Carbonaceous material for electric furnace and manufacturing method of carbonaceous material for electric furnace

Abstract

To provide a carbonaceous material for an electric furnace capable of accelerating foaming of slag when refining a molten metal using an electric furnace and a manufacturing method of a carbonaceous material for an electric furnace.SOLUTION: A carbonaceous material for an electric furnace has a composition of a fixed carbon content 50 mass% or more, a volatile component 40 mass% or less, a sulfur content 0.5 mass% or less with a balance ash content in a dry state excluding a water content, and separately the water content is 8.4 to 15 mass%, inclusive, based on the mass in the dry state excluding the water content. More preferably, a median particle diameter of the carbonaceous material for an electric furnace is 5 mm or less. Further preferably, the carbonaceous material for an electric furnace includes biomass charcoal obtained by carbonizing biomass.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a carbonaceous material for an electric furnace and a method for producing the same. [Background technology]

[0002] When refining molten iron in an electric furnace, carbonaceous material is added to the furnace as a heat source, a source of carbon for the molten iron, and to promote foaming of the slag. Powdered coal is typically used as the carbonaceous material and is injected into the furnace. Stable foaming of slag in an electric furnace effectively reduces the power consumption by improving thermal efficiency through the envelopment of the electrode tip (arc). This also reduces refractory wear and prevents contamination of the molten iron by entrained air.

[0003] Regarding the moisture content of materials fed into electric furnaces, it is generally believed that excess moisture leads to heat loss and poor powder transportability. The same is true for carbonaceous materials. When carbonaceous materials are injected into electric furnaces, they have traditionally been injected in a dry state. Furthermore, the particle size of the carbonaceous materials used is an important indicator, as it affects their reactivity within the electric furnace and their transportability to the furnace. Furthermore, in recent years, attention has been focused on using biomass-derived carbonaceous materials (hereinafter referred to as biomass charcoal) as a carbonaceous material raw material, rather than coal-derived, in order to reduce the actual generation of carbon dioxide, a greenhouse gas.

[0004] For example, Patent Document 1 relates to a method for increasing the amount of carbon dissolved in molten metal when producing molten metal using a cold iron source in an electric furnace, and discloses a carbonaceous material injection method. It states that the carbonaceous material is preferably derived from biomass. Palm kernel shell (PKS) charcoal is injected from a carbonaceous material injection immersion lance. However, there is no mention whatsoever of the moisture content or particle size of the carbonaceous material.

[0005] Patent Document 2 discloses an example in which carbonized plant biomass is used as a carbon source for a converter heating material. The particle size of the carbonized carbonized material is 3 mm or less. This carbon source is used as a raw material and molded into briquettes, which are then dried to a moisture content of 5% by mass or less to produce a converter heating material. First, these conditions are for a converter carbon material, whose main purpose is to heat up the material. Furthermore, the mechanism and expected range of foaming promotion by the carbon material differ significantly between converters and electric furnaces. Therefore, the conditions that should be met for an electric furnace cannot be easily derived from the converter carbon material. Note that the moisture range indicated in this invention is lower than the range discovered by the inventors, which will be described later, and is in a range where woody biomass requires drying.

[0006] Patent Document 3 discloses a method in which coconut shell charcoal is a carbonized product obtained by dry distillation of coconut shell biomass, and the coconut shell charcoal is used in an electric furnace as a supplementary fuel or recarburizer. While the requirements for the moisture content of the coconut shell charcoal are not specified, a maximum moisture content of 7.8% is mentioned as an example. However, this invention separates the carbonaceous material to be mixed with the scrap and charged from the carbonaceous material to be injected, and does not clearly disclose how to control the moisture content of these materials. It is generally thought that fine-grained carbonaceous material is used for injection, and the moisture content of these fine grains is said to be around 2%.

[0007] Patent Document 4 discloses the same conditions for coconut shell-derived carbonaceous material as Patent Document 3, and states that the maximum moisture content is 7.7% as an example of moisture measurement. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-145393 [Patent Document 2] International Publication No. WO2013-128786 [Patent Document 3] International Publication No. WO2009-047297 [Patent Document 4] Japanese Patent Application Laid-Open No. 2009-046726 Summary of the Invention [Problem to be solved by the invention]

[0009] When refining molten metal using an electric furnace, carbonaceous material is added to the electric furnace to promote slag foaming, but it is desired to further promote foaming with the same amount of carbonaceous material charged, or to achieve sufficient foaming with a smaller amount of carbonaceous material charged.The object of the present invention is to provide a carbonaceous material for electric furnaces that can promote slag foaming and a method for producing the same carbonaceous material. [Means for solving the problem]

[0010] That is, the gist of the present invention is as follows. [1] A carbonaceous material for electric furnaces, the composition of which in a dry state excluding moisture is 50% by mass or more of fixed carbon, 40% by mass or less of volatile components, 0.5% by mass or less of sulfur, and the remainder is ash, and the moisture content is 8.4% by mass or more and 15% by mass or less of the mass in a dry state excluding moisture. [2] The carbonaceous material for electric furnaces according to [1], wherein the median particle size of the carbonaceous material for electric furnaces is 5 mm or less. [3] The carbonaceous material for electric furnaces according to [1] or [2], wherein the carbonaceous material for electric furnaces is made of biomass charcoal obtained by carbonizing biomass. [4] The carbonaceous material for electric furnaces according to [3], wherein the biomass is wood-based. [5] A method for producing carbonaceous material for electric furnaces by crushing raw materials for carbonaceous material for electric furnaces, The method for producing a carbonaceous material for an electric furnace according to any one of [1] to [4], wherein the moisture content of the raw material of the carbonaceous material for an electric furnace during pulverization is set to 8.4 mass % or more and 15 mass % or less. [Effects of the Invention]

[0011] The present invention provides an electric furnace carbonaceous material having a moisture content of 8.4 mass % or more and 15 mass % or less, more preferably an electric furnace carbonaceous material having a median particle size of 5 mm or less, thereby facilitating slag foaming when refining molten metal using an electric furnace. DETAILED DESCRIPTION OF THE INVENTION

[0012] <Appropriate moisture content range in carbon materials for electric furnaces> Through laboratory and actual equipment tests, the inventors discovered that injecting carbonaceous material containing a certain amount of moisture into an electric furnace can promote slag foaming. Although excessive moisture can lead to heat loss, there is a range in which the aforementioned benefits of promoting foaming outweigh the effects, so the moisture content of the carbonaceous material is an important indicator.

[0013] Although moisture supplied to the electric furnace leads to heat loss within the furnace, as mentioned above, it also promotes foaming, thereby improving the furnace's thermal efficiency. Furthermore, when pre-drying the carbonaceous material, the energy consumption for drying must be considered, so unnecessary drying of the carbonaceous material is not necessarily economically rational. For these reasons, we focused on the importance of properly managing the moisture content of carbonaceous material. Laboratory and field tests revealed that the appropriate moisture content range is between 8.4% and 15% by mass of the carbonaceous material in a dry state excluding moisture. This test also demonstrated that similar effects could be achieved with agglomerated carbonaceous material, in addition to sprayed powder. The moisture content here was measured by drying the material to constant mass at 107°C according to JIS M 8820, and the dry mass was calculated using the mass after drying.

[0014] If the moisture content of the carbonaceous material is less than 8.4% by mass, the foaming promotion effect is insufficient. If it exceeds 15% by mass, heat loss increases, and gas generation becomes too rapid, occasionally resulting in large slag repulsions, saturating the foaming promotion effect. Therefore, the moisture content of the carbonaceous material is set to be between 8.4% and 15% by mass of the mass of the carbonaceous material in a dry state. The volatile content and sulfur content are also important in terms of their effect on foaming properties. The moisture range mentioned above was confirmed for carbonaceous material whose dry composition, excluding moisture, is 50% or more by mass of fixed carbon, 40% or less by mass of volatile components, 0.5% or less by mass of sulfur, and the remainder is ash.

[0015] To control the moisture content of carbonaceous material within the above range, if the moisture content is insufficient, there are two methods: adding moisture and then homogenizing it by stirring, or holding it in an atmosphere maintained at a certain humidity level for a certain period of time to allow it to absorb moisture.If the moisture content is excessive, there is another method: holding it in an atmosphere maintained at a humidity level below a certain level for a certain period of time to dehumidify it.In either case, the amount of moisture added, humidity, stirring, and holding time can be determined using the moisture measurement results of samples taken at regular intervals.

[0016] <Particle size of carbon material for electric furnaces> While particle size is important from the perspective of powder transportability of the carbonaceous material, for the moisture in the carbonaceous material to effectively promote foaming, it must enter the slag and react over a certain period of time. Therefore, the surface area where moisture is generated is crucial. In other words, particle size is the second most important factor after moisture content. Specifically, it was found that a median particle size of 5 mm or less is desirable. Here, the median particle size is the value at which the relative particle amount is 50% of the total mass.

[0017] 《Raw material for carbon for electric furnaces》 Furthermore, gas transportability is important when injecting carbon into an electric furnace. Generally, excessive moisture impairs gas transportability. We have found that biomass charcoal, obtained by carbonizing biomass, is an effective carbon material that can retain a certain amount of moisture without impeding gas transportability. Here, biomass is defined as "renewable, biologically derived organic resources, excluding fossil resources," as defined in the "Biomass Japan Comprehensive Strategy," approved by the Cabinet in December 2002 and supervised by the Ministry of Agriculture, Forestry and Fisheries. Specifically, it refers to waste and unused materials such as wood, food, and sewage sludge. Among these, plant biomass is suitable for use as charcoal in electric furnaces due to its moisture content, size, and homogeneity at the time of generation (delivery). It is also particularly desirable due to its excellent carbon dioxide circulation. Plant biomass specifically refers to coffee grounds, seaweed, and wood. Plant biomass is also desirable due to its ability to retain moisture through its pores. Biomass charcoal is produced by carbonizing biomass as a raw material. Biomass carbonization is a process in which biomass is heated in a non-oxidizing atmosphere to decompose carbon compounds.

[0018] It is desirable to first carbonize the raw material in its raw form and then pulverize or agglomerate it to the desired particle size, but there is no impediment to the practice of the present invention if the raw material is pulverized, granulated or agglomerated once before carbonization to adjust the particle size, and then carbonized and blown in as is or after further pulverization or granulation.

[0019] 《Wood type》 Furthermore, in terms of porosity and strength, it is desirable that the biomass be wood-based. Biocoal, particularly wood-based biomass, is porous and can retain a certain amount of moisture as adsorbed water, and a moisture content within the range specified in the present invention is desirable from the perspective of powder transport. Furthermore, wood-based biomass has the advantage of maintaining particle size distribution after carbonization because it has a higher strength than other biomass. Specific examples of wood-based biomass include construction waste wood, sawmill residues, and forest residues. Examples of tree species include, but are not limited to, evergreen tall trees such as cedar, cypress, pine, and eucalyptus.

[0020] <Method for manufacturing carbon material for electric furnaces> The carbonaceous material for electric furnaces of the present invention needs to contain a certain amount of moisture even when blown into an electric furnace, so it is not essential to dry it before crushing or transporting. To reduce the energy consumption for drying, it is possible to retain the same level of moisture as when it is used. That is, by setting the moisture content during crushing to 8.4% by mass or more and 15% by mass or less, it is possible to omit the drying and moisture adjustment steps before crushing without any problems. On the other hand, ensuring moisture during crushing also prevents aggregation due to static electricity, which has the advantage of increasing the uniformity of particle size.

[0021] Here, the moisture content during grinding can be measured before grinding. However, before grinding does not necessarily mean immediately before the grinding start time, such as within seconds or minutes. In other words, the value at the start of grinding can be used, which predicts the moisture content change from the measurement time, taking into account the atmosphere and residence time of the grinding equipment and the transport route to it. It is desirable that this prediction be based on the results of a prior investigation into the effects of the atmosphere and time. [Example]

[0022] In the following description, percentages are by mass unless otherwise specified, and are the proportions relative to the mass in a dry state excluding moisture.

[0023] <Evaluation of the effects of moisture content and particle size of carbonaceous materials (crucible test)> After molten electric furnace simulated slag was kept at 1600°C in a crucible with an inner diameter of 60 mm, the foaming state of the slag was evaluated when carbonaceous materials with different water content and particle size were added from above.

[0024] The carbonaceous material was obtained by crushing coal (anthracite) to 3 mm or less, and had a fixed carbon content of 88%, a volatile component of 8%, a sulfur content of 0.1%, and the remainder being ash. The moisture content of the carbonaceous material was varied from 0.5% to 20% by first holding the coal at 107±2°C in a drying oven with nitrogen reflux until it reached a constant weight, then cooling it, and then holding it under a constant atmospheric humidity or by stirring it while spraying moisture. The moisture content of the carbonaceous material was measured immediately after evaluation (specifically, approximately 10 minutes before evaluation). The production conditions and evaluation results are shown in Table 1. In Table 1 and Table 2 described below, values ​​outside the range of the present invention are underlined.

[0025] [Table 1]

[0026] The median diameter of the carbonaceous material was measured in a dry state and evaluated to be 2.2 mm for Examples 1 to 4 and Comparative Examples 1 to 6 in Table 1. For Examples 5 and 6 in Table 1, carbonaceous material was used that had been crushed or the amount of water added adjusted so that the median diameter of the carbonaceous material was 4.5 and 6.2 mm, respectively, and the moisture content was 8.4% in both cases.

[0027] The composition of the slag, expressed by mass, is a CaO / SiO2 ratio of 0.8, an iron oxide concentration (the sum of FeO and Fe2O3) of 25%, and contains a total of about 10% Al2O3 and MgO.

[0028] The foaming condition of the slag was evaluated by inserting a measuring rod from the bottom of the crucible to the surface of the slag at regular intervals from before the addition of the carbonaceous material, and the point at which the height reached its maximum was evaluated as the maximum foaming height. Each value is the average of three experiments, and the height before the addition of the carbonaceous material was 50 mm.

[0029] The relationship between the moisture content and median diameter of the carbonaceous material and the maximum foaming height of the slag is summarized in Table 1. When a maximum foaming height of 80 mm or more but less than 85 mm was obtained, a good foaming promotion effect was obtained and marked with a circle, when a maximum foaming height of 85 mm or more was obtained, a better foaming promotion effect was obtained and marked with a double circle, and when the maximum foaming height was less than 80 mm, a cross was marked. Note that in Comparative Example 6, in which the moisture content was 20%, the exact height could not be measured, because the slag surface had solidified, and no foaming promotion effect was observed.

[0030] Examples 1 to 6, in which the moisture content of the carbonaceous material was 8.4% or more and 15% or less, were rated as ○ or ◎, while Comparative Examples 1 to 5, in which the moisture content was less than 8.4% or more than 15%, were rated as ×.

[0031] Next, the carbonaceous materials were similarly evaluated after being crushed and the amount of water added was adjusted so that the median diameter of the carbonaceous materials was 4.5 and 6.2 mm, respectively, and the moisture content was 8.4%. As shown in Examples 5 and 6 in Table 1, the maximum foaming heights of all the carbonaceous materials were higher than those of the comparative examples. When Examples 1, 5, and 6, which have the same moisture content, were compared in terms of median diameter, Examples 1 and 5, which have a median diameter of 5 mm or less, exhibited a higher foaming promotion effect than Example 6.

[0032] Examples 1 to 5, in which the moisture content was 8.4% or more and the median diameter was 5 mm or less, were evaluated as ⊚.

[0033] <Evaluation of the effect of moisture content of charcoal material (actual machine test)> The same carbonaceous material as used in the crucible evaluation above was adjusted to a median diameter of 2.2 mm, and the moisture content was adjusted to 2.0 and 8.4% using the same method as above. The electricity consumption was also evaluated when the carbonaceous material was used in a 175-ton electric furnace. The amount used was 18 kg of fixed carbon per ton of steel tapped.

[0034] As mentioned above, the higher the slag foaming height, the longer the electrode tip remains immersed in the slag, and the entire arc is enveloped in the slag, suppressing heat dissipation and improving thermal efficiency. The input heat amount is proportional to the melting of the materials and the temperature rise to a predetermined level, and improved thermal efficiency results in a lower power consumption rate. Therefore, the degree of foaming promotion was evaluated using the power consumption rate.

[0035] From the viewpoint of cost and production speed, cases where the power consumption rate was lower than 400 kWh / t were evaluated as ◯, and cases where it was not 400 kWh / t were evaluated as ×. The results are shown in Table 2. When a carbonaceous material with a moisture content within the range of the present invention was used (Example 7), foaming was promoted and the power consumption rate was reduced compared to Comparative Example 7.

[0036] [Table 2]

[0037] <Evaluation of the impact of carbonaceous material types> Table 3 shows the results of evaluation of carbonaceous materials produced using raw materials other than anthracite in a 175 ton electric furnace similar to that in Example 7.

[0038] The coal used was purchased from general coal, and upon arrival, only the particle size and moisture content were adjusted. The coffee grounds were purchased and carbonized upon arrival to adjust the components (except moisture) shown in Table 3. After carbonization, the particle size was adjusted to 5 mm or less, and the moisture content was also adjusted. The cedar chips were purchased and crushed to 10 mm or less upon arrival, then carbonized to adjust the components (except moisture) shown in Table 3. After carbonization, the particle size was adjusted and the moisture content was also adjusted by crushing and classification. The amount used was 20 kg of fixed carbon per ton of steel tapped. Compared to the coal-based Example 8, Examples 9 and 10, which were biomass charcoal, achieved better reductions in electricity consumption. Example 10, which used cedar wood, showed the lowest electricity consumption.

[0039] [Table 3]

[0040] <<Evaluation of the impact of manufacturing methods>> The influence of the manufacturing method was evaluated by comparing the moisture content in the crushing process (moisture content measured before crushing) with that in Example 11, which was different from that in Example 10 in terms of moisture content during use. Examples 10 and 11 are shown in Table 4. The electricity consumption rate was evaluated using the same 175 ton electric furnace as in Example 7.

[0041] [Table 4]

[0042] In Example 10, the particle size and moisture content after carbonization were adjusted in the following manner in the production of carbon material for electric furnaces using cedar as a raw material. Specifically, the moisture content was approximately 4% after carbonization and storage indoors under controlled humidity conditions. Before pulverization and classification, the material was dried once, and the moisture content was adjusted to 0.5% before pulverization. The material was then pulverized and classified to a median diameter of 2.4 mm. The moisture content was then adjusted to 9.0% using the method described above. Here, "before pulverization" refers to one hour before pulverization began. It was confirmed that the moisture content of this carbon material fluctuated by no more than 0.1% per hour in the environment used for evaluation.

[0043] In Example 11, the moisture content was adjusted to the value in Table 4 using the method described above before the pulverization process, and then pulverized and classified. The moisture content before pulverization was 9.7%. This measurement was taken one hour before the start of pulverization. It was confirmed that the moisture value of this carbonaceous material fluctuated by no more than 0.1% per hour in the environment used for evaluation. The moisture value before use was slightly lower than the measured value before pulverization because moisture was lost during the pulverization and classification process.

[0044] Table 4 shows the costs required for drying, crushing, and classification, with Example 10 set as 100. Example 11 does not require the cost of drying, so the costs required after carbonization in carbon material production were significantly lower than Example 10. Note that although Examples 10 and 11 have the same moisture content at the time of use, they contained a certain amount of moisture during crushing, which resulted in a more uniform particle size after crushing, and as a result, Example 11 also had a lower electricity consumption rate.

Claims

1. A carbonaceous material for electric furnaces, the composition of which in a dry state excluding moisture is 50% by mass or more of fixed carbon, 40% by mass or less of volatile components, 0.5% by mass or less of sulfur, and the remainder being ash, and the moisture content is 8.4% by mass or more and 15% by mass or less of the mass of the carbonaceous material in a dry state excluding moisture.

2. 2. The carbonaceous material for electric furnaces according to claim 1, wherein the median particle size of the carbonaceous material for electric furnaces is 5 mm or less.

3. 3. The carbonaceous material for an electric furnace according to claim 1, wherein the carbonaceous material for an electric furnace is made of biomass charcoal obtained by carbonizing biomass.

4. The carbonaceous material for an electric furnace according to claim 3, wherein the biomass is wood-based.

5. A method for producing carbonaceous material for electric furnaces by pulverizing raw materials for carbonaceous material for electric furnaces, 3. The method for producing a carbonaceous material for an electric furnace according to claim 1, wherein the moisture content of the raw material of the carbonaceous material for an electric furnace during pulverization is set to 8.4 mass % or more and 15 mass % or less.

6. A method for producing carbonaceous material for electric furnaces by pulverizing raw materials for carbonaceous material for electric furnaces, The method for producing a carbonaceous material for an electric furnace according to claim 3, wherein the moisture content of the raw material of the carbonaceous material for an electric furnace during pulverization is set to 8.4 mass % or more and 15 mass % or less.

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