A method for reducing hexavalent chromium in chromium-contaminated soil or chromium residue

Abstract

The present application relates to the technical field of metal smelting, and particularly relates to a method for reducing hexavalent chromium in chromium-contaminated soil or chromium residue. The method comprises the following steps: S1, crushing the chromium-contaminated soil or the chromium residue, mixing the chromium-contaminated soil or the chromium residue with a detoxifying agent and water, and then feeding the mixture into a stirring tank to obtain slurry 1; S2, feeding the slurry 1 into a mechano-chemical reactor to perform mechano-chemical treatment for 5-60 min to obtain slurry 2; and S3, feeding the slurry 2 into a high-pressure filter to perform pressure filtration to obtain a filter cake and pressure filtrate; the filter cake is scattered by a silt machine and stored for aging, and the completely detoxified chromium-contaminated soil or chromium residue is obtained after the aging is completed. The present application realizes long-term removal of hexavalent chromium in high-concentration chromium-contaminated soil or chromium residue through high-concentration mechano-chemical treatment of the chromium-contaminated soil or the chromium residue and a specific compound detoxifying agent, and then through an aging reaction, and the detoxified product has no risk of resolubilization.

Description

Technical Field

[0001] This invention relates to the field of metal smelting technology, and more particularly to a method for reducing hexavalent chromium in chromium-contaminated soil or chromium slag. Background Technology

[0002] Chromium is a common trace element in soil and is frequently used in industries such as chemical, metallurgical, electronics, and light industry. In areas containing chromium minerals, the chromium in the soil mainly originates from leaks during the production and use of hexavalent chromium compounds, including industries such as chromium salt production, leather making, electroplating, and wood preservation. These can be broadly categorized as follows:

[0003] 1) Leakage of hexavalent chromium during the production process of chromium chemical enterprises leads to soil pollution, especially in leaching workshops and acid crystallization workshops.

[0004] 2) Inadequate implementation of seepage and rain protection measures at chromium slag storage sites has led to soil pollution. During the production process involving calcium roasting, approximately 3 tons of chromium slag are generated per ton of product. This slag is primarily stored temporarily through haphazard dumping. While earlier cleanup and harmless disposal of residual chromium slag have been completed, the remaining chromium slag storage sites constitute another source of chromium pollution in China.

[0005] 3) Pollution of electroplating sites caused by electroplating tank leaks. Electroplating tanks are the main section in the chromium plating process that uses Cr(VI). Due to their long service life, electroplating tanks are prone to leaks, which is one of the major pollution risk sources for electroplating enterprises.

[0006] 4) Site pollution caused by the production process and sludge storage areas of old leather tanning enterprises. Cr(VI) contamination leaks are easily detected during the leather tanning process. Furthermore, the treatment facilities for chromium-containing wastewater generated during tanning are inadequate, and there are instances of illegal discharge.

[0007] Furthermore, the production of chromium salts generates a large amount of solid waste, namely calcium-free roasted chromium slag. This slag is strongly alkaline, with a pH value of approximately 11–12.5, and contains 1130–8500 mg / kg of hexavalent chromium. Hexavalent chromium is characterized by strong oxidizing power, high solubility, and rapid migration; its toxicity is approximately 100 times that of trivalent chromium, and it is one of the three internationally recognized carcinogenic metals. Improper disposal of calcium-free roasted chromium slag poses a serious threat to human health and the ecological environment. Therefore, the effective treatment of calcium-free roasted chromium slag is of great significance.

[0008] Traditional chromium slag detoxification strategies can be divided into dry and wet methods. Dry methods primarily achieve chromium slag detoxification through high-temperature reduction or stabilization / solidification. Previous studies have used high-temperature roasting combined with materials such as coal, magnesium chloride, cellulose, sludge, and sucrose for dry reduction. However, high-temperature roasting is not economical for chromium slag treatment due to high energy consumption and greenhouse gas emissions. Solidification / stabilization studies have used materials such as glass-ceramic matrices, red mud, alkali-activated blast furnace slag, and nano-zero-valent iron synthesized from green tea to achieve the reduction and stabilization of Cr(VI) in chromium slag. Solidification / stabilization of chromium slag can reduce the migration of Cr(VI) into the environment, but after solidification, the volume of solid waste increases, leading to increased slag volume and occupying a large amount of land. Furthermore, there is a risk of secondary pollution during long-term storage and landfilling.

[0009] Currently, there are two main approaches to remediating chromium pollution in soil: one is to alter the form of chromium in soil or sediments, reducing Cr(VI) to the relatively less toxic Cr(III), thus decreasing its bioavailability in the soil environment; the other is to remove chromium from soil or sediments. Based on these two approaches, a series of remediation technologies have been developed both domestically and internationally, such as immobilization / stabilization, leaching, soil washing, electrokinetic remediation, chemical reduction, phytoremediation, and microbial remediation. These methods are adaptable to low to medium concentrations (<100 ppm) in soil, but they cannot achieve limited removal for high concentrations of contaminated soil, making it difficult to meet the environmental remediation requirements of <5 ppm.

[0010] Chinese patent CN111889500B discloses a method for detoxifying chromium-containing slag, chromium-containing waste residue, or chromium-containing soil. The method involves mixing crushed chromium-containing slag, chromium-containing waste residue, or chromium-containing soil with reducing agents, a release accelerator, and water, thoroughly stirring and mixing the mixture, placing it in a ball mill jar for wet ball milling, and then directly applying microwave irradiation to the mixture after the wet ball milling reaction to complete the remediation. The hexavalent chromium reduction rate reaches over 99%. However, this method primarily targets chromium-containing slag with a hexavalent chromium content of 4995 mg / kg; the effectiveness for treating higher hexavalent chromium contents is not documented. Summary of the Invention

[0011] This invention provides a method for reducing hexavalent chromium in chromium-contaminated soil or chromium slag, comprising the following steps:

[0012] S1. The chromium-contaminated soil or chromium slag is crushed, mixed with an antidote and water, and then added to a mixing tank to obtain slurry 1;

[0013] S2. Put slurry 1 into a mechanochemical reactor and perform mechanochemical treatment for 5-60 min to obtain slurry 2;

[0014] S3. The slurry 2 is fed into a high-pressure filter for filtration to obtain filter cake and filtrate; the filter cake is broken up by a pulverizer and stored for aging. After aging, completely detoxified chromium-contaminated soil or chromium slag is obtained.

[0015] In some embodiments, the hexavalent chromium content in the chromium-contaminated soil or chromium slag is greater than 5000 ppm.

[0016] In some embodiments, the mass ratio of the chromium-contaminated soil or chromium slag to water is (50-70):100.

[0017] In some embodiments, the amount of the antidote added is 5-13 wt% of the mass of the chromium-contaminated soil or chromium slag.

[0018] In some embodiments, the antidote includes at least one of ferrous sulfate, lime-sulfur mixture, pyrite, pyrrhotite containing more than 40% sulfur, glucose, glycerol, oxalic acid, citric acid, and sodium metabisulfite.

[0019] Furthermore, the applicant discovered that the ferrous sulfate, lime-sulfur mixture, and sodium metabisulfite can exert a rapid detoxification effect, while pyrite, pyrrhotite containing more than 40% sulfur, glucose, glycerol, oxalic acid, and citric acid can exert a long-lasting detoxification effect, and the detoxification effect is better when they are combined.

[0020] Furthermore, the antidote can be a mixture of ferrous sulfate, lime-sulfur mixture, glucose, and glycerol in a mass ratio of 10:2:0.5:0.1.

[0021] Furthermore, the antidote can be ferrous sulfate, pyrite, oxalic acid, and sodium metabisulfite in a mass ratio of 3:2:1:0.5.

[0022] Furthermore, the antidote can be a mixture of lime and sulfur, pyrrhotite containing more than 40% sulfur, glycerol, and citric acid in a mass ratio of 3:2:0.3:0.5.

[0023] Furthermore, the antidote may be ferrous sulfate, pyrrhotite containing more than 40% sulfur, glycerol, and oxalic acid in a mass ratio of 4:2.6:0.1:0.7.

[0024] The lime-sulfur mixture described in this invention is synthesized in-house, and its specific component is calcium polysulfide.

[0025] The method for synthesizing the lime-sulfur mixture is as follows: lime and sulfur are mixed with water to form a slurry, heated at 80-90℃ for 1 hour, and the mass ratio is quicklime:sulfur:water = 1:2:10.

[0026] In some embodiments, the moisture content of the filter cake is <20%.

[0027] In some embodiments, the mechanochemical reactor is one of a sand mill, a stirred mill, a planetary ball mill, or a vibratory mill.

[0028] In some implementations, the high-pressure filter is one of a diaphragm high-pressure filter or a stainless steel hydraulic filter.

[0029] In some implementations, the aging time is 1 to 30 days.

[0030] In some embodiments, the content of hexavalent chromium in the fully detoxified chromium-contaminated soil or chromium slag is less than 5 ppm.

[0031] In some embodiments, the fully detoxified chromium-contaminated soil or chromium slag can be landfilled.

[0032] In some embodiments, the filtrate is returned to S1 and fed into a stirring tank.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] 1. Existing technologies such as leaching and ball milling cannot treat soil contaminated with ultra-high concentrations of chromium. This invention achieves long-term removal of hexavalent chromium from soil contaminated with ultra-high concentrations of chromium or chromium slag by mechanochemical treatment with a specific compound detoxifying agent followed by aging reaction, and the detoxified products have no risk of re-dissolution.

[0035] 2. By reacting high-concentration chromium-contaminated soil or chromium slag with an antidote in a mechanochemical reactor, not only can long-term removal be achieved, but also the inclusions and spinel phase hexavalent chromium can be fully exposed and come into complete contact with the antidote, thus achieving complete detoxification of hexavalent chromium. The mechanochemical reactor is a device that uses a high-energy ball mill for chemical reactions. Conventional drum ball mills can only perform crushing and grinding operations, and cannot achieve a complete chemical reaction between the antidote and hexavalent chromium, nor can they expose the hexavalent chromium in the inclusions and spinel phase.

[0036] 3. The detoxifying agent of the present invention is a compound detoxifying agent. The inorganic chemical components ensure rapid detoxification ability, the natural sulfide minerals ensure medium-term detoxification ability, and the organic components ensure long-term detoxification ability. The combination of multiple reagent components can effectively and continuously detoxify the slow-release hexavalent chromium in solid waste. The multiple components work synergistically to achieve a good detoxification effect. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the method described in this invention. Detailed Implementation

[0038] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] The synthesis method of the lime-sulfur mixture used in the following examples and comparative examples is as follows: lime and sulfur are mixed with water to form a slurry, heated at 85°C for 1 hour, and the mass ratio is quicklime:sulfur:water = 1:2:10.

[0040] Example 1

[0041] This embodiment provides a method for reducing hexavalent chromium in chromium slag, including the following steps:

[0042] S1. A chromium slag from Changsha, Hunan Province was crushed, mixed with an antidote and water, and then added to a mixing tank to obtain slurry 1;

[0043] S2. Put slurry 1 into a mechanochemical reactor (sand mill) for mechanochemical treatment for 20 min to obtain slurry 2;

[0044] S3. The slurry 2 is fed into a diaphragm high-pressure filter for pressure filtration to obtain filter cake and filtrate; the filter cake is broken up by a pulverizer and stored for aging, and after aging, fully detoxified chromium slag is obtained.

[0045] The mass ratio of the chromium slag to water is 50:100.

[0046] The antidote is a mixture of ferrous sulfate, lime-sulfur mixture, glucose, and glycerol in a mass ratio of 10:2:0.5:0.1, and the total amount of the antidote added is 7.6 wt% of the mass of the chromium slag.

[0047] The filter cake has a moisture content of 18.6%.

[0048] The aging time is 1 day.

[0049] The fully detoxified chromium slag is landfilled, and the filtrate is returned to S1 and added to the mixing tank.

[0050] Example 2

[0051] This embodiment provides a method for reducing hexavalent chromium in chromium-contaminated soil, including the following steps:

[0052] S1. A high-concentration chromium-contaminated soil in Shandong was crushed, mixed with an antidote and water, and then added to a mixing tank to obtain slurry 1;

[0053] S2. Put slurry 1 into a mechanochemical reactor (stirred mill) for mechanochemical treatment for 20 min to obtain slurry 2;

[0054] S3. The slurry 2 is fed into a diaphragm high-pressure filter for filtration to obtain filter cake and filtrate; the filter cake is broken up and stored for aging by a soil pulverizer, and after aging, the completely detoxified chromium-contaminated soil is obtained.

[0055] The mass ratio of the chromium slag to water is 70:100.

[0056] The antidote is ferrous sulfate, pyrite, oxalic acid, and sodium metabisulfite in a mass ratio of 3:2:1:0.5, and the total amount of antidote added is 9.5 wt% of the mass of chromium slag.

[0057] The moisture content of the filter cake is 15.3%.

[0058] The aging period is 30 days.

[0059] The completely detoxified chromium-contaminated soil is landfilled, and the filtrate is returned to S1 and added to the mixing tank.

[0060] Example 3

[0061] This embodiment provides a method for reducing hexavalent chromium in chromium-contaminated soil, including the following steps:

[0062] S1. A high-concentration chromium-contaminated soil in Hubei Province was crushed, mixed with an antidote and water, and then added to a mixing tank to obtain slurry 1;

[0063] S2. Put slurry 1 into a mechanochemical reactor (planetary ball mill) for mechanochemical treatment for 20 min to obtain slurry 2;

[0064] S3. The slurry 2 is fed into a diaphragm high-pressure filter for filtration to obtain filter cake and filtrate; the filter cake is broken up and stored for aging by a soil pulverizer, and after aging, the completely detoxified chromium-contaminated soil is obtained.

[0065] The mass ratio of the chromium slag to water is 60:100.

[0066] The antidote is a mixture of lime and sulfur, pyrrhotite containing more than 40% sulfur, glycerol, and citric acid, in a mass ratio of 3:2:0.3:0.5. The total amount of the antidote added is 12.8 wt% of the mass of the chromium slag.

[0067] The filter cake has a moisture content of 15.7%.

[0068] The aging period is 30 days.

[0069] The completely detoxified high-concentration chromium-contaminated soil is landfilled, and the filtrate is returned to S1 and added to the mixing tank.

[0070] Example 4

[0071] This embodiment provides a method for reducing hexavalent chromium in chromium-contaminated soil, including the following steps:

[0072] S1. A high-concentration chromium-contaminated soil in Jiangsu Province was crushed, mixed with an antidote and water, and then added to a mixing tank to obtain slurry 1;

[0073] S2. Put slurry 1 into a mechanochemical reactor (vibrating ball mill) for mechanochemical treatment for 20 min to obtain slurry 2;

[0074] S3. The slurry 2 is fed into a diaphragm high-pressure filter for filtration to obtain filter cake and filtrate; the filter cake is broken up and stored for aging by a soil pulverizer, and after aging, the completely detoxified chromium-contaminated soil is obtained.

[0075] The mass ratio of the chromium slag to water is 65:100.

[0076] The antidote is ferrous sulfate, pyrrhotite containing more than 40% sulfur, glycerol, and oxalic acid, in a mass ratio of 4:2.6:0.1:0.7, and the total amount of antidote added is 5.4 wt% of the mass of chromium slag.

[0077] The filter cake has a moisture content of 18.3%.

[0078] The aging period is 15 days.

[0079] The completely detoxified high-concentration chromium-contaminated soil is landfilled, and the filtrate is returned to S1 and added to the mixing tank.

[0080] Comparative Example 1

[0081] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as in Example 1, except that the detoxifying agent is a mixture of ferrous sulfate, lime-sulfur mixture, glucose, and glycerol in a mass ratio of 2:10:0.5:0.1, and the total amount of detoxifying agent added is 7.6 wt% of the mass of chromium slag.

[0082] Comparative Example 2

[0083] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as in Example 1, except that the detoxifying agent is a mixture of ferrous sulfate and lime-sulfur in a mass ratio of 10:2.6, and the total amount of detoxifying agent added is 7.6 wt% of the mass of chromium slag.

[0084] Comparative Example 3

[0085] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as in Example 1, except that the detoxifying agent is a mixture of ferrous sulfate, lime-sulfur mixture, iron powder, and sodium sulfite in a mass ratio of 10:2:0.5:0.1, and the total amount of detoxifying agent added is 7.6 wt% of the mass of chromium slag.

[0086] Comparative Example 4

[0087] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as that in Example 2, except that the detoxifying agent is ferrous sulfate, pyrite, oxalic acid, and sodium metabisulfite in a mass ratio of 1:4:1:0.5, and the total amount of detoxifying agent added is 9.5 wt% of the mass of chromium slag.

[0088] Comparative Example 5

[0089] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as that in Example 2, except that the detoxifying agent is ferrous sulfate and pyrite in a mass ratio of 4.5:2, and the total amount of detoxifying agent added is 9.5 wt% of the mass of chromium slag.

[0090] Comparative Example 6

[0091] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as that in Example 2, except that the detoxifying agent is ferrous sulfate, pyrite, iron powder and sodium sulfite in a mass ratio of 3:2:1:0.5, and the total amount of detoxifying agent added is 9.5 wt% of the mass of chromium slag.

[0092] Comparative Example 7

[0093] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as that in Example 3, except that the detoxifying agent is a mixture of lime and sulfur, pyrrhotite containing more than 40% sulfur, iron powder, and sodium sulfite in a mass ratio of 4:1:0.3:0.5, and the total amount of detoxifying agent added is 12.8 wt% of the mass of chromium slag.

[0094] Comparative Example 8

[0095] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as that in Example 3, except that the detoxifying agent is a lime-sulfur mixture containing more than 40% sulfur in pyrrhotite, with a mass ratio of 3:2.8, and the total amount of detoxifying agent added is 12.8 wt% of the mass of chromium slag.

[0096] Comparative Example 9

[0097] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as that in Example 3, except that the detoxifying agent is ferrous sulfate, lime-sulfur mixture, pyrrhotite containing more than 40% sulfur, and iron powder, with a mass ratio of 3:2:0.3:0.5, and the total amount of detoxifying agent added is 12.8 wt% of the mass of chromium slag.

[0098] Comparative Example 10

[0099] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as that in Example 4, except that the detoxifying agent is ferrous sulfate, pyrrhotite containing more than 40% sulfur, glycerol, and oxalic acid in a mass ratio of 2:2:2.1:1.3, and the total amount of detoxifying agent added is 5.4 wt% of the mass of chromium slag.

[0100] Comparative Example 11

[0101] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as that in Example 4, except that the detoxifying agent is ferrous sulfate and pyrrhotite in a mass ratio of 4.8:2.6, and the total amount of detoxifying agent added is 5.4 wt% of the mass of chromium slag.

[0102] Comparative Example 12

[0103] This comparative example provides a method for reducing hexavalent chromium in chromium slag. The specific implementation method is the same as that in Example 4, except that the detoxifying agent is ferrous sulfate, pyrrhotite, sodium sulfite, and iron powder in a mass ratio of 4:2.6:0.1:0.7, and the total amount of detoxifying agent added is 5.4 wt% of the mass of chromium slag.

[0104] Performance testing

[0105] High-concentration chromium-contaminated soil or chromium slag was defined as the original sample, and the fully detoxified chromium-contaminated soil or fully detoxified chromium slag obtained in S3 was defined as the detoxified sample. The hexavalent chromium content of the products of the above examples and comparative examples was determined, and the hexavalent chromium content of the samples after 360 days of aging was also determined. The test method referred to HJ1082 / 2019, Determination of Hexavalent Chromium in Soil and Sediments. The experimental results are shown in Table 1.

[0106] Table 1

[0107]

[0108] The data from the examples demonstrate that the present invention not only has a long-lasting detoxification effect on soil or chromium slag contaminated with ultra-high concentrations of chromium, but also exhibits excellent detoxification effects on samples contaminated with medium to low concentrations of chromium. Through a unique formulation, complete detoxification of hexavalent chromium can be achieved, as well as the removal of slowly released, highly toxic hexavalent chromium from the crystal lattice. Comparative experimental data fully illustrate that the combination of rapid and long-lasting detoxifiers is key to achieving detoxification of samples contaminated with ultra-high concentrations of chromium. The rapid detoxifier ensures that the sample reaches the quality standard of <5 ppm immediately after the treatment process and also guarantees that the hexavalent chromium in the sample will not re-dissolve and increase after one year. Comparative examples 1, 4, 5, 10, and 12 show that the reagent formulation has a significant impact on the detoxification effect. Comparative examples 2, 3, 6, 7, 8, 9, and 11 demonstrate that only the combination of rapid and long-lasting detoxifiers can guarantee that the treated chromium-contaminated soil samples meet the standards.

[0109] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for reducing hexavalent chromium in chromium-contaminated soil or chromium slag, characterized in that, Includes the following steps: S1. The chromium-contaminated soil or chromium slag is crushed, mixed with an antidote and water, and then added to a mixing tank to obtain slurry 1; the hexavalent chromium content in the chromium-contaminated soil or chromium slag is greater than 5000 ppm; the amount of antidote added is 5-13 wt% of the mass of the chromium-contaminated soil or chromium slag. S2. Put slurry 1 into a mechanochemical reactor and perform mechanochemical treatment for 5-60 minutes to obtain slurry 2; S3. The slurry 2 is fed into a high-pressure filter for filtration to obtain filter cake and filtrate; the filter cake is broken up by a pulverizer and stored for aging. After aging, fully detoxified chromium-contaminated soil or chromium slag is obtained; the content of hexavalent chromium in the fully detoxified chromium-contaminated soil or chromium slag is less than 5 ppm. The antidote is a mixture of ferrous sulfate, lime-sulfur, glucose, and glycerol in a mass ratio of 10:2:0.5:0.

1. Alternatively, the antidote is ferrous sulfate, pyrite, oxalic acid, and sodium metabisulfite in a mass ratio of 3:2:1:0.

5. Alternatively, the antidote is a mixture of lime and sulfur, pyrrhotite containing more than 40% sulfur, glycerol, and citric acid in a mass ratio of 3:2:0.3:0.

5. Alternatively, the antidote is ferrous sulfate, pyrrhotite containing more than 40% sulfur, glycerol, and oxalic acid, in a mass ratio of 4:2.6:0.1:0.

7. The synthesis method of lime-sulfur mixture is as follows: lime and sulfur are mixed with water to form a slurry, heated at 80-90℃ for 1 hour, and the mass ratio is quicklime:sulfur:water = 1:2:

10.

2. The method according to claim 1, characterized in that, The mass ratio of the chromium-contaminated soil or chromium slag to water is (50-70):

100.

3. The method according to claim 2, characterized in that, The moisture content of the filter cake is <20%.

4. The method according to claim 3, characterized in that, The aging period is 1 to 30 days.

5. The method according to claim 4, characterized in that, The completely detoxified chromium-contaminated soil or chromium slag is then landfilled.

6. The method according to claim 5, characterized in that, The filtrate is returned to S1 and added to the stirring tank.

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