A method for co-processing dioxin-contaminated soil in a cement kiln

Abstract

This invention provides a method for co-processing dioxin-contaminated soil in a cement kiln, relating to the field of dioxin treatment technology. The method includes the following specific steps: First, the cement clinker raw materials / batches and pulverized coal are pre-homogenized. During the pre-treatment process, the dioxin-contaminated soil is first uniformly mixed with a TiO₂ composite carrier (TiO₂ / pulverized coal), and then treated using a medium-low temperature microwave plasma fluidized bed reactor. Finally, it is pneumatically conveyed to a decomposition furnace for co-processing in the cement kiln. The pre-treatment process effectively solves the problem of low efficiency in co-processing high-concentration dioxin-contaminated soil in cement kilns. Simultaneously, the TiO₂ added during the pre-treatment process acts as a catalyst to lower the temperature during dioxin decomposition, effectively mitigating the risk of secondary dioxin synthesis at the low-temperature end of the cement rotary kiln.

Description

Technical Field

[0001] This invention relates to the field of dioxin treatment technology, specifically a method for co-treating dioxin-contaminated soil in a cement kiln. Background Technology

[0002] Dioxins were among the first persistent organic pollutants (POPs) listed by the United Nations Environment Programme (UNEP) as priority targets for control and reduction under the Stockholm Convention on Persistent Organic Pollutants. They are chlorinated tricyclic aromatic compounds, and due to variations in the number and position of chlorine atoms, they constitute 75 polychlorinated dibenzo-p-dioxins and 135 polychlorinated dibenzofurans. Of these 210 compounds, 17 are considered to pose significant risks to human health, exhibiting acute lethal toxicity, high carcinogenicity, teratogenicity, and mutagenicity, and are classified as Group 1 carcinogens. Dioxins are virtually non-existent in nature and primarily originate from the incineration of waste (including municipal solid waste, medical waste, hazardous waste, and sewage sludge) and from industries such as metal smelting, pulp and paper making, pesticide production, and the production of certain organochlorine chemical products.

[0003] Waste incineration and metal smelting are the main sources of dioxin emissions in my country, accounting for more than 62% of the country's total dioxin emissions. Dioxins released into the atmosphere enter the soil through dry and wet deposition, causing soil contamination and posing a threat to human health. Once soil is contaminated with dioxins and the risk is unacceptable, it is essential to treat and remediate the contaminated soil.

[0004] Currently, the main methods applied to the treatment of dioxin-contaminated soil include thermal desorption, photodegradation, microbial remediation, and co-processing in cement kilns. However, thermal desorption is extremely energy-intensive (continuous heating above 800℃), time-consuming, and costly, and requires proper treatment of the collected dioxin exhaust gas to avoid secondary pollution. Dioxins can absorb near-ultraviolet electromagnetic radiation and undergo photodegradation, but the depth of photodegradation in the soil layer is extremely limited, generally within a few millimeters of the surface, and the organic matter composition also has a significant impact on the photodegradation effect. Microbial remediation of dioxin-contaminated soil has a long treatment cycle and can only degrade low-chlorinated homologues, with an efficiency of less than 70%. Conventional co-processing of dioxin-contaminated soil in cement kilns is energy-intensive, faces the barrier of low treatment efficiency for high-concentration dioxin-contaminated soil, and carries the risk of secondary dioxin synthesis at low-temperature ends (such as in kiln tail waste heat utilization systems), resulting in excessive dioxin concentrations in the flue gas emitted from the exhaust stack. Summary of the Invention

[0005] (a) Technical problems to be solved

[0006] To address the shortcomings of existing technologies, this invention provides a method for co-processing dioxin-contaminated soil in cement kilns. This method solves the problem of low efficiency in co-processing high-concentration dioxin-contaminated soil in existing cement kilns, and also eliminates the risk of dioxin re-synthesis at low temperatures during the co-processing of dioxins in existing cement kilns, which could lead to substandard emissions. This method achieves simultaneous treatment of complex pollution and also realizes a closed loop of low carbon and resource utilization.

[0007] (II) Technical Solution

[0008] To achieve the above objectives, the present invention provides the following technical solution: a method for co-treating dioxin-contaminated soil in a cement kiln, comprising the following specific steps:

[0009] S1. First, the raw materials / batches of cement clinker and pulverized coal will be pre-homogenized.

[0010] S2. After pre-homogenization, the raw materials / batches of cement clinker are sent to the batching station and weighed at the batching station, i.e., the weight of the raw meal entering the mill. The weighed raw meal entering the mill is then ground and sent to the rotary kiln after being treated by the preheater.

[0011] S3. After pre-homogenization, the pulverized coal is ground and stored. During use, it is injected into the rotary kiln through the burner.

[0012] S4. The dioxin-contaminated soil is transported to the cement plant by a transfer vehicle and weighed. The weighed dioxin-contaminated soil is then crushed in a sealed environment and stored separately. Each separately stored dioxin-contaminated soil must be labeled with its specific weight, i.e., the weight of the dioxin-contaminated soil entering the kiln.

[0013] S5. Before being added to the rotary kiln, dioxin-contaminated soil needs to be pretreated. First, a fully enclosed twin-shaft mixer (micro negative pressure) is used to mix the dioxin-contaminated soil with TiO2 composite carrier evenly. Then, the dioxin-contaminated soil mixed with TiO2 composite carrier is transported to the microwave plasma fluidized bed reactor for treatment via a closed conveyor belt. The treatment time is 15-30 minutes. The reactor is fully enclosed, and the generated waste gas is connected to the decomposition furnace through a pipeline via a negative pressure system.

[0014] S6. The pretreated dioxin-contaminated soil is sent into the decomposition furnace and then into the rotary kiln through a pneumatic conveying device in a closed manner, and the addition ratio is not higher than 5%. The formula for calculating the addition ratio is as follows: Addition ratio = weight of dioxin-contaminated soil entering the kiln / weight of raw meal entering the mill × 100%.

[0015] S7 and dioxin-contaminated soil are completely decomposed through low-temperature microwave plasma degradation and co-processing in cement kilns. Finally, they are combined with cement clinker raw materials / batches to form cement clinker. After cooling, the cement clinker is temporarily stored in a clinker silo.

[0016] Preferably, the pre-homogenization process in S1 mainly adopts either stockpile pre-homogenization or warehouse pre-homogenization.

[0017] Preferably, the dioxin-contaminated soil in S4 must be clearly labeled before being stored separately in the pretreatment workshop, and its name, properties, state, quantity, and storage time must be recorded and archived. The contaminated soil should be stored separately from cement raw materials / materials.

[0018] Preferably, the TiO2 composite carrier in S5 is 1.5-2.0 wt% TiO2 / coal powder, wherein the TiO2 particle size is <50 nm. The microwave plasma fluidized bed reactor has a frequency of 915 MHz, a power of 500-1500 W, a gas of 1-3 L / min air, and a temperature of 300-400 °C.

[0019] Preferably, the kiln tail exhaust gas generated in the preheater is discharged into an external SP waste heat boiler for waste heat recovery.

[0020] Preferably, the kiln head exhaust gas generated in the grate cooler is discharged into the external AQC waste heat boiler for waste heat recovery.

[0021] (III) Beneficial Effects

[0022] This invention provides a method for co-treating dioxin-contaminated soil in a cement kiln. It has the following beneficial effects:

[0023] The pretreatment process involves first uniformly mixing dioxin-contaminated soil with a TiO2 composite carrier (TiO2 / coal powder), then treating it using a medium-low temperature microwave plasma fluidized bed reactor; finally, it is pneumatically conveyed to a decomposition furnace for co-processing in a cement kiln. This pretreatment effectively addresses the low efficiency of co-processing high-concentration dioxin-contaminated soil in cement kilns. Furthermore, the added TiO2 acts as a catalyst to lower the temperature during dioxin decomposition, effectively mitigating the risk of secondary dioxin synthesis at the low-temperature end of the cement rotary kiln. Attached Figure Description

[0024] Figure 1 This is a process flow diagram of a method for co-treating dioxin-contaminated soil in a cement kiln, as proposed in this invention. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0026] Example:

[0027] like Figure 1 As shown in the figure, this invention provides a method for co-treating dioxin-contaminated soil in a cement kiln, comprising the following specific steps:

[0028] S1. First, the raw materials / batches of cement clinker and pulverized coal will be pre-homogenized. The pre-homogenization process in S1 mainly adopts stockpile pre-homogenization. The crushed raw materials are piled and retrieved in the pre-homogenization stockpile. The stacker piles the raw materials in a certain way on the stockpile to form overlapping herringbone-shaped material layers. The reclaimer cuts the material from each layer at the same time along the longitudinal direction perpendicular to the material pile, completing the "flat laying and direct retrieval" to achieve the mixing and homogenization of the materials. The pre-homogenization stockpile has two layout methods: rectangular and circular.

[0029] S2. After pre-homogenization, the raw materials / batches of cement clinker are sent to the batching station and weighed at the batching station, i.e., the weight of the raw meal entering the mill. The weighed raw meal entering the mill is then ground and sent to the rotary kiln after being treated by the preheater.

[0030] S3. After pre-homogenization, the pulverized coal is ground and stored. During use, it is injected into the rotary kiln through the burner.

[0031] S4. Dioxin-contaminated soil is transported to the cement plant by transfer vehicle and weighed. The weighed dioxin-contaminated soil is then sealed, crushed, and stored separately. Each separately stored dioxin-contaminated soil must be labeled with its specific weight, i.e., the weight of the dioxin-contaminated soil entering the kiln. Before the dioxin-contaminated soil in S4 is stored separately in the pretreatment workshop, it must be clearly labeled, and its name, properties, state, quantity, and storage time must be recorded and archived. The contaminated soil should be stored separately from cement raw materials / batches.

[0032] Before being added to the rotary kiln, dioxin-contaminated soil (S5) requires pretreatment. First, a fully enclosed twin-shaft mixer (with slight negative pressure) is used to uniformly mix the dioxin-contaminated soil with the TiO2 composite carrier. Then, the dioxin-contaminated soil mixed with the TiO2 composite carrier is transported to a microwave plasma fluidized bed reactor for treatment. The treatment time is 15–30 minutes. This reactor is fully enclosed, and the generated waste gas is piped to the decomposition furnace through a negative pressure system. In S5, the TiO2 composite carrier is 1.5–2.0 wt% TiO2 / coal powder, with TiO2 particle size <50 nm. The microwave plasma fluidized bed reactor has the following parameters: frequency: 915 MHz; power: 500–1500 W; gas: air 1–3 L / min; temperature: 300℃–400℃.

[0033] S6. The pretreated dioxin-contaminated soil is sent into the decomposition furnace and then into the rotary kiln through a pneumatic conveying device in a closed manner, and the addition ratio is not higher than 5%. The formula for calculating the addition ratio is as follows: Addition ratio = weight of dioxin-contaminated soil entering the kiln / weight of raw meal entering the mill × 100%.

[0034] S7 and dioxin-contaminated soil are completely decomposed through low-temperature microwave plasma degradation and co-processing in cement kilns. Finally, they are combined with cement clinker raw materials / batches to form cement clinker. After cooling, the cement clinker is temporarily stored in a clinker silo.

[0035] The kiln tail exhaust gas generated in the preheater is discharged into the external SP waste heat boiler for waste heat recovery, and the kiln head exhaust gas generated in the grate cooler is discharged into the external AQC waste heat boiler for waste heat recovery.

[0036] The above method was used to co-treat dioxin-contaminated soil in cement kilns via production lines 1, 2, 3, and 4. The relevant parameter table is as follows:

[0037] Table 1 shows the relevant parameters of cement rotary kilns.

[0038]

[0039] Note: The dioxin toxicity equivalent mass concentrations in the contaminated soil in this table are toxicity equivalent (TEQ) mass concentrations calculated using the International Toxicity Equivalent Factor (I-TEF).

[0040] Table 2 is a list of raw materials and dioxin-contaminated soil dosage.

[0041]

[0042] Table 3 summarizes the dioxin monitoring results of the exhaust stack of the waste heat utilization system at the tail of the cement rotary kiln during random time periods.

[0043]

[0044] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for co-treating dioxin-contaminated soil in a cement kiln, characterized in that: The specific steps include the following: S1. First, the raw materials / batches of cement clinker and pulverized coal will be pre-homogenized. S2. After pre-homogenization, the raw materials / batches of cement clinker are sent to the batching station and weighed at the batching station, i.e., the weight of the raw meal entering the mill. The weighed raw meal entering the mill is then ground and sent to the rotary kiln after being treated by the preheater. S3. After pre-homogenization, the pulverized coal is ground and stored. During use, it is injected into the rotary kiln through the burner. S4. The dioxin-contaminated soil is transported to the cement plant by a transfer vehicle and weighed. The weighed dioxin-contaminated soil is then crushed in a sealed environment and stored separately. Each separately stored dioxin-contaminated soil must be labeled with its specific weight, i.e., the weight of the dioxin-contaminated soil entering the kiln. S5. Before being added to the rotary kiln, dioxin-contaminated soil needs to be pretreated. First, a fully enclosed twin-shaft mixer is used to mix the dioxin-contaminated soil with TiO2 composite carrier evenly. Then, the dioxin-contaminated soil mixed with TiO2 composite carrier is transported to the microwave plasma fluidized bed reactor for treatment via a closed conveyor belt. The treatment time is 15-30 minutes. The reactor is fully enclosed, and the generated waste gas is connected to the decomposition furnace through a pipeline via a negative pressure system. S6. The pretreated dioxin-contaminated soil is conveyed into the decomposition furnace and then into the rotary kiln via a pneumatic conveying device in a closed manner, and the addition ratio is not higher than 5%. The formula for calculating the addition ratio is as follows: Addition ratio = weight of dioxin-contaminated soil entering the kiln / weight of raw meal entering the mill × 100%; S7 and dioxin contaminated soil are completely decomposed through low-temperature microwave plasma degradation and co-processing in cement kilns. Finally, they are combined with cement clinker raw materials / batches to form cement clinker. After cooling, the cement clinker is temporarily stored in a clinker silo. The TiO2 composite carrier in S5 is 1.5~2.0 wt% TiO2 / coal powder, wherein the TiO2 particle size is <50 nm. The microwave plasma fluidized bed reactor has a frequency of 915 MHz and a power of 500~1500 W. Gas: Air 1~3 L / min; Temperature: 300℃~400℃. The kiln tail exhaust gas generated in the preheater is discharged into the external SP waste heat boiler for waste heat recovery, and the kiln head exhaust gas generated in the grate cooler is discharged into the external AQC waste heat boiler for waste heat recovery.

2. The method for co-processing dioxin-contaminated soil in a cement kiln according to claim 1, characterized in that: The pre-homogenization process in S1 can be either stockpile pre-homogenization or warehouse pre-homogenization.

3. The method for co-processing dioxin-contaminated soil in a cement kiln according to claim 1, characterized in that: Before storing the dioxin-contaminated soil in the pretreatment workshop, it must be clearly labeled, and its name, properties, state, quantity, and storage time must be recorded and archived. The contaminated soil should be stored separately from cement raw materials / materials.

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