CN110117732B - Method for coating MgO protective layer on surface of hollow microsphere - Google Patents
Method for coating MgO protective layer on surface of hollow microsphere Download PDFInfo
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- CN110117732B CN110117732B CN201910474206.0A CN201910474206A CN110117732B CN 110117732 B CN110117732 B CN 110117732B CN 201910474206 A CN201910474206 A CN 201910474206A CN 110117732 B CN110117732 B CN 110117732B
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- C22C1/00—Making non-ferrous alloys
- C22C1/08—Alloys with open or closed pores
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C1/00—Making non-ferrous alloys
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Abstract
The invention discloses a method for coating a MgO protective layer on the surface of a cenosphere, which aims to solve the problem that the cenosphere is easy to crack in the preparation process of a cenosphere/magnesium alloy porous composite material and belongs to the field of surface modification of superfine powder materials. The method comprises the steps of firstly carrying out acid washing, alkali washing, sorting and drying on the cenosphere, then carrying out coupling and coating treatment, and finally carrying out calcination to obtain the cenosphere coated with the MgO protective layer. The MgO coating layer prepared by the invention is uniform and compact, has high bonding strength with the cenosphere, is not easy to fall off, and can isolate the cenosphere from the magnesium alloy melt in the preparation process of the composite material, thereby preventing the cenosphere from cracking. In addition, the MgO coating layer and the magnesium alloy melt have good wettability, so that the hollow microspheres are uniformly dispersed in the magnesium alloy, and the performance of the porous composite material is further improved. The preparation process adopted by the invention is simple, the operation is convenient, the raw materials are low in price, and the industrial production is easy to realize.
Description
Technical Field
The invention belongs to the field of surface modification of superfine powder materials, and relates to a method for coating a MgO protective layer on the surface of a hollow microsphere.
Background
The porous metal material is a new material which is emerging in recent years, and has a plurality of excellent characteristics in performance due to the special structure of the metal matrix and the pores. It not only inherits the excellent performance of metal material, but also has the excellent characteristics of porous material. Compared with non-metal materials such as high polymer, ceramic and the like, the material has the advantages of high temperature resistance, good heat conduction and electric conduction performance and high specific strength; compared with the traditional compact metal, the metal has low density, large specific surface area, and a plurality of excellent performances of sound attenuation, vibration resistance, electromagnetic shielding and the like. Therefore, the porous metal material is widely applied to the fields of aerospace, national defense, energy, chemical engineering, traffic and the like. The preparation methods of the porous metal material mainly comprise a solid phase method, a liquid phase method, a gas phase method and an electrodeposition method. The liquid phase method is a method of preparing a porous metal material by blowing gas or adding a foaming agent, a solid filler, or the like before the liquid metal is solidified. The solid phase method is a method of sintering a solid phase metal to prepare a porous metal material, and the metal remains in a solid state during the preparation process. The vapor phase method is a method in which metal is heated to be evaporated under a vacuum condition, and is deposited on a matrix material such as a polymer after being cooled, and then the matrix material is removed to obtain a porous metal material. The porous metal material prepared by the electrodeposition method has the advantages of uniform structure, excellent mechanical property and the like, but has high preparation cost and is easy to cause environmental pollution. The liquid phase method is the most widely applied method at present due to the advantages of simple operation process, low preparation cost, easy realization of large-scale production and the like.
The hollow microsphere is a thin-wall hollow microsphere with extremely small particle size, and mainly comprises a fly ash floating bead which is a byproduct of a thermal power plant and artificially manufactured hollow glass microspheres. The preparation of the hollow microsphere metal matrix composite material by adding the hollow microsphere into a metal matrix (such as magnesium alloy and the like) is a new method for preparing the porous metal material. Because the hollow microspheres have higher strength, the hollow microspheres can support a metal matrix to a certain degree, and compared with the traditional porous metal material, the hollow microsphere metal matrix composite material usually has higher strength. In addition, the hollow microspheres with uniform particle sizes can be obtained through screening, so that the hollow microsphere metal-based composite material is prepared, and various properties (such as strength, damping property, electromagnetic shielding property and the like) of the composite material can be further improved due to uniform and consistent pores of the composite material. Magnesium and magnesium alloys are the lightest metallic structural materials. The density of the pure magnesium is 1.74g/cm3About 2/3 for aluminum only. The magnesium alloy has high specific strength and specific rigidity and good heat conduction, electromagnetic shielding and damping performances, and is widely concerned by people. The hollow microspheres are added into the magnesium alloy as the reinforcement to prepare the magnesium-based porous composite material, so that the excellent performance of the magnesium alloy can be further improved, and the magnesium alloy has wider application rangeAnd (5) landscape.
However, the problem of cenosphere breakage occurs during the preparation of cenosphere magnesium-based porous composite materials by the liquid phase method. The hollow microsphere mainly comprises SiO2Easily react with magnesium melt at high temperature to generate Mg2And the Si phase reacts to thin the wall of the hollow micro-bead, so that the strength of the hollow micro-bead is reduced, the hollow micro-bead is broken in the melt stirring process, the hollow micro-bead is filled with the magnesium alloy melt, and finally the composite material loses the porous characteristic. Some solutions have been proposed by researchers to address this problem. K.N. Braszczynski-Malik et al (K.N. Braszczynski-Malik, J.Kamieniak.analysis of interface beta components in AZ91 magnesium alloy foam composites with Ni-P coated flash cenospheres. journal of Alloys and composites, 2017, (720): 352.) of Czestochowa University of Technology prepares a floating bead/AZ 91 magnesium alloy porous composite with a density as low as 1.1893g/cm by electroless plating on the surface of the floating bead of fly ash and then adding the whole floating bead to the magnesium alloy by pressure infiltration3However, the raw materials are expensive, and the preparation of the porous composite material needs special equipment, so that the preparation cost is greatly increased. In the prior art, the tension of Harbin university (application No. 201410653334.9 and application No. 201310275451.1) adopts a sol-gel method to coat Al on the surfaces of hollow microspheres2O3The protective film and the MgO protective film are used for solving the problem that the hollow microspheres are easy to react and break in the preparation process of the hollow microsphere/aluminum-magnesium alloy porous composite material, so that the material loses the porous characteristic, but the sol-gel method has complex preparation process, needs special equipment and has higher preparation cost, and the hollow microspheres coated by the method are easy to agglomerate. In addition, the application effect of the cenospheres treated by the technology in the cenospheres/magnesium alloy porous composite material is not reported.
The invention adopts the process of precipitation and calcination to coat the MgO protective layer on the surface of the hollow microsphere. The MgO protective layer is coated on the surfaces of the cenospheres by regulating and controlling the technological parameters of the pretreatment process, the precipitation chemical reaction and the calcination of the cenospheres, so that the problem that the cenospheres are broken to cause the porous characteristic of the composite material to be lost in the preparation process of the cenospheres/magnesium alloy porous composite material is solved, and a foundation is laid for preparing the porous magnesium-based composite material with excellent performance.
Disclosure of Invention
The invention aims to solve the problem that hollow microspheres are easy to crack in the preparation process of a hollow microsphere/magnesium alloy porous composite material, and the surface of the hollow microspheres is coated with a protective layer, in particular to a method for coating the surface of the hollow microspheres with an MgO protective layer.
In order to achieve the purpose, the invention adopts the technical scheme that: firstly, carrying out acid washing and alkali washing on the hollow microspheres to remove oil stains and impurities on the surfaces; secondly, sorting and drying the hollow microspheres, selecting the hollow microspheres with proper particle size and removing incomplete microspheres; then carrying out coupling and coating treatment to obtain the hollow microspheres coated with magnesium carbonate or basic magnesium carbonate; and finally, calcining the magnesium carbonate or basic magnesium carbonate coated hollow microspheres to obtain the MgO coated hollow microspheres. The steps of alkali washing, acid washing, sorting and drying can refer to the prior art, and the following specific process steps can also be adopted:
(1) acid washing: adding a certain amount of hollow microspheres into 0.5-1.5mol/L sulfuric acid solution, stirring for 10-50min at the stirring speed of 150-500r/min at the solid-liquid mass ratio of the hollow microspheres to the sulfuric acid solution of 1:5-1:25, repeatedly rinsing the microspheres with deionized water after stirring until the pH value is 7, and performing suction filtration;
(2) alkali washing: replacing sulfuric acid solution with 0.5-1.5mol/L NaOH solution to repeat the step (1);
after the hollow microspheres are subjected to acid washing and alkali washing, impurities and oil stains on the surfaces can be removed, agglomerated hollow microspheres can be scattered, hydroxylation on the surfaces of the hollow microspheres can be enhanced, and therefore the capability of adsorbing a silane coupling agent on the surfaces of the hollow microspheres is improved.
(3) Sorting and drying: putting the cenospheres treated in the step (2) into deionized water, standing for 30min, removing settled powder, taking out suspended cenospheres, performing suction filtration, drying for 12h at 80 ℃, and then selecting the cenospheres with a certain particle size range by using a sample separation sieve to obtain clean cenospheres with a certain particle size;
the following steps are the biggest difference between the present invention and the prior art, and the inventors particularly provide the following steps for the cenospheres to be treated by the present invention:
(4) coupling: adding a certain amount of 3-aminopropyltrimethoxysilane into a mixed solution of ethanol and deionized water, wherein the volume ratio of the ethanol to the deionized water is 90:10, and the volume ratio of the 3-aminopropyltrimethoxysilane to the ethanol solution is 0.5:100-2:100, stirring and hydrolyzing for 20min at room temperature, adding the hollow microspheres treated in the step (3), stirring for 1-2h under the water bath heating condition at 75 ℃, and stirring at the speed of 150-; after stirring, taking out the hollow microspheres for suction filtration, and drying for 12h at the temperature of 80 ℃;
after the treatment of the step (4), the-OCH in the 3-aminopropyl trimethoxy silane molecule3The group is combined with the surface of the hollow microsphere through hydrolysis, condensation and other reactions, and the-NH at the other end of the molecule2The groups extend outside and can react with magnesium ions in the coating solution, so that the capability of the magnesium ions to be adsorbed on the surface of the aminated hollow microsphere is improved, and the binding force of the hollow microsphere and the coating layer is further enhanced. In addition, the surface energy of the hollow microspheres after coupling treatment is greatly reduced, and the agglomeration tendency of the hollow microspheres is further reduced.
(5) Coating: adding the hollow microspheres treated in the step (4) into deionized water, wherein the solid-liquid mass ratio of the hollow microspheres to the deionized water is 1:50, then adding an emulsifier OP-10 while stirring, wherein the volume ratio of the emulsifier to the deionized water is 0.5:100-2:100, the stirring speed is 150-; secondly, coating the hollow microspheres by a precipitation method, and adding a certain amount of MgCl2·6H2O is added into the mixed solution and heated to 30-99 ℃, and simultaneously stirred for 30min at the speed of 150-2·6H2The amount of O added is MgCl2·6H2Adding O and the hollow microspheres in a mass ratio of 1:1-15: 1; then a certain amount of NaHCO 2mol/L3Gradually dropping the solution into the mixed solution at a dropping speed of 1-5mL/min while continuing stirring at a rate of 500r/min for 180min, and adding NaHCO3Is added according to NaHCO3With MgCl2·6H2Adding O in a mass ratio of 1:1-4: 1; after stirring is stopped, standing the mixed solution for 60min, taking out the hollow microspheres, repeatedly rinsing the hollow microspheres for 5 times by using deionized water, performing suction filtration, and finally drying the hollow microspheres for 12h at the temperature of 80 ℃ to obtain the hollow microspheres coated by magnesium carbonate or basic magnesium carbonate;
(6) and (3) calcining: and (4) putting the hollow microspheres processed in the step (5) into a resistance furnace, heating to 350-700 ℃ along with the furnace, preserving heat for 3-10h, and cooling to room temperature along with the furnace after the heat preservation is finished to obtain the hollow microspheres coated with the MgO protective layer.
Compared with the prior art, the invention has the beneficial effects that:
(1) the method adopts the processes of precipitation and calcination to coat the MgO protective layer on the surface of the hollow microsphere, has simple process and convenient operation, does not need expensive equipment and raw materials, and is easy for industrial production;
(2) according to the invention, the hollow microspheres are subjected to coupling treatment before coating, and the emulsifier is added in the coating process, so that the hollow microspheres can be uniformly dispersed in the mixed solution, and are prevented from agglomerating in the coating process, and thus the coating layer is uniform and compact;
(3) compared with coating layers of other components, the MgO coating layer has good wettability with the magnesium alloy melt, and can effectively avoid the agglomeration phenomenon of the cenospheres in the magnesium alloy, so that the cenospheres are uniformly dispersed in the magnesium alloy;
(4) the MgO coating layer prepared by the invention has high bonding strength with the hollow microspheres and is not easy to fall off; in the process of preparing the hollow microsphere/magnesium alloy porous composite material by adopting the MgO-coated hollow microsphere, the MgO protective layer can isolate the hollow microsphere and the magnesium alloy melt, thereby preventing the hollow microsphere from cracking;
(5) the preparation process adopted by the invention has wide applicability, can coat the MgO protective layer on the surface of the hollow microsphere and also can coat the MgO protective layer on the surface of other inorganic ultrafine powder, and has wide application prospect.
Drawings
FIG. 1 is an SEM image of cenospheres before acid and alkali washing;
FIG. 2 is an SEM image of cenospheres after acid and base washing;
FIG. 3 is an SEM photograph of MgO-coated cenospheres obtained in example 1 of the present invention;
FIG. 4 is XRD patterns of MgO-coated cenospheres and uncoated MgO cenospheres obtained in example 1 of the present invention;
FIG. 5 is a metallographic photograph of a cenosphere/magnesium alloy composite material prepared using cenospheres not coated with MgO;
FIG. 6 is a metallographic photograph of a cenosphere/magnesium alloy composite material prepared using the MgO-coated cenosphere obtained in example 1;
FIG. 7 is an SEM photograph of MgO-coated cenospheres obtained in example 2 of the present invention;
FIG. 8 is an SEM photograph of MgO-coated cenospheres obtained in example 3 of the present invention;
FIG. 9 is an SEM photograph of MgO-coated cenospheres obtained in example 4 of the present invention;
FIG. 10 is an SEM photograph of MgO-coated cenospheres obtained in example 5 of the present invention.
In the figure, FIG. 1 is an SEM image (scanning electron microscope image) of cenospheres which are not subjected to acid washing and alkali washing, and it can be seen that more impurities exist on the surface of cenospheres; FIG. 2 is an SEM image of the cenosphere after acid washing and alkali washing, and it can be seen that the cenosphere is cleaned after acid washing and alkali washing, and no impurities are attached to the surface; FIG. 3 is an SEM photograph of MgO-coated cenospheres obtained in example 1 of the present invention, and it can be seen that MgO has been uniformly coated on the surfaces of the cenospheres; as can be seen from FIG. 4, the main component of the cenospheres is amorphous SiO2The coated hollow microspheres have MgO phase, and further prove that the substance coated on the surfaces of the hollow microspheres is MgO; FIG. 5 is a metallographic photograph of a cenosphere/magnesium alloy composite material prepared using cenosphere not coated with MgO, and it can be seen that the cenosphere is ruptured and filled with a magnesium alloy melt during the preparation of the composite material, and the composite material melt is solidified to obtain a set as shown in FIG. 5The composite material loses the porous characteristic due to the rupture of the hollow microspheres; the MgO coating layer prepared by the invention has high bonding strength with the hollow microspheres, the coating layer is not easy to fall off in the preparation process of the composite material, and the hollow microspheres are protected, as shown in figure 6, the MgO coated hollow microspheres are not broken, still keep a hollow structure and are uniformly dispersed in a matrix, so that the composite material has a plurality of excellent properties of a porous material; FIGS. 7 to 10 are SEM images of the MgO-coated cenospheres obtained in examples 2 to 5 of the present invention, respectively, and it can be seen that, when the coating process parameters are changed within a certain range, a uniform and dense MgO coating layer can still be obtained on the surface of the cenospheres, and the MgO coating layer can also protect the cenospheres and prevent the cenospheres from being cracked during the preparation of the porous composite material.
Detailed Description
The present invention will be further described with reference to specific embodiments, in which the pretreatment is as follows:
firstly, adding a certain amount of hollow microspheres into 0.5-1.5mol/L sulfuric acid solution, wherein the solid-liquid mass ratio of the hollow microspheres to the sulfuric acid solution is 1:5-1:25, stirring for 10-50min at the stirring speed of 150-500r/min, repeatedly rinsing the microspheres with deionized water after stirring until the pH value is 7, and performing suction filtration; then, the above steps are repeated by using 0.5-1.5mol/L NaOH solution instead of sulfuric acid solution.
And then putting the hollow microspheres subjected to alkali washing into deionized water, standing for 30min, removing settled powder, taking out suspended hollow microspheres, performing suction filtration, drying at 80 ℃ for 12h, and finally selecting hollow microspheres with a certain particle size range by using a sample separation sieve to obtain clean hollow microspheres with a certain particle size.
Specific example 1: method for coating MgO protective layer on surface of hollow microsphere
After the pretreatment, the method comprises the following specific steps:
(1) coupling: adding a certain amount of 3-aminopropyltrimethoxysilane into a mixed solution of ethanol and deionized water, wherein the volume ratio of the ethanol to the deionized water is 90:10, and the volume ratio of the 3-aminopropyltrimethoxysilane to the ethanol solution is 1:100, stirring and hydrolyzing for 20min at room temperature, adding the pretreated hollow microspheres, and stirring for 2h under the water bath heating condition of 75 ℃, wherein the stirring speed is 400 r/min; after stirring, taking out the hollow microspheres for suction filtration, and drying for 12h at the temperature of 80 ℃;
(2) coating: adding the cenospheres treated in the step (1) into deionized water, wherein the solid-liquid mass ratio of the cenospheres to the deionized water is 1:50, then adding an emulsifier OP-10 while stirring, the volume ratio of the emulsifier to the deionized water is 1:100, the stirring speed is 400r/min, and stirring for 30min to uniformly disperse the cenospheres in the solution; secondly, coating the hollow microspheres by a precipitation method, and adding a certain amount of MgCl2·6H2Adding O into the mixed solution, heating to 90 ℃, and stirring at the speed of 400r/min for 30min, wherein the mixture is MgCl2·6H2The amount of O added is MgCl2·6H2Adding the O and the hollow microspheres in a mass ratio of 10: 1; then a certain amount of NaHCO 2mol/L3Gradually dropwise adding the solution into the mixed solution at a dropwise adding speed of 2mL/min while continuing stirring at a speed of 400r/min for 180min, and adding NaHCO3Is added according to NaHCO3With MgCl2·6H2The amount ratio of O is 2.5: 1; after stirring is stopped, standing the mixed solution for 60min, taking out the cenospheres, repeatedly rinsing the cenospheres for 5 times by using deionized water, performing suction filtration, and finally drying the cenospheres for 12h at the temperature of 80 ℃ to obtain the cenospheres coated by basic magnesium carbonate;
(3) and (3) calcining: and (3) putting the hollow microspheres processed in the step (2) into a resistance furnace, heating to 550 ℃ along with the furnace, preserving heat for 5 hours, and cooling to room temperature along with the furnace after the heat preservation is finished to obtain the hollow microspheres coated with the MgO protective layer.
The pretreatment mainly comprises acid washing, alkali washing, sorting and drying, and aims to obtain clean hollow microspheres with a certain particle size. After the hollow microspheres are subjected to acid washing and alkali washing, impurities and oil stains on the surfaces can be removed, and hydroxylation on the surfaces of the hollow microspheres can be strengthened, so that the capability of the hollow microspheres for adsorbing a silane coupling agent is improved. As can be seen from a comparison of FIGS. 1 and 2, the impurities and oil stains on the surfaces of the cenospheres were removed after the acid washing and the alkali washing, and clean cenospheres were obtained.
FIG. 3 is an SEM image of MgO-coated cenospheres, which shows that MgO has been uniformly and densely coated on the surface of the cenospheres. FIG. 4 is XRD patterns of MgO-coated cenospheres and uncoated MgO cenospheres, and it can be seen that the main component of the cenospheres is amorphous SiO2And the MgO phase is generated after coating, and further proves that the substance coated on the surface of the hollow microsphere is MgO. In order to examine the bonding strength between the MgO coating layer and the cenospheres, the cenospheres not coated with MgO and the cenospheres coated with MgO were respectively added to the magnesium alloy to prepare a porous composite material, and the metallographic structure of the obtained composite material was as shown in fig. 5 and 6. It can be seen that the hollow micro-beads which are not coated with MgO are broken and filled with magnesium alloy in the preparation process of the composite material, so that the composite material loses the porous characteristic; the MgO-coated cenosphere does not crack, and still keeps a hollow structure, which shows that the bonding strength of the MgO coating layer and the cenosphere is high, and the MgO coating layer and the cenosphere are not easy to fall off, and the cenosphere and the magnesium alloy melt can be isolated in the preparation process of the composite material, so that the cenosphere is prevented from cracking. In addition, as the MgO and the magnesium alloy melt have good wettability, the MgO-coated cenospheres can be uniformly dispersed in the magnesium alloy matrix, and the performance of the porous composite material is further improved.
Specific example 2:
this example differs from the specific example 1 in that the volume ratio of 3-aminopropyltrimethoxysilane to the ethanol solution in step (1) is 2: 100. The rest is the same as in embodiment 1.
Specific example 3:
this example differs from the specific example 1 in that MgCl is added in step (2)2·6H2The mixture of O was heated to 70 ℃. The rest is the same as in embodiment 1.
Specific example 4:
this example differs from the specific example 1 in that NaHCO is used in step (2)3The dropping rate of the solution was 5 mL/min. The rest is the same as in embodiment 1.
Specific example 5:
this example differs from the specific example 1 in that MgCl is used in step (2)2·6H2The amount of O added is MgCl2·6H2Adding the O and the cenospheres in a mass ratio of 5:1, and adding NaHCO3Is added according to NaHCO3With MgCl2·6H2The mass ratio of O was 3: 1. The rest is the same as in embodiment 1.
SEM topography maps of the MgO-coated cenospheres obtained in specific examples 2-5 are shown in FIGS. 7-10. It can be seen that the surface of the hollow microsphere can still be coated with a uniform and compact MgO protective layer by adopting different process parameters. The MgO-coated cenospheres obtained in examples 2 to 5 were added to a magnesium alloy to prepare a porous composite material, and it was found that the MgO coating layers and the cenospheres prepared in each example have high bonding strength, do not fall off during the preparation of the composite material, and can protect the cenospheres, thereby preventing the cenospheres from being broken.
Claims (2)
1. A method for coating a MgO protective layer on the surface of a hollow microsphere comprises acid washing, alkali washing, sorting and drying, and is characterized by further comprising the following steps after sorting and drying:
(1) coupling: adding a certain amount of 3-aminopropyltrimethoxysilane into a mixed solution of ethanol and deionized water, wherein the volume ratio of the ethanol to the deionized water is 90:10, and the volume ratio of the 3-aminopropyltrimethoxysilane to the ethanol solution is 0.5:100-2:100, stirring and hydrolyzing for 20min at room temperature, adding the sorted and dried hollow microspheres, stirring for 1-2h under the water bath heating condition of 75 ℃, and the stirring speed is 150-; after stirring, taking out the hollow microspheres for suction filtration, and drying for 12h at the temperature of 80 ℃;
(2) coating: adding the hollow microspheres treated in the step (1) into deionized water, wherein the solid-liquid mass ratio of the hollow microspheres to the deionized water is 1:50, then adding an emulsifier OP-10 while stirring, wherein the volume ratio of the emulsifier to the deionized water is 0.5:100-2:100, the stirring speed is 150-; secondly, a certain amount of MgCl2·6H2O is added to the above mixture, MgCl2·6H2The mass ratio of the O to the hollow microspheres is 1:1-15:1, the mixture is heated to 30-99 ℃, and the mixture is stirred for 30min at the speed of 150-; then a certain amount of NaHCO 2mol/L3Gradually adding dropwise the solution into the above mixed solution at a dropwise rate of 1-5mL/min, wherein NaHCO is3With MgCl2·6H2The mass ratio of O is 1:1-4:1, and the stirring is continued for 180min at the speed of 150-500 r/min; after stirring is stopped, standing the mixed solution for 60min, taking out the hollow microspheres, repeatedly rinsing the hollow microspheres for 5 times by using deionized water, performing suction filtration, and finally drying the hollow microspheres for 12h at the temperature of 80 ℃ to obtain the hollow microspheres coated by magnesium carbonate or basic magnesium carbonate;
(3) and (3) calcining: and (3) putting the hollow microspheres processed in the step (2) into a resistance furnace, heating to 350-700 ℃ along with the furnace, preserving heat for 3-10h, and cooling to room temperature along with the furnace after the heat preservation is finished to obtain the hollow microspheres coated with the MgO protective layer.
2. The method for coating the surface of the cenosphere with the MgO protective layer according to claim 1, wherein the steps of acid washing, alkali washing, sorting and drying are as follows:
(1) acid washing: adding a certain amount of hollow microspheres into 0.5-1.5mol/L sulfuric acid solution, stirring for 10-50min at the stirring speed of 150-500r/min at the solid-liquid mass ratio of the hollow microspheres to the sulfuric acid solution of 1:5-1:25, repeatedly rinsing the microspheres with deionized water after stirring until the pH value is 7, and performing suction filtration;
(2) alkali washing: replacing sulfuric acid solution with 0.5-1.5mol/L NaOH solution to repeat the step (1);
(3) sorting and drying: and (3) putting the cenospheres treated in the step (2) into deionized water, standing for 30min, removing settled powder, taking out suspended cenospheres, performing suction filtration, drying for 12h at the temperature of 80 ℃, and then selecting the cenospheres with a certain particle size range by using a sample separation sieve to obtain clean cenospheres with a certain particle size.
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