Preparation and dechlorination method of red-mud-based chlorine-containing hybrid plastic cracking composite dechlorination agent

Abstract

The invention belongs to the technical field of chlorine-containing mixed plastic pyrolysis dechlorination, and particularly relates to a preparation method and a dechlorination method of a red mud-based chlorine-containing mixed plastic pyrolysis composite dechlorination agent. The method comprises the following steps: firstly, converting coffee grounds into biochar through a hydrothermal carbonization reaction, and reducing part of hematite in red mud into Fe3O4 by using reducing gas released in a coffee grounds carbonization process; residual hematite in the red mud is completely reduced into Fe3O4 through a high-temperature carbon thermal reaction, meanwhile, compounding with additional CaO is achieved, and finally the composite dechlorinating agent composed of the modified red mud, the biochar and the CaO is obtained. The dechlorinating agent has a proper specific surface area and an adjustable pore structure, and can effectively fix HCl in the cracking process of chlorine-containing mixed plastic, so that plastic pyrolytic oil with ultralow chlorine content is obtained, and high-valued utilization of red mud and coffee residue solid waste is realized.

Description

Technical Field

[0001] The invention belongs to the technical field of cracking and dechlorinating chlorine-containing mixed plastics, and in particular relates to a preparation method and a dechlorination method of a red mud-based pyrolysis composite dechlorinating agent for chlorine-containing mixed plastics. Background Art

[0002] As one of the three major synthetic materials, plastic is widely used in industry, agriculture, medical treatment, furniture and home appliances, packaging and other fields due to its characteristics of easy shaping, low price and durability. With the rapid development of social economy, the production of plastics and the amount of waste plastics generated have increased rapidly. At present, the main methods for dealing with plastic waste are landfill and incineration. The landfill method wastes land resources, and the harmful elements in plastics may also cause water and soil pollution. Although incineration can convert waste plastics into heat energy for utilization, the dioxins produced during the incineration process will cause secondary pollution to the environment and release a large amount of CO2.

[0003] The waste plastic pyrolysis oil technology converts waste plastics into valuable resources such as pyrolysis oil, which can completely achieve the harmlessness and reduction of waste plastics while obtaining products with high economic benefits. It is an important means of chemical recycling of waste plastics. The raw materials of waste plastic pyrolysis technology are mostly limited to polymerized waste plastics, such as polyethylene, polypropylene, etc. However, my country's waste plastics contain a considerable proportion of polyvinyl chloride. During the pyrolysis process, some chlorine elements will enter the pyrolysis oil molecules, which will have a great impact on the subsequent processing and utilization of the pyrolysis oil. The chlorine-containing gas generated during the pyrolysis process will also corrode the equipment and pollute the environment. Therefore, dechlorination during the pyrolysis process of chlorine-containing waste plastics is crucial.

[0004] Red mud is an industrial waste residue generated during the industrial production of alumina. About 1.0-1.8 tons of red mud are emitted for every ton of alumina produced. The strong alkalinity of red mud makes it difficult to be used directly. When it is piled in the open air, the harmful impurities in the red mud will seep out and cause land salinization and pollute surface water and groundwater. Due to the small particle size and poor agglomeration performance of red mud, the dry red mud powder will spread with the wind and cause air pollution. At present, the use of red mud is limited to the preparation of cement building materials and roadbed materials, and there is no feasible method for fine or high-value utilization.

[0005] The dechlorinating agent for plastic pyrolysis gas is mainly prepared by direct mixing or loading the solid chlorine component on a carrier material. Commonly used dechlorinating agents are mainly alkali-impregnated alumina, calcium-based dechlorinating agents and copper-based dechlorinating agents. The main components of red mud are SiO2, Al2O3, CaO and Fe2O3, among which SiO2 and Al2O3 have a good skeleton support effect and can be used as carriers of the active components of the dechlorinating agent, while CaO and Fe2O3 are the main solid chlorine components. Therefore, the use of red mud solid waste to prepare dechlorinating agents has the advantage of low cost and also opens up a new path for the high-value utilization of red mud solid waste. Invention patent CN115414904A proposes a low-temperature dechlorinating agent for waste lubricating oil with limestone, lime and red mud as the main raw materials. The obtained dechlorinating agent has a high chlorine capacity and can also be washed and regenerated. Invention patent CN115477962A proposes a high-temperature flue gas dechlorinating agent with red mud, white mud and carbide slag as raw materials. Invention patent CN113372963A discloses a blast furnace gas dechlorinating agent using red mud, carbide slag, sodium hydroxide, zinc oxide, etc. as raw materials. Analysis of existing published patents shows that dechlorinating agents using red mud as raw materials can achieve a certain dechlorination effect, but the red mud content in the dechlorinating agent is relatively low and is not the main component of the dechlorinating agent, and none of them are used for dechlorination in the pyrolysis process of chlorine-containing waste plastics. Summary of the invention

[0006] The object of the present invention is to provide a chlorine-containing mixed plastic pyrolysis composite dechlorination agent with red mud as the main component, which has low cost, high chlorine capacity and good dechlorination effect, so as to solve the problems existing in the prior art.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] The first technical purpose of the present invention is to provide a method for preparing a chlorine-containing mixed plastic pyrolysis composite dechlorination agent, comprising the following steps:

[0009] (1) Raw material pretreatment: industrial solid waste red mud and coffee grounds are dried, then ground and passed through a 100-mesh sieve for later use;

[0010] (2) Hydrothermal carbonization: red mud, coffee grounds and potassium hydroxide are added to deionized water, stirred thoroughly and placed in a reactor for hydrothermal carbonization reaction. After the hydrothermal reaction is completed, the biochar-modified red mud composite is obtained by centrifugation and drying;

[0011] (3) Carbothermal reduction: The biochar-modified red mud composite was fully mixed with CaO and subjected to high-temperature carbothermal reduction reaction under a nitrogen atmosphere to obtain a CaO-red mud composite dechlorination agent.

[0012] Preferably, in step (2), the mass ratio of coffee grounds to red mud is 1:1-2, the solid-liquid ratio is 10 g:50 mL, and the potassium hydroxide concentration is 0.1-0.5 mol / L.

[0013] Preferably, the temperature of the hydrothermal reaction in step (2) is 160-250° C. and the time is 8-12 h.

[0014] Preferably, in step (3), the mass ratio of CaO to the biochar-modified red mud composite is 1:10.

[0015] Preferably, in step (3), the carbon thermal reduction temperature is 500-700° C., the heating rate is 5° C. / min, and the holding time is 3 h.

[0016] Preferably, the dechlorinating agent, based on 100% by mass, includes: on a dry basis, a red mud content of 70-80%, a biochar content of 10-20%, and a CaO content of 10%.

[0017] The second technical purpose of the present invention is to provide a dechlorination method for cracking chlorine-containing mixed plastics.

[0018] Preferably, the cracking reaction of the chlorine-containing mixed plastic adopts a tubular fixed bed reactor, nitrogen is used as the purge gas, the dechlorinating agent is loaded in the rear section of the plastic, and the mass ratio of the dechlorinating agent to the mixed plastic is 2:5.

[0019] Preferably, the pyrolysis reaction adopts a two-stage heat preservation pyrolysis, firstly heating from room temperature to 280-320°C and keeping it for 30 minutes, and then heating to 500°C and keeping it for 30 minutes, and the heating rate is 3-5°C / min.

[0020] Preferably, the mixed plastic is a mixture of one or more of low-density polyethylene, high-density polyethylene, polypropylene, polystyrene and polyvinyl chloride, wherein the mass proportion of polyvinyl chloride is 10-20%.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The present invention first converts coffee grounds into biochar through a hydrothermal carbonization reaction, and simultaneously utilizes the reducing gas released during the carbonization of the coffee grounds to reduce part of the hematite (Fe2O3) in the red mud to Fe3O4; then the hematite remaining in the red mud is completely reduced to Fe3O4 through a high-temperature carbon thermal reaction, and simultaneously realizes compounding with external CaO, and finally obtains a composite dechlorinating agent composed of modified red mud, biochar and CaO. The dechlorinating agent has a suitable specific surface area and an adjustable pore structure, and can effectively fix HCl during the cracking process of chlorine-containing mixed plastics, thereby obtaining plastic pyrolysis oil with ultra-low chlorine content, and realizing the high-value utilization of red mud and coffee grounds solid waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the X-ray diffraction spectrum of the composite dechlorinating agent prepared in Example 1 and the raw red mud.

[0024] Figure 2 This is a transmission electron microscope photograph of the composite dechlorinating agent prepared in Example 1.

[0025] Figure 3 This is a schematic diagram of the plastic cracking and dechlorination reactor used in the present invention. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments, but the present invention is not limited to these embodiments.

[0027] Example 1

[0028] 4.0 g of coffee grounds dried to constant weight and 6.0 g of red mud were ground and mixed evenly, added into 50 mL of 0.1 mol / L potassium hydroxide aqueous solution, stirred for 30 min, placed in a reactor, hydrothermally kept at 180 °C for 8 h, taken out, centrifuged and dried to obtain a biochar-modified red mud composite; 0.6 g of CaO was taken and mixed thoroughly with the red mud composite, placed in a tubular furnace, heated to 500 °C at a rate of 5 °C / min under a nitrogen atmosphere, and kept warm for 3 h to obtain a composite dechlorination agent A composed of modified red mud, biochar and CaO.

[0029] Figure 1 The X-ray diffraction spectra of the composite dechlorination agent and raw red mud prepared in this example show that the iron species in the raw red mud is mainly hematite (Fe2O3), while Fe2O3 is reduced to Fe3O4 in the composite dechlorination agent.

[0030] Example 2

[0031] 4.0 g of coffee grounds dried to constant weight and 6.0 g of red mud were ground and mixed evenly, added into 50 mL of 0.1 mol / L potassium hydroxide aqueous solution, stirred for 30 min, placed in a reactor, hydrothermally kept at 180 °C for 8 h, taken out, centrifuged and dried to obtain a biochar-modified red mud composite; 0.6 g of CaO was taken and mixed thoroughly with the red mud composite, placed in a tubular furnace, heated to 600 °C at a rate of 5 °C / min under a nitrogen atmosphere, and kept warm for 3 h to obtain a composite dechlorination agent B composed of modified red mud, biochar and CaO.

[0032] Example 3

[0033] 4.0 g of coffee grounds dried to constant weight and 6.0 g of red mud were ground and mixed evenly, added into 50 mL of 0.1 mol / L potassium hydroxide aqueous solution, stirred for 30 min, placed in a reactor, hydrothermally kept at 180 °C for 8 h, taken out, centrifuged and dried to obtain a biochar-modified red mud composite; 0.6 g of CaO was taken and fully mixed with the red mud composite, placed in a tubular furnace, heated to 700 °C at a rate of 5 °C / min under a nitrogen atmosphere, and kept warm for 3 h to obtain a composite dechlorination agent C composed of modified red mud, biochar and CaO.

[0034] Example 4

[0035] 4.0 g of coffee grounds dried to constant weight and 6.0 g of red mud were ground and mixed evenly, added into 50 mL of 0.1 mol / L potassium hydroxide aqueous solution, stirred for 30 min, placed in a reactor, hydrothermally kept at 220 °C for 8 h, taken out, centrifuged and dried to obtain a biochar-modified red mud composite; 0.6 g of CaO was taken and mixed thoroughly with the red mud composite, placed in a tubular furnace, heated to 700 °C at a rate of 5 °C / min under a nitrogen atmosphere, and kept warm for 3 h to obtain a composite dechlorination agent D composed of modified red mud, biochar and CaO.

[0036] The dechlorination activities of the dechlorinating agents prepared in Examples 1 to 4 above were evaluated.

[0037] Figure 3 A schematic diagram of a plastic cracking and dechlorination reactor used in the present invention is provided.

[0038] Mix high-density polyethylene, low-density polyethylene, polypropylene, polystyrene and polyvinyl chloride in a mass ratio of 3.0:3.0:2.5:0.5:1.0. Take 5g of the mixed plastic and 2g of the dechlorinating agent and fill them in a fixed bed reactor in sections, add asbestos to fix them. For specific filling methods, see Figure 3 The front end of the cracking reactor is connected to nitrogen, and the rear end is connected to a U-tube condenser to collect pyrolysis oil. The condenser is connected to a scrubber, and the scrubber solution is a NaOH solution.

[0039] When performing the cracking reaction, the temperature was first raised to 300°C at a rate of 5°C / min and kept at that temperature for 30 minutes, and then raised to 500°C at a rate of 3°C / min and kept at that temperature for 30 minutes. After the reaction, the oil sample in the U-shaped tube was collected and the chlorine content of the oil sample was tested.

[0040] The evaluation results of the dechlorination of mixed plastics by pyrolysis of the dechlorination agents prepared in Examples 1 to 4 are shown in Table 1.

[0041] Table 1 Chlorine content of pyrolysis oil obtained by dechlorination agent in various examples for mixed plastic cracking

[0042] Dechlorination agent Dechlorinating Agent A Dechlorination agent B Dechlorinating agent C Dechlorination agent D Chlorine content of pyrolysis oil, ppm 32 25 13 9.5

[0043] The above embodiments are only used to illustrate the technical solutions of the present invention, but not to limit the scope of the present invention in any way. Although the present invention has been described in detail with reference to the above embodiments, it is still possible for a person skilled in the art to modify the technical solutions described in the above embodiments, or to replace some of the technical features therein by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions claimed to be protected by the present invention.

Claims

1. A method for preparing a red mud-based chlorine-containing mixed plastic cracking composite dechlorination agent, characterized in that: The preparation method comprises the following steps: (1) Raw material pretreatment: industrial solid waste red mud and coffee grounds are dried, then ground and passed through a 100-mesh sieve for later use; (2) Hydrothermal carbonization: red mud, coffee grounds and potassium hydroxide are added to deionized water, stirred thoroughly and placed in a reactor for hydrothermal carbonization reaction. After the hydrothermal reaction is completed, the biochar-modified red mud composite is obtained by centrifugation and drying; (3) Carbothermal reduction: The biochar-modified red mud composite was fully mixed with CaO and subjected to high-temperature carbothermal reduction reaction under a nitrogen atmosphere to obtain a CaO-red mud composite dechlorination agent.

2. The preparation method according to claim 1, characterized in that: In step (2), the mass ratio of coffee grounds to red mud is 1:1-2, the solid-liquid ratio is 10 g:50 mL, and the potassium hydroxide concentration is 0.1-0.5 mol / L.

3. The preparation method according to claim 1, characterized in that: The temperature of the hydrothermal reaction in step (2) is 160-250° C. and the time is 8-12 hours.

4. The preparation method according to claim 1, characterized in that: In step (3), the mass ratio of CaO to the biochar-modified red mud composite is 1:

10.

5. The preparation method according to claim 1, characterized in that: In step (3), the carbon thermal reduction temperature is 500-700°C, the heating rate is 5°C / min, and the holding time is 3h.

6. A red mud-based chlorine-containing mixed plastic cracking composite dechlorination agent, which is prepared by the method according to any one of claims 1 to 5, characterized in that: The dechlorinating agent, based on 100% by mass, includes: on a dry basis, a red mud content of 70-80%, a biochar content of 10-20%, and a CaO content of 10%.

7. A method for dechlorinating chlorine-containing mixed plastics by cracking, characterized in that: The composite dechlorinating agent obtained by the preparation method according to any one of claims 1 to 6 is used in the cracking and dechlorination process of chlorine-containing mixed plastics.

8. A method for dechlorinating chlorine-containing mixed plastics by cracking according to claim 7, characterized in that: The cracking reaction of chlorine-containing mixed plastics adopts a tubular fixed bed reactor, nitrogen is used as the purge gas, and the dechlorinating agent is loaded in the rear section of the plastic. The mass ratio of the dechlorinating agent to the mixed plastic is 2:

5.

9. The method for dechlorinating chlorine-containing mixed plastics by cracking according to claim 7, characterized in that: The cracking reaction adopts a two-stage heat preservation cracking, firstly the temperature is raised from room temperature to 280-320°C for 30 min, and then the temperature is raised to 500°C for 30 min, and the heating rate is 3-5°C / min.

10. The method for dechlorinating chlorine-containing mixed plastics by cracking according to claim 7, characterized in that: The mixed plastic is a mixture of one or more of low-density polyethylene, high-density polyethylene, polypropylene, polystyrene and polyvinyl chloride, wherein the mass proportion of polyvinyl chloride is 10-20%.

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