substitute, the use of this composite material makes it possible to reduce the CO2 footprint of the steelmaking process by replacing fossil carbon with renewable carbon. It also allows the sulphur content of the resulting molten metal to be reduced. Coke is made from coal, which contains sulphur, which remains in the coke and must be removed in special desulphurisation steps.Trials

[0034] In a first example biochar crushed at a granulometry where 90% of the particles have a size inferior to 3mm (90% < 3mm) was mixed with fine grain size (<1mm) iron ore concentrate comprising 65.8% by weight of oxidized iron, 94% by weight of which being Fe2O3 and with an organic bio-binder. Straw pellets as biomass material were pyrolyzed to produce the biochar. Pyrolysis oil coming from pyrolysis of beech wood was mixed with 15wt.% of expanded Polystyrene (EPS) to produce the organic bio binder. The molar Fe / C ratio was set at 3.11.

[0035] This mix of material comprising 15% by weight of bio-binder, 19.55% by weight of biochar and 65.45% by weight of iron material was subjected to the briquetting stepwith a pressing force of 1.31 / cm2and crude composite briquettes of 45x55x35 mm size and a weight of 100g were obtained and analyzed to determine their mechanical properties.

[0036] Compressive strength of the briquettes was 1600N and apparent density 1 ,80g / cm3. The strength is sufficient for handling and charging of the briquettes for subsequent ironmaking / steelmaking steps.Trial A - Direct reduction

[0037] 1.68kg dry basis of these briquettes were subjected to a direct reduction step at solid stage, wherein the pellets were placed in an oven and heated with a heating ramp at 3°C / minutes up to 1020°C under an inert atmosphere. This inert atmosphere was added to avoid reoxidation of the iron, this is linked to the small size of the trial. For same process applied to more than a hundred kilograms of composite material this inerting would be inherently created by the gas produced by the reaction of reduction.

[0038] After 3 hours at 1020°C, 1.06kg dry basis of DRI were obtained with 74% of Fe metal and a metallization rate of 97%. Metallization rate of DRI is the extent of conversion of iron oxide into metallic iron during reduction. It is defined in percentage of the mass of metallic iron divided by the mass of total iron.Trial B - Smelting

[0039] 360kg of these briquettes were charged in 6 tons smelting furnace. Briquettes were able to penetrate the slag layer and descend in the bath of liquid metal. A carburetion yield of liquid metal from 38 to 49% was observed with the briquette according to the invention, compared to 21% observed for coke addition. Moreover, as illustrated in figures 3 and 4, a strong slag foaming was observed during the trial with loading of the briquettes compared to no specific addition. Figure 3 is a picture of a sample of solidified slag without any addition while figure 4 is a picture of a sample of solidified slag obtained with the charging of the briquettes produced with a method according to the invention. One may observe a higher porosity of the second slag samples which results from the foaming.

Claims

CLAIMS1 ) A method to produce a briquette of composite material comprising the steps of:A. Providing biochar (2) and an iron material (1 ) comprising at least 30% by weight of iron oxides,B. Mixing the biochar (2) and the iron material (1 ) in appropriate amounts to obtain a predefined molar Fe / C ratio in the range from 3 to 6.5,C. Briquetting the obtained mixture to form a briquette of composite material (6).2) A method according to claim 1 wherein the iron material (1 ) is chosen among at least one of iron ore, iron oxides concentrate, direct reduction fines, oily mill sludge, iron mine tailings, electric arc furnace dust and sintering fines.3) A method according to any one of the claims 1 or 2 wherein the mixing step further comprises the addition of a binder to the biochar (2) and to the iron material (1 ).4) A method according to any one of the claims 1 to 3 comprising a preliminary step of pyrolyzing biomass (10) to produce a biochar (2), a biogas (12) and a pyrolysis oil (13), the biochar (2) being then provided in step A.5) A method according to claim 4 wherein the biomass (10) is lignocellulosic biomass.6) A method according to claim 5 wherein the lignocellulosic biomass (10) is chosen among wheat straw, miscanthus, sawdust, eucalyptus, apple bagasse, orout-of-use wood.7) A method according to any one of the claims 4 to 6, taken in combination with claim 3, wherein the binder comprises pyrolysis oil (13).8) A method according to any one of the claims 1 to 7 wherein the ratio Fe / C is from 3 to 4.4.9) A method according to any one of the claims 1 to 7 wherein the ratio Fe / C is from4.5 to 5.9.10) A method according to any one of the claims 1 to 7 wherein the ratio Fe / C is from 6.0 to 6.5.11) A method according to anyone of the previous claims wherein, before step A, the biochar is subjected to a crushing step.12) A method according to anyone of the previous claims wherein the biochar is provided at a granulometry where 90% of the particles have a size inferior to 3mm.13) A briquette consisting of biochar and an iron material comprising at least 30% by weight of iron oxides, the amount of biochar and iron material in said briquette being such that the molar ratio Fe / C of the briquette is from 3 to 6.5.14) A briquette according to claim 13 wherein the molar ratio Fe / C of the briquette is from 3 to 4.4.15) A briquette according to claim 13 wherein the molar ratio Fe / C of the briquette is from 4.5 to 5.9.16) A briquette according to claim 13 wherein the molar ratio Fe / C of the briquette is from 6.0 to 6.5.

Images