Method for pressure leaching of construction waste contaminated with hexavalent chromium

Abstract

The application relates to the field of waste treatment, and discloses a method for changing pressure and leaching of construction waste polluted by hexavalent chromium. The method comprises the following steps: (1) mixing the construction waste polluted by hexavalent chromium and a first solution, and ensuring that the pH of the obtained mixture is not higher than 6; (2) performing at least one low-pressure-normal-pressure treatment on the obtained mixture in step (1); and (3) performing first solid-liquid separation on the obtained mixture in step (2). The method can more fully and efficiently remove hexavalent chromium in the construction waste.

Description

Technical Field

[0001] This invention relates to the field of waste treatment, specifically to a pressure immersion method for hexavalent chromium-contaminated construction waste. Background Technology

[0002] Production enterprises involving hexavalent chromium often face the problem of hexavalent chromium contamination of buildings and structures. Leaking hexavalent chromium typically rises along building walls via rainwater and wash water through capillary action, seeping into the building materials over time. Some of this hexavalent chromium is converted to trivalent chromium, but the hexavalent chromium content remains high. Hexavalent chromium in construction waste generated after demolition is released into the environment. Because hexavalent chromium has strong oxidizing and carcinogenic properties, hexavalent chromium-contaminated construction waste must be properly disposed of to eliminate the risk of harm.

[0003] Immersion washing is a common method for treating hexavalent chromium solid pollutants. It involves soaking construction waste in water or chemical solutions (such as acids, alkalis, ferrous sulfate, and calcium polysulfide) to transfer hexavalent chromium from the solid phase to the liquid phase, or to reduce hexavalent chromium to trivalent chromium. The difficulty in immersion washing of construction waste for hexavalent chromium lies in the fact that construction waste has a porous structure, and air in the pores cannot escape during the immersion process, creating air barriers that prevent the immersion solution from penetrating the interior of the construction waste and dissolving the hexavalent chromium.

[0004] To remove hexavalent chromium from construction waste to a manageable level, it is usually necessary to increase the amount of reducing agent and extend the soaking time. Although this measure increases the amount of agent or extends the treatment time, it is still difficult to achieve a good removal effect for construction waste with high hexavalent chromium content, and the treatment efficiency is also low.

[0005] Therefore, more efficient methods are needed to remove hexavalent chromium from construction waste more completely. Summary of the Invention

[0006] The purpose of this invention is to overcome the above-mentioned problems existing in the prior art and to provide a pressure immersion method for hexavalent chromium contaminated construction waste, which can fully and efficiently remove hexavalent chromium from construction waste.

[0007] To achieve the above objectives, the present invention provides a method for pressure immersion washing of hexavalent chromium-contaminated construction waste, the method comprising:

[0008] (1) Mix hexavalent chromium-contaminated construction waste and the first solution in a first mixing process, such that the pH of the material obtained from the first mixing is not higher than 6;

[0009] The first solution contains a first salt, which is selected from at least one of the sulfates of Group IA metals, the nitrates of Group IA metals, and the nitrates of Group IIA metals; the concentration of the first salt in the first solution is 0.3-0.7 mol / L.

[0010] (2) The material obtained in step (1) is subjected to at least one low-pressure-atmospheric pressure treatment;

[0011] The low-pressure-normal-pressure treatment method includes: placing the material under low-pressure treatment at 0.1-50 kPa, and then placing the material under normal-pressure treatment.

[0012] (3) Perform a first solid-liquid separation on the material obtained in step (2).

[0013] The above technical solution can effectively remove hexavalent chromium from construction waste, especially hexavalent chromium that has penetrated into the interior of construction waste and is generally difficult to remove. After treatment by the method provided by this invention, the content and leaching concentration of hexavalent chromium in construction waste are very low, reaching a level that is undetectable. Detailed Implementation

[0014] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0015] Unless otherwise specified, the pressure mentioned in this article refers to absolute pressure.

[0016] This invention provides a method for treating construction waste contaminated with hexavalent chromium, the method comprising:

[0017] (1) Mix hexavalent chromium-contaminated construction waste and the first solution in a first mixing process, such that the pH of the material obtained from the first mixing is not higher than 6;

[0018] The first solution contains a first salt, which is selected from at least one of the sulfates of Group IA metals, the nitrates of Group IA metals, and the nitrates of Group IIA metals; the concentration of the first salt in the first solution is 0.3-0.7 mol / L.

[0019] (2) The material obtained in step (1) is subjected to at least one low-pressure-atmospheric pressure treatment;

[0020] The low-pressure-normal-pressure treatment method includes: placing the material under low-pressure treatment at 0.1-50 kPa, and then placing the material under normal-pressure treatment.

[0021] (3) Perform a first solid-liquid separation on the material obtained in step (2).

[0022] Traditional methods for treating construction waste contaminated with hexavalent chromium typically require increasing the amount of reducing agent and extending the soaking time. However, these methods are inefficient and fail to completely remove the hexavalent chromium, especially that that has penetrated into the construction waste. The inventors of this invention have discovered that, compared to traditional leaching methods, the method provided by this invention can shorten the treatment time by 40-60% and reduce the amount of reagents used by 20-40%. Furthermore, it can effectively remove hexavalent chromium from construction waste, particularly removing hexavalent chromium that has penetrated into the waste and is difficult to remove. After treatment, the hexavalent chromium content and leaching concentration are extremely low, reaching undetectable levels.

[0023] The construction waste can be in the form of cement blocks, concrete blocks, or bricks, etc.

[0024] According to the present invention, preferably, the hexavalent chromium content in the hexavalent chromium-contaminated construction waste is 100-1500 mg / kg (for example, it can be 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500 and any two of the above values, and any value within the range), and the leaching concentration of hexavalent chromium is 10-100 mg / L (for example, it can be 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 and any two of the above values, and any value within the range).

[0025] Specifically, the mass content of hexavalent chromium in construction waste contaminated with hexavalent chromium can be determined according to the method in "Determination of Hexavalent Chromium in Solid Waste by Alkali Digestion Flame Atomic Absorption Spectrophotometry (HJ687-2014)"; the leaching concentration of hexavalent chromium in construction waste contaminated with hexavalent chromium can be leached according to the method in "Leaching Toxicity Method for Solid Waste by Sulfuric Acid and Nitric Acid Method (HJT299-2007)" and the concentration of hexavalent chromium in the leachate can be detected according to the method in "Determination of Hexavalent Chromium in Water by Flow Injection-Diphenylcarbazide Spectrophotometry (HJ 908-2017)".

[0026] According to the present invention, preferably, in the first mixture, the diameter of the hexavalent chromium-contaminated construction waste is ≤3 cm, more preferably ≤2.5 cm. The target diameter can be obtained by sieving; for example, material that can pass through a sieve with 3 cm apertures is considered to have a diameter ≤3 cm. Generally, if the diameter of the hexavalent chromium-contaminated construction waste obtained from the contaminated site is larger than the above range, it can be crushed to the above range first. The solution of the present invention is particularly suitable for the treatment of the aforementioned hexavalent chromium-contaminated construction waste.

[0027] According to the present invention, preferably, the concentration of the first salt in the first solution is 0.35-0.65 mol / L, more preferably 0.4-0.6 mol / L.

[0028] According to the present invention, preferably, the mass ratio of hexavalent chromium-contaminated construction waste to the first solution is 1:(3-5).

[0029] According to the present invention, preferably, the first salt is selected from at least one of sodium sulfate, potassium sulfate, sodium nitrate, potassium nitrate, magnesium nitrate and calcium nitrate, and more preferably from at least one of sodium sulfate, potassium sulfate, sodium nitrate and potassium nitrate.

[0030] According to the present invention, preferably, the method further includes: in step (1), the pH of the first mixture is 3.5-5.5, preferably 4-5. For example, concentrated nitric acid or concentrated sulfuric acid can be used to adjust the liquid phase pH to the above range.

[0031] Under the above conditions, the removal effect of hexavalent chromium can be further guaranteed.

[0032] According to the present invention, preferably, in step (2), the low-pressure-atmospheric pressure treatment is performed for a treatment time of at least 20 hours, preferably 20-48 hours, more preferably 20-36 hours (for example, it can be 20, 22, 24, 26, 28, 30, 32, 34, 36, and any value within the range formed by any two of the above values). It is understood that the low-pressure-atmospheric pressure treatment can be performed multiple times, that is, the low-pressure treatment can be performed immediately after the current atmospheric pressure treatment is completed. This is repeated multiple times until the total treatment time of the multiple low-pressure-atmospheric pressure treatments meets the above requirements.

[0033] This further ensures the removal effect of hexavalent chromium. The material containing hexavalent chromium-contaminated construction waste and the first solution can be placed in a container, and then a vacuum pump is used to control the pressure to reach a low level. The vacuum pump is turned off when the pressure is at normal.

[0034] According to the present invention, preferably, in step (2), in each low-pressure-atmospheric-pressure treatment, the pressure of the low-pressure treatment is independently 2-4 kPa, and the time of the low-pressure treatment is independently 3-6 min. Low-pressure treatment means placing the material in the above-mentioned low-pressure environment and maintaining it for the above-mentioned time.

[0035] In each low-pressure-atmospheric pressure treatment, the pressure of the low-pressure treatment can be the same or different. According to a particularly preferred embodiment of the present invention, the pressure and time of the low-pressure treatment are the same in each low-pressure-atmospheric pressure treatment. This makes operation easier.

[0036] According to the present invention, preferably, in step (2), the time for atmospheric pressure treatment in each low-pressure-atmospheric pressure treatment is independently 20-40 minutes. Atmospheric pressure treatment means placing the material in the above-mentioned atmospheric pressure environment and maintaining it for the above-mentioned time.

[0037] According to the present invention, preferably, the method further includes: in step (2), the atmospheric pressure treatment is performed under a first ultrasound. Each atmospheric pressure treatment can be performed under the first ultrasound. In each atmospheric pressure treatment, the entire process can be performed under the first ultrasound.

[0038] Preferably, the frequency of the first ultrasound is 20-30 kHz (for example, it can be 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, or any two of the above values, or any value within that range), and the power is 0.15-0.25 W / g of the first mixed material. In each atmospheric pressure treatment, the frequency and time of the ultrasound can be the same or different, but it is preferred that they be the same, which facilitates operation.

[0039] The inventors of this invention further discovered that when treatment is performed under normal pressure under the first ultrasound, it can better complement the first solution and low-pressure treatment used in this invention, thereby achieving better treatment results.

[0040] According to the present invention, preferably, the method further includes: after the first solid-liquid separation, the resulting solid phase and water are mixed a second time, and the material obtained from the second mixture is subjected to a second ultrasonic treatment. This further ensures the removal of residual hexavalent chromium. The first solid-liquid separation can be carried out by filtration.

[0041] In this process, the mass of water used in the second mixture can be 2-6 kg (for example, 2, 3, 4, 5, or 6 kg) relative to the 1 kg of solid phase obtained from the first solid-liquid separation.

[0042] According to the present invention, preferably, the frequency of the second ultrasound is 20-30 kHz (for example, it can be 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30), the power is 0.15-0.25 w / g of the second mixed material, and the time is 5-10 min.

[0043] According to the present invention, preferably, the method further includes: performing a second solid-liquid separation on the material after the second ultrasonication. The second solid-liquid separation (e.g., filtration) is performed to obtain treated hexavalent chromium-contaminated construction waste. The second solid-liquid separation can be performed after the second ultrasonication is completed.

[0044] The present invention will be described in detail below through embodiments.

[0045] In the following embodiments, the hexavalent chromium-contaminated construction waste A and B were both taken from a decommissioned chromium salt production site in Jinan City, Shandong Province. They were concrete blocks with a yellow surface and a pale yellow interior when broken open.

[0046] For untreated hexavalent chromium-contaminated construction waste, and for hexavalent chromium-contaminated construction waste treated using the method provided in this invention:

[0047] The mass content of hexavalent chromium was determined according to the method of "Determination of Hexavalent Chromium in Solid Waste by Alkali Digestion Flame Atomic Absorption Spectrophotometry (HJ687-2014)"; the leaching concentration of hexavalent chromium was determined according to the method of "Leaching Toxicity of Solid Waste by Sulfuric Acid and Nitric Acid Method (HJT299-2007)" and the concentration of hexavalent chromium in the leachate was determined according to the method of "Determination of Hexavalent Chromium in Water by Flow Injection-Diphenylcarbazide Spectrophotometry (HJ 908-2017)".

[0048] The parameters of A and B were determined as shown in Table 1 below:

[0049] Table 1

[0050] A 458 43.6 B 652 56.8

[0051] Examples 1-9 and Comparative Examples 1-2

[0052] According to the conditions in Table 2-3, the following processing shall be performed:

[0053] (1) Crushing and sieving the hexavalent chromium-contaminated construction waste A to obtain specific block sizes;

[0054] Hexavalent chromium-contaminated construction waste was mixed with a first solution containing the first salt, and the pH of the solution was adjusted using concentrated sulfuric acid.

[0055] (2) The material obtained in step (1) is subjected to low-pressure-atmospheric pressure treatment. The pressure and time of each low-pressure treatment are the same as those of the first low-pressure treatment. The time of each medium-atmospheric pressure treatment is the same as that of the first atmospheric pressure treatment. The entire process of each atmospheric pressure treatment is carried out under the first ultrasound. The frequency and power of the first ultrasound are the same each time.

[0056] (3) Filter the material obtained in step (2), mix the obtained solid phase and water (the mass ratio of solid phase and water is 1:5), and perform a second ultrasonic test on the second mixed material; the frequency of the second ultrasonic test is 20kHz, the power is 0.2w / g of the second mixed material, and the time is 10min.

[0057] (4) The material after the second ultrasound is filtered, and the resulting solid phase is the treated hexavalent chromium-contaminated construction waste.

[0058] In Table 2, "-" indicates none. The "Total Time" in Table 2 refers to the total processing time for multiple low-pressure-atmospheric-pressure treatments.

[0059] In Comparative Example 1, the pressure of the low-pressure treatment was 101 kPa, meaning that a vacuum pump was not actually used to control the pressure.

[0060] Comparative Example 2 did not use the first salt, but in the first mixture, the ratio of construction waste to water was the same as that in Example 1;

[0061] ND, or NOT DETECTED, means not detected. For hexavalent chromium content, it means below the detection limit of 2 mg / kg; for hexavalent chromium leaching concentration, it means below the detection limit of 0.004 mg / L.

[0062] Table 2

[0063]

[0064]

[0065] Table 3

[0066]

[0067] Example 10

[0068] The method is the same as in Example 1, except that hexavalent chromium-contaminated construction waste is replaced with B.

[0069] Example 11

[0070] The method is the same as in Example 3, except that ultrasound was not activated during the atmospheric pressure treatment.

[0071] For the hexavalent chromium-contaminated construction waste treated in the above embodiments and comparative examples, the mass content and leaching concentration of hexavalent chromium were tested, as shown in Table 4.

[0072] Table 4

[0073] Example 1 ND ND Example 2 ND ND Example 3 ND ND Example 4 ND ND Example 5 ND ND Example 6 2.2 0.05 Example 7 6.7 0.52 Example 8 5.8 0.46 Example 9 6.2 0.49 Example 10 ND ND Example 11 15.6 1.8 Comparative Example 1 106 11.3 Comparative Example 2 20.5 2.3

[0074] The results above show that the embodiments of the present invention can remove hexavalent chromium from construction waste more fully and efficiently.

[0075] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for pressure leaching of hexavalent chromium-contaminated construction waste, characterized in that, The method includes: (1) Mix hexavalent chromium-contaminated construction waste and the first solution in a first mixing process, such that the pH of the material obtained from the first mixing is not higher than 6; The first solution contains a first salt, which is selected from at least one of the sulfates of Group IA metals, the nitrates of Group IA metals, and the nitrates of Group IIA metals; the concentration of the first salt in the first solution is 0.3-0.7 mol / L. (2) The material obtained in step (1) is subjected to at least one low-pressure-atmospheric pressure treatment to remove the hexavalent chromium that has penetrated into the porous construction waste and is difficult to remove. The low-pressure-atmospheric-pressure treatment method includes: placing the material under 2-4 kPa for low-pressure treatment, and then placing the material under atmospheric pressure for atmospheric-pressure treatment; the low-pressure-atmospheric-pressure treatment is carried out for a total treatment time of at least 20 hours, and in each low-pressure-atmospheric-pressure treatment, the low-pressure treatment time is 3-6 minutes and the atmospheric-pressure treatment time is 20-40 minutes; the atmospheric-pressure treatment is carried out under first ultrasound. (3) Perform a first solid-liquid separation on the material obtained in step (2).

2. The method according to claim 1, wherein, In construction waste contaminated with hexavalent chromium, the mass content of hexavalent chromium is 100-1500 mg / kg, and the leaching concentration of hexavalent chromium is 10-100 mg / L.

3. The method according to claim 1, wherein, In the first mixture, the diameter of the hexavalent chromium-contaminated construction waste is ≤3cm.

4. The method according to claim 1, wherein, In the first mixture, the diameter of the hexavalent chromium-contaminated construction waste is ≤2.5cm.

5. The method according to claim 1, wherein, In the first solution, the concentration of the first salt is 0.35-0.65 mol / L; And / or, the mass ratio of hexavalent chromium-contaminated construction waste to the first solution is 1:(3-5); And / or, the first salt is selected from at least one of sodium sulfate, potassium sulfate, sodium nitrate, potassium nitrate, magnesium nitrate, and calcium nitrate.

6. The method according to claim 5, wherein, In the first solution, the concentration of the first salt is 0.4-0.6 mol / L; And / or, the first salt is selected from at least one of sodium sulfate, potassium sulfate, sodium nitrate, and potassium nitrate.

7. The method according to any one of claims 1-6, wherein, The method further includes: in step (1), the pH of the first mixture is 3.5-5.

5.

8. The method according to claim 7, wherein, The method further includes: in step (1), making the pH of the first mixture obtained 4-5.

9. The method according to claim 1, wherein, In step (2), the low-pressure to atmospheric pressure treatment is carried out for a treatment time of 20-48 hours.

10. The method according to claim 9, wherein, In step (2), the low-pressure to atmospheric pressure treatment is carried out for a treatment time of 20-36 hours.

11. The method according to claim 1, wherein, The frequency of the first ultrasound is 20-30kHz, and the power is 0.15-0.25w / g of the first mixed material.

12. The method according to any one of claims 1-6, wherein, The method further includes: after the first solid-liquid separation, the resulting solid phase and water are mixed a second time, and the material obtained from the second mixture is subjected to a second ultrasonic treatment.

13. The method according to claim 12, wherein, The second ultrasound is conducted at a frequency of 20-30 kHz and a power of 0.15-0.25 W / g of the second mixed material for 5-10 minutes.

14. The method according to claim 12, wherein, The method also includes performing a second solid-liquid separation on the material after the second ultrasound.

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