A method for preparing chromium salts by liquid-phase oxidation of chromite

Abstract

The present invention relates to a method for preparing chromate by liquid-phase oxidation of chromite. The method for preparing chromate by liquid-phase oxidation of chromite comprises the following steps: under an oxygen atmosphere condition, mixing chromite, sodium hydroxide, sodium salt and water to carry out a liquid-phase oxidation reaction, relieving pressure and cooling, keeping warm and settling, and performing solid-liquid separation to obtain a supernatant and reaction slag; leaching and separating the reaction slag to obtain a leaching residue solution and iron-rich slag; adding barium nitrate to the leaching residue solution to carry out a precipitation reaction, and separating to obtain barium chromate and dilute alkali solution; adding nitric acid to the barium chromate, and then adding ethanol to carry out a reduction reaction to obtain a mixed solution of chromium nitrate and barium nitrate; adjusting the pH value of the mixed solution, and separating to obtain chromium hydroxide and a solution rich in barium nitrate; and calcining the chromium hydroxide at a high temperature to obtain a chromate product. The present invention solves the problems of low product purity, poor economy of by-products, complex process and low production efficiency existing in the production of chromate by the existing liquid-phase oxidation method of chromite.

Description

Technical Field

[0001] The present invention relates to the technical field of chromate production, and particularly relates to a method for preparing chromate by liquid-phase oxidation of chromite. Background Art

[0002] Chromates (such as sodium dichromate, chromium oxide, chromic anhydride, chromium powder, etc.) are important inorganic chemical products, known as the "industrial monosodium glutamate", and are mainly applied in various fields such as high-performance alloys, electroplating, leather, pigments, printing and dyeing, ceramics, anti-corrosion, catalysis, medicine, etc. Sodium dichromate is an important platform compound for producing a series of chromate products. It is usually prepared by processes such as high-temperature roasting, extraction of clinker, and purification of chromite under aerobic conditions. Depending on the different fillers used during the oxidative roasting of chromite, the production process of sodium dichromate has gone through three generations: calcium-containing roasting, less-calcium roasting, and calcium-free roasting. Under the positive role and influence of national macro-control policies, China fully replaced the calcium-containing roasting process with the calcium-free roasting process in 2013, achieving the green technology upgrade of the traditional heavy-pollution chromate industry in China. Although the current mainstream calcium-free chromate production technology is more advanced than the previous two generations of processes in terms of economy and environmental protection, there are still technical defects such as a relatively low extraction rate of chromium resources in the ore (about 85%), a relatively high overall energy consumption (roasting temperature 1100 - 1300 °C), and a large amount of solid waste emissions (about 0.8 tons of chromium slag are discharged per ton of chromate produced). The chromate industry is facing huge pressure for energy conservation, emission reduction, and efficient utilization of chromium resources.

[0003] In order to achieve a conversion process that is compatible with the natural environment and has efficient resource utilization, since the 1990s, many enterprises, universities, and research institutions have started to conduct pilot tests and actively explore new ways for green and clean production of chromates. In this regard, the potassium-based liquid-phase oxidation process for producing chromates in sub-molten salts is the most representative. Compared with the calcium-free roasting process, the reaction temperature of this process is significantly reduced (250 - 400 °C), and the chromium conversion rate is significantly increased to nearly 100%. However, the product produced by this method is potassium dichromate, which has a small share in the chromate market and low economic benefits. In addition, the amount of potassium hydroxide consumed by this method is huge, and most of the domestic potassium resources rely on imports, which further restricts the industrial application of this technology. Therefore, using cheaper alkali metals as reaction media to produce bulk chromate products that meet market demand has become the development direction of the sub-molten salt liquid-phase oxidation method. On this basis, the process for preparing chromates by sodium-based sub-molten salt liquid-phase oxidation was proposed. The sodium-based process is highly similar to the potassium-based process, and many achievements have been made in the research on this process, especially in the aspect of strengthening the leaching of chromite. However, due to the small particle size of the leaching residue, the high viscosity of the reaction medium, and the high concentration of caustic soda, the problems of efficient separation of the leaching slurry / leaching residue and low-cost separation between high-concentration caustic soda / chromate have not been broken through, becoming the technical bottleneck and difficulty restricting the industrialization of the chromite liquid-phase oxidation process for producing chromates.

[0004] A clean production method for continuously preparing chromate by liquid-phase oxidation is disclosed in CN 106186068 A. In this production method, the liquid-phase oxidation leaching slurry is first diluted and cooled, and then subjected to liquid-solid separation to obtain a mixed solution containing chromate and caustic alkali and wet leaching residues respectively. The mixed solution is subjected to a series of steps such as evaporation and crystallization to obtain chromate crystals and an alkali-containing solution. The wet leaching residues are subjected to countercurrent washing to recover chromate and caustic alkali solution, thereby realizing the separation of the main components in the leaching slurry. However, since the original alkali concentration is diluted by adding water, the dilute alkali solution needs to be further concentrated before it can be recycled back to the system. In addition, when using the evaporation and crystallization process to separate the crude sodium chromate product from the chromium / alkali leaching solution, there are problems of impure products and high energy consumption and time consumption.

[0005] A method for separating caustic alkali and chromate in a solid mixture containing caustic alkali and chromate is disclosed in CN 104512929 A. This method proposes a technical route for separating chromium / alkali by methanol extraction and distillation. Specifically, taking advantage of the characteristics that caustic alkali is easily soluble in methanol while chromate is hardly soluble in methanol, using methanol as the separation medium, the leaching slurry is mixed with alcohol in a certain proportion, and after extraction for a certain time, solid-liquid separation is carried out. The chromate remains in the solid phase, and the liquid phase is a mixed solution of caustic alkali and alcohol. The alcohol distilled out after distillation is recycled after condensation, and the remaining is caustic alkali. However, in this method, the amount of methanol used is relatively large, resulting in high energy consumption for methanol recovery, and methanol has high requirements for the device sealing performance, greatly increasing the equipment investment cost.

[0006] In view of the above two key technical problems, the inventor of this case has developed a new process for heat preservation sedimentation separation of small particle leaching residues / reaction media and leaching solutions in a high-viscosity system (CN110947319A) and an efficient separation process between high-concentration caustic alkali / chromium salts based on Ba(OH)2 (CN106987733B), which not only greatly reduces the separation time and process energy consumption, but also avoids diluting the reaction solution and ensures the concentration of recycled alkali to the greatest extent. On this basis, the inventor has also developed a process for producing chromium oxide green from barium chromate intermediate (CN110407253A). Through the coupling of the above several key technologies, the bottlenecks in the existing process of sodium-based liquid-phase oxidation of chromite to produce chromate salts are basically cleared, laying a technical foundation for its industrial application. However, in further practical practice, it is found that the above process still has the following technical defects: ① After heat preservation sedimentation separation of the leaching residues / liquids, if the supernatant is directly returned to the autoclave for reuse without chromium extraction, the unseparated chromate salts will greatly inhibit the chromium leaching rate during the subsequent liquid-phase oxidation of chromite. Therefore, before the supernatant is recycled, chromium / alkali separation must be carried out, which complicates the process; ② When using solid-phase Ba(OH)2 as the transfer reagent for chromium / alkali separation, due to the common ion effect, the solubility of Ba(OH)2 in a high-alkali medium decreases, resulting in CrO4 2- and Ba 2+The precipitation conversion reaction rate between them is relatively slow, taking a long time, and the chromium / alkali separation efficiency needs to be further improved; ③ Using the process of dissolving with hydrochloric acid and reducing barium chromate with ethanol to produce chromium salts, while obtaining the main product chromium hydroxide, the by-product BaSO4 is obtained, but the added value of BaSO4 is lower than that of Ba(OH)2, and its outlet is narrow, and the technical economy is poor; ④ Using HCl as the dissolving medium, due to its strong corrosion ability, the requirements for equipment materials are high, resulting in a large investment cost, and it is easy to introduce Cl - into it, and Cl - will be brought into the downstream derivative products (such as chromium oxide green), thus affecting the product quality. SUMMARY OF THE INVENTION

[0007] The purpose of the present invention is to provide a method for preparing chromium salts by liquid-phase oxidation of chromite, so as to solve the problems of low product purity, poor by-product economy, complex process and low production efficiency existing in the current liquid-phase oxidation production of chromium salts from chromite.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0009] A method for preparing chromium salts by liquid-phase oxidation of chromite, comprising the following steps:

[0010] S1. Under the condition of an oxygen atmosphere with an oxygen partial pressure greater than 1 MPa, mix chromite, sodium hydroxide, sodium salt and water for liquid-phase oxidation reaction, relieve pressure and cool, keep warm and settle, and separate the solid and liquid to obtain supernatant liquid and reaction slag;

[0011] S2. Wash and separate the reaction slag to obtain wash slag liquid and iron-rich slag;

[0012] S3. Add barium nitrate to the wash slag liquid for precipitation reaction, and separate to obtain barium chromate and dilute alkali liquid;

[0013] S4. Add nitric acid to barium chromate, and then add ethanol for reduction reaction to obtain a mixed solution of chromium nitrate and barium nitrate;

[0014] S5. Adjust the pH value of the mixed solution, and separate to obtain chromium hydroxide and a solution rich in barium nitrate;

[0015] S6. Calcinate chromium hydroxide at high temperature to obtain chromium salt products.

[0016] Among them, the reaction medium of the liquid-phase oxidation reaction is a ternary sub-molten salt mixed system of NaOH-NaNO3-H2O.

[0017] Preferably, in S1, the mass ratio of sodium hydroxide to chromite is 3:1 to 6:1; the mass ratio of sodium salt to chromite is 0.4:1 to 1:1; in the mixed solution, the mass concentration of sodium hydroxide is 45% to 60%.

[0018] Preferably, in the step S1, the temperature of the liquid-phase oxidation reaction is between 220°C and 260°C; the oxygen partial pressure is between 1.4 MPa and 2.2 MPa; and the reaction time is between 2 h and 5 h.

[0019] Preferably, in the step S1, the sodium salt is sodium nitrate, and the mass ratio of sodium nitrate to chromite is 0.8:1; the mass ratio of sodium hydroxide to chromite is 4:1; in the mixed solution, the mass concentration of sodium hydroxide is 50%.

[0020] Preferably, in the step S1, the temperature of the liquid-phase oxidation reaction is 240°C; the oxygen partial pressure is 2 MPa; and the reaction time is 3 h.

[0021] Preferably, in the step S1, stirring is also required during the liquid-phase oxidation reaction, and the stirring speed is 700 rpm to 900 rpm.

[0022] Preferably, in the step S1, the stirring speed for the liquid-phase oxidation reaction is 800 rpm.

[0023] Preferably, in the step S3, the molar ratio of sodium chromate to barium nitrate in the slag washing liquid is 0.9:1 to 1.1:1; the temperature of the precipitation reaction is between 30°C and 70°C.

[0024] Preferably, in the step S3, the molar ratio of sodium chromate to barium nitrate in the slag washing liquid is 1:1, and the temperature of the precipitation reaction is 60°C.

[0025] Preferably, in the step S3, the stirring speed for the precipitation reaction is 100 rpm to 400 rpm, and the precipitation reaction time is 5 to 30 min.

[0026] Preferably, in the step S3, the stirring speed for the precipitation reaction is 300 rpm, and the precipitation reaction time is 20 min.

[0027] Preferably, in the step S4, the molar ratio of nitric acid to barium chromate is 4:1 to 6:1; the liquid-solid ratio of nitric acid and ethanol to barium chromate is 4:1 to 7:1 L / kg; the temperature of the reduction reaction is 30°C to 70°C.

[0028] Preferably, in the step S4, the molar ratio of nitric acid to barium chromate is 5:1; the liquid-solid ratio of nitric acid and ethanol to barium chromate is 5:1 L / kg; the temperature of the reduction reaction is 60°C.

[0029] Preferably, in the step S4, the ethanol addition amount is 1 to 4 times the theoretical amount. Here, the theoretical amount refers to the ethanol amount required to reduce 1 mol of barium chromate according to the equation CH3CH2OH + 4BaCrO4 + 20HNO3 = 2CO2↑ + 4Cr(NO3)3 + 13H2O + 4Ba(NO3)2.

[0030] Preferably, in S4, the addition amount of ethanol is 3 times the theoretical amount.

[0031] Preferably, in S3, the dilute alkali solution is a dilute alkali solution rich in sodium aluminate and sodium nitrate;

[0032] The method further includes: adding a sodium silicate slurry to the dilute alkali solution rich in sodium aluminate and sodium nitrate for an aluminum removal precipitation reaction, and separating to obtain a dilute alkali solution rich in sodium nitrate and a sodium aluminosilicate product;

[0033] The separated dilute alkali solution rich in sodium nitrate is evaporated and concentrated to precipitate a sodium nitrate product and an evaporation crystallization mother liquor. The evaporation crystallization mother liquor is combined with the supernatant in S1, and after adding alkali, it is returned to participate in the liquid-phase oxidation reaction.

[0034] Preferably, in S5, a sodium hydroxide solution with a mass fraction of 10% is used to adjust the pH value of the mixed solution to 8 - 9.

[0035] Preferably, in S5, the solution rich in barium nitrate is evaporated and concentrated, and the obtained barium nitrate is used as a precipitating agent for converting sodium chromate to barium chromate in the S3 slag washing solution.

[0036] Advantages of the present invention:

[0037] 1) By introducing NaNO3 into the traditional sodium-based liquid-phase oxidation reaction system of chromite, a ternary sub-molten salt mixed system with a reaction medium of NaOH - NaNO3 - H2O is constructed; during the entire leaching process, sodium nitrate is first reduced to sodium nitrite, and as the reaction proceeds, sodium nitrite is oxidized to sodium nitrate by oxygen. Sodium nitrate is not consumed during the whole reaction but serves as a carrier for oxygen transportation; the synergistic effect of NaNO3 and O2 accelerates the destruction and decomposition of the chromite spinel structure, strengthens the leaching process of chromite, and compared with the NaOH - H2O binary sub-molten salt mixed medium, the chromium leaching rate can be increased by about 10% - 35% under the same conditions;

[0038] 2) After introducing NaNO3 into the NaOH - H2O reaction medium, the properties of this mixed system are changed, resulting in a fundamental change in the distribution of sodium chromate leached from chromite between the slag / liquid two phases. Sodium chromate is transferred from being present in the leaching solution to being present in the slag phase, so that it is no longer necessary to extract chromium first from the leaching solution after heat preservation sedimentation separation to achieve chromium / alkali separation. The small amount of chromium salt remaining in the leaching solution will not inhibit the chromium leaching rate of the recycled leaching solution for the next batch of incoming chromite, ensuring the effective utilization of the medium;

[0039] 3) By replacing slightly soluble Ba(OH)₂ with more soluble Ba(NO₃)₂ as the transfer reagent for chromium salts, the defect that the precipitation conversion reaction rate between CrO₄ 2- and Ba 2+ is slow and time-consuming due to the common ion effect causing the decreased solubility of Ba(OH)₂ in a high-alkali medium is overcome, and the separation efficiency of chromium / alkali is significantly improved;

[0040] 4) By adopting the process of dissolving with nitric acid and reducing barium chromate with ethanol to prepare chromium salts, that is, using HNO₃ instead of HCl as the dissolution medium, the material requirements for equipment are more lenient; and the NO₃ - entrained when chromium precipitates in the form of chromium hydroxide can be decomposed and removed synchronously during the preparation of subsequent products (such as calcining chromium hydroxide to prepare chromium oxide green), making the obtained product of better quality;

[0041] 5) After completely precipitating the chromium nitrate generated during the dissolution and reduction process in the form of chromium hydroxide, the liquid phase obtained by solid-liquid separation is concentrated, and the obtained barium nitrate can be recycled for use as a precipitating agent for converting sodium chromate to barium chromate in the slag washing liquid, effectively realizing the recycling of barium salts;

[0042] 6) The method for preparing chromium salts by liquid-phase oxidation of chromite in the present invention no longer produces by-products of barium salts, breaks through the restriction of the technical defect of the poor outlet of chromium-containing barium salt hazardous waste, realizes the recycling of barium resources and caustic alkali in the reaction system, and sodium nitrate only acts as a carrier for transporting oxygen without consumption, meeting the requirements of efficient, stable and continuous leaching of chromite, and the process flow is more compact and simple, with lower labor intensity, meeting the strategic requirements of the national green circular economy, having a more promising industrial application prospect, and having promotional application value in the field of chromium salt cleaner production technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 is a flow chart of the method for preparing chromium salts by liquid-phase oxidation of chromite in the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] The following will illustrate the embodiments of the present invention with reference to the drawings and preferred embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are only for illustrating the present invention, rather than for limiting the protection scope of the present invention.

[0045] Example 1

[0046] As Figure 1As shown, a method for preparing chromium salts by liquid-phase oxidation of chromite includes the following steps:

[0047] S1. Add chromite with a particle size of 300 mesh, sodium hydroxide, sodium nitrate, and water into a high-pressure reactor for mixing. While stirring, introduce oxygen into the high-pressure reactor for liquid-phase oxidation reaction. Among them, the mass ratio of sodium nitrate to chromite is 0.8:1, the mass ratio of sodium hydroxide to chromite is 4:1, the mass concentration of sodium hydroxide in the mixed solution is 50 wt.%, the temperature of the liquid-phase oxidation reaction is 240 °C, the stirring speed is 800 rpm, the oxygen partial pressure is 2.0 Mpa, and the reaction time is 3 h. After the reaction, relieve the pressure and cool down. Then transfer the materials in the high-pressure reactor to a constant-temperature oven at 90 °C for heat preservation and sedimentation for 4 h, and perform solid-liquid separation on the sedimented materials to obtain the supernatant and reaction slag.

[0048] S2. According to the ratio of water to reaction slag of 1:1 L / kg, wash the reaction slag with water and then separate it to obtain the wash slag solution and iron-rich slag.

[0049] S3. Add barium nitrate to the wash slag solution for precipitation reaction. Among them, the molar ratio of sodium chromate to barium nitrate in the wash slag solution is 1:1, the reaction temperature is 60 °C. During the precipitation reaction, stirring is required, the stirring speed is 300 rpm, the precipitation reaction time is 20 min. After the reaction, separate to obtain barium chromate precipitate, and dilute alkali solution of sodium aluminate and sodium nitrate;

[0050] Add sodium silicate slurry to the dilute alkali solution rich in sodium aluminate and sodium nitrate for precipitation reaction, and separate to obtain the dilute alkali solution rich in sodium nitrate and sodium aluminosilicate product;

[0051] Evaporate and concentrate the dilute alkali solution rich in sodium nitrate to obtain sodium nitrate product and evaporation crystallization mother liquor. The evaporation crystallization mother liquor is combined with the supernatant in S1, supplemented with alkali, and then returned to participate in the liquid-phase oxidation reaction;

[0052] S4. Add nitric acid to dissolve the barium chromate precipitate, and then add the reducing agent ethanol for reduction reaction to obtain a mixed solution of chromium nitrate and barium nitrate. Among them, the molar ratio of nitric acid to barium chromate is 5:1, the liquid-solid ratio of nitric acid and ethanol to barium chromate is 5:1 L / kg, the ethanol addition amount is 3 times the theoretical amount, and the reduction reaction temperature is 60 °C;

[0053] S5. Adjust the pH value of the mixed solution to 8 - 9 with a 10% sodium hydroxide solution by mass, so that chromium is completely precipitated in the form of chromium hydroxide, and separate to obtain chromium hydroxide and a solution rich in barium nitrate;

[0054] The solution rich in barium nitrate is evaporated and concentrated, and the obtained barium nitrate participates in the precipitation reaction of the wash slag solution in S3;

[0055] S6. Calcinate chromium hydroxide at high temperature to obtain chromium oxide green product.

[0056] Example 2

[0057] A method for preparing chromium salts by liquid-phase oxidation of chromite, comprising the following steps:

[0058] S1. Add chromite with a particle size of 300 mesh, sodium hydroxide, sodium nitrate and water into a high-pressure reactor for mixing. Under stirring conditions, introduce oxygen into the high-pressure reactor for liquid-phase oxidation reaction. Among them, the mass ratio of sodium nitrate to chromite is 0.4:1, the mass ratio of sodium hydroxide to chromite is 6:1, the mass concentration of sodium hydroxide in the mixed solution is 45 wt.%, the temperature of the liquid-phase oxidation reaction is 220 °C, the stirring speed is 800 rpm, the oxygen partial pressure is 2.2 Mpa, and the reaction time is 5 h; after the reaction, relieve pressure and cool down; then transfer the materials in the high-pressure reactor to a constant temperature oven at 90 °C for heat preservation and sedimentation for 4 h, and perform solid-liquid separation on the sedimented materials to obtain supernatant and reaction slag;

[0059] S2. Wash the reaction slag with water according to the ratio of water to reaction slag of 1:1 L / kg and separate to obtain wash slag liquid and iron-rich slag;

[0060] S3. Add barium nitrate to the wash slag liquid for precipitation reaction. Among them, the molar ratio of sodium chromate to barium nitrate in the wash slag liquid is 1.1:1, the reaction temperature is 70 °C, stirring is required during the precipitation reaction, the stirring speed is 300 rpm, the precipitation reaction time is 20 min, and after the reaction, separate to obtain barium chromate precipitate, and dilute alkali liquid of sodium aluminate and sodium nitrate;

[0061] Add sodium silicate slurry to the dilute alkali liquid rich in sodium aluminate and sodium nitrate for precipitation reaction, and separate to obtain dilute alkali liquid rich in sodium nitrate and aluminosilicate product;

[0062] Evaporate and concentrate the dilute alkali liquid rich in sodium nitrate to obtain sodium nitrate product and evaporation crystallization mother liquor. The evaporation crystallization mother liquor is combined with the supernatant in S1, supplemented with alkali, and then returned to participate in the liquid-phase oxidation reaction;

[0063] S4. Add nitric acid to the barium chromate precipitate for dissolution, and then add reducing agent ethanol for reduction reaction to obtain a mixed solution of chromium nitrate and barium nitrate. Among them, the molar ratio of nitric acid to barium chromate is 6:1, the liquid-solid ratio of nitric acid and ethanol to barium chromate is 4:1 L / kg, the ethanol addition amount is 3 times of the theoretical amount, and the reduction reaction temperature is 30 °C;

[0064] S5. Adjust the pH value of the mixed solution to 8-9 with 10% sodium hydroxide solution by mass fraction to completely precipitate chromium in the form of chromium hydroxide, and separate to obtain chromium hydroxide and a solution rich in barium nitrate;

[0065] The solution rich in barium nitrate is evaporated and concentrated, and the obtained barium nitrate participates in the precipitation reaction of the slag washing solution in S3;

[0066] S6. Calcine chromium hydroxide at high temperature to obtain chromium oxide green product.

[0067] Example 3

[0068] A method for preparing chromium salts by liquid-phase oxidation of chromite, comprising the following steps:

[0069] S1. Add chromite with a particle size of 300 mesh, sodium hydroxide, sodium nitrate and water into a high-pressure reaction kettle for mixing, and introduce oxygen into the high-pressure reaction kettle under stirring conditions for liquid-phase oxidation reaction. Among them, the mass ratio of sodium nitrate to chromite is 1:1, the mass ratio of sodium hydroxide to chromite is 3:1, the mass concentration of sodium hydroxide in the mixed solution is 60 wt.%, the temperature of the liquid-phase oxidation reaction is 260 °C, the stirring speed is 800 rpm, the oxygen partial pressure is 1.4 Mpa, and the reaction time is 2 h; after the reaction, relieve pressure and cool; then transfer the materials in the high-pressure reaction kettle to a constant temperature oven at 90 °C for heat preservation and sedimentation for 4 h, and carry out solid-liquid separation on the sedimented materials to obtain supernatant and reaction slag;

[0070] S2. Wash the reaction slag with water according to the ratio of water to reaction slag of 1:1 L / kg and separate to obtain slag washing solution and iron-rich slag;

[0071] S3. Add barium nitrate to the slag washing solution for precipitation reaction. Among them, the molar ratio of sodium chromate to barium nitrate in the slag washing solution is 0.9:1, the reaction temperature is 30 °C, stirring is required during the precipitation reaction, the stirring speed is 300 rpm, the precipitation reaction time is 20 min, and after the reaction, separate to obtain barium chromate precipitate, and dilute alkali solution rich in sodium aluminate and sodium nitrate;

[0072] Add sodium silicate slurry to the dilute alkali solution rich in sodium aluminate and sodium nitrate for precipitation reaction, and separate to obtain dilute alkali solution rich in sodium nitrate and sodium aluminosilicate product;

[0073] Evaporate and concentrate the dilute alkali solution rich in sodium nitrate to obtain sodium nitrate product and evaporation crystallization mother liquor. The evaporation crystallization mother liquor is combined with the supernatant in S1, supplemented with alkali and then returned to participate in the liquid-phase oxidation reaction;

[0074] S4. Add nitric acid to the barium chromate precipitate for dissolution, and then add reducing agent ethanol for reduction reaction to obtain a mixed solution of chromium nitrate and barium nitrate. Among them, the molar ratio of nitric acid to barium chromate is 4:1, the liquid-solid ratio of nitric acid and ethanol to barium chromate is 7:1 L / kg, the ethanol addition amount is 3 times the theoretical amount, and the reduction reaction temperature is 70 °C;

[0075] S5. Adjust the pH value of the mixed solution to 8 - 9 with a 10% sodium hydroxide solution by mass fraction to completely precipitate chromium in the form of chromium hydroxide, and separate to obtain chromium hydroxide and a solution rich in barium nitrate;

[0076] The solution rich in barium nitrate is concentrated by evaporation, and the obtained barium nitrate participates in the precipitation reaction of the slag washing solution in S3;

[0077] S6. Calcinate chromium hydroxide at high temperature to obtain chromium oxide green products.

[0078] Control Example 1

[0079] A conventional method for preparing chromium salts by liquid - phase oxidation of chromite includes the following steps:

[0080] S1. Add chromite with a particle size of 300 mesh, sodium hydroxide, and water into a high - pressure reactor for mixing. Under stirring conditions, introduce oxygen into the high - pressure reactor for liquid - phase oxidation reaction. Among them, the mass ratio of sodium hydroxide to chromite is 4:1, the mass concentration of sodium hydroxide in the mixed solution is 50 wt.%, the temperature of the liquid - phase oxidation reaction is 240 °C, the stirring speed is 800 rpm, the oxygen partial pressure is 2.0 Mpa, and the reaction time is 3 h; after the reaction, relieve pressure and cool; then transfer the materials in the high - pressure reactor to a 90 °C constant - temperature oven for heat preservation and sedimentation for 4 h, and perform solid - liquid separation on the sedimented materials to obtain a supernatant and reaction slag;

[0081] S2. According to the ratio of water to reaction slag of 1:1 L / kg, wash the reaction slag with water and then separate to obtain a slag washing solution and iron - rich slag;

[0082] S3. Add barium hydroxide to the slag washing solution for precipitation reaction. Among them, the molar ratio of sodium chromate to barium hydroxide in the slag washing solution is 1:1, the reaction temperature is 60 °C, stirring is required during the precipitation reaction, the stirring speed is 300 rpm, the precipitation reaction time is 20 min, and after the reaction, separate to obtain barium chromate precipitate and an alkaline solution containing sodium aluminate;

[0083] S4. Add hydrochloric acid to dissolve the barium chromate precipitate, and then add a reducing agent ethanol for reduction reaction to obtain a mixed solution of chromium chloride and barium chloride. Among them, the molar ratio of hydrochloric acid to barium chromate is 5:1, the liquid - solid ratio of hydrochloric acid and ethanol to barium nitrate is 5:1, the ethanol addition amount is 3 times the theoretical amount, the reduction reaction temperature is 60 °C, and the molar ratio of hydrochloric acid to ethanol is 20:3;

[0084] S5. Adjust the pH value of the mixed solution to 8 - 9 with a 10% sodium hydroxide solution by mass fraction to completely precipitate chromium in the form of chromium hydroxide, and separate to obtain chromium hydroxide and a solution rich in barium chloride;

[0085] S6. Calcinate chromium hydroxide at high temperature to obtain chromium oxide green product.

[0086] Detection and analysis

[0087] 1) Detection of the leaching rate of chromite:

[0088] Determination of hexavalent chromium content: Determine the content of Cr(VI) in the leaching solution of chromite by diphenylcarbazide spectrophotometry. First, take an appropriate amount of the leaching solution of chromite and make it up to the mark in a volumetric flask. Add an appropriate amount of diluent to a 50 mL colorimetric tube, add water to the scale line, then add 0.5 mL of sulfuric acid (1:1), phosphoric acid (1:1) and 2 mL of chromogenic reagent in sequence, shake well, let it stand for 10 min for color development, then measure the absorbance value, calculate the content according to the standard curve, and then calculate the chromium conversion rate η according to the dilution factor: η = (n × m / V1 × V2 × 10 -6 ) / M × 100%

[0089] In the formula, n - dilution factor of the solution, m - mass of Cr(VI) in the colorimetric tube (μg), V1 - volume of the test solution (mL), V2 - total volume of the leaching solution (mL), M - Cr content in chromite (g).

[0090] The chromium leaching rate is the ratio of the measured mass of hexavalent chromium to the mass of chromium in chromite.

[0091] The measured chromium leaching rate in Example 1 is 97.97%. The chromium leaching rates in Example 2 and Example 3 are 80.3% and 89.6% respectively. The chromium leaching rate in Control Example 1 is 64.76%.

[0092] 2) Detection of the conversion rate of sodium chromate to barium chromate in the washing residue solution

[0093] According to the determination method of hexavalent chromium in 1), respectively determine the content of Cr(VI) in the washing residue solution before the conversion reaction and the content of Cr(VI) in the supernatant after the conversion of sodium chromate to barium chromate, and thus the conversion rate of sodium chromate to barium chromate can be determined.

[0094] The measured conversion rate of sodium chromate to barium chromate in the S3 washing residue solution of Example 1 is 98.1%. The measured conversion rates of sodium chromate to barium chromate in the S3 washing residue solutions of Example 2 and Example 3 are 98.4% and 90.3% respectively. The measured conversion rate of sodium chromate to barium chromate in the S3 washing residue solution of Control Example 1 is 73.6%.

[0095] 3) Detection of the chromium conversion rate in barium chromate

[0096] The total Cr content of the sample was determined by the potassium permanganate oxidation-diphenylcarbazide spectrophotometric method (GB / T 7466-1987). Add the water sample to be tested into a 150 mL conical flask, adjust it to neutral, put in a magnetic stirrer, and successively add 0.5 mL of sulfuric acid (1:1) and phosphoric acid (1:1). Add deionized water to about 50 mL, add 2 drops of 4% potassium permanganate solution, shake well, place it on a heating electric furnace, and boil until it is about 20 mL. Take it off, let it stand and cool, add 1 mL of 20% urea solution with a pipette, shake well, and add 2% sodium nitrite solution drop by drop with a dropper until the purple-red color just fades. Transfer it to a 50 mL colorimetric tube, add deionized water to the mark, add 2 mL of the color-developing agent, shake well, let it stand and develop color for 10 min, then measure the absorbance and calculate the total chromium content. As shown in the following formula:

[0097] Total Cr content = n×m / V1×V2×10 -6

[0098] In the formula, V1 - volume of the test solution (mL), n - dilution factor of the solution, V2 - total volume of the original solution (mL), m - mass of Cr in the colorimetric tube (μg).

[0099] The chromium conversion rate is the ratio of the total Cr content in the liquid phase after the reaction to the Cr content in barium chromate.

[0100] It was measured that in S4 of Example 1, the chromium conversion rate was 91.2%. In S4 of Example 2 and Example 3, the chromium conversion rates were 73.4% and 92.6% respectively.

[0101] 4) After the supernatant was used for the cyclic liquid-phase oxidation reaction, the leaching rate determination steps of chromite were the same as those in 1).

[0102] The material after heat preservation and sedimentation in Example 1 was subjected to solid-liquid separation to obtain the supernatant and the reaction residue. The supernatant was directly returned to the high-pressure reaction kettle for recycling in the leaching of chromite. It was measured that the leaching rate of chromium in ten consecutive cycles was above 96%. The supernatant returned in Example 2 and Example 3 was used for the cyclic leaching of chromite, and the leaching rate of chromium in ten consecutive cycles was above 93%.

[0103] The material after heat preservation and sedimentation in Comparative Example 1 was subjected to solid-liquid separation to obtain the supernatant and the reaction residue. The supernatant was directly returned to the high-pressure reaction kettle for recycling in the leaching of chromite. It was measured that the decline in the leaching rate of chromium in three consecutive cycles was significant, which were 68.5%, 48.3% and 22.5% in turn, and the leaching of chromite could no longer be achieved from the fourth time.

[0104] 5) Determination of chromium content in chromium oxide green products

[0105] According to the national standard GB / T20785-2006, the chromium content (calculated as Cr2O3) in the chromium oxide green produced in Example 1 was 99.58%. The chromium contents in the chromium oxide green produced in Example 2 and Example 3 also exceeded 99.3%, and the product quality was excellent. The chromium content in the chromium oxide green produced in Comparative Example 1 was only 97.83%.

[0106] In summary, for the method for preparing chromium salts by liquid-phase oxidation of chromite in the present invention, firstly, by introducing NaNO3 into the traditional sodium-based sub-molten salt liquid-phase oxidation reaction system of chromite, a ternary sub-molten salt mixed system with the reaction medium of NaOH-NaNO3-H2O was constructed; during the whole leaching process, sodium nitrate was first reduced to sodium nitrite, and as the reaction proceeded, sodium nitrite was oxidized to sodium nitrate by oxygen, and sodium nitrate was not consumed during the whole reaction but served as a carrier for oxygen transportation; the synergistic effect of NaNO3 and O2 accelerated the destruction and decomposition of the chromite spinel structure and strengthened the leaching process of chromite. Compared with the binary sub-molten salt mixed medium of NaOH-H2O, the chromium leaching rate could be increased by about 10% to 35% under the same conditions; secondly, after introducing NaNO3 into the NaOH-H2O reaction medium, the properties of the mixed system were changed, resulting in a change in the distribution of sodium chromate leached from chromite between the slag / liquid two phases. Sodium chromate was transferred from being present in the leaching solution to being present in the slag phase, so that it was no longer necessary to first extract chromium from the leaching solution after heat preservation sedimentation separation to achieve chromium / alkali separation. The small amount of chromium salts remaining in the leaching solution would not inhibit the leaching rate of chromium in the next batch of incoming chromite by the recycled leaching solution, ensuring the effective utilization of the medium; thirdly, by using Ba(NO3)2 with greater solubility to replace slightly soluble Ba(OH)2 as the transfer reagent for chromium salts, the defect that the precipitation conversion reaction rate between CrO4 2- and Ba 2+ was slow and time-consuming due to the common ion effect causing the decrease in the solubility of Ba(OH)2 in a high-alkali medium was overcome, and the separation efficiency of chromium / alkali was greatly improved; fourthly, by adopting the process of dissolving with nitric acid and reducing barium chromate with ethanol to prepare chromium salts, that is, using HNO3 instead of HCl as the dissolution medium, the requirements for the material of the equipment were more relaxed; and when chromium precipitated in the form of chromium hydroxide, the entrained NO3 -It can be synchronously decomposed and removed during the preparation of subsequent products (such as when chromium hydroxide is calcined to produce chromium oxide green), making the obtained product of better quality. Fifthly, after completely precipitating chromium in the form of chromium hydroxide and concentrating the liquid phase obtained by solid-liquid separation, the obtained barium nitrate can be recycled and used as a precipitant for converting sodium chromate to barium chromate in the slag washing liquid, effectively realizing recycling. Finally, in the method for preparing chromium salts by liquid-phase oxidation of chromite of the present invention, no barium salt products are by-produced, breaking through the restriction of the technical defect of the poor outlet of chromium-containing barium salt hazardous waste, realizing the recycling of barium resources and caustic alkali in the reaction system, and sodium nitrate is only used as a carrier for oxygen transportation without consumption, meeting the requirements of efficient, stable and continuous leaching of chromite, and the process flow is more compact and simple, with lower labor intensity, meeting the strategic requirements of the national green circular economy. Generally speaking, this improved process inputs the bulk chemical nitric acid with low price and by-produces sodium nitrate products with higher added value, with reasonable technical economy and more promising industrial application prospects, and has the value of popularization and application in the field of chromium salt production technology.

[0107] The above embodiments are only preferred embodiments given to fully illustrate the present invention, and the protection scope of the present invention is not limited thereto. Equivalent substitutions or transformations made by those skilled in the art on the basis of the present invention are all within the protection scope of the present invention.

Claims

1. A method for preparing chromium salts by liquid-phase oxidation of chromite, characterized in that, It includes the following steps: S1. Under the condition of an oxygen atmosphere with an oxygen partial pressure greater than 1 MPa, chromite, sodium hydroxide, sodium nitrate and water are mixed for a liquid-phase oxidation reaction. After depressurization and cooling, heat preservation and sedimentation are carried out, and solid-liquid separation is performed to obtain supernatant and reaction slag. Among them, the reaction medium of the liquid-phase oxidation reaction is a ternary sub-molten salt mixed system of NaOH-NaNO3-H2O, and the temperature of the liquid-phase oxidation reaction is between 220 °C and 260 °C. The supernatant is directly returned to the high-pressure reaction kettle for recycling in the leaching of chromite, and it is measured that the leaching rate of chromium in ten consecutive cycles is above 96%. S2. The reaction slag is leached and separated to obtain a leached slag solution and an iron-rich slag. S3. Barium nitrate is added to the leached slag solution for a precipitation reaction, and barium chromate and dilute alkali solution are separated. The temperature of the precipitation reaction is between 30 °C and 70 °C. S4. Nitric acid is added to barium chromate, and then ethanol is added for a reduction reaction to obtain a mixed solution of chromium nitrate and barium nitrate. The temperature of the reduction reaction is 30 °C to 70 °C. S5. The pH value of the mixed solution is adjusted, and chromium hydroxide and a solution rich in barium nitrate are separated. S6. The chromium hydroxide is calcined at high temperature to obtain a chromium salt product.

2. The method for preparing chromium salts by liquid-phase oxidation of chromite according to claim 1, characterized in that, In S1, the mass ratio of sodium hydroxide to chromite is 3:1 to 6:1; the mass ratio of sodium salt to chromite is 0.4:1 to 1:1; in the mixed solution, the mass concentration of sodium hydroxide is 45% to 60%.

3. The method for preparing chromate by liquid-phase oxidation of chromite according to claim 1, wherein In S1, the oxygen partial pressure is between 1.4 MPa and 2.2 MPa; the reaction time is between 2 h and 5 h.

4. The method for preparing chromium salts by liquid-phase oxidation of chromite according to claim 1, characterized in that, In S1, the mass ratio of sodium nitrate to chromite is 0.8:1; the mass ratio of sodium hydroxide to chromite is 4:1; in the mixed solution, the mass concentration of sodium hydroxide is 50%.

5. The method for preparing chromate by liquid-phase oxidation of chromite according to claim 4, wherein In S1, the temperature of the liquid-phase oxidation reaction is 240 °C; the oxygen partial pressure is 2 MPa; the reaction time is 3 h.

6. The method for preparing chromium salts by liquid-phase oxidation of chromite according to claim 1, characterized in that, In S3, the molar ratio of sodium chromate to barium nitrate in the leached slag solution is 0.9:1 to 1.1:

1.

7. The method for preparing chromium salts by liquid-phase oxidation of chromite according to claim 1, characterized in that, In S4, the molar ratio of nitric acid to barium chromate is 4:1 to 6:1; the liquid-solid ratio of nitric acid and ethanol to barium chromate is 4:1 to 7:1 L / kg.

8. The method for preparing chromate by liquid-phase oxidation of chromite according to claim 1, characterized in that, In S3, the dilute alkali solution is a dilute alkali solution rich in sodium aluminate and sodium nitrate. The method further includes: adding a sodium silicate slurry to the dilute alkali solution rich in sodium aluminate and sodium nitrate for a precipitation reaction, and separating to obtain a dilute alkali solution rich in sodium nitrate and a sodium aluminosilicate product. The dilute alkali solution rich in sodium nitrate is evaporated and concentrated to obtain a sodium nitrate product and an evaporation crystallization mother liquor. The evaporation crystallization mother liquor is merged with the supernatant in S1, supplemented with alkali, and then returned to participate in the liquid-phase oxidation reaction.

9. The method for preparing chromate by liquid-phase oxidation of chromite according to claim 1, wherein, In S5, the pH value of the mixed solution is adjusted to 8 - 9 using a sodium hydroxide solution.

10. The method for preparing chromate by liquid-phase oxidation of chromite according to claim 1, wherein, In S5, the solution rich in barium nitrate is evaporated and concentrated, and the obtained barium nitrate participates in the precipitation reaction of the leached slag solution in S3.

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