Research on the Damping Performance of Mining Highly Efficient Water-Retaining Colloidal Material Against the Spontaneous Combustion of Coal

2.1 Materials and reagents

In this paper, the base materials for the preparation of highly efficient water-retaining colloidal material include sodium alginate (SA), light calcium carbonate (PCC), glucono-δ-lactone (GDL) and water, of which SA, as a natural polysaccharide, is non-toxic and harmless, raw materials are abundant and easy to obtain and inexpensive, and has a certain viscosity due to the phenomenon of polyanion in its aqueous solution. The main materials and physical diagrams are shown in Table 1, Table 2 and Figure 1.

Table 1. Main materials for the trial

Table 2. Test instruments and specifications

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Figure 1. Physical view of the material

2.2 Principle of the test

SA can form a stable cross-linking structure with Ca²⁺ in a certain temperature range. Free Ca²⁺ will replace Na⁺ at the carboxyl group of SA solution, and the carboxyl group of the other chain is connected with Ca²⁺ again, finally forming a complete cross-linking structure. Because the carboxylic acid radicals at both ends of the crosslinking process can be connected first, the formed crosslinking structure is close to spherical and can wrap a large number of water molecules, so the crosslink has the characteristics of slow water release [32], so it is called highly efficient water-retaining colloidal material.

The PCC-GDL was used to prepare the composite cross-linked system, using GDL to slowly release H⁺ after dissolving in water, and the H+ first reacted with PCC to slowly release calcium ions, and the released Ca²⁺ cross-linked with sodium alginate molecules to form an overall more uniform high-water colloidal material, avoiding the uneven structure of the high-water colloidal material caused by the rapid reaction of sodium alginate and calcium ions [33]. The formation mechanism is shown in Figure 2.

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Figure 2. Mechanisms for the formation of highly hydrated colloidal material

2.3 Material preparation

According to the pre-test, when the SA dosing is less than 1% (the mass of water is 100g), the solution is not viscous because the viscosity is too low, while when the SA dosing reaches 3%, the solution is too viscous and the liquidity is poor, and the pipeline is easy to cause plugging and other phenomena when conveying, so the SA solutions with the dosing of 1%, 1.5%, 2% and 2.5% are selected for the subsequent test, as shown in Figure 3.

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Figure 3. SA solution

Different masses of SA powder were weighed and added to the water slowly and uniformly, and an electric stirrer was used to stir at a constant speed during the process. The stirring was stopped when there were no large flocs in the solution and then left to stand at room temperature for about 2 to 3 hours until the SA was completely dissolved. After the SA solution was prepared, a certain amount of PCC and GDL mixture was first added and stirred thoroughly.

2.4 Coal sample preparation

Fresh coal samples were collected from the Shigetai coal mine and stored under seal. The coal samples were crushed under nitrogen protection and sieved to a particle size of 63-75 μm. The screened pulverised coal samples were unified in a vacuum drying oven. In order to prevent dry distillation of the coal samples and cracking of the coal properties under high temperature environment, the drying temperature was set to 50°C and dried at constant temperature for 6 h. The MAGEL-6700 fully automatic coal quality analyser was used to analyse the raw coal samples and the treated samples with 2.5% SA + 1% PCC + 1% GDL added. The test results are shown in Table 3.

Table 3. Proximate analysis and ultimate analysis of pulverized coal

4.1 Indicator gas resistivity testing

Two samples of 80g each were selected, one without treatment and the other with 8g of highly efficient water-retaining colloidal material, and subjected to a programmed temperature rise test, with the test temperature range set at 30°C to 170°C, the gas atmosphere set to standard air, the flow rate set to 50.0mL/min and the rate of rise set to 0.3°C/min. A gas sample was taken every 10°C and subjected to Chromatographic gas analysis. The results for the marker gases CO and C₂H₄ are shown in Figure 6.

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Figure 6. Graph of changes in indicator gases

The addition of highly efficient water-retaining colloidal material to the coal sample, due to the highly efficient water-retaining colloidal material contains more water, in the process of following the coal sample warming highly efficient water-retaining colloidal material will take away most of the heat by way of water evaporation, thus the coal body itself is slowed down the rate of warming, the amount of CO production decreased, C₂H₄ generation temperature point is increased and the amount of production decreased. At the same time, the highly efficient water-retaining colloidal material can wrap part of the coal body to reduce the coal oxygen contact area and also play a role in inhibiting the heating of coal spontaneous combustion, so the highly efficient water-retaining colloidal material can play an effective role in the oxidation process of coal spontaneous combustion.

The barrier properties of highly efficient water-retaining colloidal material vary at different stages of the coal spontaneous combustion process, but the general trend is that the barrier properties between the critical temperature and the dry cracking temperature are greater than the barrier properties before the critical temperature. This reduces the rate and degree of oxidation.

4.2 Characteristic temperature and activation energy studies

4.2.1 Thermogravimetric tests

Two samples were taken, both weighing 10 mg, one without treatment and the other with 1 mg of a 2% SA highly efficient water-retaining colloidal material. The two samples were then spread evenly on the crucible, and 5 ml/min of air was passed through the crucible, and the heating rate was set at 5 K/min. The TG-DTG curves of the coal samples were obtained by processing the data using origin software, and the five characteristic temperature points of the original coal samples and the treated samples were determined according to the classification of characteristic temperature points, and the results are shown in Figure7 and Figure 8.

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Figure 7. TG-DTG curve of raw coal sample

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Figure 8. TG-DTG curve of treated sample

Compared with the raw coal sample, the critical temperature T₁ of the treated sample is delayed by 15.7℃. In this low temperature stage, the highly efficient water-retaining colloidal material mainly wraps the coal sample to reduce the coal oxygen composite area of the coal sample and inhibit the coal spontaneous combustion. As the highly efficient water-retaining colloidal material contains a large amount of water, with the temperature rising, the oxidation reaction will begin when the water in the plateau coal sample evaporates, and the treated sample can still take away the heat with the help of the evaporation of water in the highly efficient water-retaining colloidal material. Compared with the raw coal sample, the dry crack temperature T₂ of the treated sample is delayed by 25.5 degrees. At the stage of dry crack temperature T₃, in the early stage, the highly efficient water-retaining colloidal material still has a certain amount of water, which can absorb heat and take away heat, but the water loss rate will be accelerated. Therefore, when reaching the T3 temperature point, the temperature difference between the two samples is not significantly increased compared with T₂. At the later T₄ and T₅ temperature points, there is little difference between the treated sample and the raw coal sample. At this time, the highly efficient water-retaining colloid material has completely lost water and shrunk, and the temperature is high. At this time, the highly efficient water-retaining colloid material basically loses its function as an organic polymer fire-fighting material.

4.2.2 oxidation kinetics analysis

In the process of coal oxidation and temperature rise, a certain activation energy is required. Only when the activation energy meets the energy required for coal oxidation reaction can it promote the further oxidation of coal [34], and the activation energy required for coal oxidation also represents the difficulty of coal oxidation reaction. The greater the activation energy, the more difficult it is to oxidize coal [35]. The activation energy during coal oxidation can be obtained by thermal analysis technology combined with oxidation kinetics method, so as to test the inhibition ability of highly efficient water-retaining colloidal material.

In the non isothermal experimental process in thermal equilibrium, the mass of the sample is regarded as a function of temperature. According to Arrhenius formula [36] and law of mass action, the decomposition rate of the sample can be expressed as:

$\frac{d\alpha }{dt}=kf\left( \alpha  \right)$     (1)

$k=A\exp \left( \frac{-E}{RT} \right)$     (2)

where: k is Arrhenius rate constant (min⁻¹); E is the activation energy of the reaction (KJ·mol⁻¹); R is the gas constant, R=8.414 Jmol^-1·K^-1; A is the frequency factor (min⁻¹); T is the absolute temperature (K).

The form of f(α) depends on the type of reaction or reaction mechanism. It is generally assumed that f(α) has nothing to do with temperature T and reaction t, but only with reaction degree α. The heating rate β can be expressed as:

$\beta \text{=}\frac{dT}{dt}$     (3)

Substitute (2) and (3) into (1) and rearrange the formula:

$\frac{d\alpha }{dT}=\frac{A}{\beta }\exp \left( \frac{-E}{RT} \right)f\left( \alpha  \right)$     (4)

The integral form of equation (4) can be expressed as:

$G\left( \alpha  \right)=\int_{0}^{\alpha }{\frac{d\alpha }{f\left( \alpha  \right)}}=\frac{A}{\beta }\int_{{{T}_{0}}}^{T}{\exp \left( \frac{-E}{RT} \right)}dT$      (5)

Coats Redfern integral formula [37] is adopted:

$\ln \left[ \frac{G\left( \alpha  \right)}{{{T}^{2}}} \right]=\ln \frac{AR}{\beta E}-\frac{E}{RT}$      (6)

Therefore, $\ln \left[\frac{G(\alpha)}{\tau^{2}}\right]$ has a linear relationship with $\frac{1}{\tau}$, $\ln \left[\frac{G(\alpha)}{\tau^{2}}\right]$ is the $\mathrm{y}$-axis, $\frac{1}{T}$ is the $\mathrm{x}$-axis, and it is fitted. The slope of the fitting line is $\frac{-E}{R}$, so the activation energy $E$ can be solved.

The oxidative spontaneous combustion process of coal is a gas-solid two-phase reaction [38], and common mechanistic function models are shown in Table 4.

Table 4. Commonly used mechanistic functions for gas-solid reactions

This section focuses on the resistances of highly efficient water-retaining colloidal material prior to the dry cracking temperature of the coal body, so by using the different basis functions described above for the two stages T₀ to T₂ and T₂ to T₃, respectively, the values of E and R² are obtained as shown in Tables 5 and 6.

Table 5. E and ln A calculated by different mechanism functions in T₀ ~ T₂ stages

Table 6. E and ln A calculated by different mechanism functions in T₂ ~ T₃ stages

By selecting different basis functions for calculation and comparison, the first order chemical reaction mechanism function was preferred for the calculation of activation energy. The results of fitting the kinetic curves of the coal samples at different temperature stages are shown in Figure 9, and the activation energies of the coal samples in the untreated case and after the addition of highly efficient water-retaining colloidal material for the corresponding stages are derived as shown in Table 7.

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Figure 9. Kinetic curves for raw and treated coal samples at T0~T3

Table 7. Calculation of activation energy of coal samples at different stages (KJ·mol^-1)

According to Table 7, the activation energies of the raw coal samples in the weight loss and oxygen uptake weight gain stages were significantly different, and the value of the activation energy required increased as the reaction proceeded. After the addition of highly efficient water-retaining colloidal material, the activation energy of both stages also changed, where the activation energy of T0~T2 stage increased by 5.3 KJ·mol^-1, and the activation energy of T₂~T₃ stage increased from the original 66.9 KJ·mol^-1 to 75.4 KJ·mol^-1, indicating that the highly efficient water-retaining colloidal material played a stable blocking effect, in addition, when the temperature of the coal body exceeded the dry cracking temperature, its oxidative spontaneous combustion capacity has become very strong, so the most effective time period for materials that inhibit coal spontaneous combustion is still to control coal spontaneous combustion at low temperatures. Comparing the activation energy of the raw and treated coal samples at both stages, the highly efficient water-retaining colloidal material can exert a better dampening effect before the dry cracking temperature of the coal.

4.3 Effect of highly efficient water retaining colloidal material on functional groups in coal

Studies on the spontaneous combustion of coal have shown that the action of functional groups of varying nature present in the molecular structure of coal is a fundamental factor in the oxidative warming and hence spontaneous combustion of coal [39]. In this experiment, the changes in the functional groups of the selected test coal samples at different temperatures were investigated and analysed.

Six samples of 50 g each, three without treatment and three with 5 g of highly efficient water-retaining colloidal material in the ratio of 2% SA + 0.5% PCC + 1% GDL, were heated at 40°C, 80°C and 120°C for 12 h. After heating, the six samples were ground using a mortar and pestle and scanned using an infrared spectrometer with a wave number of 4000 ~500cm^-1, resolution 4cm^-1, 32 scans/sec. The IR spectra of the raw and treated coal samples were fitted to the peaks using origin software, where the peaks of the oxygen-containing functional groups and hydroxyl groups are shown in Figures 10-11.

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Figure 10. Fitting of oxygen-containing functional group peaks

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Figure 11. Fitting of hydroxyl peaks

The strong absorption peaks of the lignite raw coal samples selected for this experiment were phenols, alcohols, ethers and ester-oxygen bonds (Ar-C-O-, 1330~1060) and -COO- (1410, 1590~1560) in the oxygen-containing functional groups, -CH₃ and -CH₂ in the aliphatic hydrocarbons, C=C stretching vibration and -OH in the aromatic rings. The areas of the absorption peaks of the above major functional group surfaces in the raw coal samples and in the treated samples of highly efficient water-retaining colloidal material at three temperatures are shown in Table 8.

Table 8. Absorption peak areas of functional groups

As can be seen from Table 8, the overall content of each functional group in the treated sample is higher than that in the original coal sample, mainly because the highly efficient water-retaining colloidal material is an organic material, which contains a certain number of functional groups itself. The amount of Ar-C-O- in the raw coal decreases and then increases when the temperature rises, as the oxygen-containing functional groups in the raw coal have started to be oxidised to produce gases such as CO and CO₂; as the temperature rises the water molecules start to evaporate and the amount of -OH decreases. The Ar-C-O- in the treated samples decreased slightly with increasing coal temperature, but the magnitude of the change was obviously smaller than that of the original coal samples. The highly efficient water-retaining colloidal material inhibited the activity of Ar-C-O-, thus the fluctuation of the functional group content was reduced. evaporation of water and inhibit the oxidative warming process of the coal.

The C=C in the original coal sample decreased gradually with increasing temperature, which is due to the low degree of lignite deterioration and the activation of aromatic functional groups even at the low temperature stage, followed by the reaction with oxygen; however, the C=C structure in the treated sample did not decrease but slightly increased, which is mainly due to the accumulation of active functional groups in the initial stage of coal oxidation, and the oxidation reaction will occur only when the accumulation reaches the threshold, and C=C in the treated samples did not decrease significantly at 120°C, indicating that the decomposition of the coal structure was delayed backwards at this time.

In addition, the -CH₃ and -CH₂ content of the raw coal samples was low, but the effect of the highly efficient water-retaining colloidal material on -CH₃, -CH₂ and -COO- was approximately the same, while the content in the raw coal decreased and the amount in the treated samples increased when the temperature was increased to 80°C. This is due to the reduction in the number of functional groups in the raw coal as the reactive groups begin to oxidise, while some of the organic molecules contained in the highly efficient water-retaining colloidal material react with the reactive structures in the coal molecules resulting in an increase in the number of functional groups in the treated samples.

In summary, highly efficient water-retaining colloidal materials have a greater influence on the Ar-C-O-, -COO-, -CH₃, -CH₂, C=C stretching vibration and -OH during the low temperature oxidation of lignite, on the one hand, because highly efficient water-retaining colloidal materials are organic materials will increase the content of functional groups, on the other hand, highly efficient water-retaining colloidal materials can also inhibit the activity of several functional groups, thus weakening the oxidation reaction ability of coal molecules, and ultimately The spontaneous combustion of coal is delayed.