Method for solidifying chromium-containing sludge based on coal gasification coarse slag

Abstract

The invention discloses a method for solidifying chromium-containing sludge based on coal gasification coarse slag, which comprises the following steps: fully drying the coal gasification coarse slag and the chromium-containing sludge, grinding and mixing according to a weight ratio of 1: (1-5), and calcining at high temperature in an air atmosphere. After calcination treatment, heavy metal chromium in the sludge is fixed and converted into a stable uvarovite microcrystalline phase, and meanwhile, the addition of the coal gasification coarse slag also inhibits the oxidation of Cr (III), so that the problem of formation of Cr (VI) which is high in toxicity, easy to migrate and carcinogenic and teratogenic in a sludge sintering product is solved. The method has the multiple technical advantages of large solid waste treatment capacity, no chemical reagent consumption, low treatment cost and excellent treatment effect. According to the leaching result of the solid waste leaching toxicity leaching method-acetic acid buffer solution method (HJ / T HJ / 300-2007), the requirements of the national standard of hazardous waste identification standard-leaching toxicity identification (GB5085.3-2007) are met, that is, the concentration of Cr (VI) is lower than 5 mg / L, the concentration of total chromium is lower than 15 mg / L, and waste treatment with waste and waste recycling are achieved.

Description

Technical Field

[0001] This invention belongs to the field of industrial hazardous waste heavy metal sludge treatment technology, specifically relating to a method for solidifying chromium-containing sludge based on coal gasification coarse slag. Background Technology

[0002] Statistics show that the global annual production of raw hides reaches approximately 69.453 million tons, generating a large amount of solid waste during the leather tanning process and subsequent wastewater treatment. Currently, over 90% of animal hides worldwide are tanned using chromium-containing chemicals, such as chromium sulfate. However, only about 60% of the chromium is effectively utilized during the tanning process, with the remainder precipitating into the sludge. Besides the heavy metal chromium, tanning sludge also contains recalcitrant organic pollutants and pathogenic microorganisms, classifying it as hazardous industrial waste and posing a significant threat to ecosystems and human health. China is a major leather producer globally, generating millions of tons of tanning sludge annually. Its safe and harmless treatment remains a challenge and has attracted increasing public attention. Currently, many sludge treatment methods have been developed, such as landfill, composting, incineration, vitrification, and supercritical water oxidation. However, due to potential environmental pollution risks, an international convention in 1998 prohibited landfilling. The application of sludge composting leads to a significant increase in heavy metal concentrations in the soil. Supercritical water oxidation of sludge easily causes pipe blockage, equipment corrosion, and high-temperature, high-pressure problems. The vitrification process requires high temperatures (above approximately 1400°C), leading to high costs and energy consumption in sludge treatment. Furthermore, recovering chromium from low-chromium-content sludge is practically uneconomical. Therefore, exploring a safe and harmless alternative method for treating the increasing amount of chromium-containing sludge is extremely urgent.

[0003] Incineration is an effective method for treating tannery sludge, serving as the first step in its detoxification. It is the most mature thermal treatment process, offering numerous advantages such as significantly reducing sludge volume, destroying toxic organic compounds, removing odors and pathogens, and recovering energy from the sludge. However, direct incineration leads to the oxidation of Cr(III) in tannery sludge into highly toxic, easily migrating, carcinogenic, and teratogenic Cr(VI), and the leaching of heavy metal chromium from the sintered ash slag is easily leached, requiring secondary safe treatment. Given the continuously increasing production of heavy metal industrial solid waste, incorporating specific precursor materials and achieving heavy metal solidification and stabilization at suitable temperatures is the mainstream technical means for the safe and harmless disposal of hazardous waste. Coal gasification coarse slag is an industrial solid waste generated during coal gasification. Currently, my country's coal gasification coarse slag suffers from problems such as large production volume, lack of scientific management, and low comprehensive utilization rate. This not only occupies a large amount of land and harms the ecological environment but also results in the waste of a large amount of resources. Summary of the Invention

[0004] To address the aforementioned issues, this invention, from the perspective of waste co-treatment, utilizes the coarse slag from coal gasification of bulk industrial coal-based solid waste as a solidifying agent for the heavy metal chromium. Through synergistic thermal treatment, it inhibits the oxidation of Cr(III) in sludge, preventing the formation of highly toxic and easily migrating Cr(VI), while simultaneously achieving the detoxification of chromium-containing sludge and the stabilization and harmless treatment of chromium. This effectively reduces the risk of leaching from sintering ash and the resulting environmental hazards. This method is highly practical, reproducible, easy to operate, and has low processing costs. By "treating waste with waste," it simultaneously achieves the safe treatment of hazardous waste and the resource-based reuse of waste.

[0005] To achieve the above objectives, the method for solidifying chromium-containing sludge based on coal gasification coarse slag provided by the present invention includes the following steps:

[0006] Step 1: After thoroughly drying the chromium-containing sludge and coal gasification coarse slag, ball mill them and then sieve them.

[0007] Step 2: Mix the chromium-containing sludge and coal gasification coarse slag after sieving in Step 1 at a weight ratio of 1:1 to 5.

[0008] Step 3: Calcine the mixture obtained in Step 2 at 1000-1150℃ in air for 2-5 hours, and then allow it to cool naturally to obtain microcrystals of calcium chromate garnet mineral encapsulated in aluminosilicate glass.

[0009] In step 1 above, it is preferable to dry the chromium-containing sludge and the coarse coal gasification slag at 110-130°C for 6-12 hours respectively, so that their moisture content is less than 1%.

[0010] In step 2 above, it is preferable to mix the chromium-containing sludge and coal gasification coarse slag after sieving in step 1 evenly at a weight ratio of 1:3 to 4.

[0011] In step 3 above, it is preferable to calcine the mixture obtained in step 2 at 1050-1100°C for 2-3 hours in an air atmosphere.

[0012] Furthermore, in step 3 above, the preferred heating rate for calcination is 5–10 °C / min.

[0013] In step 3 above, the calcined mixture is naturally cooled to room temperature in an air atmosphere.

[0014] The beneficial effects of this invention are as follows:

[0015] 1. This invention significantly inhibits the oxidation of Cr(III) in chromium-containing sludge by co-calcining two types of waste: coal gasification slag and chromium-containing sludge. Simultaneously, it crystallizes the heavy metal chromium in the sludge, effectively preventing its migration into the environment, achieving a chromium fixation rate of over 99.9%. Furthermore, the aluminosilicate glassy phase generated by calcination of this solidification system encapsulates the calcium chromate mineral phase, achieving dual stabilization through physical encapsulation and chemical solidification of heavy metal ions. This provides a double barrier to inhibit chromium migration, thereby achieving chromium stabilization and detoxification. The calcined product was tested using the solid waste leaching toxicity leaching method—acetic acid buffer solution method (HJ / T HJ / 300-2007). Its Cr(VI) leaching concentration was far below 1 mg / L and the total chromium leaching concentration was below 6 mg / L, meeting the relevant requirements of the national standard—"Standard for Identification of Leaching Toxicity of Hazardous Waste" (GB5085.3-2007), namely, the total chromium concentration is below 15 mg / L and the Cr(VI) concentration is below 5 mg / L.

[0016] 2. This invention can remove recalcitrant organic pollutants and pathogens from chromium-containing sludge, and simultaneously achieve the immobilization of chromium in the sludge. This method has the advantages of not consuming chemical reagents and not generating secondary pollution. It is a more economical, safer, easier-to-operate, and environmentally sustainable alternative treatment technology, enabling its industrial application in the safe, efficient, and large-scale disposal of tannery sludge.

[0017] 3. This invention utilizes a technical approach to fix chromium, a heavy metal, in chromium-containing sludge using coal gasification coarse slag, highlighting its unique environmental and resource value. This method is essentially a practice of "treating waste with waste and recycling." This process not only consumes a large amount of untreated coal gasification coarse slag, solving the environmental and resource waste problems caused by its accumulation, but also provides a low-cost, highly adaptable solution for the harmless treatment of chromium-containing sludge. Ultimately, it achieves simultaneous reduction and resource utilization of two types of industrial solid waste, providing a new approach to industrial solid waste recycling and environmental pollution control. Attached Figure Description

[0018] Figure 1 This is the XRD pattern of coarse coal gasification slag.

[0019] Figure 2 This is the XRD pattern of tannery sludge.

[0020] Figure 3 These are XRD patterns of the calcined products of tannery sludge and coal gasification slag mixed at different calcination temperatures.

[0021] Figure 4 This is an SEM image of the product from the mixed calcination of tannery sludge and coal gasification slag in Example 1. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below through specific embodiments. These embodiments are merely descriptive and not limiting. Obviously, the described embodiments are only a part of the embodiments of this invention, not all of them, and should not be used to limit the scope of protection of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0023] The composition of the tannery sludge and coal gasification slag in the examples was determined by X-ray fluorescence spectrometry; the surface morphology and crystal phase composition of the calcined products were characterized by scanning electron microscopy and X-ray diffraction, respectively; the total chromium concentration of the calcined products was determined by the Chinese standard method for identification of leaching toxicity of hazardous waste, "Leaching Method for Leaching Toxicity of Solid Waste - Acetic Acid Buffer Solution Method" (HJ / T HJ / 300-2007) and flame atomic absorption spectrophotometry; the leached Cr(VI) concentration was determined by alkaline solution extraction-flame atomic absorption spectrophotometry (HJ 1082-2019); all results are the average of three tests.

[0024] The composition and content of tanning sludge and coal gasification slag in the embodiments are shown in Table 1.

[0025] Table 1. Composition and content of tannery sludge and coal gasification coarse slag

[0026] Note: In the table, nd indicates that the component was not detected.

[0027] Example 1

[0028] Step 1: Dry the tannery sludge and coal gasification slag thoroughly at 120℃ for 12 hours until the moisture content of the materials is less than 1%. The two materials are then ball-milled and passed through a 100-mesh sieve.

[0029] Step 2: Mix the tanning sludge and coal gasification coarse slag after sieving in Step 1 at a weight ratio of 1:4, and then grind them into homogenate.

[0030] Step 3: The mixture obtained in Step 2 was heated to 1100℃ in air at a heating rate of 5℃ / min and calcined at this temperature for 2 hours. After calcination, it was naturally cooled to room temperature in air. Characterization showed that Cr(III) in the tannery sludge was fixed in the calcium chromate (3CaO·3SiO2·Cr2O3) mineral phase, which has a very stable structure, high melting point, and corrosion resistance. In addition, the aluminosilicate glassy phase generated after calcination encapsulates the calcium chromate mineral phase, achieving dual stabilization of heavy metal ions through physical encapsulation and chemical solidification, further providing a double barrier to inhibit chromium migration, thus achieving detoxification of the tannery sludge and chromium stabilization. In this process, the oxidation of Cr(III) was effectively inhibited, while the formation of highly toxic Cr(VI) was also solved. The calcined product was evaluated by leaching tests, and the concentration of Cr(VI) in the leachate was 0.27 mg / L, and the total chromium concentration was 1.93 mg / L.

[0031] Following the method of Example 1, only the calcination temperature in step 3 was changed to investigate the effect of calcination temperature on the calcined product. Figure 3 This reflects the changes in various phases in the mixed calcination products of tannery sludge and coal gasification slag during the solidification of heavy metal chromium, as the calcination temperature increases. (Comparison) Figures 1-3 It was observed that a chromium-containing crystalline phase, uvarovite, was present at 900℃, with chromium existing in the form of Cr(III). Further increasing the calcination temperature to above 1000℃ resulted in the appearance of numerous uvarovite mineral microcrystals, with their peak intensity and crystallinity gradually increasing with calcination temperature. This demonstrates that Cr(III) in the tannery sludge was successfully crystallized, preventing chromium migration. Elements such as silicon, aluminum, and iron in the tannery sludge and coal gasification slag combine with alkali / alkaline earth metals potassium, sodium, calcium, and magnesium to form mineral crystals, which can inhibit the formation of highly toxic Cr(VI).

[0032] from Figure 4 It can be seen that the calcination product of the tannery sludge and coal gasification slag in Example 1 consists of a large number of mineral crystalline phases of different shapes, which are tightly bound together, proving that chromium is crystallized and immobilized during the heat treatment process.

[0033] Example 2

[0034] Step 1: Dry the tannery sludge and coal gasification slag thoroughly at 120℃ for 12 hours until the moisture content of the materials is less than 1%. The two materials are then ball-milled and passed through a 100-mesh sieve.

[0035] Step 2: Mix the tanning sludge and coal gasification coarse slag after sieving in Step 1 at a weight ratio of 3:7, and then grind them into homogenate.

[0036] Step 3: The mixture obtained in Step 2 was heated to 1100℃ in air at a heating rate of 5℃ / min and calcined at this temperature for 2 hours. After calcination, it was allowed to cool naturally to room temperature in air. Characterization showed that Cr(III) in the tannery sludge was immobilized in the calcium chromate (3CaO·3SiO2·Cr2O3) mineral phase, while effectively inhibiting the formation of Cr(VI). The calcined product was evaluated by leaching tests, and the Cr(VI) leaching concentration was 0.43 mg / L, and the total chromium leaching concentration was 2.72 mg / L.

[0037] Example 3

[0038] Step 1: Dry the tannery sludge and coal gasification slag thoroughly at 120℃ for 12 hours until the moisture content of the materials is less than 1%. The two materials are then ball-milled and passed through a 100-mesh sieve.

[0039] Step 2: Mix the tanning sludge and coal gasification coarse slag after sieving in Step 1 at a weight ratio of 2:3, and then grind them into homogenate.

[0040] Step 3: The mixture obtained in Step 2 was heated to 1100℃ in air at a heating rate of 5℃ / min and calcined at this temperature for 2 hours. After calcination, it was allowed to cool naturally to room temperature in air. Characterization showed that Cr(III) in the tannery sludge was immobilized in the calcium chromate (3CaO·3SiO2·Cr2O3) mineral phase, while effectively inhibiting the formation of Cr(VI). The calcined product was evaluated by leaching tests, and the Cr(VI) leaching concentration was 0.62 mg / L, and the total chromium leaching concentration was 3.41 mg / L.

[0041] Example 4

[0042] Step 1: Dry the tannery sludge and coal gasification slag thoroughly at 120℃ for 12 hours until the moisture content of the materials is less than 1%. The two materials are then ball-milled and passed through a 100-mesh sieve.

[0043] Step 2: Mix the tanning sludge and coal gasification coarse slag after sieving in Step 1 at a weight ratio of 1:1, and then grind them into homogenate.

[0044] Step 3: The mixture obtained in Step 2 was heated to 1100℃ in air at a heating rate of 5℃ / min and calcined at this temperature for 2 hours. After calcination, it was allowed to cool naturally to room temperature in air. Characterization showed that Cr(III) in the tannery sludge was immobilized in the calcium chromate (3CaO·3SiO2·Cr2O3) mineral phase, while effectively inhibiting the formation of Cr(VI). The calcined product was evaluated by leaching tests, and the Cr(VI) leaching concentration was 0.98 mg / L, and the total chromium leaching concentration was 4.97 mg / L.

[0045] Comparative Example 1

[0046] Step 1: Dry the tannery sludge thoroughly at 120℃ for 12 hours until the moisture content of the material is less than 1%. The resulting material is then ball-milled and passed through a 100-mesh sieve.

[0047] Step 2: The tannery sludge sieved in Step 1 was heated to 1100℃ in air at a heating rate of 5℃ / min and calcined at this temperature for 2 hours. After calcination, it was allowed to cool naturally to room temperature in air. Characterization showed that the calcination products were CaCrO4 and Cr5O. 12 Chromium crystalline phases such as Cr2O3 and MgCr2O4 were present. Due to the lack of a solidifying agent, a large amount of Cr(III) was oxidized to Cr(VI), and Cr2O3 was not completely immobilized into a stable mineral phase. Leaching tests showed that the Cr(VI) leaching concentration was 173.56 mg / L, and the total chromium leaching concentration was 982.75 mg / L, far exceeding the leaching toxicity limits for total chromium and Cr(VI) in the national hazardous waste identification standards.

[0048] Comparative Example 2

[0049] Step 1: Dry the tannery sludge and coal gasification slag thoroughly at 120℃ for 12 hours until the moisture content of the materials is less than 1%. The two materials are then ball-milled and passed through a 100-mesh sieve.

[0050] Step 2: Mix the tanning sludge and coal gasification coarse slag after sieving in Step 1 at a weight ratio of 4:1, and then grind them into homogenate.

[0051] Step 3: The mixture obtained in Step 2 was heated to 1100℃ in air at a heating rate of 5℃ / min and calcined at this temperature for 2 hours. After calcination, it was allowed to cool naturally to room temperature in air. Characterization showed that the calcined product was a calcium chromate mineral phase, and some Cr(III) from the tannery sludge was fixed into the mineral phase; however, the formation of Cr(VI) was not sufficiently inhibited. Leaching tests showed that the Cr(VI) leaching concentration was 52.77 mg / L, and the total chromium leaching concentration was 128.62 mg / L, still far exceeding the national standard limit.

[0052] Comparative Example 3

[0053] Step 1: Dry the tannery sludge and coal gasification slag thoroughly at 120℃ for 12 hours until the moisture content of the materials is less than 1%. The two materials are then ball-milled and passed through a 100-mesh sieve.

[0054] Step 2: Mix the tanning sludge and coal gasification coarse slag after sieving in Step 1 at a weight ratio of 9:1, and then grind them into homogenate.

[0055] Step 3: The mixture obtained in Step 2 was heated to 1100℃ in air at a heating rate of 5℃ / min and calcined at this temperature for 2 hours. After calcination, it was allowed to cool naturally to room temperature in air. Characterization showed that the calcined product was a calcium chromate mineral phase, and some Cr(III) from the tannery sludge was fixed into the mineral phase; however, the formation of Cr(VI) was not sufficiently inhibited. Leaching tests showed that the Cr(VI) leaching concentration was 76.23 mg / L, and the total chromium leaching concentration was 189.31 mg / L, still far exceeding the national standard limit.

[0056] In the above embodiments, the tanning sludge is alkaline. The coarse slag from coal gasification is used to solidify the chromium in the tanning sludge. On one hand, the acidic substances in the coarse slag react with the alkaline substances in the tanning sludge at high temperatures, reducing the alkalinity of the mixture system and preventing the alkaline substances in the system from inducing the oxidation of Cr(III) to Cr(VI), thereby inhibiting the oxidation and migration of Cr(III). On the other hand, the coarse slag from coal gasification converts the heavy metal chromium in the tanning sludge into stable and acid- and alkali-resistant calcium chromate garnet mineral microcrystals, greatly reducing the leaching concentration of total chromium and Cr(VI) in the sintering ash, meeting the relevant requirements of the national standard "Identification Standard for Leaching Toxicity of Hazardous Waste" (GB 5085.3-2007), and achieving the requirements for solid waste landfill and the preparation of ceramics, building materials, and refractory materials, thus realizing the resource utilization of waste.

Claims

1. A method for solidifying chromium-containing sludge based on coal gasification coarse slag, characterized in that... The method includes the following steps: Step 1: After thoroughly drying the chromium-containing sludge and coal gasification coarse slag, ball mill them and then sieve them; Step 2: Mix the chromium-containing sludge and coal gasification coarse slag after sieving in Step 1 evenly at a weight ratio of 1:1 to 5; Step 3: Calcine the mixture obtained in Step 2 at 1000-1150℃ in air for 2-5 hours, and then allow it to cool naturally to obtain microcrystals of calcium chromate garnet mineral encapsulated in aluminosilicate glass.

2. The method for solidifying chromium-containing sludge based on coal gasification coarse slag according to claim 1, characterized in that: In step 1, the chromium-containing sludge and the coarse coal gasification slag are dried at 110-130℃ for 6-12 hours to ensure that their moisture content is less than 1%.

3. The method for solidifying chromium-containing sludge based on coal gasification coarse slag according to claim 1, characterized in that: In step 2, the chromium-containing sludge and coal gasification coarse slag after sieving in step 1 are mixed evenly at a weight ratio of 1:3 to 4.

4. The method for solidifying chromium-containing sludge based on coal gasification coarse slag according to claim 1, characterized in that: In step 3, the mixture obtained in step 2 is calcined at 1050-1100℃ in air for 2-3 hours.

5. The method for solidifying chromium-containing sludge based on coal gasification coarse slag according to claim 1 or 4, characterized in that: In step 3, the heating rate of the calcination is 5-10℃ / min.

6. The method for solidifying chromium-containing sludge based on coal gasification coarse slag according to claim 1, characterized in that: In step 3, the calcined mixture is allowed to cool naturally to room temperature in an air atmosphere.

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