Method for reduction treatment of oxide scale

Abstract

The invention belongs to the technical field of ferrous metallurgy, and particularly relates to a method for reduction treatment of oxide scales, which comprises the following steps: screening the oxide scales to obtain the oxide scales with the particle size in the range of 0.2-4mm; the iron oxide scale is preheated and then added into a fluidized bed reduction system, iron oxide is reduced into metal iron under the heating reduction effect of high-temperature reducing gas in a fluidized bed, and sponge iron powder particles are generated. And the obtained sponge iron powder is subjected to hot pressing treatment under nitrogen protection, and the directly-reduced iron hot-pressed block is obtained. According to the method, only the scale needs to be simply processed, fine grinding and agglomerating treatment are not needed, the scale is used as a fluidized ironmaking raw material, and reduction treatment is carried out by adopting a fluidized bed process, so that low-carbon and low-consumption scale recycling is realized, and meanwhile, a direct reduction iron product with purer components is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron and steel metallurgy, and particularly relates to a method for reducing and treating mill scale. Background Art

[0002] Mill scale is a by-product in iron and steel production, mostly originating from steel forging and hot rolling processes. It is formed due to the oxidation reaction of iron and steel with oxygen in the air at high temperatures. Its main components are iron oxides, a small amount of metallic iron, and other impurity elements. As a kind of solid waste in steel mills, its TFe content is relatively high, usually reaching more than 70%, so it has high recycling value. At present, steel enterprises mainly use mill scale as miscellaneous materials in sintering, and the produced sinter is sent to the blast furnace for smelting. For example, patent (CN201910502677.8) discloses a method for preparing sinter using mill scale from rolling mills and the prepared sinter; there are also cases where it is pressed into pellets with other iron-containing materials, and after natural curing or low-temperature roasting, cold-bound pellets with high strength are obtained, which can be used in blast furnace production or as a coolant for steelmaking. For example, patent (CN202111507292.4) discloses a method for reprocessing and utilizing mill scale, and the mill scale made into lump minerals has a certain compressive strength and is used for slag melting and temperature adjustment in steelmaking; or it can be used in processes such as rotary hearth furnaces to produce metallized pellets. For example, patent (CN201110141818.1) discloses an iron-making method for directly reducing and grinding mill scale in a rotary hearth furnace; in addition to the above utilization methods, mill scale can also be further processed to produce reduced iron powder. Generally, after being mixed with reducing agents such as coke powder and roughly reduced in a tunnel kiln, it is further pulverized and finely reduced. For example, patent (CN201110251677.9) discloses a two-step reduction method for recycling stainless steel mill scale; in addition, there are also applications of powder metallurgy technologies such as using mill scale to prepare iron red pigments in the chemical industry and producing soft magnetic ferrite materials from mill scale. For example, patent (CN202111175022.8) discloses a device for preparing raw materials for magnetic materials using mill scale and its usage method.

[0003] In summary, the mill scale has a high TFe content and a low impurity content, making it a secondary resource with extremely high recycling value. However, at present, most of the treatment methods are to recycle it for sintering to prepare sintered ore. Although resource recycling is achieved, the added value of the product is low, and the blending ratio is also limited. Some other iron reduction technologies require the addition of coke, pulverized coal, etc. to form pellets, and later grinding, magnetic separation, etc. are needed. The treatment process is complex and does not conform to the future development trend of low-carbon metallurgy. Or it is completely used as solid waste and mixed with other iron-containing materials to form agglomerates for rotary hearth furnace recycling treatment, etc., greatly weakening its own advantage of high TFe content. Other technologies for preparing chemical products or new materials often require acid leaching or other cumbersome processes, and secondary pollution may be generated during the recycling process. Generally speaking, it is very necessary to research and develop new technologies for high added value utilization of mill scale.

[0004] As a by-product in the production of iron and steel enterprises, mill scale has a high recycling value due to its high TFe content and low impurity content. However, at present, most of the utilization methods are to recycle it for sintering to prepare sintered ore. In addition, some iron reduction technologies require mixing with other iron-containing materials, adding coke, pulverized coal, etc. to form pellets and then reducing. Although these technologies achieve resource recycling, the added value of the products is low, and the inherent advantages of high TFe and low impurities in mill scale are greatly weakened. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for reducing and treating mill scale. This method only requires simple processing of the mill scale without fine grinding and pelletizing. By using it as a fluidized ironmaking raw material and adopting a fluidized bed process for reduction treatment, low-carbon and low-consumption recycling of mill scale can be achieved, and at the same time, a direct reduced iron product with a purer composition can be obtained.

[0006] In order to achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A method for reducing and treating mill scale specifically includes the following method steps:

[0008] 1) Dry and screen the mill scale to obtain mill scale with a particle size in the range of 0.2 - 4 mm; transport it to the silo for storage and standby; the fine powder under 0.2 mm can be sent to the sintering or pelletizing workshop for comprehensive utilization; the oversize material over 4 mm can be simply crushed and re-screened after removing impurities.

[0009] 2) Add the mill scale obtained in step 1) to the fluidized bed reduction system after preheating. Under the action of high-temperature reducing gas heating and reduction in the fluidized bed, the iron oxide is reduced to metallic iron, generating sponge iron powder particles.

[0010] 3) The flue gas generated by the fluidized bed reduction system is first preheated and heat-exchanged with mill scale and then dust-removed. The dust-removed ash can be sent to the sintering or pelletizing workshop for comprehensive utilization. The flue gas after dust removal enters the circulating purification system. The purified reducing gas is supplemented with new reducing gas and then heated. The heated high-temperature reducing gas enters the fluidized bed reduction system.

[0011] 4) The sponge iron powder obtained in step 2) is hot-pressed under nitrogen protection to obtain direct reduced iron hot briquettes (HBI).

[0012] The content of TFe in the chemical composition of the mill scale is not less than 67%, the content of FeO is not less than 50%, the content of SiO2 is not more than 2%, and the content of S is not more than 0.02%.

[0013] In step 1), the moisture content of the mill scale does not exceed 2%.

[0014] As an alternative, in step 1), the mill scale in the particle size range of 0.2 - 4 mm is subjected to surface coating pretreatment. The coating material is at least one of cement or lime, and the usage amount of the coating material is not more than 0.5% of the mass of the mill scale.

[0015] In step 3), the flue gas generated by the fluidized bed reduction system is preheated and heat-exchanged with the mill scale in a cyclone heat exchanger.

[0016] The fluidized bed reduction system consists of 2 - 4 stages of fluidized beds. The residence time of the mill scale in the fluidized bed reduction system is 30 - 300 min, the pressure in the fluidized bed reduction system is 0.3 - 0.8 MPa, the fluidized bed reducing gas is hydrogen or hydrogen-rich gas, the total volume content of CO and H2 in the hydrogen-rich gas is not less than 95%, and the volume content of H2 in the hydrogen-rich gas is not less than 60%.

[0017] The circulating purification system includes corresponding heat exchange units, purification units and pressurization units according to the composition of the reducing gas. The purification unit includes a dehydration device and a CO2 removal device. The by-products generated by the purification of the flue gas, namely the removed water, CO2, etc., are sent for comprehensive treatment.

[0018] In step 3), the temperature for heating the purified reducing gas and the supplemented new reducing gas is 550 - 750 °C.

[0019] The metalization rate of the reduced iron hot briquette is not less than 92%, and the density is not less than 5 g / cm 3 .

[0020] In the present invention, scale is used as the raw material for direct reduction ironmaking, and the fluidized bed process is adopted for reduction. The inherent advantages of scale with high TFe and low impurities are fully utilized, and sponge iron products with more stable composition and higher purity can be produced compared with conventional iron ore raw materials. As an important branch of the gas-based direct reduction technology, the fluidized bed process has many advantages such as a large gas-solid contact area and good heat and mass transfer conditions. However, agglomeration and loss of fluidity are likely to occur in the fluidized bed, and raw materials at the concentrate level cannot be processed. It is necessary to require the raw materials to have a certain particle size and strength. Scale itself has a certain particle size and high strength, and can be used in the fluidized bed process with only simple treatment, without the need for fine grinding and pelletizing, reducing the processing cost. The reaction process can be controlled by reasonable fluidized bed process parameters to avoid the occurrence of agglomeration and loss of fluidity. An alternative solution can also be used to appropriately coat the scale to enhance the anti-agglomeration effect. The method for reducing and treating scale in the present invention fully utilizes the inherent advantages of scale with high TFe and low impurities, adopts the fluidized bed process with hydrogen or hydrogen-rich gas as the reducing gas, omits processes such as sintering and coking, is more green and low-carbon, the process method is simple, and sponge iron products with higher purity can be produced at low cost.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] A method for reducing and treating scale provided by the present invention uses scale as the raw material for direct reduction ironmaking, adopts the fluidized bed process for reduction, and uses hydrogen or hydrogen-rich gas as the reducing gas, which is more green and low-carbon, the process method is simple, fully utilizes the inherent advantages of scale with high TFe and low impurities, and can produce sponge iron products with higher purity at low cost compared with conventional iron ore raw materials, thus proposing a new technology with high added value for the recycling and treatment of scale. Description of the Drawings

[0023] Figure 1 It is a schematic diagram of the scale reduction process of the present invention. Detailed Embodiments

[0024] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further illustrates the detailed embodiments of the present invention in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of the present invention. These embodiments are only general descriptions of the content of the present invention and do not limit the content of the present invention.

[0025] Throughout the specification, unless otherwise specifically stated, the terms used herein should be understood as having the meaning as commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as the general understanding of those skilled in the art to which the present invention belongs. In case of contradiction, this specification shall prevail.

[0026] Example 1:

[0027] The iron oxide scale feedstock (with moisture content greater than 10%) is fully dried (moisture content 2%) and then screened to obtain iron oxide scale with a particle size in the range of 0.2 - 4 mm. It is transported to a storage bin for standby. Table 1 shows the sampled composition of the iron oxide scale; the fine powder passing through the sieve with a particle size less than 0.2 mm is sent to the sintering or pelletizing workshop for comprehensive utilization; the oversize material with a particle size greater than 4 mm is removed of impurities and then simply crushed and rescreened. The screened iron oxide scale is preheated by a cyclone heat exchanger and then added to a two-stage fluidized bed reduction system. The iron oxide scale stays in the fluidized bed reduction system for 300 min. The pressure in the fluidized bed reduction system is 0.3 - 0.5 MPa. The fluidized bed reduction gas is hydrogen, and the fluidized bed reduction temperature is 550 - 650 °C. Under the action of hydrogen heating and reduction, the iron oxide scale is reduced to sponge iron powder particles. The flue gas generated by the fluidized bed reduction system is first heat-exchanged with the iron oxide scale and then dust-removed. The dust is sent to the sintering or pelletizing workshop for comprehensive utilization. The flue gas after dust removal enters the circulating purification system, where it undergoes further heat exchange, condensation and dehydration, and pressurization treatment. At the same time, new hydrogen is supplemented. After heating, the high-temperature hydrogen is introduced into the fluidized bed reduction system. During the reaction process, the fluidization performance of the material is good, and there is no phenomenon of adhesion and agglomeration. The obtained reduction product is hot-pressed under nitrogen protection to obtain a hot briquetted iron (HBI) product with a metallization rate of 92.5% and a density of 5.1 g / cm 3 ³.

[0028] Table 1 Chemical composition of iron oxide scale

[0029]

[0030] Example 2:

[0031] The iron scale feedstock (dry material, moisture content 1.5%) is screened to obtain iron scale with a particle size in the range of 0.2 - 4 mm, which is transported to a silo for storage and standby. Table 2 shows the sampled composition of the iron scale; the undersize fines less than 0.2 mm are sent to the sintering or pelletizing workshop for comprehensive utilization; the oversize materials larger than 4 mm are removed of impurities and then simply crushed and rescreened. The iron scale obtained after screening is preheated by a cyclone heat exchanger and then added to a 3-stage fluidized bed reduction system. The residence time of the iron scale in the fluidized bed reduction system is 60 min, the pressure in the fluidized bed reduction system is 0.6 - 0.8 MPa, the fluidized bed reduction gas is hydrogen-rich gas (the total volume content of CO and H2 is 95%, and the volume content of H2 is 61.5%), and the fluidized bed reduction temperature is 650 - 750 °C. Under the action of hydrogen-rich gas heating and reduction, the iron scale is reduced to sponge iron powder particles. The flue gas generated by the fluidized bed reduction system is first heat-exchanged with the iron scale and then dust-removed. The dust is sent to the sintering or pelletizing workshop for comprehensive utilization. The flue gas after dust removal enters the circulating purification system, where it undergoes further heat exchange, condensation and dehydration, pressurization, and CO2 removal treatment. At the same time, new hydrogen-rich gas is supplemented, and the high-temperature reduction gas after heating is introduced into the fluidized bed reduction system. During the reaction process, the fluidization performance of the material is good, and no agglomeration phenomenon occurs. The obtained reduction product is hot-pressed under nitrogen protection to obtain a direct reduced iron hot briquette (HBI) with a metallization rate of 93% and a density of 5.2 g / cm 3 of the product, direct reduced iron hot briquette (HBI).

[0032] Table 2 Chemical Composition of Iron Scale

[0033]

[0034] Example 3:

[0035] The iron scale feedstock (with moisture content greater than 10%) is fully dried (moisture content 2%) and then screened to obtain iron scale with a particle size range of 0.2 - 4 mm, which is transported to the silo for storage and standby. Table 3 shows the sampling composition of the iron scale; the fine powder passing through the sieve with a size less than 0.2 mm is sent to the sintering or pelletizing workshop for comprehensive utilization; the oversize material with a size greater than 4 mm is removed of impurities and then simply crushed and re-screened. The iron scale obtained after screening is subjected to surface coating pretreatment. The coating material is cement, and the usage amount of the coating material is 0.4% of the mass of the iron scale. After being preheated by a cyclone heat exchanger, it is added to the secondary fluidized bed reduction system. The residence time of the iron scale in the fluidized bed reduction system is 30 min, the pressure in the fluidized bed reduction system is 0.6 - 0.8 MPa, the fluidized bed reduction gas is hydrogen, and the fluidized bed reduction temperature is 700 - 750 °C. Under the action of hydrogen heating and reduction, the iron scale is reduced to sponge iron powder particles. The flue gas generated by the fluidized bed reduction system is first heat-exchanged with the iron scale and then dust-removed. The dust is sent to the sintering or pelletizing workshop for comprehensive utilization. The flue gas after dust removal enters the circulating purification system, where it undergoes further heat exchange, condensation dehydration, and pressurization treatment. At the same time, new hydrogen is supplemented, and the heated high-temperature hydrogen is introduced into the fluidized bed reduction system. During the reaction process, the fluidization performance of the material is good, and no agglomeration phenomenon occurs. The obtained reduction product is subjected to hot pressing treatment under nitrogen protection to obtain a direct reduced iron hot briquette (HBI) with a metallization rate of 92.8% and a density of 5.1 g / cm 3 of the product direct reduced iron hot briquette (HBI).

[0036] Table 3 Chemical Composition of Iron Scale

[0037]

[0038] Example 4:

[0039] The iron scale feedstock (with moisture content greater than 10%) is fully dried (moisture content 1.5%) and then screened to obtain iron scale with a particle size range of 0.2 - 4 mm, which is transported to a storage bin for standby. Table 4 shows the sampled composition of the iron scale; the fine powder passing through the sieve with a particle size less than 0.2 mm is sent to the sintering or pelletizing workshop for comprehensive utilization; the oversize material with a particle size greater than 4 mm is removed of impurities and then simply crushed and re-screened. The iron scale obtained after screening is subjected to surface coating pretreatment, and the coating material is lime, with the usage amount of the coating material being 0.3% of the mass of the iron scale. After being preheated by a cyclone heat exchanger, it is added to a 4-stage fluidized bed reduction system. The residence time of the iron scale in the fluidized bed reduction system is 120 min, the pressure in the fluidized bed reduction system is 0.4 - 0.6 MPa, the fluidized bed reduction gas is hydrogen-rich gas (the total volume content of CO and H2 is 95.5%, and the volume content of H2 is 60.5%), and the fluidized bed reduction temperature is 550 - 750 °C. Under the action of heating and reduction by the hydrogen-rich gas, the iron scale is reduced to sponge iron powder particles. The flue gas generated by the fluidized bed reduction system is first heat-exchanged with the iron scale and then dust-removed. The dust is sent to the sintering or pelletizing workshop for comprehensive utilization, and the flue gas after dust removal enters the circulating purification system. After further heat-exchange, condensation and dehydration, pressurization, and CO2 removal treatment, and at the same time, new hydrogen-rich gas is supplemented, and the high-temperature hydrogen-rich gas after heating is introduced into the fluidized bed reduction system. During the reaction process, the fluidization performance of the material is good, and there is no phenomenon of adhesion and agglomeration. The obtained reduced product is subjected to hot pressing treatment under nitrogen protection to obtain a hot briquetted iron (HBI) product with a metallization rate of 93.2% and a density of 5.2 g / cm 3 of the direct reduced iron hot briquette (HBI).

[0040] Table 4 Chemical Composition of Iron Scale

[0041]

[0042] Example 5:

[0043] The iron scale feedstock (with moisture content greater than 10%) is fully dried (moisture content 1.8%) and then screened to obtain iron scale with a particle size in the range of 0.2 - 4 mm, which is transported to a silo for storage and standby. Table 5 shows the sampled composition of the iron scale; the fine powder passing through the sieve with a size less than 0.2 mm is sent to the sintering or pelletizing workshop for comprehensive utilization; the oversize material with a size greater than 4 mm is removed of impurities and then simply crushed and re-screened. The screened iron scale is preheated by a cyclone heat exchanger and then added to the 2-stage fluidized bed reduction system. The iron scale stays in the fluidized bed reduction system for 270 min. The pressure in the fluidized bed reduction system is 0.3 - 0.5 MPa. The fluidized bed reduction gas is hydrogen-rich gas (the total volume content of CO and H2 is 95.5%, and the volume content of H2 is 60.5%). The fluidized bed reduction temperature is 550 - 650 °C. Under the action of hydrogen-rich gas heating and reduction, the iron scale is reduced to sponge iron powder particles. The flue gas generated by the fluidized bed reduction system is first heat-exchanged with the iron scale and then dust-removed. The dust is sent to the sintering or pelletizing workshop for comprehensive utilization. The flue gas after dust removal enters the circulating purification system, where it undergoes further heat exchange, condensation and dehydration, pressurization, and CO2 removal treatment. At the same time, new hydrogen-rich gas is supplemented. After heating, the high-temperature hydrogen-rich gas is introduced into the fluidized bed reduction system. During the reaction process, the fluidization performance of the material is good, and no agglomeration phenomenon occurs. The obtained reduced product is hot-pressed under nitrogen protection to obtain a direct reduced iron hot briquette (HBI) with a metallization rate of 92% and a density of 5.06 g / cm 3 of the product, direct reduced iron hot briquette (HBI).

[0044] Table 5 Chemical Composition of Iron Scale

[0045]

[0046] Example 6:

[0047] The iron scale feedstock (dry material, moisture content 1.5%) is screened to obtain iron scale with a particle size range of 0.2 - 4 mm, which is transported to a silo for storage and standby. Table 6 shows the sampling composition of the iron scale; the fine powder under 0.2 mm is sent to the sintering or pelletizing workshop for comprehensive utilization; the oversize material over 4 mm is removed of impurities and then simply crushed and re-screened. The iron scale obtained after screening is preheated by a cyclone heat exchanger and then added to a 4-stage fluidized bed reduction system. The residence time of the iron scale in the fluidized bed reduction system is 120 min, the pressure in the fluidized bed reduction system is 0.4 - 0.8 MPa, the fluidized bed reduction gas is hydrogen, and the fluidized bed reduction temperature is 550 - 700 °C. Under the action of hydrogen heating and reduction, the iron scale is reduced to sponge iron powder particles. The flue gas generated by the fluidized bed reduction system is first heat-exchanged with the iron scale and then dust-removed. The dust is sent to the sintering or pelletizing workshop for comprehensive utilization. The flue gas after dust removal enters the circulating purification system, where it undergoes further heat exchange, condensation and dehydration, and pressurization treatment. At the same time, new hydrogen is supplemented, and the high-temperature reduction gas after heating is introduced into the fluidized bed reduction system. During the reaction process, the fluidization performance of the material is good, and there is no phenomenon of adhesion and agglomeration. The obtained reduction product is hot-pressed under nitrogen protection to obtain a direct reduced iron hot briquette (HBI) with a metallization rate of 92% and a density of 5.2 g / cm 3 of the product direct reduced iron hot briquette (HBI).

[0048] Table 6 Chemical composition of iron scale

[0049]

[0050] The serial numbers of the above embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments. In the above embodiments of the present invention, the descriptions of each embodiment have their own emphases. For the parts not detailed in a certain embodiment, reference can be made to the relevant descriptions of other embodiments. In the several embodiments provided in this application, it should be understood that the disclosed technical content can be implemented in other ways.

Claims

1. A method for reducing and treating iron oxide scale, characterized in that: The specific steps include the following: 1) Screening the iron oxide scale to obtain iron oxide scale with a particle size range of 0.2 to 4 mm; 2) adding the iron oxide scale obtained in step 1) into a fluidized bed reduction system after preheating, and reducing the iron oxide into metallic iron to generate sponge iron powder particles; 3) The flue gas generated by the fluidized bed reduction system is first preheated with the iron oxide scale for heat exchange and then subjected to dust removal treatment. The dust-removed flue gas enters the circulating purification system. The purified reducing gas is then supplemented with new reducing gas and heated. The heated reducing gas enters the fluidized bed reduction system; 4) The sponge iron powder obtained in step 2) is hot pressed under nitrogen protection to obtain direct reduced iron hot pressed blocks.

2. The method for reducing and treating iron oxide scale according to claim 1, characterized in that: The chemical composition of the iron oxide scale comprises TFe content not less than 67%, FeO content not less than 50%, SiO2 content not more than 2%, and S content not more than 0.02%.

3. The method for reducing and treating iron oxide scale according to claim 1, characterized in that: In the step 1), the moisture content of the iron oxide scale does not exceed 2%.

4. The method for reducing and treating iron oxide scale according to claim 1, characterized in that: In the step 1), the iron oxide scale with a particle size range of 0.2 to 4 mm is pre-treated by surface coating, the coating material is at least one of cement or lime, and the amount of the coating material used is not higher than 0.5% of the mass of the iron oxide scale.

5. The method for reducing and treating iron oxide scale according to claim 1, characterized in that: In the step 3), the flue gas generated by the fluidized bed reduction system is preheated and heat-exchanged with the iron oxide in the cyclone heat exchanger.

6. The method for reducing and treating iron oxide scale according to claim 1, characterized in that: The fluidized bed reduction system is composed of 2 to 4 stages of fluidized beds, the residence time of iron oxide in the fluidized bed reduction system is 30 to 300 minutes, the pressure in the fluidized bed reduction system is 0.3 to 0.8 MPa, and the fluidized bed reduction gas is hydrogen or hydrogen-rich coal gas. The total volume content of CO and H2 in the hydrogen-rich coal gas is not less than 95%, and the volume content of H2 in the hydrogen-rich coal gas is not less than 60%.

7. The method for reducing and treating iron oxide scale according to claim 1, characterized in that: In the step 3), the circulating purification system includes a dehydration device and a CO2 removal device; the temperature for heating the purified reducing gas and the supplemented new reducing gas is 550-750°C.

8. The method for reducing and treating iron oxide scale according to claim 1, characterized in that: The reduced iron hot pressed block has a metallization rate of not less than 92% and a density of not less than 5g / cm 3 .

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