A method for realizing efficient leaching of heavy metal elements in contaminated soil based on co-pyrolysis of chlorine-containing plastics

Abstract

The application provides a method for realizing efficient leaching of heavy metal elements in contaminated soil based on co-pyrolysis of chlorine-containing plastics, and relates to the technical field of metal element extraction.The HCl generated by the combination of chlorine free radicals and hydrogen free radicals in the thermal degradation process of the chlorine-containing plastics is ingeniously combined with the leaching of heavy metals in contaminated soil, so that the dissociation and leaching of the heavy metals combined with the soil mineral lattice are promoted, the shortage of a large amount of acid and alkali reagents consumed in the existing chemical leaching process is made up, and thus the green leaching of heavy metal elements is realized without adding chemical reagents.Furthermore, the chlorine-containing plastics can be selected from waste plastics, and the resource disposal of plastic waste is also facilitated.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal element extraction, and in particular to a method for realizing efficient leaching of heavy metal elements in contaminated soil based on co-pyrolysis of chlorine-containing plastics. BACKGROUND

[0002] Soil heavy metal pollution has always been a concern, and pollution remediation is one of the effective ways to achieve safe use of soil. However, the soil around industrial sites, mining areas and tailings ponds is often heavily contaminated with heavy metals. Due to the long remediation period, high cost and difficulty in completely remediation of heavily contaminated soil, its remediation value is difficult to cover the remediation cost. Therefore, for heavily contaminated soil, researchers rarely deal with it from the remediation and treatment level, but from the perspective of high-value metal element separation and recovery for resource utilization. This way not only reduces the harmfulness of heavily contaminated soil, but also reduces the mining of natural minerals, in line with the concepts of environmental protection and circular economy, and has significant environmental and economic benefits.

[0003] Heavy metal separation and recovery technologies mainly include thermal separation technology, hydrometallurgical technology (including chemical leaching, biological leaching) and electrochemical technology. Among them, the chemical leaching process of hydrometallurgical technology is the most commonly used heavy metal separation and extraction technology, and it is also the only technology that has been industrialized. Chemical leaching technology is to mix soil with strong acid or strong alkaline solution, and through chemical reaction to transfer heavy metals to liquid phase, and then further recover heavy metals by precipitation method, crystallization method, extraction method or electrochemistry method. Acid is the most commonly used leaching agent for chemical leaching, especially high-concentration hydrochloric acid (HCl). Because HCl has good extraction effect and is suitable for simultaneous leaching of multiple heavy metals, it is often used as the best leaching agent for heavy metal extraction and recovery. Although the HCl leaching process is simple and mature, has high extraction rate and short reaction period, it consumes a large amount of water and chemical reagents, and generates high-salinity wastewater that needs secondary treatment, so the comprehensive disposal cost is high. Therefore, it is necessary to improve the current chemical leaching technology of heavy metals to realize green and efficient leaching-extraction-recovery of multiple metals without using acid and alkali chemical reagents as much as possible. SUMMARY

[0004] The purpose of the present application is to provide a method for realizing efficient leaching of heavy metal elements in contaminated soil based on co-pyrolysis of chlorine-containing plastics, which can efficiently leach heavy metals in soil without using acid and alkali chemical reagents, and then realize efficient recovery of heavy metals.

[0005] In order to achieve the above-mentioned purpose of the application, the present application provides the following technical solutions:

[0006] The present application provides a method for realizing efficient leaching of heavy metal elements in contaminated soil based on co-pyrolysis of chlorine-containing plastics, comprising the following steps:

[0007] The heavy metal contaminated soil is mixed with the chlorine-containing plastic to obtain a mixture; when the pH value of the heavy metal contaminated soil is > 6.5, the mass of the chlorine-containing plastic accounts for 15-80% of the mass of the mixture; when the pH value of the heavy metal contaminated soil is ≤ 6.5, the mass of the chlorine-containing plastic accounts for 1-60% of the mass of the mixture.

[0008] The mixture is subjected to co-pyrolysis, and the temperature of the co-pyrolysis is 300-600 ℃.

[0009] Preferably, the content of chlorine element in the chlorine-containing plastic is > 50 wt%.

[0010] Preferably, when the pH value of the heavy metal contaminated soil is > 6.5, the mass of the chlorine-containing plastic accounts for 20-60% of the mass of the mixture.

[0011] Further preferably, when the pH value of the heavy metal contaminated soil is > 6.5, the mass of the chlorine-containing plastic accounts for 25-40% of the mass of the mixture.

[0012] Preferably, when the pH value of the heavy metal contaminated soil is ≤ 6.5, the mass of the chlorine-containing plastic accounts for 1-40% of the mass of the mixture.

[0013] Further preferably, when the pH value of the heavy metal contaminated soil is ≤ 6.5, the mass of the chlorine-containing plastic accounts for 3-15% of the mass of the mixture.

[0014] Preferably, when the pH value of the heavy metal contaminated soil is > 6.5, the temperature of the co-pyrolysis is 400-500 ℃; when the pH value of the heavy metal contaminated soil is ≤ 6.5, the temperature of the co-pyrolysis is 300-500 ℃.

[0015] Preferably, the heavy metal contaminated soil is a heavy heavy metal contaminated soil.

[0016] Preferably, the time of the co-pyrolysis is 1-2 h.

[0017] Preferably, after the mixture is subjected to the co-pyrolysis, the obtained solid pyrolysis product is further placed in an extraction solution for leaching, and through solid-liquid separation, a heavy metal extraction solution is obtained.

[0018] Compared with the prior art, the present application has the following advantages:

[0019] The present application utilizes the HCl generated by the combination of chlorine free radicals and hydrogen free radicals in the thermal degradation process of chlorine-containing plastics, and ingeniously combines with the heavy metal leaching of contaminated soil, promotes the dissociation-leaching of heavy metals combined with soil mineral lattice, makes up for the deficiency of the existing chemical leaching process which consumes a large amount of acid and alkali reagents, and thus realizes the green leaching of heavy metal elements without adding chemical reagents.

[0020] Moreover, the chlorine-containing plastics can be selected from waste plastics, which is also helpful for the resource disposal of plastic waste.

[0021] In addition, the existing thermal separation technology needs to separate and recover metal elements in solid materials by gasification at a high temperature of about 1000 DEG C, which has high energy consumption and large equipment investment. Compared with the thermal separation technology, the present application only needs to pyrolyze the chlorine-containing plastics, and based on the free radical reaction in the pyrolysis process, the soil mineral lattice combined heavy metals are dissolved to the maximum extent, and the highest temperature is only 600 DEG C.

[0022] Compared with the technologies such as biological leaching, electrochemical leaching and supercritical extraction, the present application has the advantages of long reaction period, low extraction rate and complex process. The present application only needs one step of co-pyrolysis, which can activate the soil heavy metals and promote the efficient dissolution of the soil heavy metals, and has the advantages of simple operation, short reaction period and high extraction rate.

[0023] The present application uses calcium chloride (CaCl2) solution to extract the soluble heavy metal elements in the contaminated soil. The higher the content of the extracted soluble heavy metal elements, the greater the potential of the heavy metal leaching from the soil lattice, and the easier the extraction and recovery. Compared with the method of directly using CaCl2 solution leaching without adding chlorine-containing plastics co-pyrolysis, the co-pyrolysis reaction in the present application can increase the leaching rate of the soil cadmium, lead, copper and zinc heavy metal elements by 6-440 times, 138-530 times, 26-483 times and 5-672 times, respectively. The co-pyrolysis effect of adding chlorine-containing plastics can significantly increase the extraction rate of the soil heavy metal elements, which indicates that the leaching and recovery potential of the soil heavy metals is greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 Figure is the influence diagram of the addition amount of chlorine-containing plastics on the leaching rate of heavy metals in alkaline soil;

[0025] Figure 2 Figure is the scanning electron microscope diagram of the co-pyrolysis products of the chlorine-containing plastics with zero addition amount, 8% addition amount and 30% addition amount, and the un-pyrolyzed alkaline soil;

[0026] Figure 3 Figure is the influence diagram of the addition amount of chlorine-containing plastics on the leaching rate of heavy metals in acidic soil. DETAILED DESCRIPTION

[0027] The application provides a method for realizing efficient leaching of heavy metal elements in contaminated soil based on co-pyrolysis of chlorine-containing plastics, and comprises the following steps:

[0028] The heavy metal contaminated soil is mixed with chlorine-containing plastics to obtain a mixture; when the pH value of the heavy metal contaminated soil is > 6.5, the mass of the chlorine-containing plastics accounts for 15-80% of the mass of the mixture; when the pH value of the heavy metal contaminated soil is ≤ 6.5, the mass of the chlorine-containing plastics accounts for 1-60% of the mass of the mixture.

[0029] The mixture is co-pyrolyzed at a temperature of 300-600 DEG C.

[0030] The heavy metal contaminated soil is mixed with chlorine-containing plastics to obtain a mixture.

[0031] The application does not have special requirements for the source of the heavy metal contaminated soil. In the application, the heavy metal contaminated soil is preferably heavily heavy metal contaminated soil. The application has higher application value by using heavily heavy metal contaminated soil, which can realize resource utilization of pollutants and reduce the harm of soil heavy metals. In the application, the heavy metal contaminated soil contains at least one of Cd, Pb, Cu and Zn.

[0032] The application does not have special requirements for the source of the chlorine-containing plastics, and can use commercially available products or chlorine-containing plastic waste. The application does not have special requirements for the type of chlorine-containing plastics, and all chlorine-containing plastics known in the art can be used, such as polyvinyl chloride (PVC) and chlorinated polyethylene (CPE). In the application, the content of chlorine element in the chlorine-containing plastics is preferably > 50 wt%, and in the examples of the application, the content of chlorine element in the chlorine-containing plastics is specifically 62.7 wt%.

[0033] Before mixing the heavy metal contaminated soil with the chlorine-containing plastics, the application preferably crushes, grinds and sieves the chlorine-containing plastics and the contaminated soil respectively to obtain granular materials with a particle size of less than 100 mesh.

[0034] The application does not have special requirements for the mixing process, and any method known in the art that can uniformly mix the heavy metal contaminated soil with the chlorine-containing plastics can be used.

[0035] In the application, when the pH value of the heavy metal contaminated soil is > 6.5, the mass of the chlorine-containing plastics accounts for 15-80% of the mass of the mixture, further preferably 20-60%, and more preferably 25-40%.

[0036] When the pH value of the heavy metal contaminated soil is ≤6.5, the mass of the chlorine-containing plastic accounts for 1-60% of the mass of the mixture, further preferably 1-40%, more preferably 3-15%, and most preferably 3-10%.

[0037] After obtaining the mixture, the mixture is co-pyrolyzed in the present application to further obtain a solid pyrolysis product.

[0038] In the present application, the temperature of the co-pyrolysis is 300-600℃, preferably 350-550℃. As a more preferred solution, when the pH value of the heavy metal contaminated soil is >6.5, the temperature of the co-pyrolysis is further preferably 400-500℃; when the pH value of the heavy metal contaminated soil is ≤6.5, the temperature of the co-pyrolysis is further preferably 300-500℃, and most preferably 300-400℃.

[0039] In the present application, the time of the pyrolysis is preferably 1-2h, and can be specifically 1h, 1.5h or 2h; the pyrolysis is preferably carried out under limited oxygen conditions, and the present application does not make special requirements for the limited oxygen pyrolysis conditions, and the commonly known limited oxygen pyrolysis conditions can be used, for example, the co-pyrolysis is carried out under the condition of passing inert gas (such as nitrogen), or the co-pyrolysis is carried out under sealed conditions. In the embodiments of the present application, the mixture is placed in a crucible, wrapped and sealed with multiple layers of tin foil paper, and then the sealed crucible is placed in a muffle furnace for limited oxygen co-pyrolysis.

[0040] In the present application, the rate of temperature rise to the pyrolysis temperature is preferably 5℃ / min. In the co-pyrolysis process of the present application, the chlorine-containing plastic decomposes to generate chlorine radicals and hydrogen radicals, which combine to form HCl, which can dissociate the heavy metals in the soil mineral lattice and convert them into ion exchange state heavy metals, thereby greatly improving the leaching potential of the soil heavy metals.

[0041] After obtaining the solid pyrolysis product, the present application preferably further comprises placing the solid pyrolysis product in an extraction solution for leaching, and obtaining a heavy metal extraction liquid through solid-liquid separation.

[0042] In the present application, the extraction solution is obtained by dissolving an extractant in deionized water. The present application does not have special requirements for the category and concentration of the extractant, and the concentration capable of extracting and simulating the main cations in soil pore water known in the art can be used. Specifically, the extractant can be CaCl2, NaNO3, NH4NO3 or DTPA. In the embodiments of the present application, 0.1M CaCl2 solution is used as the extraction solution. In the present application, the solid-liquid ratio of the solid pyrolysis product to the extraction solution is preferably 1:5 to 1:10 (m / v). In the present application, the leaching process is preferably carried out under normal temperature oscillation conditions. In the present application, the leaching time is preferably 1-2h; the present application does not have special requirements for the oscillation conditions, and the oscillation extraction conditions known in the art can be used, and in the embodiments of the present application, the oscillation speed is specifically 200rpm.

[0043] The present application does not have special requirements for the solid-liquid separation method, and the solid-liquid separation method known in the art can be used, such as filtration or centrifugation. The present application transfers the ion exchange state heavy metals in the solid pyrolysis product to the extraction solution through leaching-solid-liquid separation. In the embodiments of the present application, the centrifugal separation speed is 3500rpm, and the time is 10min.

[0044] After obtaining the heavy metal extraction solution, the present application preferably further comprises separating the heavy metal extraction solution by step crystallization or precipitation method to obtain cadmium, lead, copper, zinc and other products. The present application does not have special requirements for the crystallization and precipitation process of the heavy metal extraction solution, and the metal element crystallization and precipitation separation method known in the art can be used.

[0045] The method for efficiently leaching heavy metal elements from contaminated soil based on co-pyrolysis of chloroplasts provided by the present application will be described in detail in conjunction with the embodiments below, but they should not be understood as limiting the scope of protection of the present application.

[0046] Example 1

[0047] The heavily contaminated soil collected from the electronic waste disassembly site contains heavy metals lead, cadmium, copper and zinc with concentrations of 9459.4, 10.7, 2692.1 and 5069.7mg / kg, respectively, and the soil pH value is 8.19, which is alkaline. If the remediation of such heavily contaminated soil is implemented, the remediation cost is difficult to cover the remediation value. Therefore, the angle needs to be changed to dispose of such highly contaminated soil, and the present application proposes to recover these heavy metals in the heavily contaminated soil from the perspective of separation and extraction to realize the disposal and resource utilization of pollutants.

[0048] The contaminated soil is dried, ground, and sieved to obtain soil powder with a particle size of less than 100 mesh for standby; meanwhile, the waste chloroplastics (chlorine element content of 62.7wt%) are crushed and ground to plastic powder with a particle size of less than 100 mesh for standby;

[0049] The soil powder and the chloroplastics powder are weighed and uniformly mixed to obtain a mixture; wherein, the mass of the chloroplastics accounts for 15% of the total mass of the mixture;

[0050] The mixture is placed in a crucible, the crucible is wrapped and sealed with multiple layers of tin foil, and then the sealed crucible is placed in a muffle furnace for limited oxygen pyrolysis, the temperature is raised to 500°C at a rate of 5°C / min, and the pyrolysis is carried out at this temperature for 1h, and then cooled to room temperature. After the pyrolysis product is ground and sieved, the activated heavy metal contaminated product is obtained;

[0051] The heavy metal elements in the pyrolysis product are leached with 0.1M CaCl2 solution: 1.0g of pyrolysis product is weighed and added with 10mL of 0.1M CaCl2 solution. The solid-liquid mixture is oscillated and leached at 200rpm and 25°C for 2h, and then the solid-liquid separation is carried out by centrifugation at a speed of 3500rpm for 10min to obtain a heavy metal extraction solution. The concentration of heavy metals in the extraction solution is determined by inductively coupled plasma spectrometer. The 0.1M CaCl2 solution mainly extracts the dissolved heavy metals in the pyrolysis product, which reflects the leachability of heavy metals. The higher the solubility of soil heavy metals, the greater the potential of heavy metals leaching from the soil lattice, the higher the extraction rate, and the easier the recycling;

[0052] The above leaching solution is a mixed solution containing heavy metals lead, cadmium, copper and zinc. Lead, cadmium, copper or zinc precipitates are obtained by step-by-step precipitation method known to those skilled in the art.

[0053] Examples 2-7

[0054] The difference from Example 1 is only that the addition amount of chloroplastics is changed, and the mass of chloroplastics in the mixture accounts for 20% (Example 2), 25% (Example 3), 30% (Example 4), 40% (Example 5), 60% (Example 6) and 80% (Example 7) respectively.

[0055] Comparative Example 1

[0056] The difference from Example 1 is only that no chloroplastics are added (i.e. the addition amount of chloroplastics is 0%), and only the contaminated soil is pyrolyzed.

[0057] Comparative Examples 2-4

[0058] The difference from Example 1 is only that the adding amount of the chlorine-containing plastic is changed, and the mass ratio of the chlorine-containing plastic in the mixture is 3% (Comparative Example 2), 8% (Comparative Example 3) and 10% (Comparative Example 4) respectively.

[0059] The effects of different adding amounts of the chlorine-containing plastic on the heavy metal leaching rate of the alkaline soil are compared in Examples 1-7 and Comparative Examples 1-4, and the results are shown in Table 1 and Figure 1 The results show that when the adding amount of the chlorine-containing plastic is ≥15%, the leaching rates of the heavy metals cadmium, lead, copper and zinc are obviously improved compared with the heavy metal leaching rate of the original alkaline soil, which indicates that the heavy metal contaminated soil and a certain amount of the chlorine-containing plastic are co-pyrolyzed to activate the heavy metals in the alkaline soil and promote the dissolution of the heavy metals in the alkaline soil. Especially, when the adding amount of the chlorine-containing plastic is 25-40%, the leaching rates of the heavy metals cadmium, lead, copper and zinc are the largest (when the adding amount of the chlorine-containing plastic is 30%, they are increased by 159, 499, 383 and 488 times respectively compared with the heavy metal leaching rate of the original soil without adding the chlorine-containing plastic), which indicates that the co-pyrolysis of the adding amount of 25-40% of the chlorine-containing plastic greatly promotes the leaching of various heavy metals in the soil mineral lattice.

[0060] Table 1 Effects of the adding amount of the chlorine-containing plastic on the heavy metal leaching rate (%) of the alkaline soil

[0061]

[0062] The co-pyrolysis products of the chlorine-containing plastic with zero adding amount, 8% adding amount and 30% adding amount and the original alkaline soil without pyrolysis are observed by a scanning electron microscope. As shown in Figure 2 the adding of the chlorine-containing plastic significantly promotes the formation of loose porous structure on the surface of the soil mineral particles, and with the increase of the adding amount of the chlorine-containing plastic, more porous structures are formed on the surface, which is more conducive to the dissolution of the heavy metals from the soil mineral lattice structure.

[0063] Example 8

[0064] For the activation and leaching of the heavy metals in the acid contaminated soil, the acid contaminated soil collected from an electronic waste disassembly site is taken as an example, the pH value of the soil is 4.31, which is acidic, and the concentrations of the heavy metals lead, cadmium, copper and zinc in the soil are 3262.7, 9.5, 2883.5 and 3302.8 mg / kg respectively;

[0065] The above contaminated soil is dried, ground and sieved to obtain soil powder with a particle size of less than 100 mesh for standby; at the same time, the waste chlorine-containing plastic (the content of chlorine element is 62.7wt%) is crushed and ground to a plastic powder with a particle size of less than 100 mesh for standby;

[0066] The soil powder and the chlorine-containing plastic powder are weighed and mixed to obtain a mixture; the mass of the chlorine-containing plastic accounts for 1% of the total mass of the mixture;

[0067] The mixture is placed in a crucible, the crucible is wrapped and sealed with multiple layers of tin foil, and then the sealed crucible is placed in a muffle furnace for limited oxygen pyrolysis. The temperature is raised to 500°C at a rate of 5°C / min, and the pyrolysis is carried out at this temperature for 1 h. Then, the pyrolysis product is cooled to room temperature. After grinding and sieving treatment, the activated heavy metal contaminated product is obtained.

[0068] The heavy metal elements in the pyrolysis product are leached with 0.1M CaCl2 solution: 1.0g of the pyrolysis product is weighed and added with 10mL of 0.1M CaCl2 solution. The solid-liquid mixture is oscillated at 200rpm and 25°C for 2h. Then, the solid-liquid separation is carried out by centrifugation at a speed of 3500rpm for 10min. The heavy metal extraction solution is obtained. The concentration of heavy metals in the extraction solution is determined by inductively coupled plasma spectrometer.

[0069] The above leaching solution is a mixed solution containing heavy metals lead, cadmium, copper and zinc. The step-by-step precipitation method known to those skilled in the art is used to obtain the precipitates of lead, cadmium, copper or zinc, respectively.

[0070] Examples 9-15

[0071] The difference between Example 8 and the present examples is only the change of the addition amount of chlorine-containing plastic. The mass of the chlorine-containing plastic in the mixture accounts for 3% (Example 9), 5% (Example 10), 8% (Example 11), 10% (Example 12), 20% (Example 13), 30% (Example 14) and 40% (Example 15), respectively. The other treatments are the same as those in Example 8.

[0072] Comparative Example 5

[0073] Referring to Example 8, the heavy metals in the acid-contaminated soil in Example 8 are activated and leached. The difference is that no chlorine-containing plastic is added.

[0074] Comparative Example 6

[0075] The difference between Example 8 and the present examples is only the change of the addition amount of chlorine-containing plastic. The mass of the chlorine-containing plastic in the mixture accounts for 80%. The other treatments are the same as those in Example 8.

[0076] The effects of different addition amounts of chlorine-containing plastic on the leaching rate of heavy metals in acid-contaminated soil are compared in Examples 8-15 and Comparative Examples 5-6. The results are shown in Tables 2 and Figure 3 . From Tables 2 and Figure 3It can be seen that when the adding amount of the chlorine-containing plastic is 1-60%, the leaching rates of the heavy metals Cd, Pb, Cu and Zn are obviously improved compared to the leaching rates of the heavy metals in the original acid soil, indicating that the co-pyrolysis of the heavy metal contaminated soil and a certain amount of chlorine-containing plastic can promote the leaching of various heavy metals in the mineral crystal lattice of the acid contaminated soil. Especially when the adding amount of the chlorine-containing plastic is 3-10%, the leaching rates of the heavy metals Cd, Pb, Cu and Zn are the largest (when the adding amount of the chlorine-containing plastic is 8%, the leaching rates of Pb, Cd, Cu and Zn are increased by 6, 138, 30 and 5 times respectively compared to the leaching rates of Pb, Cd, Cu and Zn in the original soil without adding the chlorine-containing plastic), indicating that the co-pyrolysis of the acid soil with 3-10% of the chlorine-containing plastic can significantly improve the leaching potential of the heavy metals in the acid soil.

[0077] Table 2 Influence of the adding amount of the chlorine-containing plastic on the leaching rate of the heavy metals in the acid soil (%)

[0078]

[0079] Example 16

[0080] The difference from Example 4 is that the co-pyrolysis temperature is changed from “500℃” to “400℃”, and the other treatments are the same as those in Example 4.

[0081] Example 17

[0082] The co-pyrolysis temperature of the mixture in Example 4 is changed to 300℃, and the rest of the treatment is the same as that in Example 4.

[0083] Example 18

[0084] The co-pyrolysis temperature of the mixture in Example 4 is changed to 600℃, and the rest of the treatment is the same as that in Example 4.

[0085] Comparative Example 7

[0086] Under the condition of no pyrolysis treatment, 0.1M CaCl2 is directly used to extract the heavy metals from the alkaline soil in Example 4.

[0087] The treatment conditions and the leaching results of the heavy metals in the soil of Example 4, Examples 16-18, Comparative Example 1 and Comparative Example 7 are shown in Table 3. The leaching results show that compared to the treatment without adding the chlorine-containing plastic (Comparative Example 1), the co-pyrolysis of the alkaline soil with 30% of the chlorine-containing plastic at 400℃ (Example 16) increases the leaching rates of the heavy metals Pb, Cd, Cu and Zn in the alkaline soil by 161, 530, 483 and 614 times respectively, which significantly increases the leaching of the heavy metals in the alkaline soil; compared to the co-pyrolysis at 500℃ (Example 4), the co-pyrolysis at 400℃ (Example 16) slightly increases the leaching of various heavy metals in the alkaline soil.

[0088] Example 19

[0089] The mixture was pyrolyzed-leached-recovered according to the method of Example 11, except that the co-pyrolysis temperature was changed from "500°C" to "400°C", and other treatments were the same as those of Example 11.

[0090] Example 20

[0091] The co-pyrolysis temperature of the mixture in Example 11 was changed to 300°C, and other treatments were the same as those of Example 11.

[0092] Example 21

[0093] The co-pyrolysis temperature of the mixture in Example 11 was changed to 600°C, and other treatments were the same as those of Example 11.

[0094] Comparative Example 8

[0095] The acidic soil in Example 11 was directly subjected to heavy metal extraction using 0.1M CaCl2 without pyrolysis treatment.

[0096] The treatment conditions and heavy metal leaching results of the soil in Example 11, Examples 19-21, Comparative Example 5 and Comparative Example 8 are shown in Table 3. The leaching results show that, compared with the treatment without adding chlorine-containing plastics (Comparative Example 5), the addition of 8% chlorine-containing plastics to the co-pyrolysis at 400°C (Example 19) increased the leaching rates of heavy metals lead, cadmium, copper and zinc in the acidic soil by 6, 235, 26 and 7 times, respectively, significantly increasing the leaching of heavy metals in the acidic soil; compared with the co-pyrolysis at 500°C (Example 11), the co-pyrolysis at 400°C (Example 19) to some extent increased the leaching of various heavy metals in the acidic soil.

[0097] Example 22

[0098] According to Example 4, the contaminated soil collected from a lead-zinc mine area was subjected to heavy metal activation leaching, except that the soil used in this example was different, and the soil selected in this example was from a tailings pond in a lead-zinc mine area, which was alkaline (pH = 8.54) and contained heavy metals lead, cadmium, copper and zinc at concentrations of 4483.6, 11.9, 4804.8 and 5404.5 mg / kg, respectively; and in this example, the amount of chlorine-containing plastics added was set to 30%, and other treatments were the same as those of Example 4. The leaching results show that, compared with the treatment without pyrolysis (Comparative Example 9), the addition of 30% chlorine-containing plastics to the co-pyrolysis increased the leaching rates of heavy metals lead, cadmium, copper and zinc in the soil by 440, 222, 169 and 672 times, respectively, significantly increasing the leaching of heavy metals in the alkaline soil (see Table 3).

[0099] Comparative Example 9

[0100] The heavy metals in the alkaline soil in Example 22 were extracted directly by 0.1M CaCl2 without pyrolysis treatment.

[0101] Example 23

[0102] Referring to Example 11, the heavy metals in the electroplating sludge were activated and leached, except that the soil used in this example was different. The soil used in this example was from the electroplating sludge generated in the electroplating wastewater treatment process, and was acidic (pH = 6.04) and contained copper and zinc at concentrations of 15586 and 11725 mg / kg, respectively. In addition, the amount of the chloroplastics added in this example was 8%, and other treatments were the same as in Example 11. The leaching results showed that, compared with no pyrolysis treatment (Comparative Example 10), the co-pyrolysis of the electroplating sludge with 8% chloroplastics increased the leaching rates of copper and zinc by 43 and 56 times, respectively, and significantly promoted the leaching of the heavy metals in the electroplating sludge (see Table 3).

[0103] Comparative Example 10

[0104] The heavy metals in the acidic sludge in Example 23 were extracted directly by 0.1M CaCl2 without pyrolysis treatment.

[0105] Table 3 Leaching rates (%) of the heavy metals extracted from the soil in Examples 4, 11, 16-23 and Comparative Examples 1, 5, 7-10

[0106]

[0107] As shown in Table 3, compared with no pyrolysis treatment and no addition of chloroplastics, the co-pyrolysis of the original heavy metal contaminated soil with chloroplastics significantly increased the leaching rates of various heavy metals. By comparing the effects of different pyrolysis temperatures on the extraction and leaching of the heavy metals from the soil, it was found that the optimal pyrolysis temperature for the alkaline soil was 400-500°C, and the optimal pyrolysis temperature for the acidic soil was 300-400°C.

[0108] The above only describes the preferred embodiments of the present application, and it should be noted that, for those skilled in the art, several improvements and refinements can be made without departing from the principles of the present application, and these improvements and refinements should also be considered as falling within the scope of the present application.

Claims

1. A method for efficient leaching of heavy metal elements from contaminated soil based on co-pyrolysis of chlorine-containing plastics, comprising the following steps: mixing heavy metal contaminated soil with chlorine-containing plastics to obtain a mixture; when the pH value of the heavy metal contaminated soil is > 6.5, the mass of the chlorine-containing plastics accounts for 25-40% of the mass of the mixture; when the pH value of the heavy metal contaminated soil is ≤ 6.5, the mass of the chlorine-containing plastics accounts for 3-10% of the mass of the mixture; co-pyrolyzing the mixture; during the co-pyrolysis, chlorine-containing plastics decompose to generate chlorine radicals and hydrogen radicals, which combine to form HCl, which can dissociate heavy metals in the soil mineral lattice and convert them into ion exchange state heavy metals; the content of chlorine in the chlorine-containing plastics is > 50wt%; when the pH value of the heavy metal contaminated soil is > 6.5, the temperature of the co-pyrolysis is 400-500℃; when the pH value of the heavy metal contaminated soil is ≤ 6.5, the temperature of the co-pyrolysis is 300-500℃; the time of the co-pyrolysis is 1-2h; the heavy metal contaminated soil contains at least one of Cd, Pb, Cu and Zn.

2. The method of claim 1, wherein, the heavy metal contaminated soil is heavily contaminated with heavy metals.

3. The method of claim 1, wherein, after co-pyrolyzing the mixture, the method further comprises placing the obtained solid pyrolysis product in an extraction solution for leaching, solid-liquid separation, and obtaining a heavy metal extraction solution.

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