Refining iron ore

Abstract: An iron ore refining method in which a secondary reactor (24) is employed for partly reducing iron ore and partly oxidising a carbon containing substance to form partly reduced secondary iron ore, coal char and a calorific gas containing carbon dioxide and carbon monoxide in a ratio of not less than about 0.25. The calorific gas is separated from the partly reduced secondary iron ore and the coal char and the resulting heated solids are introduced into the primary reactor (10) without substantial cooling. All or part of the iron product is formed from the partly reduced iron ore produced in the primary reactor (10). Since the secondary reactor (24) operates at a lower temperature than the primary reactor, part of the iron product is processed at a lower temperature to reduce oxygen requirements for the refining and to increase thermal efficiency.

Description: The present invention relates to a method of refining iron ore in which the iron ore is reduced to iron containing products. More particularly, the present invention relates to such a method in which char formed from a carbon containing substance, such as coal, is gassified in the reduction of the iron ore. Even more particularly, the present invention relates to such a method in which the char is formed and all or part of the iron ore to be refined is partly reduced within a secondary reactor and the resultant char and partly reduced iron ore are fed into a higher temperature primary reactor for production of the iron containing products. There are various iron refining processes that include the use of electric arc, open hearth, blast furnaces, the COREX® Process, and primary bath smelting reactors such as are used in the ROMELT process. In all such reactors, a charge of iron ore and a carbon containing substance such as coal or coke is introduced into a reactor. Oxygen or oxygen enriched air is also introduced into the reactor to gassify the carbon containing material to produce a reducing gas containing hydrogen and carbon monoxide. The reducing gas acts to reduce the ore, thereby to produce an iron containing substance such as a mixture of steel or iron and a separate slag phase. In the COREX® process, iron ore and flux are introduced into a shaft furnace to reduce the iron to the solid metal and calcine the flux with a reducing gas produced in a melter-gassifier. Coal or coal that is preheated by combustion of the coal volatiles, known as hot char, is introduced into a melter-gassifier along with the directly reduced iron and flux from the shaft furnace and oxygen . The char is gassified and the ore from the shaft furnace is further reduced and melted to produce a molten mixture of iron and slag that can be withdrawn as a product stream. In a primary bath smelting reactor, such as used in the ROMELT process, oxygen, iron ore and coal are introduced into a reactor to produce iron and slag products. In any of the above-mentioned processes it is desired to maximise the productivity of the reactor. Further, all of the above-mentioned processes use processed oxygen, namely oxygen having a purity of about 95% or greater or oxygen enriched air. Such processed oxygen contributes to the cost of production. Thus, a reduction in required processed oxygen usage allows either reduced production costs or more productivity with no increase in cost, at least with respect to oxygen usage. The abstract from Patent Abstracts of Japan corresponding to JP 62 228415 A discloses a method to produce molten pig iron comprising : feeding iron ore fines, coal and high-temperature air into a prereducing furnace to produce products comprising gas and hot solids containing char and prereduced iron are; and feeding the hot solids into a melting furnace, where they are melted and reduced to form molten pig iron, wherein the melting furnace is operated at a temperature less than that of the prereducing furnace. Accordingly, the present invention provides a method of refining iron comprising: feeding an iron ore feed, a carbon containing substance, and an oxygen containing gas into a secondary reactor to produce products comprising a calorific gas comprising carbon dioxide and carbon monoxide in a ratio not less than 0.25 and hot solids containing char and partly reduced iron ore; separating and removing at least about 90% of the calorific gas from the products produced within the secondary reactor to form an intermediate feed; introducing the intermediate feed into a primary reactor without substantial cooling of the hot solids; and reducing the partly reduced iron ore within the primary reactor to form at least part of an iron containing product, wherein the secondary reactor is operated at a temperature less than that of the primary reactor. The present invention provides a method which can be used in connection with any of the above-mentioned processes, and which allows for an increase in reactor productivity accompanied by a reduction in the usage of processed oxygen. The present invention relates to an iron refining method in which an iron ore feed, a carbon containing substance, and an oxygen containing gas are fed a secondary reactor to produce products. The products comprise a calorific gas comprising of carbon dioxide and carbon monoxide in a ratio of no less than about 0.25 and heated solids containing char and partly reduced iron ore. In this regard, the term, "partly reduced iron ore" as used herein and in the claims means a mixture of some ferric oxides, predominately ferrous oxides, and some metallic iron produced from the iron ore (predominately ferric oxides) by reduction . The term "calorific gas", as used herein in the claims means a gas predominately containing water, carbon monoxide, carbon dioxide, and some hydrogen and hydrocarbons. At least about 90% of the calorific gas is separated from the products produced within the secondary reactor to form an intermediate feed. The intermediate feed is introduced into a primary reactor without substantial cooling of the hot solids and the partly reduced iron ore is further reduced within the primary reactor to form at least part of an iron containing liquid product. The secondary reactor operates at a temperature less than that of the primary reactor. It is to be recognised that since the present invention has application to a process such as the COREX® process, it is contemplated that the iron product is formed from iron ore and flux feeds that are split between the shaft furnace and the secondary reactor. In such a case, the resultant intermediate feed from the secondary reactor to the primary reactor only forms part of the iron containing product. In a primary bath smelting reactor all of the partly reduced iron ore to be processed would normally be supplied from the secondary reactor. A further point to be mentioned here is the term "substantial cooling", as used herein and in the claims, means cooling beyond 200°C from the temperature of the secondary reactor or within 200°C of ambient temperature. The productivity and thermal efficiency the primary reactor is limited by its heat transfer rate and efficiency. The secondary reactor increases the capacity of the primary reactor by decreasing the total energy requirement to reduce the ore and melt the flux and reduced ore to produce the molten metal and slag phase products. The secondary reactor also increases the thermal efficiency of the primary process by removing the coal volatiles and partially reducing the iron ore feed at a lower temperature within the confines of a reasonable carbon dioxide and carbon monoxide ratio. Because of this, air or at the very least oxygen enriched air at a lower oxygen enrichment than that required in the primary reactor, can be used at the oxidant within the secondary reactor and as such will decrease the requirement for processed oxygen. The invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic view of an apparatus for carrying out a method in accordance with the present invention; and Figures 2a, 2b and 2c are alternative embodiments of a secondary reactor used in the apparatus shown in Figure 1. With reference to Figure 1, the primary reactor 10 is provided which can be an open hearth furnace which could have separate inputs in the form of thermal energy. Other possibilities would be the tuyere area melting zone of a blast furnace, the melter-gassifier of the COREX® process or a primary bath smelting process such as a ROMELT primary bath reactor. In this regard, the term "melter-gassifier" is any reactor in which partly reduced or substantially reduced iron is reduced and melted to form a liquid metallic iron rich phase in the presence of coal coke or char which is undergoing gassification. The "tuyere area of a blast furnace", as that term is used herein and in the claims would be a species of a melter-gassifier. Lastly a "bath-type smelting reactor", as used herein and in the claims, is any reactor in which iron ore, a carbon source and oxygen are added to a liquid melt. For exemplary purposes the present invention will be described by reference to a primary bath smelting reactor in which ore, coal, and flux are fed as a feed 12 is fed into primary reactor 10. Oxygen-enriched air or even high purity oxygen as a feed 14 is also introduced into primary reactor 10 to produce an off gas stream 16 and an iron ore and slag products stream 18. Iron ore and optionally flux (calcium carbonate) as a feed 20 (which would constitute part of the total feed of iron ore to be refined and fluxed) and a coal feed 22 are fed into a secondary reactor 24. The coal is party oxidised by air which is introduced as an air feed 26 to form char. The volatile content of the coal is preferentially decreased. Thus, the ore can be partially reduced, the flux calcined, and the coal preheated with minimum loss of fixed carbon in the original coal feed, which is a very desirable feedstock for the primary reactor. The iron ore introduced into secondary reactor 24 is partly reduced by the coal char and calorific gas produced by the partial oxidation of the coal, so that a hot and partially reduced solid ore is produced. The reaction is conducted within secondary reactor 24 such that the calorific gas contains carbon dioxide and carbon monoxide in a ratio of no less than 0.25. The calorific gas is separated from the heated solids comprising in the main, partly reduced iron ore, calcined flux and char, and is thereafter discharged from secondary reactor 24 as a calorific gas stream 27. As will be discussed, such separation is effected by a fluidised bed reactor and should separate greater than about 90% of the calorific gas from the heated solids. The heated solids and any part of the calorific gas not separated from the heated solids are fed into primary reactor 10 as an intermediate feed 28. In primary reactor 10, the hot char gassifies and thus produces heat to melt the ore and flux, and a reducing gas containing primarily hydrogen, carbon monoxide and much smaller quantities of carbon dioxide and water products. Carbon monoxide and hydrogen reduce the partially oxidised iron as well as the iron ore that has not yet been reduced. The heat of reaction melts the resultant iron and slag which is removed as an iron ore and slag product stream 18. The secondary ore contained within feed 20 is preferably in a size range of between about 100 and about 5,000 microns in diameter. Relatively large coal particles are the preferred coal feed 22 and are preferably in a size range of between about 1mm and about 50mm in diameter. The use of such coal eliminates the need for expensive coal grinding facilities. The air feed 26 can be contacted with a coal and iron ore by a variety of methods. The secondary reactor 20 can be an external recirculating flow reactor, as illustrated in Figure 2A, a fluidised solid bed as shown in Figure 2B or an internal circulating fluidised bed as shown in Figure 2C. Alternatively, the air feed 26 can be advantageously fed as two subsidiary streams 26A and 26B above and below an expanded solids bed 30 formed within each of the reactor types illustrated in Figures 2A, 2B, and 2C. Carbon monoxide in the calorific gas from the ore, flux, and coal char bed will react with the oxygen supplied by subsidiary stream 26A to produce a more oxidised secondary reactor calorific gas, and will increase the thermal efficiency of secondary reactor 20. The resultant heat will be transferred to the expanded solids bed 30 primarily by conduction to the walls of the secondary reactor 24 and also by radiation from the secondary reactor walls. The ratio of carbon dioxide to carbon monoxide within the calorific gas produced in the secondary reactor 24 should be no less than about 0.25. Such a ratio is necessary to strike a balance between thermal efficiency and the need to reduce iron ore. The separate oxygen feeds substantially increase the optimum carbon dioxide to carbon monoxide ratio. Put another way, the more carbon dioxide, the more thermally efficient the process, but the less carbon monoxide is produced to accomplish the reduction of iron and vice-versa. The ratio can be controlled by suitably adjusting the air and coal feed ratio and the gas-solid contacting time. The bed height then becomes important, because it influences the gas-solid contacting time. The higher the bed, the higher the mass transfer efficiency between gas and solids. Higher mass transfer efficiency decreases the carbon dioxide concentration, which is advantageous because it improves the efficiency of ore reduction. However, high bed heights are not advantageous from a process heat balance standpoint. A compromise between these factors dictates a bed height from between about 1 and about 5 metres, with a bed height to vessel diameter ratio generally between about 0.2 and 5. The bed height to vessel diameter ratio is much less important than the absolute value of the bed height. Secondary reactor 24 should operate at a pressure roughly equal to or greater than the operating pressure of the primary reactor and the means used to transport solids. Where the intermediate feed 28 is accomplished by a short solids transfer line or a mechanical device, such as a hot screw feeder or star valve, the pressure within the two reactors would be about the same. In case of gas transport, the pressure within secondary reactor 24 would have to be set higher than primary reactor 10 to compensate for pressure losses in piping between the reactors. In any type of transport used, it is important that solids be fed into primary reactor 10 without appreciable heat loss from their origination within secondary reactor 24, or in other words, without a temperature loss during transport between reactors of more than about 200°C. The operating temperature of the secondary reactor 24 must be greater than the autothermal ignition temperature of coal but less than the sintering temperature of any feed component. Thus, the temperature range of secondary reactor operation is preferably in a range of between about 400°C and about 1200°C. A preferred temperature range is between about 700°C and about 950°C. The operational temperatures of secondary reactor 24 should be compared to the temperature at which primary reactor 10 would operate which would be in a range of between about 1500°C and about 1800°C. The lower temperature of the secondary reactor allows air to be used for secondary reactor 26 and can substantially reduce the requirements for processed oxygen.

Claims: A method of refining iron comprising: feeding an iron ore feed, a carbon-containing substance, and an oxygen containing gas into a secondary reactor to produce products comprising gas and hot solids containing char and partly reduced iron ore; separating and removing at least about 90% of the calorific gas from the products produced within the secondary reactor to form an intermediate feed; introducing the intermediate feed into a primary reactor without substantial cooling of the hot solids; and reducing the partly reduced iron ore within the primary reactor to form at least part of an iron containing product, wherein the secondary reactor is operated at a temperature less than that of the primary reactor; characterised in that the gas produced in the secondary reactor is a calorific gas comprising carbon dioxide and carbon monoxide in a ratio not less than 0.25. A method as claimed in Claim 1, wherein the iron ore feed and the carbon containing substance form a bed within the secondary reactor and wherein an oxygen containing gas is fed to the secondary reactor at locations thereof above and below the bed to allow the carbon monoxide to react with oxygen, the heat of reaction being transferred to the bed. A method as claimed in Claim 1 or Claim 2, wherein the temperature of operation of the secondary reactor is between 400°C and 1200°C. A method as claimed in Claim 1, Claim 2 or Claim 3, wherein the temperature of operation of the secondary reactor is between 700°C and 950°C. A method as claimed in any preceding Claim, wherein the iron ore feed comprises fine iron ore in an iron ore size range of between about 100 and about 5000 microns, and wherein the carbon containing substance comprises granular coal in a coal size range of between 1mm and about 50mm. A method as claimed in any preceding Claim, wherein the secondary reactor is a fluidised bed reactor, an internal circulating fluidised bed reactor, or a circulating flow reactor. A method as claimed in any one of Claims 1 to 5, wherein the primary reactor is a melter-gassifier or a bath-type smelter. A method as claimed in any preceding Claim, further comprising feeding flux into the secondary reactor.

Inventors: SATCHELL DONALD PRENTICE, SATCHELL, DONALD PRENTICE
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1998
Main classification: C21B13
IPC codes: C22B5, C21B11, C21B3, C21B13