CN113372672A

CN113372672A - Modified antioxidant for inhibiting spontaneous combustion of coal and preparation method thereof

Info

Publication number: CN113372672A
Application number: CN202110554648.3A
Authority: CN
Inventors: 黄志安; 宋东鸿; 高玉坤; 张英华; 姬玉成; 尹义超; 权赛南
Original assignee: University of Science and Technology Beijing USTB
Current assignee: University of Science and Technology Beijing USTB
Priority date: 2021-05-21
Filing date: 2021-05-21
Publication date: 2021-09-10
Anticipated expiration: 2041-05-21
Also published as: CN113372672B

Abstract

The invention provides a nano modified antioxidant for inhibiting coal spontaneous combustion, which belongs to a composite stopping agent and mainly comprises an antioxidant, a nano flame retardant and a gel stopping agent. The antioxidant is tert-butyl hydroquinone; the nano flame retardant is O-MMT intercalated soil prepared by modification; the gel inhibitor is high molecular hydrogel prepared by neutralization solution of sodium hydroxide and acrylic acid through a cross-linking polymerization method. The three inhibiting materials are prepared into the modified antioxidant by adopting an intercalation polymerization method. The preparation cost of the inhibitor is low, the inhibitor has the advantages of good gel fluidity and coverage, high thermal stability and good water retention performance, and simultaneously has good oxidation resistance of an antioxidant, and the added nano flame-retardant material further improves various physical properties of the inhibitor. Compared with the traditional stopping agent, the modified antioxidant has stronger universality and higher stopping performance in complex underground environment, provides a new idea for preventing and controlling coal spontaneous combustion, and has important practical significance for the development of the coal mine industry.

Description

Modified antioxidant for inhibiting spontaneous combustion of coal and preparation method thereof

Technical Field

The invention relates to the technical field of composite stopping agents, in particular to a modified antioxidant for inhibiting spontaneous combustion of coal.

Background

Sodium polyacrylate is a highly water-absorbent resin. It has good water absorption, can keep the coal body moist, and absorbs heat through the evaporation of water and the chemical reaction of the water, reduces the temperature of the coal, and prevents the coal from being heated and oxidized. However, most gel stopping agents are unstable, are easy to crack after water loss, lose the oxygen isolation capability, have short service life, are easy to pyrolyze after the coal body is heated, lose the stopping effect, and even generate flammable gas after pyrolysis to intensify the spontaneous combustion of the coal.

Tert-butyl hydroquinone is a novel phenolic oil antioxidant. The benzene ring of the tertiary butyl hydroquinone has two para-phenolic hydroxyl groups, one of which is a hindered phenolic hydroxyl group and the other is an unpositioned phenolic hydroxyl group. The steric hindrance effect enables tert-butyl hydroquinone to have higher antioxidant activity. However, tert-butyl hydroquinone is easily volatilized and decomposed at high temperature, and loses the inhibition performance.

The sodium polyacrylate and the tert-butyl hydroquinone are both single-performance stopping agents, namely the sodium polyacrylate only has physical stopping performance, and the tert-butyl hydroquinone only has chemical stopping performance, so that the problem of poor overall stopping performance exists.

At present, researches for improving the resistance performance of the coal mine resistance material through the nano material are few, and a system mature application scheme is lacked. Meanwhile, the gel time of the composite material is not easy to control, great obstacles and difficulties are brought to grouting, and raw material waste is easy to cause or the fire extinguishing rate is delayed.

At present, in practical application, a novel composite stopping agent capable of better exerting the stopping performance of each component needs to be researched and developed urgently, and the physical properties of the composite stopping agent, such as heat resistance, mechanical property, barrier property and the like, are superior to those of the traditional polymer material, so that the overall stopping efficiency is improved.

Disclosure of Invention

The invention aims to prepare a modified antioxidant for inhibiting coal spontaneous combustion, which combines the antioxidant with a gel material, and mainly comprises the antioxidant, a nano flame retardant and a gel inhibitor. The three materials are compounded into the modified antioxidant through an intercalation polymerization method, so that the physical properties of the modified antioxidant are improved, the modified antioxidant has various physical properties such as good thermal stability, fluidity, viscosity, elasticity and the like, and meanwhile, the prepared material is economic and environment-friendly and can adapt to complex underground environments.

The modified antioxidant mainly comprises a gel inhibitor, an antioxidant and a nano flame retardant, wherein the dosage of the antioxidant is 2-3% and the dosage of the nano flame retardant is 20-30% according to the mass ratio of the gel inhibitor, the antioxidant and the nano flame retardant to acrylic acid.

The gel stopping agent is sodium polyacrylate.

The antioxidant is tert-butylhydroquinone.

The nano flame retardant is O-MMT intercalated montmorillonite (intercalated soil for short).

The preparation method of the modified antioxidant comprises the following steps:

under the condition of constant-temperature water bath, according to the mass ratio of the antioxidant to acrylic acid, the using amount of the antioxidant is 2-3%, the using amount of the nano flame retardant is 20-30%, the using amount of the initiator is 1.5-2%, the using amount of the cross-linking agent is 0.5-2%, and the balance of distilled water, the antioxidant, the initiator, the nano flame retardant and the cross-linking agent are sequentially added into a sodium polyacrylate solution to carry out intercalation solution polymerization reaction, so as to prepare the modified antioxidant.

The temperature of the intercalation solution polymerization reaction is 65-75 ℃.

The nano flame retardant is O-MMT intercalated montmorillonite which is prepared according to the mass ratio of CTMAB (cetyl trimethyl ammonium bromide)/montmorillonite of 4-5 percent, namely the montmorillonite is dissolved in distilled water and stirred vigorously and heated to 65-75 ℃, and the CTMAB is added, stirred vigorously and dried to prepare the O-MMT intercalated montmorillonite.

The sodium polyacrylate is prepared by neutralizing acrylic acid with sodium hydroxide and performing cross-linking polymerization, wherein the neutralization degree is 75-85% of sodium hydroxide/acrylic acid.

The antioxidant is tert-butyl hydroquinone, is prepared by initiating a free radical reaction through an initiator KPS, and is grafted on a macromolecular carbon chain of the sodium polyacrylate through a graft polymerization reaction in a cross-linking polymerization process.

The initiator is potassium persulfate (KPS).

The cross-linking agent is N, N-methylene bisacrylamide.

The invention inserts each monomer in the solution into the intercalation soil lamellar structure by intercalation polymerization, and disintegrates the lamellar structure of the montmorillonite into nano lamellar which is uniformly distributed in the gel while forming macromolecular carbon chains by cross-linking polymerization.

The invention combines the antioxidant with the gel material, and then improves the physical property of the gel material through the modification of the nano material, thereby preparing the modified antioxidant. The material can simultaneously exert the inhibition performance of the antioxidant and the gel material, has obvious effect of inhibiting the spontaneous combustion of coal, and solves the problem of poor integral inhibition performance of a single inhibition material. The addition of the montmorillonite nano material can reduce the addition of the antioxidant, improve the thermal stability of the montmorillonite nano material, improve the fluidity of the gel, improve the covering capability of the gel on the surface of coal and reduce the cost of the inhibitor. The material solves the problems of single inhibition performance, high cost, poor fluidity and easy volatile decomposition of an antioxidant at high temperature of the traditional inhibition material, has stronger universality in a complex underground environment, and is an economic, environment-friendly and efficient inhibition agent.

Drawings

FIG. 1 is a flow diagram of the overall preparation of the present invention;

FIG. 2 is a reaction mechanism diagram of the intercalation polymerization reaction of the present invention;

FIG. 3(a) is a chemical reaction equation diagram of the process of initiating a radical reaction according to the present invention;

FIG. 3(b) is a chemical reaction equation of the neutralization process of sodium hydroxide and acrylic acid according to the present invention;

FIG. 3(c) is a chemical reaction equation of the cross-linking polymerization reaction of the present invention;

FIG. 4 is a diagram of the physical inhibition mechanism of the modified antioxidant of the present invention;

FIG. 5 is a diagram of the chemical inhibition mechanism of the modified antioxidant of the present invention;

FIG. 6 is an XRD plot of a montmorillonite of the present invention;

FIG. 7 is an XRD plot of O-MMT intercalated soil of the present invention;

FIG. 8(a) is a scanning electron microscope image at 500 times magnification of a raw coal sample according to the present invention;

FIG. 8(b) is a scanning electron microscope image of 10000 times magnification of the raw coal sample of the present invention;

FIG. 8(c) is a scanning electron microscope image at 500 times magnification of the hindered coal sample of the present invention;

FIG. 8(d) is a scanning electron microscope image at 10000 times magnification of the hindered coal sample of the present invention;

FIG. 9 is an XRD spectrum of a raw coal sample, a gel and a hindered coal sample of the present invention;

FIG. 10 is a thermogravimetric plot of a raw coal sample of the present invention;

FIG. 11 is a thermogravimetric plot of a hindered coal sample of the present invention.

Detailed Description

In order to make the technical problems, technical solutions and advantages of the present invention more apparent, the following detailed description is given with reference to the accompanying drawings and specific embodiments.

The invention provides a modified antioxidant for inhibiting spontaneous combustion of coal.

The gel comprises sodium polyacrylate, O-MMT intercalated montmorillonite and tert-butyl hydroquinone. The preparation of the gel can be divided into two steps, firstly, the preparation of O-MMT intercalated montmorillonite is carried out, and the O-MMT intercalated montmorillonite is prepared by cetyl trimethyl ammonium bromide and montmorillonite; and then preparing a modified antioxidant, wherein the modified antioxidant is prepared from O-MMT intercalated montmorillonite, monomer sodium acrylate (prepared by neutralizing acrylic acid with sodium hydroxide), tert-butyl hydroquinone, an initiator and a cross-linking agent, wherein the using amount of the antioxidant is 2.39% and the using amount of intercalated soil is 25% in mass ratio with the acrylic acid. The overall preparation scheme of the modified antioxidant is shown in figure 1.

The preparation process of the modified antioxidant comprises the following steps:

1) preparing intercalated montmorillonite

Dissolving 10g of montmorillonite in 250ml of distilled water in a beaker, placing the beaker in a water bath kettle, and vigorously stirring the mixture for 2 hours at room temperature by using a DF-101 heat collection type constant-temperature heating magnetic stirrer to form uniform suspension. Then the solution is heated to 70 ℃, 0.437g of aqueous solution of hexadecyl trimethyl ammonium bromide (CTMAB for short) is added dropwise (namely the ratio of the montmorillonite to the CTMAB in the solution is 100: 4.3734), and the mixture is stirred vigorously for 2 hours at constant temperature.

Standing for a period of time after stirring is finished, and naturally cooling to room temperature to obtain stable intercalated soil suspension. And then the intercalated soil suspension is placed into a drying oven to be dried to constant weight at 100 ℃, so that intercalated soil powder can be obtained, and the intercalated soil is sealed and stored for later use.

2) Dissolution and mixing of polymer monomers and clay

0.239g of tert-butyl hydroquinone (TBHQ) is weighed and dissolved in a proper amount of distilled water to prepare TBHQ aqueous solution with proper concentration. 2.5g of the dried O-MMT intercalated soil prepared in the previous step (according to the proportion of 4:1 of the acrylic acid to the O-MMT intercalated soil) is added into the TBHQ solution and fully stirred. 0.164g of potassium persulfate initiator KPS is added to initiate TBHQ to generate free radical reaction.

Subsequently, a certain amount of sodium hydroxide was dissolved in distilled water in a beaker to prepare a 25% aqueous solution, and 10g of acrylic acid was weighed in the beaker. Acrylic acid was neutralized with aqueous NaOH solution under ice-water bath conditions to 80% neutralization degree.

And (3) uniformly mixing the neutralized solution with TBHQ solution after initiating free radical reaction, adding 0.1g of cross-linking agent MBA, carrying out ultrasonic treatment on the solution for 1min by using an ultrasonic dispersion instrument, and fully stirring for 20-60 min by using a magnetic stirrer.

4) Graft polymerization and intercalation

After stirring, the solution is placed in a constant-temperature water bath at 75 ℃ for sealing graft polymerization reaction for 3 hours, slightly stirred at the beginning of the polymerization reaction, and stopped stirring after the mixed solution is uniform and stable.

5) Cleaning, drying and grinding

The obtained gel is placed in a ventilated place for natural cooling and the irritant gas is dissipated. After cooling to room temperature, the gel was chopped with scissors and washed with deionized water, and then dried to constant weight in a vacuum oven at 80 ℃. And (3) after the sample is completely dehydrated, grinding the sample into powder by using a ball mill, thus obtaining the modified antioxidant gel powder.

The preparation mechanism of the modified antioxidant is as follows:

1) monomer solution intercalation polymerization method of montmorillonite

The research utilizes a monomer solution intercalation polymerization method to modify the stopping agent, and mainly comprises two processes: (1) the monomer enters the clay layer. Firstly, preparing the solution of each polymer monomer and montmorillonite respectively. Then mixing the solutions together and stirring under appropriate conditions to allow the monomer to be inserted between the silicate crystals; (2) the monomers are polymerized. The solution is subjected to in-situ solution polymerization under the action of light, heat, an initiator and the like, energy is released, and the montmorillonite lamella is disintegrated and decomposed into nano-scale lamellae which are dispersed in a polymer matrix, so that intercalation polymerization can be realized, a nano composite material is prepared, and modification of a stopping agent is realized. As shown in fig. 2.

2) Cross-linking polymerization

The cross-linking polymerization process is shown in FIG. 2. The tert-butyl hydroquinone is subjected to a radical reaction under the action of a certain amount of potassium persulfate initiator, so that hydrogen bonds in hydroxyl groups are broken to generate active centers, as shown in fig. 3 (a).

The neutralization reaction of acrylic acid with sodium hydroxide produces sodium acrylate, as shown in FIG. 3 (b). The speed of solution polymerization is related to the pH value of the solution, and the neutralization degree is controlled to be 80%, so that the pH value of the solution can be adjusted within a certain range, and the gelling time of the solution is controlled to a certain extent.

Mixing the solutions of (a) and (b), adding a cross-linking agent, and carrying out cross-linking polymerization at 70 ℃. The active center generated by TBHQ free radical reaction and methyl and methylene in acrylic acid/sodium acrylate aliphatic group produce chemical reaction to produce ether bond, and TBHQ is grafted onto acrylic acid/sodium acrylate molecule. The molecular monomers are subsequently polymerized to form longer carbon chains under the conditions of an initiator KPS and high temperature. Finally, the cross-linking agent interacts with the generated carbon chains to form bridges between the polymer molecular chains, so as to form a polymer hydrogel with a three-dimensional network structure, as shown in fig. 3 (c).

The flame retardant mechanism of the modified antioxidant is analyzed below.

(1) Mechanism of physical inhibition

The physical inhibition mechanism of the modified antioxidant is shown in fig. 4, and the physical inhibition mechanism mainly comprises three items:

1) covering and filling coal bodies to exclude oxygen

The modified antioxidant has good adhesion performance, can form a layer of film, firmly covers the surface of the coal rock and isolates oxygen. Meanwhile, due to the good fluidity of the gel, the gel can penetrate into small cracks and pores of the coal rock, so that oxygen in the coal rock is discharged, and the oxygen adsorption in seams of a coal bed is reduced.

2) Absorb water, evaporate and dissipate heat

The modified antioxidant prepared by crosslinking polymerization is a three-dimensional network structure formed by connecting a plurality of polymer molecular chains by a bridge bond. The structure ensures that the gel has good water absorbability and water retention property, and takes away a large amount of heat generated by spontaneous combustion of coal through evaporation of water. The structure can also ensure that the gel formed by the gel has good flexibility, is not easy to crack after being heated and dehydrated, reduces the deep water loss of the gel and prevents coal rocks from being exposed in the air due to the cracking of the gel film.

3) Montmorillonite nano-layer sheet is uniformly distributed

The lamellar structure in the montmorillonite is disintegrated in the cross-linking polymerization process, and the layered silicate is decomposed into nano-scale lamellar. The process overcomes the defects that the montmorillonite with the original lamellar structure is easy to agglomerate in the solution and is difficult to combine with other matrix materials, and the nano-scale lamellar prepared by modification has good dispersibility and can be uniformly distributed in the gel. Compared with the nano-scale layer sheet before modification, the specific surface area of the nano-scale layer sheet is obviously improved, the contact area of the inhibition component in the montmorillonite and coal is improved, and the inhibition effect is more easily exerted.

(2) Chemical inhibition mechanism

1) Reduction of chemisorption

Chemisorption refers to the process in which oxygen is continuously adsorbed on the surface of a coal sample during the oxidation process of the coal. The rate of chemisorption has a great influence on the rate of oxidation of coal. The good water absorption of the gel can store a large amount of water, and a large amount of water vapor is generated by evaporation in the temperature rising process, so that the oxygen concentration in the air is reduced, and the chemical adsorption rate is slowed down.

2) Interrupting the chain reaction of free radicals

Spontaneous combustion of coal relies on the chain reaction of free radicals. That is, the free radicals in the coal react with oxygen to generate new free radicals, and the new free radicals react with oxidation reaction or decomposition reaction, etc., to generate more heat and free radicals. The heat is continuously accumulated in such a reciprocating way until spontaneous combustion occurs.

As shown in FIG. 5, t-butyl p-diphenol is a phenolic antioxidant having two phenolic hydroxyl groups and a t-butyl group on the benzene ring. The hydrogen atoms on the two phenolic hydroxyl groups are easily abstracted to generate semiquinone free radicals. Due to the steric effect of the tertiary butyl on the benzene ring, the free radical has low activity and cannot cause new chain reaction, but can be combined with other free radicals to generate stable compounds, and the process is the interruption of the chain reaction of the free radicals.

In the context of a particular application, the term,

firstly, according to the actual situation of underground coal spontaneous combustion prevention, considering various factors such as inhibition effect, economic cost and the like, and screening hydrogel, antioxidant and nano flame retardant material. The hydrogel is prepared from monomer sodium acrylate by solution polymerization, the initiator is tert-butyl hydroquinone, the nano flame-retardant material is montmorillonite (O-MMT intercalated soil) modified by cetyl trimethyl ammonium bromide, and polymerization is initiated at a certain temperature.

The XRD experiment shows that the interlayer spacing of the montmorillonite after the modifier treatment is enlarged from 1.433nm to 1.898nm, and no obvious diffraction peak can be seen in the modified gel, which indicates that the layered structure of the montmorillonite is damaged due to the insertion and polymerization of each monomer, and becomes a nano-scale lamella which is uniformly distributed in the colloid, improves the dispersibility, and can better play the inhibition effect.

The microscopic surface morphology of the modified antioxidant inhibition coal sample and the raw coal sample is observed through a scanning electron microscope, and the modified antioxidant gel bonds scattered fragments in coal together after inhibition treatment, so that the good fluidity helps the modified antioxidant gel to permeate into gaps of coal rocks to block the gaps, and a layer of film is formed to wrap the coal sample, so that the air can be well isolated from contacting with coal bodies.

The XPS experiment is used for analyzing the surface element content of the hindered coal sample and the raw coal sample, and the results show that the C1s peak and the O1s peak in the surface layer of the hindered coal sample are reduced due to the addition of the gel, the carbon atom content of the surface of the coal sample is reduced from 82.41% to 36.76%, the oxygen atom content is increased from 17.56% to 33.50%, and the sodium atom content of the surface of the hindered coal sample is 17.69%, so that the gel is proved to be well covered on the surface of the coal sample.

And measuring the characteristic temperature points of the raw coal sample and the inhibition coal sample at a certain heating rate through a thermogravimetric experiment. The measured six characteristic temperature points of the coal sample after the inhibition treatment have obvious effect of improving compared with the original coal. And the oxygen absorption weight gain mass percentage M of the inhibition coal sample in the oxygen absorption weight gain stage_△The temperature is reduced from 2.12 percent to 1.2 percent, and the maximum thermal weight loss rate DTG of the coal in the thermal decomposition and combustion stages_mThe temperature is reduced from 7.31% to 6.27%. The modified antioxidant can obviously delay the process of coal oxidation spontaneous combustion and reduce the intensity of coal combustion, namely the gel has good inhibition performance on coal samples.

The following description is given with reference to specific examples.

Example 1: x-ray diffraction experiments (XRD)

In this example, an X-ray diffractometer was used to analyze montmorillonite and O-MMT intercalated soil, and study the intercalation of montmorillonite treated with cetyltrimethylammonium bromide.

The X-ray diffraction patterns of XRD of the montmorillonite (a) and the modified antioxidant (b) are shown in figures 6 and 7.

The value of d can be calculated according to the Bragg equation from the value of theta angle obtained by XPS, so that the position of a diffraction peak in an XRD pattern can reflect the size of the interlayer spacing. The calculation formula is as follows:

2dsinθ＝nλ (1)

wherein d is interplanar spacing;

theta is the half diffraction angle, the included angle between the incident ray, the reflected ray and the reflection crystal face;

n is the number of reflection stages, and n is 1;

λ -incident X-ray wavelength, λ 0.154 nm;

as can be seen from fig. 6, montmorillonite has a strong diffraction peak at 2 θ ═ 6.16 °, and the interlayer spacing of montmorillonite is 1.433nm as determined by Bragg's equation. And the strong diffraction peak of the interlayer montmorillonite in the graph 7 is positioned at 4.65 degrees 2 theta, and the interlayer spacing is expanded to 1.898nm according to the Bragg equation. The successful intercalation treatment of the montmorillonite is proved, and the interlayer spacing is enlarged, which is beneficial to the monomer entering the layered structure of the montmorillonite more easily in the subsequent experiment.

Example 2: field emission scanning electron microscope experiment

In the embodiment, pure hydrogel and composite gel are dried in a drying oven to constant weight, a layer of thin gold is sprayed on the lower surface of a dried block-shaped gel sample under argon atmosphere and high pressure, then the surfaces of the two types of hydrogel are scanned by using an s-3500n Scanning Electron Microscope (SEM), and the microscopic morphology of the sample is observed.

FIG. 8 is a scanning electron microscope image of a raw coal (ab) and a hindered coal (cd), wherein the images a and c are images of the coal sample magnified 500 times, and the images b and d are images of the coal sample magnified 10k times. In fig. 8a, it can be seen that the coal blocks of the raw coal sample are different in size, a large number of extremely small coal dust is scattered among the coal blocks or even in gaps among the coal blocks in the picture except for a few large coal blocks, and the surface of the coal sample is uneven and distributed with a plurality of pits and gaps. The structures increase the contact area between the coal sample and air, so that the oxidation speed of the coal sample is accelerated, and meanwhile, the air in gaps among a large amount of fine coal scraps can store the heat of oxidation radiation of the coal sample, so that the heat dissipation of a coal pile is poor, and the oxidation spontaneous combustion is easy to occur. In fig. 8c, the modified antioxidant gel binds the fine coal dust and the coal briquettes together and fills the gap between the coal dust and the coal briquettes, reducing the surface area of the coal sample. In addition, the gel covers the surface of the coal sample in a film form, so that the coal sample is isolated from contacting with air, and the coal sample is prevented from being oxidized. Comparing fig. 8b with fig. 8d, it can be seen that in fig. 8b, a plurality of fine holes and cracks exist on the surface of the coal sample under the high-magnification lens, and a plurality of fine coal cinder are distributed on the surface of the coal sample, which appears coarse and uneven; in FIG. 8d, the fine coal slag on the surface of the coal sample after the gel coating was adhered together by the gel, and almost no fine cracks were found in each coal.

Example 3: x-ray photoelectron spectroscopy experiment

In the embodiment, an X-ray photoelectron spectrometer is used for testing raw coal, gel and a hindered coal sample, detecting the element content of the surface layer of the coal sample and observing the covering condition of the gel on the surface of the coal sample.

The results of X-ray photoelectron spectroscopy (XPS) experiments are shown in FIG. 9. The raw coal mainly has an O1s peak and a C1s peak, and the gel mainly has a C1s peak, an O1s peak, a Na 1s peak and a Na KL1 peak. By comparing the inhibition coal sample with the raw coal, the Na 1s peak and the Na KL1 peak which do not exist in the raw coal appear in the coal sample after the inhibition treatment, which shows that the gel is detected on the surface layer of the coal sample by X-ray photoelectron spectroscopy, and proves that the gel is successfully covered on the surface of the coal sample.

By observing the intensity of the peak, the peak intensity of the C1s peak in the hindered coal sample is reduced compared with that in the raw coal and is higher than that of the C1s peak in the gel, which indicates that the coal sample and the gel are simultaneously detected in the surface layer part of the hindered coal sample by the X-ray photoelectron spectrum of the hindered coal sample. The peak intensity of the O1s peak in the inhibition coal sample is slightly lower than that of the raw coal sample and is lower than that of the gel, because the gel is combined with active groups on the surface of the coal bed, such as hydroxyl OH < - >, O1s is reduced, and the peak intensity is reduced.

The main elemental compositions of the coal sample surface before and after the inhibition treatment are shown in table 1. After the inhibition treatment, the carbon atom content of the coal sample surface is reduced from 82.41% to 36.76%, the oxygen atom content is increased from 17.56% to 33.50%, and the sodium atom content after the gel is added is 29.73%. Further proves that the modified antioxidant is successfully coated on the surface of the coal sample.

TABLE 1 elemental content (%)

Example 4: verification of coal spontaneous combustion inhibiting effect of modified antioxidant

Before the experiment, the raw coal and the inhibition coal sample are ground to the particle size of 120-140 meshes, 10mg is taken and put into Al₂O₃In the crucible, the crucible was placed on a balance in a thermogravimetric analyzer (TGA). Synthetic air (21% O) was used₂/79％N₂) The reaction atmosphere of the entire thermogravimeter was replaced by aeration at 50ml/min for 5min at room temperature. The sample was heated from room temperature to 800 ℃ with a temperature rise rate of 10 ℃/min.

The thermogravimetric curves of the raw coal and the hindered coal samples are shown in fig. 10 and fig. 11. The TG and DTG curves in the graph are respectively a thermogravimetric curve and a differential quotient thermogravimetric curve.

According to the trend of the curve in the graph, the characteristic values and the stages in the coal oxidation process are divided as follows. T is₁: the initial critical temperature point is approximately the temperature point at the first maximum value of the DTG curve, namely the maximum value in the water evaporation and desorption stages; t is₂: the dry cracking temperature point, which is the temperature point on the curve at which the coal has the lowest mass before ignition, at which point moisture evaporation and desorption have been completed; t is₃: the mass maximum temperature point is the maximum temperature point of the weight gain of the coal sample caused by a series of physical and chemical reactions in the early stage; t is₄: the ignition temperature point is the temperature of the coal sample during initial combustion; t is₅: the maximum thermal weight loss rate temperature point is the point of coal component reduction and the maximum combustion rate; t is₆: the burnout temperature point can be regarded as that the weight of the coal sample gradually tends to be stable; maximum weight loss rate DTG_m(ii) a Oxygen uptake weight gain mass percentage M_△。

The oxidation process of coal is further classified into the following according to the characteristic valuesSeveral stages. The quality change of the water evaporation and desorption stage mainly comes from the evaporation of water and CO in the coal sample₂、N₂、CH₄And (4) desorbing the gas. And in the oxygen absorption weight increasing stage, the quality change in the oxygen absorption weight increasing stage mainly comes from the chemical adsorption of coal. The decomposition and combustion stage can be divided into a thermal decomposition stage before the ignition point temperature and a combustion stage after the ignition point temperature. A burn-out phase, in which the coal is substantially completely burned and the mass change is almost 0.

The parameters can quantify the process of coal oxidation spontaneous combustion, and provide indexes for evaluating the coal spontaneous combustion inhibition performance of the gel. The conditions of the characteristic values of the raw coal and the hindered coal samples obtained by the experiment in the temperature rise process of 10 ℃/min under the air atmosphere are shown in Table 2.

TABLE 2 characteristic values of raw coal and hindered coal samples during temperature rise

As can be seen from Table 2, the coal samples all have significant changes in the characteristic values after the inhibition treatment. In the water evaporation and desorption phase, T₁From 46.8 ℃ to 68.6 ℃, T₂The temperature is increased from 135.9 ℃ to 188.7 ℃, the gel is proved to have good water retention property, and the spontaneous combustion process of the coal can be obviously delayed by prolonging the time required by the evaporation of the water in the early oxidation stage of the coal to ensure that the coal absorbs heat as much as possible. In the oxygen uptake weight gain stage, T₃The temperature is increased from 316.9 ℃ to 325.3 ℃, and the oxygen absorption weight gain mass percentage M_△The reduction from 2.12% to 1.20% demonstrates that the modified antioxidant is able to slow down the rate of chemisorption of coal at this stage, which is a combination of gel oxygen barrier and chemical resistance properties. In the stage of coal pyrolysis and combustion, the maximum thermal weight loss rate DTG_mReduced from 7.31% to 6.27%, T₆The temperature is increased from 649.8 ℃ to 676.1 ℃, and the gel is proved to be capable of slowing down the oxidation rate of coal in the thermal decomposition and combustion process of the coal, reducing the intensity of coal combustion and prolonging the time required by the coal to be burnt out.

In a word, the modified antioxidant prepared by the intercalation polymerization method provided by the invention uses an X-ray diffraction experiment to observe XRD curves of montmorillonite and O-MMT intercalation montmorillonite, and the successful preparation of the modified antioxidant is proved by calculating the interlayer spacing before and after modification of the montmorillonite. Meanwhile, the material disclosed by the invention is used for carrying out inhibition treatment on a raw coal sample to obtain an inhibition coal sample, and the inhibition coal sample and the raw coal sample are researched by a field emission scanning electron microscope and an X-ray photoelectron spectroscopy, so that the gel is proved to have good covering capability. The characteristic temperature points and the characteristic values of the inhibition coal sample and the raw coal sample in the spontaneous combustion process at a certain heating rate are researched through thermogravimetric experiments, and the modified antioxidant is verified to have a good inhibiting effect on the spontaneous combustion of coal.

While the foregoing is directed to the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. The modified antioxidant is characterized by comprising a gel inhibitor, an antioxidant and a nano flame retardant, wherein the amount of the antioxidant is 2-3% and the amount of the nano flame retardant is 20-30% in mass ratio to acrylic acid.

2. The modified antioxidant for suppressing spontaneous combustion of coal as claimed in claim 1 wherein the gel retardant is sodium polyacrylate.

3. The modified antioxidant for suppressing spontaneous combustion of coal of claim 1 wherein the antioxidant is t-butylhydroquinone.

4. The modified antioxidant for suppressing spontaneous combustion of coal as claimed in claim 1 wherein the nano flame retardant is O-MMT intercalated montmorillonite.

5. The method for preparing the modified antioxidant for inhibiting the spontaneous combustion of coal as claimed in claim 1, wherein under the condition of constant temperature water bath, according to the mass ratio of the modified antioxidant to acrylic acid, the antioxidant, the initiator, the nano flame retardant and the crosslinking agent are sequentially added into a sodium polyacrylate solution, the antioxidant, the initiator, the nano flame retardant and the crosslinking agent are sequentially added into the sodium polyacrylate solution, and the modified antioxidant is prepared by intercalation solution polymerization reaction.

6. The method of claim 5, wherein the temperature of the intercalation solution polymerization reaction is between 65 ℃ and 75 ℃.

7. The method for preparing the modified antioxidant according to claim 5, wherein the nano flame retardant is O-MMT intercalated montmorillonite which is prepared by 4-5% by mass of cetyl trimethyl ammonium bromide/montmorillonite, that is, the O-MMT intercalated montmorillonite is prepared by dissolving montmorillonite in distilled water, vigorously stirring and heating to 65-75 ℃, adding cetyl trimethyl ammonium bromide, vigorously stirring and drying.

8. The method for preparing modified antioxidant as claimed in claim 5, wherein the sodium polyacrylate is prepared by neutralizing acrylic acid with sodium hydroxide and cross-linking polymerization, and the neutralization degree is 75% -85% of sodium hydroxide/acrylic acid.

9. The method of claim 5, wherein the antioxidant is t-butylhydroquinone.

10. The method of claim 5, wherein the initiator is potassium persulfate and the crosslinking agent is N, N-methylenebisacrylamide.