Preparation of red mud-based chlorinated mixed plastic cracking composite dechlorination agent and dechlorination method

Abstract

The present application belongs to the technical field of cracking and dechlorination of chlorinated mixed plastics, and particularly relates to a preparation method of a red mud-based chlorinated mixed plastic pyrolysis composite dechlorination agent and a dechlorination method. First, the coffee grounds are converted into biochar through a hydrothermal carbonization reaction, and at the same time, the reducing gas released during the carbonization of the coffee grounds is used to reduce part of the hematite in the red mud to Fe3O4; then, through a high-temperature carbon thermal reaction, the residual hematite in the red mud is completely reduced to Fe3O4, and at the same time, the composite with the added CaO is realized, and finally, the composite dechlorination agent composed of the modified red mud, the biochar and the CaO is obtained. The dechlorination agent has a suitable specific surface area and an adjustable pore structure, can effectively fix HCl during the cracking of chlorinated mixed plastics, and can further obtain plastic pyrolysis oil with ultra-low chlorine content, and realize the high-value utilization of the red mud and coffee grounds solid waste.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of cracking and dechlorination of chlorinated mixed plastics, and particularly relates to a preparation method of a red mud-based chlorinated mixed plastic pyrolysis composite dechlorination agent and a dechlorination method. BACKGROUND

[0002] Plastics, as one of the three synthetic materials, are widely used in the fields of industry, agriculture, medicine, furniture and household appliances, packaging, etc. due to their characteristics of easy shaping, low price and durability. With the rapid development of social economy, the production and waste plastic production of plastics are growing rapidly. At present, the main methods for treating plastic waste are landfill and incineration. Landfill wastes land resources, and harmful elements in plastics may cause water and soil pollution. Although incineration can convert waste plastics into heat energy for use, dioxins generated during incineration will cause secondary pollution to the environment, and release a large amount of CO2.

[0003] Waste plastic pyrolysis oil technology converts waste plastics into valuable resources such as pyrolysis oil, which not only realizes the harmlessness and reduction of waste plastics, but also obtains products with high economic benefits, and is an important means of chemical recycling of waste plastics. The raw materials of waste plastic pyrolysis technology are mostly limited to addition polymerization plastics such as polyethylene and polypropylene. However, a considerable proportion of waste plastics in China contains polyvinyl chloride, and part of the chlorine elements will enter the pyrolysis oil molecules during the pyrolysis process, which will greatly affect the subsequent processing and utilization of pyrolysis oil. The chlorine-containing gas generated during pyrolysis will also cause corrosion to the equipment and pollution to the environment. Therefore, dechlorination during the pyrolysis process of chlorinated waste plastics is crucial.

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

[0005] The dechlorination agent for plastic pyrolysis gas is mainly prepared by directly mixing or loading the chlorine fixation component on the carrier material. The commonly used dechlorination agent mainly includes impregnated alkali alumina, calcium-based dechlorination agent and copper-based dechlorination agent. The main components of red mud are SiO2, Al2O3, CaO and Fe2O3, etc., among which SiO2 and Al2O3 have a good skeleton support effect and can be used as the carrier of the active component of the dechlorination agent, and CaO and Fe2O3 are the main chlorine fixation components. Therefore, the preparation of the dechlorination agent from the red mud solid waste has the advantages of low cost, and opens up a new path for the high-value utilization of the red mud solid waste. The invention patent CN115414904A proposes a low-temperature dechlorination agent for waste lubricating oil, which is prepared from limestone, lime and red mud as main raw materials. The obtained dechlorination agent has a high chlorine capacity, and can also be regenerated by water washing. The invention patent CN115477962A proposes a high-temperature flue gas dechlorination agent prepared from red mud, white mud and carbide slag as raw materials. The invention patent CN113372963A discloses a blast furnace gas dechlorination agent prepared from red mud, carbide slag, sodium hydroxide, zinc oxide and the like. Analysis of the existing patents shows that the dechlorination agent prepared from the red mud can achieve a certain dechlorination effect, but the content of the red mud in the dechlorination agent is low, and the red mud is not the main component of the dechlorination agent, and the dechlorination agent is not used in the dechlorination of the chlorinated waste plastic pyrolysis process. SUMMARY

[0006] The purpose of the present application is to provide a low-cost, high-chlorine-content, good-dechlorination-effect chlorinated mixed plastic pyrolysis composite dechlorination agent with red mud as the main component, so as to solve the problems existing in the prior art.

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

[0008] The first technical purpose of the present application is to provide a preparation method of a chlorinated mixed plastic pyrolysis composite dechlorination agent, which comprises the following steps:

[0009] (1) Pretreatment of raw materials: dry the industrial solid waste red mud and coffee grounds, and then grind them through a 100-mesh sieve for use;

[0010] (2) Hydrothermal carbonization: add the red mud, coffee grounds and potassium hydroxide to deionized water, stir thoroughly, and then place them in a reaction kettle for hydrothermal carbonization reaction. After the hydrothermal reaction is completed, the mixture is centrifuged and dried to obtain a biochar modified red mud composite;

[0011] (3) Carbon thermal reduction: mix the biochar modified red mud composite with CaO thoroughly, and perform high-temperature carbon thermal 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 10g:50mL, and the concentration of potassium hydroxide is 0.1-0.5mol / L.

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

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

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

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

[0017] The second technical object of the present application is to provide a dechlorination method for chlorinated mixed plastic pyrolysis.

[0018] Preferably, the chlorinated mixed plastic pyrolysis reaction uses a tubular fixed bed reactor, nitrogen is used as the purge gas, the dechlorination agent is loaded in the rear section of the plastic, and the mass ratio of the dechlorination agent to the mixed plastic is 2:5.

[0019] Preferably, the pyrolysis reaction uses two-stage holding pyrolysis, first heating from room temperature to 280-320 DEG C for 30 min, and then heating to 500 DEG C for 30 min, with a heating rate of 3-5 DEG C / min.

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

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

[0022] The present application first converts coffee grounds into biochar through a hydrothermal carbonization reaction, and simultaneously reduces part of the hematite (Fe2O3) in the red mud to Fe3O4 using the reducing gas released during the carbonization of the coffee grounds; then completely reduces the residual hematite in the red mud to Fe3O4 through a high-temperature carbothermic reaction, and simultaneously realizes compounding with additional CaO, to finally obtain a composite dechlorination agent composed of modified red mud, biochar, and CaO. The dechlorination agent has a suitable specific surface area and adjustable pore structure, can effectively fix HCl during the pyrolysis of chlorinated mixed plastic, and thus obtain plastic pyrolysis oil with ultra-low chlorine content, and realizes high-value utilization of red mud and coffee grounds solid waste. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 X-ray diffraction spectra of the composite dechlorination agent prepared in Example 1 and the raw material red mud.

[0024] Figure 2 Transmission electron microscope photo of the composite dechlorination agent prepared in Example 1.

[0025] Figure 3 Schematic diagram of the plastic pyrolysis dechlorination reactor used in the present application. DETAILED DESCRIPTION

[0026] The technical solutions of the present application are further described in detail below in combination with the drawings and specific examples, and the present application is not limited to these examples.

[0027] Example 1

[0028] 4.0 g of coffee grounds and 6.0 g of red mud were ground and mixed uniformly, 50 mL of 0.1 mol / L potassium hydroxide aqueous solution was added, and after stirring for 30 min, the mixture was placed in a reaction kettle and hydrothermally incubated at 180℃ for 8 h. After taking out, centrifugal separation and drying, a biochar-modified red mud composite was obtained. 0.6 g of CaO was mixed with the red mud composite, and the mixture was placed in a tube furnace and heated to 500℃ at a rate of 5℃ / min under a nitrogen atmosphere, and incubated for 3 h to prepare a composite dechlorination agent A composed of modified red mud, biochar and CaO.

[0029] Figure 1 X-ray diffraction spectrum of the composite dechlorination agent prepared in the present example and the raw material red mud. It can be seen that the iron species in the raw material red mud is mainly hematite (Fe2O3), and in the composite dechlorination agent, Fe2O3 is reduced to Fe3O4.

[0030] Example 2

[0031] 4.0 g of coffee grounds and 6.0 g of red mud were ground and mixed uniformly, 50 mL of 0.1 mol / L potassium hydroxide aqueous solution was added, and after stirring for 30 min, the mixture was placed in a reaction kettle and hydrothermally incubated at 180℃ for 8 h. After taking out, centrifugal separation and drying, a biochar-modified red mud composite was obtained. 0.6 g of CaO was mixed with the red mud composite, and the mixture was placed in a tube furnace and heated to 600℃ at a rate of 5℃ / min under a nitrogen atmosphere, and incubated for 3 h to prepare a composite dechlorination agent B composed of modified red mud, biochar and CaO.

[0032] Example 3

[0033] The 4.0 g coffee residue dried to constant weight is uniformly mixed with 6.0 g red mud, and 50 mL of 0.1 mol / L potassium hydroxide aqueous solution is added. After stirring for 30 min, it is placed in a reaction kettle and hydrothermally treated at 180°C for 8 h. After removal, it is centrifuged and dried to obtain a biochar modified red mud composite. 0.6 g of CaO is uniformly mixed with the red mud composite, and placed in a tube furnace. Under a nitrogen atmosphere, it is heated to 700°C at a rate of 5°C / min and held for 3 h to obtain a composite dechlorination agent C composed of modified red mud, biochar and CaO.

[0034] Example 4

[0035] The 4.0 g coffee residue dried to constant weight is uniformly mixed with 6.0 g red mud, and 50 mL of 0.1 mol / L potassium hydroxide aqueous solution is added. After stirring for 30 min, it is placed in a reaction kettle and hydrothermally treated at 180°C for 8 h. After removal, it is centrifuged and dried to obtain a biochar modified red mud composite. 0.6 g of CaO is uniformly mixed with the red mud composite, and placed in a tube furnace. Under a nitrogen atmosphere, it is heated to 700°C at a rate of 5°C / min and held for 3 h to obtain a composite dechlorination agent C composed of modified red mud, biochar and CaO.

[0036] The dechlorination activity of the dechlorination agents prepared in Examples 1 to 4 above is evaluated.

[0037] Figure 3 A schematic diagram of the plastic pyrolysis dechlorination reactor used in the present application is provided.

[0038] High-density polyethylene, low-density polyethylene, polypropylene, polystyrene and polyvinyl chloride are mixed in a mass ratio of 3.0:3.0:2.5:0.5:1.0. 5 g of mixed plastic and 2 g of dechlorination agent are filled into a fixed bed reactor in sections and fixed with asbestos. The specific loading method is shown in Figure 3 The front end of the pyrolysis reactor is connected to nitrogen, and the back end is connected to a U-shaped tube condensing device to collect pyrolysis oil. The condensing device is connected to a gas washing device, and the gas washing solution is NaOH solution.

[0039] During the pyrolysis reaction, first heat to 300°C at a rate of 5°C / min, hold for 30 min, then heat to 500°C at a rate of 3°C / min, hold for 30 min. After the reaction is completed, the oil sample in the U-shaped tube is collected, and the chlorine content of the oil sample is detected.

[0040] The dechlorination agents prepared in Examples 1 to 4 are used for mixed plastic pyrolysis dechlorination evaluation results are shown in Table 1.

[0041] Table 1 Chlorine content of pyrolysis oil prepared by using the dechlorination agent of each example for mixed plastic pyrolysis

[0042] Dechlorination agent Dechlorination agent A Dechlorination agent B Dechlorination agent C Dechlorination agent D Pyrolysis oil chlorine content, ppm 32 25 13 9.5

[0043] The above examples are only used to illustrate the technical solutions of the present application, and are not intended to limit the scope of the present application in any way. Although the present application has been described in detail with reference to the foregoing embodiments, modifications or substitutions of the technical solutions described in the foregoing embodiments, or equivalent replacements of some of the technical features, can still be made by those of ordinary skill in the art without departing from the spirit and scope of the technical solutions claimed by the present application.

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 were 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 then placed in a reactor for hydrothermal carbonization. After the hydrothermal reaction is completed, the mixture is centrifuged and dried to obtain a biochar-modified red mud composite. (3) Carbothermal reduction: The biochar-modified red mud composite is 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, wherein 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, wherein: The temperature of the hydrothermal reaction in step (2) is 160-250° C., and the time is 8-12 h.

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 dechlorination 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 dechlorination method for cracking chlorine-containing mixed plastics, characterized in that: The composite dechlorinating agent obtained by the preparation method according to any one of claims 1 to 5 is used in the cracking and dechlorination process of chlorine-containing mixed plastics.

8. The dechlorination method of chlorine-containing mixed plastic 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 dechlorination agent is loaded in the rear section of the plastic. The mass ratio of the dechlorination agent to the mixed plastic is 2:

5.

9. The dechlorination method of chlorine-containing mixed plastics by cracking according to claim 7, characterized in that: The cracking reaction adopts a two-stage heat preservation cracking method, firstly heating from room temperature to 280~320℃ and keeping it for 30 min, and then heating to 500℃ and keeping it for 30 min, with a heating rate of 3~5℃ / min.

10. The dechlorination method of 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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