Patent Application
20240376559
The invention pertains to a method for generating slag and for making controlled adjustments to its characteristics, for use as mineral building material, for example. The characteristics of the slag encompass the physical and chemical composition and also the mineralogical properties of the slag.
In nature, iron is present only in the form of iron oxide. Pig iron is produced, for example, in blast furnaces. These are shaft furnaces operating on the countercurrent principle, where the charge—that is, lump ore, pellets or sinter together with coke as reducing agent and limestone and possibly further additives—is fed in at the top side and a flow of hot gases is passed through this charge from the bottom side. In this way, the feed material is steadily heated during the transient time. The substoichiometric combustion of the coke results in the formation of the reduction-capable gas carbon monoxide (CO), which reduces the iron oxides contained in the charge into iron; the CO may oxidize to carbon dioxide (CO2) in the process. Because of the temperatures prevailing at the bottom of the blast furnace, the iron is present in the liquid state.
In the lower part of the smelting furnace, pig iron and slag are tapped off at periodic intervals. Rapid cooling of the smelting furnace slag, possibly leading to a glassy solidification, can be used to produce slag sand. Sand properties can be positively influenced by the milling-in of cement. Furthermore, by substituting the slag sand for cement clinker, the CO2 footprint of cement can be improved as well.
The composition of the slag here is dictated substantially by the gangue in the iron ore, the limestone fraction, and secondary constituents and additives in the feed material.
The problem which exists, however, is that the composition of the slag and hence also of the slag sand can in principle be different at every tapping if the feed materials alter. In this case it is still possible to influence the limestone fraction and the secondary constituents and additives, but not the amount and natural composition of the gangue in the iron oxide. This means that the precise final composition of the slag is unknown only when it is tapped off. Given that the materials remain in the smelting furnace for many hours, the possibilities for short-term adaptation to the composition of the slag by way of the feed material, if they exist at all, are limited. Furthermore, the generation of slag sand in the blast furnace is contingent on the formation, during the smelting procedure, of a eutectic which has a low melting point and can therefore be tapped off more rapidly.
EP 1 354 969 B1 and EP 632 791 B1, then, show methods for admixing the slag post-tapping with additives to optimize the composition of the slag for the slag sand. The disadvantage of this method, however, is that the addition of the additives may cause the slag to cool down more quickly, so making it no longer possible for the additives to bind optimally with the slag. The greater the amount of additives admixed to the slag, the more rapid the cooling of the slag. A skewed composition of the slag may arise, with an accumulation of the additives in one region and a deficiency of the additives in another region.
DE 197 08 034 A1 discloses a method for producing liquid pig iron or liquid steel precursors. EP 1 198 599 B1 discloses a method for slag conditioning with introduction of smelting residues. EP 1 627 084 B1 discloses a method for profitably utilizing slag. DE 103 40 880 A1 discloses a method and an apparatus for atomizing slag. DE 10 2020 205 493 A1 discloses a method for establishing a controlled slag phase in a smelting assembly.
Furthermore, direct reduction plants and electric arc furnaces/smelters are also known in principle. Not known, however, is an analysis of the slag and/or the intermediates for the purpose of optimized adjustment of the slag characteristics.
The object of the present invention, therefore, is to create an improved concept for a method for generating slag and for adjusting the characteristics of the slag arising in the smelting furnace.
The object is achieved by the subject matter of the independent claim. Further advantageous embodiments are the subject matter of the dependent claims.
In accordance with the invention, a method is disclosed for generating slag of desired characteristics in the production of pig iron, with steps as follows: a) heating iron oxide in a first reactor, so that with a reducing agent present a majority of the iron oxide is reduced to iron and an iron-containing intermediate is formed. This is done using a direct reduction plant. As the end product of the direct reduction plant, the iron-containing intermediate is then present, and may also be referred to as sponge iron.
b) Thereafter, the iron-containing intermediate is heated in a second reactor, to give pig iron and the slag. The heating takes place preferably in a smelter in a reducing atmosphere. Hence it is possible to obtain a slag having an iron fraction of less than 10%, preferably less than 7%, more preferably less than 4%, which can be used for the cement industry. Slag with a higher iron fraction cannot be used for the cement industry, for quality reasons.
c) In a further step, which may also take place before or parallel to step b), the iron-containing intermediate and/or the slag which is deposited during the further heating of the iron-containing intermediate are/is analyzed. The iron-containing intermediate is taken for analysis at the end of the direct reduction plant, between direct reduction plant and smelter (i.e., in both cases, before the heating in the smelter) or in the smelter (i.e., during the heating). Additionally or alternatively, a sample of the slag heated to the final temperature may also be taken from the smelter.
d1) Depending on the analysis, a property of an additive to be added to the iron-containing intermediate is determined in order to alter the composition of the slag. In other words, a sample is taken of the iron-containing intermediate and/or of the slag. Since in the iron-containing intermediate, the iron oxide is already present in a very far-reduced state, the characteristics of the later slag can already be determined very well from it. The analysis reflects the actual state of the slag.
The intermediate may comprise, for example, pyrites, dolomite, ilmenite or bauxite, or the basic form of the oxidic compounds, or any desired combination of the stated substances. These substances are also referred to as adjuvants and they influence the properties of the slag. Hence it is advantageous for the slag to have good reception capability for extraneous substances, to have a suitable velocity (preferably between 1.05 Pa*s and 1.15 Pa*s (pascal times second)) and to have a temperature at which, as a result of the solidification, the slag breaks down into the appropriate particle size fraction and there is sufficient formation of glassy phase, and to have good binding capacity for cement production. The good reception capacity of the slag ensures effective reception of the additives and hence high homogeneity of the slag. The viscosity enables the slag to flow through the tapping hole. The temperature/the composition and also the binding capacity are relevant to the quality of the end product—for example, slag sand, Portland cement, or similar.
The additive thus determined is introduced into the second reactor, such as the smelter, for example, during the heating of the slag, to give the slag of the desired characteristics. The desired characteristics are also referred to as target characteristics.
d2) Additionally or alternatively, a control unit, for example, may recognize from the analysis that the slag requires a thermal treatment to give the slag having the desired characteristics, and may initiate the thermal treatment.
The liquid metal and the slag can be tapped off through tapping holes in the smelter. After the slag has exited the tapping hole, it is quenched, preferably with water, and atomized and therefore granulated. The aim is to obtain more than 90% glassy solidification. After that, the granules are ready for further use. As a check, and in order to be able to correct any faulty assumptions, the final granules may also be analyzed to ascertain whether they have the desired properties.
For example, by means of the analysis conducted in d1), a thermal treatment of the slag may also be derived, in particular a defined cooling rate to give the desired property of the slag.
It is not possible to withdraw an iron-containing intermediate from a conventional blast furnace, nor to add the additive to the iron-containing intermediate in the conventional blast furnace. The only possibility in the conventional blast furnace is to introduce material at the start and to withdraw the slag and the pig iron at the end. If it is found that the slag does not have the correct composition, the composition can only be altered when the additive added at the start has reached the end. This may take between half a day and a whole day. Accordingly, the method proposed cannot be transposed to the blast furnace.
A smelting furnace is disclosed for the generation of pig iron and slag having desired characteristics. The smelting furnace comprises a direct reduction plant which is configured to heat iron oxide so that with a reducing agent present a majority of the iron oxide is reduced to iron and an iron-containing intermediate is formed. The iron oxide is heated, for example, to a temperature of between 900° C. and 1100° C. The reducing agent, preferably hydrogen, which may be recovered, for example, from the electrolysis of water using regenerative energies (wind, water, solar) for the provision of the required power, for reducing CO2 emissions, may be heated to the necessary reaction temperature for the operation of the direct reduction plant, before being introduced into the direct reduction plant. The iron-containing intermediate is also referred to as sponge iron.
Downstream of the direct reduction plant is a reactor arrangement. The reactor arrangement receives the iron-containing intermediate and heats it to give pig iron and slag.
The reactor arrangement may comprise one reactor or a plurality of reactors. The reactor, or one reactor of the plurality of reactors, for heating the iron-containing intermediate may be an electric arc furnace or a smelter or an induction furnace. An electric arc furnace is an electrical furnace which heats a substance in an oxidizing atmosphere, typically discontinuously. Discontinuously means that a quantity of the substance is heated and the substance after the heating is withdrawn before a new amount of the substance is heated. A smelter is an electric furnace which heats a substance in a reducing atmosphere, typically continuously. Continuously means that a portion of the substance in the smelter is tapped off regularly, while the smelter is fed to new substance. Regularly, therefore, iron-containing intermediate is introduced into the smelter and, likewise regularly, portions of the pig iron and of the slag are tapped off. In a smelter, for example, a residual melt may be left which may be seen in turn as a starting point for the smelting of further substance. The smelter is also referred to as a melting-reduction furnace, low-shaft furnace or submerged arc furnace (SAF). In particular, terms such as open slag bath furnace (OSBF) are also customary.
When the iron-containing intermediate is heated in a smelter, the temperature up to which the heating takes place is, for example, 1500° C. to 1600° C. if the slag is being used to produce slag sand. This is the temperature at which the slag is tapped off. For the production of other mineral building materials, the maximum slag temperature may also be higher, since in this case the eutectic of the slag is no longer attained. The temperature at which the iron is tapped off is somewhat lower and is, for example, between 1400° C. and 1500° C. In particular, therefore, the tapping temperature of the pig iron is, for example, between 80° C. and 120° C. lower than the tapping temperature of the slag. The heating of the melt and the presence of the reducing agent, such as carbon and/or hydrogen, for example, cause the iron to be further reduced, and so the iron fraction in the slag is lowered. The atmosphere, more particularly reducing atmosphere, in the smelter is obtained, for example, by the reaction of carbon, dissolved in sufficient quantity in the melt, with oxidic constituents of the intermediate, owing to the prevailing chemical/physical conditions, to give a reducing gas, for example. If carbon is not present in sufficient amount in the melt, this reducing atmosphere may be generated by supplying a reduction gas and/or reduction gas-forming substance.
The smelting furnace further comprises an analysis unit which is configured to analyze the iron-containing intermediate and/or the slag. In particular, the analyzing may take place during tapping or by sampling before the tapping, or in situ. The analyzing may preferably take place online. The slag may be determined for the production of mineral building materials, such as for the production of slag sand or Portland cement, for example, by analysis of the concentration ratio of calcium, silicon, aluminum and iron. Furthermore, however, by means of the smelting furnace presented and of the corresponding production method, the production of any desired mineral building materials is possible. The mineral building materials may differ in their composition and in their properties from slag sand. For example, but not exclusively, the mineral building material may differ in its chemical and/or physical and/or mineralogical properties from conventional slag sand. The analysis unit may be a laboratory located in the vicinity of the smelting furnace, for the laboratory results to be utilized for rapid influencing of the product, in particular.
Furthermore, the smelting furnace has a control unit which is configured, depending on an analytical result, to determine a property of the added additive so as to alter a composition of the slag (actual composition) and to obtain the slag with a desired composition (target composition). Additionally or alternatively, the control unit may also recognize whether the slag requires a thermal treatment to obtain the slag having the desired characteristics.
The additive is understood to refer in particular to a mixture of different substances. Substances used may include pyrites, dolomite, ilmenite and bauxite. The selection of the substances, for example, is then regarded as a property of the additive. Furthermore, the fraction of the selected substances within the total amount of the additive may be viewed as a property of the additive. Moreover, the total amount of the additive or the amount of the selected substances may also be viewed as a property of the additive. The amount, for example, denotes the mass or the volume of the substance. Typically, however, the property of the additive encompasses not only the selection of the substances but also their respective fractions, i.e., the composition of the additive, and also the amount of the additive.
The thermal treatment may arise from the analysis of the actual state of the slag or else the desired characteristics of the slag. The thermal treatment is, for example, the operation of a defined temperature curve of the reactor arrangement for heating or else cooling the slag. For the generation of slag sand, for instance, it is necessary to cool the slag very rapidly in order to obtain at least 90% glassy solidification. Different mineral building materials, however, may have different requirements with regard to a temperature profile.
In other words, a further possibility for control or regulation, as well as the adding of the additive, is to introduce heat into the reactor arrangement or else remove heat in a target-oriented manner on the basis of the measurement results obtained using the analysis unit. This supply or removal of heat may be variable over time—that is, it may have the goal, for example, of pursuing a temperature profile for slag and/or melt that necessitates a supply of heat at certain times and a removal of heat at other times, and the leaving of the process to itself, in thermal terms, at still other times.
By means of such a procedure, it is possible for example to set a goal temperature for slag and/or melt in order to influence properties of slag and/or melt in a goal-directed way. In addition to this establishment of an individual goal temperature, it is known that the properties of slags and melts may be influenced not only by individual temperatures but also by the traversal of temperature profiles in order to obtain or to avoid particular material phases. In the area of slag, this might include the cooling of the melt phase in the rotary kiln furnace during production of cement clinker, which is required to operate at a rate such that the tricalcium silicate does not break down into dicalcium silicate and free lime and the tricalcium aluminate crystallizes in a fine-grained form, but at the same time not so quickly that the melt phase undergoes glasslike solidification.
The desired characteristics for the slag are the characteristics whereby a mineral building material, formed after the granulation of the slag, has a desired chemical composition and/or a desired physical property and/or a mineralogical property. In the case of slag sand, granulation encompasses, for example, the rapid cooling (quenching) and atomization of the slag after tapping. For different mineral building materials, a different thermal treatment may occur in order to obtain the granules. The desired characteristics of the slag, especially in terms of mineralogical phase formation, elution behavior, etc., may accordingly be selected such as to form, for example, slag sand or Portland cement or any other mineral building material.
The smelting furnace disclosed, therefore, is a response to the concern that the current efforts to replace coke by hydrogen as a reducing agent, owing to the high CO2 emissions in steelmaking, mean that, as a result of the switchover to the direct reduction process, the production of slag sand will be lost—such production in Germany alone accounts annularly for around six million metric tons. The smelting furnace described, more specifically the first reactor, is therefore already designed for the direct reduction process as well, and can be operated with (natural) gas or, advantageously, with hydrogen as reducing agent. It also enables the smelting furnace to produce mineralogical building materials other than the conventional slag sand, as well.
The idea is to use a direct reduction plant and a reactor arrangement with, for example, a smelter. In the first reactor, the iron oxide is reduced via direct reduction methods. The iron can then be present at the end of the direct reduction plant as an iron-containing intermediate in solid form, in the form of sponge iron, for example. In the reactor arrangement, the smelter for example, the iron-containing intermediate is then heated to the preset temperature at which the liquid iron is tapped off.
The separation of the overall process into two method segments with two or more individual steps (essentially the shaft furnace with a porous bed of material in the upper part, and the melting region with liquid phases in the lower part) likewise increases the number of degrees of freedom for the design of the atmosphere in the smelting furnace. Whereas in the conventional process, owing to the narrow connection, this atmosphere cannot be chosen in practice independently, or can be so chosen only to a small extent, the possibility exists in the process disclosed here for the atmosphere to be freely chosen. Correspondingly, in principle, any desired gas composition may be chosen in order, in particular, to ensure optimal conditions for the goal-directed production of products from the slag, but in particular not exclusively in terms of their chemical, physical and mineralogical properties.
As far as the analysis is concerned, there are diverse possibilities, which are described illustratively on the basis of the smelter as (part of) the reactor arrangement. During the heating, for example, no further iron-containing intermediate is added to the reactor arrangement, more particularly to the smelter. In that case, the characteristics of the future slag may be ascertained via analysis of the iron-containing intermediate. From this it is possible to ascertain for example, what physical composition the additive should have and what amount of the additive should be added to the reactor arrangement in order to obtain desired characteristics for the slag. It is, however, also possible to carry out cyclical tapping only of a part of the iron and/or of the slag, while, again cyclically, new iron-containing intermediate is added and hence a part of the slag or of the iron always remains in the reactor arrangement, more particularly in the smelter. On the assumption that the slag in the smelter already has the desired characteristics, the additive may also be determined on the basis of the analysis of the iron-containing intermediate. In other words, only the characteristics of the newly arrived slag fraction need be adjusted. Furthermore, however, for verification, it is also possible to determine the characteristics of the slag in the smelter and, in the event of deviations from the desired characteristics, to adapt them by addition of the additive.
In other words, in the analysis, the analysis unit can determine actual characteristics of the iron-containing intermediate and/or the slag and compare them with desired target characteristics of the slag, and adjust the properties of the additive or the thermal treatment depending on the difference between actual composition and target composition.
In the reactor arrangement, the additives are heated with the iron-containing intermediate and may therefore mix or bind fully with or to the slag. The result is a uniform slag having the desired characteristics.
In exemplary embodiments, the reactor arrangement comprises a first reactor, preferably the smelter, and a second reactor. The first reactor receives the iron-containing intermediate and heats it, to give the iron and the slag. The second reactor receives the liquid slag and subjects it via the control unit to a further treatment, to give the desired characteristics of the slag. The further treatment by the control unit has already been comprehensively described and comprises the addition of the additive, adjusted in terms of its properties. Additionally or alternatively, the further treatment comprises a thermal treatment of the slag. In this case, the slag is adjusted to the desired characteristics only after the iron has been tapped off, so that the process of pig iron production does not need to be altered.
In further exemplary embodiments, the reactor arrangement, preferably the first reactor or the smelter, has an opening for introducing raw material, especially furnace dust, into the reactor unit. In this way, the furnace dust swelled up and captured in the direct reduction plant may be introduced, or else any other raw materials, especially those which can be airborne. These raw materials need not necessarily be attained in production of iron; instead, raw materials (with the capacity to be airborne) from other industries, such as from clay production, for example, may also be added. In particular, the raw material may be processed, for example dried and/or granulated, before being introduced into the reactor arrangement. The characteristics of the slag are altered as a result and are analyzed by the analysis unit after the raw material has been added. The advantage of adding furnace dust is that the furnace dust contains a not inconsiderable fraction of bound iron (in the lower single-digit percentage range) which is currently lost in production of iron. Through the introduction of the furnace dust into the reactor unit, the iron it contains is likewise melted and is therefore not lost.
Instead of furnace dust, any desired raw material may also, generally, be introduced into the reactor arrangement. If the raw material is too small, it may be granulated for greater ease of introduction into the reactor arrangement. The granulation or pelletizing of raw material is advantageous if it has airborne capability. A material is regarded as having airborne capability if its particle size is less than 5 mm, preferably less than 3 mm or less than 1.5 mm. Raw materials with airborne capability can also be introduced into the reactor arrangement, but only by means of a carrier gas, which, however, is typically unwanted in the reactor arrangement.
In other words, a further advantage of the method is the use also of fine-grain feedstocks (raw material) in the new process. With the process customary at present, fine-grain feedstocks are carried by the gas stream in the bed material and so do not enter the melt. Accordingly, this dust is lost to production. In the process presented here, it is now possible, with circumvension of traversal through the upper part of the process, for the dust to be introduced directly into the smelting furnace, optionally mixed with other substances and/or already generally pretreated—for example, but not exclusively, by means of heat, comminution or agglomeration. The only limitation on the selection of such dusts fundamentally is that they do not detract from the quality of melt and/or slag to the point where it becomes unusable. It is possible accordingly to make practical use, for example, of dusts from the immediate environment of the iron and steelmaking process and also from the production of mineral building materials, for logistical reasons.
Exemplary embodiments show that the control unit is configured to select the amount of the additive such that the slag has a basicity of 1 to 5.5, preferably of 1.13 to 2. This is advantageous for the generation of mineral building materials.
In further exemplary embodiments, the second reactor is configured to atomize the slag to give atomized slag, the atomized slag having a particle size of 1 to 100 μm, preferably 1 to 40 μm. The atomizing enables rapid cooling of the slag, in order to obtain the required glassy solidification for production of slag sand, for example. The atomizing may take place in the second reactor as part of the thermal treatment.
Further exemplary embodiments show that the second reactor generates a mineral building material, a binder for example. It is possible, for example, for the control unit, as the additive or as part of the additive, to introduce cement into the second reactor, the second reactor being configured to mix the atomized slag and the cement with one another, with the atomized slag being mixed with cement in a ratio of 36:64 to 95:5, preferably 60:40 to 80:20, to give the mineral building material having a 28-d standard strength of at least 30 N/mm2.
Preferred exemplary embodiments of the present invention are elucidated below with reference to the attached drawings, in which:
Ahead of further elucidation below of exemplary embodiments of the present invention in detail, using the drawings, it is noted that identical, functionally alike or equivalent elements, objects and/or structures across the various figures have been provided with the same reference signs, and so the descriptions of these elements depicted in different exemplary embodiments can be interchanged with one another and/or applied to one another.
The blast furnace 20a additionally has a feed 34 for hot blasts, while the direct reduction plant 21a has a feed 36a, 36b for a reduction gas, hydrogen or carbon monoxide for example. The smelter 21b comprises a main opening 38, through which an iron-containing intermediate 39 passes from the direct reduction plant into the smelter 21b. The smelter 21b additionally comprises an opening 40, through which an additive can be introduced into the smelter. If the additive is to contain different substances, there may be one opening provided per substance. Alternatively, the substances may be mixed beforehand to give the additive, and then enter the smelter through an opening in the form of a mixed additive. On the bottom of the smelter, furthermore, there is a pool of slag 42 and iron 44. The openings, however, are advantageously designed such that the smelter 21b carries out the heating in the absence of air. This means that the direct reduction plant can be connected securely to the smelter so that the iron-containing intermediate enters the smelter without air contact.
Because the smelter is a separate assembly from the direct reduction plant, it is now possible, in contrast to the blast furnace, to withdraw a sample of the slag and/or of the iron-containing intermediate 39 directly in the smelter before the withdrawal of the slag. Alternatively, the sample may even be taken from the direct reduction plant itself. The sample can be analyzed for its characteristics in an analysis unit 43. Based on the result of analysis, a control unit 45 ascertains the quality of the additive. Via signal line 51a, the control unit is able to produce the additive and introduce it into the reactor arrangement, more particularly the smelter. Additionally or alternatively, the control unit 45 may also, via a further signal line 51a, set a temperature of the smelter. It is thus possible to carry out thermal treatment of the melt by operation of a predetermined temperature curve, for example.
The smelting furnace 20b has the advantage, in contrast to a direct reduction plant in combination with an electric arc furnace operating under an oxidizing atmosphere, for example, that the further processing operation of an iron works attached to the blast furnace can also be used for the smelting furnace. Hence the iron can be refined into steel in a converter. The liquid steel may be desulfurized in a ladle furnace and adjusted in its quality and then shaped by means of a continuous casting plant.
Furthermore, it is also possible to combine the feed for the raw material from
An advantage of the smelting furnace disclosed and of the corresponding method is that the existing limitation on slag composition to a composition characterized by a particularly low melting temperature is removed. It is now possible to operate the smelting furnace in principle without confinement of its degrees of freedom, in particular, but not limited to chemical, physical and mineralogical characteristics of the slag, both in the fixed time profile and in the time profile. Accordingly, the arrows in
Certain aspects have been described in connection with an apparatus. It should nevertheless be understood that these aspects also constitute a description of the corresponding method, and so a block or a component of an apparatus is also understood to be a corresponding method step or a feature of a method step. In analogy to this, aspects described in connection with one or as one method step also constitute a description of a corresponding block or detail or feature of a corresponding apparatus.
The exemplary embodiments described above represent merely an illustration of the principles of the present invention. It will be appreciated that modifications and variations of the details and arrangements described herein will be clear to others in the art. The intention is therefore that the invention should be restricted solely by the scope of protection of the subsequent claims and not by the specific details presented herein on the basis of the description and the elucidation of the exemplary embodiments.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2021 204 258.9 | Apr 2021 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/060867 | 4/25/2022 | WO |
