WO2011092516A1

WO2011092516A1 - Novel method for steel production

Info

Publication number: WO2011092516A1
Application number: PCT/GB2011/050156
Authority: WO
Inventors: Robert Glyn Lewin
Original assignee: National Nuclear Laboratory Limited
Priority date: 2010-02-01
Filing date: 2011-01-31
Publication date: 2011-08-04
Also published as: GB201001599D0

Abstract

The invention provides a method for the preparation of iron or iron alloys from iron ore, the method comprising the steps of electrolysing dissolved iron ore in an electrolytic bath comprising at least one molten salt and optionally including dissolved metals, and separating the resulting iron metal or steel. The at least one molten salt is preferably chosen from salts of alkali metals, alkaline earth metals and transition metals, and most preferably includes the oxides and halides of the alkali metals and alkaline earth metals and the halides of iron. In particularly preferred embodiments of the invention, the method comprises either an electrowinning process or a liquid/liquid metal extraction process. By the use of the method of the invention, it is possible to achieve the extraction of iron or iron alloys from iron ores whilst avoiding the generation of any significant quantities of carbon dioxide or carbon monoxide.

Description

NOVEL METHOD FOR STEEL PRODUCTION

Field of the Invention

[0001] The invention relates to the production of steel. Most particularly, it concerns a novel method for the production of steel from iron ore which avoids the use of coke and thereby avoids the production of large volumes of carbon monoxide and, most particularly, carbon dioxide.

Background to the Invention

[0002] Commercial process routes which are currently employed for the production of iron from iron ore employ an oxide reduction step using carbon, typically in the form of coke and, as a consequence, these processes generate large volumes of C0₂. Indeed, in the light of present concerns regarding the potential causes of climate change, these volumes may be regarded as being unacceptably large, especially when viewed in the light of the fact that the global steel production industry and is estimated to account for around 93% of the metals production industry worldwide. Consequently, there would be considerable environmental and commercial benefit from a process for steel production which avoided the generation of copious volumes of carbon dioxide.

[0003] The use of molten salts in the production of various metals has been considered in the prior art. In the aluminium industry, aluminium metal is recovered electrochemically from a fluoride salt bath at temperatures of up to 950 °C, the electrochemical production route being favoured in view of the electropositive nature of aluminium metal. Further, CN-A-1 1801 12 teaches a method for extracting tungsten powder by fusion electrolysis using an electrolyte composed of a mixture of NaCI and KCI which contains Na₂W0₄ and W0₃. The process is carried out at a temperature in the range of from 560 to 750 °C with an anode current density:cathode current density ratio of 0.15-1 .2 A/cm². The cathode material can be selected from iron, iron-based alloy, metal tungsten, metal tungsten alloy and any kind of composite material with electrical conductivity except a carbonaceous material, whilst the anode material is selected from a carbonaceous material or an inert metal.

[0004] JP-A-6128786 is concerned with the production of samarium metal or samarium alloy by electrolytic reduction of samarium monoxide using a molten salt electrolyte containing samarium difluoride and more than one fluoride salt of an alkali metal or alkaline earth metal. Carbon is used anode material, whilst the cathode is typically selected from carbon or iron.

[0005] In JP-A-2101 186 there is disclosed a method for obtaining high-grade neodymium-iron alloy and metallic neodymium by molten salt electrolysis in a LiF-NdF₃ electrolytic bath using a carbon anode and an iron cathode. The bath in an electrolytic cell is heated to an optimum temperature and electrolysis is carried out in an oxidising atmosphere. The formed Nd-Fe alloy drips from the bottom of the cathode as droplets and is charged into an alloy receiver and deposited.

[0006] A further method for the production of neodymium-iron alloy is taught in JP-A- 61291988, which relates to the formation of a base alloy using an Fe cathode and carbon anodes in an electrolytic bath comprising molten salts by adjusting the immersed depth of the Fe cathode on the basis of an electrolytic voltage value, and controlling the bath temperature. The bath is incorporated in an electrolytic cell and comprises NdF₃ and LiF as essential components and optionally also contains BaF₂ and CaF₂. The temperature of the electrolyte is controlled in the range of 770-950 °C and the alloy is formed by electrolytic reduction of the NdF₃, with the produced Nd being deposited on the Fe cathode.

[0007] JP-A-2057694 discloses the production of a praseodymium-iron or praseodymium- neodymium-iron alloy at high current density with high current efficiency by electrolysis with an iron cathode in a molten salt bath consisting of PrF₃, or a PrF₃-NdF₃ mixture, and LiF.

[0008] A method and apparatus for the production of a neodymium-dysprosium-iron alloy is described in JP-A-62222095. The process comprises electrolytically reducing neodymium fluoride and dysprosium fluoride in a molten salt electrolytic bath with an iron cathode and a carbon anode, depositing the resulting neodymium and dysprosium on the cathode, and and alloying with the iron forming the cathode to produce the required alloy. The molten salt electrolytic bath is composed of a mixture of neodymium fluoride with dysprosium fluoride, lithium fluoride, barium fluoride and potassium fluoride.

[0009] However, whilst the above processes all concern the production of a metal from an electrolytic bath comprising a molten salt, none of these processes is concerned with the production of a metal by the use of a molten salt in an attempt to reduce the production of carbon dioxide to an absolute minimum. Indeed it is, of course, the case that none of the cited processes is concerned with the production of steel from iron ore. This must be viewed in the light of the long held prejudice in the art of steel production in favour of smelting processes involving the use of coke, such that the problem of excessive production of carbon dioxide, with the associated environmental disadvantages, has failed to be addressed.

[0010] Thus, the present inventors have sought to provide a method for the production of steel which does confront the serious environmental issues associated with the standard methods of steel production and have attempted to provide a radical solution to this problem by the use of an electrolytic manufacturing process which involves the use of molten salts and which allows for the separation of iron from its ore and the production of steel whilst reducing emissions of carbon dioxide - and carbon monoxide to negligible or zero levels.

Summary of the Invention

[0011] Thus, according to the present invention there is provided a method for the preparation of iron from iron ore, said method comprising the steps of electrolysing the iron ore in an electrolytic bath comprising at least one molten salt and separating the resulting iron metal.

[0012] Said iron may optionally comprise alloys of iron, i.e. steel.

[0013] Typically, said at least one molten salt comprises a mixture of salts which is liquid only at elevated temperatures significantly greater than the ambient. Generally, molten salts melt above 80 °C and can readily be used in applications up to 950 °C. Said salts usually comprise inorganic cations.

[0014] Said iron ore comprises mixed oxides of iron.

[0015] Examples of such molten salts may comprise the salts of alkali metals, alkaline earth metals, transition metals and lanthanides. Preferred examples include the oxides and halides of the alkali metals and alkaline earth metals and, particularly in the context of the present invention, the halides of iron, especially those prepared from iron ore.

[0016] Thus, a preferred combination of molten salts for the performance of the method of the present invention comprises at least one iron halide and at least one alkali metal halide or alkaline earth metal halide. Especially suitable alkali metal halides are selected from the chloride salts of sodium, lithium and potassium, whilst calcium chloride is a preferred example of an alkaline earth metal halide. Preferred halides of iron comprise iron(lll) halides and, most preferably, iron (I I) halides, in view of their lower volatility than the iron(lll) species. A particularly preferred material is iron(ll) chloride.

[0017] In preferred embodiments of the method of the invention wherein the iron ore is dissolved in an electrolytic bath comprising at least one molten salt, said electrolysis is carried out in an electrolytic cell at a temperature in the range of 450 ° to 850 °C. Preferred cathode materials include iron or steel in the event that a solid cathode is employed; a preferred material for a liquid cathode system is bismuth. A further preferred liquid cathode material is lead, since it has a redox potential value which advantageously stabilises the iron(ll) chloride species in solution. The anode is preferably comprised of a carbonaceous material such as graphite, vitreous carbon or a dimensionally stable inert material such as an electrically conducting metal oxide.

[0018] In particularly preferred embodiments of the invention, said method comprises either an electrowinning process or a liquid/liquid metal extraction process. Both of these processes comprise the collection of iron or steel from the surface of a liquid metal which has been deposited in the form of metal dendrites. Both processes also have the capability for depositing more than one metal at a time, thus allowing for the production of selected steels (iron alloys) via the addition of selected dissolved metals to the molten salt bath. Said metals may either be added as the corresponding metal salts, or as the metals themselves. In the latter case, solubilisation is achieved either by electrochemical oxidation, wherein the metal is anodically oxidised, or by chemical oxidation, involving the addition of a suitable oxidising agent, a suitable example of which would be a metal chloride with a redox potential more positive than that of the metal to be oxidised.

[0019] Thus, a particularly preferred embodiment of the invention comprises the production of an alloy of iron, such a process being facilitated by the ability to dissolve selected metals in the molten salts which are present in the electrolytic bath, so that the iron and the selected alloying metal may be reduced to metal form together, thereby allowing for the desired alloy of steel to be directly produced from the electrolytic bath.

Brief Description of the Drawings

[0020] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a method according to the invention which comprises an electrowinning process for the production of iron;

Figure 2 is a schematic representation of a method according to the invention which comprises a liquid/liquid metal extraction process for the production of iron;

Figure 3 is a schematic representation of a method according to the invention which comprises an electrowinning process for the production of iron wherein chlorine is recycled from the electrolyser in order to enhance iron oxide dissolution; and

Figure 4 is a schematic representation of a method according to the invention which comprises an electrowinning process for the production of iron wherein hydrogen chloride is added in order to enhance iron oxide dissolution.

Description of the Invention

[0021] In a first preferred embodiment of the invention, the claimed method comprises an electrowinning process wherein iron ore comprising mixed iron oxides is dissolved in at least one molten salt, and the iron is then harvested through electrowinning. A suitable process is illustrated in Figure 1 , wherein iron ore is dissolved in a molten salt mixture of sodium chloride and iron(lll) chloride and impurities are separated out. Subsequently, electrolysis takes place, allowing for particulate iron to be produced at the cathode whilst oxygen is generated at the anode. It is possible that small amounts of CO or C0₂ may also be formed at the anode, but even these small amounts may be avoided by the use of a non-consumable anode, wherein CO and C0₂ emissions could be reduced to zero.

[0022] Preferred anode materials comprise carbonaceous material, vitreous carbon or a dimensionally stable metal oxide, whilst the cathode preferably comprises a liquid metal cathode. Suitable liquid metals may include, for example, bismuth or lead. The iron which is produced is typically in the form of metal dendrites as a finely divided solid or Fe particulates floating on the surface of the liquid metal cathode. Separation of the dendrites from the liquid metal and salt is required and may, for example, be achieved by simply scraping the dendrites from the surface and heating with a flux (such as CaO + NaCI) to form consolidated iron metal in the form of a metal ingot.

[0023] In a second preferred embodiment of the invention, the claimed method comprises a liquid/liquid (metal/salt) extraction process, a suitable example of which is illustrated in Figure 2. Thus, following dissolution of iron ore in a molten salt mixture of sodium chloride and iron(lll) chloride and separation of impurities, electrolysis takes place, as with the electrowinning process.

[0024] In this embodiment, however, an electropositive metal, for example sodium, lithium or calcium, alloyed with a metal which is liquid at the operating temperatures employed, is then mixed intimately with the molten salt mixture, such that iron in the salt chemically exchanges with the electropositive metal alloy, thereby resulting in metallic iron being formed at the liquid metal surface. The formation of iron on the liquid metal surface in this way is achieved by careful selection of the liquid metal, in which iron solubility should be very low. Preferred examples of liquid metals include bismuth or lead.

[0025] The liquid/liquid (metal/salt) extraction process offers the potential for the system to be engineered such that the iron floating on the liquid metal surface would make the Fe/salt/liquid metal separation step relatively easy, as described in respect of the electrowinning process.

[0026] When using a process according to the second preferred embodiment of the invention, the electropositive metal may be recovered into the cathode liquid metal by means of a second electrolysis process. In this case, sodium is recovered from the salt bath in situ, in order to maximise the overall efficiency of the system.

[0027] In a third preferred embodiment of the invention, the claimed method comprises an electrowinning process analogous to that of the first preferred embodiment, as illustrated in Figure 1 , but wherein the dissolution efficiency of the iron ore is increased by recycling chlorine produced during the electrolysis process. Such a process is illustrated in Figure 3. In this embodiment, chlorine will displace the oxygen from the iron ore to produce iron (111) chloride, which is the halide of choice for this process.

[0028] In a fourth preferred embodiment of the invention, the claimed method comprises an electrowinning process analogous to that of the first preferred embodiment, as illustrated in Figure 1 , but wherein hydrogen chloride is added to the process in order to enhance the dissolution of the iron ore. An example of such a process is illustrated in Figure 4. In this embodiment, oxygen will be displaced from the ore by the chloride ion with net generation of water as a by-product; this may easily be removed from the system.

[0029] By the use of the method according to the invention, it is possible to achieve the extraction of iron or iron alloy (steel) from iron ores, and thereby facilitate the production of steel, without any requirement for the use of coke. Consequently, the process does not generate any significant quantities of carbon dioxide or carbon monoxide and thereby avoids the detrimental environmental consequences which are associated with the use of traditional prior art methods of steel production.

[0030] Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0031] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. [0032] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

1 . A method for the preparation of iron from iron ore, said method comprising the step of electrolysing the iron ore in an electrolytic bath comprising at least one molten salt.

2. A method as claimed in claim 1 wherein said iron comprises alloys of iron.

3. A method as claimed in claim 1 or 2 wherein said iron ore comprises mixed oxides of iron.

4. A method as claimed in claim 1 , 2 or 3 wherein said at least one molten salt comprises a mixture of salts which is liquid only at temperatures significantly greater than the ambient.

5. A method as claimed in any one of claims 1 to 4 wherein said at least one molten salt comprises a mixture of salts which is liquid at temperatures above 80 °C.

6. A method as claimed in any preceding claim wherein said molten salts comprise inorganic cations.

7. A method as claimed in any preceding claim wherein said molten salts are selected from the salts of alkali metals, alkaline earth metals, transition metals and lanthanides.

8. A method as claimed in claim 7 wherein said molten salts comprise the oxides and halides of the alkali metals and alkaline earth metals and the halides of iron.

9. A method as claimed in any preceding claim wherein said at least one molten salt comprises an iron halide and at least one alkali metal halide.

10. A method as claimed in any one of claims 1 to 8 wherein said at least one molten salt comprises at least one iron halide and at least one alkaline earth metal halide.

1 1 . A method as claimed in 7, 8 or 9 wherein said alkali metal halide is selected from the chloride salts of sodium, lithium and potassium.

12. A method as claimed in claim 7, 8 or 10 wherein said alkaline earth metal halide is calcium chloride.

13. A method as claimed in any one of claims 8 to 12 wherein said halide of iron comprises an iron(ll) halide or an iron(lll) halide.

14. A method as claimed in claim 13 wherein said iron(ll) halide or iron(lll) halide comprises iron(l l) chloride or iron(lll) chloride.

15. A method as claimed in any preceding claim wherein said electrolysis is carried out in an electrolytic cell at a temperature in the range of from 450 ° to 850 °C.

16. A method as claimed in any preceding claim wherein said cathode is formed from iron, steel, bismuth or lead.

17. A method as claimed in any preceding claim wherein said anode comprises a carbonaceous material, vitreous carbon or a dimensionally stable inert material.

18. A method as claimed in claim 17 wherein said carbonaceous material comprises graphite.

19. A method as claimed in claim 17 wherein said dimensionally stable inert material comprises an electrically conducting metal oxide.

20. A method as claimed in any preceding claim wherein the molten salt mixture comprises a mixture of sodium chloride, iron (I I) chloride and iron(lll) chloride.

21 . A method as claimed in any preceding claim which comprises an electrowinning process.

22. A method as claimed in any one of claims 1 to 20 which comprises a liquid/liquid metal extraction process.

23. A method as claimed in claim 21 wherein the cathode comprises a liquid metal cathode.

24. A method as claimed in claim 23 wherein said liquid metal comprises bismuth or lead.

25. A method as claimed in claim 23 or 24 wherein the iron is produced in the form of metal dendrites which are separated from the liquid metal and salt by scraping from the surface and heating with a flux.

26. A method as claimed in claim 25 wherein said flux comprises a mixture of calcium oxide and sodium chloride.

27. A method as claimed in claim 22 wherein an electropositive metal alloyed with a metal which is liquid at the operating temperature employed is mixed intimately with the molten salt mixture, such that iron in the salt chemically exchanges with the electropositive metal alloy and results in metallic iron being formed at the liquid metal surface.

28. A method as claimed in claim 27 wherein said electropositive metal comprises sodium, lithium or calcium.

29. A method as claimed in claim 27 or 28 wherein said liquid metal comprises bismuth or lead.

30. A method as claimed in any one of claims 2 to 29 wherein the formation of said alloys of iron is facilitated by the addition of dissolved alloying metals to the electrolytic bath.

31 . A method as claimed in claim 30 wherein said addition of dissolved alloying metals to the electrolytic bath comprises the addition of metal salts to the electrolytic bath.

32. A method as claimed in claim 30 wherein said addition of dissolved alloying metals to the electrolytic bath comprises the addition of metals to the electrolytic bath and oxidation of said metals.

33. A method as claimed in claim 32 wherein said oxidation of said metals comprises electrochemical anodic oxidation of said metals.

34. A method as claimed in claim 32 wherein said oxidation of said metals comprises chemical oxidation of said metals.

35. A method as claimed in claim 34 wherein said chemical oxidation of said metals is effected by the use of a metal chloride with a redox potential more positive than that of the metal to be oxidised.

36. Iron metal or iron alloys whenever produced by the method as claimed in any one of claims 1 to 35.