A method for utilizing chromium slag

Abstract

This invention belongs to the field of solid waste resource utilization in the battery industry, specifically disclosing a method for utilizing chromium slag. Through steps such as mixing and grinding chromium slag and sodium-containing alkaline substances, low-temperature roasting and crystallization, water leaching, crystallization, and separation, this invention fully extracts Cr and P from the chromium slag, and separates and prepares sodium phosphate, sodium chromate, and industrial iron ore products. This achieves complete resource utilization of valuable elements in the chromium slag and solves the environmental pollution problem of chromium slag in the nickel salt production process of nickel-iron alloys.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization in the battery industry, and specifically relates to a method for utilizing chromium slag. Background Technology

[0002] In the battery industry, nickel salts are produced using a nickel-iron alloy process. The impurity removal process in this process generates a large amount of high-phosphorus iron-chromium slag, or chromium slag for short. Since chromium slag is a residue generated during the smelting process, valuable metals and impurities undergo recombination and distribution during slag formation, forming stable, mixed compounds. The main components of chromium slag are compounds composed of metal cations such as iron, nickel, and chromium, and phosphorus-containing anions. For example, phosphorus is typically present as one or more of orthophosphate, hypophosphatemia, phosphite, metaphosphate, polyphosphate, or pyrophosphate. The compound composition of chromium slag is very complex, and conventional ore beneficiation methods are insufficient to purify the various compounds or extract valuable elements (iron, phosphorus, nickel, and chromium). Therefore, although chromium slag contains high levels of iron phosphate and valuable elements, its resource utilization is difficult, and it has long been stored as hazardous waste, or solidified or stabilized before landfilling. It not only occupies a large amount of land but also poses a high environmental risk.

[0003] Currently, calcium-free roasting can oxidize chromium in iron-chromium slag under alkaline and high-temperature conditions, converting low-valent chromium into hexavalent chromium for extraction. However, this method mainly relies on high-temperature roasting above 1000℃ to destroy the iron-chromium spinel structure, which is unsuitable for high-phosphorus iron-chromium slag. At temperatures above 900℃, the slag melts, making subsequent leaching extraction more difficult. Furthermore, this technology suffers from drawbacks such as low chromium content and insufficient contact between chromium and alkali, leading to poor chromium encapsulation, poor mass transfer, and low chromium extraction rates (generally below 80%). It also presents challenges in separating and purifying sodium phosphate and sodium chromate. In addition, the chromium extracted using this method is currently treated with traditional crystallization to precipitate chromium salts and reduce the chromium. This process suffers from incomplete chromium reduction, low reduction efficiency, and the generation of chromium slag again, resulting in chromium resource waste, new chromium pollution, incomplete resource utilization, low product economic benefits, and limited application areas. Summary of the Invention

[0004] In view of the problems of low chromium extraction rate, incomplete resource utilization of chromium slag, and low economic benefits of products in the existing technology, the present invention will provide a method for utilizing chromium slag. The method has a high chromium extraction rate in the chromium slag, converting it all into sodium chromate, and the chromium slag is completely utilized as a resource. The process can generate no wastewater or waste residue.

[0005] To achieve the above objectives, the following technical solutions are specifically included:

[0006] A method for utilizing chromium slag includes the following steps:

[0007] (1) The chromium slag was ball-milled with a sodium-containing alkaline substance to obtain a mixture;

[0008] (2) The mixture is roasted to obtain a roasted product; the roasting temperature is 650-800℃;

[0009] (3) The roasted material is immersed in water, and after solid-liquid separation, iron ore and leachate are obtained;

[0010] (4) The leachate is crystallized, and after solid-liquid separation, sodium phosphate and separation liquid are obtained;

[0011] (5) Evaporate and crystallize the separated liquid to obtain sodium chromate.

[0012] In the method of this invention, the chromium slag and sodium-containing alkaline substances are mixed and ball-milled to change the crystal phase structure of the chromium slag, which can improve the oxidation efficiency of chromium during roasting. Then, a low-temperature oxidation roasting method is used to achieve crystal transformation, so that the main phase substance releases chromium ions after crystal transformation. During the crystal transformation process, the chromium ions are exposed to an alkaline environment for full oxidation, which improves the oxidation rate of chromium. The combination of ball milling and low-temperature oxidation roasting solves the problems of chromium encapsulation, low mass transfer between chromium and alkaline substances, and incomplete contact that lead to insufficient oxidation in the calcium-free roasting method, thereby increasing the chromium extraction rate. The method of this invention also uses crystallization to separate sodium phosphate and sodium chromate by solubility difference, instead of reducing chromium after crystallization in the traditional method, thus avoiding the problem of insufficient chromium reduction and the generation of chromium slag again.

[0013] Preferably, in step (1), the sodium-containing alkaline substance includes at least one of sodium hydroxide, sodium carbonate, and sodium bicarbonate.

[0014] Preferably, in step (1), the chromium slag comprises the following components by mass percentage: chromium 1%-5%, iron 30%-35%, and phosphorus 15%-20%.

[0015] The method of the present invention is more suitable for the above-mentioned chromium slag containing high iron and phosphorus.

[0016] Preferably, in step (1), the mass ratio of the chromium slag to the sodium-containing alkaline substance is 1:(0.8-1.3).

[0017] Preferably, in step (1), the ball milling time is 30-150 min.

[0018] Preferably, in step (2), the roasting temperature is 700-750℃.

[0019] Preferably, in step (2), the roasting time is 180-270 min.

[0020] Preferably, in step (3), the number of immersions is 1-3 times, the temperature of each immersion is selected from 50-95℃, and the time of each immersion is selected from 60-240min.

[0021] Preferably, in step (3), the mass ratio of the roasted material to water is 1:(1-3).

[0022] Preferably, in step (4), the crystallization temperature is 20-30℃.

[0023] At the above-mentioned cooling temperature, no sodium chromate will precipitate; instead, sodium phosphate will precipitate.

[0024] Preferably, in step (5), the evaporation temperature is 80-90℃ and the crystallization temperature is 15-25℃.

[0025] Given the high solubility of sodium chromate, the density of the sodium chromate solution obtained after evaporation should be between 1.8 and 2.0 g / cm³ for separation and purification. 3 By crystallizing at a temperature of 15-25℃, industrial-grade sodium chromate can be obtained. Furthermore, the distilled water generated during evaporation and cooling crystallization is collected and used for subsequent water leaching of the roasted material, achieving water recycling.

[0026] Compared with the prior art, the present invention has the following beneficial effects: The present invention fully extracts Cr and P from chromium slag through steps such as mixing and grinding of chromium slag and sodium-containing alkaline substances, low-temperature roasting and crystallization, water leaching, crystallization and separation, and separates and prepares phosphate, sodium chromate and industrial iron ore products, realizing the full resource utilization of valuable elements in chromium slag, solving the environmental pollution problem of chromium slag in the nickel salt production process of nickel-iron alloy, and solving the defects of traditional calcium-free roasting process, which is due to the small proportion of chromium and insufficient contact between chromium and alkali, resulting in chromium encapsulation, poor mass transfer effect, low chromium extraction rate, low chromium reduction efficiency, introduction of new impurities, waste of chromium resources, and the generation of new chromium pollution, and the difficulty in separating and purifying sodium phosphate and sodium chromate. Attached Figure Description

[0027] Figure 1 A flowchart of the method for utilizing chromium slag according to the present invention.

[0028] Figure 2 The XRD patterns of the mixture of Comparative Example 1, Example 1-3, and chromium slag in step (1) are shown.

[0029] Figure 3 The image shows a partial XRD pattern of the mixture of chromium slag from step (1) of Comparative Example 1 and Examples 1-3.

[0030] Figure 4 The XRD patterns of the calcined product and chromium slag from step (2) in Examples 1, 4-6 are shown.

[0031] Figure 1-4 The chromium slag, chromium slag sample, and chromium slag raw material mentioned herein all refer to the raw materials in the following examples and comparative examples: chromium slag (dry slag), and the chromium slag used in all examples and comparative examples is the same type. Detailed Implementation

[0032] To better illustrate the purpose, technical solution, and advantages of this invention, specific embodiments will be used to further explain the invention below. Unless otherwise specified, the test methods used in the embodiments and / or comparative examples are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available.

[0033] The chromium slag used below is a byproduct of the impurity removal process in the nickel salt production process of nickel-iron alloy in the battery industry, namely high-phosphorus iron-chromium slag. There are no special requirements. It can be purchased from the market or collected and recycled by oneself. The chromium slag used in the following examples and comparative examples has the same source. The chromium slag includes the following components in mass percentage: chromium 1%-5%, iron 30%-35%, and phosphorus 15%-20%.

[0034] Example 1

[0035] A method for utilizing chromium slag, the specific process of which is attached. Figure 1 As shown, the specific steps include the following:

[0036] (1) Take 50g of chromium slag (dry slag) and 50g of sodium carbonate, and ball mill them in a ball mill for 150min. After ball milling, the solid is passed through a 250-mesh sieve to obtain a mixture.

[0037] (2) The mixture was placed in a box-type muffle furnace and calcined in an air atmosphere at 750°C for 240 min to obtain the calcined product.

[0038] (3) The roasted material and pure water were leached at 80°C for 150 min at a solid-liquid ratio of 1:2. The solid and liquid were separated to obtain leachate and filter residue. The leaching conditions were repeated twice more. The leachates were combined and the filtered solid was dried to obtain industrial iron ore.

[0039] (4) Place the leachate obtained in step (3) in a crystallizer. Because the temperature of the leachate is high, the temperature of the leachate needs to be cooled to 25°C for crystallization. After crystallization, centrifuge to separate the sodium phosphate solid and the separated liquid. The dried sodium phosphate solid meets the standard of "Industrial Trisodium Phosphate HG / T 2517".

[0040] (5) The separated liquid obtained in step (4) is then placed in an evaporator and evaporated at 85°C. The distilled water produced during this process is returned to the leaching stage, ensuring that the entire chromium slag process generates no wastewater or solid waste. The final remaining liquid after evaporation is a sodium chromate solution with a density of 1.9 g / cm³.3 The sodium chromate solution was then cooled to 18°C ​​to crystallize, and the dried sample was sodium chromate solid, which conformed to the standard of "Industrial Sodium Chromate HG / T 4312".

[0041] Example 2

[0042] The difference between this embodiment and embodiment 1 is that in step (1), the ball milling time is 30 minutes.

[0043] Example 3

[0044] The difference between this embodiment and embodiment 1 is that in step (1), the ball milling time is 90 minutes.

[0045] Example 4

[0046] The difference between this embodiment and embodiment 1 is that in step (2), the roasting temperature is 650°C.

[0047] Example 5

[0048] The difference between this embodiment and embodiment 1 is that in step (2), the roasting temperature is 700℃.

[0049] Example 6

[0050] The difference between this embodiment and embodiment 1 is that in step (2), the roasting temperature is 800℃.

[0051] Example 7

[0052] A method for utilizing chromium slag specifically includes the following steps:

[0053] (1) Take 50g of chromium slag (dry slag) and 40g of sodium carbonate, and ball mill them in a ball mill for 150min. After ball milling, the solid is passed through a 250-mesh sieve to obtain a mixture.

[0054] (2) The mixture was placed in a box-type muffle furnace and calcined in an air atmosphere at 750°C for 270 min to obtain the calcined product;

[0055] (3) The roasted material and pure water were leached at 95°C for 120 min at a solid-liquid ratio of 1:3. The solid and liquid were separated to obtain leachate and filter residue. The leaching conditions were repeated twice more. The leachates were combined and the filtered solid was dried to obtain industrial iron ore.

[0056] (4) Place the leachate obtained in step (3) in a crystallizer and cool it at 20°C to crystallize. After crystallization, centrifuge to separate the sodium phosphate solid and the separated liquid. The dried sodium phosphate solid meets the standard of "Industrial Trisodium Phosphate HG / T 2517".

[0057] (5) The separated liquid obtained in step (4) is then placed in an evaporator and evaporated at 90°C. The distilled water produced during this process is returned to the leaching stage, ensuring that the entire chromium slag process generates no wastewater or solid waste. The final remaining liquid after evaporation is a sodium chromate solution with a density of 1.9 g / cm³. 3 The sodium chromate solution was then cooled and crystallized at room temperature (25°C). After drying, the sample was solid sodium chromate, which conformed to the standard of "Industrial Sodium Chromate HG / T 4312".

[0058] Example 8

[0059] A method for utilizing chromium slag specifically includes the following steps:

[0060] (1) Take 50g of chromium slag (dry slag) and 65g of sodium carbonate, and ball mill them in a ball mill for 120min. After ball milling, the solid is passed through a 250-mesh sieve to obtain a mixture.

[0061] (2) The mixture was placed in a box-type muffle furnace and calcined in an air atmosphere at 750°C for 180 min to obtain the calcined product.

[0062] (3) The roasted material and pure water were leached at 50°C for 240 min at a solid-liquid ratio of 1:3. The solid and liquid were separated to obtain leachate and filter residue. The leaching conditions were repeated twice more. The leachates were combined and the filtered solid was dried to obtain industrial iron ore.

[0063] (4) Place the leachate obtained in step (3) in a crystallizer and cool it at 30°C to crystallize. After crystallization, centrifuge to separate the sodium phosphate solid and the separated liquid. The dried sodium phosphate solid meets the standard of "Industrial Trisodium Phosphate HG / T 2517".

[0064] (5) The separated liquid obtained in step (4) is then placed in an evaporator and evaporated at 85°C. The distilled water produced during this process is returned to the leaching stage, ensuring that the entire chromium slag process generates no wastewater or solid waste. The final remaining liquid after evaporation is a sodium chromate solution with a density of 1.9 g / cm³. 3 The sodium chromate solution was then cooled and crystallized at room temperature (15°C). After drying, the sample was solid sodium chromate, which conformed to the standard of "Industrial Sodium Chromate HG / T 4312".

[0065] Comparative Example 1

[0066] The difference between this comparative example and Example 1 is that step (1) of ball milling in a ball mill was not performed. In this comparative example, the mixture of chromium slag and sodium carbonate was subjected to steps (2)-(5).

[0067] Comparative Example 2

[0068] The difference between this comparative example and Example 1 is that the calcination temperature in step (2) is 900°C.

[0069] The chromium slag melted, making it impossible to effectively leach chromium and phosphorus.

[0070] Comparative Example 3

[0071] The difference between this comparative example and Example 1 is that the calcination temperature in step (2) is 600°C.

[0072] The chromium slag, the mixture obtained in step (1), and the calcined product obtained in step (2) were characterized by XRD. The chromium and phosphorus content in the leachate of step (3) was measured by ICP to calculate the leaching rate of chromium and phosphorus.

[0073] Table 1

[0074]

[0075]

[0076] In the method of this invention, the chromium slag is roasted (low-temperature oxidative roasting) under air and alkaline conditions, during which the substances in the chromium slag undergo a crystal phase transformation, wherein the chromium element is oxidized, and the chromium and phosphorus elements in the chromium slag exist in the form of sodium chromate and sodium phosphate. Since these two sodium salts are soluble in water, they can be effectively leached out during the water immersion process. Furthermore, in the method of this invention, before the low-temperature oxidative roasting, the chromium slag is mixed with sodium-containing alkaline substances and ball-milled to pre-change the crystal phase structure of the chromium slag, which can further improve the oxidation efficiency of chromium during low-temperature oxidative roasting and further improve the leaching rate of chromium and phosphorus.

[0077] From the appendix Figure 2-3 As can be seen from the XRD pattern, although the mineral phase of the chromium slag remained unchanged with increasing grinding time, the diffraction peaks gradually broadened, indicating a change in the microstructure of the chromium slag crystals. With increasing grinding time, the main peak of the mixed-grind sample shifted to the left compared to the original chromium slag sample, suggesting possible partial oxidation in the mixed phase, leading to surface cracking and grain reduction. Table 1 shows that, compared to the ungrinding and roasting in Comparative Example 1, the grinding in Examples 2-4 facilitated the leaching of chromium and phosphorus. Grinding for 150 minutes before roasting allowed for full utilization of the elements in the chromium slag, which is beneficial for the subsequent cooling and crystallization.

[0078] From the appendix Figure 4As shown in Table 1, the main phase of the original chromium slag is Fe5(PO4)4(OH)3·2H2O. In this invention, the chromium slag is ground with sodium carbonate and then subjected to solid-phase roasting to allow the crystals to transform and release chromium ions. As the temperature increases, only the ferric oxide crystal form transforms at 650℃ and 700℃, without the transformation of the trisodium phosphate crystal form. Under these conditions, the leaching rate is low because the chromium ions are not completely released into the alkaline environment. Combined with Comparative Example 3, it can be seen that the roasting temperature should not be lower than 650℃. As the temperature increases, at 750℃ and 800℃, the main phase of the chromium slag gradually transforms into ferric oxide and trisodium phosphate, allowing chromium to be completely released into the alkaline conditions and improving the oxidation rate of chromium. Compared to Example 750℃, the mixed slag roasted at 800℃ has more impurity peaks, which is not conducive to chromium leaching. Combined with Comparative Example 2, it can be seen that the roasting temperature should not be too high. If the temperature is too high, the chromium slag will melt, making the chromium slag more tightly wrapped, making it more difficult to leach chromium and phosphorus, and thus unable to effectively leach chromium and phosphorus.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for utilizing chromium slag, characterized in that, Includes the following steps: (1) The chromium slag is ball-milled with a sodium-containing alkaline substance to obtain a mixture; the chromium slag comprises the following components in mass percentage: chromium 1%-5%, iron 30%-35%, and phosphorus 15%-20%; (2) The mixture is calcined to obtain a calcined product; the calcination temperature is 650-800℃; (3) The roasted material is immersed in water, and after solid-liquid separation, iron ore and leachate are obtained; (4) The leachate is crystallized, and after solid-liquid separation, sodium phosphate and a separation liquid are obtained; (5) Evaporate and crystallize the separated liquid to obtain sodium chromate.

2. The method for utilizing chromium slag as described in claim 1, characterized in that, In step (1), the ball milling time is 30-150 min.

3. The method for utilizing chromium slag as described in claim 1, characterized in that, In step (2), the roasting temperature is 700-750℃.

4. The method for utilizing chromium slag as described in claim 1, characterized in that, In step (2), the roasting time is 180-270 min.

5. The method for utilizing chromium slag as described in claim 1, characterized in that, In step (1), the sodium-containing alkaline substance includes at least one of sodium hydroxide, sodium carbonate, and sodium bicarbonate.

6. The method for utilizing chromium slag as described in claim 1, characterized in that, In step (1), the mass ratio of the chromium slag to the sodium-containing alkaline substance is 1:(0.8-1.3).

7. The method for utilizing chromium slag as described in claim 1, characterized in that, In step (3), the number of immersions is 1-3 times, the temperature of each immersion is selected from 50-95℃, and the time of each immersion is selected from 60-240min.

8. The method for utilizing chromium slag as described in claim 1, characterized in that, In step (3), the mass ratio of the roasted material to water is 1:(1-3).

9. The method for utilizing chromium slag as described in claim 1, characterized in that, In step (4), the crystallization temperature is 20-30℃.

10. The method for utilizing chromium slag as described in claim 1, characterized in that, In step (5), the evaporation temperature is 80-90℃ and the crystallization temperature is 15-25℃.

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