GAS ATOMIZATION OF MOLTEN MATERIALS USING BY-PRODUCT OFF-GASES

Abstract

Metallurgical processes and systems for gas atomization of molten slag and/or molten metals from a metallurgical furnace are integrated with off-gas handling processes and equipment, such that the off-gases are fed to the gas atomization plant for atomizing the molten slag and/or molten metal. The use of by-product off-gases for atomizing molten slag and/or molten metals provides a number of benefits, including elimination of off-gas handling and treatment equipment, centralization and upgrading of heat via atomization to improve heat recovery, prevention of oxidation of granular products of atomization, and reduction of CO.sub.2 emissions. A process for preparing a granular product comprises: feeding a molten material and a by-product off-gas to a dispersion apparatus; and contacting the gas with the molten material in the dispersion apparatus, whereby the molten material is dispersed and solidified by contact with the gas to form the granular product.

Claims

1. A process for preparing a granular product, comprising: (a) providing a molten material; (b) feeding the molten material to a dispersion apparatus; (c) feeding a gas to the dispersion apparatus, wherein the gas is a by-product off-gas; (d) contacting the gas with the molten material in the dispersion apparatus, whereby the molten material is dispersed and solidified by contact with the gas to form said granular product.

2. The process of claim 1, wherein the molten material is molten metal or molten slag produced by a metallurgical process conducted in a metallurgical furnace.

3. The process of claim 2, wherein the molten material is molten metal and wherein the granular product comprises metal granules.

4. The process of claim 2, wherein the molten material is molten slag and wherein the granular product comprises slag granules.

5. The process of claim 2, wherein the dispersion apparatus comprises a gas atomization plant.

6. The process of claim 5, further comprising an induced draft (ID) fan for blowing the by-product off-gas to the dispersion apparatus.

7. The process of claim 6, wherein the metallurgical furnace includes a furnace cooling system, wherein the by-product off-gas comprises air exhausted by the furnace cooling system, and whereby the ID fan extracts the air from the furnace cooling system.

8. The process of claim 6, wherein the by-product off-gas comprises a furnace off-gas from an interior of the metallurgical furnace, and whereby the ID fan extracts the furnace off-gas from the metallurgical furnace.

9. The process of claim 6, wherein the gas atomization plant produces a hot off-gas, and wherein the hot off-gas is used in downstream process equipment for energy transfer.

10. The process of claim 9, wherein the downstream process equipment comprises a drying unit or a pre-heating unit.

11. The process of claim 10, wherein the by-product off-gas supplied to the gas atomization plant is laden with particulates.

12. The process of claim 3, wherein the off-gas supplied to the gas atomization plant is substantially oxygen-free.

13. The process of claim 1, wherein the by-product off-gas is substantially oxygen-free and is enriched in one or more gases selected from the group consisting of N.sub.2, H.sub.2O, CO.sub.2, CO, H.sub.2 and SO.sub.2.

14. The process of claim 12, wherein the molten material atomized using the by-product off-gas becomes less oxidized than material that would have otherwise been atomized by air.

15. The process of claim 1, wherein the dispersion apparatus comprises a gas atomization plant; wherein the gas atomization plant produces a hot off-gas; wherein the off-gas supplied to the gas atomization plant includes an undesirable component; and wherein the undesirable component contained in the off-gas supplied to the gas atomization plant is contacted with the molten material during the formation of said granular product, such that a concentration of the undesirable component in the hot off-gas is less than a concentration of the undesirable component in the off-gas supplied to the gas atomization plant.

16. The process of claim 15, wherein the undesirable component is a gas or a particulate solid.

17. The process of claim 15, wherein the undesirable component is CO.sub.2, a sulphur-containing species such as SO.sub.2, SO.sub.3, and H.sub.2S; a nitrogen oxide selected from NO and NO.sub.2, a phosphorus-containing gas; a gaseous organic species; fluorides (such as HF and SiF.sub.4), and combinations of one or more thereof; and wherein at least a portion of the undesirable component is dissolved in the molten material during formation of said granular product.

18. The process of claim 17, wherein the gaseous organic species is selected from furans and dioxins, and combinations thereof; and wherein at least a portion of the gaseous organic species is combusted and/or dissolved in the molten material during formation of said granular product.

19. The process of claim 15, wherein the undesirable component is a particulate solid.

20. The process of claim 19, wherein the particulate solid is dust; and wherein the dust becomes incorporated into the granular product during the formation of the granular product.

21. The process of claim 15, wherein the undesirable component is CO.sub.2; and wherein a portion of the CO.sub.2 in the off-gas supplied to the gas atomization plant is captured in the granular product.

22. The process of claim 21, wherein the hot off-gas produced by the gas atomization plant is converted to synthetic gas in a downstream reformer.

23. A system for preparing a granular product, comprising: (a) a metallurgical furnace containing a molten material selected from one or more of molten metal and molten slag; (b) a gas atomization plant located proximate to the metallurgical furnace; (c) a gas supply system for supplying a by-product off-gas to the gas atomization plant; (d) a molten material supply system for transporting the molten material from the metallurgical furnace to the gas atomization plant.

24. The system of claim 23, wherein the molten material comprises slag, and wherein the molten material supply system comprises a slag vessel or launder.

25. The system of claim 23, further comprising an ID fan for blowing the by-product off-gas to the gas atomization apparatus.

26. The system of claim 25, wherein the system further comprises an air cooling system for cooling the metallurgical furnace.

27. The system of claim 26, wherein the by-product off-gas comprises air exhausted by the furnace air cooling system, and wherein the system further comprises a conduit for conveying the by-product off-gas from the furnace to the gas atomization plant.

28. The system of claim 23, wherein the by-product off-gas comprises off-gases exhausted from the metallurgical furnace, and wherein the system further comprises a conduit for conveying the by-product off-gas from the metallurgical furnace to the gas atomization plant.

29. The system of claim 28, further comprising a fume and dust capture hood for collecting the off-gases exhausted from the metallurgical furnace, wherein the conduit for conveying the by-product off-gas to the gas atomization plant is adapted to receive the off-gases from the fume and dust capture hood.

30. The system of claim 23, wherein the molten material atomized using the by-product off-gas becomes less oxidized than material that would have otherwise been atomized by air.

31. A product from off-gas atomization that is less oxidized than the same product atomized through an air atomization plant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The invention will now be described, by way of example only, with reference to the attached drawings, in which:

[0025] FIG. 1 illustrates a portion of a process flow diagram in accordance with a first embodiment disclosed herein; and

[0026] FIG. 2 illustrates a portion of a process flow diagram in accordance with a second embodiment disclosed herein.

DETAILED DESCRIPTION

[0027] The following is a detailed description of metallurgical processes and facilities which include a process and equipment for gas atomization of molten slag and/or molten metals, and in which there is integration of at least one off-gas handling process with at least one gas atomization process, and the equipment associated with these processes.

[0028] The inventors have discovered that by-product off-gases from metallurgical processes can be used for gas atomization of slag and metals from metallurgical furnaces, to produce one or more of the following benefits:

[0029] A. Elimination of off-gas handling equipment, and centralization of off-gas treatment facilities to those required solely for atomization of molten slag and/or molten metal.

[0030] B. Elimination of furnace, metal tapping, calcine transfer, dust control, and reduction equipment fans. Instead, the atomization fan can evacuate gases from these processes and use them for atomization.

[0031] C. Elimination of operating expenditures associated with the use of furnace, metal tapping, calcine transfer, dust control, and reduction equipment fans, in contrast to system that atomize slag and/or metal with air.

[0032] D. Centralization of heat from the above mentioned processes, and upgrading of this heat via slag and/or metal atomization to provide more economic slag and molten metal heat (energy) recovery.

[0033] E. Production of off-gases with high temperatures (versus air with lower temperature) for process units requiring heat (drying units, water pre-heating, etc.).

[0034] F. Elimination of off-gas treatment systems and capital expenditures associated with granulation and atomization since the high temperature off-gases can now be re-used in processes that are already equipped with such equipment. A specific example is drying in FeNi rotary kiln-electric furnace (RKEF) plants.

[0035] G. Utilization of inert gases to atomize metals or slags into shot or powders in order to prevent their oxidation.

[0036] H. Improved metal recovery efficiencies vs. current slag crushing, sorting and jigging operations and the reduced use or elimination of such by-product post treatment plants.

[0037] I. If atomization is carried out using a CO.sub.2-rich off-gas from the process: A) a fraction of carbon-containing gaseous species can be dissolved into the molten material (as dispersion increases the specific surface area enhancing dissolution); and B) temperature of the CO.sub.2-rich gas will increase as a result of contact with molten material, which facilitates the reforming process with less preheating or without any extra preheating.

[0038] It can be appreciated that the above advantages lead to significant cost-downs over the current state of the art that make: [0039] Energy recovery from slag more viable; [0040] Energy recovery from smelter off-gases more viable; [0041] Smelter costs significantly lower through down-sizing or elimination of currently utilized off-gas, slag, and metal processing systems; [0042] Reducing the greenhouse gas emission from a smelter in a more cost effective manner.

[0043] FIG. 1 illustrates a portion of a process flow diagram according to a first embodiment of the invention. The flow diagram partially illustrates a process and system for production of metal by a metallurgical furnace 10 which includes a plurality of electrodes 12 for supplying heat to produce and maintain a layer of molten metal 14 and a layer of molten slag 16 within the furnace chamber. FIG. 1 shows that the furnace includes a slag tap hole 18 which communicates with the molten slag layer 16 and a molten metal tap hole 20 which communicates with the molten metal layer 14.

[0044] Molten slag is periodically tapped from the furnace 10 through the slag tap hole 18, and is tapped directly into a movable slag vessel or a slag launder or runner, in which the molten slag is transported to another area of the plant. During transport, the slag is maintained in a molten state. The transportation of the molten slag in a slag vessel or launder is represented by arrow 22 in FIG. 1.

[0045] The furnace 10 in FIG. 1 is at least partially cooled by air. More specifically, the bottom wall and lower side wall of furnace 10 are cooled by air. In known processes and systems, furnace cooling air is provided to the furnace by a fan or blower. However, the process and system according to the present embodiment eliminates such fans or blowers.

[0046] As will be appreciated, the furnace cooling air becomes heated as it cools the bottom wall and side wall of the furnace. In FIG. 1, the flow of exhausted cooling air from the furnace bottom wall and lower side wall is represented by arrows 24 and 26, respectively. In typical processes and systems, the heated cooling air is exhausted to the atmosphere.

[0047] The system according to the present embodiment also includes a gas atomization plant 28 to atomize the molten slag and produce slag granules suitable for use in commercial products such as proppants and/or roofing granules. The gas atomization plant 28 is located in close proximity to the metallurgical furnace 10, and receives the molten slag from furnace 10 via a slag vessel or launder, as represented by arrow 22.

[0048] The molten slag is atomized inside plant 28 by a gas flow from an induced draft (ID) fan 30, wherein the supply of atomizing gas from the ID fan 30 to gas atomization plant 28 is represented by arrow 32 in FIG. 1. When the gas stream supplied by ID fan 30 contacts a falling stream of molten slag in the atomization chamber of gas atomization plant 28, the molten slag is simultaneously separated into droplets and cooled to a solid state, thereby forming solid slag granules which fall to the bottom of the chamber.

[0049] In typical installations, the gas input to gas atomization plant 28 is instead routed through an air blower (not shown but replaces the ID fan) and may comprise air at ambient temperature and pressure. However, in the process and system according to the present embodiment, the gas input to the ID fan 30 comprises exhausted off-gases from the furnace bottom wall and/or the lower side wall, as represented by arrow 24 and/or 26. Such exhausted off-gases could not be fed to the blower utilized in current air atomization plants as they are hot and dirty. In the present embodiment, the gas input to ID fan 30 comprises the combined off-gas from the furnace bottom wall and the lower side wall, and therefore FIG. 1 shows arrows 24 and 26 being combined to form arrow 34 which represents the off-gas input to the ID fan 30. As will be appreciated, the off-gas supplied to the gas atomization plant 28 by ID fan 30 includes heat extracted from the furnace, and therefore is at a temperature greater than ambient temperature.

[0050] According to the present embodiment, the ID fan 30 for supplying off-gas to the gas atomization plant 28 also draws air into the furnace cooling system. This permits the elimination of any fans for supplying cooling air to the furnace 10, thereby providing a reduction in capital expenses and operating costs. In addition, the use of the off-gas for atomization permits the elimination of a separate off-gas treatment system for the off-gas, and providing a further reduction in capital expenses and operating costs.

[0051] Therefore, in the present embodiment, air circulation through the furnace air cooling system and off-gas circulation to the gas atomization plant is provided by the same ID fan 30. It will be appreciated that the ID fan 30 may not necessarily need to be located between the furnace 10 and the gas atomization plant 28, but may instead be located upstream of the furnace 10 so as to blow cooling air to the furnace walls and to blow the heated air to the gas atomization plant 28.

[0052] In FIG. 1 the solid slag granules produced by gas atomization plant 28 are represented by box 36 and their removal from the gas atomization plant 28 is represented by arrow 38.

[0053] The off-gas used for atomization is exhausted by the gas atomization plant 28 as a slag granulation off-gas which is represented by box 40, and the removal of the off-gas from the gas atomization plant 28 is represented by arrow 42. The off-gas 40 contains heat extracted from the furnace side wall and bottom wall, and heat extracted from the molten slag. Therefore, use of the off-gas exhausted from the furnace cooling system in the gas atomization plant 28 upgrades the heat in the off-gas, thereby permitting its use in downstream process equipment for energy transfer. For example, the hot off-gas 40 from gas atomization plant 28 may be processed to recover heat therefrom, or it may be fed to other process units which require heat, such as drying units, water pre-heating units, etc. These other process units are represented by box 44 in the drawings.

[0054] Although the above description relates to an embodiment where the off-gas used for atomization is air from the furnace cooling system, and wherein molten slag is the material being atomized, this is not necessarily the case. For example, FIG. 2 illustrates a portion of a process flow diagram according to a second embodiment, in which the off-gas instead comprises off-gases exhausted from the interior of the furnace and/or from fume and dust capture hoods. The first and second embodiments have a number of common elements, and these common elements are identified in FIG. 2 using like reference numerals. Furthermore, the above descriptions of these common elements apply equally to the second embodiment, unless otherwise indicated below.

[0055] The embodiment of FIG. 2 includes a metallurgical furnace 10 as described above, except that it also includes an off-gas port 50 for venting a by-product off-gas from the interior of the furnace 10. The vented furnace off-gas is represented by arrow 52 in FIG. 2. In the present embodiment, at least a portion of the furnace off-gas may be collected in a fume and dust capture hood 54, and withdrawn therefrom to be conveyed to the gas atomization plant 28 through ID fan 30. For this purpose, FIG. 2 shows arrows 56, 58, 34 to represent the flow of the off-gas from the fume and dust capture hood 54 to the ID fan 30.

[0056] Instead of being collected in hood 54, at least a portion of the off-gas vented from furnace 10 may be conveyed directly to the gas atomization plant 28 through ID fan 30. For this purpose, FIG. 2 shows arrows 52, 60, 58, 34 to represent the flow of the off-gas from the off-gas port 50 to the ID fan. It will be appreciated that all the off-gas, or a portion thereof, may be collected in hood 54 before it is conveyed to the gas atomization plant 28, and/or all or a portion of the off-gas may be directly conveyed from the furnace 10 to the gas atomization plant (through ID fan 30), or any combination thereof.

[0057] Due to the variability in the processes carried out in the metallurgical furnace, the off-gas may be of varying composition, and this is further discussed below. Regardless of the composition of the off-gas used for atomization, it is important to emphasize that an important aspect of the present invention is the use of an induced draft fan 30 to withdraw off-gases (for example from the interior of the furnace or from the bottom of the furnace) and to supply the off-gases for atomization. This replaces a conventional blower which draws in ambient air and blows the air for atomization. This improvement is applicable to all embodiments disclosed herein.

[0058] It is also important to emphasize that the use of off-gases for the purpose of granulating may result in less oxidation of the molten material than would be the case if ambient air were used as the atomizing gas. In this regard, the off-gas may be depleted in oxidative species such as oxygen or may be substantially free of oxidative species. The extent of the oxidation is dependent on the composition of the off-gas, but this reduction in the level of oxidation can be realized from a variety of off-gases from a number of sources within the system, such as the off-gases which are vented from the furnace as in FIG. 2, which may be depleted in oxygen and/or may include one or more gaseous by-products.

[0059] Another important aspect of the present invention is that the use of the off-gas for atomization results in upgrading of the off-gas heat. This is an important benefit where it is desired to recover heat from the off-gas or use the off-gas in another process step where heat is required, e.g. in drying or preheating, and is realized in all embodiments disclosed herein.

[0060] In some embodiments, the off-gas supplied to the gas atomization plant 28 for the purpose of atomizing molten metal or slag may be laden with particulates. Use of the process and system as described herein permit the elimination of a separate off-gas treatment system, and the particulate laden off-gas is supplied to the gas atomization plant 28 by ID fan 30 as described above in the first and second embodiments. The off-gas from the gas atomization plant 28 is treated as discussed above.

[0061] Another important aspect of the present invention is that atomization of metal/slag may use CO.sub.2-rich off-gases as the atomizing gas. Thus, according to this aspect, not only is the atomization of the metal/slag accomplished by using an off-gas, but the CO.sub.2-emission from the plant is simultaneously reduced. The CO.sub.2 reduction is brought about by partial CO.sub.2-capture by dissolution of the CO.sub.2 in the metal/slag. Also, the CO.sub.2-rich off-gas is heated in the atomization plant, and therefore the off-gas from the atomization plant is sufficiently pre-heated so that it can be directly (or with minimum preheating) converted to synthetic gas (CO+H.sub.2) in a reformer. Accordingly, in this aspect of the invention, the other process units represented by box 44 comprise a reformer for producing synthetic gas.

[0062] Atomization of the metal/slag may also be conducted with off-gases containing one or more other undesirable components, defined herein as including gases and particulate solids which are of concern when released into the environment, and which typically must either be partially or completely removed by treatment of the off-gases.

[0063] For example, where the undesirable components include one or more gases such as CO.sub.2, sulphur-containing species such as SO.sub.2, SO.sub.3, and H.sub.2S nitrogen oxides (NO.sub.x) such as NO and NO.sub.2, phosphorus-containing gases, fluorides (such as HF and SiF.sub.4) and/or organic species such as furans and dioxins. The reduction of the concentration of these gases in the off-gas is accomplished as discussed above with reference to CO.sub.2, i.e. the reduction is brought about by dissolution of the undesirable component(s) in the molten metal/slag during the formation of said granular product in the atomization plant, such that a concentration of the undesirable component(s) in the hot off-gas of the gas atomization plant is less than a concentration of the undesirable component(s) in the off-gas supplied to the gas atomization plant. This may reduce the need for expensive equipment and processes to remove these undesirable components from the off-gas.

[0064] Where the undesirable components include one or more gaseous organic species such as furans and dioxins. The reduction of the concentration of these organic species in the off-gas may be accomplished by combustion of the organic species by contact of the organic species with the molten metal/slag in the presence of oxygen in the off-gas and/or in the metal/slag. Furthermore, at least a portion of the gases produced by combusting the organic species in the gas atomization plant may become dissolved in the molten metal/slag during the formation of the granular product. Thus, where some or all of the organic species are combusted to form carbon dioxide and water, the gas atomization plant may further function as an afterburner.

[0065] Where the undesirable components include particulate solids such as dust, the dust particles may become incorporated into the granular product during atomization of the molten metal/slag in the gas atomization plant. This may reduce the need for expensive equipment and processes to remove dust from the off-gas.

[0066] Finally, the present invention permits the integration of one or more off-gas streams into the off-gas from the gas atomization plant. This permits a reduction in the off-gas treatment equipment in the system, resulting in reductions in both capital and operating expenditures, and this benefit is realized by all embodiments disclosed herein.

[0067] In a further embodiment, molten metal tapped from furnace 10 through tap hole 20 may be granulated by gas atomization plant 28, rather than slag. In this regard, FIG. 2 includes a dotted line 62 representing the conveyance of molten metal from the tap hole 20 to the gas atomization apparatus 28, where the molten metal is granulated in exactly the same manner as described above for molten slag. In such an embodiment, it will be appreciated that there would be no flow of slag to the gas atomization apparatus 28, as represented by arrow 22. It will also be appreciated that this modification may also apply to the process flow diagram of FIG. 1, relating to the first embodiment.

[0068] The composition of the metal tapped from furnace 10 will of course depend on the specific metallurgical process being conducted therein. For example, where the furnace 10 is a ferronickel smelting furnace, the molten metal tapped through tap hole 20 may comprise ferronickel (FeNi). However, it will be appreciated that the processes and systems disclosed herein are not limited to any specific metallurgical processes. For example, the processes and systems disclosed herein can be applied to the production of pig iron in an ironmaking blast furnace.

[0069] The use of hot and/or dirty off-gases for atomization may require the ID fan 30 to comprise a dirty gas fan with radial blades which are capable of handling dirty off-gases rather than fresh air blowers with backwardly curved impeller blades. This applies to all embodiments disclosed herein.

[0070] In some embodiments, the off-gas supplied to the gas atomization plant 28 may be substantially oxygen-free. For example, in some embodiments, the off-gas may be rich in gases such as N.sub.2, H.sub.2O or CO.sub.2 which will result in little or no oxidation of metals contained in the molten slag or molten metal during atomization. For example, the furnace off-gases from FeCr smelters are rich in CO.sub.2 and their use as atomizing gases may be particularly beneficial in the production of pig iron or other metals in which oxidation during atomization is to be avoided. In other embodiments, the off-gas may be rich in fuel such as CO or H.sub.2, or may be rich in sulfur-containing species such as SO.sub.2.

[0071] Although the invention has been described with reference to certain specific embodiments, it is not limited thereto. Rather, the invention includes all embodiments which may fall within the scope of the following claims.