Method for treating soil contaminated by organic matter and heavy metals by ex situ indirect thermal desorption

Abstract

The invention relates to the technical field of soil regeneration treatment, and in particular to a method for treating soil contaminated by organic matter and heavy metals by ex situ indirect thermal desorption; the method comprises the following steps: S1, pretreatment of contaminated soil, S2, ex situ indirect thermal desorption treatment, and S3, solidification / stabilization treatment; the method adopts ex situ indirect thermal desorption and solidification / stabilization agent composite treatment to effectively treat soil contaminated by organic matter and heavy metals, so that the soil meets the backfill standard; and the treatment method of the invention can effectively reduce the mobility and bioavailability of pollutants, thereby achieving the purpose of harmless treatment of contaminated soil.

Description

Technical Field

[0001] The invention relates to the technical field of soil regeneration treatment, and in particular to a method for treating soil contaminated by organic matter and heavy metals by ex situ indirect thermal desorption. Background Art

[0002] Organic and heavy metal compound contaminated soil refers to the presence of two or more pollutants of different types and properties, organic and heavy metals, in the same soil environment, and these pollutants interact or react with each other, thus causing pollution. This compound pollution phenomenon is relatively common in soil and has serious impacts on the soil environment, ecosystem and human health.

[0003] First, the interaction between organic matter and heavy metals will lead to changes in soil physical and chemical properties, such as significant changes in pH, organic matter, particle size distribution, carbonate content, dissolved organic carbon, etc. Second, compound pollution will affect soil microbial activity and ecological functions, destroying the ecological balance of the soil. Finally, these pollutants will enter the human body through the food chain and other pathways, causing serious impacts on human health, such as causing chronic poisoning.

[0004] For soils contaminated by organic matter and heavy metals, current research mainly focuses on environmental ecology and environmental toxicology, as well as the study of combined remediation technology. Combined remediation technology is more complex than single remediation technology because the combined pollution system is more complex, and there will be interactions between pollutants and between pollutants and organisms. Among them, chemical oxidation remediation technology is one of the commonly used methods for remediating soil contaminated by organic matter, but this method may affect the environmental behavior of heavy metals in the soil and its potential environmental risks while removing organic pollutants.

[0005] Ex situ indirect thermal desorption technology heats the contaminated soil to a set temperature through indirect heating, causing the pollutants to vaporize and volatilize, thereby separating the pollutants from the soil particles. This technology is particularly suitable for treating high-concentration, difficult-to-degrade organic pollutants and heavy metal-contaminated soil. For soil contaminated by organic matter and heavy metals, ex situ indirect thermal desorption technology can effectively remove pollutants from the soil and make it meet the standards for reuse. Compared with traditional soil remediation methods, ex situ indirect thermal desorption technology does not use hazardous substances such as chemicals, and has less harm to the environment and human body. At the same time, the soil treated by this technology can be backfilled in situ or used as a resource for greening and roadbed soil, and the indirect economic benefits are relatively high. Ex situ indirect thermal desorption technology can control parameters such as temperature and treatment time, and can be customized according to different pollutants to achieve better remediation effects. Summary of the invention

[0006] In order to solve the above problems, the present invention provides a method for treating soil contaminated with organic matter and heavy metals by ex situ indirect thermal desorption to solve the problem of soil contamination.

[0007] The technical solution of the present invention is: a method for treating soil contaminated by organic matter and heavy metals by ex situ indirect thermal desorption, comprising the following steps:

[0008] S1. Pretreatment of contaminated soil

[0009] The soil contaminated by organic matter and heavy metals is used as the soil to be treated. After the soil to be treated is dug out and transported to the pretreatment area, large stones with a particle size of >2 cm in the soil to be treated are removed, and then the soil to be treated is sieved and crushed to a particle size of 1-20 mm, and then quicklime accounting for 1-2wt% of the weight of the soil to be treated is added, mixed evenly, to obtain a first mixed soil, and the moisture content of the first mixed soil is adjusted to less than 20% to obtain pretreated contaminated soil; the lower limit depth of the organic matter and heavy metal contaminated soil object treated in this application is -4m and above;

[0010] S2. Ex situ indirect thermal desorption treatment

[0011] S2-1, the pretreated contaminated soil is sent to the feeding conveyor with an electromagnetic iron remover at the front end by the main conveyor belt, and then the feeding conveyor belt transports the pretreated contaminated soil to the feeding auger, and finally the pretreated contaminated soil is transferred to the upper part of the feeding port of the hollow spiral rod of the thermal desorption unit by the throwing belt conveyor in the inclined feeding auger;

[0012] S2-2, the contaminated soil after pretreatment is introduced into the feeding unit along the feeding port of the hollow spiral rod, the burner is turned on to heat the outer wall of the heating chamber, the parameters in the heating chamber are monitored, and when the detection data meets the ignition conditions, the air and thermal desorption fuel are ignited and supplied to obtain a second mixed soil, and after the second mixed soil temperature is 640-660°C, the ex situ thermal desorption is carried out for 250-260 days, the treatment is completed, and the soil treated by ex situ indirect thermal desorption is obtained, and the gas after thermal desorption is obtained at the same time, and the gas is purified and discharged;

[0013] S2-3, spraying water accounting for 7-10% of the mass of the soil after the ex situ indirect thermal desorption treatment on the soil, and then excavating the soil to obtain high-temperature soil, and discharging the high-temperature soil from the discharge port of the hollow spiral rod;

[0014] S3, curing / stabilization treatment

[0015] The soil discharged from S2-3 is sprayed with water until the moisture content is 24-26%, and a solidifying / stabilizing agent is added at 1-3wt%, and the mixture is stirred for 3-5 minutes to obtain a third mixed soil, which is then transported to a curing plant for 7-15 days of curing. During the curing period, water is added in time to maintain the moisture content of the third mixed soil at 24-26%, to obtain the treated soil that meets the standards, and the treated soil that meets the standards is backfilled into the original excavated foundation pit.

[0016] Note: Before the operation of the thermal desorption unit, the induced draft fan must be used to purge the inside of the equipment and replace it with fresh air to ensure safe operation; the induced draft fan is arranged at the front end of the thermal desorption unit. In addition to the purging function, it can also supply air to the hollow spiral rod burner and the high-temperature oxidizer burner to meet the oxygen required for combustion and promote uniform heating temperature; the function of the electromagnetic iron remover is to remove metal objects in the contaminated soil after pretreatment, to prevent metal objects from entering the hollow spiral rod and causing damage to the dragon at high temperature;

[0017] The physical and chemical properties of soil have a great influence on the efficiency and results of thermal desorption treatment. Before thermal desorption treatment, contaminated soil needs to be pretreated. By adjusting the soil moisture and soil particle size, the heat transfer efficiency and heat transfer area of ​​the contaminated soil can be increased to ensure stable and efficient operation of soil thermal desorption treatment. First, using ex situ indirect desorption technology to treat contaminated soil can transfer pollutants in the soil from the solid phase, thereby effectively separating the pollutants from the soil. This process can significantly improve the removal efficiency of pollutants and reduce the burden of subsequent solidification / stabilization treatment. In the subsequent solidification / stabilization process, due to the low pollutant content, the risk of secondary pollution will be reduced accordingly. At the same time, solidification / stabilization treatment can further ensure the long-term stability of pollutants and prevent them from being re-released in the environment, thereby converting pollutants into harmless or low-toxic substances.

[0018] Furthermore, in S2-1, a belt scale for continuously weighing the feed is provided on the feeding transmission belt, and the belt scale is electrically connected to the main conveyor belt. The signal of the belt scale is sent to the frequency conversion controller of the main conveyor belt, and the frequency of the main conveyor belt is controlled by the frequency conversion controller to adjust the feed amount, and the feed amount is 28~32t / h;

[0019] Note: Controlling the feed rate within the above range can achieve better treatment of contaminated soil and better ex situ desorption effect.

[0020] Furthermore, in S2-2, a burner and an ignition device are provided at the bottom of the hollow spiral rod. The burner has a feed pipeline and an air intake pipeline. The air intake pipeline is provided with a flow rate and temperature detector, and the feed pipeline is provided with a flow rate and pressure detector; a flame detector is provided behind the burner to detect the flame, and a signal can be sent in time when the flame is extinguished to avoid accidents;

[0021] The ignition conditions are: the temperature of the air intake line reaches 200-550°C, the flow rate of the air intake line reaches 3-10L / min, the flow rate of the feed line reaches 3-5 tons / min, and the pressure of the feed line reaches 0.2-0.3MPa;

[0022] Note: The flow meter is a thermal measurement, and the measurement results will not be affected by changes in gas temperature and pressure. The right amount of intake flow and appropriate pressure can ensure that there is enough oxygen supply inside the burner to promote stable combustion, and the appropriate feed flow can prevent uneven temperature distribution or premature extinguishing due to excessive flow.

[0023] Furthermore, in S2-2, the thermal desorption fuel is liquefied natural gas, and the amount of thermal desorption fuel introduced is 400-500 Nm 3 / h, the air supply pressure is 28~32kPa;

[0024] Note: Natural gas is used as fuel for thermal desorption. Taking into account the surrounding fuel supply conditions, liquefied natural gas is selected for the operation of thermal desorption equipment. Considering that the amount of natural gas used for thermal desorption is large, various safety issues must be paid attention to when storing and using it on site, and on-site gas tank storage is selected.

[0025] Furthermore, in S2-2, the gas purification method is: cooling the gas after thermal desorption to a temperature of 60-70°C by dust removal, and then reducing the temperature to 20-35°C by condensation using a medium plate heat exchanger, and then separating the water mist entrained in the gas from the gas using a demister to obtain separated water mist and gas, and the gas is treated by primary activated carbon fiber and secondary activated carbon adsorption, and then concentrated to the burner flame of the heating unit under the action of an induced draft fan for incineration for 3-10 hours, and the separated water mist is sent to a wastewater treatment facility for purification;

[0026] Note: During the condensation process of the exhaust gas and tail water after cooling, the content of benzene and chlorobenzene in the condensed water is much lower than that in the exhaust gas, accounting for about 0.7% of the total organic pollutants. After the exhaust gas after removing water vapor is adsorbed by the primary activated carbon fiber and then by the secondary activated carbon adsorption, the benzene and chlorobenzene pollutants can be effectively removed (the removal rate can reach more than 99%), and the emission meets the standards.

[0027] Further, in S3, the solidifying / stabilizing agent is composed of agent A and agent B in a mass ratio of 1:1; in terms of weight percentage, agent A includes 94-96% of iron salts and phosphates and the remainder of calcium lignin sulfonate, wherein the weight ratio of the iron salts and the phosphates is 1:1; agent B includes 5-8% of dolomite, 5-8% of hydroxyapatite, 5-8% of quicklime, 15-20% of water glass, 15-20% of humic acid and 20-30% of natural clay minerals and the remainder of straw biochar, the pH of the solidifying / stabilizing agent is 7-8, and the particle size of the solidifying / stabilizing agent is 10-20 nm;

[0028] Description: The above ingredients as solidifying / stabilizing agents can fix mercury and nickel heavy metal pollutants in the soil or treat toxic pollutants into non-toxic or low-toxic immobilized substances through chemical reactions, coordination reactions, physical adsorption, chemical adsorption, biological stabilization and micromorphological sealing, thereby reducing the mobility and bioavailability of pollutants and achieving the purpose of harmless treatment of contaminated soil; calcium lignin sulfonate as an antifreeze agent can improve the stability and effect of the agent; straw biochar can effectively reduce the effectiveness of heavy metals in the soil due to its rich porous structure, large specific surface area, strong cation exchange capacity, multiple functional groups on the surface, good adsorption capacity and other characteristics; the combined use of multiple materials can produce a synergistic effect and improve the overall treatment effect. Calcium lignin sulfonate not only acts as an adsorbent, but can also form more stable complexes or precipitates with iron salts and phosphates. Quicklime can adjust the pH value of the soil and help the precipitation and solidification of heavy metal ions. Components such as water glass and humic acid can improve soil structure and enhance the dispersibility and stability of the agent. The agent cost is relatively low and is easy to obtain, which helps reduce the cost of treatment.

[0029] Further, in S3, during the curing period, the third mixed soil is covered with tarpaulin, and before covering, the third mixed soil is sprayed with water until the moisture content is maintained at 25%;

[0030] Note: Sprinkling water into the soil to control the moisture content at around 25% can allow the agent to fully react. The tarpaulin is set up to prevent the volatilization of moisture during the curing process from affecting the curing effect.

[0031] Furthermore, in S3, during the stirring and mixing, light treatment is added to achieve synergistic effects, specifically:

[0032] During the stirring and mixing, heavy metal activating bacteria are added to the soil every 30-40 seconds, and the soil is treated with ultraviolet radiation for 3-5 seconds, and then with electron beam radiation for 5-8 seconds. After the stirring is completed, the soil is tossed for 15-30 seconds and the soil is tossed with ultraviolet radiation for 10-15 seconds.

[0033] The ultraviolet radiation parameters are: ultraviolet wavelength is 280~320nm, irradiation intensity is 200~220μW / cm·s; the electron beam radiation parameters are: electron beam energy is 1~1.8MeV, beam power is 18~20kW, beam density is: 5×10 4 ~5×10 9 p / cm 2 ·s; the amount of the heavy metal activated bacteria added each time is 5-10wt%;

[0034] Note: Ultraviolet irradiation is more suitable for treating the surface of the soil, while electron beam radiation can penetrate deep into the soil to treat heavy metals in the deep soil. This combination ensures that heavy metals in the soil are treated comprehensively and effectively; in addition, these two methods will not produce secondary pollution during the treatment process, and have less impact on the environment and organisms. In addition, ultraviolet irradiation itself can trigger photocatalytic reactions in the soil, making the photosensitizer in the soil more susceptible to electron beam radiation, making it easier to interact with the electron beam, further improving the treatment effect.

[0035] Furthermore, the heavy metal activating bacteria is composed of purified oxidase: Bacillus: Pseudomonas fluorescens: nitrogen-fixing bacteria in a volume ratio of 0.5-0.8:1:1:1.2-1.5;

[0036] Description: Nitrogen-fixing bacteria have the ability to secrete substances such as Fe carriers and organic acids. They can change the biological activity of heavy metal ions in the soil through methylation and chelation of metabolites, thereby promoting the absorption and fixation of heavy metals by plants; Bacillus and Pseudomonas fluorescens can lower the soil pH by secreting organic acids, thereby promoting the dissolution and activation of heavy metals; purified oxidases can catalyze the redox reactions of heavy metal ions, converting them from a more toxic form to a less toxic form, and can also dissociate heavy metal ions from the soil, facilitating the absorption and conversion of microorganisms.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] (1) The present invention adopts ex situ indirect thermal desorption and solidification / stabilization agent combined treatment to effectively treat soil contaminated by organic matter and heavy metals to meet the backfill standard; compared with in situ desorption, ex situ indirect thermal desorption treatment can effectively remove volatile organic matter, and can also effectively remove semi-volatile or non-volatile, high-boiling point, and difficult-to-decompose organic pollutants. For organic pollutants containing multiple different boiling points, they can be heated once to achieve the remediation target value. In addition, under the action of high temperature, some heavy metals will also be separated from the soil due to volatilization, and then collected and treated.

[0039] (2) The present invention proposes a solidification / stabilization agent that can fix mercury and nickel heavy metal pollutants in the soil or treat toxic pollutants into non-toxic or low-toxic fixed substances through multiple effects such as chemical reaction, coordination reaction, physical adsorption, chemical adsorption, biological stabilization and micro-morphological sealing, thereby reducing the mobility and bioavailability of pollutants, thereby achieving the purpose of harmless treatment of contaminated soil.

[0040] (3) The present invention effectively utilizes ultraviolet radiation and electron beam radiation to ensure that heavy metals in the soil are comprehensively and effectively treated by adding light treatment synergistically during the material stirring process of the solidification / stabilization treatment. No secondary pollution will be generated during the treatment process, and the impact on the environment and organisms is relatively small. Ultraviolet radiation itself can trigger a photocatalytic reaction in the soil, thereby making the photosensitizer in the soil more susceptible to electron beam radiation, making it easier to react with the electron beam, further improving the treatment effect. By adding heavy metal activating bacteria to further improve the heavy metal activation effect, the soil can also be better solidified, thereby effectively improving the treatment effect of soil contaminated with organic matter and heavy metals. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a flow chart of the method of the present invention;

[0042] Figure 2 is a comparison chart of Hg leaching rates in soils contaminated with organic matter and heavy metals in Examples 1 to 15 of the present invention and Control Groups 1 to 2;

[0043] Figure 3 This is a comparison chart of Ni leaching rates in soils contaminated with organic matter and heavy metals in Examples 1 to 15 of the present invention and Control Groups 1 to 2;

[0044] Figure 4 1 is a comparison chart of benzene removal rates in soils contaminated with organic matter and heavy metals in Examples 1 to 15 of the present invention and Control Groups 1 to 2;

[0045] Figure 5 This is a comparison chart of chlorobenzene removal rates in soils contaminated with organic matter and heavy metals in Examples 1 to 15 of the present invention and Control Groups 1 to 2;

[0046] Figure 6 It is a comparison chart of Hg leaching rate in soil contaminated by organic matter and heavy metals in Example 1, Example 16 to Example 22 and Control Groups 3 to 5 of the present invention;

[0047] Figure 7 This is a comparison chart of Ni leaching rates in soils contaminated with organic matter and heavy metals in Example 1, Example 16 to Example 22 and Control Groups 3 to 5 of the present invention;

[0048] Figure 8 1 is a comparison chart of benzene removal rates in soil contaminated by organic matter and heavy metals in Example 1, Example 16 to Example 22 and Control Groups 3 to 5 of the present invention;

[0049] Fig. 9 It is a comparison chart of chlorobenzene removal rates in soil contaminated with organic matter and heavy metals in Example 1, Example 16 to Example 22 of the present invention and Control Groups 3 to 5. DETAILED DESCRIPTION

[0050] In order to further illustrate the method adopted by the present invention and the effect achieved, the technical solution of the present invention will be clearly and completely described in combination with experiments below.

[0051] Example 1: A method for treating soil contaminated with organic matter and heavy metals by ex situ indirect thermal desorption, comprising the following steps:

[0052] S1. Pretreatment of contaminated soil

[0053] Taking organic matter and heavy metal composite contaminated soil as the soil to be treated, digging out the soil to be treated and transporting it to the pretreatment area, removing large stones with a particle size of more than 2 cm in the soil to be treated, and then sieving and crushing the soil to be treated to a particle size of 10-15 mm, and then adding quicklime accounting for 1.5wt% of the weight of the soil to be treated, mixing evenly to obtain a first mixed soil, and adjusting the moisture content of the first mixed soil to 18% to obtain pretreated contaminated soil;

[0054] S2. Ex situ indirect thermal desorption treatment

[0055] S2-1, the pretreated contaminated soil is sent to the feeding conveyor with an electromagnetic iron remover at the front end by the main conveyor belt, and then the feeding conveyor belt transports the pretreated contaminated soil to the feeding auger, and finally the pretreated contaminated soil is transferred to the upper part of the feeding port of the hollow spiral rod of the thermal desorption unit by the throwing belt conveyor in the inclined feeding auger; wherein, a belt scale for continuous weighing of the feed is provided on the feeding transmission belt, and the belt scale is electrically connected to the main conveyor belt, and the signal of the belt scale is sent to the frequency converter of the main conveyor belt, and the frequency converter controls the frequency of the main conveyor belt to adjust the feed amount, and the feed amount is 30t / h;

[0056] S2-2, the contaminated soil after pretreatment is introduced into the feeding unit along the feeding port of the hollow spiral rod, the burner is turned on to heat the outer wall of the heating chamber, the parameters in the heating chamber are monitored, and when the detection data meets the ignition conditions, the second mixed soil is ignited and air and thermal desorption fuel are supplied to obtain the second mixed soil. After 255 days of ex situ indirect thermal desorption under the condition that the temperature of the second mixed soil is 650°C, the treatment is completed to obtain the soil treated by ex situ indirect thermal desorption, and the gas after thermal desorption is obtained at the same time, and the gas is purified and discharged;

[0057] The thermal desorption fuel is liquefied natural gas, and the amount of thermal desorption fuel introduced is 450Nm 3 / h, the air supply pressure is 30kPa; a burner and an ignition device are provided at the bottom of the hollow spiral rod, the burner has a feed pipeline and an air intake pipeline, a flow rate and temperature detector is provided on the air intake pipeline, and a flow rate and pressure detector is provided on the feed pipeline;

[0058] The ignition conditions are: the temperature of the air intake line reaches 400°C, the flow rate of the air intake line reaches 6L / min, the flow rate of the feed line reaches 4 tons / min, and the pressure of the feed line reaches 0.25MPa;

[0059] In S2-2, the gas purification method is: the gas after thermal desorption is dusted and cooled to a temperature of 65°C, and then the temperature is reduced to 27°C by condensation using a medium plate heat exchanger, and then the water mist entrained in the gas is separated from the gas by a demister to obtain separated water mist and gas. The gas is treated by primary activated carbon fiber and secondary activated carbon adsorption, and then concentrated to the burner flame of the heating unit under the action of an induced draft fan for incineration treatment for 5 hours, and the separated water mist is sent to a wastewater treatment facility (the wastewater treatment facility is a prior art) for purification treatment;

[0060] S2-3, spraying water accounting for 8% of the mass of the soil after the ex situ indirect thermal desorption treatment into the soil after the ex situ indirect thermal desorption treatment to obtain high-temperature soil, and discharging the high-temperature soil from the discharge port of the hollow spiral rod;

[0061] S3, curing / stabilization treatment

[0062] Spray water on the soil discharged from S2-3 until the moisture content is 25%, add a solidifying / stabilizing agent at 2wt%, stir and mix, the stirring time is 4min, and obtain a third mixed soil, which is then transported to a curing plant for 10 days of curing. During the curing period, the third mixed soil is covered with tarpaulin. Before covering, the third mixed soil is sprayed with water until the moisture content is maintained at 25%. During the curing period, water is replenished in time to maintain the moisture content of the third mixed soil at 25%, and the treated soil that meets the standards is obtained, and the treated soil that meets the standards is backfilled into the original excavated foundation pit;

[0063] The solidifying / stabilizing agent is composed of agent A and agent B in a mass ratio of 1:1; in terms of weight percentage, agent A includes 95% of iron salts and phosphates and the remainder of calcium lignin sulfonate, wherein the weight ratio of iron salts and phosphates is 1:1; agent B includes 7% of dolomite, 7% of hydroxyapatite, 7% of quicklime, 18% of water glass, 17% of humic acid and 25% of natural clay minerals and the remainder of straw biochar, the pH of the solidifying / stabilizing agent is 7.5, and the particle size of the solidifying / stabilizing agent is 13-17 nm;

[0064] The iron salt is one of FeS and FeCl2. In this embodiment, FeCl2 is used. It can reduce high-valent heavy metals (such as hexavalent chromium Cr (VI)) to low-valent states with low toxicity through reduction reactions. The iron ions in the iron salt can form precipitation or complexes with heavy metal ions, and fix the heavy metals in the soil through adsorption. The phosphate is one of KH2PO4 and NH4H2PO4. In this embodiment, KH2PO4 is used. It reacts with heavy metal ions (such as Cd 2+ 、Zn 2+ ) to form insoluble phosphate precipitation, thereby reducing the bioavailability of heavy metals; the molecular structure of calcium lignin sulfonate contains a variety of functional groups, such as hydroxyl and carboxyl groups, which can react with heavy metal ions (such as mercury ions, nickel ions, etc.) or undergo adsorption to fix heavy metals in the soil; the calcium ions and carbonate ions in dolomite can form complexes or precipitations with heavy metal ions, reducing the mobility and bioavailability of heavy metals. The surface of dolomite powder can also catalyze the redox reaction of heavy metal ions and reduce the toxicity of heavy metal ions; hydroxyapatite reacts with heavy metal ions (such as Pb 2+ ) undergo ion exchange and complexation reactions, adsorbing heavy metals on its surface or inside, thereby fixing them in the soil. Quicklime (mainly composed of calcium oxide) has strong alkalinity, which can cause heavy metal ions (except mercury ions) in the soil to undergo precipitation reactions, thereby reducing their mobility and bioavailability. The silicate ions in water glass can form silicate precipitates or complexes with heavy metal ions, fixing heavy metals in the soil through adsorption. Humic acid molecules contain a large number of functional groups such as carboxyl and phenolic hydroxyl groups, which can undergo coordination reactions or adsorption with heavy metal ions to fix heavy metals in the soil. Natural clay minerals such as montmorillonite and illite have a large specific surface area and layered structure, which can fix heavy metal ions on their surface or between layers through adsorption. Straw biochar has a porous structure and rich surface functional groups, which can fix heavy metal ions in its pores or surface through adsorption. The pore structure of straw biochar can also seal heavy metal ions in micropores to prevent them from being released into the environment.

[0065] Example 2: Different from Example 1, in S1, the soil to be treated is sieved and crushed to a particle size of 1-9 mm, and then quicklime accounting for 1 wt% of the weight of the soil to be treated is added and mixed evenly to obtain a first mixed soil, and the moisture content of the first mixed soil is adjusted to 15% to obtain pretreated contaminated soil.

[0066] Example 3: Different from Example 1, in S1, the soil to be treated is sieved and crushed to a particle size of 16-20 mm, and then quicklime accounting for 2 wt% of the weight of the soil to be treated is added and mixed evenly to obtain a first mixed soil, and the moisture content of the first mixed soil is adjusted to 20% to obtain pretreated contaminated soil.

[0067] Example 4: Different from Example 1, in S2-1, the feed rate of the pretreated contaminated soil is 28 t / h.

[0068] Example 5: Different from Example 1, in S2-1, the feed rate of the pretreated contaminated soil is 32 t / h.

[0069] Example 6: Different from Example 1, in S2-2, the treatment was completed after 260 days of ex situ thermal desorption at a second mixed soil temperature of 640°C.

[0070] Example 7: Different from Example 1, in S2-2, the treatment was completed after 250 days of ex situ thermal desorption at a second mixed soil temperature of 660°C.

[0071] Example 8: Different from Example 1, in S2-2, the amount of thermal desorption fuel introduced is 400 Nm 3 / h, the air supply pressure is 28kPa.

[0072] Example 9: Different from Example 1, in S2-2, the amount of thermal desorption fuel introduced is 500 Nm 3 / h, the air supply pressure is 32kPa.

[0073] Example 10: Different from Example 1, in the S2-2 gas purification method, the gas after thermal desorption is cooled to 60°C by dust removal, and then the temperature is reduced to 20°C by condensation using a medium plate heat exchanger.

[0074] Example 11: Different from Example 1, in the S2-2 gas purification method, the gas after thermal desorption is cooled to 70°C by dust removal, and then the temperature is reduced to 35°C by condensation using a medium plate heat exchanger.

[0075] Example 12: Different from Example 1, in S3, water is sprayed on the soil discharged from S2-3 until the moisture content is 24%, and a solidifying / stabilizing agent is added at 1wt%, and the mixture is stirred for 3 minutes to obtain a third mixed soil, which is then transported to a maintenance plant for 7 days of maintenance.

[0076] Example 13: Different from Example 1, in S3, water is sprayed on the soil discharged from S2-3 until the moisture content is 26%, and a solidifying / stabilizing agent is added at 3wt%, and the mixture is stirred for 5 minutes to obtain a third mixed soil, which is then transported to a curing plant for 15 days of curing.

[0077] Example 14: Different from Example 1, in S3, by weight percentage, agent A includes 94% of iron salts and phosphates and the remainder calcium lignin sulfonate, wherein the weight ratio of iron salts to phosphates is 1:1; agent B includes 5% dolomite, 5% hydroxyapatite, 5% quicklime, 15% water glass, 15% humic acid and 20% natural clay minerals and the remainder straw biochar, the pH of the solidifying / stabilizing agent is 7, and the particle size of the solidifying / stabilizing agent is 10~12nm.

[0078] Example 15: Different from Example 1, in S3, by weight percentage, agent A includes 96% of iron salts and phosphates and the remainder calcium lignin sulfonate, wherein the weight ratio of iron salts to phosphates is 1:1; agent B includes 8% dolomite, 8% hydroxyapatite, 8% quicklime, 20% water glass, 20% humic acid and 30% natural clay minerals and the remainder straw biochar, the pH of the solidifying / stabilizing agent is 8, and the particle size of the solidifying / stabilizing agent is 18~20nm.

[0079] Embodiment 16: Different from Embodiment 1, in S3, during the stirring and mixing, light treatment is added to achieve synergistic effect, specifically:

[0080] During the stirring and mixing, heavy metal activating bacteria were added to the soil every 35 seconds, and the soil was irradiated with ultraviolet light for 4 seconds, and then irradiated with electron beams for 7 seconds. After the stirring was completed, the soil was tossed for 22 seconds and irradiated with ultraviolet light for 13 seconds during the tossing.

[0081] The ultraviolet radiation parameters are: ultraviolet wavelength is 300nm, irradiation intensity is 210μW / cm·s; the electron beam radiation parameters are: electron beam energy is 1.4MeV, beam power is 19kW, beam density is: 5×10 7 p / cm 2 ·s; the amount of heavy metal activating bacteria added each time accounts for 8wt% of the soil weight;

[0082] The heavy metal activating bacteria is composed of purified oxidase: Bacillus: Pseudomonas fluorescens: nitrogen-fixing bacteria in a volume ratio of 0.6:1:1:1.3.

[0083] Example 17: Different from Example 16, heavy metal activating bacteria are added to the soil every 30 seconds during stirring and mixing, and the soil is treated with ultraviolet radiation for 3 seconds and then with electron beam radiation for 5 seconds. After stirring, the soil is turned over for 15 seconds and treated with ultraviolet radiation for 10 seconds during the turning and mixing.

[0084] Example 18: Different from Example 16, heavy metal activating bacteria are added to the soil every 40 seconds during stirring and mixing, and the soil is treated with ultraviolet radiation for 5 seconds and then with electron beam radiation for 8 seconds. After stirring, the soil is turned over for 30 seconds and treated with ultraviolet radiation for 15 seconds during the turning and mixing.

[0085] Example 20: Different from Example 16, the ultraviolet radiation parameters are: ultraviolet wavelength is 320nm, irradiation intensity is 220μW / cm·s; electron beam radiation parameters are: electron beam energy is 1.8MeV, beam power is 20kW, beam density is: 5×10 9 p / cm 2 ·s; the amount of heavy metal activating bacteria added each time accounts for 10wt% of the soil weight.

[0086] Example 21: Different from Example 16, the heavy metal activated bacterial solution is composed of purified oxidase: Bacillus: Pseudomonas fluorescens: nitrogen-fixing bacteria in a volume ratio of 0.5:1:1:1.2.

[0087] Example 22: Different from Example 16, the heavy metal activated bacterial solution is composed of purified oxidase: Bacillus: Pseudomonas fluorescens: nitrogen-fixing bacteria in a volume ratio of 0.8:1:1:1.5.

[0088] Experimental Example: The description of this experimental example is based on the scheme described in Example 1, and is intended to illustrate the practical application effect of the present invention. This experimental example targets organic and heavy metal composite pollutants, and prepares composite simulated contaminated soil containing heavy metals mercury and nickel and organic substances benzene and chlorobenzene as the test object, conducts a contaminated soil remediation treatment test, and measures the removal rate of organic matter in the soil and the leaching rate of heavy metals after the reaction.

[0089] 1. To explore the effect of the ex situ indirect thermal desorption step on the treatment of soil contaminated by organic matter and heavy metals.

[0090] Control group 1: Different from Example 1, the curing / stabilizing agent is a commercially available heavy metal stabilized chelating agent curing agent, specifically KR-01 heavy metal stabilized chelating agent curing agent produced by Wuxi KeRong Environmental Technology Co., Ltd.

[0091] Control group 2: Different from Example 1, the amount of solidification / stabilization agent added was 3 wt% of the mass of the soil discharged from S2-3.

[0092] Conclusion: Figure 2~Figure 5 From the data comparison of Example 1 to Example 3, it can be seen that excessive quicklime will cause the soil pH value to rise sharply, which may increase the precipitation of Hg and Ni, reduce their leaching rate, and may also cause soil alkalization, affecting the ecological function of the soil and the subsequent treatment effect. When the quicklime content is insufficient, the soil pH value may not be effectively adjusted to reduce the leaching concentration of heavy metals; an appropriate amount of quicklime can effectively promote the desorption and volatilization of benzene and chlorobenzene in the soil by increasing the soil temperature and reducing the soil water content, thereby improving its removal rate; excessive quicklime may cause serious soil alkalization, which not only affects the ecological function of the soil, but also may inhibit the activity of certain microorganisms, thereby affecting the biodegradation process of organic matter. Insufficient quicklime content may not provide enough heat or reduce the effect of water content, resulting in a reduced organic matter removal rate. However, from the comparison of the data of Example 1 and Example 4 to Example 15, it can be seen that if the parameters in the ex situ indirect thermal desorption process are too large or too small, the treatment effect on the soil contaminated with organic matter and heavy metals will be weakened. From the comparison of Example 1, Example 12 to Example 13 and Control Group 1 and Control Group 2, it can be found that although the treatment effect of Control Group 1 on mercury and nickel metals is slightly different from that of the present invention, the treatment effect on the organic substances benzene and chlorobenzene shows a significant downward trend, and the excessive addition of the solidifying / stabilizing agent in Control Group 2 will also affect the treatment effect of the soil contaminated with organic matter and heavy metals. This shows that the solidifying / stabilizing agent prepared in the present application has a significant effect on improving the effect of the soil contaminated with organic matter and heavy metals, mainly because the calcium wood sulfonate in the present invention can form a more stable complex or precipitate with the iron salt and phosphate in the solidifying / stabilizing agent, and the combined use of multiple materials can produce a synergistic effect, improve the stability and dispersibility of the agent, and thus improve the overall treatment effect.

[0093] 2. Explore the effects of solidification / stabilization process steps on the treatment of soil contaminated by organic matter and heavy metals

[0094] Control group 3: Different from Example 16, no ultraviolet radiation treatment was performed after the electron beam radiation treatment.

[0095] Control group 4: Different from Example 16, the heavy metal activated bacteria was replaced with deionized water during the stirring process.

[0096] Control group 5: Different from Example 16, the heavy metal-activated bacteria does not contain purified oxidase.

[0097] Conclusion: Figure 6~Figure 9 Comparison of Example 1 and Example 16 to Example 22 shows that the synergistic effect of stirring / light treatment during the solidification / stabilization process in Example 16 to Example 22 has an enhanced effect on the treatment of soil contaminated with organic matter and heavy metals. Example 16 is the optimal solution. The content of heavy metals and organic matter in the soil after electron beam radiation treatment will be significantly reduced. The ultraviolet radiation at this time can further reduce the number of microorganisms in the soil. The ultraviolet radiation treatment during the turning period has a wider range of action, promotes the decomposition and mineralization of certain organic matter through photochemical reactions, and can further ensure the stability and durability of the treatment effect. In the control group 3, if ultraviolet radiation treatment is not performed, the removal of organic matter will be affected to a certain extent. Comparison of Example 16, Example 17-Example 18 and Control Group 4 shows that in Example 16, the addition of heavy metal activating bacteria at regular intervals helps to promote the dissolution, migration and transformation of heavy metals in the soil, improve the bioavailability of heavy metals, provide more effective target substances for subsequent physical and chemical treatments, and can also keep soil moisture within a specific range, which is beneficial to the activity and metabolism of microorganisms, thereby accelerating the transformation and degradation process of heavy metals. Although the soil moisture can be maintained in Control Group 4, the leaching effect on heavy metals in the soil is limited. By comparing Example 16, Example 21 to Example 22 and Control Group 5, it can be seen that the treatment effect of Control Group 5 on soil contaminated with organic matter and heavy metals is significantly weakened. Bacillus usually has a strong ability to decompose organic matter, Pseudomonas fluorescens may have the ability to absorb or transform heavy metals, and nitrogen-fixing bacteria can convert nitrogen in the atmosphere into a nitrogen source that can be used by plants. In the present application, purified oxidase is added to further catalyze the redox reaction of heavy metal ions, so that they are converted from a more toxic form to a less toxic form, and heavy metal ions can be dissociated from the soil to facilitate absorption and transformation by microorganisms. In addition, the microorganisms in the composite bacterial liquid can improve soil structure, soil fertility and biodiversity while treating heavy metals and organic matter. Therefore, the method of ex situ indirect thermal desorption for treating soil contaminated with organic matter and heavy metals proposed in the present application provides a new concept for solving the problem of composite soil pollution.

Claims

1. A method for treating soil contaminated by organic matter and heavy metals by ex situ indirect thermal desorption, characterized in that: The following steps are involved: S1. Pretreatment of contaminated soil After the contaminated soil is dug out as the soil to be treated and transported to the pretreatment area, large stones with a particle size of more than 2 cm in the soil to be treated are removed, and the soil to be treated is sieved and crushed to a particle size of 1 to 20 mm, and then quicklime accounting for 1 to 2 wt% of the weight of the soil to be treated is added, mixed evenly to obtain a first mixed soil, and the moisture content of the first mixed soil is adjusted to less than 20% to obtain pretreated contaminated soil; S2. Ex situ indirect thermal desorption treatment S2-1, the contaminated soil pretreated by S1 is sent to a feeding conveyor with an electromagnetic iron remover at the front end by a main conveyor belt, and then the feeding conveyor belt transports the pretreated contaminated soil to a feeding auger, and the pretreated contaminated soil is transferred to the upper part of the feeding port of the hollow spiral rod of the thermal desorption unit by tilting the throwing belt conveyor in the feeding auger; S2-2, the contaminated soil after pretreatment is introduced into the feeding unit along the feeding port of the hollow spiral rod, the burner is turned on to heat the outer wall of the heating chamber, the parameters in the heating chamber are monitored, and when the monitoring data meets the ignition conditions, the second mixed soil is ignited and air and thermal desorption fuel are supplied to obtain the second mixed soil. After 250 to 260 days of ex situ indirect thermal desorption at a temperature of 640 to 660°C, the soil treated by ex situ indirect thermal desorption is obtained, and the gas after thermal desorption is obtained, and the gas is purified and discharged; S2-3, spraying water accounting for 7-10% of the mass of the soil after the ex situ indirect thermal desorption treatment into the soil after the ex situ indirect thermal desorption treatment to obtain high-temperature soil, and discharging the high-temperature soil from the discharge port of the hollow screw rod; S3, curing / stabilization treatment Spray water on the soil discharged from S2-3 until the moisture content is 24-26%, add a solidifying / stabilizing agent at 1-3wt%, stir and mix evenly for 3-5min to obtain the third mixed soil, and transport it to a curing plant for 7-15 days of curing. During the curing period, timely replenish water to maintain the moisture content of the third mixed soil at 24-26%, and obtain the treated soil that meets the standards and backfill it into the original excavated foundation pit; In S3, the solidifying / stabilizing agent is composed of agent A and agent B in a mass ratio of 1:1; in terms of weight percentage, agent A includes 94-96% of iron salts and phosphates and the remainder of calcium lignin sulfonate, wherein the weight ratio of iron salts and phosphates is 1:1; agent B includes 5-8% of dolomite, 5-8% of hydroxyapatite, 5-8% of quicklime, 15-20% of water glass, 15-20% of humic acid and 20-30% of natural clay minerals and the remainder of straw biochar, and the pH of the solidifying / stabilizing agent is 7-8 and the particle size is 10-20 nm; In S3, During the stirring and mixing, heavy metal activating bacteria are added to the soil every 30 to 40 seconds, and the soil is treated with ultraviolet radiation for 3 to 5 seconds, and then with electron beam radiation for 5 to 8 seconds. After the stirring is completed, the soil is continuously tossed for 15 to 30 seconds and the soil is treated with ultraviolet radiation for 10 to 15 seconds during the tossing. The heavy metal activating bacteria are composed of purified oxidase: Bacillus: Pseudomonas fluorescens: nitrogen-fixing bacteria in a volume ratio of 0.5 to 0.8:1:1:1.2 to 1.5; The ultraviolet radiation parameters are: ultraviolet wavelength is 280~320nm, irradiation intensity is 200~220μW / cm·s; the electron beam radiation parameters are: electron beam energy is 1~1.8MeV, beam power is 18~20kW, beam density is: 5×10 4 ~5×10 9 p / cm 2 ·s; the amount of the heavy metal activated bacteria added each time is 5~10wt%.

2. The method for treating soil contaminated by organic matter and heavy metals by ex situ indirect thermal desorption according to claim 1, characterized in that: In S2-1, a belt scale for continuously weighing the feed is provided on the feeding drive belt, and the belt scale is electrically connected to the main conveyor belt. The signal of the belt scale is sent to the frequency converter of the main conveyor belt, and the frequency of the main conveyor belt is controlled by the frequency converter to adjust the feed amount, which is 28~32t / h.

3. The method for treating soil contaminated by organic matter and heavy metals by ex situ indirect thermal desorption according to claim 1, characterized in that: In S2-2, a burner and an ignition device are provided at the bottom of the hollow spiral rod, the burner has a feed pipeline and an air intake pipeline, a flow rate and temperature detector is provided on the air intake pipeline, and a flow rate and pressure detector is provided on the feed pipeline; The ignition conditions are: the temperature of the air intake line reaches 200-550°C, the flow rate of the air intake line reaches 3-10L / min, the flow rate of the feed line reaches 3-5 tons / min, and the pressure of the feed line reaches 0.2-0.3MPa.

4. The method for treating soil contaminated by organic matter and heavy metals by ex situ indirect thermal desorption according to claim 1, characterized in that: In S2-2, the thermal desorption fuel is liquefied natural gas, and the amount of thermal desorption fuel introduced is 400~500Nm 3 / h, the air supply pressure is 28~32kPa.

5. The method for treating soil contaminated by organic matter and heavy metals by ex situ indirect thermal desorption according to claim 1, characterized in that: In S2-2, the gas purification method is: dust removal and cooling of the gas after thermal desorption to a temperature of 60~70°C, and then condensation by a medium plate heat exchanger to reduce the temperature to 20~35°C, and then use a demister to separate the water mist entrained in the gas from the gas to obtain separated water mist and gas. After the gas is treated with primary activated carbon fiber and secondary activated carbon adsorption, it is concentrated to the burner flame of the heating unit under the action of an induced draft fan and burned for 3~10 hours. The separated water mist is sent to a wastewater treatment facility for purification.

6. The method for treating soil contaminated by organic matter and heavy metals by ex situ indirect thermal desorption according to claim 1, characterized in that: In S3, during the curing period, the third mixed soil is covered with tarpaulin, and before covering, the third mixed soil is sprayed with water until the moisture content is maintained at 25%.

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