阅读说明：本技术 一种球状纳米三氧化二铬的制备方法 (Preparation method of spherical nano chromium sesquioxide ) 是由 梁建 田森 冯海涛 董亚萍 李波 张波 李武 郑竹林 于 2019-11-28 设计创作，主要内容包括：本发明公开了一种球状纳米三氧化二铬的制备方法。所述制备方法包括：使铬酸铵晶体、添加剂按照摩尔比1:0.2～1:1均匀混合,之后对所获混合物于400～1000℃进行热分解反应,并保温0～12h,获得球状纳米三氧化二铬。所述添加剂包括高温易分解的盐,如硝酸铵、碳酸氢铵和尿素中的任意一种或两种以上的组合。本发明提供的球状纳米三氧化二铬的合成路线短,操作简单,仅需一步热分解即可得到目标产品；利用本发明制备的球状纳米三氧化二铬产品纯度高,三氧化二铬的主含量可以达到99％以上,杂质含量低,不含硅和钒,产品可满足氧化铬绿颜料和磨料的纯度要求,并且其平均粒径为80nm～90nm,粒径尺寸均匀,且分散性较好。(The invention discloses a preparation method of spherical nano chromium sesquioxide. The preparation method comprises the following steps: uniformly mixing ammonium chromate crystals and additives according to a molar ratio of 1: 0.2-1: 1, then carrying out thermal decomposition reaction on the obtained mixture at 400-1000 ℃, and preserving heat for 0-12 h to obtain spherical nano chromium sesquioxide. The additive comprises high-temperature easily-decomposed salt, such as one or more of ammonium nitrate, ammonium bicarbonate and urea. The spherical nano chromium sesquioxide provided by the invention has a short synthetic route and is simple to operate, and a target product can be obtained only by one-step thermal decomposition; the spherical nano chromium sesquioxide product prepared by the method has high purity, the main content of the chromium sesquioxide can reach more than 99 percent, the impurity content is low, silicon and vanadium are not contained, the product can meet the purity requirements of chromium oxide green pigment and grinding material, the average grain diameter is 80-90 nm, the grain diameter size is uniform, and the dispersity is better.)

1. A preparation method of spherical nano chromium sesquioxide is characterized by comprising the following steps:

uniformly mixing ammonium chromate crystals and additives according to a molar ratio of 1: 0.2-1: 1, then carrying out thermal decomposition reaction on the obtained mixture at 400-1000 ℃, and preserving heat for 0-12 h to obtain spherical nano chromium sesquioxide.

2. The method of claim 1, wherein: the main content of ammonium chromate in the ammonium chromate crystal is more than 99.6 percent, the ammonium chromate crystal is in a crystal form, and the color and luster are bright yellow;

and/or, the additive contains ammonium or amino, belongs to a salt which is easy to decompose at high temperature, and is preferably one or the combination of more than two of ammonium nitrate, ammonium bicarbonate and urea.

3. The method of claim 1, wherein: the molar ratio of the ammonium chromate crystals to the additive is 1: 0.5-1: 0.7.

4. The method according to claim 1, comprising: heating the mixture to 400-1000 ℃ at a heating rate of 1-10 ℃/min; preferably, the heating rate is 5-8 ℃/min.

5. The method of claim 1, wherein: the temperature of the thermal decomposition reaction is 800-900 ℃; and/or the heat preservation time is 0.5-5.5 h.

6. The production method according to claim 1, characterized by comprising:

the method comprises the following steps of (1) taking carbon ferrochrome as a working electrode, and at least constructing an electrochemical reaction system together with a counter electrode and electrolyte, wherein the electrolyte comprises ammonia water or a mixed solution of the ammonia water and ammonium chromate;

electrically connecting the working electrode and the counter electrode with the anode and the cathode of a power supply respectively, so that an electrochemical reaction is generated in the electrochemical reaction system, and mixed slurry is obtained;

carrying out solid-liquid separation on the mixed slurry, and then freezing the separated liquid phase system to obtain a solid matter;

and carrying out sublimation drying on the solid matter to obtain ammonium chromate crystals.

7. The method according to claim 6, comprising:

freezing the liquid phase system at-20 to-10 ℃ for 5 to 12 hours to obtain a solid matter;

and then, carrying out sublimation drying on the solid at-50 to-40 ℃ for 6 to 24 hours under the condition that the pressure is 5 to 20Pa to obtain ammonium chromate crystals.

8. The production method according to claim 6, characterized by comprising: the carbon ferrochrome is arranged in an anode frame, and a counter electrode serving as a cathode is arranged on at least one side of the anode frame and is not in contact with the anode frame, or the counter electrode is arranged in the anode frame and is not in contact with the anode frame; preferably, the material of the anode frame comprises titanium or nickel; preferably, the cathode is made of nickel or stainless steel; preferably, the surface of the cathode is coated with an insulating material; preferably, the shape of the cathode or anode frame includes a plate shape, a hole shape, a net shape or a rod shape;

and/or, the preparation method comprises the following steps: controlling the temperature of the electrolyte to be 20-80 ℃ during the electrochemical reaction;

and/or, the preparation method comprises the following steps: controlling the circulation flow of the electrolyte to be 0-72L/h during the electrochemical reaction;

and/or the molar ratio of the ammonia water to the ammonium chromate in the mixed solution is 1: 10-10: 1.

9. The method of claim 1, further comprising: after the thermal decomposition reaction is finished, washing, filtering and drying the obtained product to obtain the spherical nano chromium sesquioxide; preferably, the mass-to-volume ratio of the spherical nano chromium sesquioxide to the washing liquid adopted for washing treatment is 100 mL-1000 mL: 100g of the total weight of the mixture; preferably, the drying treatment temperature is 60-120 ℃, and the time is 0.5-6 h.

10. The production method according to any one of claims 1 to 9, characterized in that: the spherical nano chromium sesquioxide has uniform particle size, and the average particle size is 80-90 nm.

Technical Field

The invention relates to a preparation method of nano chromium sesquioxide, in particular to a preparation method of spherical nano chromium sesquioxide, belonging to the technical field of preparation of inorganic non-metallic materials.

Background

Chromium oxide also known as chromium oxide and chromium oxide green, Cr₂O₃(ii) a Molecular weight 152, hexagonal, dark green; density 5.21g/cm³The Mohs hardness of 9 is next to that of diamond and is similar to that of corundum and silicon carbide; the temperature is stable when being heated, the melting point is 2266 +/-25 ℃, and the boiling point is about 4000 ℃; the refractive index is 2.5, so the coverage rate of the pigment is high. Stable chemical property, insolubility in water and organic solvent, acid and alkali resistance, and capability of resisting light, atmosphere and corrosive gas SO₂、H₂S, and the like, and is stable, i.e., has good weatherability.

Chromium oxide (Cr)₂O₃) The crystal structure of (a) is similar to corundum. Chromium sesquioxide has stable chemical properties and does not react with acid or alkali under ordinary conditions. Chromium sesquioxide, as an important chromium salt product, has wide application in national economy, and can be used as an abrasive, a metallurgical raw material, a thermal spraying material, a catalyst and a refractory material. Some chromium sesquioxide with special shapes have other special purposes, such as high-grade coloring agents, hydrogen absorption materials, sensing electrodes and solar absorbers. The chromium sesquioxide with special appearance comprises large-particle chromium sesquioxide, porous chromium sesquioxide, chromium sesquioxide nanowires and spherical nano chromium sesquioxide。

The nanometer chromium sesquioxide is the most important product, has the most extensive application and is widely applied in the fields of metallurgy and fine chemistry industry. This is because the nano chromium sesquioxide has three significant characteristics, one of which is that it has higher hardness and rigidity than its bulk, and when applied to the nano crystal ceramic, it can provide better hardness and toughness. Secondly, the nano chromium sesquioxide also shows characteristic super paramagnetic performance and can be used as a ferroelectric material. Thirdly, nano chromium sesquioxide is used as a catalyst for many reactions, such as ammonia decomposition, toluene oxidation and dehydrogenation, and plays an active role in redox reactions.

The chromic oxide, sodium dichromate, chromic anhydride and basic chromic sulfate are also four major products of chromium salt. Commercial chromium sesquioxide is mostly made from sodium dichromate, and the yield thereof accounts for about 20% of the consumption of sodium dichromate. The total production capacity of chromium sesquioxide in all countries of the world is about 10 million tons/year.

At present, the industrial production methods of chromium sesquioxide are various, and the large-scale production mainly comprises three methods: chromic anhydride thermal decomposition, chromic hydroxide thermal decomposition and sodium dichromate and ammonium sulfate double decomposition; the small-scale methods include potassium dichromate sulfur reduction, ammonium dichromate thermal decomposition, sodium dichromate carbon reduction, chromium chloride oxidation, and the like. The preparation method of the nano chromium sesquioxide mainly comprises a hydrothermal reduction method, a solid thermal decomposition method, a sol-gel method, a laser induced deposition method, a mechanochemical method and the like. These production methods all use sodium-based or potassium-based chromates as raw materials, and the production method using ammonium chromates is rare.

1. Thermal decomposition method of sodium dichromate and ammonium sulfate

Sodium dichromate and ammonium sulfate are mixed uniformly according to the stoichiometric proportion, and are delivered into a rotary kiln to be roasted at the temperature of 850-1050 ℃, and then are dissolved in water, pressed and filtered, washed, dried and crushed to obtain chromium sesquioxide and a byproduct sodium sulfate, wherein the reaction equation is as follows.

Na₂Cr₂O₇+(NH₄)₂SO₄＝Cr₂O₃+Na₂SO₄+N₂↑+4H₂O↑

2. Thermal decomposition method of chromic anhydride

The chromic anhydride thermal decomposition method is the most important production method of chromic oxide in China, has simple process and higher cost, is mainly used for producing the chromic oxide for pigment and grinding material with higher price, and has the following reaction equation:

4CrO₃＝2Cr₂O₃+3O₂↑

in order to ensure that the complete decomposition temperature of chromic anhydride is higher than 800 ℃, chromic oxide crystal grains are gradually formed and grown in chromic anhydride solution in the temperature rising process, the crystal defects are few, and the finished product has high hardness, dark color, strong tinting strength, high refractive index and good covering power. However, since the chromic anhydride as the raw material contains a small amount of sodium salt, the strongly basic sodium salt can oxidize a small amount of trivalent chromium into sodium chromate in high-temperature air, and 100ppm of hexavalent chromium remains even if the thermal decomposition temperature is as high as 1000 ℃.

3. Sodium chromate alkaline liquid sulfur reduction method

Sodium chromate alkali solution (containing NaOH and Na with concentration of 30g/L or less)₂CO₃) Reacting with sulfur at 90 deg.C to reduce sodium chromate into chromium hydroxide, and simultaneously producing sodium thiosulfate and sodium hydroxide as byproducts, reacting free alkali with sulfur to produce sodium sulfide, sodium sulfite and sodium thiosulfate, wherein the sulfur-containing compounds can further react with sodium chromate to generate chromium hydroxide. The obtained chromium hydroxide is calcined at the high temperature of 1300 ℃, and a chromium sesquioxide product is finally obtained, wherein the reaction equation is as follows.

4Na₂CrO₄+6S+7H₂O＝4Cr(OH)₃↓+3Na₂S₂O₃+2NaOH

2Cr(OH)₃＝Cr₂O₃+O₂↑

Although the sulfur reduction method has low cost, the prepared chromium sesquioxide has low content (more than 95 percent), more impurities, defective crystals and grey green color, and is not suitable for pigments and abrasives.

At present, sodium (potassium) chromate or sodium (potassium) dichromate is used as a raw material in various industrial preparation methods, the preparation method is complex, the production process flow is long, the particle size of a product cannot reach the nanometer level, a large amount of low-value byproducts are produced, and the technology for specially preparing nano-scale spherical chromium sesquioxide is not reported in a public way at present.

Disclosure of Invention

The invention aims to provide a preparation method of spherical nano chromium sesquioxide, thereby overcoming the defects of the prior art.

In order to achieve the purpose, the technical scheme adopted by the invention comprises the following steps:

the embodiment of the invention provides a preparation method of spherical nano chromium sesquioxide, which comprises the following steps:

uniformly mixing ammonium chromate crystals and additives according to a molar ratio of 1: 0.2-1: 1, then carrying out thermal decomposition reaction on the obtained mixture at 400-1000 ℃, and preserving heat for 0-12 h to obtain spherical nano chromium sesquioxide.

In some preferred embodiments, the additive must contain ammonium groups or amino groups and belongs to a high temperature-decomposable salt, and for example, the additive may be any one or a combination of two or more of ammonium nitrate, ammonium bicarbonate, urea and the like, but is not limited thereto.

In some preferred embodiments, the preparation method comprises:

the method comprises the following steps of (1) taking carbon ferrochrome as a working electrode, and at least constructing an electrochemical reaction system together with a counter electrode and electrolyte, wherein the electrolyte comprises ammonia water or a mixed solution of the ammonia water and ammonium chromate;

electrically connecting the working electrode and the counter electrode with the anode and the cathode of a power supply respectively, so that an electrochemical reaction is generated in the electrochemical reaction system, and mixed slurry is obtained;

carrying out solid-liquid separation on the mixed slurry, and then freezing the separated liquid phase system to obtain a solid matter;

and carrying out sublimation drying on the solid matter to obtain ammonium chromate crystals.

Furthermore, the spherical nano chromium sesquioxide has uniform particle size, and the average particle size is 80-90 nm.

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

1) the spherical nano chromium sesquioxide provided by the invention has a short synthetic route and is simple to operate, and a target product can be obtained only by one-step thermal decomposition;

2) the spherical nano chromium oxide product prepared by the technology has high purity, the main content of the chromium oxide can reach more than 99 percent, the impurity content is low, silicon and vanadium are not contained, and the product can meet the purity requirements of chromium oxide green pigment and grinding material;

3) the preparation method provided by the invention has no solid by-product, thereby greatly reducing the pollution to the environment;

4) the average particle size of the spherical nano chromium sesquioxide prepared by the method is 80-90 nm, the particle size distribution is uniform, and the dispersity is good;

5) the spherical nano chromium sesquioxide provided by the invention has two strong reflection peaks respectively at about 400nm and 550nm in an ultraviolet visible diffuse reflection spectrum, and has good color performance.

Drawings

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

FIGS. 1a and 1b are the thermogravimetric plots of ammonium chromate and the XRD patterns of decomposition products at various temperatures, respectively.

FIGS. 2a to 2h are SEM pictures of the thermal decomposition products of ammonium chromate at different temperatures, respectively.

Fig. 3a to 3d are SEM images of thermal decomposition products after mixing ammonium chromate crystals with ammonium nitrate, respectively.

Fig. 4 is an X-ray diffraction pattern of spherical nano-chromium sesquioxide prepared in an exemplary embodiment of the present invention.

Fig. 5 is a scanning electron microscope photograph of spherical nano-chromium sesquioxide prepared in an exemplary embodiment of the present invention.

Fig. 6 is a statistical view of the particle size distribution of spherical nano-chromium sesquioxide prepared in an exemplary embodiment of the present invention.

Fig. 7 is a uv-vis diffuse reflectance spectrum of spherical nano-chromium sesquioxide prepared in an exemplary embodiment of the present invention.

FIG. 8 is an SEM photograph of spherical chromium sesquioxide obtained in comparative example 1 of the present invention.

Detailed Description

In view of the defects of the prior art, the inventor of the present invention provides a technical scheme of the present invention through long-term research and a great deal of practice, and provides a preparation method of spherical nanometer chromium sesquioxide. The technical solution, its implementation and principles, etc. will be further explained as follows.

As one aspect of the technical solution of the present invention, a method for preparing spherical nano chromium oxide is provided, which comprises:

uniformly mixing ammonium chromate crystals and additives according to a molar ratio of 1: 0.2-1: 1, then carrying out thermal decomposition reaction on the obtained mixture at 400-1000 ℃, and preserving heat for 0-12 h to obtain spherical nano chromium sesquioxide.

In some preferred embodiments, the ammonium chromate crystal must meet the following three criteria, the main content of ammonium chromate is greater than or equal to 99.6%, the crystal form is a bright yellow color.

In some preferred embodiments, the additive must contain ammonium groups or amino groups and belongs to a high temperature-decomposable salt, and for example, the additive may be any one or a combination of two or more of ammonium nitrate, ammonium bicarbonate, urea and the like, but is not limited thereto.

In some preferred embodiments, the molar ratio of ammonium chromate crystals to additive is 1:0.5 to 1: 0.7.

In some preferred embodiments, the preparation method specifically comprises: heating the mixture to 400-1000 ℃ at a heating rate of 1-10 ℃/min.

Further, the heating rate is 5-8 ℃/min.

Further, the temperature of the thermal decomposition reaction is 800-900 ℃.

Further, the heat preservation time is 0.5-5.5 h.

In some preferred embodiments, the preparation method comprises:

the method comprises the following steps of (1) taking carbon ferrochrome as a working electrode, and at least constructing an electrochemical reaction system together with a counter electrode and electrolyte, wherein the electrolyte comprises ammonia water or a mixed solution of the ammonia water and ammonium chromate;

electrically connecting the working electrode and the counter electrode with the anode and the cathode of a power supply respectively, so that an electrochemical reaction is generated in the electrochemical reaction system, and mixed slurry is obtained;

carrying out solid-liquid separation on the mixed slurry, and then freezing the separated liquid phase system to obtain a solid matter;

and carrying out sublimation drying on the solid matter to obtain ammonium chromate crystals.

In some preferred embodiments, the preparation method specifically comprises:

freezing the liquid phase system at-20 to-10 ℃ for 5 to 12 hours to obtain a solid matter;

and then, carrying out sublimation drying on the solid at-50 to-40 ℃ for 6 to 24 hours under the condition that the pressure is 5 to 20Pa to obtain ammonium chromate crystals.

In some preferred embodiments, the preparation method comprises: the carbon ferrochrome is arranged in the anode frame, and a counter electrode serving as a cathode is arranged on at least one side of the anode frame and is not in contact with the anode frame, or the counter electrode is arranged in the anode frame and is not in contact with the anode frame.

Further, the cathodes are arranged on two sides of the anode frame or in the middle of the anode frame.

Further, the material of the anode frame includes, but is not limited to, titanium or nickel.

Furthermore, the surface of the cathode is coated with an insulating material, the cathode can also be made into a rod shape and coated with the insulating material, and then the cathode is placed in the anode frame to form a counter electrode, so that the resistance can be reduced.

Further, the material of the cathode includes nickel or stainless steel, but is not limited thereto.

Further, the cathode or anode frame may have a plate shape, a hole shape, a mesh shape, a rod shape, or the like, but is not limited thereto. The hole shape here means that a plate-shaped material is punched to form a perforated plate, and the opening ratio (hole diameter, pitch) can be set by itself.

In some preferred embodiments, the preparation method comprises: and controlling the temperature of the electrolyte to be 20-80 ℃ during the electrochemical reaction.

In some preferred embodiments, the preparation method comprises: and controlling the circulation flow of the electrolyte to be 0-72L/h during the electrochemical reaction.

Further, the molar ratio of the ammonia water to the ammonium chromate solution in the mixed solution is 1: 10-10: 1.

In some preferred embodiments, the preparation method further comprises: and after the thermal decomposition reaction is finished, washing, filtering and drying the obtained product to obtain the spherical nano chromium sesquioxide.

Further, the mass-volume ratio of the spherical nano chromium sesquioxide to the washing liquid adopted for washing treatment is 100 mL-1000 mL: 100 g.

Further, the drying treatment temperature is 60-120 ℃, and the drying treatment time is 0.5-6 h.

Furthermore, the spherical nano chromium sesquioxide has uniform particle size distribution and good dispersity, and the average particle size is 80-90 nm. The chromium oxide product prepared by the technology has high purity, the main content of the chromium oxide can reach more than 99 percent, the impurity content is low, silicon and vanadium are not contained, and the product can meet the purity requirements of chromium oxide green pigment and grinding material.

In some more preferred embodiments, the specific steps of the preparation method of the spherical nano chromium oxide are detailed as follows:

step one, preparing an ammonium chromate solution from carbon ferrochrome serving as a raw material in an ammonium salt or ammonia water electrolyte by an electrochemical oxidation technology, and then obtaining an ammonium chromate crystal by utilizing a freeze drying technology and equipment;

and step two, fully and uniformly mixing the ammonium chromate crystal obtained in the step one with additives (ammonium nitrate, ammonium carbonate, urea and the like) in a certain proportion, then placing the mixed sample in a crucible, carrying out thermal decomposition for a certain time in a muffle furnace, after the high-temperature treatment is finished, cooling the sample, then carrying out washing, filtering, drying and other operations, and finally obtaining the spherical nano chromium sesquioxide product.

Further, the ammonium chromate crystal is obtained by electrolyzing carbon ferrochrome, and the preparation steps are as follows:

① placing the carbon ferrochromium block in an anode frame, the carbon ferrochromium block is connected with the positive electrode of a power supply, the cathode plate is connected with the negative electrode of the power supply, so that two cathode plates can be placed on both sides of the anode, or the cathode plates are placed in the anode frame, and the ferrochromium block is isolated by an insulating net, wherein the anode frame is titanium or nickel, and the cathode is stainless steel or nickel;

② putting electrolyte solution into the electrolytic cell, wherein the electrolyte solution is ammonia water or mixed solution of ammonia water and ammonium chromate, and the temperature of the electrolyte solution is 20-80 ℃;

③, switching on a power supply, starting an electrochemical reaction, slowly dissolving ferrochrome in the electrolyte, wherein chromium is electro-oxidized into hexavalent chromate, iron is oxidized into trivalent iron oxide to be precipitated, and obtaining solid-liquid mixed slurry in an electrolytic bath;

④ freezing the ammonium chromate solution in a refrigerator at-20 deg.C to-10 deg.C for 5-12 h, and freezing the solution to solid;

⑤ and placing the obtained solid in a freeze dryer to carry out sublimation drying, wherein the sublimation temperature is-50 ℃ to-40 ℃, the pressure is 5Pa to 20Pa, and the time is 6h to 24h, and the ammonium chromate crystal powder can be obtained after the drying is finished.

Furthermore, the anode in the step (1) is in a frame structure, and the cathode plate can be positioned at two sides of the anode frame or in the middle of the anode frame and is provided with holes for facilitating the circulation of the solution. The ferrochrome is common carbon ferrochrome. The method directly uses carbon ferrochrome as a raw material, can obtain the ammonium chromate solution through one-step electrolysis, does not need to prepare ammonium chromate through sodium chromate, and shortens the preparation process of the ammonium chromate.

Further, the ratio of the ammonia water to the ammonium chromate in the step (2) is 1: 10-10: 1, the temperature of the electrolytic cell is 20-80 ℃, and the flow rate of a peristaltic pump is controlled to be 0-72L/h.

Further, the freezing time in the step (4) is 5-12 h, the cooling temperature is-20 ℃ to-10 ℃, and the solution is ensured to be completely cooled and consolidated. The ammonium chromate is crystallized by adopting a freeze drying mode, the operation temperature is-20 ℃ to-10 ℃, and the operation temperature is far lower than the decomposition temperature of the ammonium chromate, so that the ammonium chromate product with the purity of more than 99 percent can be obtained.

Further, the freeze drying temperature in the step (5) is-50 to-40 ℃, the pressure is 5Pa to 20Pa, and the drying time is 6h to 24h, so that the water in the solid is completely sublimated to obtain the porous ammonium chromate crystal. In addition, no aqueous solution can be obtained during the vacuum drying process of step (5) to obtain bright yellow ammonium chromate crystals. The ammonium chromate is crystallized by freeze drying, so that a plurality of washing processes after the traditional ammonium chromate crystal is crystallized are omitted, and the water consumption is saved.

In some preferred embodiments, the ammonium chromate crystals produced by the present invention are porous and have a bright yellow color.

The basic principle of the invention may be that:

referring to FIGS. 1a and 1b, the Thermogravimetric (TG) curves of ammonium chromate and XRD patterns of decomposition products at various temperatures are shown. As can be seen from the TG curve, the first-step decomposition temperature range is 90-120 ℃, and the weight loss rate is 16.86%; the second step is carried out at the decomposition temperature of 180-240 ℃ and the weight loss rate of 17.39%; third step decompositionThe temperature range is 240-260 ℃, and the weight loss rate is 10.74%; and fourthly, the decomposition temperature ranges from 430 ℃ to 444 ℃, and the weight loss rate is 4.17%. As can be seen from the DSC curve, four exothermic peaks were present at T108 ℃, T194 ℃, T243 ℃ and T439 ℃, and an exothermic reaction occurred. It was concluded from the calculated weight loss and the literature that the thermal decomposition of ammonium chromate followed Cr⁶⁺→Cr⁴⁺→Cr³⁺The thermal decomposition end product is Cr₂O₃There are three intermediates in the decomposition process: (NH)₄)₂Cr₂O₇、CrO₃、CrO₂The thermal decomposition process is as follows:

please refer to fig. 2 a-2 h, which are SEM pictures of thermal decomposition products of ammonium chromate at different temperatures. As can be seen from XRD and SEM images of the thermal decomposition products, the particle size of the decomposition products increases and the morphology changes as the thermal decomposition temperature increases. The chromium sesquioxide prepared under the thermal decomposition condition of 450-650 ℃ has incomplete crystal morphology and obvious particle agglomeration. The chromic oxide prepared under the thermal decomposition condition of 750-950 ℃ has smooth particles, uniform particle size distribution, good dispersibility and regular sphericity. The surface of the chromium sesquioxide prepared under the thermal decomposition condition of 1050-1150 ℃ is smooth, but some particles begin to develop from a spherical shape to a spindle shape. According to the shape result, the chromium oxide formed by thermal decomposition at 850 ℃ has a regular shape, no obvious agglomeration and a better thermal decomposition temperature. Therefore, the calcination temperature of the ammonium chromate is determined to be between 400 and 1000 ℃, and the optimized data is 800 to 900 ℃. In addition, it can be seen from the SEM image that the grain size of the ammonium chromate alone thermal decomposition product is concentrated between 200-300nm, and the nanoparticles with intact crystal form cannot be obtained.

The inventor also adds ammonium nitrate into the ammonium chromate crystal according to different proportions, and the mass ratio of the ammonium nitrate to the ammonium chromate crystal is 1: 1. 1: 0.625, 1:0.5 and 1: 0.375, then keeping the temperature for 0.5h at the thermal decomposition temperature of 850 ℃, and raising the temperature at the rate of 4 ℃ min^-1The thermal decomposition is carried out under the conditions, and the morphology of the product is shown in figures 3 a-3 d.

As can be seen from fig. 3a to fig. 3d, the added ammonium nitrate significantly affects the morphology of the thermal decomposition product of ammonium chromate, and the particle size of chromium oxide becomes smaller, especially when the addition ratio is 1: 0.625 and 1: at 0.5, the particles of the product are spherical, uniformly distributed and clear in interface, and the average particle size reaches about 70nm, so that the spherical nano-particle chromium oxide is obtained.

The inventors speculate that the mechanism of influence of the additive on the thermal decomposition process of ammonium chromate has three aspects: firstly, the selected additives all contain ammonium groups or amino groups, and the introduction of the additives can change the quantity ratio of ammonium chromium substances during the thermal decomposition of ammonium chromate, thereby influencing the nucleation and growth processes of the ammonium chromium substances and controlling the grain size; secondly, the additive is diffused and dissolved into the raw material matrix in the thermal decomposition process, so that the lattice stability is reduced, and the additive acts on the particle size and the morphology of the chromic oxide; thirdly, the additive continuously releases gas in the temperature rising process, the additive can continuously impact the matrix, the matrix has a certain crushing effect, and the thinning of matrix particles finally acts on products.

By the preparation process, the spherical nano chromium sesquioxide provided by the invention has the advantages of short synthetic route and simple operation, and the target product can be obtained only by one-step thermal decomposition; the spherical nano chromium oxide product prepared by the method has high purity, the main content of the chromium oxide can reach more than 99 percent, the impurity content is low, silicon and vanadium are not contained, the product can meet the purity requirements of chromium oxide green pigment and grinding material, the average grain diameter is 80-90 nm, the grain diameter distribution is uniform, and the dispersity is good.

In order to make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions of the present invention are described in further detail below with reference to the accompanying drawings and several preferred embodiments, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention. The test methods in the following examples are carried out under conventional conditions without specifying the specific conditions. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.