A method for treating cadmium-containing sludge

Abstract

This invention discloses a method for treating cadmium-containing sludge. First, diatomaceous earth is placed in an alkaline solution to form a suspension, which is then thoroughly stirred to allow the diatomaceous earth to react with the alkaline solution. Then, the cadmium-containing sludge is added, and stirring continues, resulting in solid-liquid separation to obtain a solid product. Finally, the obtained solid product is heated and calcined to obtain diatomaceous earth mineral crystals with cadmium internally sealed within them. This method not only has a good adsorption effect on cadmium in cadmium-containing sludge but also achieves the sealing of cadmium within the diatomaceous earth mineral crystals. By sealing cadmium within the crystal lattice of the diatomaceous earth mineral crystals, cadmium is difficult to leach into water bodies and crops, thus preventing harm. This method for treating cadmium-containing sludge is simple to operate, low in cost, and very suitable for industrial implementation.

Description

Technical Field

[0001] This invention belongs to the field of sludge treatment technology, specifically relating to a method for treating cadmium-containing sludge, particularly a method for treating cadmium-containing sludge generated during the production of pickled mustard tubers. Background Technology

[0002] With the rapid economic development of the Three Gorges Reservoir area, the production scale of its pillar industry, pickled mustard tuber, has also grown significantly. Pickled mustard tuber wastewater refers to the pickling wastewater generated during the processing of pickled mustard tuber, a byproduct of the production process. Each year, approximately 5 million cubic meters of untreated pickled mustard tuber wastewater, along with sludge containing high cadmium content, are discharged into the Three Gorges Reservoir area during production, posing a serious threat to the reservoir's aquatic environment.

[0003] Currently, the main methods for treating excess cadmium in sludge are as follows:

[0004] I. Chemical Precipitation Method

[0005] For cadmium pollution, precipitants such as barium sulfate and sodium hydroxide can be added to form cadmium precipitates, thus achieving purification. However, chemical precipitation methods may produce toxic precipitates during the process, requiring proper handling to avoid secondary pollution. Furthermore, the effectiveness of this method is affected by factors such as the degree of pollution, the type of precipitant, and the dosage, and may not achieve the desired results.

[0006] II. Bioremediation

[0007] Plants capable of accumulating cadmium, such as kidney beans and rapeseed, can be planted. These plants can absorb cadmium from sludge and accumulate it within their bodies. Alternatively, certain microorganisms, such as sulfur-oxidizing bacteria, can be used to convert cadmium into an inactive form, reducing its harm to the ecosystem. However, this remediation method takes longer than other physical or chemical treatments, potentially requiring months or even years to achieve the desired remediation effect. Furthermore, the effectiveness of bioremediation is affected by factors such as climate and pollutant concentration; different environmental conditions may influence biological growth and remediation outcomes.

[0008] III. Adsorption Method

[0009] While adsorbents such as activated carbon can adsorb cadmium from sludge, the physical adsorption of activated carbon is a reversible process. This reversible process is a microscopic phenomenon, meaning that activated carbon is simultaneously adsorbing and desorbing substances at every moment. When the adsorption rate exceeds the desorption rate, it manifests as macroscopic adsorption; when the two are roughly equal, the activated carbon becomes saturated; and when the desorption rate exceeds the adsorption rate, it manifests as macroscopic desorption. Therefore, using adsorption to treat cadmium in sludge carries the possibility of cadmium desorption, leading to cadmium dissolution and preventing the achievement of ideal treatment results.

[0010] Therefore, methods such as chemical precipitation, bioremediation, and adsorption are difficult to apply on a large scale to treat sludge with high cadmium content, and cannot achieve the desired treatment effect. Summary of the Invention

[0011] To address the aforementioned problems, this invention is proposed.

[0012] This invention utilizes the unique adsorption properties of silanol groups formed after diatomaceous earth is treated with alkali for cadmium, and also proposes a method for treating cadmium-containing sludge based on the principle of mineral crystallization.

[0013] This invention provides a method for treating cadmium-containing sludge, comprising the following steps:

[0014] (1) Place diatomaceous earth in an alkaline solution to form a suspension, stir thoroughly to allow the diatomaceous earth to react with the alkaline solution.

[0015] (2) Add cadmium-containing sludge to the system in step (1), continue stirring, and separate the solid and liquid to obtain a solid product.

[0016] (3) The solid product obtained in step (2) is heated and calcined to obtain diatomaceous earth mineral crystals containing cadmium inside.

[0017] Preferably, the alkaline solution is one or more of sodium hydroxide, potassium hydroxide, ammonia, sodium carbonate, and potassium carbonate.

[0018] Preferably, the concentration of the alkaline solution is 0.2 mol / L to 1.2 mol / L.

[0019] Preferably, the amount of alkaline solution added is calculated as the ratio of the mass of diatomaceous earth to the volume of alkaline solution: 1g / 15mL to 1g / 35mL.

[0020] Preferably, the diatomaceous earth contains 65%-75% diatomaceous opal.

[0021] Preferably, the stirring time in step (1) is 6-24 hours.

[0022] Preferably, the stirring time in step (2) is 8-12 hours.

[0023] Preferably, the heating and calcination temperature in step (3) is 100-500℃, and the heating time is 4-10h.

[0024] Preferably, the ratio of diatomaceous earth to cadmium-containing sludge is 1:5 by mass.

[0025] Preferably, the cadmium-containing sludge is cadmium-containing sludge generated during the production of pickled mustard tubers.

[0026] The principle of this invention:

[0027] The principle of this invention lies in the following: First, after alkali treatment, diatomaceous earth forms a large number of silanol groups and amorphous silica. Some impurities remain in the pores of the diatomaceous earth mineral; alkali treatment reduces these impurities, thus expanding the pores of the diatomaceous earth. Alkali treatment also increases the number of Si-OH groups on the surface of the diatomaceous earth. When cadmium-containing sludge is added, the Si-OH groups on the diatomaceous earth surface adsorb cadmium, resulting in better adsorption of cadmium from the sludge. Then, solid-liquid separation is performed to obtain a solid product. Finally, under heating and calcination conditions, the amorphous silica gradually crystallizes into crystals, sealing the cadmium adsorbed on the diatomaceous earth surface within the crystal lattice, making it difficult for cadmium to dissolve into water bodies and crops, thus preventing harm.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. This invention unexpectedly discovered that alkali treatment of diatomaceous earth significantly improves its adsorption effect on cadmium in cadmium-containing sludge. Alkali treatment of diatomaceous earth sludge revealed that within just one day, 20 kg of alkali-treated diatomaceous earth could reduce the cadmium content in 100 kg of cadmium-containing sludge with a moisture content of 30% by up to 85%. In contrast, untreated diatomaceous earth only reduced the cadmium content by 10%. This demonstrates that directly adding diatomaceous earth to sludge can also adsorb cadmium, but its adsorption capacity is limited. Furthermore, cadmium merely transfers from the sludge to the diatomaceous earth, adsorbing onto its surface; cadmium still poses a threat to the environment. More importantly, this invention achieves the sequestration of cadmium within the diatomaceous earth mineral crystals. Through the crystallization of diatomaceous earth, cadmium is sealed within the gradually crystallizing crystal lattice, making it difficult for cadmium to dissolve into water bodies and crops, thus preventing harm.

[0030] 2. The method for treating cadmium-containing sludge of the present invention is simple to operate, low in cost, and very suitable for industrial implementation. Attached Figure Description

[0031] Figure 1 Electron micrograph of diatomaceous earth after alkali treatment to adsorb cadmium.

[0032] Figure 2 The distribution of Si element after cadmium adsorption on alkali-treated diatomaceous earth.

[0033] Figure 3 A diagram showing the distribution of cadmium in diatomaceous earth after cadmium adsorption under alkali treatment. Detailed Implementation

[0034] The technical solution of the present invention will be described in detail below with reference to embodiments. These embodiments are for illustrative purposes only and should not be considered as limiting the scope of protection of the present invention. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used without specified manufacturers are all commercially available conventional products.

[0035] Diatomite is a natural porous silica mineral aggregate, mainly composed of diatomite opal (porous silica) and a very small amount of clay minerals, quartz and other minerals. The diatomite opal content is generally greater than 65%, and in some mineral samples it is even higher than 90%.

[0036] It should be noted that diatomaceous earth from different places or manufacturers can be used as raw materials. In this embodiment, the diatomaceous earth is sourced from Miyi, Sichuan.

[0037] Example 1

[0038] 20 kg of diatomaceous earth (Sichuan Miyi diatomaceous earth mineral, with an opal content of approximately 75%) was placed in 500 L of 0.2 mol / L sodium hydroxide solution to form a suspension. After stirring thoroughly for 12 h, 100 kg of cadmium-containing sludge with a water content of 30% was added to the system, and stirring was continued for another 12 h. Solid-liquid separation was performed to obtain a solid product. 50 g of the obtained solid product was evaluated by scanning electron microscopy and particle size analysis, and the single particle size distribution was 15–25 μm. The obtained solid product was then calcined at 500 °C for 10 h to crystallize, thus obtaining diatomaceous earth mineral crystals with cadmium trapped inside.

[0039] Tests have shown that treating cadmium-containing sludge with alkali-treated diatomaceous earth minerals can reduce the cadmium content in 100 kg of cadmium-containing sludge by 76% in just one day (while untreated diatomaceous earth minerals can only reduce the cadmium content by 10%).

[0040] Take 100g of the solid product obtained before heating and calcination, add 500ml of 0.1mol / L dilute sulfuric acid, stir for 10min, and determine the cadmium content of the resulting solution using atomic absorption spectrophotometry. The cadmium content is 0.028g / L. Take 100g of the diatomaceous earth mineral crystals obtained after heating and calcination, add 500ml of 0.1mol / L dilute sulfuric acid, stir for 10min, and determine the cadmium content of the resulting solution using atomic absorption spectrophotometry. The cadmium content is 0.003g / L. It can be seen that because cadmium is encapsulated within the crystal lattice of the diatomaceous earth mineral crystals, it is much more difficult for cadmium to dissolve compared to cadmium adsorbed on the surface of diatomaceous earth after alkali treatment.

[0041] Example 2

[0042] 20 kg of diatomaceous earth (Sichuan Miyi diatomaceous earth mineral, with an opal content of approximately 75%) was placed in 300 L of 1.2 mol / L sodium hydroxide solution to form a suspension. After stirring thoroughly for 24 h, 100 kg of cadmium-containing sludge with a water content of 30% was added to the system, and stirring was continued for 12 h. Solid-liquid separation was performed to obtain a solid product. 50 g of the obtained solid product was evaluated by scanning electron microscopy and particle size analysis, and the single particle size distribution was 10–20 μm. The obtained solid product was then calcined at 400 °C for 8 h to crystallize, thus obtaining diatomaceous earth mineral crystals with cadmium trapped inside.

[0043] Tests have shown that treating cadmium-containing sludge with alkali-treated diatomaceous earth minerals can reduce the cadmium content in 100 kg of cadmium-containing sludge by 85% in just one day (while untreated diatomaceous earth minerals can only reduce the cadmium content by 10%).

[0044] Take 100g of the solid product obtained before heating and calcination, add 500ml of 0.1mol / L dilute sulfuric acid, stir for 10min, and determine the cadmium content of the resulting solution using atomic absorption spectrophotometry. The cadmium content is 0.018g / L. Take 100g of the diatomaceous earth mineral crystals obtained after heating and calcination, add 500ml of 0.1mol / L dilute sulfuric acid, stir for 10min, and determine the cadmium content of the resulting solution using atomic absorption spectrophotometry. The cadmium content is 0.001g / L. Therefore, because cadmium is encapsulated within the crystal lattice of the diatomaceous earth mineral crystals, it is much more difficult for cadmium to dissolve compared to cadmium adsorbed on the surface of diatomaceous earth after alkali treatment.

[0045] Example 3

[0046] 20 kg of diatomaceous earth (Sichuan Miyi diatomaceous earth mineral, with an opal content of approximately 75%) was placed in 500 L of 0.5 mol / L ammonia solution to form a suspension. After stirring thoroughly for 12 h, 100 kg of cadmium-containing sludge with a water content of 30% was added to the system, and stirring was continued for another 12 h. Solid-liquid separation was performed to obtain a solid product. 50 g of the obtained solid product was evaluated by scanning electron microscopy and particle size analysis, and the single particle size distribution was 15–30 μm. The obtained solid product was then calcined at 300 °C for 6 h to crystallize, thus obtaining diatomaceous earth mineral crystals with cadmium trapped inside.

[0047] Tests have shown that treating cadmium-containing sludge with alkali-treated diatomaceous earth minerals can reduce the cadmium content in 100 kg of cadmium-containing sludge by 60% within just one day (while untreated diatomaceous earth minerals can only reduce the cadmium content by 10%).

[0048] Take 100g of the solid product obtained before heating and calcination, add 500ml of 0.1mol / L dilute sulfuric acid, stir for 10min, and determine the cadmium content of the resulting solution using atomic absorption spectrophotometry. The cadmium content is 0.035g / L. Take 100g of the diatomaceous earth mineral crystals obtained after heating and calcination, add 500ml of 0.1mol / L dilute sulfuric acid, stir for 10min, and determine the cadmium content of the resulting solution using atomic absorption spectrophotometry. The cadmium content is 0.008g / L. Therefore, because cadmium is encapsulated within the crystal lattice of the diatomaceous earth mineral crystals, it is much more difficult for cadmium to dissolve compared to cadmium adsorbed on the surface of diatomaceous earth after alkali treatment.

[0049] Example 4

[0050] 20 kg of diatomaceous earth (Sichuan Miyi diatomaceous earth mineral, with an opal content of approximately 75%) was placed in 300 L of 1.2 mol / L sodium carbonate solution to form a suspension. After stirring thoroughly for 6 hours, 100 kg of cadmium-containing sludge with a water content of 30% was added to the system, and stirring was continued for 12 hours. Solid-liquid separation was performed to obtain a solid product. 50 g of the obtained solid product was evaluated by scanning electron microscopy and particle size analysis, and the single particle size distribution was 15–25 μm. The obtained solid product was then calcined at 200℃ for 4 hours to crystallize, thus obtaining diatomaceous earth mineral crystals with cadmium trapped inside.

[0051] Tests have shown that treating cadmium-containing sludge with alkali-treated diatomaceous earth minerals can reduce the cadmium content in 100 kg of cadmium-containing sludge by 75% within just one day (while untreated diatomaceous earth minerals can only reduce the cadmium content by 10%).

[0052] Take 100g of the solid product obtained before heating and calcination, add 500ml of 0.1mol / L dilute sulfuric acid, stir for 10min, and determine the cadmium content of the resulting solution using atomic absorption spectrophotometry. The cadmium content is 0.029g / L. Take 100g of the diatomaceous earth mineral crystals obtained after heating and calcination, add 500ml of 0.1mol / L dilute sulfuric acid, stir for 10min, and determine the cadmium content of the resulting solution using atomic absorption spectrophotometry. The cadmium content is 0.004g / L. Therefore, because cadmium is encapsulated within the crystal lattice of the diatomaceous earth mineral crystals, it is much more difficult for cadmium to dissolve compared to cadmium adsorbed on the surface of diatomaceous earth after alkali treatment.

[0053] Example 5

[0054] 20 kg of diatomaceous earth (Sichuan Miyi diatomaceous earth mineral, with an opal content of approximately 75%) was placed in 700 L of a mixed solution of 0.8 mol / L sodium hydroxide and potassium hydroxide to form a suspension. After stirring thoroughly for 10 h, 100 kg of cadmium-containing sludge with a water content of 30% was added to the system, and stirring was continued for 12 h. Solid-liquid separation was performed to obtain a solid product. 50 g of the obtained solid product was evaluated by scanning electron microscopy and particle size analysis, and the single particle size distribution was 10–25 μm. The obtained solid product was then calcined at 100 °C for 10 h to crystallize, thus obtaining diatomaceous earth mineral crystals with cadmium trapped inside.

[0055] Tests have shown that treating cadmium-containing sludge with alkali-treated diatomaceous earth minerals can reduce the cadmium content in 100 kg of cadmium-containing sludge by 83% in just one day (while untreated diatomaceous earth minerals can only reduce the cadmium content by 10%).

[0056] Take 100g of the solid product obtained before heating and calcination, add 500ml of 0.1mol / L dilute sulfuric acid, stir for 10min, and determine the cadmium content of the resulting solution using atomic absorption spectrophotometry. The cadmium content is 0.024g / L. Take 100g of the diatomaceous earth mineral crystals obtained after heating and calcination, add 500ml of 0.1mol / L dilute sulfuric acid, stir for 10min, and determine the cadmium content of the resulting solution using atomic absorption spectrophotometry. The cadmium content is 0.002g / L. It can be seen that because cadmium is encapsulated within the crystal lattice of the diatomaceous earth mineral crystals, it is much more difficult for cadmium to dissolve compared to cadmium adsorbed on the surface of diatomaceous earth after alkali treatment.

[0057] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for treating cadmium-containing sludge, characterized in that, Includes the following steps: (1) Place diatomaceous earth in an alkaline solution to form a suspension, stir thoroughly to allow the diatomaceous earth to react with the alkaline solution; (2) Add cadmium-containing sludge to the system in step (1), continue stirring, and separate the solid and liquid to obtain a solid product; (3) The solid product obtained in step (2) is heated and calcined to obtain diatomaceous earth mineral crystals containing cadmium inside; The alkaline solution is one or more of sodium hydroxide, potassium hydroxide, ammonia, sodium carbonate, and potassium carbonate. The concentration of the alkaline solution is 0.2 mol / L to 1.2 mol / L; The amount of alkaline solution added is calculated as the ratio of the mass of diatomaceous earth to the volume of alkaline solution: 1g / 15mL to 1g / 35mL. The diatomaceous earth contains 65%-75% diatomite opal; The stirring time in step (1) is 6-24 hours; The stirring time in step (2) is 8-12 hours; The heating and calcination temperature in step (3) is 100-500℃, and the heating time is 4-10h; The ratio of diatomaceous earth to cadmium-containing sludge is 1:5 by mass.

2. The method according to claim 1, characterized in that, The cadmium-containing sludge is the cadmium-containing sludge generated during the production of pickled mustard tubers.

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