Method for disposing of hexavalent chromium-containing slag

Abstract

The objective is to provide a method for treating hexavalent chromium-containing slag, including steelmaking slag, that can reliably remove hexavalent chromium from various types of slag by utilizing leaching with carbonated water and precipitation by degassing. [Solution] Slag containing hexavalent chromium is leached with carbonated water, and the resulting filtrate is degassed to precipitate calcium components as carbonates, while hexavalent chromium migrates to and separates from the precipitate. The treated liquid after separation is recycled and reused in the leaching process. Furthermore, after releaching the obtained precipitates, high-purity carbonates are recovered after selective removal of hexavalent chromium.

Description

[Technical Field]

[0001] This invention relates to a method for treating hexavalent chromium-containing slag. More specifically, it relates to a method for treating hexavalent chromium-containing slag that can reliably remove hexavalent chromium from various types of slag, including steelmaking slag, by utilizing leaching with carbonated water and precipitation by degassing. [Background technology]

[0002] Steelmaking slag, particularly slag generated during the manufacturing process of stainless steel, contains valuable metals such as nickel and chromium, making its recovery highly desirable. However, slag can contain harmful substances such as hexavalent chromium, and concerns about their leaching restrict the use of the residue, posing a challenge.

[0003] Therefore, various technologies have been developed to recover valuable metals contained in slag. For example, Patent Document 1 discloses a method for processing stainless steel slag.

[0004] The technology disclosed in Patent Document 1 is characterized by solidifying stainless steel slag, then crushing it in multiple stages using primary, secondary, and tertiary crushers, and recovering useful metals such as nickel and chromium contained in the slag by physically separating the resulting crushed material through gravity separation and then magnetic separation. This has been shown to contribute to the recovery of valuable metals, reducing manufacturing costs and the efficient use of resources. The remaining material after the metals have been separated is to be used for landfill or as ballast in asphalt construction, etc.

[0005] Furthermore, Patent Document 2 discloses a wet method for recovering chromium from chromium-containing slag, aiming to achieve both the economic value of chromium recovery and the environmental value of preventing hexavalent chromium leaching from the residue. According to Patent Document 2, while the amount of hexavalent chromium leached from the slag before treatment is 0.08 ml to 1.00 ml, it is shown that by applying the treatment, the amount of hexavalent chromium leached can be significantly reduced to 0.01 ml to 0.04 ml. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 135533 / 1983 [Patent Document 2] Patent No. 49320309 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, the stainless steel slag processing method disclosed in Patent Document 1 focuses primarily on recovering useful metals embedded in the slag through physical methods such as crushing and sorting, and does not address the environmental issue of hexavalent chromium leaching.

[0008] On the other hand, the amount of hexavalent chromium leached out as shown in Patent Document 2 is only about 0.01 ml to 0.04 ml, and its removal effect is not sufficient to reliably meet the stricter environmental standards. There is a need for a technology that can achieve environmental standards more reliably and inexpensively. Furthermore, wet treatments like those in Patent Document 2 generally require large amounts of acid, alkali, or other chemicals, and have the problem of discharging large amounts of wastewater during the treatment process. This wastewater may contain harmful hexavalent chromium, and its treatment and disposal are costly and time-consuming, which significantly impairs the economic efficiency of the entire process and has been a major factor hindering its industrial adoption.

[0009] In response to the above-mentioned problems, the inventors of this invention have developed a method for treating hexavalent chromium-containing slag that achieves both reliable removal of hexavalent chromium and economic efficiency through the recycling and reuse of the treatment solution. This is achieved by leaching the slag containing hexavalent chromium with carbonated water to dissolve the hexavalent chromium and calcium components contained in the slag at a high concentration, and then degassing the filtrate.

[0010] This invention was conceived in view of the above points, and proposes a method for treating hexavalent chromium-containing slag that can reliably remove hexavalent chromium from various types of slag, including steelmaking slag, by utilizing leaching with carbonated water and precipitation by degassing. [Means for solving the problem]

[0011] To achieve the above objective, the present invention provides a method for treating hexavalent chromium-containing slag, comprising the steps of: leaching slag containing hexavalent chromium with carbonated water in which carbon dioxide gas is dissolved to obtain a first suspension in which the hexavalent chromium and calcium components are dissolved from the slag; solid-liquid separation of the first suspension into a first filtrate and a first residue; degassing the first filtrate to obtain a second suspension containing a first precipitate consisting of the hexavalent chromium and the calcium components precipitated as carbonates; and solid-liquid separation of the second suspension into a first treatment solution and the first precipitate.

[0012] This process includes a step in which slag containing hexavalent chromium is leached with carbonated water containing dissolved carbon dioxide to obtain a first suspension in which hexavalent chromium and calcium components are dissolved from the slag. This significantly increases the solubility of components such as calcium oxide (CaO) and calcium chromate (CaCrO4) in the slag, allowing hexavalent chromium and calcium components to dissolve efficiently and simultaneously in the leachate. Furthermore, since the dissolved calcium component is stably retained as a water-soluble bicarbonate, a reaction base is established that reliably induces the phenomenon of calcium components precipitating as carbonates and hexavalent chromium migrating to the precipitate during the subsequent degassing operation.

[0013] Furthermore, by including a solid-liquid separation step of the first suspension into a first filtrate and a first residue, undissolved slag residue (first residue) is quickly separated from the first suspension obtained by leaching, and a first filtrate containing dissolved hexavalent chromium and calcium components is obtained. This allows for the early removal of unwanted residue, thereby improving the reaction efficiency and product quality in the subsequent degassing step.

[0014] Furthermore, by including a step of degassing the first filtrate to obtain a second suspension containing a first precipitate consisting of hexavalent chromium and calcium components precipitated as carbonates, the calcium components in the first filtrate precipitate as carbonates, and hexavalent chromium migrates to these precipitates. This simultaneously achieves reliable separation of harmful hexavalent chromium into the precipitates and efficient recovery of carbonates, and the treated liquid can be purified and reused later.

[0015] Furthermore, by including a step to separate the second suspension into a first processing liquid and a first precipitate, the first processing liquid and the first precipitate are reliably separated from the second suspension containing the carbonate and the first precipitate containing hexavalent chromium. This makes it possible to efficiently recover carbonate, which has high value as a raw material for steel, with high purity.

[0016] Furthermore, when degassing is performed by reducing the pressure, heating, or a combination of reducing pressure and heating of the first filtrate, carbon dioxide can be efficiently removed from the liquid. This reduces the concentration of dissolved carbon dioxide in the first filtrate, increasing the reaction rate of calcium carbonate precipitation and further promoting the transfer of hexavalent chromium to the precipitate.

[0017] Furthermore, if the process for obtaining the first suspension includes a step in which the first treatment liquid is reused as a solvent in carbonated water with dissolved carbon dioxide gas, the supply of water and solvent from external sources can be minimized, resource efficiency can be improved, and the amount of waste liquid generated can be suppressed, significantly reducing the environmental burden. By repeatedly reusing this first treatment liquid, a sustainable and economically efficient treatment cycle can be established.

[0018] Furthermore, it further includes a step of removing hexavalent chromium from the first precipitate, a step of re-leaching the first precipitate with carbonated water in which carbon dioxide gas is dissolved to obtain a third suspension in which hexavalent chromium and calcium components are dissolved, a step of solid-liquid separating the third suspension into a second filtrate and a second residue, a step of selectively removing hexavalent chromium in the second filtrate to obtain a third filtrate, and a step of obtaining a fourth suspension containing a second precipitate composed of a calcium component precipitated as a carbonate by degassing the third filtrate, and a step of solid-liquid separating the fourth suspension into a second treatment liquid and a second precipitate. In this case, it is possible to constitute a purification process for completely removing hexavalent chromium, which is a harmful component remaining in the first precipitate recovered in the degassing and precipitation steps of the main process, using the first precipitate as a raw material.

[0019] In this step, after the precipitate is re-leached with carbonated water, high-purity carbonate (second precipitate) is precipitated and recovered again from the filtrate (third filtrate) from which hexavalent chromium has been removed by a selective separation technique. As a result, the carbonate obtained from the slag can be enhanced to a quality that can be safely reused as a steelmaking raw material, and the resource value can be maximally improved.

[0020] In addition, when the step of selectively removing hexavalent chromium in the second filtrate includes a step of passing the second filtrate through an anion exchange resin, the hexavalent chromium ions present in the second filtrate are selectively and highly efficiently adsorbed and removed from the coexisting high-concentration hydrogen carbonate ions. As a result, only the harmful hexavalent chromium can be separated while the calcium component, which is the main component of the carbonate, remains in the solution, so that the purity of the second precipitate (carbonate) recovered by subsequent degassing and precipitation is dramatically improved.

[0021] In order to achieve the above object, the method for treating hexavalent chromium-containing slag according to the present invention comprises: leaching a first precipitate containing hexavalent chromium and a calcium component precipitated as a carbonate, which is obtained by treating slag containing hexavalent chromium, with carbonated water in which carbon dioxide gas is dissolved to obtain a suspension in which the hexavalent chromium and the calcium component are dissolved; a step of solid-liquid separation of the suspension in which the hexavalent chromium and the calcium component are dissolved into a filtrate and a residue; a step of selectively removing the hexavalent chromium in the filtrate; a step of obtaining a suspension containing a second precipitate composed of the calcium component precipitated as a carbonate by degassing the filtrate; and a step of solid-liquid separation of the suspension containing the second precipitate into a treatment liquid and the second precipitate.

[0022] By the above treatment method, it is possible to establish a unique purification cycle for stably recovering high-purity carbonate using, as a raw material, a first precipitate containing hexavalent chromium obtained in the process of treating slag containing hexavalent chromium. And by passing through a multi-step process of leaching, solid-liquid separation, and degassing, it is possible to improve the quality of the first precipitate containing hexavalent chromium recovered in the main process to a quality that can be safely reused as a steel raw material.

Effects of the Invention

[0023] The method for treating hexavalent chromium-containing slag according to the present invention utilizes leaching with carbonated water and precipitation by degassing, and can surely remove hexavalent chromium from various slags including steelmaking slag.

Brief Description of the Drawings

[0024] [Figure 1] It is a diagram showing a primary treatment process, which is a method for treating hexavalent chromium-containing slag according to an embodiment of the present invention. [Figure 2] It is a diagram showing a secondary treatment process, which is a method for treating hexavalent chromium-containing slag according to an embodiment of the present invention. [Figure 3] It is a graph showing the relationship between the leaching time and the leaching concentration of each component of Ca, Si, and Mg in Example 1. [Figure 4] This graph shows the relationship between the leaching time and leaching concentration of each component, Cr and Al, in Example 1. [Figure 5] This graph shows the effect of the degassing method and degassing time on the precipitation of each component, Ca, Mg, Cr, and Si, in Example 1. [Figure 6] This graph shows the changes in the Ca component after water is passed through the anion exchange resin and after degassing in Example 2. [Figure 7] This graph shows the change in the Cr component after water is passed through the anion exchange resin and after degassing in Example 2. [Modes for carrying out the invention]

[0025] The following describes in detail, with reference to drawings and other materials, a method for treating hexavalent chromium-containing slag according to an embodiment of the present invention, in order to facilitate understanding of the present invention.

[0026] Figures 1 and 2 are process diagrams showing a method for processing hexavalent chromium-containing slag according to an embodiment of the present invention. The method for processing hexavalent chromium-containing slag according to this embodiment consists of a primary processing process (Figure 1) and a secondary processing process (Figure 2). The primary processing process involves treating slag containing hexavalent chromium, such as steelmaking slag, with carbonated water, and precipitating the hexavalent chromium and calcium components contained in the slag as precipitates while circulating the processing liquid. The secondary processing process involves redissolving the precipitate obtained in the primary processing process, selectively removing hexavalent chromium, and then recovering a precipitate mainly composed of calcium carbonate. Details of each process will be described below.

[0027] [Primary Processing] As shown in the process diagram in Figure 1, in the primary processing process, the slag is first leached with carbonated water containing dissolved carbon dioxide (step 11), and the hexavalent chromium and calcium components in the slag are dissolved in a highly concentrated, water-soluble form to obtain a suspension (first suspension). Next, the first suspension is subjected to solid-liquid separation (step 12) to ensure that undissolved residues are removed and a filtrate (first filtrate) is obtained. Degassing (step 13) is performed on this first filtrate to volatilize the carbon dioxide in the liquid, causing the calcium components to precipitate as carbonates and simultaneously transferring the hexavalent chromium to the precipitate (first precipitate). Finally, the suspension containing the first precipitate (second suspension) is subjected to solid-liquid separation (step 14) to separate and recover the first precipitate. The processing liquid obtained by this solid-liquid separation (first processing liquid) has had a significant amount of hexavalent chromium and calcium components removed and is reused as a solvent for new carbonated water.

[0028] (Step 11: Carbonated water extraction) Step 11 is a step in which slag containing hexavalent chromium is leached with carbonated water in which carbon dioxide gas is dissolved to obtain a first suspension in which hexavalent chromium and calcium components are dissolved. When slag is brought into contact with water, calcium oxide and calcium chromate in the slag dissolve, but by using carbonated water in which carbon dioxide is dissolved, the pH of the leachate decreases. This improves the solubility of hexavalent chromium and calcium components. In particular, as shown in equation (1), the calcium component, which is poorly soluble in water, is efficiently dissolved as water-soluble calcium bicarbonate by the action of carbon dioxide and is stably maintained in the liquid at a high solubility concentration. Ca(OH)2 + 2CO2 = Ca(HCO3)2 (1)

[0029] Regarding the leaching conditions in step 11, for example, the water temperature is preferably in the range of 10°C to 50°C from the viewpoint of leaching efficiency. The leaching time can be appropriately changed depending on the type and particle size of the slag, or the amount of slag to be processed. Furthermore, since the dissolution of calcium components is promoted as the carbon dioxide concentration in the carbonated water used to leach the slag increases, it is preferable to carry out the leaching while continuously supplying carbon dioxide gas.

[0030] (Step 12: Solid-liquid separation) Step 12 is a process of separating the first suspension obtained in Step 11 into a liquid phase filtrate (first filtrate) and a solid phase residue (first residue). As a result of solid-liquid separation, the first filtrate consists of a liquid phase containing water-soluble hexavalent chromium and calcium components, and the first residue mainly consists of unreacted slag residue. The purpose of this step is to obtain a clear first filtrate by quickly and reliably removing this slag residue.

[0031] Removing this residue is crucial for supplying a clear filtrate to the subsequent degassing process and ensuring a stable precipitation reaction. If solids remain in the first filtrate, there is a risk of operational problems with the equipment due to clogging caused by the sedimentation of solids in the degassing equipment and piping. Therefore, in this separation process, it is desirable to reduce the concentration of solids in the first filtrate after the solid-liquid separation operation as much as possible. The solid-liquid separation method can be centrifugal separation, sedimentation separation, filtration (pressure filtration, vacuum filtration, etc.), or a combination of these.

[0032] (Step 13: Degassing) Step 13 is a step in which the first filtrate obtained in step 12 is degassed to obtain a second suspension containing a first precipitate consisting of hexavalent chromium and calcium components precipitated as carbonates. This step is the core separation technology of the present invention and simultaneously achieves the separation of harmful hexavalent chromium and the recovery of useful calcium components (carbonates).

[0033] As a result of step 11 described above, carbon dioxide is dissolved in the liquid at a high concentration, and the calcium component exists stably in the form of water-soluble calcium bicarbonate. When the carbon dioxide in the liquid is degassed in this step, the concentration of carbon dioxide dissolved in the aqueous solution decreases rapidly. Due to this decrease in carbon dioxide concentration, as shown in equation (2), the chemical equilibrium shifts and the water-soluble calcium bicarbonate changes into sparingly soluble carbonate (mainly calcium carbonate) and precipitates. Ca(HCO3)2→CaCO3↓+H2O+CO2↑ (2)

[0034] Simultaneously with the precipitation reaction, hexavalent chromium dissolved in the liquid is incorporated into or adsorbed by the crystalline particles of the precipitated carbonate, thereby migrating from the liquid phase to the solid phase. This phenomenon ensures the reliable and highly efficient separation of hexavalent chromium. The resulting first precipitate will primarily consist of carbonate and hexavalent chromium.

[0035] Here, degassing can be efficiently carried out by combining one or more of the following methods. (1) Reduced pressure: By reducing the pressure of the gas phase portion of the first filtrate, the solubility of carbon dioxide decreases and volatilization is promoted. (2) Heating: Heating the first filtrate reduces the solubility of carbon dioxide and promotes its volatilization. (3) Combination of reduced pressure and heating: By combining (1) reduced pressure and (2) heating, it is possible to remove carbon dioxide efficiently in the shortest time and significantly improve the reaction rate of the precipitation reaction.

[0036] As described above, (1) to (3) can be appropriately selected as degassing methods, but in particular, using heating or a combination of reduced pressure and heating maximizes the volatilization efficiency of carbon dioxide, enabling rapid precipitation and separation.

[0037] (Step 14: Solid-liquid separation) Step 14 is a step in which the second suspension obtained in step 13 is separated into a solid phase, the first precipitate, and a liquid phase, the first treatment solution. The primary purpose of this step is to reliably recover the first precipitate precipitated by degassing. The first precipitate is an intermediate product mainly composed of carbonates, with hexavalent chromium incorporated into it, and is used as a raw material in the subsequent secondary treatment process.

[0038] The second objective of this process is to ensure the quality of the recovered first treatment liquid. In the first treatment liquid, hexavalent chromium and calcium components have been largely removed as a result of their migration to the solid phase in process 13. By reusing this liquid as a solvent for the carbonated water in process 11, harmful components are not introduced into process 11, enabling efficient resource utilization throughout the entire processing flow. The separated first precipitate is washed with water as needed and then sent to the subsequent secondary processing process.

[0039] [Secondary processing] Next, the secondary processing process will be explained. The secondary processing process is a purification cycle that uses the first precipitate recovered in the primary processing process as raw material and completely removes residual hexavalent chromium to obtain a high-value-added calcium carbonate product.

[0040] First, the first precipitate is re-leached with carbonated water containing dissolved carbon dioxide (step 21) to redissolve hexavalent chromium and calcium components and obtain a suspension (third suspension). Next, undissolved components are removed from the third suspension by solid-liquid separation (step 22) to obtain a clear filtrate (second filtrate). Hexavalent chromium is selectively removed from this second filtrate to ensure that only hexavalent chromium is removed (step 23) and obtain the third filtrate. Subsequently, the third filtrate is degassed (step 24) to volatilize the carbon dioxide in the liquid, thereby precipitating a high-purity carbonate precipitate (second precipitate) that does not contain hexavalent chromium. Finally, the suspension containing the second precipitate (second suspension) is separated by solid-liquid separation (step 25) to separate and recover the second precipitate, making it a final product that can be reused as a raw material for steel.

[0041] (Step 21: Carbonated water extraction) Step 21 is a process in which the first precipitate recovered in the primary processing process is used as a raw material, and the first precipitate containing hexavalent chromium and calcium components precipitated as carbonates is leached again with carbonated water in which carbon dioxide gas is dissolved to obtain a third suspension in which these components are dissolved.

[0042] The purpose of this step is to redissolve the hexavalent chromium and the main component, calcium carbonate, incorporated in the first precipitate into the second filtrate, in order to enable their selective removal later. Calcium carbonate, the main component of the first precipitate, is sparingly soluble in water. However, by using carbonated water in which carbon dioxide is dissolved, the calcium carbonate undergoes a chemical change to water-soluble calcium bicarbonate, similar to the primary treatment process, as shown in equation (3), and dissolves efficiently. CaCO3 + H2O + CO2 → Ca(HCO3)2(3)

[0043] In step 21, the entire first precipitate transitions to the liquid phase, and the hexavalent chromium that was held as a solid phase in the precipitate also transitions to the liquid phase, the third suspension (later the second filtrate). This establishes a reaction base for separating and removing only hexavalent chromium from the liquid in the next step. The leaching conditions are preferably the same as in step 11 of the primary treatment process, with a continuous supply of carbonated water with a high carbon dioxide concentration and stirring at an appropriate temperature.

[0044] (Step 22: Solid-liquid separation) Step 22 is a process of separating the third suspension obtained in Step 21 into a liquid phase, the second filtrate, and a solid phase, the residue (second residue). The purpose of this step is to ensure that any small amount of undissolved components or insoluble impurities contained in the third suspension are removed before being subjected to the subsequent step (Step 23) for the selective removal of hexavalent chromium. The second residue mainly consists of insoluble fine impurities contained in the first precipitate and trace amounts of carbonates that were not completely dissolved in the re-leaching step.

[0045] (Step 23: Selective removal of hexavalent chromium) Step 23 aims to obtain a third filtrate from which only hexavalent chromium has been efficiently removed from the second filtrate obtained in Step 22. The performance of this step determines the product purity of the second precipitate recovered in the subsequent Step 24. In the second filtrate, a large amount of calcium components are dissolved as calcium bicarbonate, along with trace amounts of hexavalent chromium present as anions such as chromate ions. Unless this hexavalent chromium is reliably removed, the usability of the final product will be significantly impaired.

[0046] The most preferable method for removing hexavalent chromium is to pass it through an anion exchange resin. Since hexavalent chromium exists as anions in the liquid, anion exchange resins have the property of selectively adsorbing them. In contrast, the main component, calcium, exists as cations or bicarbonates, so it hardly interacts with the anion exchange resin and passes through the column unchanged.

[0047] By utilizing this principle, it becomes possible to separate hexavalent chromium with high purity and in a simple manner from a solution containing a high concentration of calcium components. This selective removal allows the hexavalent chromium concentration in the third filtrate to be reduced to a level that sufficiently exceeds the predetermined standard value. Because the anion exchange resin has a very high adsorption capacity for specific anions, stable processing can be carried out continuously, and the high purity of the final obtained second precipitate (calcium carbonate) is achieved.

[0048] Furthermore, the method for removing hexavalent chromium is not limited to the anion exchange resin described above. Various known methods can be employed, such as a method in which hexavalent chromium is converted to harmless trivalent chromium using a reducing agent and then precipitated and separated by pH adjustment, or a method using other adsorbents.

[0049] (Step 24: Degassing) Step 24 is a step in which the third filtrate obtained in step 23 is degassed to obtain a fourth suspension containing a second precipitate consisting mainly of carbonates (mainly calcium carbonate). The basic principle of this step is the same as that of step 13 in the primary treatment process. That is, by volatilizing the carbon dioxide in the liquid, the carbon dioxide concentration in the aqueous solution decreases rapidly, and the chemical equilibrium shifts. As a result, water-soluble calcium bicarbonate changes to sparingly soluble calcium carbonate and precipitates.

[0050] However, unlike the first precipitate obtained in step 13, the third filtrate subjected to this precipitation step has already had hexavalent chromium selectively removed. Therefore, the second precipitate that precipitates here contains almost no hexavalent chromium and is guaranteed to be recovered as a high-purity carbonate that meets the standard values. This second precipitate becomes the final product in this process.

[0051] (Step 25: Solid-liquid separation) Step 25 is the final step of the secondary processing process, separating the fourth suspension obtained in Step 24 into a solid phase, the second precipitate, and a liquid phase, the second processing solution. The main purpose of this step is to reliably recover the second precipitate, which is substantially free of hexavalent chromium and mainly consists of high-purity calcium carbonate, as the final product. Because it has gone through Step 23 (selective removal of hexavalent chromium), this second precipitate has a quality that makes it suitable for use as a high-value-added product such as a raw material for steel or building materials. The recovered precipitate is washed with a small amount of water as needed to remove as much of the second processing solution adhering to the surface as possible and to improve product purity.

[0052] On the other hand, the second treatment liquid, separated as a liquid phase, has an extremely low concentration of hexavalent chromium, and calcium components are also largely removed by precipitation. This second treatment liquid can be reused as a solvent for carbonated water in step 11 of the primary treatment process or in step 21 of the secondary treatment process, as needed. This maximizes the environmental advantages of this treatment method, such as further reduction in water usage and minimization of wastewater from the entire process.

[0053] As described above, the present invention's method for treating hexavalent chromium-containing slag centers on a primary treatment process in which hexavalent chromium-containing slag is leached with carbonated water, and the hexavalent chromium is precipitated together with carbonate by utilizing the shift in chemical equilibrium through a degassing operation. Furthermore, a secondary treatment process is carried out in which this precipitate is releached using an anion exchange resin or the like, to selectively remove hexavalent chromium and recover high-purity calcium carbonate. In addition, since the treatment liquid is recycled throughout the entire process, water resource consumption is reduced, and a sustainable treatment system with an extremely low environmental impact can be realized.

[0054] In this embodiment, the primary and secondary processing steps have been described as a series of processes, but the present invention is not limited thereto. For example, only the primary processing step may be performed, and the first precipitate may be used or discarded as is. It is also possible to perform the secondary processing step alone, in which case the secondary processing step can use the first precipitate containing hexavalent chromium and calcium components, produced by a method other than the primary processing step of the present invention, as a raw material. [Examples]

[0055] Next, Example 1 of the present invention will be described. In Example 1, the flow of the primary treatment process (leaching → solid-liquid separation → degassing → solid-liquid separation) was applied to stainless steel slag containing hexavalent chromium, and the effects of the leaching time and degassing method on the behavior of each component, as well as the possibility of recycling the treatment liquid, were investigated.

[0056] 1. Composition of the sample The main component composition of the stainless steel slag used in the experiment is shown in Table 1 below. It is presumed that hexavalent chromium exists primarily in the forms of CaCrO4 and MgCrO4.

[0057] [Table 1]

[0058] 2. Examination of leaching conditions and leaching time (Step 11) Figures 3 and 4 show the results of a study investigating the effect of leaching time on the concentration of each component. The slag particle size was set to φ < 32 μm, the solid-liquid ratio L / S = 100 (50 mL of distilled water, 0.5 g of slag), and the water temperature was 20°C. As shown in Figure 3, the concentrations of Ca, Si, and Mg reached their maximum values ​​approximately 30 minutes after the start of leaching. Also, as shown in Figure 4, the concentration of Cr reached its maximum value approximately 10 minutes after the start of leaching.

[0059] 3. Solid-liquid separation (Step 12) The suspension after carbonic acid leaching (first suspension) was separated into solid-liquid by filtration or centrifugation, resulting in a solid-phase residue (first residue) and a liquid-phase filtrate (first filtrate). This process removed Al and insoluble Cr from the slag. 3+ Components such as Si were separated as the first residue. The first filtrate after solid-liquid separation mainly consisted of Ca(HCO3)2 and CrO4. 2- It contained Mg and Si, and was subjected to the following degassing step (step 13).

[0060] 4. Degassing of the first filtrate (Step 13) The first filtrate obtained in the above process was degassed. Degassing was performed under three conditions: reduced pressure degassing (atm < 0.1 MPa), heating degassing (70°C), and reduced pressure + heating degassing (0.015 MPa, 70°C), and the precipitation behavior was compared.

[0061] As shown in Figure 5, reduced pressure + heating degassing was the most efficient method, and under these conditions, almost all of the Cr and Ca in the liquid precipitated within approximately one hour. On the other hand, Mg tended to precipitate more slowly compared to the other components. This degassing operation resulted in Cr 6+ It can be seen that it is removed from the liquid as a precipitate along with Ca.

[0062] Table 2 shows the results of the analysis of the composition of the precipitate produced by reduced pressure + heat degassing (degassing time 1 hour). The precipitate contained 19.91 mass% Ca, which corresponds to a composition of 49.71 mass% in terms of CaCO3. The Cr content was 0.05 mass%, confirming that Cr was effectively distributed in the precipitate.

[0063] [Table 2]

[0064] 5. Solid-liquid separation of the second suspension (Step 14) By separating the solid-liquid from the suspension after degassing (second suspension), Cr 6+ The first precipitate, mainly composed of CaCO3, was separated and recovered. Then, the Cr of the liquid phase (first treatment solution) after separation was obtained. 6+ The concentration was reduced to a level that meets the environmental standard of 0.02 mg / L or less, and it was also shown that the first treatment solution can be reused as a solvent for carbonated water.

[0065] From the results above, it has been demonstrated that the primary treatment process of the present invention is an environmentally and economically advantageous treatment method that effectively separates and recovers hexavalent chromium from hexavalent chromium-containing slag and enables the recycling and reuse of the treatment liquid. [Examples]

[0066] In Example 2, the intermediate product obtained in the primary processing process is Cr 6+ The first precipitate contained is used as the raw material, and the flow of the secondary processing process is (leaching → solid-liquid separation → Cr 6+ We investigated the possibility of improving product purity by applying the following steps: removal → degassing → solid-liquid separation.

[0067] 1. Composition of the sample The precipitate used in the experiment was the first precipitate obtained in Example 1, consisting of the components shown in Table 2.

[0068] 2. Re-leaching of precipitates (Step 21) Using the above-mentioned first precipitate as a raw material, carbonic acid leaching was carried out while blowing CO2 gas into a flask under the conditions of a solid-liquid ratio L / S = 100 (50 mL of distilled water, 0.5 g of slag) and a water temperature of 20 °C. By this re-leaching, Ca and Cr 6+ completely dissolved suspension (the third suspension) was obtained.

[0069] 3. Solid-liquid separation (Step 22) The suspension after carbonic acid leaching was separated into a solid-liquid state by filtration or centrifugation, and separated into a solid-phase residue (the second residue) and a liquid-phase filtrate (the second filtrate). By this operation, the second residue, which is an undissolved component, was surely removed, and a filtrate (the second filtrate) containing Ca and Cr 6+ was obtained.

[0070] 4. Selective removal of hexavalent chromium (Step 23) The second filtrate obtained in Step 22 was treated with an anion exchange resin (SA20A: manufactured by Mitsubishi Chemical Corporation). As a result of passing the second filtrate through the anion exchange resin, as shown in Figure 6, the main component, the Ca component, was retained in the liquid and there was no significant change in its concentration. On the other hand, as shown in Figure 7, the harmful impurity, the Cr component, was selectively adsorbed by the resin and its concentration was significantly reduced.

[0071] 5. Degassing of the second filtrate (Step 24) Cr 6+ After Cr was removed, the third filtrate was degassed. Degassing was carried out under the conditions of reduced pressure + heating (0.015 MPa, 70 °C). As a result of degassing, the Ca component in the liquid was precipitated as a carbonate and a precipitate (the second precipitate) was formed.

[0072] 6. Solid-liquid separation of the fourth suspension filtrate (Step 25) By separating the suspension (the fourth suspension) after degassing precipitation, the second precipitate mainly composed of CaCO3 was separated and recovered. As shown in Table 3, it was confirmed that this second precipitate substantially did not contain hexavalent chromium.

[0073]

Table 3

[0074] From these results, it has been demonstrated that the secondary treatment process of the present invention can completely remove hexavalent chromium from the precipitate obtained in the primary treatment process and recover high-purity calcium carbonate as a product.

[0075] As described above, the hexavalent chromium-containing slag treatment method according to the present invention utilizes leaching with carbonated water and precipitation by degassing, and can reliably remove hexavalent chromium from various types of slag, including steelmaking slag.

Claims

1. A step of leaching slag containing hexavalent chromium with carbonated water in which carbon dioxide gas is dissolved, to obtain a first suspension in which the hexavalent chromium and calcium components are dissolved from the slag, The first suspension is subjected to a solid-liquid separation into a first filtrate and a first residue, The process involves degassing the first filtrate to obtain a second suspension containing a first precipitate comprising the calcium component precipitated as a carbonate and the hexavalent chromium, The process includes a step of solid-liquid separation of the second suspension into a first processing liquid and the first precipitate. A method for processing hexavalent chromium-containing slag.

2. The degassing is performed by reducing the pressure, heating, or a combination of reducing the pressure and heating the first filtrate. A method for treating hexavalent chromium-containing slag according to claim 1.

3. The step of obtaining the first suspension includes a step of reusing the first processing liquid as a solvent for the carbonated water in which the carbon dioxide gas is dissolved. A method for treating hexavalent chromium-containing slag according to claim 1 or claim 2.

4. The process further comprises the step of removing the hexavalent chromium from the first precipitate, The first precipitate is re-leached with carbonated water in which carbon dioxide gas is dissolved to obtain a third suspension in which the hexavalent chromium and calcium components are dissolved, The third suspension is subjected to a solid-liquid separation into a second filtrate and a second residue, A step of obtaining a third filtrate from which the hexavalent chromium has been selectively removed from the second filtrate, The process involves degassing the third filtrate to obtain a fourth suspension containing a second precipitate consisting of the calcium component precipitated as a carbonate, The process includes a step of solid-liquid separation of the fourth suspension into a second processing liquid and a second precipitate. A method for treating hexavalent chromium-containing slag according to claim 1 or claim 2.

5. The step of selectively removing the hexavalent chromium from the second filtrate is: The process includes passing the second filtrate through an anion exchange resin. The method for treating hexavalent chromium-containing slag according to claim 4.

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