Method for recovering phosphorus

Abstract

This method for recovering phosphorus includes adding a reducing agent containing at least one of metallic silicon and metallic aluminum and an alkali metal compound to steelmaking slag containing a phosphorus oxide to reduce the phosphorus oxide with the reducing agent in a state including the liquid phase.

Description

Phosphorus recovery method

[0001] The present disclosure relates to a method for recovering phosphorus.

[0002] Phosphorus is widely used in chemical fertilizers, industrial chemicals, semiconductors, battery materials, and more. However, in recent years, concerns have arisen about the decline in the quality and depletion of phosphate rock, making securing resources a challenge. Meanwhile, because phosphorus has a negative effect on the properties of steel during the steel manufacturing process, it is discharged as phosphorus-containing steelmaking slag (a by-product). Because steelmaking slag does not contain harmful heavy metals or radioactive elements, it has attracted attention as a promising alternative resource to phosphate rock.

[0003] For this reason, research into the recovery and recycling of phosphorus contained in steelmaking slag has been progressing. Patent Document 1 discloses a method of reducing iron oxides and phosphorus oxides in steelmaking slag using a reducing agent, dissolving the phosphorus produced by the reduction of the phosphorus oxides in iron produced by the reduction of the iron oxides, and recovering the phosphorus-containing molten iron in a molten state from the steelmaking slag.

[0004] Patent No. 5660166

[0005] In conventional methods such as those described in Patent Document 1, when reducing phosphorus oxides, it is necessary to heat the steelmaking slag to a temperature (1000°C or higher) at which the steelmaking slag melts, which requires high equipment costs and running costs. Therefore, in order to realize an economical phosphorus recovery process, it has been an issue to suppress the reaction temperature for reducing phosphorus oxides.

[0006] An object of the present disclosure is to provide a phosphorus recovery method that enables the reduction of phosphorus oxides in steelmaking slag at low temperatures.

[0007] The phosphorus recovery method according to the present disclosure involves adding a reducing agent containing at least one of metallic silicon and metallic aluminum and an alkali metal compound to steelmaking slag containing phosphorus oxides, thereby reducing the phosphorus oxides with the reducing agent in a state that includes a liquid phase.

[0008] The alkali metal compound may contain at least one selected from the group consisting of sulfates, carbonates, nitrates, oxides, and hydroxides.

[0009] The ratio of the amount of the alkali metal compound added to the total amount of the steelmaking slag and the reducing agent added may be 0.02 or more and 0.60 or less.

[0010] When the phosphorus oxide is reduced by the reducing agent in a state containing a liquid phase, the reaction temperature may be lower than when no alkali metal compound is added.

[0011] The reducing agent may contain metallic silicon.

[0012] According to the present disclosure, it is possible to provide a phosphorus recovery method that enables reduction of phosphorus oxides in steelmaking slag at low temperatures.

[0013] Figure 1 is a photograph of an experiment in which an alkali metal compound is added to steelmaking slag and heated. Figure 2 is a schematic diagram of an experimental apparatus for a phosphorus recovery experiment in which an alkali metal compound and a reducing agent are added to steelmaking slag and heated. Figure 3 is a graph showing the relationship between the ratio of the amount of alkali metal compound added to the reaction raw materials and the phosphorus yield. Figure 4 is a graph showing the relationship between the ratio of the amount of alkali metal compound added to the reaction raw materials and the phosphorus yield. Figure 5 is a graph showing the results of calculations performed using calculation software on the relationship between the ratio of the amount of alkali metal compound added to the reaction raw materials and the liquid phase ratio.

[0014] Hereinafter, several exemplary embodiments will be described with reference to the drawings. Note that the dimensional proportions of the drawings are exaggerated for the convenience of explanation and may differ from the actual proportions.

[0015] In the phosphorus recovery method of this embodiment, a reducing agent containing at least one of metallic silicon and metallic aluminum and an alkali metal compound is added to steelmaking slag containing phosphorus oxides, and the phosphorus oxides are reduced by the reducing agent in a state containing a liquid phase.

[0016] Steelmaking slag generated in the steel manufacturing process contains phosphorus oxide (P ２ O ５ ) in steelmaking slag. ２ O ５ The content of ) varies greatly depending on the process, but is generally about 1 to 5 mass %.

[0017] In the phosphorus recovery method of this embodiment, a reducing agent is added to steelmaking slag containing phosphorus oxides. ２ O ５ The reducing agent contains at least one of metallic silicon and metallic aluminum, and preferably contains metallic silicon from the viewpoint of reducing power.

[0018] The ratio of the amount of reducing agent added to the steelmaking slag is preferably 0.15 to 8.0, more preferably 0.2 to 1.5, and even more preferably 0.2 to 1.0. By setting the ratio of the amount of reducing agent added to the steelmaking slag within this range, a sufficient phosphorus yield can be ensured without inhibiting the effect of the alkali metal compound in lowering the melting point of the steelmaking slag.

[0019] Conventional phosphorus recovery methods using steelmaking slag as a raw material also involve the addition of a reducing agent. For example, when metallic silicon is used as the reducing agent, yellow phosphorus is recovered by gasification, as shown in the following reaction formula (1): 2(3CaO.P ２ O ５ ) + 5Si → 2P ２ +5SiO ２ +6CaO (1)

[0020] Conventional phosphorus recovery methods have had the problem of high reduction reaction temperatures. Because this reduction reaction proceeds in a liquid phase, a reaction temperature of 1000°C or higher is required, at which point the steelmaking slag is sufficiently melted to contain the liquid phase.

[0021] The reason why the reduction reaction of phosphorus oxide in steelmaking slag does not proceed sufficiently at temperatures below 1000°C is thought to be that the steelmaking slag exists in a solid state below 1000°C, resulting in poor contact between the steelmaking slag and the reducing agent (hereinafter referred to as the reaction raw material). Therefore, it was necessary to raise the temperature to a temperature at which the proportion of the reaction raw material that melts becomes high, i.e., a temperature at which the liquid phase fraction falls within a predetermined range. The liquid phase fraction refers to the proportion of the liquid phase in the reaction raw material.

[0022] In the phosphorus recovery method of this embodiment, not only a reducing agent but also an alkali metal compound is added to steelmaking slag containing phosphorus oxide. The alkali metal compound has the effect of lowering the melting point of the steelmaking slag containing phosphorus oxide. Therefore, the reaction temperature when reducing phosphorus oxide with a reducing agent in a state containing a liquid phase may be lower than when no alkali metal compound is added. The alkali metal compound preferably contains at least one selected from the group consisting of sulfates, carbonates, nitrates, oxides, and hydroxides. Examples of sulfates include potassium sulfate, sodium sulfate, and lithium sulfate. Examples of carbonates include potassium carbonate, sodium carbonate, and lithium carbonate. Examples of nitrates include potassium nitrate, sodium nitrate, and lithium nitrate. Examples of oxides include potassium oxide, sodium oxide, and lithium oxide. Examples of hydroxides include potassium hydroxide, sodium hydroxide, and lithium hydroxide.

[0023] One of the reasons for the high melting point of steelmaking slag is that the silicon dioxide contained in steelmaking slag is a covalently bonded crystal with a mesh-like network, providing a stable structure. On the other hand, alkali metals, which are network modifier oxides, have strong ionic bonds and act to break the mesh structure formed by silicon dioxide. Therefore, adding alkali metal compounds to steelmaking slag is thought to have the effect of lowering the melting point of steelmaking slag.

[0024] The ratio of the amount of the alkali metal compound added to the reaction raw materials is preferably 0.02 to 0.60, more preferably 0.10 to 0.50, and even more preferably 0.20 to 0.40. By setting the ratio of the amount of the alkali metal compound added to the reaction raw materials within this range, the liquid phase ratio of the reaction raw materials can be increased even at low temperatures, and phosphorus oxide in the steelmaking slag can be reduced at low temperatures.

[0025] In the phosphorus recovery method of this embodiment, a reducing agent containing at least one of metallic silicon and metallic aluminum and an alkali metal compound are added to steelmaking slag containing phosphorus oxide. The order of mixing the reducing agent and alkali metal compound when adding them to the steelmaking slag is not particularly limited. The steelmaking slag, reducing agent, and alkali metal compound may be mixed simultaneously, or the steelmaking slag and reducing agent may be mixed first, followed by the alkali metal compound. Furthermore, in order to improve contact between the reaction raw materials and promote the reduction reaction of the phosphorus oxide, the steelmaking slag, reducing agent, and alkali metal compound may be mixed and pulverized using a ball mill, bead mill, jet mill, or the like.

[0026] In the phosphorus recovery method of this embodiment, a reducing agent containing at least one of metallic silicon and metallic aluminum and an alkali metal compound is added to steelmaking slag containing phosphorus oxides, and the phosphorus oxides are reduced by the reducing agent in a state containing a liquid phase. The melting point of the steelmaking slag is then lowered by adding the alkali metal compound to the steelmaking slag. This allows the liquid phase ratio of the reaction raw materials to be increased even at low temperatures, providing a phosphorus recovery method that enables the reduction of phosphorus oxides in the steelmaking slag at low temperatures.

[0027] <Alkali Metal Compound Addition Experiment> The effect of adding an alkali metal compound to steelmaking slag on the melting point of the steelmaking slag was investigated. Figure 1 shows the results of adding potassium carbonate, an alkali metal compound, to 1.0 g of steelmaking slag containing phosphorus oxide, as shown in Table 1, and mixing the slag at 800°C for 2 hours (Samples No. 1 to No. 4). When no potassium carbonate was added to 1.0 g of steelmaking slag (Sample No. 1) or when 0.1 g of potassium carbonate was added (Sample No. 2), the slag did not melt. On the other hand, when 0.3 g of potassium carbonate was added to 1.0 g of steelmaking slag (Sample No. 3) or when 0.6 g of potassium carbonate was added (Sample No. 4), the slag melted. This indicates that adding 0.3 g or more of potassium carbonate to 1.0 g of steelmaking slag, i.e., adding 30 mass% or more of potassium carbonate to the steelmaking slag, lowers the melting point of the steelmaking slag.

[0028]

[0029] <Phosphorus Recovery Experiment> Next, the effect on phosphorus yield of adding an alkali metal compound to the reaction raw materials was confirmed. First, steelmaking slag containing phosphorus oxide and metallic silicon as a reducing agent were mixed and pulverized in a ball mill at a mass ratio of 1:2 as the reaction raw materials. Then, potassium carbonate, an alkali metal compound, was added to the reaction raw materials as shown in Table 2 and mixed to adjust the total amount of the reaction raw materials and potassium carbonate to 5.0 g, thereby producing Samples No. 5 to No. 8.

[0030]

[0031] Furthermore, steelmaking slag and metallic silicon were mixed as reaction raw materials in a mass ratio of 1:1 and pulverized in a ball mill. Potassium carbonate was then added to the reaction raw materials as shown in Table 3 and mixed to adjust the total amount of the reaction raw materials and potassium carbonate to 5.0 g, thereby producing Samples No. 9 to No. 15.

[0032]

[0033] Then, samples No. 5 to No. 15 prepared as described above were heated to conduct a phosphorus recovery experiment. Specifically, using the experimental apparatus 10 shown in FIG. 2 , sample 12 was placed in a quartz tube 11 (inner diameter 42 mm) and heated from the outside of the quartz tube 11 in an electric furnace 13 at 800°C for 2 hours. Samples 12 were samples No. 5 to No. 15 prepared as described above. Both ends of the quartz tube 11 were plugged with silicone plugs 14 penetrated by glass tubes 15. On the left side of FIG. 2 , argon gas supplied from an argon gas cylinder 20 was passed through a polyethylene tube 16 connected to one of the glass tubes 15 via a pressure regulator 22 and a flowmeter 21, filling the electric furnace 13 with argon gas. On the right side of FIG. 2 , phosphorus-containing gas generated in the quartz tube 11 was passed through the polyethylene tube 16 connected to the other glass tube 15, and the generated gas was blown into water stored in a container 30.

[0034] The phosphorus yield in the phosphorus recovery experiment can be calculated from the following formula (2): Phosphorus yield (wt%) = (W ａ X ａ -W ｂ X ｂ ) / W ａ X ａ ×100 (2)

[0035] In the above formula (2), the weight of the sample 12 before the heat treatment is W ａ (g), the weight of the sample 12 after the heat treatment is W ｂ (g) Phosphorus oxide (P) in sample 12 before heating ２ O ５ ) weight fraction X ａ , phosphorus oxide (P ２ O ５ ) weight fraction X b It was decided.

[0036] Figure 3 shows a graph illustrating the relationship between the ratio of potassium carbonate added to the reactant materials (steelmaking slag and metallic silicon) and the phosphorus yield. The dots in the graph correspond to Samples No. 5 to No. 8 in Table 2. As shown in Figure 3, when no potassium carbonate was added (Sample No. 5), the phosphorus yield was 8.1 wt %. However, when the ratio of potassium carbonate added to the reactant materials was 0.3 wt % (Sample No. 7), the phosphorus yield increased to 27.3 wt %, an increase of approximately 3.4 times. The reason for the improved phosphorus yield is thought to be that, as described above, the addition of potassium carbonate lowers the melting point of the steelmaking slag, increasing the liquid phase fraction of the reactant materials even at a low temperature of 800°C. This facilitates contact between the phosphorus oxide and metallic silicon in the reactant materials, thereby accelerating the reaction.

[0037] On the other hand, Figure 4 shows a graph illustrating the relationship between the ratio of potassium carbonate to the reactant and the phosphorus yield. The dots in the graph correspond to Samples No. 9 to No. 12 (n = 1 sample) and Samples No. 13 to No. 15 (n = 2 sample) in Table 3, respectively. The dotted line in Figure 4 represents an approximate curve. As shown in Figure 4, the phosphorus yield was 26.1 wt% when potassium carbonate was not added (Sample No. 9). However, when the ratio of potassium carbonate to the reactant was 0.3 wt% (Sample No. 11), the phosphorus yield increased by approximately 2.5 times to 64.0 wt%. The phosphorus yield reached its maximum when the ratio of potassium carbonate to the reactant was 0.3 wt%. The improved phosphorus yield is believed to be due to the fact that the addition of potassium carbonate lowers the melting point of the steelmaking slag, increasing the liquid phase fraction of the reactant even at a low temperature of 800°C, which facilitates contact between the phosphorus oxide and metallic silicon in the reactant, thereby accelerating the reaction. Furthermore, when the mass ratio of steelmaking slag to metallic silicon was 1:1 as shown in Figure 4, the phosphorus yield was significantly improved compared to when the mass ratio was 1:2 as shown in Figure 3. Thus, it was found that the ratio of the amount of reducing agent added to the steelmaking slag is preferably 0.15 to 8.0, and more preferably 0.2 to 1.5.

[0038] <Calculation of Liquid Phase Fraction> Furthermore, it was verified that the addition of an alkali metal compound to the reactant materials enabled the reduction of phosphorus oxide in steelmaking slag at a low temperature of 800°C, due to the increased liquid phase fraction of the reactant materials. Figure 5 shows the results of calculations using calculation software on the relationship between the ratio of potassium carbonate added to the reactant materials (steelmaking slag and metallic silicon) and the liquid phase fraction. Specifically, the liquid phase fraction was calculated using calculation software FactSage 8.1 (manufactured by Computational Mechanics Research Center, Inc.) when the ratio of potassium carbonate added to the reactant materials was changed.

[0039] FactSage 8.1 is software for predicting the thermodynamic equilibrium state of a multi-component system. To calculate the liquid phase ratio, the composition, temperature, and pressure of a given object were first input into FactSage 8.1 to calculate the equilibrium composition. The amount of solid phase Y of component j was calculated as the equilibrium composition. ｊ , the amount of liquid phase of component k Z ｋ The liquid phase ratio (mol%) was then calculated from the formula (3). Liquid phase ratio (mol%) = (Σ ｋ Z ｋ ) / (Σ ｊ Y ｊ +Σ ｋ Z ｋ ) x 100 (3)

[0040] As shown in Figure 5, the liquid fraction increased sharply when the ratio of potassium carbonate to the reactant exceeded 0.15 (liquid fraction: 30%). The maximum liquid fraction reached 88% to 89% when the ratio of potassium carbonate to the reactant was 0.30 to 0.37. This, combined with the results of the phosphorus recovery experiment, indicates that the phosphorus yield improves with an increase in the liquid fraction of the reactant. In particular, the ratio of potassium carbonate to the reactant is preferably 0.02 to 0.60, more preferably 0.10 to 0.50, and even more preferably 0.20 to 0.40. Furthermore, the liquid fraction is preferably 4% to 90%, more preferably 10% to 90%, and even more preferably 70% to 90%. By adjusting the ratio within these ranges, the liquid fraction of the reactant can be increased even at temperatures as low as 800°C, enabling the reduction of phosphorus oxides in steelmaking slag. When the ratio of the amount of potassium carbonate added to the reaction raw materials is 0.60, the phosphorus yield in Fig. 3 is as high as 23% by weight, despite the liquid phase ratio being 0% in Fig. 5. Similarly, when the ratio of the amount of potassium carbonate added to the reaction raw materials is 0.50, the phosphorus yield in Fig. 4 is as high as 53% by weight or 57% by mass, despite the liquid phase ratio being 0% in Fig. 5. This is thought to be because the reaction raw materials and potassium carbonate were not mixed completely uniformly, resulting in localized areas with a high liquid phase ratio, which promoted the reduction reaction.

[0041] As described above, by adding an alkali metal compound to the reaction raw materials, the melting point of the steelmaking slag can be lowered. Furthermore, at a low temperature of 800°C, the liquid phase ratio of the reaction raw materials can be increased compared to conventional methods that do not add an alkali metal compound, making it possible to reduce phosphorus oxides in the steelmaking slag at low temperatures.

[0042] Although several embodiments have been described, the embodiments can be modified or varied based on the above disclosure. All components of the above embodiments and all features described in the claims may be individually extracted and combined, unless they contradict each other.

[0043] The present disclosure can contribute, for example, to Goal 7 of the Sustainable Development Goals (SDGs) led by the United Nations, "Ensure access to affordable, reliable, sustainable and modern energy for all," and Goal 12, "Ensure sustainable consumption and production patterns."

[0044] The entire contents of Japanese Patent Application No. 2023-190933 (filing date: November 8, 2023) are incorporated herein by reference.

Claims

1. A method for recovering phosphorus, comprising adding a reducing agent containing at least one of metallic silicon and metallic aluminum, and an alkali metal compound, to steelmaking slag containing phosphorus oxide, thereby reducing the phosphorus oxide with the reducing agent in a liquid phase.

2. The phosphorus recovery method according to claim 1, wherein the alkali metal compound contains at least one selected from the group consisting of sulfates, carbonates, nitrates, oxides, and hydroxides.

3. The phosphorus recovery method according to claim 1 or 2, wherein the ratio of the amount of alkali metal compound added to the total amount of steelmaking slag and the reducing agent added is 0.02 or more and 0.60 or less.

4. The phosphorus recovery method according to claim 1 or 2, wherein the reaction temperature when reducing the phosphorus oxide with the reducing agent in a liquid phase is lower than that when the alkali metal compound is not added.

5. The phosphorus recovery method according to claim 1 or 2, wherein the reducing agent contains metallic silicon.