US20150013497A1

US20150013497A1 - Process for recovery of iron/steel from mill scales and fines

Info

Publication number: US20150013497A1
Application number: US14/505,773
Authority: US
Inventors: Henry H. HOFFMAN; Jeffrey J. Hoffman
Original assignee: GLOBAL IRON & STEEL SERVICES LLC
Current assignee: GLOBAL IRON & STEEL SERVICES LLC
Priority date: 2012-04-05
Filing date: 2014-10-03
Publication date: 2015-01-15
Also published as: CA2869553A1; MX2014012005A; WO2013151711A1; EP2834380A1; EP2834380A4

Abstract

A process for recovering iron/steel from scrap mill scales and fines is disclosed. A process in accordance with embodiments of the present invention comprises mixing mill scales and fines with coke or other carbon containing fines and sealing the mixture in a container. The container is charged into a cupola or blast furnace, where the components react to form molten iron/steel. The recovery process in accordance with an embodiment of the present invention is performed integral to cupola or blast furnace operations and the recovered metal is collected at the base of the furnace as molten iron/steel.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of PCT International Application No. PCT/US13/31189, titled “Process for Recovery of Iron/Steel from Mill Scales and Fines,” filed on Mar. 14, 2013, which claims priority to U.S. Provisional Application Ser. No. 61/620,498 filed on Apr. 5, 2012, each of which is incorporated herein, in its entirety, by this reference.

TECHNICAL FIELD

The present invention relates to a process for the recovery of iron/steel from scrap mill scales and fines, and more particularly to the refining and cooling of molten steel with mill scale particles recovered from downstream operations of a steel manufacturing process.

BACKGROUND ART

Steel is produced by refining hot metal from a blast furnace. After molten steel has been refined, it is solidified and then subjected to successive rolling operations during which the steel may be successively heated and cooled. During the successive heating and cooling of steel, or during storage between various rolling operations, the surface of the steel oxidizes to form mill scale. This mill scale flakes off during handling and rolling of the steel.
Large amounts of waste materials are produced in steel-making operations and mill scale tend to be a large portion of such materials. Mill scales include base metal chips, platelets and other fines, which are essentially oxides of iron. Such oxides are primarily composed of ferrous/ferric iron (Fe₃O₄) surrounding a core of the iron base material. Mill scale is a prime candidate for recycling in steel making or blast furnace operations because mill scale is a relatively coarse, dense, waste oxide material of relatively high iron content and low in impurities such as alumina or silica.
In addition, fines are generated from both the manufacture and the use of Direct Reduced Iron (DRI) pellets. DRI pellets are a primary material used by electric arc furnaces in the making of steel and a secondary material used by blast furnaces and cupolas. Many of the pellets are crushed during transportation to mill sites from production plants and during normal charging operations.
Numerous processes have been developed to treat and recycle mill scales but have met with varying success and are often costly. Some of these processes use plasma arc furnaces, briquetting machines and pelletizing systems. These processes are very expensive, inefficient, and require relatively clean mill scale. Thus, there is a need for an efficient and inexpensive process for recovering iron/steel from scrap mill scales.

SUMMARY OF THE INVENTION

The present invention relates to a process for enhancing the efficiency of iron/steel reclamation from mill scales and DRI fines. It is an object of the present invention to recover base metal iron/steel from mill scale platelets and DRI fines. In one embodiment, the process uses byproducts of iron and steel melting operations, such as coke and fines, to effect iron/steel recovery. The recovery process in accordance with an embodiment of the present invention is performed integral to cupola or blast furnace operations, and the recovered metal is collected at the base of the furnace as molten iron/steel. A process in accordance with an embodiment of the present invention improves energy utilization of recovery operations by taking advantage of the inherent solubility of carbon in iron and other associated metallurgical interactions. A process in accordance with an embodiment of the present invention generates additional heat energy when coke constituents in a feed mixture ignite and burn using oxygen released from the iron/steel mill scale components. A process in accordance with an embodiment of the present invention also improves molten base metal output from earlier melting of mill scale constituents at the upper levels of a blast furnace.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an exemplary container used in the process for recovering iron/steel from scrap mill scales and fines in accordance with an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

The present invention relates to a process for recovering iron/steel from scrap mill scales and fines. In embodiments of the present invention, the process includes the step of mixing of iron/steel components, such as iron/steel chips, platelets and fines (for example, pig iron fines), with coke or other carbon-containing fines to ensure maximum surface contact of all components. For example, the iron/steel components and carbon-containing components can be mixed using known industrial dry blenders to produce a homogenous mixture or substantially homogenous mixture. In another embodiment, the iron/steel components and the carbon-containing fines can be added to a container in alternating layers, in which increasing numbers of thin layers may be preferred in order to maximize contact between the iron/steel components and the carbon-containing fines. In still another embodiment, layers of containers that separately contain iron/steel components and carbon-containing fines can be added to a furnace.
In one embodiment of the present invention, components are mixed with coke comprising from about 50% carbon to about 99% carbon. In another embodiment of the present invention, components are mixed with coke comprising about 92% carbon. The quantity of coke mixed with the components is preferably sufficient to ensure maximum contact with the surfaces of the iron/steel component. In some embodiments of the present invention, the ratio of carbon to mill scale is about one carbon to about eight mill scale. For example, when using mill scale having about 72% iron oxide, the ratio of carbon can be about 12.5% by weight or about 33% by volume. Variance of mill scale can go higher depending on iron content of mill scale. In other embodiments of the present invention, the ratio of DRI fines to carbon is about one carbon to about twenty five DRI fines. For example, new DRI fines are generally between about 85% and about 95% metallic iron. In such an example, the DRI will melt to metallic iron. DRI tends to oxidize rapidly. If the DRI is oxidized (or “seasoned”), the metallic iron may drop below below 85%. In that case, carbon can be added to increase the level of reduced iron during melting. In one embodiment, between about 2% and about 5% carbon is added. Depending on the furnace (and particularly in electric arc furnaces), the percentage of DRI can increase to one hundred percent, whereby only DRI fines are inserted into a container. The coke facilitates the recovery process through interaction between the carbon present in the coke with oxygen and iron atoms. In varying embodiments, the process can be used with gallon cans of mill scale and coke in a cupola furnace. Alternatively, the process can be used with one half gallon cans of 100 percent DRI fines in an electric arc furnace. In one embodiment, the weight ratio for mill scale and coke is about 87.5 percent mill scale and about 12.5 percent coke. In an alternative embodiment, blast furnace flue dust is used as the carbon carrier, replacing coke. In a further embodiment between about 5 percent and about 20 percent or 12.5 percent, by weight of blast furnace flue dust is used. In varying embodiments, the containers (also known as “mini-furnaces”) are dropped into the cupola or blast furnace from the top of the furnace.
In at least one embodiment where the process is used with half gallon cans of 100 percent DRI fines, the fines are charged with the primary charge. Essentially, they are placed near the bottom of the furnace, on top of a layer of scrap metal, then covered with DRI and additional scrap metal. At that point, the furnace operator administers a primary electric charge to the materials and melting begins. The optimal can size is between half gallon cans, three quarter gallon cans, and gallon cans.
In a cupola or blast furnace, coke reacts with oxygen in the air to produce carbon monoxide:
2 C+O₂→2 CO
The carbon monoxide reduces iron oxide (Fe₂O₃) to molten iron, and transforms to carbon dioxide in the process:
Fe₂O₃+3 CO→2 Fe+3 CO₂
During operation of cupolas and blast furnaces, elevated temperatures existing at the upper levels of cupolas and blast furnaces increase the rotational and vibrational energies of carbon, oxygen and iron atoms and their electrons causing accelerated interaction rates. Carbon has four electrons available in its 2sp⁴orbitals. Oxygen in iron oxide (e.g., FeO and Fe₂O₃) shares iron's two valence electrons, but readily transfers to carbon to share its higher energy electrons to form carbon monoxide (CO). This release of oxygen from iron oxide results in the formation of elemental iron. CO retains the two additional electrons to break down additional iron oxide to elemental iron, and forms carbon dioxide (CO₂). Excess carbon dissolves in the elemental iron to form delta and austenitic phases of the iron/carbon alloy system. In one embodiment of the present invention, an addition of 4.3 percent carbon lowers the melting point of the liquid/solid iron/carbon mixture from about 2780° C. to about 2090° C. The austenitic phase is derived from the peritectic reaction of delta ferrite and molten iron-carbon solution. During cooling to room temperature, austenite undergoes a eutectoid reaction, which at room temperature produces a variety of useful steel structures. Higher carbon contents of the iron/carbon alloy system increases the melting point of the system and results in formation of cementite (Fe₃C). Such increase in the melting point is undesirable for an economical recovery of iron/steel from mill scales and fines. However, additional heat energy can be derived from the ignition and burning of coke as appropriate furnace temperatures are attained. Molten iron/steel is produced in the upper levels of the furnace, thereby permitting frequent loading of recoverable scrap/coke mix and a resultant increase in production output.
In a process in accordance with an embodiment of the present invention, mill scales components are mixed with coke or other carbon containing fines to form a mixture. The mixture can provide all components with maximum contact surface necessary for efficient reaction. The mixture can be poured into a container and sealed.
Referring now to FIG. 1, there is shown an exemplary container, generally designated 100, used in the process for recovering iron/steel from scrap mill scales and fines in accordance with an embodiment of the present invention. In container 100 of FIG. 1, a container body 102 is provided with a side wall 104 leading to a body end 106 which is covered, in the depicted embodiment, with a ring 106. Methods of forming container bodies and container ends and of attaching or coupling the two, to form the depicted device, are well known in the art. Ring 106 forms a flange over body end such that a cross section shaped as an inverted “U” defines a container rim 108. Ring 106 extends inward from container rim 108 to form a ring edge 110. Diameter of ring edge 110 is smaller than the diameter of container rim 108 such that it forms a concentric ring edge 110 from container rim 110. Ring 106 forms a curl depression 112 having a U-shaped cross-section as it extends inward from container rim 108. Ring edge 110 curls inward to form a ring lip 114.
The opening at or near the container rim 108 is covered by a container plug 116 generally extending laterally across the container opening up to container rim 110 such that it resembles a disc covering the container opening. Viewing from the center towards the edge, container plug 116 forms a series of alternating depressions and ridges. First, container plug 116 forms a curl depression 118 having a shallow U-shaped cross-section. Immediately following curl depression 118, container plug 116 forms a ridge 120 having an inverted U-shaped cross section. Following ridge 120, container plug 116 forms a second curl depression 122 having a U-shaped cross-section. The edge following curl depression 122 curls outward to form plug lip 124. When container plug 116 is placed over the container opening, container plug 116 covers the entire container opening and curl depression 122 contacts the inner surface of curl depression 112 such that curl depression 122 forms a tight fit with curl depression 112. The tight fit between curl depression 122 and curl depression 112 provides a gas-tight seal between container body 102 and container plug 116. In some embodiments of the present invention, 0.33 g of tin solder paste is placed in the curl depression 112 prior to placement of the container plug 116 to ensure a gas tight seal. Post closure, the mini-furnace container 100 is placed such that a heat induction system can heat the top 1/16 inch of the mini-furnace container 100 to 550° F. in 4.5 seconds. The tin solder flows through the curl depression 112 and upon cooling assures a tight and very secure seal.
In varying embodiments, containers 100 are closed or sealed using a seaming machine. This eliminates the need for a heat induction system. The seamer essentially folds the edge of the top of the can over and under the top lip of the can making an effective seal. The can lid can be vented prior to seaming (e.g., by drilling, punching, molding, machining, and the like). For example, the lid can include two 3/16″ diameter vent holes.
Container 100 can be made from a material having a composition that is compatible with the core components of the mill scales. In one embodiment of the present invention, container 100 is a flat rolled steel can having a composition of about 0.15% carbon by weight. Container wall 104 can be of sufficient thickness so as to withstand the weight of mill scales, fines and coke. The container wall 104 can be of sufficient thickness so as to withstand high pressures from gases generated with the container 100. In one embodiment of the present invention, container wall 104 thickness is from about 0.12 inches to about 0.015 inches.
The internal surface of container 100 can be coated with a material that assists in sealing container 100 and enhances the mill scale conversion reaction. The material used for coating the internal surface of container 100 can have a low melting point, attaches or bonds to the internal surface of container 100, be ductile, and/or be resistant to corrosion from oils present in contaminants in the mill scales and coke. Container 100 internal surface coating material combines with sulfur, phosphorous, silicon and other contaminants present in the mill scales and coke to form compounds that minimize the dissolution of the contaminants in the recovered molten iron/steel. The contaminants combine with coating material to form compounds or eutectics that limit the solubility of the contaminants in molten iron/steel. The resulting slag segregates from the molten iron/steel upon melting of the container 100 and the contents thereof, and the coating material can be recovered from the slag. Exemplary materials that can be used to coat the internal surface of container 100 include tin, zinc, aluminum, and the like. In some embodiments of the present invention, the internal surface of container 100 is coated with tin having a thickness of from about 0.00003 inches to about 0.0001 inches. Container 100 capacity can be chosen such that it facilitates melting and recovery of mill scale base metal. For example, containers of smaller size are useful in furnaces having a fixed opening. Containers of smaller capacity are also useful for controlling the quantity of mill scale fed into the furnace without disrupting furnace operating conditions. Containers of smaller capacity minimize production losses resulting from damage to a container (e.g., breakage during handling). In one embodiment of the present invention, the volume of container 100 is from about 0.01 gallons to about 5 gallons (e.g., about 0.25 gallons, about 0.5 gallons, about 0.75 gallons, about 1 gallon, about 2 gallons, about 2.5 gallons, and the like). In another embodiment of the present invention, the volume of container 100 is from about 0.01 gallons to about 1 gallon.
Container 100 can include a plurality of vents to release carbon monoxide and/or carbon dioxide gases generated by reactions between components within container 100. In one embodiment of the present invention, vents are located on the top surface of container 100. In embodiments of the present invention where vents located on the top surface of container 100, the diameters of the vents are about 0.1875 inches. In another embodiment of the present invention, vents are located along the sides of container 100. Vents advantageously lower internal pressures sufficiently to prevent cans from mechanically exploding and slow the cooking process to allow molten iron to form without the vaporization of the fines. The container will soften before it melts, thus the vent holes remain functioning up to the melt temperature.
In a process for recovering iron/steel from scrap mill scales, DRI fines and fines in accordance with an embodiment of the present invention, containers 100 are fed into the furnace from the top of a furnace. Containers 100 bearing the mill scale-coke mixture can be brought to the top of the furnace along with raw materials via a mechanical device, such as a crane, a skip car powered by winches or conveyor belts, and the like. Containers 100 can be charged into the furnace in a manner similar to charging raw materials into the furnace.
In some embodiments of the process in accordance with the present invention, containers 100 can be used in blast furnaces having a double bell system, where two bells are used to control the entry of raw material into the blast furnace. In such blast furnaces, containers 100 are placed into the upper or small bell. The small bell can be then rotated to position container 100 more accurately, and then opens to drop container 100 into the large bell. The small bell then closes, to seal the blast furnace, while the large bell dispenses container 100 into the blast furnace.
In other embodiments of the process in accordance with the present invention, container 100 can be used in blast furnaces having a bell-less system, where multiple hoppers containing multiple containers are discharged into the blast furnace through valves. In such blast furnaces having bell-less systems, a chute can also be implemented in order to precisely control where container 100 is placed.
As container 100 travels towards the bottom of the furnace, its contents and the container structure are consumed. Elevated temperatures existing at the upper levels of furnaces and additional heat obtained from furnace operations permit the contents of container 100 to react and form molten metal while containers 100 are still at the upper levels of the furnaces. Charge levels are monitored using instruments that determine whether a flat surface of molten metal is reestablished at the bottom. Once a reasonably flat surface is reestablished at the bottom, additional containers can be charged into the furnace. In one embodiment of the present invention, additional containers can be charged into a cupola at one hour intervals. In another embodiment of the present invention, additional containers can be charged into a blast furnace at every 12 hour intervals. Thus, the formation of molten iron/steel at the upper levels of the furnace permit frequent loading of recoverable scrap/coke mix and a resultant increase in production output. Molten metal collects at the base of the furnace and can be withdrawn into ladles. The molten metal in ladles are then transferred for downstream processing. For example, the ladles contents can be poured into ingots or other product molds, allowed to solidify and prepared for processing into desired configurations.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that equivalent constructions insofar as they do not depart from the spirit and scope of the present invention, are included in the present invention.
For example, embodiments of the invention can be applied to recover other materials typically recovered through pyrometallurgy such as manganese, copper, zinc, chromium, tin, and the like.

Claims

What is claimed is:

1. A process for recovering iron from direct-reduced iron (DRI) fines, the process comprising:

mixing mill scale with DRI fines in a container comprising a plurality of vents;

exposing the container to heat from a metallurgical furnace, wherein heat from the metallurgical furnace causes the mixture of the mill scale and DRI fines in the container to form a molten metal; and

collecting the molten metal at a base of the furnace.

2. The process of claim 1, wherein the metallurgical furnace is selected from a group consisting of: a blast furnace, an electric arc furnace, and a cupola furnace.

3. The process of claim 1, further comprising:

monitoring charge levels to the metallurgical furnace to determine whether a flat surface of the molten metal is maintained at the bottom of the furnace; and

charging additional containers when the flat surface of the molten metal is not maintained at the bottom of the furnace.

4. The process of claim 1, further comprising the step of recovering metallic iron from the molten metal collected at the base of the furnace.

5. The process of claim 1, further comprising the step of venting the container to release a plurality of gases generated by the reactions of the mill scale and the DRI fines, in the container at the lower level of the furnace, wherein the plurality of gases are released through the plurality of vents comprised in the container.

6. The process of claim 1, wherein the container is heated to a temperature from about 2090° C. to about 2780° C.

7. The process of claim 1, wherein the container is sealed to provide a gas-tight chamber.

8. The process of claim 1, wherein the DRI fines are mixed with the mill scale at a ratio of about one carbon comprised in the DRI fines to about eight mill scales.

9. The process of claim 1, wherein the DRI fines are obtained from DRI pellets.

10. The process of claim 8, wherein the carbon-containing fines are mixed with the DRI at a ratio of about one carbon comprised in the carbon-containing fines to at least twenty five DRI.

11. A process for recovering iron from mill scale, the process comprising:

mixing the mill scale with carbon-containing fines in a container, wherein the carbon-containing fines comprise from about 50 weight % to about 90 weight % of carbon;

heating the mixture of the mill scale and carbon-containing fines in the container to form a molten metal; and

recovering iron from the molten metal.

12. The process of claim 11, wherein the mill scale comprises one or more selected from the group consisting of: iron/steel chips, platelets, and fines.

13. The process of claim 11, wherein the carbon-containing fines is coke.

14. The process of claim 11, said container further comprising vents, said process further comprising a step of releasing carbon monoxide and/or carbon dioxide gases generated by the reactions of the mill scale and the carbon-containing fines in said container.

15. The process of claim 11, wherein the carbon-containing fines are mixed with the mill scales at a ratio of about one carbon comprised in the carbon-containing fines to about eight mill scales.

16. A metallurgical process comprising:

mixing mill scale with carbon-containing fines, wherein the carbon-containing fines comprise from about 50 weight % to about 90 weight % of carbon, wherein the mill scale comprises oxides of iron;

heating the mixture of the mill scale and the carbon-containing fines to form carbon monoxide;

reacting the mixture of the mill scale with the carbon monoxide to form a molten metal comprising metallic iron; and

recovering the metallic iron from the molten metal.

17. The process of claim 16, wherein the mill scale comprises oxides of iron.

18. The process of claim 16, wherein the carbon-containing fines is coke.

19. The process of claim 16, wherein the mixture is heated to between about 2090° C. and about 2780° C.

20. The process of claim 16, wherein the carbon-containing fines are mixed with the mill scales at a ratio of about one carbon comprised in the carbon-containing fines to about eight mill scales.