US20180320246A1

US20180320246A1 - Electric arc furnace dust as coating material for iron ore pellets for use in direct reduction processes

Info

Publication number: US20180320246A1
Application number: US15/768,422
Authority: US
Inventors: Hamad S. AL-TASSAN; Mohamed Bahgat Saddik; Sayed Niaz AHSAN; Hesham A. HANAFY
Original assignee: SABIC Global Technologies BV
Current assignee: SABIC Global Technologies BV
Priority date: 2015-10-23
Filing date: 2016-09-27
Publication date: 2018-11-08
Also published as: CN108474060A; WO2017068446A1; EP3365470A1; WO2017068445A1

Abstract

Disclosed are coating compositions and methods for their use, comprising 90 wt. % electric arc furnace dust based on the total dry weight of the coating composition. The electric arc furnace dust includes at least 40 wt. % of Fe₂O₃and at least 30 wt. % of CaO and CaCO₃combined, based on the total dry weight of the electric arc furnace dust.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 62/245,760, filed Oct. 23, 2015, and U.S. Provisional Application No. 62/245,759, filed Oct. 23, 2015. The contents of the referenced applications are incorporated into the present application by reference.

BACKGROUND OF THE DISCLOSURE

A. Technical Field

The present disclosure relates to coating compositions for iron ore pellets comprising electric arc furnace dust. Also disclosed are iron ore pellets comprising a first coating and a second coating comprising electric arc furnace dust, a process of manufacturing the iron ore pellets, and a process of reducing the iron ore pellets to form reduced iron pellets with reduced agglomeration of the iron ore pellets.

B. Description of the Related Art

Direct reduction (DR) of iron ores is a fundamental step in commercial manufacture of iron. Several direct reduction processes, including those using fine ore, lump ore and pellets, have been developed. Some processes use natural gas as fuel reductant, whereas others are based on coal. Approximately 90% of directly reduced iron (DRI) in the world is produced by gas-based vertical shaft furnace processes owing to their low energy consumption and high productivity. Two of the common vertical shaft furnace processes are the Midrex (USA) and Tenova HYL (Mexico) processes, both of which use pellets and/or lumps of iron ore as feed stock.
The direct reduced iron (DRI) productivity is dependent on several factors including the iron ore pellet reduction properties, the reducing gasses concentration and the reaction temperature. Higher temperature, in general leads to higher productivity or faster reduction of the pellets but is limited by the sticking tendency of the pellets at high temperatures which leads to cluster formation and an uneven flow of ore pellets and gases. One drawback encountered with gaseous shaft furnaces is sticking or agglomerating of iron ore pellets. This unintended agglomeration of pellets can make continuous operation difficult. In moving-bed shaft reduction processes, such as Midrex and HYL III, the avoidance of sticking is essential. The sticking tendency imposes an upper limit on the reduction temperature and, hence, on the productivity of the reduction process.
In direct reduction processes the product is freshly reduced iron in a solid state. Therefore, it is crucial for the material flow in the reducing module that the solid product does not agglomerate or form aggregates that block the material flow within and out of the reactor [Direct reduced iron: Technology and Economics of Production and Use, ed. by J. Feinman and D. R. Mac Rae, ISS, Warrendale, Pa., (1999).—incorporated herein by reference in its entirety]. If the pellets have little or no tendency to stick then the reduction temperature and therefore throughput can be increased. An increase of 100° C. in the reduction temperature can significantly increase throughput [Wong P L M, Kim M J, Kim H S, Choi C H. Ironmaking Steelmaking, 1999: 26: 53-57.—incorporated herein by reference in its entirety]. High reduction temperature also minimizes degradation and re-oxidation of the reduced product.
Previous methods to prevent and/or lessen the tendency for agglomeration and sticking of iron ore pellets have included lower temperatures, higher basicity, and changes in gangue content. However, decreasing the reducing temperature of the DRI process to avoid this problem can cause a significant drop in throughput. As an example, a decrease from 850° C. to 750° C. can result in a decrease of 30-40% in throughput [L. G. Henderickson and J. A. Sandoval: Iron Steel Soc. AIME, 1980, 35-48.—incorporated herein by reference in its entirety]. High basicity and gangue content may also result in larger slag volume and less metal throughput leading to unfavorable economic and operation conditions.
One way to prevent pellet agglomeration is to coat the iron surfaces with a coating material that is inactive under the reducing conditions in the shaft furnace. However, a single coating has drawbacks such as ineffective agglomerate prevention during reduction and the loss of the coating prematurely during shipment or movement prior to reduction [Jerker Sterneland and Par G. Jonsson ISIJ International, Vol. 43 (2003), No. 1, pp. 26-35; and Cano JAM, Wendling F. Mining Eng 1993: 45: 633-636; and Jianhua Shao, Zhancheng Guo, and Huiqing Tang, Steel research int., 84 (2013) No. 2, 111-118. —each incorporated herein by reference in its entirety]. For this reason, the iron ore pellets are often coated with materials to minimize adhesion tendencies. Usually these materials are sprayed in solution form so that a thin layer is formed and bonded with the surface of the pellets which then acts as a barrier between the surface of adjacent pellets during high temperature exposure, thus allowing for more free movement of the pellets during downward movement in the shaft and at the same time allowing for more uniform upward flow of the reducing gases during reduction processes. The suitability or effectivity of the coating is dependent on its ability to adhere with the pellet surface to such an extent that it is not removed during shipment, movement on a conveyor belt or charging hopper as well as inside the shaft while rolling downwards and rubbing with each other.
In the production of steel, large amounts of material are consumed, but only a fraction is incorporated into the final product. For example, integrated mills use 2.4 tons of iron ore and other inputs for each ton of crude steel produced. This waste carries an economic and environmental impact. All steel production processes form waste materials that contain oxidized iron and other oxidized metals such as calcium, zinc, magnesium, silicon, lead, chromium, and cadmium. This waste material is usually in form of dust in the gas waste streams, the gases are filtered and the dust is collected in bag houses. For example, during the electric arc furnace process, the high temperatures required to melt the feed material produce a byproduct referred to as electric arc furnace (EAF) dust. This dust is difficult to process because of its fine particle size and despite its substantial metal content is essentially worthless.
The Environmental Protection Agency classifies this EAF dust as a hazardous waste due to the presence of toxic oxides. Accordingly, its disposal has become a major problem for steel producers. Significant effort has been focused on developing an economical treatment process for EAF dust that renders it nonionic or nontoxic by removing the toxic heavy metals or by immobilizing the toxic materials in a stabilized composition for disposal or as a recycled product. These processes are not economical and the toxicity of their products is unclear. Thus, a need exists to reclaim and/or repurpose EAF dust in a cost-effective and environmentally safe way.

BRIEF SUMMARY OF THE DISCLOSURE

In view of the forgoing, one aspect of the present disclosure is to provide coating compositions comprising electric arc furnace dust that allow for the utilization of a waste material and provide sustainability and cost saving measures in steel plant operation. A second aspect of the present disclosure is to provide iron ore pellets comprising an iron ore core that is coated with a first coating and a second coating comprising the coating composition that reclaims waste electric arc furnace dust. A third aspect of the present disclosure is a process for manufacturing the iron ore pellets. A fourth aspect of the present disclosure is a process for reducing the iron ore pellets at high temperature and throughput with reduced agglomeration to efficiently and economically form reduced iron.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of the reduction under load apparatus.

FIG. 2A is a schematic diagram of the tumble drum apparatus in front view.

FIG. 2B is a schematic diagram of the tumble drum apparatus in side view.

FIG. 3 is an X-ray diffraction (XRD) analysis of iron ore pellets.

FIG. 4A is a scanning electron microscope (SEM) micrograph of iron ore pellets.

FIG. 4B is a SEM micrograph of iron ore pellets.

FIG. 5 is a SEM photo of electric arc furnace dust.

FIG. 6 is an energy-dispersive X-ray spectroscopy (EDX) analysis of electric arc furnace dust.

FIG. 7 is a SEM photo of the electric arc furnace dust coating layer of iron ore pellets.

FIG. 8 is an EDX analysis of the electric arc furnace dust coating layer of iron ore pellets.

FIG. 9 is a SEM photo of the electric arc furnace dust coating layer of iron ore pellets after a rubbing test.

FIG. 10 is an EDX analysis of the electric arc furnace dust coating layer of iron ore pellets after a rubbing test.

FIG. 11 is a SEM photo of the cement coating layer of iron ore pellets.

FIG. 12 is an EDX analysis of the cement coating layer of iron ore pellets.

FIG. 13 is a SEM photo of the cement coating layer of iron ore pellets after a rubbing test.

FIG. 14 is an EDX analysis of the cement coating layer of iron ore pellets after a rubbing test.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views.
According to a first aspect, the present disclosure relates to a coating composition comprising electric arc furnace dust in an amount of at least 90% based on the total dry weight of the coating composition.
Electric arc furnace (EAF) dust, or lime dust, is the solid material recovered from the off-gases from the production of molten steel and/or iron including electric arc furnaces. An electric arc furnace is a furnace that heats charged material by means of an electric arc. It allows steel to be made from 100% scrap metal feedstock. EAF dust is generated during the melting of materials in an electric arc furnace and collected by a de-dusting system such as bag filters or electrostatic precipitators and stored. Generally, the EAF dust is a complex material comprising small fines of mostly metal oxides. The predominant material is iron oxide with the remainder comprising oxides of calcium, zinc, chromium, lead, magnesium, manganese, sodium, nickel, and/or potassium. The composition of the dust is directly associated with the chemistry of the metallic charge used in the electric arc furnace. For example, processes that recycle scrap metal from sources as varied as automobiles, railroad rails or discarded structural steel generate EAF dust with larger proportions of zinc, iron and lead and smaller proportions of tin, cadmium, chromium, copper, silica, lime, and/or alumina.
In one embodiment, the coating composition and the electric arc furnace dust substantially comprises Fe₂O₃, CaO, and CaCO₃. In a preferred embodiment, other materials are present in less than 10 wt. %, preferably less than 5 wt. %, preferably less than 3 wt. %, preferably less than 2 wt. %, preferably less than 1 wt. %, preferably less than 0.5 wt. % relative to the total weight of the coating composition and the electric arc furnace dust.
In one embodiment, the electric arc furnace dust comprises greater than 40 wt. % of Fe₂O₃, preferably greater than 45%, preferably greater than 50%, preferably greater than 55%, preferably greater than 60%, preferably greater than 65%, preferably greater than 66%, preferably greater than 67%, preferably greater than 68%, preferably greater than 69 wt. % of Fe₂O₃relative to the total weight of the electric arc furnace dust. The Fe₂O₃present in the electric arc furnace dust is consistent with the description of Fe₂O₃in the iron ore core provided herein below. In another preferred aspect, the electric arc furnace dust comprises 45 wt. % to 60 wt. % Fe₂O₃or 50 wt. % to 55 wt. % Fe₂O₃based on the total weight of the dust.
The coating composition preferably comprises at least 90% by weight of electric arc furnace dust. Preferably the coating composition comprises at least 95% by weight, at least 96% by weight, at least 97% by weight, at least 98% by weight, at least 99% by weight of electric arc furnace dust. The coating composition preferably consists of electric arc furnace dust.
“Slag” as used herein, refers to the by-product left over after a desired metal has been separated (i.e., smelted) from its raw ore. Slag is usually a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and elemental metals. While slags are generally used to remove waste in metal smelting, they can also assist in the temperature control of the smelting and minimize any re-oxidation of the final liquid metal product before the molten metal is removed from the furnace and used to make solid metal.
In one embodiment, steel production involves an oxidation process where lime is used to form a liquid slag which absorbs impurities from the liquid metal through formation of complex oxides in the liquid slag. Oxidation is simply the addition of oxygen into the furnace, causing metals and non-metallics to form oxides that are lighter than the liquid steel and as such float to the surface of the bath. As some metallic oxides are acidic in nature, they can react with the basic refractories of the furnace. A basic slag is made using lime and dolomitic lime which protects the furnace refractory. Basic slag practice is utilized for most grades of steel. Electric arc furnace dust, or lime dust, is generated during steel making operations in an electric arc furnace where the charging of lime is used towards the formation of “slag.” The amount of lime addition is based on the silicon and aluminum levels of the steel bath and can affect the composition of the electric arc furnace dust.
Lime is calcium-containing inorganic material in which carbonates, oxides, and hydroxides predominate. Lime may refer to quicklime or burnt lime, which is calcium oxide that has been derived from calcining limestone. Lime may also refer to hydrated lime or slaked lime, which is calcium hydroxide which has been derived from the hydration of quicklime. Therefore, “lime” as used herein, may refer to calcium carbonate, calcium oxide or calcium hydroxide containing materials including, but not limited to, dololime, lump lime or special lime, and mixtures thereof.
In one embodiment, the coating composition and the electric arc furnace dust comprise greater than 30 wt. % of CaO and CaCO₃combined, preferably greater than 35%, preferably greater than 40%, preferably greater than 45%, preferably greater than 50%, preferably greater than 55%, preferably greater than 56%, preferably greater than 57%, preferably greater than 58%, preferably greater than 59 wt. % of CaO and CaCO₃combined relative to the total weight of the electric arc furnace dust. In another preferred embodiment, the electric arc furnace dust comprises 20 wt. % to 30 wt. % CaCO₃and 10 wt. % to 20 wt. % CaO, preferably 23 wt. % to 27 wt. % CaCO₃and 12 wt. % to 17 wt. % CaO.
In another embodiment, the electric arc furnace dust comprises lime as described above. In one embodiment, the coating composition and electric arc furnace dust comprises primarily Fe₂O₃, CaO, and CaCO₃, and portions of MgO and SiO₂.
Magnesium oxide or magnesia is a white hygroscopic solid mineral that occurs naturally as periclase. It consists of a lattice of Mg²⁺ions and O²⁻ ions held together by ionic binding. In one embodiment, the coating composition comprises electric arc furnace dust and the electric arc furnace dust and coating composition comprise less than 5 wt. % of MgO, preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1 wt. % of MgO relative to the total weight of the electric arc furnace dust.
Silicon dioxide or silica is an oxide of silicon most commonly found in nature as quartz. Silica is one of the most complex and most abundant families of materials existing both as several minerals and synthetics. Examples include fused quartz, crystal, fumed silica, silica gel and aerogels. In one embodiment, the coating composition comprises electric arc furnace dust and the coating composition and electric arc furnace dust comprise less than 5 wt. % of SiO₂, preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1 wt. % of SiO₂relative to the total weight of the electric arc furnace dust.
In one embodiment, the coating composition comprises electric arc furnace dust and the coating composition and electric arc furnace dust are substantially free of zinc, chromium, manganese, lead, nickel, sodium, and/or potassium. These compounds are generally present in less than 1 wt. %, preferably less than 0.5 wt. %, preferably less than 0.1 wt. %, preferably less than 0.01 wt. %, preferably less than 0.001 wt. % relative to the total weight of the electric arc furnace dust.
Other inorganic compounds may be present in the electric arc furnace dust and coating composition including, but not limited to, aluminum as Al₂O₃, antimony, arsenic, barium, boron, copper, mercury, selenium, silver, molybdenum, thorium, uranium, vanadium, strontium, cadmium, lithium, sulphate or chloride, and oxides and mixtures thereof. These compounds are generally present in less than 0.5 wt. % or even 0 wt. % relative to the total weight % of the electric arc furnace dust.
In one embodiment, the coating composition comprising electric arc furnace dust is substantially free of reducing agents including, but not limited to, ferrous chloride and/or ferrous sulfate. These agents are sometimes used to solidify the electric arc furnace dust before, during, or after storage. In another embodiment, the coating composition may further comprise a binder material. The “binder” material refers to any material or substance that holds or draws other materials together to form a cohesive whole mechanically, chemically or as an adhesive. In a preferred embodiment, the binder material may refer to any portion of the coating composition that can harden or solidify onto the particles of the coating composition holding the coating mixture in place on any surface. In a preferred embodiment, the binding material is at least one selected from the group consisting of a cement material including, but not limited to, non-hydraulic cements, hydraulic cements, Portland cement, Portland cement blends (i.e., Portland blast furnace slag cement, Portland fly ash cement, Portland pozzolan cement, Portland silica fume cement, masonry cements, expansive cements, white blended cements, colored cements, very finely ground cements), other cements (i.e., pozzolan-lime cements, slag-lime cements, supersulfated cements, calcium sulfoaluminate cements, “natural” cements, geopolymer cements) and mixtures thereof and a clay material including, but not limited to, kaolinites, montmorillonite-smectites, illites, chlorites, and mixtures thereof. A further detailed exemplary chemical analysis of electric arc furnace dust that may be used as the coating composition is shown in Table 2.
It is envisioned that other types of metallurgic dusts may be used in lieu of electric arc furnace dust or as a further portion of the coating composition including, but not limited to, iron scrap, iron rouge recovered from steel cleaning lines, mill scale, iron containing minerals and low grade iron based pigments and other solids recovered from electric arc furnaces, basic oxygen furnaces, and blast furnaces. As used herein, “metallurgic dust” refers to any unpurified metal composition comprising a significant portion of iron compositions.
In one embodiment, the coating composition is substantially granular and comprises grains with an average particle size of 1-20 μm, preferably 1-15 μm, more preferably 2-10 μm. It is additionally envisaged that the coating composition may comprise some coarse grains with an average particle size of 1-10 mm. In a preferred embodiment, these coarse grains comprise less than 10 wt. %, preferably less than 5 wt. %, preferably less than 4 wt. %, preferably less than 3 wt. %, preferably less than 2 wt. %, preferably less than 1 wt. % of the coating composition by weight relative to the total weight of the coating composition. In a preferred embodiment, the coating composition is a dry powdered material. The coating composition is equally envisaged as a slurry, solution, suspension, dispersion, and/or emulsion. In one embodiment, the slurry comprises 10-30 wt. %, preferably 15-25 wt. %, preferably 18-22 wt. % of the coating composition relative to the total weight of the slurry. The weight percentages described above generally refer to dry weights of the coating composition.
According to a second aspect, the present disclosure relates to iron ore pellets including an iron ore core. Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are typically rich in iron oxides and vary in color from dark grey, bright yellow, deep purple to rusty red. The iron itself is usually found in the form of magnetite (Fe₃O₄, 72.4% Fe), hematite (Fe₂O₃, 69.9% Fe), goethite (FeO(OH), 62.9% Fe), limonite (FeO(OH).n(H2O)) or siderite (FeCO₃, 48.2% Fe) and mixtures thereof. Ores containing very high quantities of hematite or magnetite (greater than ˜60% iron) are known as natural ore or direct shipping ore. These ores can be fed directly into iron-making blast furnaces. Iron ore is the raw material used to make pig iron, which is one of the main raw materials to make steel.
Iron (III) oxide or ferric oxide is the inorganic compound with formula Fe₂O₃. It is one of the three main oxides of iron, the other two being iron (II) oxide (FeO) which is rare, and iron (II, III) oxide (Fe₃O₄) which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe₂O₃is the main source of iron for the steel industry. Fe₂O₃is ferromagnetic, dark red, and readily attacked by acids.
Fe₂O₃can be obtained in various polymorphs. In the major polymorphs, α and γ, iron adopts an octahedral coordination geometry, each Fe center is bound to six oxygen ligands. α-Fe₂O₃has the rhombohedral corundum (α-Al₂O₃) structure and is the most common form. It occurs naturally as the mineral hematite which is mined as the main ore of iron. γ-Fe₂O₃has a cubic structure, is metastable and converted to the alpha phase at high temperatures. It is also ferromagnetic. Several other phases have been identified, including the β-phase, which is cubic body centered, metastable, and at temperatures above 500° C. converts to alpha phase, and the epsilon phase, which is rhombic, and shows properties intermediate between alpha and gamma phase. This phase is also metastable, transforming to the alpha phase between 500 and 750° C. Additionally, at high pressure an iron oxide can exist in an amorphous form. The ore in the iron ore core may have an α polymorph, a β polymorph, a γ polymorph, an ε polymorph or mixtures thereof.
The iron (III) oxide in the iron ore core may also be in the form of an iron hydrate. When alkali is added to solutions of soluble Fe(III) salts a red-brown gelatinous precipitate forms which is Fe₂O₃.H₂O (also written as Fe(O)OH). Several forms of the hydrate oxide of Fe(III) exist as well.
The term “iron ore core” as used herein refers to an iron rich material (i.e., greater than 40 wt. %, preferably greater than 50 wt. %, more preferably greater than 60 wt. % elemental iron by weight), onto which a single or a plurality of coatings are added to form a surface coated iron ore core.
The iron ore core may be a porous starting material that becomes coated, and the interface between the iron ore core and the coating material may also form pores. In this disclosure, “porosity” is an index showing a ratio of void volume with respect to an entire volume of a structure (e.g., the iron ore core, the first coating, the second coating). The porosity can be calculated, for example, by taking a photograph of the cross sectional structure, measuring a total void area using the photograph, and calculating the porosity as a ratio of void area with respect to an entire cross sectional area of the structure. In one embodiment, the iron ore core has a porosity of 1-40%, preferably 5-35%, more preferably 10-30%.
In the present disclosure, the general shape and size of the iron ore core may dictate the shape and size of the iron ore pellets described herein. In a preferred embodiment, the iron ore cores of the present disclosure are in the form of a pellet, which is spherical or substantially spherical (e.g., oval, oblong, etc.) in shape. However, the iron ore cores disclosed herein may have various shapes other than spheres. For instance, it is envisaged that iron ore cores may be in the shape of a “lump” or a “briquette.” Lumps or briquettes tend to have a more cubical or rectangular shape when compared to pellet forms. Therefore, the iron ore cores of the present disclosure may also be generally spherical, cubic, or rectangular in shape. The size of the iron ore core may also dictate the size of the iron ore pellets herein. In one embodiment, the iron ore core has an average diameter of 5-20 mm, preferably 8-18 mm, more preferably 10-16 mm, although the size may vary from these ranges and still provide acceptable iron ore pellets.
In addition to iron and/or iron oxide, various non-ferrous materials (i.e., metals and non-metals) may be present in the iron ore core including, but not limited to, aluminum, copper, lead, nickel, tin, titanium, zinc, bronze, metal oxides thereof, metal sulfides thereof, calcium oxide, magnesium oxide, magnesite, dolomite, aluminum oxide, manganese oxide, silica, sulfur, phosphorous, and combinations thereof. The total weight % of these non-ferrous materials relative to the total wt. % of the iron ore core is typically no more than 40%, preferably no more than 30%, preferably no more than 20%, preferably no more than 15%, preferably no more than 10%, preferably no more than 5%, preferably no more than 4%, preferably no more than 3%, preferably no more than 2%, more preferably no more than 1%.
The conventional route for making steel includes using one or more sintering or pelletization plants, coke ovens, blast furnaces, and basic oxygen furnaces. Such plants require high capital investment and raw materials of stringent specifications. Direct reduction, an alternative route of iron making, has been developed to overcome some of these difficulties of conventional blast furnaces. Iron ore is reduced in solid state to form direct reduced iron (DRI). The most important reaction of iron (III) oxide is its carbothermal reduction, which gives iron used in steel-making (formula I):
Fe₂O₃+3CO→2Fe+3CO₂ (I):
The investment and operating costs of direct reduction plants are low compared to integrated steel plants. As used herein, direct-reduced iron (DRI), also known as sponge iron, is produced from the direct reduction of iron ore in the form of lumps, pellets, or fines by a reducing gas produced from natural gas or coal. The reducing gas is a mixture, the majority of which is hydrogen (H₂) and carbon monoxide (CO) which act as reducing agents. Direct reduced iron has about the same iron content as pig iron, typically 90-94%.
The direct reduction of iron ore pellets at high temperature (e.g., greater than 400° C.) may lead to the formation of agglomerates. As used herein, the term “agglomerates” or “agglomerated” refers to two or more iron ore pellets, either coated (i.e., a first coating, a second coating, or both) or non-coated (i.e., the iron ore core itself), which are attached to form a pellet cluster that has a longest length of at least 25 mm in any measurable direction. For spherical or substantially spherical pellet agglomerates, longest length refers to the longest linear diameter of the pellet agglomerate. For non-spherical pellet agglomerates, such as pellet agglomerates that form a cubic shape, the longest length may refer to any of the length, width, or height of the agglomerate. The iron ore pellets may be attached to each other in any reasonable manner, including attached through surface coating interactions (e.g., glued, tacked, cemented, pasted, etc.), attached by highly connected or integral interactions (e.g., melted together, fused, amalgamated, etc.), or entrapped within a cluster (e.g., sandwiched between a plurality of attached pellets). The iron ore pellets may also be attached as a result of interlocking fibrous iron precipitates (iron whiskers). For instance, growth of iron whiskers may lead to pellets that are hooked or entangled to each other through the fibrous iron whiskers. Therefore, one object of the present disclosure is to provide a coating for iron ore that prevents the formation of agglomerates before, during and/or after direct reduction processes.
The iron ore pellets of the present disclosure also include a first coating comprising at least one selected from the group consisting of bauxite, bentonite, and dolomite. The iron ore core coated with a first coating is referred to herein as a “coated iron core” or “coated core.”
Bauxite is an aluminum ore and the predominant source of aluminum throughout the world. It consists mostly of the minerals gibbsite Al(OH)₃, boehmite γ-AlO(OH), and diaspore α-AlO(OH), mixed with the two iron oxides goethite FeO(OH) and hematite (Fe₂O₃), the clay mineral kaolinite Al₂Si₂O₅(OH)₄, and small amounts of anatase TiO₂. Lateritic bauxites (silicate bauxites) are distinguished from karst bauxite ores (carbonate bauxites). In one embodiment, the first coating comprises bauxite and the bauxite first coating comprises 40-60% Al₂O₃, 10-30% Fe₂O₃, 0.1-10% SiO₂, and 1-3% TiO₂. Other inorganic compounds may be present in the bauxite first coating including, but not limited to, P₂O₅, MnO, MgO, CaO, etc. These compounds are generally present in less than 5% or even 0% relative to the total weight % of the bauxite.
Bentonite is an absorbent aluminum phyllosilicate, impure clay consisting primarily of montmorillonite. Phyllosilicates are sheet silicate minerals formed by parallel sheets of silicate tetrahedra with Si₂O₅or a 2:5 ratio, they may be hydrated with either water or hydroxyl groups attached. Montmorillonite generally comprises sodium, calcium, aluminum, magnesium and silicon and oxides and hydrates thereof. Other compounds may also be present in the bentonite of the present disclosure including, but not limited to, potassium-containing compounds and/or iron-containing compounds. There are different types of bentonite, named for the respective dominant element, such as potassium (K), sodium (Na), calcium (Ca), and aluminum (Al). For industrial purposes, two main classes of bentonite exist: sodium and calcium bentonite. Therefore, in terms of the present disclosure bentonite may refer to potassium bentonite, sodium bentonite, calcium bentonite, aluminum bentonite, and mixtures thereof, depending on the relative amounts of potassium, sodium, calcium, and/or aluminum in the bentonite first coating.
Dolomite is an anhydrous carbonate mineral composed of calcium magnesium carbonate, e.g., CaMg(CO₃)₂. Dolomite can also describe the sedimentary carbonate rock composed primarily of mineral dolomite, known as dolostone or dolomitic limestone. The mineral dolomite crystallizes in the trigonal-rhombohedral system and forms white, tan gray or pink crystals. Dolomite is a double carbonate, having an alternating structural arrangement of calcium and magnesium ions. In one embodiment, the first coating comprises dolomite and the dolomite first coating comprises 15-25% Ca, 10-20% Mg, 10-20% C, and 40-60% O, with the calcium and magnesium being present primarily as oxides or hydroxides. Other inorganic compounds may be present in the dolomite first coating including, but not limited to, Al₂O₃, MnO, Fe₂O₃, etc. These compounds are generally present in less than 5% or even 0% relative to the total weight % of the dolomite.
It is envisioned that other types of sedimentary rock sources may be used in lieu of bauxite, bentonite, and/or dolomite as material in the first coating including, but not limited to, limestone, calcite, vaterite, aragonite, magnesite, taconite, gypsum, quartz, marble, hematite, limonite, magnetite, andesite, serpentinite, garnet, basalt, dacite, nesosilicates or orthosilicates, sorosilicates, cyclosilicates, inosilicates, phyllosilicates, tectosilicates, and the like.
“Coating,” “coat,” or “coated” as used herein, refers to a covering that is applied to a surface of the iron ore core or a coated iron ore core. The coating may “substantially cover” the surface, whereby the % surface area coverage of the surface being coated is at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%. In some cases, the coating may “incompletely cover,” or only cover portions of the surface being coated, whereby the % surface area coverage of the surface being coated is less than 75%, less than 65%, less than 60%, less than 55%, less than 50%, less than 45%, less than 40%, less than 355, less than 30%, less than 25%, less than 20%, less than 15%, less than 10%. The “coating” or “coat” may refer to one material (i.e., dolomite, bauxite, bentonite, electric arc furnace dust, etc.) that covers a surface being coated, or alternatively, the coating may refer to a plurality of materials (i.e., mixtures) that cover a surface being coated. The plurality of materials may be applied to a surface as a mixture or sequential applications of the individual materials. With sequential applications of individual materials, it may be possible to form distinct layers. These distinct layers may have a defined interface. The coating thickness of the present disclosure may be varied depending on the coating materials and the process for applying the coating. The term “coating” may also refer to a single application of a material, or a plurality of applications of the same material.
In one embodiment, the first coating substantially covers the iron ore core, where the first coating covers greater than 75%, preferably greater than 85%, preferably greater than 90%, preferably greater than 95% of the surface of the iron ore core. Alternatively, the first coating may be applied to only a portion of the surface of the iron ore core (i.e., incompletely cover), and the applied coating may still prevent agglomeration. This first coating may be sufficient to prevent agglomeration of the iron ore pellets.
In one embodiment, the iron ore pellets have a wt. % of the first coating ranging from 0.05-2%, preferably 0.1-1.5%, more preferably 0.2%-1.0% relative to the total weight of the iron ore pellets.
In one embodiment, an average thickness of the first coating is 50-150 μm, preferably 60-100 μm, more preferably 70-80 μm. In one embodiment, the first coating is uniform. Alternatively, the first coating may be non-uniform. The term “uniform” refers to an average coating thickness that differs by no more than 50%, by no more than 25%, by no more than 10%, by no more than 5%, by no more than 4%, by no more than 3%, by no more than 2%, by no more than 1% at any given location on the surface of the coated material. The term “non-uniform” refers to an average coating thickness that differs by more than 5% at any given location on the surface of the coated material.
The iron ore pellets of the present disclosure further include a second coating comprising the coating composition described herein in any of its embodiments comprising electric arc furnace (EAF) dust in an amount of at least 90% based on the total dry weight of the second coating, preferably at least 95%, preferably at least 97%, preferably at least 98%, preferably at least 99%. In a preferred embodiment, the first coating is disposed between a surface of the iron ore core and the second coating.
In one embodiment, the second coating substantially covers the first coating. In this scenario, the second coating covers at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99% of the surface of the first coating. Alternatively, the second coating may be applied to only a portion of the surface of the first coating (i.e., incompletely cover the first coating). In the scenario where the first coating incompletely covers the iron ore core, the second coating may cover the iron ore core rather than, or in addition to covering the first coating. In one embodiment, the iron ore pellets have a wt. % of the second coating ranging from 0.05-2%, preferably 0.1-1.5%, more preferably 0.2-1.0% relative to the total weight of the iron ore pellets. In one embodiment, an average thickness of the second coating is 50-150 μm, preferably 60-100 μm, more preferably 70-80 μm. Similar to the coverage of the first coating, the second coating may cover the first coating and/or the iron ore core in a uniform fashion, or alternatively in a non-uniform fashion.
In a preferred embodiment, the first and second coatings form distinct layers with distinct and identifiable interfaces between the two layers. In one embodiment, the first and second coatings form distinct layers, although the interface between the two layers is a mixture of both the first and second layer. For example, in one embodiment the first layer contains at least one of bauxite, bentonite, and dolomite, and the second layer contains electric arc furnace dust. Preferably the major component of the first layer is not present in the second layer and the major component of the second layer is not present in the first layer. In one embodiment the iron ore pellets of the present disclosure have a porosity of 1-35%, preferably 5-30%, more preferably 10-25%.
The average thickness of both coatings (the first coating and the second coating) on the iron ore core is about 100-300 μm, preferably 120-200 μm, more preferably 140-160 μm. Further, the total weight percent of the sum of the first coating and second coating is 0.1-4%, 0.2-3.5%, preferably 0.3-3%, preferably 0.4-2.5%, more preferably 0.5-2% relative to the total weight of the iron ore pellets. The iron ore pellets may have an average pellet diameter of 5-20 mm, preferably 7-18 mm, more preferably 9-16 mm.
In one embodiment, the first and second coating reduce the formation of agglomerated iron ore pellets at temperatures in the range of 20° C. to 1100° C., preferably 500° C. to 1000° C., preferably 750° C. to 950° C., preferably 800° C. to 900° C. compared to a substantially similar iron ore pellet or iron ore core without the first coating, the second coating, or both.
In one embodiment, the iron ore pellets have a % agglomeration of less than 5%, preferably less than 4%, preferably less than 3%, preferably less than 2%, preferably less than 1% in terms of the wt. % of agglomerated iron ore pellets with a longest length of at least 25 mm relative to the total weight of the iron ore pellets.
In one embodiment, the thickness of the first and second coating decreases by no more than 60%, by no more than 50%, by no more than 40%, by no more than 30%, by no more than 20%, by no more than 10% after rotating the iron ore pellets at 10-30 rpm, in terms of the average coating thickness of the sum of the first and second coating [ASTM E376—incorporated herein by reference in its entirety].
According to a third aspect, the present disclosure relates to a process for manufacturing the iron ore pellets of the present disclosure, in one or more of their embodiments, including applying at least one selected from the group consisting of bauxite, bentonite, and dolomite to an iron ore core to form a coated iron ore core coated with a first coating. In one embodiment, the applying involves coating the iron ore core with a first coating, where the first coating covers greater than 75%, preferably greater than 85%, preferably greater than 90%, preferably greater than 95% of the surface of the iron ore core.
In one embodiment, the first coating is applied to the iron ore core as a slurry, preferably aqueous, comprising 10-30 wt. %, preferably 15-25 wt. %, more preferably 18-22 wt. % of bauxite, bentonite and/or dolomite relative to the total weight of the slurry. “Slurry” as used herein refers to a semiliquid mixture typically of particles or particulates of the coating material suspended in liquid. The liquid used in the slurry is not envisioned as particularly limiting and is preferably water. In one embodiment, the slurry has a pH of 4-8, although the pH of the slurry may be more acidic or more basic depending on the application. The slurry may also refer to a suspension, a dispersion, or an emulsion, etc.
The slurry preferably comprises a solids concentration of no more than 15 kg of coating material per ton of iron ore pellets to be coated, preferably no more than 10 kg/ton, preferably no more than 5 kg/ton, preferably no more than 4 kg/ton, preferably no more than 3 kg/ton, preferably no more 2 kg/ton, preferably no more than 1 kg/ton, preferably no more than 0.5 kg/ton, preferably no more than 0.25 kg/ton.
In one embodiment, the slurry may further comprise binder materials including, but not limited to, clay materials, cement materials, concrete materials, acrylic polymers or copolymers, polymers or copolymers of vinyl acetate or synthetic oils which can harden on the particles holding the coating mixture in place on the surface.
Several methods may be used to coat the iron ore core including, but not limited to, spray coating, dip coating, brush coating and spin coating. Spray coating is a process whereby the slurry is applied through the air to a surface as atomized particles using a spray coating device. A spray coating device may employ compressed gas, such as air, to atomize and direct the slurry.
Dip coating is a process whereby the pellet is inserted and removed from a bath of the slurry. The pellet is immersed in the slurry and the coating deposits itself on the pellet while being removed from the bath. The excess liquid can be drained from the pellet during this process, and the liquid of the slurry can then be evaporated.
Brush coating is a process whereby a slurry is smoothed on the surface by a brush or by multiple brushes. Spin coating is a process whereby a slurry is applied to the center of the pellet and the pellet is then rotated at high speed to spread the coating material by centrifugal force.
It is envisaged that the coating may be applied manually or through automation and that the applications of coatings may be done to individual iron ore cores or coated iron ore cores or in parallel to a plurality of iron ore cores or coated iron ore cores at the same time.
In one embodiment, the process for manufacturing the iron ore pellets also includes measuring a surface area coverage of the first coating on the iron ore core. In one embodiment, the surface area coverage is measured with at least one instrument selected from the group consisting of an optical microscope, an X-ray diffractometer, an X-ray fluorescence spectrometer, and a scanning electron microscope. Further, the surface area coverage may be measured upon visual inspection.
In addition to measuring the surface area coverage, other coating characteristics may be measured to determine if an acceptable amount of coating has been applied. For instance, the thickness of the coating can be measured using one or more of these techniques. Further, the measuring may involve an analysis of the porosity and/or surface roughness of the coating surface, for instance by measuring a specific surface area (i.e., BET surface area) through BET adsorption or gas permeability techniques.
In a preferred embodiment, the process further comprises drying the coated iron ore core for 0.5-24 hours, preferably 0.5-12 hours, more preferably 1-8 hours, even more preferably 1-6 hours prior to applying the second coating. By drying the first coating prior to applying the second coating, the formation of two distinct coating layers may be obtained. The formation of two distinct layers may be advantageous to prevent pellet agglomeration and to prevent premature removal of the coatings prior to an iron reduction process.
Further, the process of applying the first coating and measuring the coating characteristics (i.e., surface area coverage, thickness, etc.) can be repeated a plurality of times in an iterative fashion until an acceptable level of coating is achieved (e.g., greater than 75% surface area coverage of the iron ore core).
The process for manufacturing the iron ore pellets also involves applying the coating composition comprising electric arc furnace (EAF) dust described herein in any of its embodiments to the coated iron ore core to form the iron ore pellets coated with the first coating and the second coating. In one embodiment, the applying involves coating the coated iron ore core with the second coating, where the second coating covers greater than 50%, preferably greater than 60%, preferably greater than 70%, preferably greater than 75%, preferably greater than 85%, preferably greater than 90%, preferably greater than 95% of the surface of the coated iron ore core.
In one embodiment, the second coating is applied to the iron ore core coated with a first coating as a slurry comprising 10-30 wt. %, preferably 15-25 wt. %, more preferably 18-22 wt. % of EAF dust relative to the total weight of the slurry. The second coating may be applied using the techniques used to apply the first coating (e.g., spray coating, dip coating, brush coating and spin coating).
The slurry preferably comprises a solids concentration of no more than 15 kg of EAF dust coating composition per ton of iron ore pellets to be coated, preferably no more than 10 kg/ton, preferably no more than 5 kg/ton, preferably no more than 4 kg/ton, preferably no more than 3 kg/ton, preferably no more 2 kg/ton, preferably no more than 1 kg/ton, preferably no more than 0.5 kg/ton, preferably no more than 0.25 kg/ton, most preferably 5-0.25 kg/ton.
In another embodiment, the process for manufacturing the iron ore pellets also includes measuring a surface area coverage of the second coating on the first coating. The second coating surface area coverage and coating characteristics can be measured using methods of analysis used to measure the first coating.
Further, the process may also include drying the second coating, and repeating the application of the second coating a plurality of times in an iterative fashion until an acceptable level of coating is achieved (e.g., greater than 75% surface area coverage of the coated iron ore core).
In a preferred embodiment, the process for manufacturing the iron ore pellets of the present disclosure in any of their embodiments further comprises determining a clustering index and/or performing a reduction under load test in accordance with international standard [ISO 11256—incorporated herein by reference in its entirety] to evaluate the quality of the iron ore pellets as a feedstock for a direct reduction process. ISO 11256 specifies a method to provide a relative measure for evaluating the formation of cluster of iron ore pellets when reduced under conditions resembling those prevailing in shaft direct reduction processes.
A schematic diagram of an exemplary reduction under load (ISO 11256) apparatus is shown in FIG. 1. In a preferred embodiment, the apparatus comprises a reduction tube, a loading device, waste gas, a furnace, and a gas supply system. The reduction tube comprises an outer reduction tube 1, an inner reduction tube 2, upper and lower perforated plates comprising a test portion 3, a gas inlet 4, a gas outlet 5, and a thermocouple exit 6. The loading device comprises a compressed air inlet 7, a pressure cylinder 8, a frame for the pressure cylinder 9, and a loading ram 10. The waste gas comprises a throttle valve 11 and a waste gas fan 12. The gas supply system comprises gas cylinders 15, gas flowmeters 16, and a mixing vessel 17.
In a preferred embodiment, the apparatus is constituted of a vertical oven divided into five heating zones starting from the bottom. One thermocouple is placed in the oven and a triple thermocouple is placed inside the reaction tube. Reducing gas and nitrogen flow rate is controlled by a mass flow meter and controller. The vertical electrical oven is equipped with a weighing system. In a preferred embodiment, The system is capable of applying a total static load of up to 150 kPa on a bed of the test portion. The test portion is a 500-2500 g sample of pellets. The test portion comprises 50% pellets having a size in the range 20-12.5 mm and 50% having a size in the range of 12.5-5 mm. The pellet sample is isothermally reduced in a fixed bed at 700-1100° C., preferably 750-1000° C., preferably 800-900° C., or 850° C. under static load using a reducing gas consisting of 30% CO, 15% CO₂, 45% H₂, and 10% N₂in a flow rate of 30-50 L/min, preferably 40 L/min until a degree of reduction of 95% was achieved.
In a preferred embodiment, the reduced test portion (cluster) is disaggregated by tumbling, using a specific tumbling drum. The percentage of clusters is determined on the cooled sample. The clustered pellets consisting of more than two pellets are applied to the tumbler test. A schematic diagram of an exemplary tumble drum apparatus is shown in FIG. 2A (front view) and FIG. 2B (side view). It comprises a revolution counter 18, a door with handle 19, a stub axle 20 with no through shaft, two lifters 21 (generally 50 mm×50 mm×5 mm), a direction of rotation 22, and a plate 23.
In a preferred embodiment, the tendency for cluster formation or agglomeration will decrease by increasing the coating amount in kg per ton of iron ore core or iron ore pellet. In a preferred embodiment the iron ore pellets have a coating index measurement that achieves the Midrex process standard requirement. As used herein, this means the cluster mass comprising agglomerates of more than one pellet with a longest length of greater than 25 mm after ten revolutions was zero or 0%. This confirms that the utilization of the coating composition described herein in any of its embodiments as a secondary coating material for iron ore pellets is highly effective in reducing the formation of clusters in the iron ore feed for a reduction furnace. An exemplary determination of clustering index is further detailed below.
According to a fourth aspect, the present disclosure relates to a process for manufacturing reduced iron pellets involving i) applying at least one selected from the group consisting of bauxite, bentonite, and dolomite to an iron ore core to form a coated iron ore core coated with a first coating, ii) applying the coating composition comprising electric arc furnace (EAF) dust described herein in any of its embodiments to the coated iron ore core to form the iron ore pellets coated with the first coating and the second coating, iii) feeding the coated iron ore pellets into a reduction furnace, and iv) reducing the iron ore pellets with a reducing gas to form reduced iron pellets. The techniques used to apply the first and second coating, as well as the measurement techniques used to analyze the coating characteristics of the applied coatings have been mentioned previously.
In one embodiment, the process further comprises drying the coated iron ore core for 0.5-24 hours, preferably 0.5-12 hours, more preferably 1-8 hours, even more preferably 1-6 hours prior to applying the second coating. By drying the first coating prior to applying the second coating, the formation of two distinct coating layers may be obtained. The formation of two distinct layers may be advantageous to prevent pellet agglomeration and to prevent premature removal of the coatings prior to an iron reduction process.
In one embodiment, the temperature for the reduction is up to 1100° C., preferably up to 1000° C., more preferably up to 950° C. The reducing may be performed isothermally, or alternatively, a temperature gradient may be used to reduce the iron ore throughout the reduction process. In one embodiment, the reducing gas is hydrogen (H₂). In one embodiment, the reducing gas is carbon monoxide (CO). In a preferred embodiment, the reducing gas comprises both hydrogen and carbon monoxide. In this scenario, other gases may be present in the reducing gas, including carbon dioxide, nitrogen and the like. The ratio of hydrogen to carbon monoxide may be about 10:1, 9:1, 8:1, 7:1, 6:1, 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9 or 1:10. The reducing gas of the present disclosure may be derived from natural gas, coal or both.
In one embodiment, the iron ore pellets are reduced in a direct reduction apparatus. In one embodiment, the direct reduction apparatus is a fixed-bed reactor. Alternatively, in one embodiment, the direct reduction apparatus is a moving-bed shaft. In a preferred embodiment, the direct reduction apparatus is a vertical moving-bed shaft. In a vertical moving-bed shaft apparatus, the iron ore pellets, in one or more of their embodiments, are placed proximal to the top of the moving-bed shaft, where the iron ore pellets are heated and allowed to move towards the bottom of the moving-bed shaft gradually as they are reduced. The reducing gas is flowed countercurrent to the movement and feeding of the iron ore pellets. Then the reduced iron pellets are collected proximal to the bottom of the shaft apparatus. In a vertical moving-bed shaft reduction apparatus, the avoidance of agglomerated iron ore pellets is essential to allow the downward movement of the iron ore pellets for reduction and to allow for efficient flow of the reducing gas upwardly. Therefore, the first and second coating of the iron ore pellets may provide a more efficient direct reduction process by minimizing the formation of agglomerates. The wt. % of iron in the reduced iron pellets is greater than 90%, greater than 91%, greater than 92%, greater than 93%, greater than 94%, greater than 95%, relative to the total weight of the reduced iron pellet.
In one embodiment, the process further comprises tumbling the iron ore pellets and/or the reduced iron pellets and weighting agglomerated iron ore pellets and/or reduced iron pellets with a longest length of at least 25 mm relative to the total weight of the iron ore pellets and/or the reduced iron pellets to determine % agglomeration.
It is envisaged that the reduced iron pellets of the present disclosure may be used for the manufacture of steel and steel related products. The type of steel produced using the reduced iron pellets of the present disclosure may vary depending on added alloying elements. Steel is an alloy of iron and carbon that is widely used in construction and other applications because of its high tensile strength and low cost. Carbon, other elements, and inclusions within iron act as hardening agents that prevent the movement of dislocations that naturally exist in the iron atom crystal lattices. The carbon in typical steel alloys may contribute up to 2.1% of its weight. The steel material of the present disclosure may be any of the broadly categorized steel compositions, including carbon steels, alloy steels, stainless steels and tool steels. Carbon steels contain trace amounts of alloying elements and account for 90% of total steel production. Carbon steels can be further categorized into three groups depending on their carbon content: low carbon steels/mild steels contain up to 0.3% carbon, medium carbon steels contain 0.3-0.6% carbon, and high carbon steels contain more than 0.6% carbon. Alloy steels contain alloying elements (e.g., manganese, silicon, nickel, titanium, copper, chromium, and/or aluminum) in varying proportions in order to manipulate the steel's properties, such as its hardenability, corrosion resistance, strength, formability, weldability, or ductility. Stainless steels generally contain between 10-20 wt. % chromium as the main alloying element and are valued for high corrosion resistance. With over 11 wt. % chromium, steel is about 200 times more resistant to corrosion than mild steel. These steels can be divided into three groups based on their crystalline structure: austenitic steels; ferritic steels; and martensitic steels. Tool steels contain tungsten, molybdenum, cobalt and vanadium in varying quantities to increase heat resistance and durability, making them ideal for cutting and drilling equipment.
In one embodiment, the reduced iron pellets manufactured by the direct reduction process are maintained at or near the temperature used during the reducing, and are transferred at this elevated temperature to a steelmaking apparatus (e.g., blast furnace, etc.), such that less heat is required to melt the reduced iron pellets during a steelmaking process.
The examples below are intended to further illustrate protocols for preparing and assessing the coated iron ore pellets and reduced iron pellets described herein, and are not intended to limit the scope of the claims.

Example 1

Raw Materials

SAMARCO iron ore pellets were used in the experiments. This ore is practically used in iron making processes in the Saudi Iron and Steel Company (HADEED). Electric arc furnace (EAF) dust (lime dust) generated from an electric arc furnace during charging of dolo-lime, lump lime, and special lime for slag formation was used. The iron ore and EAF dust were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM).
The various characterization tests of the iron ore pellets showed that iron oxide (Fe₂O₃) is the major phase with the presence of SiO₂, CaO, and Al₂O₃as minor components (FIG. 3 and Table 1).

TABLE 1

X-ray fluorescence (XRF) chemical analysis of SAMARCO
iron ore pellets

	COMPOUNDS	Conc. %	Elements	Conc. %

		O	30.8850
Na₂O	0.1100	Na	0.0835
MgO	0.1350	Mg	0.0815
Al₂O₃	0.3000	Al	0.1615
SiO₂	1.8150	Si	0.8484
P₂O₅	0.0710	P	0.0315
K₂O	0.0082	K	0.0069
CaO	0.7810	Ca	0.5655
TiO₂	0.0345	Ti	0.0210
V₂O₅	0.0040	V	0.0023
Cr₂O₃	0.0328	Cr	0.0226
MnO	0.0554	Mn	0.0436
Fe₂O₃	Balance	Fe	Balance
NiO	0.1040	Ni	0.0827

The SEM photos for SAMARCO iron ore samples are shown in FIG. 4A and FIG. 4B. It was observed that grain coalescence with very low micropores and many macropores took place in a dense structure.
The characterization of EAF dust was also performed and is given in Table 2. The EAF dust is mainly Fe₂O₃(53.09%) with CaO and CaCO₃(39.13%). The morphological examination under SEM with EDX analysis, as given in FIG. 5 and FIG. 6, show the average grain size of EAF dust as 2.0-10.0 μm while visual observation of the EAF dust indicates that it contains some coarse grains in the range of 1.0-9.0 mm.

TABLE 2

Chemical analysis for electric arc furnace (EAF) dust

PARAMETER	METHOD	RESULT	UNIT

Aluminum as Al₂O₃	Acid Digestion/ICP	0.29	%
Antimony (Sb)	Acid Digestion/ICP - MS	<0.001	%
Arsenic (As)	Acid Digestion/ICP - MS	<0.001	%
Barium (Ba)	Acid Digestion/ICP	0.041	%
Boron (B)	Acid Digestion/ICP	0.014	%
Chromium (Cr)	Acid Digestion/ICP - MS	<0.001	%
Copper (Cu)	Acid Digestion/ICP - MS	0.0017	%
Lead (Pb)	Acid Digestion/ICP - MS	<0.001	%
Manganese (Mn)	Acid Digestion/ICP	0.48	%
Mercury (Hg)	Acid Digestion/ICP - MS	<0.001	%
Nickel (Ni)	Acid Digestion/ICP - MS	0.0011	%
Selenium (Se)	Acid Digestion/ICP - MS	0.0056	%
Silver (Ag)	Acid Digestion/ICP - MS	<0.001	%
Zinc (Zn)	Acid Digestion/ICP - MS	0.0085	%
Molybdenum (Mo)	Acid Digestion/ICP - MS	<0.001	%
Thorium (Th)	Acid Digestion/ICP - MS	<0.001	%
Uranium (U)	Acid Digestion/ICP - MS	<0.001	%
Vanadium (V)	Acid Digestion/ICP - MS	0.0127	%
Strontium (Sr)	Acid Digestion/ICP	0.004	%
Cadmium (Cd)	Acid Digestion/ICP - MS	<0.001	%
Silica as SiO₂	Gravimetry	3.02	%
Lithium (Li)	Acid Digestion/ICP	<0.001	%
Iron Oxide as Fe2O3	Acid Digestion/ICP	53.09	%
Calcium Oxide	Acid Digestion/ICP	14.03	%
Magnesium Oxide	Acid Digestion/ICP	3.47	%
Sulphate	Water Extraction/IC	0.06	%
Chloride	Water Extraction/IC	0.05	%
Sodium	Water Extraction/IC	0.09	%
Potassium	Water Extraction/IC	0.04	%
Carbonate as CaCO3	Volumetry	25.1	%
Moisture	ASTM D 2974	0.05	%
Carbon Content	ASTM D 2974	0.18	%
LOI @ 550° C.	ASTM D 2974	0.38	%
LOI @ 800° C.	ASTM D 2974	0.64	%

Example 2

Coating of Iron Ore Pellets and Adhesive Characterization

SAMARCO iron ore pellets were coated comparatively with various concentrations of cement and electric arc furnace (EAF) dust suspensions. 5000 g of SAMARCO iron ore pellets were used in each coating test. Pellets were placed in a disc pelletizer of 50 cm in diameter rotating at 20 rpm. The coating was applied by spraying a suspension of coating material (cement or EAF dust). Solid concentrations (2.0 Kg cement or EAF dust per ton of iron ore) using 20% suspension concentration were applied.
Coated pellets were left to air-dry for 4 hrs followed by rotation of the pellets in a disc pelletizer for 10 min at a predetermined speed (20 rpm) and angle to cause rubbing of the pellets against each other and removal of the loose and/or un-adhered particles. The remaining coating and its uniformity on the surface of the pellets was evaluated under a microscope at standard magnification. The coating index of the particles is expressed as the percentage of the remaining coating thickness after the rubbing test: Coating Index=(T₂/T₁)×100, wherein T₁=coating thickness before the rotation test and T₂=coating thickness after the rotation test.
For EAF dust coated pellets, SAMARCO pellets before and after the EAF dust coating and after the rubbing test were visually inspected. The surface and internal layers of the coated pellets before and after rubbing were analyzed by energy-dispersive X-ray spectroscopy (EDX) as shown in FIG. 8 and FIG. 10 respectively. It was found that calcium and carbon have a higher percentage on the surface layer compared to the internal core confirming the formation of a coating layer comprising lime (FIG. 8). Also, after the rubbing test a higher percentage of Ca and C are observed on the surface layer compared to the internal core to confirm the presence of the coating layer after rubbing. The residual powder of the EAF dust coating layer after rotation for 10 min in the disc pelletizer is less than 0.1 g.
Comparatively similar results were observed in the case of cement coating. SAMARCO pellets before and after the cement coating and after the rubbing test were visually inspected. The surface and internal layers of the coated pellets before and after rubbing were analyzed by EDX as shown in FIG. 12 and FIG. 14, respectively. It was found that calcium, aluminum, and silicon have a higher percentage on the surface layer compared to the internal one confirming the formation of a cement coating layer (FIG. 14). Also, after the rubbing test a higher percentage of Ca, Al, and Si are observed on the surface layer compared to the internal one confirming the presence of the coating layer after rubbing. The residual powder of the cement coating layer after rotation for 10 min in the disc pelletizer is less than 0.1 g.
Thus, the comparative results of the adhesive characterization reflected that EAF dust has relatively acceptable adhesive nature to be used as a coating material for iron ore pellets during production of direct reduced iron (DRI).

Example 3

Determination of the Clustering Index (Reduction Under Load Test)—ISO 11256

ISO 11256 specifies a method to provide a relative measure for evaluating the formation of clusters of iron ore pellets when reduced under conditions resembling those prevailing in shaft direct-reduction processes.
The clustering or sticking index was measured for SAMARCO iron ore pellets coated with various concentrations of electric arc furnace (EAF) dust slurry. A schematic diagram of the reduction under load (ISO 11256) apparatus is shown in FIG. 1. The apparatus comprises a reduction tube, a loading device, waste gas, a furnace, and a gas supply system. The reduction tube comprises an outer reduction tube 1, an inner reduction tube 2, upper and lower perforated plates comprising a test portion 3, a gas inlet 4, a gas outlet 5, and a thermocouple exit 6. The loading device comprises a compressed air inlet 7, a pressure cylinder 8, a frame for the pressure cylinder 9, and a loading ram 10. The waste gas comprises a throttle valve 11 and a waste gas fan 12. The gas supply system comprises gas cylinders 15, gas flowmeters 16, and a mixing vessel 17.
The apparatus is constituted of a vertical oven divided into five heating zones starting from the bottom. One thermocouple is placed in the oven and a triple thermocouple is placed inside the reaction tube. Reducing gas and nitrogen flow rate is controlled by a mass flow meter and controller. The vertical electrical oven is equipped with a weighing system.
The system is capable of applying a total static load of 147 kPa on a bed of the test portion. The test portion is a 2000 g sample of pellets. The test portion comprises 50% pellets having a size in the range 16.0-12.5 mm and 50% having a size in the range of 12.5-10 mm. The pellet sample is isothermally reduced in a fixed bed at 850° C. under static load using a reducing gas consisting of 30% CO, 15% CO₂, 45% H₂and 10% N₂in a flow rate of 40 L/min until a degree of reduction of 95% was achieved.
The reduced test portion (cluster) is disaggregated by tumbling, using a specific tumbling drum. The percentage of clusters is determined on the cooled sample. The clustered pellets containing more than two pellets are applied to the tumbler test. A schematic diagram of the tumble drum apparatus is shown in FIG. 2A (front view) and FIG. 2B (side view). It comprises a revolution counter 18, a door with handle 19, a stub axle 20 with no through shaft, two lifters 21 (50 mm×50 mm×5 mm), a direction of rotation 22, and a plate 23.
The tumble drum is made of a steel plate at least 5 mm in thickness, having an internal diameter of 1000 mm and an internal length of 500 mm. Two equally spaced L-shaped steel lifters, 50 mm flat by 50 mm high by 5 mm thick and 500 mm long are solidly attached longitudinally inside the drum by welding, so as to prevent accumulation of material between the lifter and drum. Each lifter is fastened so that it points towards the axis of the drum, with its attached leg pointing away from the direction of rotation, thus providing a clear unobstructed shelf for lifting the iron ore pellets sample. The door is constructed so as to fit into the drum forming a smooth inner surface. During the test, the door is rigidly fastened and sealed to prevent any loss of sample. The drum is rotated on stub axles attached to its ends by flanges welded to provide smooth inner surfaces. The drum is replaced when the thickness of the plate is reduced to 3 mm in any area. The lifters are replaced when the height of the shelf is reduced to less than 47 mm.
All material is removed from the reduction tube. The mass of the reduced material is determined (m_r). During this operation, some individual pellets usually separate from the clustered material. These pellets are removed and the mass of the clustered material is recorded (m_e, 1). This step is considered as the first disaggregation operation. The removal of the test portion from the reduction tube is a critical step and care must be taken to avoid its untimely disaggregation. The clustered material is placed inside the tumble drum and rotated for a total of 35 revolutions, divided into 7 disaggregation operations of 5 revolutions each. After each disaggregation operation, the mass of the remaining clusters is measured and recorded as a series (m_c, 2, m_c, 3 . . . m_c, 8). Any individual pellets that are separated from the clustered material shall be removed prior to the next disaggregation operation.
The clustering index (CI) is expressed as a percentage and is calculated from the following equation where m_ris the total mass, in grams, of the test portion after reduction and m_c, i is the mass, in grams, of the clusters after the i^thdisaggregation operation expressed by formula (II):
$\begin{matrix} C I = \frac{100}{8 \times m_{r}} \times \sum_{i = 1}^{8} m_{c}, i & (II) \end{matrix}$
The clustering index measurement (ISO11256) was applied comparatively on SAMARCO iron ore pellets coated with various electric arc furnace (EAF) dust coating conditions, including 20% EAF slurry concentrations with a coated material amount of 0.5, 2.0 and 4.0 Kg per ton of iron ore. The results of the clustering index measurements are show in tables 3, 4 and 5.

TABLE 3

Clustering index measurement for SAMARCO iron ore pellets coated
with 20% electric arc furnace dust slurry concentration and 0.5
Kg electric arc furnace dust per ton of iron ore pellets

	No	Cluster Mass (g)

1	1114	(after Red.)
2	53	(after 05 rev.)
3	29	(after 10 rev.)
4	12	(after 15 rev.)
5	5	(after 20 rev.)
6	5	(after 25 rev.)
7	0	(after 30 rev.)
8	0	(after 35 rev.)

TABLE 4

Clustering index measurement for SAMARCO iron ore pellets coated
with 20% electric arc furnace dust slurry concentration and 2.0
Kg electric arc furnace dust per ton of iron ore pellets

	No	Cluster Mass (g)

1	900	(after Red.)
2	47	(after 05 rev.)
3	32	(after 10 rev.)
4	21	(after 15 rev.)
5	21	(after 20 rev.)
6	21	(after 25 rev.)
7	21	(after 30 rev.)
8	18	(after 35 rev.)

TABLE 5

Clustering index measurement for SAMARCO iron ore pellets coated
with 20% electric arc furnace dust slurry concentration and 4.0
Kg electric arc furnace dust per ton of iron ore pellets

	No	Cluster Mass (g)

1	583	(after Red.)
2	17	(after 05 rev.)
3	0	(after 10 rev.)
4	0	(after 15 rev.)
5	0	(after 20 rev.)
6	0	(after 25 rev.)
7	0	(after 30 rev.)
8	0	(after 35 rev.)

It was noticed that the tendency for cluster formation decreased by increasing the coating amount from 0.5, 2.0 to 4.0 Kg per ton of iron ore. Also it was found that the coating index measurement for iron ore pellets coated with 20% EAF dust slurry concentrations in 4.0 Kg per ton of iron ore achieved the Midrex process standard requirement. As used herein, this means the cluster mass comprising agglomerates of more than one pellet with a longest length of greater than 25 mm after 10 revolutions was zero or 0%. These obtained results confirm that the utilization of electric arc furnace (EAF) dust as a coating material for iron ore pellets is highly promising.
Thus, the foregoing discussion discloses and describes merely exemplary embodiments of the present invention. As will be understood by those skilled in the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting of the scope of the invention, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, defines, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public.

Claims

1. A coating composition comprising:

(a) at least 90 wt. % of electric arc furnace (EAF) dust based on the total dry weight of the coating composition, wherein the electric arc furnace dust comprises greater than 40 wt. % of iron (III) oxide (Fe₂O₃) and greater than 30 wt. % of calcium oxide (CaO) and calcium carbonate (CaCO₃) combined, based on the total dry weight of the electric arc furnace dust; and

(b) at least one binder material selected from a clay material or a cement material or both.

2. The coating composition of claim 1, wherein the electric arc furnace dust comprises at least 20 wt. % CaCO₃and at least 10 wt. % CaO.

3. The coating composition of claim 2, wherein the electric arc furnace dust comprises 45 wt. % to 60 wt. %, preferably 50 wt. % to 55 wt. % Fe₂O₃, 20 wt. % to 30 wt. %, preferably 23 wt. % to 27 wt. % CaCO₃, and 10 wt. % to 20 wt. %, preferably 12 wt. % to 17 wt. % CaO.

4. The coating composition of claim 3, wherein the electric arc furnace dust comprises 50 wt. % to 55 wt. % Fe₂O₃, 23 wt. % to 27 wt. % CaCO₃, and 12 wt. % to 17 wt. % CaO.

5. The coating composition of any one of claims 1 to 4, further comprising 0.5-5 wt. % of MgO and 0.5-5 wt. % of SiO₂relative to the total dry weight of the coating composition.

6. The coating composition of any one of claims 1 to 4, which is substantially free of zinc, chromium, manganese, lead, nickel, sodium and potassium.

7. The coating composition of any one of claims 1 to 4, which is substantially free of reducing agents comprising ferrous chloride and/or ferrous sulfate.

8. The coating composition of any one of claims 1 to 4, wherein the coating composition is a granular powder having an average particle size of 1-20 μm.

9. Iron ore pellets comprising:

an iron ore core;

a first coating comprising at least one selected from the group consisting of bauxite, bentonite, and dolomite; and

a second coating comprising the coating composition of any one of claims 1 to 9 in an amount of at least 90% based on the total dry weight of the second coating,

wherein the first coating is disposed between a surface of the iron ore core and the second coating.

10. The iron ore pellets of claim 9, wherein the average diameter or average longest length of the pellets ranges from 5-20 mm.

11. The iron ore pellets of claim 9, wherein the iron ore pellets comprise from 0.05-2 wt. % of the first coating relative to the total weight of the iron ore pellets and from 0.05-2 wt. % of the second coating relative to the total weight of the iron ore pellets.

12. The iron ore pellets of claim 9, wherein the first coating covers greater than 75% of the surface of the iron ore core and the second coating covers greater than 75% of the surface of the first coating.

13. The iron ore pellets of claim 9, wherein the average thickness of the first coating is 50-150 μm and the average thickness of the second coating is 50-150 μm.

14. The iron ore pellets of claim 9, wherein the first and second coating reduce the formation of agglomerated iron ore pellets at temperatures in the range of 20° C. to 1100° C. compared to a substantially similar iron ore pellet without the first coating, the second coating, or both coatings.

15. A process for manufacturing iron ore pellets, the process comprising:

applying a first coating as a slurry comprising at least one selected from the group consisting of bauxite, bentonite, and dolomite in an amount of 10-30 wt. % based on the total weight of the slurry to an iron ore core to form a coated iron ore core coated with a first coating; and

applying a second coating as a slurry comprising the coating composition of any one of claims 1 to 8 in an amount of 10-30 wt. % based on the total weight of the slurry to the coated iron ore core to form the iron ore pellets coated with the first coating and the second coating,

wherein the second coating is applied as a slurry comprising a solid concentration of 0.25-5 kg of the coating composition per ton of coated iron ore cores.

16. The process of claim 15, further comprising determining a clustering index of the iron ore pellets in accordance with ISO 11256 to evaluate the quality of the iron ore pellets as a feedstock for a direct reduction process.

17. The process of claim 15, wherein the iron ore pellets have a Midrex standard requirement of 0% agglomerated pellets with a longest length of greater than 25 mm relative to the total weight of iron ore pellets after 10 or fewer tumbling revolutions.

18. A process for manufacturing reduced iron pellets, the process comprising:

applying a first coating comprising at least one selected from the group consisting of bauxite, bentonite, or dolomite to an iron ore core to form a coated iron ore core;

applying a second coating comprising the coating composition of any one of claims 1 to 8 to the coated iron ore core to form iron ore pellets;

feeding the iron ore pellets into a reduction furnace; and

reducing the iron ore pellets with a reducing gas at temperatures up to 1100° C. to form reduced iron pellets.

19. The process of claim 18, wherein the first coating is applied to the iron ore core as a slurry comprising 10-30 wt. % of bauxite, bentonite, or dolomite based on the total weight of the slurry, the second coating is applied to the coated iron ore core as a slurry comprising 10-30 wt. % of the coating composition based on the total weight of the slurry, and wherein the second coating is applied as a slurry comprising a solid concentration of 0.25-5 kg of the coating composition per ton of iron ore pellets to be coated.