METHOD FOR TREATING STEEL MILL SLAG
20250146091 · 2025-05-08
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Inventors
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Abstract
The method according to the invention for treating steel mill slag includes the steps of A) providing steel mill slag in solid form having a fineness such that the steel mill slag has a specific surface area according to BET of 0.1 m2/g or more; step B) treating the steel mill slag by heating it to a temperature of at least 800 Celsius, while adding oxidizing agents and auxiliary agents; and step C) separating iron (III) oxide and iron (II,III) oxide out of the treated steel mill slag.
Claims
1. A method for treating steel mill slag comprising the following steps: A) providing steel mill slag, which, when solidified, has iron (II) oxide (FeO) and, if solidified, has a fineness such that the steel mill slag has a specific surface area according to BET of 0.1 m.sup.2/g or more; B) treating the steel mill slag at a temperature of at least 600 C., while adding oxidising agents and auxiliary agents, a) wherein the oxidising agents oxidise at least a part of the iron (II) oxide (FeO) present to iron (III) oxide (Fe.sub.2O.sub.3) and/or iron (II,III) oxide (Fe.sub.3O.sub.4) and b) wherein the auxiliary agents form a bond at least with calcium from calcium-iron compound present and bound in the steel mill slag and b1) thereby release iron (III) oxide (Fe.sub.2O.sub.3) and/or iron (II,III) oxide (Fe.sub.3O.sub.4) and b2) further increase the proportion of 2 CaO.Math.SiO.sub.2 (belite) and/or other calcium silicates, such as alite, wollastonite and/or rankinite, in the treated steel mill slag, C) separating iron (III) oxide (Fe.sub.2O.sub.3) and iron (II,III) oxide (Fe.sub.3O.sub.4) out of the treated steel mill slag.
2. The method according to claim 1, wherein the method further includes: D) further treating the steel mill slag treated at a temperature of at least 600 C. in a reducing atmosphere by means of adding reducing agents to the heated, treated steel mill slag to reduce iron (III) oxide (Fe.sub.2O.sub.3) to iron (II,III) oxide (Fe.sub.3O.sub.4).
3. The method according to claim 2, wherein step D) is carried out after step B) and before step C).
4. The method according to claim 1, wherein the warm, treated steel mill slag is cooled, solidified and crushed, in particular ground up, before step C) so that the cooled, solidified and crushed steel mill slag has a specific surface area according to BET of 0.1 m.sup.2/g or more.
5. The method according to claim 1, wherein the separation in step C) is based on magnetic properties, density and/or by means of flotation.
6. The method according to claim 1, wherein the method further includes: E) treating the treated steel mill slag with CO.sub.2 to convert 2 CaO.Math.SiO.sub.2 (belite) and/or other calcium silicates, such as alite, wollastonite and/or rankinite, into CaCO.sub.3 and SiO.sub.2 and/or other corresponding reaction products.
7. The method according to claim 6, wherein step E) is carried out in an aqueous suspension of cooled, solidified and crushed steel mill slag and a gas, which contains CO.sub.2, is injected.
8. The method according to claim 6, wherein the method further includes: F) separating CaCO.sub.3-rich material after step E), in particular by means of flotation.
9. The method according to claim 6, wherein the gas, which contains CO.sub.2, originates from steel production.
10. The method according to claim 8, wherein a CaCO.sub.3-rich material is separated and/or recovered and fed into the steel or pig iron production process or cement production process for further utilization.
11. The method according to claim 1, wherein an SiO.sub.2-rich material is separated and/or recovered and fed into the cement production process for further utilization.
12. The method according to claim 1, wherein the recovered iron (III) oxide (Fe.sub.2O.sub.3) and/or iron (II,III) oxide (Fe.sub.3O.sub.4) is fed into the steel production process and/or blast furnace processes for further utilization.
13. The method according to claim 1, wherein a 2 CaO.Math.SiO.sub.2 (belite)-rich material and/or a material which is rich in other calcium silicates such as alite, wollastonite and/or rankinite is separated and/or recovered and fed into the cement production process for further utilization.
14. The method according to claim 1, wherein before step A), in a step 0), the steel mill slag is prepared to the desired particle size by means of grinding and/or granulation.
15. The method according to claim 1, wherein in step 0) or after step A) metallic iron (Fe) and/or iron (II,III) oxide (Fe.sub.3O.sub.4) are separated from the steel mill slag.
Description
[0046] The invention is explained in more detail below by means of an exemplary embodiment and a schematic flow diagram. In this drawing:
[0047]
[0048]
[0049] In step I, steel mill slag is provided. This has various ferrous compounds in its solidified state, such as metallic iron (Fe), wstite (iron (II) oxide, FeO), haematite (iron (III) oxide, Fe.sub.2O.sub.3), magnetite (iron (II,III) oxide, Fe.sub.3O.sub.4), srebrodolskite (Ca.sub.2Fe.sub.2O5, C.sub.2F), tetracalcium aluminate ferrite (brownmillerite, Ca.sub.2(Al,Fe).sub.2O.sub.5, C.sub.4AF). Converted to iron (III) oxide, the average iron content is around 30%.
[0050] The steel mill slag provided in step I is then ground in step II to a sufficient fineness so that it has, for example, a specific area surface according to BET of 0.1 m.sup.2/g, preferably 0.5 m.sup.2/g, in particular 1.0 m.sup.2/g or more. This grinding can be carried out using a vertical roller mill, for example. Alternatively, the steel mill slag can already be present as granulate before step I, so that it no longer needs to be crushed further and step II can be omitted.
[0051] In a step III, elemental iron (Fe) and iron (II,III) oxide (Fe.sub.3O.sub.4) can already be separated from the ground or crushed steel mill slag by means of magnetic separation. These two components have good ferromagnetic properties, such that magnetic separation is possible. This is supported by the presence in the described fineness, as the materials are usually no longer intergrown with other phases. This step is also optional.
[0052] The steel mill slag is then heated in step IV. Instead of the method steps described so far, liquid slag from upstream processes can also be used. The treatment, wherein solidified slag is also heated, takes place in a normal atmosphere, as with ambient air. Additional, even more ambient air can be injected. It is essential here that an oxidation reaction takes place in the steel mill slag as described in equation (1).
2 FeO+O.sub.2.fwdarw.Fe.sub.2O.sub.3(1)
[0053] Alternatively, Fe.sub.3O.sub.4 can be obtained:
3 FeO+O.sub.2.fwdarw.Fe.sub.3O.sub.4(2)
[0054] Here, the O.sub.2 present in the air can serve as an oxidizing agent. However, alternatively or additionally, the steel mill slag can also be mixed or treated with other oxidizing agents such as H.sub.2O.sub.2 and other peroxides, ozone, N.sub.2O or pure oxygen.
[0055] In addition, additives are added, for example in the form of SiO.sub.2. This addition can also take place in step II, so that homogenization takes place during grinding if the slag is solid.
[0056] When SiO.sub.2 is added to the slag, the reactions described in equation (4) take place:
2 CaO.Math.Fe.sub.2O.sub.3+SiO.sub.2.fwdarw.2 CaO.Math.SiO.sub.2+Fe.sub.2O.sub.3(4)
[0057] The SiO.sub.2 source can be used rock flour, for example from sandstone or quartzite, coal fly ash, sand, silica dust, pozzolana and/or fired clay, as well as the SiO.sub.2-rich residue from this method.
[0058] For separation using a magnetic separator, it is advantageous if the iron (III) oxide is converted into iron (II,III) oxide, as this enables better separation using a magnetic separator. Therefore, without additional cooling, the heated steel mill slag can be exposed to a reducing atmosphere in step V, so that a conversion of the iron (III) oxide to iron (II,III) oxide takes place, as described in equation (3). It should be noted here that this reaction has already taken place as an oxidation in the reverse direction in step IV and iron (II,III) oxide has been oxidized to iron (III) oxide. Accordingly, this step can be omitted if only enough oxidizing agent has been added to the slag to produce mainly magnetite and little or no haematite.
2 Fe.sub.3O.sub.4+O.sub.2.Math.3 Fe.sub.2O.sub.3(3)
[0059] The steel mill slag is then cooled down again in a step VI, wherein the heat energy present can be recovered.
[0060] Iron (II,III) oxide and, if still present, iron (III) oxide can then be separated from the cooled slag in step VIIa, for example by using a magnetic separator or density separation. For this purpose, it is advantageous if the cooled slag is ground again so that belite components intergrown with iron (II,III) oxide are separated.
[0061] The remaining slag, which has a high belite content, can then be supplied to the cement industry for use as an additional main component or as a raw meal component.
[0062] Alternatively or additionally to this, the slag can be further treated in step VIIb, in which water is added to the slag to produce an aqueous suspension. The treated slag can also be finely ground for this purpose, but this is not absolutely necessary. Air or another gas containing CO.sub.2, for example exhaust air from steel production, can be injected into the suspension, causing the belite and/or other calcium silicates such as alite, wollastonite and/or rankinite to decompose into calcium carbonate (CaCO.sub.3) and silicon dioxide (SiO.sub.2) or other reaction products. This separates the belite from intergrown iron oxides so that they can be separated more easily later. The reaction on which this is based is described in equation (5).
2 CaO.Math.SiO.sub.2+2 CO.sub.2.fwdarw.2 CaCO.sub.3+SiO.sub.2(5)
[0063] The iron oxides formerly intergrown with other phases can be further separated in step VIII by means of flotation, for example, wherein this step can also be carried out simultaneously with step VIIb. The SiO.sub.2-rich residual material and the calcium carbonate (CaCO.sub.3) can also be separated in this process.
[0064] The recovered iron, both in metallic form (Fe) and in the form of iron (III) or iron (II,III) oxides, can then be reused in steel production process. Similarly, the calcium carbonate (CaCO.sub.3) can also be fed into the steel production process, which reduces the amount of limestone required there accordingly.
[0065] As described above, the recovered belite can be used directly in the cement production process. The same also applies to the SiO.sub.2-rich residue, which can also be reused in step II. The CaCO.sub.3-rich residue and the unseparated mixture containing SiO.sub.2 and CaCO.sub.3 can also be used in the cement production process as a raw meal component and/or as an alternative main component.
[0066] The CO.sub.2 required in step VIIb preferably comes from the steel production process and can thus significantly improve the environmental impact with regard to CO.sub.2 production in the steel production process, as it can be bound in this process according to the invention.
[0067] The method according to the invention is explained in more detail below using a specific example.
[0068] A steel mill slag with the following composition was used for the investigations, which was determined using quantitative X-ray diffraction: 17% C.sub.2F, 45% -C.sub.2S, 2% -C.sub.2S, 5% CaO, 3% metallic iron, 4% portlandite, 24% wstite and 1% magnetite. The figures are given in percentage by mass and refer to the crystalline components. Amorphous phases are also included, but these have not been quantified. This also applies to the analysis results below, which were also determined using quantitative X-ray diffraction.
[0069] The material was ground in a vibrating disc mill and then in a McCrone mill with the addition of water. The ferromagnetic components were then separated using a permanent magnet in an aqueous suspension and the sample was subsequently dried. The amount separated was 5% of the material used. The deposited product consisted of 6% C.sub.2F, 4% -C.sub.2S, 73% metallic iron, 7% magnetite and 10% wstite. It is therefore an iron-rich material with low impurities of CaO, SiO.sub.2 and other oxides, which can be used for the production of pig iron and steel due to its composition.
[0070] The material remaining after magnetic separation was composed as follows: 15% C.sub.2F, 46% -C.sub.2S, 4% -C.sub.2S, 3% calcite, 1% metallic iron, 4% portlandite, 25% wstite and 2% magnetite. This material still contains large amounts of iron, but not as metallic iron. The iron is bound in various mineral phases, in particular as C.sub.2F and wstite. Neither of the two phases are ferromagnetic and can therefore hardly be separated with a magnetic separator.
[0071] To enable separation of these iron quantities, the material was mixed with an auxiliary agent (SiO.sub.2 ultrafine flour Sikron SF 6000) in a ratio of SiO.sub.2: modified steel mill slag of 1:14. The SiO.sub.2 fine material consisted entirely of cristobalite. The two substances were homogenized by grinding them together in the vibratory disc mill for 2 minutes. Part of the mixture was then fired for 4 hours at 1,100 C. in a muffle furnace. The material was in an open crucible and was therefore in constant contact with the atmosphere and the oxygen it contained.
[0072] Oxygen uptake and chemical reactions occurred during the high-temperature treatment. After cooling, the phase composition was determined by using quantitative X-ray diffraction. This amounted to 6% C.sub.2F, 48% -C.sub.2S, 4% -C.sub.2S, 9% C.sub.3A, 9% rankinite, 3% wstite and 22% magnetite. As a result, extensive oxidation of the wustite to magnetite occurred during the thermal treatment, as the wstite concentration fell from 23% to 3% and the magnetite concentration rose from 2% to 22%.
[0073] At the same time, the C.sub.2F was converted into C.sub.2S, consuming cristobalite and releasing iron oxide, which also contributed to an increase in the magnetite concentration in the in the majority of the iron being bound in a phase that can be separated by magnetic separation. Obviously, the oxygen supply was not high enough for further oxidation to haematite. Furthermore, the concentration of calcium silicates in the sample increased, as the belite concentration rose from 46% to 52% and the rankinite concentration increased from 0% to 9%. sample. Thus, the thermal treatment after the addition of the additives has resulted
[0074] After thermal treatment, a large proportion of the iron is bound as magnetite and can be separated. To facilitate the separation process, the material was ground again in a McCrone mill with the addition of water for 5 minutes and then treated with CO.sub.2. For this purpose, 5 grams of the treated steel mill slag was added to 400 ml of water and stirred continuously while simultaneously introducing CO.sub.2 into the beaker containing the sample. Phase separation was facilitated by the use of ultrasound and the addition of nucleating agents (CaCO.sub.3, Merck). After a treatment period of 3 hours, the sample was filtered, dried and analyzed. After the reaction of the modified steel mill slag with carbon dioxide, the material no longer contained any belite and only C.sub.2F, rankinite, magnetite and calcite could be detected as crystalline compounds. Other reaction products such as amorphous SiO.sub.2, magnesium carbonate and dolomite were also formed.
[0075] The magnetite was separated using a permanent magnet in an aqueous suspension. Flotation was then carried out to separate calcium carbonate and SiO.sub.2. Dodecylamine was used as a collector and starch was used as a depressor, causing the calcium carbonate to rise with the introduced air bubbles and the SiO.sub.2 and other phases to sink. This meant that the calcium carbonate could be removed from the top and the SiO.sub.2-rich residue from the bottom, and after treatment they were separated and could be dried.
[0076] The method according to the invention can thus be used to separate iron from steel mill slag in a simple and efficient manner and even the other components can be used to reduce the costs of upstream methods. In addition, CO.sub.2 binding is possible with the method according to the invention, which significantly improves the environmental impact.