Method for preparing chromic oxide and sodium sulfate solution from chromium-containing sodium hydrogen sulfate

Abstract

The invention discloses a method for preparing a chromic oxide and sodium sulfate solution from chromium-containing sodium hydrogen sulfate, which comprises the following steps: preparing chromium-containing sodium hydrogen sulfate into a sheet by a sheet preparation machine, and calcining to obtain a calcined product; adding the calcined product into water to prepare a feed liquid, then adding sodium hydroxide for neutralization to prepare a mixed solution of chromic hydroxide and sodium sulfate, carrying out heat preservation and curing, then carrying out filter pressing to obtain a filter cake, then washing and carrying out filter pressing on the filter cake to obtain a chromic hydroxide filter cake, and carrying out filter pressing to obtain a filtrate which is a sodium sulfate solution; and calcining the chromium hydroxide filter cake to obtain chromium sesquioxide. According to the resource utilization method for the by-product chromium-containing sodium hydrogen sulfate in the chromic anhydride production process, the chromium-containing sodium hydrogen sulfate is converted into chromium sesquioxide and sodium sulfate solutions with high additional values, the problem of poison of waste is solved, and efficient recycling of chromium and sodium hydrogen sulfate is achieved; in addition, the method is simple in process, low in cost and suitable for industrial production.

Description

Technical Field

[0001] This invention belongs to the field of waste utilization technology. Specifically, this invention relates to a method for preparing chromium trioxide and sodium sulfate solutions from chromium-containing sodium bisulfate. Background Technology

[0002] Chromium trioxide (Cr₂O₃) is a dark green hexagonal or trigonal crystal with a metallic luster. It is insoluble in water and ethanol, slightly soluble in acids, and soluble in sulfuric acid to form purple chromium sulfate; it dissolves in strong alkalis to form dark green chromite, and also in hot alkali metal bromate solutions. It is mainly used in the coatings industry, serving as the primary coloring pigment in most green paints. It is also used in paints, plastics, ceramics, refractory materials, pigments, and as a catalyst in organic synthesis. Below are some common methods for preparing chromium trioxide:

[0003] Oxidation-reduction method: Potassium dichromate and sulfur are mixed and then subjected to a reduction reaction, followed by wet grinding, hot water washing, pressure filtration, drying, and pulverization.

[0004] Chromic anhydride decomposition method: Chromic anhydride can be obtained by thermal decomposition at 1100℃, followed by cooling and high-speed pulverization.

[0005] Chromium hydroxide decomposition method: Chromium hydroxide is thermally decomposed at high temperature, and then washed, dried, and pulverized to obtain the product.

[0006] Chromic anhydride, a key raw material in the chemical industry, plays a crucial role in metal surface treatment and pigment manufacturing. However, the production of chromic anhydride inevitably generates sodium bisulfate, a byproduct containing chromium. Sodium bisulfate is enriched with large amounts of impurities such as iron, aluminum, and vanadium, which significantly impacts product quality and limits its recycling.

[0007] In conclusion, researching how to prepare chromium trioxide from chromium-containing sodium bisulfate and achieving efficient resource recovery and utilization of chromium-containing sodium bisulfate is of great significance. Summary of the Invention

[0008] One object of the present invention is to solve at least the above-mentioned problems and / or defects, and to provide at least the advantages described below.

[0009] To achieve these objectives and other advantages of the present invention, a method for preparing a solution of chromium trioxide and sodium sulfate from sodium bisulfate containing chromium is provided, comprising the following steps:

[0010] Step 1: Chromium-containing sodium bisulfate is made into flakes using a flake-making machine, and then calcined to obtain the calcined product;

[0011] Step 2: Add the calcined product to water to prepare a liquid, neutralize it, keep it warm and mature, and then filter it to obtain a filter cake. Then wash the filter cake and filter it again to obtain a chromium hydroxide filter cake. The filtrate obtained by filtration is a sodium sulfate solution.

[0012] Step 3: Calcining the chromium hydroxide filter cake to obtain chromium trioxide.

[0013] Preferably, in step one, the calcination temperature is 600–1000℃ and the calcination time is 10–60 min.

[0014] Preferably, in step one, the rotation speed of the film-making machine is 30-80 r / min.

[0015] Preferably, in step two, a liquid solution with a concentration of 150-300 g / L is prepared; more preferably, a liquid solution with a concentration of 200-250 g / L is prepared.

[0016] Preferably, in step two, neutralization is achieved by adding sodium hydroxide to adjust the solution to neutral, i.e., pH = 7.

[0017] Preferably, in step two, the temperature for heat preservation and curing is 60–95°C, and the time is 2–4 hours.

[0018] Preferably, in step two, the pressure of the filter press is 0.1 to 0.3 MPa.

[0019] Preferably, in step three, the calcination temperature is 1200–1500℃ and the calcination time is 1–4 h; more preferably, the calcination temperature is 1300–1400℃ and the calcination time is 2–3 h.

[0020] Preferably, step two is replaced by: adding the calcined product to water to prepare a liquid, placing the liquid at a certain distance from the ultraviolet lamp for irradiation, adding sodium metabisulfite to the liquid during irradiation to reduce hexavalent chromium in the liquid to trivalent chromium, then adding sodium hydroxide for neutralization to obtain a mixture of chromium hydroxide and sodium sulfate, stopping the irradiation, keeping it warm for aging, obtaining a filter cake by pressure filtration, washing the filter cake and pressure filtration to obtain a chromium hydroxide filter cake, and the filtrate obtained by pressure filtration is a sodium sulfate solution.

[0021] Preferably, step three is replaced by: calcining the chromium hydroxide filter cake, cooling it, grinding it to 100-300 mesh, calcining it at 500-700℃ for 0.5-2 hours, and naturally cooling it to room temperature to obtain chromium trioxide.

[0022] Preferably, the ultraviolet lamp is a high-pressure mercury lamp with a wavelength of 320-380nm, a power of 300-1000W, and a distance of 5-30cm.

[0023] Preferably, the amount of sodium metabisulfite added is 1.1 to 1.5 times the theoretical amount required to completely reduce hexavalent chromium in the solution to trivalent chromium.

[0024] Preferably, the chromium-containing sodium bisulfate contains: 1.92–4.02 wt% sodium dichromate, 5.6–10.2 wt% total chromium, 0.10–0.19 wt% sulfate, and 0.20–0.28 wt% Fe2O3.

[0025] The present invention has at least the following beneficial effects: The present invention provides a method for the resource utilization of sodium bisulfate containing chromium, a byproduct in the production of chromic anhydride, which converts sodium bisulfate containing chromium into a high-value-added solution of chromium trioxide and sodium sulfate, solving the toxicity problem of waste and realizing the efficient recycling of chromium and sodium bisulfate; in addition, the method of the present invention is simple, low-cost, and suitable for industrial production.

[0026] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Detailed Implementation

[0027] The present invention will now be described in further detail so that those skilled in the art can implement it based on the description.

[0028] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0029] The sodium bisulfate containing chromium used in the following examples is a byproduct of the chromium anhydride workshop, containing 3.02 wt% sodium dichromate, 8.6 wt% total chromium, 0.17 wt% sulfate, and 0.24 wt% Fe2O3.

[0030] Example 1

[0031] A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate includes the following steps:

[0032] Step 1: The high-temperature molten sodium bisulfate containing chromium, a byproduct of the sulfuric acid process for producing chromic anhydride, is shaped into flakes by a flake-making machine and then calcined in a rotary kiln for 20 minutes to obtain the calcined product. The flake-making machine rotates at 40 r / min, the temperature inside the rotary kiln is 800℃, and the kiln speed is 50 r / h.

[0033] Step 2: Add the calcined product to water to prepare a solution with a concentration of 200 g / L, then add sodium hydroxide for neutralization to obtain a mixed solution of chromium hydroxide and sodium sulfate. Keep it at 80℃ for 3 hours and filter it under 0.2 MPa pressure to obtain a filter cake. Then wash the filter cake with water and filter it under pressure to obtain a chromium hydroxide filter cake. The filtrate obtained by pressure filtration is a sodium sulfate solution.

[0034] Step 3: The chromium hydroxide filter cake is fed into a rotary kiln at 1300℃ and calcined for 2 hours to obtain chromium trioxide product, in which the Cr2O3 content is 95.56%.

[0035] Example 2

[0036] A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate includes the following steps:

[0037] Step 1: The high-temperature molten sodium bisulfate containing chromium, a byproduct of the sulfuric acid process for producing chromic anhydride, is shaped into flakes by a flake-making machine and then calcined in a rotary kiln for 20 minutes to obtain the calcined product. The flake-making machine rotates at 40 r / min, the temperature inside the rotary kiln is 850℃, and the kiln speed is 50 r / h.

[0038] Step 2: Add the calcined product to water to prepare a solution with a concentration of 200 g / L, then add sodium hydroxide for neutralization to obtain a mixed solution of chromium hydroxide and sodium sulfate. Keep it at 80℃ for 3 hours and filter it under 0.2 MPa pressure to obtain a filter cake. Then wash the filter cake with water and filter it under pressure to obtain a chromium hydroxide filter cake. The filtrate obtained by pressure filtration is a sodium sulfate solution.

[0039] Step 3: The chromium hydroxide filter cake is fed into a rotary kiln at 1300℃ and calcined for 2 hours to obtain chromium trioxide product, in which the Cr2O3 content is 95.98%.

[0040] Example 3

[0041] A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate includes the following steps:

[0042] Step 1: The high-temperature molten sodium bisulfate containing chromium, a byproduct of the sulfuric acid process for producing chromic anhydride, is shaped into flakes by a flake-making machine and then calcined in a rotary kiln for 30 minutes to obtain the calcined product. The flake-making machine rotates at 40 r / min, the temperature inside the rotary kiln is 800℃, and the kiln speed is 50 r / h.

[0043] Step 2: Add the calcined product to water to prepare a solution with a concentration of 200 g / L, then add sodium hydroxide for neutralization to obtain a mixed solution of chromium hydroxide and sodium sulfate. Keep it at 80℃ for 3 hours and filter it under 0.2 MPa pressure to obtain a filter cake. Then wash the filter cake with water and filter it under pressure to obtain a chromium hydroxide filter cake. The filtrate obtained by pressure filtration is a sodium sulfate solution.

[0044] Step 3: The chromium hydroxide filter cake is fed into a rotary kiln at 1300℃ and calcined for 2 hours to obtain chromium trioxide product, in which the Cr2O3 content is 95.53%.

[0045] Example 4

[0046] A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate includes the following steps:

[0047] Step 1: The high-temperature molten sodium bisulfate containing chromium, a byproduct of the sulfuric acid process for producing chromic anhydride, is shaped into flakes by a flake-making machine and then calcined in a rotary kiln for 20 minutes to obtain the calcined product. The flake-making machine rotates at 40 r / min, the temperature inside the rotary kiln is 800℃, and the kiln speed is 70 r / h.

[0048] Step 2: Add the calcined product to water to prepare a solution with a concentration of 200 g / L, then add sodium hydroxide for neutralization to obtain a mixed solution of chromium hydroxide and sodium sulfate. Keep it at 80℃ for 3 hours and filter it under 0.2 MPa pressure to obtain a filter cake. Then wash the filter cake with water and filter it under pressure to obtain a chromium hydroxide filter cake. The filtrate obtained by pressure filtration is a sodium sulfate solution.

[0049] Step 3: The chromium hydroxide filter cake is fed into a rotary kiln at 1300℃ and calcined for 2 hours to obtain chromium trioxide product, in which the Cr2O3 content is 94.76%.

[0050] Example 5

[0051] A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate includes the following steps:

[0052] Step 1: The high-temperature molten sodium bisulfate containing chromium, a byproduct of the sulfuric acid process for producing chromic anhydride, is shaped into flakes by a flake-making machine and then calcined in a rotary kiln for 20 minutes to obtain the calcined product. The flake-making machine rotates at 40 r / min, the temperature inside the rotary kiln is 800℃, and the kiln speed is 50 r / h.

[0053] Step 2: Add the calcined product to water to prepare a solution with a concentration of 250 g / L, then add sodium hydroxide for neutralization to obtain a mixed solution of chromium hydroxide and sodium sulfate. Keep it at 80℃ for 3 hours and filter it under 0.2 MPa pressure to obtain a filter cake. Then wash the filter cake with water and filter it under pressure to obtain a chromium hydroxide filter cake. The filtrate obtained by pressure filtration is a sodium sulfate solution.

[0054] Step 3: The chromium hydroxide filter cake is fed into a rotary kiln at 1300℃ and calcined for 2 hours to obtain chromium trioxide product, in which the Cr2O3 content is 94.16%.

[0055] Example 6

[0056] A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate includes the following steps:

[0057] Step 1: The high-temperature molten sodium bisulfate containing chromium, a byproduct of the sulfuric acid process for producing chromic anhydride, is shaped into flakes by a flake-making machine and then calcined in a rotary kiln for 20 minutes to obtain the calcined product. The flake-making machine rotates at 40 r / min, the temperature inside the rotary kiln is 800℃, and the kiln speed is 50 r / h.

[0058] Step 2: Add the calcined product to water to prepare a solution with a concentration of 200 g / L, then add sodium hydroxide for neutralization to obtain a mixed solution of chromium hydroxide and sodium sulfate. Keep it at 80℃ for 3 hours and filter it under 0.2 MPa pressure to obtain a filter cake. Then wash the filter cake with water and filter it under pressure to obtain a chromium hydroxide filter cake. The filtrate obtained by pressure filtration is a sodium sulfate solution.

[0059] Step 3: The chromium hydroxide filter cake is fed into a rotary kiln at 1400℃ and calcined for 2 hours to obtain chromium trioxide product, in which the Cr2O3 content is 95.43%.

[0060] Example 7

[0061] A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate includes the following steps:

[0062] Step 1: The high-temperature molten sodium bisulfate containing chromium, a byproduct of the sulfuric acid process for producing chromic anhydride, is shaped into flakes by a flake-making machine and then calcined in a rotary kiln for 20 minutes to obtain the calcined product. The flake-making machine rotates at 40 r / min, the temperature inside the rotary kiln is 800℃, and the kiln speed is 50 r / h.

[0063] Step 2: Add the calcined product to water to prepare a solution with a concentration of 200 g / L, then add sodium hydroxide for neutralization to obtain a mixed solution of chromium hydroxide and sodium sulfate. Keep it at 80℃ for 3 hours and filter it under 0.2 MPa pressure to obtain a filter cake. Then wash the filter cake with water and filter it under pressure to obtain a chromium hydroxide filter cake. The filtrate obtained by pressure filtration is a sodium sulfate solution.

[0064] Step 3: The chromium hydroxide filter cake is fed into a rotary kiln at 1300℃ and calcined for 3 hours to obtain chromium trioxide product, in which the Cr2O3 content is 95.12%.

[0065] Example 8

[0066] A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate includes the following steps:

[0067] Step 1: The high-temperature molten sodium bisulfate containing chromium, a byproduct of the sulfuric acid process for producing chromic anhydride, is shaped into flakes by a flake-making machine and then calcined in a rotary kiln for 20 minutes to obtain the calcined product. The flake-making machine rotates at 70 r / min, the temperature inside the rotary kiln is 800℃, and the kiln speed is 50 r / h.

[0068] Step 2: Add the calcined product to water to prepare a solution with a concentration of 200 g / L, then add sodium hydroxide for neutralization to obtain a mixed solution of chromium hydroxide and sodium sulfate. Keep it at 80℃ for 3 hours and filter it under 0.2 MPa pressure to obtain a filter cake. Then wash the filter cake with water and filter it under pressure to obtain a chromium hydroxide filter cake. The filtrate obtained by pressure filtration is a sodium sulfate solution.

[0069] Step 3: The chromium hydroxide filter cake is fed into a rotary kiln at 1300℃ and calcined for 2 hours to obtain chromium trioxide product, in which the Cr2O3 content is 95.28%.

[0070] Example 9

[0071] A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate includes the following steps:

[0072] Step 1: The high-temperature molten sodium bisulfate containing chromium, a byproduct of the sulfuric acid process for producing chromic anhydride, is shaped into flakes by a flake-making machine and then calcined in a rotary kiln for 20 minutes to obtain the calcined product. The flake-making machine rotates at 40 r / min, the temperature inside the rotary kiln is 850℃, and the kiln speed is 50 r / h.

[0073] Step 2: Add the calcined product to water to prepare a solution with a concentration of 200 g / L. Place the solution 20 cm away from an ultraviolet lamp (high-pressure mercury lamp, wavelength 365 nm, power 500 W) for irradiation. While irradiating, add sodium metabisulfite to the solution to reduce hexavalent chromium to trivalent chromium. Then add sodium hydroxide for neutralization to obtain a mixture of chromium hydroxide and sodium sulfate. Stop irradiation and mature at 80℃ for 3 hours. Filter the solution under 0.2 MPa pressure to obtain a filter cake. Wash the filter cake with water and filter it again to obtain a chromium hydroxide filter cake. The filtrate obtained by filtration is the sodium sulfate solution. The amount of sodium metabisulfite added is 1.2 times the amount required to completely reduce hexavalent chromium to trivalent chromium in the solution.

[0074] Step 3: The chromium hydroxide filter cake is fed into a rotary kiln at 1300℃ and calcined for 2 hours. After cooling, it is ground to 200 mesh and then calcined at 600℃ for 1 hour. After natural cooling to room temperature, chromium trioxide product is obtained, in which the Cr2O3 content is 98.72%.

[0075] In this embodiment, in step two, sodium metabisulfite is used as a reducing agent to reduce the residual hexavalent chromium in the liquid to trivalent chromium. At the same time, ultraviolet irradiation is used during the reduction and precipitation process to promote the reduction of hexavalent chromium and the precipitation of chromium hydroxide, thereby improving the reaction efficiency. Then, in step three, the product after high-temperature calcination is ground and then calcined again at low temperature (600°C). Compared with Example 2, the chromium trioxide product obtained in Example 9 has a higher Cr2O3 content.

[0076] Example 10

[0077] A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate includes the following steps:

[0078] Step 1: The high-temperature molten sodium bisulfate containing chromium, a byproduct of the sulfuric acid process for producing chromic anhydride, is shaped into flakes by a flake-making machine and then calcined in a rotary kiln for 20 minutes to obtain the calcined product. The flake-making machine rotates at 40 r / min, the temperature inside the rotary kiln is 850℃, and the kiln speed is 50 r / h.

[0079] Step 2: Add the calcined product to water to prepare a solution with a concentration of 200 g / L. Add sodium metabisulfite to reduce hexavalent chromium in the solution to trivalent chromium. Then add sodium hydroxide for neutralization to obtain a mixed solution of chromium hydroxide and sodium sulfate. Keep it at 80℃ for 3 hours and filter it under 0.2 MPa pressure to obtain a filter cake. Wash the filter cake with water and filter it under pressure to obtain a chromium hydroxide filter cake. The filtrate obtained by pressure filtration is the sodium sulfate solution. The amount of sodium metabisulfite added is 1.2 times the amount required to completely reduce hexavalent chromium in the solution to trivalent chromium.

[0080] Step 3: The chromium hydroxide filter cake is fed into a rotary kiln at 1300℃ and calcined for 2 hours. After cooling, it is ground to 200 mesh and then calcined at 600℃ for 1 hour. After natural cooling to room temperature, chromium trioxide product is obtained, in which the Cr2O3 content is 98.04%.

[0081] Example 11

[0082] A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate includes the following steps:

[0083] Step 1: The high-temperature molten sodium bisulfate containing chromium, a byproduct of the sulfuric acid process for producing chromic anhydride, is shaped into flakes by a flake-making machine and then calcined in a rotary kiln for 20 minutes to obtain the calcined product. The flake-making machine rotates at 40 r / min, the temperature inside the rotary kiln is 850℃, and the kiln speed is 50 r / h.

[0084] Step 2: Add the calcined product to water to prepare a solution with a concentration of 200 g / L, then add sodium hydroxide for neutralization to obtain a mixed solution of chromium hydroxide and sodium sulfate. Keep it at 80℃ for 3 hours and filter it under 0.2 MPa pressure to obtain a filter cake. Then wash the filter cake with water and filter it under pressure to obtain a chromium hydroxide filter cake. The filtrate obtained by pressure filtration is a sodium sulfate solution.

[0085] Step 3: The chromium hydroxide filter cake is fed into a rotary kiln at 1300℃ and calcined for 2 hours. After cooling, it is ground to 200 mesh and then calcined at 600℃ for 1 hour. After natural cooling to room temperature, chromium trioxide product is obtained, in which the Cr2O3 content is 96.58%.

[0086] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and embodiments shown and described herein.

Claims

1. A method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate, characterized in that, Includes the following steps: Step 1: Chromium-containing sodium bisulfate is made into flakes using a flake-making machine, and then calcined to obtain the calcined product; Step 2: Add the calcined product to water to prepare a liquid, neutralize it, keep it warm and mature, and then filter it to obtain a filter cake. Then wash the filter cake and filter it again to obtain a chromium hydroxide filter cake. The filtrate obtained by filtration is a sodium sulfate solution. Step 3: Calcine the chromium hydroxide filter cake to obtain chromium trioxide.

2. The method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate as described in claim 1, characterized in that, In step one, the calcination temperature is 600–1000℃ and the calcination time is 10–60 min.

3. The method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate as described in claim 1, characterized in that, In step one, the rotation speed of the film-making machine is 30-80 r / min.

4. The method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate as described in claim 1, characterized in that, In step two, a liquid solution with a concentration of 150–300 g / L is prepared.

5. The method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate as described in claim 1, characterized in that, In step two, neutralization is achieved by adding sodium hydroxide to adjust the pH of the solution to neutral.

6. The method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate as described in claim 1, characterized in that, In step two, the temperature for heat preservation and maturation is 60–95°C, and the time is 2–4 hours.

7. The method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate as described in claim 1, characterized in that, In step two, the pressure for pressure filtration is 0.1 to 0.3 MPa.

8. The method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate as described in claim 1, characterized in that, In step three, the calcination temperature is 1200–1500℃ and the calcination time is 1–4 hours.

9. The method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate as described in claim 1, characterized in that, Step two is replaced by: adding the calcined product to water to prepare a liquid, placing the liquid at a certain distance from the ultraviolet lamp for irradiation, adding sodium metabisulfite to the liquid while irradiating to reduce hexavalent chromium in the liquid to trivalent chromium, adding sodium hydroxide for neutralization, stopping the irradiation, keeping it warm and ripening, obtaining a filter cake by pressure filtration, washing the filter cake and pressure filtration to obtain chromium hydroxide filter cake, and the filtrate obtained by pressure filtration is sodium sulfate solution.

10. The method for preparing chromium trioxide and sodium sulfate solution from chromium-containing sodium bisulfate as described in claim 1, characterized in that, Step three is replaced by: calcining the chromium hydroxide filter cake, cooling it, grinding it to 100-300 mesh, calcining it at 500-700℃ for 0.5-2 hours, and then naturally cooling it to room temperature to obtain chromium trioxide.