RU2720416C1 - Method of producing a foamed hydrogel of silicic acid - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to production of dense foamed structures with low heat conductivity, specifically to production of foamed hydrogel of silicic acid. Described is a method of producing a foamed hydrogel of silicic acid, through which a colloidal solution formed during hydrolysis of a mixture of an aqueous solution of an alkali metal silicate and a hydrocarbon surfactant, is subjected to a phase sol-gel transition by mixing said colloidal solution with a gelation activator, characterized in that mixing the colloidal solution with an activator is carried out in a mixing volume, then formed in the volume of mixing hydrosol of silicic acid is compression-fed to foam generating grids with formation of a foamed hydrogel of silicic acid, wherein in the mixing volume, the process is carried out at a volume ratio of the colloidal solution to activator of 1:(30–100), at activation temperature from minus 20 to 0 °C or from +50 to +110 °C, activator used is a gaseous non-metal oxide, for the process of mixing in the bulk and a colloidal solution, and the gaseous activator is supplied compression at pressure of 3–10 atm, and the gas source is a cylinder with compressed gas or a solid-fuel gas generator.

EFFECT: improved characteristics of the obtained foamed hydrogel of silicic acid.

8 cl, 2 dwg, 3 tbl, 8 ex

Description

The invention relates to the field of obtaining dense foamed structures with low thermal conductivity, namely, the production of foamed silicic acid hydrogel, which can be used to extinguish fires of solid combustible materials, accompanied and not accompanied by decay, as well as to prevent re-ignition, localization and isolation of fires flammable substances.

The invention is known "Autonomous installation of foam fire extinguishing, the fire extinguishing system of large tanks" [RF patent No. 2674710 published in 2018, IPC А62С 3/00], which is a sealed container with water, inside which there is a tank for storing a foaming agent with a gas generator for displacing it into a container with water and the formation of a foaming solution that is saturated with the products of combustion of the charge. Under the pressure of the gases generated in the displacement gas generators, the extinguishing agent is supplied to the fire object with the formation of low-level foam. The description of the patent describes the features of a method for producing gas-filled foam inside a container by vigorously mixing a solution with saturation by the combustion products of charges from an aerosol-forming composition. The foam obtained inside the tank is fed through a pipeline and a nozzle to the fire.

The disadvantage of this method is the inability to obtain quick-hardening foams, since a fire extinguishing agent (compression foam) is formed inside the device’s case and, if hardened quickly, would remain in the case without having to leave it.

The invention is known "Foamed silica gel, the use of foamed silica gel as a fire extinguishing agent and a sol-gel method for its preparation" [US Pat. RF №2590379, publ. 2015, IPC С01В 33/16). This patent describes a method that consists in air-mechanical foaming of a mixture of an aqueous solution of 20-50% sodium silicate and a 6% synthetic hydrocarbon foaming agent with 1-3.5% aqueous solution of acetic acid with a mass ratio of an aqueous solution of sodium silicate with a foaming surface active substance and an aqueous solution of acetic acid 35: 1 with the formation of a hardening foam of mainly low and medium multiplicity with the occurrence in the foam medium of the reactions of ash formation of silica and polycondensation of sol cream ezema sol-gel transition silica to obtain foamed silica gel with its hardness set within from 2 seconds to 2 minutes, and change in its volume of not more than 20% within 24 hours.

It should be noted in paragraph 1 of the claims to this patent states: "Foamed silica gel used in explosion and fire prevention, characterized in that ...", that is, the purpose of the declared gel is indicated. However, the term “fire prevention” refers to preventive measures during equipment operation. It can also be assumed that the composition claimed in the patent is intended for use at the facilities for the production and transportation of combustible cryogenic liquids (article "Methods, means and technologies of explosion and fire prevention at the facilities for the production and transportation of combustible cryogenic liquids" in the journal "Fires and emergencies: prevention, elimination" No. 3-2015 p. 14-25 ").

However, in the description of the patent there is no evidence (or even indications) of the achieved result when using the patented composition of quick-hardening foam (BTP) for the above cases.

The well-known "Method of fire prevention and solid extinguishing" [RF patent No. 2672945 publ. 11.21.2018, IPC A62C 13/04, A62C 5/02 C-1 B 33/14, B01F 3/08], which includes the preparation of foamed silica gel in the form of quick-hardening foam by mixing component A in the form of an aqueous solution of a mixture of silicate alkali metal and foaming surfactant, mainly synthetic hydrocarbon foaming agent, in the ratio, mass. %: 10-70, mainly 20-50 sodium silicate, 1-15, mainly 3-6 foaming surfactant, the rest is water, with component B (ash activating agent), comprising 20-60%, mainly from 30-50 % aqueous solution of acetic acid. In this case, foaming is carried out by means of a foam generator with ejection of atmospheric air into a mixture of components. Thus, it is not entirely correct to call a two-component system (only A and B), since the third component is involved in the foaming process - ejected air.

The disadvantage of this method is the lack of control over the process (or degree) of hardening of the obtained BTP, since the gelation rate of silica is very sensitive to various factors, including the pH of the activator (hardener). The disadvantage inherent in the invention according to patent No. 2590379 regarding explosion and fire prevention can rightfully be attributed to this patent No. 2672945, since in the description of the patent there is no evidence of the achievement of technical results when it is used for explosion and fire prevention. This patent should be considered only for solid-state quenching.

The closest in technical essence and the achieved technical result is the "Method of explosion fire prevention and solid extinguishing" RF patent No. 2672945, which is selected as a prototype.

The present invention is aimed at solving the problem of increasing the stability of the formation of quick-hardening foam based on foamed silicic acid hydrogel and eliminating the sensitivity of the process to the pH of the activator, which may impede the necessary rate of formation of BTP.

When using the proposed method, the following technical result is achieved:

- increasing the stability of the formation of BTP in the range from 1 to 10 seconds, due to additional controlled factors of ash and gel formation (activation temperature and the ratio of mixed components);

- obtaining a dense BTP with a micro- and macroporous structure (pore size of not more than 250 microns), which is clearly confirmed by the pore size on the BTP section.

The achievement of the specified technical result is ensured by the fact that the method of producing foamed silicic acid hydrogel, in which the colloidal solution formed during the hydrolysis of a mixture of an aqueous solution of alkali metal silicate and hydrocarbon surfactant, is subjected to a phase sol-gel transition by mixing the specified colloidal solution with a gel activator, characterized the fact that the mixing of the colloidal solution with the activator is carried out in the mixing volume, then the hydrosol cr formed in the mixing volume henic acid is compressed to foam networks with the formation of foamed hydrogel of silicic acid, while in the volume of mixing the process is carried out at a volume ratio of colloidal solution: activator as 1: (30-100), at an activation temperature of minus 20 to 0 degrees C or +50 to +110 degrees C, gaseous non-metal oxide is used as an activator, and a colloidal solution and a gaseous activator are supplied compression at a pressure of 3-10 atm for the volume mixing process, and a cylinder with compressed gas or solid fuel gas generator.

To implement this method, carbon monoxide (IV), or nitric oxide, or sulfur oxide can be used as gaseous non-metal oxide. But the most accessible and harmless is carbon monoxide (IV).

It is preferable to use a single gas source at all stages of the preparation of foamed silicic acid hydrogel, for example, in the form of a cylinder with a compressed gas or a solid fuel gas generator, which ensures interchangeability of apparatuses throughout the process.

The best results are achieved if gelation in a negative range of activation temperatures mainly uses compressed gas from a cylinder, and in a positive range of activation temperatures mainly uses gas from a solid fuel gas generator.

As the alkali metal silicate, sodium silicate or potassium silicate, or a mixture thereof, is used, since these silicates are produced in large volumes by the domestic industry.

The use in the composition of the colloidal solution of an additional nonionic surfactant, for example, phenoxol in an amount of 0.5-1.5 mass. % contributes to the controllability of the gelation process.

The resulting foamed hydrogel of silicic acid has a micro- and macroporous structure with a pore size of not more than 250 microns, which contributes to an increase in the strength and density of the resulting BTP and affects the range and accuracy of the foam.

The pores of the resulting foamed silica hydrogel contain oxide of the corresponding non-metal and chemically bound water.

The implementation of the invention

The process of obtaining the foamed structure of silicic acid hydrogel includes the step of preparing a colloidal solution, then the stage of mixing the resulting colloidal solution with a gaseous activator of gelation to provide a phase sol-gel transition and the process of formation of foamed silicic acid hydrogel.

The preparation of a colloidal solution is carried out by mixing alkali metal silicates, water and a surfactant (surfactant), if necessary add an additional nonionic surfactant, for example, phenoxol.

The phase sol-gel transition is carried out in the volume (chamber) of mixing the resulting colloidal solution with a gel activator in the form of gaseous non-metal oxide, mainly with carbon monoxide (IV). As a source of carbon dioxide, a compressed gas cylinder or a solid fuel gas generator can act.

The preparation of foamed silicic acid hydrogel was carried out in an experimental setup using, for example, a solid fuel gas generator. The installation has two tanks. One container is filled with a colloidal solution of alkali metal silicate, water and a hydrocarbon surfactant (if necessary, additional nonionic surfactants, for example, phenoxol, are added). The second tank is a gas generator with a solid fuel pyrotechnic charge (checker), which, when burned, forms a mixture of gases (CO ₂ , water vapor and a small amount of nitrogen) with a pronounced predominance of CO ₂ . When the igniter device is activated, the checker lights up and releases the indicated gases, СО ₂ is an activator of gelation. Part of the gas is used to push the colloidal solution from the first tank into the mixing chamber (volume), another part of the gas is fed directly to the mixing chamber. The pressure in the entire system is from 3 to 10 atm. The temperature of the emitted gases ranges from plus 50 to plus 110 degrees Celsius, and can vary in the indicated range using chemical heat absorbers.

The experimental setup with a gas cylinder is similar to the above with the exception of the fact that a cylinder with compressed CO _{2 is} used as the second tank, and the pressure and components are fed into the mixing chamber using shut-off and start-up valves and control and regulating devices. The pressure in the system is manually controlled from 3 to 10 atm. According to thermocouples, the temperature of the outgoing gas CO ₂ can vary in the range from minus 10 to minus 20 degrees C.

The hydrosol of silicic acid formed in the mixing chamber is compressed to the foam generating grids. As a result, a foamed hydrogel of silicic acid is formed, the solidification of which, caused by the sol-gel transition, occurs within 1 to 10 seconds. The foamed structure is capable of chemically absorbing the resulting moisture with the formation of monohydrates, which leads to mechanical and thermal stability.

It is important that the formation of a hydrosol of silicic acid occurs at the moment of mixing the colloidal solution with gaseous non-metal oxide. Early ash formation in a container with a working solution can lead to delamination due to coagulation of silicic acid particles.

The inventors have investigated the effect of foaming anionic and nonionic surfactants, as well as mixtures thereof, on premature ash formation in an aqueous solution of an alkali metal silicate with a high silicate modulus. As an ionic (anionic) surfactant, we used a biodegradable foaming agent for special purposes with increased fire extinguishing ability PO-6CT, which is a mixture of alkyl sulfonic acids. Phenoxol, which is a nonionic polymer, was used as a nonionic surfactant. For this purpose, a surfactant or a mixture thereof in an amount of up to 6% was added to a freshly prepared 25% aqueous solution of alkali metal silicate. Data on the investigated compositions are presented in table 1.

The resulting compositions were tested for foaming (visually compared 0 - no foaming, 1 - good foaming, 2 - excellent foaming), as well as stability for 1.5 months at room temperature. Initially, samples No. 1,2,3,5 were a homogeneous mixture, while sample 4 consisted of two phases. After 1.5 months, a small amount of white sediment settled on the bottom was noted in sample No. 1, samples No. 2 and 3 remained homogeneous, sample No. 4 was initially two-phase and its condition did not change, and a significant amount of loose sediment formed in sample No. 5 probably silicic acid. The relevant data are presented in table 1.

Thus, sample No. 1 (without phenoxol) showed good results, and samples No. 2 and No. 3 containing 0.5 and 1.5% phenoxol showed the best results.

The micro- and macroporous structure of the foamed silicic acid hydrogel is confirmed by scanning electron microscopy (SEM) at 100-fold magnification (Fig. 1). Obtained by this method, foamed hydrogel of silicic acid visually differs from the foam of the prototype (Fig. 2), the SEM results of which are indicated in the article of the authors of the prototype "Improving the efficiency of explosion and fire prevention by using quick-hardening foam based on structured particles of silica" in the journal "Safety". - 2015. - No. 2. - S. 8-10.

In the prototype, the formation of sol with the subsequent formation of a gel of silicic acid occurs by lowering the pH level in the direction of the phase sol-gel transition, which is a well-known technique. In the proposed method, the formation of a hydrosol is observed when dispersed particles begin to grow and after passing through the foam-generating nets, a silicic acid hydrogel is formed. Thus, it turns out that the hardening time of BTP depends on the volumetric ratio of gas-liquid. The second factor contributing to the sol and gel formation is the activation temperature. When using a cylinder with compressed carbon monoxide (IV), the temperature of the outflow of CO ₂ at the outlet of the cylinder varies from minus 10 to minus 20 degrees C. This is due to the throttling effect. At this temperature, gas enters the mixing chamber. The gas temperature is much lower than the crystallization temperature of the solvent, which probably contributes to the crystallization of part of the solvent and a shift in the concentration of the dispersed phase of silicic acid to a larger side, which leads to the growth of particles with their subsequent polymerization. It was unexpected that the high temperature of the gas-generating gases accelerates the growth of colloidal particles, i.e. intensifies the process of hydrosol formation, and also contributes to the formation of foamed silicic acid hydrogel in the time range from 1 to 10 seconds, despite the fact that at high temperature the solubility of CO ₂ decreases. Thus, the temperature of the activating gas eliminates the differences in the flow rate of the supplied fluid.

The inventors conducted a study on the effect of the temperature of the gaseous activator and the colloidal solution: gas ratios during the formation of the silicic acid hydrogel.

The criterion for the experiment was chosen the time during which the formed foam released from the exhaust device of the model installation remained liquid, fell on the protected object, spread over it and hardened. Usually, this time did not exceed 10 seconds. The foam hardness was evaluated organoleptically.

To obtain foamed hydrogel of silicic acid, widely used by the domestic industry were used: sodium liquid glass according to TU 2145-044-05744685-01 amended. 1, biodegradable synthetic hydrocarbon blowing agent PO-6CT according to TU 0258-148-05744685-98 as amended. 1-5, highly concentrated low-foaming biologically soft nonionic wetting agent Phenoxol 9/10 BV according to TU 2484-097-05744685-95, gaseous carbon dioxide according to GOST 8050-8. As well as drinking water in accordance with GOST R 51232-98.

Table 2 in the examples:

No. 2 and 5 tested the composition under No. 1 of the table. 1; -№№1,3,6,8 and 9 tested the composition under No. 2 of the table. 1; - No. 4.7 and 10 tested the composition under No. 3 of the table. 1.

Examples No. 4 and 5 of the table. 1 were not subject to tests to determine the time of formation of the foamed silicic acid hydrogel (Table 2), as they did not provide foaming.

Table 2 in the examples:

- No. 1-4, a gaseous activator was supplied at a pressure of 3-4 atm .;

- No. 5-7, a gaseous activator was supplied at a pressure of 5-6 atm .;

- No. 8-10, a gaseous activator was supplied at a pressure of 9-10 atm .;

When the supply pressure of the gaseous activator is more than 10 atm. a stream of atomized colloidal solution was observed without the formation of a foamed silica hydrogel structure. When the supply pressure of the gaseous activator is below 3 atm. the range of the foam structure stream was not provided.

At any ratio at an activation temperature of -30 and +120 degrees C. With a dense foamed structure of a hydrogel of silicic acid is not formed. In tests 1 (A-D), this is apparently due to excessive crystallization of water and blocking foam-generating nets in the form of ice, and in tests 10 (A-D) a flow of atomized liquid in a gas stream was observed without the formation of a three-dimensional foam structure. In tests A (2–9), B5, and B5, the gas volumetric flow rate was too small for the sol – gel phase transition to occur in the range up to 300 seconds. In tests D (2-9), the phase sol-gel transition occurred too quickly in the mixing chamber or in the foam-generating nozzle, which led to clogging of the mesh cells and disruption of the normal mode of operation of the system until the exhaust device was deformed. Excessive gas pressure knocked out foam-generating nets. In tests 5G and 6G, the formation time of the foamed silicic acid hydrogel is almost identical. The ratio of colloidal solution: gas, as 1: 100 slightly exceeds the stoichiometric, which is 1:85. In this case, when the amount of activator deviates (tests B (2-4) and 6 (B-C)) from the stoichiometric one to a smaller side, an increase in the time of the phase sol-gel transition is observed. In the remaining tests, the gel time did not exceed 10 seconds.

The increased temperature of the gas-generating gases and the lowered temperature of СО ₂ from the compressed gas cylinder make it possible to obtain foamed silicic acid hydrogel in the time range from 1 to 10 seconds, and also reduce the activator consumption.

The resulting foamed silicic acid hydrogel has a monodispersed porous structure with a pore size of not more than 250 μm, as evidenced by the photo of the foam section shown in FIG. 1. For comparison, in FIG. 2 shows a section of the foam of the prototype, having a significantly larger pore size (more than 250 microns) and not differing in monodispersity.

In further practical tests, samples were taken under No.№ЗВ and 9Б from table. 2, since the configuration of the model installation at such temperatures and colloidal solution: gas ratios is most optimal. In example No. 3B, there is no need to use special control and regulatory equipment, and example No. 9B allows you to reduce the size of the tank with a solid fuel gas-generating charge due to the use of fewer chemical heat absorbers.

Practical tests of the possibility of achieving a technical result and industrial implementation of the method were carried out using a model installation in the open air under normal climatic conditions, and a wind speed not exceeding 5 m / s, as well as in the absence of precipitation.

According to GOST 51057-2001, the model fire site 1A was a wooden stack in the form of a cube placed on a solid support in such a way that the distance from the base of the stack to the supporting surface was 400 mm.

As a combustible material, 72 softwood bars were used no lower than the third grade according to GOST 8486-86 with a section of 40 mm, a length of 500 mm, and a moisture content of 10-20%. The stack contained 12 layers of 6 bars in each layer, laid out so that the bars of each subsequent layer were perpendicular to the bars of the underlying layer with the formation of rectangular channels throughout the stack. The free surface area of the model focus was 4.7 m ² .

Under the stack was a metal tray for flammable liquid with a size of 400 × 400 × 100 mm, into which 5.0 dm ³ was poured to form a continuous smooth surface and 1.1 dm ^{3 of} summer type gasoline meeting the requirements of GOST R 51105-97.

A pallet with a flammable liquid was placed under the stack so that the centers of the stack and the pallet coincided.

They set fire to gasoline in the pan and after 8-10 minutes from the moment of the start of burning, when the stack was covered with flames from all sides, they began to extinguish the model fire with various fire extinguishing agents. For comparison, we used a stream of water and air-mechanical foam based on a 6% solution of PO-6Ts foaming agent in water.

During extinguishing, the fire site was rotated at a speed of 3-5 rpm, which made it possible to supply extinguishing agents to each side of the fire chamber sequentially and without operator intervention, excluding the influence of the human factor.

Extinguishing using a model unit was carried out with the addition of foamed silicic acid hydrogel at a displacement pressure of 0.6 MPa and at a distance from the barrel to the fire site of 6 m to 8 m. The model unit was placed stationary. Extinguishing at the indicated distance was positive.

Quenching of the model site was evaluated visually. After the supply of extinguishing agents, the time of reignition was recorded. The test results were considered positive if re-ignition did not occur within 10 min followed by steady burning of the stack.

Calculation of the extinguishing efficiency indicator was calculated by the formula:

где:

Where:

S is the area of the model center, m ² ;

Q - total consumption of extinguishing agent, l;

t is the quenching time of the model center, s

Table 3 presents the results of comparative tests.

Thus, the implementation of the proposed method confirms the achievement of the specified technical result.

Moreover, the achieved result is not obvious, given that the proposed method, in addition to other distinguishing features from the prototype, is based on a heterogeneous mixture of components, and, according to the authors, meets all the eligibility criteria under current law.

Claims (8)

1. A method of producing a foamed silicic acid hydrogel, in which a colloidal solution formed during the hydrolysis of a mixture of an aqueous solution of an alkali metal silicate and a hydrocarbon surfactant is subjected to a phase sol-gel transition by mixing said colloidal solution with a gel activator, characterized in that the mixture is a colloidal solution with the activator is carried out in the volume of mixing, then the hydrosol of silicic acid formed in the volume of mixing is applied to compression foam grids with the expansion of the foamed hydrogel of silicic acid, while in the volume of mixing the process is carried out at a volume ratio of colloidal solution: activator as 1: (30-100), at an activation temperature from minus 20 to 0 ° C or from +50 to + 110 ° C, Gaseous non-metal oxide is used as an activator, and a colloidal solution and a gaseous activator are supplied compression at a pressure of 3-10 atm for the volume mixing process, and a compressed gas cylinder or solid fuel gas generator is used as a gas source. 2. The method according to p. 1, characterized in that as the gaseous oxide of non-metal using carbon monoxide, or nitric oxide, or sulfur oxide. 3. The method according to p. 1, characterized in that at all stages of the production of foamed silicic acid hydrogel, one gas source is used in the form of a cylinder with a compressed gas or a solid fuel gas generator. 4. The method according to p. 1, characterized in that the compressed gas from the cylinder is mainly used in the negative range of the activation temperature, and gas from the solid fuel gas generator is mainly used in the positive range of the activation temperature. 5. The method according to p. 1, characterized in that as the alkali metal silicate use sodium silicate, or potassium silicate, or a mixture thereof. 6. The method according to p. 1, characterized in that the colloidal solution further comprises a nonionic surfactant, for example phenoxol, in an amount of 0.5-1.5 wt.%. 7. The method according to p. 1, characterized in that when using compressed gas from a cylinder, the mixing process is preferably carried out at a volume ratio of colloidal solution: activator as 1: (50-100). 8. The method according to p. 1, characterized in that the obtained foamed hydrogel of silicic acid has a micro- and macroporous structure with a pore size of not more than 250 microns.

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