Phosphorus recovery method

Abstract

This invention provides a phosphorus recovery method that enables the reduction of phosphorus oxides in steelmaking slag at low temperatures. [Solution] The phosphorus recovery method includes a step of reducing phosphorus oxides with a reducing agent containing at least one of metallic silicon and metallic aluminum, and optionally an additive containing aluminum, in a liquid phase, by adding the reducing agent to steelmaking slag containing phosphorus oxides, wherein the content of aluminum relative to the total of the steelmaking slag, reducing agent and additive is 0.8 mol% or more.

Description

[Technical Field]

[0001] This disclosure relates to a method for recovering phosphorus. [Background technology]

[0002] Phosphorus is widely used in chemical fertilizers, industrial chemicals, semiconductors, and battery materials. However, in recent years, concerns have arisen regarding the declining quality and depletion of phosphate ore, making resource securing a challenge. On the other hand, in the steel manufacturing process, phosphorus negatively affects the properties of steel, so it is discharged as a phosphorus-containing steelmaking slag (by-product). Since steelmaking slag does not contain harmful heavy metals or radioactive elements, it is attracting attention as a promising alternative resource to phosphate ore.

[0003] Therefore, research is underway on the recovery and recycling of phosphorus contained in steelmaking slag. Patent Document 1 discloses a method in which iron oxides and phosphorus oxides in steelmaking slag are reduced using a reducing agent, and the phosphorus produced by the reduction of phosphorus oxides is dissolved in iron produced by the reduction of iron oxides, thereby recovering it from the steelmaking slag as molten phosphorus-containing molten iron. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 5660166 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Conventional methods, such as those described in Patent Document 1, require heating the steelmaking slag to a temperature at which it melts when reducing phosphorus oxides, resulting in high equipment costs and running costs for high-temperature heating. Therefore, a challenge has been to lower the reaction temperature for reducing phosphorus oxides in order to make the phosphorus recovery process economically viable.

[0006] This disclosure aims to provide a phosphorus recovery method that can reduce phosphorus oxides in steelmaking slag at low temperatures. [Means for solving the problem]

[0007] The phosphorus recovery method according to this disclosure includes a step of reducing phosphorus oxides with a reducing agent containing at least one of metallic silicon and metallic aluminum, and optionally an additive containing aluminum, in a liquid phase, to steelmaking slag containing phosphorus oxides. The content of aluminum relative to the total of the steelmaking slag, reducing agent, and additive is 0.8 mol% or more.

[0008] Additives may be added during the reduction process.

[0009] The ratio of the amount of reducing agent added to the steelmaking slag may be 0.15 to 8 by weight.

[0010] The reducing agent may contain metallic silicon.

[0011] The reducing agent may contain metallic silicon contained in silicon sludge.

[0012] The additive may contain at least one selected from the group consisting of aluminum oxide, metallic aluminum, aluminum hydroxide, aluminum dross, and combustion ash. [Effects of the Invention]

[0013] This disclosure provides a phosphorus recovery method that can reduce phosphorus oxides in steelmaking slag at low temperatures. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic diagram showing an experimental apparatus for recovering phosphorus by heating a sample containing steelmaking slag. [Figure 2] This graph shows the relationship between the heating time of the sample and the phosphorus yield. [Figure 3] It is a graph showing the relationship between the temperature of the sample and the liquid phase ratio. [Figure 4] It is a graph showing the relationship between the weight ratio of metallic silicon and the phosphorus recovery rate.

Embodiments for Carrying Out the Invention

[0015] Hereinafter, several exemplary embodiments will be described with reference to the drawings. Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation and may be different from the actual ratios.

[0016] Conventionally, even in the phosphorus recovery method using steelmaking slag as a raw material, a reducing agent has been added. For example, when metallic silicon is used as a reducing agent, phosphorus has been recovered by gasifying yellow phosphorus as represented by the following reaction formula (1). 2(3CaO·P2O5)+5Si→2P2+5SiO2+6CaO (1)

[0017] However, in the conventional phosphorus recovery method, a problem has been that the reaction temperature for reduction is high. Since such a reduction reaction proceeds in a state including a liquid phase, it is preferable that the steelmaking slag melts and is in a state including a liquid phase.

[0018] The reason why the reduction reaction of the phosphoric oxide in the steelmaking slag does not proceed sufficiently at low temperatures is considered to be that at low temperatures, the steelmaking slag exists in a solid state and the contact between the steelmaking slag and the reducing agent is poor. Therefore, it is preferable to raise the temperature until the ratio at which the steelmaking slag melts becomes high, that is, the temperature at which the liquid phase ratio is within a predetermined range. Note that the liquid phase ratio indicates the proportion of the liquid phase in the reaction raw materials including the steelmaking slag, the reducing agent, and additives added as necessary.

[0019] Therefore, the phosphorus recovery method according to this embodiment includes a step of reducing the phosphorus oxides with the reducing agent in a liquid phase by adding a reducing agent and optionally an additive containing aluminum to steelmaking slag containing phosphorus oxides. The molar ratio of the content of metallic aluminum to the total of the steelmaking slag, reducing agent, and additive is 0.8 mol% or more. By this method, phosphorus oxides in steelmaking slag can be reduced at low temperatures. The phosphorus recovery method according to this embodiment will be described in detail below.

[0020] Steelmaking slag is slag produced simultaneously when steel is manufactured by refining molten iron, scrap, etc. Steelmaking slag may include at least one selected from the group consisting of converter slag, electric furnace slag, molten iron pretreatment slag, and secondary refining slag. Converter slag is slag produced simultaneously when steel is manufactured by refining pig iron in a converter. Electric furnace slag is slag produced simultaneously when steel is manufactured in an electric furnace using scrap as the main raw material. Molten iron pretreatment slag is slag produced when molten iron is treated with desulfurization, desiliconization, or dephosphorization before being placed in a converter, etc. Molten iron pretreatment slag may include at least one slag selected from the group consisting of desulfurization slag, desiliconization slag, and dephosphorization slag. Secondary refining slag is slag produced when molten steel tapped from a converter, etc. is treated with desulfurization, dephosphorization, or degassing.

[0021] Steelmaking slag, generated during the steel manufacturing process, contains phosphorus oxides (P2O5). The phosphorus oxide (P2O5) content in steelmaking slag varies significantly depending on the process, but is generally around 0.05 to 5% by weight. Steelmaking slag may also contain aluminum oxide (Al2O3).

[0022] The phosphorus recovery method of this embodiment involves adding a reducing agent to steelmaking slag containing phosphorus oxides. The reducing agent is added to reduce the phosphorus oxides (P2O5) contained in the steelmaking slag and generate phosphorus. To improve contact between the steelmaking slag and the reducing agent and promote the reduction reaction of phosphorus oxides, the steelmaking slag and the reducing agent may be mixed and crushed using a ball mill, bead mill, jet mill, or the like.

[0023] The reducing agent contains at least one of metallic silicon and metallic aluminum. From the viewpoint of reducing power, the reducing agent preferably contains metallic silicon. The reducing agent may contain 95% by weight of at least one of metallic silicon and metallic aluminum, 97% or more by weight, or 99% or more by weight. The reducing agent may contain 100% by weight or less of at least one of metallic silicon and metallic aluminum.

[0024] The reducing agent may contain metallic silicon contained in the silicon sludge. Silicon sludge is the cutting debris of silicon wafers discharged from the silicon wafer cutting process. The silicon sludge may contain 95% by weight or more of metallic silicon relative to the total content of metal elements. The silicon sludge may contain 99% by weight or less, or 98% by weight or less, of metallic silicon relative to the total content of metal elements. The silicon sludge may contain less than 5% by weight, less than 4% by weight, less than 3% by weight, less than 2% by weight, or less than 1% by weight of aluminum oxide relative to the total content of metal elements. The reducing agent may contain 50% by weight or more, 60% by weight or more, 70% by weight or more, 80% by weight or more, or 90% by weight or more of silicon sludge. The reducing agent may also contain 100% by weight or less of silicon sludge. For example, if the silicon sludge contains aluminum oxide, the aluminum oxide contained in the silicon sludge may be used as at least part of the additive having the element aluminum.

[0025] The ratio of the amount of reducing agent added to the steelmaking slag is preferably 0.15 to 8 by weight. By setting the ratio of the amount of reducing agent added to 0.15 or more by weight, a sufficient phosphorus yield can be ensured without hindering the effect of lowering the melting point of the steelmaking slag. Furthermore, by setting the ratio of the amount of reducing agent added to 8 or less by weight, the amount of reducing agent used can be reduced, and the energy required to heat the additive from room temperature to the heat treatment temperature can be reduced. In particular, since there is little surplus metallic silicon circulating in the market, reducing the amount of reducing agent used can facilitate the implementation of the phosphorus recovery method according to this embodiment. The ratio of the amount of reducing agent added to the steelmaking slag may be 0.2 or more by weight, or 0.23 or more. Furthermore, the ratio of the amount of reducing agent added to the steelmaking slag may be 5 or less by weight, 3 or less, 1.5 or less, 1 or less, 0.5 or less, or 0.3 or less.

[0026] In the phosphorus recovery method according to this embodiment, an additive containing aluminum is optionally added to the steelmaking slag containing phosphorus oxides. By adding such an additive to the steelmaking slag, the liquid phase ratio of the reaction raw materials, including the steelmaking slag, reducing agent, and additive, can be increased even at low temperatures, and phosphorus oxides in the steelmaking slag can be reduced at low temperatures. However, as will be described later, for example, if the steelmaking slag contains a sufficient amount of aluminum oxide, it is not necessary to add the additive to the steelmaking slag.

[0027] Additives containing the element aluminum may include at least one selected from the group consisting of aluminum oxide (Al2O3), metallic aluminum, aluminum hydroxide (Al(OH3)), aluminum dross, and combustion ash. By using these additives, the liquid phase ratio of the reaction raw materials can be efficiently increased. Aluminum dross is the residue that floats on the surface of the molten metal and the residue at the bottom of the furnace when aluminum or aluminum alloys are melted. Combustion ash may be, for example, ash produced by burning biomass or coal.

[0028] The amount of additive added per 100 parts by weight of the total steelmaking slag and reducing agent is not particularly limited, but is preferably 6 parts by weight or less. By adding 6 parts by weight or less of additive, the amount of additive used can be reduced, and the energy required to heat the additive from room temperature to the heat treatment temperature can be reduced. The amount of additive added may be 5 parts by weight or less, 4 parts by weight or less, 3 parts by weight or less, 2 parts by weight or less, or 1 part by weight or less. In addition, the amount of additive added per 100 parts by weight of the total steelmaking slag and reducing agent may be 0 parts by weight or more, 1 part by weight or more, 2 parts by weight or more, 3 parts by weight or more, 4 parts by weight or more, or 5 parts by weight or more. The amount of additive added may be determined considering the aluminum content contained in the steelmaking slag. In addition, the content of aluminum element in the additive per 100 moles of the total steelmaking slag and reducing agent may be 0.1 moles or more, 0.2 moles or more, 0.3 moles or more, 0.5 moles or more, 0.9 moles or more, or 1.5 moles or more. The amount of aluminum element in the additive relative to 100 moles of steelmaking slag and reducing agent may be 3 moles or less, 2 moles or less, 1 mole or less, or 0.5 moles or less.

[0029] In the phosphorus recovery method according to this embodiment, the content of aluminum element relative to the total of steelmaking slag, reducing agent, and additives is 0.8 mol% or more. By setting the aluminum element content to 0.8 mol% or more, the liquid phase ratio can be improved. The aluminum element content may be 1 mol% or more, 1.5 mol% or more, or 2 mol% or more. There is no particular upper limit to the aluminum element content. For example, the aluminum element content may be 8 mol% or less, 6 mol% or less, 4 mol% or less, or 3 mol% or less.

[0030] It should be noted that typical steelmaking slag contains aluminum oxide, and the aluminum in the aluminum oxide contained in the steelmaking slag can also contribute to the reduction of phosphorus oxides. However, the aluminum content in steelmaking slag varies depending on the conditions under which the steelmaking slag is produced. Therefore, if the amount of aluminum in the steelmaking slag is low, the liquid phase ratio can be improved by adding an additive containing aluminum.

[0031] As described above, the phosphorus recovery method according to this embodiment includes a step of reducing phosphorus oxides with a reducing agent containing at least one of metallic silicon and metallic aluminum, and optionally an additive containing aluminum, in a liquid phase, to steelmaking slag containing phosphorus oxides. The content of aluminum relative to the total of the steelmaking slag, reducing agent, and additive is 0.8 mol% or more.

[0032] Therefore, according to the phosphorus recovery method of this embodiment, the liquid phase ratio of the reaction raw materials can be increased even at low temperatures, and phosphorus oxides in steelmaking slag can be reduced at low temperatures. [Examples]

[0033] The embodiments will be described in more detail below, but the embodiments are not limited to these examples.

[0034] [Test Example 1] First, steelmaking slag containing phosphorus oxides, metallic silicon as a reducing agent, and potassium carbonate (K2CO3) were weighed in a weight ratio of steelmaking slag:metallic silicon:potassium carbonate (K2CO3) = 1:0.25:0.3. These were then mixed and ground using a ball mill to prepare the sample. The steelmaking slag contained 1.32 mol% (2.2 wt%) of aluminum oxide (Al2O3). As raw materials for metallic silicon, a reaction reagent (Fujifilm Wako Pure Chemical Industries, Ltd., silicon: purity 99.9%) and silicon sludge were used. The composition of the silicon sludge was SiO2: 96.3 wt%, Al2O3: 2.6 wt%, and K2O: 0.5 wt%. That is, the metallic composition of the silicon sludge was Si: 95.6 wt%, Al: 2.9 wt%, and K: 0.9 wt%.

[0035] Next, the sample prepared as described above was heated using the experimental apparatus 10 shown in FIG. 1 to conduct a phosphorus recovery experiment. Specifically, the sample 12 was placed in a quartz tube 11 (inner diameter 42 mm), and the sample was heated at 1200 °C for 0 hours, 0.25 hours, 0.5 hours, or 1 hour using an electric furnace 13 arranged outside the quartz tube 11. Both sides of the quartz tube 11 were sealed with silicon stoppers 14 penetrated by glass tubes 15. A polyethylene tube 16 was connected to the glass tube 15 on the upstream side of the quartz tube 11. An argon gas cylinder 20 was connected to one end of the polyethylene tube 16, and the argon gas supplied from the argon gas cylinder 20 was filled into the electric furnace 13 after adjusting the flow rate with a flow meter 21 and a pressure regulator 22. A polyethylene tube 16 was connected to the glass tube 15 on the downstream side of the quartz tube 11, and the gas containing phosphorus generated in the quartz tube 11 was passed through the polyethylene tube 16. The gas containing phosphorus was blown into the water stored in the container 30 for recovery.

[0036] The phosphorus recovery rate in the phosphorus recovery experiment was calculated from the following formula (1). Phosphorus recovery rate (wt%) = (W a X a -W b X b ) / W a X a × 100 (1)

[0037] In the above formula (1), the weight of the sample 12 before heating is W a (g), the weight of the sample 12 after heating is W b (g), the weight fraction of phosphorus oxide (P2O5) in the sample 12 before heating is X a , and the weight fraction of phosphorus oxide (P2O5) in the sample 12 after heating is X b .

[0038] <(g), the weight of the sample 12 after heating is W Figure 2 is a graph showing the relationship between the heating time of the sample and the phosphorus yield. As shown in Figure 2, when the heating time was 0.25 hours and 0.5 hours, it was confirmed that the phosphorus yield was higher when phosphorus was recovered using silicon sludge than when phosphorus was recovered using silicon reagent. On the other hand, when the heating time was 1 hour, there was no significant difference in phosphorus yield between silicon sludge and silicon reagent, and it is thought that the upper limit of the phosphorus yield that can be recovered in the phosphorus recovery experiment was reached. From these results, it was estimated that the reason why the phosphorus yield was higher when using silicon sludge was that the aluminum oxide contained in the silicon sludge reduced the liquid phase fraction of the sample.

[0039] [Test Example 2] To confirm whether the higher phosphorus yield when using silicon sludge in the phosphorus recovery experiment is due to the aluminum oxide contained in the silicon sludge reducing the liquid phase fraction of the sample, the sample heated in Test Example 1 was evaluated using SEM-EDX. The evaluation results when using silicon reagent are shown in Table 1. The evaluation results when using silicon sludge are shown in Table 2.

[0040] [Table 1]

[0041] [Table 2]

[0042] Regarding the spherical particles of the heated sample, it was confirmed that the coexisting elements after rapid cooling for 30 minutes or more were Ca-P when using a silicon reagent, while they were Fe-P when using silicon sludge. This is presumed to be because the Ca-P originally present in the steelmaking slag remains coexisting as Ca-P when using a silicon reagent. On the other hand, when using silicon sludge, it is presumed that the phosphorus is reduced, separating the Ca-P, and Fe, which has a high affinity for P in the steelmaking slag, combines with it, resulting in the confirmation of the coexistence of Fe-P. Furthermore, regarding the spherical particles of the heated sample, when using a silicon reagent, the diameter of the spherical particles became 40 μm after 30 minutes of rapid cooling and 50 μm after 60 minutes of rapid cooling. On the other hand, regarding the spherical particles of the heated sample, when using silicon sludge, the diameter of the spherical particles became 100 μm after 30 minutes of rapid cooling and 200 μm after 60 minutes of rapid cooling. These results suggest that using silicon sludge tends to result in a higher liquid phase fraction of the sample and promotes phosphorus reduction compared to using silicon reagents. In other words, it is suggested that the aluminum oxide contained in the silicon sludge contributes to the decrease in the liquid phase fraction of the sample and promotes phosphorus reduction.

[0043] [Test Example 3] Next, the liquid phase fraction was calculated for samples prepared by adding metallic silicon as a reducing agent and aluminum oxide (Al2O3) as an additive to steelmaking slag containing phosphorus oxide. Specifically, a mixed raw material was prepared so that the weight ratio of steelmaking slag to metallic silicon was 1:0.25. Then, the liquid phase fraction was calculated for samples to which 0 parts by weight (Comparative Example 1), 1 part by weight (Example 1), 3 parts by weight (Example 2), and 5 parts by weight (Example 3) of aluminum oxide were added per 100 parts by weight of the mixed raw material. That is, the liquid phase fraction was calculated for samples to which 0 moles (0 moles of aluminum), 0.6 moles (0.32 moles of aluminum), 1.8 moles (0.96 moles of aluminum), and 3 moles (1.6 moles of aluminum) of aluminum oxide were added per 100 moles of the mixed raw material. It was assumed that the steelmaking slag contained 1.32 mol% (0.70 mol%) of aluminum oxide (Al2O3). Therefore, the content of elemental aluminum relative to the total of steelmaking slag, reducing agent, and additives was 0.70 mol% in Comparative Example 1, 1.02 mol% in Example 1, 1.66 mol% in Example 2, and 2.3 mol% in Example 3. The liquid phase fraction was calculated using FactSage 8.1, a calculation software manufactured by the Computational Mechanics Research Center Co., Ltd.

[0044] [Table 3]

[0045] FactSage 8.1 is software for predicting the thermodynamic equilibrium state of a multi-component system. To calculate the liquid phase fraction, the composition, temperature, and pressure of a given object were input into FactSage 8.1 to calculate the equilibrium composition. The amount of solid phase Y of component j was then defined as the equilibrium composition. j , amount of component k in the liquid phase Z k These were obtained. Then, the liquid phase ratio (mol%) was obtained from equation (2). Liquid phase ratio (mol%) = (Σ k Z k ) / (Σ j Y j +Σ k Z k ) × 100 (2)

[0046] Figure 3 is a graph showing the relationship between the temperature of the sample and the liquidus fraction. From the results in Figure 3, it can be seen that when aluminum is added to the steelmaking slag, the temperature at which the liquid phase begins to form in the sample is lower compared to when aluminum is not added to the steelmaking slag. Furthermore, the liquidus fraction at 1000°C was 38.5% in Comparative Example 1, while it was approximately 51% in Example 1. In Example 2, the liquidus fraction was 75.3%, which was nearly twice as high as when aluminum was not added. From these results, it was confirmed that the liquidus fraction of the sample can be increased and phosphorus oxides in steelmaking slag can be reduced at low temperatures even without adding alkali metal compounds such as potassium carbonate (K2CO3).

[0047] [Test Example 4] Next, steelmaking slag containing phosphorus oxide and potassium carbonate (K2CO3) were weighed in a weight ratio of steelmaking slag:potassium carbonate (K2CO3) = 1:0.3. These were mixed and ground using a ball mill to prepare a mixed raw material. In addition, the mixed raw material and metallic silicon were weighed in a weight ratio of 1 part steelmaking slag to 0, 0.25, 0.5, 0.75, or 1 part metallic silicon. These were then mixed and ground using a ball mill to prepare a sample. The steelmaking slag contains 1.32 mol% (2.2 wt%) aluminum oxide (Al2O3). The metallic silicon raw material used was a reaction reagent (Fujifilm Wako Pure Chemical Industries, Ltd., silicon: purity 99.9%). The materials prepared in this way were subjected to a phosphorus recovery experiment in the same manner as in Test Example 1, and the phosphorus yield was calculated.

[0048] Figure 4 is a graph showing the relationship between the weight ratio of metallic silicon to steelmaking slag (weight ratio 1) and potassium carbonate (weight ratio 0.3) in the sample, and the phosphorus yield. As shown in Figure 4, it was confirmed that the phosphorus yield increased when metallic silicon was added. In this test example, the relationship between the weight ratio of metallic silicon and the phosphorus yield was evaluated when potassium carbonate was added. However, potassium carbonate plays a role in reducing the liquid phase fraction of the sample. Therefore, it is expected that a similar trend will be observed even if potassium carbonate is replaced with an additive containing aluminum elements such as aluminum oxide. In other words, it is expected that the phosphorus yield will increase when metallic silicon is added to a weight ratio of steelmaking slag of 1.

[0049] Although several embodiments have been described, it is possible to modify or transform the embodiments based on the above disclosure. All components of the above embodiments, and all features described in the claims, may be taken individually and combined, provided that they do not conflict with each other.

[0050] This disclosure can contribute, for example, to United Nations Sustainable Development Goal (SDG) 7, "Ensure access to affordable, reliable, sustainable, and modern energy for all," and Goal 12, "Ensure sustainable consumption and production patterns."

Claims

1. The process includes adding a reducing agent containing at least one of metallic silicon and metallic aluminum, and optionally an additive containing aluminum, to steelmaking slag containing phosphorus oxide, thereby reducing the phosphorus oxide with the reducing agent in a liquid phase. A method for recovering phosphorus, wherein the content of aluminum element relative to the total of the steelmaking slag, the reducing agent, and the additive is 0.8 mol% or more.

2. The phosphorus recovery method according to claim 1, wherein the additive is added in the reduction step.

3. The phosphorus recovery method according to claim 1 or 2, wherein the ratio of the amount of reducing agent added to the steelmaking slag is 0.15 or more and 8 or less by weight.

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

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

6. The phosphorus recovery method according to claim 1 or 2, wherein the additive contains at least one selected from the group consisting of aluminum oxide, metallic aluminum, aluminum hydroxide, aluminum dross, and combustion ash.

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