CN121317841A - Fire extinguishing powder based on seed crystal induced struvite and preparation method thereof - Google Patents

Abstract

The invention belongs to the technical field of environment-friendly fire extinguishing materials, and particularly relates to fire extinguishing powder based on seed crystal induced struvite and a preparation method thereof. The MAP fire extinguishing powder with good fire extinguishing performance is prepared by taking supernatant liquid rich in nitrogen and phosphorus of a sewage treatment plant as a raw material, removing heavy metal ions in sewage through a cation selective permeation membrane, then adjusting the pH value and the nitrogen and phosphorus ratio of the supernatant liquid, introducing seed crystals to induce struvite (MAP) to crystallize and precipitate, filtering and drying to obtain MAP particles synthesized by seed crystal induction, and further adopting a horizontal planetary ball mill to uniformly mix MAP and grind the MAP and a modifier. The invention innovatively adopts Layered Double Hydroxide (LDH) as seed crystal inducer, which can obviously raise the pyrolysis initiation temperature and optimize the thermal decomposition kinetics process, thereby greatly enhancing the thermal stability and the extinguishing efficiency of the dry powder extinguishing agent.

Description

Fire extinguishing powder based on seed crystal induced struvite and preparation method thereof

Technical Field

The invention belongs to the technical field of environment-friendly fire extinguishing materials, and particularly relates to a fire extinguishing powder based on seed crystal induced struvite (MgNH ₄PO₄·6H₂ O, MAP), a preparation method and application thereof, which are particularly suitable for high-efficiency and environment-friendly dry powder fire extinguishing agents.

Background

The particle size of the fire extinguishing component of the common dry powder is generally 10-75 mu m, and the mass of single particles is relatively large, so that the defects of rapid sedimentation, poor dispersity, small specific surface area and the like exist, and further the capability of capturing free radicals and active groups in the process of exerting a chemical fire extinguishing mechanism is relatively poor, and the fire extinguishing capability is limited. Therefore, the preparation of the fire extinguishing agent which has smaller particle size, better fluidity, large specific surface area, high activity, stable dispersion and long-time suspension in the air becomes a key target for the design of the high-efficiency dry powder fire extinguishing agent.

MAP, the chemical formula of which is MgNH ₄PO₄·6H₂ O, commonly known as struvite, is first found in sewage treatment plants. When the concentration of Mg ²⁺、NH₄ ⁺ and PO ₄ ^3- in the urban wastewater is higher, the magnesium-zinc oxide can be crystallized and separated out under alkaline conditions and at proper temperature. In sewage treatment systems, MAP typically forms hard scale in the pipeline, which if not treated in time will cause pipeline blockage, and therefore, large amounts of MAP are produced by sewage treatment plants each year. At present, MAP is recycled by many centralized sewage treatment plants and is used as a slow release fertilizer, but because the MAP is rich in heavy metal elements, the MAP is directly applied to soil pollution easily caused, so that the application of the MAP in agriculture is limited. At present, MAP is recovered and is mostly used for building materials, so that the flame retardant capability of the building materials can be improved. Notably, MAP possesses chemical properties similar to the main chemical component of monoammonium phosphate (NH ₄H₂PO₄) of commercial ABC dry powder extinguishing agents, and researchers have found that MAP does possess fire extinguishing capabilities and can be used as a potential fire extinguishing material.

MAP synthesis is related to factors such as nitrogen-phosphorus-magnesium ion ratio, sewage pH, organic matters in sewage, seed crystal addition and the like. Patent CN202011305662.1 discloses a method for preparing MAP from phosphorus-containing wastewater, which selects crushed and refined or commercial sold MAP particles as seed crystals, and controls the particle size of produced MAP by controlling the particle size of the seed crystals, the seed crystal amount and the discharge period of target MAP by using a fluidized bed, so as to ensure the continuity of MAP production.

Patent CN202310713771.4 discloses a preparation method of modified MAP, which comprises adding lignosulfonate into supernatant, performing MAP precipitation process, and modifying MAP in situ to improve its thermal stability, so that it is easier to adhere to various substrate surfaces to form compact fireproof coating, and improve flame retardant effect of MAP.

Patent CN202210882495.X discloses a preparation method of MAP superfine dry powder, which uses silicone oil, talcum powder, magnesium stearate and DOPO-VST to carry out surface modification on MAP particles synthesized by non-seed crystal induction so as to enable the MAP particles to reach superfine standard, thereby enhancing the fire extinguishing performance of the MAP superfine dry powder.

The existing method for synthesizing MAP and preparing MAP into fire extinguishing powder mainly has the following problems that (1) the common MAP dry powder has low pyrolysis temperature and is not easy to preserve for a long time, (2) the common MAP powder has long fire extinguishing time and insufficient fire extinguishing efficiency, (3) the specific structural design and precipitation process improvement are carried out by adopting a fresh process around the fire extinguishing application of MAP, and (4) the modification process of MAP in the fire extinguishing field is complex, so that the industrialized production of MAP is limited.

As an inorganic nano layered material, layered Double Hydroxide (LDH) can realize high-selectivity adsorption to PO ₄ ^3- by mechanisms such as hydrogen bonding, electrostatic attraction between hydroxide and oxygen ions, ion exchange and the like so as to promote MAP generation, and the LDH is often used as a flame-retardant material due to the characteristics of environmental friendliness, high flame-retardant efficiency and the like.

Patent 202310086527.X discloses a preparation method of porous double metal hydroxide, under the protection of inert gas, adding sodium hydroxide solution and magnesium aluminum nitrate mixed solution simultaneously into sodium nitrate solution with pH=10-11, maintaining pH of the mixed solution to be 10.5-11.5, and precipitating for 8 hours to obtain double metal hydroxide precipitate.

Patent 202411166598.1 discloses a preparation method of layered double hydroxide, which uses divalent metal alkali to simultaneously adjust the pH of the solution and provide metal ions, and simultaneously dissolves the solution and trivalent metal salt in water for removing CO ₂, and adjusts the reaction temperature to 140-180 ℃ for reaction for 10-15 hours, so as to obtain the double hydroxide.

The prior LDH preparation mainly has the problems that (1) the common LDH preparation mode has complex reaction conditions and high equipment requirements, (2) the common LDH preparation mode has high reaction temperature and high energy consumption, and (3) the common LDH preparation process is complex and difficult to carry out technological production.

Therefore, the invention provides a relatively easy LDH preparation method and adopts the technical scheme of using LDH as seed crystal to induce and precipitate MAP, thereby simplifying the preparation process and preparation conditions of the LDH in the prior art and synchronously solving the problems of thermal stability, fire extinguishing efficiency, simplified process and the like of MAP fire extinguishing powder prepared by the prior art.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides a preparation method of crystal-induced MAP fire extinguishing powder, which creatively introduces LDH as crystal seeds, realizes high-efficiency adsorption of PO ₄ ^3- through interaction of hydrogen bonding, electrostatic adsorption and the like so as to improve crystallization and precipitation efficiency and pyrolysis temperature of MAP, is favorable for obtaining fire extinguishing powder with better dispersivity, larger specific surface area and higher fire extinguishing efficiency through a grinding and refining process, and solves the problems of low pyrolysis temperature, insufficient fire extinguishing efficiency and the like of common MAP fire extinguishing powder.

In order to achieve the technical purpose, the invention adopts the following technical scheme:

the first aspect of the present invention provides a process for the preparation of a Layered Double Hydroxide (LDH) comprising the steps of:

Mixing soluble divalent metal ion salt and soluble trivalent metal ion salt in deionized water, regulating pH of the mixed solution to be alkaline by using NaOH, placing the mixed solution in a hydrothermal reactor for reacting for a certain time, and centrifugally drying the precipitate product to obtain the LDH with the corresponding metal composition.

Further, the soluble divalent metal ion comprises at least one of Mg ²⁺、Zn²⁺、Ni²⁺、Co²⁺、Mn²⁺、Cu²⁺、Cd²⁺、Sn²⁺, the soluble trivalent metal ion comprises at least one of Al ³+、Fe³⁺、Cr³⁺、Bi³⁺、La³⁺、Ce³⁺, and the mole ratio of the soluble divalent metal ion to the soluble trivalent metal ion is (1-3): 1-3.

Further, the LDH preparation reaction requires an ambient pH of 7.5-14 and a reaction temperature of 80-170 ℃.

Further, the temperature rising mode of the LDH preparation reaction is programmed temperature rising at 5-10 ℃ per min, and the temperature duration is 8-15 hours.

Further, the centrifugal speed required for recovering the product obtained by the LDH preparation reaction is 6000-8000rpm, the centrifugal time is 5-10 minutes, the drying temperature of the obtained product is 30-35 ℃ and the drying time is 25-30 hours.

In a second aspect, the present invention provides a method for inducing synthesis of struvite (MAP) fire extinguishing powder, comprising the steps of:

selecting supernatant of a sewage treatment plant as a raw material, adding a solid porous medium, fully stirring uniformly, standing for precipitation, filtering to remove heavy metal ions, adding a certain amount of ammonium salt and phosphate into the supernatant to adjust the nitrogen-phosphorus ion ratio, then adding LDH obtained by the method provided by the first aspect as seed crystal, stirring to fully and uniformly mix the seed crystal with the supernatant, adjusting the pH of the supernatant by using a NaOH solution, adding magnesium salt to adjust the nitrogen-phosphorus-magnesium ion ratio, standing for precipitation for a certain time after stirring at normal temperature, centrifuging to obtain bottom precipitation, drying to obtain seed crystal induced MAP particles, mixing the obtained MAP particles with a modifier according to a certain proportion, and crushing and refining by using a horizontal planetary ball mill to obtain MAP fire extinguishing powder which meets the requirements of fire extinguishing powder and has an average particle size of 20-35 mu m.

Further, the solid porous medium used for heavy metal ion adsorption is one or a combination of at least two of active carbon, chitosan resin, montmorillonite and acidified sepiolite.

Further, the ammonium salt required for the MAP precipitation reaction is one or a combination of at least two of (NH₄)₃PO₄·3H₂O、NH₄Cl、NH₄NO₃、(NH₄)₂CO₃.

Further, the phosphate required for the MAP precipitation reaction is one or a combination of at least two of Na₃PO₄·12H₂O、(NH₄)₃PO₄·3H₂O、NaH₂PO₄、Na₂HPO₄·12H₂O、NaH₂PO₄·12H₂O、NH₄H₂PO₄、(NH₄)₂HPO₄.

Further, the magnesium salt required for the MAP precipitation reaction is one or a combination of at least two of MgCl ₂·6H₂O、Mg(NO₃)₂、MgSO₄.

Further, the mole ratio of nitrogen, phosphorus and magnesium ions in MAP precipitation reaction is N:P = (0.8-1.5): (0.8-1.8): (1-1.8).

Further, the LDH required for the MAP precipitation reaction provides one or a combination of at least two of MgAl-LDH, mgFe-LDH, niCr-LDH, znAl-LDH, coCr-LDH synthesized by the method of the first aspect.

Further, the environmental pH of MAP precipitation reaction is 8-13;

Further, the reaction temperature of MAP precipitation reaction is 25-35 ℃;

further, the precipitation time of MAP precipitation reaction is 3-5 hours;

Further, the MAP precipitation reaction product is centrifuged at 6000-8000rpm for 8-10 minutes at 30-35 ℃.

Further, the modifier of MAP particles obtained by MAP precipitation reaction is one or a combination of at least two of silicon talcum powder, hydrophobic silicon dioxide, magnesium stearate, active carbon, acidified sepiolite and aluminum zirconium pillared montmorillonite;

Further, the mass ratio of MAP particles to modifier obtained by MAP precipitation reaction is (75-95): 5-25;

further, the ball milling parameter of mixing and crushing the solid particles obtained by MAP precipitation reaction and the modifier is 300-400rpm, and the ball milling time is 30-60 minutes.

Further, the D50 of the fire extinguishing powder is 10-15 mu m, the D90 is 20-35 mu m, and the specific surface area is 25-90m ²/g.

The MAP fire extinguishing powder with good fire extinguishing performance is prepared by taking supernatant liquid rich in nitrogen and phosphorus of a sewage treatment plant as a raw material, removing heavy metal ions in sewage through a cation selective permeation membrane, then adjusting the pH value of the supernatant liquid to the nitrogen and phosphorus ratio, introducing seed crystals to induce MAP to crystallize and precipitate, filtering and drying to obtain MAP particles synthesized by seed crystal induction, and further adopting a horizontal planetary ball mill to uniformly mix MAP with modifiers such as hydrophobic nano silicon dioxide, talcum powder and magnesium stearate and grinding. According to the invention, double metal hydroxide (LDH) is innovatively introduced as seed crystals in the MAP recovery process, PO ₄ ^3- in supernatant is efficiently adsorbed by utilizing the effects of hydrogen bonding, electrostatic attraction and the like, so that the crystallization rate of MAP is remarkably improved, the yield of MAP recovered from sewage is greatly improved, in addition, the LDH has flame retardant property, the pyrolysis temperature of MAP fire extinguishing powder is remarkably improved by introducing the LDH, so that the MAP fire extinguishing powder can enter into the root of a flame and then is pyrolyzed, thereby enhancing the fire extinguishing performance, the MAP synthesized by seed crystal induction and a modifier are mixed and thinned through a ball milling process, the contact area of the fire extinguishing powder and the flame can be effectively increased, the synergistic effect of fire extinguishing and smoke suppression of the MAP fire extinguishing powder is fully exerted, and finally the MAP fire extinguishing powder with excellent comprehensive performance is obtained.

Compared with the prior art, the invention has the following beneficial effects:

(1) The invention provides a mode of separating out MAP by seed crystal induced sewage precipitation, which shortens the nucleation time of MAP by adding LDH as seed crystal, accelerates the nucleation rate and improves the yield of MAP at the same time, thereby being more beneficial to the recovery of MAP;

(2) According to the invention, LDH is innovatively used as a seed crystal inducer, so that the pyrolysis starting temperature of the LDH is remarkably improved, and the thermal decomposition kinetic process is optimized, so that the thermal stability and the fire extinguishing efficiency of the LDH serving as a dry powder extinguishing agent are greatly enhanced;

(3) The MAP synthesized by the invention is an additional product of sewage treatment, reduces the solid waste disposal cost, opens up a path of pollutant-resource, gives the MAP new functional application value, and provides a new application scene for the MAP.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required to be used in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only embodiments of the present invention, and that other drawings can be obtained according to the provided drawings without inventive effort for a person skilled in the art.

Fig. 1 is a flow chart of a preparation method of the LDH-MAP fire extinguishing powder.

FIG. 2 is a schematic diagram of a 1m ³ small fire-extinguishing platform used in the experiments of the present invention.

Fig. 3 is a graph showing comparison of SEM morphology features of LDHs of different types in the examples, wherein fig. 3 (a) is MgAl-LDH, fig. 3 (b) is MgFe-LDH, fig. 3 (c) is ZnAl-LDH, fig. 3 (d) is NiCr-LDH, fig. 3 (e) is CoCr-LDH, and fig. 3 (f) is LDH-MAP.

Fig. 4 is a comparative analysis of XRD components of different fire extinguishing powders in the examples.

Fig. 5a, b are adsorption and desorption curves and pore size distribution curves for commercial ABC dry powder, blank MAP fire-extinguishing powder, and MAP fire-extinguishing powder in examples.

FIG. 6 is a comparison of thermogravimetric data of commercial ABC dry powder, blank MAP fire extinguishing powder, and MAP fire extinguishing powder of the examples, FIG. 6a is a Thermogravimetric (TG) curve, FIG. 6b is a Differential Scanning Calorimetric (DSC) curve, and FIG. 6c is a Derivative Thermogravimetric (DTG) curve.

Fig. 7 is a fire-extinguishing video screenshot of commercial ABC dry powder, blank MAP fire-extinguishing powder, and MAP fire-extinguishing powder of example 4, fig. 7 (a), fig. 7 (B), fig. 7 (c) being class B fires, fig. 7 (d), fig. 7 (e), fig. 7 (f) being class a fires.

FIG. 8 is a graph comparing the smoke suppression effect (CO concentration) of commercial ABC dry powder, blank MAP fire extinguishing powder and MAP fire extinguishing powder in the examples in a fire scene.

FIG. 9 shows particle size distribution of commercial ABC dry powder, blank MAP fire extinguishing powder and MAP fire extinguishing powder in examples, wherein FIG. 9 (a) shows commercial ABC dry powder, FIG. 9 (b) shows blank MAP fire extinguishing powder, FIG. 9 (c) shows MgAl-MAP, FIG. 9 (d) shows MgFe-MAP, FIG. 9 (e) shows ZnAl-MAP, FIG. 9 (f) shows NiCr-MAP, and FIG. 9 (g) shows CoCr-MAP.

Detailed Description

The following will clearly and completely describe the technical solutions in the embodiments of the present invention, and it is obvious that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

The word "embodiment" as used herein does not necessarily mean that any embodiment described as "exemplary" is preferred or advantageous over other embodiments. Performance index testing in the examples of the present application, unless otherwise specified, was performed using conventional testing methods in the art. It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure.

Unless otherwise defined, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs, and other test methods and techniques not specifically mentioned herein are meant to be common to those of ordinary skill in the art.

Numerous specific details are set forth in the following examples in order to provide a better understanding of the present application. It will be understood by those skilled in the art that the present application may be practiced without some of these specific details. In the examples, some methods, means, instruments, devices, etc. well known to those skilled in the art are not described in detail in order to highlight the gist of the present application.

On the premise of no conflict, the technical features disclosed by the embodiment of the application can be combined at will, and the obtained technical scheme belongs to the disclosure of the embodiment of the application.

The embodiment of the invention discloses a method for synthesizing MAP fire extinguishing dry powder by seed crystal induction.

The present invention will be further specifically illustrated by the following examples, which are not to be construed as limiting the invention, but rather as falling within the scope of the present invention, for some non-essential modifications and adaptations of the invention that are apparent to those skilled in the art based on the foregoing disclosure.

The technical scheme of the invention will be further described below with reference to specific embodiments.

Example 1

The embodiment provides a method for synthesizing MAP fire extinguishing dry powder by seed crystal induction, which comprises the following steps:

MgCl ₂·6H₂ O and AlCl ₃·6H₂ O are dissolved in deionized water, the molar ratio of metal ions is controlled to be Mg ²⁺:Al³⁺ =3:2, the pH value of the solution is regulated to be 9 by using 5mol/L NaOH solution, the solution is placed in a hydrothermal reactor and reacted for 8 hours at 120 ℃, and the obtained product is centrifuged and then placed in a blast drying oven for drying at 35 ℃ to obtain MgAl-LDH.

After pretreatment of supernatant of wastewater treatment plant, adding orthophosphate and ammonium salt to adjust N:P=1:1, adding MgAl-LDH into the supernatant, adjusting pH=11 by using 5mol/L NaOH solution, fully stirring by using a magnetic stirrer, dissolving a certain amount of MgCl ₂·6H₂ O into deionized water according to the ion ratio of N:P=1:1:1.5, fully stirring, adding the mixture into the supernatant, reacting for 8-10h, centrifuging the obtained precipitate at 8000rpm for 10min, and drying in a blast drying box at 35 ℃ to obtain MgAl-MAP.

Mixing MgAl-MAP and nano hydrophobic silica according to the mass ratio of 95:5, and treating for 40 minutes by using a horizontal planetary ball mill under the condition of 300 r/mm to crush and refine to obtain MAP powder with certain fire extinguishing performance.

Example 2

The embodiment provides a method for synthesizing MAP fire extinguishing dry powder by seed crystal induction, which comprises the following steps:

MgCl ₂·6H₂ O and FeCl ₃·6H₂ O are dissolved in deionized water, the molar ratio of metal ions is controlled to be Mg ²⁺:Fe³⁺ =1:1, the pH value of the solution is regulated to be 10 by using 5mol/L NaOH solution, the solution is placed in a hydrothermal reactor and reacted for 10 hours at 135 ℃, and the obtained product is centrifuged and then placed in a blast drying oven for drying at 35 ℃ to obtain MgFe-LDH.

After pretreatment of supernatant of wastewater treatment plant, adding orthophosphate and ammonium salt to adjust N:P=1:1, adding MgFe-LDH into the supernatant, adjusting pH=9.5 by using 5mol/L NaOH solution, fully stirring by using a magnetic stirrer, dissolving a certain amount of MgCl ₂·6H₂ O into deionized water according to the ion ratio of N:P=1:1:1.5, fully stirring, adding the mixture into the supernatant, reacting for 8-10 hours, centrifuging the obtained precipitate at 8000rpm for 8 minutes, and drying in a blast drying box at 35 ℃ to obtain MgFe-MAP.

Mixing MgFe-MAP and magnesium stearate according to the mass ratio of 90:10, and treating for 45 minutes by using a horizontal planetary ball mill under the condition of 320 r/mm to crush and refine to obtain MAP powder with certain fire extinguishing performance.

Example 3

The embodiment provides a method for synthesizing MAP fire extinguishing dry powder by seed crystal induction, which comprises the following steps:

ZnCl ₂·6H₂ O and AlCl ₃·6H₂ O are dissolved in deionized water, the molar ratio of metal ions is controlled to be Zn ²⁺:Al³⁺ =2:3, the pH value of the solution is regulated to be 11 by using 5mol/L NaOH solution, the solution is placed in a hydrothermal reactor and reacted for 9 hours at 115 ℃, and the obtained product is centrifuged and then placed in a blast drying oven for drying at 35 ℃ to obtain the ZnAl-LDH.

After pretreatment of supernatant of wastewater treatment plant, adding orthophosphate and ammonium salt to adjust N: P=1:1 in supernatant, adding ZnAl-LDH into the supernatant, adjusting pH=12 by using 5mol/L NaOH solution, fully stirring by using a magnetic stirrer, dissolving a certain amount of MgCl ₂·6H₂ O into deionized water according to the ion ratio of N: P=1:1:1.4, fully stirring, adding the mixture into the supernatant, reacting for 8-10 hours, centrifuging the obtained precipitate at 8000rpm for 8 minutes, and drying in a blast drying oven at 35 ℃ to obtain ZnAl-MAP.

ZnAl-MAP and talcum powder are mixed according to the mass ratio of 88:12, and are treated for 35 minutes by a horizontal planetary ball mill under the condition of 350 r/mm to be crushed and refined, so that MAP powder with certain fire extinguishing performance is obtained.

Example 4

The embodiment provides a method for synthesizing MAP fire extinguishing dry powder by seed crystal induction, which comprises the following steps:

NiCl ₂·6H₂ O and CrCl ₃·6H₂ O are dissolved in deionized water, the molar ratio of metal ions is controlled to be Ni ²⁺:Cr³⁺ =2.5:3, the pH value of the solution is adjusted to be 11 by using 5mol/L NaOH solution, the solution is placed in a hydrothermal reactor and reacted for 10 hours at 145 ℃, and the obtained product is centrifuged and dried at 35 ℃ in a blast drying box to obtain the NiCr-LDH.

After pretreatment of supernatant of wastewater treatment plant, adding orthophosphate and ammonium salt to adjust N: P=1:1 in supernatant, adding NiCr-LDH into the supernatant, adjusting pH=10 by using 5mol/L NaOH solution, fully stirring by using a magnetic stirrer, dissolving a certain amount of MgCl ₂·6H₂ O into deionized water according to the ion ratio of N: P=1:1:1.3, fully stirring, adding the mixture into the supernatant, reacting for 8-10 hours, centrifuging the obtained precipitate at 8000rpm for 10 minutes, and drying in a blast drying oven at 35 ℃ to obtain NiCr-MAP.

Mixing NiCr-MAP and active carbon according to the mass ratio of 98:2, and treating for 45 minutes by using a horizontal planetary ball mill under the condition of 320 r/mm to crush and refine to obtain MAP powder with certain fire extinguishing performance.

Example 5

The embodiment provides a method for synthesizing MAP fire extinguishing dry powder by seed crystal induction, which comprises the following steps:

The CoCl ₂·6H₂ O and the CrCl ₃·6H₂ O are dissolved in deionized water, the molar ratio of metal ions is controlled to be Co ²⁺:Cr³⁺ =3:2, the pH value of the solution is adjusted to be 11 by using 5mol/L NaOH solution, the solution is placed in a hydrothermal reactor and reacted for 12 hours at 150 ℃, and the obtained product is centrifuged and then placed in a blast drying oven for drying at 35 ℃ to obtain the CoCr-LDH.

After pretreatment of supernatant of wastewater treatment plant, adding orthophosphate and ammonium salt to adjust N: P=1:1 in supernatant, adding CoCr-LDH into the supernatant, adjusting pH=9.5 by using 5mol/L NaOH solution, fully stirring by using a magnetic stirrer, dissolving a certain amount of MgCl ₂·6H₂ O into deionized water according to the ion ratio of N: mg=1:1:1.6, fully stirring, adding the mixture into the supernatant, reacting for 8-10 hours, centrifuging the obtained precipitate at 8000rpm for 10 minutes, and drying at 35 ℃ in a blast drying oven to obtain CoCr-MAP.

Mixing CoCr-MAP and acidified sepiolite according to the mass ratio of 85:15, and treating for 50min under the condition of 300 r/mm by using a horizontal planetary ball mill to crush and refine to obtain MAP powder with certain fire extinguishing performance.

The dry powder performance test experiment is divided into a class A fire experiment and a class B fire experiment, and comprises the following specific steps:

(1) Class A fire experiment

1) Weighing 50g of commercial ABC dry powder, mgAl-MAP powder, mgFe-MAP powder, znAl-MAP powder, niCr-MAP powder and CoCr-MAP powder for fire extinguishing experiments, weighing the residual weight after the fire extinguishing experiments, recording the dry powder consumption and the fire extinguishing time, repeating each group at least three times, and taking an average value;

2) The nitrogen with the pressure of 0.4MPa is used as a driving force, and the dry powder is released to a fire scene through a nozzle positioned right above a fire source;

3) Putting 30ml of n-heptane into a 200X 20mm oil basin as a uniform ignition source, placing a 4X 4 small wood pile consisting of 16 small wood strips with the length of 15X 150mm right above the oil basin, igniting the oil basin by using an ignition gun, timing for 30s after the oil basin is burnt out, and releasing a fire extinguishing agent;

4) The whole experimental process is carried out on a 1m ³ small fire-extinguishing platform as shown in figure 2;

(2) Class B fire experiment

1) Weighing 50g of commercial ABC dry powder, mgAl-MAP powder, mgFe-MAP powder, znAl-MAP powder, niCr-MAP powder and CoCr-MAP powder for fire extinguishing experiments, weighing the residual weight after the fire extinguishing experiments, recording the dry powder consumption and the fire extinguishing time, repeating each group at least three times, and taking an average value;

2) The nitrogen with the pressure of 0.4MPa is used as a driving force, and the dry powder is released to a fire scene through a nozzle positioned right above a fire source;

3) 70ml of n-heptane and 80ml of deionized water are taken to be put into a 200 multiplied by 20mm oil basin, the oil basin is ignited by a flame gun, and the flame is kept stable by precombustion for 25 seconds;

4) The whole experimental process is carried out on a 1m ³ small fire-extinguishing platform as shown in figure 2;

the relevant performance characterization and parameters of the fire extinguishing dry powder are as follows:

(1) Sample morphology observation the morphology of the different LDHs was observed using a scanning electron microscope analyzer (zeck TESCAN MIRA LMS), as shown in figure 3, and it was found that LDHs in the five groups of examples exhibited typical duplex metal hydroxide morphology. The LDH in examples 1, 2 and 3 is formed by stacking a plurality of nano sheets in a 'card house' type layered manner, the LDH in example 4 is formed by stacking a plurality of nano sheets in a staggered manner to form a flower-shaped structure, interlayer gaps are clearly visible, the material has an ordered layered structure, the LDH in example 5 is stacked by stacking a plurality of ultrathin nano sheets in a 'petal' type manner, and uniform mesopores exist between the layers, so that MAP can grow on the surface of the material. Fig. 3 (f) shows the morphology of LDH-MAP, which partially shows a lamellar structure and partially has bulk crystals, indicating that during crystallization, MAP is crystallized and coated on the surface of LDH with LDH as a core.

(2) XRD test the samples were tested using an X-ray diffractometer (Japanese Rigaku SmartLab SE), and it can be seen from FIG. 4 that the characteristic peaks of the samples in the different examples are in agreement with the characteristic peaks of the LDH standard card (PDF 14-0191) and the MAP standard card (PDF 15-0762), confirming that seed-induced synthesis of MAP does not allow chemical reaction of MAP with LDH to generate new chemical phases.

(3) BET test commercial ABC dry powder, non-seed induced MAP, mgAl-MAP powder, mgFe-MAP powder, znAl-MAP powder, niCr-MAP powder and CoCr-MAP powder were tested with an adsorption and desorption instrument (Micromeritics ASAP 2460) and specific surface area, pore size and pore volume were obtained as shown in FIG. 5 and Table 1.

TABLE 1 BET data for different samples

From the above table and the graph, the specific surface areas of different LDH-induced synthetic MAP dry powders are larger than that of commercial ABC dry powder, which means that the contact area between powder particles and flame is larger, the effective fire extinguishing components can be rapidly released and pyrolyzed more fully, and the specific surface areas of the powder in examples 1 and 4 are larger than that of MAP without seed crystal induction, which further proves that the pyrolysis is more fully and the flame inhibition effect is better.

(4) TG-DSC is used for explaining the fire extinguishing mechanism, wherein a PERKINELMER STA synchronous thermal analyzer is used for testing commercial ABC dry powder, non-seed crystal induction MAP, mgAl-MAP powder, mgFe-MAP powder, znAl-MAP, niCr-MAP and CoCr-MAP powder to obtain a TG-DSC curve, and the fire extinguishing mechanism is explained according to thermogravimetric data, as shown in figure 6.

As can be seen from the figure, the heat absorption capacity of commercial ABC dry powder is 558.79J/g, the heat absorption capacities of blank struvite and examples 1,2, 3,4 and 5 are 572.97J/g, 815.83J/g, 605.93J/g, 653.35J/g, 927.30J/g and 523.15J/g respectively, the heat absorption capacities are equivalent to those of commercial ABC, the heat absorption capacities of examples 1,2, 3 and 4 are slightly higher than those of ABC dry powder, the heat stability of the LDH-induced synthesized MAP powder is improved, and the heat stability is improved from the initial 80 ℃ to 95-105 ℃, so that the heat absorption capacity is favorable for stock storage, and the pyrolysis is favorable for restarting after the fire extinguishing powder reaches the flame root, so that the fire extinguishing effect is more fully exerted.

(5) A, B kinds of fire extinguishing video screenshot (0.4 MPa driving air pressure is taken as an example) is shown in FIG. 7, wherein the extinguishing time of NiCr-MAP powder is shortest, mgAl-MAP powder is next, and the contrast effect of commercial ABC dry powder is worst.

(6) The change in CO concentration of the fire in different fire extinguishing experiments was tested using testo 350,350 flue gas analyzer, as shown in figure 8. The highest CO concentrations of the fire sites of the commercial ABC dry powder, the non-seeding induced MAP powder, and the different samples of examples 1,2, 3, 4, and 5 were 163ppm, 138ppm, 130ppm, 126ppm, 141ppm, 122ppm, and 145ppm, respectively, which demonstrated that the introduction of LDH as a seed in the synthesis of MAP was effective in improving the smoke suppression effect of the samples.

The performance of the MAP and commercial ABC dry powders in the five examples described above is shown in Table 2 and FIG. 9, where D50 (μm) is the particle size corresponding to a sample cumulative particle size distribution percentage of 50% and D90 (μm) is the particle size corresponding to a sample cumulative particle size distribution percentage of 90%.

TABLE 2 commercial ABC Dry powders and results of class B fire Performance test for each example (taking 0.4MPa drive pressure as an example)

The data comparison shows that the relevant performance parameters of the seed crystal-induced synthetic MAP prepared by the invention reach the level of commercial ABC dry powder, but the fire extinguishing time and the dry powder consumption during fire extinguishment are obviously superior to those of commercial ABC dry powder, the effect is obvious, and the overall fire extinguishing performance is obviously improved.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (10)

1. A method for preparing a layered double hydroxide, comprising the steps of:

Mixing soluble divalent metal ion salt and soluble trivalent metal ion salt in deionized water, regulating pH value of the mixed solution to be alkaline by using NaOH, placing the mixed solution in a hydrothermal reactor for reacting for a certain time, and centrifugally drying the precipitate product to obtain the layered double hydroxide with corresponding metal composition.

2. The method for producing a layered double hydroxide according to claim 1, wherein the soluble divalent metal ion comprises at least one of Mg ²⁺、Zn²⁺、Ni²⁺、Co²⁺、Mn²⁺、Cu²⁺、Cd²⁺、Sn²⁺, the soluble trivalent metal ion comprises at least one of Al ³+、Fe³⁺、Cr³⁺、Bi³⁺、La³⁺、Ce³⁺, and the molar ratio of the soluble divalent metal ion to the soluble trivalent metal ion is (1-3): 1-3.

3. The method for preparing the layered double hydroxide according to claim 1, wherein the layered double hydroxide is prepared by the reaction at a desired pH of 7.5-14 and a desired temperature of 80-170 ℃ and a temperature-programmed temperature of 5-10 ℃ per minute for 8-15 hours.

4. The method for producing a layered double hydroxide according to claim 1, wherein the centrifugal speed required for recovery of the obtained product is 6000 to 8000rpm, the centrifugal time is 5 to 10 minutes, the drying temperature of the obtained product is 30 to 35 ℃ and the drying time is 25 to 30 hours.

5. The preparation method of the seed crystal induced synthesized struvite fire extinguishing powder is characterized by comprising the following steps of:

Selecting supernatant of a sewage treatment plant as a raw material, adding a solid porous medium, fully stirring uniformly, standing for precipitation, filtering to remove heavy metal ions, adding a certain amount of ammonium salt and phosphate into the supernatant to adjust the nitrogen-phosphorus ion ratio, then adding the layered double hydroxide obtained by the method as claimed in claim 1 as seed crystal, stirring to fully mix the seed crystal with the supernatant uniformly, adjusting the pH of the supernatant to be alkaline by using a NaOH solution, then adding a magnesium salt to adjust the nitrogen-phosphorus-magnesium ion ratio in the supernatant, stirring at normal temperature, standing for precipitation for a certain time, centrifuging to obtain bottom precipitation, drying to obtain seed crystal induced struvite particles, mixing the obtained struvite particles with a modifier according to a certain proportion, and then using a horizontal planetary ball mill to crush and refine to obtain struvite fire extinguishing powder which meets the requirements of fire extinguishing powder and has an average particle size of 20-35 mu m.

6. The preparation method according to claim 5, wherein the solid porous medium is one or a combination of at least two of activated carbon, chitosan resin, montmorillonite and acidified sepiolite, the ammonium salt is selected from (one or a combination of at least two of NH ₄)₃PO₄·3H₂O、NH₄Cl、(NH₄)₂CO₃, the phosphate is one or a combination of at least two of Na₃PO₄·12H₂O、(NH₄)₃PO₄·3H₂O、NaH₂PO₄、Na₂HPO₄·12H₂O、NaH₂PO₄·12H₂O、NH₄H₂PO₄、(NH₄)₂HPO₄, the magnesium salt is one or a combination of at least two of MgCl ₂·6H₂O、Mg(NO₃)₂、MgSO₄, and the molar ratio of nitrogen to phosphorus to magnesium ions is N: P = (0.8-1.5): (0.8-1.8): (1-1.8).

7. The process according to claim 5, wherein the precipitation reaction requires an ambient pH of 8-13, a temperature of 25-35 ℃ and a duration of 3-5 hours.

8. The method according to claim 5, wherein the centrifugal speed of the precipitated product is 6000-8000rpm, the centrifugal time is 8-10 minutes, the drying temperature is 30-35 ℃, the ball milling speed of the mixed and crushed precipitated product and modifier is 300-400rpm, and the ball milling time is 30-60 minutes.

9. The preparation method of the composite material according to claim 5, wherein the modifier is one or a combination of at least two of talcum powder, hydrophobic silicon dioxide, magnesium stearate, active carbon, acidified sepiolite and aluminum zirconium pillared montmorillonite, and the mass ratio of the struvite particles to the modifier is (75-95) (5-25).

10. A seed-induced struvite fire extinguishing powder prepared according to any one of claims 5 to 9, wherein the fire extinguishing powder has a D50 of 10 to 15 μm, a D90 of 20 to 35 μm, and a specific surface area of 25 to 90m ²/g.