A composite material comprising iron oxides and biomass

Abstract

A composite material consisting of lignocellulosic biomass having a moisture content from 5 to 10% by weight and an iron material comprising at least 30% of iron oxides, the molar ratio Fe / C of said composite material being from 3 to 6.5.

Description

A composite material comprising iron oxides and biomass

[0001] The invention is related to a composite material comprising iron oxides and biomass, to a production method of such a composite and to an ironmaking method using such a composite.

[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 alternative 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 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 composite according to the invention, consisting of lignocellulosic biomass having a moisture content from 5 to 10% by weight and an iron material comprising at least 30% of iron oxides, the molar ratio Fe / C of said composite material being from 3 to 6.5.

[0007] The composite 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, direct reduction fines, oily mill sludge, iron mine tailings, electric arc furnace dust and sintering fines, biomass is chosen among at least one of wheat straw, miscanthus, out-of-use wood or sawdust, the composite material is in form of an extruded pellet, the composite material consists of from 25 to 65 % by weight of biomass and from 35 to 75% by weight of iron material. the molar ratio Fe / C is from 3 to 4.4, the molar ratio Fe / C is from 4.5 to 5.9, the molar ratio Fe / C is from 6.0 to 6.5.

[0008] The invention is also related to a method to produce such a composite material comprising the steps of providing lignocellulosic biomass having a moisture content from 5 to 10% by weight, providing an iron material comprising at least 30% of iron oxides, mixing the biomass and the iron material in appropriate amounts to obtain a predefined molar Fe / C ratio in the range from 3 to 6.5 and agglomerating the obtained mixture to form the composite material.

[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 composite material according to the invention in the form of pellets

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

[0011] The pellets shown in Figure 1 consists of lignocellulosic biomass material having a moisture content from 5 to 10% by weight and an iron material comprising at least 30% by weight of iron oxides, molar ratio Fe / C of said composite material being from 3 to 6.5.

[0012] 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 byproducts, agricultural products and by-products (including residues such as straw and chaff waste from harvesting crops) and / or energy crops such as sorghum, switchgrass and sugar cane (as sugar cane bagasse).

[0013] This biomass material is preferably chosen among wheat straw, miscanthus, sawdust, eucalyptus, apple bagasse, or out-of-use wood.

[0014] One advantage of using biomass is that it has a higher proportion of volatile matter VM compared to fossil carbon. VM is the component of coal I biomass which is driven off when the coal I biomass is rapidly heated out of contact of air. The gases released are usually a mixture of short- and long-chain hydrocarbons, aromatic hydrocarbons, and sulfur. VM is the percentage loss in mass, adjusted for moisture, when coal is heated out of contact with air under standard conditions.

[0015] The iron material comprising at least 30% by weight of iron oxides maybe a concentrate of iron ore, 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.

[0016] The inventors have found, surprisingly, that when such biomass material with specific moisture content is mixed with iron oxides, without any use of binders or additional fluxes, the resultant mixture is suitable for forming a composite material of the required strength for handling and transportation. Moreover, such a composite has proven to be suitable for a direct reduction process with a percentage of metallization of at least 93%, compared to an average metallization rate 92-95% for a DRI produced by existing technologies such as Midrex® or HYL®. 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.

[0017] A method to produce such a composite material comprises the supply of the biomass with required moisture content and the iron material, the preparation of a mixture of the biomass and the iron material to obtain a predefined ratio. The mixture is then agglomerated to form a composite material. The agglomeration is for example extrusion.

[0018] The molar Fe / C ratio of the composite according to the invention is set in a range from 3 to 6.5. Below 3, the Fe / C ratio will be too low to have a reducing effect. Above 6.5, to the iron content would be too low for the subsequent steel and / or ironmaking steps.

[0019] The main role of the carbon addition to the composite 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.

[0020] , Different reductions reactions may be considered, 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

[0021] The molar ratio of Fe / C depends also on the subsequent use of the composite material. Indeed, carbon may also be used for other actions than reducing iron oxides. In particular, when used in a direct reduction process 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 composite material 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.

[0022] This solid reduction may also be performed without the use of any reducing gas but by heating of the composite materials to a temperature from 800°C to 1000°C to bum off the carbon and oxygen and thus reduce the iron oxides to metal iron. In that case, the molar Fe / C ratio can be adapted, so that the carbon of the composite material may also be used, on top of reduction of the iron oxides, to provide heat to the process. This solid reduction may be performed in Rotary Hearth Furnace (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 this process may be provided by electrical heating and not by combustion of the carbon.

[0023] 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.

[0024] When added to an electric arc furnace (EAF), an electrical smelting furnace (ESF), a submerged arc furnace (SAF), or an oxygen bath smelting furnace (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 generate more gases than standard DRI which will enhance slag foaming. Slag foaming is essential for energy efficiency of the electric arc furnace.

[0025] The density of the composite can notably be adjusted by changing the molar ratio Fe / C.

[0026] 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.

[0027] 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.

[0028] A Fe / C molar ratio from 6.0 to 6.5 allows increasing the density of the composite and of the resulting reduced iron product but there is a higher risk of reoxidation during the reduction step due to the production of CO2.

[0029] The use of this composite material 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

[0030] In a first example, Pelletl , 2kg of wheat straw with 5% by weight of moisture were mixed with 3kg of Mount Wright iron ore comprising 67% by weight of iron oxides,remainder being gangue comprising notably alumina, silica, phosphorus and sulphur, among them being 90% by weight of hematite, and the mixture was subjected to an extrusion step in a 7.5kW extruder to produce composite extruded pellets having a diameter of 6mm and a length of 30mm. These composite pellets have a Fe / C molar ratio of 3.11. Analysis of this composite was performed, and results are illustrated in Table 1.

[0031] Volatile matter is determined as the loss in mass, adjusted for moisture, when a sample is heated at 900 degrees Celsius for a period of 7 minutes. This procedure is undertaken out of contact with air under standardized conditions defined in European Standard EN15148-2009 ("Solid biofuels - Determination of the content of volatile matter").

[0032] Other components content is indicated as dry basis, meaning that water content of the analyzed material is excluded from the weight calculation. In the present case the pellet was heated to 105°C until a constant weight was observed, meaning that all water was evaporated, before chemical analysis of the remaining components.Table 1

[0033] 1.68kg dry basis of these pellets were subjected to a direct reduction step at solid state, wherein the pellets were placed in an oven and heated with a heating ramp at 3°C / min up to 1020°C under an inert atmosphere. This inert atmosphere was used 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 the reoxidation is inherently prevented by the gas produced by the reaction of reduction.

[0034] 1.06kg dry basis of DRI were obtained with 74% of Fe metal and a metallization rate of 96%.

[0035] In a second example Pellet2, a mixture comprising 60% by weight of wheat straw having a moisture content of 5% and 40% by weight of Mount Wright iron ore comprising 67% by weight of iron oxides, among them being 90% by weight of hematite, wassubjected to an extrusion step in a 7.5kW pelletizer to produce composite extruded pellets having a diameter of 6mm and a length of 30mm. These composite pellets have a Fe / C molar ratio of 4.67. The obtained composite pellets were subjected to a direct reduction step at solid step, wherein the extruded pellets were placed in a crucible and heated in an oven at 1000°C under an inert atmosphere with a ramping of 350°C / hour up to 1000°C and a dwelling time at 1000°C of 1 hour. Metallization rate of the produced DRI is of 96%.

Claims

CLAIMS1 ) A composite material consisting of lignocellulosic biomass having a moisture content from 5 to 10% by weight and an iron material comprising at least 30% of iron oxides, the molar ratio Fe / C of said composite material being from 3 to 6.5.2) A composite material according to claim 1 wherein the iron material is chosen among at least one of iron ore, direct reduction fines, oily mill sludge, iron mine tailings, electric arc furnace dust and sintering fines.3) A composite material according to any one of claims 1 or 2 wherein biomass is chosen among at least one of wheat straw, miscanthus, out-of-use wood or sawdust.4) A composite material according to any one of claims 1 to 3 wherein said composite material is in form of an extruded pellet.5) A composite material according to any one of claims 1 to 4 consisting of from 25 to 65 % by weight of biomass and from 35 to 75% by weight of iron material.6) A composite material according to anyone of claims 1 to 5 wherein the molar ratio Fe / C is from 3 to 4.4.7) A composite material according to anyone of claims 1 to 5 wherein the molar ratio Fe / C is from 4.5 to 5.9.8) A composite material according to anyone of claims 1 to 5 wherein the molar ratio Fe / C is from 6.0 to 6.5.9) A method to produce a composite material according to anyone of claims 1 to 8 comprising the steps of: a. Providing lignocellulosic biomass having a moisture content from 5 to 10% by weight, b. Providing an iron material comprising at least 30% of iron oxides,c. Mixing the biomass and the iron material in appropriate amounts to obtain a predefined molar Fe / C ratio in the range from 3 to 6.5 d. Agglomerating the obtained mixture to form the composite material.10) A method according to claim 12 wherein the agglomerating step is an extrusion step.

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