WO1996041895A1

WO1996041895A1 - Method for producing molten iron

Info

Publication number: WO1996041895A1
Application number: PCT/US1996/010030
Authority: WO
Inventors: Jonathan Sidney Edelson
Original assignee: Borealis Technical Inc Ltd
Current assignee: Borealis Technical Inc Ltd
Priority date: 1995-06-09
Filing date: 1996-06-07
Publication date: 1996-12-27
Anticipated expiration: 1997-12-09
Also published as: AU6383796A

Abstract

A method for reducing particulate iron oxide (24) and/or other iron units to molten iron (26) utilizing natural gas (17) as the source for process heat and reduction potential, in which the ore freely falls during the melting and reduction process. Reducing gas (22) and iron oxide (24) are carried through the process in a generally counterflow relationship. Heat for melting and reduction is generally supplied by combusting a fraction of the reductant gas (22) with oxygen (23) or air. Heat may also be supplied through electric arc heating.

Description

METHOD FOR PRODUCING MOLTEN IRON

Background-Cross-Reference to Related Applications:

The present invention may be used as the reduction process identified in my patent, filed under serial number 08/258,503, titled Method for the Production of Steel, however the method of the present invention is most suited to the use of primary fuels. The present invention is related to the reduction process identified in my patent, filed under serial number 08/258,572, titled Method for Reducing Particulate Iron Ore to Molten Iron usinσ Hvdrocren as Reductant.

Background-Field of Invention:

The present invention relates to the smelting reduction of iron ore.

Background of the Invention:

The present invention is seen as a replacement for the use of the blast furnace in steel making. The purpose of the present invention is to allow the production of hot molten iron, such as one would produce using a blast furnace, while using inexpensive fuels such as natural gas, such as one would currently use to fuel a direct reduction plant.

Conventional blast furnace operation involves a counter flow of ore and/or other input iron units (hereafter referred to simply as ore) and a reducing gas. This flow occurs in a furnace at high temperature. The ore is fed as particles which may be lump ore, sinter feed, pellets, briquettes, or other agglomerates. In such a blast furnace, the ore descends as a burden, in which individual particles of ore rest atop other particles of ore, thus forming a porous mass. The reducing gas rises through channels in the burden, and exits at the top of the furnace. The reducing gas is produced by the action of a hot oxidizing gas, usually oxygen enriched air, on carbon in the form of coke. This coke is introduced along with the ore, and in addition to providing reductant potential, the coke provides for the necessary gas channels as the ore melts. Finally, some of the coke dissolves in the molten iron, producing a high carbon content product which must be decarburized in order to produce steel.

The use of ore burdens introduces several difficulties related to the mass of the burden. First, the walls of the furnace must be able to support the mass of the burden. Secondly, the ore must be able to support itself in order to provide for gas flow (as will be described below) . The thermal mass of the burden makes initiating or halting furnace operation a lengthy process involving extensive fuel use. In a furnace which melts the burden as it descends, improper shutdown can render the furnace unusable and filled with a solid fused mass of ore, slag, and metal.

The need for gas channels is a major factor in process parameters. The ore must meet strict size and strength requirements. The mass fraction present as fines must be small, and the ore must be strong enough to not degrade excessively into fines. In furnaces which melt the ore, a refractory material is needed which maintains the gas channels. In conventional blast furnace operations, the coke serves this purpose in addition to supplying fuel. Thus the coke must meet size and strength requirements as well. Coking coal of sufficient quality, and the coking process itself, are major cost factors in blast furnace operations.

The ore size requirements are problematic because many ore beneficiation processes are better suited to producing smaller ore particles. If the beneficiated ore particles are too small for the blast furnace, then an agglomeration step is needed in order to produce suitable larger particles.

Refinements in blast furnace technology have tended to reduce coke consumption. These refinements include the injection of hydrocarbon fuels with the hot blast, and the use of top gas to provide the hot blast. U.S. Patent 4,421,553 to Pongis et al. (1983) demonstrates that one can considerably reduce coke consumption rate through the use of externally generated and heated reducing gas; essentially no coke was used as fuel. Pongis et al. were also able to achieve a high level of control over blast furnace operation through the control of the reducing gas temperature, pressure, and composition.

Direct reduction processes are processes which do not melt the iron ore. The ore must maintain the gas channels, and thus must meet strength requirements. Because the ore is not melted, the need for refractory coke is eliminated. Internal reductants, if used, may be ordinary coal or fuel oils. Reductant may be entirely external, and fed as a blast over the ore. Reformed natural gas as the reductant has proven viable in regions where natural gas is plentiful.

In direct reduction operation, the ore does not melt, and in fact cannot be allowed to melt. If melting occurs, ore particles may stick to the walls of the furnace or to each other, potentially causing damaging materials flow problems.

The product of the direct reduction furnace is sponge iron, a high surface area material consisting of metallic iron intermingled with gangue from the ore. This material needs to be melted to separate the iron from the gangue, generally in a separate step involving an electric arc furnace. The material must be passivated prior to transport, and is subject to catastrophic oxidation. The product is not suitable as the primary feed for steel conversion processes other than the electric arc furnace. Specifically, direct reduction does not produce the molten hot metal that is needed by the basic oxygen furnace or by other self-powered steel conversion processes.

Direct reduction processes make use of a burden similar to that of the blast furnace process. Ore must meet similar size, strength, and stability requirements to that of the ore used in blast furnace operations. Additionally, impurities in the ore may produce low melting point materials which can cause fusion between ore particles. Substandard ore and/or incorrect charging can cause problems with gas flow, burden stability, or agglomeration.

Various processes have been proposed or attempted in order to combine the coke-free direct reduction process with melting in order to supply hot metal or pig iron. The most optimistic of these processes claims to produce steel directly from the iron ore. In general, these processes combine a direct reduction process with a melting process, using off gasses and heat from the melting process to supply reductant to the direct reduction process.

An example of a direct reduction process combined with melting is U.S. Patent 3,140,168 to Halley et al. (1964) . In this process, a fluidized bed pre-reduction zone is coupled to an arc furnace wherein the arc includes a hydrogen jet, the arc providing both the reductant and heat to the reduction process, as well as melting the reduced ore. This process is limited by the use of a burden subject to agglomeration, specifically in the fluidized bed. Additionally, this process requires the use of electric heating.

In U.S. Patent 4,248,408 to Beggs et al. (1981) a method was taught which used electrical heating to provide the necessary heat for the reduction process, as well as the use of oxy-fuel burners for melting of the product. A burden again limits the ore which may be used and the temperatures which may be used, and electrical heating is required.

The closest known prior art to the present invention are the various processes which make use of freely falling ore particles. These processes avoid the problem of having to maintain an acceptable burden by eliminating the burden entirely. The falling particle processes allow for furnace operation above the melting point of the ore. Higher temperatures enable a more rapid reduction reaction, and provide for a molten product. Additionally, a wider choice of reductants become available. Eliminating the burden also reduces the requirements for ore strength, and makes the use of fine ore particles, such as produced by beneficiation or superconcentration processes, desirable. Finally, because the melting, reduction, and reductant reformation processes are coterminous, furnace complexity and cost are greatly reduced.

U.S. Patent 2,066,665 to Baily (1937) is an early example of the freely falling reduction furnace. Baily teaches that particles of ore are caused to descend in an upward flow of reducing gas, which is produced through the partial combustion of fuel in air. The particles of ore are swiftly reduced, and allowed to fall into a pool of molten metal and slag. The reducing gas considered in the aforementioned patent is composed of carbon monoxide, generally mixed with hydrogen. No attempt is made to recycle the reducing gas, which by virtue of reaction equilibrium can only be partially used. Additionally, the temperature limits of the gas handling system will necessitate high gas flow rates. While an attempt is made to recover the heat energy entrained in the used reducing gas, much of the available energy is wasted. The fuel value of the used reducing gas is used indirectly through the generation of electricity, increasing the complexity of the system.

U.S. Patent 2,951,756 to P.E. Cavanaugh (1960) discloses a Jet Smelting Process, in which ore particles are entrained in a turbulent reducing flame consisting of natural gas and oxygen. As compared to the work by Baily, higher ore temperatures are achieved, and a more rapid reaction facilitated. However, no effort is made to recycle the 'top gas*, and the gas flow is not counter-flow to the ore flow. The reduction method is extremely simple, however it requires excessive natural gas for operation, and the lack of counterflow between reductant and ore means that full reduction cannot be achieved.

Objects and Advantages

Accordingly, besides the objects and advantages of the methods of the smelting reduction of iron ore described in my above patent, several objects and advantages of the present invention are the following:

It is an object of the present invention to provide a method by which iron ore can be reduced to molten metal without the use of coke.

An advantage of the present invention is that the use of expensive coke may be avoided.

An advantage of the present invention is that the many pollution problems associated with coke use are avoided. It is an object of the present invention to provide a method of reducing iron ore using natural gas.

An advantage of the present invention is that currently wasted natural gas resources may be profitably used.

An advantage of the present invention is that a substantial portion of the reduction potential of natural gas is provided by hydrogen, thus reducing carbon dioxide emissions substantially.

It is an object of the present invention to provide for the recycling and reuse of top gas.

An advantage of the present invention is that natural gas use may be minimized.

An advantage of the present invention is that top gas waste products may be removed from the waste stream for proper disposal.

It is an object of the present invention to provide a reduction method compatible with small ore particle sizes.

An advantage of the present invention is that beneficiated ores may be used without agglomeration.

An advantage of the present invention is that weak or friable ores may be beneficially used.

An advantage of the present invention is that small ore particle sizes will allow for the use of enhanced beneficiation techniques.

It is an object of the present invention to provide a reduction method which both allows the use of highly beneficiated ores of extreme purity, and which does not introduce impurities along with the reductant. An advantage of the present invention is that the product will be of extreme purity.

An advantage of the present invention is that the product may be simply converted to steel, without extensive purification, slagging, or decarburization steps.

It is an object of the present invention to provide a reduction method amenable to rapid startup and shutdown.

An advantage of the present invention is that reduction furnace operations can be adjusted to accommodate variation in economics and fuel supply, at little marginal cost.

It is an object of the present invention to provide a readily scaleable furnace for the smelting reduction of iron ore.

An advantage of the present invention is that a reduction furnace may be sized to requirements with little penalty for the use of a small scale furnace.

An advantage of the present invention is that the initial capital investment in the furnace may be small.

Further objects and advantages of the present invention are potentially improved steel quality owing to the higher quality iron produced, the potential of tapping steel directly from the reduction furnace without a later steel conversion step, the reduction in cost of the production of fine specialty steels, and facilitating the production of esoteric steels. Further objects and advantages of the present invention will become apparent from a consideration of the figure and ensuing descriptions.

Description of Drawing

The drawing illustrates an apparatus for carrying out the method of the invention. Figure 1 is a schematic drawing of a preferred embodiment of the present invention. List of reference numerals:

1 Refractory lined chamber

2 materials inlet

3 feed control device

4 feed hopper

5 main top gas manifold

6 main oxygen jet array

7 oxygen supply ring

8 secondary gas inlet jets

9 main reductant gas jet array

10 main reductant gas supply ring

11 heat exchanger

12 condensate water

13 gas cleaning units

14 bypass valve

15 scrubbed materials

16 reductant gas handling system

17 natural gas supply line

18 oxygen supply

19 oxygen gas handling system

22 reductant gas

23 oxygen

24 iron ore

25 fluxes

26 molten iron

27 molten slag

28 hot gas

29 molten iron ore

30 sensible heat

31 impure reductant gas

Description of Invention

Briefly described, the invention comprises melting iron ore using heat from the combustion of natural gas and oxygen in a reducing flame, the ore being allowed to fall freely through the combustion zone. The now molten ore continues to fall through a reduction zone supplied with hot natural gas. The natural gas rapidly and completely reduces the molten ore to the metallic state. The molten and reduced ore collects at the base of the furnace, either in a hearth suitably fitted for tapping, or in a ladle fitted for removal.

Referring now to the drawing, a refractory lined chamber 1 comprises the reduction and melting zones. A suitable materials inlet 2 is disposed at and connected to the top of refractory lined chamber 1, and is provided with a feed control device 3 and a feed hopper 4. All feed components are used in conventional practice, and thus such components are not described in detail. All such components are amenable to standard modifications such as screw feed or controlled atmosphere.

Refractory lined chamber 1 is provided with several gas inlet jets and exhaust ports, including a main top gas manifold 5, a main oxygen jet array 6 and an oxygen supply ring 7, numerous secondary gas inlet jets 8, and a main reductant gas jet array 9, and a main reductant gas supply ring 10. Main top gas manifold 5 is connected to a heat exchanger 11. Heat exchanger 11 is further connected to appropriate gas scrubbing units 13 for the removal of impurities in the gas. Cleaning units 13 may include, but are not limited to, filter bags for the removal of particulates, carbon dioxide removal units, etc. Removal of carbon dioxide may be effected through several means, including but not limited to: reaction with suitable organic compounds such as aniline, centrifugal separation from the other top gases, selective separation though the use of a Venturi tube, filtering of hydrogen through the use of palladium membranes, reformation to carbon monoxide, or other means. Potentially, cleaning units 13 will not be necessary depending upon ore quality, however, at a minimum, carbon dioxide will need to be removed from the top gas.

Cleaning units 13 are connected to heat exchanger 11, and heat exchanger 11 is connected to a natural gas supply line 17. The output of heat exchanger 11 is further connected by means of a suitable reductant gas handling system 16, consisting of pipes, compressors, and valves, to main reductant gas supply ring 10. The specifics of reductant gas handling system 16 will be obvious to an individual skilled in the art, the specification of such being that it carry hot reducing gas from heat exchanger 11 to supply ring 10. Gas handling system 16 may also be connected to secondary gas inlet jets 8. Reductant gas supply ring 10 is further connected to main reductant gas jet array 9, thus closing the top gas/natural gas/reductant gas cycle.

An oxygen supply 18 is connected by means of a suitable oxygen gas handling system 19 to a main oxygen supply ring 7. Gas handling system 19 may also be connected to secondary gas inlet jets 8. Oxygen supply ring 7 is connected to a main oxygen jet array 6, forming the oxygen control and inlet system.

In operation, the inputs to the process are natural gas 22, oxygen 23, iron ore 24 and fluxes 25. The outputs are molten iron 26, molten slag 27, condensate water 12, and various scrubbed impurities 15, including carbon dioxide. The process is essentially continuous, with ore 24 being fed at the top, and molten iron 26 and molten slag 27 being tapped from the base, with continuous circulation of reductant gas 22, replenishment of reductant gas via natural gas supply line 17.

To fully describe the process, it is essential to examine the flow of condensed matter and the gas flow separately. These material flows may take place in the system described above. The following presumes that the reactor has reached a steady state of operation.

The condensed matter cycle is as follows:

Iron ore 24 enters the furnace through the materials inlet 2, at a rate determined by control device 3. As necessary, fluxing materials 25 may be introduced along with the iron ore 24. Iron ore 24 encounters rising hot gas 28, and iron ore 24 is heated thereby. Hot gas 28 is maintained to be slightly reducing, preventing the oxidation of magnetite ores.

Shortly after being fed through materials inlet 2, iron ore 24 and fluxing materials 25 reach the level of the main oxygen jet array 6. Oxygen 23 blown through main oxygen jet array 6 combusts with combustible materials contained in hot gas 28, producing combustion products and heat. This heat raises the temperature of iron ore 24, and causes iron ore 24 to melt. Oxygen flow rates determine the quantity of heat generated, and thus the temperature of the molten iron ore 29; the temperature of molten iron ore 29 is selected to be in the range of 1600°C and 2000°C, or above, and appropriate quantities of oxygen 23 are injected to maintain the desired temperature. Molten iron ore 29 descends through hot gas 28. Reductant materials in hot gas 28, consisting of natural gas, as well as hydrogen and carbon monoxide, rapidly and completely reduces molten iron ore 29 to molten iron 26. Molten iron 26 is collected at the base of refractory lined chamber 1. Fluxing materials 25 as well as gangue contained in iron ore 24 additionally melt in the combustion zone, and are collected as molten slag 27 along with molten iron 26. Hot reducing gas 22, injected through reducing gas jet array 9, provides the initial material of hot gas 28.

The main gas cycle is as follows:

Reductant gas 22, heated by heat exchanger 11 is fed via reductant gas handling system 16, main reductant gas supply ring 10, and main natural gas jet array 9 into refractory-lined chamber 1. Natural gas flows upward as hot gas 28, reducing molten iron ore 29, producing molten iron 26. The by-products of said reduction step are water vapor and carbon dioxide, which become part of hot gas 28. As hot gas 28 flows upward, the fraction composed of reduction by¬ product increases, and the reduction potential of hot gas 28 decreases. The flow of reducing gas 22 must therefore be adjusted so as to be between 1.5 and 5 times greater than the chemical reductant requirements of the iron ore reduction reaction.

As hot gas 28 reaches the level of main oxygen jet array 6, it reacts with injected oxygen 23, providing heat for the initial melting or iron ore 24. Hot gas 28 now contains up to 70% reduction by-products. Hot gas 28 continues to flow upward, and exits refractory-lined chamber 1 through to gas manifold 5.

Hot gas 28 composition in top gas manifold consists of reductant materials such as hydrogen, carbon monoxide, and un-reacted natural gas, as well as reductant by-products such as water and carbon dioxide, as well as low level impurities. Hot gas 28 will also be carrying considerable sensible heat 30. Hot gas 28 is carried to heat exchanger 11, where sensible heat 30 is transferred to reductant gas 22. As hot gas 28 cools, the contained water vapor condenses out, leaving impure reductant gas 31. Impure reductant gas 31 is then purified in cleaning units 13 to remove reduction by¬ products such as carbon dioxide, as well as materials such as sulfur dioxide. The now purified reductant gas is combined with fresh natural gas via natural gas supply line 17, producing reductant gas 22 for the reduction process.

Reductant gas 22 is fed back through heat exchanger 11, where it is heated by sensible heat 30 recovered from the top gas. Hot reductant gas 22 is then fed by reductant gas handling system 16 to main reductant gas supply ring 10, and again through refractory- lined chamber 1, closing the gas loop.

Oxygen 23 may be injected through secondary gas inlet jets 8 in order to maintain the temperature of the reduction reaction. Reductant gas 22 may also be injected through secondary gas inlet jets 8 in order to control reaction parameters. Such additional heating may not be necessary, and secondary gas inlet jets 8, as well as the associated piping, may be eliminated. In the case that oxidizer ratios need to be increased for process control, water may be added to the process stream along with oxygen to increase such oxidizer ratios without increasing process temperature.

Specification of Cleaning units 13:

There are numerous devices which may be used for cleaning units 13. The specification of cleaning units 13 is that they remove reductant by-products from the gas stream such that unused reduction potential of reductant gas 22 may be recovered. In general, water will have been removed by condensation in heat exchanger 11. The specific methods of carbon dioxide will be selected for the specific implementation of the present invention. Of the systems noted below, all are known in the art, and several are in active use in the steel industry.

1) Top gas may simply be used as a source of process heat, with no attempt made to recover reductant potential. Heat exchanger 11 would then be used to heat natural gas provided by natural gas supply line 17. Such a system is totally analogous to the use of blast furnace top gas as a heating fuel .

2) Carbon dioxide may be removed from the top gas through catalytic reformation to carbon monoxide, a reductant. Such reformation would consume natural gas, and would require potentially expensive catalytic systems. However this process is again quite well known.

3) Carbon dioxide scrubbers using aniline or similar chemical fluids may be used to capture carbon dioxide for later disposal.

4) Gas centrifuge techniques may be used to separate carbon dioxide from lighter gasses such as hydrogen and methane. The use of such techniques would result in the loss of any high order hydrocarbons found in the natural gas stream.

5) Cryogenic techniques in which the natural gas and carbon dioxide are condensed out of the top gas, with the carbon dioxide fraction being selectively removed.

6) Semi-permeable membrane techniques in which a selectively permeable membrane is used to deplete the process gas stream of carbon dioxide and water vapor. This method may permit operation without the use of a heat exchanger.

7) Reversible chemical reaction techniques such as the reaction of carbon dioxide and calcium oxide to form calcium carbonate. This is a common method of carbon dioxide removal in which the calcium oxide may be regenerated by heating.

Summary, Ramifications, and Scope:

The present invention is a method for reducing particulate iron oxide to molten iron using natural gas as the source of process heat and reduction potential. While the above description contains many specificities, these should not be construed as limitations on the scope of the invention, but rather as exemplification of one preferred embodiment thereof. Many other variations are possible. For example, the process need not be operated in a continuous fashion, perhaps operated in batches using particles suspended in a micro-gravity environment. Another variation is that the ore particles could be carried into the reduction zone using a jet flame, rather than allowing the ore particles to fall through a flame zone. Another variation would make use of ore equivalent materials, such as mill scale or other iron oxide sources, in place of some or all of the iron ore. Another possible variation is having the particles move through the various reaction zones by means of some type of support, at any angle of inclination, perhaps in a horizontal furnace or rotating hearth furnace. Yet another mode of operation would place electrodes at the level of the oxygen jets in order to provide heat for melting through the use of an electric arc. Yet another alternative method would use an alternative oxidizing gas such as air. Accordingly, the scope of the invention should be determined not by the embodiment illustrated, but by the appended claims and their legal equivalents.

Claims

Claims: I claim:

1. A method of reducing particulate iron ore to molten metal comprising the steps of

(a) introducing said ore into a refractory lined furnace means along with fluxing means for the production of slag;

(b) passing said ore particles through a flame generated by combusting oxygen with excess reductant gas, so as to fully melt the ore particles;

(c) regulating the rate of said introduction of said ore particles so that said ore particles are able to freely fall through said furnace means;

(d) introducing reductant gas into said furnace means through inlet means and causing said reductant gas to flow upwardly through said furnace means, in counterflow to said molten ore particles;

(e) introducing oxygen into said furnace means through inlet means for combustion with said reductant gas;

(f) removing hot gaseous by-products from said furnace means; and

(g) collecting said molten particles in then reduced state,

the improvement wherein being compounding said reductant gas of material including natural gas, alone or in combination with reformation products of said natural gas and/or reformation products of said hot gaseous by-products .

2) A method as in claim 1 wherein said particulate iron ore identified in the preamble of said claim is any material consisting of iron in chemical combination with one or more additional chemical elements . 3) A method as in claim 1 additionally comprising the following steps limiting the improvement of 1:

(h) purifying said hot gaseous by-products from said furnace means through the removal of products of said combustion of said oxygen with said reductant gas, said products of said combustion being exemplified by water, or carbon dioxide, as well as the removal of top gas impurities such as sulfur dioxide; and

(i) returning said purified gaseous by-products to said furnace means as a fraction of said reductant gas.

4) A method as in claim 1 additionally comprising in the preamble of claim 1:

(j) preheating said reductant gas prior to said injection into said furnace means.

5) A method as in claim 1 additionally comprising in the preamble of claim 1:

(j) passing said hot gaseous by-products from said furnace means through heat exchange means for the recovery of heat from said hot gaseous by-products; and

(k) passing said reductant gas through said heat exchanger means for preheating said reductant gas prior to said injection into said furnace means.

6) A method as in claim 1 additionally comprising in the preamble of claim 1:

(1) preheating said oxygen.

7) A method as in claim 1 additionally comprising in the preamble of claim 1: (1) passing said hot gaseous by-products from said furnace means through heat exchange means for the recovery of heat from said hot gaseous by-products; and

(m) passing said oxygen gas through said heat exchanger means for preheating said oxygen gas prior to said injection into said furnace means.

8) A method as in claim 1 additionally comprising in the preamble of claim 1:

(n) introducing an electric arc into said furnace means by passing an electric current between suitable electrode means for the production of an arc.

9) A method as in claim 3 wherein said purifying of said hot gaseous by-products is the process of passing said hot gaseous by-products through gas centrifuge means for the removal of said combustion products.

10) A method as in claim 3 wherein said purifying of said hot gaseous by-products is the process of reforming said hot gaseous by-products over catalytic material means for the conversion of said combustion products into said reductant gas.

11) A method as in claim 3 wherein said purifying of said hot gaseous by-products is by the process of passing said hot gaseous by-products over a bed of calcium oxide for the removal of said combustion products.

12) A method as in claim 3 wherein said purifying of said hot gaseous by-products is by the process of passing said hot gaseous by-products through semi-permeable membrane means for the separation of said combustion products.

13) A method as in claim 3 wherein said purifying of said hot gaseous by-products is by the process of cryogenic condensation of said combustion products .