Method for preparing hydrated chromic oxide by reducing hexavalent chromium salt solution

Abstract

The invention relates to a method for preparing hydrated chromic oxide by reducing a hexavalent chromium salt solution, which comprises the following steps: (1) mixing the hexavalent chromium salt solution and a piezoelectric material in an inert atmosphere to obtain a precursor solution; (2) carrying out hydrothermal reaction on the precursor solution in a hydrogen atmosphere to obtain slurry containing hydrated chromic oxide; and (3) carrying out solid-liquid separation on the slurry containing the hydrated chromic oxide in an inert atmosphere to obtain the hydrated chromic oxide. Under the hydrothermal condition, efficient reduction of hexavalent chromium is achieved through the catalysis of the piezoelectric material and the synergistic effect of hydrogen, the reaction temperature and pressure of hexavalent chromium reduction through hydrogen are remarkably reduced, and a new method is provided for preparing hydrated chromic oxide through hexavalent chromium low-temperature reduction.

Description

Technical Field

[0001] This invention relates to the field of materials preparation technology, and in particular to a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. Background Technology

[0002] Chromium hydroxyoxide (CrOOH), also known as hydrated chromium oxide or chromite, is an important precursor for the production of Cr2O3 and one of the main products in the chromium salt industry. It possesses excellent thermal stability, corrosion resistance, and wear resistance, and is widely used in pigments, ceramics, adsorbents, building materials, catalysts, and magnetic materials.

[0003] Currently, the main methods for preparing hydrated chromium oxide include the trivalent chromium precipitation method and the hexavalent chromium reduction method. The trivalent chromium precipitation method requires the prior reduction of hexavalent chromium to prepare soluble trivalent chromium compounds (such as chromium chloride hexahydrate, chromium nitrate, etc.), but this method suffers from long processes and high costs. The hexavalent chromium reduction method requires the selection of suitable reducing agents and reduction conditions to ensure a high reduction rate while achieving efficient separation of hydrated chromium oxide from byproducts.

[0004] CN102408130A discloses a method for preparing hydrated chromium oxide. The method uses chromic anhydride and boric acid as raw materials, and urea as a leavening agent. After uniform mixing, the mixture is calcined within a certain temperature range. The resulting clinker is boiled and pulped in a boric acid solution. After filtration, the resulting material is pulped in boiling water, and then filtered, washed, and dried to obtain hydrated chromium oxide. The clinker obtained by this method needs to be treated with boiling in boric acid solution and pulping in boiling water to remove byproducts.

[0005] CN103420422A discloses a method for preparing chromium hydroxyl oxide. This method uses solid chromate as raw material and a reducing gas as a reducing agent, achieving clean production. However, the gas-solid reduction reaction temperature is as high as 550-750℃, and the product still needs to be washed with a washing solution at 60-80℃.

[0006] CN109354068A discloses a method for preparing chromium oxide, which involves the process of reducing hexavalent chromium salt solution with a reducing gas under hydrothermal conditions to obtain chromium hydroxyl oxide. The method involves adding a hexavalent chromium salt solution to a reaction apparatus, introducing a protective gas, and then heating in a sealed manner. Once the target temperature is reached, a reducing gas is continuously introduced to allow the reaction to proceed, resulting in a mixed slurry. The resulting mixed slurry is then separated into solid and liquid components to obtain chromium hydroxyl oxide powder. However, the hydrogen reduction reaction conditions for hexavalent chromium salt solution are quite stringent, requiring sophisticated equipment and resulting in high preparation costs.

[0007] Therefore, developing a method for preparing hydrated cadmium oxide using hexavalent chromium salt solution as raw material with low activation energy and milder reaction conditions has become an urgent technical problem to be solved. Summary of the Invention

[0008] To address the aforementioned technical problems, the present invention aims to provide a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. This method reduces the activation energy of the reaction, allowing the reaction to proceed under relatively mild conditions, thus solving the problem of harsh conditions in existing processes for preparing hydrated chromium oxide by hydrogen reduction of hexavalent chromium salt solution.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a method for preparing hydrated chromium oxide by reduction of a hexavalent chromium salt solution, the method comprising the following steps:

[0011] (1) Under an inert atmosphere, a hexavalent chromium salt solution and a piezoelectric material are mixed to obtain a precursor solution;

[0012] (2) Under a hydrogen atmosphere, the precursor solution undergoes a hydrothermal reaction to obtain a slurry containing hydrated chromium oxide;

[0013] (3) Under an inert atmosphere, the slurry containing hydrated chromium oxide is separated into solid and liquid components to obtain hydrated chromium oxide.

[0014] This invention employs a piezoelectric material-catalyzed synergistic hydrothermal hydrogen reduction method to prepare hydrated chromium oxide, achieving a clean preparation of hydrated chromium oxide.

[0015] The overall reaction of chromate ions with hydrogen is shown in equation (1):

[0016]

[0017] Based on the electrode reaction analysis of the hydrogen reduction reaction system, possible elementary reactions include:

[0018]

[0019] According to the two-film model, the reaction process may include the following steps: (1) Hydrogen in the gas phase is transferred to the gas-liquid interface through the gas film; (2) Hydrogen is transferred from the phase interface to the liquid film, dissociates, and reacts chemically with chromate ions transferred from the hexavalent chromium salt solution to the liquid film. At this time, the reaction and diffusion occur simultaneously; (3) Unreacted hydrogen continues to diffuse into the sodium chromate solution, dissociates, and reacts with chromate ions in the solution; (4) The generated solid product, hydrated chromium oxide, diffuses in the direction of decreasing concentration gradient. The rate of reduction reaction in the hexavalent chromium salt solution and the influence of gas-liquid mass transfer on the rate of liquid phase reduction reaction are the key factors determining the formation of hydrated chromium oxide.

[0020] Specifically, the two elementary reactions, hydrogen dissociation (Equation (2)) and chromate reduction (Equation (3)), directly affect the mass transfer at the gas-liquid interface and the rate of reduction in the liquid phase, and are key to determining the conditions and rate of hydrogen reduction in sodium chromate solution. However, hydrogen is a sparingly soluble gas, and the energy required for hydrogen molecule dissociation is very high (up to 430 kJ per mole of hydrogen molecule dissociating into hydrogen atoms). Therefore, the activation energy required for the elementary reaction of hydrogen dissociation (Equation (2)) is high, and the reaction conditions are relatively harsh. At the same time, the reaction rate of the elementary reaction of hexavalent chromium reduction (Equation (3)) is also affected by the charge generated by hydrogen dissociation. Due to the above factors, the temperature, pressure, and other conditions required for the reduction of hexavalent chromium by hydrogen in solution are relatively high, and the reaction time is relatively long, so the industrial feasibility needs to be improved.

[0021] Piezoelectric catalysis is a method of catalyzing redox reactions using the piezoelectric effect. When subjected to mechanical energy such as ultrasound or stirring, piezoelectric materials generate induced charges and an internal electric field. The generated charges can participate in the chemical reaction, lowering the activation energy and increasing the reaction rate; the generated internal electric field can promote the separation of positive and negative charges, improving the utilization efficiency of active groups. When piezoelectric materials are introduced into the reduction reaction system of sodium chromate solution and hydrogen, the induced charges generated by the piezoelectric material under mechanical action react with the CrO4 in the sodium chromate solution. 2- Hydrogen may undergo the following redox reactions: (1) The active groups such as electrons generated by piezoelectric catalysis react with CrO4 2- The reaction catalyzes the elementary reaction of hexavalent chromium reduction (Equation (3)), allowing the reaction to proceed at a lower temperature and increasing the reduction rate of hexavalent chromium; (2) the active groups such as holes generated by piezo-catalysis react with hydrogen, catalyzing the elementary reaction of hydrogen dissociation (Equation (2)), increasing the reduction rate, and simultaneously lowering the pH of the solution. In fact, for the piezo-catalyzed reduction of hexavalent chromium, hydrogen in the system can also act as a hole sacrificial agent, consuming the holes generated by piezo-catalysis, reducing the probability of recombination between electrons and holes, increasing the lifetime of active groups such as electrons, thereby further improving the efficiency of the piezo-catalyzed reduction of hexavalent chromium.

[0022] Therefore, this invention achieves efficient reduction of hexavalent chromium under hydrothermal conditions through the catalytic effect of piezoelectric materials and the synergistic effect of hydrogen, significantly reducing the reaction temperature and pressure of hydrogen reduction of hexavalent chromium, and providing a new method for the low-temperature reduction of hexavalent chromium to prepare hydrated chromium oxide.

[0023] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0024] And / or, the mass ratio of the piezoelectric material and the hexavalent chromium salt in the hexavalent chromium salt solution in step (1) is 50-1:1, for example, it can be 50:1, 40:1, 30:1, 20:1, 10:1, 5:1, 2:1 or 1:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] And / or, the temperature of the hydrothermal reaction in step (2) is 80℃-240℃, for example, it can be 80℃, 90℃, 100℃, 120℃, 150℃, 180℃, 200℃, 220℃, 230℃ or 240℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 120℃-200℃.

[0026] The present invention further controls the temperature of the hydrothermal reaction in step (2) to be 80℃-240℃. The temperature of the hydrothermal reaction affects the reaction conversion rate, reaction rate and total pressure of the system. If the temperature of the hydrothermal reaction is too high, the total pressure of the system will be too high, and the cost of the device and safety risks will increase sharply. If the temperature of the hydrothermal reaction is too low, the reaction conversion rate will be extremely low and the reaction rate will be very slow.

[0027] And / or, under the hydrogen atmosphere, the total pressure of the system is 0.5MPa-6MPa, for example, it can be 0.5MPa, 1MPa, 2MPa, 3MPa, 4MPa, 5MPa or 6MPa, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 1MPa-4MPa.

[0028] This invention further controls the total pressure of the reaction system to 0.5MPa-6MPa. The total pressure of the hydrothermal reaction affects the reaction conversion rate, reaction rate, and system safety. If the total pressure is too high, the reaction conversion rate and reaction rate will increase, but the equipment cost and safety risks will increase sharply. If the total pressure is too low, the reaction conversion rate will be extremely low and the reaction rate will be very slow.

[0029] And / or, the hydrothermal reaction time is 4h-20h, for example, it can be 4h, 5h, 6h, 8h, 10h, 12h, 15h or 20h, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 6h-12h.

[0030] And / or, the hydrothermal reaction is carried out under stirring and / or ultrasonic conditions.

[0031] And / or, the stirring speed is 500rpm-5000rpm, for example, it can be 500rpm, 1000rpm, 1500rpm, 2000rpm, 2500rpm, 3000rpm, 3500rpm, 4000rpm, 4500rpm or 5000rpm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 1000rpm-5000rpm.

[0032] And / or, the stirring includes paddle stirring and / or magnetic stirring.

[0033] And / or, the frequency of the ultrasound is 20kHz-400kHz, for example, it can be 20kHz, 50kHz, 100kHz, 150kHz, 200kHz, 250kHz, 300kHz, 350kHz or 400kHz, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0034] And / or, the power density of the ultrasound is 0.4W / mL-1.2W / mL, for example, it can be 0.4W / mL, 0.5W / mL, 0.8W / mL, 1.0W / mL, 1.1W / mL or 1.2W / mL, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0035] In the hydrothermal reaction process of this invention, a higher stirring rate and ultrasonic power density are more conducive to the piezoelectric catalytic effect and the reduction of hexavalent chromium, but the energy consumption is also higher.

[0036] And / or, the Curie temperature of the piezoelectric material is greater than 100°C, for example, it can be 110°C, 200°C, 300°C, 400°C, 500°C, 600°C, 800°C or 1000°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably greater than 300°C.

[0037] The present invention selects piezoelectric materials with high Curie temperatures, which can ensure that the piezoelectric materials will not transform into the paraelectric phase under reaction conditions.

[0038] The present invention preferably uses a piezoelectric material that has undergone polarization treatment. The piezoelectric material after polarization treatment can generate a stable charge output after being subjected to mechanical energy, which is more conducive to the reduction of hexavalent chromium.

[0039] This invention does not limit the specific types of piezoelectric materials. Commonly used piezoelectric materials in the prior art can achieve the technical effects of this application. For example, the piezoelectric materials in this invention can be barium titanate (BaTiO3), polyvinylidene fluoride (PVDF), lead antimonylate niobate modified lead zirconate titanate, or bismuth layered ceramics (Bi4Ti3O3).12 Na 0.5 Bi 4.5 Ti 3.9 Mn 0.06 Nb 0.04 O 15 Lead zirconate titanate or lead zirconate titanate-based modified materials, or any combination of at least two of them, typically but not limitingly, includes combinations of BaTiO3 and PVDF, Bi4Ti3O 12 Combination with PVDF, BaTiO3 and Bi4Ti3O 12 The combination of BaTiO3 and lead zirconate titanate, Bi4Ti3O 12 A combination with lead zirconate titanate.

[0040] And / or, the form of the piezoelectric material includes any one or a combination of at least two of the following: film, ceramic, or single crystal. Typical but non-limiting combinations include combinations of film and ceramic, ceramic and single crystal, film and single crystal, and film, ceramic, and single crystal.

[0041] And / or, the hexavalent chromium salt solution includes any one or a combination of at least two of potassium dichromate solution, sodium dichromate solution, sodium chromate solution, or potassium chromate solution. Typical but non-limiting combinations include combinations of potassium dichromate solution and potassium chromate solution, combinations of sodium dichromate solution and sodium chromate solution, combinations of potassium dichromate solution and sodium dichromate solution, and combinations of sodium chromate solution and potassium chromate solution.

[0042] And / or, the concentration of the hexavalent chromium salt solution is 0.01 mol / L to 1.2 mol / L, for example, it can be 0.01 mol / L, 0.1 mol / L, 0.5 mol / L, 0.6 mol / L, 0.8 mol / L, 1.0 mol / L, 1.1 mol / L or 1.2 mol / L, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 0.1 mol / L to 1 mol / L.

[0043] This invention further controls the concentration of the hexavalent chromium salt solution to 0.01 mol / L-1.2 mol / L. The concentration of the hexavalent chromium salt solution affects the reaction conversion rate, as well as the quality and yield of hydrated chromium oxide. If the concentration of the hexavalent chromium salt solution is too high, the reaction conversion rate is likely to be low and the hydrated chromium oxide will contain incompletely reduced hexavalent chromium. If the concentration of the hexavalent chromium salt solution is too low, the yield of hydrated chromium oxide will be affected.

[0044] And / or, the inert atmosphere described in steps (1) and (3) includes a nitrogen atmosphere.

[0045] Preferably, the process after solid-liquid separation and before obtaining hydrated chromium oxide further includes washing and drying the filter cake.

[0046] As a preferred embodiment of the method described in this invention, the method includes the following steps:

[0047] (1) Under a nitrogen atmosphere, a hexavalent chromium salt solution with a concentration of 0.1 mol / L to 1 mol / L and a piezoelectric material are mixed in a mass ratio of 50-1:1 to obtain a precursor solution;

[0048] (2) Hydrogen gas is introduced and the temperature is raised to 120℃-200℃. Under the condition that the total pressure is controlled by adjusting the amount of hydrogen gas to 1MPa-4MPa, the precursor solution is subjected to hydrothermal reaction at 120℃-200℃ for 6h-12h under stirring and / or ultrasonic conditions to obtain a slurry containing hydrated chromium oxide.

[0049] (3) Under a nitrogen atmosphere, the slurry containing hydrated chromium oxide is subjected to solid-liquid separation, washing and drying to obtain hydrated chromium oxide.

[0050] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0051] Compared with the prior art, the present invention has at least the following beneficial effects:

[0052] This invention achieves highly efficient reduction of hexavalent chromium under hydrothermal conditions through the synergistic effect of piezoelectric materials and hydrogen, significantly reducing the reaction temperature and pressure for hydrogen reduction of hexavalent chromium. This provides a novel method for the low-temperature reduction of hexavalent chromium to prepare hydrated chromium oxide. It also enables the clean preparation of hydrated chromium oxide. Attached Figure Description

[0053] Figure 1 This is the Fourier transform-infrared (FT-IR) spectrum of hydrated chromium oxide prepared by the method provided in Example 1 of this invention. Detailed Implementation

[0054] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0055] In this invention, the terms "first aspect," "second aspect," "third aspect," "fourth aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," "fourth," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on the quantity.

[0056] Unless otherwise specified, all reagents and consumables used in the following examples and comparative examples were purchased from conventional reagent manufacturers in the art; unless otherwise specified, the experimental methods and techniques used were conventional methods and techniques in the art.

[0057] Example 1

[0058] This embodiment provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution, the method comprising the following steps:

[0059] (1) Under a nitrogen atmosphere, a sodium dichromate solution with a concentration of 0.2 mol / L (c1) and a lead zirconate titanate piezoelectric ceramic sheet (10 mm long, 5 mm wide, and 0.5 mm thick) with a mass ratio of 20:1 to the piezoelectric material and sodium dichromate was mixed to obtain a precursor solution.

[0060] (2) Hydrogen gas was introduced and the temperature was raised. After the temperature reached 150°C, the amount of hydrogen gas was adjusted to control the total pressure to 4MPa. The precursor solution was subjected to hydrothermal reaction at 150°C for 7 hours under stirring at 1000rpm to obtain a slurry containing hydrated chromium oxide.

[0061] (3) Under a nitrogen atmosphere, the slurry containing hydrated chromium oxide is subjected to solid-liquid separation, and the resulting filter cake is washed and dried to obtain hydrated chromium oxide.

[0062] The FT-IR spectrum of hydrated chromium oxide prepared by piezoelectric catalytic reduction of hexavalent chromium solution is shown below. Figure 1 As shown. The sample is at 521 cm. -1 1064 cm -1 1631 cm -1 2018 cm -1 3300~3100 cm -1 An absorption peak appeared at 521 cm⁻¹. -1 The absorption peak at that location corresponds to Cr Ⅲ Antisymmetric stretching vibration of -O, 1064 cm⁻¹ -1 The absorption peak at that location is due to Cr Ⅲ Caused by the bending vibration of -OH; 1631 cm -1The absorption peak at 2018 cm⁻¹ is caused by the HOH bending vibration in non-free water molecules; -1 The broadband observed in this region corresponds to the stretching vibration of the OHO; 3300~3100 cm -1 The absorption peaks are caused by the stretching vibration of OH groups adsorbed in water. These are all characteristic absorption peaks of hydrated chromium oxide.

[0063] Example 2

[0064] This embodiment provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution, the method comprising the following steps:

[0065] (1) Under a nitrogen atmosphere, a potassium dichromate solution with a mass ratio of 10:1 (100 mL) and a concentration of 0.1 mol / L was mixed with a polyvinylidene fluoride piezoelectric film (5 mm long, 2 mm wide, and 5 μm thick) to obtain a precursor solution.

[0066] (2) Hydrogen gas was introduced and the temperature was raised. After the temperature reached 80℃, the amount of hydrogen gas was adjusted to control the total pressure to 1.0MPa. The precursor solution was ultrasonically vibrated at a frequency of 60kHz and a power density of 1.2W / mL. The hydrothermal reaction was carried out at 80℃ for 12h to obtain a slurry containing hydrated chromium oxide.

[0067] (3) Under a nitrogen atmosphere, the slurry containing hydrated chromium oxide is subjected to solid-liquid separation, and the resulting filter cake is washed and dried to obtain hydrated chromium oxide.

[0068] Example 3

[0069] This embodiment provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution, the method comprising the following steps:

[0070] (1) Under a nitrogen atmosphere, a precursor solution was obtained by mixing a 500 mL potassium chromate solution with a concentration of 1 mol / L and a pre-polarized lead antimonate niobate modified lead zirconate titanate piezoelectric ceramic sheet (10 mm in diameter and 0.5 mm in thickness) with a mass ratio of 35:1 between the piezoelectric material and hexavalent chromium.

[0071] (2) Hydrogen gas was introduced and the temperature was raised to 230°C. The amount of hydrogen gas was adjusted to control the total pressure to 4.5 MPa. The precursor solution was subjected to hydrothermal reaction at 230°C for 12 hours under stirring at 600 rpm to obtain a slurry containing hydrated chromium oxide.

[0072] (3) Under a nitrogen atmosphere, the slurry containing hydrated chromium oxide is subjected to solid-liquid separation, and the resulting filter cake is washed and dried to obtain hydrated chromium oxide.

[0073] Example 4

[0074] This embodiment provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution, the method comprising the following steps:

[0075] (1) Under a nitrogen atmosphere, a precursor solution was obtained by mixing a 500 mL sodium chromate solution with a concentration of 1.2 mol / L and a pre-polarized barium strontium titanate piezoelectric ceramic sheet (10 mm in diameter and 0.3 mm in thickness) with a mass ratio of 50:1 between the piezoelectric material and sodium chromate.

[0076] (2) Hydrogen gas was introduced and the temperature was raised to 260°C. The amount of hydrogen gas was adjusted to control the total pressure to 5 MPa. The precursor solution was subjected to hydrothermal reaction at 260°C for 6 hours under stirring at 600 rpm to obtain a slurry containing hydrated chromium oxide.

[0077] (3) Under a nitrogen atmosphere, the slurry containing hydrated chromium oxide is subjected to solid-liquid separation, and the resulting filter cake is washed and dried to obtain hydrated chromium oxide.

[0078] Example 5

[0079] This embodiment provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. The only difference from Embodiment 1 is that the temperature of the hydrothermal reaction in step (2) is 60°C, while the other steps remain unchanged.

[0080] Example 6

[0081] This embodiment provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. The only difference from Example 1 is that the total pressure of the reaction system in step (2) is 0.5 MPa, while the other steps remain unchanged.

[0082] Example 7

[0083] This embodiment provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. The only difference from Example 1 is that the total pressure of the reaction system in step (2) is 12 MPa, while the other steps remain unchanged.

[0084] Example 8

[0085] This embodiment provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. The only difference from Embodiment 1 is that the concentration of sodium dichromate solution in step (2) is 0.05 mol / L, while the other steps remain unchanged.

[0086] Example 9

[0087] This embodiment provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. The only difference from Example 1 is that the concentration of sodium dichromate solution in step (2) is 2 mol / L, while the other steps remain unchanged.

[0088] Comparative Example 1

[0089] This comparative example provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. The only difference from Example 1 is that piezoelectric material is not added in step (1).

[0090] Comparative Example 2

[0091] This comparative example provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. The only difference from Example 1 is that the piezoelectric material in step (1) is replaced with a strontium titanate paraelectric ceramic sheet (10 mm long, 5 mm wide, and 0.5 mm thick), while the other steps remain unchanged.

[0092] Comparative Example 3

[0093] This comparative example provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. The only difference from Example 1 is that the reaction atmosphere in step (2) is the same as in step (1), that is, no hydrogen gas is introduced.

[0094] Comparative Example 4

[0095] This comparative example provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. The difference from Example 1 is that no piezoelectric material is added in step (1), and the temperature of the hydrothermal reaction in step (2) is 300°C. Due to the temperature change, the total pressure of the reaction system increases to 13.3 MPa. The other steps remain unchanged.

[0096] Comparative Example 5

[0097] This comparative example provides a method for preparing hydrated chromium oxide by reduction of hexavalent chromium salt solution. The only difference from Example 2 is that piezoelectric material is not added in step (1).

[0098] test:

[0099] The hydrated chromium oxide and the filtrate after the reaction prepared in the examples and comparative examples were tested. The purity was calculated based on the chromium content in the hydrated chromium oxide using the melting and titration method of the national standard "Chromium Oxide Green Pigment GB / T 20785-2006". The hexavalent chromium concentration (c1) in the filtrate after titration was calculated as 1-c1 / c0. The test results are shown in Table 1 below.

[0100] Table 1

[0101]

[0102] The test results show that:

[0103] (1) As can be seen from Examples 1-4, under hydrothermal conditions, the present invention achieves efficient reduction of hexavalent chromium through the catalytic effect of piezoelectric materials and the synergistic effect of hydrogen, which significantly reduces the reaction temperature and pressure of hydrogen reduction of hexavalent chromium, and provides a new method for the low-temperature reduction of hexavalent chromium to prepare hydrated chromium oxide.

[0104] (2) By comparing Example 1 and Example 5, it can be seen that the present invention further controls the temperature of the hydrothermal reaction in step (2) to 80℃-240℃. The temperature of the hydrothermal reaction affects the catalytic effect of the piezoelectric material and its synergistic effect with hydrogen. If the temperature of the hydrothermal reaction is too high, the catalytic performance of the piezoelectric material and its synergistic effect with hydrogen will be weakened, affecting the reduction rate. If the temperature of the hydrothermal reaction is too low, the conversion rate of the reduction reaction itself will be affected. Even with the synergistic effect of piezoelectric catalysis and hydrogen, the final reduction rate will still be low.

[0105] (3) By comparing Example 1 with Example 6-7, it can be seen that the present invention further controls the total pressure of hydrogen to 0.5MPa-6MPa. The total pressure of the system during the hydrothermal reaction affects the reaction efficiency between hydrogen and hexavalent chromium. If the total pressure is too low, the reduction rate will be significantly reduced. Although the total pressure is too high, it has almost no effect on the reaction process, but it will cause the equipment cost to increase sharply, the cost to increase significantly, and the safety risk to increase.

[0106] (4) By comparing Example 1 with Examples 8-9, it can be seen that by further controlling the concentration of the hexavalent chromium salt solution to 0.01mol / L-1.2mol / L, the concentration of the hexavalent chromium salt solution has a slight effect on the purity and reduction rate of hydrated chromium oxide; if the concentration of the hexavalent chromium salt solution is too high, the purity and reduction rate of hydrated chromium oxide will be slightly reduced.

[0107] (5) As can be seen from Example 1 and Comparative Examples 1-2, the present invention can significantly improve the efficiency of hexavalent chromium reduction in solution by introducing piezoelectric materials to play a catalytic role. However, when piezoelectric materials are not used or are lacking, the reduction efficiency is greatly reduced.

[0108] (6) As can be seen from Example 1 and Comparative Example 3, the present invention can significantly improve the efficiency of hexavalent chromium reduction in solution by utilizing the synergistic effect of piezoelectric material and hydrogen. However, when there is no hydrogen, the amount of charge generated by the piezoelectric material is greatly reduced, and the amount of reduction electrons is insufficient for hexavalent chromium, so only a small amount of hexavalent chromium can be reduced, and the reduction rate is greatly reduced.

[0109] (7) As can be seen from Example 1 and Comparative Example 4, without the addition of piezoelectric materials, the reaction temperature must be increased to 300℃ and the total pressure of the reaction system must be increased to more than 13MPa to achieve a reduction rate comparable to that of Example 1. Therefore, this invention achieves efficient reduction of hexavalent chromium under hydrothermal conditions through the catalytic effect of piezoelectric materials and the synergistic effect of hydrogen, significantly reducing the reaction temperature and pressure of hydrogen reduction of hexavalent chromium, and providing a new method for the low-temperature reduction of hexavalent chromium to prepare hydrated chromium oxide.

[0110] (8) As can be seen from Example 2 and Comparative Example 5, the present invention can significantly improve the efficiency of hexavalent chromium reduction in solution under the same temperature and pressure conditions by introducing piezoelectric materials to play a catalytic role. However, when piezoelectric materials are not used or are lacking, the reduction efficiency is greatly reduced.

[0111] In summary, this invention achieves efficient reduction of hexavalent chromium under hydrothermal conditions through the catalytic effect of piezoelectric materials and the synergistic effect of hydrogen, significantly reducing the reaction temperature and pressure of hydrogen reduction of hexavalent chromium, and providing a new method for the low-temperature reduction of hexavalent chromium to prepare hydrated chromium oxide.

[0112] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing hydrated chromium oxide by reduction of a hexavalent chromium salt solution, characterized in that, The method includes the following steps: (1) Under an inert atmosphere, a hexavalent chromium salt solution and a piezoelectric material are mixed to obtain a precursor solution; (2) Under a hydrogen atmosphere, the precursor solution undergoes a hydrothermal reaction to obtain a slurry containing hydrated chromium oxide; (3) Under an inert atmosphere, the slurry containing hydrated chromium oxide is separated into solid and liquid components to obtain hydrated chromium oxide.

2. The method according to claim 1, characterized in that, In step (1), the mass ratio of the piezoelectric material to the hexavalent chromium salt in the hexavalent chromium salt solution is 50-1:1; And / or, the temperature of the hydrothermal reaction in step (2) is 80℃-240℃; And / or, the hydrothermal reaction time in step (2) is 4h-20h; And / or, in the hydrogen atmosphere described in step (2), the total pressure of the system is 0.5MPa-6MPa.

3. The method according to claim 1 or 2, characterized in that, The hydrothermal reaction in step (2) is carried out under stirring and / or ultrasonic conditions.

4. The method according to claim 3, characterized in that, The stirring speed is 500 rpm to 5000 rpm; And / or, the frequency of the ultrasound is 20kHz-400kHz; And / or, the power density of the ultrasound is 0.4 W / mL to 1.2 W / mL.

5. The method according to any one of claims 1-4, characterized in that, The Curie temperature of the piezoelectric material in step (1) is greater than 100°C.

6. The method according to any one of claims 1-5, characterized in that, The hexavalent chromium salt solution in step (1) includes any one or a combination of at least two of potassium dichromate solution, sodium dichromate solution, sodium chromate solution, or potassium chromate solution.

7. The method according to any one of claims 1-6, characterized in that, The concentration of the hexavalent chromium salt solution is 0.01 mol / L to 1.2 mol / L.

8. The method according to any one of claims 1-7, characterized in that, The inert atmosphere described in steps (1) and (3) includes a nitrogen atmosphere and / or an argon atmosphere.

9. The method according to any one of claims 1-8, characterized in that, After the solid-liquid separation in step (3) and before obtaining hydrated chromium oxide, the filter cake is washed and dried.

10. The method according to any one of claims 1-9, characterized in that, The method includes the following steps: (1) Under a nitrogen atmosphere, a hexavalent chromium salt solution with a concentration of 0.1 mol / L to 1 mol / L and a piezoelectric material are mixed in a mass ratio of 50-1:1 to obtain a precursor solution; (2) Hydrogen gas is introduced and the temperature is raised to 120℃-200℃. Under the condition that the total pressure is controlled by adjusting the amount of hydrogen gas to 1MPa-4MPa, the precursor solution is subjected to hydrothermal reaction at 120℃-200℃ for 6h-12h under stirring and / or ultrasonic conditions to obtain a slurry containing hydrated chromium oxide. (3) Under a nitrogen atmosphere, the slurry containing hydrated chromium oxide is subjected to solid-liquid separation, washing and drying to obtain hydrated chromium oxide.

Images